Overexpression of Dyrk1A Is Implicated in Several Cognitive,Electrophysiological and Neuromorphological Alterations Found in a Mouse Model of Down Syndrome

Down syndrome (DS) phenotypes result from the overexpression of several dosage-sensitive genes. The DYRK1A (dual-specificity tyrosine-(Y)-phosphorylation regulated kinase 1A) gene, which has been implicated in the behavioral and neuronal alterations that are characteristic of DS, plays a role in neuronal progenitor proliferation, neuronal differentiation and long-term potentiation (LTP) mechanisms that contribute to the cognitive deficits found in DS. The purpose of this study was to evaluate the effect of Dyrk1A overexpression on the behavioral and cognitive alterations in the Ts65Dn (TS) mouse model, which is the most commonly utilized mouse model of DS, as well as on several neuromorphological and electrophysiological properties proposed to underlie these deficits. In this study, we analyzed the phenotypic differences in the progeny obtained from crosses of TS females and heterozygous Dyrk1A (+/−) male mice. Our results revealed that normalization of the Dyrk1A copy number in TS mice improved working and reference memory based on the Morris water maze and contextual conditioning based on the fear conditioning test and rescued hippocampal LTP. Concomitant with these functional improvements, normalization of the Dyrk1A expression level in TS mice restored the proliferation and differentiation of hippocampal cells in the adult dentate gyrus (DG) and the density of GABAergic and glutamatergic synapse markers in the molecular layer of the hippocampus. However, normalization of the Dyrk1A gene dosage did not affect other structural (e.g., the density of mature hippocampal granule cells, the DG volume and the subgranular zone area) or behavioral (i.e., hyperactivity/attention) alterations found in the TS mouse. These results suggest that Dyrk1A overexpression is involved in some of the cognitive, electrophysiological and neuromorphological alterations, but not in the structural alterations found in DS, and suggest that pharmacological strategies targeting this gene may improve the treatment of DS-associated learning disabilities.     

Overexpression of Dyrk1A causes the defects in synaptic vesicle endocytosis

Trisomy 21-linked Dyrk1A (dual-specificity tyrosine phosphorylation-regulated kinase 1A) overexpression is implicated in pathogenic mechanisms underlying mental retardation in Down syndrome (DS). It is known to phosphorylate multiple substrates including endocytic proteins in vitro, but the functional consequence of Dyrk1A-mediated phosphorylation on endocytosis has never been investigated. Here, we show that overexpression of Dyrk1A causes defects in clathrin-mediated endocytosis and specifically, in the recruitment of endocytic proteins to clathrin-coated pits in fibroblasts. Synaptic vesicle endocytosis also significantly slowed down as a result of Dyrk1A overexpression in cultured hippocampal neurons. These effects are dependent on Dyrk1A kinase activity. The inhibitory effect of Dyrk1A on synaptic vesicle endocytosis was confirmed in neuronal cultures derived from transgenic mice overexpressing Dyrk1A at levels found in DS. Pharmacological blockade of Dyrk1A with epigallocatechin gallate rescued the endocytic phenotypes found in transgenic neurons. Together, our results suggest that aberrant Dyrk1A-mediated phosphorylation of the endocytic machinery perturbs synaptic vesicle endocytosis, which may contribute to synaptic dysfunctions and cognitive deficits associated with DS.     

Dopaminergic deficiency in mice with reduced levels of the dual-specificity tyrosine-phosphorylated and regulated kinase 1A, Dyrk1A(+/-)

The dual-specificity tyrosine-phosphorylated and regulated kinase 1A (DYRK1A) gene encodes a protein kinase known to play a critical role in neurodevelopment. Mice with one functional copy of Dyrk1A (Dyrk1A(+/-)) display a marked hypoactivity and altered gait dynamics in basal conditions and in novel environments. Dopamine (DA) is a key neurotransmitter in motor behavior and genetic deletion of certain genes directly related to the dopaminergic system has a strong impact on motor activity. We have studied the effects of reduced Dyrk1A expression on the function of the nigrostriatal dopaminergic system. To characterize the dopaminergic system in DYRK1A(+/-) mice, we have used behavioral, pharmacological, histological, neurochemical and neuroimaging (microPET) techniques in a multidisciplinary approach. Dyrk1A(+/-) mice exhibited decreased striatal DA levels, reduced number of DA neurons in the substantia nigra pars compacta, as well as altered behavioral responses to dopaminergic agents. Moreover, microdialysis experiments revealed attenuated striatal DA release and positron emission tomography scan display reduced forebrain activation when challenged with amphetamine, in Dyrk1A(+/-) compared with wild-type mice. These data indicate that Dyrk1A is essential for a proper function of nigrostriatal dopaminergic neurons and suggest that Dyrk1A(+/-) mice can be used to study the pathogenesis of motor disorders involving dopaminergic dysfunction.     

Dyrk1A Phosphorylates p53 and Inhibits Proliferation of Embryonic Neuronal Cells

Down syndrome (DS) is associated with many neural defects, including reduced brain size and impaired neuronal proliferation, highly contributing to the mental retardation. Those typical characteristics of DS are closely associated with a specific gene group “Down syndrome critical region” (DSCR) on human chromosome 21. Here we investigated the molecular mechanisms underlying impaired neuronal proliferation in DS and, more specifically, a regulatory role for dual-specificity tyrosine-(Y) phosphorylation-regulated kinase 1A (Dyrk1A), a DSCR gene product, in embryonic neuronal cell proliferation. We found that Dyrk1A phosphorylates p53 at Ser-15 in vitro and in immortalized rat embryonic hippocampal progenitor H19-7 cells. In addition, Dyrk1A-induced p53 phosphorylation at Ser-15 led to a robust induction of p53 target genes (e.g. p21^CIP1) and impaired G₁/G₀-S phase transition, resulting in attenuated proliferation of H19-7 cells and human embryonic stem cell-derived neural precursor cells. Moreover, the point mutation of p53-Ser-15 to alanine rescued the inhibitory effect of Dyrk1A on neuronal proliferation. Accordingly, brains from embryonic DYRK1A transgenic mice exhibited elevated levels of Dyrk1A, Ser-15 (mouse Ser-18)-phosphorylated p53, and p21^CIP1 as well as impaired neuronal proliferation. These findings suggest that up-regulation of Dyrk1A contributes to altered neuronal proliferation in DS through specific phosphorylation of p53 at Ser-15 and subsequent p21^CIP1 induction.     

QSAR analysis of pyrazolidine-3,5-diones derivatives as Dyrk1A inhibitors

Individuals with Down syndrome (DS) suffer from mental retardation. Overexpression and the resulting increased specific activity of Dyrk1A kinase located on chromosome 21 cause a learning and memory deficit in Dyrk1A transgenic mice. To search for therapeutic agents with Dyrk1A inhibition activity, previously we obtained HCD160 as a new hit compound for Dyrk1A inhibition. In the present study, we synthesized 34 HCD160 derivatives to investigate the quantitative structure–activity relationship (QSAR). This analysis could provide important information for novel drug discovery for treatment of DS related learning and memory deficits.     

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.     

Dyrk1A, a Serine/Threonine Kinase, is Involved in ERK and Akt Activation in the Brain of Hyperhomocysteinemic Mice

Hyperhomocysteinemia due to cystathionine beta synthase (CBS) deficiency is associated with diverse brain disease. Whereas the biological actions linking hyperhomocysteinemia to the cognitive dysfunction are not well understood, we tried to establish relationships between hyperhomocysteinemia and alterations of signaling pathways. In the brain of CBS-deficient mice, a murine model of hyperhomocysteinemia, we previously found an activation of extracellular signal-regulated kinase (ERK) pathway and an increase of Dyrk1A, a serine/threonine kinase involved in diverse functions ranging from development and growth to apoptosis. We then investigated the relationship between Dyrk1A and the signaling pathways initiated by receptor tyrosine kinases (RTK), the ERK and PI3K/Akt pathways. We found a significant increase of phospho-ERK, phospho-MEK, and phospho-Akt in the brain of CBS-deficient and Dyrk1a-overexpressing mice. This increase was abolished when CBS-deficient and Dyrk1A-transgenic mice were treated with harmine, an inhibitor of Dyrk1A kinase activity, which emphasizes the role of Dyrk1A activity on ERK and Akt activation. Sprouty 2 protein level, a negative feedback loop modulator that limits the intensity and duration of RTK activation, is decreased in the brain of CBS-deficient mice, but not in the brain of Dyrk1A transgenic mice. Furthermore, a reduced Dyrk1A and Grb2 binding on sprouty 2 and an increased interaction of Dyrk1A with Grb2 were found in the brain of Dyrk1A transgenic mice. The consequence of Dyrk1A overexpression on RTK activation seems to be a decreased interaction of sprouty 2/Grb2. These observations demonstrate ERK and Akt activation induced by Dyrk1A in the brain of hyperhomocysteinemic mice and open new perspectives to understand the basis of the cognitive defects in hyperhomocysteinemia.     

Increased dosage of Dyrk1A alters alternative splicing factor (ASF)-regulated alternative splicing of tau in Down syndrome

Two groups of tau, 3R- and 4R-tau, are generated by alternative splicing of tau exon 10. Normal adult human brain expresses equal levels of them. Disruption of the physiological balance is a common feature of several tauopathies. Very early in their life, individuals with Down syndrome (DS) develop Alzheimer-type tau pathology, the molecular basis for which is not fully understood. Here, we demonstrate that Dyrk1A, a kinase encoded by a gene in the DS critical region, phosphorylates alternative splicing factor (ASF) at Ser-227, Ser-234, and Ser-238, driving it into nuclear speckles and preventing it from facilitating tau exon 10 inclusion. The increased dosage of Dyrk1A in DS brain due to trisomy of chromosome 21 correlates to an increase in 3R-tau level, which on abnormal hyperphosphorylation and aggregation of tau results in neurofibrillary degeneration. Imbalance of 3R- and 4R-tau in DS brain by Dyrk1A-induced dysregulation of alternative splicing factor-mediated alternative splicing of tau exon 10 represents a novel mechanism of neurofibrillary degeneration and may help explain early onset tauopathy in individuals with DS.     

Dyrk1A phosphorylates alpha-synuclein and enhances intracellular inclusion formation

Lewy bodies (LBs) are pathological hallmarks of Parkinson disease (PD) but also occur in Alzheimer disease (AD) and dementia of LBs. Alpha-synuclein, the major component of LBs, is observed in the brain of Down syndrome (DS) patients with AD. Dyrk1A, a dual specificity tyrosine-regulated kinase (Dyrk) family member, is the mammalian ortholog of the Drosophila minibrain (Mnb) gene, essential for normal postembryonic neurogenesis. The Dyrk1A gene resides in the human chromosome 21q22.2 region, which is associated with DS anomalies, including mental retardation. In this study, we examined whether Dyrk1A interacts with alpha-synuclein and subsequently affects intracellular alpha-synuclein inclusion formation in immortalized hippocampal neuronal (H19-7) cells. Dyrk1A selectively binds to alpha-synuclein in transformed and primary neuronal cells. Alpha-synuclein overexpression, followed by basic fibroblast growth factor-induced neuronal differentiation, resulted in cell death. We observed that accompanying cell death was increased alpha-synuclein phosphorylation and intracytoplasmic aggregation. In addition, the transfection of kinase-inactive Dyrk1A or Dyrk1A small interfering RNA blocked alpha-synuclein phosphorylation and aggregate formation. In vitro kinase assay of anti-Dyrk1A immunocomplexes demonstrated that Dyrk1A could phosphorylate alpha-synuclein at Ser-87. Furthermore, aggregates formed by phosphorylated alpha-synuclein have a distinct morphology and are more neurotoxic compared with aggregates composed of unmodified wild type alpha-synuclein. These findings suggest alpha-synuclein inclusion formation regulated by Dyrk1A, potentially affecting neuronal cell viability.     

The App-Runx1 region is critical for birth defects and electrocardiographic dysfunctions observed in a Down syndrome mouse model

Down syndrome (DS) leads to complex phenotypes and is the main genetic cause of birth defects and heart diseases. The Ts65Dn DS mouse model is trisomic for the distal part of mouse chromosome 16 and displays similar features with post-natal lethality and cardiovascular defects. In order to better understand these defects, we defined electrocardiogram (ECG) with a precordial set-up, and we found conduction defects and modifications in wave shape, amplitudes, and durations in Ts65Dn mice. By using a genetic approach consisting of crossing Ts65Dn mice with Ms5Yah mice monosomic for the App-Runx1 genetic interval, we showed that the Ts65Dn viability and ECG were improved by this reduction of gene copy number. Whole-genome expression studies confirmed gene dosage effect in Ts65Dn, Ms5Yah, and Ts65Dn/Ms5Yah hearts and showed an overall perturbation of pathways connected to post-natal lethality (Coq7, Dyrk1a, F5, Gabpa, Hmgn1, Pde10a, Morc3, Slc5a3, and Vwf) and heart function (Tfb1m, Adam19, Slc8a1/Ncx1, and Rcan1). In addition cardiac connexins (Cx40, Cx43) and sodium channel sub-units (Scn5a, Scn1b, Scn10a) were found down-regulated in Ts65Dn atria with additional down-regulation of Cx40 in Ts65Dn ventricles and were likely contributing to conduction defects. All these data pinpoint new cardiac phenotypes in the Ts65Dn, mimicking aspects of human DS features and pathways altered in the mouse model. In addition they highlight the role of the App-Runx1 interval, including Sod1 and Tiam1, in the induction of post-natal lethality and of the cardiac conduction defects in Ts65Dn. These results might lead to new therapeutic strategies to improve the care of DS people.     

Trisomic and allelic differences influence phenotypic variability during development of Down syndrome mice

Individuals with full or partial Trisomy 21 (Ts21) present with clinical features collectively referred to as Down syndrome (DS), although DS phenotypes vary in incidence and severity between individuals. Differing genetic and phenotypic content in individuals with DS as well as mouse models of DS facilitate the understanding of the correlation between specific genes and phenotypes associated with Ts21. The Ts1Rhr mouse model is trisomic for 33 genes (the "Down syndrome critical region" or DSCR) hypothesized to be responsible for many clinical DS features, including craniofacial dysmorphology with a small mandible. Experiments with Ts1Rhr mice showed that the DSCR was not sufficient to cause all DS phenotypes by identifying uncharacteristic craniofacial abnormalities not found in individuals with DS or other DS mouse models. We hypothesized that the origins of the larger, dysmorphic mandible observed in adult Ts1Rhr mice develop from larger embryonic craniofacial precursors. Because of phenotypic variability seen in subsequent studies with Ts1Rhr mice, we also hypothesized that genetic background differences would alter Ts1Rhr developmental phenotypes. Using Ts1Rhr offspring from two genetic backgrounds, we found differences in mandibular precursor volume as well as total embryonic volume and postnatal body size of Ts1Rhr and nontrisomic littermates. Additionally, we observed increased relative expression of Dyrk1a and differential expression of Ets2 on the basis of the genetic background in the Ts1Rhr mandibular precursor. Our results suggest that trisomic gene content and allelic differences in trisomic or nontrisomic genes influence variability in gene expression and developmental phenotypes associated with DS.     

A high-performance liquid chromatography assay for Dyrk1a,a Down syndrome-associated kinase

Down syndrome is the most common aneuploidy. It is caused by the presence of an extra copy of chromosome 21. Several studies indicate that aberrant expression of the kinase Dyrk1a (dual-specificity tyrosine phosphorylation-regulated kinase 1a) is implicated in Down syndrome, in particular in the onset of mental retardation. Moreover, elevated Dyrk1a activity may also be a risk factor for other neurodegenerative disorders such as Alzheimer’s disease. Over the past years, Dyrk1a has appeared as a potential drug target. Availability of sensitive and quantitative enzyme assays is of prime importance to understand the role of Dyrk1a and to develop specific inhibitors. Here, we describe a new method to measure Dyrk1a activity based on the separation and quantification of specific fluorescent peptides (substrate and phosphorylated product) by high-performance liquid chromatography (HPLC). Kinetic and mechanistic analyses using well-known inhibitors of Dyrk1a confirmed the reliability of this approach. In addition, this assay was further validated using brain extracts of mice models expressing different copies of the Dyrk1a gene. Our results indicate that this novel Dyrk1a assay is simple, sensitive, and specific. It avoids the use of radioactivity-based approaches that, until now, have been widely employed to measure Dyrk1a activity.     

Hepatocyte-specific Dyrk1a gene transfer rescues plasma apolipoprotein A-I levels and aortic Akt/GSK3 pathways in hyperhomocysteinemic mice

Hyperhomocysteinemia, characterized by high plasma homocysteine levels, is recognized as an independent risk factor for cardiovascular diseases. The increased synthesis of homocysteine, a product of methionine metabolism involving B vitamins, and its slower intracellular utilization cause increased flux into the blood. Plasma homocysteine level is an important reflection of hepatic methionine metabolism and the rate of processes modified by B vitamins as well as different enzyme activity. Lowering homocysteine might offer therapeutic benefits. However, approximately 50% of hyperhomocysteinemic patients due to cystathionine-beta-synthase deficiency are biochemically responsive to pharmacological doses of B vitamins. Therefore, effective treatments to reduce homocysteine levels are needed, and gene therapy could provide a novel approach. We recently showed that hepatic expression of DYRK1A, a serine/threonine kinase, is negatively correlated with plasma homocysteine levels in cystathionine-beta-synthase deficient mice, a mouse model of hyperhomocysteinemia. Therefore, Dyrk1a is a good candidate for gene therapy to normalize homocysteine levels. We then used an adenoviral construct designed to restrict expression of DYRK1A to hepatocytes, and found decreased plasma homocysteine levels after hepatocyte-specific Dyrk1a gene transfer in hyperhomocysteinemic mice. The elevation of pyridoxal phosphate was consistent with the increase in cystathionine-beta-synthase activity. Commensurate with the decreased plasma homocysteine levels, targeted hepatic expression of DYRK1A resulted in elevated plasma paraoxonase-1 activity and apolipoprotein A-I levels, and rescued the Akt/GSK3 signaling pathways in aorta of mice, which can prevent homocysteine-induced endothelial dysfunction. These results demonstrate that hepatocyte-restricted Dyrk1a gene transfer can offer a useful therapeutic targets for the development of new selective homocysteine lowering therapy.     

MicroRNA-199b targets the nuclear kinase Dyrk1a in an auto-amplification loop promoting calcineurin/NFAT signalling

MicroRNAs (miRs) are a class of single-stranded, non-coding RNAs of about 22 nucleotides in length. Increasing evidence implicates miRs in myocardial disease processes. Here we show that miR-199b is a direct calcineurin/NFAT target gene that increases in expression in mouse and human heart failure, and targets the nuclear NFAT kinase dual-specificity tyrosine-(Y)-phosphorylation regulated kinase 1a (Dyrk1a), constituting a pathogenic feed forward mechanism that affects calcineurin-responsive gene expression. Mutant mice overexpressing miR-199b, or haploinsufficient for Dyrk1a, are sensitized to calcineurin/NFAT signalling or pressure overload and show stress-induced cardiomegaly through reduced Dyrk1a expression. In vivo inhibition of miR-199b by a specific antagomir normalized Dyrk1a expression, reduced nuclear NFAT activity and caused marked inhibition and even reversal of hypertrophy and fibrosis in mouse models of heart failure. Our results reveal that microRNAs affect cardiac cellular signalling and gene expression, and implicate miR-199b as a therapeutic target in heart failure.     

Over-expression of Dyrk1A affects bleeding by modulating plasma fibronectin and fibrinogen level in mice

Down syndrome is the most common chromosomal abnormality in humans. Patients with Down syndrome have hematologic disorders, including mild to moderate thrombocytopenia. In case of Down syndrome, thrombocytopenia is not associated with bleeding, and it remains poorly characterized regarding molecular mechanisms. We investigated the effects of overexpression of Dyrk1A, an important factor contributing to some major Down syndrome phenotypes, on platelet number and bleeding in mice. Mice overexpressing Dyrk1A have a decrease in platelet number by 20%. However, bleeding time was found to be reduced by 50%. The thrombocytopenia and the decreased bleeding time observed were not associated to an abnormal platelet receptors expression, to a defect of platelet activation by ADP, thrombin or convulxin, to the presence of activated platelets in the circulation or to an abnormal half-life of the platelets. To propose molecular mechanisms explaining this discrepancy, we performed a network analysis of Dyrk1A interactome and demonstrated that Dyrk1A, fibronectin and fibrinogen interact indirectly through two distinct clusters of proteins. Moreover, in mice overexpressing Dyrk1A, increased plasma fibronectin and fibrinogen levels were found, linked to an increase of the hepatic fibrinogen production. Our results indicate that overexpression of Dyrk1A in mice induces decreased bleeding consistent with increased plasma fibronectin and fibrinogen levels, revealing a new role of Dyrk1A depending on its indirect interaction with these two proteins.     

A novel inhibitor rescues cerebellar defects in a zebrafish model of Down syndrome–associated kinase Dyrk1A overexpression

The highly conserved dual-specificity tyrosine phosphorylation–regulated kinase 1A (Dyrk1A) plays crucial roles during central nervous system development and homeostasis. Furthermore, its hyperactivity is considered responsible for some neurological defects in individuals with Down syndrome. We set out to establish a zebrafish model expressing human Dyrk1A that could be further used to characterize the interaction between Dyrk1A and neurological phenotypes. First, we revealed the prominent expression of dyrk1a homologs in cerebellar neurons in the zebrafish larval and adult brains. Overexpression of human dyrk1a in postmitotic cerebellar Purkinje neurons resulted in a structural misorganization of the Purkinje cells in cerebellar hemispheres and a compaction of this cell population. This impaired Purkinje cell organization was progressive, leading to an age-dependent dispersal of Purkinje neurons throughout the cerebellar molecular layer with larval swim deficits resulting in miscoordination of swimming and reduced exploratory behavior in aged adults. We also found that the structural misorganization of the larval Purkinje cell layer could be rescued by pharmacological treatment with Dyrk1A inhibitors. We further reveal the in vivo efficiency of a novel selective Dyrk1A inhibitor, KuFal194. These findings demonstrate that the zebrafish is a well-suited vertebrate organism to genetically model severe neurological diseases with single cell type specificity. Such models can be used to relate molecular malfunction to cellular deficits, impaired tissue formation, and organismal behavior and can also be used for pharmacological compound testing and validation.     

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.