Expression of the transcription factor Isl1 in dopaminergic neurons of the mouse forebrain

The development of the bewildering assortment of neuronal types found in the vertebrate central nervous system (CNS) depends on the distribution of transcription factors and signalling molecules along the embryonic neural tube. The Islet-1 (Isl1) gene, which encodes a transcription factor of the LIM-homeodomain family, is known to be expressed in the nervous system, playing crucial funtions in the differentiation of neuronal populations located in the spinal cord, striatum, hypothalamus and retina. Here, we use immunofluorencence to trace the distribution of Isl1 protein during the development of the mouse forebrain, with an emphasis on the hypothalamic area and its neighbouring regions. Isl1 is abundantly expressed in the subpallium, most of the hypothalamus and in the prethalamus. Interestingly, we found that Isl1 is expressed in most dopaminergic neurons of the forebrain in early development (e10.5, e11.5), as revealed by colabelling with the enzyme tyrosine hydroxylase (TH). At later stages (e18.5) and adulthood, the degree of colocalisation of Isl1 with TH decreases, but the factor is still found in most dopaminergic neurones of the forebrain, in particular of the prethalamic region (A13 group), tuberal hypothalamus (A12), preoptic area (A15) and part of the periventricular area (part of the A14 group). Altogether, our observations indicate that Isl1 is a molecular marker of forebrain dopaminergic groups and might play a role in the development of these neuronal populations.