Adult Hippocampal Neurogenesis Of The APPswe/PS1dE9 Mouse Model Of Alzheimer Disease And Study For The Mechanisms Of The Local Microenvironmental Modulation

Alzheimer's disease (AD), the most common neurodegenerative disorder and cause of dementia and memory loss in the elderly, is neuropathologically marked by the extracellular deposits of beta amyloid (Aβ) plaques,and the intracellular neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau protein. Neurologically, it is initially manifested as a series of mild cognitive impairments, deficits in short-term memory, loss of spatial memory, and emotional imbalances.Several lines of evidence have suggested that the abnormal accumulation of Aβoligomers at nerve terminals in AD can lead to the synaptic damage and ultimately to neurodegeneration. More recent studies have uncovered evidence that neurodegenerative processes taking place in AD may include interference with neurogenesis in the hippocampal formation. The hippocampus is regarded as one of the most important region associated with the learning and memory,and also as the major damaged region in AD.So,the intensive investigation of the pathogenesis in the hippocampus of AD would help us to know more about AD.Adult hippocampus neurogenesis has been confirmed in birds, reptiles, rodents, and primates, including humans. Adult neurogenesis has consistently been found in the subventricular zone (SVZ) of the lateral ventricle and in the hippocampal subgranular zone. Throughout adulthood, stem cells in the subgranular zone (SGZ) of the dentate gyrus generate neuroblasts that migrate into the inner granule cell layer and differentiate into new granule cells. A critical feature of adult neurogenesis is that stem cell proliferation and cell migration are spatially restricted to a specific permissive environment . Previous reports have suggested that perturbations of neurogenesis can lead to functional changes in the hippocampus. Such findings have focused attention on the status of hippocampal neurogenesis in individuals with AD. Some studies have reported a decline in neurogenesis in AD brain, while others have reported increased neurogenesis; no clear consensus has yet emerged. Moreover, in transgenic animal models of AD, clear evidence has been presented of perturbed neurogenesis with both decreases and increases being reported.The differences in the human population of AD patients,the difference in expression of the AD-related gene between the family early-onset AD and dispersed AD,the differences in the genetic background of the mouse model of AD,all these differences might account for these contradictory results mentioned above.Much effort has been made on the molecular mechanisms that regulate adult hippocampal neurogenesis in recent years. Stem cell self-renewal and progenitor differentiation is regulated by the specialized microenviroment-or"niche"-in which these cells reside. Such niches are composed of soluble factors as well as membrane bound molecules and extracellular matrix (ECM). Adult SVZ cells themselves produce bone morphogenetic proteins (BMPs) and their receptors. Noggin, a secreted BMP antagonist, is strongly expressed in ependymal cells. Noggin encodes a BMP antagonist that binds with high affinity to BMP2/4; it is thought to bind to other BMPs but with lower affinity. Noggin binding to BMPs inhibits their activation of BMP receptors. It has been demonstrated that noggin antagonizes BMP4 to create a niche for subventricular zone neurogenesis.Precise control of BMP4 signaling in the extracellular space appears to play a critical role in multiple events during development, including neural induction,tissue patterning, epithelial- mesenchymal interactions, and in the creation of stem cell "niches" in developing and adult organs. BMPs and Noggin are known to play important roles in the brain, including the regulation of neural induction and of the stem cell"niche"in adult organs. Previous reports showed that both Noggin and BMP4 are expressed in the proliferative SGZ of postnatal rats; an antisense Noggin oligodeoxynucleotide was found to decrease cell proliferation in the dentate gyrus (DG) of adult rats. Increased BMP4 expression in the hippocampal DG might therefore contribute to the decrease in cell proliferation in the DG of the APP/PS1 transgenic mouse model. Although experimental evidence demonstrates that BMP can modulate dentate neurogenesis under pathological conditions, little is known of the role that Noggin might play in this process. On the other hand,the Aβand activated astrocyte in the senile plaques, also act as the important composition of the microenvironment in the brain of AD.Whether and how are involved in the regulation of neural stem cells in the hippocampus? So,we hypothesis that the BMP4/Noggin,the Aβand astrocytes are involved in the modulation of the adult hippocampal neurogenesis of AD at the molecular and cellular levels.To test this hypothesis , the experiments were designed as followed:1. Decreased adult neurogenesis in hippocampus of the APPswe/PS1dE9 mouse model of Alzheimer's disease.BrdU labeled technique is applied to observe the status of the adult hippocampal neurogenesis in the APPswe/PS1dE9 mouse model of AD. Then, the capability of spatial learning and memory was tested by the Morris water maze,and the classical pathological changes were also tested.All these research were designed to explore the relationship among the adult hippocampal neurogenesis, the behavior and the pathogenesis.The data showed that adult hippocampal neurogenesis is decreased.The brain of this AD mouse is defined as the Aβdeposition and activated astrocyte,while no neural loss was observed yet.The mouse also showed a deficit of the spatial recognition. These results implied an intensive correlation among the abnormal adult neurogenesis and the progressive deposition of Aβpalques,and the possibility of the Aβpeptides involved the abnormal adult hippocampal neurogenesis.2. Involvement of BMP4 and Noggin in the regulation of the adult hippocampal neurogenesis in the APPswe/PS1dE9 mouse model of AD.The in situ hybridization(ISH) and immonuhistochemistry are used to examine that expression of BMP4 and Noggin in hippocampal DG region at the mRNA and protein levels respectively,and the Noggin/Fc chimera were injected into the lateral ventricle to investigate the role of the Noggin played in the proliferation and differentiation of the neural stem cells(NSCs) in the DG of hippocampus. We planed to explore the possible relationship between the expression of BMP4/Noggin and the adult hippocampal neurogenesis. The results showed that the higher expression of BMP4 and lower expression of Noggin are proved to be age dependent,and obviously to be correlated with each other. We firstly reported that there existed a significant positive correlation between the higher expression of BMP4 and the decreased expression of the adult neurogenesis,and also a significant negative correlation between the lower expression of Noggin and the decreased expression of the adult neurogenesis.The application of the antagonism of the Noggin to BMP4 would promote the proliferation and differentiation of the hippocampal NSCs.These might imply that BMP4 and Noggin might co-modulate the adult neurogenesis.We isolated and purified the hippocampal NSCs of mouse and added different dosage of the BMP4 into the cultural medium of NSCs to observe the influence of BMP4 on the proliferation and differentiation of the hippocampal NSCs.The data showed that the BMP4 inhibited the proliferation and differentiation of the hippocampal NSCs,and inhibited its differentiation in neurons. The Noggin- recombinated Adenovirus was used to infect the NSCs to prove the effect of Noggin on the proliferation and differentiation of the hippocampal NSCs.Results showed that Noggin could effectively promote the proliferation and differentiation of the hippocampal NSCs,and help the induction of neuronal differentiation.3. Involvement of the Aβand astrocyte in the modulation of the proliferation and differentiation of the hippocampal NSCs.The mouse primary astrocytes were isolated and co-cultured with astrocyte in Transwell system.The aged Aβ1-42 peptides were also used to test the induced migration of the mouse NSCs and the proliferation,neural differentiation of NSCs.The in vitro study demonstrates that the aged Aβ1-42 peptides could induce the significant migration of hippocampal NSCs compared with the NS control group,and the astrocyte combined with the aged Aβ1-42 peptides could promote the percentage of the S phase in the cell cycle. The flow cytometry(FCM) also showed more proliferative NSCs,and enhance the differentiation of NSCs in vitro,although the NSCs-drived astroglia accounted for most of the new born cells. We might hypothesis that the Aβand astrocyte might play a positive role in the regulation of the proliferation,neural differentiation of NSCs under early pathological process of AD.In conclusion, the present study indicates the adult hippocampal neurogenesis in the APPswe/PS1dE9 mouse model of AD is decreased,and this status is significantly associated with the abnormal expression of BMP4 and Noggin in the hippocampal DG region.The in vitro study also proved that the aged- Aβand astrocyte might activate the proliferation and differentiation of NSCs..All results demonstrated that the decreased adult hippocampal neurogenesis might be responsible for the development of pathogenesis of AD.The microenvironmental factors are also involved in the regulation of the adult neurogenesis in the neural stem cell niche. Our study support us with more knowledge about the possible mechanism of the adult hippocampal neurogenesis in AD,and also suggest a prospect for the endogenous NSCs therapy for AD in future.