Neurovascular Unit Dysfunction and Neurodegenerative Disorders

1Neurodegenerative disorders, like Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS), are severe neurological disorders that severely affect the quality of life of patients and result in a heavy burden on the economy and society. AD symptoms include memory loss and cognitive impairment. The prevalence of AD patients is high. In 2020, there were approximately 4.7 million people with AD in the United States, and by 2050, the number is expected to triple. PD is the second most common neurodegenerative disorder. Common clinical manifestations include tremor, rigidity, and bradykinesia, which result in a heavy burden on patients and society. ALS is a common degenerative disease, which affects motor neurons and causes progressive atrophy of skeletal muscles, paralysis, and death. However, our understanding of the pathogenesis of these disorders is still limited.

2Numerous research studies have demonstrated that the diseases mentioned above are related to disruption of the neurovascular unit (NVU). The NVU, which is composed of vascular cells, glial cells, and neurons, plays an important role in maintaining the functional integrity of the blood–brain barrier (BBB) and regulating the volume of the cerebral blood flow (CBF). Disruption of the NVU may induce dysfunction of the BBB and decrease the CBF, which may contribute to the pathogenesis of neurodegenerative disorders. When the NVU is disrupted and CBF decreases, not only is the supply of oxygen and nutrients to the brain reduced but also the clearance of neurotoxic substance, such as β-amyloid (Aβ) and α-synuclein from brain parenchyma is diminished.

3In this review, we will discuss the roles of the NVU in the pathogenesis of neurodegenerative diseases, AD, PD, and ALS. Because the prevalence of Huntington’s disease, multiple sclerosis, and other neurodegenerative diseases is relatively low, we do not discuss research related to those diseases in this review. We will especially focus on AD because of significant research related to the close interaction between NVU dysfunction and AD. In this article, we describe the composition of the NVU as well as its function in molecular transport and CBF regulation. Moreover, in this study, we review changes in the NVU with respect to the pathogenesis of neurodegenerative disorders, including specific mechanisms in different neurodegenerative diseases. Finally, we review potential therapeutic targets associated with these neurovascular deficits.

4The NVU is composed of vascular cells (including endothelial cells, pericytes, and vascular smooth muscle cells), glial cells (astrocytes, microglia, and oligodendroglia), and neurons (Figure 1; ). The tube structure of the capillaries in the brain is formed by endothelial cells. The outside of the endothelial tubes is surrounded by pericytes and astrocyte end-feet. Moreover, endothelial tubes are surrounded by extracellular matrix that forms the basement membrane. Combined with neurons, all those mentioned above comprise the NVU. Tight junctions and adherens junctions connect endothelial cells and tight junctions limit the paracellular permeability of the BBB. There are several transmembrane proteins involved in constructing tight junctions, including claudin, occludin, junctional adhesion molecule, and zonula occludens-1 (ZO-1). Vascular endothelial (VE) cadherin is the principal cadherin that forms the adherens junction and mediates intercellular adhesion. Both tight and adherens junctions play key roles in the control of endothelial permeability. Tight junctions prevent free diffusion of proteins and seal the paracellular cleft between endothelial cells, whereas adherens junctions play a key role in cell-to-cell contacts and promote cell maturation.

5Pericytes cover the abluminal surface of capillaries and regulate blood flow by controlling the capillary diameter. Pericytes also clear toxic proteins to maintain the stability of the central nervous system (CNS) and play a key role in the formation of tight junctions.

6Astrocytes are the most abundant glial cells in CNS. In the NVU, astrocytes communicate with endothelial cells through their end-feet. Astrocytes combine neuronal activity with blood vessels in a process, termed neurovascular coupling. They respond to neuronal activity and deliver signals to regulate the CBF. Furthermore, astrocytes play a vital role in molecular transport and BBB integrity.

7The NVU is crucial for stabilizing the environment of the brain. Firstly, the continuous endothelial cells with tight junction, basement membrane, and end-feet of astrocytes in the NVU form the BBB. The BBB regulates which molecules or cells enter the brain and clear detrimental proteins from brain parenchyma to the peripheral circulatory system. Secondly, the NVU regulates the CBF in response to neuronal activity through neurovascular coupling, which ensures sufficient oxygen and nutrient delivery to brain tissue where they are needed.

8The most widely accepted pathophysiologic mechanism of AD is insoluble Aβ deposition in senile plaques. Aβ accumulation in the brain is highly related to the dysfunction of the NVU and is thought to be caused by the following two mechanisms, increased permeability of the BBB and reduced CBF.

9As mentioned above, adjacent endothelial cells are connected by tight and adherens junctions, which form the BBB to maintain homeostasis of the brain. Various studies using different experimental methods have shown that the BBB integrity is impaired in AD.

10The detection of plasma-derived proteins in brain parenchyma is an extensively used approach for detecting BBB breakdown. Plasma proteins, including prothrombin, are found in postmortem cortical tissues of AD patients, and leakage of proteins is more common in patients with at least one APOE4 allele. Using a novel dynamic contrast-enhanced MRI protocol to quantify BBB permeability, showed that BBB permeability was increased in patients with mild impaired cognitive function than in healthy controls. Furthermore, BBB dysfunction leads to decreased Aβ clearance in AD. There are several mechanisms related to BBB dysfunction, which may lead to amyloid burden in the brain (Figure 2).

11Firstly, decreased expression of low-density lipoprotein receptor-related protein 1 (LRP1) and P-glycoprotein (P-gp), together with increased expression of the receptor for advanced glycation end products (RAGE), is are observed in endothelial cells in AD patients. All these proteins are crucial in Aβ transport across the BBB. LRP1 is expressed on endothelial cells and can internalize Aβ on the abluminal side. The internalized Aβ is then transported into lysosome in endothelial cells for further degradation, and some internalized Aβ would be transferred to the luminal side by receptor-mediated transcytosis. P-gp is an ATP-dependent efflux transporter that is located on the luminal surface of endothelial cells. In a previous animal study, it was concluded that deficient expression of P-gp decreased Aβ clearance and increased Aβ deposition in the brain. RAGE is a member of immunoglobulin superfamily and can bind Aβ. RAGE mediates the entry of Aβ from peripheral vessels to the brain through the BBB. RAGE immunoreactivity in endothelial cells was significantly increased in postmortem AD brains compared with healthy controls. Increased expression of RAGE in endothelial cells leads to more influx of Aβ from the peripheral circulatory system to brain parenchyma.

12Secondly, tight junction proteins such as occludins, claudins, and ZO-1 are reduced in endothelial cells. As reported in previous studies, Aβ was responsible for changes in tight junction protein expression. It has been revealed that Aβ1-42 oligomers disrupt tight junctions and increase permeability of the BBB through reduction in the expression of occludin, claudin-5, and ZO-1 in endothelial cells.

13Decades before the onset of clinical symptoms, CBF in the cortex changed in AD patients. In AD and mild cognitive impairment patients, arterial spin-labeling MRI demonstrated reduced CBF in temporal and parietal cortices.

14The most widely accepted cause of CBF reduction in AD is the cholinergic-vascular hypothesis. This hypothesis postulates that CBF changes are due to changes in vascular innervation caused by neuronal loss, especially the loss of cholinergic innervation. In a previous study, an extensive reduction in cholinergic neurons in the temporal lobe cortex and hippocampus of postmortem AD brains was shown. Cholinergic neurons have a critical role in controlling the vascular tone in the brain. Acetylcholine binding to muscarinic receptors in vascular smooth muscle cells dilates arterioles. The deficit in neurovascular coupling leads to decreased CBF in the brain.

15In addition to disruption of neurovascular coupling, vascular abnormalities may lead to decreased CBF in AD. Vascular abnormalities, such as tortuous, kinking, looping, or twisting arterioles, are common in AD. Such morphological changes in arterioles are due to vessel wall thinning and vascular smooth muscle cells loss. String vessels, which are composed of connective tissue and lack endothelial cells, are not functional in maintaining BBB integrity. The density of string vessels is remarkably increased in the brain gray matter of AD patients, and this is related to decreased CBF in the brain.

16A decreased CBF also decreases clearance and increases the deposition of Aβ. It has been reported that parenchymal Aβ deposition and cerebral amyloid angiopathy burden are increased in animal models of cerebral hypoperfusion. Consistent with the data from animal experiments, postmortem human brains showed that the severity of cerebral amyloid angiopathy significantly correlated to cortical microinfarcts. Similar to the role of hypoperfusion in promoting Aβ accumulation, it has been reported that plasma Aβ increased after cardiac arrest in humans. Hypoperfusion can trigger accelerated deposition of Aβ. Hypoperfusion in the brain has been shown to dramatically increase the cleavage of Aβ from the amyloid precursor protein (APP), through upregulation of β- and γ-secretase, two enzymes that are required for the production of Aβ. Moreover, in addition to increased production of Aβ, decreased CBF leads to insufficient clearance of Aβ. It was found that about half of the Aβ clearance could be attributed to CBF and vascular-perivascular pathways.

17The effects of brain hypoxia and Aβ deposition are mutual. The accumulation of Aβ deteriorates cerebrovascular function, increases arterial vasoconstriction, and reduces CBF.

18The widely accepted pathological mechanism of PD involves the loss of dopaminergic neurons in the ventral tier mesencephalon. However, it is demonstrated that PD pathology not only is restricted to the dopaminergic system but also influences the noradrenergic, serotonergic, and cholinergic systems. Increasing evidence has shown that disruption of the BBB may play a critical role in the pathological mechanism of PD.

19Previous research has revealed that the BBB is disrupted in various toxin-induced PD models, including 6-OHDA and MPTP-treated mice. Other studies have demonstrated that the expression of α-synuclein is associated with increased permeability of the BBB. Several mechanisms can explain the BBB disruption in PD patients. Firstly, with increasing age, senile astrocytes and microglia produce various cytokines, chemokines (e.g., IL-6, IL-1β, and TNF-α), and reactive oxygen species (ROS), which disrupt the integrity of the BBB and lead to the rearrangement of tight junctions. Secondly, PD patients show reduced expression of P-gp in the midbrain, which is related to BBB dysfunction. It is thought that the decreased P-gp maybe related to an accumulation of α-synuclein and other neurotoxic substances in the brain. Decreased efflux membrane transport through P-gp leads to brain damage due to the accumulation of harmful substances. Furthermore, in PD patients, angiogenesis occurs; however, the newly created vessels are less likely to display the restrictive properties of the BBB. Hence, the poorly developed BBB cannot protect the parenchyma from toxic factors in the peripheral circulation.

20Amyotrophic lateral sclerosis is a lethal neurological disease involving rapid and progressive degeneration of motor neurons in the brain and spinal cord. Most patients die within 24–48 months after symptom onset. A small number of ALS patients have familial ALS, and of those, with a familial etiology, 20% have inherited superoxide dismutase-1 (SOD1) mutations, which induce the disease. For sporadic ALS, various pathological mechanisms have been proposed; however, the precise pathogenesis is still unclear. One widely accepted pathogenic mechanism of ALS is related to BBB and blood–spinal cord barrier (BSCB) impairment, which leads to motor neuron damage.

21Mice with ALS-linked SOD1 mutations have reduced levels of tight junction proteins, including ZO-1, occludin, and claudin-5, which disrupt the BBB and BSCB functions. Human lumbar spinal cords from ALS patients also demonstrate diminished expression of ZO-1 and occludin, which corresponds with the finding in animals. Another study showed dramatically reduced perivascular occludin, collagen IV, and astrocyte end-feet surrounded with endothelial cells in the postmortem spinal cord of ALS patients. Furthermore, the disruption of the BBB and BSCB occurs prior to motor neuron degeneration. In the NVU, the degeneration of tight junctions, impairment of endothelial cells, and reduction in astrocytic end-feet contribute to dysfunction of the BBB and BSCB. This leads to vascular leakage and the entry of harmful substances from the peripheral blood into the CNS parenchyma. The expression of two proteins important for endothelial cell function, GLUT-1 and CD146, is decreased in ALS. Additionally, in SOD1 mutated mice, blood flow in the cervical and lumbar spinal cord is decreased by about 30–45% before the onset of symptoms.

22In short, the NVU is impaired in ALS before the onset of clinical symptoms, and damage to the NVU plays a key role in the pathogenesis of ALS.

23Increasing evidence has shown the important role of the NVU in the pathogenesis of neurodegenerative diseases, including AD, PD, and ALS. Most of these neurodegenerative disorders are intractable, especially AD and ALS. Therefore, looking to vascular cells as potential drug targets for neurodegenerative disorders is a promising avenue of research.

24For instance, BBB breakdown and CBF reductions are critical in the pathogenesis of AD. As discussed before, downregulation of LRP1 expression is found in endothelial cells and pericytes of AD patients, and this significantly affects the clearance of Aβ across the BBB. Thus, increasing expression of LRP1 may be a promising therapeutic target for AD patients. Statins can reduce the risk of AD by decreasing Aβ levels. In addition, the molecular mechanism of how statins influence Aβ metabolism involves increasing the expression of LRP1 and accelerating Aβ clearance. Therefore, statins may be recommended for upregulation of LRP1. Recent research has found that treatment with 1,25-(OH)2-vitamin D3 increased the expression of LRP1 significantly both in vivo and in vitro. Therefore, sufficient vitamin D supplementation may be beneficial for Aβ clearance from the brain parenchyma to blood. Furthermore, loss of cholinergic innervation leads to decreased CBF in AD patients. Acetylcholinesterase inhibitors (AChEIs), like tacrine, galantamine, and donepezil, increase the acetylcholine concentrations and have been widely used for decades. AChEIs have been shown to increase blood perfusion in the frontal lobe and prevent the progression of cognitive impairment after 1 year of treatment.

25For abnormal angiogenesis, Aβ immunization therapy in Tg2576 mice has been shown to dramatically reduce the formation of non-functional vessels, which increase vascular permeability and lead to brain damage. In Aβ-immunized mice, amyloidogenesis-triggered angiogenesis was decreased, and the vascular density reverted to normal levels. However, early clinical trials using the Aβ vaccine in human AD showed unexpected negative side effects. Therefore, additional studies are required prior to the application of Aβ immunization therapy in AD patients.

26In a mouse model of AD, implantation of encapsulated VE growth factor (VEGF)-secreting cells resulted in increased vascularization and reduced Aβ deposition in the cerebral cortex. Furthermore, cognitive behavior improved after implantation of VEGF microcapsules. A recent study in mice showed that the transplantation of endothelial progenitor cells into the hippocampus increased microvessel density, whereas the deposition of Aβ senile plaque and hippocampal cell apoptosis was decreased. Transplantation of endothelial progenitor cells could also upregulate the expression of the tight junction proteins ZO-1, occludin, and claudin-5.

27Given that oxidative stress plays an important role in the underlying mechanism of PD, astrocytes as a part of NVU secrete several beneficial antioxidant compounds, including glutathione (GSH), superoxide dismutases (SODs), and ascorbate. These molecules are important for neuron survival during the neurodegenerative processes. Several studies have showed that GSH was critical for the protection of BBB integrity. Thus, supplementary of these molecules may be an effective treatment for PD.

28In this review, we describe the role of NVU dysfunction in the pathogenesis of several neurodegenerative diseases, including AD, PD, and ALS. BBB breakdown and CBF reduction influence the removal of harmful substances from the brain and play an important role in the onset of neurodegenerative disorders. A focus on the NVU for potential drug targets may be helpful to inform novel vascular-targeted therapies. Nevertheless, the mechanisms of BBB breakdown during neurodegenerative diseases need to be further elucidated.

29XY and CJ drafted the manuscript. AS reviewed and modified the manuscript. All authors agreed on the final version.

30The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.