James R Bartles

Biography

The research in my lab is centered on the “espins,” a family of multifunctional actin-bundling proteins, and the elucidation of their roles in the organization and function of hair cell stereocilia. Espins are produced in different sized isoforms from a single gene. They are present in the parallel actin bundle of hair cell stereocilia and are the target of mutations that cause deafness and vestibular dysfunction in mice and humans. For example, the jerker mutation in the espin gene of mice causes deafness and vestibular dysfunction accompanied by abnormal stereocilia morphogenesis and accelerated stereocilia degeneration. In particular, the stereocilia of jerker homozygotes, which lack espin proteins, fail to increase in diameter during morphogenesis, suggesting that espins are required for the appositional growth of the stereocilia parallel actin bundle. Espins are also present in the microvilli of taste receptor cells, solitary chemoreceptor cells, vomeronasal sensory neurons and Merkel cells, raising the possibility that these proteins play general roles in the microvillar projections of vertebrate sensory cells. Unlike the mammalian plastin/fimbrin actin-bundling proteins found in stereocilia and microvilli, espins are not inhibited by calcium ion. In addition, espins are much more effective than fascin-1 at over-twisting actin filaments in parallel actin bundles and can do so even at low stoichiometry. In tranfected epithelial cells, espins dramatically elongate microvillar parallel actin bundles and could, therefore, also help determine the length of stereocilia and microvilli. Espins can bind actin monomer via their Wiskott-Aldrich Syndrome protein homology 2 (WH2) domain in vitro and in vivo and can assemble large actin bundles de novo when targeted to specific locations in transfected cells. These remarkable biological activities distinguish espins from other actin-bundling proteins and may make them especially well-suited to sensory cells.

Research Interest

multifunctional actin-bundling proteins, Espins