Blood-spinal cord barrier pericyte reductions contribute to increased capillary permeability

Chronic Arsenic Exposure Increases Aβ(1-42) Production and Receptor for Advanced Glycation End Products Expression in Rat Brain

Chronic arsenic exposure during development is associated with alterations of chemical transmission and demyelination, which result in cognitive deficits and peripheral neuropathies. At the cellular level, arsenic toxicity involves increased generation of reactive species that induce severe cellular alterations such as DNA fragmentation, apoptosis, and lipid peroxidation. It has been proposed that arsenic-associated neurodegeneration could evolve to Alzheimer disease in later life.1,2 In this study, the effects of chronic exposure to inorganic arsenic (3 ppm by drinking water) in Wistar rats on the production and elimination of Amyloid-β (Aβ) were evaluated. Male Wistar rats were exposed to 3 ppm of arsenic in drinking water from fetal development until 4 months of age. After behavioral deficits induced by arsenic exposure through contextual fear conditioning were verified, the brains were collected for the determination of total arsenic by inductively coupled plasma-mass spectrometry, the levels of amyloid precursor protein and receptor for advanced glycation end products (RAGE) by Western blot analysis as well as their transcript levels by RT-qPCR, Aβ_(1-42) estimation by ELISA assay and the enzymatic activity of β-secretase (BACE1). Our results demonstrate that chronic arsenic exposure induces behavioral deficits accompanied of higher levels of soluble and membranal RAGE and the increase of Aβ_(1-42) cleaved. In addition, BACE1 enzymatic activity was increased, while immunoblot assays showed no differences in the low-density lipoprotein receptor-related protein 1 (LRP1) receptor among groups. These results provide evidence of the effects of arsenic exposure on the production of Aβ_(1-42) and cerebral amyloid clearance through RAGE in an in vivo model that displays behavioral alterations. This work supports the hypothesis that early exposure to metals may contribute to neurodegeneration associated with amyloid accumulation.

Region-specific permeability of the blood-brain barrier upon pericyte loss

The blood-brain barrier (BBB) regulates differing needs of the various brain regions by controlling transport of blood-borne components from the neurovascular circulation into the brain parenchyma. The mechanisms underlying region-specific transport across the BBB are not completely understood. Previous work showed that pericytes are key regulators of BBB function. Here we investigated whether pericytes influence BBB permeability in a region-specific manner by analysing the regional permeability of the BBB in the pdgf-b ^ret/ret mouse model of pericyte depletion. We show that BBB permeability is heterogeneous in pdgf-b ^ret/ret mice, being significantly higher in the cortex, striatum and hippocampus compared to the interbrain and midbrain. However, we show that this regional heterogeneity in BBB permeability is not explained by local differences in pericyte coverage. Region-specific differences in permeability were not associated with disruption of tight junctions but may result from changes in transcytosis across brain endothelial cells. Our data show that certain brain regions are able to maintain low BBB permeability despite substantial pericyte loss and suggest that additional, locally-acting mechanisms may contribute to control of transport.

Oxidative stress and DNA damage after cerebral ischemia: Potential therapeutic targets to repair the genome and improve stroke recovery

The past two decades have witnessed remarkable advances in oxidative stress research, particularly in the context of ischemic brain injury. Oxidative stress in ischemic tissues compromises the integrity of the genome, resulting in DNA lesions, cell death in neurons, glial cells, and vascular cells, and impairments in neurological recovery after stroke. As DNA is particularly vulnerable to oxidative attack, cells have evolved the ability to induce multiple DNA repair mechanisms, including base excision repair (BER), nucleotide excision repair (NER) and non-homogenous endpoint jointing (NHEJ). Defective DNA repair is tightly correlated with worse neurological outcomes after stroke, whereas upregulation of DNA repair enzymes, such as APE1, OGG1, and XRCC1, improves long-term functional recovery following stroke. Indeed, DNA damage and repair are now known to play critical roles in fundamental aspects of stroke recovery, such as neurogenesis, white matter recovery, and neurovascular unit remodeling. Several DNA repair enzymes are essential for comprehensive neural repair mechanisms after stroke, including Polβ and NEIL3 for neurogenesis, APE1 for white matter repair, Gadd45b for axonal regeneration, and DNA-PKs for neurovascular remodeling. This review discusses the emerging role of DNA damage and repair in functional recovery after stroke and highlights the contribution of DNA repair to regenerative elements after stroke. This article is part of the Special Issue entitled 'Cerebral Ischemia'.

Promising neuroprotective strategies for traumatic spinal cord injury with a focus on the differential effects among anatomical levels of injury

Traumatic spinal cord injury (SCI) is a devastating condition of motor, sensory, and autonomic dysfunction. The significant cost associated with the management and lifetime care of patients with SCI also presents a major economic burden. For these reasons, there is a need to develop and translate strategies that can improve outcomes following SCI. Given the challenges in achieving regeneration of the injured spinal cord, neuroprotection has been at the forefront of clinical translation. Yet, despite many preclinical advances, there has been limited translation into the clinic apart from methylprednisolone (which remains controversial), hypertensive therapy to maintain spinal cord perfusion, and early decompressive surgery. While there are several factors related to the limited translational success, including the clinical and mechanistic heterogeneity of human SCI, the misalignment between animal models of SCI and clinical reality continues to be an important factor. Whereas most clinical cases are at the cervical level, only a small fraction of preclinical research is conducted in cervical models of SCI. Therefore, this review highlights the most promising neuroprotective and neural reparative therapeutic strategies undergoing clinical assessment, including riluzole, hypothermia, granulocyte colony-stimulating factor, glibenclamide, minocycline, Cethrin (VX-210), and anti-Nogo-A antibody, and emphasizes their efficacy in relation to the anatomical level of injury. Our hope is that more basic research will be conducted in clinically relevant cervical SCI models in order to expedite the transition of important laboratory discoveries into meaningful treatment options for patients with SCI.

Three-dimensional biomimetic vascular model reveals a RhoA, Rac1, and N-cadherin balance in mural cell-endothelial cell-regulated barrier function

The integrity of the endothelial barrier between circulating blood and tissue is important for blood vessel function and, ultimately, for organ homeostasis. Here, we developed a vessel-on-a-chip with perfused endothelialized channels lined with human bone marrow stromal cells, which adopt a mural cell-like phenotype that recapitulates barrier function of the vasculature. In this model, barrier function is compromised upon exposure to inflammatory factors such as LPS, thrombin, and TNFα, as has been observed in vivo. Interestingly, we observed a rapid physical withdrawal of mural cells from the endothelium that was accompanied by an inhibition of endogenous Rac1 activity and increase in RhoA activity in the mural cells themselves upon inflammation. Using a system to chemically induce activity in exogenously expressed Rac1 or RhoA within minutes of stimulation, we demonstrated RhoA activation induced loss of mural cell coverage on the endothelium and reduced endothelial barrier function, and this effect was abrogated when Rac1 was simultaneously activated. We further showed that N-cadherin expression in mural cells plays a key role in barrier function, as CRISPR-mediated knockout of N-cadherin in the mural cells led to loss of barrier function, and overexpression of N-cadherin in CHO cells promoted barrier function. In summary, this bicellular model demonstrates the continuous and rapid modulation of adhesive interactions between endothelial and mural cells and its impact on vascular barrier function and highlights an in vitro platform to study the biology of perivascular-endothelial interactions.

Pericytes of the neurovascular unit: key functions and signaling pathways

Pericytes are vascular mural cells embedded in the basement membrane of blood microvessels. They extend their processes along capillaries, pre-capillary arterioles and post-capillary venules. CNS pericytes are uniquely positioned in the neurovascular unit between endothelial cells, astrocytes and neurons. They integrate, coordinate and process signals from their neighboring cells to generate diverse functional responses that are critical for CNS functions in health and disease, including regulation of the blood-brain barrier permeability, angiogenesis, clearance of toxic metabolites, capillary hemodynamic responses, neuroinflammation and stem cell activity. Here we examine the key signaling pathways between pericytes and their neighboring endothelial cells, astrocytes and neurons that control neurovascular functions. We also review the role of pericytes in CNS disorders including rare monogenic diseases and complex neurological disorders such as Alzheimer's disease and brain tumors. Finally, we discuss directions for future studies.

Cerebral blood flow regulation and neurovascular dysfunction in Alzheimer disease

Cerebral blood flow (CBF) regulation is essential for normal brain function. The mammalian brain has evolved a unique mechanism for CBF control known as neurovascular coupling. This mechanism ensures a rapid increase in the rate of CBF and oxygen delivery to activated brain structures. The neurovascular unit is composed of astrocytes, mural vascular smooth muscle cells and pericytes, and endothelia, and regulates neurovascular coupling. This Review article examines the cellular and molecular mechanisms within the neurovascular unit that contribute to CBF control, and neurovascular dysfunction in neurodegenerative disorders such as Alzheimer disease.

Hyperbaric oxygen therapy of spinal cord injury

Spinal cord injury (SCI) is a complex disease process that involves both primary and secondary mechanisms of injury and can leave patients with devastating functional impairment as well as psychological debilitation. While no curative treatment is available for spinal cord injury, current therapeutic approaches focus on reducing the secondary injury that follows SCI. Hyperbaric oxygen (HBO) therapy has shown promising neuroprotective effects in several experimental studies, but the limited number of clinical reports have shown mixed findings. This review will provide an overview of the potential mechanisms by which HBO therapy may exert neuroprotection, provide a summary of the clinical application of HBO therapy in patients with SCI, and discuss avenues for future studies.

Novel method to study pericyte contractility and responses to ischaemia in vitro using electrical impedance

Pericytes are contractile vascular mural cells overlying capillary endothelium, and they have been implicated in a variety of functions including regulation of cerebral blood flow. Recent work has suggested that both in vivo and ex vivo, ischaemia causes pericytes to constrict and die, which has implications for microvascular reperfusion. Assessing pericyte contractility in tissue slices and in vivo is technically challenging, while in vitro techniques remain unreliable. Here, we used isolated cultures of human brain vascular pericytes to examine their contractile potential in vitro using the iCelligence electrical impedance system. Contraction was induced using the vasoactive peptide endothelin-1, and relaxation was demonstrated using adenosine and sodium nitroprusside. Endothelin-1 treatment also resulted in increased proliferation, which we were able to monitor in the same cell population from which we recorded contractile responses. Finally, the observation of pericyte contraction in stroke was reproduced using chemical ischaemia, which caused a profound and irreversible contraction clearly preceding cell death. These data demonstrate that isolated pericytes retain a contractile phenotype in vitro, and that it is possible to quantify this contraction using real-time electrical impedance recordings, providing a significant new platform for assessing the effects of vasoactive and vasculoprotective compounds on pericyte contractility.

Endothelial and Astrocytic Support by Human Bone Marrow Stem Cell Grafts into Symptomatic ALS Mice towards Blood-Spinal Cord Barrier Repair

Vascular pathology, including blood-CNS barrier (B-CNS-B) damage via endothelial cell (EC) degeneration, is a recently recognized hallmark of Amyotrophic Lateral Sclerosis (ALS) pathogenesis. B-CNS-B repair may be a new therapeutic approach for ALS. This study aimed to determine effects of transplanted unmodified human bone marrow CD34+ (hBM34+) cells into symptomatic G93A mice towards blood-spinal cord barrier (BSCB) repair. Thirteen weeks old G93A mice intravenously received one of three different doses of hBM34+ cells. Cell-treated, media-treated, and control mice were euthanized at 17 weeks of age. Immunohistochemical (anti-human vWF, CD45, GFAP, and Iba-1) and motor neuron histological analyses were performed in cervical and lumbar spinal cords. EB levels in spinal cord parenchyma determined capillary permeability. Transplanted hBM34+ cells improved behavioral disease outcomes and enhanced motor neuron survival, mainly in high-cell-dose mice. Transplanted cells differentiated into ECs and engrafted within numerous capillaries. Reduced astrogliosis, microgliosis, and enhanced perivascular end-feet astrocytes were also determined in spinal cords, mostly in high-cell-dose mice. These mice also showed significantly decreased parenchymal EB levels. EC differentiation, capillary engraftment, reduced capillary permeability, and re-established perivascular end-feet astrocytes in symptomatic ALS mice may represent BSCB repair processes, supporting hBM34+ cell transplantation as a future therapeutic strategy for ALS patients.

Wnt/TLR Dialog in Neuroinflammation, Relevance in Alzheimer's Disease

The innate immune system (IIS) represents the first line of defense against exogenous and endogenous harmful stimuli. Different types of pathogens and diverse molecules can activate the IIS via a ligand-receptor mechanism. Cytokine release, recruitment of immunocompetent cells, and inflammation constitute the initial steps in an IIS-mediated response. While balanced IIS activity can resolve a harmful event, an altered response, such as deficient or persistent IIS activity, will have a critical effect on organism homeostasis. In this regard, chronic IIS activation has been associated with a wide range of diseases, including chronic inflammatory disorders (inflammatory bowel disease, arthritis, chronic obstructive pulmonary disease, among others), cancer and, more recently, neurodegenerative disorders. The relevance of the immune response, particularly inflammation, in the context of neurodegeneration has motivated rigorous research focused on unveiling the mechanisms underlying this response. Knowledge regarding the molecular hallmarks of the innate immune response and understanding signaling pathway cross talk are critical for developing new therapeutic strategies aimed at modulating the neuroinflammatory response within the brain. In the present review, we discuss the IIS in the central nervous system, particularly the cross talk between the toll-like receptor-signaling cascade and the wingless-related MMTV integration site (Wnt) signaling pathway and its relevance in neurodegenerative disorders such as Alzheimer's disease.

Pericyte degeneration leads to neurovascular uncoupling and limits oxygen supply to brain

Pericytes are perivascular mural cells of brain capillaries that are positioned centrally within the neurovascular unit between endothelial cells, astrocytes and neurons. This unique position allows them to play a major role in regulating key neurovascular functions of the brain. The role of pericytes in the regulation of cerebral blood flow (CBF) and neurovascular coupling remains, however, debatable. Using loss-of-function pericyte-deficient mice, here we show that pericyte degeneration diminishes global and individual capillary CBF responses to neuronal stimulus resulting in neurovascular uncoupling, reduced oxygen supply to brain and metabolic stress. We show that these neurovascular deficits lead over time to impaired neuronal excitability and neurodegenerative changes. Thus, pericyte degeneration as seen in neurological disorders such as Alzheimer’s disease may contribute to neurovascular dysfunction and neurodegeneration associated with human disease.

Mechanisms responsible for the inhibitory effects of epothilone B on scar formation after spinal cord injury

Scar formation after spinal cord injury is regarded as an obstacle to axonal regeneration and functional recovery. Epothilone B provides moderate microtubule stabilization and is mainly used for anti-tumor therapy. It also reduces scar tissue formation and promotes axonal regeneration after spinal cord injury. The aim of the present study was to investigate the effect and mechanism of the microtubule-stabilizing reagent epothilone B in decreasing fibrotic scarring through its action on pericytes after spinal cord injury. A rat model of spinal cord injury was established via dorsal complete transection at the T10 vertebra. The rats received an intraperitoneal injection of epothilone B (0.75 mg/kg) at 1 and 15 days post-injury in the epothilone B group or normal saline in the vehicle group. Neuron-glial antigen 2, platelet-derived growth factor receptor β, and fibronectin protein expression were dramatically lower in the epothilone B group than in the vehicle group, but β-tubulin protein expression was greater. Glial fibrillary acidic protein at the injury site was not affected by epothilone B treatment. The Basso, Beattie, and Bresnahan locomotor scores were significantly higher in the epothilone B group than in the vehicle group. The results of this study demonstrated that epothilone B reduced the number of pericytes, inhibited extracellular matrix formation, and suppressed scar formation after spinal cord injury.

Regional early and progressive loss of brain pericytes but not vascular smooth muscle cells in adult mice with disrupted platelet-derived growth factor receptor-β signaling

Pericytes regulate key neurovascular functions of the brain. Studies in pericyte-deficient transgenic mice with aberrant signaling between endothelial-derived platelet-derived growth factor BB (PDGF-BB) and platelet-derived growth factor receptor β (PDGFRβ) in pericytes have contributed to better understanding of the role of pericytes in the brain. Here, we studied PdgfrβF7/F7 mice, which carry seven point mutations that disrupt PDGFRβ signaling causing loss of pericytes and vascular smooth muscle cells (VSMCs) in the developing brain. We asked whether these mice have a stable or progressive vascular phenotype after birth, and whether both pericyte and VSMCs populations are affected in the adult brain. We found an early and progressive region-dependent loss of brain pericytes, microvascular reductions and blood-brain barrier (BBB) breakdown, which were more pronounced in the cortex, hippocampus and striatum than in the thalamus, whereas VSMCs population remained unaffected at the time when pericyte loss was already established. For example, compared to age-matched controls, PdgfrβF7/F7 mice between 4-6 and 36-48 weeks of age developed a region-dependent loss in pericyte coverage (22-46, 24-44 and 4-31%) and cell numbers (36-49, 34-64 and 11-36%), reduction in capillary length (20-39, 13-46 and 1-30%), and an increase in extravascular fibrinogen-derived deposits (3.4-5.2, 2.8-4.1 and 0-3.6-fold) demonstrating BBB breakdown in the cortex, hippocampus and thalamus, respectively. Capillary reductions and BBB breakdown correlated with loss of pericyte coverage. Our data suggest that PdgfrβF7/F7 mice develop an aggressive and rapid vascular phenotype without appreciable early involvement of VSMCs, therefore providing a valuable model to study regional effects of pericyte loss on brain vascular and neuronal functions. This model could be a useful tool for future studies directed at understanding the role of pericytes in the pathogenesis of neurological disorders associated with pericyte loss such as vascular dementia, Alzheimer's disease, amyotrophic lateral sclerosis, stroke and human immunodeficiency virus-associated neurocognitive disorder.

The distinct abilities of tube-formation and migration between brain and spinal cord microvascular pericytes in rats

Pericytes are contractile cells that wrap around the endothelial cells of capillaries throughout the body. They play an important role in regulating the blood brain barrier (BBB) and blood spinal cord barrier (BSCB). The differences between brain and spinal cord microvascular endothelial cells have been investigated. However, no report has elucidated the similarities and differences between brain microvascular pericytes (BMPs) and spinal cord microvascular pericytes (SCMPs) in vitro. The similarities were found between the two types of pericytes not only in the proliferation ability but also in the expression of toll like receptor 4. On the other hand, BMPs showed more than 2 fold in tubular length formation compared with SCMPs. The number of migratory SCMPs was larger than that of migratory BMPs. The expressions of connexin 43 and vascular endothelial growth factor (VEGF) in BMPs were increased compared with those in SCMPs, while SCMPs expressed more desmin and N-cadherin than BMPs. The abilities of tube-formation and migration between BMPs and SCMPs were markedly different, which might be mediated by VEGF, connexin 43, N-cadherin and desmin. These distinguishing features may reflect the more widespread differences between the BBB and BSCB which directly impact pathophysiological processes in various major diseases.

Pericytes in Brain Injury and Repair After Ischemic Stroke

Pericytes are functional components of the neurovascular unit (NVU). They provide support to other NVU components and maintain normal physiological functions of the blood-brain barrier (BBB). The brain ischemia and reperfusion result in pathological alterations in pericytes. The intimate anatomical and functional interactions between pericytes and other NVU components play pivotal roles in the progression of stroke pathology. In this review, we depict the biology and functions of pericytes in the normal brain and discuss their effects in brain injury and repair after ischemia/reperfusion. Since ischemic stroke occurs mostly in elderly people, we also review age-related changes in pericytes and how these changes predispose aged brains to ischemic/reperfusion injury. Strategies targeting pericyte responses after ischemia and reperfusion may provide new therapies for ischemic stroke.

Gene expression comparison reveals distinct basal expression of HOX members and differential TNF-induced response between brain- and spinal cord-derived microvascular endothelial cells

Background

The heterogeneity of endothelial cell types underlies their remarkable ability to sub-specialize and provide specific requirements for a given vascular bed. Here, we compared rat microvascular endothelial cells (MECs) derived from the brain and spinal cord in both basal and inflammatory conditions.

Methods

We used whole rat genome microarrays to compare, at different time points, basal and TNF-α-induced gene expression of rat MECs from in vitro models of the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB). Validation at both messenger RNA (mRNA) and protein levels was performed on freshly extracted microvessels (MVs) from the brain and spinal cord (BMVs and SCMVs, respectively), as these were considered the closest in vivo tissues to cultured MECs.

Results

Most of the genes encoding adhesion/tight junction molecules and known endothelial markers were similarly expressed in brain and spinal cord MECs (BMECs and SCMECs, respectively). However, one striking finding was the higher expression of several Hox genes, which encode transcription factors involved in positional identity. The differential expression of Hoxa9 and Hoxb7 at the mRNA levels as well as protein levels was confirmed in BMVs and SCMVs. Although the TNF-α response was in general higher in BMECs than in SCMECs at 12 h, the opposite was observed at 48 h. Furthermore, we found that expression of Tnfrsf1a and Tnfrsf1b encoding the TNF receptor super-family member 1a/TNFR1 and 1b/TNFR2, respectively, were constitutively higher in BMVs compared to SCMVs. However, only Tnfrsf1b was induced in SCMECs in response to TNF-α at 24 and 48 h.

Conclusions

Our results support a role for HOX members in defining the positional identities of MECs in vivo. Our data also suggest that the delayed transcriptional activation upon TNF-α treatment in SCMECs results from the requirement of the TNF-induced expression of Tnfrsf1b. In contrast, its high basal expression in BMECs might be sufficient to confer an immediate and efficient TNF-α response.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0749-6) contains supplementary material, which is available to authorized users.

Interferon-γ blocks signalling through PDGFRβ in human brain pericytes

BACKGROUND

Neuroinflammation and blood-brain barrier (BBB) disruption are common features of many brain disorders, including Alzheimer's disease, epilepsy, and motor neuron disease. Inflammation is thought to be a driver of BBB breakdown, but the underlying mechanisms for this are unclear. Brain pericytes are critical cells for maintaining the BBB and are immunologically active. We sought to test the hypothesis that inflammation regulates the BBB by altering pericyte biology.

METHODS

We exposed primary adult human brain pericytes to chronic interferon-gamma (IFNγ) for 4 days and measured associated functional aspects of pericyte biology. Specifically, we examined the influence of inflammation on platelet-derived growth factor receptor-beta (PDGFRβ) expression and signalling, as well as pericyte proliferation and migration by qRT-PCR, immunocytochemistry, flow cytometry, and western blotting.

RESULTS

Chronic IFNγ treatment had marked effects on pericyte biology most notably through the PDGFRβ, by enhancing agonist (PDGF-BB)-induced receptor phosphorylation, internalization, and subsequent degradation. Functionally, chronic IFNγ prevented PDGF-BB-mediated pericyte proliferation and migration.

CONCLUSIONS

Because PDGFRβ is critical for pericyte function and its removal leads to BBB leakage, our results pinpoint a mechanism linking chronic brain inflammation to BBB dysfunction.

Deficiency in matrix metalloproteinase-2 results in long-term vascular instability and regression in the injured mouse spinal cord

Angiogenesis plays a critical role in wound healing after spinal cord injury. Therefore, understanding the events that regulate angiogenesis has considerable relevance from a therapeutic standpoint. We evaluated the contribution of matrix metalloproteinase (MMP)-2 to angiogenesis and vascular stability in spinal cord injured MMP-2 knockout and wildtype (WT) littermates. While MMP-2 deficiency resulted in reduced endothelial cell division within the lesioned epicenter, there were no genotypic differences in vascularity (vascular density, vascular area, and endothelial cell number) over the first two weeks post-injury. However, by 21days post-injury MMP-2 deficiency resulted in a sharp decline in vascularity, indicative of vascular regression. Complementary in vitro studies of brain capillary endothelial cells confirmed MMP-2 dependent proliferation and tube formation. As deficiency in MMP-2 led to prolonged MMP-9 expression in the injured spinal cord, we examined both short-term and long-term exposure to MMP-9 in vitro. While MMP-9 supported endothelial tube formation and proliferation, prolonged exposure resulted in loss of tubes, findings consistent with vascular regression. Vascular instability is frequently associated with pericyte dissociation and precedes vascular regression. Quantification of PDGFrβ+ pericyte coverage of mature vessels within the glial scar (the reactive gliosis zone), a known source of MMP-9, revealed reduced coverage in MMP-2 deficient animals. These findings suggest that acting in the absence of MMP-2, MMP-9 transiently supports angiogenesis during the early phase of wound healing while its prolonged expression leads to vascular instability and regression. These findings should be considered while developing therapeutic interventions that block MMPs.

Inhibition of p21-Activated Kinase 1 by IPA-3 Promotes Locomotor Recovery After Spinal Cord Injury in Mice

STUDY DESIGN

Ninety-six male adult CD-1 mice were randomly divided into sham, spinal cord injury (SCI) + vehicle, and SCI + IPA-3 groups. Expression of matrix metalloproteinase (MMP)-2 and MMP-9, production of tumor necrosis factors (TNF)-α and interleukin (IL)-1β, tissue edema, blood-spinal cord barrier penetrability, neural cell apoptosis, and neurological function recovery were measured.

OBJECTIVE

The aim of the study was to evaluate the effect of specific inhibition of p21-activated kinase 1 (PAK1) by IPA-3 on SCI and the underlying mechanisms thereof.

SUMMARY OF BACKGROUND DATA

SCI is a devastating clinical condition that may result in long-lasting and deteriorating functional deficits. The major goal of SCI treatment is to limit the development of secondary injury. IPA-3, a PAK1 inhibitor, exhibited neuroprotection against secondary damage after traumatic brain injury and subarachnoid hemorrhage (SAH).

METHODS

MMP-2, MMP-9, and cleaved caspase-3 expression were assessed by Western blot. Inflammatory cytokines TNF-α and IL-1β were detected by enzyme-linked immunosorbent assay (ELISA). The blood-spinal cord barrier disruption was measured by water content and Evans blue extravasation of the spinal cord. Neuronal apoptosis was evaluated by Nissl staining and Terminal-deoxynucleoitidyl Transferase Mediated Nick End Labeling (TUNEL) assay. The locomotor behavior of hind limb was evaluated by Basso Mouse Scale (BMS) at 1, 3, 7, 14, and 28 days post-injury.

RESULTS

Compared with SCI + vehicle mice, IPA-3 treatment showed decreased p-PAK1, MMP-2, MMP-9, cleaved caspase-3, TNF-α, and IL-1β expression. Moreover, inhibition of PAK1 by IPA-3 reduced spinal cord water content and Evans blue extravasation, increased neuronal survival, and reduced TUNEL-positive cells at 24 hours after SCI. Furthermore, IPA-3 improved spinal cord functional recovery 7 days after SCI.

CONCLUSION

Inhibition of PAK1 by IPA-3 promoted recovery of neurological function, possibly by downregulating the expression of MMP-2, MMP-9, TNF-α, and IL-1β. Our data suggest that PAK1 may be a potential therapeutic target in patients with SCI.

LEVEL OF EVIDENCE

1.

Focused ultrasound-mediated drug delivery through the blood-brain barrier

Despite recent advances in blood-brain barrier (BBB) research, it remains a significant hurdle for the pharmaceutical treatment of brain diseases. Focused ultrasound (FUS) is one method to transiently increase permeability of the BBB to promote drug delivery to specific brain regions. An introduction to the BBB and a brief overview of the methods, which can be used to circumvent the BBB to promote drug delivery, is provided. In particular, we discuss the advantages and limitations of FUS technology and the efficacy of FUS-mediated drug delivery in models of disease. MRI for targeting and evaluating FUS treatments, combined with administration of microbubbles, allows for transient, reproducible BBB opening. The integration of a real-time acoustic feedback controller has improved treatment safety. Successful clinical translation of FUS has the potential to transform the treatment of brain disease worldwide without requiring the development of new pharmaceutical agents.

Heterogeneity of the blood-brain barrier

The brain microvascular network is comprised of capillaries, arterioles and venules, all of which retain - although to a different extent - blood-brain barrier (BBB) properties. Capillaries constitute the largest and tightest microvasculature. In contrast, venules have a looser junctional arrangement, while arterioles have a lower expression of P-gp. Development and maintenance of the BBB depends on the interaction of cerebral endothelial cells with pericytes and astrocytes, which are all heterogeneous in different regions of the central nervous system. At the level of circumventricular organs microvessels are permeable, containing fenestrations and discontinuous tight junctions. In addition, the blood-spinal cord barrier - where the number of pericytes is lower and expression of junctional proteins is reduced - is also more permeable than the BBB. However, much less is known about the cellular, molecular and functional differences among other regions of the brain. This review summarizes our current knowledge on the heterogeneity of the brain microvasculature.

Pericyte structure and distribution in the cerebral cortex revealed by high-resolution imaging of transgenic mice

Pericytes are essential for normal brain function, but many aspects of their physiology remain enigmatic due to a lack of tools to genetically target this cell population. Here, we characterize brain pericytes using two existing Cre-recombinase driver mouse lines that can serve distinct purposes in cerebrovascular research. One line expresses an inducible version of Cre under the NG2 proteoglycan promoter, which provides the sparse labeling necessary to define the morphology of single cells. These mice reveal structural differences between pericytes adjacent to arterioles versus those broadly distributed in the capillary bed that may underlie differential roles in control of vessel caliber. A second line expresses Cre constitutively under the platelet-derived growth factor receptor β promoter and provides continuous, highly specific and near-complete labeling of pericytes and myocytes along the entire cerebrovasculature. This line provides a three-dimensional view of pericyte distribution along the cortical angioarchitecture following optical clearing of brain tissue. In combination with recent reporter lines for expression of optogenetic actuators and activity-sensitive probes, these mice may be key tools for studying pericyte biology in the intact brain.

Shedding of soluble platelet-derived growth factor receptor-β from human brain pericytes

Platelet-derived growth factor receptor-β (PDGFRβ) is expressed in the brain by vascular mural cells-brain capillary pericytes and arterial vascular smooth muscle cells (VSMCs). Recent evidence shows that blood-brain barrier (BBB) disruption and increased permeability, especially in the hippocampus, positively correlates with elevated levels of soluble PDGFRβ (sPDGFRβ) in cerebrospinal fluid (CSF) in patients with mild dementia. To determine which vascular cell type(s) contributes to increased sPDGFRβ in CSF, we compared PDGFRβ expression and sPDGFRβ shedding in response to injury in early passage primary cultures of human brain pericytes, brain arterial VSMCs, and brain endothelial cells. PDGFRβ protein was undetectable in endothelial cells, but was found both in pericytes and VSMCs. PDGFRβ relative protein abundance was by 4.2-fold (p<0.05) higher in pericytes compared to VSMCs. Hypoxia (1% O2) or amyloid-β peptide (25 μM) compared to normoxia (21% O2) both increased over 48 h shedding of sPDGFRβ and its levels in the culture medium from pericytes cultures, but not from VSMCs cultures, by 4.3-fold and 4.6-fold, respectively, compared to the basal sPDGFRβ levels in the medium (1.43±0.15 ng/ml). This was associated with the corresponding loss of cell-associated PDGFRβ from pericytes and no change in cellular levels of PDGFRβ in VSMCs. Thus, sPDGFRβ is a biomarker of pericyte injury, and elevated sPDGFRβ levels in biofluids in patients with dementia and/or other neurodegenerative disorders likely reflects pericyte injury, which supports the potential for sPDGFRβ to be developed and validated as a biomarker of brain pericyte injury and BBB dysfunction.

Traumatic brain injury results in rapid pericyte loss followed by reactive pericytosis in the cerebral cortex

Accumulating evidence suggests a pivotal role of PDGFRß positive cells, a specific marker for central nervous system (CNS) pericytes, in tissue scarring. Identification of cells that contribute to tissue reorganization in the CNS upon injury is a crucial step to develop novel treatment strategies in regenerative medicine. It has been shown that pericytes contribute to scar formation in the spinal cord. It is further known that ischemia initially triggers pericyte loss in vivo, whilst brain trauma is capable of inducing pericyte detachment from cerebral vessels. These data point towards a significant role of pericytes in CNS injury. The temporal and spatial dynamics of PDGFRß cells and their responses in traumatic brain injury are poorly understood. Here we show that PDGFRß positive cells initially decline in the acute phase following experimental traumatic brain injury. However, PDGFRß positive cells increase significantly in the trauma zone days after brain injury. Using various pericyte markers we identify these cells to be pericytes that are demarcated by reactive gliosis. Our data indicate that brain trauma causes a biphasic response of pericytes in the early phase of brain trauma that may be of relevance for the understanding of pathological cellular responses in traumatic brain injury.

The pericyte: a forgotten cell type with important implications for Alzheimer's disease?

Pericytes are cells in the blood-brain barrier (BBB) that degenerate in Alzheimer's disease (AD), a neurodegenerative disorder characterized by early neurovascular dysfunction, elevation of amyloid β-peptide (Aβ), tau pathology and neuronal loss, leading to progressive cognitive decline and dementia. Pericytes are uniquely positioned within the neurovascular unit between endothelial cells of brain capillaries, astrocytes and neurons. Recent studies have shown that pericytes regulate key neurovascular functions including BBB formation and maintenance, vascular stability and angioarchitecture, regulation of capillary blood flow, and clearance of toxic cellular by-products necessary for normal functioning of the central nervous system (CNS). Here, we review the concept of the neurovascular unit and neurovascular functions of CNS pericytes. Next, we discuss vascular contributions to AD and review new roles of pericytes in the pathogenesis of AD such as vascular-mediated Aβ-independent neurodegeneration, regulation of Aβ clearance and contributions to tau pathology, neuronal loss and cognitive decline. We conclude that future studies should focus on molecular mechanisms and pathways underlying aberrant signal transduction between pericytes and its neighboring cells within the neurovascular unit, that is, endothelial cells, astrocytes and neurons, which could represent potential therapeutic targets to control pericyte degeneration in AD and the resulting secondary vascular and neuronal degeneration.

Cerebrospinal fluid biomarkers of neurovascular dysfunction in mild dementia and Alzheimer's disease

Alzheimer's disease (AD) is the most common form of age-related dementias. In addition to genetics, environment, and lifestyle, growing evidence supports vascular contributions to dementias including dementia because of AD. Alzheimer's disease affects multiple cell types within the neurovascular unit (NVU), including brain vascular cells (endothelial cells, pericytes, and vascular smooth muscle cells), glial cells (astrocytes and microglia), and neurons. Thus, identifying and integrating biomarkers of the NVU cell-specific responses and injury with established AD biomarkers, amyloid-β (Aβ) and tau, has a potential to contribute to better understanding of the disease process in dementias including AD. Here, we discuss the existing literature on cerebrospinal fluid biomarkers of the NVU cell-specific responses during early stages of dementia and AD. We suggest that the clinical usefulness of established AD biomarkers, Aβ and tau, could be further improved by developing an algorithm that will incorporate biomarkers of the NVU cell-specific responses and injury. Such biomarker algorithm could aid in early detection and intervention as well as identify novel treatment targets to delay disease onset, slow progression, and/or prevent AD.

Expression of the ALS-causing variant hSOD1(G93A) leads to an impaired integrity and altered regulation of claudin-5 expression in an in vitro blood-spinal cord barrier model

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive paralysis due to the loss of primary and secondary motor neurons. Mutations in the Cu/Zn-superoxide dismutase (SOD1) gene are associated with familial ALS and to date numerous hypotheses for ALS pathology exist including impairment of the blood-spinal cord barrier. In transgenic mice carrying mutated SOD1 genes, a disrupted blood-spinal cord barrier as well as decreased levels of tight junction (TJ) proteins ZO-1, occludin, and claudin-5 were detected. Here, we examined TJ protein levels and barrier function of primary blood-spinal cord barrier endothelial cells of presymptomatic hSOD1(G93A) mice and bEnd.3 cells stably expressing hSOD1(G93A). In both cellular systems, we observed reduced claudin-5 levels and a decreased transendothelial resistance (TER) as well as an increased apparent permeability. Analysis of the β-catenin/AKT/forkhead box protein O1 (FoxO1) pathway and the FoxO1-regulated activity of the claudin-5 promoter revealed a repression of the claudin-5 gene expression in hSOD1(G93A) cells, which was depended on the phosphorylation status of FoxO1. These results strongly indicate that mutated SOD1 affects the expression and localization of TJ proteins leading to impaired integrity and breakdown of the blood-spinal cord barrier.

Alterations in the blood-spinal cord barrier in TDP-43 conditional knockout mice

We investigated whether the loss of motor neuron-specific TDP-43 protein causes any change in the blood-spinal cord barrier (BSCB) in the spinal cord of TDP-43 conditional knockout (TDP CKO) mice. The TDP CKO mice were divided into four groups: early presymptomatic, late presymptomatic, early symptomatic, and late symptomatic stages. The spinal cords were pathologically examined. TDP CKO mice showed the activation of MAC-2 (macrophages/microglia) and fibrinogen exclusively in the anterior horn from the early symptomatic through the late symptomatic stages. Immunohistochemical and western blot analyses detected no reduction in tight junction proteins in TDP CKO mice as compared to age-matched wild-type mice at any stage. Electron-microscopically, TDP CKO mice showed vacuoles in the cytoplasm of most endothelial cells at the early symptomatic stage. The endothelium occasionally exhibited swollen cytoplasm by edematous fluid with the intact tight junction. The cytoplasm of the pericytes was relatively well preserved in contrast to the endothelial disruption. Extravascular or perivascular spaces were frequently edematous and vacuolated. At other stages, the BSCB was well preserved as in the controls. Thus, the temporary and reversible breakdown of the BSCB with leakage or increased permeability at the early symptomatic stage observed in this study could be a direct pathogenic consequence of the loss of TDP-43 protein, and the temporal impairment of BSCB, in turn, might contribute to the motor neuron degeneration in TDP CKO mice.

Blood-brain barrier breakdown in the aging human hippocampus

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 Alzheimer's 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

Prostacyclin prevents pericyte loss and demyelination induced by lysophosphatidylcholine in the central nervous system

Pericytes play pivotal roles in physiological and pathophysiological conditions in the central nervous system. As pericytes prevent vascular leakage, they can halt neuronal damage stemming from a compromised blood-brain barrier. Therefore, pericytes may be a good target for the treatment of neurodegenerative disorders, although evidence is lacking. In this study, we show that prostacyclin attenuates lysophosphatidylcholine (LPC)-mediated vascular dysfunction through pericyte protection in the adult mouse spinal cord. LPC decreased the number of pericytes in an in vitro blood-brain barrier model, and this decrease was prevented by iloprost treatment, a prostacyclin analog. Intrathecal administration of iloprost attenuated vascular barrier disruption after LPC injection in the mouse spinal cord. Furthermore, iloprost treatment diminished demyelination and motor function deficits in mice injected with LPC. These results support the notion that prostacyclin acts on pericytes to maintain vascular barrier integrity.

Cerebral Cavernous Malformation-1 Protein Controls DLL4-Notch3 Signaling Between the Endothelium and Pericytes

BACKGROUND AND PURPOSE

Cerebral cavernous malformation (CCM) is a neurovascular dysplasia characterized by conglomerates of enlarged endothelial channels in the central nervous system, which are almost devoid of pericytes or smooth muscle cells. This disease is caused by loss-of-function mutations in CCM1, CCM2, or CCM3 genes in endothelial cells, making blood vessels highly susceptible to angiogenic stimuli. CCM1- and CCM3-silenced endothelial cells have a reduced expression of the Notch ligand Delta-like 4 (DLL4) resulting in impaired Notch signaling and irregular sprouting angiogenesis. This study aimed to address if DLL4, which is exclusively expressed on endothelial cells, may influence interactions of endothelial cells with pericytes, which express Notch3 as the predominant Notch receptor.

METHODS

Genetic manipulation of primary human endothelial cells and brain pericytes. Transgenic mouse models were also used.

RESULTS

Endothelial cell-specific ablation of Ccm1 and Ccm2 in different mouse models led to the formation of CCM-like lesions, which were poorly covered by periendothelial cells. CCM1 silencing in endothelial cells caused decreased Notch3 activity in cocultured pericytes. DLL4 proteins stimulated Notch3 receptors on human brain pericytes. Active Notch3 induced expression of PDGFRB2, N-Cadherin, HBEGF, TGFB1, NG2, and S1P genes. Notch3 signaling in pericytes enhanced the adhesion strength of pericytes to endothelial cells, limited their migratory and invasive behavior, and enhanced their antiangiogenic function. Pericytes silenced for Notch3 expression were more motile and could not efficiently repress angiogenesis.

CONCLUSIONS

The data suggest that Notch signaling in pericytes is important to maintain the quiescent vascular phenotype. Deregulated Notch signaling may, therefore, contribute to the pathogenesis of CCM.

Molecular properties determining unbound intracellular and extracellular brain exposure of CNS drug candidates

In the present work we sought to gain a mechanistic understanding of the physicochemical properties that influence the transport of unbound drug across the blood-brain barrier (BBB) as well as the intra- and extracellular drug exposure in the brain. Interpretable molecular descriptors that significantly contribute to the three key neuropharmacokinetic properties related to BBB drug transport (Kp,uu,brain), intracellular accumulation (Kp,uu,cell), and binding and distribution in the brain (Vu,brain) for a set of 40 compounds were identified using partial least-squares (PLS) analysis. The tailoring of drug properties for improved brain exposure includes decreasing the polarity and/or hydrogen bonding capacity. The design of CNS drug candidates with intracellular targets may benefit from an increase in basicity and/or the number of hydrogen bond donors. Applying this knowledge in drug discovery chemistry programs will allow designing compounds with more desirable CNS pharmacokinetic properties.

p38 MAP kinase mediates transforming-growth factor-β1-induced upregulation of matrix metalloproteinase-9 but not -2 in human brain pericytes

Pericytes are vascular mural cells embedded within the basal lamina of blood micro-vessels. Within the neurovascular unit, pericytes play important roles in regulating neurovascular homeostasis by secreting soluble factors, such as matrix metalloproteinases (MMPs). However, little is known about the regulatory signaling pathways in brain pericytes. Here we show that transforming growth factor-β1 (TGF-β1) induces MMP-9 upregulation in pericytes via p38 mitogen-activated protein (MAP) kinase signaling. Cultured human brain vascular pericytes were used in this study. When the brain pericytes were treated with purified human TGF-β1 (0.1-10ng/mL for 24h), the levels of MMP-2 and MMP-9 in culture media were significantly increased in a concentration dependent manner as measured by gelatin zymography. WST assay confirmed that TGF-β1 did not affect cell survival of the brain pericytes. A TGF-β-receptor inhibitor SB431542 (0.5-5μM) decreased the TGF-β1-induced upregulation of MMP-2 and MMP-9. To assess the underlying intracellular mechanisms, we focused on p38 MAP kinase signaling, which is one of the major downstream kinases for TGF-β1. A well-validated p38 MAP kinase inhibitor SB203580 (0.5-5μM) cancelled the effect of TGF-β1 in upregulation of MMP-9 but not MMP-2. Western blotting confirmed that TGF-β1 treatment increased the level of p38 MAP kinase phosphorylation in pericytes, and again, the TGF-β-receptor inhibitor SB431542 (0.5-5μM) blocked the TGF-β1-induced phosphorylation of p38 MAP kinase. Both TGF-β1 and MMP-9 are major neurovascular mediators, and therefore, our current finding may suggest a novel mechanism for how pericytes regulate neurovascular homeostasis.

Interferon-γ safeguards blood-brain barrier during experimental autoimmune encephalomyelitis

The function of blood-brain barrier is often disrupted during the progression of multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE). However, the molecular mechanism of blood-brain barrier modulation during neuroinflammation remains unclear. Herein, we show that the expression of interferon-γ (IFNγ) receptor on endothelial cells (ECs) protected mice from the brain inflammation during EAE. IFNγ stabilized the integrity of the cerebral endothelium and prevented the infiltration of leukocytes into the brain. Further analysis revealed that IFNγ increased the expression of tight junction proteins zonula occludens protein 1 and occludin, as well as membranous distribution of claudin-5, in brain ECs. Silencing claudin-5 abolished the IFNγ-mediated improvement of EC integrity. Taken together, our results show that IFNγ, a pleiotropic proinflammatory cytokine, stabilizes blood-brain barrier integrity and, therefore, prevents brain inflammation during EAE.

Recruiting specialized macrophages across the borders to restore brain functions

Although is well accepted that the central nervous system has an immune privilege protected by the blood–brain barrier (BBB) and maintained by the glia, it is also known that in homeostatic conditions, peripheral immune cells are able to penetrate to the deepest regions of brain without altering the structural integrity of the BBB. Nearly all neurological diseases, including degenerative, autoimmune or infectious ones, compromising brain functions, develop with a common pattern of inflammation in which macrophages and microglia activation have been regarded often as the “bad guys.” However, recognizing the huge heterogeneity of macrophage populations and also the different expression properties of microglia, there is increasing evidence of alternative conditions in which these cells, if primed and addressed in the correct direction, could be essential for reparative and regenerative functions. The main proposal of this review is to integrate studies about macrophage’s biology at the brain borders where the ultimate challenge is to penetrate through the BBB and contribute to change or even stop the course of disease. Thanks to the efforts made in the last century, this special wall is currently recognized as a highly regulated cooperative structure, in which their components form neurovascular units. This new scenario prompted us to review the precise cross-talk between the mind and body modes of immune response.

Redistribution of PDGFRβ cells and NG2DsRed pericytes at the cerebrovasculature after status epilepticus

PURPOSE

The role of cerebrovascular dysfunction in seizure disorders is recognized. Blood-brain barrier (BBB) damage in epilepsy has been linked to endothelial and glial pathophysiological changes. Little is known about the involvement of pericytes, a cell type that contributes to BBB function.

METHODS

NG2DsRed mice were used to visualize cerebrovascular pericytes. The pattern of vascular and parenchymal distributions of platelet-derived growth factor receptor beta (PDGFRβ) cells was evaluated by immunohistochemistry. Status epilepticus was induced in NG2DsRed or C57BL/6J mice by intraperitoneal kainic acid (KA). Animals were perfused intracardially using FITC-Dextran or FITC-Albumin to visualize the cerebrovasculature. Colocalization was performed between NG2DsRed, PDGFRβ and microglia IBA-1. Confocal 3D vessel reconstruction was used to visualize changes in cell morphology and position. PDGFRβ expression was also evaluated in vitro using organotypic hippocampal cultures (OHC) treated with kainic acid to induce seizure-like activity. Co-localization of PDGFRβ with the vascular marker RECA-1 and NG2 was performed. Finally, we assessed the expression of PDGFRβ in brain specimens obtained from a cohort of patients affected by drug resistant epilepsy compared to available autoptic brain.

RESULTS

In vivo, severe status epilepticus (SE) altered NG2DsRed vascular coverage. We found dishomogenous NG2DsRed perivascular ramifications after SE and compared to control. Concomitantly, PDGFRβ(+) cells re-distributed towards the cerebrovasculature after severe SE. Cerebrovascular NG2DsRed partially colocalized with PDGFRβ(+) while parenchymal PDGFRβ(+) cells did not colocalize with IBA-1(+) microglia. Using in vitro OHC we found decreased NG2 vascular staining and increased PDGFRβ(+) ramifications associated with RECA-1(+) microvessels after seizure-like activity. Cellular PDGFRβ and NG2(+) colocalization was observed in the parenchyma. Finally, analysis of human TLE brains revealed perivascular and parenchymal PDGFRβ(+) cell distributions resembling the murine in vivo and in vitro results. PDGFRβ(+) cells at the cerebrovasculature were more frequent in TLE brain tissues as compared to the autoptic control.

CONCLUSIONS

The rearrangement of PDGFRβ(+) and vascular NG2DsRed cells after SE suggests a possible involvement of pericytes in the cerebrovascular modifications observed in epilepsy. The functional role of vascular-parenchymal PDGFRβ(+) cell redistribution and the relevance of a pericyte response to SE remain to be fully elucidated.

Mechanism of West Nile virus neuroinvasion: a critical appraisal

West Nile virus (WNV) is an important emerging neurotropic virus, responsible for increasingly severe encephalitis outbreaks in humans and horses worldwide. However, the mechanism by which the virus gains entry to the brain (neuroinvasion) remains poorly understood. Hypotheses of hematogenous and transneural entry have been proposed for WNV neuroinvasion, which revolve mainly around the concepts of blood-brain barrier (BBB) disruption and retrograde axonal transport, respectively. However, an over‑representation of in vitro studies without adequate in vivo validation continues to obscure our understanding of the mechanism(s). Furthermore, WNV infection in the current rodent models does not generate a similar viremia and character of CNS infection, as seen in the common target hosts, humans and horses. These differences ultimately question the applicability of rodent models for pathogenesis investigations. Finally, the role of several barriers against CNS insults, such as the blood-cerebrospinal fluid (CSF), the CSF-brain and the blood-spinal cord barriers, remain largely unexplored, highlighting the infancy of this field. In this review, a systematic and critical appraisal of the current evidence relevant to the possible mechanism(s) of WNV neuroinvasion is conducted.

Biphasic mechanisms of neurovascular unit injury and protection in CNS diseases

In the past decade, evidence has emerged that there is a variety of bidirectional cell-cell and/or cell-extracellular matrix interactions within the neurovascular unit (NVU), which is composed of neuronal, glial, and vascular cells along with extracellular matrix. Many central nervous system diseases, which lead to NVU dysfunction, have common features such as glial activation/transformation and vascular/blood-brain-barrier alteration. These phenomena show dual opposite roles, harmful at acute phase and beneficial at chronic phase. This diverse heterogeneity may induce biphasic clinical courses, i.e. degenerative and regenerative processes in the context of dynamically coordinated cellcell/ cell-matrix interactions in the NVU. A deeper understanding of the seemingly contradictory actions in cellular levels is essential for NVU protection or regeneration to suppress the deleterious inflammatory reactions and promote adaptive remodeling after central nervous system injury. This mini-review will present an overview of recent progress in the biphasic roles of the NVU and discuss the clinical relevance of NVU responses associated with central nervous system diseases, such as stroke and other chronic neurodegenerative diseases.

Physical exercise training and neurovascular unit in ischemic stroke

Physical exercise could exert a neuroprotective effect in both clinical studies and animal experiments. A series of related studies have indicated that physical exercise could reduce infarct volume, alleviate neurological deficits, decrease blood-brain barrier dysfunction, promote angiogenesis in cerebral vascular system and increase the survival rate after ischemic stroke. In this review, we summarized the protective effects of physical exercise on neurovascular unit (NVU), including neurons, astrocytes, pericytes and the extracellular matrix. Furthermore, it was demonstrated that exercise training could decrease the blood-brain barrier dysfunction and promote angiogenesis in cerebral vascular system. An awareness of the exercise intervention benefits pre- and post stroke may lead more stroke patients and people with high-risk factors to accept exercise therapy for the prevention and treatment of stroke.

The role of pericytes in neurovascular unit remodeling in brain disorders

Neurons are extremely vulnerable cells that tightly rely on the brain’s highly dynamic and complex vascular network that assures an accurate and adequate distribution of nutrients and oxygen. The neurovascular unit (NVU) couples neuronal activity to vascular function, controls brain homeostasis, and maintains an optimal brain microenvironment adequate for neuronal survival by adjusting blood-brain barrier (BBB) parameters based on brain needs. The NVU is a heterogeneous structure constituted by different cell types that includes pericytes. Pericytes are localized at the abluminal side of brain microvessels and contribute to NVU function. Pericytes play essential roles in the development and maturation of the neurovascular system during embryogenesis and stability during adulthood. Initially, pericytes were described as contractile cells involved in controlling neurovascular tone. However, recent reports have shown that pericytes dynamically respond to stress induced by injury upon brain diseases, by chemically and physically communicating with neighboring cells, by their immune properties and by their potential pluripotent nature within the neurovascular niche. As such, in this paper, we would like to review the role of pericytes in NVU remodeling, and their potential as targets for NVU repair strategies and consequently neuroprotection in two pathophysiologically distinct brain disorders: ischemic stroke and Alzheimer’s disease (AD).

Blood-CNS Barrier Impairment in ALS patients versus an animal model

Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disease with a complicated and poorly understood pathogenesis. Recently, alterations in the blood-Central Nervous System barrier (B-CNS-B) have been recognized as a key factor possibly aggravating motor neuron damage. The majority of findings on ALS microvascular pathology have been determined in mutant superoxide dismutase (SOD1) rodent models, identifying barrier damage during disease development which might similarly occur in familial ALS patients carrying the SOD1 mutation. However, our knowledge of B-CNS-B competence in sporadic ALS (SALS) has been limited. We recently showed structural and functional impairment in postmortem gray and white matter microvessels of medulla and spinal cord tissue from SALS patients, suggesting pervasive barrier damage. Although numerous signs of barrier impairment (endothelial cell degeneration, capillary leakage, perivascular edema, downregulation of tight junction proteins, and microhemorrhages) are indicated in both mutant SOD1 animal models of ALS and SALS patients, other pathogenic barrier alterations have as yet only been identified in SALS patients. Pericyte degeneration, perivascular collagen IV expansion, and white matter capillary abnormalities in SALS patients are significant barrier related pathologies yet to be noted in ALS SOD1 animal models. In the current review, these important differences in blood-CNS barrier damage between ALS patients and animal models, which may signify altered barrier transport mechanisms, are discussed. Understanding discrepancies in barrier condition between ALS patients and animal models may be crucial for developing effective therapies.