Tissue Barriers

Increased aquaporin-1 and Na+ -K+ -2Cl- cotransporter 1 expression in choroid plexus leads to blood-cerebrospinal fluid barrier disruption and necrosis of hippocampal CA1 cells in acute rat models of hyponatremia

Hyponatremia is a metabolic disorder characterized by increased cerebrospinal fluid (CSF) volume and pressure, although the site of brain insult is unclear. Specifically, the hippocampus, which is in direct contact with expanding CSF ventricles, may be involved. The present study was undertaken to investigate the possible roles of choroid plexus aquaporin-1 (AQP1) and of cation chloride transporters (Na(+) -K(+) -2Cl(-) cotransporter 1 [NKCC1] and K(+) -Cl(-) cotransporter 4 [KCC4]) in the underlying hippocampal pathophysiology of hyponatremia in acute (6 and 12 hr duration) experimental models. It was found that the expressions of AQP1 and NKCC1 proteins in choroid plexus were significantly increased, whereas the expression of KCC4 protein was unchanged vs. control values after 6 and 12 hr of hyponatremia. Choroid plexuses with increased AQP1 and NKCC1 after 6 hr of hyponatremia showed caspase 3-dependent apoptosis and disruption of the blood-CSF barrier. Furthermore, necrotic changes in CA1 neuronal cells were observed after 6 and 12 hr of hyponatremia. Overall, these data suggest that increases in AQP1 and NKCC1 expression under hyposmotic stress may be one of the molecular mechanisms underlying the pathophysiology of acute hyponatremia, such as the necrotic cell death of hippocampal CA1 region by increasing water transport across the blood-CSF barrier. Furthermore, we suggest that opening of the blood-CSF barrier after acute hyponatremia may be triggered the secondary adverse conditions that are capable of enhancing selective necrosis in hippocampal CA1 cells.

Human choroid plexus growth factors: What are the implications for CSF dynamics in Alzheimer's disease?

The choroid plexus plays a key role in supporting neuronal function by secreting cerebrospinal fluid (CSF) and may be involved in the regulation of various soluble factors. Because the choroid plexus is involved in growth factor secretion as well as CSF dynamics, it is important to understand how growth factors in CSF interact with the brain parenchyma as well as with cells in direct contact with the flowing CSF, i.e., choroid plexus and arachnoid villi. While the existence of growth factors in the choroid plexus has been documented in several animal models, the presence and distribution of growth factors in the human choroid plexus has not been extensively examined. This study describes the general distribution and possible functions of a number of key proteins in the human choroid plexus and arachnoid villi, including basic fibroblast growth factor, FGF receptor, and vascular endothelial growth factor. FGF and VEGF could both be readily demonstrated in choroid plexus epithelial cells. The presence of FGF and VEGF within the choroid plexus was also confirmed by ELISA analysis. Since Alzheimer's disease (AD) is known to be associated with a number of growth factor abnormalities, we examined the choroid plexus and arachnoid villi from AD patients. Immunohistochemical studies revealed the presence of FGF and VEGF within the AD choroid plexus and an increased density of FGFr in both the choroid plexus and the arachnoid villi of AD patients. No qualitative changes in the distribution of FGF and VEGF were observed in the AD choroid plexus. The appearance of FGFr in AD arachnoid was associated with robust amyloid and vimentin immunoreactivity. These findings confirm the presence of FGF and VEGF within the normal and AD choroid plexus and suggest that the alteration of growth factors and their receptors may contribute to the pathogenesis of the hydrocephalus ex vacuo that is characteristically seen in AD.

Rapid distribution of intraventricularly administered sucrose into cerebrospinal fluid cisterns via subarachnoid velae in rat

The intracranial distribution of [14C]sucrose, an extracellular marker infused for 30 s into one lateral ventricle, was determined by autoradiography of frozen-dried brain sections. Within 3.5 min [14C]sucrose appeared in: (i) the third ventricle, including optic, infundibular and mammillary recesses; (ii) the aqueduct of Sylvius; (iii) the velum interpositum, a part of the subarachnoid space that runs along the roof of the third ventricle and contains many blood vessels; (iv) the mesencephalic and fourth ventricles; and (v) the superior medullary velum, a highly vascular extension of the subarachnoid space that terminates at the walls of the mesencephalic and fourth ventricles. Within 5 min, radioactivity was present in the interpeduncular, ambient and quadrigeminal cisterns, which encircle the midbrain. By 10 min, approximately 11% of the radioactivity had passed into the subarachnoid space via a previously undescribed flow pathway that included the velum interpositum and superior medullary velum. At many places along the ventricular system, [14C]sucrose appeared to move from cerebrospinal fluid into the adjacent tissue by simple diffusion, as reported previously (Blasberg R. G. et al. (1974) J. Pharmac. exp. Ther. 195, 73-83; Levin V. A. et al. (1970) Am. J. Physiol. 219, 1528-1533; Patlak C. and Fenstermacher J.D. (1975) Am. J. Physiol. 229, 877-884; Rosenberg G. A. and Kyner W.T. (1980) Brain Res. 193, 56-66; Rosenberg G. A. et al. (1986) Am. J. Physiol 251, F485-F489). Little sucrose was, however, taken up by: (i) circumventricular organs such as the subfornical organ; (ii) medullary and cerebellar tissue next to the lateral recesses; and (iii) the superior and inferior colliculi and cerebral peduncles. For the latter two groups of structures, entry from cerebrospinal fluid was apparently blocked by a thick, multilayered glia limitans. Although [14C]sucrose was virtually absent from the rest of the subarachnoid system after 1 h, it remained in the perivascular spaces and/or walls of pial arteries and arterioles for more than 3 h. Certain transport proteins, protease inhibitors, growth factors and other neurobiologically active materials are present in cerebrospinal fluid, and their distribution to the brain and its blood vessels may be important. The present work shows, in the rat, that the flow of cerebrospinal fluid and the disposition of its constituents is fairly complex and differs among regions. Flow was rapid throughout the ventricular system and into various subarachnoid velae and cisterns, but was surprisingly slow and slight over the cerebral and cerebellar cortices. The cerebrospinal fluid-to-tissue flux of material was relatively free at many interfaces, but was greatly restricted at others, the latter indicating that the old concept of a "cerebrospinal fluid-brain barrier" may hold at such places. Finally, radiolabeled sucrose was retained longer within the walls and perivascular spaces of pial arteries and arterioles than in other subarachnoid tissues; one function of the cerebrospinal fluid system or "third circulation" may thus be delivering factors and agents to these pial blood vessels.

Cystatin C, a protease inhibitor, in degenerating rat hippocampal neurons following transient forebrain ischemia

Cystatin C, a cysteine protease inhibitor produced by the choroid plexus and found in CSF at high concentrations, may have an important role in brain injury. We used the two-vessel occlusion model with hypotension to induce transient forebrain ischemia (TFI) in rats for 10 min and then examined cystatin C immuno-like reactivity (CC-IR) after 1, 3, 7 and 14 days of recovery. Our results reveal that CC-IR was minimal or absent in the hippocampus of normal and 1 day post-ischemic animals. However, CC-IR was present in CA1 pyramidal cells and a small number of reactive glia of the stratum radiatum (SR) and stratum oriens (SO) at 3, 7 and 14 days post-ischemia. Histological assessment of the hippocampus indicates that CC-IR was localized in morphologically degenerative neurons. This distinct temporal expression of cystatin C in the rat hippocampus is concurrent with delayed neuronal death following TFI. Thus, these results indicate that cystatin C and/or its substrates may be important components of the post-ischemic neurodegenerative and repair process.

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.