Cerebral blood flow regulation and neurovascular dysfunction in Alzheimer disease

Abstract

Cerebral blood flow (CBF) regulation is essential for normal brain function. The mammalian brain has evolved a unique mechanism for CBF control known as neurovascular coupling. This mechanism ensures a rapid increase in the rate of CBF and oxygen delivery to activated brain structures. The neurovascular unit is composed of astrocytes, mural vascular smooth muscle cells and pericytes, and endothelia, and regulates neurovascular coupling. This Review article examines the cellular and molecular mechanisms within the neurovascular unit that contribute to CBF control, and neurovascular dysfunction in neurodegenerative disorders such as Alzheimer disease.

Figure 1. A schematic representation of the neurovascular unit showing cellular elements regulating cerebral blood flow along the vascular tree

Different cell types of the neurovascular unit (NVU) including neurons, astrocytes, mural cells – vascular smooth muscle cells (VSMCs) and pericytes, and endothelium, regulate cerebral blood flow at different levels of the vascular tree. The cellular composition of the NVU differs along the vascular tree, but the principal cellular components all remain represented, as illustrated here. a) At the level of penetrating arteries, the NVU is composed of endothelial cells making up the inner layer of the vessel wall, covered by a thin extracellular basement membrane and ringed by one to three layers of VSMCs, and ensheathed by pia. The Virchow-Robin space containing the cerebrospinal fluid is between pia and the glia limitans formed by astrocytic endfeet. Both VSMCs and astrocytes are innervated by local neurons. b) Arterioles differ in that there is only one layer of VSMCs, and astrocyte coverage and innervation of the vessel wall and endothelial inner layer display continuity with penetrating arteries, and brain capillaries, above and below the arteriole level, respectively. In addition to VSMCs, precapillary arterioles may also contain transitional pericytes, a cell type between pericyte and VSMCs. c) At the capillary level, the NVU is composed of endothelial cells that share a common basement membrane with pericytes. Pericytes stretch their processes along and around capillaries and make direct interdigitated or “peg-socket”-like contacts with endothelial cells. Pericytes and endothelial cells are covered by astrocyte endfeet. Both astrocytes and pericytes are innervated by local neurons similar as shown for astrocytes and VSMCs in the upper segments of the vascular tree.

Figure 2. Arteriolar regulation of cerebral blood flow

Neurons. Nitric oxide (NO) is a major moderator of functional hyperemia. NO produced by neurons acts directly on VSMCs, leading to VSMC hyperpolarization and relaxation. Adenosine triphosphate (ATP) and adenosine released by neuronal activity can also act directly on VSMCs through purinergic P2XR and P2YR receptors resulting in constriction, or adenosine 2A receptors (A2R) resulting in relaxation, respectively. Neuronal-mediated large increases in extracellular [K⁺] activate VSMC voltage-gated calcium channels (VGCCs), resulting in VSMCs intracellular [Ca²⁺] increases, leading to depolarization and contraction. Astrocytes. The role of astrocytes in neurovascular coupling to arterioles is controversial. Glutamate or ATP released from neurons are postulated to act on metabotropic glutamate receptors (mGluR) or P2YR on astrocytes, respectively, to initiate 1,4,5-trisphosphate (IP₃)-dependent intracellular [Ca²⁺] increase, which has been shown by some studies to contribute to neurovascular coupling, but disputed by others. According to some studies, intracellular [Ca²⁺] rise launches signaling cascades in the astrocytes and release of vasoactive ions and molecules from astrocyte endfeet to VSMCs, mainly K⁺ ions from large conductance calcium- activated potassium channels (BK_Ca), arachidonic acid (AA), through phospholipase A2 (PLA₂) pathway, and AA metabolic products epoxyeicosatetraenoic acids (EETs), and prostaglandin E2 (PGE₂), via cytochrome P450 (p450) and cyclooxygenase 1 (Cox1), respectively. Extracellular Ca²⁺ intake from transient receptor potential vanilloid channel 4 (TRPV4) provides another means of increasing intracellular [Ca²⁺]. Dashed lines and question marks indicate pathways for which there is limited or conflicting data in the literature. VSMCs. EETs and moderate increases in extracellular [K⁺] both act on VSMC potassium channels including BK_Ca and inward rectifier potassium channels (K_IR), resulting in hyperpolarization and relaxation of the VSMCs (left). However, large increases in extracellular [K⁺] activate VGCCs, resulting in intracellular [Ca²⁺] increases leading to VSMCs depolarization and contraction (right). PGE₂, which acts through prostaglandin EP₄ receptors (EP₄R) on VSMCs, generates cyclic adenosine monophosphate (cAMP) from intracellular ATP, also producing hyperpolarization and relaxation. PGE₂ levels can be modulated by extracellular lactate levels, which can block reuptake of PGE₂ by prostaglandin transporters (PGT). Lactate levels depend on the oxygen content of the tissue. Conversely, AA taken in by VSMCs can be metabolized to 20-hydroxyeicosatetraenoic acid (20-HETE), a potent VSMC depolarizer, resulting in VSMC contraction. NO released by neurons or endothelial cells can block VSMC 20-HETE production, modulating VSMC contraction and favoring relaxation through facilitation of conversion of guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP) via soluble guanylate cyclase (sGC). Adenosine released by neurons acts on A2R and blocks VSMC VGCC activation leading to VSMCs hyperpolarization and relaxation. In contrast, neuronal release of ATP to VSMCs P2XR and P2YR can increase intracellular [Ca²⁺], resulting in depolarization and contraction. Endothelium. Vasoactive compounds, including ATP, adenosine diphosphate (ADP), uridine triphosphate (UTP), acetylcholine (ACh), and bradykinin (BK), in the blood stream can bind their respective receptors (P2XR, P2YR, muscarinic M3R, and B2R) to initiate signaling pathways in endothelial cells similar to the pathways in astrocytes, with the addition of the diacyl-glycerol (DAG) AA pathway mediated by phospholipase C (PLC), generating vasoactive molecules that are released to VSMCs. Intracellular [Ca²⁺] increases initiated by receptor-mediated signaling of endothelium can produce endothelial AA, EETs, and prostacyclin (PGI₂), which act on VSMCs similarly to PGE₂, to generate VSMC hyperpolarization and relaxation. The intracellular [Ca²⁺] rise can also activate endothelial nitric oxide synthase (eNOS) leading to NO production, and endothelial K_Ca channels, releasing K⁺ that can act on VSMCs as well as hyperpolarizing the endothelium. Endothelial-derived hyperpolarizing factor (EDHF) can also trigger VSMC hyperpolarization. Additionally, shear stress on the endothelial vessel walls and red blood cells (RBCs) due to blood flow triggers ATP release from RBCs and signaling pathways, including activation of endothelial eNOS, and direct production of AA and its metabolites. Endothelial and endothelial-VSMCs gap junctions (GJs) facilitate retrograde endothelial signal propagation and signaling to VSMCs.

Figure 3. Capillary regulation of cerebral blood flow

Neuronal ATP can activate P2XRs on astrocytes to produce AA via phospholipase D 2 (PLD2)-mediated production of diacylglycerol (DAG) and subsequent metabolism by DAG lipase (DAGL). Cox1 can metabolize AA to produce PGE₂ via prostaglandin H2 (PGH2) and prostaglandin E synthase (PGES), triggering EP₄R on pericytes and leading to pericyte relaxation. AA produced in astrocytes can also diffuse into pericytes and form 20-HETE via p450 leading to pericyte depolarization and contraction. Neuronal activity releases glutamate, which has been shown to activate astrocytic mGluRs, triggering an IP₃–dependent increase in intracellular [Ca²⁺] leading to AA production via the PLA2 pathway. However, recent studies contradict this finding (indicated by dashed lines and question mark. Neurotransmitters adenosine, norepinephrine, and ATP have been demonstrated to alter pericyte contractile state. Specifically, adenosine binds A1 and A2 receptors (A1R, A2R) and activates KATP channels leading to hyperpolarization and relaxation. Also, activation of potassium channels including KCa and possibly KIR results in hyperpolarization of pericytes, decreasing Ca²⁺ entry through voltage-gated channels. Neuronal release of NO, which in pericytes inhibits AA metabolism to 20-HETE, can lead to pericyte relaxation. However, the role of NO in capillary dilation has been questioned (indicated by dashed lines and question mark). Norepinephrine acts through α2-adrenergic receptors (α2AR) leading to increased intracellular [Ca²⁺], depolarization and contraction. ATP activation of the purinergic receptors P2XR or P2YR on pericytes induces depolarizing currents, and increases intracellular [Ca²⁺] and pericyte contraction. Additionally, neuronal-mediated large increases in extracellular [K⁺] activates VGCCs, resulting in pericyte intracellular [Ca²⁺] increases, depolarization and contraction. Furthermore, several vasoconstrictors, including endothelin-1 (ET-1), platelet-derived growth factor-B (PDGF-BB), both secreted by vascular endothelial cells, or blood-derived insulin-like growth factor (IGF-1), act on their respective receptors (endothelin A receptors (ETAR), Pdgfrβ and IGFR-1) leading to depolarization of pericytes and Ca²⁺ entry into the cell. Endothelial-endothelial and pericyte-endothelial gap junctions allow fast and direct exchange of small molecules.

Figure 4. Neurovascular dysfunction in Alzheimer's disease: two-hit vascular hypothesis

Several genetic risk factors for Alzheimer's disease (AD) (e.g., apolipoprotein E4 gene (APOE4), Presenilin-1 mutations (PSEN1), Phosphatidylinositol Binding Clathrin Assembly Protein (PICALM), Clusterin (CLU), vascular factors (e.g., hypertension (HTN), diabetes mellitus (DM)), and environmental factors (e.g., pollution) lead to neurovascular dysfunction and damage to small arteries, arterioles and brain capillaries via amyloid-β (Aβ)-independent (hit 1, red) and/or Aβ-dependent (hit 2, blue) pathway. Both pathways interact and converge on blood vessels, and can independently or synergistically (purple lines) lead to neuronal injury, synaptic dysfunction and neurodegeneration contributing to dementia. Lifestyle can modify the effects of these hits; for example a moderate exercise and diet have beneficial effects on cardiovascular and cerebrovascular system. AD affects different cell types of the neurovascular unit. For example, VSMCs hypercontractility and degeneration leads to aberrant responses of small arteries and arterioles, cerebral blood flow (CBF) dysregulation and reductions independently of Aβ or in Aβ-dependent manner. In the Aβ pathway, damage to small arteries and arterioles are often associated with amyloid angiopathy, and rupture of the vessel wall with microhemorrhages. Degeneration of pericytes leads to loss of capillary dilation in response to neuronal stimuli, hypoperfusion and blood-brain barrier (BBB) breakdown with accumulation of blood-derived toxins and fluid in the perivascular spaces. Both, Aβ-independent (e.g., hypoxia, ischemia) and Aβ-dependent mechanisms contribute to changes in capillary circulation. Endothelial damage leads to loss of endothelial-dependent vasodilation, CBF dysregulation and reductions. Activation of astrocytes and microglia mediates inflammatory response and release of vasoactive cytokines and chemokines, further comprising CBF regulation and BBB integrity. Damage to blood vessels can initiate a cascade of events leading to Aβ accumulation in brain (hit 1), which accelerates the Aβ-dependent pathway of neurodegeneration (hit 2). For example, brain ischemic changes (hit 1) lead to increased Aβ production by stimulating expression of α and γ secretases, enzymes mediating Aβ generation, whereas BBB dysfunction in Aβ clearance receptors lipoprotein receptor and multidrug resistance protein 1 leads to faulty Aβ clearance and retention in brain. Reduced CBF (hit 1) and elevated Aβ (hit 2) can each independently or synergistically lead to tau phosphorylation (Phospho-tau) and tau pathology in neurons, and worsen neuroinflammation. When combined, they accelerate neuronal damage and injury.