SMN–ANXA2 MRNA INTERACTION REGULATES AXONAL LOCALIZATION IN A DEVELOPMENTAL STAGE-DEPENDENT MANNERSMN–ANXA2 MRNA INTERACTION REGULATES AXONAL LOCALIZATION IN A DEVELOPMENTAL STAGE-DEPENDENT MANNER

Local translation of mRNAs in neuronal compartments is a fundamental mechanism supporting neuronal polarity, synaptic plasticity, and maintenance. Dysregulation of axonal mRNA trafficking and local protein synthesis has emerged as a common pathological mechanism in several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), and spinal muscular atrophy (SMA). Although these disorders differ in their genetic origins and affected neuronal populations, they often share alterations in RNA-binding proteins and localized translation pathways critical for axonal integrity. Using SMA as a model, we explored how disruption of RNA trafficking impacts cytoskeletal dynamics in human motoneurons. SMA results from loss of the ubiquitously expressed SMN protein, which, beyond its canonical role in snRNP assembly, functions in axonal mRNA transport. Among transcripts associated with SMN, Anxa2 mRNA encodes Annexin A2 (ANXA2), a phospholipid-binding protein involved in actin remodeling and microtubule stabilization. We employed single-molecule fluorescence in situ hybridization (smFISH) in human iPSC-derived motoneurons to quantify axonal Anxa2 mRNA. SMA neurons displayed a developmental stage–dependent reduction of axonal Anxa2 localization, coinciding with periods of rapid axon growth. Complementary RNA–protein interaction assays confirmed that SMN binds Anxa2 mRNA transiently during early development, and restoring SMN levels rescued its axonal targeting only within this temporal window. These findings highlight how temporal regulation of local mRNA translation contributes to neuronal vulnerability and suggest that impaired axonal translation may represent a convergent pathway in neurodegenerative diseases.

Figure 1: Localization of Anxa2 mRNA is reduced in axons of SMA MNs cultured in microfluidic chambers.