Cell type- and brain structure-specific patterns of distribution of minibrain kinase in human brain

Truncated Dyrk1A aggravates neuronal apoptosis by inhibiting ASF-mediated Bcl-x exon 2b inclusion

AIM

Aggravated neuronal loss, caused mainly by neuronal apoptosis, is observed in the brain of patients with Alzheimer's disease (AD) and animal models of AD. A truncated form of Dual-specific and tyrosine phosphorylation-regulated protein kinase 1A (Dyrk1A) plays a vital role in AD pathogenesis. Downregulation of anti-apoptotic Bcl-xL is tightly correlated with neuronal loss in AD. However, the molecular regulation of neuronal apoptosis and Bcl-x expression by Dyrk1A in AD remains largely elusive. Here, we aimed to explore the role and molecular mechanism of Dyrk1A in apoptosis.

METHODS

Cell Counting Kit-8 (CCK8), flow cytometry, and TdT-mediated dUTP Nick-End Labeling (TUNEL) were used to check apoptosis. The cells, transfected with Dyrk1A or/and ASF with Bcl-x minigene, were used to assay Bcl-x expression by RT-PCR and Western blots. Co-immunoprecipitation, autoradiography, and immunofluorescence were conducted to check the interaction of ASF and Dyrk1A. Gene set enrichment analysis (GSEA) of apoptosis-related genes was performed in mice overexpressing Dyrk1A (TgDyrk1A) and AD model 5xFAD mice.

RESULTS

Dyrk1A promoted Bcl-xS expression and apoptosis. Splicing factor ASF promoted Bcl-x exon 2b inclusion, leading to increased Bcl-xL expression. Dyrk1A suppressed ASF-mediated Bcl-x exon 2b inclusion via phosphorylation. The C-terminus deletion of Dyrk1A facilitated its binding and kinase activity to ASF. Moreover, Dyrk1a1-483 further suppressed the ASF-mediated Bcl-x exon 2b inclusion and aggravated apoptosis. The truncated Dyrk1A, increased Bcl-xS, and enrichment of apoptosis-related genes was observed in the brain of 5xFAD mice.

CONCLUSIONS

We speculate that increased Dyrk1A and truncated Dyrk1A may aggravate neuronal apoptosis by decreasing the ratio of Bcl-xL/Bcl-xS via phosphorylating ASF in AD.

Insights from the protein interaction Universe of the multifunctional "Goldilocks" kinase DYRK1A

Human Dual specificity tyrosine (Y)-Regulated Kinase 1A (DYRK1A) is encoded by a dosage-dependent gene located in the Down syndrome critical region of human chromosome 21. The known substrates of DYRK1A include proteins involved in transcription, cell cycle control, DNA repair and other processes. However, the function and regulation of this kinase is not fully understood, and the current knowledge does not fully explain the dosage-dependent function of this kinase. Several recent proteomic studies identified DYRK1A interacting proteins in several human cell lines. Interestingly, several of known protein substrates of DYRK1A were undetectable in these studies, likely due to a transient nature of the kinase-substrate interaction. It is possible that the stronger-binding DYRK1A interacting proteins, many of which are poorly characterized, are involved in regulatory functions by recruiting DYRK1A to the specific subcellular compartments or distinct signaling pathways. Better understanding of these DYRK1A-interacting proteins could help to decode the cellular processes regulated by this important protein kinase during embryonic development and in the adult organism. Here, we review the current knowledge of the biochemical and functional characterization of the DYRK1A protein-protein interaction network and discuss its involvement in human disease.

Functions of SRPK, CLK and DYRK kinases in stem cells, development, and human developmental disorders

Human developmental disorders encompass a wide range of debilitating physical conditions and intellectual disabilities. Perturbation of protein kinase signalling underlies the development of some of these disorders. For example, disrupted SRPK signalling is associated with intellectual disabilities, and the gene dosage of DYRKs can dictate the pathology of disorders including Down's syndrome. Here, we review the emerging roles of the CMGC kinase families SRPK, CLK, DYRK, and sub-family HIPK during embryonic development and in developmental disorders. In particular, SRPK, CLK, and DYRK kinase families have key roles in developmental signalling and stem cell regulation, and can co-ordinate neuronal development and function. Genetic studies in model organisms reveal critical phenotypes including embryonic lethality, sterility, musculoskeletal errors, and most notably, altered neurological behaviours arising from defects of the neuroectoderm and altered neuronal signalling. Further unpicking the mechanisms of specific kinases using human stem cell models of neuronal differentiation and function will improve our understanding of human developmental disorders and may provide avenues for therapeutic strategies.

The Conserved Transcriptional Activation Activity Identified in Dual-Specificity Tyrosine-(Y)-Phosphorylation-Regulated Kinase 1

Dual-specificity tyrosine-(Y)-phosphorylation-regulated kinase 1 (DYRK1) encodes a conserved protein kinase that is indispensable to neuron development. However, whether DYRK1 possesses additional functions apart from kinase function remains poorly understood. In this study, we firstly demonstrated that the C-terminal of ascidian Ciona robusta DYRK1 (CrDYRK1) showed transcriptional activation activity independent of its kinase function. The transcriptional activation activity of CrDYRK1 could be autoinhibited by a repression domain in the N-terminal. More excitingly, both activation and repression domains were retained in HsDYRK1A in humans. The genes, activated by the activation domain of HsDYRK1A, are mainly involved in ion transport and neuroactive ligand-receptor interaction. We further found that numerous mutation sites relevant to the DYRK1A-related intellectual disability syndrome locate in the C-terminal of HsDYRK1A. Then, we identified several specific DNA motifs in the transcriptional regulation region of those activated genes. Taken together, we identified a conserved transcription activation domain in DYRK1 in urochordates and vertebrates. The activation is independent of the kinase activity of DYRK1 and can be repressed by its own N-terminal. Transcriptome and mutation data indicate that the transcriptional activation ability of HsDYRK1A is potentially involved in synaptic transmission and neuronal function related to the intellectual disability syndrome.

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.

Dyrk1a gene dosage in glutamatergic neurons has key effects in cognitive deficits observed in mouse models of MRD7 and Down syndrome

Perturbation of the excitation/inhibition (E/I) balance leads to neurodevelopmental diseases including to autism spectrum disorders, intellectual disability, and epilepsy. Loss-of-function mutations in the DYRK1A gene, located on human chromosome 21 (Hsa21,) lead to an intellectual disability syndrome associated with microcephaly, epilepsy, and autistic troubles. Overexpression of DYRK1A, on the other hand, has been linked with learning and memory defects observed in people with Down syndrome (DS). Dyrk1a is expressed in both glutamatergic and GABAergic neurons, but its impact on each neuronal population has not yet been elucidated. Here we investigated the impact of Dyrk1a gene copy number variation in glutamatergic neurons using a conditional knockout allele of Dyrk1a crossed with the Tg(Camk2-Cre)4Gsc transgenic mouse. We explored this genetic modification in homozygotes, heterozygotes and combined with the Dp(16Lipi-Zbtb21)1Yey trisomic mouse model to unravel the consequence of Dyrk1a dosage from 0 to 3, to understand its role in normal physiology, and in MRD7 and DS. Overall, Dyrk1a dosage in postnatal glutamatergic neurons did not impact locomotor activity, working memory or epileptic susceptibility, but revealed that Dyrk1a is involved in long-term explicit memory. Molecular analyses pointed at a deregulation of transcriptional activity through immediate early genes and a role of DYRK1A at the glutamatergic post-synapse by deregulating and interacting with key post-synaptic proteins implicated in mechanism leading to long-term enhanced synaptic plasticity. Altogether, our work gives important information to understand the action of DYRK1A inhibitors and have a better therapeutic approach.

The Dual Specificity Tyrosine Phosphorylation Regulated Kinase 1A, DYRK1A, drives cognitive alterations with increased dose in Down syndrome (DS) or with reduced dose in DYRK1A-related intellectual disability syndromes (ORPHA:268261; ORPHA:464311) also known as mental retardation, autosomal dominant disease 7 (MRD7; OMIM #614104). Here we report that specific and complete loss of Dyrk1a in glutamatergic neurons induced a range of specific cognitive phenotypes and alter the expression of genes involved in neurotransmission in the hippocampus. We further explored the consequences of Dyrk1a dosage in glutamatergic neurons on the cognitive phenotypes observed respectively in MRD7 and DS mouse models and we found specific roles in long-term explicit memory with no impact on motor activity, short-term working memory, and susceptibility to epilepsy. Then we demonstrated that DYRK1A is a component of the glutamatergic post-synapse and interacts with several component such as NR2B and PSD95. Altogether our work describes a new role of DYRK1A at the glutamatergic synapse that must be considered to understand the consequence of treatment targeting DYRK1A in disease.

Minibrain-related kinase/dual-specificity tyrosine-regulated kinase 1B implication in stem/cancer stem cells biology

Dual-specificity tyrosine phosphorylation-regulated kinase 1B (DYRK1B), also known as minibrain-related kinase (MIRK) is one of the best functionally studied members of the DYRK kinase family. DYRKs comprise a family of protein kinases that are emerging modulators of signal transduction pathways, cell proliferation and differentiation, survival, and cell motility. DYRKs were found to participate in several signaling pathways critical for development and cell homeostasis. In this review, we focus on the DYRK1B protein kinase from a functional point of view concerning the signaling pathways through which DYRK1B exerts its cell type-dependent function in a positive or negative manner, in development and human diseases. In particular, we focus on the physiological role of DYRK1B in behavior of stem cells in myogenesis, adipogenesis, spermatogenesis and neurogenesis, as well as in its pathological implication in cancer and metabolic syndrome. Thus, understanding of the molecular mechanisms that regulate signaling pathways is of high importance. Recent studies have identified a close regulatory connection between DYRK1B and the hedgehog (HH) signaling pathway. Here, we aim to bring together what is known about the functional integration and cross-talk between DYRK1B and several signaling pathways, such as HH, RAS and PI3K/mTOR/AKT, as well as how this might affect cellular and molecular processes in development, physiology, and pathology. Thus, this review summarizes the major known functions of DYRK1B kinase, as well as the mechanisms by which DYRK1B exerts its functions in development and human diseases focusing on the homeostasis of stem and cancer stem cells.

A Munc18-1 mutant mimicking phosphorylation by Down Syndrome-related kinase Dyrk1a supports normal synaptic transmission and promotes recovery after intense activity

Phosphorylation of Munc18-1 (Stxbp1), a presynaptic organizer of synaptic vesicle fusion, is a powerful mechanism to regulate synaptic strength. Munc18-1 is a proposed substrate for the Down Syndrome-related kinase dual-specificity tyrosine phosphorylation-regulate kinase 1a (Dyrk1a) and mutations in both genes cause intellectual disability. However, the functional consequences of Dyrk1a-dependent phosphorylation of Munc18-1 for synapse function are unknown. Here, we show that the proposed Munc18-1 phosphorylation site, T479, is among the highly constrained phosphorylation sites in the coding regions of the gene and is also located within a larger constrained coding region. We confirm that Dyrk1a phosphorylates Munc18-1 at T479. Patch-clamp physiology in conditional null mutant hippocampal neurons expressing Cre and either wildtype, or mutants mimicking or preventing phosphorylation, revealed that synaptic transmission is similar among the three groups: frequency/amplitude of mEPSCs, evoked EPSCs, paired pulse plasticity, rundown kinetics upon intense activity and the readily releasable pool. However, synapses expressing the phosphomimic mutant responded to intense activity with more pronounced facilitation. These data indicate that Dyrk1a-dependent Munc18-1 phosphorylation has a minor impact on synaptic transmission, only after intense activity, and that the role of genetic variation in both genes in intellectual disability may be through different mechanisms.

Elevation of casein kinase 1ε associated with TDP-43 and tau pathologies in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the presence of extracellular amyloid β plaques and intraneuronal neurofibrillary tangles of hyperphosphorylated microtubule-associated protein tau in the brain. Aggregation of transactive response DNA-binding protein of 43 kDa (TDP-43) in the neuronal cytoplasm is another feature of AD. However, how TDP-43 is associated with AD pathogenesis is unknown. Here, we found that casein kinase 1ε (CK1ε) phosphorylated TDP-43 at Ser403/404 and Ser409/410. In AD brains, the level of CK1ε was dramatically increased and positively correlated with the phosphorylation of TDP-43 at Ser403/404 and Ser409/410. Overexpression of CK1ε promoted its cytoplasmic aggregation and suppressed TDP-43-promoted tau mRNA instability and tau exon 10 inclusion, leading to an increase of tau and 3R-tau expressions. Levels of CK1ε and TDP-43 phosphorylation were positively correlated with the levels of total tau and 3R-tau in human brains. Furthermore, we observed, in pilot immunohistochemical studies, that the severe tau pathology was accompanied by robust TDP-43 pathology and a high level of CK1ε. Taken together, our findings suggest that the elevation of CK1ε in AD brain may phosphorylate TDP-43, promote its cytoplasmic aggregation and suppress its function in tau mRNA processing, leading to acceleration/exacerbation of tau pathology. Thus, the elevation of CK1ε may link TDP-43 to tau pathogenesis in AD brain.

Pathological Tau From Alzheimer's Brain Induces Site-Specific Hyperphosphorylation and SDS- and Reducing Agent-Resistant Aggregation of Tau in vivo

Neurofibrillary tangles (NFTs) made up of hyperphosphorylated tau are a histopathological hallmark of Alzheimer’s disease (AD) and related tauopathies. Hyperphosphorylation of tau is responsible for its loss of normal physiological function, gain of toxicity and its aggregation to form NFTs. Injection of misfolded tau seeds into mouse brain induces tau aggregation, but the nature of tau phosphorylation in pathologic tau seeded pathology is unclear. In the present study, we injected hyperphosphorylated and oligomeric tau isolated from AD brain (AD P-tau) into hippocampus of human tau transgenic mice and found that in addition to tau aggregation/pathology, tau was hyperphosphorylated at Ser202/Thr205, Thr212, Ser214, Thr217, Ser262, and Ser422 in AD P-tau injected hippocampus and at Ser422 in the contralateral hippocampus and in the ipsilateral cortex. AD P-tau-induced AD-like high molecular weight aggregation of tau that was SDS- and reducing agent-resistant and site-specifically hyperphosphorylated in the ipsilateral hippocampus. There were no detectable alterations in levels of tau phosphatases or tau kinases in AD P-tau-injected brains. Furthermore, we found that hyperphosphorylated tau was easier to be captured by AD P-tau and that aggregated tau was more difficult to be dephosphorylated than the non-aggregated tau by protein phosphatase 2A (PP2A). Based on these findings, we speculate that AD P-tau seeds hyperphosphorylated tau to form aggregates, which resist to the dephosphorylation by PP2A, resulting in hyperphosphorylation and pathology of tau.

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.

Expression of Tau Pathology-Related Proteins in Different Brain Regions: A Molecular Basis of Tau Pathogenesis

Microtubule-associated protein tau is hyperphosphorylated and aggregated in affected neurons in Alzheimer disease (AD) brains. The tau pathology starts from the entorhinal cortex (EC), spreads to the hippocampus and frontal and temporal cortices, and finally to all isocortex areas, but the cerebellum is spared from tau lesions. The molecular basis of differential vulnerability of different brain regions to tau pathology is not understood. In the present study, we analyzed brain regional expressions of tau and tau pathology-related proteins. We found that tau was hyperphosphorylated at multiple sites in the frontal cortex (FC), but not in the cerebellum, from AD brain. The level of tau expression in the cerebellum was about 1/4 of that seen in the frontal and temporal cortices in human brain. In the rat brain, the expression level of tau with three microtubule-binding repeats (3R-tau) was comparable in the hippocampus, EC, FC, parietal-temporal cortex (PTC), occipital-temporal cortex (OTC), striatum, thalamus, olfactory bulb (OB) and cerebellum. However, the expression level of 4R-tau was the highest in the EC and the lowest in the cerebellum. Tau phosphatases, kinases, microtubule-related proteins and other tau pathology-related proteins were also expressed in a region-specific manner in the rat brain. These results suggest that higher levels of tau and tau kinases in the EC and low levels of these proteins in the cerebellum may accounts for the vulnerability and resistance of these representative brain regions to the development of tau pathology, respectively. The present study provides the regional expression profiles of tau and tau pathology-related proteins in the brain, which may help understand the brain regional vulnerability to tau pathology in neurodegenerative tauopathies.

Dyrk1A overexpression leads to increase of 3R-tau expression and cognitive deficits in Ts65Dn Down syndrome mice

Alternative splicing of tau exon 10 generates tau isoforms with three or four microtubule-binding repeats, 3R-tau and 4R-tau, which is equally expressed in adult human brain. Imbalanced expression in 3R-tau and 4R-tau has been found in several sporadic and inherited tauopathies, suggesting that dysregulation of tau exon 10 is sufficient to cause neurodegenerative diseases. We previously reported that Dyrk1A, which is overexpressed in Down syndrome brains, regulates alternative splicing of exogenous tau exon 10. In the present study, we investigated the regulation of endogenous tau exon 10 splicing by Dyrk1A. We found that inhibition of Dyrk1A enhanced tau exon 10 inclusion, leading to an increase in 4R-tau/3R-tau ratio in differentiated-human neuronal progenitors and in the neonatal rat brains. Accompanied with overexpression of Dyrk1A, 3R-tau was increased and 4R-tau was decreased in the neonatal brains of Ts65Dn mice, a model of Down syndrome. Treatment with Dyrk1A inhibitor, green tea flavonol epigallocatechin-gallate (EGCG), from gestation to adulthood suppressed 3R-tau expression and rescued anxiety and memory deficits in Ts65Dn mouse brains. Thus, Dyrk1A might be an ideal therapeutic target for Alzheimer's disease, especially for Down syndrome and EGCG which inhibits Dyrk1A may have potential effect on the treatment or prevention of this disease.

DYRK1A regulates Hap1-Dcaf7/WDR68 binding with implication for delayed growth in Down syndrome

Huntingtin-associated protein 1 (Hap1) is known to be critical for postnatal hypothalamic function and growth. Hap1 forms stigmoid bodies (SBs), unique neuronal cytoplasmic inclusions of unknown function that are enriched in hypothalamic neurons. Here we developed a simple strategy to isolate the SB-enriched fraction from mouse brain. By analyzing Hap1 immunoprecipitants from this fraction, we identified a Hap1-interacting SB component, DDB1 and CUL4 associated factor 7 (Dcaf7)/WD40 repeat 68 (WDR68), whose protein level and nuclear translocation are regulated by Hap1. Moreover, we found that Hap1 bound Dcaf7 competitively in cytoplasm with dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A), a protein implicated in Down syndrome (DS). Depleting Hap1 promoted the DYRK1A-Dcaf7 interaction and increased the DYRK1A protein level. Transgenic DS mice overexpressing DYRK1A showed reduced Hap1-Dcaf7 association in the hypothalamus. Furthermore, the overexpression of DYRK1A in the hypothalamus led to delayed growth in postnatal mice, suggesting that DYRK1A regulates the Hap1-Dcaf7 interaction and postnatal growth and that targeting Hap1 or Dcaf7 could ameliorate growth retardation in DS.

An ELISA DYRK1A non-radioactive kinase assay suitable for the characterization of inhibitors

The DYRK1A (dual specificity tyrosine phosphorylation-regulated kinase 1A) gene encodes a proline-directed Ser/Thr kinase. Elevated expression and/or altered distribution of the kinase have been implicated in the neurological impairments associated with Down syndrome (DS) and Alzheimer’s disease (AD). Consequently, DYRK1A inhibition has been of significant interest as a potential strategy for therapeutic intervention of DS and AD. Many classes of novel inhibitors have been described in the past decade. Although non-radioactive methods for analyzing DYRK1A inhibition have been developed, methods employing radioactive tracers are still commonly used for quantitative characterization of DYRK1A inhibitors. Here, we present a non-radioactive ELISA assay based on the detection of DYRK1A-phosphorylated dynamin 1a fragment using a phosphorylation site-specific antibody. The assay was verified by the use of two well-characterized DYRK1A inhibitors, epigallocatechin gallate (EGCG) and harmine. The IC ₅₀s for EGCG and harmine determined by the ELISA method were found to be comparable to those previously measured by radioactive tracing methods.  Furthermore, we determined the mode of inhibition for EGCG and harmine by a modification of the ELISA assay. This assay confirms the mode of inhibition of EGCG (non-ATP-competitive) and harmine (ATP-competitive), as previously determined. We conclude that the ELISA platform demonstrated here is a viable alternative to the traditional radioactive tracer assays for analyzing DYRK1A inhibitors.

DYRK1A, a Dosage-Sensitive Gene Involved in Neurodevelopmental Disorders, Is a Target for Drug Development in Down Syndrome

Down syndrome (DS) is one of the leading causes of intellectual disability, and patients with DS face various health issues, including learning and memory deficits, congenital heart disease, Alzheimer's disease (AD), leukemia, and cancer, leading to huge medical and social costs. Remarkable advances on DS research have been made in improving cognitive function in mouse models for future therapeutic approaches in patients. Among the different approaches, DYRK1A inhibitors have emerged as promising therapeutics to reduce DS cognitive deficits. DYRK1A is a dual-specificity kinase that is overexpressed in DS and plays a key role in neurogenesis, outgrowth of axons and dendrites, neuronal trafficking and aging. Its pivotal role in the DS phenotype makes it a prime target for the development of therapeutics. Recently, disruption of DYRK1A has been found in Autosomal Dominant Mental Retardation 7 (MRD7), resulting in severe mental deficiency. Recent advances in the development of kinase inhibitors are expected, in the near future, to remove DS from the list of incurable diseases, providing certain conditions such as drug dosage and correct timing for the optimum long-term treatment. In addition the exact molecular and cellular mechanisms that are targeted by the inhibition of DYRK1A are still to be discovered.

Truncation and Activation of Dual Specificity Tyrosine Phosphorylation-regulated Kinase 1A by Calpain I: A MOLECULAR MECHANISM LINKED TO TAU PATHOLOGY IN ALZHEIMER DISEASE

Hyperphosphorylation and dysregulation of exon 10 splicing of Tau are pivotally involved in pathogenesis of Alzheimer disease (AD) and/or other tauopathies. Alternative splicing of Tau exon 10, which encodes the second microtubule-binding repeat, generates Tau isoforms containing three and four microtubule-binding repeats, termed 3R-Taus and 4R-Taus, respectively. Dual specificity tyrosine-phosphorylation-regulated kinase 1A (Dyrk1A) lies at the Down syndrome critical region of chromosome 21. Overexpression of this kinase may contribute to the early Tau pathology in Down syndrome via phosphorylation of Tau and dysregulation of Tau exon 10. Here, we report that Dyrk1A was truncated at the C terminus and was associated with overactivation of calpain I in AD brain. Calpain I proteolyzed Dyrk1A in vitro first at the C terminus and further at the N terminus and enhanced its kinase activity toward Tau via increased Vmax but not Km. C-terminal truncation of Dyrk1A resulted in stronger activity than its full-length protein in promotion of exon 10 exclusion and phosphorylation of Tau. Dyrk1A was truncated in kainic acid-induced excitotoxic mouse brains and coincided with an increase in 3R-Tau expression and phosphorylation of Tau via calpain activation. Moreover, truncation of Dyrk1A was correlated with an increase in the ratio of 3R-Tau/4R-Tau and Tau hyperphosphorylation in AD brain. Collectively, these findings suggest that truncation/activation of Dyrk1A by Ca(2+)/calpain I might contribute to Tau pathology via promotion of exon 10 exclusion and hyperphosphorylation of Tau in AD brain.

Activity-dependent facilitation of Synaptojanin and synaptic vesicle recycling by the Minibrain kinase

Phosphorylation has emerged as a crucial regulatory mechanism in the nervous system to integrate the dynamic signaling required for proper synaptic development, function, and plasticity, particularly during changes in neuronal activity. Here we present evidence that Minibrain (Mnb; also known as Dyrk1A), a serine/threonine kinase implicated in autism spectrum disorder and Down syndrome, is required presynaptically for normal synaptic growth and rapid synaptic vesicle endocytosis at the Drosophila neuromuscular junction (NMJ). We find that Mnb-dependent phosphorylation of synaptojanin (Synj) is required, in vivo, for complex endocytic protein interactions and to enhance Synj activity. Neuronal stimulation drives Mnb mobilization to endocytic zones and triggers Mnb-dependent phosphorylation of Synj. Our data identify Mnb as a synaptic kinase that promotes efficient synaptic vesicle recycling by dynamically calibrating Synj function at the Drosophila NMJ, and in turn endocytic capacity, to adapt to conditions of high synaptic activity.

Intracellular distribution of differentially phosphorylated dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A)

The gene encoding dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A) is located within the Down syndrome (DS) critical region of chromosome 21. DYRK1A interacts with a plethora of substrates in the cytosol, cytoskeleton, and nucleus. Its overexpression is a contributing factor to the developmental alterations and age-associated pathology observed in DS. We hypothesized that the intracellular distribution of DYRK1A and cell-compartment-specific functions are associated with DYRK1A posttranslational modifications. Fractionation showed that, in both human and mouse brain, almost 80% of DYRK1A was associated with the cytoskeleton, and the remaining DYRK1A was present in the cytosolic and nuclear fractions. Coimmunoprecipitation revealed that DYRK1A in the brain cytoskeleton fraction forms complexes with filamentous actin, neurofilaments, and tubulin. Two-dimensional gel analysis of the fractions revealed DYRK1A with distinct isoelectric points: 5.5-6.5 in the nucleus, 7.2-8.2 in the cytoskeleton, and 8.7 in the cytosol. Phosphate-affinity gel electrophoresis demonstrated several bands of DYRK1A with different mobility shifts for nuclear, cytoskeletal, and cytosolic DYRK1A, indicating modification by phosphorylation. Mass spectrometry analysis disclosed one phosphorylated site in the cytosolic DYRK1A and multiple phosphorylated residues in the cytoskeletal DYRK1A, including two not previously described. This study supports the hypothesis that intracellular distribution and compartment-specific functions of DYRK1A may depend on its phosphorylation pattern.

Inhibition of DYRK1A destabilizes EGFR and reduces EGFR-dependent glioblastoma growth

Glioblastomas (GBMs) are very aggressive tumors that are resistant to conventional chemo- and radiotherapy. New molecular therapeutic strategies are required to effectively eliminate the subpopulation of GBM tumor-initiating cells that are responsible for relapse. Since EGFR is altered in 50% of GBMs, it represents one of the most promising targets; however, EGFR kinase inhibitors have produced poor results in clinical assays, with no clear explanation for the observed resistance. We uncovered a fundamental role for the dual-specificity tyrosine phosphorylation-regulated kinase, DYRK1A, in regulating EGFR in GBMs. We found that DYRK1A was highly expressed in these tumors and that its expression was correlated with that of EGFR. Moreover, DYRK1A inhibition promoted EGFR degradation in primary GBM cell lines and neural progenitor cells, sharply reducing the self-renewal capacity of normal and tumorigenic cells. Most importantly, our data suggest that a subset of GBMs depends on high surface EGFR levels, as DYRK1A inhibition compromised their survival and produced a profound decrease in tumor burden. We propose that the recovery of EGFR stability is a key oncogenic event in a large proportion of gliomas and that pharmacological inhibition of DYRK1A could represent a promising therapeutic intervention for EGFR-dependent GBMs.

Dyrk1A is dynamically expressed on subsets of motor neurons and in the neuromuscular junction: possible role in Down syndrome

Individuals with Down syndrome (DS) present important motor deficits that derive from altered motor development of infants and young children. DYRK1A, a candidate gene for DS abnormalities has been implicated in motor function due to its expression in motor nuclei in the adult brain, and its overexpression in DS mouse models leads to hyperactivity and altered motor learning. However, its precise role in the adult motor system, or its possible involvement in postnatal locomotor development has not yet been clarified. During the postnatal period we observed time-specific expression of Dyrk1A in discrete subsets of brainstem nuclei and spinal cord motor neurons. Interestingly, we describe for the first time the presence of Dyrk1A in the presynaptic terminal of the neuromuscular junctions and its axonal transport from the facial nucleus, suggesting a function for Dyrk1A in these structures. Relevant to DS, Dyrk1A overexpression in transgenic mice (TgDyrk1A) produces motor developmental alterations possibly contributing to DS motor phenotypes and modifies the numbers of motor cholinergic neurons, suggesting that the kinase may have a role in the development of the brainstem and spinal cord motor system.

Developments in harmine pharmacology--implications for ayahuasca use and drug-dependence treatment

Ayahuasca is a hallucinogenic botanical mixture originating in the Amazon area where it is used ritually, but is now being taken globally. The 2 main constituents of ayahuasca are N,N-dimethyltryptamine (DMT), a hallucinogen, and harmine, a monoamine oxidase inhibitor (MAOI) which attenuates the breakdown of DMT, which would otherwise be broken down very quickly after oral consumption. Recent developments in ayahuasca use include the sale of these compounds on the internet and the substitution of related botanical (anahuasca) or synthetic (pharmahuasca) compounds to achieve the same desired hallucinogenic effects. One intriguing result of ayahuasca use appears to be improved mental health and a reduction in recidivism to alternate (alcohol, cocaine) drug use. In this review we discuss the pharmacology of ayahuasca, with a focus on harmine, and suggest pharmacological mechanisms for the putative reduction in recidivism to alcohol and cocaine misuse. These pharmacological mechanisms include MAOI, effects at 5-HT(2A) and imidazoline receptors and inhibition of dual-specificity tyrosine-phosphorylation regulated kinase 1A (DYRK1A) and the dopamine transporter. We also speculate on the therapeutic potential of harmine in other CNS conditions.

Gene dosage-dependent association of DYRK1A with the cytoskeleton in the brain and lymphocytes of down syndrome patients

The triplication of the DYRK1A gene encoding proline-directed serine/threonine kinase and located in the critical region of Down syndrome (DS) has been implicated in cognitive deficits and intellectual disability of individuals with DS. We investigated the effect of abnormal levels of this kinase on the cytoskeleton in brain and peripheral tissues of DS subjects. In DS tissues, the predictable approximately equal to 1.5-fold enhancement of the levels of DYRK1A protein was demonstrated. An association of DYRK1A with all 3 major cytoskeleton networks was identified using immunoprecipitation. We concentrated on the actin cytoskeleton because its association with DYRK1A was the most affected by the enzyme levels. As measured by coimmunoprecipitation in DS tissues, but not in fragile X lymphocytes, actin association with DYRK1A was reduced. This reduced association was dependent on the state of phosphorylation of cytoskeletal proteins and was present only in cells overproducing DYRK1A kinase; therefore, the effect was attributable to the DYRK1A gene dosage. Alterations of DYRK1A-actin assemblies were detected in newborn and infant groups, thereby linking DYRK1A overexpression with abnormal brain development of DS children. The identification of the actin cytoskeleton as one of cellular targets of DYRK1A action provides new insights into a gene dosage-sensitive mechanism by which DYRK1A could contribute to the pathogenesis of DS. In addition, the presence of this DS-specific cytoskeleton anomaly in lymphocytes attests to the systemic nature of some features of DS. To our knowledge, this is the first study conducted in human tissue that shows DYRK1A association with the cytoskeleton.

Dual-specificity tyrosine phosphorylation-regulated kinase 1A (Dyrk1A) modulates serine/arginine-rich protein 55 (SRp55)-promoted Tau exon 10 inclusion

Tau exon 10, which encodes the second microtubule-binding repeat, is regulated by alternative splicing. Its alternative splicing generates Tau isoforms with three- or four-microtubule-binding repeats, named 3R-tau and 4R-tau. Adult human brain expresses equal levels of 3R-tau and 4R-tau. Imbalance of 3R-tau and 4R-tau causes Tau aggregation and neurofibrillary degeneration. In the present study, we found that splicing factor SRp55 (serine/arginine-rich protein 55) promoted Tau exon 10 inclusion. Knockdown of SRp55 significantly promoted Tau exon 10 exclusion. The promotion of Tau exon 10 inclusion by SRp55 required the arginine/serine-rich region, which was responsible for the subnucleic speckle localization. Dyrk1A (dual specificity tyrosine-phosphorylated and regulated kinase 1A) interacted with SRp55 and mainly phosphorylated its proline-rich domain. Phosphorylation of SRp55 by Dyrk1A suppressed its ability to promote Tau exon 10 inclusion. Up-regulation of Dyrk1A as in Down syndrome could lead to neurofibrillary degeneration by shifting the alternative splicing of Tau exon 10 to an increase in the ratio of 3R-tau/4R-tau.

Effect of DYRK1A activity inhibition on development of neuronal progenitors isolated from Ts65Dn mice

Overexpression of dual-specificity tyrosine-(Y)-phosphorylation-regulated kinase 1A (DYRK1A), encoded by a gene located in the Down syndrome (DS) critical region, is considered a major contributor to developmental abnormalities in DS. DYRK1A regulates numerous genes involved in neuronal commitment, differentiation, maturation, and apoptosis. Because alterations of neurogenesis could lead to impaired brain development and mental retardation in individuals with DS, pharmacological normalization of DYRK1A activity has been postulated as DS therapy. We tested the effect of harmine, a specific DYRK1A inhibitor, on the development of neuronal progenitor cells (NPCs) isolated from the periventricular zone of newborn mice with segmental trisomy 16 (Ts65Dn mice), a mouse model for DS that overexpresses Dyrk1A by 1.5-fold. Trisomy did not affect the ability of NPCs to expand in culture. Twenty-four hours after stimulation of migration and neuronal differentiation, NPCs showed increased expression of Dyrk1A, particularly in the trisomic cultures. After 7 days, NPCs developed into a heterogeneous population of differentiating neurons and astrocytes that expressed Dyrk1A in the nuclei. In comparison with disomic cells, NPCs with trisomy showed premature neuronal differentiation and enhanced γ-aminobutyric acid (GABA)-ergic differentiation, but astrocyte development was unchanged. Harmine prevented premature neuronal maturation of trisomic NPCs but not acceleration of GABA-ergic development. In control NPCs, harmine treatment caused altered neuronal development of NPCs, similar to that in trisomic NPCs with Dyrk1A overexpression. This study suggests that pharmacological normalization of DYRK1A activity may have a potential role in DS therapy.

Phosphorylation by Dyrk1A of clathrin coated vesicle-associated proteins: identification of the substrate proteins and the effects of phosphorylation

Dyrk1A phosphorylated multiple proteins in the clathrin-coated vesicle (CCV) preparations obtained from rat brains. Mass spectrometric analysis identified MAP1A, MAP2, AP180, and α- and β-adaptins as the phosphorylated proteins in the CCVs. Each protein was subsequently confirmed by [(32)P]-labeling and immunological methods. The Dyrk1A-mediated phosphorylation released the majority of MAP1A and MAP2 and enhanced the release of AP180 and adaptin subunits from the CCVs. Furthermore, Dyrk1A displaced adaptor proteins physically from CCVs in a kinase-concentration dependent manner. The clathrin heavy chain release rate, in contrast, was not affected by Dyrk1A. Surprisingly, the Dyrk1A-mediated phosphorylation of α- and β-adaptins led to dissociation of the AP2 complex, and released only β-adaptin from the CCVs. AP180 was phosphorylated by Dyrk1A also in the membrane-free fractions, but α- and β-adaptins were not. Dyrk1A was detected in the isolated CCVs and was co-localized with clathrin in neurons from mouse brain sections and from primary cultured rat hippocampus. Previously, we proposed that Dyrk1A inhibits the onset of clathrin-mediated endocytosis in neurons by phosphorylating dynamin 1, amphiphysin 1, and synaptojanin 1. Current results suggest that besides the inhibition, Dyrk1A promotes the uncoating process of endocytosed CCVs.

Altered regulation of tau phosphorylation in a mouse model of down syndrome aging

Down syndrome (DS) results from trisomy of human chromosome 21 (Hsa21) and is associated with an increased risk of Alzheimer's disease (AD). Here, using the unique transchromosomic Tc1 mouse model of DS we investigate the influence of trisomy of Hsa21 on the protein tau, which is hyperphosphorylated in Alzheimer's disease. We show that in old, but not young, Tc1 mice increased phosphorylation of tau occurs at a site suggested to be targeted by the Hsa21 encoded kinase, dual-specificity tyrosine-(Y)-phosphorylation regulated kinase 1A (DYRK1A). We show that DYRK1A is upregulated in young and old Tc1 mice, but that young trisomic mice may be protected from accumulating aberrantly phosphorylated tau. We observe that the key tau kinase, glycogen synthase kinase3-β (GSK-3β) is aberrantly phosphorylated at an inhibitory site in the aged Tc1 brain which may reduce total glycogen synthase kinase3-β activity. It is possible that a similar mechanism may also occur in people with DS.

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

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

Regulation of the alternative splicing of tau exon 10 by SC35 and Dyrk1A

Abnormal alternative splicing of tau exon 10 results in imbalance of 3R-tau and 4R-tau expression, which is sufficient to cause neurofibrillary degeneration. Splicing factor SC35, a member of the superfamily of the serine/arginine-rich (SR) proteins, promotes tau exon 10 inclusion. The molecular mechanism by which SC35 participates in tau exon 10 splicing remains elusive. In the present study, we found that tau pre-mRNA was coprecipitated by SC35 tagged with HA. Mutation of the SC35-like exonic splicing enhancer located at exon 10 of tau affected both the binding of SC35 to tau pre-mRNA and promotion of tau exon 10 inclusion, suggesting that SC35 acts on the SC35-like exonic splicing enhancer to promote tau exon 10 inclusion. Dyrk1A (dual-specificity tyrosine-phosphorylated and regulated kinase 1A) phosphorylated SC35 in vitro and interacted with it in cultured cells. Overexpression of Dyrk1A suppressed SC35's ability to promote tau exon 10 inclusion. Downregulation of Dyrk1A promoted 4R-tau expression. Therefore, upregulation of Dyrk1A in Down syndrome brain or Alzheimer's brain may cause dysregulation of tau exon 10 splicing through SC35, and probably together with other splicing factors, leading to the imbalance in 3R-tau and 4R-tau expression, which may initiate or accelerate tau pathology and cause neurofibrillary degeneration in the diseases.

Harmine is an ATP-competitive inhibitor for dual-specificity tyrosine phosphorylation-regulated kinase 1A (Dyrk1A)

Harmine is a β-carboline alkaloid. The compound is a potent inhibitor of dual-specificity tyrosine phosphorylation-regulated kinase 1A (Dyrk1A), a kinase implicated in Down syndrome. In this study, we show that harmine functions as an ATP-competitive inhibitor against Dyrk1A. Our conclusion is supported by kinetic analysis of harmine inhibition as well as by the characterization of a Dyrk1A mutation conferring significant resistance to harmine. The mutation, V306A, is located next to the highly conserved D307 residue in kinases known to coordinate the phosphate groups of ATP through a Mg²+ ion. The V306A mutation offers harmine resistance by differentially altering Dyrk1A affinity for harmine and ATP. The V306A mutation causes no apparent alteration to Dyrk1A activity except for the reduction in ATP affinity. This deficiency could be fully compensated by supplying ATP with a concentration in the physiological range. Our results reveal that harmine inhibits Dyrk1A activity by interacting with residues in the ATP-binding pocket and displacing ATP. Our results also suggest that harmine will be a good lead compound for further designing of selective ATP-competitive Dyrk1A inhibitors through exploration of the ATP-binding pocket of Dyrk1A.

DYRK family of protein kinases: evolutionary relationships, biochemical properties, and functional roles

Dual-specificity tyrosine-regulated kinases (DYRKs) comprise a family of protein kinases within the CMGC group of the eukaryotic kinome. Members of the DYRK family are found in 4 (animalia, plantae, fungi, and protista) of the 5 main taxa or kingdoms, and all DYRK proteins studied to date share common structural, biochemical, and functional properties with their ancestors in yeast. Recent work on DYRK proteins indicates that they participate in several signaling pathways critical for developmental processes and cell homeostasis. In this review, we focus on the DYRK family of proteins from an evolutionary, biochemical, and functional point of view and discuss the most recent, relevant, and controversial contributions to the study of these kinases.

Link between DYRK1A overexpression and several-fold enhancement of neurofibrillary degeneration with 3-repeat tau protein in Down syndrome

Triplication of chromosome 21 in Down syndrome (DS) results in overexpression of the minibrain kinase/dual-specificity tyrosine phosphorylated and regulated kinase 1A gene (DYRK1A). DYRK1A phosphorylates cytoplasmic tau protein and appears in intraneuronal neurofibrillary tangles (NFTs). We have previously shown significantly more DYRK1A-positive NFTs in DS brains than in sporadic Alzheimer disease (AD) brains. This study demonstrates a gene dosage-proportional increase in the level of DYRK1A in DS in the cytoplasm and the cell nucleus, and enhanced cytoplasmic and nuclear immunoreactivity of DYRK1A in DS. The results suggest that overexpressed DYRK1A may alter both phosphorylation of tau and alternative splicing factor (ASF). Two-dimensional electrophoresis revealed modification of ASF phosphorylation in DS/AD and AD in comparison to controls. Altered phosphorylation of ASF by overexpressed nuclear DYRK1A may contribute to the alternative splicing of the tau gene and an increase by 2.68 × of the 3R/4R ratio in DS/AD, and a several-fold increase in the number of 3R tau-positive NFTs in DS/AD subjects compared with that in sporadic AD subjects. These data support the hypothesis that phosphorylation of ASF by overexpressed DYRK1A may contribute to alternative splicing of exon 10, increased expression of 3R tau, and early onset of neurofibrillary degeneration in DS.

The role of DYRK1A in neurodegenerative diseases

Recent studies indicate that the dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A) gene, which is located on chromosome 21q22.2 and is overexpressed in Down syndrome (DS), may play a significant role in developmental brain defects and in early onset neurodegeneration, neuronal loss and dementia in DS. The identification of hundreds of genes deregulated by DYRK1A overexpression and numerous cytosolic, cytoskeletal and nuclear proteins, including transcription factors, phosphorylated by DYRK1A, indicates that DYRK1A overexpression is central for the deregulation of multiple pathways in the developing and aging DS brain, with structural and functional alterations including mental retardation and dementia. DYRK1A overexpression in DS brains may contribute to early onset neurofibrillary degeneration directly through hyperphosphorylation of tau and indirectly through phosphorylation of alternative splicing factor, leading to an imbalance between 3R-tau and 4R-tau. The several-fold increases in the number of DYRK1A-positive and 3R-tau-positive neurofibrillary tangles in DS support this hypothesis. Moreover, the enhanced phosphorylation of amyloid precursor protein by overexpressed DYRK1A facilitates amyloidogenic amyloid precursor protein cleavage elevating Aβ40 and 42 levels, and leading to brain β-amyloidosis. Therefore, inhibiting DYRK1A activity in DS may serve to counteract the phenotypic effects of its overexpression and is a potential method of treatment of developmental defects and the prevention of age-associated neurodegeneration, including Alzheimer-type pathology.

Generation of a panel of antibodies against proteins encoded on human chromosome 21

BACKGROUND

Down syndrome (DS) is caused by trisomy of all or part of chromosome 21. To further understanding of DS we are working with a mouse model, the Tc1 mouse, which carries most of human chromosome 21 in addition to the normal mouse chromosome complement. This mouse is a model for human DS and recapitulates many of the features of the human syndrome such as specific heart defects, and cerebellar neuronal loss. The Tc1 mouse is mosaic for the human chromosome such that not all cells in the model carry it. Thus to help our investigations we aimed to develop a method to identify cells that carry human chromosome 21 in the Tc1 mouse. To this end, we have generated a panel of antibodies raised against proteins encoded by genes on human chromosome 21 that are known to be expressed in the adult brain of Tc1 mice

RESULTS

We attempted to generate human specific antibodies against proteins encoded by human chromosome 21. We selected proteins that are expressed in the adult brain of Tc1 mice and contain regions of moderate/low homology with the mouse ortholog. We produced antibodies to seven human chromosome 21 encoded proteins. Of these, we successfully generated three antibodies that preferentially recognise human compared with mouse SOD1 and RRP1 proteins on western blots. However, these antibodies did not specifically label cells which carry a freely segregating copy of Hsa21 in the brains of our Tc1 mouse model of DS.

CONCLUSIONS

Although we have successfully isolated new antibodies to SOD1 and RRP1 for use on western blots, in our hands these antibodies have not been successfully used for immunohistochemistry studies. These antibodies are freely available to other researchers. Our data high-light the technical difficulty of producing species-specific antibodies for both western blotting and immunohistochemistry.

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.

Dyrk1A binds to multiple endocytic proteins required for formation of clathrin-coated vesicles

In spite of a nuclear targeting sequence, a substantial amount of dual-specificity tyrosine phosphorylation-regulated kinase (Dyrk1A) is located within the cytoplasm of neurons. Analysis of fractionated rat brains revealed that the majority of Dyrk1A was in the postnuclear precipitate. The kinase in this fraction was resistant to high salt and Triton X-100 extraction at pH 6.5. Hypothesizing that Dyrk1A binds tightly with cell constituents, we searched for Dyrk1A binding proteins in the Triton X-100-insoluble fraction extracted with urea and fractionated by column chromatography. An overlay assay using the recombinant kinase revealed that multiple proteins are capable of binding to Dyrk1A. Among them, we identified clathrin heavy chain and dynamin 1 as potential candidates. An overlay assay using purified and partially purified proteins showed the binding of Dyrk1A with both proteins. Under native conditions, Dyrk1A precipitated with newly formed clathrin cages and with dynamin via the GST-amphiphysin SH3 domain. We also identified another endocytic protein, endophilin 1, as an additional Dyrk1A binding protein. We then tested whether the clathrin-coated vesicle (CCV)-associated proteins could be phosphorylated by Dyrk1A. Multiple proteins apparently distinctive from the known substrates were phosphorylated in the brain CCV. Our findings suggest a role for Dyrk1A in controlling synaptic vesicle recycling processes.

Developmental regulation of tau phosphorylation, tau kinases, and tau phosphatases

Tau is a neuronal microtubule-associated protein. Its hyperphosphorylation plays a critical role in Alzheimer disease (AD). Expression and phosphorylation of tau are regulated developmentally, but its dynamic regulation and the responsible kinases or phosphatases remain elusive. Here, we studied the developmental regulation of tau in rats during development from embryonic day 15 through the age of 24 months. We found that tau expression increased sharply during the embryonic stage and then became relatively stable, whereas tau phosphorylation was much higher in developing brain than in mature brain. However, the extent of tau phosphorylation at seven of the 14 sites studied was much less in developing brain than in AD brain. Tau phosphorylation during development matched the period of active neurite outgrowth in general. Tau phosphorylation at various sites had different topographic distributions. Several tau kinases appeared to regulate tau phosphorylation collectively at overlapping sites, and the decrease of overall tau phosphorylation in adult brain might be due to the higher levels of tau phosphatases in mature brain. These studies provide new insight into the developmental regulation of site-specific tau phosphorylation and identify the likely sites required for the abnormal hyperphosphorylation of tau in AD.

The Down syndrome candidate dual-specificity tyrosine phosphorylation-regulated kinase 1A phosphorylates the neurodegeneration-related septin 4

The dual-specific kinase DYRK1A (dual-specificity tyrosine phosphorylation-regulated kinase 1A) is the mammalian orthologue of the Drosophila minibrain (MNB) protein kinase and executes diverse roles in neuronal development and adult brain physiology. DYRK1A is overexpressed in Down syndrome (DS) and has recently been implicated in several neurodegenerative diseases. In an attempt to elucidate the molecular basis of its involvement in cognitive and neurodegeneration processes, we searched for novel proteins interacting with the kinase domain of DYRK1A in the adult mouse brain and identified septin 4 (SEPT4, also known as Pnutl2/CDCrel-2). SEPT4 is a member of the group III septin family of guanosine triphosphate hydrolases (GTPases), which has previously been found in neurofibrillary tangles of Alzheimer disease brains and in alpha-synuclein-positive cytoplasmic inclusions in Parkinson disease brains. In transfected mammalian cells, DYRK1A specifically interacts with and phosphorylates SEPT4. Phosphorylation of SEPT4 by DYRK1A was inhibited by harmine, which has recently been identified as the most specific inhibitor of DYRK1A. In support of a physiological relation in the brain, we found that Dyrk1A and Sept4 are co-expressed and co-localized in neocortical neurons. These findings suggest that SEPT4 is a substrate of DYRK1A kinase and thus provide a possible link for the involvement of DYRK1A in neurodegenerative processes and in DS neuropathologies.

The protein kinase DYRK1A regulates caspase-9-mediated apoptosis during retina development

The precise regulation of programmed cell death is critical for the normal development of the nervous system. We show here that DYRK1A (minibrain), a protein kinase essential for normal growth, is a negative regulator of the intrinsic apoptotic pathway in the developing retina. We provide evidence that changes in Dyrk1A gene dosage in the mouse strongly alter the cellularity of inner retina layers and result in severe functional alterations. We show that DYRK1A does not affect the proliferation or specification of retina progenitor cells, but rather regulates the number of cells that die by apoptosis. We demonstrate that DYRK1A phosphorylates caspase-9 on threonine residue 125, and that this phosphorylation event is crucial to protect retina cells from apoptotic cell death. Our data suggest a model in which dysregulation of the apoptotic response in differentiating neurons participates in the neuropathology of diseases that display DYRK1A gene-dosage imbalance effects, such as Down's syndrome.

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.

The role of overexpressed DYRK1A protein in the early onset of neurofibrillary degeneration in Down syndrome

The gene encoding the minibrain kinase/dual-specificity tyrosine phosphorylated and regulated kinase 1A (DYRK1A) is located in the Down syndrome (DS) critical region of chromosome 21. The third copy of DYRK1A is believed to contribute to abnormal brain development in patients with DS. In vitro studies showing that DYRK1A phosphorylates tau protein suggest that this kinase is also involved in tau protein phosphorylation in the human brain and contributes to neurofibrillary degeneration, and that this contribution might be enhanced in patients with DS. To explore this hypothesis, the brain tissue from 57 subjects including 16 control subjects, 21 patients with DS, and 20 patients with sporadic Alzheimer's disease (AD) was examined with two antibodies to the amino-terminus of DYRK1A (7F3 and G-19), as well as two polyclonal antibodies to its carboxy-terminus (X1079 and 324446). Western blots demonstrated higher levels of full-length DYRK1A in the brains of patients with DS when compared to control brains. Immunocytochemistry revealed that DYRK1A accumulates in neurofibrillary tangles (NFTs) in subjects with sporadic AD and in subjects with DS/AD. Overexpression of DYRK1A in patients with DS was associated with an increase in DYRK1A-positive NFTs in a gene dosage-dependent manner. Results support the hypothesis that overexpressed DYRK1A contributes to neurofibrillary degeneration in DS more significantly than in subjects with two copies of the DYRK1A gene and sporadic AD. Immunoreactivity with antibodies against DYRK1A not only in NFTs but also in granules in granulovacuolar degeneration and in corpora amylacea suggests that DYRK1A is involved in all three forms of degeneration and that overexpression of this kinase may contribute to the early onset of these pathologies in DS.

Overexpression of Dyrk1A contributes to neurofibrillary degeneration in Down syndrome

Adults with Down syndrome (DS) develop Alzheimer neurofibrillary degeneration in the brain, but the underlying molecular mechanism is unknown. Here, we report that the presence of an extra copy of the dual-specificity tyrosine-phosphorylated and regulated kinase 1A (Dyrk1A) gene due to trisomy 21 resulted in overexpression of Dyrk1A and elevated kinase activity in DS brain. Dyrk1A phosphorylated tau at several sites, and these sites were hyperphosphorylated in adult DS brains. Phosphorylation of tau by Dyrk1A primed its further phosphorylation by glycogen synthase kinase-3beta (GSK-3beta). Dyrk1A-induced tau phosphorylation inhibited tau's biological activity and promoted its self-aggregation. In Ts65Dn mouse brain, an extra copy of the Dyrk1A gene caused increased expression and activity of Dyrk1A and resulted in increased tau phosphorylation. These findings strongly suggest a novel mechanism by which the overexpression of Dyrk1A in DS brain causes neurofibrillary degeneration via hyperphosphorylating tau.

Dual-specificity tyrosine(Y)-phosphorylation regulated kinase 1A-mediated phosphorylation of amyloid precursor protein: evidence for a functional link between Down syndrome and Alzheimer's disease

Most individuals with Down Syndrome (DS) show an early-onset of Alzheimer's disease (AD), which potentially results from the presence of an extra copy of a segment of chromosome 21. Located on chromosome 21 are the genes that encode beta-amyloid (Abeta) precursor protein (APP ), a key protein involved in the pathogenesis of AD, and dual-specificity tyrosine(Y)-phosphorylation regulated kinase 1A (DYRK1A ), a proline-directed protein kinase that plays a critical role in neurodevelopment. Here, we describe a potential mechanism for the regulation of AD pathology in DS brains by DYRK1A-mediated phosphorylation of APP. We show that APP is phosphorylated at Thr668 by DYRK1A in vitro and in mammalian cells. The amounts of phospho-APP and Abeta are increased in the brains of transgenic mice that over-express the human DYRK1A protein. Furthermore, we show that the amounts of phospho-APP as well as those of APP and DYRK1A are elevated in human DS brains. Taken together, these results reveal a potential regulatory link between APP and DYRK1A in DS brains, and suggest that the over-expression of DYRK1A in DS may play a role in accelerating AD pathogenesis through phosphorylation of APP.

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.

Dual-specificity tyrosine phosphorylation-regulated kinase 1A does not require tyrosine phosphorylation for activity in vitro

The dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A) gene is localized in human chromosome 21, and its overexpression has been associated with the learning and memory deficits of Down syndrome. DYRK1A contains a Y319XY321 motif shared by all members of the DYRK protein kinase family. Residue Y321 in the motif is phosphorylated in DYRK1A prepared from Escherichia coli and from eukaryotic cells. It has been proposed that the YXY motif is an equivalent of the TXY motif, the activation loop, of mitogen-activated protein kinase and that phosphorylation at the motif is required for DYRK activity. In this study, the role of tyrosine phosphorylation in the activity of DYRK1A was investigated in detail. Wild-type DYRK1A with a reduced level of phosphotyrosine (pY) was prepared by treating E. coli-produced DYRK1A with two different protein tyrosine phosphatases. The resulting pY-depleted DYRK1A could not regain pY during autophosphorylation but was as active as the untreated control. These findings were further supported by the observation that DYRK1A retained significant enzymatic activity when both tyrosine residues in the YXY motif were replaced with either histidine or glutamine. Together, we conclude that tyrosine phosphorylation and tyrosine residues in the YXY motif are not directly involved in DYRK1A enzymatic activity in vitro.

DYRK1A autophosphorylation on serine residue 520 modulates its kinase activity via 14-3-3 binding

Dual-specificity tyrosine-phosphorylated and regulated kinase (DYRK) proteins are an evolutionarily conserved family of protein kinases, with members identified from yeast to humans, that participate in a variety of cellular processes. DYRKs are serine/threonine protein kinases that are activated by autophosphorylation on a tyrosine residue in the activation loop. The family member DYRK1A has been shown to phosphorylate several cytosolic proteins and a number of splicing and transcription factors, including members of the nuclear factor of activated T cells family. In the present study, we show that DYRK1A autophosphorylates, via an intramolecular mechanism, on Ser-520, in the PEST domain of the protein. We also show that phosphorylation of this residue, which we show is subjected to dynamic changes in vivo, mediates the interaction of DYRK1A with 14-3-3beta. A second 14-3-3 binding site is present within the N-terminal of the protein. In the context of the DYRK1A molecule, neither site can act independently of the other. Bacterially produced DYRK1A and the mutant DYRK1A/S520A have similar kinase activities, suggesting that Ser-520 phosphorylation does not affect the intrinsic kinase activity on its own. Instead, we demonstrate that this phosphorylation allows the binding of 14-3-3beta, which in turn stimulates the catalytic activity of DYRK1A. These findings provide evidence for a novel mechanism for the regulation of DYRK1A kinase activity.

Trisomy-driven overexpression of DYRK1A kinase in the brain of subjects with Down syndrome

Down syndrome (DS) is the most common genetic disorder associated with mental retardation (MR). It is believed that many of the phenotypic features of DS stem from enhanced expression of a set of genes located within the triplicated region on chromosome 21. Among those genes is DYRK1A encoding dual-specificity proline-directed serine/treonine kinase, which, as documented by animal studies, can potentially contribute to cognitive deficits in DS. Whether this contribution can be exerted through elevated levels of DYRK1A protein in the brain of DS subjects was the main goal of the present study. The levels of DYRK1A protein were measured by Western blotting in six brain structures that included cerebral and cerebellar cortices and white matter. The study involved large cohorts of DS subjects and age-matched controls representing infants and adults of different age, gender and ethnicity. Trisomic Ts65Dn mice, an animal model of DS, were also included in the study. Both in trisomic mice and in DS subjects, the brain levels of DYRK1A protein were increased approximately 1.5-fold, indicating that this protein is overexpressed in gene dosage-dependent manner. The exception was an infant group, in which there was no enhancement suggesting the existence of a developmentally regulated mechanism. We found DYRK1A to be present in every analyzed structure irrespective of age. This widespread occurrence and constitutive expression of DYRK1A in adult brain suggest an important, but diverse from developmental role played by this kinase in adult central nervous system. It also implies that overexpression of DYRK1A in DS may be potentially relevant to MR status of these individuals during their entire life span.

MNB/DYRK1A phosphorylation regulates the interactions of synaptojanin 1 with endocytic accessory proteins

MNB/DYRK1A is a proline-directed serine/threonine kinase implicated in Down syndrome (DS). In an earlier screening, two proteins from adult rat brain, one 100kDa and the other 140 kDa, were found to be prominently phosphorylated by the kinase. The 100-kDa protein was previously characterized as an isoform of dynamin 1. In this study, we identified the 140-kDa protein as synaptojanin 1 (SJ1). MNB/DYRK1A phosphorylates SJ1 at multiple sites and produces complex behaviors in binding to amphiphysin 1 and intersectin 1 (ITSN1). However, the phosphorylation has little effect on the phosphatidylinositol phosphatase activity of SJ1. These results suggest that MNB/DYRK1A is involved in regulating the recruitment activity but not the phosphatase activity of SJ1. Our findings may be especially important in the etiology of DS because MNB/DYRK1A, SJ1, and ITSN1 are all located at or near the region of human chromosome 21, which is postulated to be involved in the disease.

Kinetic properties of a MNB/DYRK1A mutant suitable for the elucidation of biochemical pathways

Minibrain kinase/dual-specificity tyrosine phosphorylation regulated kinase 1A (MNB/DYRK1A) is a proline/arginine-directed serine/threonine kinase implicated in the learning deficits of Down syndrome. Epigallocatechin-3-gallate (EGCG), the major tea polyphenolic compound, is a potent MNB/DYRK1A inhibitor. In this study, we investigated the mechanism of EGCG inhibition of MNB/DYRK1A using a combination of genetic and biochemical approaches. In the testing system using MNB/DYRK1A-promoted Gli 1-dependent transcription as the readout, NIH3T3 cells expressing EGCG resistant MNB/DYRK1A mutant R21 were found to acquire EGCG resistance for a wide range of drug concentrations. Mutant R21 harbors a single K465R substitution, which produces a 3-fold gain in the EGCG resistance in vitro. However, the gain in the EGCG resistance alone cannot fully interpret the effectiveness of mutant R21 in suppressing EGCG in cultured cells. Kinetic analysis suggests that EGCG functions as a noncompetitive inhibitor against ATP. Interestingly, the K465R mutation changes the mode of EGCG inhibition on MNB/DYRK1A so that it becomes a competitive inhibitor against ATP. This competitive mode of EGCG inhibition coupled with high intracellular ATP concentrations and an elevated EGCG resistance are likely to be the basis for the resistant property of mutant R21 in cultured cells. The K465R mutation apparently transforms the intramolecular interactions required for MNB/DYRK1A catalysis. This mutant would also be valuable for the elucidation of the mechanisms of MNB/DYRK1A-catalyzed reaction.

dDYRK2 and Minibrain interact with the chromatin remodelling factors SNR1 and TRX

The DYRKs (dual specificity tyrosine phosphorylation-regulated kinases) are a conserved family of protein kinases that autophosphorylate a tyrosine residue in their activation loop by an intra-molecular mechanism and phosphorylate exogenous substrates on serine/threonine residues. Little is known about the identity of true substrates for DYRK family members and their binding partners. To address this question, we used full-length dDYRK2 (Drosophila DYRK2) as bait in a yeast two-hybrid screen of a Drosophila embryo cDNA library. Of 14 independent dDYRK2 interacting clones identified, three were derived from the chromatin remodelling factor, SNR1 (Snf5-related 1), and three from the essential chromatin component, TRX (trithorax). The association of dDYRK2 with SNR1 and TRX was confirmed by co-immunoprecipitation studies. Deletion analysis showed that the C-terminus of dDYRK2 modulated the interaction with SNR1 and TRX. DYRK family member MNB (Minibrain) was also found to co-precipitate with SNR1 and TRX, associations that did not require the C-terminus of the molecule. dDYRK2 and MNB were also found to phosphorylate SNR1 at Thr102 in vitro and in vivo. This phosphorylation required the highly conserved DH-box (DYRK homology box) of dDYRK2, whereas the DH-box was not essential for phosphorylation by MNB. This is the first instance of phosphorylation of SNR1 or any of its homologues and implicates the DYRK family of kinases with a role in chromatin remodelling.