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

Motor phenotypic alterations in TgDyrk1a transgenic mice implicate DYRK1A in Down syndrome motor dysfunction

Motor deficits are among the most frequent impairments in Down syndrome (DS), but their neuropathological and molecular bases remain elusive. Here we investigate the motor profile of transgenic mice overexpressing Dyrk1a, Tg(Dyrk1a)1Cff (hereafter TgDyrk1a), a candidate gene hypothesized to cause some of the neurological defects associated with DS. We have previously shown DYRK1A expression in the cerebellum and functionally related structures, most brainstem motor nuclei and spinal cord, supporting a role for Dyrk1a in controlling motor function. Here we demonstrate that TgDyrk1a mice present DYRK1A overexpression in these areas along with specific motor dysfunction. The main finding that emerged was impairment of motor learning and alteration of the organization of locomotor behavior, which agrees with reported clinical observations in subjects with DS. These results confirm and extend previous data and provide further insight to the functional domains that might be altered in TgDyrk1a mice and underlying molecular mechanisms of DS motor dysfunction.

Phosphorylation of Ser640 in muscle glycogen synthase by DYRK family protein kinases

Glycogen synthase, a key enzyme in the regulation of glycogen synthesis by insulin, is controlled by multisite phosphorylation. Glycogen synthase kinase-3 (GSK-3) phosphorylates four serine residues in the COOH terminus of glycogen synthase. Phosphorylation of one of these residues, Ser(640) (site 3a), causes strong inactivation of glycogen synthase. In previous work, we demonstrated in cell models that site 3a can be phosphorylated by an as yet unidentified protein kinase (3a-kinase) distinct from GSK-3. In the present study, we purified the 3a-kinase from rabbit skeletal muscle and identified one constituent polypeptide as HAN11, a WD40 domain protein with unknown function. Another polypeptide was identified as DYRK1A, a member of the dual-specificity tyrosine phosphorylated and regulated protein kinase (DYRK) family. Two isoforms of DYRK, DYRK1A and DYRK1B, co-immunoprecipitate with HAN11 when coexpressed in COS cells indicating that the proteins interact in mammalian cells. Co-expression of DYRK1A, DYRK1B, or DYRK2 with a series of glycogen synthase mutants with Ser/Ala substitutions at the phosphorylation sites in COS cells revealed that protein kinases cause phosphorylation of site 3a in glycogen synthase. To confirm that DYRKs directly phosphorylate glycogen synthase, recombinant DYRK1A, DYRK2, and glycogen synthase were produced in bacterial cells. In the presence of Mg-ATP, both DYRKs inactivated glycogen synthase by more than 10-fold. The inactivation correlated with phosphorylation of site 3a in glycogen synthase. These results indicate that protein kinase(s) from the DYRK family may be involved in a new mechanism for the regulation of glycogen synthesis.

Overexpression of the human MNB/DYRK1A gene induces formation of multinucleate cells through overduplication of the centrosome

BACKGROUND

Previously we cloned the human MNB/DYRK1A gene from the "Down syndrome critical region" on chromosome 21. This gene encodes a dual specificity protein kinase that catalyzes its autophosphorylation on serine/threonine and tyrosine residues. But, the functions of the MNB/DYRK1A gene in cellular processes are unknown.

RESULTS

In this study, we examined HeLa cells transfected with cDNA encoding a green fluorescent protein (GFP)-MNB/DYRK1A fusion protein and found 2 patterns of expression: In one group of transfected cells, GFP-MNB/DYRK1A was localized as dots within the nucleus; and in the other group, it was overexpressed and had accumulated all over the nucleus. In the cells overexpressing GFP-MNB/DYRK1A, multinucleation was clearly observed; whereas in those with the nuclear dots, such aberrant nuclei were not found. Furthermore, in the latter cells, essential processes such as mitosis and cytokinesis occurred normally. Multinucleation was dependent on the kinase activity of MNB/DYRK1A, because it was not observed in cells overexpressing kinase activity-negative mutants, GFP-MNB/DYRK1A (K179R) and GFP-MNB/DYRK1A (Y310F/Y312F). Immunostaining of GFP-MNB/DYRK1A-overexpressing cells with specific antibodies against alpha- and gamma-tubulin revealed that multiple copies of centrosomes and aberrant multipolar spindles were generated in these cells.

CONCLUSIONS

These results indicate that overexpression of MNB/DYRK1A induces multinucleation in HeLa cells through overduplication of the centrosome during interphase and production of aberrant spindles and missegregation of chromosomes during mitosis.

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.

Animal models of mental retardation: from gene to cognitive function

About 2-3% of all children are affected by mental retardation, and genetic conditions rank among the leading causes of mental retardation. Alterations in the information encoded by genes that regulate critical steps of brain development can disrupt the normal course of development, and have profound consequences on mental processes. Genetically modified mouse models have helped to elucidate the contribution of specific gene alterations and gene-environment interactions to the phenotype of several forms of mental retardation. Mouse models of several neurodevelopmental pathologies, such as Down and Rett syndromes and X-linked forms of mental retardation, have been developed. Because behavior is the ultimate output of brain, behavioral phenotyping of these models provides functional information that may not be detectable using molecular, cellular or histological evaluations. In particular, the study of ontogeny of behavior is recommended in mouse models of disorders having a developmental onset. Identifying the role of specific genes in neuropathologies provides a framework in which to understand key stages of human brain development, and provides a target for potential therapeutic intervention.

The MNB/DYRK1A protein kinase: neurobiological functions and Down syndrome implications

Major attention is being paid in recent years to the genes harbored within the so called Down syndrome Critical Region of human chromosome 21. Among them, those genes with a possible brain function are becoming the focus of intense research due to the numerous neurobiological alterations and cognitive deficits that Down syndrome individuals have. MNB/DYRK1A is one of these genes. It encodes a protein kinase with unique genetic and biochemical properties, which have been evolutionarily conserved from insects to humans. MNB/DYRK1A is expressed in the developing brain where it seems to play a role in proliferation of neural progenitor cells, neurogenesis, and neuronal differentiation. Although at a lower level, MNB/DYRK1A is also expressed in the adult brain where, as judged by the phenotype of mutant and transgenic animals, it may be involved in learning and memory. Nevertheless, most of the molecular mechanisms underlying these functions remain to be unraveled. In this review we compile and discuss experimental evidences, which support the involvement of MNB/DYRK1A in several neuropathologies and cognitive deficits of Down syndrome.

Dyrk1A haploinsufficiency affects viability and causes developmental delay and abnormal brain morphology in mice

DYRK1A is the human orthologue of the Drosophila minibrain (mnb) gene, which is involved in postembryonic neurogenesis in flies. Because of its mapping position on chromosome 21 and the neurobehavioral alterations shown by mice overexpressing this gene, involvement of DYRK1A in some of the neurological defects of Down syndrome patients has been suggested. To gain insight into its physiological role, we have generated mice deficient in Dyrk1A function by gene targeting. Dyrk1A(-/-) null mutants presented a general growth delay and died during midgestation. Mice heterozygous for the mutation (Dyrk1A(+/-)) showed decreased neonatal viability and a significant body size reduction from birth to adulthood. General neurobehavioral analysis revealed preweaning developmental delay of Dyrk1A(+/-) mice and specific alterations in adults. Brains of Dyrk1A(+/-) mice were decreased in size in a region-specific manner, although the cytoarchitecture and neuronal components in most areas were not altered. Cell counts showed increased neuronal densities in some brain regions and a specific decrease in the number of neurons in the superior colliculus, which exhibited a significant size reduction. These data provide evidence about the nonredundant, vital role of Dyrk1A and suggest a conserved mode of action that determines normal growth and brain size in both mice and flies.

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.

Identification of the autophosphorylation sites and characterization of their effects in the protein kinase DYRK1A

Protein kinases of the DYRK ('dual-specificity tyrosine-regulated kinase') family are characterized by a conserved Tyr-Xaa-Tyr motif (Tyr-319-Tyr-321) in a position exactly corresponding to the activation motif of the mitogen-activated protein kinase (MAP kinase) family (Thr-Xaa-Tyr). In a molecular model of the catalytic domain of DYRK1A, the orientation of phosphorylated Tyr-321 is strikingly similar to that of Tyr-185 in the known structure of the activated MAP kinase, extracellular-signal-regulated kinase 2. Consistent with our model, substitution of Tyr-321 but not of Tyr-319 by phenylalanine markedly reduced the enzymic activity of recombinant DYRK1A expressed in either Escherichia coli or mammalian cells. Direct identification of phosphorylated residues by tandem MS confirmed that Tyr-321, but not Tyr-319, was phosphorylated. When expressed in COS-7 cells, DYRK1A was found to be fully phosphorylated on Tyr-321. A catalytically inactive mutant of DYRK1A contained no detectable phosphotyrosine, indicating that Tyr-321 is autophosphorylated by DYRK1A. MS identified Tyr-111 and Ser-97 as additional autophosphorylation sites in the non-catalytic N-terminal domain of bacterially expressed DYRK1A. Enzymic activity was not affected in the DYRK1A-Y111F mutant. The present experimental data and the molecular model indicate that the activity of DYRK1A is dependent on the autophosphorylation of a conserved tyrosine residue in the activation loop.

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.

Murine models for Down syndrome

The availability of the recently published DNA sequence of human chromosome 21 (HSA21) is a landmark contribution that will have an immediate impact on the study of the role of specific genes to Down syndrome (DS). Trisomy 21 or DS is the only autosomal aneuploidy that is not lethal in the fetal or early postnatal period. DS phenotypes show variable penetrance, affecting many different organs, including brain (mental retardation, early onset of Alzheimer's disease, AD), muscle (hypotonia), skeleton, and blood. DS phenotypes may stem directly from the cumulative effect of overexpression of specific HSA21 gene products or indirectly through the interaction of these gene products with the whole genome, transcriptome, or proteome. Mouse genetic models have played an important role in the elucidation of the contribution of specific genes to the DS phenotype. To date, the strategies used for modeling DS in mice have been three: (1) to assess single-gene contributions to DS phenotype, using transgenic techniques to create models overexpressing single or combinations of genes, (2) to assess the effects of overexpressing large foreign DNA pieces, introduced on yeast artificial chromosomes (YACs) or bacterial artificial chromosomes (BACs) into transgenic mice, and (3) mouse trisomies that carry all or part of MMU16, which has regions of conserved homology with HSA21. Here we review the existing murine models and the relevance of their contribution to DS research.

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)