Molecular mechanisms of cerebrospinal fluid production

Abstract

The epithelial cells of the choroid plexuses secrete cerebrospinal fluid (CSF), by a process which involves the transport of Na(+), Cl(-) and HCO(3)(-) from the blood to the ventricles of the brain. The unidirectional transport of ions is achieved due to the polarity of the epithelium, i.e. the ion transport proteins in the blood-facing (basolateral) membrane are different to those in the ventricular (apical) membrane. The movement of ions creates an osmotic gradient which drives the secretion of H(2)O. A variety of methods (e.g. isotope flux studies, electrophysiological, RT-PCR, in situ hybridization and immunocytochemistry) have been used to determine the expression of ion transporters and channels in the choroid plexus epithelium. Most of these transporters have now been localized to specific membranes. For example, Na(+)-K(+)ATPase, K(+) channels and Na(+)-2Cl(-)-K(+) cotransporters are expressed in the apical membrane. By contrast the basolateral membrane contains Cl(-)- HCO(3) exchangers, a variety of Na(+) coupled HCO(3)(-) transporters and K(+)-Cl(-) cotransporters. Aquaporin 1 mediates water transport at the apical membrane, but the route across the basolateral membrane is unknown. A model of CSF secretion by the mammalian choroid plexus is proposed which accommodates these proteins. The model also explains the mechanisms by which K(+) is transported from the CSF to the blood.

Fig. 1

The locations of the choroid plexuses and the distribution of CSF in the human brain. The CSF is shown as the stippled area and the choroid plexuses are shown as the solid black structures.

Fig. 2

The microscopic structure of the choroid plexus. (A) Branched structure of the choroid plexus with villi projecting into the ventricle of the brain. Each plexus consists of a network of capillaries covered by a single layer of cuboidal epithelial cells. (B) Electron micrograph of choroid plexus epithelial cells from the rat fourth ventricle. The apical membrane has many well-developed microvilli (mv), and the basolateral membrane displays multiple infoldings. Scale bar=1 μm. (C) Junctional complex linking epithelial cells. The arrow shows the position of the zona occludens and the scale bar=1 μm.

Fig. 3

KCC expression in rat choroid plexus by RT-PCR. The numbers at the top of the figure indicate RT-PCR products obtained with primers specific for (1) KCC1, (2) KCC2, (3a) KCC3a, (3b) KCC3b or (4) KCC4, and mRNA isolated from rat fourth ventricle choroid plexus. The numbers to the left indicate the position of molecular mass markers. All the products were of the expected size, and showed at least 91% identity with respective published sequences for mouse KCCs.

Fig. 4

Whole-cell conductances in rat choroid plexus epithelial cells. (A) Current profiles for the two components of the K⁺ conductance (Kv and Kir). The profiles were produced by applying voltage steps from -120 to 60 mV and using a K⁺-rich electrode solution and an artificial CSF in the bath. Currents via the time-dependent, outward-rectifying conductance (Kv) are observed at potentials greater than -20 mV. The time-independent, inward-rectifying currents (Kir) are observed at potentials of less than -80 mV. (B) The inward-rectifying anion conductance. Currents were recorded in K⁺-free solutions at Vm= 0 to -140 mV. The dashed line indicates 0 current in and (A) and (B).

Fig. 5

Kv1.1, Kv1.3 and Kv1.6 are expressed in the choroid plexus epithelium. Western analysis was performed a membrane rich fraction isolated from rat fourth ventricle choroid plexus using specific antibodies for Kv1.1, Kv1.3 and Kv1.6. +Ag refers to Western analyses performed when the primary antibody was pre-adsorbed with an excess of the antigens to which each antibody was raised.

Fig. 6

Ion transport by the choroid plexus. (A) Major fluxes of ions across the choroid plexus epithelium. (B) Ion transporters involved in Na⁺, HCO₃^- and Cl^- secretion by the choroid plexus. c.a., carbonic anhydrase. (C) Mechanism of K⁺ absorption. (D) H₂O transport in the choroid plexus epithelium.