Astrocytic Alterations and Dysfunction in Down Syndrome: Focus on Neurogenesis, Synaptogenesis, and Neural Circuits Formation.

Down syndrome (DS) is characterized by severe neurodevelopmental alterations that ultimately lead to the typical hallmark of DS: intellectual disability. In the DS brain, since the prenatal life stages, the number of astrocytes is disproportional compared to the healthy brain. This increase is due to a shift from neuron to astrocyte differentiation during brain development. Astrocytes are involved in numerous functions during brain development, including balancing pro-neurogenic and pro-gliogenic stimuli, sustaining synapse formation, regulating excitatory/inhibitory signal equilibrium, and supporting the maintenance and integration of functional neural circuits. The enhanced number of astrocytes in the brain of DS individuals leads to detrimental consequences for brain development. This review summarizes the mechanisms underlying astrocytic dysfunction in DS, and particularly the dysregulation of key signaling pathways, which promote astrogliogenesis at the expense of neurogenesis. It further examines the implications of astrocytic alterations on dendritic branching, spinogenesis and synaptogenesis, and the impact of the abnormal astrocytic number in neural excitability and in the maintenance of the inhibitory/excitatory balance. Identifying deregulated pathways and the consequences of astrocytic alterations in early DS brain development may help in identifying new therapeutic targets, with the ultimate aim of ameliorating the cognitive disability that affects individuals with DS.