BackgroundIn the SOD1G93A mutant rat model of amyotrophic lateral sclerosis (ALS), neuronal death and rapid paralysis progression are associated with the emergence of activated aberrant glial cells that proliferate in the degenerating spinal cord. Whether pharmacological downregulation of such aberrant glial cells will decrease motor neuron death and prolong survival is unknown. We hypothesized that proliferation of aberrant glial cells is dependent on kinase receptor activation, and therefore, the tyrosine kinase inhibitor masitinib (AB1010) could potentially control neuroinflammation in the rat model of ALS.MethodsThe cellular effects of pharmacological inhibition of tyrosine kinases with masitinib were analyzed in cell cultures of microglia isolated from aged symptomatic SOD1G93A rats. To determine whether masitinib prevented the appearance of aberrant glial cells or modified post-paralysis survival, the drug was orally administered at 30 mg/kg/day starting after paralysis onset.ResultsWe found that masitinib selectively inhibited the tyrosine kinase receptor colony-stimulating factor 1R (CSF-1R) at nanomolar concentrations. In microglia cultures from symptomatic SOD1G93A spinal cords, masitinib prevented CSF-induced proliferation, cell migration, and the expression of inflammatory mediators. Oral administration of masitinib to SOD1G93A rats starting after paralysis onset decreased the number of aberrant glial cells, microgliosis, and motor neuron pathology in the degenerating spinal cord, relative to vehicle-treated rats. Masitinib treatment initiated 7 days after paralysis onset prolonged post-paralysis survival by 40 %.ConclusionsThese data show that masitinib is capable of controlling microgliosis and the emergence/expansion of aberrant glial cells, thus providing a strong biological rationale for its use to control neuroinflammation in ALS. Remarkably, masitinib significantly prolonged survival when delivered after paralysis onset, an unprecedented effect in preclinical models of ALS, and therefore appears well-suited for treating ALS.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-016-0620-9) contains supplementary material, which is available to authorized users.

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Age is a recognized risk factor for amyotrophic lateral sclerosis (ALS), a paralytic disease characterized by progressive loss of motor neurons and neuroinflammation. A hallmark of aging is the accumulation of senescent cells. Yet, the pathogenic role of cellular senescence in ALS remains poorly understood. In rats bearing the ALS-linked SOD1 G93A mutation, microgliosis contribute to motor neuron death, and its pharmacologic downregulation results in increased survival. Here, we have explored whether gliosis and motor neuron loss were associated with cellular senescence in the spinal cord during paralysis progression. In the lumbar spinal cord of symptomatic SOD1 G93A rats, numerous cells displayed nuclear p16 INK4a as well as loss of nuclear Lamin B1 expression, two recognized senescence-associated markers. The number of p16 INK4a -positive nuclei increased by four-fold while Lamin B1-negative nuclei increased by 1,2-fold, respect to non-transgenic or asymptomatic transgenic rats. p16 INK4a -positive nuclei and Lamin B1-negative nuclei were typically localized in a subset of hypertrophic Iba1-positive microglia, occasionally exhibiting nuclear giant multinucleated cell aggregates and abnormal nuclear morphology. Next, we analyzed senescence markers in cell cultures of microglia obtained from the spinal cord of symptomatic SOD1 G93A rats. Although microglia actively proliferated in cultures, a subset of them developed senescence markers after few days in vitro and subsequent passages. Senescent SOD1 G93A microglia in culture conditions were characterized by large and flat morphology, senescence-associated beta-Galactosidase (SA-β-Gal) activity as well as positive labeling for p16 INK4a , p53, matrix metalloproteinase-1 (MMP-1) and nitrotyrosine, suggesting a senescent-associated secretory phenotype (SASP). Remarkably, in the degenerating lumbar spinal cord other cell types, including ChAT-positive motor neurons and GFAP-expressing astrocytes, also displayed nuclear p16 INK4a staining. These results suggest that cellular senescence is closely associated with inflammation and motor neuron loss occurring after paralysis onset in SOD1 G93A rats. The emergence of senescent cells could mediate key pathogenic mechanisms in ALS.

Distal axonopathy is a recognized pathological feature of amyotrophic lateral sclerosis (ALS). In the peripheral nerves of ALS patients, motor axon loss elicits a Wallerian‐like degeneration characterized by denervated Schwann cells (SCs) together with immune cell infiltration. However, the pathogenic significance of denervated SCs accumulating following impaired axonal growth in ALS remains unclear. Here, we analyze SC phenotypes in sciatic nerves of ALS patients and paralytic SOD1G93A rats, and identify remarkably similar and specific reactive SC phenotypes based on the pattern of S100β, GFAP, isolectin and/or p75NTR immunoreactivity. Different subsets of reactive SCs expressed colony‐stimulating factor‐1 (CSF1) and Interleukin‐34 (IL‐34) and closely interacted with numerous endoneurial CSF‐1R‐expressing monocyte/macrophages, suggesting a paracrine mechanism of myeloid cell expansion and activation. SCs bearing phagocytic phenotypes as well as endoneurial macrophages expressed stem cell factor (SCF), a trophic factor that attracts and activates mast cells through the c‐Kit receptor. Notably, a subpopulation of Ki67+ SCs expressed c‐Kit in the sciatic nerves of SOD1G93A rats, suggesting a signaling pathway that fuels SC proliferation in ALS. c‐Kit+ mast cells were also abundant in the sciatic nerve from ALS donors but not in controls. Pharmacological inhibition of CSF‐1R and c‐Kit with masitinib in SOD1G93A rats potently reduced SC reactivity and immune cell infiltration in the sciatic nerve and ventral roots, suggesting a mechanism by which the drug ameliorates peripheral nerve pathology. These findings provide strong evidence for a previously unknown inflammatory mechanism triggered by SCs in ALS peripheral nerves that has broad application in developing novel therapies.

Amyotrophic Lateral Sclerosis (ALS) is a paradigmatic neurodegenerative disease, characterized by progressive paralysis of skeletal muscles associated with motor neuron degeneration. It is well-established that glial cells play a key role in ALS pathogenesis. In transgenic rodent models for familial ALS reactive astrocytes, microglia and oligodendrocyte precursors accumulate in the degenerating spinal cord and appear to contribute to primary motor neuron death through a non-cell autonomous pathogenic mechanism. Furthermore in rats expressing the ALS-linked SOD1 mutation, rapid spread of paralysis coincides with emergence of neurotoxic and proliferating aberrant glia cells with an astrocyte-like phenotype (AbA cells) that are found surrounding damaged motor neurons. AbAs simultaneously express astrocytic markers GFAP, S100β and Connexin-43 along with microglial markers Iba-1, CD11b and CD163. Studies with cell cultures have shown that AbAs originate from inflammatory microglial cells that undergo phenotypic transition. Because AbAs appear only after paralysis onset and exponentially increase in parallel with disease progression, they appear to actively contribute to ALS progression. While several reviews have been published on the pathogenic role of glial cells in ALS, this review focuses on emergence and pro-inflammatory activity of AbAs as part of an increasingly complex neurodegenerative microenvironment during ALS disease development.

Objective: We aimed to determine the potential of aberrant glial cells (AbAs) isolated from the spinal cord of adult SOD1^G93A symptomatic rats to induce gliosis and neuronal damage following focal transplantation into the lumbar spinal cord of wild-type rats. Methods: AbAs were obtained from the spinal cords of SOD1^G93A symptomatic rats. One hundred thousand cells were injected using a glass micropipette into the lumbar spinal cords (L3-L5) of syngeneic wild-type adult rats. Equal volumes of culture medium or wild-type neonatal microglia were used as controls. Seven days after transplantation, immunohistochemistry analysis was carried out using astrocytic and microglia cell markers. Transplanted SOD1^G93A AbAs were recognized by specific antibodies to human SOD1 (hSOD1) or misfolded human SOD1. Results: Seven days after transplantation, AbAs were mainly detected in the medial region of the lumbar ventral horn as a well-limited cell cluster formed at the site of injection by their immunoreactivity to either misfolded SOD1 or normally folded hSOD1. Compared with controls, transplanted AbAs were surrounded by marked microgliosis and reactive astrocytes. Marked microgliosis was observed to extend bilaterally up to the cervical cord. Motor neurons close to AbA transplants were surrounded by activated glial cells and displayed ubiquitin aggregation. Conclusions: AbAs bearing mutant SOD1^G93A have the potential to induce neuroinflammation along the spinal cord and incipient damage to the motor neurons. The emergence of AbAs during amyotrophic lateral sclerosis pathogenesis may therefore be a mechanism to boost neuroinflammation and spread motor neuron damage along the neuroaxis.

Amyotrophic lateral sclerosis (ALS) is characterized by degeneration of upper and lower motor neurons accompanied by proliferation of reactive microglia in affected regions. However, it is unknown whether the hematopoietic marker CD34 can identify a subpopulation of proliferating microglial cells in the ALS degenerating spinal cord. Immunohistochemistry for CD34 and microglia markers was performed in lumbar spinal cords of ALS rats bearing the SOD1G93A mutation and autopsied ALS and control human subjects. Characterization of CD34-positive cells was also performed in primary cell cultures of the rat spinal cords. CD34 was expressed in a large number of cells that closely interacted with degenerating lumbar spinal cord motor neurons in symptomatic SOD1G93A rats, but not in controls. Most CD34+ cells co-expressed the myeloid marker CD11b, while only a subpopulation was stained for Iba1 or CD68. Notably, CD34+ cells actively proliferated and formed clusters adjacent to damaged motor neurons bearing misfolded SOD1. CD34+ cells were identified in the proximity of motor neurons in autopsied spinal cord from sporadic ALS subjects but not in controls. Cell culture of symptomatic SOD1G93A rat spinal cords yielded a large number of CD34+ cells exclusively in the non-adherent phase, which generated microglia after successive passaging. A yet unrecognized CD34+ cells, expressing or not the microglial marker Iba1, proliferate and accumulate adjacent to degenerating spinal motor neurons, representing an intriguing cell target for approaching ALS pathogenesis and therapeutics.