Microvascular P-glycoprotein expression at the blood-brain barrier following focal astrocyte loss and at the fenestrated vasculature of the area postrema

Morphological, histological and immunohistochemical study of the area postrema in the dog

Circumventricular organs are specialized brain structures that are located mainly at the midsagittal line, around the third and fourth ventricles, often protruding into the lumen. They are positioned at the interface between the neuroparenchyma and the ventricular system of the brain. These highly vascularized nervous tissue structures differ from the brain parenchyma, as they lack a blood-brain barrier. Circumventricular organs have specialized sensory and secretory functions. It is essential for any pathologist who evaluates brain sections to have a solid knowledge of microscopic neuroanatomy and to recognize these numerous specialized structures within the nervous system as normal and not mistake them for pathological changes. The purpose of this study was to provide, for the first time, a detailed and complete histological description of the healthy canine area postrema and to determine its resemblance to that of other mammalian species. Anatomical dissections with routine histological and immunohistochemical techniques were carried out on ten canine brains. The cellular composition of area postrema proved to be largely comparable to that of other mammal species.

Glial influences on BBB functions and molecular players in immune cell trafficking

The blood-brain barrier (BBB) constitutes an elaborate structure formed by specialized capillary endothelial cells, which together with pericytes and perivascular glial cells regulates the exchanges between the central nervous system (CNS) and the periphery. Intricate interactions between the different cellular constituents of the BBB are crucial in establishing a functional BBB and maintaining the delicate homeostasis of the CNS microenvironment. In this review, we discuss the role of astrocytes and microglia in inducing and maintaining barrier properties under physiological conditions as well as their involvement during neuroinflammatory pathologies. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

Vascular and extravascular distribution of the ATP-binding cassette transporters ABCB1 and ABCC1 in aged human brain and pituitary

ATP-binding cassette (ABC) transporters play an increasing role in the understanding of pathologic peptide deposition in neurodegenerative diseases (NDs), such as Alzheimer's and Parkinson's. To describe the location of the most important ABC transporters for NDs in human brain tissue, we investigated ABCB1 and ABCC1 immunohistologically in the adult human brain and pituitary. Both transporters have similar but not identical expression patterns. In brain regions with an established blood-brain barrier (BBB), ABCB1 and ABCC1 were ubiquitously expressed in endothelial cells of the microvasculature and in a subset of larger blood vessels (mostly venules). Remarkably, both transporters were also found in fenestrated capillaries in circumventricular organs where the BBB is absent. Moreover, ABCB1 and ABCC1 were also expressed in various non-endothelia cells such as pericytes, astrocytes, choroid plexus epithelia, ventricle ependymal cells, and neurons. With regard to their neuronal expression it was shown that both transporters are located in specific nerve cell populations, which are also immunopositive for three putative cell markers of purinergic cell signalling, namely 5'-nucleotidase, adenosine deaminase and nucleoside triphosphate diphosphohydrolase-2. Therefore, we speculate that neuronal expression of ABCB1 and ABCC1 might be linked to adenosinergic/purinergic neuromodulation. Lastly, both transporters were observed in multiple adenohypophyseal cells.

Conditional Müllercell ablation causes independent neuronal and vascular pathologies in a novel transgenic model

Müller cells are the major glia of the retina that serve numerous functions essential to retinal homeostasis, yet the contribution of Müller glial dysfunction to retinal diseases remains largely unknown. We have developed a transgenic model using a portion of the regulatory region of the retinaldehyde binding protein 1 gene for conditional Müller cell ablation and the consequences of primary Müller cell dysfunction have been studied in adult mice. We found that selective ablation of Müller cells led to photoreceptor apoptosis, vascular telangiectasis, blood-retinal barrier breakdown and, later, intraretinal neovascularization. These changes were accompanied by impaired retinal function and an imbalance between vascular endothelial growth factor-A (VEGF-A) and pigment epithelium-derived factor. Intravitreal injection of ciliary neurotrophic factor inhibited photoreceptor injury but had no effect on the vasculopathy. Conversely, inhibition of VEGF-A activity attenuated vascular leak but did not protect photoreceptors. Our findings show that Müller glial deficiency may be an important upstream cause of retinal neuronal and vascular pathologies in retinal diseases. Combined neuroprotective and anti-angiogenic therapies may be required to treat Müller cell deficiency in retinal diseases and in other parts of the CNS associated with glial dysfunction.