The HSPB2-HSPB3 complex: unraveling new functions that affect nuclear homeostasis and their implication in neuromuscular and muscular diseases.

HSPB2 and HSPB3 are poorly characterized members of the HSPB family (HSPB1-HSPB10) that form a stoichiometric complex. HSPB2 and HSPB3 are expressed during muscle differentiation and play a role in muscle maintenance. The R7S mutation of HSPB3 causes motor neuropathy type 2C (dHMN 2C). We identified in myopathic patients two yet unpublished HSPB3 mutations. One mutation affects the R116 residue of HSPB3, equivalent to the R120 and the K141 residues of HSPB5 and HSPB8, associated with myopathy and motor neuropathy, respectively. The second mutation causes a premature stop codon, generating an unstable peptide that is rapidly degraded after synthesis, thus indirectly affecting HSPB2-HSPB3 complex formation. We started to characterize in cells the stability, subcellular localization and functions of HSPB2 and HSPB3 (wildtype/wt and mutants). We found that HSPB2 and HSPB3 (wt and mutants) affect nuclear shape and integrity, which may be relevant to neuro/muscular diseases. In fact, mutations in proteins of the nuclear envelope and in lamins, the major components of the nuclear lamina, cause neuromuscular and muscular diseases. Besides regulating nuclear shape and mechanical stability, lamins regulate gene expression and RNA transcription. Thus, the HSPB3 mutants either via a toxic gain of function or a loss of function and by indirectly altering the formation of the HSPB2-HSPB3 complex may affect nuclear architecture, gene expression and/or transcription. This, in turn, may render skeletal muscles more vulnerable to e.g. stress and mechanical strain, thereby decreasing their viability. Here, we aim at further investigating in myoblasts these yet uncharacterized nuclear effects of HSPB2 and HSPB3, pinpointing how HSPB3 mutants alter them. Our approach will demonstrate whether HSPB3 mutations cause disease by affecting specific nuclear functions, identifying key targets whose future modulation may be therapeutically useful.