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Hartzell Lab

Research

Chloride channels are poorly understood. Until recently, "chloride" (or more precisely, "anion") channels have received considerably less attention than cation channels. One reason for this may be that many Cl channels perform functions that are slow and cell-biological, like fluid secretion and cell volume regulation, whereas cation channels have historically been associated with cellular excitability, which is more rapid. We are interested in two families of chloride channels that are gated open by increases in cytosolic calcium (which in turn is controlled by various hormones and neurotransmitters). These so-called calcium-activated Cl channels (CaCCs) play manifold roles in cell physiology including epithelial secretion, sensory adaptation, regulation of smooth muscle tone, control of neuronal and cardiac excitability, and nociception. CaCCs are well-suited for their diverse roles because they are dually gated by both calcium and voltage so that their activity can be tuned to the interplay between metabotropic and ionotropic inputs. Although CaCC gating has attracted attention since these channels were first described nearly 3 decades ago, a lack of consensus regarding their molecular composition has stymied a detailed understanding of how they work.

Anoctamins and Bestrophins are two kinds of calcium-activated chloride channels. Anoctamins were shown to be Ca-activated Cl channels in 2008 and are responsible for the "classical" CaCCs that are widely expressed in many tissues, especially epithelia. Bestrophins are more restricted in their expression and play more specialized roles. Bestrophin-1 is expressed in the retinal pigment epithelium and certain mutations in bestrophin-1 cause degeneration of the retina and blindness. Bestrophin-2 is expressed in goblet cells in colon and may play a role in bicarbonate secretion in this tissue. We are interested in understanding how both anoctamins and bestrophins are regulated by calcium. We are identifying the calcium binding sites and the molecular links between the calcium sensor and the channel gate using a combination of both whole-cell and single channel electrophysiological mesurements and mutagenesis. We are dissecting the signaling protein network associated with these channels using sophistocated proteomic and novel imaging methods. We are also exploring how these channels function in a variety of different tissues (including retina, kidney, colon, brain, and salivary gland) to understand their physiological and pathological roles. Read our reviews to learn more about our work on bestrophins and anoctamins.

Listen to the Chloride Channel Song by Nathan Germick and Criss Hartzell (may take a minute to load).
Cartoon of an epithelial cell expressing a CaCC channel. The CaCC can be activated by Ca influx through plasma membrane Ca channels or by Ca mobilized from internal stores by G-protein-coupled receptors (GPCR) through the phospholipase-C (PLC) - IP3 pathway. CaCCs can secrete Cl or absorb Cl depending on the membrane potential and the The predicted topology of Ano1 is shown superimposed.
CaCCs