Estudio de la neurogénesis adulta en un modelo murino de sobreexpresión condicional de la glucógeno sintasa quinasa-3 beta

Resumen

Adult neurogenesis in dentate gyrus (DG), a process of generating functional neurons from adult neural precursors, has been related to memory and learning processes. On the other hand, glycogen synthase kinase-3 (GSK-3) has been also related to memory and more recently to adult neurogenesis. Here, using a conditional transgenic model overexpressing GSK-3ß (Tet/ GSK-3ß model), we have demonstrate that DG atrophy observed in this animal model could be due in part to alterations in adult neurogenesis. These alterations mainly affect differentiation/ maturation process, causing a delay in time pattern expression of doublecortin (DCX) marker, alterations in time pattern expression of NeuN marker and changes in dendritic morphology of DCX-positive cells. We have also observed a decrease in nestin positive cells, considered as neuronal precursor cells and an increase in active microglia cells as well as changes in levels of some cytokine molecules like eotaxin, fractalkine, GM-CSF and IFN-¿. All these findings suggest that cellular environment would become detrimental contributing to the impairment of adult neurogenesis process observed in GSK-3ß overexpressing mice. Here, we also show that shutdown of GSK-3ß overexpression after doxycicline treatment leads to normal microglia and normal DCX-positive cells density, although depletion of precursor cells is a non reversible phenomenon. In another part of this work, we have tested if dorsal and ventral hippocampal regions respond in a different manner to increased GSK3ß levels. We have found reactive astrocytosis, increased neuronal death and a significant reduction in granular cell layer volume and total cell number in dorsal region but not in the ventral one. Biochemistry analysis have shown higher levels of phosphorylated GSK3ß in serine-9 that inactivate the enzyme in ventral region compared to dorsal area suggesting that the observed susceptibility is in part due to different GSK3 regulation. Previous studies have demonstrated impairment in Morris Water Maze and Object recognition tests pointing out to dorsal hippocampal atrophy. Here, we have observed that there are no changes in two tests used to evaluate emotional status, the light¿dark box and the novelty suppressed feeding test, suggesting that GSK3ß mice do not show any anxiety-related disorder. Thus, our results demonstrate that in vivo overexpression of GSK3ß results in dorsal but not ventral hippocampal DG neurodegeneration and suggest that both areas do not behave in a similar manner in neurodegenerative processes. Finally, we here investigated whether tau protein, a main substrate of GSK-3, plays a role in adult neurogenesis. First we demonstrate that new neurons generated in the subgranular zone (SGZ) express tau in a hyperphosphorylated form. Phospho-tau expression colocalized with DCX but not with GFAP, Ki67 or calbindin. The same was observed in the subventricular zone (SVZ). Tau knockout mice did not show a significant decrease in the number of DCX-positive cells, although an alteration of pattern localization of the marker was observed. These findings suggest that basal tau phosphorylation present in adult animals is in part due to neurogenesis and from Tau knockout mice it seems that tau could be involved in normal migration of new neurons.