Septin 7 interacts with Numb to preserve sarcomere structural organization and muscle contractile function

Abstract

Here, we investigated mechanisms by which aging-related diminished levels of Numb in skeletal muscle fibers contribute to loss of muscle strength and power, two critical features of sarcopenia. Numb is an adaptor protein best known for its critical roles in development including asymmetric cell division, cell-type specification and termination of intracellular signaling. Numb expression is reduced in old humans and mice. We previously showed that, in skeletal muscle fibers, Numb is localized to sarcomeres where it is concentrated near triads. Conditional inactivation of Numb in myofibers causes weakness, disorganization of sarcomeres and smaller mitochondria with impaired function. Proteomics analysis of protein complexes isolated from C2C12 myotubes by immunoprecipitation using antibodies against Numb indicated that Septin 7 is a potential Numb binding partner. Septin 7 is a member of the family of GTP-binding proteins that organize into filaments, sheets and rings, and is considered part of the cytoskeleton. Immunofluorescence evaluation revealed a partial overlap of staining for Numb and Septin 7 in myofibers. Conditional, inducible knockouts of Numb led to disorganization of Septin 7 staining in myofibers. These findings support the conclusion that Septin 7 is a Numb binding partner. Because prior reports showed that conditional inactivation of Septin 7 in skeletal muscle led to weakness and disorganization of sarcomeres, and altered mitochondrial size, we also conclude that interactions between Numb and Septin 7 are critical for proper structural organization of the sarcomere, for optimal muscle contractile function, and for control of the size and function of mitochondria.