Macrophages have been implicated in peripheral nerve regeneration for some time, supposedly through their involvement in Wallerian degeneration, the process by which the distal nerve degenerates after axotomy and is cleared by phagocytosis. Thus, in several studies in which macrophage accumulation in the distal nerve was reduced and Wallerian degeneration inhibited, regeneration was delayed. However, this interpretation ignores the more recent findings that macrophages also accumulate around axotomized cell bodies. The function of macrophage action at this second site has not been clear. In two mutant strains of mice, the slow Wallerian degeneration (Wld s ) mouse and the chemokine receptor CCR2 knock-out mouse, we report that macrophage accumulation after axotomy was abolished in both the dorsal root ganglion (DRG) and the distal sciatic nerve. To measure neurite outgrowth, DRG neurons were given a conditioning lesion, and outgrowth was measured in vitro 7 d later in the absence of the distal nerve segment. The increased growth normally seen after a conditioning lesion did not occur or was reduced in Wld s or CCR2 Ϫ/Ϫ mice. In the superior cervical ganglion (SCG), particularly in Wld s mice, macrophage accumulation was reduced but not abolished after axotomy. In SCG neurons from Wld s mice, the conditioning lesion response was unchanged; however, in CCR2 Ϫ/Ϫ mice in which the effect on macrophage accumulation was greater, SCG neurite outgrowth was significantly reduced. These results indicate that macrophages affect neurite outgrowth by acting at the level of peripheral ganglia in addition to any effects they might produce by facilitation of Wallerian degeneration.

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration. Wallerian degeneration and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

Neuroinflammation plays a critical role in the regeneration of peripheral nerves following axotomy. An injury to the sciatic nerve leads to significant macrophage accumulation in the L5 DRG, an effect not seen when the dorsal root is injured. We recently demonstrated that this accumulation around axotomized cell bodies is necessary for a peripheral conditioning lesion response to occur. Here we asked whether overexpression of the monocyte chemokine CCL2 specifically in DRG neurons of uninjured mice is sufficient to cause macrophage accumulation and to enhance regeneration or whether other injury-derived signals are required. AAV5-EF1α-CCL2 was injected intrathecally, and this injection led to a time-dependent increase in CCL2 mRNA expression and macrophage accumulation in L5 DRG, with a maximal response at 3 wk post-injection. These changes led to a conditioning-like increase in neurite outgrowth in DRG explant and dissociated cell cultures. This increase in regeneration was dependent upon CCL2 acting through its primary receptor CCR2. When CCL2 was overexpressed in CCR2 −/− mice, macrophage accumulation and enhanced regeneration were not observed. To address the mechanism by which CCL2 overexpression enhances regeneration, we tested for elevated expression of regeneration-associated genes in these animals. Surprisingly, we found that CCL2 overexpression led to a selective increase in LIF mRNA and neuronal phosphorylated STAT3 (pSTAT3) in L5 DRGs, with no change in expression seen in other RAGs such as GAP-43. Blockade of STAT3 phosphorylation by each of two different inhibitors prevented the increase in neurite outgrowth. Thus, CCL2 overexpression is sufficient to induce macrophage accumulation in uninjured L5 DRGs and increase the regenerative capacity of DRG neurons via a STAT3-dependent mechanism.

Neurite outgrowth is key to the formation of functional circuits during neuronal development. Neurotrophins, including nerve growth factor (NGF), increase neurite outgrowth in part by altering the function and expression of Ca 2؉ -permeable cation channels. Here we report that transient receptor potential vanilloid 2 (TRPV2) is an intracellular Ca 2؉ -permeable TRPV channel upregulated by NGF via the mitogen-activated protein kinase (MAPK) signaling pathway to augment neurite outgrowth. TRPV2 colocalized with Rab7, a late endosome protein, in addition to TrkA and activated extracellular signal-regulated kinase (ERK) in neurites, indicating that the channel is closely associated with signaling endosomes. In line with these results, we showed that TRPV2 acts as an ERK substrate and identified the motifs necessary for phosphorylation of TRPV2 by ERK. Furthermore, neurite length, TRPV2 expression, and TRPV2-mediated Ca 2؉ signals were reduced by mutagenesis of these key ERK phosphorylation sites. Based on these findings, we identified a previously uncharacterized mechanism by which ERK controls TRPV2-mediated Ca 2؉ signals in developing neurons and further establish TRPV2 as a critical intracellular ion channel in neuronal function.E stablishment of precise neural connections during nervous system development is essential in forming functional circuits. Neurite outgrowth allows for connection and communication between developing neurons and their targets. In the developing peripheral nervous system, nerve growth factor (NGF) is a targetderived extracellular cue necessary for outgrowth (1). Upon binding to its extracellular receptor, NGF activates the phosphoinositide 3-kinase (PI3K) signaling pathway, which is essential for the survival of developing neurons, and the mitogen-activated protein kinase (MAPK) pathway, which promotes differentiation and neurite outgrowth (2, 3). These signaling pathways have numerous downstream effectors in developing neurons, including several Ca 2ϩ -permeable transient receptor potential (TRP) channels (4-8).Thermosensitive TRP channels from the vanilloid subfamily (thermoTRPV channels) consist of four nonselective Ca 2ϩ -permeable cation channels, TRP vanilloid 1 (TRPV1) to TRPV4, originally described as pain and temperature sensors in adult sensory neurons (9-12). Recent evidence suggests, however, that only TRPV1 functions as a molecular sensor of heat and painful stimuli in vivo, while the function of TRPV2 to -4 remains unclear (13-16). TRPV2 and TRPV4 have also been detected in developing peripheral neurons, suggesting that they may play a role in growth programs during development (17, 18). Consistent with this notion, TRPV2 has been implicated in axon outgrowth (18), and critical mutations in TRPV4 result in peripheral axonal neuropathies (19,20). Despite these initial findings, the details by which thermoTRPV channels influence neuronal development remain unknown.Here we explore the molecular mechanisms by which thermoTRPV channels contribute to neurotrophin-mediated peripher...