The contribution of ion channels to shaping macrophage behaviour.

The recognised role of Na + channels in cell physiology is the propagation of an action potential and depolarization of the plasma membrane. It would, therefore, appear that these channels would not play a role in macrophage biology, see below Electrical Excitability section. However, seven of the nine voltage-gated Na + channel (Na v ) family members (Na v 1.1, Na v 1.3–1.7, and Na v 1.9) ( Carrithers et al., 2007 ; Sun et al., 2019 ) have been detected in macrophages, with many of them being associated with intracellular membranes. Na v 1.5 is expressed in late endosomes and phagolysosomes of macrophages and is reported to regulate phagocytosis and endosomal acidification through triggering Na + efflux and intra-endosomal pH reduction ( Carrithers et al., 2007 ). A later study showed Na v 1.5 to regulate mycobacteria phagocytosis and phagosome maturation through organelle polarization and localized Ca 2+ oscillations ( Carrithers et al., 2011 ). Another Na + channel, Na v 1.6, has also been reported to regulate phagocytosis in macrophages and microglia ( Craner et al., 2005 ), and it localizes to cytoskeletal filaments and endoplasmic reticulum ( Carrithers et al., 2007 ). Several studies have also suggested a role for Na v -mediated regulation of macrophage phagocytosis in inflammatory disease progression including myelin degradation in multiple sclerosis ( Black et al., 2013 ), experimental autoimmune encephalomyelitis ( Craner et al., 2005 ) and atherosclerotic lesion development ( Sun et al., 2019 ). Neosaxitoxin, a local anaesthetic and at nanomolar concentrations a selective inhibitor of six Na v isoforms (Na v 1.1–1.4, 1.6, and 1.7) ( Walker et al., 2012 ), has also been shown to inhibit LPS-induced release of NO produced by iNOS, TNFα and IL-1β from macrophages, thus indicating a role for Na v in inflammatory mediator release from macrophages ( Montero et al., 2020 ). Decrease in pH has long been associated with acute and inflammatory diseases ( Kellum et al., 2004 ). The Na + -permeable channels, acid-sensing ion channels (ASICs), ASIC1, ASIC2a, and ASIC3, have also been identified in macrophages, with ASIC1 immunolocalising to the nucleus, and ASIC2a and ASIC3 being mostly found throughout cell cytoplasm ( Ni et al., 2018 ). Of significance, extracellular acidosis and ASIC activation were found to promote migration, endocytosis, cytokine production and apoptosis in macrophages ( Ni et al., 2018 ; Kong et al., 2013 ; Foster et al., 2021 ). It is rather fitting that some classical physiological changes associated with inflammation, such as heat and low pH, may be important sensory cues for macrophages, see the following sections.

As would be predicted, intracellular Ca 2+ regulates a variety of functions in macrophages, ranging from cytokine production to phagocytosis ( Vaeth et al., 2015 ). CRAC channels are highly selective for Ca 2+ compared to other cations and are activated by depletion of ER Ca 2+ stores ( Shim et al., 2015 ). Briefly, depletion of ER Ca 2+ stores leads to conformational changes in stromal interaction molecules (STIM) 1 and 2 located on the ER. These conformational changes allow STIM 1 and 2 to oligomerize and form clusters “puncta”, into which CRAC is recruited, resulting in SOCE. ORAI1 is the gene encoding the CRAC channel protein, but the two terms are often used interchangeably. Chauhan et al. reported ORAI1/CRAC to mediate the basal Ca 2+ influx in macrophages ( Chauhan et al., 2018 ). A tight coupling between CRAC channels, SOCE, and K Ca 3.1 channels is thought to exist in macrophages, where Ca 2+ -activated K Ca 3.1 channels amplify the CRAC current, prolonging Ca 2+ signalling and store repletion ( Gao et al., 2010 ). Loss-of-function mutations in ORAI1 and STIM1 genes abolish CRAC channel function and SOCE in immune cells, resulting in CRAC channelopathies ( Shaw and Feske, 2012 ). However, controversies regarding CRAC/SOCE and macrophage functions exist. STIM1 was reported to be essential for the FcγR-mediated Ca 2+ entry, phagocytosis, inflammatory cytokine production and autoimmune inflammation ( Braun et al., 2009 ). Another study identified CRAC channels as a principal link between chemokine receptor stimulation and the activation of focal adhesion-related tyrosine kinase Pyk2 and MAPK pathways leading to inflammatory mediator secretion in macrophages ( del Corno et al., 2001 ). Loss of Ca 2+ entry via CRAC/TRPC1 was found to induce ER stress and apoptosis in macrophages ( Conceicao et al., 2019 ). However, other studies have questioned the importance of Ca 2+ ( Hishikawa et al., 1991 ; Greenberg et al., 1991 ; McNeil et al., 1986 ) and SOCE in macrophage function ( Vaeth et al., 2015 ). Recently it has been shown that M1 and M2 macrophages exhibit different Ca 2+ entry mechanisms, with M2 macrophages being associated with ORAI1, and TRPC1 channels being associated with M1 macrophages ( Nascimento Da Conceicao et al., 2021 ). Loss of either TRPC1 or ORAI1 limited the functionality of M1 and M2 macrophages respectively. Three other TRP family channels, TRPA1, TRPM7, and TRPP2, have also been reported to mediate the non-ORAI1 component of SOCE in macrophages ( Liu et al., 2021 ). This again highlights the important interplay between ion channels and macrophage functions. In addition to already discussed CRAC channels, L-type voltage-gated Ca 2+ channels have been identified in macrophages and suggested to modulate macrophage responses to infection, with roles in reactive oxygen species generation, survival and autophagy ( Antony et al., 2015 ; Sharma et al., 2016 ). Inositol 1,4,5-trisphosphate receptors, IP3Rs, are ubiquitous Ca 2+ channels mediating Ca 2+ release primarily from endoplasmic reticulum stores, have also been implicated in macrophage Ca 2+ signalling, polarization ( Liu et al., 2019 ) and cytokine production ( Staats et al., 2016 ). However, how many of these functions are dependent on SOCE activation will require further research ( Sampieri et al., 2018 ). Finally, Ca 2+ -permeable two-pore channels, TPCs, found on membranes of the endolysosomal system, together with TRPML channels (see below), have been found to regulate endolysosomal osmo-/mechano-sensation and volume regulation ( Chen et al., 2021 ) and drive macrophage phagocytosis ( Davis et al., 2020 ). TRP channels also account for a significant part of research concerning Ca 2+ -permeable ion channels in macrophages, although they can show varying degrees of selectivity for different cations ( Table 3 ). A summary of Transient Receptor Potential (TRP) ion channels identified in macrophages to date. PLC, phospholipase C; DAG, diacylglycerol; PIP 3 , phosphatidylinositol (3,4,5)-trisphosphate; PIP 2 , phosphatidylinositol 4,5-bisphosphate; (c)ADPR (cyclic) adenosine diphosphate ribose; ROS, reactive oxygen species; ATP, adenosine triphosphate. TRP channel families are; A, ankyrin; C, canonical; M, melastatin; ML, mucolipin; N, no mechanoreceptor potential C; P, polycystic; S, soromelastatin; V, vanilloid; VL, vanilloid-like.

In addition to the members of the large ion channel families a number of other ion channels have been reported to influence macrophage behaviour. Proton voltage-gated channel, H v 1, is reported to regulate the respiratory burst during phagocytosis, limit membrane depolarization and intracellular acidosis in macrophages ( DeCoursey, 2010 ) ( Ramsey et al., 2009 ). In microglia, H v 1 was reported to enhance reactive oxygen species generation contributing to brain damage at early time points after ischemic stroke ( Wu et al., 2012 ). Voltage-Dependent Anion Channels (VDAC), initially described as mitochondria-associated porins ( Schein et al., 1976 ), have been identified on plasma ( Bàthori et al., 1999 ) and phagosomal membranes as well. In addition to the pivotal roles of mitochondria in cellular metabolism ( Ramond et al., 2019 ), in macrophages VDACs appear to play a more negative role during infection, being crucial for bacterial growth and survival within phagosomes. The channels were reported to colocalize and interact with bacterial proteins and aid bacterial lipid translocation ( Danelishvili et al., 2017 ). Several studies report the presence of functional volume-regulated anion channels, VRACs, in macrophages. Burrow and others suggested VRAC to be a functional link between sphingolipid and purinergic signalling systems in macrophages, with roles in phagocytosis, migration and inflammation ( Burow et al., 2014 ). A recent study by Green et al. reports LRRC8A, a pore-forming subunit of VRAC, to be essential for hypotonicity-induced NLRP3 inflammasome activation in macrophages ( Green et al., 2020 ). Cl − intracellular channels (CLIC), in particular CLIC1, is a family of ion channels permeable to Cl − ions in macrophages, which was reported to regulate macrophage function through modulation of phagosomal acidification and reactive oxygen species production ( Jiang et al., 2012 ). Additionally, iNOS-induced nuclear translocation of another family member, CLIC4, was reported to be essential for macrophage transition from a pro- to anti-inflammatory state ( Malik et al., 2012 ). Furthermore, both CLIC1 and CLIC4 have been suggested to be involved in the NLRP3 inflammasome regulation ( Domingo-Fernández et al., 2017 ). One more Cl − channel found in macrophages is transmembrane protein 16A, TMEM16A ( Shi et al., 2020 ), which has been reported to mediate microRNA9-induced TGF-β/Smad2 signalling implicated in LPS-induced sepsis ( Zhen et al., 2019 ; Sui et al., 2020 ; Bai et al., 2021 ). A number of studies have pointed to the presence of hemichannels and gap junctions in macrophages ( Levy et al., 1976 ; Porvaznik and MacVittie, 1979 ; Eugenín et al., 2003 ; Jara et al., 1995 ). Plasma membrane hemichannels are one half of gap junction channels which allow direct ion transfer between cytoplasms of contacting cells and thus allow metabolic and electrical synchronization between the contacting cells. Hemichannels Connexin 43 ( Eugenín et al., 2003 ; Jara et al., 1995 ; Beyer and Steinberg, 1991 ; Dosch et al., 2019 ), Connexin 37 ( Kwak et al., 2002 ; Lu et al., 2018 ) and Pannexin 1 ( Chen et al., 2019 ; Adamson and Leitinger, 2014 ; Pelegrin and Surprenant, 2006 ) have been identified in macrophages to date and proposed to affect macrophage activation and a range of functions ( Rodjakovic et al., 2021 ), including macrophage migration ( Shen et al., 2018 ), ATP signalling ( Pelegrin and Surprenant, 2006 ) and cytokine production ( Dosch et al., 2019 ). Importantly, growing evidence suggests that the presence of hemichannels and gap junctions in macrophages is crucial for many physiological and pathophysiological processes, such as heartbeat propagation ( Hulsmans et al., 2017 ), immunomodulatory communication in the lungs ( Westphalen et al., 2014 ) and intercellular communication in intestine ( Al-Ghadban et al., 2016 ). Finally, a novel mechanically-activated cation channel Piezo1 has been reported in macrophages. Piezo1 roles are proposed to include mechanical stimuli sensing, potentially via Piezo1 and integrins crosstalk and modulation of macrophage polarization ( Atcha et al., 2021a ; Atcha et al., 2021b ), toll-like receptor 4-mediated enhancement of macrophage bactericidal activity ( Geng et al., 2021 ) and macrophage migration (unpublished data). Recently, this channel has also been demonstrated to be a key regulator of erythrocyte phagocytosis by macrophages and iron metabolism ( Ma et al., 2021 ). Since iron metabolism, inflammation and immunity are intimately linked ( Ganz and Nemeth, 2015 ), this area of research deserves further consideration. Piezo1 is considered to be a promising candidate for therapeutic intervention against atherosclerosis and other cardiovascular diseases ( Umar Shinge et al., 2022 ), and it will be interesting if this channel along with other mechano-sensing channels represent a mechanism by which physical stimuli together with chemical stimuli, such as PAMPs and DAMPs, can be integrated.

While the concentrations of ions on each side of a membrane are predominantly determined by transporters, passive movement of ions across the membrane is controlled by ion channels. The principal difference between channels and transporters is that “open” ion channels allow passive diffusion of ions down their electrochemical gradient, whereas transporters depend on the use of energy to move ions, usually, against their gradient ( Purves et al., 2001 ). The membrane potential is a measurement of the difference in electrical potential across a membrane. The gold standard for measuring membrane potential involves the use of electrophysiological methods, however, due to the highly dynamic nature of macrophage plasma membranes, particularly activated macrophages, electrophysiological measurements using patch-clamp electrodes can be a challenge. Nevertheless, several studies, using a variety of electrophysiological methodologies and configurations, have reported the resting plasma membrane potential for non-activated monocytes and macrophages. Gallin and Gallin reported the resting plasma membrane potential of human macrophages as −14.5 ± 5 mV ( Gallin and Gallin, 1977 ), while another study reported the membrane potential of human macrophages as −42 ± 14 mV ( Mukherjee et al., 1983 ). In both these studies cells were isolated from human peripheral blood, although it appears that in the latter study macrophages spent less time in culture. Recordings of the resting plasma membrane potential of macrophages isolated from different species, tissues and cell lines, and using various isolating protocols have also been made ( Castranova et al., 1979 ; dos Reis et al., 1979 ; Gallin and Livengood, 1980 ; Gallin, 1981 ; Ince et al., 1983 ). The picture that emerges is a high degree of variation between different macrophage populations, and it has been speculated that this may reflect the functional characteristics of the macrophage populations investigated ( Mukherjee et al., 1983 ; Zhang et al., 2010 ). In line with this idea, recent work using a differentiated macrophage cell line, THP-1 cells, has reported that M2 macrophages have a lower resting plasma membrane potential than M1 macrophages ( Li et al., 2016 ). The significance of the membrane potential is that it impacts certain biological processes, e.g. the driving force for Ca 2+ entry into a cell when channels selective for this ion are open and membrane lipid dynamics ( Zhou X. et al., 2015 ). Therefore, membrane potential can influence the activation thresholds, efficiency and magnitude of cellular responses ( Kadir et al., 2018 ), and could have significant effects on macrophage activity/function regulating cell signalling molecules and cascades located within or in close proximity to the membrane. In addition to plasma membrane, intracellular membrane potentials can also regulate cellular and organelle functions, an example being the role of the proton channel Hv1 in the regulation of NADPH oxidase-2 (NOX2) activity in granulocytes ( DeCoursey, 2016 ). In macrophages, mitochondrial membrane potential is known to regulate cellular functions ( Wang et al., 2021 ), and it is expected that the membrane potential of other organelles ( Figure 1 ) will have regulatory roles in macrophages too ( Selezneva et al., 2021 ). However, the majority of research on ion channels in macrophages is focused on the plasma membrane. Locations of functional ion channels reported in macrophages. Mitochondria and nucleus are double-membrane-bounded organelles with ion channels located in the inner and/or outer membranes. The outer nuclear membrane is continuous with the ER. VDAC, Voltage-Dependent Anion Channel; GABA-A, Gamma-Aminobutyric Acid type A receptor; 5-HT3, 5-Hydroxytryptamine3 receptor; nACh, Nicotinic Acetylcholine receptor; CRAC, Ca 2+ Release-Activated Channel; H v , Proton voltage-gated channel; VRAC, Volume-Regulated Anion Channel; Ca v , Ca 2+ voltage-gated channel; TRP, Transient Receptor Potential; K v/Ca , K + voltage-gated and/or Ca 2+ -activated channel; BK, Big K + channel; Na v , Na + voltage-gated channel; TMEM, Transmembrane Protein; IP3R, Inositol 1,4,5-trisphosphate receptor; ER, Endoplasmic Reticulum; ASIC, Acid-Sensing Ion Channel; CLIC, Cl − Intracellular Channel. Created with BioRender.com . It is evident that ion channels regulate ion flux and membrane potential and therefore signalling events at or local to membranes. These signalling outcomes will be dependent on the type of ion channel(s) being activated. Therefore, starting with potassium channels, in the next section we will review the research linking members of different ion channel families with signalling and activation states of macrophages.

The gold standard technique for identifying functional ion channels in a membrane is patch clamping electrophysiology. Being a common technique used to obtain electrophysiological data from many cell types, it can be technically challenging in cells which have dynamic membranes, as is the case for activated macrophages ( Pi et al., 2014 ). Macrophages are heterogeneous populations of cells, and, since patch clamping experiments are carried out on single cells, investigators need to be aware of sampling issues, i.e. macrophages which are amenable to electrophysiological recordings may not represent bulk phenotypic behaviour of the macrophage population. Interestingly, research in neurons has developed Patch-seq protocols, where electrophysiological and transcriptomic data can be collected from the patched cell ( Lipovsek et al., 2021 ). To the best of our knowledge this technique has not been investigated in macrophages to date, however, this could represent a powerful technique by which macrophage functional phenotypes could be linked to gene expression profiles. Cells have also been patch clamped in-situ e.g. neurons in brain slices. It could therefore be possible to patch clamp macrophages in tissue slices, e.g. spleen, granuloma, or even tumours to investigate tumour associated macrophage (TAM) activity. This methodology may also overcome some of the macrophage plasticity issues encountered when investigating these cells in-vitro . One limitation to patch clamping is that it is low throughput, although automation of this technique is possible for some cell types. To investigate ion-channel effects in multiple cells simultaneously in vitro, fluorescent dyes can be used, however, these dyes require to be of sufficient spatial and temporal resolution to characterise ion channel dynamics ( Haworth and Brackenbury, 2019 ). Furthermore, ion channels may have functions which are not dependent on their “channel” function. An example of this is the recent demonstration that the role of voltage-gated Ca 2+ channels in regulating T-lymphocyte function is independent of cation flux ( Cahalan, 2022 ). To investigate this possibility in macrophages a combination of electrophysiological, genomic, proteome and interactome methodologies will be required. Care must also be taken when using pharmacological and genetic approaches to investigate ion channel functions. Channel blockers/openers must be chosen carefully and their relative selectivity for a particular target should be considered. This is especially important in ion channel research where the selectivity of pharmacological tools is often dependent on the subunit composition and dynamics of the ion channel. Also, it must be recognised that some ion channels can be found at multiple locations within the cell, where they may play different roles in cellular pathways. This confounding factor must be taken into account when using pharmacological and genetic approaches to manipulate ion channel activities and investigate their downstream effects. The coordination of different ion channels in a single cell to orchestrate a particular cellular function is well recognised in cell types such as neurons and smooth muscle. Evidence suggests such coordinated behaviour of ion channels is also present in macrophages. One example would be K + channel opening creating a driving force for Ca 2+ entry into the cells (see above). A number of ion channels, TWIK2, TRP, P2X, VRAC, and CLIC channels, are reported to regulate NLRP3 inflammasome ( Li et al., 2021 ). Ion channels can also regulate cell volume changes which activate NLRP3. TRPV2 and TRPM7 in macrophages are thought to control the RVD ( Liedtke, 2006 ; Compan et al., 2012 ; Santoni et al., 2015 ). Moreover, TRPM2-mediated Ca 2+ influx has been suggested to be important for inflammasome activation ( Zhong et al., 2013 ). Cytosolic K + efflux is a common trigger involved in inflammasome activation. P2X7 activation was reported to promote the efflux of K + and activate NLRP3 in macrophages ( Ferrari et al., 1997 , 2006 ; Mariathasan et al., 2006 ; Piccini et al., 2008 ; Barberà-Cremades et al., 2012 ; Muñoz-Planillo et al., 2013 ) ( Pétrilli et al., 2007 ). Recently, Di et al. identified a K + channel TWIK2 as another mediator of K + efflux and inflammasome activation in macrophages ( Wu et al., 2021 ) ( Di et al., 2018 ). Finally, intracellular Cl − efflux, an event downstream of K + efflux, is also reported to be critical upstream event for inflammasome activation. Two Cl − selective ion channels, VRAC and CLIC, have been reported to participate in inflammasome regulation in macrophages ( Green et al., 2020 ) ( Domingo-Fernández et al., 2017 ). The important point to make is that combinations of ion channels are involved in inflammasome activation; and these combinations will be dependent on the stimuli. Phagocytosis and polarization are other examples of macrophage functions discussed in this review, which are modulated by integrated ion channel networks ( Table 1 ). It would not be surprising if the effects of individual ion channels on macrophage behaviour are in fact dependent on the actions of other channels. If so, the integration of these ion channel “circuits” with traditional cell signalling pathways in macrophages will need to be investigated. In neurons the different composition of voltage-gated ion channels results in different firing properties ( Berger and Crook, 2015 ) ( Drion et al., 2015 ) but as the name suggests these ion channels are regulated my membrane voltage i.e. neurons are electrically excitable. Macrophage express an array of functional ion channels, some of which are voltage-gated. This leads us to the controversial question, are macrophages electrically excitable? While macrophages are generally considered non-excitable cells, the presence of gap junction channels Connexins in macrophages allows cardiac macrophages to facilitate electrical conduction through the distal atrioventricular node, where conducting cells densely intersperse with elongated macrophages expressing Connexin 43. When coupled to spontaneously beating cardiomyocytes via connexin-43-containing gap junctions, cardiac macrophages have a negative resting membrane potential and depolarize in synchrony with cardiomyocytes ( Hulsmans et al., 2017 ). Moreover, computer simulations to quantitatively assess plausible effects of macrophages on electrotonically coupled cardiomyocytes showed that macrophages can depolarize resting cardiomyocytes, shorten early and prolong late action potential duration, with effects depending on coupling strength and individual macrophage electrophysiological properties, in particular resting membrane potential and presence/absence of K ir 2.1 channel ( Simon-Chica et al., 2021 ). To study this further, in-situ measurements of electrophysiological properties of macrophages in contact with other cells will have to be performed. As indicated in the introduction, macrophages not only have central roles in immune and inflammatory responses but participate in general homeostatic responses in the body ( Murray and Wynn, 2011 ; Wynn et al., 2013 ). Therefore, it is not surprising that macrophage dysfunction may contribute to numerous pathologies and make this cell a prime target for therapeutic intervention. However, because of the central role of the macrophage in host defence, direct inhibition of macrophage functions is of limited value. A better therapeutic strategy would be to shape the macrophage actions, thus removing its aberrant effects while maintaining its desired functions. We suggest that the balance between macrophage pro-, anti-inflammatory and homeostatic responses is partly defined by ion channel actions. Therefore, could ion channel targeting drugs be used to fine-tune macrophage response in certain diseases? Outlined above, a number of ion channels have been demonstrated to regulate macrophage biology, and a number of drugs which target these channels, predominantly in non-immune tissues/cells, have entered clinical trials or been approved for various diseases, e.g. epilepsy, cardiac dysrhythmia and anxiety ( Imbrici et al., 2013 ; Priest and McDermott, 2015 ; Cojocaru et al., 2021 ) ( Table 4 ). Illustrated in Table 4 , drugs which target ion channels can be designed to block or open ion channels, as well as act as positive allosteric modulators. This together with various channel subunit compositions (both core and auxiliary) give researchers a large chemical space to explore when developing drugs targeting ion channels in macrophages and minimizing off target/tissue effects. Table 5 introduces the reader to several examples of diseases which may be amenable to targeting macrophage ion channels for their treatment. Therapeutic opportunities for macrophage ion channel-targeted drugs.

K + channels are the most widely distributed of all the ion channels and are classified into families and subfamilies based on their structure and properties ( Alexander et al., 2011 ). The main function of membrane K + channels is to control the membrane potential. In general, it appears that monocyte-macrophage differentiation and macrophage activation increase the diversity of K + channels expressed by the cell ( Vicente et al., 2003 ; Yoshida, 2017 ). Ca 2+ is a key second messenger in cell signalling with concentration, spatial and temporal parameters all contributing to information conveyed by this divalent cation. K + channels are important in responding to and regulating Ca 2+ fluxes in cells. For example, extracellular ATP activation of P2Y receptors results in intracellular Ca 2+ oscillations and promotes IL-6 transcription in macrophages ( Hanley et al., 2004 ). Ca 2+ -activated K + channels contribute to the intermediate signalling pathways associated with this event by mediating membrane potential oscillations ( Hanley et al., 2004 ). A tight coupling between the Ca 2+ -activated K + channel, K Ca 3.1, and store-operated Ca 2+ entry (SOCE) is reported to exist in macrophages. Activated by Ca 2+ influx through Ca 2+ release-activated Ca 2+ (CRAC) channels, K Ca 3.1 amplifies CRAC current, prolongs Ca 2+ signalling and Ca 2+ store refilling ( Gao et al., 2010 ). Macrophages infiltrating into inflamed tissues ( Strøbæk et al., 2013 ), atherosclerotic lesions ( Toyama et al., 2008 ) and transplanted organs have all been shown to express elevated levels of K Ca 3.1. Toyama and others demonstrated K Ca 3.1 inhibitors, TRAM-34 and clotrimazole, to significantly reduce macrophage infiltration into atherosclerotic plaques and consequently reduce atherosclerosis development in blood vessels ( Toyama et al., 2008 ). Similarly, Xu and others showed that blocking K Ca 3.1 shifts macrophage polarization towards M2 macrophage phenotype and stabilises atherosclerotic plaques reducing rupture ( Xu et al., 2017 ). Besides atherosclerosis, Chung and others showed that voltage- and Ca 2+ - activated K + ion channels are capable of influencing macrophage migration by altering the cell volume and shape with implications for neurodegenerative disorders such as HIV1-associated dementia ( Chung et al., 2002 ). Voltage- and Ca 2+ - activated K + channel blockers, charybodotoxin, margatoxin, agatoxin and apamin, were reported to inhibit macrophage migration in the brain, therefore, possibly affecting the levels of neuronal damage ( Chung et al., 2002 ). Taken together, K Ca 3.1 channels seem to have potential to modulate the intracellular signalling responses of macrophages and contribute to M1/M2 macrophage polarization. Following several years of controversies about the expression of voltage-gated potassium channels in macrophages ( Mackenzie et al., 2003 ; Park et al., 2006 ; Felipe et al., 2010 ), the work of Felipe and others established the presence of both K v 1.3 and K v 1.5 in macrophages and microglia, a type of resident macrophage found in the central nervous system. K v 1.3 and K v 1.5 form heterotetrameric functional channels ( Vicente et al., 2006 ; Villalonga et al., 2007 ; Villalonga et al., 2010 ), and depending on the composition of K v 1.3/K v 1.5 in this tetramer, the ion channels take on biophysical and pharmacological characteristics more closely resembling either K v 1.3 or K v 1.5 ( Vicente et al., 2006 ; Vicente et al., 2008 ; Villalonga et al., 2007 ; Villalonga et al., 2010 ; Felipe et al., 2010 ). This highlights the importance of ion channel subunit composition to channel functionality. K v 1.3 and K v 1.5 have distinctive channel kinetics, i.e. conductance, activation and inactivation rates, pharmacology and functions in immune cells ( Felipe et al., 2010 ) ( Table 2 ). In macrophages K v 1.3 is involved in setting the resting membrane potential ( Mackenzie et al., 2003 ; Felipe et al., 2010 ). Vicente and others reported the reduced activity of voltage-gated K + channels in macrophages inhibits inducible nitric oxide synthase (iNOS) expression ( Vicente et al., 2003 ). In microglia K v 1.3 blockers reduce production of proinflammatory factors IL-6, TNFα and the enzyme cyclooxygenase-2, protecting from microglia-mediated brain injury ( Peng et al., 2014 ; Fordyce et al., 2005 ). One possible hypothesis is that K v 1.3 is essential for the immune cell physiology, while K v 1.5 contributes to the fine tuning of immune cell responses. A summary of potassium (K + ) ion channels identified in macrophages to date. ATP, adenosine triphosphate. Inwardly-rectifying potassium channels allow K + to more easily enter the cell than exit and play a role in setting the membrane potential and are denoted by K ir . The inwardly-rectifying K ir 2.1 was reported to be the target of memantine’s anti-inflammatory effects ( Tsai et al., 2013 ), influence macrophage functional activity and was down-regulated by LPS ( Vicente et al., 2003 ). ATP-sensitive potassium channels (K ATP ) are also expressed in macrophages are composed of four pore-forming inwardly rectifying K ir 6.1/K ir 6.2 subunits and four regulatory sulfonylurea receptor subunits. K ATP channels in macrophages have been seen to modulate cell membrane potential properties, with blockers of the channels promoting a M2 phenotype ( Ling et al., 2013 ; Li et al., 2016 ), as well as regulate microglial activity ( Ortega et al., 2012 ). A two-pore domain K + channel (TWIK2), which can be activated by a number of chemical and physical parameters including pH, oxygen tensions and stretch, was identified as an essential upstream mechanism for the ATP-induced NLRP3 inflammasome activation ( Wu et al., 2021 ; Di et al., 2018 ). Research on Ca 2+ -activated K + channel in macrophages has also shed light on the role of ion channels in non-plasma membrane locations. The large conductance Ca 2+ - and voltage-activated K + channel, K Ca 1.1, also known as BK channel, has been reported to be present in both plasma and nuclear membranes in macrophages ( Yoshida, 2017 ; Selezneva et al., 2021 ). Plasma membrane BK channels were found to regulate a disintegrin and metalloprotease domain 17 (ADAM17) enzyme and, hence, the release of TNFα and IL-6 receptor-α from activated macrophages ( Yoshida, 2017 ), while previous studies had reported BK to be central in the NF-κB dependent inflammatory response of macrophages to bacterial LPS ( Papavlassopoulos et al., 2006 ). Interestingly, BK channels appear to be predominantly intracellular in resting macrophages and appear upregulated on the plasma membrane upon LPS stimulation ( Yoshida, 2017 ). BK channels located on the nuclear envelope (NE) were shown to regulate cAMP response element binding protein (CREB) phosphorylation in macrophages ( Selezneva et al., 2021 ). Moreover, this study reported that nuclear Ca 2+ and calmodulin dependent kinases II and IV were involved in regulation of CREB by nuclear BK channels in macrophages ( Selezneva et al., 2021 ). Similar findings were previously reported in hippocampal neurones ( Li et al., 2014 ). BK channels have been also found on the NE of microglia and were proposed to be involved in microglial activation, facilitating NO and cytokine production, potentially by regulating Ca 2+ and K + fluxes in the nucleus ( Yang et al., 2019 ). A recent study reported lysosomal BK channels in macrophages to be functionally coupled with transient receptor potential (TRP)ML, discussed below, and regulate large particle phagocytosis through modulating lysosomal exocytosis ( Sun et al., 2020 ). This study highlights an important phenomenon of ion channel coordination to facilitate cell signalling events. Finally, deficiency of transmembrane protein 175 (TMEM175), a K + channel associated with the endolysosomal system has also been shown to result in accelerated autophagosome-lysosome fusion in macrophages ( Cang et al., 2015 ).

TRP channels form a large superfamily of cation channels which are important cell sensors. Split into nine families; A, C, M, ML, N, P, S, V, and VL; these can be further divided into subfamilies and display diversity in their permeability to different cations and responses to different chemical and physical stimuli, see Table 3 , ( Venkatachalam and Montell, 2007 ; Bouron et al., 2015 ; Feng et al., 2014 ). When considering the variety of cellular functions macrophages have, and the dynamic tissue environments in which they perform these functions, it is not surprising that a number of TRP subfamilies have been demonstrated to play regulatory roles in macrophages ( Table 3 ) ( Link et al., 2010 ; Zhong et al., 2013 ; Schilling et al., 2014 ; Solanki et al., 2014 ; Zhou X. et al., 2015 ; Sun et al., 2015 ; Khalil et al., 2016 ; Chauhan et al., 2018 ; Wang et al., 2020 ). Taking the TRP family in alphabetical order, TRPA1 channel was reported to regulate the expression of pro-inflammatory neuropeptides, cytokines and chemokines, exert a mucosal protection in colitis ( Kun et al., 2014 ) and reduce atherosclerosis progression by suppressing macrophage M1 polarization ( Wang et al., 2020 ). Similarly, a recent study showed that ablation of TRPA1 exacerbates M1 macrophage infiltration and renal inflammation after renal ischemia-reperfusion injury in mice ( Ma and Wang, 2021 ). TRPC1 was found to be crucial for the proinflammatory responses to bacterial infections through stimulating protein kinase C α pathway, NF-κB/Jun kinase nuclear translocation and cytokine release ( Zhou Y. et al., 2015 ) ( Py et al., 2014 ). TRPC3 also contributes to macrophage M1 polarization ( Solanki et al., 2014 ; Vazquez et al., 2016 ; Chauhan et al., 2018 ) and appears to be an obligatory component of survival mechanisms in human and murine macrophages ( Tano et al., 2014 ). TRPC6 was suggested to have roles in phagocytosis and restore microbicidal function in compromised alveolar macrophages from cystic fibrosis patients ( Riazanski et al., 2015 ). In the melastatin family of TRP channels plasma membrane TRPM2 was shown to control chemokine production in monocytes, such as macrophage inflammatory protein-2 production (CXCL2) ( Yamamoto et al., 2008 ; Zong et al., 2022 ). Zhong et al. identified TRPM2 as a key factor linking oxidative stress and NLRP3 inflammasome activation ( Zhong et al., 2013 ). In this study TRPM2 deficient macrophages displayed impaired NLRP3 inflammasome activation and subsequently IL-1β secretion ( Zhong et al., 2013 ). A recent study identified a TRPM2–CD36 axis in macrophages as a critical molecular mechanism underlying atherogenesis ( Zong et al., 2022 ). However, there is some controversy whether TRPM2 promotes inflammation or protects against it. While reactive oxygen species activate TRPM2, one study ( Di et al., 2012 ) reported an ability of TRPM2 to inhibit reactive oxygen species production in macrophages and prevent endotoxin-induced lung inflammation. Similarly, another study reported that the absence of TRPM2 results in greater reactive oxygen species and inflammatory mediator production in macrophages, promotes macrophage M1 polarization and increases gastric inflammation ( Beceiro et al., 2016 ). Moving to other members of this TRP family, TRPM4 was reported to regulate the AKT signalling pathway, phagocytic function and macrophage survival in sepsis ( Serafini et al., 2012 ). Both TRPM7 and TRPM8 have been suggested to have roles in macrophage polarization. TRPM7 blockers were reported to prevent macrophage M2 polarization and inhibitory effects of IL-4 and M-CSF on TNFα production ( Schilling et al., 2014 ). TRPM8 in macrophages determines M1 and M2 actions by regulating TNFα and IL-10 production ( Khalil et al., 2016 ). TRPML channels belong to the mucolipin TRP subfamily and primarily localize to membrane-bounded vesicles along the endocytosis and exocytosis pathways. In the endolysosomal system TRPML2 was established to be primarily expressed in recycling endosomes, whereas TRPML3 and TRPML1 in early endosomes and late endosomes/lysosomes, respectively ( Sun et al., 2015 ; Plesch et al., 2018 ). A study by Samie et al. reported that TRPML1 knockout and blockade in macrophages causes inhibition of large particle phagocytosis, whereas TRPML1 overexpression and agonists facilitate the process ( Samie et al., 2013 ). The authors suggest a mechanism, where the binding of phagocytosis target by macrophage leads to PIP2-dependent TRPML1 activation and resulting Ca 2+ release induces lysosomal exocytosis ( Samie et al., 2013 ). Furthermore, TRPML1 together with BK channels was suggested to be involved in the fusion of phagosomes with lysosomes ( Dayam et al., 2015 ; Cao et al., 2015 ; Sun et al., 2020 ) and enhance the degradative lysosome function through transcription factor TFEB activation ( Gray et al., 2016 ). Interestingly, Sun and others reported that TRPML2 expression in macrophages significantly increases upon toll-like receptor activation, while the levels of other TRPML subfamily members do not seem to change with macrophage activation ( Sun et al., 2015 ). Sun and others also showed that TRPML2 knockout leads to reduced production of chemokines such as CCL2 and impaired recruitment of peripheral macrophages ( Sun et al., 2015 ). In support of this, a later study reported that TRPML2 agonist stimulates macrophage migration and the release of CCL2 from macrophages ( Plesch et al., 2018 ). Recently, a role for TRPML2 in endolysosomal viral trafficking and viral infection has been suggested ( Santoni et al., 2020 ; Chen et al., 2020 ). TRPML3 is thought to have a role in the regulation of endocytosis, membrane trafficking and autophagy ( Kim et al., 2009 ). A recent study reported TRPML3 in alveolar macrophages to regulate elastase MMP-12 uptake, influencing chronic obstructive pulmonary disease development ( Spix et al., 2022 ). Extensive research has been carried out on TRPV1 and its role in pain, and while a number of drugs have entered clinical trials, these drugs are associated with significant side effects which has limited their utility, see Table 4 ( Moran, 2018 ). In macrophages TRPV1 inhibits M1 polarization via Ca 2+ /CaMKII/Nrf2 signalling pathway ( Lv et al., 2021 ) and influences cytokine production in models of osteoarthritis, sepsis ( Ninomiya et al., 2017 ), inflammatory tissue fibrosis ( Okada et al., 2011 ) and viral infection ( Sanjai Kumar et al., 2021 ). Interestingly, a recent study suggested that inflammation-mediated macrophage polarisation induces TRPV1/TRPA1 heteromeric channel formation with key roles in endometriosis pain ( Zhu et al., 2022 ). A second member of TRPV subfamily, TRPV2 channel, plays regulatory roles in a number of macrophage functions including chemotaxis and early phagocytosis ( Link et al., 2010 ; Nagasawa et al., 2007 ), podosome formation ( Nagasawa and Kojima, 2012 ) and cytokine production ( Yamashiro et al., 2010 ). Moreover, TRPV2 and TRPM7 in macrophages are thought to control regulatory volume decrease (RVD), which leads to NLRP3 inflammasome activation ( Santoni et al., 2015 ; Compan et al., 2012 ; Liedtke, 2006 ). During RVD TRPV2 is translocated to the plasma membrane where it induces cellular permeabilization, while TRPM7 acts as a controller of the regulatory volume decrease process ( Muraki et al., 2003 ). TRPM7 blockade was reported to delay the RVD, reduce cell permeabilization and IL-1β release in macrophages ( Numata et al., 2007 ; Penner and Fleig, 2007 ). An interesting member of TRPV family is TRPV4. It responds to mechanical stimuli and has been shown to regulate macrophage phagocytosis and cytokine secretion ( Scheraga et al., 2016 ). A more recent study by Scheraga R. et al (2020) established that TRPV4 enhances macrophage bacterial clearance and reduces proinflammatory secretion of cytokines through MAPK molecular pathway switching ( Scheraga R. et al., 2020 ). TRPV4 roles in macrophages appear to be important in inflammation and disease progression where mechanical stimuli may be of significance e.g. ventilator/chemically-induced injury ( Hamanaka et al., 2010 ; Balakrishna et al., 2014 ; Scheraga R. G. et al., 2020 ) and tuberculosis ( Naik et al., 2020 ). A role for TRPV4 in matrix stiffness-induced macrophage polarization has also been suggested ( Dutta et al., 2020 ). Finally, there are reports that TRPV4 can contribute to the formation of specialised macrophage phenotypes ( Goswami et al., 2017 , 2021 ). Ion channel-targeted drugs in clinical trials I-III or approved. The table includes a non-exhaustive list of drugs which have entered clinical trials against various ion channels. The drugs show high selectivity for the intended target, or the major therapeutic outcome is believed to be via indicated target. It is presumed that most of the effects reported are due to engaging the intended ion channel in non-macrophage cells. DMARD, disease-modifying antirheumatic drug; FEV 1 , forced expiratory volume in 1 s; MS, multiple sclerosis; RA, rheumatoid arthritis; SUR1, sulfonylurea receptor-1. Together these studies highlight the importance and complexity of these “sensory” channels in macrophages, and we refer the reader to some in-depth reviews of this channel superfamily ( Santoni et al., 2018 ; Nguyen et al., 2022 ). However, it is important to discriminate in ion channel knock-down studies if the reported effects are directly as a result of ion channel modulation in the macrophage or via an up-stream mechanism, e.g. ion channel modulation in T lymphocytes can subsequently impact macrophage activity ( Okada et al., 2011 ; Ma and Wang, 2021 ). With time, more roles for TRP channels in macrophage regulation are being established. Functional modulation of TRP channels in macrophages may represent a rich area for the development of therapies to treat a range of inflammatory diseases.

In this review we have highlighted the current state of the literature regarding the expression and roles of ion channels in macrophages ( Table 1 ; Figure 1 ). We believe that targeting ion channels in macrophages represents a viable therapeutic opportunity for the treatment of a number of diseases ( Table 5 ). Indeed, rather than inhibiting inflammatory responses, we believe targeting ion channels may be able to shape the inflammatory response and thus provide better clinical utility.

Belonging to the innate branch of the immune system, macrophages are important in the killing of invading microbes, the removal of noxious material; the initiation, propagation and resolution of inflammatory responses, the progression of tissue repair and the maintenance of homeostasis ( Murray and Wynn, 2011 ; Wynn et al., 2013 ). However, an inappropriate, exaggerated or dysfunctional macrophage response can lead to host tissue damage and contribute to a variety of pathologies ( Belchamber and Donnelly, 2017 ; Crayne et al., 2019 ). To carry out their required biological functions macrophages need to produce qualitatively, quantitatively and temporally the appropriate biomolecules. They achieve this by utilizing a complex series of interconnected, feedback rich, signalling cascades ( Raza et al., 2008 ; Drexler et al., 2008 ). These signalling pathways receive input from numerous cellular sensors, which allow the macrophage to respond in a timely and appropriate manner to extracellular and intracellular signals, which are often in a state of constant flux, to produce the appropriate response. Indeed, Mosser et al. has recently referred to macrophages as “transducers of the body” ( Mosser et al., 2020 ). This diversity of functions and flexibility of response to environmental cues results in macrophages showing phenotypic heterogeneity. How macrophage populations are classified is highly debatable and depends on various criteria including tissue location/lineage, current cellular functionality or activation state and the granularity of the classification profiles used. However, a convenient, if simplified, way to classify them is pro-inflammatory M1 macrophages and anti-inflammatory M2 macrophages (Smith et al., 2016; Gordon and Plüddemann, 2017; Gessain et al., 2020). Ion channels control the movement of ions, such as Na + , K + , and Ca 2+ , across cell membranes, both plasma and intracellular membranes such as mitochondrial, lysosomal, endoplasmic reticulum membranes and nuclear membranes, and are known to be important cellular sensors and participate in cell signalling cascades in numerous cell types ( Selezneva et al., 2022 ). Here we will provide an up-to-date overview of ion channels known to be present and functional in macrophages. First, we give a brief introduction to membrane potential and how it could impact macrophage biology. Then, beginning with potassium (K + ) channels, which are the main regulators of cell membrane potential, we review the evidence which supports a role for ion channels, Table 1 , in shaping macrophage behaviour. Finally, we discuss prospects in this area of research and the potential of macrophage ion channels as therapeutic targets. Currently identified functional ion channels in macrophages. The nomenclature for ion channels can be found in the International Union of Basic and Clinical Pharmacology (IUPHAR)/British Pharmacology Society (BPS) Guide to Pharmacology ( IUPHAR/BPS, 2011 ; Alexander et al., 2011 ). IP 3 , inositol 1,4,5-trisphosphate; PLC, phospholipase C; ATP, adenosine triphosphate; PIP2, phosphatidylinositol 4,5-bisphosphate; NAADP, nicotinic acid adenine dinucleotide phosphate.

By comparison to voltage-gated ion channels, which open to changes in membrane potential, a LGIC opens in response to the binding of certain chemical messengers. Therefore, the opening of a LGIC will not only be determined by its expression but also the concentration of the relevant ligand which has access to the ligand-binding site on LGIC. This is an important caveat when investigating LGICs in macrophages since many of the endogenous ligands for LGICs are not found in tissues at sufficient concentrations to activate the channel and are not traditionally synthesised by cells found in inflammatory lesions. Nevertheless, this does not negate the possibility that synthetic openers, blockers and allosteric modulators of macrophage LGICs may have therapeutic value. P2X receptors are a family of ATP-gated cation channels comprising seven subtypes, P2X1-7. Although several P2X receptor mRNA transcripts, P2X1, P2X3-5 and P2X7, were detected in macrophages, to date only P2X4 and P2X7 have been reported to have defined functional roles in macrophages, with their activation being associated with the onset of inflammatory reactions ( Kessler et al., 2011 ; Bowler et al., 2003 ). The stimulation of P2X4, which has approximately 100-fold higher affinity for ATP than P2X7 ( Jarvis and Khakh, 2009 ; Asatryan et al., 2018 ), in macrophages was found to trigger Ca 2+ influx and p38 MAPK phosphorylation, causing cytosolic phospholipase A 2 activation and cyclooxygenase-dependent release of the lipid mediator prostaglandin E 2 ( Ulmann et al., 2010 ). Another study reported a role for P2X4 in chemokine secretion by macrophages ( Layhadi et al., 2018 ). In microglia P2X4 was reported to trigger the release of brain-derived neurotrophic factor and initiate a core pain signalling pathway ( Beggs et al., 2012 ; Masuda et al., 2014 ). Many studies agree that P2X7 plays a prominent stimuli-dependent role in controlling inflammasome activation and macrophage function. P2X7 activation was reported to promote the efflux of K + , activate the NLRP3 inflammasome and cause release of IL-1β from macrophages ( Ferrari et al., 1997 ; Ferrari et al., 2006 ; Piccini et al., 2008 ; Barberà-Cremades et al., 2012 ; Mariathasan et al., 2006 ; Muñoz-Planillo et al., 2013 ; Pétrilli et al., 2007 ). Moreover, Franceschini et al. showed that P2X7 and NLRP3 inflammasome closely interact and colocalize at discrete subplasmalemmal sites in microglia and macrophages ( Franceschini et al., 2015 ). Absence of the P2X7 was demonstrated to result in an inability of macrophages to release IL-1β from macrophages in response to ATP and, therefore, impaired cytokine signalling cascades ( Solle et al., 2001 ). Hemichannel Pannexin 1 was found to be required for the IL-1β release by the ATP-gated P2X7 (discussed below) ( Pelegrin and Surprenant, 2006 ). There have also been reports of P2X7 receptor involvement in ATP-induced ATP release ( Suadicani et al., 2006 ), microvesicle shedding ( MacKenzie et al., 2001 ) and phagocytosis ( Gu et al., 2010 ). Nicotinic acetylcholine (nACh) α7 receptors are another ligand-gated cation-selective channel which contributes to shaping macrophage behaviour. Interestingly, these channels expressed in macrophages participate in a so-called anti-inflammatory vagal reflex or cholinergic anti-inflammatory pathway which integrates the nervous and immune system. In this reflex acetylcholine (ACh) has an anti-inflammatory effect on macrophages ( Wang et al., 2002 ). Studies show that nAChα7 receptor activation in macrophages is capable of modulating inflammation by inhibiting NF-κB nuclear translocation ( Yoshikawa et al., 2006 ), activating the JAK2/STAT3 pathway ( de Jonge et al., 2005 ; Joe et al., 2011 ) and selectively inhibiting pro-inflammatory cytokine production while leaving anti-inflammatory cytokines undisturbed ( Báez-Pagán et al., 2015 ; Borovikova et al., 2000 ; Wang et al., 2002 ). Tsoyi et al. have suggested that nAChα7 receptor activation in macrophages can mediate anti-inflammatory effects by activating the PI3K/Akt/Nrf-2 pathway and upregulating heme oxygenase-1 ( Tsoyi et al., 2011 ). Due to the high levels of cholinesterase activity in tissues a valid question to ask is the source of ACh which activates macrophage nACh receptors. Importantly, along with the splenic nerves research now indicates that immune cells act as a source of ACh in the spleen and this contributes to the anti-inflammatory vagal reflex ( Fujii et al., 2017 ), although it must be acknowledged that human nAChα7 receptors require high concentration of ACh (EC50 = 128 ± 12 μM) to be activated ( Gill et al., 2011 ). Other nACh receptor subunits have been reported in macrophages including α4β2 ( Kiguchi et al., 2012 ; Kiguchi et al., 2018 ), α2, α5 and α10 ( Kawashima et al., 2007 ), however, their functions in macrophages require further investigation. In addition to the above mentioned nACh and P2X receptors, 5-Hydroxytryptamine 3 (5-HT3) receptor is another LGIC found in macrophages where it is thought to modulate cytokine secretion ( Seidel et al., 2008 ; Maehara et al., 2015 ). The role of 5-HT (serotonin) in inflammation is historically associated with the vasculature ( Majno and Palade, 1961 ), with the majority of peripheral 5-HT being stored in platelets and released upon platelet activation. However, other possible sources of peripheral serotonin could include lymphocytes, macrophages and mast cells ( Herr et al., 2017 ). Maehara and others suggest a therapeutic potential for 5-HT3 receptor antagonists targeting peritoneal macrophages expressing the receptors in post-operative ileus ( Maehara et al., 2015 ). The presence of functional Cl − -permeable gamma-aminobutyric acid (GABA) type A receptors in macrophages has also recently been reported ( Kim et al., 2018 ). GABA is one of the principal neurotransmitters in the brain and is long associated with GABAergic cells, however, GABA can be synthesised by a number of cells including B lymphocytes ( Zhang et al., 2021 ). Interestingly, GABA has been attributed with both inhibitory effects on phagocyte activation and pro-inflammatory functions ( Bhandage and Barragan, 2021 ). While GABA appears to promote monocyte differentiation into anti-inflammatory macrophages that secrete IL-10 ( Zhang et al., 2021 ), the treatment of macrophages with GABA or GABAergic drugs was shown to promote autophagy activation, enhance phagosomal maturation and antimicrobial responses against mycobacterial infection ( Kim et al., 2018 ). Propofol, a commonly used anaesthetic agent, was reported to bind to GABA A receptors in macrophages and inhibit macrophage phagocytosis, with potential implications for post-surgical wound healing ( Shiratsuchi et al., 2009 ). Furthermore, a recent study highlights a role for GABA A receptors in macrophages in pressure overload-induced heart failure ( Bu et al., 2021 ).