Neuropathy-Associated HSPB1 Mutant Impairs Neuronal Mechanoadaptation and Axonal Regeneration

The small heat shock protein HSPB1 is a ubiquitously expressed mechanoresponsive chaperone essential for cytoskeletal remodeling under mechanical load. Mutations in HSPB1, including S135F, cause Charcot-Marie-Tooth (CMT) peripheral neuropathy, yet the mechanisms underlying the selective vulnerability of peripheral nerves remain enigmatic. Here we demonstrate that substrate stiffness is a critical determinant of HSPB1^S135F-mediated neurodegeneration. Using stiffness-tunable polydimethylsiloxane (PDMS) substrates (1 kPa, 10 kPa, 2 MPa) and uniaxial cyclic stretch, we show that primary dorsal root ganglia (DRG) neurons and SH-SY5Y cells expressing HSPB1^S135F exhibit profound deficits in mechanoadaptation. On compliant substrates (10 kPa), HSPB1^S135F causes stretch-induced axon fragmentation and neuronal death, whereas HSPB1^WT confers robust neuroprotection. HSPB1^S135F also disrupts stiffness-directed neuritogenesis in differentiated SH-SY5Y cells: HSPB1^WT-expressing cells show optimal axonal outgrowth and βIII-tubulin expression on 10 kPa substrates mimicking muscle tissue stiffness, while HSPB1^S135F mutants display disorganized focal adhesions and complete differentiation failure. Mechanistically, we uncover that HSPB1^S135F dysregulates stage-specific transglutaminase (TGase) expression-insufficient TGase during early neuritogenesis impairs filopodia stabilization, whereas aberrant TGase persistence at late stages constrains axon extension. Our findings establish HSPB1 as a biomechanical sensor that integrates ECM stiffness signals to coordinate peripheral nerve regeneration, and identify defective mechanoadaptation as a previously unrecognized pathomechanism in CMT. These results open new avenues for stiffness-targeted therapeutic strategies in peripheral neuropathy.