Quantitative trait loci modulate ventricular size in the mouse brain

Molecular Mechanisms and Risk Factors for the Pathogenesis of Hydrocephalus

Hydrocephalus is a neurological condition due to the aberrant circulation and/or obstruction of cerebrospinal fluid (CSF) flow with consequent enlargement of cerebral ventricular cavities. However, it is noticed that a lot of patients may still go through symptomatic progression despite standard shunting procedures, suggesting that hydrocephalus is far more complicated than a simple CSF circulative/obstructive disorder. Growing evidence indicates that genetic factors play a fundamental role in the pathogenesis of some hydrocephalus. Although the genetic research of hydrocephalus in humans is limited, many genetic loci of hydrocephalus have been defined in animal models. In general, the molecular abnormalities involved in the pathogenesis of hydrocephalus include brain development and ependymal cell dysfunction, apoptosis, inflammation, free radical generation, blood flow, and cerebral metabolism. Moreover, recent studies have indicated that the molecular abnormalities relevant to aberrant cerebral glymphatic drainage turn into an attractive subject in the CSF circulation disorder. Furthermore, the prevalent risk factors could facilitate the development of hydrocephalus. In this review, we elicited some possible fundamental molecular mechanisms and facilitating risk factors involved in the pathogenesis of hydrocephalus, and aimed to widen the diagnosis and therapeutic strategies for hydrocephalus management. Such knowledge could be used to improve patient care in different ways, such as early precise diagnosis and effective therapeutic regimens.

Research-Relevant Conditions and Pathology of Laboratory Mice, Rats, Gerbils, Guinea Pigs, Hamsters, Naked Mole Rats, and Rabbits

Animals are valuable resources in biomedical research in investigations of biological processes, disease pathogenesis, therapeutic interventions, safety, toxicity, and carcinogenicity. Interpretation of data from animals requires knowledge not only of the processes or diseases (pathophysiology) under study but also recognition of spontaneous conditions and background lesions (pathology) that can influence or confound the study results. Species, strain/stock, sex, age, anatomy, physiology, spontaneous diseases (noninfectious and infectious), and neoplasia impact experimental results and interpretation as well as animal welfare. This review and the references selected aim to provide a pathology resource for researchers, pathologists, and veterinary personnel who strive to achieve research rigor and validity and must understand the spectrum of "normal" and expected conditions to accurately identify research-relevant experimental phenotypes as well as unusual illness, pathology, or other conditions that can compromise studies involving laboratory mice, rats, gerbils, guinea pigs, hamsters, naked mole rats, and rabbits.

Widespread Volumetric Brain Changes following Tooth Loss in Female Mice

Tooth loss is associated with altered sensory, motor, cognitive and emotional functions. These changes vary highly in the population and are accompanied by structural and functional changes in brain regions mediating these functions. It is unclear to what extent this variability in behavior and function is caused by genetic and/or environmental determinants and which brain regions undergo structural plasticity that mediates these changes. Thus, the overall goal of our research program is to identify genetic variants that control structural and functional plasticity following tooth loss. As a step toward this goal, here our aim was to determine whether structural magnetic resonance imaging (sMRI) is sensitive to detect quantifiable volumetric differences in the brains of mice of different genetic background receiving tooth extraction or sham operation. We used 67 adult female mice of 7 strains, comprising the A/J (A) and C57BL/6J (B) strains and a randomly selected sample of 5 of the 23 AXB-BXA strains (AXB1, AXB4, AXB24, BXA14, BXA24) that were produced from the A and B parental mice by recombinations and inbreeding. This panel of 25 inbred strains of genetically diverse inbred strains of mice is used for mapping chromosomal intervals throughout the genome that harbor candidate genes controlling the phenotypic variance of any trait under study. Under general anesthesia, 39 mice received extraction of 3 right maxillary molar teeth and 28 mice received sham operation. On post-extraction day 21, post-mortem whole-brain high-resolution sMRI was used to quantify the volume of 160 brain regions. Compared to sham operation, tooth extraction was associated with a significantly reduced regional and voxel-wise volumes of cortical brain regions involved in processing somatosensory, motor, cognitive and emotional functions, and increased volumes in subcortical sensorimotor and temporal limbic forebrain regions including the amygdala. Additionally, comparison of the 10 BXA14 and 21 BXA24 mice revealed significant volumetric differences between the two strains in several brain regions. These findings highlight the utility of high-resolution sMRI for studying tooth loss-induced structural brain plasticity in mice, and provide a foundation for further phenotyping structural brain changes following tooth loss in the full AXB-BXA panel to facilitate mapping genes that control brain plasticity following orofacial injury.

GABAergic neurons regulate lateral ventricular development via transcription factor Pax5

Postmortem studies have revealed a downregulation of the transcription factor Pax5 in GABAergic neurons in bipolar disorder, a neurodevelopmental disorder, raising the question whether Pax5 in GABAergic neurons has a role in normal brain development. In a genetic approach to study functions of Pax5 in GABAergic neurons, Pax5 was specifically deleted in GABAergic neurons from Pax5 floxed mice using a novel Gad1-Cre transgenic mouse line expressing Cre recombinase in Gad1-positive, that is, GABAergic neurons. Surprisingly, these mice developed a marked enlargement of the lateral ventricles at approximately 7 weeks of age, which was lethal within 1-2 weeks of its appearance. This hydrocephalus phenotype was observed in mice homozygous or heterozygous for the Pax5 conditional knockout, with a gene dosage-dependent penetrance. By QTL (quantitative trait loci) mapping, a 3.5 Mb segment on mouse chromosome 4 flanked by markers D4Mit237 and D4Mit214 containing approximately 92 genes including Pax5 has previously been linked to differences in lateral ventricular size. Our findings are consistent with Pax5 being a relevant gene underlying this QTL phenotype and demonstrate that Pax5 in GABAergic neurons is essential for normal ventricular development.

A modifier locus on chromosome 5 contributes to L1 cell adhesion molecule X-linked hydrocephalus in mice

Humans with L1 cell adhesion molecule (L1CAM) mutations exhibit X-linked hydrocephalus, as well as other severe neurological disorders. L1-6D mutant mice, which are homozygous for a deletion that removes the sixth immunoglobulin-like domain of L1cam, seldom display hydrocephalus on the 129/Sv background. However, the same L1-6D mutation produces severe hydrocephalus on the C57BL/6J background. To begin to understand how L1cam deficiencies result in hydrocephalus and to identify modifier loci that contribute to X-linked hydrocephalus by genetically interacting with L1cam, we conducted a genome-wide scan on F2 L1-6D mice, bred from L1-6D 129S2/SvPasCrlf and C57BL/6J mice. Linkage studies, utilizing chi-square tests and quantitative trait loci mapping techniques, were performed. Candidate modifier loci were further investigated in an extension study. Linkage was confirmed for a locus on chromosome 5, which we named L1cam hydrocephalus modifier 1 (L1hydro1), p = 4.04 X 10(-11).

Genetic dissection of the mouse brain using high-field magnetic resonance microscopy

Magnetic resonance (MR) imaging has demonstrated that variation in brain structure is associated with differences in behavior and disease state. However, it has rarely been practical to prospectively test causal models that link anatomical and functional differences in humans. In the present study we have combined classical mouse genetics with high-field MR to systematically explore and test such structure-functional relations across multiple brain regions. We segmented 33 regions in two parental strains-C57BL/6J (B) and DBA/2J (D)-and in nine BXD recombinant inbred strains. All strains have been studied extensively for more than 20 years using a battery of genetic, functional, anatomical, and behavioral assays. We compared levels of variation within and between strains and sexes, by region, and by system. Average within-strain variation had a coefficient of variation (CV) of 1.6% for the whole brain; while the CV ranged from 2.3 to 3.6% for olfactory bulbs, cortex and cerebellum, and up to approximately 18% for septum and laterodorsal thalamic nucleus. Variation among strain averages ranged from 6.7% for cerebellum, 7.6% for whole brain, 9.0% for cortex, up to approximately 26% for the ventricles, laterodorsal thalamic nucleus, and the interpeduncular nucleus. Heritabilities averaged 0.60+/-0.18. Sex differences were not significant with the possible (and unexpected) exception of the pons ( approximately 20% larger in males). A correlation matrix of regional volumes revealed high correlations among functionally related parts of the CNS (e.g., components of the limbic system), and several high correlations between regions that are not anatomically connected, but that may nonetheless be functionally or genetically coupled.

Multigenic factors associated with a hydrocephalus-like phenotype found in inter-subspecific consomic mouse strains

Hydrocephalus is a significant clinical condition in humans and is known to be a multifactorial neurologic disorder. It has been thought that genetic factors are closely involved in the etiology of congenital hydrocephalus, but further investigation is required to elucidate the genetic architecture of hydrocephalus. By analyzing breeding records of a panel of inter-subspecific consomic mouse strains, we found that consomic strains with MSM/Ms (MSM) chromosomes 4, 5, 7, 11, 15, and 17 showed a significantly higher incidence of hydrocephalus, whereas both parental strains, MSM and C57BL/6J (B6), rarely showed this abnormality. Further analysis of the consomic Chr 17 strain revealed that apparently normal individuals of this strain also exhibited increased brain ventricle size compared to B6 and had larger individual variation of ventricle size within the strain. Thus, we concluded that hydrocephalus is an extreme phenotype of individual ventricle size variation. We then established and analyzed several subconsomic strains of Chr 17 to identify genetic factors related to hydrocephalus-like phenotype and successfully mapped one genetic locus around the proximal region of Chr 17.

Genetic analysis of posterior medial barrel subfield (PMBSF) size in somatosensory cortex (SI) in recombinant inbred strains of mice

BACKGROUND

Quantitative trait locus (QTL) mapping is an important tool for identifying potential candidate genes linked to complex traits. QTL mapping has been used to identify genes associated with cytoarchitecture, cell number, brain size, and brain volume. Previously, QTL mapping was utilized to examine variation of barrel field size in the somatosensory cortex in a limited number of recombinant inbred (RI) strains of mice. In order to further elucidate the underlying natural variation in mouse primary somatosensory cortex, we measured the size of the posterior medial barrel subfield (PMBSF), associated with the representation of the large mystacial vibrissae, in an expanded sample set that included 42 BXD RI strains, two parental strains (C57BL/6J and DBA/2J), and one F1 strain (B6D2F1). Cytochrome oxidase labeling was used to visualize barrels within the PMBSF.

RESULTS

We observed a 33% difference between the largest and smallest BXD RI strains with continuous variation in-between. Using QTL linkage analysis from WebQTL, we generated linkage maps of raw total PMBSF and brain weight adjusted total PMBSF areas. After removing the effects of brain weight, we detected a suggestive QTL (likelihood ratio statistic [LRS]: 14.20) on the proximal arm of chromosome 4. Candidate genes under the suggestive QTL peak for PMBSF area were selected based on the number of single nucleotide polymorphisms (SNPs) present and the biological relevance of each gene. Among the candidate genes are Car8 and Rab2. More importantly, mRNA expression profiles obtained using GeneNetwork indicated a strong correlation between total PMBSF area and two genes (Adcy1 and Gap43) known to be important in mouse cortex development. GAP43 has been shown to be critical during neurodevelopment of the somatosensory cortex, while knockout Adcy1 mice have disrupted barrel field patterns.

CONCLUSION

We detected a novel suggestive QTL on chromosome 4 that is linked to PMBSF size. The present study is an important step towards identifying genes underlying the size and possible development of cortical structures.

Loss of X-linked mental retardation gene oligophrenin1 in mice impairs spatial memory and leads to ventricular enlargement and dendritic spine immaturity

Loss of oligophrenin1 (OPHN1) function in human causes X-linked mental retardation associated with cerebellar hypoplasia and, in some cases, with lateral ventricle enlargement. In vitro studies showed that ophn1 regulates dendritic spine through the control of Rho GTPases, but its in vivo function remains unknown. We generated a mouse model of ophn1 deficiency and showed that it mimics the ventricles enlargement without affecting the cerebellum morphoanatomy. The ophn1 knock-out mice exhibit behavioral defects in spatial memory together with impairment in social behavior, lateralization, and hyperactivity. Long-term potentiation and mGluR-dependent long-term depression are normal in the CA1 hippocampal area of ophn1 mutant, whereas paired-pulse facilitation is reduced. This altered short-term plasticity that reflects changes in the release of neurotransmitters from the presynaptic processes is associated with normal synaptic density together with a reduction in mature dendritic spines. In culture, inactivation of ophn1 function increases the density and proportion of immature spines. Using a conditional model of loss of ophn1 function, we confirmed this immaturity defect and showed that ophn1 is required at all the stages of the development. These studies show that, depending of the context, ophn1 controls the maturation of dendritic spines either by maintaining the density of mature spines or by limiting the extension of new filopodia. Altogether, these observations indicate that cognitive impairment related to OPHN1 loss of function is associated with both presynaptic and postsynaptic alterations.

Quantitative trait loci linked to thalamus and cortex gray matter volumes in BXD recombinant inbred mice

To investigate whether there are separate or shared genetic influences on the development of the thalamus and cerebral cortex, we identified quantitative trait loci (QTLs) for relevant structural volumes in BXD recombinant inbred (RI) strains of mice. In 34 BXD RI strains and two parental strains (C57BL/6J and DBA/2J), we measured the volumes of the entire thalamus and cortex gray matter using point counting and Cavalieri's rule. Heritability was calculated using analysis of variance (ANOVA), and QTL analysis was carried out using WebQTL (http://www.genenetwork.org). The heritability of thalamus volume was 36%, and three suggestive QTLs for thalamus volume were identified on chromosomes 10, 11 and 16. The heritability of cortical gray matter was 43%, and four suggestive QTLs for cortex gray matter volume were identified on chromosomes 2, 8, 16 and 19. The genetic correlation between thalamus and cortex gray matter volumes was 0.64. Also, a single QTL on chromosome 16 (D16Mit100) was identified for thalamus volume, cortex gray matter volume and Morris water maze search-time preference (r=0.71). These results suggest that there are separate and shared genetic influences on the development of the thalamus and cerebral cortex.

Sexual dimorphism revealed in the structure of the mouse brain using three-dimensional magnetic resonance imaging

A large variety of sexual dimorphisms have been described in the brains of many vertebrate species, including humans. Naturally occurring sexual dimorphism has been implicated in the risk, progression and recovery from numerous neurological disorders, including head injury, multiple sclerosis and stroke. Genetically altered mice are a key tool in the study of structure-function relationships in the mammalian central nervous system and serve as models for human neuropsychiatric and neurological disorders. However, there are a limited number of quantitative three-dimensional analyses of the adult mouse brain structures. In order to address limitations in our knowledge of anatomical differences, a comprehensive study was undertaken using full 3D magnetic resonance imaging (MRI) to examine sexual dimorphisms in the C57BL/6J whole mouse brain. An expected difference in overall brain size between the sexes was found, where male brains were 2.5% larger in volume than female brains. Beyond the overall brain size differences in the sexes, the following significantly different regions were found: males were larger in the thalamus, primary motor cortex and posterior hippocampus, while females were larger in posterior hypothalamic area, entorhinal cortex and anterior hippocampus. Using high-definition 3D MRI on a normal inbred mouse strain, we have mapped in detail many sex-associated statistically significant differences in brain structures.

Independent quantitative trait loci influence ventral and dorsal hippocampal volume in recombinant inbred strains of mice

Anatomical and functional studies support segregation of the hippocampus into ventral and dorsal components along its septotemporal axis. However, it is unknown whether the development of these two components of the hippocampus is influenced by common or separate genetic factors. In this study, we used recombinant inbred strains of mice to determine whether the same or different quantitative trait loci (QTL) influence ventral and dorsal hippocampal volume. Using two sets of strains of recombinant inbred mice (BXD and AXB/BXA), we identified separate QTLs for ventral and dorsal hippocampal volume. In BXD mice, suggestive QTLs for ventral hippocampus were identified on chromosomes 2, 8 and 13, and a significant QTL for dorsal hippocampal volume was identified on chromosome 15. There was also a suggestive QTL for dorsal hippocampal volume on chromosome 13. In AXB/BXA mice, there were no significant or suggestive QTLs for ventral hippocampal volume, but a significant QTL for dorsal hippocampus was identified on chromosome 5. These findings suggest that the development of the ventral and dorsal components of the hippocampus is influenced by separate genetic loci.

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.

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.

Ventricular size mapping in a transgenic model of schizophrenia

Genetically engineered mice have been generated to model a variety of neurological disorders. The chakragati (ckr) mouse is beginning to provide valuable insights into the structural brain changes underlying certain manifestations of schizophrenia. For instance, these mice show enlargement of the lateral ventricles, an abnormality frequently reported as a structural aberration in the schizophrenic brain. As neither the anatomical pattern nor the timing of this ventricular enlargement is known, we used magnetic resonance imaging (MRI) techniques to non-invasively visualize the development of the ventricular system in 5-, 10- and 30-day-old ckr pups. High-resolution MR images obtained from these mutants showed a progressive enlargement of the lateral ventricles, starting at day 5 of postnatal life. These emerging deficits were associated with abnormalities in mid-saggital corpus callosum area and thickness, particularly in 30-day-old adolescent animals. At this time of development, aberrant behaviors that mimic certain symptoms of schizophrenia also appeared in ckr mice suggesting that structural changes in ventricular size predates the onset of psychotic-like behaviors. These results are viewed as further indication that pre- and peri-natal disturbances of the ventricular system and adjacent neural regions may be important pathogenic factors in schizophrenia. Application of MRI to the ckr mouse is relatively new but has great potential for clarifying the relationship between brain structure changes and genetically induced vulnerabilities to psychoses.

Sex-specific, postpuberty changes in mouse brain structures revealed by three-dimensional magnetic resonance microscopy

Sexual dimorphism of brain structures has been reported in some species. We report that sex-dependent developmental structure changes exist in the C57Bl/6(J) mouse, a common model for the genetic analysis of brain function. High resolution, three-dimensional (3D) magnetic resonance microscopy (MRM) images were obtained in intact brains of male and female adult and peripubertal mice. The lateral and third ventricles, hippocampus, amygdala, striatum, and total brain were reconstructed in 3D. As observed in humans, there was overall cerebral growth from peripuberty to adulthood in both sexes. After correcting for the increased brain size, the hippocampus and amygdala were disproportionately larger in adult compared to peripubertal mice. Several sexual dimorphisms were also observed. The lateral ventricles were larger, while the amygdala (the left side in particular) was smaller in females compared to males. Lateral and third ventricles were reduced over time in males only, exhibiting a sex-specific developmental profile. The striatal size was uniform among the groups studied. The surface area of the segmented structures was assayed. Possible shape distortions were detected for the lateral ventricles, hippocampus, and overall brain structure based on a lack of covariance between the surface area and volumetric measurements. Although many sexually dimorphic changes are reported perinatally, our results suggest that there are additional sex-specific transformations that occur around puberty and persist in adulthood.

Informatics center for mouse genomics: the dissection of complex traits of the nervous system

In recent years, there has been an explosion in the number of tools and techniques available to researchers interested in exploring the genetic basis of all aspects of central nervous system (CNS) development and function. Here, we exploit a powerful new reductionist approach to explore the genetic basis of the very significant structural and molecular differences between the brains of different strains of mice, called either complex trait or quantitative trait loci (QTL) analysis. Our specific focus has been to provide universal access over the web to tools for the genetic dissection of complex traits of the CNS--tools that allow researchers to map genes that modulate phenotypes at a variety of levels ranging from the molecular all the way to the anatomy of the entire brain. Our website, The Mouse Brain Library (MBL; http://mbl.org) is comprised of four interrelated components that are designed to support this goal: The Brain Library, iScope, Neurocartographer, and WebQTL. The centerpiece of the MBL is an image database of histologically prepared museum-quality slides representing nearly 2000 mice from over 120 strains--a library suitable for stereologic analysis of regional volume. The iScope provides fast access to the entire slide collection using streaming video technology, enabling neuroscientists to acquire high-magnification images of any CNS region for any of the mice in the MBL. Neurocartographer provides automatic segmentation of images from the MBL by warping precisely delineated boundaries from a 3D atlas of the mouse brain. Finally, WebQTL provides statistical and graphical analysis of linkage between phenotypes and genotypes.