Abhay P. Sagare

Pericytes Control Key Neurovascular Functions and Neuronal Phenotype in the Adult Brain and during Brain Aging

Pericytes play a key role in the development of cerebral microcirculation. The exact role of pericytes in the neurovascular unit in the adult brain and during brain aging remains, however, elusive. Using adult viable pericyte-deficient mice, we show that pericyte loss leads to brain vascular damage by two parallel pathways: (1) reduction in brain microcirculation causing diminished brain capillary perfusion, cerebral blood flow, and cerebral blood flow responses to brain activation that ultimately mediates chronic perfusion stress and hypoxia, and (2) blood-brain barrier breakdown associated with brain accumulation of serum proteins and several vasculotoxic and/or neurotoxic macromolecules ultimately leading to secondary neuronal degenerative changes. We show that age-dependent vascular damage in pericyte-deficient mice precedes neuronal degenerative changes, learning and memory impairment, and the neuroinflammatory response. Thus, pericytes control key neurovascular functions that are necessary for proper neuronal structure and function, and pericyte loss results in a progressive age-dependent vascular-mediated neurodegeneration.

LRP/Amyloid β-Peptide Interaction Mediates Differential Brain Efflux of Aβ Isoforms

Abstract LRP (low-density lipoprotein receptor-related protein) is linked to Alzheimers disease (AD). Here, we report amyloid β-peptide Aβ40 binds to immobilized LRP clusters II and IV with high affinity (K d = 0.6–1.2 nM) compared to Aβ42 and mutant Aβ, and LRP-mediated Aβ brain capillary binding, endocytosis, and transcytosis across the mouse blood-brain barrier are substantially reduced by the high β sheet content in Aβ and deletion of the receptor-associated protein gene. Despite low Aβ production in the brain, transgenic mice expressing low LRP-clearance mutant Aβ develop robust Aβ cerebral accumulations much earlier than Tg-2576 Aβ-overproducing mice. While Aβ does not affect LRP internalization and synthesis, it promotes proteasome-dependent LRP degradation in endothelium at concentrations >1 μM, consistent with reduced brain capillary LRP levels in Aβ-accumulating transgenic mice, AD, and patients with cerebrovascular β-amyloidosis. Thus, low-affinity LRP/Aβ interaction and/or Aβ-induced LRP loss at the BBB mediate brain accumulation of neurotoxic Aβ.

Apolipoprotein E controls cerebrovascular integrity via cyclophilin A

Human apolipoprotein E has three isoforms: APOE2, APOE3 and APOE4. APOE4 is a major genetic risk factor for Alzheimer’s disease and is associated with Down’s syndrome dementia and poor neurological outcome after traumatic brain injury and haemorrhage. Neurovascular dysfunction is present in normal APOE4 carriers and individuals with APOE4-associated disorders. In mice, lack of Apoe leads to blood–brain barrier (BBB) breakdown, whereas APOE4 increases BBB susceptibility to injury. How APOE genotype affects brain microcirculation remains elusive. Using different APOE transgenic mice, including mice with ablation and/or inhibition of cyclophilin A (CypA), here we show that expression of APOE4 and lack of murine Apoe, but not APOE2 and APOE3, leads to BBB breakdown by activating a proinflammatory CypA–nuclear factor-κB–matrix-metalloproteinase-9 pathway in pericytes. This, in turn, leads to neuronal uptake of multiple blood-derived neurotoxic proteins, and microvascular and cerebral blood flow reductions. We show that the vascular defects in Apoe-deficient and APOE4-expressing mice precede neuronal dysfunction and can initiate neurodegenerative changes. Astrocyte-secreted APOE3, but not APOE4, suppressed the CypA–nuclear factor-κB–matrix-metalloproteinase-9 pathway in pericytes through a lipoprotein receptor. Our data suggest that CypA is a key target for treating APOE4-mediated neurovascular injury and the resulting neuronal dysfunction and degeneration.

P-glycoprotein deficiency at the blood-brain barrier increases amyloid-β deposition in an Alzheimer disease mouse model

Accumulation of amyloid-beta (Abeta) within extracellular spaces of the brain is a hallmark of Alzheimer disease (AD). In sporadic, late-onset AD, there is little evidence for increased Abeta production, suggesting that decreased elimination from the brain may contribute to elevated levels of Abeta and plaque formation. Efflux transport of Abeta across the blood-brain barrier (BBB) contributes to Abeta removal from the brain. P-glycoprotein (Pgp) is highly expressed on the luminal surface of brain capillary endothelial cells and contributes to the BBB. In Pgp-null mice, we show that [I]Abeta40 and [I]Abeta42 microinjected into the CNS clear at half the rate that they do in WT mice. When amyloid precursor protein-transgenic (APP-transgenic) mice were administered a Pgp inhibitor, Abeta levels within the brain interstitial fluid significantly increased within hours of treatment. Furthermore, APP-transgenic, Pgp-null mice had increased levels of brain Abeta and enhanced Abeta deposition compared with APP-transgenic, Pgp WT mice. These data establish a direct link between Pgp and Abeta metabolism in vivo and suggest that Pgp activity at the BBB could affect risk for developing AD as well as provide a novel diagnostic and therapeutic target.

ApoE isoform-specific disruption of amyloid β peptide clearance from mouse brain

Neurotoxic amyloid beta peptide (Abeta) accumulates in the brains of individuals with Alzheimer disease (AD). The APOE4 allele is a major risk factor for sporadic AD and has been associated with increased brain parenchymal and vascular amyloid burden. How apoE isoforms influence Abeta accumulation in the brain has, however, remained unclear. Here, we have shown that apoE disrupts Abeta clearance across the mouse blood-brain barrier (BBB) in an isoform-specific manner (specifically, apoE4 had a greater disruptive effect than either apoE3 or apoE2). Abeta binding to apoE4 redirected the rapid clearance of free Abeta40/42 from the LDL receptor-related protein 1 (LRP1) to the VLDL receptor (VLDLR), which internalized apoE4 and Abeta-apoE4 complexes at the BBB more slowly than LRP1. In contrast, apoE2 and apoE3 as well as Abeta-apoE2 and Abeta-apoE3 complexes were cleared at the BBB via both VLDLR and LRP1 at a substantially faster rate than Abeta-apoE4 complexes. Astrocyte-secreted lipo-apoE2, lipo-apoE3, and lipo-apoE4 as well as their complexes with Abeta were cleared at the BBB by mechanisms similar to those of their respective lipid-poor isoforms but at 2- to 3-fold slower rates. Thus, apoE isoforms differentially regulate Abeta clearance from the brain, and this might contribute to the effects of APOE genotype on the disease process in both individuals with AD and animal models of AD.

Transport pathways for clearance of human Alzheimer’s amyloid β-peptide and apolipoproteins E and J in the mouse central nervous system

Amyloid β-peptide (Aβ) clearance from the central nervous system (CNS) maintains its low levels in brain. In Alzheimers disease, Aβ accumulates in brain possibly because of its faulty CNS clearance and a deficient efflux across the blood—brain barrier (BBB). By using human-specific enzyme-linked immunosorbent assays, we measured a rapid 30 mins efflux at the BBB and transport via the interstitial fluid (ISF) bulk flow of human-unlabeled Aβ and of Aβ transport proteins, apolipoprotein E (apoE) and apoJ in mice. We show (i) Aβ40 is cleared rapidly across the BBB via low-density lipoprotein receptor-related protein (LRP)1 at a rate of 0.21 pmol/min g ISF or 6-fold faster than via the ISF flow; (ii) Aβ42 is removed across the BBB at a rate 1.9-fold slower compared with Aβ40; (iii) apoE, lipid-poor isoform 3, is cleared slowly via the ISF flow and across the BBB (0.03–0.04 pmol/min g ISF), and after lipidation its transport at the BBB becomes barely detectable within 30 mins; (iv) apoJ is eliminated rapidly across the BBB (0.16 pmol/ming ISF) via LRP2. Clearance rates of unlabeled and corresponding 125I-labeled Aβ and apolipoproteins were almost identical, but could not be measured at low physiologic levels by mass spectrometry. Amyloid β-peptide 40 binding to apoE3 reduced its efflux rate at the BBB by 5.7-fold, whereas Aβ42 binding to apoJ enhanced Aβ42 BBB clearance rate by 83%. Thus, Aβ, apoE, and apoJ are cleared from brain by different transport pathways, and apoE and apoJ may critically modify Aβ clearance at the BBB.

Blood-brain barrier breakdown in the aging human hippocampus.

UNLABELLED The blood-brain barrier (BBB) limits entry of blood-derived products, pathogens, and cells into the brain that is essential for normal neuronal functioning and information processing. Post-mortem tissue analysis indicates BBB damage in Alzheimers disease (AD). The timing of BBB breakdown remains, however, elusive. Using an advanced dynamic contrast-enhanced MRI protocol with high spatial and temporal resolutions to quantify regional BBB permeability in the living human brain, we show an age-dependent BBB breakdown in the hippocampus, a region critical for learning and memory that is affected early in AD. The BBB breakdown in the hippocampus and its CA1 and dentate gyrus subdivisions worsened with mild cognitive impairment that correlated with injury to BBB-associated pericytes, as shown by the cerebrospinal fluid analysis. Our data suggest that BBB breakdown is an early event in the aging human brain that begins in the hippocampus and may contribute to cognitive impairment. VIDEO ABSTRACT

ALS-causing SOD1 mutants generate vascular changes prior to motor neuron degeneration.

We report here that amyotrophic lateral sclerosis–linked superoxide dismutase 1 (SOD1) mutants with different biochemical characteristics disrupted the blood–spinal cord barrier in mice by reducing the levels of the tight junction proteins ZO-1, occludin and claudin-5 between endothelial cells. This resulted in microhemorrhages with release of neurotoxic hemoglobin-derived products, reductions in microcirculation and hypoperfusion. SOD1 mutant–mediated endothelial damage accumulated before motor neuron degeneration and the neurovascular inflammatory response occurred, indicating that it was a central contributor to disease initiation.

A multimodal RAGE-specific inhibitor reduces amyloid β–mediated brain disorder in a mouse model of Alzheimer disease

In Alzheimer disease (AD), amyloid β peptide (Aβ) accumulates in plaques in the brain. Receptor for advanced glycation end products (RAGE) mediates Aβ-induced perturbations in cerebral vessels, neurons, and microglia in AD. Here, we identified a high-affinity RAGE-specific inhibitor (FPS-ZM1) that blocked Aβ binding to the V domain of RAGE and inhibited Aβ40- and Aβ42-induced cellular stress in RAGE-expressing cells in vitro and in the mouse brain in vivo. FPS-ZM1 was nontoxic to mice and readily crossed the blood-brain barrier (BBB). In aged APPsw/0 mice overexpressing human Aβ-precursor protein, a transgenic mouse model of AD with established Aβ pathology, FPS-ZM1 inhibited RAGE-mediated influx of circulating Aβ40 and Aβ42 into the brain. In brain, FPS-ZM1 bound exclusively to RAGE, which inhibited β-secretase activity and Aβ production and suppressed microglia activation and the neuroinflammatory response. Blockade of RAGE actions at the BBB and in the brain reduced Aβ40 and Aβ42 levels in brain markedly and normalized cognitive performance and cerebral blood flow responses in aged APPsw/0 mice. Our data suggest that FPS-ZM1 is a potent multimodal RAGE blocker that effectively controls progression of Aβ-mediated brain disorder and that it may have the potential to be a disease-modifying agent for AD.

Role of the MEOX2 homeobox gene in neurovascular dysfunction in Alzheimer disease.

Neurovascular dysfunction substantially contributes to Alzheimer disease. Here, we show that transcriptional profiling of human brain endothelial cells (BECs) defines a subset of genes whose expression is age-independent but is considerably altered in Alzheimer disease, including the homeobox gene MEOX2 (also known as GAX), a regulator of vascular differentiation, whose expression is low in Alzheimer disease. By using viral-mediated MEOX2 gene silencing and transfer, we show that restoring expression of the protein it encodes, GAX, in BECs from individuals with Alzheimer disease stimulates angiogenesis, transcriptionally suppresses AFX1 forkhead transcription factor–mediated apoptosis and increases the levels of a major amyloid-β peptide (Aβ) clearance receptor, the low-density lipoprotein receptor–related protein 1 (LRP), at the blood-brain barrier. In mice, deletion of Meox2 (also known as Gax) results in reductions in brain capillary density and resting cerebral blood flow, loss of the angiogenic response to hypoxia in the brain and an impaired Aβ efflux from brain caused by reduced LRP levels. The link of MEOX2 to neurovascular dysfunction in Alzheimer disease provides new mechanistic and therapeutic insights into this illness.