Synchrony of spontaneous Ca2+ activity in microvascular mural cells

Characterization of the cellular components of mouse collecting lymphatic vessels reveals that lymphatic muscle cells are the innate pacemaker cells regulating lymphatic contractions

Collecting lymphatic vessels (cLVs) exhibit spontaneous contractions with a pressure-dependent frequency, but the identity of the lymphatic pacemaker cell is still debated. By analogy to pacemakers in the GI and lower urinary tracts, proposed cLV pacemaker cells include interstitial cells of Cajal like cells (ICLC) or the lymphatic muscle (LMCs) cells themselves. Here we combined immunofluorescence and scRNAseq analyses with electrophysiological methods to examine the cellular constituents of the mouse cLV wall and assess whether any cell type exhibited morphological and functional processes characteristic of pacemaker cells: a continuous if not contiguous network integrated into the electrical syncytium; spontaneous Ca2+ transients; and depolarization-induced propagated contractions. We employed inducible Cre (iCre) mouse models routinely used to target these specific cell populations including: c-kitCreERT2 to target ICLC; PdgfrβCreERT2 to target pericyte-like cells; PdgfrαCreERTM to target CD34+ adventitial cells and ICLC; and Myh11CreERT2 to target LMCs directly. These specific inducible Cre lines were crossed to the fluorescent reporter ROSA26mT/mG, the genetically encoded Ca2+ sensor GCaMP6f, and the light-activated cation channel rhodopsin2 (ChR2). c-KitCreERT2 labeled both a sparse population of LECs and round adventitial cells that responded to the mast cell activator compound 48-80. PdgfrβCreERT2 drove recombination in both adventitial cells and LMCs, limiting its power to discriminate a pericyte-specific population. PdgfrαCreERTM labeled a large population of interconnected, oak leaf-shaped cells primarily along the adventitial surface of the vessel. Of these cells, only LMCs consistently, but heterogeneously, displayed spontaneous Ca2+ events during the diastolic period of the contraction cycle, and whose frequency was modulated in a pressure-dependent manner. Optogenetic depolarization through the expression of ChR2 under control of Myh11CreERT2, but not PdgfrαCreERTM or c-KitCreERT2, resulted in propagated contractions upon photo-stimulation. Membrane potential recordings in LMCs demonstrated that the rate of diastolic depolarization significantly correlated with contraction frequency. These findings support the conclusion that LMCs, or a subset of LMCs, are responsible for mouse cLV pacemaking.

A switch in the pathway of TRPC3-mediated calcium influx into brain pericytes contributes to capillary spasms after subarachnoid hemorrhage

Calcium influx and subsequent elevation of the intracellular calcium concentration ([Ca2+]i) induce contractions of brain pericytes and capillary spasms following subarachnoid hemorrhage. This calcium influx is exerted through cation channels. However, the specific calcium influx pathways in brain pericytes after subarachnoid hemorrhage remain unknown. Transient receptor potential canonical 3 (TRPC3) is the most abundant cation channel potentially involved in calcium influx into brain pericytes and is involved in calcium influx into other cell types either via store-operated calcium entry (SOCE) or receptor-operated calcium entry (ROCE). Therefore, we hypothesized that TRPC3 is associated with [Ca2+]i elevation in brain pericytes, potentially mediating brain pericyte contraction and capillary spasms after subarachnoid hemorrhage. In this study, we isolated rat brain pericytes and demonstrated increased TRPC3 expression and its currents in brain pericytes after subarachnoid hemorrhage. Calcium imaging of brain pericytes revealed that changes in TRPC3 expression mediated a switch from SOCE-dominant to ROCE-dominant calcium influx after subarachnoid hemorrhage, resulting in significantly higher [Ca2+]i levels after SAH. TRPC3 activity in brain pericytes also contributed to capillary spasms and reduction in cerebral blood flow in an in vivo rat model of subarachnoid hemorrhage. Therefore, we suggest that the switch in TRPC3-mediated calcium influx pathways plays a crucial role in the [Ca2+]i elevation in brain pericytes after subarachnoid hemorrhage, ultimately leading to capillary spasms and a reduction in cerebral blood flow.

Roles of endothelial prostaglandin I2 in maintaining synchronous spontaneous Ca2+ transients in rectal capillary pericytes

In hollow visceral organs, capillary pericytes appear to drive spontaneous Ca2+ transients in the upstream arterioles. Here, mechanisms underlying the intercellular synchrony of pericyte Ca2+ transients were explored. Ca2+ dynamics in NG2 chondroitin sulphate proteoglycan (NG2)-expressing capillary pericytes were examined using rectal mucosa-submucosa preparations of NG2-GCaMP6 mice. Spontaneous Ca2+ transients arising from endoplasmic reticulum Ca2+ release were synchronously developed amongst capillary pericytes in a gap junction blocker (3 μM carbenoxolone)-sensitive manner and could spread into upstream vascular segments. Spontaneous Ca2+ transients were suppressed by the Ca2+ -activated Cl- channel (CaCC) blocker niflumic acid and their synchrony was diminished by a TMEM16A inhibitor (3 μM Ani9) in accordance with TMEM16A immunoreactivity in pericytes. In capillaries where cyclooxygenase (COX)-2 immunoreactivity was expressed in endothelium but not pericytes, non-selective COX inhibitors (1 μM indomethacin or 10 μM diclofenac) or COX-2 inhibitor (10 μM NS 398) disrupted the synchrony of spontaneous Ca2+ transients and raised the basal Ca2+ level. Subsequent prostaglandin I2 (PGI2 ; 100 nM) or the KATP channel opener levcromakalim restored the synchrony with a reduction in the Ca2+ level. PGI2 receptor antagonist (1 μM RO1138452) also disrupted the synchrony of spontaneous Ca2+ transients and increased the basal Ca2+ level. Subsequent levcromakalim restored the synchrony and reversed the Ca2+ rise. Thus, the synchrony of spontaneous Ca2+ transients in pericytes appears to be developed by the spread of spontaneous transient depolarisations arising from the opening of TMEM16A CaCCs. Endothelial PGI2 may play a role in maintaining the synchrony, presumably by stabilising the resting membrane potential in pericytes. KEY POINTS: Capillary pericytes in the rectal mucosa generate synchronous spontaneous Ca2+ transients that could spread into the upstream vascular segment. Spontaneous Ca2+ release from the endoplasmic reticulum (ER) triggers the opening of Ca2+ -activated Cl- channel TMEM16A and resultant depolarisations that spread amongst pericytes via gap junctions, establishing the synchrony of spontaneous Ca2+ transients in pericytes. Prostaglandin I2 (PGI2 ), which is constitutively produced by the endothelium depending on cyclooxygenase-2, appears to prevent premature ER Ca2+ releases in the pericytes allowing periodic, regenerative Ca2+ releases. Endothelial PGI2 may maintain the synchrony of pericyte activity by stabilising pericyte resting membrane potential by opening of KATP channels.

Pericyte dysfunction and impaired vasomotion are hallmarks of islets during the pathogenesis of type 1 diabetes

Pancreatic islets are endocrine organs that depend on their microvasculature to function. Along with endothelial cells, pericytes comprise the islet microvascular network. These mural cells are crucial for microvascular stability and function, but it is not known if/how they are affected during the development of type 1 diabetes (T1D). Here, we investigate islet pericyte density, phenotype, and function using living pancreas slices from donors without diabetes, donors with a single T1D-associated autoantibody (GADA+), and recent onset T1D cases. Our data show that islet pericyte and capillary responses to vasoactive stimuli are impaired early on in T1D. Microvascular dysfunction is associated with a switch in the phenotype of islet pericytes toward myofibroblasts. Using publicly available RNA sequencing (RNA-seq) data, we further found that transcriptional alterations related to endothelin-1 signaling and vascular and extracellular matrix (ECM) remodeling are hallmarks of single autoantibody (Aab)+ donor pancreata. Our data show that microvascular dysfunction is present at early stages of islet autoimmunity.

Vasomotion in Retinal Arterioles Is Modified by Exercise and Flicker Stimulation

PURPOSE

Vasomotion is spontaneous oscillations in the diameter of resistance vessels with derived effects on blood flow, and it has been proposed that disturbances in vasomotion may be involved in retinal vascular disease. The purpose of this study was to investigate whether retinal vasomotion shows regional variation and is modified by activated autoregulation.

METHODS

Video recordings of the diameter of retinal arterioles previously obtained from 55 normal persons were subjected to Fourier analysis to characterize the frequencies and propagation of spontaneous diameter changes in retinal arterioles. The analyses were performed on peripapillary temporal retinal arterioles, on arteriolar branches toward the macular area and the retinal periphery, and were performed during rest, during an increase in the arterial blood pressure induced by isometric exercise, and during increased retinal metabolism induced by flickering light.

RESULTS

There was no propagation of diameter changes along the studied vascular segments. Isometric exercise constricted the arterioles significantly by (mean ± SD) 1.76% ± 3.56% (P = 0.02) and increased the power of diameter oscillations at very low frequencies (0.1-1.4 c/min). Flicker stimulation dilated the arterioles significantly by (mean ± SD) 5.10% ± 2.91% (P < 0.0001) and reduced the power of diameter oscillations at all but the very low frequencies (P < 0.006 for all comparisons). Flicker-induced dilation and changes in hydraulic conductance were lower in peripheral than in macular arterioles.

CONCLUSIONS

Retinal vasomotion in normal persons increases during increased arterial blood pressure and decreases during flicker stimulation. The findings may act as a basis for the study of vasomotion in retinal vascular disease.

The Dependence of Urinary Bladder Responses on Extracellular Calcium Varies Between Muscarinic, Histamine, 5-HT (Serotonin), Neurokinin, Prostaglandin, and Angiotensin Receptor Activation

With many common bladder diseases arising due to abnormal contractions, a greater understanding of the receptor systems involved may aid the development of future treatments. The aim of this study was to identify any difference in the involvement of extracellular calcium (Ca²⁺) across prominent contractile-mediating receptors within cells lining the bladder. Strips of porcine urothelium and lamina propria were isolated from the urinary bladder dome and mounted in isolated tissue baths containing Krebs-bicarbonate solution, perfused with carbogen gas at 37°C. Tissue contractions, as well as changes to the frequency and amplitude of spontaneous activity were recorded after the addition of muscarinic, histamine, 5-hydroxytryptamine, neurokinin-A, prostaglandin E2, and angiotensin II receptor agonists in the absence and presence of 1 µM nifedipine or nominally zero Ca²⁺ solution. The absence of extracellular Ca²⁺ influx after immersion into nominally zero Ca²⁺ solution, or the addition of nifedipine, significantly inhibited the contractile responses (p < 0.05 for all) after stimulation with carbachol (1 µM), histamine (100 µM), 5-hydroxytryptamine (100 µM), neurokinin-A (300 nM), prostaglandin E2 (10 µM), and angiotensin II (100 nM). On average, Ca²⁺ influx from extracellular sources was responsible for between 20–50% of receptor-mediated contractions. This suggests that although the specific requirement of Ca²⁺ on contractile responses varies depending on the receptor, extracellular Ca²⁺ plays a key role in mediating G protein-coupled receptor contractions of the urothelium and lamina propria.

Deregulation of Ca2+-Signaling Systems in White Adipocytes, Manifested as the Loss of Rhythmic Activity, Underlies the Development of Multiple Hormonal Resistance at Obesity and Type 2 Diabetes

Various types of cells demonstrate ubiquitous rhythmicity registered as simple and complex Ca²⁺-oscillations, spikes, waves, and triggering phenomena mediated by G-protein and tyrosine kinase coupled receptors. Phospholipase C/IP₃-receptors (PLC/IP₃R) and endothelial NO-synthase/Ryanodine receptors (NOS/RyR)–dependent Ca²⁺ signaling systems, organized as multivariate positive feedback generators (PLC-G and NOS-G), underlie this rhythmicity. Loss of rhythmicity at obesity may indicate deregulation of these signaling systems. To issue the impact of cell size, receptors’ interplay, and obesity on the regulation of PLC-G and NOS-G, we applied fluorescent microscopy, immunochemical staining, and inhibitory analysis using cultured adipocytes of epididumal white adipose tissue of mice. Acetylcholine, norepinephrine, atrial natriuretic peptide, bradykinin, cholecystokinin, angiotensin II, and insulin evoked complex [Ca²⁺]_i responses in adipocytes, implicating NOS-G or PLC-G. At low sub-threshold concentrations, acetylcholine and norepinephrine or acetylcholine and peptide hormones (in paired combinations) recruited NOS-G, based on G proteins subunits interplay and signaling amplification. Rhythmicity was cell size- dependent and disappeared in hypertrophied cells filled with lipids. Contrary to control cells, adipocytes of obese hyperglycemic and hypertensive mice, growing on glucose, did not accumulate lipids and demonstrated hormonal resistance being non responsive to any hormone applied. Preincubation of preadipocytes with palmitoyl-L-carnitine (100 nM) provided accumulation of lipids, increased expression and clustering of IP₃R and RyR proteins, and partially restored hormonal sensitivity and rhythmicity (5–15% vs. 30–80% in control cells), while adipocytes of diabetic mice were not responsive at all. Here, we presented a detailed kinetic model of NOS-G and discussed its control. Collectively, we may suggest that universal mechanisms underlie loss of rhythmicity, Ca²⁺-signaling systems deregulation, and development of general hormonal resistance to obesity.