Tripartite motif-containing protein 32 regulates Ca²⁺ movement in skeletal muscle

Mutations in tripartite motif-containing protein 32 (TRIM32), especially in NHL repeats, have been found in skeletal muscle in patients with type 2H limb-girdle muscular dystrophy (LGMD2H). However, the roles of the NHL repeats of TRIM32 in skeletal muscle functions have not been well addressed. In the present study, to examine the functional role(s) of the TRIM32 NHL repeats in skeletal muscle, TRIM32-binding proteins in skeletal muscle were first searched using a binding assay and MALDI-TOF/TOF. Sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 1a (SERCA1a) was found to be a TRIM32-binding protein. Next, a deletion mutant of TRIM32 missing the NHL repeats (NHL-Del) was expressed in mouse primary skeletal myotubes during myoblast differentiation into myotubes. Ca²⁺ movement in the myotubes was examined using single-cell Ca²⁺ imaging. Unlike wild-type (WT) TRIM32, NHL-Del did not enhance the amount of Ca²⁺ release from the sarcoplasmic reticulum (SR), Ca²⁺ release for excitation-contraction (EC) coupling, or extracellular Ca²⁺ entry via store-operated Ca²⁺ entry (SOCE). In addition, even compared with the vector control, NHL-Del resulted in reduced SOCE due to reduced expression of extracellular Ca²⁺ entry channels. Transmission electron microscopy (TEM) observation of the myotubes revealed that NHL-Del induced the formation of abnormal vacuoles and tubular structures in the cytosol. Therefore, by binding to SERCA1a via its NHL repeats, TRIM32 may participate in the regulation of Ca²⁺ movement for skeletal muscle contraction and the formation of cellular vacuoles and tubular structures in skeletal muscle. Functional defects in TRIM32 due to mutations in NHL repeats may be pathogenic toward LGMD2H.