Lily Jan, PhD

Potassium Channels and Calcium-activated Chloride Channels

Conserved among eukaryotes and prokaryotes, potassium channels modulate neuronal signaling in the brain and peripheral nervous system, regulate cell volume and the flow of salt across epithelia, and control heart rate, vascular tone, and the release of hormones such as insulin. They further protect neurons and muscles under metabolic stress. 

Calcium-activated chloride channels (CaCCs) also serve a broad range of physiological functions by regulating the flow of salt and water across epithelia, and electrical potential across the cell membrane. In the nervous system, these chloride channels may regulate neuronal excitability and synaptic efficacy. In green algae and plants that lack voltage-gated sodium channels, electric signal generation may depend on calcium-activated chloride channels.

To understand how ion channels allow only the physiologically appropriate ions to go through, how they alter channel activity in response to electrical and chemical signals, how they contribute to neuronal signaling and synaptic plasticity, and how channel malfunctions can cause a variety of diseases, it is important to determine their molecular identity so that these channels can be studied one at a time—a challenge both for potassium channels and for CaCCs, owing to the heterogeneity, low abundance, and ubiquity of these channels.

Currrent Projects

It is remarkable that the TMEM16 family of “transmembrane proteins with unknown function” includes not only channels that permeate negatively charged ions but also closely related channels that permeate positively charged ions: TMEM16A and TMEM16B form CaCCs that allow chloride ions to go through, whereas TMEM16F forms a small-conductance calcium-activated non-selective cation channel (SCAN) that permeates calcium. TMEM16F is linked to the Scott syndrome, a bleeding disorder arising from a deficiency in calcium-activated lipid scrambling that exposes phosphatidylserine on the surface of blood cells to trigger blood coagulation. Unexpectedly, we recently found that TMEM16C facilitates the sodium-activated potassium channel activity in small dorsal root ganglion neurons to modulate the excitability of these sensory neurons and pain sensitivity. So the TMEM16 family encompasses an unusually broad range of ion channels.

Potassium channel mutations are linked to diseases of the brain (epilepsy, episodic ataxia), ear (deafness), heart (arrhythmia), muscle (myokymia, periodic paralysis), kidney (hypertension), pancreas (hyperinsulinemic hypoglycemia, neonatal diabetes), as well as developmental abnormalities of neural crest–derived tissues (Andersen's syndrome). Not only do mutations that increase or decrease potassium channel activity cause diseases, alteration of potassium channel expression levels could also have a strong impact. For example, our recent study revealed that increased expression of EAG2 potassium channels in medulloblastoma is associated with malignant tumor growth. Moreover, midlife obesity involves an increase in ATP-sensitive potassium channel activity that causes reduction of hypothalamic neuronal excitability and release of peptides crucial for controlling food intake and bodyweight.

Calcium-activated chloride channels regulate neuronal excitability as well as secretion from exocrine glands and airway epithelia. In the smooth muscle, calcium release from internal store will activate CaCCs, leading to membrane potential changes that open calcium channels to sustain smooth muscle contraction. Therefore, CaCC modulators may be considered for the treatment of hypertension and pulmonary diseases such as asthma. In collaboration with Dr. Min Li at Johns Hopkins School of Medicine, we have been characterizing novel CaCC modulators that are useful in biophysical studies of CaCC channels as well as pharmacological and physiological studies of CaCCs in different tissues and brain regions.

One unique advantage in channel studies is the possibility of examining one channel at a time, with submillisecond resolution, for many seconds, in experimentally determined intracellular and extracellular environments. In addition to conducting biophysical, biochemical, and cell biological studies of channel assembly, trafficking, regulation, and function, we need to learn how these channels are targeted to specific subcellular compartments of neurons and how they respond dynamically to neuronal activity and in turn modulate neuronal signaling. A few examples of our recent studies are summarized below:

Axonal Targeting of Kv1 Channels
Axonal Kv1 channels in the Shaker family enable action potentials to invade a physiologically appropriate number of axonal branches without bouncing back from the nerve terminals; hyperexcitability caused by altered Kv1 channel activity accounts for the symptoms of patients with episodic ataxia type 1 and the shaking phenotype of Shaker mutant flies. To understand the regulation of axonal Kv1 channels, we identified an axonal targeting machinery involving components that are evolutionarily conserved: the microtubule plus end–binding protein EB1 and the KIF3 kinesin motor that physically associate with Kv1 channel beta subunits are both required for axonal targeting of Kv1 channels.

Synaptic Regulation of Local Translation of Kv1.1 and Kv4.2 Channels in dendrites
To characterize dendritically targeted mRNAs that may be under local synaptic regulation for channel production in dendrites, we found that Kv1.1 and Kv4.2 mRNAs in the dendrites are subject to neuronal activity regulation that involves signaling molecules linked to tuberous sclerosis and fragile X syndrome – diseases associated with elevated risk for autism.

Biophysical and Physiological Studies of Calcium-activated Chloride Channels (CaCCs)
To look into how calcium activates CaCCs in the TMEM16 family, we have identified and characterized a distantly related CaCC named Subdued for its critical role in host defense in Drosophila, in order to zero in on those highly conserved residues that may be involved in calcium gating. We are also looking into the unusual ability of some TMEM16 family member to permeate cations while its close relatives are anion channels. Mouse mutants have been generated for physiological studies of this novel family of ion channels.

To identify the founding members of ion channel families, we began with positional cloning of the Shaker voltage-gated potassium (Kv) channel gene in the fruit fly in the 1980s, expression cloning of a mammalian inwardly rectifying potassium (Kir) channel in the 1990s, and then expression cloning of the frog oocyte calcium-activated chloride channel (CaCC) in the 2000s. Once we have identified the founding members of two large and distantly related potassium channel families as well as calcium-activated chloride channels that belong to a novel ion channel family, we proceed to study the channel family members one at a time to learn about their functions. All three families of ion channels are highly conserved evolutionarily. Indeed, even the physiological functions are evolutionarily conserved for some of the channels.

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