DYRK-DOWN: DYRK1A, a dosage sensitive gene at the crossroad of development and brain function for treating Down syndrome – DYRK-DOWN

Previous studies on Down Syndrome (DS) and Mental retardation Disease 7 have shown that any variation in DYRK1A dosage alters the GABAergic inhibitory and glutamatergic excitatory systems, but the mechanisms involved remain to be elucidated. DYRK1A encodes a protein kinase with many substrates involved in signalling pathways and participates to gene transcription, cell survival, differentiation and endocytosis. During development, DYRK1A is expressed in pre-neural and neural precursors, both GABAergic and glutamatergic, then in new and differentiated neurons; later it is found in all types of neurons in the adult brain. Notably, Dyrk1a overdosage (1.5 fold) influences the mode of division and the differentiation of cortical progenitors, thereby regulating the neuronal output in vivo. In addition mice with increased DYRK1A level show an impaired inhibitory/excitatory balance, leading to an overactivated neuronal network.

Reducing DYRK1A kinase activity with specific pharmacological inhibitors restores different level of cognition in DS mouse models in young adult mice and in human. These rescuing experiments are very promising for treating people with DS at early stage of development. Nevertheless more detailed information about the role of DYRK1A during development and brain function is needed for treating Down syndrome with a DYRK1A inhibitor at early stage.

Here we propose to fill this gap and investigate the DYRK1A–regulated mechanisms during neuronal development and in the adult brain of DS models. To this aim, we will take advantage of L41 as a pharmacological tool to identify molecular and cellular events following DYRK1A inhibition both in normal and pathological conditions. Our project will be divided in 3 tasks answering to 3 main questions:

Task1: What are the main targets of DYRK1A kinase in neurons? Various targets of DYKR1A have been identified in different cell types. Some are relevant for neuronal development or function (TAU, APP). Nevertheless here we propose to explore new targets of DYRK1A in neurons using the L41 kinase inhibitor and phosphoproteomic approach.

Task2: How important is DYRK1A activity for brain development? The increased dosage of Dyrk1a affects the development of the glutamatergic projection neurons and the function of GABAergic interneurons. Nevertheless, how and to which extend this is controlled by deregulation of DYRK1A activity is not known. To evaluate the significance of DYRK1A in brain development, we will identify the cellular mechanisms controlled by DYRK1A by comparing 1) the progenitor’s biology (cell cycle and divisions, …) and 2) the neuronal migrations (radial for projection neurons and tangential for interneurons) in different mouse models.

Task3: What are the benefits of modulating DYRK1A kinase activity on the glutamatergic and GABAergic circuits and on cognitive function in adult DS mouse models? We previously showed rescue of specific learning and memory deficits in the Dp1Yey trisomic model a) in which Dyrk1a was specifically inactivated during development of glutamatergic or GABAergic neurons (Dp1Yey; Dyrk1acKO/+) or b) after L41 post-natal treatment in young adult mice; suggesting that overdosage of Dyrk1a is a key determinant of cognitive deficits. We want now to 1) pursue those behavioural analyses in Dp1Yey mice after L41 treatment at pre-natal stage and later to fix the best window of treatment in this preclinical model; 2) analyse changes in specific markers of glutamatergic and GABAergic pathways; 3) directly quantify the effects of DYRK1A activity on the functional interplay between excitation and inhibition in the CA3 hippocampal circuit in vivo, with monosynaptic resolution.

We therefore expect this work to pave the way for the treatment of early and late-stage DS individuals, identifying molecular targets and providing potential preventive and curating pharmacological tools against this devastating neurodevelopmental disease.