Vasopressin V1a receptor signaling in a rat choroid plexus cell line

An enriched environment prevents cognitive impairment in an Alzheimer's disease model by enhancing the secretion of exosomal microRNA-146a from the choroid plexus

Alzheimer’s disease (AD) is characterized by the extensive deposition of amyloid-β plaques and neurofibrillary tangles. We previously found that preserved function of astrocytes is associated with cognitively normal subjects with AD pathology. Here we show that an enriched environment (EE) can prevent cognitive impairment in AD model mice by ameliorating astrocytic inflammation and increasing synaptic density in the subiculum area of the hippocampus. In AD model mice treated with an EE, increased levels of microRNA (miR)-146a and down-regulation of NF-κB were observed in the hippocampus. In addition, increased levels of interferon (IFN)-γ were seen in serum from mice exposed to an EE. In vitro, enhanced miR-146a expression was observed in exosomes derived from the choroid plexus (CP) after IFN-γ treatment. In further in vitro experiments, we transfected miR-146a into Aβ/lipopolysaccharide-induced inflammatory astrocytes and showed that miR-146a ameliorated astrocytic inflammation by down-regulating tumor necrosis factor receptor-associated factor 6 and NF-κB. The present study indicates that following an EE, exosomal miR-146a derived from the CP cells is a key factor in ameliorating astrocytic inflammation, leading to synaptogenesis and correction of cognitive impairment.

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An enriched environment (EE) prevented the cognitive impairment in 5 ​× ​FAD mice.

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An EE inhibited astrocytic inflammation and increased miR-146a in hippocampus.

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An EE increased the levels of interferon-γ (IFN-γ) in serum.

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IFN-γ increased the secretion of exosomal miR-146a from cultured choroid plexus.

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Transfection of miR-146a down-regulated NF-κB in cultured astrocytes.

The blood-cerebrospinal fluid barrier: structure and functional significance

The choroid plexus (CP) of the blood-CSF barrier (BCSFB) displays fundamentally different properties than blood-brain barrier (BBB). With brisk blood flow (10 × brain) and highly permeable capillaries, the human CP provides the CNS with a high turnover rate of fluid (∼400,000 μL/day) containing micronutrients, peptides, and hormones for neuronal networks. Renal-like basement membranes in microvessel walls and underneath the epithelium filter large proteins such as ferritin and immunoglobulins. Type IV collagen (α3, α4, and α5) in the subepithelial basement membrane confers kidney-like permselectivity. As in the glomerulus, so also in CP, the basolateral membrane utrophin A and colocalized dystrophin impart structural stability, transmembrane signaling, and ion/water homeostasis. Extensive infoldings of the plasma-facing basal labyrinth together with lush microvilli at the CSF-facing membrane afford surface area, as great as that at BBB, for epithelial solute and water exchange. CSF formation occurs by basolateral carrier-mediated uptake of Na+, Cl-, and HCO3-, followed by apical release via ion channel conductance and osmotic flow of water through AQP1 channels. Transcellular epithelial active transport and secretion are energized and channeled via a highly dense organelle network of mitochondria, endoplasmic reticulum, and Golgi; bleb formation occurs at the CSF surface. Claudin-2 in tight junctions helps to modulate the lower electrical resistance and greater permeability in CP than at BBB. Still, ratio analyses of influx coefficients (Kin) for radiolabeled solutes indicate that paracellular diffusion of small nonelectrolytes (e.g., urea and mannitol) through tight junctions is restricted; molecular sieving is proportional to solute size. Protein/peptide movement across BCSFB is greatly limited, occurring by paracellular leaks through incomplete tight junctions and low-capacity transcellular pinocytosis/exocytosis. Steady-state concentration ratios, CSF/plasma, ranging from 0.003 for IgG to 0.80 for urea, provide insight on plasma solute penetrability, barrier permeability, and CSF sink action to clear substances from CNS.

Water and solute secretion by the choroid plexus

The cerebrospinal fluid (CSF) provides mechanical and chemical protection of the brain and spinal cord. This review focusses on the contribution of the choroid plexus epithelium to the water and salt homeostasis of the CSF, i.e. the secretory processes involved in CSF formation. The choroid plexus epithelium is situated in the ventricular system and is believed to be the major site of CSF production. Numerous studies have identified transport processes involved in this secretion, and recently, the underlying molecular background for some of the mechanisms have emerged. The nascent CSF consists mainly of NaCl and NaHCO(3), and the production rate is strictly coupled to the rate of Na(+) secretion. In contrast to other secreting epithelia, Na(+) is actively pumped across the luminal surface by the Na(+),K(+)-ATPase with possible contributions by other Na(+) transporters, e.g. the luminal Na(+),K(+),2Cl(-) cotransporter. The Cl(-) and HCO(3) (-) ions are likely transported by a luminal cAMP activated inward rectified anion conductance, although the responsible proteins have not been identified. Whereas Cl(-) most likely enters the cells through anion exchange, the functional as well as the molecular basis for the basolateral Na(+) entry are not yet well-defined. Water molecules follow across the epithelium mainly through the water channel, AQP1, driven by the created ionic gradient. In this article, the implications of the recent findings for the current model of CSF secretion are discussed. Finally, the clinical implications and the prospects of future advances in understanding CSF production are briefly outlined.