The blood–brain barrier (BBB) is confined to the endothelium of brain capillaries and is indispensable for fluid homeostasis and neuronal function. In this study, we show that endothelial Wnt/β-catenin (β-cat) signaling regulates induction and maintenance of BBB characteristics during embryonic and postnatal development. Endothelial specific stabilization of β-cat in vivo enhances barrier maturation, whereas inactivation of β-cat causes significant down-regulation of claudin3 (Cldn3), up-regulation of plamalemma vesicle-associated protein, and BBB breakdown. Stabilization of β-cat in primary brain endothelial cells (ECs) in vitro by N-terminal truncation or Wnt3a treatment increases Cldn3 expression, BBB-type tight junction formation, and a BBB characteristic gene signature. Loss of β-cat or inhibition of its signaling abrogates this effect. Furthermore, stabilization of β-cat also increased Cldn3 and barrier properties in nonbrain-derived ECs. These findings may open new therapeutic avenues to modulate endothelial barrier function and to limit the devastating effects of BBB breakdown.

The adult quiescent blood-brain barrier (BBB), a structure organised by endothelial cells through interactions with pericytes, astrocytes, neurons and microglia in the neurovascular unit, is highly regulated but fragile at the same time. In the past decade, there has been considerable progress in understanding not only the molecular pathways involved in BBB development, but also BBB breakdown in neurological diseases. Specifically, the Wnt/β-catenin, retinoic acid and sonic hedgehog pathways moved into the focus of BBB research. Moreover, angiopoietin/Tie2 signalling that is linked to angiogenic processes has gained attention in the BBB field. Blood vessels play an essential role in initiation and progression of many diseases, including inflammation outside the central nervous system (CNS). Therefore, the potential influence of CNS blood vessels in neurological diseases associated with BBB alterations or neuroinflammation has become a major focus of current research to understand their contribution to pathogenesis. Moreover, the BBB remains a major obstacle to pharmaceutical intervention in the CNS. The complications may either be expressed by inadequate therapeutic delivery like in brain tumours, or by poor delivery of the drug across the BBB and ineffective bioavailability. In this review, we initially describe the cellular and molecular components that contribute to the steady state of the healthy BBB. We then discuss BBB alterations in ischaemic stroke, primary and metastatic brain tumour, chronic inflammation and Alzheimer's disease. Throughout the review, we highlight common mechanisms of BBB abnormalities among these diseases, in particular the contribution of neuroinflammation to BBB dysfunction and disease progression, and emphasise unique aspects of BBB alteration in certain diseases such as brain tumours. Moreover, this review highlights novel strategies to monitor BBB function by non-invasive imaging techniques focussing on ischaemic stroke, as well as novel ways to modulate BBB permeability and function to promote treatment of brain tumours, inflammation and Alzheimer's disease. In conclusion, a deep understanding of signals that maintain the healthy BBB and promote fluctuations in BBB permeability in disease states will be key to elucidate disease mechanisms and to identify potential targets for diagnostics and therapeutic modulation of the BBB.

Several platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) family members display C-terminal protein motifs that confer retention of the secreted factors within the pericellular space. To address the role of PDGF-B retention in vivo, we deleted the retention motif by gene targeting in mice. This resulted in defective investment of pericytes in the microvessel wall and delayed formation of the renal glomerulus mesangium. Long-term effects of lack of PDGF-B retention included severe retinal deterioration, glomerulosclerosis, and proteinuria. We conclude that retention of PDGF-B in microvessels is essential for proper recruitment and organization of pericytes and for renal and retinal function in adult mice. Received April 3, 2003; revised version accepted May 23, 2003. The control of cell migration and the formation of specific patterns during embryonic development are believed to depend, at least in part, on the precise spatial distribution of secreted growth and differentiation factors (GDFs). This is achieved by strictly localized and regulated synthesis and secretion of GDFs, but also by binding of the secreted GDFs to cell surface-and extracellular matrix molecules. One type of molecule strongly implicated in the regulation of GDF activities in vivo is the heparan sulphate proteoglycans ( Certain isoforms of platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) family members display positively charged stretches of amino acids residues at the C terminus. These stretches are included or excluded depending on alternative splicing or proteolytic processing (Eriksson and Alitalo 1999; Heldin and Westermark 1999). For VEGF-A, the long splice isoforms, which carry HSPG-binding domains, accumulate on the cell surface or in the extracellular matrix, whereas short VEGF-A is diffusible following cellular release (Park et al. 1993). The developmental role of HSPG binding of VEGF-A was recently addressed using mice in which the long VEGF-A splice isoforms were selectively ablated (Carmeliet et al. 1999;Ruhrberg et al. 2002;Stalmans et al. 2002). In these mice, extracellular VEGF-A distribution becomes more widespread, leading to changes in endothelial sprouting and branching, and to the formation of abnormal vascular patterns (Ruhrberg et al. 2002). In PDGF-A and PDGF-B, the HSPGbinding motifs do not affect receptor binding or biological activity of the recombinant proteins (Östman et al. 1989). However, in transfected cells, these motifs confer retention of the secreted growth factor to the surface of the producing cells. Conversely, absence of the retention motif leads to increased secretion of a diffusible protein that readily accumulates in the cell culture medium (LaRochelle et al. 1991;Östman et al. 1991;Raines and Ross 1992;Andersson et al. 1994). The retention motif also appears to limit the action range of PDGF-B in vivo, as suggested from experiments with transplanted keratinocytes transfected with PDGF-B expression vectors (Eming et al. 1999). ...