Stephen Michael Sims

The general theme of my research is the control of ion channels in cells by neurotransmitters, hormones and other factors. There are two projects, one involving smooth muscle cells and the other osteoclasts, the cells responsible for resorption of bone.1) Smooth muscle cells. My studies are focused on transmitter regulation of currents in mammalian smooth muscle. Using the perforated patch configuration of patch-clamp, we record membrane currents from gastric and airway muscle cells while simultaneously monitoring contraction. The excitatory neurotransmitter acetylcholine (ACh) activates non-selective cation and Cl- currents, and also causes contraction, even in the absence of external Ca2+. These results are consistent with ACh releasing Ca2+ from internal stores, triggering contraction and activating channels. These data provide evidence for a positive feedback loop, where Ca2+ released from stores causes depolarization, in turn leading to opening of voltage-dependent Ca2+ channels. This model for cholinergic excitation is fundamentally different from established views, where depolarization is the primary event causing Ca2+ entry. We also use fluorescent indicator dyes to monitor Ca2+ concentration in cells, and are recording membrane currents, [Ca2+]i and contraction simultaneously in single cells. A new direction we are developing is the studies of signaling pathways in human esophageal smooth muscle cells. Tissues are obtained from surgery by Dr. Harold Preiksaitis (St. Josephs Health Centre), andin his laboratory muscle contraction as well as the molecular properties of receptors and ion channels are studied in collaboration with Dr. Laurier. In our laboratory, we disperse the cells and examine functional properties of the signaling pathways. Wehave characterized membrane currents in the cells using patch clamp recording.In addition, we have completed a series of studies using Ca2+ -sensitive dyes to characterize muscarinic signalling paths. Both studies represent the first ever study of human esophageal cells in this way. We are continuing to investigate the channels and receptor signalling pathways in esophagus, and integrating the work with molecular studies in Dr. Preiksaitis and Dr. Laurier's laboratory. These studies contribute to our understanding of smooth muscle contraction, with relevance to understanding the basis of various smooth muscle disorders, such as gastrointestinal dysmotility and asthma.2) Osteoclasts. In collaboration with Dr. Jeff Dixon (Physiology and Oral Biology, Faculty of Medicine) we are investigating electrophysiological properties of mammalian osteoclasts. In addition to patch-clamp recording methods, we are utilizing Ca2+ indicator dyes, time-lapse video recording and functional assays to investigate the control of osteoclast function. We were the first to characterize K+ and Cl- channels in mammalian osteoclasts and are presently studying their regulation by G-proteins and substrate in controlling expression of ionic currents. In other studies, we are investigating the regulation of osteoclast motility by hormones and growth factors (e.g. TGF- , M-CSF). Knowledge of the cellular basis for control of osteoclast motility and resorptive activity is essential for understanding the role of these cells in normal bone turnover and in diseases involving excessive bone loss, such as osteoporosis and some forms of arthritis. We have initiated a collaboration with Dr. Michael Underhill(Oral Biology) to use molecular techniques to study osteoclasts (a continuation of previous collaborations with Dr. Cedric Minkin, University of Southern California). We have now obtained an osteoclast cDNA library and are probing for types of receptors expressed by the cells. Our overall aim is to correlate the functional currents, and Ca2 responses with molecular details of the signaling components (e.g., subtypes of nucleotide receptor). Electrophysiological and molecular studies have identified the outward rectifier channel Kv1.3 as well as the inward rectifier channel IRK1 in rat and mouse osteoclasts. Ongoing studies are using combined patch-clamp and fluorescence to characterize the changes of [Ca2+]i and currents in osteoclasts elicited by nucleotides. These studies provide insight into the receptor signaling events in bone cells.

Academic Experience:1993-1998 Medical Research Council of Canada Scientist;1992-1995 Member of MRC Cell Physiology Grant review panel