Phenotypic transition of microglia into astrocyte-like cells associated with disease onset in a model of inherited ALS

Abstract

Microglia and reactive astrocytes accumulate in the spinal cord of rats expressing the Amyotrophic lateral sclerosis (ALS)-linked SOD1 G93A mutation. We previously reported that the rapid progression of paralysis in ALS rats is associated with the appearance of proliferative astrocyte-like cells that surround motor neurons. These cells, designated as Aberrant Astrocytes (AbA cells) because of their atypical astrocytic phenotype, exhibit high toxicity to motor neurons. However, the cellular origin of AbA cells remains unknown. Because AbA cells are labeled with the proliferation marker Ki67, we analyzed the phenotypic makers of proliferating glial cells that surround motor neurons by immunohistochemistry. The number of Ki67 +AbA cells sharply increased in symptomatic rats, displaying large cell bodies with processes embracing motor neurons. Most were co-labeled with astrocytic marker GFAP concurrently with the microglial markers Iba1 and CD163. Cultures of spinal cord prepared from symptomatic SOD1 G93A rats yielded large numbers of microglia expressing Iba1, CD11b, and CD68. Cells sorted for CD11b expression by flow cytometry transformed into AbA cells within two weeks. During these two weeks, the expression of microglial markers largely disappeared, while GFAP and S100β expression increased. The phenotypic transition to AbA cells was stimulated by forskolin. These findings provide evidence for a subpopulation of proliferating microglial cells in SOD1 G93A rats that undergo a phenotypic transition into AbA cells after onset of paralysis that may promote the fulminant disease progression. These cells could be a therapeutic target for slowing paralysis progression in ALS.

RNA-Seq Analysis of Microglia Reveals Time-Dependent Activation of Specific Genetic Programs following Spinal Cord Injury

“…However, we found no changes in microglia expression of Sox2 and BMP transcripts after injury, suggesting that other signaling pathways may be involved in SCI-induced microglia expression of astrocytic markers in vivo . Increased expression of microglial Gfap, Vim and Serpina3n transcripts was also reported in a mouse model of ALS (Chiu et al, 2013), whilst in vivo microglial transformation into astrocyte-like cells were found in a rat model of the disease (Trias et al, 2013). Co-expression of microglia and astrocytic markers has been also found in neoplastic glioblastoma multiform cells associated with increased inflammation (Huysentruyt et al, 2011).…”

Evidence for mast cells contributing to neuromuscular pathology in an inherited model of ALS

“…This result is significant, considering that in this animal model, a fulminant paralysis develops in rats aged 5-6 months, rapidly progressing from abnormal gait to complete loss of motor function in a period of 2-3 weeks (27). The therapeutic effect of masitinib in ALS rats was associated with the inhibition of CSF-1R in microglia and aberrant glial cells that surround degenerating spinal motor neurons (33,34). Because masitinib is a multifaceted drug affecting multiple immune cell types, we hypothesized that the protective effect of masitinib in ALS likely involves other inflammatory cell types such as mast cells and macrophages in addition to glial cells.…”

Neuroimmune and Inflammatory Signals in Complex Disorders of the Central Nervous System

“…During the symptomatic phase of ALS, microglia are characterized by proliferation and transformation into phagocytic cells, displaying a morphology similar to that of macrophages in the periphery [118, 119]. They can form clusters of proliferating microglia adjacent to the damaged motor neurons [120, 121], thus playing a preponderant pathogenic role during the progression of ALS [122, 123]. In a mouse model of ALS expressing mutant SOD1, genetic excision of the mutated protein only in myeloid cells and microglia results in a slower paralysis progression as compared with mice expressing the mutant protein in microglia [124].…”

“…In ALS paralytic rats expressing the SOD1 G93A mutation, overactivated microglia in the spinal cord originate an aberrant cell phenotype displaying both microglia and astrocyte markers in the ventral horn of the spinal cord [120, 128]. Moreover, such aberrant glial cells actively proliferate after the onset of paralysis and make intimate contact with degenerating motor neurons, suggesting they contribute to spread motor neuron pathology [120, 128].…”

“…Moreover, such aberrant glial cells actively proliferate after the onset of paralysis and make intimate contact with degenerating motor neurons, suggesting they contribute to spread motor neuron pathology [120, 128]. Accordingly, aberrant glial cells isolated in culture are highly toxic to motor neurons, suggesting they are key neurotoxic effectors in ALS [128].…”