Expression of the mnb (dyrk) protein in adult and embryonic mouse tissues

Case Report: Gut and spleen anomalies associated with DYRK1A syndrome

DYRK1A syndrome has been extensively studied primarily with regard to neurologic and other phenotypic features such as skeleton and craniofacial alterations. In the present paper, we aim to highlight unusual anomalies associated with a DYRK1A mutation: a 17-year-old female patient with language and cognitive delay, microcephaly, and an autistic disorder, who was operated upon for spleen torsion with anomalous gut fixation.

Dyrk1a from Gene Function in Development and Physiology to Dosage Correction across Life Span in Down Syndrome

Down syndrome is the main cause of intellectual disabilities with a large set of comorbidities from developmental origins but also that appeared across life span. Investigation of the genetic overdosage found in Down syndrome, due to the trisomy of human chromosome 21, has pointed to one main driver gene, the Dual-specificity tyrosine-regulated kinase 1A (Dyrk1a). Dyrk1a is a murine homolog of the drosophila minibrain gene. It has been found to be involved in many biological processes during development and in adulthood. Further analysis showed its haploinsufficiency in mental retardation disease 7 and its involvement in Alzheimer's disease. DYRK1A plays a role in major developmental steps of brain development, controlling the proliferation of neural progenitors, the migration of neurons, their dendritogenesis and the function of the synapse. Several strategies targeting the overdosage of DYRK1A in DS with specific kinase inhibitors have showed promising evidence that DS cognitive conditions can be alleviated. Nevertheless, providing conditions for proper temporal treatment and to tackle the neurodevelopmental and the neurodegenerative aspects of DS across life span is still an open question.

Vascular defects of DYRK1A knockouts are ameliorated by modulating calcium signaling in zebrafish

DYRK1A is a major causative gene in Down syndrome (DS). Reduced incidence of solid tumors such as neuroblastoma in DS patients and increased vascular anomalies in DS fetuses suggest a potential role of DYRK1A in angiogenic processes, but in vivo evidence is still scarce. Here, we used zebrafish dyrk1aa mutant embryos to understand DYRK1A function in cerebral vasculature formation. Zebrafish dyrk1aa mutants exhibited cerebral hemorrhage and defects in angiogenesis of central arteries in the developing hindbrain. Such phenotypes were rescued by wild-type dyrk1aa mRNA, but not by a kinase-dead form, indicating the importance of DYRK1A kinase activity. Chemical screening using a bioactive small molecule library identified a calcium chelator, EGTA, as one of the hits that most robustly rescued the hemorrhage. Vascular defects of mutants were also rescued by independent modulation of calcium signaling by FK506. Furthermore, the transcriptomic analyses supported the alterations of calcium signaling networks in dyrk1aa mutants. Together, our results suggest that DYRK1A plays an essential role in angiogenesis and in maintenance of the developing cerebral vasculature via regulation of calcium signaling, which may have therapeutic potential for DYRK1A-related vascular diseases.

Summary: The roles of DYRK1A in angiogenesis and maintenance of the developing cerebral vasculature mediated by calcium signaling were revealed using zebrafish dyrk1aa knockout mutants.

DYRK1A and cognition: A lifelong relationship

The dosage of the serine threonine kinase DYRK1A is critical in the central nervous system (CNS) during development and aging. This review analyzes the functions of this kinase by considering its interacting partners and pathways. The role of DYRK1A in controlling the differentiation of prenatal newly formed neurons is presented separately from its role at the pre- and post-synaptic levels in the adult CNS; its effects on synaptic plasticity are also discussed. Because this kinase is positioned at the crossroads of many important processes, genetic dosage errors in this protein produce devastating effects arising from DYRK1A deficiency, such as in MRD7, an autism spectrum disorder, or from DYRK1A excess, such as in Down syndrome. Effects of these errors have been shown in various animal models including Drosophila, zebrafish, and mice. Dysregulation of DYRK1A levels also occurs in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. Finally, this review describes inhibitors that have been assessed in vivo. Accurate targeting of DYRK1A levels in the brain, with either inhibitors or activators, is a future research challenge.

Targeting trisomic treatments: optimizing Dyrk1a inhibition to improve Down syndrome deficits

Overexpression of Dual-specificity tyrosine-phosphorylated regulated kinase 1A (DYRK1A), located on human chromosome 21, may alter molecular processes linked to developmental deficits in Down syndrome (DS). Trisomic DYRK1A is a rational therapeutic target, and although reductions in Dyrk1a genetic dosage have shown improvements in trisomic mouse models, attempts to reduce Dyrk1a activity by pharmacological mechanisms and correct these DS-associated phenotypes have been largely unsuccessful. Epigallocatechin-3-gallate (EGCG) inhibits DYRK1A activity in vitro and this action has been postulated to account for improvement of some DS-associated phenotypes that have been reported in preclinical studies and clinical trials. However, the beneficial effects of EGCG are inconsistent and there is no direct evidence that any observed improvement actually occurs through Dyrk1a inhibition. Inconclusive outcomes likely reflect a lack of knowledge about the tissue-specific patterns of spatial and temporal overexpression and elevated activity of Dyrk1a that may contribute to emerging DS traits during development. Emerging evidence indicates that Dyrk1a expression varies over the life span in DS mouse models, yet preclinical therapeutic treatments targeting Dyrk1a have largely not considered these developmental changes. Therapies intended to improve DS phenotypes through normalizing trisomic Dyrk1a need to optimize the timing and dose of treatment to match the spatiotemporal patterning of excessive Dyrk1a activity in relevant tissues. This will require more precise identification of developmental periods of vulnerability to enduring adverse effects of elevated Dyrk1a, representing the concurrence of increased Dyrk1a expression together with hypothesized tissue-specific-sensitive periods when Dyrk1a regulates cellular processes that shape the long-term functional properties of the tissue. Future efforts targeting inhibition of trisomic Dyrk1a should identify these putative spatiotemporally specific developmental sensitive periods and determine whether normalizing Dyrk1a activity then can lead to improved outcomes in DS phenotypes.

New Perspectives for the Rescue of Cognitive Disability in Down Syndrome

Down syndrome (DS) is a relatively common genetic condition caused by the triplication of human chromosome 21. No therapies currently exist for the rescue of neurocognitive impairment in DS. This review presents exciting findings showing that it is possible to restore brain development and cognitive performance in mouse models of DS with therapies that can also apply to humans. This knowledge provides a potential breakthrough for the prevention of intellectual disability in DS.

Preclinical toxicity of AZD7969: Effects of GSK3β inhibition in adult stem cells

AZD7969 is a potent inhibitor of glycogen synthase kinase 3 (GSK3β), which is a multifunctional serine/threonine kinase that negatively regulates the Wnt/β-catenin signaling pathway. Treatment of rats and dogs with AZD7969 for periods of up to 4 weeks resulted in a number of changes, the most significant of which was a dose-dependent, and treatment-related, increase in proliferation in a number of tissues that was thought to arise from derepression of Wnt/β-catenin signaling in the stem cell compartment. Phenotypically, this resulted in hyperplasia that either maintained normal tissue architecture in the gastrointestinal tract, liver, kidney, and adrenals or effaced normal tissue architecture within the bones, incisor teeth, and femorotibial joint. In addition to these changes, we noted a treatment-related increase in iron loading in the liver and proximal small intestines. This off-target effect was robust, potent, and occurred in both dogs and rats suggesting that AZD7969 might be a useful tool compound to study iron storage disorders in the laboratory.

NGF upregulates the plasminogen activation inhibitor-1 in neurons via the calcineurin/NFAT pathway and the Down syndrome-related proteins DYRK1A and RCAN1 attenuate this effect

BACKGROUND

Plasminogen activator inhibitor 1 (PAI-1) is a key regulator of the plasminogen activation system. Although several lines of evidence support a significant role of PAI-1 in the brain, the regulation of its expression in neurons is poorly understood. In the present study we tested the hypothesis that NGF induces the upregulation of PAI-1 via the calcineurin/nuclear factor of activated T cells (NFAT) pathway and analysed whether the overexpression of the Down syndrome-related proteins DYRK1A and RCAN1 modulated the effect of NGF on PAI-1 expression.

RESULTS

NGF upregulated PAI-1 mRNA levels in primary mouse hippocampal neurons cultured for 3 days in vitro and in the rat pheochromocytoma cell line PC12. Reporter gene assays revealed that NGF activated the calcineurin/NFAT pathway in PC12 cells. Induction of PAI-1 by NGF was sensitive to the calcineurin inhibitor FK506 and the specific inhibition of NFAT activation by the cell permeable VIVIT peptide. Activation of calcineurin/NFAT signalling through other stimuli resulted in a much weaker induction of PAI-1 expression, suggesting that other NGF-induced pathways are involved in PAI-1 upregulation. Overexpression of either DYRK1A or RCAN1 negatively regulated NFAT-dependent transcriptional activity and reduced the upregulation of PAI-1 levels by NGF.

CONCLUSION

The present results show that the calcineurin/NFAT pathway mediates the upregulation of PAI-1 by NGF. The negative effect of DYRK1A and RCAN1 overexpression on NGF signal transduction in neural cells may contribute to the altered neurodevelopment and brain function in Down syndrome.

Dyrk1A influences neuronal morphogenesis through regulation of cytoskeletal dynamics in mammalian cortical neurons

Down syndrome (DS) is the most frequent genetic cause of mental retardation. Cognitive dysfunction in these patients is correlated with reduced dendritic branching and complexity, along with fewer spines of abnormal shape that characterize the cortical neuronal profile of DS. DS phenotypes are caused by the disruptive effect of specific trisomic genes. Here, we report that overexpression of dual-specificity tyrosine phosphorylation-regulated kinase 1A, DYRK1A, is sufficient to produce the dendritic alterations observed in DS patients. Engineered changes in Dyrk1A gene dosage in vivo strongly alter the postnatal dendritic arborization processes with a similar progression than in humans. In cultured mammalian cortical neurons, we determined a reduction of neurite outgrowth and synaptogenesis. The mechanism underlying neurite dysgenesia involves changes in the dynamic reorganization of the cytoskeleton.

A patient with Down syndrome with a de novo derivative chromosome 21

Pure partial trisomy of chromosome 21 is a rare event. The patients with this aberration are very important for setting up precise karyotype-phenotype correlations particularly in Down syndrome phenotype. We present here a patient with Down syndrome with a de novo derivative chromosome 21. Karyotype of the patient was designated as 46,XY,der(21)(p13)dup(21)(q11.2q21.3)dup(21)(q22.2q22.3) with regard to cytogenetic, FISH and array-CGH analyses. Non-continuous monosomic, disomic and trisomic chromosomal segments through the derivative chromosome 21 were detected by array-CGH analysis. STR analyses revealed maternal origin of the de novo derivative chromosome 21. The dual-specificity tyrosine (Y)-phosphorylation regulated kinase 1A (DYRK1A) and Down Syndrome Critical Region 1 (DSCR1) genes that are located in Down syndrome critical region, are supposed to be responsible for most of the clinical findings of Down syndrome. However, our patient is the first patient with Down syndrome whose clinical findings were provided in detail, with a de novo derivative chromosome 21 resulting from multiple chromosome breaks excluding DYRK1A and DSCR1 gene regions.

Dyrk1A overexpression inhibits proliferation and induces premature neuronal differentiation of neural progenitor cells

Dyrk1A is a member of the mammalian Dyrk [dual-specificity tyrosine-(Y)-phosphorylation regulated kinase] family of protein kinases that is expressed at high levels in the brain, but its role in the development and function of this organ is not well understood. The human DYRK1A gene is located on trisomic chromosome 21 in Down syndrome (DS) patients, leading to its overexpression. Dyrk1A is also overexpressed in animal models of DS and in gene-specific transgenic mice that consistently exhibit cognitive impairment. To elucidate the cellular and molecular mechanisms that are affected by increased levels of Dyrk1A in the developing brain, we overexpressed this kinase in the embryonic mouse neocortex using the in utero electroporation technique. We found that Dyrk1A overexpression inhibits neural cell proliferation and promotes premature neuronal differentiation in the developing cerebral cortex without affecting cell fate and layer positioning. These effects are dependent on the Dyrk1A kinase activity and are mediated by the nuclear export and degradation of cyclin D1. This study identifies specific Dyrk1A-induced mechanisms that disrupt the normal process of corticogenesis and possibly contribute to cognitive impairment observed in DS patients and animal models.

Age-associated motor and visuo-spatial learning phenotype in Dyrk1A heterozygous mutant mice

Dual-specificity tyrosine-regulated kinase 1A (DYRK1A) is a candidate gene for the Down syndrome neurological defects and may be involved in the progression of Alzheimer's disease. Heterozygous mice for Dyrk1A (Dyrk1A+/-) exhibit decreased brain size, motor abnormalities and cognitive deficits in the adult. However, there is no information about the mutant phenotype in old ages. Here we analyze the impact of Dyrk1A dosage reduction on motor performance and hippocampal-dependent learning and memory in aged Dyrk1A+/- mice. Whereas motor tests showed marked alterations in traction ability, prehensile reflex and balance, heterozygous mice only present a slight impairment of visuo-spatial memory even though they show a robust decrease of CA1-CA3 and dentate gyrus cells.

A quantitative assessment of gene expression (QAGE) reveals differential overexpression of DOPEY2, a candidate gene for mental retardation, in Down syndrome brain regions

The brain alterations and mental retardation in Down syndrome are associated with overdosage of chromosome 21 genes. To shed light on the understanding of the molecular effect of this genetic overdosage, gene expression studies have crucial importance to quantify expression variations in Down syndrome tissues compared to normal ones. Herein, an in situ Quantitative Assessment of Gene Expression (QAGE) was used to quantify and statistically analyze, for the first time, DOPEY2 expression variations in different regions of the Down syndrome human fetal brains and to compare them to corresponding normal brains. DOPEY2, which is localized in the Down Syndrome Critical Region (DSCR) and is a candidate gene for neurological alterations in Down syndrome, showed a delimited regional and cellular expression pattern in the cortex, hippocampus and cerebellum, characterized by different transcriptional intensities in both normal and trisomic brains. DOPEY2 is overexpressed more than 50% (1.79-, 1.97- and 2.12-folds in the cortex, cerebellum and hippocampus, respectively), and showed statistically significant differences in the overexpression ratios in the three brain regions expressing DOPEY2. The demonstration of differential DOPEY2 expression and overexpression in human fetal brains suggests that this gene is submitted to a complex transcriptional control and could depend from other human chromosome 21 genes. Moreover, DOPEY2 overexpression in the brain regions, that are altered in Down syndrome patients and involved in learning and memory processes, is in agreement to the hypothesis that this gene plays a potential role in functional brain alterations and in the pathogenesis of mental retardation in Down syndrome. This new in situ QAGE approach allowed quantitative measurements of transcriptional changes and statistical evaluations of the expression and overexpression patterns of DOPEY2 at specific regions of the brain, which is a complementary approach to qRT-PCR and microarray for transcriptome study. Moreover, this approach could be a powerful tool to study the candidate chromosome 21 genes for Down syndrome and other pathologies caused by regionalized quantitative transcriptional alterations, for greater interpretation of functional processes driving gene expression.

Impaired spatial learning strategies and novel object recognition in mice haploinsufficient for the dual specificity tyrosine-regulated kinase-1A (Dyrk1A)

BACKGROUND

Pathogenic aneuploidies involve the concept of dosage-sensitive genes leading to over- and underexpression phenotypes. Monosomy 21 in human leads to mental retardation and skeletal, immune and respiratory function disturbances. Most of the human condition corresponds to partial monosomies suggesting that critical haploinsufficient genes may be responsible for the phenotypes. The DYRK1A gene is localized on the human chromosome 21q22.2 region, and has been proposed to participate in monosomy 21 phenotypes. It encodes a dual-specificity kinase involved in neuronal development and in adult brain physiology, but its possible role as critical haploinsufficient gene in cognitive function has not been explored.

METHODOLOGY/PRINCIPAL FINDINGS

We used mice heterozygous for a Dyrk1A targeted mutation (Dyrk1A+/-) to investigate the implication of this gene in the cognitive phenotypes of monosomy 21. Performance of Dyrk1A+/- mice was assayed 1/ in a navigational task using the standard hippocampally related version of the Morris water maze, 2/ in a swimming test designed to reveal potential kinesthetic and stress-related behavioral differences between control and heterozygous mice under two levels of aversiveness (25 degrees C and 17 degrees C) and 3/ in a long-term novel object recognition task, sensitive to hippocampal damage. Dyrk1A+/- mice showed impairment in the development of spatial learning strategies in a hippocampally-dependent memory task, they were impaired in their novel object recognition ability and were more sensitive to aversive conditions in the swimming test than euploid control animals.

CONCLUSIONS/SIGNIFICANCE

The present results are clear examples where removal of a single gene has a profound effect on phenotype and indicate that haploinsufficiency of DYRK1A might contribute to an impairment of cognitive functions and stress coping behavior in human monosomy 21.

Mental retardation and associated neurological dysfunctions in Down syndrome: a consequence of dysregulation in critical chromosome 21 genes and associated molecular pathways

Down syndrome (DS), affecting 1/700 live births, is the major genetic cause of mental retardation (MR), a cognitive disorder with hard impact on public health. DS brain is characterized by a reduced cerebellar volume and number of granular cells, defective cortical lamination and reduced cortical neurons, malformed dendritic trees and spines, and abnormal synapses. These neurological alterations, also found in trisomic mouse models, result from gene-dosage effects of Human Chromosome 21 (HC21) on the expression of critical developmental genes. HC21 sequencing, mouse ortholog gene identification and DS mouse model generation lead to determine HC21 gene functions and the effects of protein-dosage alterations in neurodevelopmental and metabolic pathways in DS individuals. Trisomic brain transcriptome of DS patients and trisomic mouse models identified some molecular changes determined by gene-overdosage and associated dysregulation of some disomic gene expression in DS brains. These transcriptional variations cause developmental alterations in neural patterning and signal transduction pathways that may lead to defective neuronal circuits responsible for the pathogenesis of MR in DS. Recently, the first altered molecular pathway responsible of some DS phenotypes, including neurological and cognitive disorders has been identified. In this pathway, two critical HC21 genes (DYRK1A and DSCR1) act synergistically to control the phosphorylation levels of NFATc and NFATc-regulated gene expression. Interestingly, the NFATc mice show neurological dysfunctions similar to those seen in DS patients and trisomic mouse models. Treatment of DS mouse model Ts65Dn with GABA(A) antagonists allowed post-drug rescue of cognitive defects, indicating a hopeful direction in clinical therapies for MR in children with DS.

The spatio-temporal and subcellular expression of the candidate Down syndrome gene Mnb/Dyrk1A in the developing mouse brain suggests distinct sequential roles in neuronal development

It is widely accepted that the neurological alterations in Down syndrome (DS) are principally due to modifications in developmental processes. Accordingly, a large part of the research on DS in recent years has focused on chromosome 21 genes that influence brain development. MNB/DYRK1A is one of the genes on human chromosome 21 that has raised most interest, due to its relationship with the brain functions that are altered in DS. Although a number of interesting experimental mouse models for DS are being developed, we still know little about the expression of Mnb/Dyrk1A during mouse brain development. Here, we report that Mnb/Dyrk1A displays a rather dynamic spatio-temporal expression pattern during mouse central nervous system development. Our data indicate that Mnb/Dyrk1A is specifically expressed in four sequential developmental phases: transient expression in preneurogenic progenitors, cell cycle-regulated expression in neurogenic progenitors, transient expression in recently born neurones, and persistent expression in late differentiating neurones. Our results also suggest that the subcellular localization of MNB/DYRK1A, including its translocation to the nucleus, is finely regulated. Thus, the MNB/DYRK1A protein kinase could be a key element in the molecular machinery that couples sequential events in neuronal development. This rich repertoire of potential functions in the developing central nervous system is suitable to be linked to the neurological alterations in DS through the use of mouse experimental models.

Mental retardation in Down syndrome: From gene dosage imbalance to molecular and cellular mechanisms

Down syndrome (DS), the most frequent genetic disorder leading to mental retardation (MR), is caused by three copies of human chromosome 21 (HC21). Trisomic and transgenic mouse models for DS allow genetic dissection of DS neurological and cognitive disorders in view to identify genes responsible for these phenotypes. The effects of the gene dosage imbalance on DS phenotypes are explained by two hypotheses: the "gene dosage effect" hypothesis claims that a DS critical region, containing a subset of dosage-sensitive genes, determines DS phenotypes, and the "amplified developmental instability" hypothesis holds that HC21 trisomy determines general alteration in developmental homeostasis. Transcriptome and expression studies showed different up- or down-expression levels of genes located on HC21 and the other disomic chromosomes. HC21 genes, characterized by their overexpression in brain regions affected in DS patients and by their contribution to neurological and cognitive defects when overexpressed in mouse models, are proposed herein as good candidates for MR. In this article, we propose a new molecular and cellular mechanism explaining MR pathogenesis in DS. In this model, gene dosage imbalance effects on transcriptional variations are described considering the nature of gene products and their functional relationships. These transcriptional variations may affect different aspects of neuronal differentiation and metabolism and finally, determine the brain neuropathologies and mental retardation in DS.

DYRK1A-mediated hyperphosphorylation of Tau. A functional link between Down syndrome and Alzheimer disease

Most individuals with Down syndrome show early onset of Alzheimer disease (AD), resulting from the extra copy of chromosome 21. Located on this chromosome is a gene that encodes the dual specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A). One of the pathological hallmarks in AD is the presence of neurofibrillary tangles (NFTs), which are insoluble deposits that consist of abnormally hyperphosphorylated Tau. Previously it was reported that Tau at the Thr-212 residue was phosphorylated by Dyrk1A in vitro. To determine the physiological significance of this phosphorylation, an analysis was made of the amount of phospho-Thr-212-Tau (pT212) in the brains of transgenic mice that overexpress the human DYRK1A protein (DYRK1A TG mice) that we recently generated. A significant increase in the amount of pT212 was found in the brains of DYRK1A transgenic mice when compared with age-matched littermate controls. We further examined whether Dyrk1A phosphorylates other Tau residues that are implicated in NFTs. We found that Dyrk1A also phosphorylates Tau at Ser-202 and Ser-404 in vitro. Phosphorylation by Dyrk1A strongly inhibited the ability of Tau to promote microtubule assembly. Following this, using mammalian cells and DYRK1A TG mouse brains, it was demonstrated that the amounts of phospho-Ser-202-Tau and phospho-Ser-404-Tau are enhanced when DYRK1A amounts are high. These results provide the first in vivo evidence for a physiological role of DYRK1A in the hyperphosphorylation of Tau and suggest that the extra copy of the DYRK1A gene contributes to the early onset of AD.

DYRK1A BAC transgenic mice show altered synaptic plasticity with learning and memory defects

Among the various phenotypes seen in Down syndrome (DS), mental retardation is the most common and most debilitating condition suffered by individuals with DS. The DYRK1A gene on human chromosome 21q22.2 encodes a subfamily of protein kinases that displays dual substrate specificities and is known to play a critical role in neurodevelopment. To study DS mental retardation, we have generated transgenic mice that contain only one copy of the complete human DYRK1A gene in a bacterial artificial chromosome. The transgenic mice showed significant impairment in hippocampal-dependent memory tasks in a Morris water maze. Interestingly, we observed shifts in both long-term potentiation and long-term depression, which suggests a role for DYRK1A in bidirectional synaptic plasticity. These mice represent the most clinically relevant DYRK1A mouse model to date and provide us a valuable tool for the in vivo study of mechanisms that underlie the learning and memory deficit in DS.

C21orf5, a human candidate gene for brain abnormalities and mental retardation in Down syndrome

Mental retardation represents the more invalidating pathological aspect of trisomy 21 and has a hard impact on public health. The dosage imbalance of chromosome 21 genes could be the cause of neurological alterations and mental retardation seen in Down syndrome. We studied C21orf5 that we have demonstrated to be overexpressed in Down syndrome tissues, as a candidate gene for trisomy 21. A new optical technology (Rachidi et al., 2000) was used to compare signal intensity and cell density in presumptive embryonic brain compartments, at their boundaries and in higher specialized brain centres during fetal lifespan. We showed a developmentally regulated transcriptional activity of C21orf5 and a regional and cellular specific distribution of gene transcripts during human embryonic and fetal development. A wide but differential expression was detected in the nervous system during embryogenesis with a relatively lower level in the forebrain than in the midbrain and hindbrain and the highest transcription intensity in the future cerebellum. This developmentally regulated expression is maintained during post-embryogenesis and evolves selectively in fetal cerebral, hippocampal and cerebellar areas. Differential and cellular specificity were detected in hippocampus with higher C21orf5 mRNA level in the pyramidal cells compared to granular cells of the dentate gyrus. The expression pattern detected in cortical and cerebellar structures correlates well to the altered cortical lamination and to the lower size of the cerebellum observed in Down syndrome patients. In addition, the patterned differential expression detected in the medial temporal-lobe system, including hippocampal formation and perirhinal cortex, working as control centres of the memory circuits and involved in cognitive processes and memory storage, also corresponds to abnormal brain regions seen in Down syndrome patients. The C21orf5 selective expression in the key brain structures for learning and memory suggests that C21orf5 overexpression could participate in mental retardation pathogenesis in Down syndrome patients.

DYRK1A enhances the mitogen-activated protein kinase cascade in PC12 cells by forming a complex with Ras, B-Raf, and MEK1

Dual-specificity tyrosine-phosphorylated and regulated kinase 1A (Dyrk1A) is the human homologue of the Drosophila mnb (minibrain) gene. In Drosophila, mnb is involved in postembryonic neurogenesis. In human, DYRK1A maps within the Down syndrome critical region of chromosome 21 and is overexpressed in Down syndrome embryonic brain. Despite its potential involvement in the neurobiological alterations observed in Down syndrome patients, the biological functions of the serine/threonine kinase DYRK1A have not been identified yet. Here, we report that DYRK1A overexpression potentiates nerve growth factor (NGF)-mediated PC12 neuronal differentiation by up-regulating the Ras/MAP kinase signaling pathway independently of its kinase activity. Furthermore, we show that DYRK1A prolongs the kinetics of ERK activation by interacting with Ras, B-Raf, and MEK1 to facilitate the formation of a Ras/B-Raf/MEK1 multiprotein complex. These data indicate that DYRK1A may play a critical role in Ras-dependent transducing signals that are required for promoting or maintaining neuronal differentiation and suggest that overexpression of DYRK1A may contribute to the neurological abnormalities observed in Down syndrome patients.

Regulation of the proapoptotic activity of huntingtin interacting protein 1 by Dyrk1 and caspase-3 in hippocampal neuroprogenitor cells

Dual specific protein kinase Dyrks are thought to play a key role in the regulation of cell growth in a variety of cellular systems. Interestingly, human Dyrk1 is mapped to the Down's syndrome (DS) critical region on chromosome 21, and thought to be a candidate gene responsible for the mental retardation of DS patients. Huntingtin-interacting protein 1 (Hip-1), a proapoptotic mediator, is implicated as a molecular accomplice in the pathogenesis of Huntington's disease. In the present study we found that Dyrk1 selectively binds to and phosphorylates Hip-1 during the neuronal differentiation of embryonic hippocampal neuroprogenitor (H19-7) cells. The Dyrk1-mediated phosphorylation of Hip-1, in response to bFGF, resulted in the blockade of Hip-1-mediated neuronal cell death as well as the enhancement of neurite outgrowth. Furthermore, the addition of etoposide to proliferating H19-7 cells caused the diminished binding of Hip-1 to Dyrk1 and the levels of phosphorylated Hip-1 remarkably decreased. Simultaneously, the dissociated Hip-1 from Dyrk1 bound to caspase-3 in response to etoposide, which led to its activation and consequently cell death in H19-7 cells. These data suggest that the phosphorylation of Hip-1 by Dyrk1 has a dual role in regulating neuronal differentiation and cell death. The interaction between Dyrk1 and Hip-1 appeared to be differentially modulated by different kinds of stimuli, such as bFGF and etoposide in H19-7 cells.

Dual-specificity tyrosine-phosphorylated and regulated kinase 1A (DYRK1A) interacts with the phytanoyl-CoA alpha-hydroxylase associated protein 1 (PAHX-AP1), a brain specific protein

Down syndrome (DS) is the most common genetic defect correlated with mental retardation and delayed development. The specific genes responsible for these phenotypic alterations have not yet been defined. Dyrk1A (dual-specificity tyrosine-phosphorylated and regulated kinase 1A), the human ortholog of the Drosophila minibrain gene (mnb), maps to the Down syndrome critical region of human chromosome 21 and is overexpressed in Down syndrome fetal brain. In Drosophila, minibrain is involved in postembryonic neurogenesis. In human, DYRK1A encodes a serine-threonine kinase but despite its potential involvement in the neurobiological alterations associated with Down syndrome, its physiological function has not yet been defined. To gain some insight into its biological function, we used the yeast two-hybrid approach to identify binding partners of DYRK1A. We found that the C-terminal region of DYRK1A interacts with a brain specific protein, phytanoyl-CoA alpha-hydroxylase-associated protein 1 (PAHX-AP1, also named PHYHIP) which was previously shown to interact with phytanoyl-CoA alpha-hydroxylase (PAHX, also named PHYH), a Refsum disease gene product. This interaction was confirmed by co-immunoprecipitation of PC12 cells co-transfected with DYRK1A and PAHX-AP1. Furthermore, immunofluorescence analysis of PC12 cells co-transfected with both plasmids showed a re-distribution of DYRK1A from the nucleus to the cytoplasm where it co-localized with PAHX-AP1. Finally, in PC12 cells co-transfected with both plasmids, DYRK1A was no longer able to interact with the nuclear transcription factor CREB, thereby confirming that the intracellular localization of DYRK1A was changed from the nucleus to the cytoplasm in the presence of PAHX-AP1. Therefore, these data indicate that by inducing a re-localization of DYRK1A into the cytoplasm, PAHX-AP1 may contribute to new cellular functions of DYRK1A and suggest that PAHX-AP1 may be involved in the development of neurological abnormalities observed in Down syndrome patients.

Dyrk1A potentiates steroid hormone-induced transcription via the chromatin remodeling factor Arip4

Dyrk1A, a mammalian homolog of the Drosophila minibrain gene, encodes a dual-specificity kinase, involved in neuronal development and in adult brain physiology. In humans, a third copy of DYRK1A is present in Down syndrome (trisomy 21) and has been implicated in the etiology of mental retardation. To further understand this pathology, we searched for Dyrk1A-interacting proteins and identified Arip4 (androgen receptor-interacting protein 4), a SNF2-like steroid hormone receptor cofactor. Mouse hippocampal and cerebellar neurons coexpress Dyrk1A and Arip4. In HEK293 cells and hippocampal neurons, both proteins are colocalized in a speckle-like nuclear subcompartment. The functional interaction of Dyrk1A with Arip4 was analyzed in a series of transactivation assays. Either Dyrk1A or Arip4 alone displays an activating effect on androgen receptor- and glucocorticoid receptor-mediated transactivation, and Dyrk1A and Arip4 together act synergistically. These effects are independent of the kinase activity of Dyrk1A. Inhibition of endogenous Dyrk1A and Arip4 expression by RNA interference showed that both proteins are necessary for the efficient activation of androgen receptor- and glucocorticoid receptor-dependent transcription. As Dyrk1A is an activator of steroid hormone-regulated transcription, the overexpression of DYRK1A in persons with Down syndrome may cause rather broad changes in the homeostasis of steroid hormone-controlled cellular events.

Transgenic mouse in vivo library of human Down syndrome critical region 1: association between DYRK1A overexpression, brain development abnormalities, and cell cycle protein alteration

Down syndrome is the most frequent genetic cause of mental retardation, having an incidence of 1 in 700 live births. In the present study we used a transgenic mouse in vivo library consisting of 4 yeast artificial chromosome (YAC) transgenic mouse lines, each bearing a different fragment of the Down syndrome critical region 1 (DCR-1), implicated in brain abnormalities characterizing this pathology. The 152F7 fragment, in addition to genes also located on the other DCR-1 fragments, bears the DYRK1A gene, encoding for a serine-threonine kinase. The neurobehavioral analysis of these mouse lines showed that DYRK1A overexpressing 152F7 mice but not the other lines display learning impairment and hyperactivity during development. Additionally, 152F7 mice display increased brain weight and neuronal size. At a biochemical level we found DYRK1A overexpression associated with a development-dependent increase in phosphorylation of the transcription factor FKHR and with high levels of cyclin B1, suggesting for the first time in vivo a correlation between DYRK1A overexpression and cell cycle protein alteration. In addition, we found an altered phosphorylation of transcription factors of CREB family. Our findings support a role of DYRK1A overexpression in the neuronal abnormalities seen in Down syndrome and suggest that this pathology is linked to altered levels of proteins involved in the regulation of cell cycle.

The differentially expressed C21orf5 gene in the medial temporal-lobe system could play a role in mental retardation in Down syndrome and transgenic mice

Mental retardation represents the more invalidating pathological aspect of Down syndrome, DS, and has a hard impact in public health. Modifications in DS brain, concerning abnormal size, neuronal differentiation, and cell density, cause changes in the neurophysiology and behavior of DS patients, and could be determined by dosage imbalance of genes localized in the DS critical region, DCR. Among these genes, C21orf5 showed high homology with Caenorhabditis elegans Pad1 involved in cellular differentiation and patterning. To shed light on C21orf5 role in DS, we performed molecular characterization of human and mouse orthologs, their spatio-temporal expression during development and in adult, and overexpression in DS and transgenic mice. C21orf5 was widely expressed early in embryogenesis in the nervous system. Later, its expression became differential and increased in mesencephalon and rhomboencephalon. This developmental expression profile evolves selectively in adult brain with higher signals in hippocampus, cerebellum, perirhinal, and entorhinal cortex, compared to the other cortical regions. Cellular specificity was detected in hippocampus with higher C21orf5 mRNA level in CA3 cells. Our findings appoint C21orf5 as candidate gene for mental retardation: Its overexpression in DS cells may contribute to gene imbalance in DS.Its specific expression in normal and its mirroring pattern in transgenic mice correspond to abnormal regions in DS patients and to neurological phenotype of transgenic mice. Altered cortical lamination in transgenic mice and the Pad1 ortholog function suggest a potential role of C21orf5 in cell differentiation. Its patterned differential expression in the medial temporal-lobe system, including hippocampal formation and perirhinal cortex involved in memory storage, and learning and memory defects in the transgenic mice suggest a specialized role for C21orf5 in cognitive processes. These evidences suggest that C21orf5 is an attractive candidate gene contributing to neurological alterations responsible for mental retardation in DS patients.

Expression patterns and subcellular localization of the Down syndrome candidate protein MNB/DYRK1A suggest a role in late neuronal differentiation

The Minibrain (Mnb) gene belongs to a new protein kinase family, which is evolutionarily conserved, and probably plays several roles during brain development and in adulthood. In Drosophila, mnb is involved in postembryonic neurogenesis and in learning/memory. In humans, MNB has been mapped within the Down syndrome critical region of chromosome 21 and is overexpressed in the Down syndrome embryonic brain. It has been widely proposed that MNB is involved in the neurobiological alterations associated with Down syndrome. Nevertheless, little is known about the functional role that MNB plays in vertebrate brain development. We have recently shown [Hämmerle et al. (2002) Dev. Biol., 246, 259-273] that in early vertebrate embryos, Mnb is transiently expressed in neural progenitor cells during the transition from proliferating to neurogenic divisions. Here we have studied in detail a second wave of Mnb expression, which takes place in the brain of intermediate and late vertebrate embryos. In these stages, MNB seems to be restricted to certain populations of neurons, as no consistent expression was detected in astroglial or oligodendroglial cells. Interestingly, MNB expression takes place at the time of dendritic tree differentiation and is initiated by a transient translocation from the cytoplasm to the nucleus. Afterwards, MNB protein is transported to the growing dendritic tree, where it colocalizes with Dynamin 1, a putative substrate of MNB kinases. We propose that MNB kinase is involved in the signalling mechanisms that regulate dendrite differentiation. This functional role helps to build a new hypothesis for the implication of MNB/DYRK1A in the developmental aetiology of Down syndrome neuropathologies.

Dyrk1A expression pattern supports specific roles of this kinase in the adult central nervous system

Dyrk1A and its Drosophila orthologue, the protein minibrain (mnb), belong to a family of serine/threonine kinases involved in the development of the central nervous system (CNS). However, additional roles for Dyrk1A have to be proposed, as its expression is still prominent in the adult brain. To gain insight into Dyrk1A physiological roles we have studied the distribution of this kinase in the CNS of mice in adulthood. A homogeneous diffuse immunostaining of variable intensity was detected throughout the neuropile, with the white matter displaying faint Dyrk1A immunoreactivity. Dyrk1A immunostaining was strong in the olfactory bulb, the cerebellar cortex and functionally related structures, the spinal cord and most of the motor nuclei of the midbrain and brain stem. These data agree with a possible implication of this kinase in the physiology of olfaction and motor functions. Cellular and subcellular localisation of Dyrk1A was also studied in primary cell culture of cerebellum, one of the structures showing significant Dyrk1A immunostaining in the adult. The distribution of Dyrk1A in primary cell culture showed the presence of this protein in the nucleus and the cytoplasm of both neurons and astrocytes. Moreover, studies on the subcellular distribution of Dyrk1A in whole brain homogenates of adult mice showed the presence of this protein both in nuclear and cytoplasm-enriched fractions, thus supporting selective functions of this kinase in these two subcellular compartments. The present results showing the distribution of Dyrk1A in widespread areas of the adult CNS and in different subcellular compartments, together with previous reports demonstrating its implication in developmental events concur with the idea of several spatio-temporal functional profiles.

Role for YakA, cAMP, and protein kinase A in regulation of stress responses of Dictyostelium discoideum cells

The Dictyostelium protein kinase YakA is required for the growth-to-development transition. During growth YakA controls the cell cycle, regulating the intervals between cell divisions. When starved for nutrients Dictyostelium cells arrest growth and undergo changes in gene expression, decreasing vegetative mRNAs and inducing the expression of pkaC. YakA is an effector of these changes, being necessary for the decrease of vegetative mRNA expression and the increase of protein kinase A (PKA) activity that will ultimately regulate expression of adenylyl cyclase, cAMP synthesis, and the induction of development. We report a role for this kinase in the response to nitrosoative or oxidative stress of Dictyostelium cells. Hydrogen peroxide and sodium nitroprusside arrest the growth of cells and trigger cAMP synthesis and activation of PKA in a manner similar to the well-established response to nutrient starvation. We have found that yakA null cells are hypersensitive to nitrosoative/oxidative stress and that a second-site mutation in pkaC suppresses this sensitivity. The response to different stresses has been investigated and YakA, cAMP, and PKA have been identified as components of the pathway that regulate the growth arrest that follows treatment with compounds that generate reactive oxygen species. The effect of different types of stress was evaluated in Dictyostelium and the YakA/PKA pathway was also implicated in the response to heat stress.

Mnb/Dyrk1A is transiently expressed and asymmetrically segregated in neural progenitor cells at the transition to neurogenic divisions

The Minibrain (Mnb) gene encodes a new family of protein kinases that is evolutionarily conserved from insects to humans. In Drosophila, Mnb is involved in postembryonic neurogenesis. In humans, MNB has been mapped within the Down's Syndrome (DS) critical region of chromosome 21 and is overexpressed in DS embryonic brain. In order to study a possible role of Mnb on the neurogenesis of vertebrate brain, we have cloned the chick Mnb orthologue and studied the spatiotemporal expression of Mnb in proliferative regions of the nervous system. In early embryos, Mnb is expressed before the onset of neurogenesis in the three general locations where neuronal precursors are originated: neuroepithelia of the neural tube, neural crest, and cranial placodes. Mnb is transiently expressed during a single cell cycle of neuroepithelial progenitor (NEP) cells. Mnb expression precedes and widely overlaps with the expression of Tis21, an antiproliferative gene that has been reported to be expressed in the onset of neurogenic divisions of NEP cells. Mnb transcription begins in mitosis, continues during G(1), and stops before S-phase. Very interestingly, we have found that Mnb mRNA is asymmetrically localized during the mitosis of these cells and inherited by one of the sibling cells after division. We propose that Mnb defines a transition step between proliferating and neurogenic divisions of NEP cells.

Dynamin is a minibrain kinase/dual specificity Yak1-related kinase 1A substrate

The minibrain kinase (Mnbk)/dual specificity Yak 1-related kinase 1A (Dyrk1A) gene is implicated in the mental retardation associated with Down's syndrome. It encodes a proline-directed serine/threonine kinase whose function has yet to be defined. We have used a solid-phase Mnbk/Dyrk1A kinase assay to aid in the search for the cellular Mnbk/Dyrk1A substrates. The assay revealed that rat brain contains two cytosolic proteins, one with a molecular mass of 100 kDa and one with a molecular mass of 140 kDa, that were prominently phosphorylated by Mnbk/Dyrk1A. The 100-kDa protein was purified and identified as dynamin 1. The conclusion was further supported by evidence that a recombinant glutathione S-transferase fusion protein containing dynamin isoform 1aa was phosphorylated by Mnbk/Dyrk1A. In addition to isoform 1aa, Mnbk/Dyrk1A also phosphorylated isoforms 1ab and 2aa but not human MxA protein when analyzed by the solid-phase kinase assay. Upon Mnbk/Dyrk1A phosphorylation, the interaction of dynamin 1 with the Src homology 3 domain of amphiphysin 1 was reduced. However, when Mnbk/Dyrk1A phosphorylation was allowed to proceed more extensively, the phosphorylation enhanced rather than reduced the binding of dynamin 1 to amphiphysin 1. The result suggests that Mnbk/Dyrk1A can play a dual role in regulating the interaction of dynamin 1 with amphiphysin 1. Mnbk/Dyrk1A phosphorylation also reduced the interaction of dynamin with endophilin 1, whereas the same phosphorylation enhanced the binding of dynamin 1 to Grb2. Nevertheless, the dual function of Mnbk/Dyrk1A phosphorylation was not observed for the interaction of dynamin 1 with endophilin 1 or Grb2. The interactions of dynamin with amphiphysin and endophilin are essential for the formation of endocytic complexes; our results suggest that Mnbk/Dyrk1A may function as a regulator controlling the assembly of endocytic apparatus.

Protein kinase Dyrk1 activates cAMP response element-binding protein during neuronal differentiation in hippocampal progenitor cells

Dyrk is a dual specific protein kinase thought to be involved in normal embryo neurogenesis and brain development. Defects/imperfections in this kinase have been suggested to play an important role in the mental retardation of patients with Down's syndrome. The transcriptional factor cAMP response element-binding protein (CREB) has been implicated in the formation of many types of synaptic plasticity, such as learning and memory. In the present study we show that Dyrk1 activity is markedly induced during the differentiation of immortalized hippocampal progenitor (H19-7) cells. The addition of a neurogenic factor, basic fibroblast growth factor, to the H19-7 cells results in an increased specific binding of Dyrk1 to active CREB. In addition, Dyrk1 directly phosphorylates CREB, leading to the stimulation of subsequent CRE-mediated gene transcription during the neuronal differentiation in H19-7 cells. Blockade of Dyrk1 activation significantly inhibits the neurite outgrowth as well as CREB phosphorylation induced by basic fibroblast growth factor. These findings suggest that Dyrk1 activation and subsequent CREB phosphorylation is important in the neuronal differentiation of central nervous system hippocampal cells.

mDYRK3 kinase is expressed selectively in late erythroid progenitor cells and attenuates colony-forming unit-erythroid development

DYRKs are a new subfamily of dual-specificity kinases that was originally discovered on the basis of homology to Yak1, an inhibitor of cell cycle progression in yeast. At present, mDYRK-3 and mDYRK-2 have been cloned, and mDYRK-3 has been characterized with respect to kinase activity, expression among tissues and hematopoietic cells, and possible function during erythropoiesis. In sequence, mDYRK-3 diverges markedly in noncatalytic domains from mDYRK-2 and mDYRK-1a, but is 91.3% identical overall to hDYRK-3. Catalytically, mDYRK-3 readily phosphorylated myelin basic protein (but not histone 2B) and also appeared to autophosphorylate in vitro. Expression of mDYRK-1a, mDYRK-2, and mDYRK-3 was high in testes, but unlike mDYRK1a and mDYRK 2, mDYRK-3 was not expressed at appreciable levels in other tissues examined. Among hematopoietic cells, however, mDYRK-3 expression was selectively elevated in erythroid cell lines and primary pro-erythroid cells. In developmentally synchronized erythroid progenitor cells, expression peaked sharply following exposure to erythropoietin plus stem cell factor (SCF) (but not SCF alone), and in situ hybridizations of sectioned embryos revealed selective expression of mDYRK-3 in fetal liver. Interestingly, antisense oligonucleotides to mDYRK-3 were shown to significantly and specifically enhance colony-forming unit-erythroid colony formation. Thus, it is proposed that mDYRK-3 kinase functions as a lineage-restricted, stage-specific suppressor of red cell development. (Blood. 2001;97:901-910)

High-level expression of the Mnb/Dyrk1A gene in brain and heart during rat early development

We previously isolated human MNB/DYRK1A cDNA from "the Down syndrome critical region" of human chromosome 21 (Shindoh et al., 1996, Biochem. Biophys. Res. Commun. 225: 92-99). As described herein, we prepared a polyclonal anti-MNB/DYRK1A antibody and used it in a Western blot assay to assess the expression of the MNB/DYRK1A protein during rat development. The MNB/DYRK1A protein was expressed strongly not only in the brain but also in other tissues from embryonic rats. At the early postnatal stage, expression of the protein was high in the central nervous system and heart, but low in liver, lung, spleen, and kidney. The level of MNB/DYRK1A protein in all tissues studied gradually decreased with postnatal growth. Similarly, Northern blot analysis revealed that a major 6.0-kb transcript of the Mnb/Dyrk1A gene was expressed at a high level in the brain during the early postnatal period but that its level was low in the adult. The finding that the MNB/DYRK1A protein is expressed strongly in the central nervous system and heart may indicate a significant role for this protein in the development of these organs.