Early-stage dynamics of chloride ion–pumping rhodopsin revealed by a femtosecond X-ray laser

Chloride ion–pumping rhodopsin (ClR) in some marine bacteria utilizes light energy to actively transport Cl⁻ into cells. How the ClR initiates the transport is elusive. Here, we show the dynamics of ion transport observed with time-resolved serial femtosecond (fs) crystallography using the Linac Coherent Light Source. X-ray pulses captured structural changes in ClR upon flash illumination with a 550 nm fs-pumping laser. High-resolution structures for five time points (dark to 100 ps after flashing) reveal complex and coordinated dynamics comprising retinal isomerization, water molecule rearrangement, and conformational changes of various residues. Combining data from time-resolved spectroscopy experiments and molecular dynamics simulations, this study reveals that the chloride ion close to the Schiff base undergoes a dissociation–diffusion process upon light-triggered retinal isomerization.

Chloride ion (Cl⁻) concentration in some bacterial cells is regulated by rhodopsin proteins, generally known as halorhodopsin, or hR. These proteins use light energy to pump Cl⁻ into cells (1, 2). Light is harvested by a molecule of retinal, covalently linked to an essential lysine residue in the seventh transmembrane helix of GPCR-like (G protein–coupled receptor) proteins. Light activation causes retinal to isomerize from the all-trans to the 13-cis configuration. This change triggers subsequent conformational changes throughout the rhodopsin molecule and releases chloride into the cytoplasm. Retinal thermally relaxes to the all-trans configuration within milliseconds and is then ready for the next photocycle. Cl⁻ ions are transported from the extracellular (EC) side to the cytoplasmic (CP) side during each photocycle (3, 4).Light-driven ion-pumping rhodopsin can be used to develop artificial solar energy harvesting and optogenetics (5–8), but the molecular mechanism must be understood in detail for such applications. Despite the importance of hR, our current experimental data concerning the structure and dynamics of the protein remain very limited. A related protein, proton (H⁺)-pumping bacteriorhodopsin (bR) discovered in the early 1970s, has been extensively studied by multiple methods, including time-resolved spectroscopy, crystallography, mutagenesis, and computer simulation (9–12). In particular, recent studies using time-resolved serial femtosecond crystallography (TR-SFX) methods performed at X-ray free-electron laser (XFEL) facilities allow three-dimensional (3D) visualization of retinal isomerization and associated local conformational changes. These changes are accompanied by movement of protons from a donor aspartate group to an acceptor aspartate (13–15). However, the central component of this process, the transported H⁺, is difficult to observe by X-ray crystallography and could not be directly traced in bR TR-SFX studies. Recently, a breakthrough was reported in a study on the sodium-pumping rhodopsin KR2 (K. eikastus rhodopsin 2), in which electron density signals of Na⁺ uptake were observed at Δt = 1 ms after laser illumination (16).Cl⁻, a strong X-ray scatterer, can be directly observed from electron density maps. These maps provide first-hand information on the movement of ions as being transported within short timescales after light activation. Furthermore, hR and bR presumably share a common molecular mechanism despite transporting ions in opposite directions. A close relationship is strongly implied by the interconversion of the function of two rhodopsins. Outward H⁺-pumping bR can be converted to an inward Cl⁻ pump by changing a single residue (D85T) (17), while hR from the cyanobacterium, Mastigocladopsis repens, is reported to pump protons after a single mutation (T74D) (18). The chloride pump can therefore serve as a system analogous to the proton transporter and provide valuable information that is difficult to obtain directly from bR.In this study, we focus on chloride ion–pumping rhodopsin (ClR) from the marine flavobacterium Nonlabens marinus S1-08T (19). The conserved DTD motif (Asp85-Thr89-Asp96) of the bR family, residues 85, 89, and 96, is replaced by an NTQ motif (Asn98- Thr102-Gln109) in ClR (Fig. 1). The sequence identity of ClR and canonical bR from Halobacterium salinarum is only 27%, but the two proteins, nevertheless, have highly similar structures, including the disposition of the retinal chromophore. ClR structures at cryogenic and room temperatures clearly reveal an architecture composed of seven transmembrane helices (TM A to G) (2, 20, 21). The retinal is covalently linked to the Nζ atom of the Lys235 located on TM-G. Anomalous diffraction signals of the Br⁻ identify a stable binding site near the protonated Schiff base (PSB) and a plausible exit site on the CP side (Fig. 1A). Buried water molecules and locations of cavities inside ClR suggest a pathway for Cl⁻ uptake on the EC side, but the molecular mechanism for light-triggered Cl⁻ pumping remains obscure. Upon light activation, the Cl⁻ tightly held near the PSB must break free from its hydrogen bonding network (Fig. 1B). It then passes through a hydrophobic region to reach the CP side (Fig. 1C). Crystal structures of ClR were previously determined with crystals under continuous illumination of visible laser light. Intriguingly, these steady-state models revealed unexpected movement of the retinal, without indication of photo-isomerization (22). Steady-state measurements, which show averages of mixed states, are thus of limited use in deciphering the molecular mechanism of light-driven Cl⁻ pumping.Open in a separate windowFig. 1.Structure of ClR and a plausible pathway of Cl⁻ transport. (A) Cross-sections of ClR with the backbone structure shown in cartoon representation. Transmembrane helices are marked using letters A through G, and the C-terminal helix H in the cytoplasm is also indicated. Surfaces are clipped to show the cross-section colored in yellow and the model being sliced and then opened about the axis near the helix E. Water molecules and Cl⁻ ions are shown as red- and green-colored spheres. Blue curves indicate the path of ion entering ClR and the principal pumping direction after passing retinal. (B) Key residues near the Cl⁻ ion and retinal, together with the NTQ motif shown in stick representation. (C) Residues that form a hydrophobic region between the retinal and the cytoplasm are highlighted in ball-and-stick representation. The red arrow points to a major barrier that Cl⁻ needs to overcome. ClR backbone is shown in cartoon representation, with residues colored based on hydrophobicity (the blue to red spectrum corresponds to the hydrophobicity scale from hydrophilic to hydrophobic).     

Impaired endothelium-dependent vasodilatation in subclinical hypothyroidism: beneficial effect of levothyroxine therapy

Subclinical hypothyroidism (sHT) is associated with enhanced cardiovascular risk. To test the hypothesis that patients with sHT are characterized by endothelial dysfunction and impaired nitric oxide (NO) availability, in 14 patients [serum cholesterol, 218 +/- 41 mg/dl (5.6 +/- 0.9 mM)] and 28 euthyroid subjects, subdivided into groups A and B [serum cholesterol, 170 +/- 19 mg/dl (4.4 +/- 0.5 mM) and 217 +/- 21 mg/dl (5.6 +/- 0.5 mM), respectively], we studied the forearm blood flow (strain-gauge plethysmography) response to intrabrachial acetylcholine, an endothelium-dependent vasodilator, at baseline and during infusion of N(G)-monomethyl-L-arginine (L-NMMA), a NO synthase inhibitor. Response to sodium nitroprusside and minimal forearm vascular resistances were also evaluated. In sHT patients, vasodilation to acetylcholine was reduced, compared with group B (+358 +/- 29% vs. +503 +/- 19%, P = 0.0003) and group A (663 +/- 65%, P = 0.02 vs. group B and P = 0.0002 vs. sHT). L-NMMA blunted the vasodilation to acetylcholine in groups A and B (49.1 +/- 6.3% and 42.7 +/- 5.5% maximal forearm blood flow reduction, respectively, P < 0.0001 vs. acetylcholine), whereas it was ineffective in sHT patients (12.8 +/- 2.5%). Response to sodium nitroprusside and minimal vascular resistances were similar. In sHT (n = 9) patients, 6 months of euthyroidism by levothyroxine replacement increased acetylcholine-vasodilation and restored L-NMMA inhibition. Patients with sHT are characterized by endothelial dysfunction resulting from a reduction in NO availability, an alteration partially independent of dyslipidemia and reversed by levothyroxine supplementation.     

Plasma levels of cell adhesion molecules during hyperinsulinemia and modulation of vasoactive mediators

Endothelial expression of cell adhesion molecules (CAMs) plays an important role in atherosclerosis. Atherosclerosis is increased in hyperinsulinemic states, but whether insulin per se is proatherogenic remains unclear. To investigate the effects of hyperinsulinemia on CAM expression, plasma levels of ICAM-1, VCAM-1 and E-selectin were measured before and after forearm infusion of insulin in healthy subjects. Insulin administration for 2h resulted in significant hyperinsulinemia, whereas no significant change was observed in soluble CAMs (all p > 0.05). Because insulin stimulates endothelial release of both endothelin-1 (ET-1) and nitric oxide (NO), which may modulate the expression of CAMs, we also investigated the response of CAMs to ET-1 receptor blockade, alone and in combination with NO synthesis inhibition. ET-1 receptor blockade during hyperinsulinemia resulted in a vasodilator response, but did not affect soluble CAMs (all p > 0.05). Superimposition of NO inhibition by L-NMMA reversed the vasodilator effect of ET-1 blockade, without affecting soluble CAMs (all p > 0.05). In conclusion, acute hyperinsulinemia, alone or during ET-1 and NO pathway blockade, does not affect soluble CAMs. These results do not support a direct effect of insulin on endothelial cells to affect leukocyte adhesiveness to the vascular wall.     

Chronic endogenous fungal endophthalmitis: diagnostic and treatment challenges: A case report

Rationale:Endogenous fungal endophthalmitis (EFE) is a sight-threatening complication of systemic fungemia. As the prevalence rises, treatment remains a challenge especially when there is a failure in first-line treatment or drug-resistant fungus. This case report studies a case of chronic EFE, focusing on the diagnostic procedures, treatment options, monitoring parameters and the treatment outcomePatient concerns:A 64-year-old man with underlying well controlled diabetes mellitus was treated with 2 weeks’ course of intravenous antifungal fluconazole for pyelonephritis as his blood culture grew Candida albicans. Concurrently, he complained of 3 months of bilateral painless progressive blurring of vision. At presentation, his visual acuity (VA) was light perception both eyes. Ocular examination revealed non granulomatous inflammation with dense vitritis of both eyes.Diagnosis:He was diagnosed with EFE but the condition responded poorly with the medications.Interventions:He was treated with intravitreal (IVT) amphotericin B and fluconazole was continued. Vitrectomy was performed and intraoperative findings included bilateral fungal balls in the vitreous and retina with foveal traction in the left eye. Postoperatively, vision acuity was 6/24, N8 right eye and 2/60, N unable for left eye with extensive left macular scar and hole. Vitreous cultures were negative. He received multiple IVT amphotericin B and was started on topical steroid eye drops for persistent panuveitis with systemic fluconazole. Ocular improvement was seen after switching to IVT and topical voriconazole. Despite this, his ocular condition deteriorated and he developed neovascular glaucoma requiring 3 topical antiglaucoma agents. Panretinal photocoagulation was subsequently performed.Outcomes:At 3 months’ follow-up, his vision acuity remained at 6/24 for right eye and 2/60 for the left eye. There was no recurrence of inflammation or infection in both eyes.Lessons:Voriconazole could serve as a promising broad spectrum tri-azole agent in cases of failure in first-line treatment or drug-resistant fungus.     

Shortened Telomere Length in Sputum Cells of Bronchiectasis Patients is Associated with Dysfunctional Inflammatory Pathways

Telomere attrition is an established ageing biomarker and shorter peripheral blood leukocyte telomere length has been associated with increased risks of respiratory diseases. However, whether telomere length in disease-relevant sputum immune cells of chronic respiratory disease patients is shortened and which pathways are dysfunctional are not clear. Here we measured telomere length from sputum samples of bronchiectasis and asthmatic subjects and determined that telomere length in sputum of bronchiectasis subjects was significantly shorter (Beta?=????1.167, P_Adj?=?2.75?×?10^?4). We further performed global gene expression analysis and identified genes involved in processes such as NLRP3 inflammasome activation and regulation of adaptive immune cells when bronchiectasis sputum telomere length was shortened. Our study provides insights on dysfunctions related to shortened telomere length in sputum immune cells of bronchiectasis patients.

     

Engagement of the inhibitory receptor CD158a interrupts TCR signaling,preventing dynamic membrane reorganization in CTL/tumor cell interaction

Renal cell carcinoma (RCC) infiltrating lymphocytes (TILs) express killer cell immunoglobulinlike receptors (KIRs) that inhibit the antitumor CD8(+) T-cell lysis. In the present study, to better examine the functional consequences of KIR engagement on cytotoxic T lymphocyte (CTL)/tumor interaction, we have investigated the influence of KIR CD158a on early steps of T-cell activation. We show that coengagement of T-cell receptor (TCR) and CD158a by tumor cells inhibited tyrosine phosphorylation of early signaling proteins ZAP-70 and LAT, lipid raft coalescence, and TCR/CD3 accumulation at the CTL/tumor cell interface. In addition, the guanine exchange factor Vav was not phosphorylated, and no actin cytoskeleton rearrangement was observed. Our data indicate a role of KIR CD158a in the dynamic events induced by TCR triggering, preventing CTL membrane reorganization, and subsequent completion of CTL activation program. Accordingly, the expression of CD158 by TILs may favor tumor cell escape to the immune response.     

Carcinoma of colon presenting as fever of unknown origin

Fever of unknown origin (FUO) is defined as fever of more than 38.3 degrees C, the cause of which remains elusive after 1 week of intensive investigation. Most cases of FUO are restricted to infections, malignancies, and inflammatory diseases. FUO was previously reported as the presenting symptom of a few solid tumors such as lymphoma, renal cell carcinoma, and hepatocellular carcinoma. Colon carcinoma manifesting as FUO has been rarely reported. We describe three female patients who presented with classical FUO and microcytic anemia. As a control, we retrospectively evaluated 28 matched patients with carcinoma of colon and no fever. The evaluation included review of patient files, clinical and laboratory data, and pathologic specimens. In the three patients (mean age, 58 years) who presented with FUO and had left-sided colon carcinoma and microcytic anemia, pathologic evaluation of the tumor tissues demonstrated a severe organized inflammatory process forming abscesses in the pericolic fat. The 28 control matched patients showed no such histopathologic changes. In patients presenting with FUO, especially those who present with microcytic anemia, even with no bowel disturbances or elevated carcinoembryonic antigen levels, diagnostic workup should include a search for occult colorectal carcinoma. In our three cases, it appears that microabscesses in the pericolic fat are the cause of fever.