Ion Channel Structure-Function

At the interface of genetics, biophysics and biochemistry, we seek to unravel the molecular nature of excitation in the cochlea. Our laboratory is particularly interested in the structure and function of ion channels, with an emphasis on how these channels are regulated and modulated in the mature animal, in development and following trauma. In excitable cells, membrane depolarization initiates the flux of ions like potassium, calcium and sodium across the cell membrane, regulating neurotransmitter release, homeostasis, a host of cell signaling pathways and even cell survival. Below, we highlight one of our active projects in this area:

Cochlea hair cells are highly polarized and their membrane processes are exquisitely compartmentalized. The generic hair cell shown here illustrates efferent (SK/AChR) and afferent (BK/CaV) synapses, each of which contain a large number of proteins clustered into discrete domains.

Lipid Rafts and Hearing Loss

In our search for the mechanisms that organize ion channels into discrete compartments in the cell membrane, we uncovered a new ototoxin, that is, a drug that is used therapeutically but carries with it an unfortunate risk to hearing health. This drug is a simple ring of sugars called cyclodextrin (specifically, 2-hydroxypropyl-beta-cyclodextrin, or HPBCD). Cyclodextrins are widely used in biophysics to manipulate cell membranes by mobilizing cholesterol. Originally, we used these compounds to test whether ion channel clustering in hair cells occurred in lipid microdomains. We were surprised to find that cyclodextrin enhanced calcium current while inhibiting potassium current. In Motor City terms, this would be equivalent to stepping on the gas while releasing the brake, a citation that could lead to excitotoxicity in cells. While generally recognized as safe, we found that high doses of HBPCD were capable of rapidly inducing hearing loss in mice by destroying cochlear hair cells. Clinical trials using HPBCD to treat lipid disorders are seeing increased hearing loss risk in human patients. Current projects seek to understand the relationship between cholesterol, excitability and hair cell health in order to gain insight into basic membrane biology and design ways to mitigate the risk to hearing with HPBCD use in the clinic.

As an example, two hair cells are shown below with antibody stains to the BK-type potassium channel, clustered into hot spots at the base of each cell. 


When exposed to cyclodextrin, channel function is altered, possibly through disorganization of the clustered domain and rearrangement of the lipid microenvironment around these ion channels.  Below, we show example whole-cell electrical recordings from cochlear hair cells in normal saline with and without cyclodextrin treatment.              

When cyclodextrin (HPBCD) is given subcutaneously to normal mice, we see widespread loss of outer hair cells within 3 days of treatment.  Current projects are looking at the mechanisms and mitigation of this devastating effect.