Reactive astrocytes in ALS display dysregulated intron retention

Reactive astrocytes are implicated in amyotrophic lateral sclerosis (ALS), although the mechanisms controlling reactive transformation are unknown. We show that decreased intron retention (IR) is common to human iPSC-derived astrocytes carrying VCP, C9orf72 and SOD1 ALS-causing mutations as well as astrocytes stimulated to undergo reactive transformation. Notably, transcripts with decreased IR and increased expression are overrepresented in reactivity processes including cell-adhesion, stress-response, and immune-activation. We examined translatome sequencing (TRAP-seq) of astrocytes from a SOD1 mouse model, which revealed that transcripts upregulated in translation significantly overlap with transcripts with decreased IR. Using nucleo-cytoplasmic fractionation of VCP astrocytes coupled with mRNA sequencing and proteomics, we identify that decreased IR in nuclear-detained transcripts is associated with increased cytoplasmic expression of genes and proteins encoding regulators of reactivity - indicating nuclear-to-cytoplasmic translocation and translation of spliced reactivity-related transcripts. Our study provides important insights into the molecular regulation of reactive transformation of ALS astrocytes.