Macrophage colony stimulating factor is a crucial factor for the intrinsic macrophage response in mice heterozygously deficient for the myelin protein P0

Presymptomatic macrophage targeting has a long-lasting therapeutic effect on treatment termination

Macrophage-mediated inflammation is a potent driver of disease progression in mouse models of Charcot-Marie-Tooth (CMT) 1 diseases. This leads to the possibility to consider these cells as therapeutic targets to dampen disease outcome in the so far non-treatable neuropathies. As a pharmacological proof-of-principle study, long-term targeting of nerve macrophages with the orally applied CSF-1 receptor specific kinase (c-FMS) inhibitor PLX5622 showed a substantial alleviation of the neuropathy in distinct CMT1 mouse models. However, regarding translational options, clinically relevant questions emerged regarding treatment onset, duration and termination. Corroborating previous data, we here show that in a model for CMT1B, peripheral neuropathy was substantially alleviated after early continuous PLX5622 treatment in CMT1B mice, leading to preserved motor function. However, late-onset treatment failed to mitigate histopathological and clinical features, despite a similar reduction in the number of macrophages. Surprisingly, in CMT1B mice, terminating early PLX5622 treatment at six months was still sufficient to preserve motor function at 12 months of age, suggesting a long-lasting, therapeutic effect of early macrophage depletion. This novel and unexpected finding may have important translational implications, since we here show that continuous macrophage targeting appears not to be necessary for disease alleviation, provided that the treatment starts within an early, critical time window.

Targeting macrophage and microglia activation with colony stimulating factor 1 receptor inhibitor is an effective strategy to treat injury-triggered neuropathic pain

Introduction Neuropathic pain is a debilitating condition. The importance of neuroimmune interactions in neuropathic pain has been evidenced by the involvement of different immune cells in peripheral and central sensitization of pathological pain. Macrophages and microglia are the most abundant immune cells activated in injured nerves and spinal cord, respectively. Several lines of evidence showed that macrophage/microglia survival, activation, proliferation, and differentiation require the involvement of macrophage-colony stimulating factor. In this study, we investigated whether blocking macrophage-colony stimulating factor/colony stimulating factor 1 receptor signaling can be effective in relieving neuropathic pain. Materials and methods Partial sciatic nerve ligation was performed in mice to induce neuropathic pain behavior. Mice were orally treated with a selective colony stimulating factor 1 receptor inhibitor, PLX5622, daily in both preventive (two days prior to surgery until D14 post-partial sciatic nerve ligation) and reversal paradigms (D28-D33 post-partial sciatic nerve ligation). Animal neuropathic pain behavior was monitored using von Frey hairs and acetone application. Phenotype of macrophages in injured nerves was analyzed at D3 and D33 post-injury using flow cytometry analysis. The effect of PLX5622 on microglia activation in lumbar spinal cord was further examined by immunohistochemistry using Iba-1 antibody. Results Significant alleviation of both mechanical and cold allodynia was observed in PLX5622-treated animals, both in preventive and reversal paradigms. PLX5622 treatment reduced the total number of macrophages in injured nerves, it appears colony stimulating factor 1 receptor inhibition affected more specifically CD86⁺ (M1 like) macrophages. Consequently, the expression of various pro-inflammatory cytokines (TNF-α, IL-1β) was reduced. Microglia activation in dorsal horn of lumbar spinal cord following partial sciatic nerve ligation was significantly inhibited with PLX5622 treatment in both preventive and reversal paradigms. Conclusion Macrophages in peripheral nerve and microglia in the spinal cord are required in the generation and maintenance of injury-associated neuropathic pain. Blocking macrophage-colony stimulating factor/colony stimulating factor 1 receptor signaling on these myeloid cells along the pain transmission pathway is an effective strategy to alleviate neuropathic pain.

Cell-Surface and Secreted Isoforms of CSF-1 Exert Opposing Roles in Macrophage-Mediated Neural Damage in Cx32-Deficient Mice

Previous studies in myelin-mutant mouse models of the inherited and incurable nerve disorder, Charcot-Marie-Tooth (CMT) neuropathy, have demonstrated that low-grade secondary inflammation implicating phagocytosing macrophages amplifies demyelination, Schwann cell dedifferentiation and perturbation of axons. The cytokine colony stimulating factor-1 (CSF-1) acts as an important regulator of these macrophage-related disease mechanisms, as genetic and pharmacologic approaches to block the CSF-1/CSF-1R signaling result in a significant alleviation of pathological alterations in mutant peripheral nerves. In mouse models of CMT1A and CMT1X, as well as in human biopsies, CSF-1 is predominantly expressed by endoneurial fibroblasts, which are closely associated with macrophages, suggesting local stimulatory mechanisms. Here we investigated the impact of cell-surface and secreted isoforms of CSF-1 on macrophage-related disease in connexin32-deficient (Cx32def) mice, a mouse model of CMT1X. Our present observations suggest that the secreted proteoglycan isoform (spCSF-1) is predominantly expressed by fibroblasts, whereas the membrane-spanning cell-surface isoform (csCSF-1) is expressed by macrophages. Using crossbreeding approaches to selectively restore or overexpress distinct isoforms in CSF-1-deficient (osteopetrotic) Cx32def mice, we demonstrate that both isoforms equally regulate macrophage numbers dose-dependently. However, spCSF-1 mediates macrophage activation and macrophage-related neural damage, whereas csCSF-1 inhibits macrophage activation and attenuates neuropathy. These results further corroborate the important role of secondary inflammation in mouse models of CMT1 and might identify specific targets for therapeutic approaches to modulate innate immune reactions.

SIGNIFICANCE STATEMENT

Mouse models of Charcot-Marie-Tooth neuropathy have indicated that low-grade secondary inflammation involving phagocytosing macrophages amplifies demyelination, Schwann cell dedifferentiation, and perturbation of axons. The recruitment and pathogenic activation of detrimental macrophages is regulated by CSF-1, a cytokine that is mostly expressed by fibroblasts in the diseased nerve and exists in three isoforms. We show that the cell-surface and secreted isoforms of CSF-1 have opposing effects on macrophage activation and disease progression in a mouse model of CMT1X. These insights into opposing functions of disease-modulating cytokine isoforms might enable the development of specific therapeutic approaches.

Targeting the colony stimulating factor 1 receptor alleviates two forms of Charcot-Marie-Tooth disease in mice

See Scherer (doi:10.1093/awv279) for a scientific commentary on this article.Charcot-Marie-Tooth type 1 neuropathies are inherited disorders of the peripheral nervous system caused by mutations in Schwann cell-related genes. Typically, no causative cure is presently available. Previous preclinical data of our group highlight the low grade, secondary inflammation common to distinct Charcot-Marie-Tooth type 1 neuropathies as a disease amplifier. In the current study, we have tested one of several available clinical agents targeting macrophages through its inhibition of the colony stimulating factor 1 receptor (CSF1R). We here show that in two distinct mouse models of Charcot-Marie-Tooth type 1 neuropathies, the systemic short- and long-term inhibition of CSF1R by oral administration leads to a robust decline in nerve macrophage numbers by ∼70% and substantial reduction of the typical histopathological and functional alterations. Interestingly, in a model for the dominant X-linked form of Charcot-Marie-Tooth type 1 neuropathy, the second most common form of the inherited neuropathies, macrophage ablation favours maintenance of axonal integrity and axonal resprouting, leading to preserved muscle innervation, increased muscle action potential amplitudes and muscle strengths in the range of wild-type mice. In another model mimicking a mild, demyelination-related Charcot-Marie-Tooth type 1 neuropathy caused by reduced P0 (MPZ) gene dosage, macrophage blockade causes an improved preservation of myelin, increased muscle action potential amplitudes, improved nerve conduction velocities and ameliorated muscle strength. These observations suggest that disease-amplifying macrophages can produce multiple adverse effects in the affected nerves which likely funnel down to common clinical features. Surprisingly, treatment of mouse models mimicking Charcot-Marie-Tooth type 1A neuropathy also caused macrophage blockade, but did not result in neuropathic or clinical improvements, most likely due to the late start of treatment of this early onset disease model. In summary, our study shows that targeting peripheral nerve macrophages by an orally administered inhibitor of CSF1R may offer a highly efficacious and safe treatment option for at least two distinct forms of the presently non-treatable Charcot-Marie-Tooth type 1 neuropathies.

Proteolipid protein modulates preservation of peripheral axons and premature death when myelin protein zero is lacking

Protein zero (P0) is the major structural component of peripheral myelin. Lack of this adhesion protein from Schwann cells causes a severe dysmyelinating neuropathy with secondary axonal degeneration in humans with the neuropathy Dejerine-Sottas syndrome (DSS) and in the corresponding mouse model (P0(null)-mice). In the mammalian CNS, the tetraspan-membrane protein PLP is the major structural myelin constituent and required for the long-term preservation of myelinated axons, which fails in hereditary spastic paraplegia (SPG type-2) and the relevant mouse model (Plp(null)-mice). The Plp-gene is also expressed in Schwann cells but PLP is of very low abundance in normal peripheral myelin; its function has thus remained enigmatic. Here we show that the abundance of PLP but not of other tetraspan myelin proteins is strongly increased in compact peripheral myelin of P0(null)-mice. To determine the functional relevance of PLP expression in the absence of P0, we generated P0(null)*Plp(null)-double-mutant mice. Compared with either single-mutant, P0(null)*Plp(null)-mice display impaired nerve conduction, reduced motor functions, and premature death. At the morphological level, axonal segments were frequently non-myelinated but in a one-to-one relationship with a hypertrophic Schwann cell. Importantly, axonal numbers were reduced in the vital phrenic nerve of P0(null)*Plp(null)-mice. In the absence of P0, thus, PLP also contributes to myelination by Schwann cells and to the preservation of peripheral axons. These data provide a link between the Schwann cell-dependent support of peripheral axons and the oligodendrocyte-dependent support of central axons.

CSF-1-activated macrophages are target-directed and essential mediators of Schwann cell dedifferentiation and dysfunction in Cx32-deficient mice

We investigated connexin 32 (Cx32)-deficient mice, a model for the X-linked form of Charcot-Marie-Tooth neuropathy (CMT1X), regarding the impact of low-grade inflammation on Schwann cell phenotype. Whereas we previously identified macrophages as amplifiers of the neuropathy, we now explicitly focus on the impact of the phagocytes on Schwann cell dedifferentiation, a so far not-yet addressed disease-related mechanism for CMT1X. Using mice heterozygously deficient for Cx32 and displaying both Cx32-positive and -negative Schwann cells in one and the same nerve, we could demonstrate that macrophage clusters rather than single macrophages precisely associate with mutant but not with Cx32-positive Schwann cells. Similarly, in an advanced stage of Schwann cell perturbation, macrophage clusters were strongly associated with NCAM- and L1-positive, dedifferentiated Schwann cells. To clarify the role of macrophages regarding Schwann cell dedifferentiation, we generated Cx32-deficient mice additionally deficient for the macrophage-directed cytokine colony-stimulating factor (CSF)-1. In the absence of CSF-1, Cx32-deficient Schwann cells not only showed the expected amelioration in myelin preservation but also failed to upregulate the Schwann cell dedifferentiation markers NCAM and L1. Another novel and unexpected finding in the double mutants was the retained activation of ERK signaling, a pathway which is detrimental for Schwann cell homeostasis in myelin mutant models. Our findings demonstrate that increased ERK signaling can be compatible with the maintenance of Schwann cell differentiation and homeostasis in vivo and identifies CSF-1-activated macrophages as crucial mediators of detrimental Schwann cell dedifferentiation in Cx32-deficient mice.

Endogenous antibodies contribute to macrophage-mediated demyelination in a mouse model for CMT1B

Background

We could previously identify components of both the innate and the adaptive immune system as disease modifiers in the pathogenesis of models for Charcot-Marie-Tooth (CMT) neuropathies type 1B and 1X. As part of the adaptive immune system, here we investigated the role of antibodies in a model for CMT1B.

Methods

Antibodies were localized and characterized in peripheral nerves of the CMT1B model by immunohistochemistry and Western blot analysis. Experimental ablation of antibodies was performed by cross breeding the CMT1B models with mutants deficient in B-lymphocytes (JHD−/− mutants). Ameliorated demyelination by antibody deficiency was reverted by intravenous injection of mouse IgG fractions. Histopathological analysis was performed by immunocytochemistry and light and quantitative electron microscopy.

Results

We demonstrate that in peripheral nerves of a mouse model for CMT1B, endogenous antibodies strongly decorate endoneurial tubes of peripheral nerves. These antibodies comprise IgG and IgM subtypes and are preferentially, but not exclusively, associated with nerve fiber aspects nearby the nodes of Ranvier. In the absence of antibodies, the early demyelinating phenotype is substantially ameliorated. Reverting the neuropathy by reconstitution with murine IgG fractions identified accumulating antibodies as potentially pathogenic at this early stage of disease.

Conclusions

Our study demonstrates that in a mouse model for CMT1B, endogenous antibodies contribute to early macrophage-mediated demyelination and disease progression. Thus, both the innate and adaptive immune system are mutually interconnected in a genetic model for demyelination. Since in Wallerian degeneration antibodies have also been shown to be involved in myelin phagocytosis, our study supports our view that inherited demyelination and Wallerian degeneration share common mechanisms, which are detrimental when activated under nonlesion conditions.

Similarities between inherited demyelinating neuropathies and Wallerian degeneration: an old repair program may cause myelin and axon perturbation under nonlesion conditions

Wallerian degeneration (WD) and inherited demyelinating neuropathies of the Charcot-Marie-Tooth type 1 (CMT1) appear to represent completely distinct events. CMT1-like diseases are chronic disorders of peripheral nerves that are genetically caused and lead to secondary neurodegenerative events, resulting in usually non-treatable disabilities, whereas WD is an acute, usually transient, reaction on injuries, aiming to allow peripheral nerve regeneration. Despite these differences, there are some striking similarities regarding molecular characteristics of neural cells in the affected peripheral nerves. The most conspicuous similarities might comprise the inflammatory component in both situations, as identified in appropriate mouse models. However, although inflammation is a beneficial component in WD, leading to removal of regrowth-repellent myelin debris, inflammation in CMT1 mouse models causes damage of initially intact nerve fibers. We hypothesize that, in CMT1 models, molecular pathways are activated that are shared with an important repair program after peripheral nerve injury, but lead to neural perturbation when activated under nonlesion conditions, as is the case in CMT1. These novel insights into the pathogenesis of CMT1 might be instrumental for the development of new therapeutic options in humans.

Myelin antigen load influences antigen presentation and severity of central nervous system autoimmunity

This study was designed to understand the impact of self-antigen load on manifestation of organ specific autoimmunity. Using a transgenic mouse model characterized by CNS hypermyelination, we show that larger myelin content results in greater severity of experimental autoimmune encephalomyelitis attributable to an increased number of microglia within the hypermyelinated brain. We conclude that a larger self-antigen load affects an increase in number of tissue resident antigen presenting cells (APCs) most likely due to compensatory antigen clearance mechanisms thereby enhancing the probability of productive T cell-APC interactions in an antigen abundant environment and results in enhanced severity of autoimmune disease.

Neuroinflammation by cytotoxic T-lymphocytes impairs retrograde axonal transport in an oligodendrocyte mutant mouse

Mice overexpressing proteolipid protein (PLP) develop a leukodystrophy-like disease involving cytotoxic, CD8+ T-lymphocytes. Here we show that these cytotoxic T-lymphocytes perturb retrograde axonal transport. Using fluorogold stereotactically injected into the colliculus superior, we found that PLP overexpression in oligodendrocytes led to significantly reduced retrograde axonal transport in retina ganglion cell axons. We also observed an accumulation of mitochondria in the juxtaparanodal axonal swellings, indicative for a disturbed axonal transport. PLP overexpression in the absence of T-lymphocytes rescued retrograde axonal transport defects and abolished axonal swellings. Bone marrow transfer from wildtype mice, but not from perforin- or granzyme B-deficient mutants, into lymphocyte-deficient PLP mutant mice led again to impaired axonal transport and the formation of axonal swellings, which are predominantly located at the juxtaparanodal region. This demonstrates that the adaptive immune system, including cytotoxic T-lymphocytes which release perforin and granzyme B, are necessary to perturb axonal integrity in the PLP-transgenic disease model. Based on our observations, so far not attended molecular and cellular players belonging to the immune system should be considered to understand pathogenesis in inherited myelin disorders with progressive axonal damage.

FMS Kinase Inhibitors: Current Status and Future Prospects

FMS, first discovered as the oncogene responsible for Feline McDonough Sarcoma, is a type III receptor tyrosine kinase that binds to the macrophage or monocyte colony-stimulating factor (M-CSF or CSF-1). Signal transduction through that binding results in survival, proliferation, and differentiation of monocyte/macrophage lineage. Overexpression of CSF-1 and/or FMS has been implicated in a number of disease states such as the growth of metastasis of certain types of cancer, in promoting osteoclast proliferation in bone osteolysis, and many inflammatory disorders. Inhibition of CSF-1 and/or FMS may help treat these pathological conditions. This article reviews FMS gene, FMS kinase, CSF-1, IL-34, and their roles in bone osteolysis, cancer biology, and inflammation. Monoclonal antibodies, FMS crystal structure, and small molecule FMS kinase inhibitors of different chemical scaffolds are also reviewed.

Colony-stimulating factor-1 mediates macrophage-related neural damage in a model for Charcot-Marie-Tooth disease type 1X

Previous studies in our laboratory have shown that in models for three distinct forms of the inherited and incurable nerve disorder, Charcot-Marie-Tooth neuropathy, low-grade inflammation implicating phagocytosing macrophages mediates demyelination and perturbation of axons. In the present study, we focus on colony-stimulating factor-1, a cytokine implicated in macrophage differentiation, activation and proliferation and fostering neural damage in a model for Charcot-Marie-Tooth neuropathy 1B. By crossbreeding a model for the X-linked form of Charcot-Marie-Tooth neuropathy with osteopetrotic mice, a spontaneous null mutant for colony-stimulating factor-1, we demonstrate a robust and persistent amelioration of demyelination and axon perturbation. Furthermore, functionally important domains of the peripheral nervous system, such as juxtaparanodes and presynaptic terminals, were preserved in the absence of colony-stimulating factor-1-dependent macrophage activation. As opposed to other Schwann cell-derived cytokines, colony-stimulating factor-1 is expressed by endoneurial fibroblasts, as revealed by in situ hybridization, immunocytochemistry and detection of β-galactosidase expression driven by the colony-stimulating factor-1 promoter. By both light and electron microscopic studies, we detected extended cell-cell contacts between the colony-stimulating factor-1-expressing fibroblasts and endoneurial macrophages as a putative prerequisite for the effective and constant activation of macrophages by fibroblasts in the chronically diseased nerve. Interestingly, in human biopsies from patients with Charcot-Marie-Tooth type 1, we also found frequent cell-cell contacts between macrophages and endoneurial fibroblasts and identified the latter as main source for colony-stimulating factor-1. Therefore, our study provides strong evidence for a similarly pathogenic role of colony-stimulating factor-1 in genetically mediated demyelination in mice and Charcot-Marie-Tooth type 1 disease in humans. Thus, colony-stimulating factor-1 or its cognate receptor are promising target molecules for treating the detrimental, low-grade inflammation of several inherited neuropathies in humans.

Macrophages counteract demyelination in a mouse model of globoid cell leukodystrophy

Whether microglia and macrophages are beneficial or harmful in many neurological disorders, including demyelinating diseases such as multiple sclerosis and the leukodystrophies, is currently under debate. Answering this question is of special interest in globoid cell leukodystrophy (GLD), a genetic fatal demyelinating disease, because its rapidly progressive demyelination in the nervous system is accompanied by a characteristic accumulation of numerous globoid macrophages. Therefore, we cross-bred the twitcher (twi) mouse, a bona fide model of GLD, with the macrophage-deficient osteopetrotic mutant and studied the resultant macrophage-deficient twitcher (twi+op) mouse. The twi+op mouse had few microglia and macrophages in the white matter and, interestingly, showed a more severe clinical phenotype compared to the twi mouse. The number of nonmyelinated axons in the spinal cord was significantly higher in twi+op mice than in twi mice at 45 d old. The difference appeared to be due to impaired remyelination in twi+op mice rather than accelerated demyelination. Quantitative reverse transcription PCR and immunohistochemical studies revealed that the recruitment of oligodendrocyte progenitor cells in response to demyelination was compromised in twi+op mice. Increased myelin debris in the white matter parenchyma of twi+op mice suggested that phagocytosis by macrophages may play an important role in promoting remyelination. Macrophage markers for both protective and destructive phenotypes were significantly upregulated in the spinal cord of twi mice but were close to normal in twi+op mice due to the reduced macrophage number. The overall effects of macrophages in GLD appear to be beneficial to myelin by promoting myelin repair.

Attenuation of MCP-1/CCL2 expression ameliorates neuropathy in a mouse model for Charcot-Marie-Tooth 1X

The chemokine monocyte chemoattractant protein-1 (MCP-1/CCL2) has been previously shown to be an important mediator of macrophage-related neural damage in models of two distinct inherited neuropathies, Charcot-Marie-Tooth (CMT) 1A and 1B. In mice deficient in the gap junction protein connexin 32 (Cx32def), an established model for the X-chromosome-linked dominant form of CMT (CMT1X), we investigated the role of the chemokine in macrophage immigration and neural damage by crossbreeding the Cx32def mice with MCP-1 knockout mutants. In Cx32def mutants typically expressing increased levels of MCP-1, macrophage numbers were strongly elevated, caused by an MCP-1-mediated influx of haematogenous macrophages. Curiously, the complete genetic deletion of MCP-1 did not cause reduced macrophage numbers in the nerves due to compensatory proliferation of resident macrophages. In contrast, and as already seen in other CMT models, heterozygous deletion of MCP-1 led to reduced numbers of phagocytosing macrophages and an alleviation of demyelination. Whereas alleviated demyelination was transient, axonal damage was persistently improved and even robust axonal sprouting was detectable at 12 months. Other axon-related features were alleviated electrophysiological parameters, reduced muscle denervation and atrophy, and increased muscle strength. Similar to models for CMT1A and CMT1B, we identified MEK-ERK signalling as mediating MCP-1 expression in Cx32-deficient Schwann cells. Blocking this pathway by the inhibitor CI-1040 caused reduced MCP-1 expression, attenuation of macrophage increase and amelioration of myelin- and axon-related alterations. Thus, attenuation of MCP-1 upregulation by inhibiting ERK phosphorylation might be a promising approach to treat CMT1X and other so far untreatable inherited peripheral neuropathies in humans.

Effects of the CC chemokine receptor 2 in mice deficient for the myelin protein zero (P0)

In myelin protein zero (P0)-deficient mice immune cells are critically involved in the pathogenesis of a primary genetic disease. Previously it has been shown that the chemokine CCL2 affects the functional properties of endoneurial macrophages in heterozygous P0 mice. The aim of the present study was to characterize the role of the CCL2-receptor CCR2 in the pathogenesis of the neuropathy in P0 deficient mice. In demyelinating nerves of heterozygous P0 mice (P0+/-) CCR2-deficiency did not affect the number of endoneurial macrophages; there was a trend towards a higher number of activated macrophages. CCR2-deficiency resulted in an increased nerve demyelination. In dysmyelinating nerves of homozygous P0 mice (P0-/-), CCR2-deficiency led to a significant decrease of endoneurial macrophages but did not affect axonal degeneration. There was no effect of CCR2 on T-lymphocytes in both disease models. Our data confirm a functional role of the CCR2 receptor in the examined models of hereditary neuropathies. In P0+/- mutants CCR2 decreases macrophage activation and is protective against demyelination, whereas in P0-/- mice it increases the accumulation of endoneurial macrophages.

MCP-1/CCL2 modifies axon properties in a PMP22-overexpressing mouse model for Charcot-Marie-tooth 1A neuropathy

Charcot-Marie-Tooth 1A (CMT1A) neuropathy, the most common inherited peripheral neuropathy, is primarily caused by a gene duplication for the peripheral myelin protein-22 (PMP22). In an accordant mouse model, we investigated the role of monocyte chemoattractant protein-1 (MCP-1/CCL2) as a regulator of nerve macrophages and neural damage including axonopathy and demyelination. By generating PMP22tg mice with reduced levels or lack of MCP-1/CCL2, we found that MCP-1/CCL2 is involved in the increase of macrophages in mutant nerves. PMP22tg mice with wild-type levels of MCP-1/CCL2 showed strong macrophage increase in the diseased nerves, whereas either 50% reduction or total absence of MCP-1/CCL2 led to a moderate or a strong reduction of nerve macrophages, respectively. Interestingly, MCP-1/CCL2 expression level and macrophage numbers were correlated with features indicative of axon damage, such as maldistribution of K+ channels, reduced compound muscle action potentials, and muscle weakness. Demyelinating features, however, were most highly reduced when MCP-1/CCL2 was diminished by 50%, whereas complete lack of MCP-1/CCL2 showed an intermediate demyelinating phenotype. We also identified the MEK1/2-ERK1/2-pathway as being involved in MCP-1/CCL2 expression in the Schwann cells of the CMT1A model. Our data show that, in a CMT1A model, MCP-1/CCL2 activates nerve macrophages, mediates both axon damage and demyelination, and may thus be a promising target for therapeutic approaches.

Further evidence for a crucial role of resident endoneurial macrophages in peripheral nerve disorders: lessons from acrylamide-induced neuropathy

Endoneurial macrophages are crucially involved in the pathogenesis of neuropathies. Historically, the macrophage response in neuropathies is believed to be of hematogenous origin. However, recent studies could demonstrate an intrinsic generation of the early macrophage response by resident endoneurial macrophages after traumatic nerve injury and in a model of hereditary neuropathy. We hypothesized that the local macrophage response might suffice to generate an appropriate macrophage response in mild neuropathies, supplemented by infiltrating macrophages only in severe nerve pathology. To clarify this assumption, we investigated the macrophage response in acrylamide-induced neuropathy as a model of a slowly progressive neuropathy with a defined onset. We induced the neuropathy in bone marrow chimeric mice carrying green fluorescent protein transgenic bone marrow, allowing the differentiation of resident (GFP(-)) and invading hematogenous endoneurial (GFP(+)) macrophages. Quantification of GFP(-) and GFP(+) endoneurial macrophages in the sciatic nerve revealed an increase only of resident macrophages in proximal parts, whereas in distal parts a minor additional influx of hematogenous macrophages was observed. The immunohistochemical profile of GFP(-) and GFP(+) macrophages was similar but distal GFP(-) macrophages were differentially activated than their GFP(+) counterparts. Characterization of CCR2-deficient mice revealed a function for this chemokine system in attracting hematogenous macrophages but not in generating the intrinsic macrophage response. In conclusion, we provide evidence for a role of resident macrophages in acrylamide-induced neuropathy. Resident endoneurial macrophages intrinsically generate the macrophage response in this slowly progressive neuropathy, which only becomes supplemented by hematogenous macrophages in distal areas of more pronounced damage.