Microglia and motor neurons during disease progression in the SOD1G93A mouse model of amyotrophic lateral sclerosis: changes in arginase1 and inducible nitric oxide synthase

Abstract

BackgroundAmyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease affecting the motor system. Although the etiology of the disease is not fully understood, microglial activation and neuroinflammation are thought to play a role in disease progression.MethodsWe examined the immunohistochemical expression of two markers of microglial phenotype, the arginine-metabolizing enzymes inducible nitric oxide synthase (iNOS) and arginase1 (Arg1), in the spinal cord of a mouse model carrying an ALS-linked mutant human superoxide dismutase transgene (SOD1G93A) and in non-transgenic wild-type (WT) mice. Immunolabeling for iNOS and Arg1 was evaluated throughout disease progression (6 to 25 weeks), and correlated with body weight, stride pattern, wire hang duration and ubiquitin pathology. For microglia and motor neuron counts at each time point, SOD1G93A and WT animals were compared using an independent samples t-test. A Welch t-test correction was applied if Levene’s test showed that the variance in WT and SOD1G93A measurements was substantially different.ResultsDisease onset, measured as the earliest change in functional parameters compared to non-transgenic WT mice, occurred at 14 weeks of age in SOD1G93A mice. The ventral horn of the SOD1G93A spinal cord contained more microglia than WT from 14 weeks onwards. In SOD1G93A mice, Arg1-positive and iNOS-positive microglia increased 18-fold and 7-fold, respectively, between 10 and 25 weeks of age (endpoint) in the lumbar spinal cord, while no increase was observed in WT mice. An increasing trend of Arg1- and iNOS-expressing microglia was observed in the cervical spinal cords of SOD1G93A mice. Additionally, Arg1-negative motor neurons appeared to selectively decline in the spinal cord of SOD1G93A mice, suggesting that Arg1 may have a neuroprotective function.ConclusionsThis study suggests that the increase in spinal cord microglia occurs around and after disease onset and is preceded by cellular pathology. The results show that Arg1 and iNOS, thought to have opposing inflammatory properties, are upregulated in microglia during disease progression and that Arg1 in motor neurons may confer protection from disease processes. Further understanding of the neuroinflammatory response, and the Arg1/iNOS balance in motor neurons, may provide suitable therapeutic targets for ALS.

Evidence for an early innate immune response in the motor cortex of ALS

“…We further evaluated the phenotype of these MCP1+ microglia (Additional file 3: Figure S3) using known functional markers for beneficial/M2 microglia (Arginase 1, Arg1, Additional file 3: Figure S3a-b) or detrimental/M1 microglia (inducible nitric oxide synthase, iNOS, Additional file 3: Figure S3c-d) [15, 41]. Different from previously published reports in the hSOD1 G93A spinal cord [42], neither Arg1 (Additional file 3: Figure S3f-g) nor iNOS (Additional file 3: Figure S3h-i) was detected in MCP1+ microglia in the MCP1-CCR2-WT or MCP1-CCR2-hSOD1 G93A motor cortex. Finally, a subset of CCR2+ cells that co-localize with infiltrating monocyte marker Ly6C were also detected juxtaposed to the degenerating CSMN apical dendrites (Fig.…”

Fingolimod: A Disease-Modifier Drug in a Mouse Model of Amyotrophic Lateral Sclerosis

“…Regional and temporal differences in neurodegeneration and immune response have been reported in ALS animal models [28,40,41], by which the cortex is the less affected and the lumbar spinal cord the earliest and most affected region. These distinct responses have been related to specific patterns of microglial activation and lymphocyte infiltration [28], and the corresponding gradual T2 to T1 phase switch [5].…”

Neuron-Microglia Interactions in Motor Neuron Degeneration. The Inflammatory Hypothesis in Amyotrophic Lateral Sclerosis Revisited

“…In addition, chronically activated microglia may become increasingly dysfunctional and may directly participate in the development of secondary tissue injury [143]. In contrast, M2 neurorestorative microglia activity includes the production of high levels of anti-inflammatory cytokines like IL-4, IL-10, neurotrophic factors, and proteins such as insulin-like growth factor (IGF)-1, arginase-1 and Ym-1 [144][145][146].…”