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Mechanisms of Ion Channels and Pathogenesis of Cystic Fibrosis

Research Summary

Zhe Lu and colleagues are interested in understanding the fundamental mechanisms by which ion channels accomplish many important biological tasks, including controlling heart rate and coupling blood glucose levels to insulin secretion. They also investigate how disruptions in ion channel function lead to diseases such as cystic fibrosis.

Ion channels are integral membrane proteins that allow specific types of ions to pass across cell membranes in a controlled manner. The ion flow across cell membranes generates electric signals in the so-called excitable cells—including nerve, muscle, and endocrine cells—and results in salt transport across cell membranes. We are studying ion channels in the plasma membrane that preferentially conduct K+ or Cl ions as well as the Ca2+ release channel on the endoplasmic reticulum (ER). This Ca2+ release channel is commonly called the inositol trisphosphate (InsP3) receptor (InsP3R).

Enzymatic Control of Voltage-Gated Ion Channels
Voltage-gated ion channels have traditionally been viewed as the hallmarks of excitable cells. This view has been challenged, initially by the discovery that voltage-gated ion channels also exist in nonexcitable cells, such as glia and lymphocytes, and more recently by the well-known finding that bacterial genomes encode voltage-gated ion channels. A question thus arises, that is, can voltage-gated ion channels also be controlled by means other than varying the voltage difference between the solutions separated by the plasma membrane? Experimental evidence for this conjecture provides the key to two questions: How do nonexcitable cells utilize voltage-gated ion channels, and how can we control these channels for therapeutic purposes?

Figure 1: Structures of the ligand-binding domain (LBD) of inositol trisphosphate (InsP3) receptor...

We have found experimentally that a phospholipase, sphingomylinase (SMase) D from spider venom, can turn on voltage-gated K+ channels. SMase D removes the positively charged choline groups from the phospholipid sphingomyelin that is primarily present in the outer leaf of the membrane lipid bilayer. Removal of positively charged choline from sphingomyelin energetically favors outward movement of the positively charged voltage sensor during activation. Consequently, voltage-gated K+ channels become activated upon SMase D treatment at a typical resting membrane potential at which the channels otherwise remain largely closed. Thus, conceptually, voltage-gated ion channels can be activated enzymatically or through regulating lipid metabolism.

Anomalous Voltage Dependence of K+ Channels
Inward-rectifier K+ (Kir) channels function as K+-selective diodes in the cell membrane, allowing K+ to flow much more efficiently into than out of cells. This unique property—observed originally by Bernhard Katz in 1949 and commonly called anomalous or inward rectification—enables the channels to accomplish various critical physiological tasks. Inward rectification results from a voltage-dependent channel block by intracellular cationic blockers such as the polyamine spermine, which carries four positive charges. The cause of the exceedingly strong voltage dependence (valence of ~5) has been the subject of intense investigation. On the basis of our decade-long systematic study, we have hypothesized that the strong voltage dependence primarily reflects the movement of charges carried by permeant (K+) ions across the transmembrane electrical field—not by the amine blocker itself—as K+ ions and the blocker displace each other during blocking and unblocking of the channel pore. This model quantitatively accounts for the defining feature of inward rectification: the classical observation that, at a given concentration of intracellular K+, rectification is a function of the difference between the membrane potential and the equilibrium potential for K+ ions, rather than a function of the membrane potential per se.

The validity of our hypothesis was dependent on two key assumptions: 1) that five ion sites are located intracellular to the filter and 2) that the blocker can force essentially unidirectional K+ movement in a pore region generally wider than the combined dimensions of the blocker plus a K+ ion. To substantiate these assumptions, we determined a crystal structure of the cytoplasmic portion of a Kir channel with five ions bound and demonstrated that a constriction near the intracellular end of the pore, acting as a gasket, prevents K+ ions from bypassing the blocker. This heretofore unrecognized "gasket" ensures that the blocker can effectively displace K+ ions across the selectivity filter to generate exceedingly strong voltage sensitivity.

Engineering a Specific and High-Affinity Inhibitor for a Subtype of Kir Channels
Kir channels play many important biological roles, including controlling cardiac pacemaker activity, coupling insulin secretion to blood glucose levels, modulating neural transmission, and maintaining electrolyte balance. These channels are emerging as important therapeutic targets. Also, subtype-specific inhibitors would be useful tools for studying the physiological functions of these channels. Unfortunately, available K+ channel inhibitors lack the necessary specificity to be used reliably as pharmacological tools to dissect the various kinds of K+ channel currents in situ. The two main targets for inhibitors are the ion-conduction pore for all K+ channels and, for voltage-gated K+ channels, the voltage sensor. The highly conserved nature of both types of target accounts for the great difficulty in finding inhibitors specific for a given class of K+ channels or, worse, individual subtypes within a class.

By modifying a toxin from honeybee venom, we have successfully engineered an inhibitor that blocks Kir1 with high (1 nM) affinity and high (>250-fold) selectivity over other commonly studied Kir subtypes. This success not only yields a highly desirable tool but also demonstrates the practical feasibility of engineering subtype-specific K+ channel inhibitors.

Apo and InsP3-Bound Crystal Structures of the Ligand-Binding Domain of an InsP3R
InsP3 triggers a rise in the concentration of cytoplasmic free Ca2+, thereby initiating important biological processes such as learning and memory, muscle contraction, secretion, T cell proliferation, and fertilization. InsP3 binding to InsP3R in the ER membrane causes an ion pore within the InsP3R protein to open. Efflux of Ca2+ from the ER through the InsP3R pore elevates the cytoplasmic free Ca2+ concentration, a signal that triggers cellular processes by various means, ranging from direct binding to indirect regulation of gene expression. We have determined the crystal structure of the ligand-binding domain (LBD) of InsP3R1, which is formed by the ~600 N-terminal residues, in both InsP3-bound and -unbound conformations (see figure). Three structural lobes within the LBD—two β-trefoil folds (β-TF1 and β-TF2) and an armadillo repeat fold (ARF)—are arranged in a triangle. Comparison of the InsP3-bound and -unbound conformations reveals that β-TF1 and ARF, which are respectively formed by the first and last thirds of the LBD sequence, move as a rigid unit with respect to β-TF2 formed by the intervening sequence. Whereas the LBD without the InsP3-bound conformation may spontaneously transition between gating states, binding of InsP3 between β-TF2 and ARF locks it in a state that would strongly bias the gating equilibrium toward the open state of the ion pore. Our study provides the requisite structural information to understand the allosteric regulation of InsP3R.

The Antioxidant Role of Thiocyanate in the Pathogenesis of Cystic Fibrosis
Cystic fibrosis (CF), a multiorgan genetic disease, originates from mutations in the CF transmembrane conductance regulator (CFTR). Lung injuries inflicted by recurring infection and excessive inflammation cause ~90 percent of the morbidity and mortality of patients with CF. It remains unclear how CFTR mutations lead to lung illness. Although commonly known as a Cl channel, CFTR also conducts thiocyanate (SCN) ions, important because, in several ways, they can limit potentially harmful accumulations of hydrogen peroxide (H2O2) and hypochlorite (OCl). First, lactoperoxidase (LPO) in the airways catalyzes oxidation of SCN to tissue-innocuous hypothiocyanite (OSCN) while consuming H2O2. Second, SCN, even at low concentrations, competes effectively with Cl for myeloperoxidase (MPO) from white blood cells, thus limiting OCl production by the enzyme. Third, SCN can rapidly reduce OCl without catalysis. The reported SCN concentration in the airway fluid is 460 µM. We have shown that 400 μM SCN plus LPO effectively protect the Calu-3 lung line from injuries caused by H2O2 and that SCN at concentrations ≥100 μM protects from OCl made by MPO.

These projects were partially funded by the National Institute of General Medical Sciences of the National Institutes of Health.

As of October 08, 2012

Scientist Profile

Investigator
University of Pennsylvania
Biophysics, Physiology