ABSTRACT
We show here the role of retinoic acid receptor (RAR) beta and alpha signalling in proliferation and differentiation of endogenous adult forebrain neural progenitor cells (NPCs). RARbeta activation stimulates Sonic hedgehog signalling (Shh), and induces the proliferation of the NPCs. They can be induced to become Doublecortin (DCX) expressing migrating neuroblasts by RARalpha signalling, some of which differentiate into cholinergic neurons. The same signalling pathways cause the proliferation of embryonic forebrain NPCs. These cells express glial fibrillary acidic protein (GFAP) and are predominantly uni/bipolar, two characteristics of neuronal progenitor cells. We further show that fibroblast growth factor (FGF) signalling, induces the expression of the retinoic acid degrading enzyme cytochrome P450 (cyp) 26a1, and that one of its products, 4-oxo-RA, mimics the action of the RARalpha agonist in the differentiation of the NPCs into cholinergic neurons.
Subject(s)
Fibroblast Growth Factors/metabolism , Hedgehog Proteins/metabolism , Neurons/physiology , Receptors, Retinoic Acid/metabolism , Signal Transduction/physiology , Stem Cells/physiology , Animals , Cell Differentiation/physiology , Cells, Cultured , Doublecortin Domain Proteins , Doublecortin Protein , Enzyme Inhibitors/metabolism , Fibroblast Growth Factors/genetics , Hedgehog Proteins/genetics , Imidazoles/metabolism , Microtubule-Associated Proteins/genetics , Microtubule-Associated Proteins/metabolism , Neurons/cytology , Neuropeptides/genetics , Neuropeptides/metabolism , Prosencephalon/cytology , Pyrroles/metabolism , Rats , Rats, Wistar , Receptors, Retinoic Acid/agonists , Receptors, Retinoic Acid/genetics , Retinoic Acid Receptor alpha , Stem Cells/cytology , Tretinoin/analogs & derivatives , Tretinoin/chemistry , Tretinoin/metabolismABSTRACT
By culturing neural progenitor cells in the presence of retinoid receptor agonists, we have defined the components of the retinoid-signalling pathway that are important for the birth and maintenance of neuronal cells. We provide evidence that depending on the order and combination of retinoid receptors activated, different neuronal cells are obtained. Astrocytes and oligodendrocytes are predominantly formed in the presence of activated retinoic acid receptor (RAR) alpha, whereas motoneurons are formed when RARbeta is activated. We have looked at the regulation of two transcription factors islet-1/2 which are involved in neuronal development. We find that activated RARbeta up-regulates islet-1 expression, whereas activation of RARalpha can either act in combination with RARbeta signalling to maintain islet-1 expression or induce islet-2 expression in the absence of activated RARbeta. RARgamma cannot directly regulate islet-1/2 but can down-regulate RARbeta expression, which results in loss of islet-1 expression. We finally show that activated RARalpha is one of the final steps required for a mature motoneuron phenotype.
