Bu, Guojun

Background and Purpose— Age-related changes in the cerebrovasculature, including blood–brain barrier (BBB) disruption, are emerging as potential risks for diverse neurological conditions. Because the accumulation of senescent cells in tissues is increasingly recognized as a critical step leading to age-related organ dysfunction, we evaluated whether senescent vascular cells are associated with compromised BBB integrity. Methods— Effects of vascular cell senescence on tight junction and barrier integrity were studied using an in vitro BBB model, composed of endothelial cells, pericytes, and astrocytes. In addition, tight junction coverage in microvessels and BBB integrity in BubR1 hypomorphic (BubR1 H/H ) mice, which display senescence cell-dependent phenotypes, were examined. Results— When an in vitro BBB model was constructed with senescent endothelial cells and pericytes, tight junction structure and barrier integrity (evaluated by transendothelial electric resistance and tracer efflux assay using sodium fluorescein and Evans blue-albumin were significantly impaired. Endothelial cells and pericytes from BubR1 H/H mice had increased senescent-associated β-galactosidase activity and p16 INK4a expression, demonstrating an exacerbation of senescence. The coverage by tight junction proteins in the cortical microvessels were reduced in BubR1 H/H mice, consistent with a compromised BBB integrity from permeability assays. Importantly, the coverage of microvessels by end-feet of aquaporin 4–immunoreactive astrocytes was not altered in the cortex of the BubR1 H/H mice. Conclusions— Our results indicate that accumulation of senescent vascular cells is associated with compromised BBB integrity, providing insights into the mechanism of BBB disruption related to biological aging.

Background and purposeLow density lipoprotein receptor related proteins (LRPs) 1 and 6 have been implicated in cerebral ischaemia. In addition, genetic variation in LRP1 and LRP6 has been linked with various factors that are related to risk of ischaemic stroke. The aim of this study was to examine the association of LRP1 and LRP6 gene variants with risk of ischaemic stroke as part of the Ischemic Stroke Genetics Study (ISGS).MethodsA Caucasian series (434 stroke patients, 319 controls) and an African American series (161 stroke patients, 116 controls) were included. Fourteen LRP6 variants and three LRP1 variants were genotyped and assessed for association with ischaemic stroke.ResultsIn the Caucasian series, significant associations with ischaemic stroke were observed for LRP6 rs2075241 [odds ratio (OR) 0.42, P = 0.023], rs2302685 (OR 0.44, P = 0.049), rs7975614 (OR 0.07, P = 0.017), rs10492120 (OR 0.62, P = 0.036) and rs10743980 (OR 0.66, P = 0.037). Risk of ischaemic stroke was significantly lower for carriers of any of these five protective LRP6 variants (24.0% of subjects) compared to non‐carriers (OR 0.57, P = 0.003). The protective association for LRP6 rs2075241 was observed at a similar magnitude across ischaemic stroke subtypes, whilst the effects of rs23022685, rs10492120 and rs10743980 were most apparent for cardioembolic and large vessel stroke. In the African American series, LRP1 rs11172113 was associated with an increased risk of stroke (OR 1.89, P = 0.006).ConclusionsThe results of our preliminary study provide evidence that LRP6 and LRP1 variants may be associated with risk of ischaemic stroke. Validation in larger studies is warranted.

Significance Endocytic sorting of amyloid precursor protein (APP) governed by the vacuolar protein sorting (Vps10) family of receptors plays a decisive role in controlling the outcome of APP proteolytic processing and the generation of amyloid β (Aβ) peptides, the key pathogenic factor of Alzheimer's disease (AD). This study provides the first evidence to our knowledge that a G protein-coupled receptor, namely, the α 2A adrenergic receptor, modulates APP endocytic sorting and promotes Aβ generation through disrupting APP interaction with a Vps10 family protein, sorting-related receptor with A repeat. Significantly, blockade of α 2A AR by a clinical antagonist reduces AD-related pathology and ameliorates cognitive deficits in an AD transgenic model, suggesting repurposing clinical α 2 AR antagonists as a new direction for AD treatment.

Significance It has been proposed that differential physical interactions of apolipoprotein E (apoE) isoforms with soluble amyloid-β (Aβ) in brain fluids influence the metabolism of Aβ, providing a major mechanism to account for how APOE influences Alzheimer’s disease risk. The current study challenges this proposal and clearly shows that lipoproteins containing apoE isoforms are unlikely to play a significant role in Aβ metabolism by binding directly to Aβ in physiological fluids such as cerebrospinal fluid or interstitial fluid. Our in vitro and in vivo results suggest that apoE isoforms influence Aβ metabolism by competing for the same clearance pathways within the brain.

Sorting nexin 17 (SNX17) is an adaptor protein present in early endosomal antigen 1 (EEA1)‐positive sorting endosomes that promotes the efficient recycling of low‐density lipoprotein receptor‐related protein 1 (LRP1) to the plasma membrane through recognition of the first NPxY motif in the cytoplasmic tail of this receptor. The interaction of LRP1 with SNX17 also regulates the basolateral recycling of the receptor from the basolateral sorting endosome (BSE). In contrast, megalin, which is apically distributed in polarized epithelial cells and localizes poorly to EEA1‐positive sorting endosomes, does not interact with SNX17, despite containing three NPxY motifs, indicating that this motif is not sufficient for receptor recognition by SNX17. Here, we identified a cluster of 32 amino acids within the cytoplasmic domain of LRP1 that is both necessary and sufficient for SNX17 binding. To delineate the function of this SNX17‐binding domain, we generated chimeric proteins in which the SNX17‐binding domain was inserted into the cytoplasmic tail of megalin. This insertion mediated the binding of megalin to SNX17 and modified the cell surface expression and recycling of megalin in non‐polarized cells. However, the polarized localization of chimeric megalin was not modified in polarized Madin‐Darby canine kidney cells. These results provide evidence regarding the molecular and cellular mechanisms underlying the specificity of SNX17‐binding receptors and the restricted function of SNX17 in the BSE.

Blood-brain barrier (BBB) dysfunction in acute liver failure (ALF) results in increased BBB permeability that often precludes the patients from obtaining a life-saving liver transplantation. It remains controversial whether matrix metalloproteinase-9 (MMP-9) from the injured liver contributes to the deregulation of BBB function in ALF. We selectively upregulated a physiologic inhibitor of MMP-9 (TIMP-1) with a single intracerebroventricular injection of TIMP-1 cDNA plasmids at 48 and 72 hours, or with pegylated-TIMP-1 protein. Acute liver failure was induced with tumor necrosis factor-α and D-(+)-galactosamine in mice. Permeability of BBB was assessed with sodium fluorescein (NaF) extravasation. We found a significant increase in TIMP-1 within the central nervous system (CNS) after the administration of TIMP-1 cDNA plasmids and that increased TIMP-1 within the CNS resulted in an attenuation of BBB permeability, a reduction in activation of epidermal growth factor receptor and p38 mitogen-activated protein kinase signals, and a restoration of the tight junction protein occludin in mice with experimental ALF. Pegylated TIMP-1 provided similar protection against BBB permeability in mice with ALF. Our results provided a proof of principle that MMP-9 contributes to the BBB dysfunction in ALF and suggests a potential therapeutic role of TIMP-1 in ALF.

Rationale: Among the extracellular modulators of Bmp (bone morphogenetic protein) signaling, Bmper (Bmp endothelial cell precursor-derived regulator) both enhances and inhibits Bmp signaling. Recently we found that Bmper modulates Bmp4 activity via a concentration-dependent, endocytic trap-and–sink mechanism. Objective: To investigate the molecular mechanisms required for endocytosis of the Bmper/Bmp4 and signaling complex and determine the mechanism of Bmper's differential effects on Bmp4 signaling. Methods and Results: Using an array of biochemical and cell biology techniques, we report that LRP1 (LDL receptor-related protein 1), a member of the LDL receptor family, acts as an endocytic receptor for Bmper and a coreceptor of Bmp4 to mediate the endocytosis of the Bmper/Bmp4 signaling complex. Furthermore, we demonstrate that LRP1-dependent Bmper/Bmp4 endocytosis is essential for Bmp4 signaling, as evidenced by the phenotype of lrp1-deficient zebrafish, which have abnormal cardiovascular development and decreased Smad1/5/8 activity in key vasculogenic structures. Conclusions: Together, these data reveal a novel role for LRP1 in the regulation of Bmp4 signaling by regulating receptor complex endocytosis. In addition, these data introduce LRP1 as a critical regulator of vascular development. These observations demonstrate Bmper's ability to fine-tune Bmp4 signaling at the single-cell level, unlike the spatial regulatory mechanisms applied by other Bmp modulators.