Biomedical subjects

Timothy D Houle

Publications and source records attributed to Timothy D Houle.

3 recordsLinked to original sources

PubMed2006

Different endoplasmic reticulum trafficking and processing pathways for calsequestrin (CSQ) and epitope-tagged CSQ.

Cardiac calsequestrin (CSQ) is a protein that traffics to and concentrates inside sarcoplasmic reticulum (SR) terminal cisternae, a protein secretory compartment of uncertain origin. To investigate trafficking of CSQ within standard ER compartments, we expressed CSQ in nonmuscle cell lines and examined its localization by immunofluorescence and its molecular structure from the mass spectrum of total cellular CSQ. In all cells examined, CSQ was a highly phosphorylated protein with a glycan structure predictive of ER-retained proteins: Man9,8GlcNAc2 lacking terminal GlcNAc. Immunostaining was restricted to polymeric ER cisternae. Secretory pathway disruption by brefeldin A and thapsigargin led to altered CSQ glycosylation and phosphorylation consistent with post-ER trafficking. When epitope-tagged forms of CSQ were expressed in the same cells, mannose trimming of CSQ glycans was far more extensive, and C-terminal phosphorylation sites were nearly devoid of phosphate, in complete contrast to the highly phosphorylated wild-type protein that concentrates in all cells tested. Epitope-tagged CSQ also showed a reduced ER staining compared to wild-type protein, with significant staining in juxta-Golgi compartments. Loss of ER retention due to epitope tags or thapsigargin and resultant changes in protein structure or levels of bound Ca(2+) point to CSQ polymerization as an ER/SR retention mechanism.

Animals↗

PubMed2005

Triadin overexpression stimulates excitation-contraction coupling and increases predisposition to cellular arrhythmia in cardiac myocytes.

Triadin 1 (TRD) is an integral membrane protein that associates with the ryanodine receptor (RyR2), calsequestrin (CASQ2) and junctin to form a macromolecular Ca signaling complex in the cardiac junctional sarcoplasmic reticulum (SR). To define the functional role of TRD, we examined the effects of adenoviral-mediated overexpression of the wild-type protein (TRD(WT)) or a TRD mutant lacking the putative CASQ2 interaction domain residues 200 to 224 (TRD(Del.200-224)) on intracellular Ca signaling in adult rat ventricular myocytes. Overexpression of TRD(WT) reduced the amplitude of I(Ca)- induced Ca transients (at 0 mV) but voltage dependency of the Ca transients was markedly widened and flattened, such that even small I(Ca) at low and high depolarizations triggered maximal Ca transients. The frequency of spontaneous Ca sparks was significantly increased in TRD(WT) myocytes, whereas the amplitude of individual sparks was reduced. Consistent with these changes in Ca release signals, SR Ca content was decreased in TRD(WT) myocytes. Periodic electrical stimulation of TRD(WT) myocytes resulted in irregular, spontaneous Ca transients and arrhythmic oscillations of the membrane potential. Expression of TRD(Del.200-224) failed to produce any of the effects of the wild-type protein. The lipid bilayer technique was used to record the activity of single RyR2 channels using microsome samples obtained from control, TRD(WT) and TRD(Del.200-224) myocytes. Elevation of TRD(WT) levels increased the open probability of RyR2 channels, whereas expression of the mutant protein did not affect RyR2 activity. We conclude that TRD enhances cardiac excitation-contraction coupling by directly stimulating the RyR2. Interaction of TRD with RyR2 may involve amino acids 200 to 224 in C-terminal domain of TRD.

Adenoviridae↗

PubMed2004

Calsequestrin mutant D307H exhibits depressed binding to its protein targets and a depressed response to calcium.

OBJECTIVE: A point mutation in human cardiac calsequestrin (CSQ-D307H) is responsible for a form of polymorphic ventricular tachycardia (PVT). When overexpressed in heart cells, the mutated CSQ leads to diminished Ca(2+) transients, consistent with defective regulation of intralumenal sarcoplasmic reticulum (SR) Ca(2+). METHODS: To analyze the D307H mutant and determine whether the D307H mutation results in loss of normal protein-protein interactions, we prepared recombinant human wild-type (WT) and D307H forms of CSQ in mammalian cells. RESULTS: Although we found the two proteins to undergo similar glycosylation and phosphorylation, we discovered that Ca(2+)-dependent binding of the D307H mutant to both triadin-1 and junctin was reduced by greater than 50% compared to WT. Reduced binding of the D307H mutant CSQ to target proteins was similar throughout a complete range of Ca(2+) concentrations. To investigate the mechanism of reduced Ca(2+)-dependent binding, Ca(2+)-dependent changes in intrinsic fluorescence emission for the two protein forms were compared. Intrinsic fluorescence of the D307H mutant was highly reduced, reflecting significant alteration in the tertiary protein structure. Moreover, the changes in fluorescence caused by increasing the Ca(2+) concentration were very significantly blunted, indicating that the Ca(2+)-dependent conformational change was virtually lost. CONCLUSIONS: We conclude that the point mutation D307H leads to a profoundly altered conformation that no longer responds normally to Ca(2+) and fails to bind normally to triadin and junctin.

Animals↗