The survival motor neuron protein: structure, functions, stability, and therapeutic targeting
The survival motor neuron (SMN) protein is an essential and highly versatile assembly factor that coordinates RNA metabolism and ribonucleoprotein (RNP) complex formation across multiple cellular compartments. Although SMN is required for the survival of virtually all cell types, its deficiency disproportionately affects α -motor neurons, causing their selective degeneration and giving rise to spinal muscular atrophy (SMA). Once viewed primarily as a motor neuron disease, SMA is now understood to be a multi-systemic disorder in which cell-intrinsic dysfunction extends to skeletal muscle, inflammatory glial cells, and metabolic organs. This review examines the regulatory mechanisms that control SMN protein stability, collectively termed proteostasis, with a focus on how post-translational modifications coordinate with the ubiquitin-proteasome system and the autophagy-lysosomal pathway to govern protein turnover and clearance. We also address the emerging concept of gene dosage sensitivity, including the underappreciated paradox that therapeutic SMN overexpression can be as harmful as deficiency, producing distinct toxicities in both neuromuscular and peripheral tissues. Finally, we highlight the need for next-generation combination therapies that integrate genetic modifiers, targeted degradation strategies, and post-translational regulators to maintain SMN levels within the narrow physiological range required for safety and efficacy.