Developmental genetics

Cytoplasmic tail of transmembrane Dscam controls antibacterial responses by regulating cell proliferation-related genes in hemocytes of Chinese mitten crab (Eriocheir sinensis)

In arthropods, the involvement of Dscam (Down syndrome cell adhesion molecule) in innate immunity has been extensively demonstrated. Its cytoplasmic tail contains multiple conserved functional sites, which indicates its involvement in different intracellular signaling pathways. In this study, we focused on the role of the cytoplasmic tail of Dscam in the Chinese mitten crab (Eriocheir sinensis) immune defense. In the group with cytoplasmic tail knockdown (the site was located on constant exons 37 and 38), 3885 differentially expressed genes (DEGs) were identified. The DEGs were enriched in small molecule binding, protein-containing complex binding, and immunity-related pathways. The expression of selected genes were validated using quantitative real-time reverse transcription PCR. We identified key Cell cycle, Janus kinase (JAK)-signal transducer, activator of transcription (STAT) and mitogen-activated protein kinase (MAPK) signaling pathway genes, the results indicated that the cytoplasmic tail of Dscam controls antibacterial responses by regulating cell proliferation-related genes in hemocytes.

Alternatively spliced exon 33 in Dscam controls antibacterial responses through regulating cellular endocytosis and regulation of actin cytoskeleton gene expression in the hemocytes of the Chinese mitten crab (Eriocheir sinensis)

It has been widely established that Down syndrome cell adhesion molecule (Dscam) regulates arthropod cellular endocytosis. However, the signal transduction pathways and molecular mechanisms of the regulatory process remain unclear. Our previous study identified a Dscam-mediated immune signal transduction pathway that regulates cellular antimicrobial peptide expression, and a conserved endocytosis motif encoded by exon 33 in the cytoplasmic tail of transmembrane Dscam. Therefore, the present study aimed to determine the transcriptional response of the Chinese mitten crab (Eriocheir sinensis) Dscam with a cytoplasmic tail encoded by different exons. In the group of exon 32 knockdown, 306 differentially expressed genes (DEGs) were identified, and 3579 differentially expressed genes (DEGs) were identified in the group of exon 33 knockdown (green fluorescent protein, (GFP) as control). The DEGs were enriched in small molecule binding, protein-containing complex binding, and immunity-related pathways. Quantitative real-time reverse transcription PCR validated the data for selected genes. Our study contributes to the understanding of the immune defense mechanism in E. sinensis and the development of the innate immune system, thus providing insights into disease control and prevention in aquaculture.

Long-term voluntary running modifies the levels of proteins of the excitatory/inhibitory system and reduces reactive astrogliosis in the brain of Ts65Dn mouse model for Down syndrome

We showed previously that voluntary long-term running improved cognition and motor skills, but in an age-dependent manner, in the Ts65Dn mouse model for Down syndrome (DS). Presently, we investigated the effect of running on the levels of some key proteins of the excitatory/inhibitory system, which is impaired in the trisomic brain, and on astroglia, a vital component of this system. Ts65Dn mice had free access to a running wheel for 9-13 months either from weaning or from the age of 7 months. Sedentary Ts65Dn mice served as controls. We found that running modified the levels of four of the seven proteins we tested that are associated with the glutamatergic/GABA-ergic system. Thus, Ts65Dn runners demonstrated increased levels of glutamine synthetase and metabotropic glutamate receptor 1 and decreased levels of glutamate transporter 1 and glutamic acid decarboxylase 65 (GAD65) versus sedentary mice, but of metabotropic glutamate receptor 1 and GAD65 only in the post-weaning cohort. GAD67, ionotropic N-methyl-D-aspartate type receptor subunit 1, and GABA_Aα5 receptors' levels were similar in runners and sedentary animals. The number of glial fibrillary acidic protein (GFAP)-positive astrocytes and the levels of GFAP were significantly reduced in runners relative to sedentary mice. Our study provides new insight into the mechanisms underlying the beneficial effect of voluntary, sustained running on function of the trisomic brain by identifying the involvement of proteins associated with glutamatergic and GABAergic systems and reduction in reactive astrogliosis.

Metabolic Diseases and Down Syndrome: How Are They Linked Together?

Down syndrome is a genetic disorder caused by the presence of a third copy of chromosome 21, associated with intellectual disabilities. Down syndrome is associated with anomalies of both the nervous and endocrine systems. Over the past decades, dramatic advances in Down syndrome research and treatment have helped to extend the life expectancy of these patients. Improved life expectancy is obviously a positive outcome, but it is accompanied with the need to address previously overlooked complications and comorbidities of Down syndrome, including obesity and diabetes, in order to improve the quality of life of Down syndrome patients. In this focused review, we describe the associations between Down syndrome and comorbidities, obesity and diabetes, and we discuss the understanding of proposed mechanisms for the association of Down syndrome with metabolic disorders. Drawing molecular mechanisms through which Type 1 diabetes and Type 2 diabetes could be linked to Down syndrome could allow identification of novel drug targets and provide therapeutic solutions to limit the development of metabolic and cognitive disorders.

The Usage of Exon-Exon Splice Junctions for the Detection of Alternative Splicing using the REIDS model

Alternative gene splicing is a common phenomenon in which a single gene gives rise to multiple transcript isoforms. The process is strictly guided and involves a multitude of proteins and regulatory complexes. Unfortunately, aberrant splicing events have been linked to genetic disorders. Therefore, understanding mechanisms of alternative splicing regulation and differences in splicing events between diseased and healthy tissues is crucial in advancing personalized medicine and drug developments. We propose a linear mixed model, Random Effects for the Identification of Differential Splicing (REIDS), for the identification of alternative splicing events using Human Transcriptome Arrays (HTA). For each exon, a splicing score is calculated based on two scores, an exon score and an array score. The junction information is used to rank the identified exons from strongly confident to less confident candidates for alternative splicing. The design of junctions was also discussed to highlight the complexity of exon-exon and exon-junction interactions. Based on a list of Rt-PCR validated probe sets, REIDS outperforms AltAnalyze and iGems in the % recall rate.

Modeling Down Syndrome with Patient iPSCs Reveals Cellular and Migration Deficits of GABAergic Neurons

The brain of Down syndrome (DS) patients exhibits fewer interneurons in the cerebral cortex, but its underlying mechanism remains unknown. By morphometric analysis of cortical interneurons generated from DS and euploid induced pluripotent stem cells (iPSCs), we found that DS GABA neurons are smaller and with fewer neuronal processes. The proportion of calretinin over calbindin GABA neurons is reduced, and the neuronal migration capacity is decreased. Such phenotypes were replicated following transplantation of the DS GABAergic progenitors into the mouse medial septum. Gene expression profiling revealed altered cell migratory pathways, and correction of the PAK1 pathway mitigated the cell migration deficit in vitro. These results suggest that impaired migration of DS GABAergic neurons may contribute to the reduced number of interneurons in the cerebral cortex and hippocampus in DS patients.

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DS iPSC-derived GABA interneurons show cellular deficits

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DS GABA interneurons exhibit decreased migration in vitro and in vivo

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RNA-seq reveals that expression of PAK1 is disrupted in the DS interneurons

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Regulation of PAK1 pathway rescues the defects of migration

A random effects model for the identification of differential splicing (REIDS) using exon and HTA arrays

Background

Alternative gene splicing is a common phenomenon in which a single gene gives rise to multiple transcript isoforms. The process is strictly guided and involves a multitude of proteins and regulatory complexes. Unfortunately, aberrant splicing events do occur which have been linked to genetic disorders, such as several types of cancer and neurodegenerative diseases (Fan et al., Theor Biol Med Model 3:19, 2006). Therefore, understanding the mechanism of alternative splicing and identifying the difference in splicing events between diseased and healthy tissue is crucial in biomedical research with the potential of applications in personalized medicine as well as in drug development.

Results

We propose a linear mixed model, Random Effects for the Identification of Differential Splicing (REIDS), for the identification of alternative splicing events. Based on a set of scores, an exon score and an array score, a decision regarding alternative splicing can be made. The model enables the ability to distinguish a differential expressed gene from a differential spliced exon. The proposed model was applied to three case studies concerning both exon and HTA arrays.

Conclusion

The REIDS model provides a work flow for the identification of alternative splicing events relying on the established linear mixed model. The model can be applied to different types of arrays.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-017-1687-8) contains supplementary material, which is available to authorized users.

Contribution of the two genes encoding histone variant h3.3 to viability and fertility in mice

Histones package DNA and regulate epigenetic states. For the latter, probably the most important histone is H3. Mammals have three near-identical H3 isoforms: canonical H3.1 and H3.2, and the replication-independent variant H3.3. This variant can accumulate in slowly dividing somatic cells, replacing canonical H3. Some replication-independent histones, through their ability to incorporate outside S-phase, are functionally important in the very slowly dividing mammalian germ line. Much remains to be learned of H3.3 functions in germ cell development. Histone H3.3 presents a unique genetic paradigm in that two conventional intron-containing genes encode the identical protein. Here, we present a comprehensive analysis of the developmental effects of null mutations in each of these genes. H3f3a mutants were viable to adulthood. Females were fertile, while males were subfertile with dysmorphic spermatozoa. H3f3b mutants were growth-deficient, dying at birth. H3f3b heterozygotes were also growth-deficient, with males being sterile because of arrest of round spermatids. This sterility was not accompanied by abnormalities in sex chromosome inactivation in meiosis I. Conditional ablation of H3f3b at the beginning of folliculogenesis resulted in zygote cleavage failure, establishing H3f3b as a maternal-effect gene, and revealing a requirement for H3.3 in the first mitosis. Simultaneous ablation of H3f3a and H3f3b in folliculogenesis resulted in early primary oocyte death, demonstrating a crucial role for H3.3 in oogenesis. These findings reveal a heavy reliance on H3.3 for growth, gametogenesis, and fertilization, identifying developmental processes that are particularly susceptible to H3.3 deficiency. They also reveal partial redundancy in function of H3f3a and H3f3b, with the latter gene being generally the most important.

An investigation of the molecular mechanisms engaged before and after the development of Alzheimer disease neuropathology in Down syndrome: a proteomics approach

Down syndrome (DS) is one of the most common causes of intellectual disability, owing to trisomy of all or part of chromosome 21. DS is also associated with the development of Alzheimer disease (AD) neuropathology after the age of 40 years. To better clarify the cellular and metabolic pathways that could contribute to the differences in DS brain, in particular those involved in the onset of neurodegeneration, we analyzed the frontal cortex of DS subjects with or without significant AD pathology in comparison with age-matched controls, using a proteomics approach. Proteomics represents an advantageous tool to investigate the molecular mechanisms underlying the disease. From these analyses, we investigated the effects that age, DS, and AD neuropathology could have on protein expression levels. Our results show overlapping and independent molecular pathways (including energy metabolism, oxidative damage, protein synthesis, and autophagy) contributing to DS, to aging, and to the presence of AD pathology in DS. Investigation of pathomechanisms involved in DS with AD may provide putative targets for therapeutic approaches to slow the development of AD.

Sex differences in the cholinergic basal forebrain in the Ts65Dn mouse model of Down syndrome and Alzheimer's disease

In the Down syndrome (DS) population, there is an early incidence of dementia and neuropathology similar to that seen in sporadic Alzheimer's disease (AD), including dysfunction of the basal forebrain cholinergic neuron (BFCN) system. Using Ts65Dn mice, a model of DS and AD, we examined differences in the BFCN system between male and female segmentally trisomic (Ts65Dn) and disomic (2N) mice at ages 5-8 months. Quantitative stereology was applied to BFCN subfields immunolabeled for choline acetyltransferase (ChAT) within the medial septum/vertical limb of the diagonal band (MS/VDB), horizontal limb of the diagonal band (HDB) and nucleus basalis of Meynert/substantia innominata (NBM/SI). We found no sex differences in neuron number or subregion area measurement in the MS/VDB or HDB. However, 2N and Ts65Dn females showed an average 34% decrease in BFCN number and an average 20% smaller NBM/SI region area compared with genotype-matched males. Further, relative to genotype-matched males, female mice had smaller BFCNs in all subregions. These findings demonstrate that differences between the sexes in BFCNs of young adult Ts65Dn and 2N mice are region and genotype specific. In addition, changes in post-processing tissue thickness suggest altered parenchymal characteristics between male and female Ts65Dn mice.

Role of tau protein in neuronal damage in Alzheimer's disease and Down syndrome

Neurodegenerative disorders constitute a growing concern worldwide. Their incidence has increased steadily, in particular among the elderly, a high-risk population that is becoming an important segment of society. Neurodegenerative mechanisms underlie many ailments such as Parkinson's disease, Huntington's disease, Alzheimer's disease (AD) and Down syndrome (DS, trisomy 21). Interestingly, there is increasing evidence suggesting that many such diseases share pathogenic mechanisms at the cellular and subcellular levels. These include altered protein misfolding, impaired autophagy, mitochondrial dysfunction, membrane damage, and altered axonal transport. Regarding AD and DS, the first common link comes from observations that DS patients undergo AD-like pathology early in adulthood. Also, the gene encoding for the amyloid precursor protein is present in human autosome 21 and in murine chromosome 16, an animal model of DS. Important functions related to preservation of normal neuronal architecture are impaired in both conditions. In particular, the stable assembly of microtubules, which is critical for the cytoskeleton, is impaired in AD and DS. In this process, tau protein plays a pivotal role in controlling microtubule stability. Abnormal tau expression and hyperphosphorylation are common features in both conditions, yet the mechanisms leading to these phenomena remain obscure. In the present report we review possible common mechanisms that may alter tau expression and function, in particular in relation to the effect of certain overexpressed DS-related genes, using cellular models of human DS. The latter contributes to the identification of possible therapeutic targets that could aid in the treatment of both AD and DS.

Form of dual-specificity tyrosine-(Y)-phosphorylation-regulated kinase 1A nonphosphorylated at tyrosine 145 and 147 is enriched in the nuclei of astroglial cells, adult hippocampal progenitors, and some cholinergic axon terminals

Compelling lines of evidence indicate that overexpression of dual-specificity tyrosine-(Y)-phosphorylation regulated kinase 1A (DYRK1A) in subjects with trisomy 21 (Down syndrome[DS]) contributes to the abnormal structure and function of the DS brain. In the present study, we used a novel, phospho-dependent antibody recognizing DYRK1A only with nonphosphorylated tyrosine 145 and 147 (DYRK1A Tyr-145/147P(-)), to investigate the expression pattern of this DYRK1A species in trisomic and disomic human and mouse brains. Immunoblotting and dephosphorylation experiments demonstrated higher levels of DYRK1A Tyr-145/147P(-) in postnatal trisomic brains in comparison with controls (by ∼40%) than those of the DYRK1A visualized by three other N- and C-terminally directed antibodies to DYRK1A. By immunofluorescence, the immunoreactivity to DYRK1A Tyr-145/147P(-) was the strongest in the nuclei of astroglial cells, which contrasted with the predominantly neuronal localization of DYRK1A visualized by the three other antibodies to DYRK1A we used. In addition, DYRK1A Tyr-145/147P(-) was enriched in the nuclei of neuronal progenitors and newly born neurons in the adult hippocampal proliferative zone and also occurred in some cholinergic axonal terminals. Our data show a distinctive expression pattern of DYRK1A forms nonphosphorylated at Tyr-145 and Tyr-147 in the brain tissue and suggest that DS subjects may exhibit not only upregulation of total DYRK1A, but also more subtle differences in phosphorylation levels of this kinase in comparison with control individuals.

Engineering DYRK1A overdosage yields Down syndrome-characteristic cortical splicing aberrations

Down syndrome (DS) associates with impaired brain functions, but the underlying mechanism(s) are yet unclear. The "gene dosage" hypothesis predicts that in DS, overexpression of a single gene can impair multiple brain functions through a signal amplification effect due to impaired regulatory mechanism(s). Here, we report findings attributing to impairments in the splicing process such a regulatory role. We have used DS fetal brain samples in search for initial evidence and employed engineered mice with MMU16 partial trisomy (Ts65Dn) or direct excess of the splicing-associated nuclear kinase Dyrk1A, overdosed in DS for further analyses. We present specific albeit modest changes in the DS brain's splicing machinery with subsequently amplified effects in target transcripts; and we demonstrate that engineered excess of Dyrk1A can largely recapitulate these changes. Specifically, in both the fetal DS brains and the Dyrk1A overdose models, we found ample modestly modified splicing-associated transcripts which apparently induced secondary enhancement in exon inclusion of key synaptic transcripts. Thus, DS-reduced levels of the dominant-negative TRKBT1 transcript, but not other TRKB mRNA transcripts, were accompanied by corresponding decreases in BDNF. In addition, the DS brains and Dyrk1A overdosage models showed selective changes in the transcripts composition of neuroligin mRNAs as well as reductions in the "synaptic" acetylcholinesterase variant AChE-S mRNA and corresponding increases in the stress-inducible AChE-R mRNA variant, yielding key synaptic proteins with unusual features. In cotransfected cells, Dyrk1A overdosage caused parallel changes in the splicing pattern of an AChE mini-gene, suggesting that Dyrk1A overdosage is both essential and sufficient to induce the observed change in the composition of AChE mRNA variants. Furthermore, the Dyrk1A overdosage animal models showed pronounced changes in the structure of neuronal nuclear speckles, where splicing events take place and in SR proteins phosphorylation known to be required for the splicing process. Together, our findings demonstrate DS-like brain splicing machinery malfunctioning in Dyrk1A overexpressing mice. Since individual splicing choices may alter cell fate determination, axon guidance, and synaptogenesis, these findings suggest the retrieval of balanced splicing as a goal for DS therapeutic manipulations early in DS development.

Olig1 and Olig2 triplication causes developmental brain defects in Down syndrome

Over-inhibition is thought to be one of the underlying causes of the cognitive deficits in Ts65Dn mice, the most widely used model of Down syndrome. We found a direct link between gene triplication and defects in neuron production during embryonic development. These neurogenesis defects led to an imbalance between excitatory and inhibitory neurons and to increased inhibitory drive in the Ts65Dn forebrain. We discovered that Olig1 and Olig2, two genes that are triplicated in Down syndrome and in Ts65Dn mice, were overexpressed in the Ts65Dn forebrain. To test the hypothesis that Olig triplication causes the neurological phenotype, we used a genetic approach to normalize the dosage of these two genes and thereby rescued the inhibitory neuron phenotype in the Ts65Dn brain. These data identify seminal alterations during brain development and suggest a mechanistic relationship between triplicated genes and these brain abnormalities in the Ts65Dn mouse.

Aneuploidy: from a physiological mechanism of variance to Down syndrome

Quantitative differences in gene expression emerge as a significant source of variation in natural populations, representing an important substrate for evolution and accounting for a considerable fraction of phenotypic diversity. However, perturbation of gene expression is also the main factor in determining the molecular pathogenesis of numerous aneuploid disorders. In this review, we focus on Down syndrome (DS) as the prototype of "genomic disorder" induced by copy number change. The understanding of the pathogenicity of the extra genomic material in trisomy 21 has accelerated in the last years due to the recent advances in genome sequencing, comparative genome analysis, functional genome exploration, and the use of model organisms. We present recent data on the role of genome-altering processes in the generation of diversity in DS neural phenotypes focusing on the impact of trisomy on brain structure and mental retardation and on biological pathways and cell types in target brain regions (including prefrontal cortex, hippocampus, cerebellum, and basal ganglia). We also review the potential that genetically engineered mouse models of DS bring into the understanding of the molecular biology of human learning disorders.

Altered expression of mitochondrial and extracellular matrix genes in the heart of human fetuses with chromosome 21 trisomy

BACKGROUND

The Down syndrome phenotype has been attributed to overexpression of chromosome 21 (Hsa21) genes. However, the expression profile of Hsa21 genes in trisomic human subjects as well as their effects on genes located on different chromosomes are largely unknown. Using oligonucleotide microarrays we compared the gene expression profiles of hearts of human fetuses with and without Hsa21 trisomy.

RESULTS

Approximately half of the 15,000 genes examined (87 of the 168 genes on Hsa21) were expressed in the heart at 18-22 weeks of gestation. Hsa21 gene expression was globally upregulated 1.5 fold in trisomic samples. However, not all genes were equally dysregulated and 25 genes were not upregulated at all. Genes located on other chromosomes were also significantly dysregulated. Functional class scoring and gene set enrichment analyses of 473 genes, differentially expressed between trisomic and non-trisomic hearts, revealed downregulation of genes encoding mitochondrial enzymes and upregulation of genes encoding extracellular matrix proteins. There were no significant differences between trisomic fetuses with and without heart defects.

CONCLUSION

We conclude that dosage-dependent upregulation of Hsa21 genes causes dysregulation of the genes responsible for mitochondrial function and for the extracellular matrix organization in the fetal heart of trisomic subjects. These alterations might be harbingers of the heart defects associated with Hsa21 trisomy, which could be based on elusive mechanisms involving genetic variability, environmental factors and/or stochastic events.

Tracking the evolution of alternatively spliced exons within the Dscam family

Background

The Dscam gene in the fruit fly, Drosophila melanogaster, contains twenty-four exons, four of which are composed of tandem arrays that each undergo mutually exclusive alternative splicing (4, 6, 9 and 17), potentially generating 38,016 protein isoforms. This degree of transcript diversity has not been found in mammalian homologs of Dscam. We examined the molecular evolution of exons within this gene family to locate the point of divergence for this alternative splicing pattern.

Results

Using the fruit fly Dscam exons 4, 6, 9 and 17 as seed sequences, we iteratively searched sixteen genomes for homologs, and then performed phylogenetic analyses of the resulting sequences to examine their evolutionary history. We found homologs in the nematode, arthropod and vertebrate genomes, including homologs in several vertebrates where Dscam had not been previously annotated. Among these, only the arthropods contain homologs arranged in tandem arrays indicative of mutually exclusive splicing. We found no homologs to these exons within the Arabidopsis, yeast, tunicate or sea urchin genomes but homologs to several constitutive exons from fly Dscam were present within tunicate and sea urchin. Comparing the rate of turnover within the tandem arrays of the insect taxa (fruit fly, mosquito and honeybee), we found the variants within exons 4 and 17 are well conserved in number and spatial arrangement despite 248–283 million years of divergence. In contrast, the variants within exons 6 and 9 have undergone considerable turnover since these taxa diverged, as indicated by deeply branching taxon-specific lineages.

Conclusion

Our results suggest that at least one Dscam exon array may be an ancient duplication that predates the divergence of deuterostomes from protostomes but that there is no evidence for the presence of arrays in the common ancestor of vertebrates. The different patterns of conservation and turnover among the Dscam exon arrays provide a striking example of how a gene can evolve in a modular fashion rather than as a single unit.

Abnormal APP, cholinergic and cognitive function in Ts65Dn Down's model mice

We evaluated Ts65Dn Down's syndrome mice and their littermates (LM) at 1-2, 4, and 12 months of age to determine amyloid precursor protein (APP)-related cellular and biochemical changes associated with cognitive deficits. Ts65Dn mice showed cognitive deficits in the Morris water maze compared to LM mice at 4 and 12 months of age. Ts65Dn, but not LM mice, developed a septohippocampal cholinergic neuronal degeneration of choline acetyltransferase (ChAT)-positive neurons at 12 months of age. These cellular changes were compensated by increases in ChAT enzyme activity of remaining cholinergic terminals in the hippocampus. By 12 months of age, Ts65Dn mice had elevations of APP protein levels in the hippocampus compared to their LM. At this age, both Ts65Dn mice and their LM abnormally responded to cholinergic muscarinic M1 agonist treatment in terms of hippocampal APP, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) levels compared to young adult C57BL/6 mice. In summary, the Ts65Dn mice show developmental and progressive age-related behavioral deficits, hippocampal APP, and cholinergic pathology. The relatively better cognitive spatial performance in LM compared to Ts65Dn mice suggests that high APP levels combined with progressive degeneration of the cholinergic system are critical to the pathology and cognitive deficits seen in Ts65Dn mice.

The Down syndrome cell adhesion molecule (DSCAM) interacts with and activates Pak

The Down syndrome cell adhesion molecule (DSCAM) is a member of the immunoglobulin superfamily that maps to a Down syndrome region of chromosome 21q22.2-22.3. In Drosophila, Dscam functions as an axon guidance receptor regulating targeting and branching. Genetic and biochemical studies have shown that in Drosophila, Dscam activates Pak1 via the Dock adaptor molecule. The extracellular domain of human DSCAM is highly homologous to the Drosophila protein; however, the intracellular domains of both human and Drosophila DSCAM share no obvious sequence identity. To study the signaling mechanisms of human DSCAM, we investigated the interaction between DSCAM and potential downstream molecules. We found that DSCAM directly binds to Pak1 and stimulates Pak1 phosphorylation and activity, unlike Drosophila where an adaptor protein Dock mediates the interaction between Dscam and Pak1. We also observed that DSCAM activates both JNK and p38 MAP kinases. Furthermore, expression of the cytoplasmic domain of DSCAM induces a morphological change in cultured cells that is JNK-dependent. These observations suggest that human DSCAM also signals through Pak1 and may function in axon guidance similar to the Drosophila Dscam.

Understanding mental retardation in Down's syndrome using trisomy 16 mouse models

Mental retardation in Down's syndrome, human trisomy 21, is characterized by developmental delays, language and memory deficits and other cognitive abnormalities. Neurophysiological and functional information is needed to understand the mechanisms of mental retardation in Down's syndrome. The trisomy mouse models provide windows into the molecular and developmental effects associated with abnormal chromosome numbers. The distal segment of mouse chromosome 16 is homologous to nearly the entire long arm of human chromosome 21. Therefore, mice with full or segmental trisomy 16 (Ts65Dn) are considered reliable animal models of Down's syndrome. Ts65Dn mice demonstrate impaired learning in spatial tests and abnormalities in hippocampal synaptic plasticity. We hypothesize that the physiological impairments in the Ts65Dn mouse hippocampus can model the suboptimal brain function occuring at various levels of Down's syndrome brain hierarchy, starting at a single neuron, and then affecting simple and complex neuronal networks. Once these elements create the gross brain structure, their dysfunctional activity cannot be overcome by extensive plasticity and redundancy, and therefore, at the end of the maturation period the mind inside this brain remains deficient and delayed in its capabilities. The complicated interactions that govern this aberrant developmental process cannot be rescued through existing compensatory mechanisms. In summary, overexpression of genes from chromosome 21 shifts biological homeostasis in the Down's syndrome brain to a new less functional state.

On the cause of mental retardation in Down syndrome: extrapolation from full and segmental trisomy 16 mouse models

Down syndrome (DS, trisomy 21, Ts21) is the most common known cause of mental retardation. In vivo structural brain imaging in young DS adults, and post-mortem studies, indicate a normal brain size after correction for height, and the absence of neuropathology. Functional imaging with positron emission tomography (PET) shows normal brain glucose metabolism, but fewer significant correlations between metabolic rates in different brain regions than in controls, suggesting reduced functional connections between brain circuit elements. Cultured neurons from Ts21 fetuses and from fetuses of an animal model for DS, the trisomy 16 (Ts16) mouse, do not differ from controls with regard to passive electrical membrane properties, including resting potential and membrane resistance. On the other hand, the trisomic neurons demonstrate abnormal active electrical and biochemical properties (duration of action potential and its rates of depolarization and repolarization, altered kinetics of active Na(+), Ca(2+) and K(+) currents, altered membrane densities of Na(+) and Ca(2+) channels). Another animal model, the adult segmental trisomy 16 mouse (Ts65Dn), demonstrates reduced long-term potentiation and increased long-term depression (models for learning and memory related to synaptic plasticity) in the CA1 region of the hippocampus. Evidence suggests that the abnormalities in the trisomy mouse models are related to defective signal transduction pathways involving the phosphoinositide cycle, protein kinase A and protein kinase C. The phenotypes of DS and its mouse models do not involve abnormal gene products due to mutations or deletions, but result from altered expression of genes on human chromosome 21 or mouse chromosome 16, respectively. To the extent that the defects in signal transduction and in active electrical properties, including synaptic plasticity, that are found in the Ts16 and Ts65Dn mouse models, are found in the brain of DS subjects, we postulate that mental retardation in DS results from such abnormalities. Changes in timing and synaptic interaction between neurons during development can lead to less than optimal functioning of neural circuitry and signaling then and in later life.

Down syndrome cell adhesion molecule DSCAM mediates homophilic intercellular adhesion

Down Syndrome (DS) caused by trisomy 21 is the most common birth defect associated with mental retardation. Recently, a novel gene named, DSCAM, has been identified in the DS critical region. DSCAM is predicted to be a transmembrane protein with a very high structural and sequence homology to Ig superfamily of cell adhesion molecules and is expressed in the developing nervous system with the highest level in fetal brain. Diverse glycoproteins of cell surfaces and extracellular matrices operationally termed as 'adhesion molecule' are important in the specification of cell interactions during development, maintenance and regeneration of the nervous system. To understand the cellular function of DSCAM protein, we transfected human DSCAM cDNA into mouse fibroblast L cells and analysed its expression. On Western blot analysis, antibodies raised against recombinant DSCAM-Ig3 recognized a 198 kDa protein band in the membrane fraction of DSCAM transfected L cells. Stable transformants expressing DSCAM showed uniform surface expression. DSCAM-expressing transfectants exhibited enhanced adhesive properties, aggregating with faster kinetics and forming aggregates in a homophilic manner. Divalent cations are not required for this cell aggregation. These results demonstrate that DSCAM is a cell adhesion molecule that can mediate cation-independent homophilic binding activity between DSCAM expressing cells.

Taurodontism: an anomaly of teeth reflecting disruptive developmental homeostasis

Two models concerning morphometric traits occurring frequently in aneuploidy states posit, respectively, 1) that they reflect the expression of specific major oligogenes for that trait on the chromosome involved or 2) that they result from a generalized disruption of developmental homeostasis. In contrast to previous studies that have investigated variations in morphometric traits in a single aneuploidy state, this study investigates a single morphometric trait, taurodontism, as it occurs in otherwise normal individuals, in nonchromosomal syndromes, and in aneuploidy syndromes to determine whether the trait best fits the oligogene or the disrupted developmental homeostasis model. Taurodontism is diagnosed from dental radiographs. It is an extreme variation in tooth form seen in multirooted teeth in which the bifurcation or trifurcation of the roots is displaced toward the apex of the root, resulting in increased size of the pulp chamber. The point of furcation, and consequently the size of the pulp chamber, is a quasicontinuously distributed trait. The results indicate that taurodontism most likely is the result of disrupted developmental homeostasis.