Spinal but not cortical microglia acquire an atypical phenotype with high VEGF, galectin-3 and osteopontin, and blunted inflammatory responses in ALS rats.

Activation of microglia, CNS resident immune cells, is a pathological hallmark of amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder affecting motor neurons. Despite evidence that microglia contribute to disease progression, the exact role of these cells in ALS pathology remains unknown. We immunomagnetically isolated microglia from different CNS regions of SOD1(G93A) rats at three different points in disease progression: presymptomatic, symptom onset and end-stage. We observed no differences in microglial number or phenotype in presymptomatic rats compared to wild-type controls. Although after disease onset there was no macrophage infiltration, there were significant increases in microglial numbers in the spinal cord, but not cortex. At disease end-stage, microglia were characterized by high expression of galectin-3, osteopontin and VEGF, and concomitant downregulated expression of TNFα, IL-6, BDNF and arginase-1. Flow cytometry revealed the presence of at least two phenotypically distinct microglial populations in the spinal cord. Immunohistochemistry showed that galectin-3/osteopontin positive microglia were restricted to the ventral horns of the spinal cord, regions with severe motor neuron degeneration. End-stage SOD1(G93A) microglia from the cortex, a less affected region, displayed similar gene expression profiles to microglia from wild-type rats, and displayed normal responses to systemic inflammation induced by LPS. On the other hand, end-stage SOD1(G93A) spinal microglia had blunted responses to systemic LPS suggesting that in addition to their phenotypic changes, they may also be functionally impaired. Thus, after disease onset, microglia acquired unique characteristics that do not conform to typical M1 (inflammatory) or M2 (anti-inflammatory) phenotypes. This transformation was observed only in the most affected CNS regions, suggesting that overexpression of mutated hSOD1 is not sufficient to trigger these changes in microglia. These novel observations suggest that microglial regional and phenotypic heterogeneity may be an important consideration when designing new therapeutic strategies targeting microglia and neuroinflammation in ALS.