International Journal of Molecular Sciences Review Structure and Junctional Complexes of Endothelial, Epithelial and Glial Brain Barriers Mariana Castro Dias *, Josephine A. Mapunda, Mykhailo Vladymyrov and Britta Engelhardt * Theodor Kocher Institute, University of Bern, 3012 Bern, Switzerland; [email protected] (J.A.M.); [email protected] (M.V.) * Correspondence: [email protected] (M.C.D.); [email protected] (B.E.) Received: 14 October 2019; Accepted: 26 October 2019; Published: 29 October 2019 Abstract: The homeostasis of the central nervous system (CNS) is ensured by the endothelial, epithelial, mesothelial and glial brain barriers, which strictly control the passage of molecules, solutes and immune cells. While the endothelial blood-brain barrier (BBB) and the epithelial blood-cerebrospinal fluid barrier (BCSFB) have been extensively investigated, less is known about the epithelial and mesothelial arachnoid barrier and the glia limitans. Here, we summarize current knowledge of the cellular composition of the brain barriers with a specific focus on describing the molecular constituents of their junctional complexes. We propose that the brain barriers maintain CNS immune privilege by dividing the CNS into compartments that differ with regard to their role in immune surveillance of the CNS. We close by providing a brief overview on experimental tools allowing for reliable in vivo visualization of the brain barriers and their junctional complexes and thus the respective CNS compartments. Keywords: brain barriers; blood-brain barrier; neurovascular unit; blood-cerebrospinal fluid barrier; arachnoid barrier; glia limitans; tight junctions; adherens junctions 1. Introduction The brain barriers established by the endothelial blood-brain barrier (BBB), the epithelial blood-cerebrospinal fluid barrier (BCSFB), the meningeal brain barriers and the blood spinal cord barrier are essential for maintaining central nervous system (CNS) homeostasis [1]. While the structural and junctional components of the BBB and of the BCSFB of the choroid plexus (ChP) have been vastly explored and described, much less is known about the cells and junctional complexes establishing the meningeal barriers and the glia limitans. Development of the CNS vasculature begins with the process of vasculogenesis, where angioblasts, which originate from the mesoderm, move to the head region and form the primary perineural vascular plexus around the developing brain. The proliferating neuroectodermal tissue is invaded by vascular sprouts from the primary perineural plexus establishing the brain vasculature by a process called angiogenesis. The unique barrier properties of the brain endothelial cells are not intrinsic and are rather induced by a process referred to as barriergenesis by the continuous crosstalk with the developing neuroectodermal tissue [2–5]. The ChP develops from different locations along the dorsal axis of the neural tube, with the hindbrain ChP of the fourth ventricle being the first to be formed, once the neural tube is closed [6]. While the ChP stroma develops from mesenchymal cells, the ChP epithelium derives from neuroepithelial cells. Despite the different origin along the rostral-caudal axis of the developing nervous system, the mature ChP epithelia are morphologically similar (reviewed in Reference [6]). The meningeal barriers comprise three layers: the dura mater, the arachnoid mater and the pia mater, and cover the brain and the spinal cord from its earliest stages of development, establishing Int. J. Mol. Sci. 2019, 20, 5372; doi:10.3390/ijms20215372 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2019, 20, 5372 2 of 27 Int. J. Mol. Sci. 2019, 20, x FOR PEER REVIEW 2 of 26 meningesa protective has covering not much of theadvanced CNS in beyond the adult. identi Ourfying current the understandingorigins of pial, of arachnoid the development and dural of fibroblaststhe meninges in the has frontal not much brain advanced from neural beyond crest, identifyingand in the midbrain, the origins hindbrain of pial, arachnoidand spinal andcord duralfrom mesodermfibroblasts in[7]. the Despite frontal their brain differences from neural in crest,developmental and in the midbrain,origin, the hindbraincells of the and pia, spinal arachnoid cord from and duramesoderm mater [ 7have]. Despite been theirdescribed differences to establish in developmental functionally origin, similar the ba cellsrriers. of the However, pia, arachnoid the respective and dura barriermater have function been may described be established to establish by different functionally molecular similar compon barriers.ents However, of their thejunctional respective complexes barrier [8–11].function may be established by different molecular components of their junctional complexes [8–11]. Taken together, junctionaljunctional complexes established between cells of the brain barriers might not only didifferffer betweenbetween the the di differentfferent barriers barriers but but also also within within one respectiveone respective barrier barrier along along the rostral-caudal the rostral- caudalaxis. Here, axis. weHere, summarize we summarize the current the current knowledge knowledg of thee of brain the brain barriers, barriers, with with a particular a particular focus focus on onthe molecularthe molecular components components of their of junctional their junction complexes.al complexes. Additionally, Additionally, we suggest we that suggest visualization that visualizationof brain barriers of brain junctional barriers complexes junctional provides complexes useful provides landmarks useful forlandmarksin vivo imaging for in vivo of immuneimaging ofsurveillance immune surveillance of the CNS. of the CNS. 2. Blood-Brain Blood-Brain Barrier and Its Central Players: Cellular Cellular and and Acellular Acellular Components Components of the Neurovascular Unit (NVU) The brainbrain vasculature vasculature is ais big a andbig complexand complex network network composed composed of arteries of andarteries arterioles, and capillaries,arterioles, venulescapillaries, and venules veins, whichand veins, allows which for theallows vital for distribution the vital distribution of nutrients of and nutrients oxygen and to the oxygen CNS to at the CNSlevel at of the level microvasculature, of the microvasculature, namely arterioles, namely capillaries arterioles, andcapillaries post-capillary and post-capillary venules [12 ].venules Brain [12].microvascular Brain microvascular endothelial cells,endothelial referred cells, to as referr the blood-brained to as the barrier blood-brain (BBB), possessbarrier unique(BBB), featurespossess uniquerestricting features the free restricting passage ofthe ions, free molecules passage of and ions, cells molecules from the and blood cells into from the CNSthe blood parenchyma, into the whileCNS parenchyma,facilitating the while transport facilitating of toxic compoundsthe transport out ofof thetoxic CNS compounds [13,14]. Development out of the and CNS maintenance [13,14],. Developmentof BBB characteristics and maintenance in CNS microvascular of BBB characteristics endothelium in CNS relies microvascular on the continuous endothelium crosstalk relies between on thecellular continuous and acellular crosstalk elements between of cellular the CNS. and In acellula the adult,r elements the entity of the of BBBCNS. endothelium, In the adult, the pericytes, entity ofastrocytes, BBB endothelium, and the basal pericytes, membranes astrocytes, are thus and collectively the basal membranes referred to as are the thus neurovascular collectively unitreferred (NVU) to (representedas the neurovascular in Figure unit1)[ 15 (NVU)]. Maintenance (represented of a in functional Figure 1) NVU [15]. isMaintenance a prerequisite of fora functional BBB function NVU and is arequires prerequisite understanding for BBB function of the specific and requires contributions understa ofnding these of cells the tospecific barrier contributions function. of these cells to barrier function. Figure 1. Schematic representation of the components of the neurovascular unit at the level of brain capillaries and post-capillary venules. Drawings Drawings of of the the individual individual cell cell types types were adapted from Servier Medical Art (http://smart.servier.com/), (http://smart.servier.com/), licensedlicensed under a Creative Common Attribution 3.0 Generic License. 2.1. Endothelial Cells 2.1. Endothelial Cells Most studies of the BBB focus on the capillary endothelial cells, due to their unique Most studies of the BBB focus on the capillary endothelial cells, due to their unique properties properties [16,17]. Besides lacking fenestrae, these endothelial cells are brought together by the [16,17]. Besides lacking fenestrae, these endothelial cells are brought together by the presence of presence of continuous and complex tight junctions that prohibit free diffusion of molecules across continuous and complex tight junctions that prohibit free diffusion of molecules across the BBB via the paracellular route [18–20]. Additionally, they have very low pinocytosis rates when compared to Int. J. Mol. Sci. 2019, 20, 5372 3 of 27 the BBB via the paracellular route [18–20]. Additionally, they have very low pinocytosis rates when compared to the peripheral endothelium [21,22]. Of notice, the major facilitator superfamily domain-containing protein 2 (Mfsd2a), a membrane transport protein, is specifically expressed in the CNS endothelium and acts as a key regulator suppressing vesicular