First proposed by Berislav Zlolovic in 2005, the essence of two hit theory is that vascular impairment precedes neurologic impairment. Vascular impairment both causing neurologic damage and causes accumulation of amyloid-beta. The increase in amyloid beta triggers hit two and hit two causes overwhelming neurologic impairment. Conventional AD theory, starts with what Zlokovic calls Hit 2. In APOE4 carriers, prevention of AD starts with intervention at stage of Hit One.
In the brain, pial arteries give rise to penetrating arteries which branch into smaller arteries, arterioles and brain capillaries distally. To ensure adequate vascular access the mammalian brain has evolved to become a dense vascular structure with the diffusion distance between neurons and adjacent capillary rarely exceeding 15 um (micrometer, one millionth of a meter). In humans the total perfusion vascular length of cerebrovascular tree is 600-700 km (@400 miles) with major contribution coming from dense capillary network.
The NVU is composed of endothelial cells, vascular mural cells, glial cells (astrocyte and microglia) and neurons. The inner lining is composed of endothelial cells which are attached to vascular basement membrane. In intracerebral ateries, on the brain side of basement membrane, vascular smooth muscle cells (VSMC) occupy most of vascular wall. At the level of brain cpillaries, pericytes replace VSMC and are attached to vascular basement membrane. At each level mural cells are further surrounded by astrocyte end feet and are in close proximity to neurons and microglia.
In the brain capillary, the endothelial cells that form the BBB are connected by tight and adherens junctions which confer the low permeability of BBB. Small lipophilic molecules, oxygen and carbon dioxide diffuse freely across endothelial cells. Larger molecules including glucose require transporters. Endothelial cells have high number of mitochondria for high-energy demand transport.
Pericytes share basement membrane with endothelium. The pericytes are comprised of a cell body and elongated, multiple finger-like cytoplasmic processes which cover the abluminal side of brain capillaries. The pericytes play the major role in maintenance of health of cerebral capillaries. Pericytes play a major role in development of AD in hit one stage in APOE4 carries.
How APOE4 triggers early pericyte dysfunction and BBB breakdown: Apoliporoteins are secreted by astrocytes. ApoE2 and ApoE3 bind well to LRP-1 (lipoprotein receptor-related protein-1), (a transport protein). ApoE4 exhibits weak binding to LRP-1 which results in pathologic elevations of CypA. Elevation of CypA increases pro-inflmmatory nuclear factor-kB (NF-kB) which causes elevation of pericyte MMP-9. Secretion and activation of MMP-9 degrades endothelial tight and adherens proteins leading to disruption of BBB. The resulting vascular injury may then lead to neuronal dysfunction and degeneration.
THIS IS MOLECULAR PATHWAY HOW APOE4 CAUSES AD.
Pericyte degeneration leads to BBB disruption and microhemorrhages and unrestricted entry and accumulation of blood-derived products in brain including RBC-derived hemoglobin and plasma-derived proteins such as albumin, plasmin, thrombin, fibrin and immunoglubulins
Plasmin and thrombin have direct neurotoxic properties, wheras fibrin accelerates neurovascular injury. Brain degradation of hemoglobin liberates free iron which catalyzes formation of reactive oxygen specie (ROS) leading to further injury. Albumin increases oncotic pressure resulting in edema, microvascular compression and reduced blood flow.
Pericyte loss also leads to endothelial cell death and microvascular regression leading to additional simultaneous reductions in blood flow. In mouse models, vascular injury in absence of AB as a result of pericyte loss is sufficient for neurodegeneration. (Figure from Zlokovic, 2014)
Brain pericyte loss as a result of disruption of the platelet derived growth factor receptor B (PDGFRB) signaling triggers early AB-independent vascular injury leading to microvascular loss and disruption which results in hypoperfusion/hypoxia and brain accumulation of toxic plasma proteins, respectively (black).
Loss of pericytes also leads to diminished clearance of soluble AB species from brain interstitial fluid (ISF) and further elevations in brain AB levels (red). AB may then overwhelm degradation pathways in surviving pericytes resulting in further pericyte degeneration (dashed lines).
Both pericyte loss and AB acting simultaneously result in early development of all facets of Alzheimer's disease neuropathology including AB plaques and neuronal tau pathology, degeneration and loss, which are not observed at an early disease stage when AB accumulation or pericyte-driven vascular injury occurs in isolation. (From Zlokovic, 2014) (33))