Msx1 disruption leads to diencephalon defects and hydrocephalus

Congenital hydrocephalus: a review of recent advances in genetic etiology and molecular mechanisms

The global prevalence rate for congenital hydrocephalus (CH) is approximately one out of every five hundred births with multifaceted predisposing factors at play. Genetic influences stand as a major contributor to CH pathogenesis, and epidemiological evidence suggests their involvement in up to 40% of all cases observed globally. Knowledge about an individual's genetic susceptibility can significantly improve prognostic precision while aiding clinical decision-making processes. However, the precise genetic etiology has only been pinpointed in fewer than 5% of human instances. More occurrences of CH cases are required for comprehensive gene sequencing aimed at uncovering additional potential genetic loci. A deeper comprehension of its underlying genetics may offer invaluable insights into the molecular and cellular basis of this brain disorder. This review provides a summary of pertinent genes identified through gene sequencing technologies in humans, in addition to the 4 genes currently associated with CH (two X-linked genes L1CAM and AP1S2, two autosomal recessive MPDZ and CCDC88C). Others predominantly participate in aqueduct abnormalities, ciliary movement, and nervous system development. The prospective CH-related genes revealed through animal model gene-editing techniques are further outlined, focusing mainly on 4 pathways, namely cilia synthesis and movement, ion channels and transportation, Reissner's fiber (RF) synthesis, cell apoptosis, and neurogenesis. Notably, the proper functioning of motile cilia provides significant impulsion for cerebrospinal fluid (CSF) circulation within the brain ventricles while mutations in cilia-related genes constitute a primary cause underlying this condition. So far, only a limited number of CH-associated genes have been identified in humans. The integration of genotype and phenotype for disease diagnosis represents a new trend in the medical field. Animal models provide insights into the pathogenesis of CH and contribute to our understanding of its association with related complications, such as renal cysts, scoliosis, and cardiomyopathy, as these genes may also play a role in the development of these diseases. Genes discovered in animals present potential targets for new treatments but require further validation through future human studies.

Uncovering the role of the subcommissural organ in early brain development through transcriptomic analysis

BACKGROUND

The significant role of embryonic cerebrospinal fluid (eCSF) in the initial stages of brain development has been thoroughly studied. This fluid contains crucial molecules for proper brain development such as members of the Wnt and FGF families, apolipoproteins, and retinol binding protein. Nevertheless, the source of these molecules remains uncertain since they are present before the formation of the choroid plexus, which is conventionally known as the primary producer of cerebrospinal fluid. The subcommissural organ (SCO) is a highly conserved gland located in the diencephalon and is one of the earliest differentiating brain structures. The SCO secretes molecules into the eCSF, prior to the differentiation of the choroid plexus, playing a pivotal role in the homeostasis and dynamics of this fluid. One of the key molecules secreted by the SCO is SCO-spondin, a protein involved in maintenance of the normal ventricle size, straight spinal axis, neurogenesis, and axonal guidance. Furthermore, SCO secretes transthyretin and basic fibroblast growth factor 2, while other identified molecules in the eCSF could potentially be secreted by the SCO. Additionally, various transcription factors have been identified in the SCO. However, the precise mechanisms involved in the early SCO development are not fully understood.

RESULTS

To uncover key molecular players and signaling pathways involved in the role of the SCO during brain development, we conducted a transcriptomic analysis comparing the embryonic chick SCO at HH23 and HH30 stages (4 and 7 days respectively). Additionally, a public transcriptomic data from HH30 entire chick brain was used to compare expression levels between SCO and whole brain transcriptome. These analyses revealed that, at both stages, the SCO differentially expresses several members of bone morphogenic proteins, Wnt and fibroblast growth factors families, diverse proteins involved in axonal guidance, neurogenic and differentiative molecules, cell receptors and transcription factors. The secretory pathway is particularly upregulated at stage HH30 while the proliferative pathway is increased at stage HH23.

CONCLUSION

The results suggest that the SCO has the capacity to secrete several morphogenic molecules to the eCSF prior to the development of other structures, such as the choroid plexus.

SCO-spondin knockout mice exhibit small brain ventricles and mild spine deformation

Reissner's fiber (RF) is an extracellular polymer comprising the large monomeric protein SCO-spondin (SSPO) secreted by the subcommissural organ (SCO) that extends through cerebrospinal fluid (CSF)-filled ventricles into the central canal of the spinal cord. In zebrafish, RF and CSF-contacting neurons (CSF-cNs) form an axial sensory system that detects spinal curvature, instructs morphogenesis of the body axis, and enables proper alignment of the spine. In mammalian models, RF has been implicated in CSF circulation. However, challenges in manipulating Sspo, an exceptionally large gene of 15,719 nucleotides, with traditional approaches has limited progress. Here, we generated a Sspo knockout mouse model using CRISPR/Cas9-mediated genome-editing. Sspo knockout mice lacked RF-positive material in the SCO and fibrillar condensates in the brain ventricles. Remarkably, Sspo knockout brain ventricle sizes were reduced compared to littermate controls. Minor defects in thoracic spine curvature were detected in Sspo knockouts, which did not alter basic motor behaviors tested. Altogether, our work in mouse demonstrates that SSPO and RF regulate ventricle size during development but only moderately impact spine geometry.

SCO-spondin knockout mice exhibit small brain ventricles and mild spine deformation

Reissner's fiber (RF) is an extracellular polymer comprising the large monomeric protein SCO-spondin (SSPO) secreted by the subcommissural organ (SCO) that extends through cerebrospinal fluid (CSF)-filled ventricles into the central canal of the spinal cord. In zebrafish, RF and CSF-contacting neurons (CSF-cNs) form an axial sensory system that detects spinal curvature, instructs morphogenesis of the body axis, and enables proper alignment of the spine. In mammalian models, RF has been implicated in CSF circulation. However, challenges in manipulating Sspo , an exceptionally large gene of 15,719 nucleotides, with traditional approaches has limited progress. Here, we generated a Sspo knockout mouse model using CRISPR/Cas9-mediated genome-editing. Sspo knockout mice lacked RF-positive material in the SCO and fibrillar condensates in the brain ventricles. Remarkably, Sspo knockout brain ventricle sizes were reduced compared to littermate controls. Minor defects in thoracic spine curvature were detected in Sspo knockouts, which did not alter basic motor behaviors tested. Altogether, our work in mouse demonstrates that SSPO and RF regulate ventricle size during development but only moderately impact spine geometry.

A novel feature of the ancient organ: A possible involvement of the subcommissural organ in neurogenic/gliogenic potential in the adult brain

The subcommissural organ (SCO) is a circumventricular organ highly conserved in vertebrates from Cyclostomata such as lamprey to mammals including human. The SCO locates in the boundary between the third ventricle and the entrance of the aqueduct of Sylvius. The SCO functions as a secretory organ producing a variety of proteins such as SCO-spondin, transthyretin, and basic fibroblast growth factor (FGF) into the cerebrospinal fluid (CSF). A significant contribution of the SCO has been thought to maintain the homeostasis of CSF dynamics. However, evidence has shown a possible role of SCO on neurogenesis in the adult brain. This review highlights specific features of the SCO related to adult neurogenesis, suggested by the progress of understanding SCO functions. We begin with a brief history of the SCO discovery and continue to structural features, gene expression, and a possible role in adult neurogenesis suggested by the SCO transplant experiment.

The subcommissural organ maintains features of neuroepithelial cells in the adult mouse

The subcommissural organ (SCO) is a part of the circumventricular organs located in the dorsocaudal region of the third ventricle at the entrance of the aqueduct of Sylvius. The SCO comprises epithelial cells and produces high molecular weight glycoproteins, which are secreted into the third ventricle and become part of Reissner's fibre in the cerebrospinal fluid. Abnormal development of the SCO has been linked with congenital hydrocephalus, a condition characterized by excessive accumulation of cerebrospinal fluid in the brain. In the present study, we characterized the SCO cells in the adult mouse brain to gain insights into the possible role of this brain region. Immunohistochemical analyses revealed that expression of Pax6, a transcription factor essential for SCO differentiation during embryogenesis, is maintained in the SCO at postnatal stages from P0 to P84. SCO cells in the adult brain expressed known neural stem/progenitor cell (NSPC) markers, Sox2 and vimentin. The adult SCO cells also expressed proliferating marker PCNA, although expression of another proliferation marker Ki67, indicating a G₂/M phase, was not detected. The SCO cells did not incorporate BrdU, a marker for DNA synthesis in the S phase. Therefore, the SCO cells have a potential for proliferation but are quiescent for cell division in the adult. The SCO cells also expressed GFAP, a marker for astrocytes or NSPCs, but not NeuN (for neurons). A few cells positive for Iba1 (microglia), Olig2 (for oligodendrocytes) and PDGFRα (oligodendrocyte progenitors) existed within or on the periphery of the SCO. These findings revealed that the SCO cells have a unique feature as secretory yet immature neuroepithelial cells in the adult mouse brain.

The Enigmatic Reissner's Fiber and the Origin of Chordates

Reissner’s fiber (RF) is a secreted filament that floats in the neural canal of chordates. Since its discovery in 1860, there has been no agreement on its primary function, and its strong conservation across chordate species has remained a mystery for comparative neuroanatomists. Several findings, including the chemical composition and the phylogenetic history of RF, clinical observations associating RF with the development of the neural canal, and more recent studies suggesting that RF is needed to develop a straight vertebral column, may shed light on the functions of this structure across chordates. In this article, we will briefly review the evidence mentioned above to suggest a role of RF in the origin of fundamental innovations of the chordate body plan, especially the elongation of the neural tube and maintenance of the body axis. We will also mention the relevance of RF for medical conditions like hydrocephalus, scoliosis of the vertebral spine and possibly regeneration of the spinal cord.

Hydrocephalus in mouse B3glct mutants is likely caused by defects in multiple B3GLCT substrates in ependymal cells and subcommissural organ

Peters plus syndrome, characterized by defects in eye and skeletal development with isolated cases of ventriculomegaly/hydrocephalus, is caused by mutations in the β3-glucosyltransferase (B3GLCT) gene. In the endoplasmic reticulum, B3GLCT adds glucose to O-linked fucose on properly folded Thrombospondin Type 1 Repeats (TSRs). The resulting glucose-fucose disaccharide is proposed to stabilize the TSR fold and promote secretion of B3GLCT substrates, with some substrates more sensitive than others to loss of glucose. Mouse B3glct mutants develop hydrocephalus at high frequency. In this study, we demonstrated that B3glct mutant ependymal cells had fewer cilia basal bodies and altered translational polarity compared to controls. Localization of mRNA encoding A Disintegrin and Metalloproteinase with ThromboSpondin type 1 repeat 20 (ADAMTS20) and ADAMTS9, suggested that reduced function of these B3GLCT substrates contributed to ependymal cell abnormalities. In addition, we showed that multiple B3GLCT substrates (Adamts3, Adamts9, and Adamts20) are expressed by the subcommissural organ, that subcommissural organ-spondin (SSPO) TSRs were modified with O-linked glucose-fucose, and that loss of B3GLCT reduced secretion of SSPO in cultured cells. In the B3glct mutant subcommissural organ intracellular SSPO levels were reduced and BiP levels increased, suggesting a folding defect. Secreted SSPO colocalized with BiP, raising the possibility that abnormal extracellular assembly of SSPO into Reissner's fiber also contributed to impaired CSF flow in mutants. Combined, these studies underscore the complexity of the B3glct mutant hydrocephalus phenotype and demonstrate that impaired cerebrospinal fluid (CSF) flow likely stems from the collective effects of the mutation on multiple processes.

Loss of homeoprotein Msx1 and Msx2 leading to athletic and kinematic impairment related to the increasing neural excitability of neurons in aberrant neocortex in mice

Although homeoproteins Msx1 and Msx2, the cell-specific transcription regulators, have been proven to play multiple roles in the embryogenesis of bone, muscle and tooth, the functions and mechanisms of Msx1 and Msx2 in the development of the central nervous system of mice after birth are not clear because of the death of Msx1 and Msx1/2 germline-deleted embryo at late gestation of mouse. In current research, Nestin-Cre mice was introduced to generate the central nervous system-specific knockout mice (Nestin-Cre;Msx1,Msx2^fl/fl). We found that besides the falling of the body mass and the brain volume, the cortical tissue sections and staining showed the decreasing thickness of layer II-IV and declining number of vertebral cells in layer V resulting from Msx1/2 deletion. In addition, electrophysiological tests revealed the aberrant action potential parameters of deep pyramidal neurons in Nestin-Cre;Msx1,2^fl/fl mice, which may be related with the ethology impairment displayed in further experiments. We discovered Nestin-Cre;Msx1,2^fl/fl mice had severe impairment in their athletic ability and kinematic learning ability in rotate test, and exhibited hyperactivity in open-field test. Above all, our results revealed that deletion of homeoproteins Msx1 and Msx2 could lead to behavioral disorders and suggested that Msx1 and Msx2 played a crucial role in regulating the development and function of the neocortex. In addition, our current research provided a new mouse model for understanding the pathogenesis of human central nervous system disease.

An inactivating mutation in the histone deacetylase SIRT6 causes human perinatal lethality

Ferrer et al. demonstrate that a homozygous inactivating mutation in the histone deacetylase SIRT6 results in severe congenital anomalies and perinatal lethality in four affected fetuses. Human induced pluripotent stem cells derived from D63H homozygous fetuses fail to differentiate into embryoid bodies, functional cardiomyocytes, and neural progenitor cells due to a failure to repress pluripotent genes.

It has been well established that histone and DNA modifications are critical to maintaining the equilibrium between pluripotency and differentiation during early embryogenesis. Mutations in key regulators of DNA methylation have shown that the balance between gene regulation and function is critical during neural development in early years of life. However, there have been no identified cases linking epigenetic regulators to aberrant human development and fetal demise. Here, we demonstrate that a homozygous inactivating mutation in the histone deacetylase SIRT6 results in severe congenital anomalies and perinatal lethality in four affected fetuses. In vitro, the amino acid change at Asp63 to a histidine results in virtually complete loss of H3K9 deacetylase and demyristoylase functions. Functionally, SIRT6 D63H mouse embryonic stem cells (mESCs) fail to repress pluripotent gene expression, direct targets of SIRT6, and exhibit an even more severe phenotype than Sirt6-deficient ESCs when differentiated into embryoid bodies (EBs). When terminally differentiated toward cardiomyocyte lineage, D63H mutant mESCs maintain expression of pluripotent genes and fail to form functional cardiomyocyte foci. Last, human induced pluripotent stem cells (iPSCs) derived from D63H homozygous fetuses fail to differentiate into EBs, functional cardiomyocytes, and neural progenitor cells due to a failure to repress pluripotent genes. Altogether, our study described a germline mutation in SIRT6 as a cause for fetal demise, defining SIRT6 as a key factor in human development and identifying the first mutation in a chromatin factor behind a human syndrome of perinatal lethality.

Application of Weighted Gene Co-Expression Network Analysis to Explore the Key Genes in Alzheimer's Disease

BACKGROUND

Weighted co-expression network analysis (WGCNA) is a powerful systems biology method to describe the correlation of gene expression based on the microarray database, which can be used to facilitate the discovery of therapeutic targets or candidate biomarkers in diseases.

OBJECTIVE

To explore the key genes in the development of Alzheimer's disease (AD) by using WGCNA.

METHODS

The whole gene expression data GSE1297 from AD and control human hippocampus was obtained from the GEO database in NCBI. Co-expressed genes were clustered into different modules. Modules of interest were identified through calculating the correlation coefficient between the module and phenotypic traits. GO and pathway enrichment analyses were conducted, and the central players (key hub genes) within the modules of interest were identified through network analysis. The expression of the identified key genes was confirmed in AD transgenic mice through using qRT-PCR.

RESULTS

Two modules were found to be associated with AD clinical severity, which functioning mainly in mineral absorption, NF-κB signaling, and cGMP-PKG signaling pathways. Through analysis of the two modules, we found that metallothionein (MT), Notch2, MSX1, ADD3, and RAB31 were highly correlated with AD phenotype. Increase in expression of these genes was confirmed in aged AD transgenic mice.

CONCLUSION

WGCNA analysis can be used to analyze and predict the key genes in AD. MT1, MT2, MSX1, NOTCH2, ADD3, and RAB31 are identified to be the most relevant genes, which may be potential targets for AD therapy.

Expression Patterns of Extracellular Matrix Proteins during Posterior Commissure Development

Extracellular matrix (ECM) molecules are pivotal for central nervous system (CNS) development, facilitating cell migration, axonal growth, myelination, dendritic spine formation, and synaptic plasticity, among other processes. During axon guidance, the ECM not only acts as a permissive or non-permissive substrate for navigating axons, but also modulates the effects of classical guidance cues, such as netrin or Eph/ephrin family members. Despite being highly important, little is known about the expression of ECM molecules during CNS development. Therefore, this study assessed the molecular expression patterns of tenascin, HNK-1, laminin, fibronectin, perlecan, decorin, and osteopontin along chick embryo prosomere 1 during posterior commissure development. The posterior commissure is the first transversal axonal tract of the embryonic vertebrate brain. Located in the dorso-caudal portion of prosomere 1, posterior commissure axons primarily arise from the neurons of basal pretectal nuclei that run dorsally to the roof plate midline, where some turn toward the ipsilateral side. Expressional analysis of ECM molecules in this area these revealed to be highly arranged, and molecule interactions with axon fascicles suggested involvement in processes other than structural support. In particular, tenascin and the HNK-1 epitope extended in ventro-dorsal columns and enclosed axons during navigation to the roof plate. Laminin and osteopontin were expressed in the midline, very close to axons that at this point must decide between extending to the contralateral side or turning to the ipsilateral side. Finally, fibronectin, decorin, and perlecan appeared unrelated to axonal pathfinding in this region and were instead restricted to the external limiting membrane. In summary, the present report provides evidence for an intricate expression of different extracellular molecules that may cooperate in guiding posterior commissure axons.

Adaptation of the targeted capture Methyl-Seq platform for the mouse genome identifies novel tissue-specific DNA methylation patterns of genes involved in neurodevelopment

Methyl-Seq was recently developed as a targeted approach to assess DNA methylation (DNAm) at a genome-wide level in human. We adapted it for mouse and sought to examine DNAm differences across liver and 2 brain regions: cortex and hippocampus. A custom hybridization array was designed to isolate 99 Mb of CpG islands, shores, shelves, and regulatory elements in the mouse genome. This was followed by bisulfite conversion and sequencing on the Illumina HiSeq2000. The majority of differentially methylated cytosines (DMCs) were present at greater than expected frequency in introns, intergenic regions, near CpG islands, and transcriptional enhancers. Liver-specific enhancers were observed to be methylated in cortex, while cortex specific enhancers were methylated in the liver. Interestingly, commonly shared enhancers were differentially methylated between the liver and cortex. Gene ontology and pathway analysis showed that genes that were hypomethylated in the cortex and hippocampus were enriched for neuronal components and neuronal function. In contrast, genes that were hypomethylated in the liver were enriched for cellular components important for liver function. Bisulfite-pyrosequencing validation of 75 DMCs from 19 different loci showed a correlation of r = 0.87 with Methyl-Seq data. We also identified genes involved in neurodevelopment that were not previously reported to be differentially methylated across brain regions. This platform constitutes a valuable tool for future genome-wide studies involving mouse models of disease.

In Vivo Expression of the PTB-deleted Odin Mutant Results in Hydrocephalus

Odin has been implicated in the downstream signaling pathway of receptor tyrosine kinases, such as the epidermal growth factor and Eph receptors. However, the physiologically relevant function of Odin needs to be further determined. In this study, we used Odin heterozygous mice to analyze the Odin expression pattern; the targeted allele contained a β-geo gene trap vector inserted into the 14th intron of the Odin gene. Interestingly, we found that Odin was exclusively expressed in ependymal cells along the brain ventricles. In particular, Odin was highly expressed in the subcommissural organ, a small ependymal glandular tissue. However, we did not observe any morphological abnormalities in the brain ventricles or ependymal cells of Odin null-mutant mice. We also generated BAC transgenic mice that expressed the PTB-deleted Odin (dPTB) after a floxed GFP-STOP cassette was excised by tissue-specific Cre expression. Strikingly, Odin-dPTB expression played a causative role in the development of the hydrocephalic phenotype, primarily in the midbrain. In addition, Odin-dPTB expression disrupted proper development of the subcommissural organ and interfered with ependymal cell maturation in the cerebral aqueduct. Taken together, our findings strongly suggest that Odin plays a role in the differentiation of ependymal cells during early postnatal brain development.

Anatomical, molecular and pathological consideration of the circumventricular organs

BACKGROUND AND PURPOSE

Circumventricular organs (CVOs) are a diverse group of specialised structures characterized by peculiar vascular and position around the third and fourth ventricles of the brain. In humans, these organs are present during the fetal period and some become vestigial after birth. Some, such as the pineal gland (PG), subcommissural organ (SCO) and organum vasculosum of the lamina terminalis (OVLT), which are located around the third ventricle, might be the site of origin of periventricular tumours. In contrast to humans, CVOs are present in the adult rat and can be dissected by laser capture microdissection (LCM).

METHODS

In this study, we used LCM and microarrays to analyse the transcriptomes of three CVOs, the SCO, the subfornical organ (SFO) and the PG and the third ventricle ependyma of the adult rat, in order to better characterise these organs at the molecular level. Furthermore, an immunohistochemical study of Claudin-3 (CLDN3), a membrane protein involved in forming cellular tight junctions, was performed at the level of the SCO.

RESULTS

This study highlighted some potentially new or already described specific markers of these structures as Erbb2 and Col11a1 in ependyma, Epcam and CLDN3 in the SCO, Ren1 and Slc22a3 in the SFO and Tph, Anat and Asmt in the PG. Moreover, we found that CLDN3 expression was restricted to the apical pole of ependymocytes in the SCO.

Role of the subcommissural organ in the pathogenesis of congenital hydrocephalus in the HTx rat

The present investigation was designed to clarify the role of the subcommissural organ (SCO) in the pathogenesis of hydrocephalus occurring in the HTx rat. The brains of non-affected and hydrocephalic HTx rats from embryonic day 15 (E15) to postnatal day 10 (PN10) were processed for electron microscopy, lectin binding and immunocytochemistry by using a series of antibodies. Cerebrospinal fluid (CSF) samples of non-affected and hydrocephalic HTx rats were collected at PN1, PN7 and PN30 and analysed by one- and two-dimensional electrophoresis, immunoblotting and nanoLC-ESI-MS/MS. A distinct malformation of the SCO is present as early as E15. Since stenosis of the Sylvius aqueduct (SA) occurs at E18 and dilation of the lateral ventricles starts at E19, the malformation of the SCO clearly precedes the onset of hydrocephalus. In the affected rats, the cephalic and caudal thirds of the SCO showed high secretory activity with all methods used, whereas the middle third showed no signs of secretion. At E18, the middle non-secretory third of the SCO progressively fused with the ventral wall of SA, resulting in marked aqueduct stenosis and severe hydrocephalus. The abnormal development of the SCO resulted in the permanent absence of Reissner's fibre (RF) and led to changes in the protein composition of the CSF. Since the SCO is the source of a large mass of sialilated glycoproteins that form the RF and of those that remain CSF-soluble, we hypothesize that the absence of this large mass of negatively charged molecules from the SA domain results in SA stenosis and impairs the bulk flow of CSF through the aqueduct.

Wnt/β-catenin signaling and Msx1 promote outgrowth of the maxillary prominences

Facial morphogenesis requires a series of precisely orchestrated molecular events to promote the growth and fusion of the facial prominences. Cleft palate (CP) results from perturbations in this process. The transcriptional repressor Msx1 is a key participant in these molecular events, as demonstrated by the palatal clefting phenotype observed in Msx1^−/− embryos. Here, we exploited the high degree of conservation that exists in the gene regulatory networks that shape the faces of birds and mice, to gain a deeper understanding of Msx1 function in CP. Histomorphometric analyses indicated that facial development was disrupted as early as E12.5 in Msx1^−/− embryos, long before the palatal shelves have formed. By mapping the expression domain of Msx1 in E11.5 and E12.5 embryos, we found the structures most affected by loss of Msx1 function were the maxillary prominences. Maxillary growth retardation was accompanied by perturbations in angiogenesis that preceded the CP phenotype. Experimental chick manipulations and in vitro assays showed that the regulation of Msx1 expression by the Wnt/β-catenin pathway is highly specific. Our data in mice and chicks indicate a conserved role for Msx1 in regulating the outgrowth of the maxillary prominences, and underscore how imbalances in Msx1 function can lead of growth disruptions that manifest as CP.

Congenital Hydrocephalus in Genetically Engineered Mice

There is evidence that genetic factors play a role in the complex multifactorial pathogenesis of hydrocephalus. Identification of the genes involved in the development of this neurologic disorder in animal models may elucidate factors responsible for the excessive accumulation of cerebrospinal fluid in hydrocephalic humans. The authors report here a brief summary of findings from 12 lines of genetically engineered mice that presented with autosomal recessive congenital hydrocephalus. This study illustrates the value of knockout mice in identifying genetic factors involved in the development of congenital hydrocephalus. Findings suggest that dysfunctional motile cilia represent the underlying pathogenetic mechanism in 8 of the 12 lines ( Ulk4, Nme5, Nme7, Kif27, Stk36, Dpcd, Ak7, and Ak8). The likely underlying cause in the remaining 4 lines ( RIKEN 4930444A02, Celsr2, Mboat7, and transgenic FZD3) was not determined, but it is possible that some of these could also have ciliary defects. For example, the cerebellar malformations observed in RIKEN 4930444A02 knockout mice show similarities to a number of developmental disorders, such as Joubert, Meckel-Gruber, and Bardet-Biedl syndromes, which involve mutations in cilia-related genes. Even though the direct relevance of mouse models to hydrocephalus in humans remains uncertain, the high prevalence of familial patterns of inheritance for congenital hydrocephalus in humans suggests that identification of genes responsible for development of hydrocephalus in mice may lead to the identification of homologous modifier genes and susceptibility alleles in humans. Also, characterization of mouse models can enhance understanding of important cell signaling and developmental pathways involved in the pathogenesis of hydrocephalus.

Subdivisions of chick diencephalic roof plate: implication in the formation of the posterior commissure

The subcommissural organ (SCO) is a roof plate differentiation located in the caudal diencephalon under the posterior commissure (PC). A role for SCO and its secretory product, SCO-spondin, in the formation of the PC has been proposed. Here, we provide immunohistochemical evidence to suggest that SCO is anatomically divided in a bilateral region positive for SCO-spondin that surrounds a negative medial region. Remarkably, axons contacting the lateral region are highly fasciculated, in sharp contrast with the defasciculated axons of the medial region. In addition, lateral axon fascicles run toward the midline inside of tunnels limited by the basal prolongations of SCO cells and extracellular SCO-spondin. Our in vitro data in collagen gel matrices show that SCO-spondin induces axonal growth and fasciculation of pretectal explants. Together, our findings support the idea that SCO-spondin participates in the guidance and fasciculation of axons of the PC.

The expression of twisted gastrulation in postnatal mouse brain and functional implications

Twisted gastrulation (TWSG1), an extracellular regulator of bone morphogenetic protein (BMP) signaling, is critical for embryonic brain development. Mice deficient in TWSG1 have abnormal forebrain development manifesting as holoprosencephaly. The expression and potential roles of TWSG1 in postnatal brain development are less well understood. We show that Twsg1 is expressed in the adult mouse brain in the choroid plexus (CP), hippocampus, and other regions, with the strongest expression observed in CP. TWSG1 was also detected in a human fetal brain at mid-gestation, with highest levels in the epithelium of CP. Bmp1, Bmp2, Bmp4-Bmp7 as well as BmprIA and BmprII, but not BmprIB, were expressed in CP. BMP antagonists Chordin (Chrd) and Noggin were not detected in CP, however Chrd-like 1 and brain-specific Chrd-like (Brorin) were expressed. Electrophysiological study of synaptic plasticity revealed normal paired-pulse facilitation and long-term potentiation in the CA1 region of hippocampus in Twsg1(-/-) mice. Among the homozygous mutants that survive beyond the first 2 weeks, the prevalence of hydrocephalus was 4.3%, compared to 1.5% in a wild type colony (P=0.0133) between 3 and 10 weeks of life. We detected a high level of BMP signaling in CP in wild type adult mice that was 17-fold higher than in the hippocampus (P=0.005). In contrast, transforming growth factor beta (TGFbeta) signaling was predominant in the hippocampus. Both BMP signaling and the expression of BMP downstream targets Msx1 and Msx2 were reduced in CP in Twsg1(-/-) mice. In summary, we show that Twsg1 is expressed in the adult mouse and human fetal CP. We also show that BMP is a branch of TGFbeta superfamily that is dominant in CP. This presents an interesting avenue for future research in light of the novel roles of CP in neural progenitor differentiation and neuronal repair, especially since TWSG1 appears to be the main regulator of BMP present in CP.

Polarized expression of integrin beta1 in diencephalic roof plate during chick development, a possible receptor for SCO-spondin

The roof plate of the caudal diencephalon is formed by the posterior commissure (PC) and the underlying secretory ependyma, the subcommissural organ (SCO). The SCO is composed by radial glial cells bearing processes that cross the PC and attach to the meningeal basement membrane. Since early development, the SCO synthesizes SCO-spondin, a glycoprotein that shares similarities to axonal guidance proteins. In vitro, SCO-spondin promotes neuritic outgrowth through a mechanism mediated by integrin beta1. However, the secretion of SCO-spondin toward the extracellular matrix that surrounds the PC axons and the expression of integrins throughout PC development have not been addressed. Here we provide immunohistochemical evidence to suggest that during chick development SCO cells secrete SCO-spondin through their basal domain, where it is deposited into the extracellular matrix in close contact with axons of the PC that express integrin beta1. Our results suggest that SCO-spondin has a role in the development of the PC through its interaction with integrin beta1.

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.

[Msx1 and its influence on craniofacial growth]

Many genes that interact in a complex and interdependent manner participate in the development of the craniofacial complex. One of them, the Msxl homeobox gene, a transcription factor, is expressed from early developmental stages to adulthood in accordance with specific spatio-temporal patterns. When it is suppressed, transgenic mice exhibit craniofacial abnormalities that demonstrate what is its function in normal growth, just as it has been shown that certain Msxl mutations in humans are commonly associated with tooth agenesis.

Expression and function of the columnar patterning gene msh in late embryonic brain development of Drosophila

In Drosophila, specification of neural identity requires a network of conserved transcription factors, such as the columnar genes for dorsoventral patterning. Here, we analyze the expression and function of the columnar patterning gene muscle specific homeobox (msh) in late embryonic brain development. Expression of msh is observed in all brain neuromeres, including neurons and neuropile glia. Functional analysis demonstrates that msh is essential for proper development of the tritocerebral neuromere and brain neuropile glia. Thus, msh mutants display a severe loss of neural and glial tissue together with axonal patterning defects. This gap-like phenotype initially correlates with defects in neural and glial cell formation and during later embryonic development is associated with increased apoptotic activity. Taken together, our results provide evidence that the columnar patterning gene msh is required for correct tritocerebral neuromere development, as well as for neuropile glia formation and axogenesis in embryonic brain development of Drosophila.

Bone morphogenetic protein signalling and vertebrate nervous system development

Transforming growth factor-beta (TGFbeta) signalling, particularly signalling from the bone morphogenetic protein (BMP) members of this protein family, is crucial for the development of both the central and peripheral nervous systems in vertebrates. Experimental embryology and genetics performed in a range of organisms are providing insights into how BMPs establish the neural tissue and control the types and numbers of neurons formed. These studies also highlight the interactions between different developmental signals that are necessary to form a functional nervous system. The challenges ahead will be to uncover functions of TGFbeta signalling in later stages of CNS development, as well as to determine possible associations with neurological diseases.

Glypicans are differentially expressed during patterning and neurogenesis of early mouse brain

Glypicans are essential modulators of cell signalling during embryogenesis. Little is known about their functions in brain development. We show here that mouse glypicans (gpc-1 to gpc-6) are differentially expressed in embryonic brains during key morphogenetic events. In gastrulating embryos, gpc-4 is the only glypican expressed in anterior visceral endoderm. During neural tube closure, gpc-4 transcripts are restricted to the anterior neural ridge and telencephalon. At this stage, gpc-1 expression shifts from trunk and head mesenchyme to neural tube. Gpc-3 mRNA appears across the ventral neural tube, then in the lamina terminalis and hypothalamus. Gpc-2 and gpc-6 transcripts are in all brain compartments. Gpc-5 is found in ventral brains as neurogenesis starts. Onset of neurogenesis also coincides with differential expression of glypican genes either in neural progenitors or in differentiating neurons. The novel expression sites of glypicans shown here contribute to the identification of signalling molecules involved in brain patterning.

The secretory ependymal cells of the subcommissural organ: which role in hydrocephalus?

Ependyma in the central nervous system gives rise to several specialized cell types, including the secretory ependymal cells located in the subcommissural organ. These elongated cells show large cisternae in their cytoplasm, which are filled with material secreted into the cerebrospinal fluid and toward the leptomeningeal spaces. A specific secretion of the subcommissural organ was named SCO-spondin, regarding its marked homology with developmental proteins of the thrombospondin superfamily (presence of thrombospondin type 1 repeats). The ependymal cells of the subcommissural organ and SCO-spondin secretion are suspected to play a crucial role in cerebrospinal fluid flow and/or homeostasis. There is a close correlation between absence of the subcommissural organ and hydrocephalus in rat and mouse strains exhibiting congenital hydrocephalus, and in a number of mice transgenic for developmental genes. The ependymal cells of the subcommissural organ are under research as a key factor in several developmental processes of the central nervous system.

Expression of the human PAC1 receptor leads to dose-dependent hydrocephalus-related abnormalities in mice

Hydrocephalus is a common and potentially devastating birth defect affecting the CNS, and its relationship with G protein-coupled receptors (GPCRs) is unknown. We have expressed 2, 4, or 6 copies of a GPCR--the human PAC1 receptor with a 130-kb transgene in the mouse nervous system in a pattern closely resembling that of the endogenous gene. Consistent with PAC1 actions, PKA and PKC activity were elevated in the brains of Tg mice. Remarkably, Tg mice developed dose-dependent hydrocephalus-like characteristics, including enlarged third and lateral ventricles and reduced cerebral cortex, corpus callosum, and subcommissural organ (SCO). Neuronal proliferation and apoptosis were implicated in hydrocephalus, and we observed significantly reduced neuronal proliferation and massively increased neuronal apoptosis in the developing cortex and SCO of Tg embryos, while neurite outgrowth and neuronal migration in vitro remain uncompromised. Ventricular ependymal cilia are crucial for directing cerebrospinal fluid flow, and ependyma of Tg mice exhibited disrupted cilia with increased phospho-CREB immunoreactivity. These data demonstrate that altered neuronal proliferation/apoptosis and disrupted ependymal cilia are the main factors contributing to hydrocephalus in PAC1-overexpressing mice. This is the first report to our knowledge demonstrating that misregulation of GPCRs can be involved in hydrocephalus-related neurodevelopmental disorders.

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.

Dose-dependent functions of Fgf8 in regulating telencephalic patterning centers

Mouse embryos bearing hypomorphic and conditional null Fgf8mutations have small and abnormally patterned telencephalons. We provide evidence that the hypoplasia results from decreased Foxg1 expression,reduced cell proliferation and increased cell death. In addition, alterations in the expression of Bmp4, Wnt8b, Nkx2.1 and Shh are associated with abnormal development of dorsal and ventral structures. Furthermore, nonlinear effects of Fgf8 gene dose on the expression of a subset of genes, including Bmp4 and Msx1, correlate with a holoprosencephaly phenotype and with the nonlinear expression of transcription factors that regulate neocortical patterning. These data suggest that Fgf8 functions to coordinate multiple patterning centers, and that modifications in the relative strength of FGF signaling can have profound effects on the relative size and nature of telencephalic subdivisions.

msh/Msx gene family in neural development

The involvement of Msx homeobox genes in skull and tooth formation has received a great deal of attention. Recent studies also indicate a role for the msh/Msx gene family in development of the nervous system. In this article, we discuss the functions of these transcription factors in neural-tissue organogenesis. We will deal mainly with the interactions of the Drosophila muscle segment homeobox (msh) gene with other homeobox genes and the repressive cascade that leads to neuroectoderm patterning; the role of Msx genes in neural-crest induction, focusing especially on the differences between lower and higher vertebrates; their implication in patterning of the vertebrate neural tube, particularly in diencephalon midline formation. Finally, we will examine the distinct activities of Msx1, Msx2 and Msx3 genes during neurogenesis, taking into account their relationships with signalling molecules such as BMP.