Hypoxic stress upregulates Kir2.1 expression by a pathway including hypoxic-inducible factor-1α and dynamin2 in brain capillary endothelial cells

Electro-metabolic signaling

Precise matching of energy substrate delivery to local metabolic needs is essential for the health and function of all tissues. Here, we outline a mechanistic framework for understanding this critical process, which we refer to as electro-metabolic signaling (EMS). All tissues exhibit changes in metabolism over varying spatiotemporal scales and have widely varying energetic needs and reserves. We propose that across tissues, common signatures of elevated metabolism or increases in energy substrate usage that exceed key local thresholds rapidly engage mechanisms that generate hyperpolarizing electrical signals in capillaries that then relax contractile elements throughout the vasculature to quickly adjust blood flow to meet changing needs. The attendant increase in energy substrate delivery serves to meet local metabolic requirements and thus avoids a mismatch in supply and demand and prevents metabolic stress. We discuss in detail key examples of EMS that our laboratories have discovered in the brain and the heart, and we outline potential further EMS mechanisms operating in tissues such as skeletal muscle, pancreas, and kidney. We suggest that the energy imbalance evoked by EMS uncoupling may be central to cellular dysfunction from which the hallmarks of aging and metabolic diseases emerge and may lead to generalized organ failure states-such as diverse flavors of heart failure and dementia. Understanding and manipulating EMS may be key to preventing or reversing these dysfunctions.

Molecular basis and clinical implications of HIFs in cardiovascular diseases

Oxygen maintains the homeostasis of an organism in a delicate balance in different tissues and organs. Under hypoxic conditions, hypoxia-inducible factors (HIFs) are specific and dominant factors in the spatiotemporal regulation of oxygen homeostasis. As the most basic functional unit of the heart at the cellular level, the cardiomyocyte relies on oxygen and nutrients delivered by the microvasculature to keep the heart functioning properly. Under hypoxic stress, HIFs are involved in acute and chronic myocardial pathology because of their spatiotemporal specificity, thus granting them therapeutic potential. Most adult animals lack the ability to regenerate their myocardium entirely following injury, and complete regeneration has long been a goal of clinical treatment for heart failure. The precise manipulation of HIFs (considering their dynamic balance and transformation) and the development of HIF-targeted drugs is therefore an extremely attractive cardioprotective therapy for protecting against myocardial ischemic and hypoxic injury, avoiding myocardial remodeling and heart failure, and promoting recovery of cardiac function.

Reciprocal Relationship between Ca2+ Signaling and Ca2+-Gated Ion Channels as a Potential Target for Drug Discovery

Cellular Ca²⁺ signaling functions as one of the most common second messengers of various signal transduction pathways in cells and mediates a number of physiological roles in a cell-type dependent manner. Ca²⁺ signaling also regulates more general and fundamental cellular activities, including cell proliferation and apoptosis. Among ion channels, Ca²⁺-permeable channels in the plasma membrane as well as endo- and sarcoplasmic reticulum membranes play important roles in Ca²⁺ signaling by directly contributing to the influx of Ca²⁺ from extracellular spaces or its release from storage sites, respectively. Furthermore, Ca²⁺-gated ion channels in the plasma membrane often crosstalk reciprocally with Ca²⁺ signals and are central to the regulation of cellular functions. This review focuses on the physiological and pharmacological impact of i) Ca²⁺-gated ion channels as an apparatus for the conversion of cellular Ca²⁺ signals to intercellularly propagative electrical signals and ii) the opposite feedback regulation of Ca²⁺ signaling by Ca²⁺-gated ion channel activities in excitable and non-excitable cells.

Hypoxia increases the proliferation of brain capillary endothelial cells via upregulation of TMEM16A Ca2+-activated Cl- channels

The blood-brain barrier (BBB) is mainly formed by brain capillary endothelial cells (BCECs) and is exposed to hypoxic environments under pathological conditions. The effects of hypoxia on the expression and activity of Ca²⁺-activated Cl^- (Cl_Ca) channels, TMEM16A, were examined in bovine brain endothelial t-BBEC117 cells and mouse BCECs. The expression of TMEM16A was upregulated by hypoxia. Whole-cell Cl_Ca currents increased under hypoxia. Hypoxia also increased cell proliferation and trans-endothelial permeability, which were attenuated by Cl_Ca channel blockers or TMEM16A siRNA. These findings are useful for elucidating the pathological role of TMEM16A Cl_Ca channels in the BBB during cerebral ischemia.

TMEM16A Ca2+-Activated Cl- Channel Regulates the Proliferation and Migration of Brain Capillary Endothelial Cells

The blood-brain barrier (BBB) is essential for the maintenance of homeostasis in the brain. Brain capillary endothelial cells (BCECs) comprise the BBB, and thus a delicate balance between their proliferation and death is required. Although the activity of ion channels in BCECs is involved in BBB functions, the underlying molecular mechanisms remain unclear. In the present study, the molecular components of Ca²⁺-activated Cl^- (Cl_Ca) channels and their physiological roles were examined using mouse BCECs (mBCECs) and a cell line derived from bovine BCECs, t-BBEC117. Expression analyses revealed that TMEM16A was strongly expressed in mBCECs and t-BBEC117 cells. In t-BBEC117 cells, whole-cell Cl^- currents were sensitive to the Cl_Ca channel blockers, 100 μM niflumic acid and 10 μM T16A_inh-A01, and were also reduced markedly by small-interfering RNA (siRNA) knockdown of TMEM16A. Importantly, block of Cl_Ca currents with Cl_Ca channel blockers or TMEM16A siRNA induced membrane hyperpolarization. Moreover, treatment with TMEM16A siRNA caused an increase in resting cytosolic Ca²⁺ concentration ([Ca²⁺]_cyt). T16A_inh-A01 reduced cell viability in a concentration-dependent manner. Either Cl_Ca channel blockers or TMEM16A siRNA also curtailed cell proliferation and migration. Furthermore, Cl_Ca channel blockers attenuated the trans-endothelial permeability. In combination, these results strongly suggest that TMEM16A contributes to Cl_Ca channel conductance and can regulate both the resting membrane potential and [Ca²⁺]_cyt in BCECs. Our data also reveal how these BCECs may be involved in the maintenance of BBB functions, as both the proliferation and migration are altered following changes in channel activity. SIGNIFICANCE STATEMENT: In brain capillary endothelial cells (BCECs) of the blood-brain barrier (BBB), TMEM16A is responsible for Ca²⁺-activated Cl^- channels and can regulate both the resting membrane potential and cytosolic Ca²⁺ concentration, contributing to the proliferation and migration of BCECs. The present study provides novel information on the molecular mechanisms underlying the physiological functions of BCECs in the BBB and a novel target for therapeutic drugs for disorders associated with dysfunctions in the BBB.

Oxidative stress facilitates cell death by inhibiting Orai1-mediated Ca2+ entry in brain capillary endothelial cells

Brain capillary endothelial cells (BCECs) form the blood-brain barrier (BBB) and play an essential role in the regulation of its functions. Oxidative stress accumulates excessive reactive oxygen species (ROS) and facilitates the death of BCECs, leading to a dysfunctional BBB. However, the mechanisms underlying the death of BCECs under oxidative stress remain unclear. In the present study, the effects of oxidative stress on cell viability, ROS production, intracellular Ca²⁺ concentration, and protein expression were examined using a cell line derived from bovine BCECs, t-BBEC117. When t-BBEC117 cells were exposed to oxidative stress induced by hydrogen peroxide (H₂O₂, 10-100 μM), cell growth was inhibited in a dose-dependent manner. Oxidative stress by 30 μM H₂O₂ increased the production of ROS and its effects were blocked by the ROS scavenger, 10 mM N-acetyl-l-cysteine (NAC). In addition, oxidative stress reduced store-operated Ca²⁺ entry (SOCE) and this decrease was recovered by NAC or the Orai channel activator, 5 μM 2-aminoethyl diphenylborinate (2-APB). The siRNA knockdown of Orai1 revealed that Orai1 was mainly responsible for SOCE channels and its activity was decreased by oxidative stress. However, the protein expression of Orai1 and STIM1 was not affected by oxidative stress. Oxidative stress-induced cell death was rescued by 2-APB, NAC, or the STIM-Orai activating region. In conclusion, oxidative stress reduces Orai1-mediated SOCE and, thus, facilitates the death of BCECs.

Abnormal neurovascular coupling as a cause of excess cerebral vasodilation in familial migraine

Aims

Acute migraine attack in familial hemiplegic migraine type 2 (FHM2) patients is characterized by sequential hypo- and hyperperfusion. FHM2 is associated with mutations in the Na, K-ATPase α2 isoform. Heterozygous mice bearing one of these mutations (α2+/G301R mice) were shown to have elevated cerebrovascular tone and, thus, hypoperfusion that might lead to elevated concentrations of local metabolites. We hypothesize that these α2+/G301R mice also have increased cerebrovascular hyperaemic responses to these local metabolites leading to hyperperfusion in the affected part of the brain.

Methods and results

Neurovascular coupling was compared in α2+/G301R and matching wild-type (WT) mice using Laser Speckle Contrast Imaging. In brain slices, parenchymal arteriole diameter and intracellular calcium changes in neuronal tissue, astrocytic endfeet, and smooth muscle cells in response to neuronal excitation were assessed. Wall tension and smooth muscle membrane potential were measured in isolated middle cerebral arteries. Quantitative polymerase chain reaction, western blot, and immunohistochemistry were used to assess the molecular background underlying the functional changes. Whisker stimulation induced larger increase in blood perfusion, i.e. hyperaemic response, of the somatosensory cortex of α2+/G301R than WT mice. Neuronal excitation was associated with larger parenchymal arteriole dilation in brain slices from α2+/G301R than WT mice. These hyperaemic responses in vivo and ex vivo were inhibited by BaCl2, suggesting involvement of inward-rectifying K+ channels (Kir). Relaxation to elevated bath K+ was larger in arteries from α2+/G301R compared to WT mice. This difference was endothelium-dependent. Endothelial Kir2.1 channel expression was higher in arteries from α2+/G301R mice. No sex difference in functional responses and Kir2.1 expression was found.

Conclusion

This study suggests that an abnormally high cerebrovascular hyperaemic response in α2+/G301R mice is a result of increased endothelial Kir2.1 channel expression. This may be initiated by vasospasm-induced accumulation of local metabolites and underlie the hyperperfusion seen in FHM2 patients during migraine attack.

Propofol improved hypoxia-impaired integrity of blood-brain barrier via modulating the expression and phosphorylation of zonula occludens-1

Aims

Hypoxia may damage blood‐brain barrier (BBB). The neuroprotective effect of propofol has been reported. We aimed to identify whether and how propofol improved hypoxia‐induced impairment of BBB integrity.

Methods

Mouse brain microvascular endothelial cells (MBMECs) and astrocytes were cocultured to establish in vitro BBB model. The effects of hypoxia and propofol on BBB integrity were examined. Further, zonula occludens‐1 (ZO‐1) expression and phosphorylation, hypoxia‐inducible factor‐1α (HIF‐1α) and vascular endothelial growth factor (VEGF) expression, intracellular calcium concentration and Ca²⁺/calmodulin‐dependent protein kinase II (CAMKII) activation were measured.

Results

Hypoxia‐impaired BBB integrity, which was protected by propofol. Hypoxia‐reduced ZO‐1 expression, while induced ZO‐1 phosphorylation. These effects were attenuated by propofol. The expression of HIF‐1α and VEGF was increased by hypoxia and was alleviated by propofol. The hypoxia‐mediated suppression of ZO‐1 and impaired BBB integrity was reversed by HIF‐α inhibitor and VEGF inhibitor. In addition, hypoxia increased the intracellular calcium concentration and induced the phosphorylation of CAMKII, which were mitigated by propofol. The hypoxia‐induced phosphorylation of ZO‐1 and impaired BBB integrity was ameliorated by calcium chelator and CAMKII inhibitor.

Conclusion

Propofol could protect against hypoxia‐mediated impairment of BBB integrity. The underlying mechanisms may involve the expression and phosphorylation of ZO‐1.