The French scientist Charles Benjamin Rouget identified the pericyte nearly 140 years ago. Since that time the role of the pericyte in vascular function has been difficult to elucidate. It was not until the development of techniques to isolate and culture pericytes that scientists have begun to understand the true impact of this unique cell in the maintenance of tissue homeostasis. In the brain the pericyte is an integral cellular component of the blood-brain barrier and, together with other cells of the neurovascular unit (endothelial cells, astrocytes and neurons) the pericyte makes fine-tuned regulatory adjustments and adaptations to promote tissue survival. These regulatory changes involve trans-cellular communication networks between cells. In this review we consider evidence for cell-to-cell crosstalk between pericytes and astrocytes during development and in adult brain.

While the pathologic events associated with multiple sclerosis (MS), diffuse axonal injury, cognitive damage, and white matter plaques, have been known for some time, the etiology of MS is still unknown and therapeutic efforts are somewhat disappointing. This may be due to a lack of fundamental knowledge on how to buffer the brain from secondary injury following immune attack. Maintenance of central nervous system (CNS) homeostasis is a complex set of regulatory adjustments by the neurovascular unit that includes induction of adaptive angiogenesis. Although aspects of adaptive angiogenesis are induced in MS and experimental autoimmune encephalomyelitis (EAE), vascular remodeling is ineffective and the balance between metabolic need and oxygen (O(2)) and glucose availability is disrupted.We hypothesized that restoration of metabolic homeostasis in the CNS would ameliorate tissue damage and promote repair in myelin oligodendrocyte glycoprotein(MOG)-induced EAE in mice. Exposure of animals to chronic mild hypoxia (10% O(2)) increased vascular density and significantly delayed onset and severity of clinical EAE. When animals were exposed to hypoxia after the onset of clinical symptoms, the severity of chronic inflammatory disease was reduced or even inhibited. In addition, spinal cord pathology was decreased.While the mechanism of protection is unclear, results suggest that hypoxia has therapeutic potential in EAE.

Microvascular adaptation to metabolic stress is important in the maintenance of tissue homeostasis. Nowhere is this more important than in the central nervous system (CNS) where the cellular constituents of the neurovascularture including endothelial cells, pericytes and some astroglia must make fine-tuned autoregulatory modulations that maintain the delicate balance between oxygen availability and metabolic demand. miRNAs have been reported to play an important regulatory role in many cellular functions including cell differentiation, growth and proliferation, lineage determination, and metabolism. In this study, we investigated the possible role of miRNAs in the CNS capillary pericyte response to hypoxic stress. Micro-array analysis was used to examine the expression of 388 rat miRNAs in primary rat cortical pericytes with and without exposure to low oxygen (1%) after 24 or 48 hr. Pericytes subjected to hypoxia showed 27 miRNAs that were higher than control and 31 that were lower. Validation and quantification was performed by Real Time RT-PCR on pericytes subjected to 2 hr, 24 hr, or 48 hr of hypoxia. Hypoxia induced changes included physiological pathways governing the stress response, angiogenesis, migration and cell cycle regulation. miRNAs associated with HIF-1α (miR−322[1], miR−199a [2]), TGF-β1 (miR−140[3], miR−145[4], miR−376b−3p[5]) and VEGF (miR−126a[6], miR−297[7], miR−16[8], miR−17−5p[9]) were differentially regulated. Systematic and integrative analysis of possible gene targets analyzed by DAVID bioinformatics resource (http://david.abcc.ncifcrf.gov) and MetaSearch 2.0 (GeneGo) for some of these miRNAs was conducted to determine possible gene targets and pathways that may be affected by the post-transcriptional changes after hypoxic insult.

BackgroundIn the brain, chronic inflammatory activity may lead to compromised delivery of oxygen and glucose suggesting that therapeutic approaches aimed at restoring metabolic balance may be useful. In vivo exposure to chronic mild normobaric hypoxia (10 % oxygen) leads to a number of endogenous adaptations that includes vascular remodeling (angioplasticity). Angioplasticity promotes tissue survival. We have previously shown that induction of adaptive angioplasticity modulates the disease pattern in myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). In the present study, we define mechanisms by which adaptation to low oxygen functionally ameliorates the signs and symptoms of EAE and for the first time show that tissue hypoxia may fundamentally alter neurodegenerative disease.MethodsC57BL/6 mice were immunized with MOG, and some of them were kept in the hypoxia chambers (day 0) and exposed to 10 % oxygen for 3 weeks, while the others were kept at normoxic environment. Sham-immunized controls were included in both hypoxic and normoxic groups. Animals were sacrificed at pre-clinical and peak disease periods for tissue collection and analysis.ResultsExposure to mild hypoxia decreased histological evidence of inflammation. Decreased numbers of cluster of differentiation (CD)4+ T cells were found in the hypoxic spinal cords associated with a delayed Th17-specific cytokine response. Hypoxia-induced changes did not alter the sensitization of peripheral T cells to the MOG peptide. Exposure to mild hypoxia induced significant increases in anti-inflammatory IL-10 levels and an increase in the number of spinal cord CD25+FoxP3+ T-regulatory cells.ConclusionsAcclimatization to mild hypoxia incites a number of endogenous adaptations that induces an anti-inflammatory milieu. Further understanding of these mechanisms system may pinpoint possible new therapeutic targets to treat neurodegenerative disease.

The microvascular pericyte is an integral component of the blood-brain barrier and the neurovascular unit. Most model systems that have been developed to study the functional parameters of these systems have not incorporated the pericyte. In this chapter, we consider pericyte coculture and triple culture systems and detail the methodology, suggestions, and problems with isolation of these unique cells. We also present data to show that triple cultures are ideal to study the role of the CNS pericyte in CNS angiogenesis.