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

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

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.

Constitutive Dyrk1A is abnormally expressed in Alzheimer disease, Down syndrome, Pick disease, and related transgenic models.

DYRK1A, dual-specificity tyrosine-regulated kinase 1A, maps to human chromosome 21 within the Down syndrome (DS) critical region. Dyrk1 phosphorylates the human microtubule-associated protein tau at Thr212 in vitro, a residue that is phosphorylated in fetal tau and hyper-phosphorylated in Alzheimer disease (AD) and tauopathies, including Pick disease (PiD). Furthermore, phosphorylation of Thr212 primes tau for phosphorylation by glycogen synthase kinase 3 (GSK-3). The present study examines Dyrk1A in the cerebral cortex of sporadic AD, adult DS with associated AD, and PiD. Increased Dyrk1A immunoreactivity has been found in the cytoplasm and nuclei of scattered neurons of the neocortex, entorhinal cortex, and hippocampus in AD, DS, and PiD. Dyrk1A is found in sarkosyl-insoluble fractions which are enriched in phosphorylated tau in AD brains, thus suggesting a possible association of Dyrk1A with neurofibrillary tangle pathology. Yet, no clear relationship has been observed between tau phosphorylation at Thr212, and GSK-3 and Dyrk1A expression in diseased brains. Transgenic mice bearing a triple tau mutation (G272V, P301L, and R406W) and expressing hyper-phosphoyrylated tau in neurons of the entorhinal cortex, hippocampus, and cerebral neocortex show increased expression of Dyrk1A in individual neurons in the same regions. However, transgenic mice over-expressing Dyrk1A do not show increased phosphorylation of tau at Thr212, thus suggesting that Dyrk1A over-expression does not trigger per se hyper-phosphorylation of tau at Thr212 in vivo. The present observations indicate modifications in the expression of constitutive Dyrk1A in the cytoplasm and nuclei of neurons in various neurodegenerative diseases associated with tau phosphorylation.

Alterations in the phenotype of neocortical pyramidal cells in the Dyrk1A+/- mouse.

The gene encoding the dual-specificity tyrosine-regulated kinase DYRK1A maps to the chromosomal segment HSA21q22.2, which lies within the Down syndrome critical region. The reduction in brain size and behavioral defects observed in mice lacking one copy of the murine homologue Dyrk1A (Dyrk1A+/-) support the idea that this kinase may be involved in monosomy 21 associated mental retardation. However, the structural basis of these behavioral defects remains unclear. In the present work, we have analyzed the microstructure of cortical circuitry in the Dyrk1A+/- mouse and control littermates by intracellular injection of Lucifer Yellow in fixed cortical tissue. We found that labeled pyramidal cells were considerably smaller, less branched and less spinous in the cortex of Dyrk1A+/- mice than in control littermates. These results suggest that Dyrk1A influences the size and complexity of pyramidal cells, and thus their capability to integrate information.

Haploinsufficiency of Dyrk1A in mice leads to specific alterations in the development and regulation of motor activity.

DYRK1A is a protein kinase proposed to be involved in neurogenesis. Gene-targeting disruption of Dyrk1A in mice leads to decreased body and brain size, with no severe disturbance of behavior. In this study, the authors focused on the motor profile of Dyrk1A(+/-) mice. These mice presented impairment of neuromotor development with decreased activity, suggesting a physiological role of Dyrk1A in the maturation of the neuromotor system. In the adult, a marked hypoactivity and alteration of specific motor parameters were detected. These results are in agreement with the significant expression of Dyrk1A in structures related to motor function and support a role of Dyrk1A in the control of motor function.

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

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

Neurobehavioral development of two mouse lines commonly used in transgenic studies.

The present study was aimed at establishing the differences in the neurodevelopmental profile between two F2 lines derived from two F1 hybrid mouse strains (129 x C57BL/6 and C57BL/6 x SJL). The choice of the given strains was based on the frequent use of these mice in transgenic research. For the neurodevelopment phenotyping, we employed a test battery consisting of 23 somatometric, sensorial and motor tests. Significant variations between the strains were established in different functional domains. Some specific delays in the appearance of developmental landmarks were observed in F2 mice derived from crosses of F1 C57BL/6 x 129, whereas they acquired early developmental functions, such as the righting reflex, sooner than C57BL/6 x SJL-derived mice. C57BL/6 x 129 F2 offspring were spontaneously hypoactive, and their poorer motor performance was confirmed by low performance in the negative geotaxis test. However, there were no differences in the general psychomotor development as shown by the good performance in the homing test in both F2 lines. Both strains were susceptible to the handling procedures used, presenting a similar alteration in the response observed in the homing test as compared to nonhandled control mice. In conclusion, our work highlights the importance of the genetic background for transgenesis experiments and also the need for well-established testing protocols to obtain sufficient information at the first stage of behavioral screening of genetically modified mice.