Gregory H. Hockerman, Ph.D. Associate Professor of Medicinal Chemistry and Molecular Pharmacology Phone: (765) 496-3874 Fax: (765) 494-1414 E-mail: gregh@purdue.edu Full directory listing

Specialization: Molecular pharmacology

Education

Research: Molecular pharmacology

Voltage-gated calcium channels are key players in a large array of physiological processes including contraction of cardiac, vascular and skeletal muscle, release of neurotransmitters from nerve terminals, gene expression, and hormone secretion. The long-range goal of our studies is to contribute to the development of drugs that can modulate voltage-gated calcium channels in a tissue and type selective manner to treat cardiovascular disease and type II diabetes. Work in our laboratory is focused on two general questions.

Cardiovascular Disease: How do small molecule drugs modulate voltage-gated calcium channels? Three distinct classes of small molecule drugs that modulate L-type calcium channels are currently in clinical use: dihydropyridines (DHP), phenylalkylamines (PAA) and benzothiazepines (BZP). These drugs are used extensively to treat cardiovascular disorders such as hypertension, angina pectoris and some arrhythmias. Using site-directed mutagenesis, expression of recombinant channels in tsA 201 cells, and whole-cell patch-clamp recording techniques, we have identified key amino acid residues for the interaction of each class of drugs with L-type channels. Using this information, we were able to construct a dihydropyridine binding site in a normally DHP-insensitive P/Q- type calcium channel by making only 9 amino acid substitutions. The resulting mutant channel exhibited dihydropyridine modulation nearly identical to that of L-type channels. This result suggests that DHPs may be a useful starting point in the search for calcium channel type selective drugs. Recent work in my lab has centered on describing the binding site for the BZP diltiazem in the Ca_v1.2 L-type channel, as well as the Ca²⁺ binding site in the channel that modulates the affinity for diltiazem. We are currently using chemical reagents in conjunction with mutations in the drug binding sites to understand how drug binding modulates channel function.

Type II Diabetes: What roles do L-type calcium channels play in insulin-secreting cells? Drugs acting on L-type calcium channels are used mainly for their effects on the cardiovascular system. L-type channels are, however, located in many tissues outside of the cardiovascular system, most notably in neurons and endocrine cells. Two distinct L-type channels, Ca_v1.2 and Ca_v1.3, are often present in the same cells, and distinct roles for each have been difficult to identify, since they are blocked by the same drugs. However, Ca_v1.2 and 1.3 channels are divergent in regions accessible to the cytoplasm, suggesting distinct roles for these channels in cell signaling. Our current research is focused on the role of Ca_v1.2 and 1.3 channels in insulin secreting cells. My lab has developed mutant versions of Ca_v1.2 and 1.3 that are resistant to the DHP class of L-type channel blockers (Ca_v1.2/DHPi and Ca_v1.3/DHPi), but normally sensitive to the BZP diltiazem. After introducing these mutant channels into insulin-secreting INS-1 cells, we are able to functionally isolate either Ca_v1.2 or Ca_v1.3 channels by "turning off" endogenous L-type channels with the DHP nifedipine. Using this approach, we found that Ca_v1.3, but not Ca_v1.2, channels are coupled to glucose-stimulated insulin secretion from INS-1 cells. Further, we found that Ca_v1.3, but not Ca_v1.2, channels are coupled to glucose-stimulated oscillations in intracellular Ca²⁺ concentrations. We are currently testing a series of chimeric Ca_v1.2/1.3 channels to determine which regions of Ca_v1.3 are critical for coupling to insulin secretion.

Representative Publications

Jacobo, S.M.P., Guerra, M.L., Jarrard, R.E., Przybyla, J.A., Liu, G., Watts, V.J., and Hockerman, G.H. The intracellular II-III loops of Ca_v1.2 and Ca_v1.3 uncouple L-type voltage-gated Ca²⁺ channels from glucagon-like peptide-1 potentiation of insulin secretion in INS-1 cells via displacement from lipid rafts J. Pharmacol. Exp. Ther. 330:283-293 (2009).

Walsh, K.B., Zhang, J., Fuseler, J.W., Hilliard, N., and Hockerman, G.H. Adenoviral-mediated expression of dihydropyridine-insensitive L-type calcium channels in cardiac ventricular myocytes and fibroblasts Eur. J. Pharmacol. 565:7-16 (2007).

Liu, G., Jacobo, S.M.P., Hilliard, N., and Hockerman, G.H. Cyclic AMP potentiates coupling of both Ca_v1.2 and Ca_v1.3 to glucose-stimulated insulin secretion at sub-maximal glucose concentration through EPAC and PKA in INS-1 cells. J. Pharmacol. Exp. Ther. 318:152-160 (2006).

Dilmac, N., Hilliard, N., and Hockerman, G.H. Molecular determinants of frequency-dependence and Ca²⁺ potentiation of verapamil block in the pore region of Ca_v1.2. Mol. Pharmacol. 66:1236-1247 (2004).

Liu, G., Hilliard, N., and Hockerman, G.H. Preferential coupling of Ca_v1.3 to glucose-induced [Ca²⁺]i in the pancreatic beta cell line INS-1. Mol. Pharmacol. 65:1269-1277 (2004).

Dilmac, N., Hilliard, N., and Hockerman, G.H. Molecular determinants of Ca²⁺ potentiation of diltiazem block and Ca²⁺-dependent inactivation in the pore region of Ca_v1.2 Mol. Pharmacol. 64:491-501 (2003).

Liu, G., Dilmac, N., Hilliard, N., and Hockerman, G.H. Ca_v1.3 is preferentially coupled to glucose-stimulated insulin secretion in the pancreatic beta cell line INS-1. J. Pharmacol. Exp. Ther. 305, 271-278 (2003).

Hockerman, G.H., Dilmac, N., Scheuer, T., and Catterall, W.A. Molecular determinants of diltiazem block in domains IIIS6 and IVS6 of L-type calcium channels. Mol. Pharmacol. 58, 1264-1270 (2000).

Hockerman, G.H., Peterson, B.Z., Tanada, T.N., Sharp, E.M., Scheuer, T., and Catterall, W.A. Construction of a high affinity receptor site for dihydropyridine agonists and antagonists by single amino acid substitutions in a non-L-type Ca²⁺ channel. Proc. Natl. Acad. Sci. USA 94, 14906-14911 (1997).

Hockerman, G.H., Johnson, B.D., Abbott, M.R., Scheuer, T., and Catterall, W.A . Molecular determinants of high affinity phenylalkylamine block of L-type calcium channels in transmembrane segment IIIS6 and the pore region of the Alpha1 subunit. J. Biol. Chem. 272, 18759-18765 (1997).