Zur Funktion der Kinase Dyrk1A im Gehirn der adulten Maus

The Drosophila gene minibrain codes for a protein kinase. Mutants
with a reduced expression of minibrain are characterised by a
strongly reduced brain size and learning and memory deficits.
Dyrk1A is the mammalian homologue of minibrain; the encoded protein
is the founding member of the family of Dyrk protein kinases. The
human DYRK1A gene is implicated in the emergence of the cognitive
deficits in Down syndrome, due to its location on chromosome 21 in
the so-called Down syndrome critical region (DSCR), its
overexpression in Down syndrome individuals and also because
Dyrk1A-overexpressing mice show learning and memory deficits. Taken
together, these data point to a gene dosage dependent function of
Dyrk1A/minibrain in processes that are associated with cognitive
functions such as learning and memory. The aim of this study was to
elucidate the function of Dyrk1A in the adult mammalian brain.
Three aproaches were chosen: First, an analysis of the expression
of Dyrk1A and of the intracellular localisation of the respective
protein in the adult mouse brain was performed. Dyrk1A is expressed
in several brain areas, e.g. in the neocortex, in the hippocampal
formation and in the cerebellum. In neurons, Dyrk1A is located
either mostly nuclear, or predominantly or even exclusively in the
cytoplasm, depending on the region and on the type of neuron.
Second, to characterise the gene dosis dependent function of
Dyrk1A, it was intended to generate transgenic mice that express a
third copy of Dyrk1A, as it is the case in human trisomy 21 (Down
syndrome). These transgenic mice should express the third copy of
Dyrk1A in a region- and time-specific manner, e.g. in principal
neurons in the adult forebrain. This should avoid developmental
defects and disturbances of, e.g., the motor system and should
allow for the specific analysis of the role of Dyrk1A in learning
and memory and in brain regions involved in cognitive processes. A
BAC (bacterial artificial chromosome) clone containing the whole
Dyrk1A locus was isolated. This BAC was modified by homologous
recombination in E. coli (GET recombination) to render it suitable
for the generation of transgenic mice. Furthermore, a recombination
cassette for a second modification step was constructed to allow
for the region- and time-specific expression of the Dyrk1A gene on
the BAC. Third, a yeast two-hybrid screen to identify proteins
interacting with Dyrk1A was performed. Two of six clones obtained
were shown to code for proteins interacting specifically with
Dyrk1A: Sept4 (Pnutl2/H5/CDCrel-2), a GTPase of the Septin family,
and Arip4, a steroid hormone receptor cofactor with ATP-dependent
chromatin-remodelling activity. These interactions were confirmed
in mammalian cells by coimmunoprecipitation. By in
situ-hybridisation, the coexpression of Dyrk1A with the genes of
both interactors was shown in various brain regions. In addition,
Dyrk1A and Arip4 were shown to be colocalised in a speckle-like
nuclear subcompartment in primary rat hippocampal neurons and in
mammalian cell lines. These results point to an interaction of
Dyrk1A with Arip4 also in vivo and implicate Dyrk1A in steroid
hormone-mediated signalling. Furthermore, an interaction between
Dyrk1A and Sept5/CDCrel-1, a close relative of Sept4, was shown.
Sept5 is thought to play an inhibitory role in the fusion of
synaptic vesicles with the presynaptic membrane and transmitter
release. The results of this study contribute to the understanding
of the role of Dyrk1A in physiological and pathophysiological
contexts. Eventually, this should help (i) to elucidate the
mechanisms that lead to mental retardation in Down syndrome and
(ii) to develop tools to interfere with the function of Dyrk1A and
to establish novel strategies for a therapy to alleviate mental
retardation in Down syndrome.