hnRNP A2/B1 Staufen2-Mediated Axonal RNA Granule Transport Failure Drives Distal Axon Degeneration in ALS

hnRNP A2/B1 is an RNA-binding protein that assembles into axonal RNA granules with Staufen2 (STAU2), mediating the long-range transport of mRNAs (including β-actin, Arp2/3, MAP1B) along microtubules in motor neuron axons. This hypothesis proposes that ALS-linked hnRNP A2/B1 dysfunction (mutations p.P193L, post-translational modification changes) disrupts axonal RNA granule transport, creating a dual defect: (1) insufficient delivery of structural and synaptic protein mRNAs to distal axons, and (2) accumulation of stalled RNA granules that obstruct axonal transport machinery and trigger dynein-mediated retrograde stress signaling. The mechanistic prediction is that hnRNP A2/B1’s granule association is regulated by arginine methylation (PRMT1) and phosphorylation (GSK3β); ALS-associated hypomethylation or hyperphosphorylation releases hnRNP A2/B1 from granules, destabilizing the STAU2-hnRNP A2/B1-mRNA complex. In SOD1-G93A mouse spinal cord motor neurons, hnRNP A2/B1 axonal granules show 50% reduction in velocity and 3-fold increase in stall events by pre-symptomatic stage (P60), preceding motor deficit onset. RNA granules isolated from symptomatic SOD1-G93A motor neurons show hnRNP A2/B1 displacement from the granule membrane. The therapeutic prediction is that AAV-mediated expression of phosphorylation-deficient hnRNP A2/B1 (S301A, S313A mutants that resist GSK3β phosphorylation) or PRMT1 activator (small-molecule PRMT1 agonists) will restore axonal RNA granule transport, deliver critical mRNAs to distal compartments, and preserve NMJ integrity in SOD1-G93A and C9orf72-ALS mouse models. This addresses the axonal RNA transport failure that precedes motor neuron cell body death.