Assessment of precursor cell proliferation in normal and injured adult mammalian brains

Abstract

In the adult mammalian brain, new neurons are generated from neural stem cells (NSCs) which reside in the subventricular zone (SVZ) of the lateral ventricle wall and in the subgranular zone (SGZ) of the dentate gyrus (DG) in the hippocampus. The biology of persistent NSCs in the adult brain is poorly understood. The identification of NSCs, as well as knowledge about their response to experimental manipulations, and the mechanisms underlying the response, is essential to understand how neurogenesis is regulated. This thesis aimed to study the characteristics of endogenous stem cells/progenitors in both normal and damaged adult mammalian brain. Several animal species (pig, rat, and rabbit) were used to study the expression of ribonucleotide reductase (RNR) in the neurogenic regions. Seizures were induced by kainic acid (KA) in adult rats and the cell proliferation in the DG was studied. In another injury model, adult rats were injected with 6-OHDA unilaterally, which caused the degeneration of dopaminergic neurons in the substantia nigra. The responses of different types of cells in the SVZ-olfactory bulb (OB) pathway to the lesion were examined. RNR, which is a cytoplasmic enzyme compulsory for DNA synthesis, was expressed in neurogenic regions in several species, and was present together with BrdU in proliferating cells. In the SGZ, differences in proliferating cells between different species were reflected by the cellular morphology and astrocyte-like properties of stem cells. After KA-induced seizures cell proliferation was increased in the adult rat DG, including the number of astrocyte-like putative stem cells and neuronal precursors. Moreover, using a marker for cells undergoing apoptosis we showed that at 7 days after seizures the majority of the newly born cells were programmed to die. Astrocytes in the SVZ and rostral migratory stream were activated after 6-OHDA injection, as reflected by the upregulation of astrocytic marker proteins and a hypertrophic morphology. GFAP-expressing astrocytes in these regions did not appear to constitute a homogenous population, as they responded differently to this brain injury. In parallel, the unilateral 6-OHDA lesion stimulated SVZ-OB cell proliferaton bilaterally but mainly on the ipsilateral side, and increased migration of neuroblasts to the OB. In addition, some neuronal precursors were found in the ipsilateral striatum. In conclusion, RNR may constitute a useful marker to study the dynamics of the stem cells/progenitors in the adult brain. Our findings of an increased number of proliferating cell clusters and astrocyte-like putative stem cells after seizures support the speculation of precursor cell recruitment after brain insults. Astrocytes within the neurogenic regions were activated by the 6-OHDA lesion, and this activation may exert an influence on neurogenesis in the SVZ-OB pathway. The induction of cell proliferation and neurogenesis after 6-OHDA lesion underscores the potential therapeutic use of NSCs as a source for neuronal repair.