Homeostatic capabilities of the choroid plexus epithelium in Alzheimer's disease

Abstract

As the secretory source of vitamins, peptides and hormones for neurons, the choroid plexus (CP) epithelium critically provides substances for brain homeostasis. This distributive process of cerebrospinal fluid (CSF) volume transmission reaches many cellular targets in the CNS. In ageing and ageing-related dementias, the CP-CSF system is less able to regulate brain interstitial fluid. CP primarily generates CSF bulk flow, and so its malfunctioning exacerbates Alzheimers disease (AD). Considerable attention has been devoted to the blood-brain barrier in AD, but more insight is needed on regulatory systems at the human blood-CSF barrier in order to improve epithelial function in severe disease. Using autopsied CP specimens from AD patients, we immunocytochemically examined expression of heat shock proteins (HSP90 and GRP94), fibroblast growth factor receptors (FGFr) and a fluid-regulatory protein (NaK2Cl cotransporter isoform 1 or NKCC1). CP upregulated HSP90, FGFr and NKCC1, even in end-stage AD. These CP adjustments involve growth factors and neuropeptides that help to buffer perturbations in CNS water balance and metabolism. They shed light on CP-CSF system responses to ventriculomegaly and the altered intracranial pressure that occurs in AD and normal pressure hydrocephalus. The ability of injured CP to express key regulatory proteins even at Braak stage V/VI, points to plasticity and function that may be boosted by drug treatment to expedite CSF dynamics. The enhanced expression of human CP 'homeostatic proteins' in AD dementia is discussed in relation to brain deficits and pharmacology.

Figure 1

Schema for CSF convection of water and solutes: Arterial blood perfusing the choroid plexus continually provides water, ions and organic substrates for the CP epithelial cells to form the CSF that underlies volume transmission. Manufactured by the CP epithelium as the result of numerous transport processes, the CSF is actively secreted into the ventricles. In transit through the ventricular cavities to the arachnoid drainage sites, the CSF exchanges anabolites and catabolites with brain interstitial fluid. As a result, trophic and signaling molecules are delivered (blue arrow) to the neurons and, concurrently, toxic waste products and unneeded proteins are removed (red arrow) by CSF 'sink action' on the brain. The permeable ependymal membrane allows bi-directional diffusion of beneficial and harmful molecules. Bulk flow or volume transmission of CSF is thus essential in effecting the homeostasis of fluid composition.

Figure 2

Heat shock (stress) protein expression in human CP: Formalin-fixed, paraffin-embedded specimens (8–10 micrometers thick) were de-paraffinized and rehydrated. Sections were incubated overnight with antibodies against HSP90 (1/500; SPA830) and GRP94 (1/500;SPA850) and stained by the ABC technique (Vectastain Elite ABC peroxidase). Deposition of the brown chromogen (diaminobenzidine) reaction product was either substantially reduced by preabsorption blocking or virtually eliminated by omission of either primary or secondary antibodies. Slides were assessed blindly for staining intensity and distribution [37]. See text for description of localization and interpretation. For each HSP, images are representative of 10 AD specimens and 5 age-matched controls.

Figure 3

FGF receptor expression in human CP: Lateral ventricle plexuses obtained from pathologically-confirmed AD subjects were immersion-fixed in paraformaldehyde, cryoprotected and stored at -70°C. CP segments were free-floated for 72–96 hr in a 1/500 polyclonal antibody against the FGFr, which recognizes all FGF receptor subtypes. Specific staining was established by antibody omission/preabsorption [36]. ABC peroxidase/diaminobenzidine technique was used. Arrowhead points to small punctated dots of immunoreactivity. Localization of FGFr is described in text. Images are typical of those obtained from 8 controls and 8 AD specimens, respectively.

Figure 4

NaK2Cl cotransporter expression in human CP: Lateral ventricle plexuses were incubated with T4 (not thyroxine) antibody, which stains the secretory isoform 1 of the NaK2Cl (NKCC1) cotransporter protein. The T4 antibody (mouse monoclonal; 1:100) was from the University of Iowa Developmental Studies Hybridoma Bank (Iowa City, IA); the biotinylated secondary was a rat-absorbed horse antibody. Diaminobenzidine was used to develop the brown reaction product. Controls (negative staining results; not shown) involved omission of secondary and/or primary antibody. AD tissues were from patients at Braak stage V/VI (top right) and III/IV (bottom right). Images are representative of 6 CPs analyzed for AD (mean age of 76 yr) and 6 for age-matched controls (mean age of 76 yr). On average, the staining intensity of AD specimens was 50% greater than controls. The text describes staining localization. All photographs are at the same magnification.

Figure 5

A working model for hydrocephalus-induced alterations in NKCC1 cotransporter expression in CP: Ion transport across basolateral and apical surfaces is driven by transmembrane ion gradients [74, 83]. Normally the net transport of Na, K, Cl, and HCO3 from choroid cell into CSF is integral to CSF production [5, 7]. However, we postulate that when CSF formation is inhibited by various neurohumoral agents there is stimulated (+) inward NaK2Cl flux from CSF into the cell; this would increase cytoplasmic Na and Cl concentration [77] thus creating a less favorable ion gradient for basolateral uptake of Na and Cl from plasma. Consequently, there is inhibited (-) basolateral ion uptake, and sequentially, reduced apical extrusion of Na into CSF [40]. Net effect = decreased CSF formation. Consistent with this idea are observations of enhanced expression of CP NKCCl in congenital hydrocephalus [87] and AD (Fig. 4), both of which are generally associated with lower rates of CSF formation. Agents in Table 3 (e.g., Ang II) simultaneously stimulate the inward and outward arms of NKCC1, but the former three times the latter, resulting in net inward flux of ions [discussed in ref. 93]. The model thus vectorially emphasizes the inward arm of the NKCC1 (large arrowheads) as the one primarily stimulated by agents that suppress CSF formation. We hypothesize that in hydrocephalus, with increased intracranial pressure and/or ventriculomegaly, there is an associated attenuation of fluid output by CP.