HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) All Versions of this Article: 19/1/76 04-1711fjev1    most recent Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Oshio, K. Articles by Manley, G. T. Search for Related Content PubMed PubMed Citation Articles by Oshio, K. Articles by Manley, G. T.

Reduced cerebrospinal fluid production and intracranial pressure in mice lacking choroid plexus water channel Aquaporin-1

Kotaro Oshio^*, Hiroyuki Watanabe^*, Yaunlin Song, A. S. Verkman and Geoffrey T. Manley^*,1

^* Department of Neurosurgery and
Departments of Medicine and Physiology, University of California, San Francisco, California, USA

¹Correspondence: Department of Neurological Surgery, University of California, San Francisco, 1001 Potrero Ave., Bldg. 1, Room 101, San Francisco, CA 94110, USA. E-mail: manley{at}itsa.ucsf.edu

SPECIFIC AIMS

Aquaporin-1 (AQP1) is a water channel strongly expressed at the ventricular facing surface of choroid plexus epithelium (CPE), where it is thought to play an important role in facilitating water transport across the CPE apical membrane during secretion of cerebrospinal fluid (CSF). The purpose of this study was to define the role of AQP1 in CPE osmotic water permeability, CSF production and absorption, and intracranial pressure (ICP) regulation.

PRINCIPAL FINDINGS

1. Decreased osmotic water permeability in choroid plexus from AQP1 null mice
Relative CPE cell volume was measured using a spatial filtering light microscopy method. Changing the CSF perfusate osmolality from 300 to 150 mOsm in wild-type mice resulted in a rapid increase in cell volume that was reversible upon perfusion with a 300 mOsm solution. Both swelling and shrinking were slowed by 4.8-fold in AQP1-deficient CPE.

2. Decreased intracranial pressure (ICP) in AQP1 null mice
A method was developed to obtain reproducible, dynamic ICP measurements in mice. Measurements in a series of wild-type and AQP1 null mice demonstrated a significant 56% reduction in ICP in AQP1 null mice (4.2±0.8 cm H₂O vs. 9.5±3.2 cm H₂O for wild-type mice). The central venous pressure (CVP) was measured via canulation of the jugular vein. CVP was 4.3 ± 0.7 cm H₂O in wild-type mice and significantly reduced to 0.8 ± 0.3 cm H₂O in AQP1 null mice, secondary to the effects of AQP1 on the kidney. To establish whether the reduction in ICP was caused by the absence of AQP1 in the choroid plexus or the systemic effects of AQP1 deletion in the kidney, ICP and CVP were measured in mice lacking AQP3. Mice deficient in AQP3 also manifest marked polyuria and volume depletion due to reduced water permeability in the collecting duct. Similar to AQP1-deficient mice, AQP3-deficient mice had significantly reduced CVP (0.1±0.2 cm H₂O). However, the ICP in AQP3-deficient mice was reduced by only 24%.

3. Decreased CSF production in AQP1 null mice
The rate of CSF production was quantified using a modified Pappenheimer indicator dilution method (Fig. 1 A). CSF production was reduced by 20% in AQP1 null mice (Fig. 1B ). Acetazolamide, a carbonic anhydrase inhibitor of CSF formation, reduced CSF production to similar levels in wild-type and AQP1 null mice. The cAMP agonist forskolin increased CSF production in wild-type and AQP1 null mice. The absolute rate of CSF production AQP1 null mice after forskolin was 25% less than that in wild-type mice. The rate of pressure-dependent CSF outflow was measured by continuous infusion (Fig. 1C ). Infusion of aCSF into the lateral ventricle resulted in an increase in ICP that reached a constant value that depended on the infusion rate. The slope indicates that pressure-dependent outflow was not different in wild-type and AQP1 null mice. The intercept of the regression line on the pressure axis (extrapolated to zero infusion) was computed for each experiment as an alternative measure of resting ICP. The extrapolated ICP was reduced by 3-fold in AQP1 null mice in agreement with previous resting measurements.

View larger version (30K): [in this window] [in a new window]   Figure 1. CSF dynamics. A) Micropipettes are introduced into the lateral ventricle (LV) and the cisterna magna (CM). Left: representative time course of fluorescence ratio (outflow/inflow) (top) and ICP (bottom) during ventriculo-cisternal perfusion. B) CSF formation in wild-type and AQP1 null mice measured by indicator dilution method. The CSF formation rates at baseline were significantly decreased in AQP1 null mice. The CSF production stimulated with forskolin was reduced by 25% in AQP1 null mice (*P<0.01, ANOVA). Right-side scale indicates the fluorescence ratio (outflow/inflow). C) CSF outflow resistance analyzed by the constant rate infusion method. Left: representative experiment showing an ICP recording in response to continuous fluid infusion. The infusion rate was transiently increased from 1 to 10 µL min–1, resulting in a stepwise increase of ICP. Inset: magnified view of ICP at 8 µL min–1. Right: plots of plateau pressure vs. infusion rate for wild-type mice (open circles) and AQP1 null mice (filled circles) determined from experiments shown at left. The slopes give CSF outflow resistance. Differences between slopes are not significant. The extrapolated ICP was significantly reduced in AQP1 null mice (P<0.01, ANOVA).

4. Decreased ICP and improved survival after cold injury brain trauma in AQP1 null mice
A cortical-freeze brain injury model was used to examine the effects of a focal mass lesion on ICP and clinical outcome. As shown in Fig. 2 A, this insult produced a significant increase in the tissue water content of the injured ipsilateral vs. contralateral hemispheres but no differences in the increase in brain water content in wild-type mice and AQP1 null mice. However, there was a significant difference in the peak ICP after cold injury (Fig. 2B ). In the wild-type mouse, severe cold injury produced a steady increase in ICP that resulted in brain herniation and respiratory arrest as evidenced by the sharp drop in the continuous pressure measurement (Fig. 2C , top). In the AQP1 null mouse, ICP increased after injury but plateaued at a much lower pressure with no death (Fig. 2C , bottom). At 4 h, only 25% of the wild-type mice were alive (Fig. 2D ) vs. 87% of the AQP1 null mice.

View larger version (25K): [in this window] [in a new window]   Figure 2. Tissue water content, intracranial pressure (ICP), and survival after cold-induced brain injury. A) Tissue water content of the contralateral control hemisphere and ipsilateral injured hemisphere 60 min after focal insult in wild-type (+/+) and AQP1 null (–/–) mice (mean ±SE, n=6). B) Peak ICP after cold-induced brain injury demonstrates a 60% reduction in AQP1 null mice. *P <0.002 (ANOVA). C) Representative time courses of ICP after cold-induced injury. D) Effect of severe cold-induced injury on survival rate. Mice were followed for 3 h.

CONCLUSIONS AND SIGNIFICANCE

The morphology of the choroid plexus and its location within the cerebral ventricles suggest it is the principal site of CSF production (Fig. 3 ). Early studies showed a high ratio of osmotic-to-diffusional water permeability of the choroid epithelium, suggesting the presence of an aqueous pore-like pathway. The location of the AQP1 water channel in the apical membrane of the choroid plexus provides an explanation of these findings. Furthermore, deletion of AQP1 reduced osmotically driven water permeability by nearly 5-fold across the choroidal epithelium. However, CSF production was reduced in AQP1-deficient mice by only 20–25%. There are several possible explanations for the modest reduction in CSF production. The simplest is that AQP1-facilitated transcellular water transport accounts for only part of the total CSF production, with the balance of water transport occurring through paracellular and non-AQP1-mediated transcellular routes (Fig. 3) . A more controversial possibility is that the choroid plexus may not be the principal site of CSF production and that extrachoroidal CSF production by the brain parenchyma may be significant. Milhorat reported in 1971 that CSF production measured after removal of the choroid plexus is reduced by only 31%. Thus, it is possible that, in vivo, the choroid plexus may only account for 30–50% of CSF production. If so, then water transport by AQP1 may account for a substantial percentage of choroidal CSF production.

View larger version (31K): [in this window] [in a new window]   Figure 3. A) Schematic of CSF production and steady-state ICP regulation. CSF fluid is produced by transchoroidal (arrow) and extrachoroidal routes (dot arrow). Left upper schema indicates molecular channels of water and ions in choroid plexus epithelium. Water can enter the ventricle in transcellular and paracellular routes via choroid plexus. B) Systemic and cerebral effects of AQP1 deletion act synergistically to decrease ICP.

Perhaps the most intriguing observation in these experiments was the significant reduction of ICP in AQP1 null mice. ICP was reduced >50% in AQP1 null mice as measured by two different methods. ICP is the dynamic result of the interaction of the CSF, cerebral blood, and brain tissue compartments (Fig. 3) . However, the individual effect of each compartment on steady-state ICP is difficult to assess due to the complexity of the physiologic relationship between these compartments and their nonlinear behavior. The steady-state ICP can be described by a simplified equation: ICp = I_f x R_out + P_ss, where I_f is CSF formation rate, R_out is outflow resistance, and P_ss is sagittal sinus pressure. Assuming there is no effect of AQP1 deletion on outflow resistance, 25% of the reduced ICP in AQP1 null mice is a consequence of reduced CSF production. Although the sagittal sinus pressure was not measured directly, measurements from the jugular vein (the major draining vein of the brain in continuity with the sagittal sinus) revealed a mean 3.5 mmHg reduction in pressure in AQP1 null mice. Thus, the effect on venous pressure appears to account for the balance of the reduction in ICP in AQP1 null mice. The decrease in central venous pressure is likely due to the urinary concentrating defect previously reported for the AQP1 null mice. To verify that decreased ICP in AQP1 null mice was not due solely to the secondary effect of volume depletion, ICP and CVP were measured in AQP3 null mice, which have hypovolemia secondary to a urinary concentrating defect. AQP3 null mice had significantly decreased CVP, but the ICP was reduced only about half as much as the AQP1 null mice. Together, these results demonstrate that two different AQP1-dependent mechanisms act synergistically to significantly decrease in ICP.

Elevation of ICP can result in irreversible injury to the central nervous system. Increased ICP is a common pathophysiological mechanism for many life-threatening neurological disorders. The ability of AQP1 deletion to decrease 2 of the 3 principal determinants of steady-state ICP, CSF production, and venous pressure suggests that AQP1deletion might be protective in a model of brain trauma. Here we report that AQP1 null mice have significantly lower ICP and improved survival after cold-induced brain injury. Measurements of tissue water content showed that these results could not be accounted for by differences in the extent of the focal lesions. Because brain compliance is nonlinear, small increments in water volume produce large changes in ICP. This is why in clinical practice drainage of even small amounts of CSF is recommended for the treatment of elevated ICP in traumatic brain injury. Thus, the decrease in CSF production and reduced CVP in AQP1 null mice could have a significant effect on elevated ICP. There is currently no consensus on treatment for increased ICP. Results presented here provide evidence for the importance of CSF production and CVP on ICP under normal and pathological conditions and suggest a novel application of AQP1 inhibitors for pharmacological treatment of elevated ICP.

FOOTNOTES

To read the full text of this article, go to http://www.fasebj.org/cgi/doi/10.1096/fj.04-1711fje;

This article has been cited by other articles:

				M. J. Wiegman, L. V. Bullinger, M. M. Kohlmeyer, T. C. Hunter, and M. J. Cipolla Regional Expression of Aquaporin 1, 4, and 9 in the Brain During Pregnancy Reproductive Sciences, May 1, 2008; 15(5): 506 - 516. [Abstract] [PDF]

				M. R. Hodges, G. J. Tattersall, M. B. Harris, S. D. McEvoy, D. N. Richerson, E. S. Deneris, R. L. Johnson, Z.-F. Chen, and G. B. Richerson Defects in Breathing and Thermoregulation in Mice with Near-Complete Absence of Central Serotonin Neurons J. Neurosci., March 5, 2008; 28(10): 2495 - 2505. [Abstract] [Full Text] [PDF]

				S. Jacobs, E. Ruusuvuori, S. T. Sipila, A. Haapanen, H. H. Damkier, I. Kurth, M. Hentschke, M. Schweizer, Y. Rudhard, L. M. Laatikainen, et al. Mice with targeted Slc4a10 gene disruption have small brain ventricles and show reduced neuronal excitability PNAS, January 8, 2008; 105(1): 311 - 316. [Abstract] [Full Text] [PDF]

				A. J. Yool Aquaporins: Multiple Roles in the Central Nervous System Neuroscientist, October 1, 2007; 13(5): 470 - 485. [Abstract] [PDF]

				J. G. Kim, Y. J. Son, C. H. Yun, Y. I. Kim, I. S. Nam-goong, J. H. Park, S. K. Park, S. R. Ojeda, A. V. D'Elia, G. Damante, et al. Thyroid Transcription Factor-1 Facilitates Cerebrospinal Fluid Formation by Regulating Aquaporin-1 Synthesis in the Brain J. Biol. Chem., May 18, 2007; 282(20): 14923 - 14931. [Abstract] [Full Text] [PDF]

				M. H. Levin and A. S. Verkman Aquaporin-3-Dependent Cell Migration and Proliferation during Corneal Re-epithelialization. Invest. Ophthalmol. Vis. Sci., October 1, 2006; 47(10): 4365 - 4372. [Abstract] [Full Text] [PDF]

				J. Praetorius and S. Nielsen Distribution of sodium transporters and aquaporin-1 in the human choroid plexus Am J Physiol Cell Physiol, July 1, 2006; 291(1): C59 - C67. [Abstract] [Full Text] [PDF]

				M. Hara-Chikuma and A.S. Verkman Aquaporin-1 Facilitates Epithelial Cell Migration in Kidney Proximal Tubule J. Am. Soc. Nephrol., January 1, 2006; 17(1): 39 - 45. [Abstract] [Full Text] [PDF]

				A. S. Verkman More than just water channels: unexpected cellular roles of aquaporins J. Cell Sci., August 1, 2005; 118(15): 3225 - 3232. [Abstract] [Full Text] [PDF]

				M. C. Papadopoulos and A. S. Verkman Aquaporin-4 Gene Disruption in Mice Reduces Brain Swelling and Mortality in Pneumococcal Meningitis J. Biol. Chem., April 8, 2005; 280(14): 13906 - 13912. [Abstract] [Full Text] [PDF]

This Article Full Text (PDF) All Versions of this Article: 19/1/76 04-1711fjev1    most recent Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Oshio, K. Articles by Manley, G. T. Search for Related Content PubMed PubMed Citation Articles by Oshio, K. Articles by Manley, G. T.