Membrane transporters control cerebrospinal fluid formation independently of conventional osmosis to modulate intracranial pressure

Abstract

Background Disturbances in the brain fluid balance can lead to life-threatening elevation in the intracranial pressure (ICP), which represents a vast clinical challenge. Nevertheless, the details underlying the molecular mechanisms governing cerebrospinal fluid (CSF) secretion are largely unresolved, thus preventing targeted and efficient pharmaceutical therapy of cerebral pathologies involving elevated ICP. Methods Experimental rats were employed for in vivo determinations of CSF secretion rates, ICP, blood pressure and ex vivo excised choroid plexus for morphological analysis and quantification of expression and activity of various transport proteins. CSF and blood extractions from rats, pigs, and humans were employed for osmolality determinations and a mathematical model employed to determine a contribution from potential local gradients at the surface of choroid plexus. Results We demonstrate that CSF secretion can occur independently of conventional osmosis and that local osmotic gradients do not suffice to support CSF secretion. Instead, the CSF secretion across the luminal membrane of choroid plexus relies approximately equally on the Na+/K+/2Cl− cotransporter NKCC1, the Na+/HCO3− cotransporter NBCe2, and the Na+/K+-ATPase, but not on the Na+/H+ exchanger NHE1. We demonstrate that pharmacological modulation of CSF secretion directly affects the ICP. Conclusions CSF secretion appears to not rely on conventional osmosis, but rather occur by a concerted effort of different choroidal transporters, possibly via a molecular mode of water transport inherent in the proteins themselves. Therapeutic modulation of the rate of CSF secretion may be employed as a strategy to modulate ICP. These insights identify new promising therapeutic targets against brain pathologies associated with elevated ICP.

Modelling idiopathic intracranial hypertension in rats: contributions of high fat diet and testosterone to intracranial pressure and cerebrospinal fluid production

“…To resolve the molecular mechanisms underlying the testosterone-induced elevated CSF secretion, the activity of two choroid plexus transport mechanisms, the Na + /K + -ATPase and the Na + /K + /2Cl - cotransporter (NKCC1), both known to contribute to CSF secretion [54] was assessed. To determine the contribution of the Na + /K + -ATPase to the increased CSF production observed in the testosterone-treated rats, its transport activity was determined in acutely excised choroid plexus from testosterone-treated rats and their control counterparts by influx assays with the radioisotope 86 Rb + , as a congener of K + , in the absence and presence of the Na + /K + -ATPase inhibitor ouabain (Fig.…”

“…Adjuvant testosterone treatment of lean female rats was then employed to mimic the elevated levels of testosterone in the IIH patients. The elevated CSF levels of testosterone caused an increased rate of CSF production in these rats, which was, at least in part, associated with augmented activity of the choroid plexus transport protein, NKCC1, which has been demonstrated as a key contributor to CSF secretion in mice, rats, and dogs [51, 54, 76, 77]. The testosterone-mediated effect on the choroid plexus transport protein and the CSF secretion rate only came about with prolonged treatment with testosterone (4 weeks) and was absent with acute (1 h) testosterone exposure of the excised choroid plexus.…”

“…To determine if the elevated testosterone modulated the rate of CSF secretion, the LI-COR imaging system was used to acutely assess the CSF flow in testosterone-treated female Wistar rats. This experimental approach relies on injection of a fluorescent dye into the lateral ventricle of the anaesthetized rat, after which the rat is swiftly transferred to the whole animal fluorescent reader and the fluorescence dispersion rate obtained as a proxy for the CSF secretion rate [51,[54][55][56] (Fig. 5A).…”

“…0.13 µg/g bodyweight, n = 5 vs control: 7.94 ± 0.23 µg/g bodyweight, n = 5, p < 0.01, Fig.5F), likely due to the excess bodyweight gain induced by testosterone treatment.Transport activity of NKCC1, not the Na + /K + -ATPase, is affected by testosterone treatment To resolve the molecular mechanisms underlying the testosterone-induced elevated CSF secretion, the activity of two choroid plexus transport mechanisms, the Na ), both known to contribute to CSF secretion[54] was assessed. To determine the ATPase to the increased CSF production observed in the testosteronetreated rats, its transport activity was determined in acutely excised choroid plexus from testosterone-treated rats and their control counterparts by influx assays with the activity, which was not significantly different between the two experimental groups (TT: 14.7 ± 6.4 × 10 3 cpm, n = 4 vs control: 19.2 ± 3.8 × 10 3 cpm, n = 4, p = 0.25, Fig.…”

Nocturnal increase in cerebrospinal fluid secretion as a regulator of intracranial pressure

“…Some researchers have expressed concern about this technique [40–42], one of which based on an assumption that the dextran enters the brain parenchyma during the experimental procedure, which would cause an artificially low amount of dextran to exit the cisterna magna puncture. We cannot detect any dextran in the brain parenchyma of rats following execution of a full experimental procedure [36] and suspect that Liu et al [43] detected such parenchymal tracer entry due to the lack of a cisterna magna puncture in their experimental approach. Their constant tracer infusion in a mouse lateral ventricle (42 μl in total) with no experimentally-induced exit path for the excess brain fluid could well promote their observed fluorescent entry into the brain tissue.…”