MATR3 Nuclear Body Disruption Impairs RNA Processing Hubs and Triggers Splicing Defects in ALS Motor Neurons

MATR3 (Matrin-3) is a nuclear matrix protein that forms distinct nuclear bodies (MATR3-NBs) functioning as RNA processing hubs for spliceosome recycling and transcription termination. This hypothesis proposes that ALS-linked MATR3 mutations (p.S85C, p.F115C, p.G497E) disrupt MATR3-NB integrity, causing aberrant spliceosome dynamics, intron retention accumulation, and nuclear RNA export defects that trigger motor neuron death. The mechanistic prediction is that MATR3-NBs serve as transient storage and assembly platforms for U snRNP components; their disruption by disease mutations disperses spliceosome machinery, causing widespread splicing dysregulation including cryptic splice site activation. In iPSC-derived motor neurons from MATR3-ALS patients (p.S85C), MATR3-NBs are reduced in number (3.2 vs 8.1 per nucleus in controls) and show dispersed, irregular morphology by super-resolution microscopy. RNA-seq of these motor neurons reveals significant intron retention (RI values elevated 2.3-fold) and exon skipping events affecting synaptic function transcripts (SCN2A, GRIA1, GRIK2). MATR3 knockdown in wild-type motor neurons recapitulates the splicing defect, confirming specificity. The therapeutic prediction is that AAV-mediated MATR3 overexpression (wild-type, using a neuronal-specific promoter) will restore MATR3-NB frequency and splicing fidelity, reduce intron retention to baseline levels, and prevent motor neuron death in MATR3-ALS patient-derived motor neurons. Additionally, spliceosome-targeting small molecules (e.g., pladienolide B, part of a new antisense oligonucleotide approach) may compensate for MATR3-related splicing defects.