Effect Of Hemopexin On Adult Neurogenesis In The Subventricular Zone-olfactory Bulb Pathway

Neurogenesis is one of the most important issues of neuroscience research. It had longbeen thought that nerve cells have no ability to regenerate in the adult mammalian centralnervous system (CNS). However, recent studies show that there are neural stem cells in theCNS, which mainly distributed in two specific brain regions, the subgranular zone (SGZ) inthe dentate gyrus of the hippocampus and the subventricular zone (SVZ) of the lateralventricles. Neural stem cells migrate for a long distance from anterior subventricular zone(SVZa) to the olfactory bulb (OB) through the rostral migratory stream (RMS), and becomenew interneurons after proliferation and differentiation. The new interneurons then integrateinto local circuitry in the OB to replace the apoptosis or damaged cells. This is called thesubventricular zone-olfactory bulb pathway (SVZ-OB pathway). Neurogenesis in theSVZ-OB pathway continues throughout life in the adult mammalian CNS, so that the pathwayis an excellent model for studing adult neurogenesis. Neurogenesis in the SVZ-OB pathwayplays a key role on the development and maintenance of olfactory system. So, to clarify theunderlying mechenisms of neurogenesis in the pathway is also an important issue in the fieldof olfactory developmental biology. Thus, researches on the SVZ-OB pathway will help toclarify the normal building process of the nervous system, and provide a new theoretical basisfor the treatment of diseases of the nervous system.Neurogenesis in the SVZ-OB pathway is a sophisticated and complex process, which isregulated not only by many intracellular signaling molecules, but also by the adjacent cellsand their expressed extracellular factors. The understanding on the mechanisms of thisprocess is still very limited. Known factors, which can regulate neurogenesis, do not wellexplain the problems how the stem cells maintain its fixed migration patterns, how theydifferentiate into different olfactory interneurons, and how they maintain a fixed quantitativeproportion between the interneuron subtypes, etc.Studies have shown that there are olfactory ensheathing cells (OECs) in the outerglomerular layer and the olfactory nerve layer, which is close to the glomerular layer, of theolfactory bulb. OECs are a specific type of glial cells in the olfactory system, with the abilityto promote the regeneration of olfactory receptor neurons. OECs play an important role in themaintenance of normal structure and function of the olfactory system. The exist of OECs makes the microenvironment in the olfactory system conducive to development andregeneration, which is different from the microenvironment in other part of CNS.Unfortunately, the understanding on the roles that OECs play in the olfactory system duringdevelopment is still limited. Temporal and spatial evidence both hinds that OECs may takepart in the regulation of neurogenesis in SVZ-OB pathway. Our previous study found thatOECs attract SVZa neural progenitors to the olfactory bulb through the secretion of solubleproteins. However, it is still unclear which molecules play the key role in this course, andwhat is the underlying mechanisms. These are the key issues which this project try to clearify.Using a combination of the methods and techniques of cell biology, molecular biologyand morphology, this subject discussed the effect of OEC-secreted protein hemopexin duringneurogenesis in the SVZ-OB pathway. The study consists of four parts: in the first part, byusing two-dimensional electrophoresis and tandem mass spectrometry, we detected OECspecific secreted proteins through a comparative analysis of OEC-secreted proteins andastrocyte-secreted proteins. In the second part, we detected the expression of hemopexin andits receptor LRP1in olfactory system by immunohistochemistry. In the third part, we studiedthe effect of hemopexin on migration, proliferation, apoptosis, and differentiation of SVZastem cell in vitro. Finally, we clarified the function of hemopexin on the process ofneurogenesis in SVZ-OB pathway in vivo using morphological analysis.The results of our studies are as follows:1. Using two-dimensional electrophoresis and tandem mass spectrometry analysis tocompare OEC secreted proteins and astrocyte secreted proteins,48OEC sepcific expressedproteins were found. Protein hemopexin was chosen as the object to be study.2. In the olfactory system of adult mice, hemopexin is highly expressed in cultured andinternal OECs, while its receptor LRP1is expressed in the SVZa neural stem cells and the OBinterneurons.3. By culturing SVZa stem cells in vitro, we found that OEC-secreted protein hemopexinpromoted migration and cytoskeletal reorganization of cultured SVZa stem cells, andaccelerated them to differentiated into neurons and oligodendrocytes, but did not change theirfinal quantitative proportion; and did not affect their proliferation and apoptosis.4. By morphologic analysis of hemopexin-deficient mice, we found that the defect ofhemopexin makes the stem cells in chain migration in the RMS decreased in density, and became loose in arrangement; the mitral cells in OB were perturbed and the quantity of whichdecreased; the subtype differentiation of olfactory interneurons became abnormal inhemopexin-deficient mice.In summary, we found that OEC specific secreted protein hemopexin can not onlypromote the migration of stem cells from SVZa towards the OB and maintain their chainmigration, but also take part in the regulation of differentiation to olfactory interneurons.These data hinds that hemopexin plays an important part during neurogenesis in the SVZ-OBpathway. This research will help us to deeply understand the mechanisms of neurogenesis ofSVZ neural stem cells, and the roles that OECs play in the structure formation and functionregulation of the olfactory system.