Salt Bridges and Gating in the COOH-terminal Region of HCN2 and CNGA1 Channels

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels and cyclic nucleotide-gated (CNG) channels are activated by the direct binding of cyclic nucleotides. The intracellular COOH-terminal regions exhibit high sequence similarity in all HCN and CNG channels. This region contains the cyclic nucleotide-binding domain (CNBD) and the C-linker region, which connects the CNBD to the pore. Recently, the structure of the HCN2 COOH-terminal region was solved and shown to contain intersubunit interactions between C-linker regions. To explore the role of these intersubunit interactions in intact channels, we studied two salt bridges in the C-linker region: an intersubunit interaction between C-linkers of neighboring subunits, and an intrasubunit interaction between the C-linker and its CNBD. We show that breaking these salt bridges in both HCN2 and CNGA1 channels through mutation causes an increase in the favorability of channel opening. The wild-type behavior of both HCN2 and CNGA1 channels is rescued by switching the position of the positive and negative residues, thus restoring the salt bridges. These results suggest that the salt bridges seen in the HCN2 COOH-terminal crystal structure are also present in the intact HCN2 channel. Furthermore, the similar effects of the mutations on HCN2 and CNGA1 channels suggest that these salt bridge interactions are also present in the intact CNGA1 channel. As disrupting the interactions leads to channels with more favorable opening transitions, the salt bridges appear to stabilize a closed conformation in both the HCN2 and CNGA1 channels. These results suggest that the HCN2 COOH-terminal crystal structure contains the C-linker regions in the resting configuration even though the CNBD is ligand bound, and channel opening involves a rearrangement of the C-linkers and, thus, disruption of the salt bridges. Discovering that one portion of the COOH terminus, the CNBD, can be in the activated configuration while the other portion, the C-linker, is not activated has lead us to suggest a novel modular gating scheme for HCN and CNG channels

Multiple Phosphorylation of Rhodopsin and the In Vivo Chemistry Underlying Rod Photoreceptor Dark Adaptation

AbstractDark adaptation requires timely deactivation of phototransduction and efficient regeneration of visual pigment. No previous study has directly compared the kinetics of dark adaptation with rates of the various chemical reactions that influence it. To accomplish this, we developed a novel rapid-quench/mass spectrometry-based method to establish the initial kinetics and site specificity of light-stimulated rhodopsin phosphorylation in mouse retinas. We also measured phosphorylation and dephosphorylation, regeneration of rhodopsin, and reduction of all-trans retinal all under identical in vivo conditions. Dark adaptation was monitored by electroretinography. We found that rhodopsin is multiply phosphorylated and then dephosphorylated in an ordered fashion following exposure to light. Initially during dark adaptation, transduction activity wanes as multiple phosphates accumulate. Thereafter, full recovery of photosensitivity coincides with regeneration and dephosphorylation of rhodopsin

Mesenteric artery disease in the elderly

AbstractPurposeThe purpose of this study was to estimate the population-based prevalence of mesenteric artery stenosis (MAS) and occlusion among independent elderly Americans.MethodAs part of an ancillary investigation to the Cardiovascular Health Study (CHS), participants in the Forsyth County, NC cohort had visceral duplex sonography of the celiac arteries and superior mesenteric arteries (SMAs). Critical MAS was defined by celiac peak systolic velocity ≥2.0 m/s and/or SMA peak systolic velocity ≥2.7 m/s. Occlusion of either vessel was defined by lack of a Doppler-shifted signal within the imaged artery. Demographic data, blood pressures, and blood lipid levels were collected as part of the baseline CHS examination. Participants' weights were measured at baseline and before the duplex exam. Univariate tests of association were performed with two-way contingency tables, Student t tests, and Fisher exact tests. Multivariate associations were examined with logistic regression analysis.ResultsA total of 553 CHS participants had visceral duplex sonography technically adequate to define the presence or absence of MAS. The study group had a mean age of 77.2 ± 4.9 years and comprised 63% women and 37% men. Participant race was 76% white and 23% African-American. Ninety-seven participants (17.5%) had MAS. There was no significant difference in age, race, gender, body mass index, blood pressure, cholesterol, or low-density lipoproteins for participants with or without MAS. Forward stepwise variable selection found renal artery stenosis (P = .008; odds ratio [OR], 2.85; 95% confidence interval [CI], 1.31, 6.21) and high-density lipoprotein >40 (P = .02; OR, 3.03; 95% CI, 1.17, 7.81) significantly associated with MAS in a multivariate logistic regression model. Eighty-three of the 97 participants with MAS (15.0% of the cohort) had isolated celiac stenosis. Seven participants (1.3% of the cohort) had combined celiac and SMA stenosis. Five participants (0.9% of the cohort) had isolated SMA stenosis. Two participants (0.4% of the cohort) had celiac occlusion. Considering all participants with MAS, there was no association with weight change. However, SMA stenosis and celiac occlusion demonstrated an independent association with annualized weight loss (P = .028; OR, 1.54; 95% CI, 1.05, 2.26) and with renal artery stenosis (P =.001; OR, 9.48; 95% CI, 2.62, 34.47).ConclusionThis investigation provides the first population-based estimate of the prevalence of MAS among independent elderly Americans. MAS existed in 17.5% of the study cohort. The majority had isolated celiac disease. SMA stenosis and celiac artery occlusion demonstrated a significant and independent association with weight loss and concurrent renal artery disease

C-terminal Movement during Gating in Cyclic Nucleotide-modulated Channels*

Activation of cyclic nucleotide-modulated channels such as CNG and HCN channels is promoted by ligand-induced conformational changes in their C-terminal regions. The primary intersubunit interface of these C termini includes two salt bridges per subunit, formed between three residues (one positively charged and two negatively charged amino acids) that we term the SB triad. We previously hypothesized that the SB triad is formed in the closed channel and breaks when the channel opens. Here we tested this hypothesis by dynamically manipulating the SB triad in functioning CNGA1 channels. Reversing the charge at positions Arg-431 and Glu-462, two of the SB triad residues, by either mutation or application of charged reagents increased the favorability of channel opening. To determine how a charge reversal mutation in the SB triad structurally affects the channel, we solved the crystal structure of the HCN2 C-terminal region with the equivalent E462R mutation. The backbone structure of this mutant was very similar to that of wild type, but the SB triad was rearranged such that both salt bridges did not always form simultaneously, suggesting a mechanism for the increased ease of opening of the mutant channels. To prevent movement in the SB triad, we tethered two components of the SB triad region together with cysteine-reactive cross-linkers. Preventing normal movement of the SB triad region with short cross-linkers inhibited channel opening, whereas longer cross-linkers did not. These results support our hypothesis that the SB triad forms in the closed channel and indicate that this region expands as the channel opens