MYC-Driven Gliosis Impairs Neuronal Survival and Contributes to Early Senescence in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease where glial cells play a key role in disease progression through non-cell-autonomous mechanisms. In the early 2000s, seminal and consistent studies demonstrated that glial cells play a key role in modulating disease progression in a mouse model expressing mutant SOD1. Chronic activation of glial cells leads to the dysfunction and degeneration of motor and cortical neurons in ALS and frontotemporal dementia (FTD) with an unknown mechanism. To shed light on the molecular pathogenetic processes underlying the exordium and the contribution of gliosis to disease onset and progression, we took advantage of a slow progression transgenic mouse expressing low levels of TDP-43 with the Q331K mutation. The model showed increased astrogliosis at disease onset that was not retained in later disease stages. Through RNA sequencing, we identify an initial timeframe, concomitant to disease onset, marked by increased astrocyte proliferation, followed by a typical activation stage with elevated expression of pro-inflammatory genes. Here, we show that GFAP increase in ALS happens concomitantly with motor degeneration and is associated with a propensity of astrocytes to proliferate. This was demonstrated by Ki-67 labelling in vivo, and EdU incorporation in primary astrocytes derived from our mouse model. To study the effect of gliosis on disease progression, we selectively removed mutant TDP-43 in astrocytes at two months of age. At 6 months, GFAP levels were comparable with WT littermates and neuronal loss milder, indicating that the expression of mutant TDP-43 in astrocytes is necessary to drive gliosis. This was further confirmed by behavioral tests, where we saw a delayed onset of motor symptoms, while at a cognitive level, we saw a rescue. We next took advantage of genetics to look for any genetic predisposition to gliosis in ALS patients. By interrogating the genomes of ALS patients in the Project MinE cohort, we revealed the enrichment of single-nucleotide polymorphisms (SNPs) in the responsive elements of transcription factors linked to proliferation, with a hub centered around MYC. RNA and protein levels were unaffected; however, the phosphorylated, active form of MYC was higher in primary astrocytes expressing TDP-43Q331K and SOD1G93A, as well as in patient-derived astrocytes modeling C9orf72-linked and sporadic ALS. Interestingly, in vivo overexpression of MYC in astrocytes without TDP-43 again led to neuronal loss, mimicking the mutant TDP-43 model. Conversely, MYC inhibition with Omomyc revealed MYC-dependent transcriptional control over Socs3, which is directly involved in the activation of the JAK-STAT3 pathway, implicated in astrocyte reactivity and gliosis. Proliferation correlated with increased levels of phosphorylated MYC, but induce early senescence both in vitro and in vivo. Treatment with senolytic compounds improved cognitive outcomes, although motor deficits persisted. These data highlighted the role of glial cell dysfunction in ALS progression, suggesting MYC as a driven transcriptional factor in reactive gliosis, neuroinflammation, and early senescence.