New aspects in fenestrated capillary and tissue dynamics in the sensory circumventricular organs of adult brains

Abstract

The blood-brain barrier (BBB) generally consists of endothelial tight junction barriers that prevent the free entry of blood-derived substances, thereby maintaining the extracellular environment of the brain. However, the circumventricular organs (CVOs), which are located along the midlines of the brain ventricles, lack these endothelial barriers and have fenestrated capillaries; therefore, they have a number of essential functions, including the transduction of information between the blood circulation and brain. Previous studies have demonstrated the extensive contribution of the CVOs to body fluid and thermal homeostasis, energy balance, the chemoreception of blood-derived substances, and neuroinflammation. In this review, recent advances have been discussed in fenestrated capillary characterization and dynamic tissue reconstruction accompanied by angiogenesis and neurogliogenesis in the sensory CVOs of adult brains. The sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), have size-selective and heterogeneous vascular permeabilities. Astrocyte-/tanycyte-like neural stem cells (NSCs) sense blood- and cerebrospinal fluid-derived information through the transient receptor potential vanilloid 1, a mechanical/osmotic receptor, Toll-like receptor 4, a lipopolysaccharide receptor, and Nax, a Na-sensing Na channel. They also express tight junction proteins and densely and tightly surround mature neurons to protect them from blood-derived neurotoxic substances, indicating that the NSCs of the CVOs perform BBB functions while maintaining the capacity to differentiate into new neurons and glial cells. In addition to neurogliogenesis, the density of fenestrated capillaries is regulated by angiogenesis, which is accompanied by the active proliferation and sprouting of endothelial cells. Vascular endothelial growth factor (VEGF) signaling may be involved in angiogenesis and neurogliogenesis, both of which affect vascular permeability. Thus, recent findings advocate novel concepts for the CVOs, which have the dynamic features of vascular and parenchymal tissues.

Keywords: TLR4; TRPV1; VEGF; angiogenesis; blood-brain barrier (BBB); homeostasis; inflammation; neural stem cells (NSCs).

Figure 1

Schematic illustration showing localization of the sensory and secretory CVOs in adult rodent brains.

Figure 2

Different vascular permeabilities between the sensory and secretory CVOs. The extravascular fluorescence of the LMW fluorescent tracer FITC was stronger in the secretory CVOs (C,E) than in sensory CVOs (A,B,D). Scale bar = 50 μm. Lam, laminin; DAPI, 4',6-diamidino-2-phenylindole. Confocal micrographs are rearranged with permission from Springer-Verlag (Morita and Miyata, 2012).

Figure 3

Electron micrographs showing coverage of fenestrated capillaries by cellular processes of astrocyte-/tanycyte-like NSCs and dendrites and a wide perivascular space in the adult mouse. The fenestrated capillaries of the AP were surrounded by the cellular processes of astrocyte-/tanycyte-like NSCs and dendrites (A). There was a wide perivascular space including pericytes between the inner and outer basement membranes of the OVLT. Cellular processes of astrocyte-/tanycyte-like NSCs were often found to make contact with the outer basement membrane (B). Solid arrows and asterisks show the cellular processes of astrocyte-/tanycyte-like NSCs and dendrites, respectively. Open arrows and inset revealed that cellular membranes were tightly juxtaposed with each other. Open and solid arrowheads indicate the inner and outer basement membranes, respectively. E, endothelial cell; P, pericyte. Scale bars = 1 μm. Electron micrographs are rearranged with permission from Springer-Verlag (Morita et al., 2015a).

Figure 4

Heterogeneous vascular permeability of the LMW fluorescent tracer FITC and diffusion barrier of GFAP-positive NSCs in the sensory CVOs of adult mice. The fluorescent intensity of blood-derived FITC was stronger at the central part of the OVLT (A), SFO (B), and AP (C) than at the distal part. The cellular processes of GFAP-positive NSCs were very dense at the distal part of each CVO, and FITC did not diffuse to the outside of the sensory CVOs beyond GFAP-positive NSCs. Scale bars = 50 μm. cp, capillary plexus; cz, central zone; lz, lateral zone; os, outer shell; pz, periventricular zone; vc, ventromedial core. Photomicrographs are rearranged with permission from Springer-Verlag (Morita et al., 2015a).

Figure 5

Brain infusion of the TRPV1 agonist resiniferatoxin induced Fos expression by GFAP-positive NSCs and neurons in the sensory CVOs of adult mice. A large number of Fos-positive nuclei were observed in the OVLT (A), SFO (B), and AP (C) after the intracerebroventricular infusion of resiniferatoxin. Fos-positive nuclei were detected in GFAP-positive NSCs (arrowheads) and HuC/D-positive mature neurons (arrows). 3D images confirmed the presence of Fos-positive nuclei in HuC/D-positive neurons and GFAP-positive NSCs. Scale bars = 10 (bottom panels) and 50 (top panels) μm. Photographs are rearranged with permission from John Wiley and Sons Inc. (Mannari et al., 2013).

Figure 6

Expression of the LPS receptor TLR4 by GFAP-positive NSCs in the sensory CVOs of adult mice. TLR4 was strongly expressed in the OVLT (A), SFO (B), and AP (C). The expression of TLR4 was detected in GFAP-positive NSCs in the sensory CVOs (D–F). The expression of TLR4 was also observed in CD45-positive microglia in the solitary nucleus around the central canal (G). CC, central canal; oc, optic chiasma; 3V, 3rd ventricle. Scale bars = 50 μm. Photographs are reconstructed with permission from Elsevier Inc. (Nakano et al., 2015).

Figure 7

mRNA and protein expression of the angiogenesis-inducing factor VEGF-A in the sensory CVOs of adult mouse brains. In situ hybridization histochemistry shows stronger Vegf-a mRNA signals in the OVLT, MPA, SFO, AP, and solitary nucleus than in the adjacent brain regions (A–C). Triple labeling immunohistochemistry shows that the immunoreactivity of VEGF-A was detected in GFAP-positive NSCs (arrows) and MAP2-positive mature neurons (arrowheads) (D–F). CC, central canal; oc, optic chiasma; 3V, 3rd ventricle. Scale bars = 50 μm. Photographs are rearranged with permission from Springer-Verlag (Furube et al., ; Morita et al., 2015b).

Figure 8

Filopodia of endothelial cells in the sensory CVOs of the adult mouse. Laminin-positive vascular filopodia (arrowheads) extended from the existing thick capillariesin the OVLT (A), SFO (B), and AP (C). Scale bars = 10 μm. Confocal micrographs are rearranged with permission from Springer-Verlag (Morita et al., 2015b).

Figure 9

Proliferation and apoptosis of endothelial cells in the NH of adult mice. Mice were orally administered the VEGF signaling inhibitor AZD2171 for 6 days and were then kept for 6 days. The number of BrdU-labeled endothelial cells was significantly higher after the withdrawal of the VEGF signaling inhibitor (B) than that of the control (A). The expression of the apoptotic marker caspase-3 was induced in endothelial cells after a 2-day treatment with the VEGF signaling inhibitor (C). A high magnification view reveals the continuous distribution of caspase-3-positive endothelial cells (D). Scale bars = 50 (A–D) and 5 (inset in B) μm. Data are rearranged with permission from BioScientifica Limited (Furube et al., 2014).

Figure 10

Plasminogen expression and vascular niche of Math1-positive NPCs in the ME and AP of adult mice. Strong immunoreactivity for plasminogen was detected in Math1-positive NPCs (arrowheads) in the ME (A) and AP (B). Math1-positive NPCs typically localized in close contact with the vascular matrix in the AP (arrows). DAPI, diamidino-2-phenylindole; Plg, plasminogen. Scale bars = 50 (A,B) and 10 (C, insets in A,B) μm. Photographs are reconstructed with permission from John Wiley and Sons Inc. (Hourai and Miyata, 2013).

Figure 11

The fate of NSCs in the sensory CVOs using Nestin-CreERT2/CAG-CAT^loxP∕loxP-EGFP transgenic adult mice. Nestin-CreERT2/CAG-CAT^loxP∕loxP-EGFP transgenic mice were sacrificed 60 days after the final administration of tamoxifen. A large number of EGFP-expressing cells were detected in the OVLT, whereas only a few were observed in the median preoptic area and medial preoptic nucleus (A). EGFP-expressing cells were observed in the vhc as well as in the SFO (B). EGFP-expressing cells were detected in the AP and its neighboring brain regions such as the solitary nucleus, 10N, and 12 N (C). MnPO, median preoptic area; MPA, medial preoptic area; Sol, solitary nucleus; vhc, ventral hippocampal commissure. Scale bar = 50 μm. Data are rearranged with permission from Springer-Verlag (Furube et al., 2015).