Neuronal chemokines: new insights into neuronal communication after injury Article Swipe

Classically, chemokines were described as small proteins driving leukocyte migration. Nonetheless, more and more studies are showing the great variety of cell functions and tissues in which they participate, including neural cells. During the last years, research has highlighted the importance of chemokines in the nervous system, governing a wide range of processes (Mesquida-Veny et al., 2021). This is evidenced for example by the crucial role played by CXCL12 during cortical development, or the homeostatic role of neuronal CX3CL1, preventing microglial activation. We are now certain that many chemokines and their receptors are widely expressed in neurons, and growing evidence has shown them as fundamental players in direct neuronal communication, both during homeostasis and after insult. In line with this, many recent studies have helped in defining the complexity of immune activation in response to neuronal signals. Neuronal chemokines were first thought to act as mere chemoattractants of leukocytes, however, recent evidence shows us that they also play a role in their phenotype modulation, ultimately affecting their signaling to other cell types, including neurons, orchestrating in that manner complex responses such as wound healing or nerve regeneration. Neurons alter their secretome when exposed to different stimuli and according to their physiological state. Numerous stimuli, such as metabolic changes, injuries, or neuronal activity have been shown to induce changes in neuronal secretion (Niemi et al., 2016; Mesquida-Veny et al., 2021). This, in turn, allows them to further tune their communication signaling to other cells, both between neurons and among other cells including glial cells such as astrocytes, pericytes, or microglia as well as migrating and tissue-resident monocytes. In the context of axonal injuries, several neuronal chemokines are released, however, many of them have been associated with the induction of neuropathic pain. As such, DLK-dependent, neuronally derived CCL2, CCL7 and CCL12 promote pain after injury, because of the infiltration and activation of leukocytes (Figure 1A; Hu et al., 2019). In this setting, a similar role has also been described for neuronal CCL21, via microglial activation. Furthermore, in this paradigm, CCL21 was shown to travel along the axon inside vesicles, opening the possibility of remote communication. Nevertheless, later studies have shown that particular neuronal chemokines, such as CCL2, CX3CL1, or CCL21 among others may also be associated with many other cellular responses, including the activation of pathways associated with axonal regeneration, including actin cytoskeleton dynamics, expression of regeneration associated genes or catabolic shift (Niemi et al., 2016; Hervera et al., 2018; Mesquida-Veny et al., 2022).Figure 1: Summary of the main known neuronal chemokines in the context of axonal injury and their underlying mechanisms.(A) Injury induced neuronal CCL2 recruits infiltrating macrophages that in turn activate neuronal regenerative programme through Interleukin-6 family cytokines and gp-130 (Niemi et al., 2016). (B) Injury induced neuronal release of CX3CL1, promotes the recruiting and activation of peripheral CX3CR1+ macrophages that signal neurons back via exosome-based redox signals, modifying different key neuronal pathways that in turn activate their intrinsic regenerative programme (Hervera et al., 2018). (C) Excitotoxic injuries promote an increased neuronal expression of CCL2 and CXCL2 that stimulate astrocytes, promoting neuronal survival and differentiation through a mechanism based on basic fibroblast growth factor (Kalehua et al., 2004). (D) Aged mice display neuronal overexpression of the chemokine CXCL13 after injury. This overexpressed chemokine recruits CXCR5+CD8+ T-cells, which interact with injured neurons, repressing axonal regeneration of DRG neurons via caspase 3 activation (Zhou et al., 2022). (E) Specific activation of transient receptor potential cation channel subfamily V member 1 (TRPV1)+ nociceptors induces the production and release of CCL21, which acts on actin cytoskeleton remodeling in the growth cone of proprioceptors, promoting neurite outgrowth via CCR7-MEK-ERK pathway (Mesquida-Veny et al., 2022). (F) CCL5 is needed to promote the regenerative effects induced by CNTF after a central nervous system injury (Xie et al., 2021). (G) CXCL12 promotes axonal growth via its receptor CXCR4 (Zilkha-Falb et al., 2016). Akt: Protein kinase B; cAMP: cyclic adenosine monophosphate; CCL: CC chemokine ligand; CCR: CC chemokine receptor; CD8: cluster of differentiation 8; CNTF: ciliary neurotrophic factor; CNTFR: CNTF receptor; CX3CL1: CX3C chemokine ligand 1; CX3CR1: CX3C chemokine Receptor 1; CXCL: CXC chemokine ligand; CXCL: CXC chemokine receptor; ERK: extracellular signal-regulated kinase; FGF: fibroblast growth factor; FGFR: FGF receptor; JAK: Janus kinase; LIF: leukemia inhibitory factor; LIFR: LIF receptor; MEK: mitogen-activated protein kinase kinase; MHC1: major histocompatibility complex 1; NOX2: nicotinamide adenine dinucleotide phosphate oxidase 2; oxPTEN: oxidized PTEN; PI3K: phosphatidylinositol-3 kinase; PTEN: phosphatase and tensin homolog; STAT3: signal transducer and activator of transcription 3; TCR: T-cell receptor; TRPV1: transient receptor potential cation channel subfamily V member 1. Created with BioRender.com.For instance, injury-induced neuronal CCL2 overexpression is sufficient to increase neuronal regenerative capacity through a paracrine mechanism (Figure 1A). This secreted CCL2 recruits infiltrating macrophages that in turn activate neuronal gp-130 receptor through interleukin-6 family cytokines such as leukemia inhibitory factor or ciliary neurotrophic factor (Niemi et al., 2016). Similarly, dorsal root ganglia (DRG) neurons trigger the cleavage and release of CX3CL1, recruiting and activating peripheral CX3CR1+ macrophages (Figure 1B; Hervera et al., 2018). This process has been shown to be essential during peripheral nerve regeneration after injury, as well as during the activation of the conditioning effect in DRG neurons. Briefly, recruited macrophages signal back to DRG neurons through exosome-based redox signals, modifying different key neuronal pathways that in turn activate their intrinsic regenerative program. Alternatively, after excitotoxic injuries the increased expression of CCL2 and CXCL2 has long been described to promote neuronal apoptosis; however, evidence showed that CCL2/CXCL2-stimulated astrocytes could promote neuronal survival and differentiation through a mechanism based on basic fibroblast growth factor (Figure 1C; Kalehua et al., 2004). These data support the duality of autocrine and paracrine roles for these chemokines following central nervous system injury. Interestingly, a recent study has shown how dietary interventions such as intermittent fasting, might alter the neuronal secretome through changes in the gut microbiome, leading to enhanced regenerative capacity in a global response to metabolic changes that comprise the gut microbiome, possibly the neuronal secretome, and the immune response after injury (Serger et al., 2022). On the negative side, T-cell signaling after an injury has been recently shown to be exacerbated during aging, leading to lymphotoxin dependant neuronal overexpression of the chemokine CXCL13. This chemokine in turn recruits CXCR5+CD8+ T-cells, which interact with injured neurons, repressing axonal regeneration of DRG neurons via caspase 3 activation (Figure 1D; Zhou et al., 2022). Additionally, several chemokine receptors have also been reported to be widely expressed in different neuronal types, however, their functions remain less investigated. In that sense, we have recently described how specific activation of transient receptor potential cation channel subfamily V member 1 (TRPV1)+ nociceptors induces the production and release of CCL21. In turn, this CCL21 acts on proprioceptors, promoting neurite outgrowth through activation of the receptor CCR7, leading to MEK-ERK pathway activation and reorganization of the growth cone dynamics (Figure 1E; Mesquida-Veny et al., 2022). While injured nociceptors were already described to release CCL21, its effects were mainly attributed to immune cell recruitment and neuropathic pain. Our results contributed to the new implications that have been emerging for this chemokine, such as after cartilage damage, where it can be released to promote regeneration, by increasing the migration of mesenchymal stem cells (Joutoku et al., 2019). Recent studies have also shown that increased activity in nociceptive neurons after tissue damage, which is typically associated with pathological neuropathic pain, and, in our study, through the release of CCL21, can also be fundamental in the healing process, such as promoting adipose tissue regeneration via calcitonin gene-related peptide production, a process which was found to be required for proper regeneration in that context (Rabiller et al., 2021). These, together with our findings unravel a novel role for nociceptive signaling in the healing process, highlighting the need for deeper exploration of the precise mechanisms involved in this response in order to identify new therapeutic targets for regeneration, including the nervous system. These latest findings also stress the importance of the physiological-like activation of nociceptors, which seems to play an important role in wound healing and nerve regeneration. This also indicates that injury-induced pain, which was typically presumed to be a deleterious response, may initiate an intricate dialogue among cell types that may in fact be fundamental to drive the healing process in different tissues. While the entirety of different components implicated in this mechanism remains to be discovered, neuronal chemokines have already showcased their role among the main actors. Nociception and pain signaling may therefore be a more complex evolutionary mechanism than we had previously anticipated, as it is increasingly described in several processes orchestrating neuronal plasticity and the immune response, functions that go beyond their pre-established alerting role against hazardous situations. This is yet another demonstration of the complex and still unresolved pleiotropy of existing survival mechanisms, in line with what has already been observed for inflammation in the central nervous system, where the failure of proper transitions between the different phases of healing leads to exacerbated inflammation and further damage after injury (Mesquida-Veny et al., 2021). Moreover, this novel concept also questions the indiscriminate use of strong analgesics, after an injury, which may be hindering our physiological response and thus the adequate transitions during the healing process, similarly to the current practice of using modulatory rather than indiscriminate immunosuppressive drugs after spinal cord injuries. While nociceptor activation lays out of scope as a therapeutic approach, further investigation and characterization of the signaling pathways initiated by nociceptor stimulation may increase our understanding of the molecular mechanisms driving the healing process and may enable the design of new strategies to foster tissue regeneration. Interestingly, while most of the underlying mechanisms remain to be further characterized, other neuronally-derived chemokines, CCL5, CCL20, and CXCL2, have already been described to promote axonal growth (Bhardwaj et al., 2013). These chemokines were also shown to be released in response to hepatocyte growth factor, which participates in axon morphogenesis. Similarly, a recent study highlights the need of CCL5 to promote the regenerative effects induced by ciliary neurotrophic factor in a central nervous system injury model such as the optic nerve (Figure 1F; Xie et al., 2021). Finally, and while its neuronal origin remains controversial, CXCL12 has also been shown to promote axonal growth (Figure 1G), as well as to drive recovery after a demyelinating injury via its receptor CXCR4 (Zilkha-Falb et al., 2016). While each day there is more and more evidence on the role of neuronal chemokines as paracrine messengers between neurons and other cell types, our and other studies are starting to unravel them as novel signs of neuron-neuron communication, beyond classical neurotransmission. Nonetheless, a better characterization of their impact on healing, and the underlying mechanisms at different stages after injury would help to optimize their applications and tailor current therapies. This work was supported by HDAC3-EAE-SCI Project with ref. PID2020-119769RA-I00 from MCIN/AEI/10.13039/501100011033 to AH. C-Editors: Zhao M, Liu WJ, Qiu Y; T-Editor: Jia Y