The Roles Of Reelin And Notch Signaling Pathways In The Nervous System Development And The Pathogenesis Of Alzheimer's Disease

Reelin is a glycoprotein, which mainly is involved in neural migration and cortical lamination in development. Reelin also has important roles to adjust neuronal and synaptic plasticity in adulthood. Recent studies have showed that Reelin can be engaged in the pathogenesis of Alzheimer's disease through enhancing the synaptic plasticity and inhibiting Tau protein phosphorylation. Notch signaling pathway is a highly conserved mechanism in biological evolution, which can regulate cell proliferation and differentiation and determine cell fate during neural development. Notch receptors also play important roles in neural development and neural pathogenesis as well. ?-secretase is necessary in both the activation of Notch signaling pathway and the hydrolysis of amyloid-? precursor protein(APP), suggesting that Notch signaling pathway may play an important role in the pathogenesis of Alzheimer's disease(AD). Recently, relationship between the notch pathway and AD pathogenesis has caused widespread concern gradually, and it becomes a hot topic in neuroscience.Objective: With APP/PS1 double transgenic and reeler mice, we tried to investigate the effects of Reelin and Notch pathway in neural development and the pathogenesis of Alzheimer's disease. Our study will offer us more knowledge about the nervous system development and the pathogenesis of AD.Methods: Immunoctytochemistry, Western blot, DiI diolistic assay, transmission electron microscopy and Golgi staining were used to investigate the Reelin's functions in neural development, synaptic aging, and AD pathogenesis.Results:(1) Reeler mice were used to study the functions of Reelin on neuronal migration in cerebellum. We found that Reelin was expressed mainly in granule neuron precursors in external granular layer(ECL) and granule cells in internal granular layer(IGL). In reeler mice, the cerebellum was usually atrophy without cortical sulcus. In addition, compared with WT mice, Sox2-positive stem cells in reeler mice migrated into internal granular layer(IGL) with time-lag and scattered distribution in IGL and PCL. On the other hand, Purkinje cells could not reach their destination and accumulated in internal granular layer as well. We also found that reeler mice have reduced expression level of Notch1 receptors.(2) In the study, the newborn, youth and aged mice at various ages were used to observe the synaptic morphology changes in cerebellar cortex during the mouse development and aging. In early development, the density of dendritic spines and synaptic buttons increased fast, and the synapses matured as well, for instance, under electron microscope, the synaptic vesicles and mitochondia increased in presynaptic terminal, and the thick specialized zones appeared in pre- and post-synaptic membranes. The number of synapses and the morphological structures were gradually stable in adulthood. With synaptic aging, the dendritic spines and synaptic buttons lost, and the synaptic ultrastuctures altered as well, such as the reduction of synaptic vesicles and mitochondia in presynaptic terminal and the thin specialized zones in pre- and post-synaptic membranes.(3) APP/PS1 double transgenic mice were used as AD model to observe the pathological changes, and abnormal expressions of Reelin and Notch were also investigated as well. We found that the pathological changes occurred after P6M(postnatal 6 months) in the hippocampus and neocortex of AD mice, such as amyloid plaques and neurofibrillary tangles. In addition, we found that astrocytes and microglia usually clustered around amyloid plaques in AD mice. With age increasing, Reelin also accumulated around amyloid plaques and form amyloid-like deposits. In AD mice, the expression of Notch receptor with both full-length Notch1 and Notch1 intracellular domain decreased. DNA methylation decreased in AD hippocampus and neocortex. Especially in amyloid plaques, the DNA methylation almost disappeared, and the expression of Dnmt3 a and Dnmt1 were decreased in AD mice.Conclusion:(1) Reelin plays an important role in the development of radial glial cells(Bergmann cells), cell migration and neural proliferation. In addition, Notch1 signaling pathway is involved in cerebellar development, via the regulation to Reelin pathway.(2) The density of dendritic spines and synaptic buttons increases with development, and then decreases with aging. Aging synapses can disorder synaptic morphology and function, and it is probably an important pathologic cause of neural degenerative diseases.(3) The amyloid plaques in AD mice can induce the cluster of astrocytes and microglia and the accumulation of Reelin. The abnormal expression of Notch receptor and DNA methyltransferase probably are the causes of the neural dysfunction in AD.