The essential function of the circulatory system is to continuously and efficiently supply the O2 and nutrients necessary to meet the metabolic demands of every cell in the body, a function in which vast capillary networks play a key role. Capillary networks serve an additional important function in the central nervous system: acting as a sensory network, they detect neuronal activity in the form of elevated extracellular K+ and initiate a retrograde, propagating, hyperpolarizing signal that dilates upstream arterioles to rapidly increase local blood flow. Yet, little is known about how blood entering this network is distributed on a branch-to-branch basis to reach specific neurons in need. Here, we demonstrate that capillary-enwrapping projections of junctional, contractile pericytes within a postarteriole transitional region differentially constrict to structurally and dynamically determine the morphology of capillary junctions and thereby regulate branch-specific blood flow. We further found that these contractile pericytes are capable of receiving propagating K+-induced hyperpolarizing signals propagating through the capillary network and dynamically channeling red blood cells toward the initiating signal. By controlling blood flow at junctions, contractile pericytes within a functionally distinct postarteriole transitional region maintain the efficiency and effectiveness of the capillary network, enabling optimal perfusion of the brain.

Neuronal activity leads to an increase in local cerebral blood flow (CBF) to allow adequate supply of oxygen and nutrients to active neurons, a process termed neurovascular coupling (NVC). We have previously shown that capillary endothelial cell (cEC) inwardly rectifying K+ (Kir) channels can sense neuronally evoked increases in interstitial K+ and induce rapid and robust dilations of upstream parenchymal arterioles, suggesting a key role of cECs in NVC. The requirements of this signal conduction remain elusive. Here, we utilize mathematical modeling to investigate how small outward currents in stimulated cECs can elicit physiologically relevant spread of vasodilatory signals within the highly interconnected brain microvascular network to increase local CBF. Our model shows that the Kir channel can act as an “on–off” switch in cECs to hyperpolarize the cell membrane as extracellular K+ increases. A local hyperpolarization can be amplified by the voltage-dependent activation of Kir in neighboring cECs. Sufficient Kir density enables robust amplification of the hyperpolarizing stimulus and produces responses that resemble action potentials in excitable cells. This Kir-mediated excitability can remain localized in the stimulated region or regeneratively propagate over significant distances in the microvascular network, thus dramatically increasing the efficacy of K+ for eliciting local hyperemia. Modeling results show how changes in cEC transmembrane current densities and gap junctional resistances can affect K+-mediated NVC and suggest a key role for Kir as a sensor of neuronal activity and an amplifier of retrograde electrical signaling in the cerebral vasculature.

No abstract

No abstract

Upon inositol trisphosphate (IP3) stimulation of non-excitable cells, including vascular endothelial cells, calcium (Ca2+) shuttling between the endoplasmic reticulum (ER) and mitochondria, facilitated by complexes called Mitochondria-Associated ER Membranes (MAMs), is known to play an important role in the occurrence of cytosolic Ca2+ concentration ([Ca2+]Cyt) oscillations. A mathematical compartmental closed-cell model of Ca2+ dynamics was developed that accounts for ER-mitochondria Ca2+ microdomains as the µd compartment (besides the cytosol, ER and mitochondria), Ca2+ influx to/efflux from each compartment and Ca2+ buffering. Varying the distribution of functional receptors in MAMs vs. the rest of ER/mitochondrial membranes, a parameter called the channel connectivity coefficient (to the µd), allowed for generation of [Ca2+]Cytoscillations driven by distinct mechanisms at various levels of IP3 stimulation. Oscillations could be initiated by the transient opening of IP3 receptors facing either the cytosol or the µd, and subsequent refilling of the respective compartment by Ca2+ efflux from the ER and/or the mitochondria. Only under conditions where the µd became the oscillation-driving compartment, silencing the Mitochondrial Ca2+ Uniporter led to oscillation inhibition. Thus, the model predicts that alternative mechanisms can yield [Ca2+]Cyt oscillations in non-excitable cells, and, under certain conditions, the ER-mitochondria µd can play a regulatory role.

Cerebral SVDs encompass a group of genetic and sporadic pathological processes leading to brain lesions, cognitive decline, and stroke. There is no specific treatment for SVDs, which progress silently for years before becoming clinically symptomatic. Here, we examine parallels in the functional defects of PAs in CADASIL, a monogenic form of SVD, and in response to SAH, a common type of hemorrhagic stroke that also targets the brain microvasculature. Both animal models exhibit dysregulation of the voltage‐gated potassium channel, KV1, in arteriolar myocytes, an impairment that compromises responses to vasoactive stimuli and impacts CBF autoregulation and local dilatory responses to neuronal activity (NVC). However, the extent to which this channelopathy‐like defect ultimately contributes to these pathologies is unknown. Combining experimental data with computational modeling, we describe the role of KV1 channels in the regulation of myocyte membrane potential at rest and during the modest increase in extracellular potassium associated with NVC. We conclude that PA resting membrane potential and myogenic tone depend strongly on KV1.2/1.5 channel density, and that reciprocal changes in KV channel density in CADASIL and SAH produce opposite effects on extracellular potassium‐mediated dilation during NVC.

No abstract

26Control of astrocytes via modulation of Ca 2+ oscillations using techniques like optogenetics can prove to be crucial 27 in therapeutic intervention of a variety of neurological disorders. However, a systematic study quantifying the 28 effect of optogenetic stimulation in astrocytes is yet to be performed. Here, we propose a novel stochastic 29 Ca 2+ dynamics model that incorporates the light sensitive component -channelrhodopsin 2 (ChR2). Utilizing this 30 model, we studied the effect of various pulsed light stimulation paradigms on astrocytes for select variants of 31 ChR2 (wild type, ChETA, and ChRET/TC) in both an individual and a network of cells. Our results exhibited a 32 consistent pattern of Ca 2+ activity among individual cells in response to optogenetic stimulation, i.e., showing 33 steady state regimes with increased Ca 2+ basal level and Ca 2+ spiking probability. Furthermore, we performed a 34 global sensitivity analysis to assess the effect of stochasticity and variation of model parameters on astrocytic 35 Ca 2+ dynamics in the presence and absence of light stimulation, respectively. Results indicated that directing 36 variants towards the first open state of the photo-cycle of ChR2 (o 1 ) enhances spiking activity in astrocytes during 37 optical stimulation. Evaluation of the effect of astrocytic ChR2 expression (heterogeneity) on Ca 2+ signaling 38 revealed that the optimal stimulation paradigm of a network does not necessarily coincide with that of an 39 individual cell. Simulation for ChETA-incorporated astrocytes suggest that maximal activity of a single cell 40 reduced the spiking probability of the network of astrocytes at higher degrees of ChR2 expression efficiency due 41 to an elevation of basal Ca 2+ beyond physiological levels. Collectively, the framework presented in this study 42 provides valuable information for the selection of light stimulation paradigms that elicit optimal astrocytic activity 43 using existing ChR2 constructs, as well as aids in the engineering of future optogenetic constructs. 44 45 3 46 Author summary 47Optogenetics -an avant-garde technique involves targeted delivery of light sensitive ion channels to cells. 48Channelrhodopsin 2 (ChR2), an algal derived light sensitive ion channel has extensively been used in 49 neuroscience to manipulate various cell types in a guided and controlled manner. Despite being predominantly 50 used in neurons, recent advancements have led to the expansion of the application of optogenetics in non-neuronal 51 cell types, like astrocytes. These cells play a key role in various aspects of the central nervous system and 52 alteration of their signaling is associated with various disorders, including epilepsy, stroke and Alzheimer's 53 disease. Hence, invaluable information for therapeutic intervention can be obtained from using optogenetics to 54 regulate astrocytic activity in a strategic manner. Here, we propose a novel computational model to assess 55 astrocytic response to optogenetic stimulation which implicitly accounts for ...