Z-disc-associated, Alternatively Spliced, PDZ Motif-containing Protein ( ZASP) Mutations in the Actin-binding Domain Cause Disruption of Skeletal Muscle Actin Filaments in Myofibrillar Myopathy

Background: The binding partners of the ZASP internal region that is mutated in zaspopathy are not yet known. Results: The internal region of ZASP binds to skeletal muscle -actin, and zaspopathy mutations cause actin disruption. Conclusion: ZASP mutations in the actin-binding domain are deleterious to the muscle Z-disc structure. Significance: ZASP-actin interaction expands the role of ZASP and defines the mechanism of zaspopathy. The core of skeletal muscle Z-discs consists of actin filaments from adjacent sarcomeres that are cross-linked by -actinin homodimers. Z-disc-associated, alternatively spliced, PDZ motif-containing protein (ZASP)/Cypher interacts with -actinin, myotilin, and other Z-disc proteins via the PDZ domain. However, these interactions are not sufficient to maintain the Z-disc structure. We show that ZASP directly interacts with skeletal actin filaments. The actin-binding domain is between the modular PDZ and LIM domains. This ZASP region is alternatively spliced so that each isoform has unique actin-binding domains. All ZASP isoforms contain the exon 6-encoded ZASP-like motif that is mutated in zaspopathy, a myofibrillar myopathy (MFM), whereas the exon 8-11 junction-encoded peptide is exclusive to the postnatal long ZASP isoform (ZASP-Lex10). MFM is characterized by disruption of skeletal muscle Z-discs and accumulation of myofibrillar degradation products. Wild-type and mutant ZASP interact with -actin, -actinin, and myotilin. Expression of mutant, but not wild-type, ZASP leads to Z-disc disruption and F-actin accumulation in mouse skeletal muscle, as in MFM. Mutations in the actin-binding domain of ZASP-Lex10, but not other isoforms, cause disruption of the actin cytoskeleton in muscle cells. These isoform-specific mutation effects highlight the essential role of the ZASP-Lex10 isoform in F-actin organization. Our results show that MFM-associated ZASP mutations in the actin-binding domain have deleterious effects on the core structure of the Z-discs in skeletal muscle.