Effects of three deafness-causing gamma-actin mutations on actin structure and function

Hearing requires proper function of the auditory hair cell, which is critically dependent upon its actin-based cytoskeletal structure. Eleven point mutations in gamma (γ) nonmuscle actin have been identified as causing progressive autosomal dominant nonsyndromic hearing loss (DFNA20/26); however, exactly why these mutations lead to deafness is unclear. Organization, stability, and repair of the hair cell cytoskeleton are highly regulated by actin binding proteins (ABPs), and two of the mutations, K118M and K118N, are located near an area of the actin monomer believed to be important in actin-ABP interactions. A third mutation, D51N, is located in a region of the actin monomer believed to be important for polymerization dynamics and stability in filamentous actin. I therefore hypothesized that the K118M/N mutations cause hearing loss due to impaired regulation of the actin cytoskeleton within the hair cell, whereas the D51N mutation likely interferes with polymerization dynamics and actin filament stability or flexibility.

The goal of my thesis was to investigate the effects of these three mutations, K118M, K118N, and D51N, on actin dynamics and regulation. I show in Chapter 2 that the K118M/N mutations differentially affect regulation of actin by the Arp2/3 complex, but also, surprisingly, that the K118N mutation accelerates polymerization dynamics. Chapter 3 details a continued investigation of the K118M/N mutations to ascertain their effects on actin structure and dynamics, particularly with regard to how they may affect polymerization. Chapter 4 provides both an in vivo and in vitro characterization of the D51N mutation, which revealed that not only does the mutation significantly accelerate actin polymerization, it also causes significant effects on yeast mitochondrial morphology and cytoskeletal regulation.

The work detailed within this thesis provides new insight into how the K118M/N and D51N mutations affect actin structure and dynamics and how these effects could lead to deafness. More importantly, this work provides a strong foundation for many future studies, ranging from structural investigation of the K118N and D51N actins as F-actin mimics, to the potential role of mitochondria in actin-based disease.