Uncoupling of Ca2+ sparks from BK channels in cerebral arteries underlies hypoperfusion in hypertension-induced vascular dementia

Ca2+ microdomains in vascular smooth muscle cells: Roles in vascular tone regulation and hypertension

Vascular smooth muscle cells (VSMCs) modulate blood pressure by adjusting vascular contractility. Specific families of ion channels that are expressed in VSMCs regulate membrane potential and intracellular Ca2+ concentration ([Ca2+]cyt). Subsets of them are known to form molecular complexes with Ca2+-sensitive molecules via scaffolding proteins such as caveolin and junctophilin. This enables localized and molecular complex-specific signal transduction to regulate vascular contractility. This intracellular region is referred to as a Ca2+ microdomain. When hypertensive stimuli are applied to blood vessels, gene expression of ion channels and scaffold proteins in vascular cells changes dramatically, often leading to membrane depolarization and increased [Ca2+]cyt. As a result, blood vessels undergo functional remodeling characterized by enhanced contractility. In addition, the transcription of inflammatory genes in vascular cells is also upregulated. This induces leukocyte infiltration into the vascular wall and structural remodeling mediated by VSMC proliferation and extracellular matrix remodeling. This functional and structural remodeling perpetuates the hypertensive state, leading to progressive damage to systemic organs. This review summarizes recent findings on the mechanisms by which Ca2+ microdomains in VSMCs regulate contractility. In addition, the changes in Ca2+ microdomains due to hypertensive stimuli and their contributions to both functional and structural remodeling are summarized.

Blood pressure and the brain: the conundrum of hypertension and dementia

As the population ages, the anticipated rates of dementia worldwide are likely to increase dramatically, especially in low- and middle-income countries; thus, any opportunity to modify dementia risk is especially critical. Hypertension is one risk factor that is highly prevalent, consistently important for late-life brain health, and which could represent a target for prevention of dementia. Furthermore, hypertension is the most significant modifiable risk factor for stroke. This review will summarize existing literature linking hypertension with dementia and brain health more broadly, will discuss potential mechanisms linking hypertension with brain health, and will consider specific factors that may impact not only the relationship between hypertension and the brain but also the importance of treatment, including different associations over the life course.

Sex-specific mechanisms of cerebral microvascular BKCa dysfunction in a mouse model of Alzheimer's disease

INTRODUCTION

Cerebrovascular dysfunction occurs in Alzheimer's disease (AD), impairing hemodynamic regulation. Large conductance Ca2+-activated K+ channels (BKCa) regulate cerebrovascular reactivity and are impaired in AD. BKCa activity depends on intracellular Ca2+ (Ca2+ sparks) and nitro-oxidative post-translational modifications. However, whether these mechanisms underlie BKCa impairment in AD remains unknown.

METHODS

Cerebral arteries from 5x-FAD and wild-type (WT) littermates were used for molecular biology, electrophysiology, ex vivo, and in vivo experiments.

RESULTS

Arterial BKCa activity is reduced in 5x-FAD via sex-dependent mechanisms: in males, there is lower BKα subunit expression and less Ca2+ sparks. In females, we observed reversible nitro-oxidative modification of BKCa. Further, BKCa is involved in hemodynamic regulation in WT mice, and its dysfunction is associated with vascular deficits in 5x-FAD.

DISCUSSION

Our data highlight the central role played by BKCa in cerebral hemodynamic regulation and that molecular mechanisms of its impairment diverge based on sex in 5x-FAD.

HIGHLIGHTS

Cerebral microvascular BKCa dysfunction occurs in both female and male 5x-FAD. Reduction in BKα subunit protein and Ca2+ sparks drive the dysfunction in males. Nitro-oxidative stress is present in females, but not males, 5x-FAD. Reversible nitro-oxidation of BKα underlies BKCa dysfunction in female 5x-FAD.

Combining In Vivo Two-Photon and Laser Speckle Microscopy With the Ex Vivo Capillary-Parenchymal Arteriole Preparation as a Novel Approach to Study Neurovascular Coupling

OBJECTIVE

Cerebral blood flow (CBF) decline is increasingly recognized as an area of importance for targeting neurodegenerative disorders, yet full understanding of the mechanisms that underlie CBF changes are lacking. Animal models are crucial for expanding our knowledge as methods for studying global CBF and neurovascular coupling in humans are limited and require expensive specialized scanners.

METHODS

Use of appropriate animal models can increase our understanding of cerebrovascular function, so we have combined chronic cranial windows with in vivo two-photon and laser speckle microscopy and ex vivo capillary-parenchymal arteriole (CaPA) preparations. Chronic cranial windows allow for longitudinal direct observation of the cerebral microvasculature and surrounding parenchyma while the CaPA preparation can assess capillary and arteriole function in isolation of the neuronal tissue.

RESULTS

Here, we found that extra-dural cranial windows and related imaging protocols do not affect vascular function in the CaPA preparation. Cortical vessels from animals that have undergone imaging can therefore be taken to discover physiological alterations in the cerebral vasculature that contribute to any observed in vivo changes.

CONCLUSION

This approach will enhance neurodegenerative research with the benefit of limiting animal usage.

Ca2+ signaling in vascular smooth muscle and endothelial cells in blood vessel remodeling: a review

Vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) act together to regulate blood pressure and systemic blood flow by appropriately adjusting blood vessel diameter in response to biochemical or biomechanical stimuli. Ion channels that are expressed in these cells regulate membrane potential and cytosolic Ca2+ concentration ([Ca2+]cyt) in response to such stimuli. The subsets of these ion channels involved in Ca2+ signaling often form molecular complexes with intracellular molecules via scaffolding proteins. This allows Ca2+ signaling to be tightly controlled in localized areas within the cell, resulting in a balanced vascular tone. When hypertensive stimuli are applied to blood vessels for extended periods, gene expression in these vascular cells can change dramatically. For example, alteration in ion channel expression often induces electrical remodeling that produces a depolarization of the membrane potential and elevated [Ca2+]cyt. Coupled with endothelial dysfunction blood vessels undergo functional remodeling characterized by enhanced vasoconstriction. In addition, pathological challenges to vascular cells can induce inflammatory gene products that may promote leukocyte infiltration, in part through Ca2+-dependent pathways. Macrophages accumulating in the vascular adventitia promote fibrosis through extracellular matrix turnover, and cause structural remodeling of blood vessels. This functional and structural remodeling often leads to chronic hypertension affecting not only blood vessels, but also multiple organs including the brain, kidneys, and heart, thus increasing the risk of severe cardiovascular events. In this review, we outline recent advances in multidisciplinary research concerning Ca2+ signaling in VSMCs and ECs, with an emphasis on the mechanisms underlying functional and structural vascular remodeling.

The life cycle of a capillary: Mechanisms of angiogenesis and rarefaction in microvascular physiology and pathologies

Capillaries are the smallest blood vessels (<10 μm in diameter) in the body and their walls are lined by endothelial cells. These microvessels play a crucial role in nutrient and gas exchange between blood and tissues. Capillary endothelial cells also produce vasoactive molecules and initiate the electrical signals that underlie functional hyperemia and neurovascular coupling. Accordingly, capillary function and density are critical for all cell types to match blood flow to cellular activity. This begins with the process of angiogenesis, when new capillary blood vessels emerge from pre-existing vessels, and ends with rarefaction, the loss of these microvascular structures. This review explores the mechanisms behind these processes, emphasizing their roles in various microvascular diseases and their impact on surrounding cells in health and disease. We discuss recent work on the mechanisms controlling endothelial cell proliferation, migration, and tube formation that underlie angiogenesis under physiological and pathological conditions. The mechanisms underlying functional and anatomical rarefaction and the role of pericytes in this process are also discussed. Based on this work, a model is proposed in which the balance of angiogenic and rarefaction signaling pathways in a particular tissue match microvascular density to the metabolic demands of the surrounding cells. This negative feedback loop becomes disrupted during microvascular rarefaction: angiogenic mechanisms are blunted, reactive oxygen species accumulate, capillary function declines and eventually, capillaries disappear. This, we propose, forms the foundation of the reciprocal relationship between vascular density, blood flow, and metabolic needs and functionality of nearby cells.

Dysregulation of Ion Channels and Transporters and Blood-Brain Barrier Dysfunction in Alzheimer's Disease and Vascular Dementia

The blood-brain barrier (BBB) plays a critical role in maintaining ion and fluid homeostasis, essential for brain metabolism and neuronal function. Regulation of nutrient, water, and ion transport across the BBB is tightly controlled by specialized ion transporters and channels located within its unique cellular components. These dynamic transport processes not only influence the BBB's structure but also impact vital signaling mechanisms, essential for its optimal function. Disruption in ion, pH, and fluid balance at the BBB is associated with brain pathology and has been implicated in various neurological conditions, including stroke, epilepsy, trauma, and neurodegenerative diseases such as Alzheimer's disease (AD). However, knowledge gaps exist regarding the impact of ion transport dysregulation on BBB function in neurodegenerative dementias. Several factors contribute to this gap: the complex nature of these conditions, historical research focus on neuronal mechanisms and technical challenges in studying the ion transport mechanisms in in vivo models and the lack of efficient in vitro BBB dementia models. This review provides an overview of current research on the roles of ion transporters and channels at the BBB and poses specific research questions: 1) How are the expression and activity of key ion transporters altered in AD and vascular dementia (VaD); 2) Do these changes contribute to BBB dysfunction and disease progression; and 3) Can restoring ion transport function mitigate BBB dysfunction and improve clinical outcomes. Addressing these gaps will provide a greater insight into the vascular pathology of neurodegenerative disorders.

Loss of anoctamin 1 reveals a subtle role for BK channels in lymphatic muscle action potentials

Ca2+ signalling plays a crucial role in determining lymphatic muscle cell excitability and contractility through its interaction with the Ca2+-activated Cl- channel anoctamin 1 (ANO1). In contrast, the large-conductance (BK) Ca2+-activated K+ channel (KCa) and other KCa channels have prominent vasodilatory actions by hyperpolarizing vascular smooth muscle cells. Here, we assessed the expression and contribution of the KCa family to mouse and rat lymphatic collecting vessel contractile function. The BK channel was the only KCa channel consistently expressed in fluorescence-activated cell sorting-purified mouse lymphatic muscle cell lymphatic muscle cells. We used a pharmacological inhibitor of BK channels, iberiotoxin, and small-conductance Ca2+-activated K+ channels, apamin, to inhibit KCa channels acutely in ex vivo isobaric myography experiments and intracellular membrane potential recordings. In basal conditions, BK channel inhibition had little to no effect on either mouse inguinal-axillary lymphatic vessel (MIALV) or rat mesenteric lymphatic vessel contractions or action potentials (APs). We also tested BK channel inhibition under loss of ANO1 either by genetic ablation (Myh11CreERT2-Ano1 fl/fl, Ano1ismKO) or by pharmacological inhibition with Ani9. In both Ano1ismKO MIALVs and Ani9-pretreated MIALVs, inhibition of BK channels increased contraction amplitude, increased peak AP and broadened the peak of the AP spike. In rat mesenteric lymphatic vessels, BK channel inhibition also abolished the characteristic post-spike notch, which was exaggerated with ANO1 inhibition, and significantly increased the peak potential and broadened the AP spike. We conclude that BK channels are present and functional on mouse and rat lymphatic muscle cells but are otherwise masked by the dominance of ANO1. KEY POINTS: Mouse and rat lymphatic muscle cells express functional BK channels. BK channels make little contribution to either rat or mouse lymphatic collecting vessel contractile function in basal conditions across a physiological pressure range. ANO1 limits the peak membrane potential achieved in the action potential and sets a plateau potential limiting the voltage-dependent activation of BK. BK channels are activated when ANO1 is absent or blocked and slightly impair contractile strength by reducing the peak membrane potential achieved in the action potential spike and accelerating the post-spike repolarization.

Challenges in the Therapeutic Targeting of KCa Channels: From Basic Physiology to Clinical Applications

Calcium-activated potassium (KCa) channels are ubiquitously expressed throughout the body and are able to regulate membrane potential and intracellular calcium concentrations, thereby playing key roles in cellular physiology and signal transmission. Consequently, it is unsurprising that KCa channels have been implicated in various diseases, making them potential targets for pharmaceutical interventions. Over the past two decades, numerous studies have been conducted to develop KCa channel-targeting drugs, including those for disorders of the central and peripheral nervous, cardiovascular, and urinary systems and for cancer. In this review, we synthesize recent findings regarding the structure and activating mechanisms of KCa channels. We also discuss the role of KCa channel modulators in therapeutic medicine. Finally, we identify the major reasons behind the delay in bringing these modulators to the pharmaceutical market and propose new strategies to promote their application.

Impact of aging on vascular ion channels: perspectives and knowledge gaps across major organ systems

Individuals aged ≥65 yr will comprise ∼20% of the global population by 2030. Cardiovascular disease remains the leading cause of death in the world with age-related endothelial "dysfunction" as a key risk factor. As an organ in and of itself, vascular endothelium courses throughout the mammalian body to coordinate blood flow to all other organs and tissues (e.g., brain, heart, lung, skeletal muscle, gut, kidney, skin) in accord with metabolic demand. In turn, emerging evidence demonstrates that vascular aging and its comorbidities (e.g., neurodegeneration, diabetes, hypertension, kidney disease, heart failure, and cancer) are "channelopathies" in large part. With an emphasis on distinct functional traits and common arrangements across major organs systems, the present literature review encompasses regulation of vascular ion channels that underlie blood flow control throughout the body. The regulation of myoendothelial coupling and local versus conducted signaling are discussed with new perspectives for aging and the development of chronic diseases. Although equipped with an awareness of knowledge gaps in the vascular aging field, a section has been included to encompass general feasibility, role of biological sex, and additional conceptual and experimental considerations (e.g., cell regression and proliferation, gene profile analyses). The ultimate goal is for the reader to see and understand major points of deterioration in vascular function while gaining the ability to think of potential mechanistic and therapeutic strategies to sustain organ perfusion and whole body health with aging.