S-acylation regulates the membrane association and activity of Calpain-5

Calpain-5 regulates muscle-specific protein expression and nuclear positioning during myoblast differentiation

Intracellular calcium dynamics is key to regulating various physiological events. Myotube formation by myoblast fusion is controlled by the release of Ca2+ from the endoplasmic reticulum (ER), and the calpain (CAPN) family is postulated to be an executioner of the process. However, the activation of a specific member of the family or its physiological substrates is unclear. In this study, we explore the involvement of a CAPN in myoblast differentiation. Time-course experiments showed that the reduction in potential substrates of calpains, c-Myc and STAT3 (signal transducer and activator of transcription 3) and generation of STAT3 fragments occurred multiple times at an early stage of myoblast differentiation. Inhibition of the ER Ca2+ release suppressed these phenomena, suggesting that the reduction was dependent on the cleavage by a CAPN. CAPN5 knockdown suppressed the reduction. In vitro reconstitution assay showed Ca2+- and CAPN5-dependent degradation of c-Myc and STAT3. These results suggest the activation of CAPN5 in differentiating myoblasts. Fusion of the Capn5 knockdown myoblast efficiently occurred; however, the upregulation of muscle-specific proteins (myosin and actinin) was suppressed. Myofibrils were poorly formed in the fused cells with a bulge where nuclei formed a cluster, suggesting that the myonuclear positioning was abnormal. STAT3 was hyperactivated in those fused cells, possibly inhibiting the upregulation of muscle-specific proteins necessary for the maturation of myotubes. These results suggest that the CAPN5 activity is essential in myoblast differentiation.

Impaired activity and membrane association of most calpain-5 mutants causal for neovascular inflammatory vitreoretinopathy

Neovascular inflammatory vitreoretinopathy (NIV) is a rare eye disease that ultimately leads to complete blindness and is caused by mutations in the gene encoding calpain-5 (CAPN5), with six pathogenic mutations identified. In transfected SH-SY5Y cells, five of the mutations resulted in decreased membrane association, diminished S-acylation, and reduced calcium-induced autoproteolysis of CAPN5. CAPN5 proteolysis of the autoimmune regulator AIRE was impacted by several NIV mutations. R243, L244, K250 and the adjacent V249 are on β-strands in the protease core 2 domain. Conformational changes induced by Ca2+binding result in these β-strands forming a β-sheet and a hydrophobic pocket which docks W286 side chain away from the catalytic cleft, enabling calpain activation based on comparison with the Ca2+-bound CAPN1 protease core. The pathologic variants R243L, L244P, K250N, and R289W are predicted to disrupt the β-strands, β-sheet, and hydrophobic pocket, impairing calpain activation. The mechanism by which these variants impair membrane association is unclear. G376S impacts a conserved residue in the CBSW domain and is predicted to disrupt a loop containing acidic residues which may contribute to membrane binding. G267S did not impair membrane association and resulted in a slight but significant increase in autoproteolytic and proteolytic activity. However, G267S is also identified in individuals without NIV. Combined with the autosomal dominant pattern of NIV inheritance and evidence that CAPN5 may dimerize, the results are consistent with a dominant negative mechanism for the five pathogenic variants which resulted in impaired CAPN5 activity and membrane association and a gain-of-function for the G267S variant.