Cellular mechanisms involved in the stenosis and obliteration of the cerebral aqueduct of hyh mutant mice developing congenital hydrocephalus

Hydrocephalus and abnormal subcommissural organ in mice lacking presenilin-1 in Wnt1 cell lineages

Presenilin-1 (PS1) is a transmembrane protein that is in many cases responsible for the development of familial Alzheimer's disease. PS1 is widely expressed in embryogenesis and is essential for neurogenesis, somitogenesis, angiogenesis, and cardiac morphogenesis. To further investigate the role of PS1 in the brain, we inactivated the PS1 gene in Wnt1 cell lineages using the Cre-loxP recombination system. Here we show that conditional inactivation of PS1 in Wnt1 cell lineages results in congenital hydrocephalus and subcommissural organ abnormalities, suggesting a possible role of PS1 in the regulation of cerebrospinal fluid homeostasis.

Ependymal cells: biology and pathology

The literature was reviewed to summarize the current understanding of the role of ciliated ependymal cells in the mammalian brain. Previous reviews were summarized. Publications from the past 10 years highlight interactions between ependymal cells and the subventricular zone and the possible role of restricted ependymal populations in neurogenesis. Ependymal cells provide trophic support and possibly metabolic support for progenitor cells. Channel proteins such as aquaporins may be important for determining water fluxes at the ventricle wall. The junctional and anchoring proteins are now fairly well understood, as are proteins related to cilia function. Defects in ependymal adhesion and cilia function can cause hydrocephalus through several different mechanisms, one possibility being loss of patency of the cerebral aqueduct. Ependymal cells are susceptible to infection by a wide range of common viruses; while they may act as a line of first defense, they eventually succumb to repeated attacks in long-lived organisms. Ciliated ependymal cells are almost certainly important during brain development. However, the widespread absence of ependymal cells from the adult human lateral ventricles suggests that they may have only regionally restricted value in the mature brain of large size.

Disruption of the neurogenic niche in the subventricular zone of postnatal hydrocephalic hyh mice

Neural stem cells persist after embryonic development in the subventricular zone (SVZ) niche and produce new neural cells during postnatal life; ependymal cells are a key component associated with this neurogenic niche. In the animal model of human hydrocephalus, the hyh mouse, the ependyma of the lateral ventricles is progressively lost during late embryonic and early postnatal life and disappears from most of the ventricular surface throughout its life span. To determine the potential consequences of this loss on the SVZ, we characterized the abnormalities in this neurogenic niche in hyh mice. There was overall disorganization and a marked reduction of proliferative cells in the SVZ of both newborn and adult hyh hydrocephalic mice in vivo; neuroblasts were displaced to the ventricular surface, and their migration through the rostral migratory stream was reduced. The numbers of resident neural progenitor cells in hyh mice were also markedly reduced, but they were capable of proliferating, forming neurospheres, and differentiating into neurons and glia in vitro in a manner indistinguishable from that of wild-type progenitor cells. These findings suggest that the reduction of proliferative activity observed in vivo is not caused by a cell autonomous defect of SVZ progenitors but is a consequence of a reduced number of these cells. Furthermore, the overall tissue disorganization of the SVZ and displacement of neuroblasts imply alterations in the neurogenic niche of postnatal hyh mice.

A simple PCR-based genotyping method for M105I mutation of alpha-SNAP enhances the study of early pathological changes in hyh phenotype

alpha-SNAP is an essential component of the protein machinery responsible for membrane fusion events in different cell types. The hyh (hydrocephalus with hop gait) mouse carries a missense mutation in Napa gene that results in a point mutation (M105I) in alpha-SNAP protein. Homozygous animals for the mutant allele have been identified by the clinical and/or neuropathological phenotype, or by direct sequencing of PCR products. The aims of the present study were (i) to develop a high-throughput technique to genotype hyh mice, (ii) to correlate genotype-phenotype, and (iii) to analyze the earliest pathological changes of hyh mutant mice. As no restriction sites are affected by the hyh mutation, we resolved this problem by creating a BspHI restriction site with a modified (mismatch) polymerase chain reaction (PCR) primer in wild-type allele. This artificially created restriction site (ACRS)-PCR technique is a simple, rapid and reliable method to genotype hyh mice in a day-work procedure. Biochemical and histological analysis of genotyped hyh embryos at different developmental stages allowed us to identify and characterize the earliest brain pathological changes of the hyh phenotype, including the first signs of neuroepithelial disruption and neuronal ectopia. In addition, genotype-phenotype analysis of 327 animals confirmed that (i) hyh is a single-gene autosomal recessive disorder, and (ii) the disorder has 100% penetrance (i.e., the mutation was only present in affected mice). The genotyping method described here enhances the potentiality of hyh mouse as a unique in vivo model to study the role of membrane trafficking in different developmental and physiological processes.

SCO-ping out the mechanisms underlying the etiology of hydrocephalus

The heterogeneous nature of congenital hydrocephalus has hampered our understanding of the molecular basis of this common clinical problem. However, disease gene identification and characterization of multiple transgenic mouse models has highlighted the importance of the subcommissural organ (SCO) and the ventricular ependymal (vel) cells. Here, we review how altered development and function of the SCO and vel cells contributes to hydrocephalus.

Sporadic obstructive hydrocephalus in Aqp4 null mice

Aquaporin-4 (Aqp4) is a water transport protein expressed in glia and ependymocytes in brain. We report here the unexpected occurrence of severe obstructive hydrocephalus in a random subset of Aqp4 knockout mice. Of 612 Aqp4 knockout mice produced by heterozygote-heterozygote or knockout-knockout breedings, 9.6% of offspring manifested progressive encephalomegaly. Encephalomegaly was never seen in wild-type or Aqp4 heterozygous mice. Examination of the subset encephalomegalic mice revealed marked triventricular hydrocephalus (lateral ventricle size approximately 500 mm(3)), elevated intracranial pressure (19 +/- 3 vs. 6.1 +/- 0.6 mm Hg), and death by age 6 weeks, with a median survival of 28 days. Intraventricular dye injection studies revealed total obstruction of the cerebral aqueduct. Evans blue extravasation studies indicated an intact blood-brain barrier in the hydrocephalic mice. Brain histology revealed reduced ventricular size and ependymocyte disorganization in some nonhydrocephalic Aqp4 null mice. Our studies establish Aqp4 deletion as a predisposing factor for the development of congenital obstructive hydrocephalus in mice. We suggest that AQP4 polymorphisms might also contribute to the development of aqueduct stenosis in humans.

Priorities for hydrocephalus research: report from a National Institutes of Health-sponsored workshop

OBJECT

Treatment for hydrocephalus has not advanced appreciably since the advent of cerebrospinal fluid (CSF) shunts more than 50 years ago. Many questions remain that clinical and basic research could address, which in turn could improve therapeutic options. To clarify the main issues facing hydrocephalus research and to identify critical advances necessary to improve outcomes for patients with hydrocephalus, the National Institutes of Health (NIH) sponsored a workshop titled "Hydrocephalus: Myths, New Facts, and Clear Directions." The purpose of this paper is to report on the recommendations that resulted from that workshop.

METHODS

The workshop convened from September 29 to October 1, 2005, in Bethesda, Maryland. Among the 150 attendees was an international group of participants, including experts in pediatric and adult hydrocephalus as well as scientists working in related fields, neurosurgeons, laboratory-based neuroscientists, neurologists, patient advocates, individuals with hydrocephalus, parents, and NIH program and intramural staff. Plenary and breakout sessions covered injury and recovery mechanisms, modeling, biomechanics, diagnosis, current treatment and outcomes, complications, quality of life, future treatments, medical devices, development of research networks and information sharing, and education and career development.

RESULTS

The conclusions were as follows: 1) current methods of diagnosis, treatment, and outcomes monitoring need improvement; 2) frequent complications, poor rate of shunt survival, and poor quality of life for patients lead to unsatisfactory outcomes; 3) investigators and caregivers need additional methods to monitor neurocognitive function and control of CSF variables such as pressure, flow, or pulsatility; 4) research warrants novel interdisciplinary approaches; 5) understanding of the pathophysiological and recovery mechanisms of neuronal function in hydrocephalus is poor, warranting further investigation; and 6) both basic and clinical aspects warrant expanded and innovative training programs.

CONCLUSIONS

The research priorities of this workshop provide critical guidance for future research in hydrocephalus, which should result in advances in knowledge, and ultimately in the treatment for this important disorder and improved outcomes in patients of all ages.

Disruption of neural progenitors along the ventricular and subventricular zones in periventricular heterotopia

Periventricular heterotopia (PH) is a disorder characterized by neuronal nodules, ectopically positioned along the lateral ventricles of the cerebral cortex. Mutations in either of two human genes, Filamin A (FLNA) or ADP-ribosylation factor guanine exchange factor 2 (ARFGEF2), cause PH (Fox et al. in 'Mutations in filamin 1 prevent migration of cerebral cortical neurons in human periventricular heterotopia'. Neuron, 21, 1315-1325, 1998; Sheen et al. in 'Mutations in ARFGEF2 implicate vesicle trafficking in neural progenitor proliferation and migration in the human cerebral cortex'. Nat. Genet., 36, 69-76, 2004). Recent studies have shown that mutations in mitogen-activated protein kinase kinase kinase-4 (Mekk4), an indirect interactor with FlnA, also lead to periventricular nodule formation in mice (Sarkisian et al. in 'MEKK4 signaling regulates filamin expression and neuronal migration'. Neuron, 52, 789-801, 2006). Here we show that neurons in post-mortem human PH brains migrated appropriately into the cortex, that periventricular nodules were primarily composed of later-born neurons, and that the neuroependyma was disrupted in all PH cases. As studied in the mouse, loss of FlnA or Big2 function in neural precursors impaired neuronal migration from the germinal zone, disrupted cell adhesion and compromised neuroepithelial integrity. Finally, the hydrocephalus with hop gait (hyh) mouse, which harbors a mutation in Napa [encoding N-ethylmaleimide-sensitive factor attachment protein alpha (alpha-SNAP)], also develops a progressive denudation of the neuroepithelium, leading to periventricular nodule formation. Previous studies have shown that Arfgef2 and Napa direct vesicle trafficking and fusion, whereas FlnA associates dynamically with the Golgi membranes during budding and trafficking of transport vesicles. Our current findings suggest that PH formation arises from a final common pathway involving disruption of vesicle trafficking, leading to impaired cell adhesion and loss of neuroependymal integrity.

Congenital hydrocephalus associated with abnormal subcommissural organ in mice lacking huntingtin in Wnt1 cell lineages

Huntingtin (htt) is a 350 kDa protein of unknown function, with no homologies with other known proteins. Expansion of a polyglutamine stretch at the N-terminus of htt causes Huntington's disease (HD), a dominant neurodegenerative disorder. Although it is generally accepted that HD is caused primarily by a gain-of-function mechanism, recent studies suggest that loss-of-function may also be part of HD pathogenesis. Huntingtin is an essential protein in the mouse since inactivation of the mouse HD homolog (Hdh) gene results in early embryonic lethality. Huntingtin is widely expressed in embryogenesis, and associated with a number of interacting proteins suggesting that htt may be involved in several processes including morphogenesis, neurogenesis and neuronal survival. To further investigate the role of htt in these processes, we have inactivated the Hdh gene in Wnt1 cell lineages using the Cre-loxP system of recombination. Here we show that conditional inactivation of the Hdh gene in Wnt1 cell lineages results in congenital hydrocephalus, implicating huntingtin for the first time in the regulation of cerebral spinal fluid (CSF) homeostasis. Our results show that hydrocephalus in mice lacking htt in Wnt1 cell lineages is associated with increase in CSF production by the choroid plexus, and abnormal subcommissural organ.

Spinal hemorrhages are associated with early neonatal motor function loss in human spina bifida aperta

BACKGROUND

In spina bifida aperta (SBA), leg movements caudal to the meningomyelocele are present in utero, but they disappear shortly after birth. It is unclear whether leg movements disappear by impact of the neuro-developmental malformation or by superimposed traumatic damage. If superimposed traumatic damage is involved, targeted fetal intervention could improve motor outcome.

AIM

To characterize neuromuscular pathology in association with perinatal motor function loss in SBA.

PATIENTS/METHODS

In fetal SBA (n=8; 16-40 weeks GA), the median time interval between ultrasound registrations of fetal motor behavior and post-mortem histology was 1 week. Histology was assessed cranial, at and caudal to the meningomyelocele and compared with findings in fetal controls (n=4).

RESULTS

Despite fetal movements caudal to the meningomyelocele (5/6), histology indicated muscle fiber alterations (6/6) that concurred with neuro-developmental and traumatic spinal defects [Neuro-developmental defects: spinal ependymal denudation (3/8), reduced amount of (caspase3-negative) lower motor neurons (LMNs; 8/8), aberrant spinal vascularization (8/8). Traumatic defects: gliosis (7/8), acute/fresh spinal hemorrhages near LMNs (8/8)].

CONCLUSION

In all delivered SBA patients, recent spinal hemorrhages were superimposed upon pre-existing defects. If early therapeutic strategies can prevent these superimposed secondary spinal hemorrhages, motor outcome may improve.

Pathogenesis of cerebral malformations in human fetuses with meningomyelocele

Background

Fetal spina bifida aperta (SBA) is characterized by a spinal meningomyelocele (MMC) and associated with cerebral pathology, such as hydrocephalus and Chiari II malformation. In various animal models, it has been suggested that a loss of ventricular lining (neuroepithelial/ependymal denudation) may trigger cerebral pathology. In fetuses with MMC, little is known about neuroepithelial/ependymal denudation and the initiating pathological events.

The objective of this study was to investigate whether neuroepithelial/ependymal denudation occurs in human fetuses and neonates with MMC, and if so, whether it is associated with the onset of hydrocephalus.

Methods

Seven fetuses and 1 neonate (16–40 week gestational age, GA) with MMC and 6 fetuses with normal cerebral development (22–41 week GA) were included in the study. Identification of fetal MMC and clinical surveillance of fetal head circumference and ventricular width was performed by ultrasound (US). After birth, MMC was confirmed by histology. We characterized hydrocephalus by increased head circumference in association with ventriculomegaly. The median time interval between fetal cerebral ultrasound and fixing tissue for histology was four days.

Results

At 16 weeks GA, we observed neuroepithelial/ependymal denudation in the aqueduct and telencephalon together with sub-cortical heterotopias in absence of hydrocephalus and/or Chiari II malformation. At 21–34 weeks GA, we observed concurrence of aqueductal neuroepithelial/ependymal denudation and progenitor cell loss with the Chiari II malformation, whereas hydrocephalus was absent. At 37–40 weeks GA, neuroepithelial/ependymal denudation coincided with Chiari II malformation and hydrocephalus. Sub-arachnoidal fibrosis at the convexity was absent in all fetuses but present in the neonate.

Conclusion

In fetal SBA, neuroepithelial/ependymal denudation in the telencephalon and the aqueduct can occur before Chiari II malformation and/or hydrocephalus. Since denuded areas cannot re-establish cell function, neuro-developmental consequences could induce permanent cerebral pathology.

Patterned neuropathologic events occurring in hyh congenital hydrocephalic mutant mice

Hyh mutant mice develop long-lasting hydrocephalus and represent a good model for investigating neuropathologic events associated with hydrocephalus. The study of their brains by use of lectin binding, bromodeoxyuridine labeling, immunochemistry, and scanning electron microscopy revealed that certain events related to hydrocephalus followed a well-defined pattern. A program of neuroepithelium/ependyma denudation was initiated at embryonic day 12 and terminated at the end of the second postnatal week. After the third postnatal week the denuded areas remained permanently devoid of ependyma. In contrast, a selective group of ependymal areas resisted denudation throughout the lifespan. Ependymal denudation triggered neighboring astrocytes to proliferate. These astrocytes expressed particular glial markers and formed a superficial cell layer replacing the lost ependyma. The loss of the neuroepithelium/ependyma layer at specific regions of the ventricular walls and at specific stages of brain development would explain the fact that only certain brain structures had abnormal development. Therefore, commissural axons forming the corpus callosum and the hippocampal commissure displayed abnormalities, whereas those forming the anterior and posterior commissures did not; and the brain cortex was not homogenously affected, with the cingular and frontal cortices being the most altered regions. All of these telencephalic alterations developed at stages when hydrocephalus was not yet patent at the lateral ventricles, indicating that abnormal neural development and hydrocephalus are linked at the etiologic level, rather than the former being a consequence of the latter. All evidence collected on hydrocephalic hyh mutant mice indicates that a primary alteration in the neuroepithelium/ependyma cell lineage triggers both hydrocephalus and abnormalities in telencephalic development.

Cyclophosphamide-induced agenesis of cerebral aqueduct resulting in hydrocephalus in mice

The present work was undertaken to reveal the mechanism of cerebral aqueduct agenesis found to result in hydrocephalus following intrauterine exposure to model teratogen, cyclophosphamide, in murine fetuses. A single dose of 10-mg/kg body weight cyclophosphamide was injected intaperitoneally to pregnant mice on day 10, 11 or 12 of gestation. Fetuses were collected through abdominal incision on day 18 and studied for various malformations of brain and cranium including hydrocephalus. Incomplete development and failure of canalization of the cerebral aqueduct were detected when serial sections of brain in coronal and transverse planes were studied under the microscope. Biotechnological investigations such as % DNA fragmentation, % viable cell count and cell proliferation assay were carried out on brain cells for further studies. Agenesis and non-canalization of the cerebral aqueduct resulted in increased pressure of CSF, which led to rupture of the aqueduct complicated by leakage and accumulation of CSF in brain substance forming a cavity containing CSF parallel and lateral to the unopened part of the cerebral aqueduct. Incomplete development along with non-canalization of the cerebral aqueduct resulted in blockage of CSF flow through the ventricles that manifest as internal hydrocephalus. External hydrocephalus on the other hand was detected where the CSF accumulated in the cavity formed inside the brain substance and established communication with the CSF in the subarachnoid space. Cyclophosphamide induced inhibition of mitosis and cell differentiation of ependymal cells reflecting a decreased % viable cell count and cell proliferation assay along with augmentation of apoptosis of brain cells quantified as increased % DNA fragmentation count, which were identified as the contributing factors underlying the agenesis and incomplete development of the cerebral aqueduct. The study also suggests that cell survival, proliferation, migration or differentiation of ependymal cells might have been affected, and we speculate that CSF may have an inducing role in the development and canalization of the cerebral aqueduct.

Development of hydrocephalus in mice expressing the G(i)-coupled GPCR Ro1 RASSL receptor in astrocytes

We developed a transgenic mouse line that expresses the G(i)-coupled RASSL (receptor activated solely by synthetic ligand) Ro1 in astrocytes to study astrocyte-neuronal communication. Surprisingly, we found that all transgenics expressing Ro1 developed hydrocephalus. We analyzed these mice in an effort to develop a new model of hydrocephalus that will further our understanding of the pathophysiology of the disease. Expression of Ro1 was restricted to astrocytes by crossing the transgenic hGFAP-tTA (tet transactivator behind the human glial fibrillary acidic protein promoter) mouse line with the transgenic tetO-Ro1/tetO-LacZ mouse line. This cross produced double-transgenic mice that expressed Ro1 in astrocytes. All double transgenics developed hydrocephalus by postnatal day 15, whereas single-transgenic littermate controls appeared normal. Hydrocephalic Ro1 mice displayed enlarged ventricles, partial denudation of the ependymal cell layer, altered subcommissural organ morphology, and obliteration of the cerebral aqueduct. Severely hydrocephalic mice also had increased levels of phospho-Erk and GFAP expression. Administration of doxycycline to breeding pairs suppressed Ro1 expression and the onset of hydrocephalus in double-transgenic offspring. Ro1 animals maintained on dox did not develop hydrocephalus; however, if taken off doxycycline at weaning, double-transgenic mice developed enlarged ventricles within 7 weeks, indicating that Ro1 expression also induces hydrocephalus in adults. This study discovered a new model of hydrocephalus in which the rate of pathogenesis can be controlled enabling the study of the pathogenesis of both juvenile and adult onset hydrocephalus.

High levels of Cre expression in neuronal progenitors cause defects in brain development leading to microencephaly and hydrocephaly

Hydrocephalus is a common and variegated pathology often emerging in newborn children after genotoxic insults during pregnancy (Hicks and D'Amato, 1980). Cre recombinase is known to have possible toxic effects that can compromise normal cell cycle and survival. Here we show, by using three independent nestin Cre transgenic lines, that high levels of Cre recombinase expression into the nucleus of neuronal progenitors can compromise normal brain development. The transgenics analyzed are the nestin Cre Balancer (Bal1) line, expressing the Cre recombinase with a nuclear localization signal, and two nestin CreER(T2) (Cre recombinase fused with a truncated estrogen receptor) mice lines with different levels of expression of a hybrid CreER(T2) recombinase that translocates into the nucleus after tamoxifen treatment. All homozygous Bal1 nestin Cre embryos displayed reduced neuronal proliferation, increased aneuploidy and cell death, as well as defects in ependymal lining and lamination of the cortex, leading to microencephaly and to a form of communicating hydrocephalus. An essentially overlapping phenotype was observed in the two nestin CreER(T2) transgenic lines after tamoxifen mediated-CreER(T2) translocation into the nucleus. Neither tamoxifen-treated wild-type nor nestin CreER(T2) oil-treated control mice displayed these defects. These results indicate that some forms of hydrocephalus may derive from a defect in neuronal precursors proliferation. Furthermore, they underscore the potential risks for developmental studies of high levels of nuclear Cre in neurogenic cells.

Heterogeneous expression of hydrocephalic phenotype in the hyh mice carrying a point mutation in α-SNAP

The hyh mouse carrying a point mutation in the gene encoding for soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein alpha (alpha-SNAP) develops inherited hydrocephalus. The investigation was designed to study: (i) the clinical evolution of hyh mice; (ii) factors other than the alpha-SNAP mutation that may influence the expression of hydrocephalus; (iii) the neuropathological features underlying the different forms of clinical evolution. The study included 3017 mice, 22.4% of which were hydrocephalic. The neuropathological study was performed in 112 mice by use of light and electron microscopy. It was found that maternal- and sex-related factors are involved in the heterogeneous expression of hyh phenotype. The clinical evolution recorded throughout a 4-year period also revealed a heterogeneous expression of the hydrocephalic phenotype. Two subpopulations were distinguished: (i) 70% of mice underwent a rapidly progressive hydrocephalus and died during the first 2 months of life; they presented macrocephaly, extremely large expansion of the ventricles, equilibrium impairment and decreased motor activity. (ii) Mice with slowly progressive hydrocephalus (30%) survived for periods ranging between 2 months and 2 years. They had no or moderate macrocephaly; moderate ventricular dilatation and preserved general motor activity; they all presented spontaneous ventriculostomies communicating the ventricles with the subarachnoid space, indicating that such communications play a key role in the long survival of these mice. The hyh mutant represents an ideal animal model to investigate how do the brain "adapt" to a virtually life-lasting hydrocephalus.

Genetics of human hydrocephalus

Human hydrocephalus is a common medical condition that is characterized by abnormalities in the flow or resorption of cerebrospinal fluid (CSF), resulting in ventricular dilatation. Human hydrocephalus can be classified into two clinical forms, congenital and acquired. Hydrocephalus is one of the complex and multifactorial neurological disorders.

A growing body of evidence indicates that genetic factors play a major role in the pathogenesis of hydrocephalus. An understanding of the genetic components and mechanism of this complex disorder may offer us significant insights into the molecular etiology of impaired brain development and an accumulation of the cerebrospinal fluid in cerebral compartments during the pathogenesis of hydrocephalus. Genetic studies in animal models have started to open the way for understanding the underlying pathology of hydrocephalus. At least 43 mutants/loci linked to hereditary hydrocephalus have been identified in animal models and humans. Up to date, 9 genes associated with hydrocephalus have been identified in animal models. In contrast, only one such gene has been identified in humans. Most of known hydrocephalus gene products are the important cytokines, growth factors or related molecules in the cellular signal pathways during early brain development. The current molecular genetic evidence from animal models indicate that in the early development stage, impaired and abnormal brain development caused by abnormal cellular signaling and functioning, all these cellular and developmental events would eventually lead to the congenital hydrocephalus.

Owing to our very primitive knowledge of the genetics and molecular pathogenesis of human hydrocephalus, it is difficult to evaluate whether data gained from animal models can be extrapolated to humans. Initiation of a large population genetics study in humans will certainly provide invaluable information about the molecular and cellular etiology and the developmental mechanisms of human hydrocephalus.

This review summarizes the recent findings on this issue among human and animal models, especially with reference to the molecular genetics, pathological, physiological and cellular studies, and identifies future research directions.

Function of the neuron-specific alternatively spliced isoforms of nonmuscle myosin II-B during mouse brain development

We report that the alternatively spliced isoforms of nonmuscle myosin heavy chain II-B (NHMC II-B) play distinct roles during mouse brain development. The B1-inserted isoform of NMHC II-B, which contains an insert of 10 amino acids near the ATP-binding region (loop 1) of the myosin heavy chain, is involved in normal migration of facial neurons. In contrast, the B2-inserted isoform, which contains an insert of 21 amino acids near the actin-binding region (loop 2), is important for postnatal development of cerebellar Purkinje cells. Deletion of the B1 alternative exon, together with reduced expression of myosin II-B, results in abnormal migration and consequent protrusion of facial neurons into the fourth ventricle. This protrusion is associated with the development of hydrocephalus. Restoring the amount of myosin II-B expression to wild-type levels prevents these defects, showing the importance of total myosin activity in facial neuron migration. In contrast, deletion of the B2 alternative exon results in abnormal development of cerebellar Purkinje cells. Cells lacking the B2-inserted isoform show reduced numbers of dendritic spines and branches. Some of the B2-ablated Purkinje cells are misplaced in the cerebellar molecular layer. All of the B2-ablated mice demonstrated impaired motor coordination.

Single and multiple congenic strains for hydrocephalus in the H-Tx rat

The H-Tx rat has fetal-onset hydrocephalus with a complex mode of inheritance. Previously, quantitative trait locus mapping using a backcross with Fischer F344 rats demonstrated genetic loci significantly linked to hydrocephalus on Chromosomes 10, 11, and 17. Hydrocephalus was preferentially associated with heterozygous alleles on Chrs 10 and 11 and with homozygous alleles on Chr 17. This study aimed to determine the phenotypic contribution of each locus by constructing single and multiple congenic strains. Single congenic rats were constructed using Fischer F344 as the recipient strain and a marker-assisted protocol. The homozygous strains were maintained for eight generations and the brains examined for dilated ventricles indicative for hydrocephalus. No congenic rats had severe (overt) hydrocephalus. A few pups and a significant number of adults had mild disease. The incidence was significantly higher in the C10 and C17 congenic strains than in the nonhydrocephalic F344 strain. Breeding to F344 to make F.H-Tx C10 or C11 rats heterozygous for the hydrocephalus locus failed to produce progeny with severe disease. Both bicongenic and tricongenic rats of different genotype combinations were constructed by crossing congenic rats. None had severe disease but the frequency of mild hydrocephalus in adults was similar to congenic rats and significantly higher than in the F344 strain. Rats with severe hydrocephalus were recovered in low numbers when single congenic or bicongenic rats were crossed with the parental H-Tx strain. It is concluded that the genetic and epigenetic factors contributing to severe hydrocephalus in the H-Tx strain are more complex than originally anticipated.

Ependymal Denudation and Alterations of the Subventricular Zone Occur in Human Fetuses With a Moderate Communicating Hydrocephalus

In mutant rodents, ependymal denudation occurs early in fetal life, preceding the onset of a communicating hydrocephalus, and is a key event in the etiology of this disease. The present investigation was designed to obtain evidence whether or not ependymal denudation occurs in 16- to 40-week-old human fetuses developing a communicating hydrocephalus (n = 8) as compared to fetuses of similar ages with no neuropathologic alterations (n = 15). Sections through the walls of the cerebral aqueduct and lateral ventricles were processed for lectin binding and immunocytochemistry using antibodies against ependyma, astroglia, neuroblasts, and macrophages markers. Anticaveolin was used as a functional marker of the fetal ependyma. The structural and functional molecular markers are differentially expressed throughout the differentiation of the human fetal ependyma. Denudation of the ependyma of the aqueduct and lateral ventricles occurred in all fetuses developing a communicating hydrocephalus, including the youngest ones studied. The denuded surface area increased in parallel with the fetus age. The possibility is advanced that in many or most cases of human fetal hydrocephalus there is a common defect at the ependymal cell lineage leading to ependymal detachment. Evidence was obtained that in hydrocephalic human fetuses a process to repair the denuded areas takes place during the fetal life. In hydrocephalic fetuses, detachment of the ependyma of the lateral ventricles resulted in the (i) loss of the germinal ependymal zone, (ii) disorganization of the subventricular zone and, (iii) abnormal migration of neuroblasts into the ventricular cavity. Thus, detachment of the ependymal layer in hydrocephalic fetuses would not only be associated with the pathogenesis of hydrocephalus but also to abnormal neurogenesis.

Genetic analysis of inherited hydrocephalus in a rat model

Congenital hydrocephalus is a serious neurological disorder with a diverse etiology. Although there is strong evidence for genetic causes, few genes have been identified in humans. The rodent model, the H-Tx rat, has hydrocephalus with an onset in late gestation and a complex mode of inheritance. Ventricular dilatation is associated with abnormalities in the cerebral aqueduct and subcommissural organ. Quantitative trait locus (QTL) mapping was performed on DNA from the progeny of a backcross with the non-hydrocephalic Fischer F344 strain, using DNA microsatellite markers. The hydrocephalus trait was quantified by measuring the severity of the ventricular dilatation. Four chromosomes, each with a locus for hydrocephalus (Chrs 9, 10, 11, and 17), were mapped using additional markers and DNA from four subsets of backcross progeny with allelic recombination at or near each locus. The genetic positions for the markers and the loci were located using the Ensemble Rat Genome Browser. For each chromosome studied, the interval containing the locus was examined for known rat genes and for human genes identified from human-rat homology. Genes expressed in brain and with a function associated with known causes of hydrocephalus were identified as possible candidate genes. Future studies to characterize the causative genes in this animal model will improve the understanding of genetic causes in humans.