Molecular Physiology of TMEM16/Anoctamin Proteins

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Understanding the mechanisms by which small molecules are transported across cell membranes is a fundamental challenge in cell physiology. This application focuses on one family of transport proteins, the Anoctamins / TMEM16s, because they play diverse and indispensable roles in cellular physiology. The founding members of the Anoctamin (ANO) family are Ca2+-activated Cl- channels (ANO1 and ANO2). These channels are ubiquitously expressed and are intimately engaged in keeping our epithelia moist by driving the secretion of bodily fluids, controlling gut motility, facilitating the secretion of hormones, and regulating neuronal excitability and smooth muscle contractility, among other functions. Dysfunction of ANO1 has been implicated in a variety of human disease states including hypertension, colitis, asthma, and lung disease. Genetic disruption of the ANO1 gene in mice causes major developmental abnormalities, behavioral disorders, altered gastrointestinal motility, and ability to sense pain. Because ANO1 and ANO2 play such varied but essential roles in cell physiology, they represent novel targets for therapeutic drug development, but as yet ANOs as drug targets have received relatively little attention. Recently, 3-D structures of various ANOs including ANO1 have provided valuable insights into how these proteins work, but major questions remain. The long-range goal of our research is to understand the structure and function of ANO1 (TMEM16A) and ANO2 (TMEM16B). Specifically in this application, we focus on the regulation of ANO1 by the phospholipid phosphatidylinositol-(4,5)bisphosphate (PI(4,5)P2). While PI(4,5)P2 is a minor lipid in the cell membrane, it is clear that it plays a critical, but scantily understood, role in ANO1 and ANO2 function. We will use a combination of single-cell electrophysiology, directed mutagenesis, and computational molecular dynamics modeling to elucidate how the opening and closing of ANO1 is controlled by PI(4,5)P2 and calcium ions. There are 3 aims: (1) We will characterize the biophysical mechanisms of ANO1 and ANO2 regulation by PI(4,5)P2, the functional interactions between PI(4,5)P2 and calcium, and the structural requirements of phosphoinositides and inositol phosphates for channel regulation. (2) We will identify the amino acids involved in PI(4,5)P2 regulation and locate the PI(4,5)P2 binding sites in ANO1 and ANO2. Preliminary data provides strong support for the existence of 3 different PI(4,5)P2 binding sites in ANO1. (3) We will determine the functional roles for each of the 3 different PI(4,5)P2 binding sites in regulating ANO1 Ca2+ sensitivity, gating, and inactivation. A compelling reason for comparing ANO1 and ANO2 is that although these 2 proteins are 70% similar (57% identical) in sequence, ANO1 is stimulated by PI(4,5)P2 while ANO2 is inhibited. This difference provides a rich opportunity to understand how PI(4,5)P2 binding is coupled to channel function. These studies will answer pressing outstanding questions about the regulation of these channels that are crucial to human health and disease.