Hydrocephalus and aquaporins: lessons learned from the bench

Generation of Periventricular Reactive Astrocytes Overexpressing Aquaporin 4 Is Stimulated by Mesenchymal Stem Cell Therapy

Aquaporin-4 (AQP4) plays a crucial role in brain water circulation and is considered a therapeutic target in hydrocephalus. Congenital hydrocephalus is associated with a reaction of astrocytes in the periventricular white matter both in experimental models and human cases. A previous report showed that bone marrow-derived mesenchymal stem cells (BM-MSCs) transplanted into the lateral ventricles of hyh mice exhibiting severe congenital hydrocephalus are attracted by the periventricular astrocyte reaction, and the cerebral tissue displays recovery. The present investigation aimed to test the effect of BM-MSC treatment on astrocyte reaction formation. BM-MSCs were injected into the lateral ventricles of four-day-old hyh mice, and the periventricular reaction was detected two weeks later. A protein expression analysis of the cerebral tissue differentiated the BM-MSC-treated mice from the controls and revealed effects on neural development. In in vivo and in vitro experiments, BM-MSCs stimulated the generation of periventricular reactive astrocytes overexpressing AQP4 and its regulatory protein kinase D-interacting substrate of 220 kDa (Kidins220). In the cerebral tissue, mRNA overexpression of nerve growth factor (NGF), vascular endothelial growth factor (VEGF), hypoxia-inducible factor-1 (HIF1α), and transforming growth factor beta 1 (TGFβ1) could be related to the regulation of the astrocyte reaction and AQP4 expression. In conclusion, BM-MSC treatment in hydrocephalus can stimulate a key developmental process such as the periventricular astrocyte reaction, where AQP4 overexpression could be implicated in tissue recovery.

A comparison of Adult and Pediatric Hydrocephalus

Hydrocephalus is a common clinical problem encountered in neurosurgical practice. With greater subspecialisation, pediatric neurosurgery has emerged as a special discipline in several countries. However, in the developing world, which inhabits a large pediatric population, a limited number of neurosurgeons manage all types of hydrocephalus across all ages. There are some essential differences in pediatric and adult hydrocephalus. The spectrum of hydrocephalus of dysgenetic origin in a neonate and that of normal pressure hydrocephalus of the old age has a completely different strategy of management. Endoscopic third ventriculostomy outcomes are known to be closely associated with age at presentation and surgery. Efficacy of alternative pathways of CSF absorption also differs according to age. Managing this disease in various age groups is challenging because of these differences in etiopathology, tempo of the disease, modalities of investigations and various treatment protocols as well as prognosis.

Evaluation of aquaporins in the cerebrospinal fluid in patients with idiopathic normal pressure hydrocephalus

Brain aquaporin 1 (AQP1) and AQP4 are involved in cerebrospinal fluid (CSF) homeostasis and might participate in the origin of hydrocephalus. Studies have shown alterations of perivascular AQP4 expression in idiopathic normal pressure hydrocephalus (iNPH) and Alzheimer's disease (AD). Due to the overlapping of clinical signs between iNPH and certain neurological conditions, mainly AD, specific biomarkers might improve the diagnostic accuracy for iNPH. The goal of the present study was to analyze and quantify the presence of AQP1 and AQP4 in the CSF of patients with iNPH and AD to determine whether these proteins can be used as biomarkers of iNPH. We examined AQP1 and AQP4 protein levels in the CSF of 179 participants (88 women) classified into 5 groups: possible iNPH (81 participants), hydrocephalus associated with other neurological disorders (13 participants), AD (41 participants), non-AD dementia (32 participants) and healthy controls (12 participants). We recorded each participant's demographic and clinical variables and indicated, when available in the clinical history, the record of cardiovascular and respiratory complications. An ELISA showed virtually no AQP content in the CSF. Information on the vascular risk factors (available for 61 patients) confirmed some type of vascular risk factor in 86% of the patients with possible iNPH and 58% of the patients with AD. In conclusion, the ELISA analysis showed insufficient sensitivity to detect the presence of AQP1 and AQP4 in CSF, ruling out the possible use of these proteins as biomarkers for diagnosing iNPH.

A Rat Model of Neurocysticercosis-Induced Hydrocephalus: Chronic Progressive Hydrocephalus with Mild Clinical Impairment

BACKGROUND

Hydrocephalus is the most common complication of extraparenchymal neurocysticercosis, combining obstructive and inflammatory mechanisms that impair cerebrospinal fluid circulation.

METHODS

We studied the long-term progression of neurocysticercosis-induced hydrocephalus in a rat model. We generated an experimental rat model of neurocysticercosis-induced hydrocephalus by cisternal inoculation of cysts or antigens of Taenia crassiceps and compared it with the classic model of kaolin-induced hydrocephalus. We used 52 animals divided into 4 groups: 1) control, 2) neurocysticercosis-induced hydrocephalus by cysts or 3) by antigens, and 4) kaolin-induced hydrocephalus. We studied behavioral, radiologic, and morphologic alterations at 1 and 6 months after inoculation by open field test, magnetic resonance imaging, and immunohistochemical localization of aquaporin-4 (AQP-4).

RESULTS

Behavioral changes were observed later in neurocysticercosis-induced than in kaolin-induced hydrocephalic rats (P = 0.023). The ventricular volume of hydrocephalus induced by experimental neurocysticercosis progressively evolved, with the magnetic resonance imaging changes being similar to those observed in humans. Periventricular inflammatory and astrocytic reactions were also observed. AQP-4 expression was higher in the sixth than in the first month after inoculation (P = 0.016) and also occurred in animals that received antigen inoculation but did not develop hydrocephalus, suggesting that AQP-4 may constitute an alternative route of cerebrospinal fluid absorption under inflammatory conditions.

CONCLUSIONS

Our neurocysticercosis-induced hydrocephalus model allows for the long-term maintenance of hydrocephalic animals, involving mild clinical performance impairments, including body weight and behavioral changes.

Erythropoietin-mediated activation of aquaporin-4 channel for the treatment of experimental hydrocephalus

OBJECTIVE

In this study, we investigate a neuroprotective agent, erythropoietin (EPO), in animal hydrocephalus model and its potential reversal effects on hydrocephalus by altering the expression of aquaporin-4 (AQP4).

METHODS

Obstructive hydrocephalus was induced in 2-week-old rat pups by injecting kaolin (50 μl, 10 mg/ml in saline) into the cisterna magna, while the control pups received only saline. Kaolin-injected pups were divided into two groups on the fifth day after kaolin injection; one group received intra-peritoneal (i.p.) EPO (1 μg/pup) for 5 consecutive days, while other group received i.p. saline for 5 days. The effects of EPO on hydrocephalus were investigated by studying cerebral ventricle size and structural ependymal changes. We examined also the EPO effects on AQP4 expression and microRNA expression.

RESULTS

EPO treatment significantly reduced dilation of the cerebral ventricle and denudation of ependymal line in hydrocephalic pups comparing with the control group. Increased expression of AQP4 in periventricular ependymal lining and cultured astrocytes and increased vascular formation were noted after EPO treatment. Additionally, we identified miR-668 as an endogenous regulator of AQP4 in response to EPO. Anti-miR-668 dampened EPO-induced activation of AQP4 expression.

CONCLUSIONS

Together, our results show that EPO-mediated upregulation of AQP4 significantly reduces dilation of the cerebral ventricles in obstructive hydrocephalus pups and may lead to potential therapeutic options for hydrocephalus.

Acetazolamide Mitigates Intracranial Pressure Spikes Without Affecting Functional Outcome After Experimental Hemorrhagic Stroke

Increased intracranial pressure (ICP) after stroke can lead to poor outcome and death. Novel treatments to combat ICP rises are needed. The carbonic anhydrase inhibitor acetazolamide diminishes cerebrospinal fluid (CSF) production, reduces ICP in healthy animals, and is beneficial for idiopathic intracranial hypertension patients. We tested whether acetazolamide mitigates ICP elevations by presumably decreasing CSF volume after collagenase-induced striatal hemorrhage in rats. We confirmed that acetazolamide did not adversely affect hematoma formation in this model or physiological variables, such as temperature. Then, we assessed the effects of acetazolamide on ICP. Lastly, we tested the effects of acetazolamide on behavioral and histological outcome. Acetazolamide reduced the magnitude and occurrence of short-timescale ICP spikes, assessed as disproportionate increases in ICP (sudden ICP increases > 10 mmHg), 1-min peak ICP, and the magnitude of spikes > 20 mmHg. However, mean ICP was unaffected. In addition, acetazolamide reduced ICP variability, reflecting improved intracranial compliance. Compliance measures were strongly correlated with high peak and mean ICP, whereas ipsilateral hemisphere water content was not correlated with ICP. Despite effects on ICP, acetazolamide did not improve behavioral function or affect lesion size. In summary, we show that intracerebral hemorrhage creates an impaired compliance state within the cranial space that can result in large, transient ICP spikes. Acetazolamide ameliorates intracranial compliance and mitigates ICP spikes, but does not improve functional outcome, at least for moderate-severity ICH in rats.

Extracranial outflow of particles solved in cerebrospinal fluid: Fluorescein injection study

Cerebrospinal fluid is thought to be mainly absorbed into arachnoid granules in the subarachnoid space and drained into the sagittal sinus. However, some observations such as late outbreak of arachnoid granules in fetus brain and recent cerebrospinal fluid movements study by magnetic resonance images, conflict with this hypothesis. In this study, we investigated the movement of cerebrospinal fluid in fetuses. Several kinds of fluorescent probes with different molecular weights were injected into the lateral ventricle or subarachnoid space in mouse fetuses at a gestational age of 13 days. The movements of the probes were monitored by live imaging under fluorescent microscope. Following intraventricular injection, the probes dispersed into the 3rd ventricle and aqueduct immediately, but did not move into the 4th ventricle and spinal canal. After injection of low and high molecular weight conjugated probes, both probes dispersed into the brain but only the low molecular weight probe dispersed into the whole body. Following intra-subarachnoid injection, both probes diffused into the spinal canal gradually. Neither probe dispersed into the brain and body. The probe injected into the lateral ventricle moved into the spinal central canal by the fetus head compression, and returned into the aqueduct by its release. We conclude this study as follows: (i) The movement of metabolites in cerebrospinal fluid in the ventricles will be restricted by molecular weight; (ii) Cerebrospinal fluid in the ventricle and in the subarachnoid space move differently; and (iii) Cerebrospinal fluid may not appear to circulate. In the event of high intracranial pressure, the fluid may move into the spinal canal.

Blood-brain barrier and foetal-onset hydrocephalus, with a view on potential novel treatments beyond managing CSF flow

Despite decades of research, no compelling non-surgical therapies have been developed for foetal hydrocephalus. So far, most efforts have pointed to repairing disturbances in the cerebrospinal fluid (CSF) flow and to avoid further brain damage. There are no reports trying to prevent or diminish abnormalities in brain development which are inseparably associated with hydrocephalus. A key problem in the treatment of hydrocephalus is the blood–brain barrier that restricts the access to the brain for therapeutic compounds or systemically grafted cells. Recent investigations have started to open an avenue for the development of a cell therapy for foetal-onset hydrocephalus. Potential cells to be used for brain grafting include: (1) pluripotential neural stem cells; (2) mesenchymal stem cells; (3) genetically-engineered stem cells; (4) choroid plexus cells and (5) subcommissural organ cells. Expected outcomes are a proper microenvironment for the embryonic neurogenic niche and, consequent normal brain development.

Cerebrospinal fluid hypersecretion in pediatric hydrocephalus

Hydrocephalus, despite its heterogeneous causes, is ultimately a disease of disordered CSF homeostasis that results in pathological expansion of the cerebral ventricles. Our current understanding of the pathophysiology of hydrocephalus is inadequate but evolving. Over this past century, the majority of hydrocephalus cases has been explained by functional or anatomical obstructions to bulk CSF flow. More recently, hydrodynamic models of hydrocephalus have emphasized the role of abnormal intracranial pulsations in disease pathogenesis. Here, the authors review the molecular mechanisms of CSF secretion by the choroid plexus epithelium, the most efficient and actively secreting epithelium in the human body, and provide experimental and clinical evidence for the role of increased CSF production in hydrocephalus. Although the choroid plexus epithelium might have only an indirect influence on the pathogenesis of many types of pediatric hydrocephalus, the ability to modify CSF secretion with drugs newer than acetazolamide or furosemide would be an invaluable component of future therapies to alleviate permanent shunt dependence. Investigation into the human genetics of developmental hydrocephalus and choroid plexus hyperplasia, and the molecular physiology of the ion channels and transporters responsible for CSF secretion, might yield novel targets that could be exploited for pharmacotherapeutic intervention.

Hydrocephalus: the role of cerebral aquaporin-4 channels and computational modeling considerations of cerebrospinal fluid

Aquaporin-4 (AQP4) channels play an important role in brain water homeostasis. Water transport across plasma membranes has a critical role in brain water exchange of the normal and the diseased brain. AQP4 channels are implicated in the pathophysiology of hydrocephalus, a disease of water imbalance that leads to CSF accumulation in the ventricular system. Many molecular aspects of fluid exchange during hydrocephalus have yet to be firmly elucidated, but review of the literature suggests that modulation of AQP4 channel activity is a potentially attractive future pharmaceutical therapy. Drug therapy targeting AQP channels may enable control over water exchange to remove excess CSF through a molecular intervention instead of by mechanical shunting. This article is a review of a vast body of literature on the current understanding of AQP4 channels in relation to hydrocephalus, details regarding molecular aspects of AQP4 channels, possible drug development strategies, and limitations. Advances in medical imaging and computational modeling of CSF dynamics in the setting of hydrocephalus are summarized. Algorithmic developments in computational modeling continue to deepen the understanding of the hydrocephalus disease process and display promising potential benefit as a tool for physicians to evaluate patients with hydrocephalus.

Current concepts on the pathophysiology of idiopathic chronic adult hydrocephalus: Are we facing another neurodegenerative disease?

INTRODUCTION

Since its description five decades ago, the pathophysiology of idiopathic chronic adult hydrocephalus (iCAH) has been traditionally related to the effect that ventricular dilatation exerts on the structures surrounding the ventricular system. However, altered cerebral blood flow, especially a reduction in the CSF turnover rate, are starting to be considered the main pathophysiological elements of this disease.

DEVELOPMENT

Compression of the pyramidal tract, the frontostriatal and frontoreticular circuits, and the paraventricular fibres of the superior longitudinal fasciculus have all been reported in iCAH. At the level of the corpus callosum, gliosis replaces a number of commissural tracts. Cerebral blood flow is also altered, showing a periventricular watershed region limited by the subependymal arteries and the perforating branches of the major arteries of the anterior cerebral circulation. The CSF turnover rate is decreased by 75%, leading to the reduced clearance of neurotoxins and the interruption of neuroendocrine and paracrine signalling in the CSF.

CONCLUSIONS

iCAH presents as a complex nosological entity, in which the effects of subcortical microangiopathy and reduced CSF turnover play a key role. According to its pathophysiology, it is simpler to think of iCAH more as a neurodegenerative disease, such as Alzheimer disease or Binswanger disease than as the classical concept of hydrocephalus.

Hydrocephalus in children

Hydrocephalus is a common disorder of cerebral spinal fluid (CSF) physiology resulting in abnormal expansion of the cerebral ventricles. Infants commonly present with progressive macrocephaly whereas children older than 2 years generally present with signs and symptoms of intracranial hypertension. The classic understanding of hydrocephalus as the result of obstruction to bulk flow of CSF is evolving to models that incorporate dysfunctional cerebral pulsations, brain compliance, and newly characterised water-transport mechanisms. Hydrocephalus has many causes. Congenital hydrocephalus, most commonly involving aqueduct stenosis, has been linked to genes that regulate brain growth and development. Hydrocephalus can also be acquired, mostly from pathological processes that affect ventricular outflow, subarachnoid space function, or cerebral venous compliance. Treatment options include shunt and endoscopic approaches, which should be individualised to the child. The long-term outcome for children that have received treatment for hydrocephalus varies. Advances in brain imaging, technology, and understanding of the pathophysiology should ultimately lead to improved treatment of the disorder.

Morinda citrifolia L. (noni) and memantine attenuate periventricular tissue injury of the fourth ventricle in hydrocephalic rabbits

This study was designed to evaluate the neuroprotective effects of Morinda citrifolia L. (Rubiaceae), commonly known as noni, and memantine (a N-methy-D-aspartate receptor inhibitor) on hydrocephalus-induced neurodegenerative disorders. Kaolin was injected into the cistern magna of male adult New Zealand rabbits to establish a hydrocephalus animal model. Memantine (20 mg/kg, intraperitoneally; memantine-treated group) or noni (5 mL/kg, intragastrically; noni-treated group) was administered daily for 2 weeks. Microtubule-associated protein-2 and caspase-3 immunohistochemistry were performed to detect neuronal degeneration and apoptosis in the periventricular tissue of the fourth ventricle of rabbits. Microtubule-associated protein-2 staining density was significantly decreased in the hydrocephalic group, while the staining density was significantly increased in the memantine- and noni-treated groups, especially in the noni-treated group. Noni treatment decreased the number of caspase-3-positive cells in rabbits with hydrocephalus, while memantine had no effect. These findings suggest that noni exhibits more obvious inhibitory effects on hydrocephalus-induced neurodegenerative disorders than memantine in periventricular tissue of the fourth ventricle.

Idiopathic cerebrospinal fluid overproduction: case-based review of the pathophysiological mechanism implied in the cerebrospinal fluid production

Cerebrospinal fluid (CSF) overproduction results from either CSF infection or choroid plexus hypertrophy or tumor, with only a single idiopathic case described so far. We report a unique case of a male infant with Crouzon syndrome who presented with intracranial hypertension, caused by up to 4-fold increase in CSF daily production. Conditions related to CSF overproduction, namely central nervous system infections and choroid plexus hypertrophy or tumor, were ruled out by repeated magnetic resonance imaging and CSF samples. Medical therapy failed to reduce CSF production and the patient underwent several shunting procedures, cranial expansion, and endoscopic coagulation of the choroid plexus. This article thoroughly reviews pertinent literature on CSF production mechanisms and possible therapeutic implications.

VEGF: a potential target for hydrocephalus

Growth factors are primarily responsible for the genesis, differentiation and proliferation of cells and maintenance of tissues. Given the central role of growth factors in signaling between cells in health and in disease, it is understandable that disruption of growth factor-mediated molecular signaling can cause diverse phenotypic consequences including cancer and neurological conditions. This review will focus on the specific questions of enlarged cerebral ventricles and hydrocephalus. It is also well known that angiogenic factors, such as vascular endothelial growth factor (VEGF), affect tissue permeability through activation of receptors and adhesion molecules; hence, recent studies showing elevations of this factor in pediatric hydrocephalus led to the demonstration that VEGF can induce ventriculomegaly and altered ependyma when infused in animals. In this review, we discuss recent findings implicating the involvement of biochemical and biophysical factors that can induce a VEGF-mimicking effect in communicating hydrocephalus and pay particular attention to the role of the VEGF system as a potential pharmacological target in the treatment of some cases of hydrocephalus. The source of VEGF secretion in the cerebral ventricles, in periventricular regions and during pathologic events including hydrocephalus following hypoxia and hemorrhage is sought. The review is concluded with a summary of potential non-surgical treatments in preclinical studies suggesting several molecular targets including VEGF for hydrocephalus and related neurological disorders.

Structural defects in cilia of the choroid plexus, subfornical organ and ventricular ependyma are associated with ventriculomegaly

Background

Hydrocephalus is a heterogeneous disorder with multiple etiologies that are not yet fully understood. Animal models have implicated dysfunctional cilia of the ependyma and choroid plexus in the development of the disorder. In this report, we sought to determine the origin of the ventriculomegaly in four Bardet Biedl syndrome (BBS) mutant mouse strains as models of a ciliopathy.

Methods

Evans Blue dye was injected into the lateral ventricle of wild- type and BBS mutant mice to determine whether obstruction of intra- or extra-ventricular CSF flow contributed to ventriculomegaly. Transmission electron microscopy (TEM) was used to examine the ultrastructure of the choroid plexus, subfornical organ (SFO), subcommisural organ (SCO), and ventricular ependyma to evaluate their ultrastructure and the morphology of their primary and motile cilia.

Results and discussion

No obstruction of intra- or extra-ventricular CSF flow was observed, implying a communicating form of hydrocephalus in BBS mutant mice. TEM analyses of the mutants showed no evidence of choroidal papillomas or breakdown of the blood:CSF barrier. In contrast, structural defects were observed in a subpopulation of cilia lining the choroid plexus, SFO, and ventricular ependyma. These included disruptions of the microtubular structure of the axoneme and the presence of electron-dense vesicular-like material along the ciliary shaft and at the tips of cilia.

Conclusions

Abnormalities in cilia structure and function have the potential to influence ciliary intraflagellar transport (IFT), cilia maintenance, protein trafficking, and regulation of CSF production. Ciliary structural defects are the only consistent pathological features associated with CSF-related structures in BBS mutant mice. These defects are observed from an early age, and may contribute to the underlying pathophysiology of ventriculomegaly.

Widespread contribution of Gdf7 lineage to cerebellar cell types and implications for hedgehog-driven medulloblastoma formation

The roof plate is a specialized embryonic midline tissue of the central nervous system that functions as a signaling center regulating dorsal neural patterning. In the developing hindbrain, roof plate cells express Gdf7 and previous genetic fate mapping studies showed that these cells contribute mostly to non-neural choroid plexus epithelium. We demonstrate here that constitutive activation of the Sonic hedgehog signaling pathway in the Gdf7 lineage invariably leads to medulloblastoma. Lineage tracing analysis reveals that Gdf7-lineage cells not only are a source of choroid plexus epithelial cells, but are also present in the cerebellar rhombic lip and contribute to a subset of cerebellar granule neuron precursors, the presumed cell-of-origin for Sonic hedgehog-driven medulloblastoma. We further show that Gdf7-lineage cells also contribute to multiple neuronal and glial cell types in the cerebellum, including glutamatergic granule neurons, unipolar brush cells, Purkinje neurons, GABAergic interneurons, Bergmann glial cells, and white matter astrocytes. These findings establish hindbrain roof plate as a novel source of diverse neural cell types in the cerebellum that is also susceptible to oncogenic transformation by deregulated Sonic hedgehog signaling.

Permeability of the arachnoid and pia mater. The role of ion channels in the leptomeningeal physiology

PURPOSE

The purpose of this paper is to study the ionic permeability of the leptomeninges related to the effect of ouabain (sodium-potassium-ATPase inhibitor) and amiloride (epithelial sodium channel (ENaC) inhibitor) on the tissue, as well as identify the presence of ion channels.

METHODS

Cranial leptomeningeal samples from 26 adult sheep were isolated. Electrophysiological measurements were performed with Ussing system and transmembrane resistance values (R(TM) in Ω*cm(2)) obtained over time. Experiments were conducted with the application of ouabain 10(-3) M or amiloride 10(-5) M at the arachnoidal and pial sides. Immunohistochemical studies of leptomeningeal tissue were prepared with alpha-1 sodium-potassium-ATPase (ATP1A1), beta-ENaC, and delta-ENaC subunit antibodies.

RESULTS

The application of ouabain at the arachnoidal side raised the transmembrane resistance statistically significantly and thus decreased its ionic permeability. The addition of ouabain at the pial side led also to a significant but less profound increment in transmembrane resistance. The addition of amiloride at the arachnoidal or pial side did not produce any statistical significant change in the R(TM) from controls (p > 0.05). Immunohistochemistry confirmed the presence of the ATP1A1 and beta- and delta-ENaC subunits at the leptomeninges.

CONCLUSIONS

In summary, leptomeningeal tissue possesses sodium-potassium-ATPase and ENaC ion channels. The application of ouabain alters the ionic permeability of the leptomeninges thus reflecting the role of sodium-potassium-ATPase. Amiloride application did not alter the ionic permeability of leptomeninges possibly due to localization of ENaC channels towards the subarachnoid space, away from the experimental application sites. The above properties of the tissue could potentially be related to cerebrospinal fluid turnover at this interface.

Expression and function of aquaporins in peripheral nervous system

The expression and role of the aquaporin (AQP) family water channels in the peripheral nervous system was less investigated. Since 2004, however, significant progress has been made in the immunolocalization, regulation and function of AQPs in the peripheral nervous system. These studies showed selective localization of three AQPs (AQP1, AQP2, and AQP4) in dorsal root ganglion neurons, enteric neurons and glial cells, periodontal Ruffini endings, trigeminal ganglion neurons and vomeronasal sensory neurons. Functional characterization in transgenic knockout mouse model revealed important role of AQP1 in pain perception. This review will summarize the progress in this field and discuss possible involvement of AQPs in peripheral neuropathies and their potential as novel drug targets.

Hydrocephalus induces dynamic spatiotemporal regulation of aquaporin-4 expression in the rat brain

BACKGROUND

The water channel protein aquaporin-4 (AQP4) is reported to be of possible major importance for accessory cerebrospinal fluid (CSF) circulation pathways. We hypothesized that changes in AQP4 expression in specific brain regions correspond to the severity and duration of hydrocephalus.

METHODS

Hydrocephalus was induced in adult rats (~8 weeks) by intracisternal kaolin injection and evaluated after two days, one week and two weeks. Using magnetic resonance imaging (MRI) we quantified lateral ventricular volume, water diffusion and blood-brain barrier properties in hydrocephalic and control animals. The brains were analysed for AQP4 density by western blotting and localisation by immunohistochemistry. Double fluorescence labelling was used to study cell specific origin of AQP4.

RESULTS

Lateral ventricular volume was significantly increased over control at all time points after induction and the periventricular apparent diffusion coefficient (ADC) value significantly increased after one and two weeks of hydrocephalus. Relative AQP4 density was significantly decreased in both cortex and periventricular region after two days and normalized after one week. After two weeks, periventricular AQP4 expression was significantly increased. Relative periventricular AQP4 density was significantly correlated to lateral ventricular volume. AQP4 immunohistochemical analysis demonstrated the morphological expression pattern of AQP4 in hydrocephalus in astrocytes and ventricular ependyma. AQP4 co-localized with astrocytic glial fibrillary acidic protein (GFAP) in glia limitans. In vascular structures, AQP4 co-localized to astroglia but not to microglia or endothelial cells.

CONCLUSIONS

AQP4 levels are significantly altered in a time and region dependent manner in kaolin-induced hydrocephalus. The presented data suggest that AQP4 could play an important neurodefensive role, and may be a promising future pharmaceutical target in hydrocephalus and CSF disorders.

Reactive astrocytes as therapeutic targets for CNS disorders

Reactive astrogliosis has long been recognized as a ubiquitous feature of CNS pathologies. Although its roles in CNS pathology are only beginning to be defined, genetic tools are enabling molecular dissection of the functions and mechanisms of reactive astrogliosis in vivo. It is now clear that reactive astrogliosis is not simply an all-or-nothing phenomenon but, rather, is a finely gradated continuum of molecular, cellular, and functional changes that range from subtle alterations in gene expression to scar formation. These changes can exert both beneficial and detrimental effects in a context-dependent manner determined by specific molecular signaling cascades. Dysfunction of either astrocytes or the process of reactive astrogliosis is emerging as an important potential source of mechanisms that might contribute to, or play primary roles in, a host of CNS disorders via loss of normal or gain of abnormal astrocyte activities. A rapidly growing understanding of the mechanisms underlying astrocyte signaling and reactive astrogliosis has the potential to open doors to identifying many molecules that might serve as novel therapeutic targets for a wide range of neurological disorders. This review considers general principles and examines selected examples regarding the potential of targeting specific molecular aspects of reactive astrogliosis for therapeutic manipulations, including regulation of glutamate, reactive oxygen species, and cytokines.