RETRACTED: Formyl Peptide Receptor 1 Signaling in Acute Inflammation and Neural Differentiation Induced by Traumatic Brain Injury

Adult formyl peptide receptor 1 gene-deficient mice on the C57BL/6 genetic background and C57BL/6 animals were used as the wild-type controls [ 33 , 34 , 35 ]. Male, 10-week-old mice of the strains Fpr1−/− and wild-type C57/BL6 (from William Harvey Research Institute, Barts and The London School of Medicine, London, UK, purchased from Envigo, Milan, Italy) were used in this study. The University of Messina Review Board for animals’ care approved the study. All in vivo experiments followed the new regulations of the USA (Animal Welfare Assurance No A5594-01), Europe (EU Directive 2010/63), Italy (D.Lgs 2014/26) and the ARRIVE guidelines. Mice were anesthetized with an intraperitoneal injection (i.p.) of ketamine and xylazine (2.6 and 0.16 mg/kg body weight, respectively). Traumatic brain injury was performed by a controlled cortical impactor (CCI) as already described [ 36 ]. Briefly, a craniotomy was made encompassing bregma and lambda and between the sagittal suture and the coronal ridge of the right hemisphere, using a Micro motor hand piece and drill. The ensuing bone flap was removed and on the exposed cortex a cortical contusion was produced using the controlled impactor device Impact OneTM Stereotaxic impactor a for CCI (Leica, Milan, Italy) [ 37 ]. Subsequently, the skin incision was sutured, and 2% lidocaine jelly was applied to the lesion to minimize any possible discomfort. Both the WT and FPR1-deficient animals were treated with penicillin (40,000 U/kg) for five days after TBI to rule out confounding effects by bacterial infections (in the long-term series) [ 38 ]. The mice were randomly divided into four groups ( n = 25): TBI WT group: mice were subjected to CCI as described above. TBI Fpr1 KO group: Fpr1 KO mice were subjected to CCI as well as the WT group. Sham WT group: Mice were subjected to the surgical procedures as per the above group (anesthesia and craniotomy) except that the impact tip was not applied. Sham Fpr1 KO group: Mice were subjected to the surgical procedures as per the above group (anesthesia and craniotomy) except that the impact tip was not applied. TBI WT group: mice were subjected to CCI as described above. TBI Fpr1 KO group: Fpr1 KO mice were subjected to CCI as well as the WT group. Sham WT group: Mice were subjected to the surgical procedures as per the above group (anesthesia and craniotomy) except that the impact tip was not applied. Sham Fpr1 KO group: Mice were subjected to the surgical procedures as per the above group (anesthesia and craniotomy) except that the impact tip was not applied. In order to analyze the effect of the Fpr1 gene deletion of animals subjected to traumatic brain injury, two experiments were carried out ( n = 10): Exp 1—to investigate the early stage of acute inflammation, animals were sacrificed at 24 h after TBI. Exp 2—to investigate the neurogenesis, animals were sacrificed four weeks after the injury. Exp 1—to investigate the early stage of acute inflammation, animals were sacrificed at 24 h after TBI. Exp 2—to investigate the neurogenesis, animals were sacrificed four weeks after the injury. Twenty-four days after the traumatic brain injury, the Open Field Test was used for evaluating locomotor activity [ 39 ]. The apparatus consisted of a Plexiglas box 50 cm × 50 cm with its floor separated into 16 squares. The center was defined by four squares and the squares along the wall defined the periphery. During the test, the mouse was located in the center of the box, and the movement of the mouse was observed for 5 min. The movement was scored as a line crossing when a mouse removed all paws from one square and entered another. The number of crossings and the time spent in the center were calculated and scored. The social interaction test was performed twenty-three days after the traumatic brain injury using a three-chambered apparatus (polycarbonate 80 cm × 31.5 cm) divided into three compartments. It consisted of three trials of ten minutes. Initially, a mouse was acclimated in an empty arena for 5 min. In the second phase, the experimental mouse was exposed to an object, one of the empty wired cages and a wired cage covering a stimulus mouse. Time spent engaging in investigatory behavior with the novel mouse and the frequency of the investigatory behavior with the novel mouse was recorded. All testing happened during the dark phase (21:00–03:00 h) under illumination with red light. The experiment was conducted as previously described [ 40 ] and performed in a black empty box in a quiet environment. The mouse was replaced in the box, and its behavior was observed for 10 min. The total time the mouse spent exploring each object was recorded. The exploration time included the distance between the object and the nose tip when the mouse sniffed the object from less than 2 cm, and the times the front paw or nose directly touched the object. Walking near the object was not considered exploratory behavior. A solution of 90% ethanol was used to eliminate odors between different animals (to avoid olfactory cues from affecting the exploratory behavior of other animals). The water maze test was conducted as previously described [ 41 ] after twenty-four days from TBI. The device was a stainless-steel sink, with a height of 50 cm and diameter of 100 cm, containing four quadrants. A circular platform with a height of 27 cm and a diameter of 9 cm was in the center of the platform quadrant, and the position did not change throughout the experiment. Milk was added to make the water opaque and the temperature was kept at 23 °C. On the first day, a visual platform experiment was performed. During the following 2–5 days, the navigation experiment was performed. One day after the navigation experiment, the platform was removed for the test. The mouse was located in the water in the same quadrant. The time spent in the target quadrant and the number of entries into it were recorded. All experiments were carried out between 9.00 a.m. and 5.00 p.m. Brain tissues were fixed in formalin solution, dehydrated by graded ethanol and embedded in paraffin. Sections of 5-μm thickness were collected on the glass slides, deparaffinized, and then stained with hematoxylin and eosin (H&E) [ 42 , 43 ]. Histopathologic changes of the gray matter were evaluated on a six-point scale [ 44 ]. At 24 h and four weeks after CCI, animals were euthanized, and the brain tissues were frozen and sectioned in coronal sections (300 μm). Samples were stained, and the area of injured hemisphere was scored using image analysis software. The hemispheric volume was evaluated by summing the area of each section and multiplying it by 0.5. Lesion volume (mm 3 ) was showed as the difference between the injured and uninjured hemisphere volume. For the analysis, n = 5 animals from each group were employed. Myeloperoxidase (MPO) activity, an indicator of neutrophils accumulation, was determined as previously published [ 45 , 46 ]. MPO activity was expressed in U per gram weight of wet tissue and was defined as the quantity of enzyme degrading 1 µmol of peroxide min −1 at 37 °C. For the analysis, n = 5 animals from each group were employed. Western blot analysis was executed on tissues harvested 24 h and four weeks after TBI [ 46 ]. Cytosolic and nuclear extracts were divided as described previously [ 47 , 48 ]. Membranes were probed with specific Abs: anti-NF-kB p-65 (1:1000; Santa Cruz Biotechnology, Heidelberg, Germany) or with IkB-α (1:1000; Santa Cruz Biotechnology), or with anti-NLRP3(1:500; Santa Cruz Biotechnology), or with anti-ASC (1:500; Santa Cruz Biotechnology), or with anti-Caspase-1 (1:500; Santa Cruz Biotechnology), or with iNOS (1:500; Transduction Laboratories, Milan, Italy), or with anti-MnSod (1:500 Millipore, Milan, Italy), or with anti p-AKT (1:500; Santa Cruz Biotechnology), or with anti AKT (1:500; Santa Cruz Biotechnology), or with anti β-III tubulin (1:500; Santa Cruz Biotechnology), or with anti GFAP (1:500; Santa Cruz Biotechnology) or with anti-pp38 (1:500; Santa Cruz Biotechnology) or with anti-p38 (1:500; Santa Cruz Biotechnology) or with anti-pERK (1:500; Santa Cruz Biotechnology) or anti-ERK (1:500; Santa Cruz Biotechnology) or with anti-Cox-2 (1:500; Santa Cruz Biotechnology) or anti-PGE synthase (1:500; Santa Cruz Biotecnology) or anti-PGD2 synthase (1:500; Santa Cruz Biotecnology) in 1xPBS, 5% w/v nonfat dried milk, 0.1% Tween-20 at 4 °C, overnight. To control that, the blots were loaded with equal amounts of proteins and also were probed with antibodies against b-actin protein (cytosolic fraction 1:500; Santa Cruz Biotechnology) or lamin A/C (nuclear fraction 1:500 Sigma-Aldrich Corp., Milan, Italy). Signals were examined with an enhanced chemiluminescence (ECL) detection system reagent according to the manufacturer’s instructions (Thermo, Monza, Italy). The relative expression of the protein bands was quantified by densitometry with BIORAD ChemiDocTM XRS + software and standardized to the b-actin and lamin A/C levels. (All original western blots can be available in Figure S1 ). To assess newly-generated neurons and proliferated cells in Dentate Gyrus (DG), mice received BrdU (50 mg/kg, i.p. dissolved in saline) every day for seven days after TBI [ 49 ]. BrdU incorporation into cell nuclei was assessed by immunohistochemistry. Immunohistochemical analysis was performed as already described [ 50 ]. Subsequently, the sections were incubated overnight with an anti-Bromodeoxyuridine (BrdU) antibody (1:100; Santa Cruz Biotechnology) or anti-MPO antibody (1:250; Santa Cruz Biotechnology) or anti-NRLP3 antibody (1:250; Santa Cruz Biotechnology) or anti-COX-2 antibody (1:250; Santa Cruz Biotechnology) or anti-Iba-1 antibody (1:250; Santa Cruz Biotechnology) or anti-GFAP antibody (1:450; Santa Cruz Biotechnology). Sections were washed with PBS and incubated with peroxidase-conjugated bovine anti-mouse IgG, secondary antibody (1:2000 Jackson Immuno Research, West Grove, PA, USA). Specific labeling was provided with a biotin-conjugated goat anti-mouse IgG and avidin-biotin peroxidase complex (Vector Laboratories, Burlingame, CA, USA). Images were collected using a Leica DM6 microscope associated with Leica LAS X Navigator software. The number of positive cells was counted in three sections per animal and presented as the number of positive cells per high-power field. Brains were collected and rinsed with PBS to remove excess blood, chopped into 1–2 mm pieces and homogenized with a tissue homogenizer. A total of 1.0 mL of Lysis Buffer (R&D Systems) was added. Brains were lysed at room temperature for 30 min with gentle agitation and centrifuged to remove debris. An aliquot of each tissue lysate was removed and assayed for levels of IL-1β and IL 18. The levels of IL-1β and IL 18 in tissues surrounding the cortical contusion site were performed by the specific ELISA kits according to the manufacturer’s instructions (R&D Systems, Inc., Minneapolis, MN, USA). All compounds used in this study, except where differently specified, were purchased from Sigma-Aldrich Company Ltd. All values in the figures and text are expressed as the mean ± standard error of the mean (SEM) of N = 5 number of animals. Results were analyzed by two-way ANOVA followed by a Bonferroni post-hoc test for multiple comparisons. Non-parametric data were analyzed with the Fisher’s exact test. A p -value < 0.05 was considered significant. * p < 0.05 vs. Sham WT; ° p < 0.05 vs. TBI WT.

N-formylpeptide receptor 1 (FPR1) is a member of a small family of 7-transmembrane G protein-coupled receptors (GPCRs), called the FPR family, that is responsible of several host defense reactions [ 1 , 2 ]. N-formylpeptide receptor 1 is mainly expressed on sentinel cells with chemotactic or phagocytic activity, like monocytes [ 3 , 4 ], neutrophils [ 3 , 5 ], dendritic cells [ 4 , 6 ] and macrophages [ 4 , 7 ]. Moreover, it is expressed also on non-phagocytic and non-mobile sentinel cells like endothelial cells [ 8 , 9 ], epithelial cells [ 10 , 11 ], neurons [ 12 ] and glia [ 13 , 14 , 15 ]. It binds several ligands, such as the mitochondrial and pro-inflammatory bacterial N-formylpeptides, as well as the anti-inflammatory agonists lipoxin A4 and annexin-1 [ 2 ]. Furthermore, ligands for Fpr1 were detected during inflammatory processes and may stimulate several responses, such as phagocytosis, chemotactic migration, degranulation and free oxygen species production [ 2 ]. Traumatic brain injury (TBI) is one of the most important public health issues, clinically considered a “silent epidemic” because the derived problems are not immediately visible [ 16 ]. Research has greatly clarified the mechanisms underlying the TBI pathology [ 17 ]. These involve a primary insult caused by direct biomechanical forces and a secondary insult that results in brain damage and death following TBI [ 17 , 18 , 19 ]. TBI can produce physical and behavioral symptoms and result in complete recovery or permanent disability. In addition, secondary events associated with TBI can influence the life quality of patients [ 20 ]. Patients may have a decline in cognitive functions [ 21 , 22 ], risk of cerebral atrophy and dementia [ 23 ]. Traumatic injury is a widely used animal model that directly relate to the common clinical problem in humans. After controlled cortical impact, the acute inflammatory response induces the activation of nuclear transcriptional factors, assembly of pro-inflammatory complexes, release of inflammation mediators and increase in oxidative stress [ 24 , 25 ]. In animals, TBI also induces secondary processes, leading to neurological disorders such as learning and memory impairment, and seizures [ 26 ]. In particular, four weeks after injury, TBI increases neurogenesis [ 27 , 28 , 29 ]. Several studies have found that FPR1 was expressed in neurons [ 12 , 30 , 31 ]. In particular, it has been described that the FPR1 expression was induced during neuronal differentiation. FPR1 activation triggers the activation NFkB and STAT3 transcriptional factors and signaling molecules, such MAPK, PLC and PLD. Beyond the inflammatory functions, evidence for different jobs has been lacking for Fpr1 [ 32 ]. In this regard, we decided to investigate the effect of the absence of Fpr1 gene expression in mice subjected to traumatic brain injury from an early stage of acute inflammation to neurogenesis and systematic behavioral testing four weeks after injury.

No significant histological and macroscopic differences were detected in the brain tissue of Sham WT and Fpr1 KO animals ( Figure 1 A,C,E for histological score and Figure 1 F for lesion volume). Twenty-four hours after TBI injury, the histological analysis of the perilesional area showed in the TBI WT group significant tissue damage, ischemic changes, thickened blood vessels, and gliosis in the brain parenchyma ( Figure 1 B,E for histological score and Figure 1 F for lesion volume). In the TBI Fpr1 KO group, the histological analysis showed a significantly reduced degree of brain injury compared to the TBI WT group ( Figure 1 D,E for histological score). Moreover, the absence of an Fpr1 receptor led to a reduction in the lesion volume compared to the TBI WT group ( Figure 1 F). Traumatic brain injury was characterized by neutrophil infiltration in the tissue, quantified through measurement of MPO activity ( Figure 2 A). Mice lacking Fpr1 gene expression subjected to TBI showed reduced MPO activity ( Figure 2 E), compared to the TBI WT group ( Figure 2 D). No positive staining for MPO was identified in the sham groups ( Figure 2 B,C). P-p38 expression levels, monitored by Western blotting, were considerably increased in tissue collected from TBI WT mice compared to the Sham WT and Fpr1 KO animals. The absence of Fpr1 decreased p-p38 expression in TBI animals ( Figure 3 A,B). Moreover, traumatic brain injury induced increased ERK phosphorylation in WT animals, while it was remarkably reduced in samples from TBI Fpr1 KO animals 24 h after injury. ( Figure 3 C,D). Immunohistochemical analysis showed increased COX-2 expression in TBI WT mice ( Figure 4 C) compared to the sham WT ( Figure 4 A) and Fpr1 KO animals ( Figure 4 B), while TBI Fpr1 KO animals did not show any upregulation ( Figure 4 D). Western blot analysis also displayed an increased expression of COX-2 ( Figure 4 E,F) and PGE2 synthase ( Figure 4 G,H) in brains from TBI WT mice compared to the Sham WT and Fpr1 KO animals, while TBI Fpr1 KO animals did not show any increase. PGD2 synthase expression levels decreased in WT animals 24 h after TBI, compared to the Sham WT and Fpr1 KO animals. In TBI Fpr1 KO animals, PGD2 synthase expression remained at the basal levels ( Figure 4 I,J). Nuclear upregulation of NF-κB is a hallmark of inflammatory brain diseases [ 51 ]. In order to investigate the pathway whereby Fpr1 gene deletion could moderate the inflammatory response induced by traumatic brain injury 24 h afterwards, we checked the IκB-α expression in the cytosol and NF-κB expression into the nucleus ( Figure 5 A,C). Western blot analysis showed basal cytosolic expression of IκB-α in brain samples from Sham WT and Fpr1 KO mice, while IκB-α expression was remarkably decreased in samples from TBI WT animals 24 days after injury. In Fpr1 KO animals, a reduction in TBI-induced IκB-α expression was detected ( Figure 5 A,B). In parallel, NF-κB expression in the brain nuclear fractions were substantially upregulated 24 h after TBI, compared to the sham WT and Fpr1 KO animals, which was reduced in Fpr1 KO mice ( Figure 5 C,D). The activation of the NF-κB pathway due to the traumatic brain injury led to an increase expression of the inflammasome complex in brain tissue. Immunohistochemical analysis showed increased NLRP3 expression in the TBI WT mice ( Figure 6 C) compared to the Sham WT ( Figure 6 A) and Fpr1 KO animals ( Figure 6 B), while TBI Fpr1 KO animals did not show any upregulation ( Figure 6 D). In particular, 24 h after injury, the TBI WT animals showed an upregulation of NLRP3 ( Figure 6 E,H), ASC ( Figure 6 F,I) and Caspase-1 ( Figure 6 G,J) levels compared to the Sham WT and Fpr1 KO animals. Tissues collected from TBI Fpr1 KO mice showed a reduced expression of all the proteins of the inflammasome complex. We also evaluated by ELISA analysis the expression of IL-1β and IL-18 activated by the NLRP3 pathway. Both the IL-1β and IL-18 levels were upregulated in TBI Fpr1 KO mice compared to the Sham WT and Fpr1 KO animals ( Figure 7 A,B). The absence of the Fpr1 receptor also reduced the activation of this interleukins as well as expression of the NLRP3 complex. Next, we evaluated the anti-neuroinflammatory effect of the absence of Fpr1 on oxidative stress activation 24 h post traumatic brain injury. Twenty-four hours after TBI, the Western blot analysis for iNOS expression displayed an upregulation of its level in WT animals compared to the Sham WT and Fpr1 KO mice, while Fpr1 KO animals subjected to TBI showed significantly less activation ( Figure 7 C,E). To test whether Fpr1 modulates the oxidative process, we investigated the brain expression of the anti-oxidant enzyme Mn-SOD. A basal expression of Mn-SOD was found in samples from Sham WT and Fpr1 KO mice. TBI reduced its expression in WT animals while the absence of the Fpr1 receptor significantly restored brain Mn-SOD expression ( Figure 7 D,F). Astrocytes activation plays a critical role in neuroinflammation. When compared to the Sham WT ( Figure 8 A,E) and Sham FPR1 KO group ( Figure 8 B,F), immunohistochemical evaluation of the glial fibrillary acidic protein (GFAP) revealed a significant increase in the TBI WT group in both the cortex and hippocampus, as shown in Figure 8 C,G, respectively. In the TBI Fpr1 KO group, there were no significant increase in GFAP-positive cells ( Figure 8 D,H). In order to evaluate the effect of the Fpr1 gene deletion on microglia activation 24 h after traumatic brain injury, an immunohistochemical analysis was performed. We observed that ionized calcium binding adaptor molecule 1 (Iba1) expression was very low in the Sham WT cortex ( Figure 9 A) and hippocampus ( Figure 9 E) and Sham FPR1 KO cortex ( Figure 9 B) and hippocampus ( Figure 9 F), while it was increased in the TBI WT cortex ( Figure 9 C) and hippocampus ( Figure 9 G). In the TBI Fpr1 KO group there were no significant increase in Iba1-positive cells in both the cortex ( Figure 9 D) and hippocampus ( Figure 9 H). No significant histological and macroscopic differences were detected in the brain tissue of Sham WT and Fpr1 KO animals ( Figure 10 A,C,E for histological score and Figure 10 F for lesion volume). Four weeks after TBI injury, the histological analysis showed in the TBI WT group significant tissue damage ( Figure 10 B,E for histological score and Figure 10 F for lesion volume). The TBI Fpr1 KO group showed a significantly reduced degree of brain injury compared to the TBI WT group ( Figure 10 D,E for histological score). Moreover, the absence of the Fpr1 receptor led to a reduction in the lesion volume compared to the TBI WT group ( Figure 10 F). Immunohistochemical analysis showed increased COX-2 expression in TBI WT mice ( Figure 11 C) compared to the Sham WT ( Figure 11 A) and Fpr1 KO animals ( Figure 11 B), while TBI Fpr1 KO animals did not show any upregulation ( Figure 11 D). Western blot analysis of COX-2 confirmed this data ( Figure 11 E,F). Molecular analysis displayed increased expression of iNOS in TBI WT mice compared to the Sham WT and Fpr1 KO animals, while TBI Fpr1 KO animals did not show any upregulation ( Figure 12 A,D). Western blot analysis showed upregulated levels of PGE2 synthase ( Figure 12 B,E) in tissue samples collected from TBI WT mice compared to the Sham WT and Fpr1 KO animals, while TBI Fpr1 KO animals did not show increased levels. PGD2 synthase expression was downregulated in TBI WT animals, compared to the Sham WT and Fpr1 KO animals. In TBI Fpr1 KO animals, PGD2 synthase expression did not increase ( Figure 12 C,F). BrdU (50 mg/Kg) was administered for seven days after the TBI, and then the mice were sacrificed after four weeks from the injury to label the proliferating neural progenitors in the DG. Sham WT and FPR1 KO animals showed the baseline of proliferating cells in the DG ( Figure 13 A,B,E). In the TBI WT group, the density of the surviving proliferated cells was elevated ( Figure 13 C,E) while the TBI Fpr1 KO group did not show any significant upregulation ( Figure 13 D,E). Four weeks after traumatic brain injury induced by a controlled cortical impact, Morris water maze results showed defects in learning and memory abilities in TBI Fpr1 KO animals. During the training and the probe test, the TBI Fpr1 KO group took a longer time than the TBI WT group to find the hidden platform ( Figure 14 A,B). To further assess the locomotor activity in mutant mice, the Open Field test was performed. We found the number of crossings increased in TBI Fpr1 KO mice compared to the WT animals ( Figure 14 C,D); in contrast, the TBI Fpr1 KO group decreased time spent in the center. In order to evaluate impairments in their social interaction and exploring behavior, important clinical features of dementia, we performed the social interaction test and the novel object recognition test. Rodents have the natural habit of exploring new objects and interacting with other mice. In the social interaction test, we observed that the total duration of the contacts was decreased in the TBI Fpr1 KO animals compared to the TBI WT group, while the number of contacts was significantly increased in the TBI Fpr1 KO animals compared to the TBI WT group ( Figure 14 E,F). The novel object recognition test was used to evaluate changes in cognitive function. Fpr1 KO animals subjected to traumatic brain injury had less exploratory behavior and spent significantly less time with the novel object, indicating compromise of cognitive function, while in TBI WT mice the function within the novel object recognition test returned to normal values ( Figure 14 G). To determine whether the absence of the Fpr1 receptor influence the neuronal differentiation, four weeks after traumatic brain injury a Western blot analysis for p-AKT, AKT, β-III tubulin and GFAP was performed. The Western blot showed that the p-AKT/AKT ratio significantly increased in WT animals four weeks after injury, compared to the TBI Fpr1 KO animals ( Figure 15 A,D). In WT animals subjected to TBI, the expression of the neuron marker β-III tubulin also increased ( Figure 15 B,E); meanwhile, levels of glial fibrillary acidic protein (GFAP) gradually decreased ( Figure 15 C,F). Four weeks after the TBI, in the absence of Fpr1 β-III, the tubulin levels did not increase ( Figure 15 B,E), while the GFAP levels were significantly upregulated compared to the WT animals ( Figure 15 C,F).

In this study, we analyzed the effect of the Fpr1 gene deletion on animals subjected to traumatic brain injury, from the early stage of acute inflammation to the neurogenesis four weeks from the injury. Controlled cortical impact is one of the most used animal models to induce traumatic brain injury thanks to its ability to reproduce what happens in humans [ 52 ]. Brain trauma induces tissue injury, release of inflammatory mediators and alteration of the blood brain barrier, which, in turn, allows neutrophils, macrophages and lymphocytes to access to lesion site [ 53 ]. Once recruited, inflammatory cells are activated and triggers multiple pathways, such as increasing gene transcription, assembly of intracellular pro-inflammatory complexes, release of reactive oxygen species and nitric oxide [ 54 ]. Neutrophils are one of the most important cells in the inflammatory response in the central nervous system (CNS) after traumatic brain injury [ 55 , 56 ]. Fpr1 are constitutively expressed on quiescent neutrophils and rapidly upregulated in response to inflammatory stimuli. The important pathogenic role of Fpr1 has been already described in experimental colitis and endometriosis, showing modulation of immune cell recruitment together with a modulation of local cellular activation and survival [ 50 , 57 ]. In particular, Fpr1 synthesis happens late in neutrophil maturation, with storage in azurophilic granules and secretory vesicles [ 58 , 59 ]. Neutrophil activation induces Fpr1 translocation on cell surfaces and upregulates Fpr1 protein synthesis. Once activated, as G-protein-coupled receptors, Fpr1 induces guanosine diphosphate (GDP) transformation into guanosine triphosphate (GTP). GTP catalyzes the dissociation of α from the βγ subunits, stimulating phosphoinositide 3-kinaseγ (PI3Kγ) phospholipase C β (PLCβ) and the MAPK signaling pathway [ 50 , 60 , 61 ]. The first part of the study demonstrates the beneficial effect of Fpr1 gene deletion in acute response in a model of cerebral trauma. Twenty-four hours after traumatic brain injury, the myeloperoxidase (MPO) test, used as a specific and sensitive method to quantify neutrophil accumulation, display a reduction in MPO activity in animals lacking the Fpr1 gene, compared to the WT animals [ 62 ]. In particular, Fpr1 KO mice showed reduced infiltration of neutrophils, macrophages and immune cells to the site of injury. In absence of the Fpr1 gene, the second messengers p38 MAPK and the extracellular signal-regulated kinase (ERK), normally increased twenty-four hours after injury [ 63 ], were not phosphorylated. Fpr1 gene deletion display a positive effect against trauma through the inhibition of the MAPK pathway. The ERK1/2, PI-3K and p38 MAPK signaling pathways are directly involved in the induction of COX-2 in neutrophils [ 64 ]. After brain injury, the COX 2 gene expression levels and PGE2 synthase increased while the PGD2 synthase decreased, suggesting that PGE2 and PGD2 afforded contraindicative effects of inflammation and anti-inflammation, respectively [ 65 ]. In the absence of Fpr1, the COX-2 levels were decreased, as was PGE2 synthase, while PGD2 synthase showed increased levels compared to the WT animals, indicating that Fpr1 could modulate acute inflammation acting through the COX-2 pathway. PLCβ hydrolyses phosphoinositol-4,5-bisphosphate (PIP 2 ) into inositol 1,4,5-trisphosphate (IP 3 ) and diacylglycerol (DAG) [ 11 ]. IP 3 induces calcium release from the endoplasmic reticulum. An increase in calcium concentration triggers the calmodulin (CaM)/calcineurin pathway. DAG remains localized on the membrane and mediates the association of protein kinase C (PKC) to the cell membrane, starting a chain of phosphorylation, which will contribute to the functional responses of the cell. PKC isoforms are responsible of the transcription factor nuclear factor kappa B (NF-κB) translocation to the nucleus. NF-kB is involved in different cellular processes, such as cell proliferation, oxidative stress apoptosis and secretion of cytokines [ 66 ]. Usually, it is bound by the inhibitor protein IkB α into the cytoplasm. Several stimuli, including the brain tissue injury, induce the degradation of IkB α, freeing NF-kB. The positive effects of the Fpr1 gene deletion may be attributed, in part, to the downregulation of the NF-kB pathway. Brain tissues collected from Fpr1 KO mice showed a reduced IkB α degradation into the cytoplasm and NF-κB translocation into the nucleus induced by the injury. Through this inhibition, the absence of Fpr1 reduces the transcription of the NF-κB target genes involved in the enhancement of the inflammatory process, such as the NLRP3 inflammasome. Recently, the master role of the NLRP3 inflammasome in the inflammatory response to a traumatic brain injury has been underlined [ 36 , 67 , 68 ]; its inhibition can ameliorate the secondary events associated with traumatic brain injury [ 69 ]. NLRP3 inflammasome is a multiprotein complex, constituted by an adaptor protein containing a caspase-recruitment domain (ASC) and the serine protease caspase 1 (Casp1) [ 70 ]. The assembly of this complex catalyzes the activation of Casp1, which in turn cleaves and activates the pro-inflammatory cytokines IL-1β and IL-18, supporting the inflammatory process. Once again, the Frp1 gene deletion may reduce the NLRP3 transcription and the resulting assembly of the inflammasome complex by reducing the activation of the acute phase of inflammation. Moreover, the upregulation of proinflammatory cytokines induces generation of free oxygen radicals, immune cells proteases and toxic metabolites [ 71 , 72 ]. Generation of ROS lead to oxidative stress, causing alteration of several signaling pathways [ 36 ]. One effect of the increased oxidative stress is the modulation of the redox-sensitive protein expression [ 73 ]. In our study, we underlined that the absence of the Fpr1 gene expression stabilized the levels of the anti-oxidant enzyme Mn-SOD induced by traumatic brain injury. Therefore, the upregulation of iNOS expression can enhance the oxidative stress associated with brain injury [ 74 , 75 ] Fpr1 KO animals displayed a reduced iNOS expression. After TBI, the accumulation of circulating macrophages, neutrophils, lymphocytes and the release of inflammatory mediators to the lesion site activates the glial and inflammatory cells, thus supporting the inflammatory process [ 53 , 76 , 77 , 78 ]. Our results show that the absence of Fpr1 gene expression leads to a reduction of Iba1 and GFAP expression at the early phase of inflammation. Interestingly, the major pro-inflammatory markers continue to be overexpressed in the hippocampus and brain cortex, even four weeks after a traumatic brain injury [ 79 ], while mice lacking the Fpr1 gene showed a reduction of neuroinflammation compared to the WT animals. In order to evaluate the inflammatory course four weeks after traumatic brain injury, we evaluated several inflammatory markers also at this timepoint. Neuroinflammation and oxidative stress were significantly increased at this time point [ 80 , 81 ]. Our data confirmed the persistent inflammation four weeks after injury in WT animals, while the absence of the Fpr1 showed a reduction in oxidative stress, cyclooxygenase-2 and prostaglandin synthase expression. It is well described that TBI induces secondary processes, leading to neurological disorders, such as learning and memory impairment [ 26 ]. In contrast, four weeks after injury, severe controlled cortical impact significantly promoted neuron differentiation and cell proliferation [ 82 ]. In particular, it promotes neurogenesis at three stages: immature neurons, NSC proliferation and newly-generated mature neurons [ 28 , 29 , 82 ]. Neural stem cells care for adult neurogenesis and represent the regenerative potential of the brain through a lifetime. This proliferation after traumatic brain injury brings about the possibility of repair injury by controlled cortical impact. Recently, Fprs have been found in stem cells [ 83 , 84 , 85 ], the spinal cord, human brain, hypoglossal nucleus neurons and anterior horn cells [ 12 ]. Moreover, protein levels of Fpr1 increased during the differentiation process in the neural stem cells [ 27 ]. In particular, neural stem cells can differentiate into astrocytes, neurons and oligodendrocytes, expressing GFAP, beta-III tubulin and Olig2 [ 86 ]. The second part of the study evaluated the effect of the absence of the Fpr1 gene expression on neurological disorders and neurogenesis associated with the traumatic brain injury. The number of BrdU-incorporating cells showed an increased neurogenesis in WT animals subjected to TBI four weeks after injury compared to the Fpr1 KO animals. To address whether the observed difference between the Fpr1 KO mice and WT mice was also shown in animal behavior, several tests were performed. Behavioral tests showed the impaired cognitive and social functions of the Fpr1 KO mice subjected to TBI compared to the WT animals with the same lesion. In particular, in the Morris water maze test, Fpr1 KO animals displayed reduced memory and learning capacity; moreover, in the Open Field, social interaction and novel object recognition tests, they showed reduced exploratory activity and anxiety-like behavior. To further comprehend the mechanism by which Fpr1 influenced neurogenesis, the molecules associated with neuronal differentiation were assessed. A recent study underlined that self-renewal and neurogenesis are regulated through the PI3K/Akt pathway [ 87 ]. In this work, for the first time we show that, four weeks after traumatic brain injury, neurogenesis and neuronal differentiation are activated by Fpr1 signaling. This pathway involves PI3K upregulation and AKT phosphorylation. In the absence of Fpr1 gene expression, animals subjected to traumatic brain injury displayed reduced neural differentiation while showing upregulation of astrocyte differentiation.

Twenty-four hours after traumatic brain injury, the absence of the Fpr1 gene showed a positive effect by reducing the acute inflammation. In particular, we observed the reduction of the p-38 and ERK phosphorylation, a reduced expression of NFkB into the nucleus and a reduced expression of the components of the NLRP3 inflammasome complex, which also led to a reduction in the levels of IL-1beta and IL-18. We investigated iNOS, COX-2, PGE synthase and PGD2 synthase expression levels at 24 h and a chronic timepoint as a marker of inflammation. The positive effects of the absence of Fpr1 gene expression on inflammation are due to its priming effect of the inflammatory pathway. In our experiment, four weeks after injury, the severe controlled cortical impact significantly promoted neuron differentiation and cell proliferation. Behavioral tests showed the impaired cognitive and social functions of the Fpr1 KO mice subjected to TBI compared to the WT animals with the same lesion. From a molecular point of view, self-renewal and neurogenesis are regulated through the PI3K/Akt pathway. It was found that the activation of FPRs promoted the neuronal differentiation of neural stem cells and inhibited their differentiation into astrocytes, and this process was dependent on the PI3K-AKT signaling pathway. We showed that in the absence of the Fpr1 gene expression a reduced AKT phosphorylation was observed. This led to a reduced differentiation into the neurons, showed by a reduction in beta-III tubulin expression, while GFAP expression was increased, showing no inhibition of astrocyte differentiation. Collectively, our study reported that the absence of Fpr1 gene expression reduced inflammation and oxidative stress immediately after traumatic brain injury and at a chronic timepoint. Four weeks after traumatic brain injury, Fpr1 promoted the differentiation of neuronal stem cells into neurons and reduced their differentiation into astrocytes via the PI3K/Akt pathway.