Overexpression of the monocyte chemokine CCL2 in dorsal root ganglion neurons causes a conditioning-like increase in neurite outgrowth and does so via a STAT3 dependent mechanism

Abstract

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.

Dynamic impact of brief electrical nerve stimulation on the neural immune axis—polarization of macrophages toward a pro‐repair phenotype in demyelinated peripheral nerve

“…The concept that manipulating the immune response may be beneficial for remyelination is not new (Pavelko et al, ). Past studies report both beneficial and detrimental outcomes (Triarhou and Herndon, ) and current studies link polarization of macrophages to an M2 phenotype as being beneficial to both CNS and PNS repair (Kigerl et al, ; Mokarram et al, ; Niemi et al, ; Wang et al, ; Ydens et al, ). However, our study is first to show that brief increases in neuronal activity can drive the polarization of immune cells toward an M2 pro‐repair phenotype, coincident with faster and more robust remyelination (M c Lean et al, ).…”

“…This is similar to the response we have observed in the 8 day focally demyelinated nerves subjected to ES 3 days previously, with higher levels of Schwann cell-expressed MCP1 being observed in conjunction with increased macrophage numbers and the majority of these macrophages polarized to an M2 phenotype. Increased CCL2 has also been shown to drive pro-regenerative responses in sensory neurons (Boivin et al, 2007;Kwon et al, 2015;Niemi et al, 2016), suggesting that the ability of ES to shift the balance in favor of the M2 phenotype in our focal demyelination model is likely key to our observed improvements in remyelination (M c Lean et al, 2014).…”

“…The latter implies that ES may be altering the chemokine signaling pathway responsible for recruitment of circulating monocytes to the affected region. Chemokine (C‐C motif) ligand 2 (CCL2; aka MCP‐1) induced recruitment of peripheral immune cells is a central feature in many demyelinating and peripheral nerve pathologies (Kim et al, ; Siebert et al, ; Subang and Richardson, ), with alteration of this axis ameliorating the impacts of demyelinating pathologies and promoting peripheral nerve repair (Groh et al, ; Kim et al, ; Kwon et al, ; Niemi et al, ; Yuan et al, ).…”

Atf3 mutant mice show reduced axon regeneration and impaired regeneration-associated gene induction after peripheral nerve injury

“…PNS injury upregulates Ccl2 in DRGs [27,74,75] and FMNs [76]. So far, CCL2 has been attributed functions during nerve injury-associated immune responses mainly by regulating monocyte rather than neuronal responses [23–29].…”

Intrinsic mechanisms of neuronal axon regeneration

“…Although it is known that JAKs are activated via membrane glycoprotein 130 (gp130; also known as IL6Rβ) signalling 68 , it is unclear which ligands are responsible for gp130 activation and their source. Several studies have suggested that interleukin 6 (IL-6) 69–71 and CC-chemokine ligand 2 (CCL2) produced by macrophages 72 are responsible for gp130 activation in DRG neurons, whereas astrocytic expression of ciliary neurotrophic factor (CNTF) 73 and leukaemia inhibitory factor (LIF) 74 activate gp130 in RGCs. pSTAT3 is retrogradely transported by the molecular motor dynein to the cell soma 57 .…”