Neurovascular unit

The neurovascular unit (NVU) comprises the components of the brain that collectively regulate cerebral blood flow in order to deliver the requisite nutrients to activated neurons.^[1] The NVU addresses the brain's unique dilemma of having high energy demands yet low energy storage capacity. In order to function properly, the brain must receive substrates for energy metabolism–mainly glucose–in specific areas, quantities, and times.^[2] Neurons do not have the same ability as, for example, muscle cells, which can use up their energy reserves and refill them later; therefore, cerebral metabolism must be driven in the moment. The neurovascular unit facilitates this ad hoc delivery and, thus, ensures that neuronal activity can continue seamlessly.^[2]

The neurovascular unit was formalized as a concept in 2001, at the inaugural Stroke Progress Review Group of the National Institute of Neurological Disorders and Stroke (NINDS).^[1] In prior years, the importance of both neurons and cerebral vasculature was well known; however, their interconnected relationship was not. The two were long considered distinct entities which, for the most part, operated independently. Since 2001, though, the rapid increase of scientific papers citing the neurovascular unit represents the growing understanding of the interactions that occur between the brain’s cells and blood vessels.^[1]

The neurovascular unit consists of neurons, astrocytes, vasculature (endothelial and vascular mural cells), the vasomotor apparatus (smooth muscle cells and pericytes), and microglia.^[1] Together these function in the homeostatic haemodynamic response of cerebral hyperaemia.^[3] Cerebral hyperaemia is a fundamental central nervous system mechanism of homeostasis that increases blood supply to neural tissue when necessary.^[3] This mechanism controls oxygen and nutrient levels using vasodilation and vasoconstriction in a multidimensional process involving the many cells of the neurovascular unit, along with multiple signaling molecules.^[1] The interactions between the components of the NVU allow it to sense neurons' needs of oxygen and glucose and, in turn, trigger the appropriate vasodilatory or vasoconstrictive responses.^[3] Neuronal activity^[4] as well as astrocytes^[5] can therefore participate in CNV, both by inducing vasodilation and vasoconstriction^[6].Thus, the NVU provides the architecture behind neurovascular coupling, which connects neuronal activity to cerebral blood flow and highlights the interdependence of their development, structure, and function.^[1]

The temporal and spatial link between cerebral blood flow and neuronal activity allows the former to serve as a proxy for the latter. Neuroimaging techniques that directly or indirectly monitor blood flow, such as fMRI and PET scans, can, thus, measure and locate activity in the brain with precision.^[1] Imaging of the brain also allows researchers to better understand the neurovascular unit and its many complexities. Furthermore, any impediments to the function of the neurovascular system will prevent neurons from receiving the appropriate nutrients. A complete stoppage for only a few minutes, which could be caused by arterial occlusion or heart failure, can result in permanent damage and death. Dysfunction in the NVU is also associated with neurodegenerative diseases including Alzheimer's and Huntington's disease.^[1]

The neurovascular unit is made up of vascular cells (including endothelium, pericytes, and smooth muscle cells), glia (astrocytes and microglia), and neurons with synaptic junctions for signaling.^[1] Cerebral vessels, namely arterioles and the perivascular compartment, form the network of the NVU.^[7] Arterioles are made up of pial vessels and arterioles, and the perivascular compartment includes perivascular macrophages in addition to Mato, pial, and mast cells. Cerebral blood flow is a critical component of this overall system and it is facilitated by the neck arteries. Segmented vascular resistance, or the amount of flow control that each section of the brain maintains, is measured as the ratio of the blood pressure gradient to blood flow volume.^[8] The blood flow within the NVU is a low resistance channel that allows blood to be distributed to different parts of the body.^[9] The cells of the NVU sense the needs of neural tissue and release many different mediators that engage in signaling pathways and initiate effector systems such as the myogenic effect; these mediators trigger the vascular smooth muscle cells to increase blood flow through vasodilation or to reduce blood flow by vasoconstriction.^[3][1][10] This is recognized as a multidimensional response that operates across the cerebrovascular network as a whole.^[1]

The cells of the neurovascular unit also make up the blood–brain barrier (BBB), which plays an important role in maintaining the microenvironment of the brain.^[11] In addition to regulating the exit and entrance of blood, the blood–brain barrier also filters toxins that may cause inflammation, injury, and disease.^[12] The overall microvasculature unit functions as a defense for the central nervous system.^[11] Encompassed within the BBB are two types of blood vessels: endothelial and mural cells. Endothelial cells form the wall of the BBB, while mural cells exist on the outer surface of this layer of endothelial cells. The mural cells also have their own abluminal layer which hosts pericytes that work to maintain the permeability of the barrier, and the epithelial cells filter the amount of toxins entering. These cells connect to different segments of the vascular tree that exist within the brain.^[12]

Cellular processes critically rely on the production of adenosine triphosphate (ATP), which requires glucose and oxygen.^[13] These need to be delivered to areas in the brain with consistency via cerebral blood flow. In order for the brain to receive enough blood flow when in high demand, coupling occurs between neurons and CBF. Neurovascular coupling encompasses the changes in cerebral blood flow that occur in response to the level of neuronal activity.^[1][14][15] When the brain needs to exert more energy, there is an associated increase in the level of blood flow to compensate for this. The brain does not have a place where it stores energy, and, therefore, the response of blood flow has to be immediate so that crucial functions for continued life can persist. Difficulties arise when angiotensin proteins are present in higher concentrations, as there is an associated increase in blood flow that leads to hypertension and potential disorders.^[8] Furthermore, modern imaging techniques have allowed researchers to view and study cerebral blood flow in a noninvasive manner. However, imaging deep brain structures in vivo is challenging. Therefore, NVC can be studied on ex vivo brain slices maintained in survival conditions.^[16][17] Ultimately, neurovascular coupling promotes brain health by moderating proper cerebral blood flow. There is still much more to be discovered about it, though; and, due to the difficulty of in vivo research, the growing body of knowledge on neurovascular coupling relies heavily on ex vivo techniques for imaging the neurovascular unit.

The neurovascular unit enables imaging techniques to measure neuronal activity by tracking blood flow. Various other types of neuroimaging also allow the NVU itself to be studied by providing visual insights into the complex interactions between neurons, glial cells, and blood vessels in the brain.

Fluorescence microscopy is a widely used imaging technique that utilizes fluorescent probes to visualize specific molecules or structures within the neurovascular unit.^[18] It allows researchers to label and track cellular components, such as neurons, astrocytes, and blood vessel markers, with high specificity.^[19] Fluorescence imaging offers excellent spatial resolution, allowing for detailed visualization of cellular morphology and localized molecular interactions.^[20] By using different fluorophores, researchers can simultaneously examine multiple cellular components and molecular pathways of the neurovascular unit. However, limited tissue penetration depth, photobleaching, and phototoxicity negatively impact the potential for long-term imaging studies.^[20]

Electron microscopy provides details of the neurovascular unit at the nanometer scale by using a focused beam of electrons instead of light, enabling higher resolution imaging. Transmission electron microscopy images thin tissue sections, providing detailed information about the fine cellular structures, including synapses and organelles.^[21] Scanning electron microscopy, on the other hand, provides 3D information by scanning a focused electron beam across the sample's surface, allowing for the visualization of the topography of neurovascular unit components.^[22] Electron microscopy techniques are, thus, invaluable for studying the precise cellular and subcellular interactions within the NVU.^[23] However, it requires sample preparation involving fixation, dehydration, and staining, which can introduce artifacts, and it is not suitable for live or large-scale imaging due to its time-consuming nature.

Magnetic resonance imaging (MRI) is a non-invasive imaging technique that uses strong magnetic fields and radio waves to generate detailed images of the brain's anatomy and function.^[24] It can provide information about blood flow, oxygenation levels, and structural characteristics of the neurovascular unit. The functional MRI (fMRI) allows researchers to study brain activity by measuring changes in blood oxygenation associated with neural activity, thus classifying it as a blood-oxygen-level-dependent imaging (BOLD imaging) technique. Diffusion MRI (dMRI) provides insights into the brain's structural connectivity by tracking the diffusion of water molecules in its tissue.^[25] MRI, in general, has excellent spatial resolution and can be used for both human and animal studies, making it a valuable tool for studying the neurovascular unit in vivo. It has limited temporal resolution, though, and its ability to visualize finer cellular and molecular details within the neurovascular unit is relatively lower compared to microscopy techniques.

Optical coherence tomography (OCT) is an imaging technique that utilizes low-coherence interferometry to generate high-resolution cross-sectional images of biological tissues.^[26] It can, thus, provide information about the microstructure and vascular network of the neurovascular unit.^[27] More specifically, OCT has been used to study cerebral blood flow dynamics, changes in vessel diameter, and blood–brain barrier integrity. It also has real-time imaging capabilities and can, thus, be effectively applied in both clinical and preclinical settings.^[27] Downsides of optical coherence tomography include limited depth penetration in highly scattering tissues and a lower resolution in increasing depth, which can limit its application in deep brain regions.^[26]

Neurovascular failure, or neurovascular disease, refers to a range of conditions that negatively affect the function of blood vessels in the brain and spinal cord.^[28] While the exact mechanisms behind neurovascular disease are unknown, people with inherited conditions (such as a family history of heart disease, diabetes, and/or high cholesterol), poor lifestyle choices, genetic changes during pregnancy, physical trauma, and other specific genetic characteristics are generally at higher risk.^[28] In particular, neurovascular failure can be caused by problems arising in the blood vessels, including blockages (embolism), clot formation (thrombosis), narrowing (stenosis), and rupture (hemorrhage). In response to pathogenic stimuli, such as tissue hypoxia, signaling pathways involved in neurovascular coupling are impaired.^[29][30] Neuronal injury is often preceded by the expression and release of pro-angiogenic factors, such as vascular endothelial growth factor (VEGF); in addition to this, the upregulation of astrocyte receptors in endothelial cells can stimulate endothelial proliferation and migration, which can dangerously increase blood–brain barrier (BBB) permeability.^[29] Ultimately, vascular dysfunction results in decreased cerebral blood flow and abnormalities in the blood–brain barrier, which poses a threat to the normal functioning of the brain.^[31]

Efficient blood supply to the brain is extremely significant to its normal functioning, and improper blood flow can lead to potentially devastating neurological consequences.^[31] Alterations of vascular regulatory mechanisms lead to brain dysfunction and disease. The emerging view is that neurovascular dysfunction is a feature not only of cerebrovascular pathologies, such as stroke, but also of neurodegenerative conditions, such as Alzheimer's disease.^[32] While studies are still ongoing to determine the precise effects of neurovascular failure, there is emerging evidence that neurovascular dysfunction plays a pivotal role in the degeneration of the nervous system, which contrasts the typical view that neurodegeneration is caused by intrinsic neuronal effects.^[29][33] The breakdown of neurovascular coupling (e.g., modulations in neuronal activity that cause changes in local blood flow^[8]) and the pathophysiology of the NVU is commonly observed across a wide variety of neurological and psychiatric disorders, including Alzheimer’s disease.^[29] The combination of recent hypotheses and evidence suggests that the pathophysiology of the NVU may contribute to cognitive impairment and be an initiating trigger for neurological manifestations of diseases such as Alzheimer's and dementia.^[34][30] Ultimately, despite the vast amount of current literature supporting vascular contributions to neurological phenotypes, there is still much to be investigated, especially with respect to the effect of neurovasculature on neurological diseases; namely, whether the initiating event occurs at the neuronal level and "mobilizes" vascular response or the vascular event triggers neuronal dysfunction.^[29][30]

Alzheimer's disease (AD) is the most common type of dementia, a neurodegenerative disease with progressive impairment of behavioral and cognitive functions.^[35] Neuropathologically, there are two major indicators of Alzheimer's: neurofibrillary tangles (NFTs) and an accumulation of amyloid β peptide (Aβ) in the brain, known as amyloid plaques, or around blood vessels, known as amyloid angiopathy.^[36] There is growing support for the vascular hypothesis of AD, which posits that blood vessels are the origin for a variety of pathogenic pathways that lead to neuronal damage and AD.^[37] Vascular risk factors can result in dysregulation of the neurovascular unit and hypoxia. Destruction of the organization of the blood–brain barrier, decreased cerebral blood flow, and the establishment of an inflammatory context often result in neuronal damage since these factors promote the aggregation of β-amyloid peptide in the brain.^[37] During a review of various consortium data, it was shown that more than 30% of AD cases exhibit cerebrovascular disease on post-mortem examination, and almost all have evidence of cerebral amyloid angiopathy, microvascular degeneration, and white matter lesions.^[38] Despite this data, it is still insufficient to reach a pathologic diagnosis, making it unclear whether AD is a cause or a consequence of neuronal dysfunction.^[32][37] However, considering that AD seems to include a combination of vascular and neurodegenerative processes and that disruption to the vascular physiology occurs early in the disease process, targeting the vascular component may help potentially decelerate the pathologic progression of AD.^[39] Currently, only a few vascular targets have been the subject of large-scale randomised controlled trials.^[39]

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by an abnormal repetition of the CAG trinucleotide repeat within the Huntingtin gene (Htt).^[40] Common features of Huntington's include involuntary movements (chorea), bradykinesia, psychiatric symptoms, and cognitive decline, all of which are accelerated through neuronal cell death.^[41][42] The idea that neurovascular impairments may contribute to early neuronal cell loss in Huntington’s disease has been attracting significant attention in the HD community. Reduced cerebral blood flow, increased small vessel density, and increased blood–brain barrier (BBB) permeability–all traits of neurovascular dysfunction–have been reported in both rodent and patient post-mortem tissue.^[43][44][45] Preliminary findings support that neurovascular alterations occur in Huntington's disease and may contribute to its early neuropathology.^[46] It has also been proposed that neurovascular dysregulation manifests earlier in Huntington's than other pathologies, triggering innate immune signaling and a reduction of protein levels critical for maintaining the blood–brain barrier.^[47] While neurovascular failure in HD pathogenesis is still being tested, recent work supports clinical application. For example, immunohistological assays revealed vessel aberrations in brain tissue, establishing the early onset of such aberrations as a potential biomarker for early Huntington's diagnosis.^[33]