Red blood cells stabilize flow in brain microvascular networks

Photoannealing of Microtissues Creates High-Density Capillary Network Containing Living Matter in a Volumetric-Independent Manner

The vascular tree is crucial for the survival and function of large living tissues. Despite breakthroughs in 3D bioprinting to endow engineered tissues with large blood vessels, there is currently no approach to engineer high-density capillary networks into living tissues in a scalable manner. Here, photoannealing of living microtissue (PALM) is presented as a scalable strategy to engineer capillary-rich tissues. Specifically, in-air microfluidics is used to produce living microtissues composed of cell-laden microgels in ultrahigh throughput, which can be photoannealed into a monolithic living matter. Annealed microtissues inherently give rise to an open and interconnected pore network within the resulting living matter. Interestingly, utilizing soft microgels enables microgel deformation, which leads to the uniform formation of capillary-sized pores. Importantly, the ultrahigh throughput nature underlying the microtissue formation uniquely facilitates scalable production of living tissues of clinically relevant sizes (>1 cm3) with an integrated high-density capillary network. In short, PALM generates monolithic, microporous, modular tissues that meet the previously unsolved need for large engineered tissues containing high-density vascular networks, which is anticipated to advance the fields of engineered organs, regenerative medicine, and drug screening.

Ionic Liquid Coating-Driven Nanoparticle Delivery to the Brain: Applications for NeuroHIV

Delivering cargo to the central nervous system (CNS) remains a pharmacological challenge. For infectious diseases such as HIV, the CNS acts as a latent reservoir that is inadequately managed by systemic antiretrovirals (ARTs). ARTs thus cannot eradicate HIV, and given CNS infection, patients experience neurological deficits collectively referred to as "neuroHIV". Herein, the development of bioinspired ionic liquid-coated nanoparticles (IL-NPs) for in situ hitchhiking on red blood cells (RBCs) is reported, which enables 48% brain delivery of intracarotid arterial- infused cargo. Moreover, IL choline trans-2-hexenoate (CA2HA 1:2) demonstrates preferential accumulation in parenchymal microglia over endothelial cells post-delivery. This study further demonstrates successful loading of abacavir (ABC), an ART that is challenging to encapsulate, into IL-NPs, and verifies retention of antiviral efficacy in vitro. IL-NPs are not cytotoxic to primary human peripheral blood mononuclear cells (PBMCs) and the CA2HA 1:2 coating itself confers notable anti-viremic capacity. In addition, in vitro cell culture assays show markedly increased uptake of IL-NPs into neural cells compared to bare PLGA nanoparticles. This work debuts bioinspired ionic liquids as promising nanoparticle coatings to assist CNS biodistribution and has the potential to revolutionize the delivery of cargos (i.e., drugs, viral vectors) through compartmental barriers such as the blood-brain-barrier (BBB).

Leptomeningeal collaterals regulate reperfusion in ischemic stroke and rescue the brain from futile recanalization

Recanalization is the mainstay of ischemic stroke treatment. However, even with timely clot removal, many stroke patients recover poorly. Leptomeningeal collaterals (LMCs) are pial anastomotic vessels with yet-unknown functions. We applied laser speckle imaging, ultrafast ultrasound, and two-photon microscopy in a thrombin-based mouse model of stroke and fibrinolytic treatment to show that LMCs maintain cerebral autoregulation and allow for gradual reperfusion, resulting in small infarcts. In mice with poor LMCs, distal arterial segments collapse, and deleterious hyperemia causes hemorrhage and mortality after recanalization. In silico analyses confirm the relevance of LMCs for preserving perfusion in the ischemic region. Accordingly, in stroke patients with poor collaterals undergoing thrombectomy, rapid reperfusion resulted in hemorrhagic transformation and unfavorable recovery. Thus, we identify LMCs as key components regulating reperfusion and preventing futile recanalization after stroke. Future therapeutic interventions should aim to enhance collateral function, allowing for beneficial reperfusion after stroke.

Network response of brain microvasculature to neuronal stimulation

Neurovascular coupling (NVC), or the adjustment of blood flow in response to local increases in neuronal activity is a hallmark of healthy brain function, and the physiological foundation for functional magnetic resonance imaging (fMRI). However, it remains only partly understood due to the high complexity of the structure and function of the cerebrovascular network. Here we set out to understand NVC at the network level, i.e. map cerebrovascular network reactivity to activation of neighbouring neurons within a 500×500×500 μm3 cortical volume (∼30 high-resolution 3-nL fMRI voxels). Using 3D two-photon fluorescence microscopy data, we quantified blood volume and flow changes in the brain vessels in response to spatially targeted optogenetic activation of cortical pyramidal neurons. We registered the vessels in a series of image stacks acquired before and after stimulations and applied a deep learning pipeline to segment the microvascular network from each time frame acquired. We then performed image analysis to extract the microvascular graphs, and graph analysis to identify the branch order of each vessel in the network, enabling the stratification of vessels by their branch order, designating branches 1-3 as precapillary arterioles and branches 4+ as capillaries. Forty-five percent of all vessels showed significant calibre changes; with 85 % of responses being dilations. The largest absolute CBV change was in the capillaries; the smallest, in the venules. Capillary CBV change was also the largest fraction of the total CBV change, but normalized to the baseline volume, arterioles and precapillary arterioles showed the biggest relative CBV change. From linescans along arteriole-venule microvascular paths, we measured red blood cell velocities and hematocrit, allowing for estimation of pressure and local resistance along these paths. While diameter changes following neuronal activation gradually declined along the paths; the pressure drops from arterioles to venules increased despite decreasing resistance: blood flow thus increased more than local resistance decreases would predict. By leveraging functional volumetric imaging and high throughput deep learning-based analysis, our study revealed distinct hemodynamic responses across the vessel types comprising the microvascular network. Our findings underscore the need for large, dense sampling of brain vessels for characterization of neurovascular coupling at the network level in health and disease.

Impaired drainage through capillary-venous networks contributes to age-related white matter loss

The gradual loss of cerebral white matter contributes to cognitive decline during aging. However, microvascular networks that support the metabolic demands of white matter remain poorly defined. We used in vivo deep multi-photon imaging to characterize microvascular networks that perfuse cortical layer 6 and corpus callosum, a highly studied region of white matter in the mouse brain. We show that these deep tissues are exclusively drained by sparse and wide-reaching venules, termed principal cortical venules, which mirror vascular architecture at the human cortical-U fiber interface. During aging, capillary networks draining into deep branches of principal cortical venules are selectively constricted, reduced in density, and diminished in pericyte numbers. This causes hypo-perfusion in deep tissues, and correlates with gliosis and demyelination, whereas superficial tissues become relatively hyper-perfused. Thus, age-related impairment of capillary-venular drainage is a key vascular deficit that contributes to the unique vulnerability of cerebral white matter during brain aging.

Partitioning of dense RBC suspensions in single microfluidic bifurcations: role of cell deformability and bifurcation angle

Red blood cells (RBCs) are a key determinant of human physiology and their behaviour becomes extremely heterogeneous as they navigate in narrow, bifurcating vessels in the microvasculature, affecting local haemodynamics. This is due to partitioning in bifurcations which is dependent on the biomechanical properties of RBCs, especially deformability. We examine the effect of deformability on the haematocrit distributions of dense RBC suspensions flowing in a single, asymmetric Y-shaped bifurcation, experimentally. Human RBC suspensions (healthy and artificially hardened) at 20% haematocrit (Ht) were perfused through the microchannels at different flow ratios between the outlet branches, and negligible inertia, and imaged to infer cell distributions. Notable differences in the shape of the haematocrit distributions were observed between healthy and hardened RBCs near the bifurcation apex. These lead to more asymmetric distributions for healthy RBCs in the daughter and outlet branches with cells accumulating near the inner channel walls, exhibiting distinct hematocrit peaks which are sharper for healthy RBCs. Although the hematocrit distributions differed locally, similar partitioning characteristics were observed for both suspensions. Comparisons with RBC distributions measured in a T-shaped bifurcation showed that the bifurcation angle affects the haematocrit characteristics of the healthy RBCs and not the hardened ones. The extent of RBC partitioning was found similar in both geometries and suspensions. The study highlights the differences between local and global characteristics which impact RBC distribution in more complex, multi-bifurcation networks.

Glomerular microcirculation: Implications for diabetes, preeclampsia, and kidney injury

This review outlines the features of tandem regulation of glomerular microcirculation by autoregulatory mechanisms and intraglomerular redistribution of blood flow. Multiple points of cooperation exist between autoregulatory and distributional mechanisms. Mutual interactions between myogenic and tubuloglomerular feedback (TGF) mechanisms regulating the inflow are briefly discussed. In addition to this, TGF operation involving purinergic, autocoid, and NO signaling affects, however, not only afferent arteriolar tone, but mesangial cell tone as well. The latter reversibly reconfigures the distribution of blood flow between the shorter and longer pathways in the glomerular tuft. I advance a hypothesis that blood flow in these pathways spontaneously alternates, and mesangial cell tonicity serves as a rheostatic shift between them. Furthermore, humoral messengers from macula densa cells, themselves dependent on myogenic mechanisms, fine-tune the secretion of renin and, subsequently, the local, intrarenal generation of angiotensin II, which, in turn, provides additional vasomotor signaling to glomerular capillaries through changing the tone of mesangial cells. This complex regulatory network may partially explain the phenomenon of renal functional reserve, as well as suggest implications for changes in renal function during pregnancy, early diabetes mellitus, and acute kidney injury.

The role of leptomeningeal collaterals in redistributing blood flow during stroke

Leptomeningeal collaterals (LMCs) connect the main cerebral arteries and provide alternative pathways for blood flow during ischaemic stroke. This is beneficial for reducing infarct size and reperfusion success after treatment. However, a better understanding of how LMCs affect blood flow distribution is indispensable to improve therapeutic strategies. Here, we present a novel in silico approach that incorporates case-specific in vivo data into a computational model to simulate blood flow in large semi-realistic microvascular networks from two different mouse strains, characterised by having many and almost no LMCs between middle and anterior cerebral artery (MCA, ACA) territories. This framework is unique because our simulations are directly aligned with in vivo data. Moreover, it allows us to analyse perfusion characteristics quantitatively across all vessel types and for networks with no, few and many LMCs. We show that the occlusion of the MCA directly caused a redistribution of blood that was characterised by increased flow in LMCs. Interestingly, the improved perfusion of MCA-sided microvessels after dilating LMCs came at the cost of a reduced blood supply in other brain areas. This effect was enhanced in regions close to the watershed line and when the number of LMCs was increased. Additional dilations of surface and penetrating arteries after stroke improved perfusion across the entire vasculature and partially recovered flow in the obstructed region, especially in networks with many LMCs, which further underlines the role of LMCs during stroke.

A three-dimensional method for morphological analysis and flow velocity estimation in microvasculature on-a-chip

Three-dimensional (3D) imaging techniques (e.g., confocal microscopy) are commonly used to visualize in vitro models, especially microvasculature on-a-chip. Conversely, 3D analysis is not the standard method to extract quantitative information from those models. We developed the μVES algorithm to analyze vascularized in vitro models leveraging 3D data. It computes morphological parameters (geometry, diameter, length, tortuosity, eccentricity) and intravascular flow velocity. μVES application to microfluidic vascularized in vitro models shows that they successfully replicate functional features of the microvasculature in vivo in terms of intravascular fluid flow velocity. However, wall shear stress is lower compared to in vivo references. The morphological analysis also highlights the model's physiological similarities (vessel length and tortuosity) and shortcomings (vessel radius and surface-over-volume ratio). The addition of the third dimension in our analysis produced significant differences in the metrics assessed compared to 2D estimations. It enabled the computation of new indices, such as vessel eccentricity. These μVES capabilities can find application in analyses of different in vitro vascular models, as well as in vivo and ex vivo microvasculature.

Stretch-activated ion channel TMEM63B associates with developmental and epileptic encephalopathies and progressive neurodegeneration

By converting physical forces into electrical signals or triggering intracellular cascades, stretch-activated ion channels allow the cell to respond to osmotic and mechanical stress. Knowledge of the pathophysiological mechanisms underlying associations of stretch-activated ion channels with human disease is limited. Here, we describe 17 unrelated individuals with severe early-onset developmental and epileptic encephalopathy (DEE), intellectual disability, and severe motor and cortical visual impairment associated with progressive neurodegenerative brain changes carrying ten distinct heterozygous variants of TMEM63B, encoding for a highly conserved stretch-activated ion channel. The variants occurred de novo in 16/17 individuals for whom parental DNA was available and either missense, including the recurrent p.Val44Met in 7/17 individuals, or in-frame, all affecting conserved residues located in transmembrane regions of the protein. In 12 individuals, hematological abnormalities co-occurred, such as macrocytosis and hemolysis, requiring blood transfusions in some. We modeled six variants (p.Val44Met, p.Arg433His, p.Thr481Asn, p.Gly580Ser, p.Arg660Thr, and p.Phe697Leu), each affecting a distinct transmembrane domain of the channel, in transfected Neuro2a cells and demonstrated inward leak cation currents across the mutated channel even in isotonic conditions, while the response to hypo-osmotic challenge was impaired, as were the Ca2+ transients generated under hypo-osmotic stimulation. Ectopic expression of the p.Val44Met and p.Gly580Cys variants in Drosophila resulted in early death. TMEM63B-associated DEE represents a recognizable clinicopathological entity in which altered cation conductivity results in a severe neurological phenotype with progressive brain damage and early-onset epilepsy associated with hematological abnormalities in most individuals.

Understanding human aging and the fundamental cell signaling link in age-related diseases: the middle-aging hypovascularity hypoxia hypothesis

Aging-related hypoxia, oxidative stress, and inflammation pathophysiology are closely associated with human age-related carcinogenesis and chronic diseases. However, the connection between hypoxia and hormonal cell signaling pathways is unclear, but such human age-related comorbid diseases do coincide with the middle-aging period of declining sex hormonal signaling. This scoping review evaluates the relevant interdisciplinary evidence to assess the systems biology of function, regulation, and homeostasis in order to discern and decipher the etiology of the connection between hypoxia and hormonal signaling in human age-related comorbid diseases. The hypothesis charts the accumulating evidence to support the development of a hypoxic milieu and oxidative stress-inflammation pathophysiology in middle-aged individuals, as well as the induction of amyloidosis, autophagy, and epithelial-to-mesenchymal transition in aging-related degeneration. Taken together, this new approach and strategy can provide the clarity of concepts and patterns to determine the causes of declining vascularity hemodynamics (blood flow) and physiological oxygenation perfusion (oxygen bioavailability) in relation to oxygen homeostasis and vascularity that cause hypoxia (hypovascularity hypoxia). The middle-aging hypovascularity hypoxia hypothesis could provide the mechanistic interface connecting the endocrine, nitric oxide, and oxygen homeostasis signaling that is closely linked to the progressive conditions of degenerative hypertrophy, atrophy, fibrosis, and neoplasm. An in-depth understanding of these intrinsic biological processes of the developing middle-aged hypoxia could provide potential new strategies for time-dependent therapies in maintaining healthspan for healthy lifestyle aging, medical cost savings, and health system sustainability.

Capillary responses to functional and pathological activations rely on the capillary states at rest

Brain capillaries play a crucial role in maintaining cellular viability and thus preventing neurodegeneration. The aim of this study was to characterize the brain capillary morphology at rest and during neural activation based on a big data analysis from three-dimensional microangiography. Neurovascular responses were measured using a genetic calcium sensor expressed in neurons and microangiography with two-photon microscopy, while neural acivity was modulated by stimulation of contralateral whiskers or by a seizure evoked by kainic acid. For whisker stimulation, 84% of the capillary sites showed no detectable diameter change. The remaining 10% and 6% were dilated and constricted, respectively. Significant differences were observed for capillaries in the diameter at rest between the locations of dilation and constriction. Even the seizures resulted in 44% of the capillaries having no detectable change in diameter, while 56% of the capillaries dilated. The extent of dilation was dependent on the diameter at rest. In conclusion, big data analysis on brain capillary morphology has identified at least two types of capillary states: capillaries with diameters that are relatively large at rest and stable over time regardless of neural activity and capillaries whose diameters are relatively small at rest and vary according to neural activity.

Ionic Liquid Coating-Driven Nanoparticle Delivery to the Brain: Applications for NeuroHIV

Delivering cargo to the central nervous system (CNS) remains a pharmacological challenge. For infectious diseases such as HIV, the CNS acts as a latent reservoir that is inadequately managed by systemic antiretrovirals (ARTs). ARTs thus cannot eradicate HIV, and given CNS infection, patients experience an array of neurological deficits that are collectively referred to as 'neuroHIV'. Herein we report the development of bioinspired ionic liquid-coated nanoparticles (IL-NPs) for in situ hitchhiking on red blood cells (RBCs), which enabled 48% delivery of intravenously infused cargo to the brain. Moreover, the ionic liquid (IL) choline trans-2-hexenoate (CA2HA 1:2) demonstrated preferential accumulation in parenchymal microglia over endothelial cells post-delivery. We further demonstrate the successful loading of abacavir (ABC), an ART that is challenging to encapsulate, into the IL-coated NPs and verify the retention of antiviral efficacy in vitro. IL-NPs were not cytotoxic to primary human peripheral blood mononuclear cells (PBMCs) and the CA2HA 1:2 coating conferred notable anti-viremic capacity on its own. In addition, in vitro cell culture assays showed markedly increased uptake of IL-coated nanoparticles into neuronal cells compared to bare nanoparticles. This work debuts bioinspired ionic liquids as promising nanoparticle coatings to assist CNS biodistribution and has the potential to revolutionize the delivery of cargos (i.e., drugs, viral vectors) through compartmental barriers such as the blood-brain-barrier (BBB), illustrated in the graphical abstract below.

Spatiotemporal analysis of blood plasma and blood cell flow fluctuations of cerebral microcirculation in anesthetized rats

Cerebral hemodynamics fluctuates spontaneously over broad frequency ranges. However, its spatiotemporal coherence of flow oscillations in cerebral microcirculation remains incompletely understood. The objective of this study was to characterize the spatiotemporal fluctuations of red blood cells (RBCs) and plasma flow in the rat cerebral microcirculation by simultaneously imaging their dynamic behaviors. Comparisons of changes in cross-section diameters between RBC and plasma flow showed dissociations in penetrating arterioles. The results indicate that vasomotion has the least effect on the lateral movement of circulating RBCs, resulting in variable changes in plasma layer thickness. Parenchymal capillaries exhibited slow fluctuations in RBC velocity (0.1 to 0.3 Hz), regardless of capillary diameter fluctuations (<0.1 Hz). Temporal fluctuations and the velocity of RBCs decreased significantly at divergent capillary bifurcations. The results indicate that a transit of RBCs generates flow resistance in the capillaries and that slow velocity fluctuations of the RBCs are subject to a number of bifurcations. In conclusion, the high-frequency oscillation of the blood flow is filtered at the bifurcation through the capillary networks. Therefore, a number of bifurcations in the cerebral microcirculation may contribute to the power of low-frequency oscillations.

Red blood cell dynamics in extravascular biological tissues modelled as canonical disordered porous media

The dynamics of blood flow in the smallest vessels and passages of the human body, where the cellular character of blood becomes prominent, plays a dominant role in the transport and exchange of solutes. Recent studies have revealed that the microhaemodynamics of a vascular network is underpinned by its interconnected structure, and certain structural alterations such as capillary dilation and blockage can substantially change blood flow patterns. However, for extravascular media with disordered microstructure (e.g. the porous intervillous space in the placenta), it remains unclear how the medium’s structure affects the haemodynamics. Here, we simulate cellular blood flow in simple models of canonical porous media representative of extravascular biological tissue, with corroborative microfluidic experiments performed for validation purposes. For the media considered here, we observe three main effects: first, the relative apparent viscosity of blood increases with the structural disorder of the medium; second, the presence of red blood cells (RBCs) dynamically alters the flow distribution in the medium; third, symmetry breaking introduced by moderate structural disorder can promote more homogeneous distribution of RBCs. Our findings contribute to a better understanding of the cell-scale haemodynamics that mediates the relationship linking the function of certain biological tissues to their microstructure.

Neurovascular coupling: motive unknown

In the brain, increases in neural activity drive changes in local blood flow via neurovascular coupling. The common explanation for increased blood flow (known as functional hyperemia) is that it supplies the metabolic needs of active neurons. However, there is a large body of evidence that is inconsistent with this idea. Baseline blood flow is adequate to supply oxygen needs even with elevated neural activity. Neurovascular coupling is irregular, absent, or inverted in many brain regions, behavioral states, and conditions. Increases in respiration can increase brain oxygenation without flow changes. Simulations show that given the architecture of the brain vasculature, areas of low blood flow are inescapable and cannot be removed by functional hyperemia. As discussed in this article, potential alternative functions of neurovascular coupling include supplying oxygen for neuromodulator synthesis, brain temperature regulation, signaling to neurons, stabilizing and optimizing the cerebral vascular structure, accommodating the non-Newtonian nature of blood, and driving the production and circulation of cerebrospinal fluid (CSF).

Pericyte remodeling is deficient in the aged brain and contributes to impaired capillary flow and structure

Deterioration of brain capillary flow and architecture is a hallmark of aging and dementia. It remains unclear how loss of brain pericytes in these conditions contributes to capillary dysfunction. Here, we conduct cause-and-effect studies by optically ablating pericytes in adult and aged mice in vivo. Focal pericyte loss induces capillary dilation without blood-brain barrier disruption. These abnormal dilations are exacerbated in the aged brain, and result in increased flow heterogeneity in capillary networks. A subset of affected capillaries experience reduced perfusion due to flow steal. Some capillaries stall in flow and regress, leading to loss of capillary connectivity. Remodeling of neighboring pericytes restores endothelial coverage and vascular tone within days. Pericyte remodeling is slower in the aged brain, resulting in regions of persistent capillary dilation. These findings link pericyte loss to disruption of capillary flow and structure. They also identify pericyte remodeling as a therapeutic target to preserve capillary flow dynamics.

Ultrafast two-photon fluorescence imaging of cerebral blood circulation in the mouse brain in vivo

SignificanceCharacterizing blood flow by tracking individual red blood cells as they move through vessels is essential for understanding vascular function. With high spatial resolution, two-photon fluorescence microscopy is the method of choice for imaging blood flow at the cellular level. However, its application is limited to a low flow speed regimen in anesthetized animals by its slow focus scanning mechanism. Using an ultrafast scanning module, we demonstrated two-photon fluorescence imaging of blood flow at 1,000 two-dimensional frames and 1,000,000 one-dimensional line scans per second in the brains of awake mice. These ultrafast measurements enabled us to study hemodynamic and fluid mechanical regimens beyond the reach of conventional methods.

Measurement of Blood Velocity With Laser Scanning Microscopy: Modeling and Comparison of Line-Scan Image-Processing Algorithms

Laser scanning microscopy is widely used to measure blood hemodynamics with line-scans in physiological and pathological vessels. With scans of broken lines, i.e., lines made of several segments with different orientations, it also allows simultaneous monitoring of vessel diameter dynamics or the activity of specific cells. Analysis of red blood cell (RBC) velocity from line-scans requires specific image-processing algorithms, as angle measurements, Line-Scanning Particle Image Velocimetry (LSPIV) or Fourier transformation of line-scan images. The conditions under which these image-processing algorithms give accurate measurements have not been fully characterized although the accuracy of measurements vary according to specific experimental parameters: the vessel type, the RBC velocity, the scanning parameters, and the image signal to noise ratio. Here, we developed mathematical models for the three previously mentioned line-scan image-processing algorithms. Our models predict the experimental conditions in which RBC velocity measurements are accurate. We illustrate the case of different vessel types and give the parameter space available for each of them. Last, we developed a software generating artificial line-scan images and used it to validate our models.

Neutrophils Obstructing Brain Capillaries Are a Major Cause of No-Reflow in Ischemic Stroke

Despite successful clot retrieval in large vessel occlusion stroke, ∼50% of patients have an unfavorable clinical outcome. The mechanisms underlying this functional reperfusion failure remain unknown, and therapeutic options are lacking. In the thrombin-model of middle cerebral artery (MCA) stroke in mice, we show that, despite successful thrombolytic recanalization of the proximal MCA, cortical blood flow does not fully recover. Using in vivo two-photon imaging, we demonstrate that this is due to microvascular obstruction of ∼20%-30% of capillaries in the infarct core and penumbra by neutrophils adhering to distal capillary segments. Depletion of circulating neutrophils using an anti-Ly6G antibody restores microvascular perfusion without increasing the rate of hemorrhagic complications. Strikingly, infarct size and functional deficits are smaller in mice treated with anti-Ly6G. Thus, we propose neutrophil stalling of brain capillaries to contribute to reperfusion failure, which offers promising therapeutic avenues for ischemic stroke.

Pericytes: Intrinsic Transportation Engineers of the CNS Microcirculation

Pericytes in the brain are candidate regulators of microcirculatory blood flow because they are strategically positioned along the microvasculature, contain contractile proteins, respond rapidly to neuronal activation, and synchronize microvascular dynamics and neurovascular coupling within the capillary network. Analyses of mice with defects in pericyte generation demonstrate that pericytes are necessary for the formation of the blood-brain barrier, development of the glymphatic system, immune homeostasis, and white matter function. The development, identity, specialization, and progeny of different subtypes of pericytes, however, remain unclear. Pericytes perform brain-wide 'transportation engineering' functions in the capillary network, instructing, integrating, and coordinating signals within the cellular communicome in the neurovascular unit to efficiently distribute oxygen and nutrients ('goods and services') throughout the microvasculature ('transportation grid'). In this review, we identify emerging challenges in pericyte biology and shed light on potential pericyte-targeted therapeutic strategies.

Distribution Networks Achieve Uniform Perfusion through Geometric Self-Organization

A generic flow distribution network typically does not deliver its load at a uniform rate across a service area, instead oversupplying regions near the nutrient source while leaving downstream regions undersupplied. In this Letter we demonstrate how a local adaptive rule coupling tissue growth with nutrient density results in a flow network that self-organizes to deliver nutrients uniformly. This geometric adaptive rule can be generalized and imported to mechanics-based adaptive models to address the effects of spatial gradients in nutrients or growth factors in tissues.

Association between erythrocyte dynamics and vessel remodelling in developmental vascular networks

Sprouting angiogenesis is an essential vascularization mechanism consisting of sprouting and remodelling. The remodelling phase is driven by rearrangements of endothelial cells (ECs) within the post-sprouting vascular plexus. Prior work has uncovered how ECs polarize and migrate in response to flow-induced wall shear stress (WSS). However, the question of how the presence of erythrocytes (widely known as red blood cells (RBCs)) and their impact on haemodynamics affect vascular remodelling remains unanswered. Here, we devise a computational framework to model cellular blood flow in developmental mouse retina. We demonstrate a previously unreported highly heterogeneous distribution of RBCs in primitive vasculature. Furthermore, we report a strong association between vessel regression and RBC hypoperfusion, and identify plasma skimming as the driving mechanism. Live imaging in a developmental zebrafish model confirms this association. Taken together, our results indicate that RBC dynamics are fundamental to establishing the regional WSS differences driving vascular remodelling via their ability to modulate effective viscosity.

The severity of microstrokes depends on local vascular topology and baseline perfusion

Cortical microinfarcts are linked to pathologies like cerebral amyloid angiopathy and dementia. Despite their relevance for disease progression, microinfarcts often remain undetected and the smallest scale of blood flow disturbance has not yet been identified. We employed blood flow simulations in realistic microvascular networks from the mouse cortex to quantify the impact of single-capillary occlusions. Our simulations reveal that the severity of a microstroke is strongly affected by the local vascular topology and the baseline flow rate in the occluded capillary. The largest changes in perfusion are observed in capillaries with two inflows and two outflows. This specific topological configuration only occurs with a frequency of 8%. The majority of capillaries have one inflow and one outflow and is likely designed to efficiently supply oxygen and nutrients. Taken together, microstrokes bear potential to induce a cascade of local disturbances in the surrounding tissue, which might accumulate and impair energy supply locally.

Red Blood Cell-Mediated S-Nitrosohemoglobin-Dependent Vasodilation: Lessons Learned from a β-Globin Cys93 Knock-In Mouse

Significance: Red blood cell (RBC)-mediated vasodilation plays an important role in oxygen delivery. This occurs through hemoglobin actions, at least in significant part, to convert heme-bound nitric oxide (NO) (in tense [T]/deoxygenated-state hemoglobin) into vasodilator S-nitrosothiol (SNO) (in relaxed [R]/oxygenated-state hemoglobin), convey SNO through the bloodstream, and release it into tissues to increase blood flow. The coupling of hemoglobin R/T state allostery, both to NO conversion into SNO and to SNO release (along with oxygen), under hypoxia supports the model of a three-gas respiratory cycle (O2/NO/CO2). Recent Advances: Oxygenation of tissues is dependent on a single, strictly conserved Cys residue in hemoglobin (βCys93). Hemoglobin couples SNO formation/release at βCys93 to O2 binding/release at hemes ("thermodynamic linkage"). Mice bearing βCys93Ala hemoglobin that is unable to generate SNO-βCys93 establish that SNO-hemoglobin is important for R/T allostery-regulated vasodilation by RBCs that couple blood flow to tissue oxygenation. Critical Issues: The model for RBC-mediated vasodilation originally proposed by Stamler et al. in 1996 has been largely validated: SNO-βCys93 forms in vivo, dilates blood vessels, and is hypoxia-regulated, and RBCs actuate vasodilation proportionate to hypoxia. Numerous compensations in βCys93Ala animals to alleviate tissue hypoxia (discussed herein) are predicted to preserve vasodilatory responses of RBCs but impair linkage to R/T transition in hemoglobin. This is borne out by loss of responsivity of mutant RBCs to oxygen, impaired blood flow responses to hypoxia, and tissue ischemia in βCys93-mutant animals. Future Directions: SNO-hemoglobin mediates hypoxic vasodilation in the respiratory cycle. This fundamental physiology promises new insights in vascular diseases and blood disorders.

Predicting Vessel Diameter Changes to Up-Regulate Biphasic Blood Flow During Activation in Realistic Microvascular Networks

A dense network of blood vessels distributes blood to different regions of the brain. To meet the temporarily and spatially varying energy demand resulting from changes in neuronal activity, the vasculature is able to locally up-regulate the blood supply. However, to which extent diameter changes of different vessel types contribute to the up-regulation, as well as the spatial and temporal characteristics of their changes, are currently unknown. Here, we present a new simulation method, which solves an inverse problem to calculate diameter changes of individual blood vessels needed to achieve predefined blood flow distributions in microvascular networks. This allows us to systematically compare the impact of different vessel types in various regulation scenarios. Moreover, the method offers the advantage that it handles the stochastic nature of blood flow originating from tracking the movement of individual red blood cells. Since the inverse problem is formulated for time-averaged pressures and flow rates, a deterministic approach for calculating the diameter changes is used, which allows us to apply the method for large realistic microvascular networks with high-dimensional parameter spaces. Our results obtained in both artificial and realistic microvascular networks reveal that diameter changes at the level of capillaries enable a very localized regulation of blood flow. In scenarios where only larger vessels, i.e., arterioles, are allowed to adapt, the flow increase cannot be confined to a specific activated region and flow changes spread into neighboring regions. Furthermore, relatively small dilations and constrictions of all vessel types can lead to substantial changes of capillary blood flow distributions. This suggests that small scale regulation is necessary to obtain a localized increase in blood flow.

Local vs. Global Blood Flow Modulation in Artificial Microvascular Networks: Effects on Red Blood Cell Distribution and Partitioning

Our understanding of cerebral blood flow (CBF) regulation during functional activation is still limited. Alongside with the accepted role of smooth muscle cells in controlling the arteriolar diameter, a new hypothesis has been recently formulated suggesting that CBF may be modulated by capillary diameter changes mediated by pericytes. In this study, we developed in vitro microvascular network models featuring a valve enabling the dilation of a specific micro-channel. This allowed us to investigate the non-uniform red blood cell (RBC) partitioning at microvascular bifurcations (phase separation) and the hematocrit distribution at rest and for two scenarios modeling capillary and arteriolar dilation. RBC partitioning showed similar phase separation behavior during baseline and activation. Results indicated that the RBCs at diverging bifurcations generally enter the high-flow branch (classical partitioning). Inverse behavior (reverse partitioning) was observed for skewed hematocrit profiles in the parent vessel of bifurcations, especially for high RBC velocity (i.e., arteriolar activation). Moreover, results revealed that a local capillary dilation, as it may be mediated in vivo by pericytes, led to a localized increase of RBC flow and a heterogeneous hematocrit redistribution within the whole network. In case of a global increase of the blood flow, as it may be achieved by dilating an arteriole, a homogeneous increase of RBC flow was observed in the whole network and the RBCs were concentrated along preferential pathways. In conclusion, overall increase of RBC flow could be obtained by arteriolar and capillary dilation, but only capillary dilation was found to alter the perfusion locally and heterogeneously.

In vitro investigations of red blood cell phase separation in a complex microchannel network

Microvascular networks feature a complex topology with multiple bifurcating vessels. Nonuniform partitioning (phase separation) of red blood cells (RBCs) occurs at diverging bifurcations, leading to a heterogeneous RBC distribution that ultimately affects the oxygen delivery to living tissues. Our understanding of the mechanisms governing RBC heterogeneity is still limited, especially in large networks where the RBC dynamics can be nonintuitive. In this study, our quantitative data for phase separation were obtained in a complex in vitro network with symmetric bifurcations and 176 microchannels. Our experiments showed that the hematocrit is heterogeneously distributed and confirmed the classical result that the branch with a higher blood fraction received an even higher RBC fraction (classical partitioning). An inversion of this classical phase separation (reverse partitioning) was observed in the case of a skewed hematocrit profile in the parent vessels of bifurcations. In agreement with a recent computational study [P. Balogh and P. Bagchi, Phys. Fluids 30,051902 (2018)], a correlation between the RBC reverse partitioning and the skewness of the hematocrit profile due to sequential converging and diverging bifurcations was reported. A flow threshold below which no RBCs enter a branch was identified. These results highlight the importance of considering the RBC flow history and the local RBC distribution to correctly describe the RBC phase separation in complex networks.