Erythrocyte flow heterogeneity in the cerebrocortical capillary network

CD44 mediates shear stress mechanotransduction in an in vitro blood-brain barrier model through small GTPases RhoA and Rac1

Fluid shear stress is an important mediator of vascular permeability, yet the molecular mechanisms underlying the effect of shear on the blood-brain barrier (BBB) have yet to be clarified in cerebral vasculature despite its importance for brain homeostasis. The goal of this study is to probe components of shear mechanotransduction within the BBB to gain a better understanding of pathologies associated with changes in cerebral perfusion including ischemic stroke. Interrogating the effects of shear stress in vivo is complicated by the complexity of factors in the brain parenchyma and the difficulty associated with modulating blood flow regimes. The in vitro model used in this study is compatible with real-time measurement of barrier function using a transendothelial electrical resistance as well as immunocytochemistry and dextran permeability assays. These experiments reveal that there is a threshold level of shear stress required for barrier formation and that the composition of the extracellular matrix, specifically the presence of high molecular weight hyaluronan, dictates the flow response. Gene editing to modulate the expression of CD44, a mechanosensitive receptor for hyaluronan, demonstrates that the receptor is required for the endothelial response to shear stress. Manipulation of small GTPase activity reveals CD44 activates Rac1 while inhibiting RhoA activation. Additionally, adducin-γ localizes to tight junctions in response to shear stress and RhoA inhibition and is required to maintain the barrier. This study identifies specific components of the mechanosensing complex associated with the BBB response to fluid shear stress and, therefore, illuminates potential targets for barrier manipulation in vivo.

Cerebral blood volume changes during brain activation

Cerebral blood volume (CBV) changes significantly with brain activation, whether measured using positron emission tomography, functional magnetic resonance imaging (fMRI), or optical microscopy. If cerebral vessels are considered to be impermeable, the contents of the skull incompressible, and the skull itself inextensible, task- and hypercapnia-related changes of CBV could produce intolerable changes of intracranial pressure. Because it is becoming clear that CBV may be useful as a well-localized marker of neural activity changes, a resolution of this apparent paradox is needed. We have explored the idea that much of the change in CBV is facilitated by exchange of water between capillaries and surrounding tissue. To this end, we developed a novel hemodynamic boundary-value model and found approximate solutions using a numerical algorithm. We also constructed a macroscopic experimental model of a single capillary to provide biophysical insight. Both experiment and theory model capillary membranes as elastic and permeable. For a realistic change of input pressure, a relative pipe volume change of 21±5% was observed when using the experimental setup, compared with the value of approximately 17±1% when this quantity was calculated from the mathematical model. Volume, axial flow, and pressure changes are in the expected range.

Cerebral microcirculation shear stress levels determine Neisseria meningitidis attachment sites along the blood-brain barrier

Neisseria meningitidis is a commensal bacterium of the human nasopharynx. Occasionally, this bacterium reaches the bloodstream and causes meningitis after crossing the blood–brain barrier by an unknown mechanism. An immunohistological study of a meningococcal sepsis case revealed that neisserial adhesion was restricted to capillaries located in low blood flow regions in the infected organs. This study led to the hypothesis that drag forces encountered by the meningococcus in the bloodstream determine its attachment site in vessels. We therefore investigated the ability of N. meningitidis to bind to endothelial cells in the presence of liquid flow mimicking the bloodstream with a laminar flow chamber. Strikingly, average blood flows reported for various organs strongly inhibited initial adhesion. As cerebral microcirculation is known to be highly heterogeneous, cerebral blood velocity was investigated at the level of individual vessels using intravital imaging of rat brain. In agreement with the histological study, shear stress levels compatible with meningococcal adhesion were only observed in capillaries, which exhibited transient reductions in flow. The flow chamber assay revealed that, after initial attachment, bacteria resisted high blood velocities and even multiplied, forming microcolonies resembling those observed in the septicemia case. These results argue that the combined mechanical properties of neisserial adhesion and blood microcirculation target meningococci to transiently underperfused cerebral capillaries and thus determine disease development.

Capillary flow and diameter changes during reperfusion after global cerebral ischemia studied by intravital video microscopy

The reaction of cerebral capillaries to ischemia is unclear. Based on Hossmann's observation of postischemic "delayed hypoperfusion," we hypothesized that capillary flow is decreased during reperfusion because of increased precapillary flow resistance. To test this hypothesis, we measured cerebral capillary erythrocyte velocity and diameter changes by intravital microscopy in gerbils. A cranial window was prepared over the frontoparietal cortex in 26 gerbils anesthetized with halothane. The animals underwent either a sham operation or fifteen minutes of bilateral carotid artery occlusion causing global cerebral ischemia. Capillary flow velocities were measured by frame-to-frame tracking of fluorescein isothiocyanate labeled erythrocytes in 1800 capillaries after 1-hour reperfusion. Capillary flow velocities were decreased compared to control (0.25 +/- 0.27 mm/s vs. 0.76 +/- 0.45 mm/s; P<0.001). Precapillary arteriole diameters in reperfused animals were reduced to 76.3 +/- 6.9% compared to baseline (P<0.05). Capillary diameters in reperfused animals (2.87 +/- 0.97 microm) were reduced (P<0.001) compared to control (4.08 +/- 1.19 microm). Similar reductions of precapillary (24%) and capillary vessel diameters (30%) and absolute capillary flow heterogeneity indicate that delayed (capillary) hypoperfusion occurs as a consequence of increased precapillary arteriole tone during reperfusion.

The Cerebral Microcirculation in Ischemia and Hypoxemia

The cerebral capillary circulation exhibits heterogenous perfusion and undergoes characteristic changes in the distribution of RBC flow in response to systemic physiological stimuli. Hypoxemia, hypercapnia and hypotension increase the homogeneity of capillary perfusion, which is thought to preserve or enhance transcapillary exchange. Redistribution of capillary RBC flow between nutritive capillaries and preferential channels may contribute to this response. Selective changes in capillary flow may be brought about by non-smooth muscle-based contractile or blood-borne mechanisms. Isovolemic hemodilution anemia increases RBC velocity and supply rate with no decrease in capillary hematocrit. The effect of cerebral ischemia on microvascular patency depends on the severity and time course of the insult and whether the injury is global or focal. Capillary plugging is not observed following transient forebrain ischemia in the rat cerebral cortex but may contribute to tissue injury prior to reperfusion and during prolonged and severe ischemia. In the future, a better understanding of the functional architecture of the cerebral capillary network and its significance in the adaptation to altered circulatory conditions will continue to be an important goal of research. More work will have to be done to (i) substantiate the postulated physiological regulation of cerebral capillary flow, (ii) determine the cellular mechanism of integration of flow-dependent and neuronal activity-dependent signals, and (iii) identify the principal mediators, their cellular sources and molecular targets. The final answer to these questions will in a large part depend on our ability to directly, i.e. microscopically, visualize microvascular, neuronal and molecular phenomena as they occur in the brain in a spatially and temporally distributed manner.

Stereological estimates of number and length of capillaries in subdivisions of the human hippocampal region

The hippocampal formation is a neuroanatomically well-defined region of the brain involved in memory processes. In view of the functional importance of the region and its involvement in a number of brain pathologies, including Alzheimer's disease and temporal lobe epilepsy, a quantitative description of its vascular supply represents an important first step in evaluating the involvement of vascular changes in these phenomena. Unbiased estimates of the length and connectivity of the vascular supply of brain regions have not been described previously. The total number, total length, and distribution of the diameters of capillaries were estimated in the five major subdivisions of the hippocampal formation (fascia dentata, hilus, CA3-2, CA1, and subiculum) in 5 normal males, 52-84 years of age. These estimates were used to derive several other structural parameters. Both the primary and the derived parameters were used to make inter- and intra-individual comparisons. For each of the five major subdivisions from each individual, the volume was estimated using the Cavalieri principle. The total capillary length was estimated on 3-microm-thick plastic isotropic uniform random sections. Using a topological definition of a capillary unit and the optical disector, total capillary number was estimated in 40-microm-thick plastic sections. Length-and number-weighted three-dimensional diameter distributions were obtained from the thin and thick plastic sections, respectively. In each subdivision the total length of capillaries was correlated with previously obtained data on the number of neurons in the same subdivisions of the same individuals. Intersubdivisional differences were observed, in that the hilus of the dentate gyrus had fewer capillaries per unit volume than the other four subdivisions. Interindividual comparisons indicate that the interindividual variances are of a magnitude suitable for sensitive group comparisons. The design-based stereological methods that were used in the analyses can provide a basis for a new unbiased approach to the estimation of vascular parameters in well-defined regions of the brain.

Dynamic in vivo measurement of erythrocyte velocity and flow in capillaries and of microvessel diameter in the rat brain by confocal laser microscopy

A new method for studying brain microcirculation is described. Both fluorescently labeled erythrocytes and plasma were visualized on-line through a closed cranial window in anesthetized rats, using laser-scanning two-dimension confocal microscopy. Video images of capillaries, arterioles, and venules were digitized off-line to measure microvessel diameter and labeled erythrocyte flow and velocity in parenchymal capillaries up to 200 microm beneath the brain surface. The method was used to analyze the rapid adaptation of microcirculation to a brief decrease in perfusion pressure. Twenty-second periods of forebrain ischemia were induced using the tour-vessel occlusion model in eight rats. EEG, arterial blood pressure, and body temperature were continuously controlled. In all conditions, labeled erythrocyte flow and velocity were both very heterogeneous in capillaries. During ischemia, capillary perfusion was close to 0, but a low blood flow persisted in arterioles and venules, while EEG was flattening. The arteriole and venule diameter did not significantly change. At the unclamping of carotid arteries, there was an instantaneous increase (by about 150%) of arteriole diameter. Capillary erythrocyte flow and velocity increased within 5 seconds, up to, respectively, 346 +/- 229% and 233 +/- 156% of their basal value. No capillary recruitment of erythrocytes was detected. All variables returned to their basal levels within less than 100 seconds after declamping. The data are discussed in terms of a possible involvement of shear stress in the reperfusion period.

Regulation of oxygen supply in the cerebral circulation

The dynamics and regulation of red blood cell flow in the cerebral microcirculation was studied by intravital fluorescence video-microscopy in a closed cranial window preparation in the rat. The studies revealed that capillary perfusion in the brain is essentially continuous but a stationary difference from capillary to capillary within the same microvascular network exists. The main mechanism of an increase in flow in cerebral capillaries is an increase in linear velocity with no or minor role for classical capillary recruitment. While cyclic opening and closing of capillaries is not evident, low frequency oscillations in capillary flow velocity are present when perfusion or oxygen supply to tissue is challenged. In hypoxic hypoxia and moderate hypercapnia, RBC velocity increases in all capillaries while in severe hypercapnia, redistribution of RBC velocity in the capillary network occurs. Both systemic hypotension and severe hypercapnia are accompanied by an increase in the homogeneity of capillary flow; this change involves the redistribution of RBC flow between thoroughfare channels and exchange capillaries. Thoroughfare channels may thus provide a recruitable flow reserve in the cerebral microcirculation. The capillary flow response to hypoxic and anemic hypoxia depends on the activity neuronal nitric oxide synthase. These findings suggest the presence of a physiological regulatory mechanism of cerebral capillary red blood cell flow and oxygen supply which may involve neuronal nitric oxide as a mediator.