Library Reader Issue 02: Source Of Clarification

Library resource awareness poster covering the difference between primary, secondary, and tertiary sources, along with UNE Library resources which carry each.https://dune.une.edu/libraryreader/1001/thumbnail.jp

Investigating the role of L-type voltage-gated calcium channels in electrical events in pancreatic Beta cells

L-type voltage-gated calcium channels play a central role in providing the influx of calcium that stimulates electrical activity and insulin secretion in pancreatic β cells. Cav1.2 and Cav1.3 are the two L-type channels that are present in β cells and insulin-secreting cell lines; however the defined role that each channel has in contributing to the electrical activity that drives insulin secretion remains to be fully understood. Cav1.2 and Cav1.3 share much similarity in amino acid sequence, yet there is a higher sequence divergence in the intracellular loops that connect the homologous transmembrane domains of the α 1 pore-forming subunit. The intracellular loop connecting domains II and III (II-III loop) of Cav1.2 and Cav1.3 share only about 40% amino acid sequence identity, making this region one of the most divergent regions of the channel. The II-III loop region of voltage-gated calcium channels has been highly studied regarding its function as a site for protein-protein interactions which positions the calcium channels in signaling complexes that modulate neurotransmitter and hormone release. To investigate the role of the Cav II-III loop in β cells, we stably expressed the II-III loop portion of each channel in INS-1 pancreatic β cells and created stable INS-1 cell lines called Cav1.2/II-III cells (II-III loop of Cav1.2 expressed) and Cav1.3/II-III cells (II-III loop of Cav1.3 expressed). We have previously reported that Cav1.2 and Cav1.3 are localized to lipid raft regions and that expressing the II-III loop peptide of either channel specifically displaces the respective channel out of lipid rafts. Displacement of Ca v1.2 or Cav1.3 could have functional consequences for the electrical activity that initiates and regulates insulin secretion. I measured whole-cell voltage-gated calcium channel activity and K ATP channel activity in INS-1, Cav1.2/II-III, and Ca v1.3/II-III cells to determine if there was any effect of channel displacement from lipid rafts on ion channel activity. I examined the current-voltage relationship of voltage-gated calcium channels and observed no difference in the three cell lines, suggesting that the endogenous channels are still activated in their normal membrane potential threshold range. In the Cav1.2/II-III cells, the whole-cell IBa density was actually significantly greater compared to INS-1 and Cav1.3/II-III cells. I measured KATP channel activity and modulation by diazoxide and sulfonylureas in each cell line. I did not detect a significant difference in tolbutamide sensitivity of KATP channel currents or gliclazide sensitivity of KATP channel-mediated membrane depolarization. Displacement of Cav 1.2 or Cav1.3 does not diminish voltage-gated calcium channel activity or affect the function of the KATP channel. I used the perforated-patch clamp recording configuration to measure glucose induced depolarization and action potentials in INS-1, Cav1.2/II-III, and Cav1.3/II-III cells to determine if there was an effect of Cav1.2 or Cav1.3 being displaced from lipid rafts. I determined that there was no significant difference in the level of depolarization induced by 18 mM glucose in each cell line, supporting my previous observations that the activity of voltage-gated calcium channels and the KATP channel in these cells has not been compromised. I observed that displacement of Cav1.2 or Cav1.3 affected action potential spiking in the Cav1.2/II-III, and Cav1.3/II-III cells. There was an increase in frequency of action potentials in the Cav1.2/II-III cells compared to INS-1 cells, suggesting Cav1.2 is integral for contributing to the Ca2+ signal that leads to SK channel activation for regulation of action potential frequency. I also observed a decrease in frequency of action potentials in the Cav1.3/II-III cells compared to the INS-1 cells, suggesting Cav1.3 is integral for contributing to the Ca2+ signal that is required for induction of action potentials. These observations provide evidence for coupling of Cav1.2 or Cav1.3 to mechanisms of action potential regulation in pancreatic β cells

The Intracellular II-III Loops of Cav1.2 and Cav1.3 Uncouple L-Type Voltage-Gated Ca2+ Channels from Glucagon-Like Peptide-1 Potentiation of Insulin Secretion in INS-1 Cells via Displacement from Lipid RaftsS⃞

L-type Ca2+ channels play a key role in the integration of physiological signals regulating insulin secretion that probably requires their localization to specific subdomains of the plasma membrane. We investigated the role of the intracellular II-III loop domains of the L-type channels Cav1.2 and 1.3 in coupling of Ca2+ influx with glucose-stimulated insulin secretion (GSIS) potentiated by the incretin hormone glucagon-like peptide (GLP)-1. In INS-1 cell lines expressing the Cav1.2/II-III or Cav1.3/II-III peptides, GLP-1 potentiation of GSIS was inhibited markedly, coincident with a decrease in GLP-1-stimulated cAMP accumulation and the redistribution of Cav1.2 and Cav1.3 out of lipid rafts. Neither the Cav1.2/II-III nor the Cav1.3/II-III peptide decreased L-type current density compared with untransfected INS-1 cells. GLP-1 potentiation of GSIS was restored by the L-type channel agonist 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester (FPL-64176). In contrast, potentiation of GSIS by 8-bromo-cAMP (8-Br-cAMP) was inhibited in Cav1.2/II-III but not Cav1.3/II-III cells. These differences may involve unique protein-protein interactions because the Cav1.2/II-III peptide, but not the Cav1.3/II-III peptide, immunoprecipitates Rab3-interacting molecule (RIM) 2 from INS-1 cell lysates. RIM2, and its binding partner Piccolo, localize to lipid rafts, and they may serve as anchors for Cav1.2 localization to lipid rafts in INS-1 cells. These findings suggest that the II-III interdomain loops of Cav1.2, and possibly Cav1.3, direct these channels to membrane microdomains in which the proteins that mediate potentiation of GSIS by GLP-1 and 8-Br-cAMP assemble

Evaluation of Difluoromethyl Ketones as Agonists of the γ‑Aminobutyric Acid Type B (GABA_B) Receptor

The design, synthesis, biological evaluation, and in vivo studies of difluoromethyl ketones as GABA_B agonists that are not structurally analogous to known GABA_B agonists, such as baclofen or 3-aminopropyl phosphinic acid, are presented. The difluoromethyl ketones were assembled in three synthetic steps using a trifluoroacetate-release aldol reaction. Following evaluation at clinically relevant GABA receptors, we have identified a difluoromethyl ketone that is a potent GABA_B agonist, obtained its X-ray structure, and presented preliminary in vivo data in alcohol-preferring mice. The behavioral studies in mice demonstrated that this compound tended to reduce the acoustic startle response, which is consistent with an anxiolytic profile. Structure–activity investigations determined that replacing the fluorines of the difluoromethyl ketone with hydrogens resulted in an inactive analogue. Resolution of the individual enantiomers of the difluoromethyl ketone provided a compound with full biological activity at concentrations less than an order of magnitude greater than the pharmaceutical, baclofen

Evaluation of Difluoromethyl Ketones as Agonists of the γ‑Aminobutyric Acid Type B (GABA_B) Receptor

The design, synthesis, biological evaluation, and in vivo studies of difluoromethyl ketones as GABA_B agonists that are not structurally analogous to known GABA_B agonists, such as baclofen or 3-aminopropyl phosphinic acid, are presented. The difluoromethyl ketones were assembled in three synthetic steps using a trifluoroacetate-release aldol reaction. Following evaluation at clinically relevant GABA receptors, we have identified a difluoromethyl ketone that is a potent GABA_B agonist, obtained its X-ray structure, and presented preliminary in vivo data in alcohol-preferring mice. The behavioral studies in mice demonstrated that this compound tended to reduce the acoustic startle response, which is consistent with an anxiolytic profile. Structure–activity investigations determined that replacing the fluorines of the difluoromethyl ketone with hydrogens resulted in an inactive analogue. Resolution of the individual enantiomers of the difluoromethyl ketone provided a compound with full biological activity at concentrations less than an order of magnitude greater than the pharmaceutical, baclofen