The nervous system: Orchestra conductor in cancer, regeneration, inflammation and immunity.

Neuroimmunology has primarily focused on the CNS and associated neuroinflammatory disorders, such as multiple sclerosis, as well as the role for neuroimmune interactions in neurodegenerative and neuropsychiatric diseases. 58 Neuroimmune interactions in the CNS can have both deleterious effects and a role in normal brain development and recovery from trauma. Multiple sclerosis is a classic example of deleterious effects of aberrant immune activation, in which T lymphocytes and other inflammatory mediators attack the myelin sheath in the CNS. 59 Neuroimmune interactions in the CNS extend to microglia and the complement system, with roles in neurodegenerative disorders, including Alzheimer's disease. 60 More recently, the study of neuroimmune interactions related to the PNS has come to the fore. 61 As opposed to oncology, research on the gastrointestinal (GI) tract has long acknowledged a role of neurons, including those of the enteric nervous system (ENS), in selected gastroenterological diseases, such as inflammatory bowel disease (IBD), and serves to illustrate general principles relevant to peripheral neuroimmunology. 4 The GI tract is characterized by a dense, complex network of nerves and neurons that coordinate gut physiological functions. 62 In addition, the GI tract is replete with a variety of immune cells that interact with nerves and neurons. Neuroimmune cross talk in the gut is critical to maintenance of normal physiology and homeostasis as well as being involved in a variety of gut perturbations including infection, food allergy, and IBD. 63 In the gut, ENS neurons and neurites are entangled and communicate with immune cells including macrophages. One premise for gut physiology is that both immune cells and neurons sense danger and communicate with each other. The PNS and immune systems serve as sentinels for dangerous pathogens, noxious agents, and other stimuli. Figure  2 illustrates a simple, pragmatic view of homeostasis and disrupted biology. Figure  2A shows a homeostatic gut with tolerogenic communication between neurons and immune cells. We designate the tolerogenic state by notations, I 0 or N 0 . When neurons and/or immune cells receive an inflammatory stimulus, they convert to an inflammatory state we depict as I 1 or N 1 . If the inflammatory stimulus is brief or inconsequential, neurons and immune cells can revert back to baseline and homeostasis. But otherwise, initial remodeling and disease initiation commence. As disease progresses, immune cells have a large role in driving inflammation and disease progression and can recruit additional neurons into the disease state as in Figure  2B . As disease progresses further and may be treated with, for example, anti‐TNFα agents, immune cells are brought back to the I 0 state, but if the enteric neuron aberration is not resolved, neurons could bring the immune cells back to the activated state I 1 as shown in Figure  2C . Activation of neural anti‐inflammatory pathways could have potential for treating IBD that is refractory to other, primarily immune, treatments. The role of neuroimmune crosstalk in maintenance of homeostasis and diseased states in inflammatory disorders. (A) Disease initiation: neurons and immune cells sense danger together or independently. (B) Disease progression: inflammatory immune cells convert neurons to inflamed state. (C) Disease flares: inflamed neurons convert immune cells to inflamed state. I 0 , tolerogenic immune cell. N 0 , tolerogenic neuron. I 1 , diseased immune cell. N 1 , diseased neuron ENS neurons that influence gut immune cells include intrinsic primary afferent neurons, vasoactive intestinal peptide neurons that project to the mucosa and cholinergic neurons that influence macrophages in the external muscle layers. 62 Canonical enteric neuropeptides, such as calcitonin gene‐related peptide, and neurotransmitter pathways, including cholinergic, influence immune cells with anti‐inflammatory potential. 61 Acetylcholine is an important neurotransmitter for communication between extrinsic/intrinsic neurons and ENS to immune cells. The muscarinic GPCRs and nicotinic ligand gated ion channels are expressed in varying patterns across the subsets of neurons and immune cells enabling specific signaling. It is expressed on multiple neuronal types and particularly on peripheral nerves including the ENS. A number of preclinical studies have confirmed the therapeutic potential of targeting alpha 7 nicotinic acetylcholine receptor‐mediated anti‐inflammatory effects through modulation of proinflammatory cytokines. 64 Alone or in combination, neuropeptide and/or neurotransmitter modulation may restore neuroimmune homeostasis with potential anti‐inflammatory benefit for IBD. Outside of the GI system, neuroimmune cross talk has a wide‐ranging role in the maintenance of the tolerogenic state, as well as a factor in variety of diseases. The role of the PNS and immune systems as sentinels for dangerous signals is common with barrier tissues, including the skin, replete with immune cells, nociceptors, and sensory neurons that serve to detect a variety of danger alerts. Innate lymphoid cells and the PNS communicate and determine the state of resident immune cells, including macrophages, and fibroblasts. 65 In fact, neuroimmune interactions appear to play a fundamental role in psoriasis, a chronic inflammatory skin disease, including dysfunction of nociceptive neurons. 66 Neuroimmune interactions in disease are not limited to the skin and the GI system. For example, the PNS plays an important role in the pathophysiology of endometriosis, a chronic debilitating condition. 67 Sensory nerves that surround and innervate endometriotic lesions not only drive the chronic and debilitating pain associated with endometriosis but also contribute to a pro‐growth phenotype by secreting neurotrophic factors and interacting with surrounding immune cells.

All authors collaborated on the paper and offered substantive revisions and approval of the final version.

The role of the nervous system in health and disease is expanding, and the emerging exploration of the neuroscience of human diseases opens a new frontier in biomedicine. From regeneration to cancer, immunity and beyond, a better understanding of the role played by the nervous system as the orchestra conductor of cellular and tissular growth and differentiation should delineate new avenues for the management of human health and diseases. Therapeutic translation of cancer neuroscience is already emerging with the targeting of adrenergic and cholinergic neurosignaling in the TME, proven to be effective in reducing tumor progression in vivo, 31 , 38 , 43 , 44 but the existing experimental and clinical evidence now need to be tested in clinical trials. Some clinical trials have already been completed, and more are on the way about the use of beta‐blockers to target adrenergic signaling in cancer. In breast cancer, β‐blockers reduced the biomarkers of metastasis in a phase II randomized trial 68 and inhibited cancer progression with reduced patient mortality. 69 It should also be noted that, at this stage, the reported neurosignaling activities in cancer are mostly adrenergic and cholinergic, but the potential role of other neurosignaling pathways should not be underestimated. For instance, dopamine receptor D2 is correlated with gastric cancer prognosis 70 and repositioning dopamine D2 receptor agonists can enhance chemotherapy and treat bone metastatic tumors 71 ; therefore, dopamine and its signaling also appear as valid targets in cancer. Aside from impairing neurosignaling, another promising approach is to target neurotrophic growth factors to prevent tumor innervation. 72 Blocking antibodies against NGF 24 and other neurotrophic growth factors, 25 or pharmacological inhibitors of their tyrosine kinase receptors Trk 39 , 43 , 44 have been demonstrated to inhibit tumor progression, and the effect of antineurotrophic growth factor strategies also extends to the inhibition of cancer‐induced pain. 73 Pain is a serious issue in oncology, and the perspective of targeting simultaneously cancer progression and cancer pain by targeting neurotrophic growth factors and their signaling pathways is particularly attractive. Aside from therapeutics, the other promising area for cancer neuroscience is tumor prognosis. Determining the outcome of the tumor at the time of diagnosis is increasingly important for treatment choice and patient segmentation, and as nerve infiltration in the TME is associated with tumor aggressiveness, 31 , 46 , 74 , 75 the assessment of nerve density may become part of routine clinicopathological analyses in oncology, as well as increasing utility through imaging, particularly in prostate cancer. 37 Similarly, neurotrophic growth factors and their receptors are overexpressed in human tumors, 76 , 77 , 78 , 79 and they could also be of value in cancer clinicopathology. Of note, the expression of neurotrophic growth factors has been shown to be associated with cancer prognosis in dogs and therefore the value of quantifying neurotrophic growth factors in clinicopathology may also be applicable to veterinary oncology. 80 In conclusion, clinical translation currently emerging in the fields of cancer neuroscience and exoneural biology is likely to pave the way for further clinical developments in immunity, inflammation, cancer, and regenerative medicine. Moreover, the nervous system, and particularly the brain, is the integration center of cognition, emotions, and social interactions. The deciphering of the psychological mechanisms involved in physical health has already been pioneered, 81 and it can be anticipated that the recent developments of neuroscience that we have described here may also lead to a better understanding of the contribution of neurophysiological, cognitive, and social inputs in human health and diseases.

Nerve involvement in limb regeneration was initially discovered in the salamander, which has the remarkable property to regenerate appendages (limbs and tail) after amputation. Starting with the formation of a cellular bud, called the blastema, regeneration of limb and tail occurs in only a few weeks and requires the infiltration of nerves into the blastema. Denervation of the stump prevents the formation and growth of the blastema, and in the absence of nerves, instead of regeneration a simple wound healing takes place. Nerve dependence in regeneration was first reported in the middle of the 19th century 6 in the context of salamander limb regeneration and was later shown to also apply to tissues other than appendages and to other species. In regeneration of the amphibian lens, neural retina, and the forebrain, regeneration can only occur in the presence of olfactory nerve projections. 8 In the fish, fin and barbel regeneration also requires innervation for the creation of a regenerative blastema and the progressive reconstitution of a fully functional structure. 8 , 9 , 10 In mammals, nerves are required for heart regeneration through the stimulation of stem cell growth, 11 and in digit tip regeneration (the remnant of limb regeneration in amphibians), denervation also blocks regeneration. 12 , 13 The understanding of nerve involvement in regeneration at the molecular level largely remains to be elucidated. On the one hand, blastema cells produce neurotrophic factors that attract nerves in the regenerative structure, and on the other hand, nerve liberate various mitogenic factors and neurotransmitters that stimulate neurosignaling in blastema cells. Neuregulin and nerve growth factor (NGF) have been shown to facilitate innervation during heart regeneration 11 and in digit tip regeneration, a Wnt‐mediated mechanism, is necessary to attract nerves that promote blastema cell growth. 13 Immune cells, and in particular macrophages, may also contribute to the attraction of nerves to the blastema. Macrophages secrete neurotrophic factors that can stimulate nerve outgrowth, 14 and this mechanism seems to be at play in regeneration. 15 Importantly, growing nerves have been shown to secrete a series of growth factors and neurotransmitters through nerve endings. Transferrin, 16 substance P, 17 fibroblast growth factors (FGFs) and bone morphogenetic protein‐2 (BMP2), 18 platelet‐derived growth factor (PDGF) and oncostatin, 19 as well as the morphogenetic factor nAG (a determinant of proximodistal position) 20 , 21 have all been shown to be released by nerve endings during regeneration and actively stimulate the growth of the regenerate. Nerves also induce the overexpression of histone deacetylase 1 (HDAC1) that contributes to the proliferation of regenerative cells. 22 Denervation causes the deprivation of the above listed nerve‐released molecules and that results in the impairment or strong reduction in regenerative capacities. Of note, it is likely that this list of trophic factors released by nerves is not exhaustive, with other molecular players likely yet to be discovered. Importantly, the cellular complexity of nerves should be taken into account in the molecular mechanisms of nerve dependence. Indeed, peripheral nerves are not only made of neurons but also include supportive Schwann cells, which are also involved in the stimulatory impact of nerves in regeneration by directly producing and releasing trophic factors such as PDGF. 19 , 20 , 21 Aside from the release of molecules by nerve endings in the blastema cells, an important mechanism of nerve dependence in regeneration has recently been discovered. In digit tip regeneration and skin repair, peripheral nerves provide a reservoir of mesenchymal precursor cells that directly contribute to regeneration. 23 Neural crest–derived mesenchymal precursor cells in the endoneurium are able to migrate to the blastema and later evolve into progenitors of nonneural cells, contributing to the growth and differentiation of the blastema, and in particular the formation of bones. 23 Similarly, neural crest–derived nerve mesenchymal cells contributed to the dermis during skin wound healing. 23 These findings support a model where peripheral nerves directly contribute precursor cells to promote repair and regeneration of injured tissues. Together, not only nerves stimulate regeneration through paracrine‐based molecular interactions but also they can provide a source of precursor cells that directly contribute to regeneration. This dual function of nerves places them in a central role as orchestrator for the cellular and molecular interactions played during regeneration and, as we will describe in the next section, there are strong similarities between the role and mechanism of action of nerves in regeneration and cancer (Figure  1 ). The orchestrating role of nerves in regeneration versus cancer. (A) In regeneration, the outgrowth of new nerves in the blastema is stimulated by the production and release of neurotrophic factors from blastema cells. In return, nerves liberate growth factors, neurotransmitters and morphogens in the blastema, leading to the stimulation of blastema growth and morphogenesis to reconstitute the missing structure. (B) In cancer, the outgrowth of new nerves in the tumor microenvironment is stimulated by the release of neurotrophic factors from tumor cells. In return, nerves liberate neurotransmitters and growth factors in the tumor microenvironment and that leads to the stimulation of tumor growth and metastasis. (C) In both regeneration and cancer, the release of neurotransmitters and growth factors by nerve endings stimulate not only the proliferation of stem and progenitor cells, but also there is an impact of nerves in the stimulation of angiogenesis and inflammation, further fueling the development of the blastema and the tumor, respectively

The peripheral nervous system (PNS) comprises nerves outside of the central nervous system (CNS), including selected cranial nerves, spinal roots, sensory and autonomic ganglia, somatic nerves, and neuromuscular junctions. The role of the PNS has been thought to be restricted to several basic functions, including control of voluntary striated muscles, conveyance of nonvisual sensory information to the CNS, and autonomic function regulation. Peripheral nerves have also been well characterized in injury and disease. Traditionally, peripheral nerve dysfunction includes peripheral neuropathies, peripheral nerve injuries, and neuromuscular disorders, such as amyotrophic lateral sclerosis, muscular dystrophy, myasthenia gravis, and spinal muscular atrophy. In recent years, the field of cancer neuroscience has emerged and has highlighted the role of nerves beyond traditional peripheral nerve diseases and to a variety of other organ systems. 1 , 2 Nerves are emerging as promoters of cancer growth and dissemination and although their mechanisms of action need to be fully elucidated, nerves appear to stimulate various signaling pathways in cancer cells and other cellular components of the tumor microenvironment (TME). 1 , 3 Apart from tumor cells and stromal cells, immune cells are a major component in the TME and also interact with nerves and tumor innervation, which should now be considered a hallmark of cancer. 3 More broadly, nerves are increasingly described for their regulatory functions in immunity and inflammation. Diseases from inflammatory bowel disease 4 to endometriosis 5 have posited neural etiologies, at least in part. Interestingly, the role of nerves in stimulating cellular growth and dissemination has long been associated with animal regeneration, where nerves are necessary for the reconstitution of lost body parts, 6 and the recent extension of the concept of nerve dependence from regeneration to cancer and immunity is an important milestone. 7 In this review, we aim to offer a perspective about exoneural biology, the role of nerves outside the nervous system in health and disease. We will first highlight the current knowledge about nerve dependence in regeneration and the emerging role of nerves in both cancer and immunity. The striking similarities in nerve activities between cancer, regeneration, and immunity emphasize the trophic impact of nerves and suggest that targeting nerves and neurosignaling is a promising therapeutic approach for the treatment of various human diseases.

Pearl S. Huang and John A. Wagner are employees of Cygnal Therapeutics and may own stock and/or stock options. Hubert Hondermarck is a member of the Scientific Advisory Board of Cygnal Therapeutics.