Blood brain barrier (BBB) breakdown is not only a consequence of, but also contributes to many neurological disorders, including stroke and Alzheimer’s disease. How the basement membrane (BM) contributes to the normal functioning of the BBB remains elusive. Here we use conditional knockout mice and an acute adenovirus-mediated knockdown model to show that lack of astrocytic laminin, a brain-specific BM component, induces BBB breakdown. Using functional blocking antibody and RNAi, we further demonstrate that astrocytic laminin, by binding to integrin α2 receptor, prevents pericyte differentiation from the BBB-stabilizing resting stage to the BBB-disrupting contractile stage, and thus maintains the integrity of BBB. Additionally, loss of astrocytic laminin decreases aquaporin-4 (AQP4) and tight junction protein expression. Altogether, we report a critical role for astrocytic laminin in BBB regulation and pericyte differentiation. These results indicate that astrocytic laminin maintains the integrity of BBB through, at least in part, regulation of pericyte differentiation.

The blood–brain barrier (BBB) is a highly complex and dynamic structure, mainly composed of brain microvascular endothelial cells, pericytes, astrocytes and the basement membrane (BM). The vast majority of BBB research focuses on its cellular constituents. Its non-cellular component, the BM, on the other hand, is largely understudied due to its intrinsic complexity and the lack of research tools. In this review, we focus on the role of the BM in BBB integrity. We first briefly introduce the biochemical composition and structure of the BM. Next, the biological functions of major components of the BM in BBB formation and maintenance are discussed. Our goal is to provide a concise overview on how the BM contributes to BBB integrity.

Blood-Brain Barrier (BBB) is a dynamic structure that maintains the homeostasis of the brain and thus proper neurological functions. BBB compromise has been found in many pathological conditions, including neuroinflammation. Monocyte chemoattractant protein-1 (MCP1), a chemokine that is transiently and significantly up-regulated during inflammation, is able to disrupt the integrity of BBB and modulate the progression of various diseases, including excitotoxic injury and hemorrhage. In this review, we first introduce the biochemistry and biology of MCP1, and then summarize the effects of MCP1 on BBB integrity as well as individual BBB components.

SummaryPrevious studies have shown that plasmin cleaves monocyte chemoattractant protein 1 (MCP1; officially known as C-C motif chemokine 2, CCL2) at K104, and this cleavage enhances its chemotactic potency significantly. Accumulating evidence reveals that MCP1 also disrupts the integrity of the blood-brain barrier (BBB). Here, we show that K104Stop-MCP1, truncated at the K104 where plasmin would normally cleave, is more efficient than the full-length protein (FL-MCP1) in compromising the integrity of the BBB in in vitro and in vivo models. K104Stop-MCP1 increases the permeability of BBB in both wild-type mice and mice deficient for tissue plasminogen activator (tPA), which converts plasminogen into active plasmin, suggesting that plasmin-mediated truncation of MCP1 plays an important role in BBB compromise. Furthermore, we show that the mechanisms underlying MCP1-induced BBB disruption involve redistribution of tight junction proteins (occludin and ZO-1) and reorganization of the actin cytoskeleton. Finally, we show that the redistribution of ZO-1 is mediated by phosphorylation of ezrin-radixin-moesin (ERM) proteins. These findings identify plasmin as a key signaling molecule in the regulation of BBB integrity and suggest that plasmin inhibitors might be used to modulate diseases accompanied by BBB compromise.

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Laminin, a major component of the basement membrane, plays an important role in blood brain barrier regulation. At the neurovascular unit, brain endothelial cells, astrocytes, and pericytes synthesize and deposit different laminin isoforms into the basement membrane. It has been shown that laminin α4 (endothelial laminin) regulates vascular integrity at embryonic/neonatal stage, while astrocytic laminin maintains vascular integrity in adulthood. Here, we investigate the function of pericyte-derived laminin in vascular integrity. Using a conditional knockout mouse line, we report that loss of pericytic laminin leads to hydrocephalus and BBB breakdown in a small percentage (10.7%) of the mutants. Interestingly, BBB disruption always goes hand-in-hand with hydrocephalus in these mutants, and neither symptom is observed in the rest 89.3% of the mutants. Further mechanistic studies show that reduced tight junction proteins, diminished AQP4 expression, and decreased pericyte coverage are responsible for the BBB disruption. Together, these data suggest that pericyte-derived laminin is involved in the maintenance of BBB integrity and regulation of ventricular size/development.

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Ablation of astrocytic laminin disrupted the interaction between vascular smooth muscle cells and astrocytes, down-regulated contractile protein expression, and weakened vascular integrity in deep brain regions, leading to hemorrhage.