Abstracts

RATIONALE: New neurons are generated in the adult mammalian hippocampus at low levels under baseline conditions and with increased frequency after status epilepticus. Little is known about the nature of the precursors responsible for neo-neuronogenesis. We attempted to identify precursor cells in cultures of embryonic rat hippocampus that might give rise to the new neurons and/or other hippocampal cell types.
METHODS: Cultures of dissociated rat hippocampal neurons were grown in serum free conditions in the presence of platelet-derived growth factor-beta (PDGF-B). After 3 weeks, the cultures were exposed either to brain morphogenetic protein (BMP) to stimulate astrocytic differentiation, or tri-iodthyronine (T3) to stimulate oligodendrocytic differentiation or brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF) or retinoic acid (RA) to stimulate neuronal differentiation.
RESULTS: After 3 weeks of culture, a population of PDGF-responsive, small, dark, multipolar cells appeared in the cultures. Similar cells could be seen in much lower numbers, and later, in cultures not treated with PDGF-B. These hippocampal precursor cells (HPCs) did not stain for markers of mature oligodendrocytes (ODC), astrocytes (AC) or neurons, but were stained with markers of glial precursor cells (A2B5, PSA-NCAM, NG2). After exposure to BMP, about 30% of these HPCs differentiated into AC. Similiarly, addition of T3 to the culture media led to the differentiation into mature ODC of about 30% of the HPCs. Exposure to BDNF, FGF or RA (three agents that have previously been shown to induce neuronal differentiation in neuronal stem cells) did not induce the differentiation of these HPCs into neurons. However, subsequent studies revealed that injuring mature neurons in the culture induced neuronal differentiation in these cells, although neither the specific factor involved nor the underlying mechanism of this induced neuronal differentiation has yet been identified.
CONCLUSIONS: In 3 week-old dissociated rat hippocampal cultures, a group of previously undescribed HPCs displays the ability to differentiate into ACs, ODCs, and neurons, depending on the stimulus applied. Neuronal differentiation was not induced by application of traditional neuronal differentiating factors, although could eventually be achieved by a novel mechanism. At the end of this activity, participants will be able to recognize an important new cell type in hippocampal cultures that may play a role in epileptogenesis.
[Supported by: NS 24260 (MAD)]