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.
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.
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