The Neurovascular Unit Coming of Age: A Journey through Neurovascular Coupling in Health and Disease

Abstract

The concept of the neurovascular unit (NVU), formalized at the 2001 Stroke Progress Review Group meeting of the National Institute of Neurological Disorders and Stroke, emphasizes the intimate relationship between the brain and its vessels. Since then, the NVU has attracted the interest of the neuroscience community, resulting in considerable advances in the field. Here the current state of knowledge of the NVU will be assessed, focusing on one of its most vital roles: the coupling between neural activity and blood flow. The evidence supports a conceptual shift in the mechanisms of neurovascular coupling, from a unidimensional process involving neuronal-astrocytic signaling to local blood vessels to a multidimensional one in which mediators released from multiple cells engage distinct signaling pathways and effector systems across the entire cerebrovascular network in a highly orchestrated manner. The recently appreciated NVU dysfunction in neurodegenerative diseases, although still poorly understood, supports emerging concepts that maintaining neurovascular health promotes brain health.

Keywords: astrocytes; cerebral blood flow; endothelium; functional hyperemia; neurodegeneration; neuroimaging; pericytes; smooth muscle cells.

Figure 1. Citations of the neurovascular unit in the literature from 1997 to 2016

The search term “neurovascular unit” was used in the Web of Science database (Thompson Reuters). Since the SPRG meeting in 2001, a dramatic rise in the number of citations was observed. Prior to 2001, the term was often used for critical care units combining neurology and neurosurgery.

Figure 2. Historical overview of the concept of neurovascular coupling

Pioneers in establishing the concept of neurovascular coupling are shown on top and examples of their work presented below the timeline. Studies in the late 1800s, by Angelo Mosso and Charles Roy and Charles Sherrington, hinted at the possibility that brain activity increases cerebral blood flow. The lower panels illustrate the apparatus used by Mosso to record changes in brain volume in patients with skull defects (Mosso, 1880), and Roy and Sherrington’s recordings of brain expansion in response to intracarotid infusion of a brain extract (Roy and Sherrington, 1890). In the 1930s Carl Schmidt recorded increases in temperature in the cat visual cortex by shining light into the eye. The original recording is shown in the lower panel (Schmidt and Hendrix, 1938). In the 1950–60s Seymour Kety and Lou Sokoloff developed autoradiographic methods to image regional CBF during neural activation. The lower panel shown the increase in CBF produced in the calcarine cortex and superior colliculus by visual stimulation (Freygang and Sokoloff, 1958). In the 1960-70s David Ingvar and Niels Lassen developed methods to measure regional CBF in the human brain using radioactive tracers and external detection. The lower panel illustrates the increase in CBF produced by hand movement in the contralateral sensory-motor cortex and supplementary motor area (Lassen et al., 1978), marking the birth of functional brain imaging.

Figure 3. Anatomy of the cerebrovascular tree and segmental vascular resistance

The internal carotid artery (ICA) enters the skull and merges with branches of the vertebral arteries to form the circle of Willis at the base of the brain. The middle cerebral artery (MCA) takes off from the circle of Willis and supplies a large territory of the cerebral cortex. The MCA gives rise to pial arteries and arterioles that run on the surface of the brain forming a heavily interconnected network from which arterioles penetrating into the substance of the brain originate (penetrating arterioles). Penetrating arterioles give rise to the capillary network, which feeds into the venous system returning the blood to the heart. The component (%) of the total vascular resistance that each cerebrovascular segment offers to blood flow, which reflects their potential for flow control, is indicated. Therefore, vessels outside the brain are responsible for 60% of the resistance, and vessels within the substance of the brain for 40% (based on data from (Stromberg and Fox, 1972) and (De Silva and Faraci, 2016)).

Figure 4. Neurovascular associations along the cerebrovascular tree: Pial arteries and penetrating arterioles

A pial arteriole on the cortical surface, giving rise to a penetrating arteriole is shown on the right. Pial arterioles have thick coat of SMC, are surrounded by the subarachnoid space, and are densely innervated by nerve fibers originating form cranial autonomic and sensory ganglia, such as the sympathetic, parasympathetic and trigeminal ganglia. Penetrating arterioles enter the substance of the brain and are surrounded by a perivascular space containing several cell types including perivascular macrophages (PVM). In Figures 3 and 4, the vasculature (green) was visualized with the lipophilic dye DiO injected into the circulation. PVM and meningeal macrophages (blue), located in the perivascular and subarachnoid space, respectively, are visualized by CD206 immunocytochemistry.

Figure 5. Neurovascular associations along the cerebrovascular tree: Intraparenchymal arterioles and capillaries

As the arterioles penetrate deeper into the brain the perivascular space disappears and the vessels become encased in astrocytic end feet (intraparenchymal arterioles). Endowed with a single layer of SMC, intraparenchymal arterioles lack perivascular nerves. Occasionally, neural processes originating from local neurons or subcortical pathways projecting to the cerebral cortex terminate near the vessel. Capillaries are devoid of SMC but are endowed with pericytes, which are fully enclosed by the basement membrane of the endothelium. Occasional neurovascular contacts similar to those seen in intraparenchymal arterioles are also observed.

Figure 6. Potential feed-forward and feed-back mechanisms driving the local vascular response evoked by synaptic activity

Glutamate released by synaptic activity activates postsynaptic glutamate receptors (GluR) leading to activation of Ca²⁺-dependent signaling pathways resulting in the release of vasoactive factors that may drive the initial “feed-forward” component (metabolism independent) of the local vascular response in arterioles and capillaries. At the same time a reduction in tissue O₂ caused by the increase energy consumption induced by activation leads to the accumulation of vasoactive metabolic byproducts that may drive a secondary feed-back component (metabolism dependent) to better match the flow response to the metabolic needs of the tissue.

Figure 7. Neurovascular coupling at the capillary level

Synaptic activity leads to release of extracellular K⁺ and increase O₂ utilization. In turn, K⁺ activates K_IR channels on endothelial cells, and pericytes leading to endothelial cell hyperpolarization, which is conducted retrogradely via gap junctions linking adjacent endothelial cells. At the same time the reduction in O₂ leads to increase deformability of red blood cells (RBC), reducing their viscosity and increasing capillary flow. The resulting shear stress on capillary endothelium may also contribute to their hyperpolarization. The role of astrocytes is less clear, but activation of K⁺ channels in astrocytes may also contribute to increase K⁺ in the vicinity of endothelial cells, and due to their link to adjacent astrocytes could also play a role in the retrograde propagation of the hyperpolarization. ATP released during neural activity could increase astrocytic Ca²⁺ via P2X1 receptors and relax contractile mural cells via reaction products of the COX pathways (Mishra et al., 2016), but as discussed in the text there is no consensus on the ability of capillaries to dilate. Neurovascular contacts from interneurons or subcortical pathways are not depicted (see figure 5).

Figure 8. Neurovascular coupling at the level of intraparenchymal arterioles

The propagated endothelial hyperpolarization arising from capillaries is transferred to smooth muscle cells (SMC) via myoendothelial junctions, resulting in their relaxation and vasodilatation. The signal continues to be transferred retrogradely to vessels upstream through gap junctions linking SMC as well as endothelial cells. The vasodilatation is complemented by vasoactive factors released by nearby activated neurons and astrocytes, which contributes to sustain the vasodilatation in its propagation to upstream vessels. In addition, the drop in intravascular pressure and increased flow velocity caused by the vasodilatation downstream may contribute to smooth muscle hyperpolarization and relaxation by activating the myogenic response/flow-mediated vasodilatation (FMV). Therefore, at the arteriolar level the vasodilatation has two components: propagated response from capillaries and local response from activated neurons and astrocytes. Neurovascular contacts from interneurons or subcortical pathways are not depicted (see figure 5). Ado: adenosine; Glu: glutamate; Prost: prostanoids.

Figure 9. Neurovascular coupling at the level of pial arteriole

At this level the vasodilation depends on the propagated endothelial and SMC hyperpolarization, and, possibly, activation of the myogenic response/flow mediated vasodilation (FMV). Therefore, at the level of pial arterioles the hemodynamic response is not directly related to synaptic activity, but depends on the retrograde propagation of vasodilatation arising from smaller arterioles and capillaries surrounding the activated area. Perivascular nerves are not depicted (see figure 4).

Figure 10. The perivascular space and its clearance pathways

The perivascular space surrounding penetrating arterioles is involved in the clearance of unwanted molecules form the brain, including, but not limited to, Aβ and tau. Products of cerebral activity and metabolism reach the perivascular space by diffusion and can be disposed of through different pathways. A transvascular pathway is thought to rely on scavenger receptors and molecular transporters that carry the unwanted molecules across the vascular wall. A perivascular pathway has long been proposed in which the molecules are transported retrogradely along vascular basement membranes reaching the subarachnoid space and eventually discharging into the cervical nodes. A paravascular (glymphatic) pathway relies on aquaporin-4 channels (AQP4) on astrocytic end-feet to allow the CSF to enter the interstitial space, creating a convective flow that clears unwanted molecules from the brain and feeds into the perivascular venous side eventually draining into dural lymphatics or the cribriform plate. Abbreviations: CD36: cluster of differentiation 36; LRP1: low-density lipoprotein receptor-related protein-1; PICALM: Phosphatidylinositol Binding Clathrin Assembly Protein; SR-B: scavenger receptor B.

Figure 11. Potential pathogenic mechanisms by which neurovascular dysfunction can cause brain dysfunction

Alterations of the NVU may lead to reductions in CBF below the threshold required for normal brain oxygenation leading to hypoxia. BBB dysfunction may alter the homeostasis of the brain internal milieu by limiting the delivery of glucose and other nutrients and impairing the clearance of unwanted metabolites through efflux transporters. Reduction in trophic factor production by NVU cells may increase neuronal and glial vulnerability and susceptibility to disease. Alterations of clearance pathways may promote the accumulation of molecules, such as Aβ and tau, leading to proteinopathies.