Math3 and NeuroD in amacrine cell fate specification

A wide variety of neurons as well as glia differentiate from common precursors. The molecular mechanisms involved in generation of such diverse cell types in the developing nervous system are still obscure. Neural retina is an ideal model system with which to investigate the mechanisms for generation of multiple cell types (Harris, 1997; Cepko, 1999), as it has a relatively simple structure, mimics normal development in isolated explant cultures and is therefore easy to analyze (Caffé et al., 1989; Sparrow et al., 1990; Tomita et al., 1996; Morrow et al., 1999). In the neural retina, seven types of cells (six types of neuron and one type of glia) form three cellular layers: the outer nuclear layer (ONL), which contains rod and cone photoreceptors; the inner nuclear layer (INL), which contains bipolar, horizontal and amacrine interneurons, and Müller glia; and the ganglion cell layer (GCL), which contains ganglion and displaced amacrine cells. It has been shown that these seven types of cells differentiate from common precursors under the control of intrinsic cues, such as transcription factors, and extrinsic signals, such as neurotrophic factors (Harris, 1997; Cepko, 1999). Recent studies have demonstrated that basic helix-loop-helix (bHLH)and homeobox-type transcription factors contribute to the intrinsic properties of retinal precursors. (Kageyama and Nakanishi, 1997; Lee, 1997; Cepko, 1999; Livesey and Cepko, 2001). These transcription factors regulate determination and differentiation of multiple cell types. For example, the bHLH gene NeuroD(Neurod1– Mouse Genome Informatics) and the homeobox gene Crx regulate generation of photoreceptors (Furukawa et al., 1997; Furukawa et al., 1999; Freund et al., 1997; Chen et al., 1997; Morrow et al., 1999). Similarly, the bHLH genes Mash1and Math3 (Ascl1and Atoh3, respectively – Mouse Genome Informatics) and the homeobox gene Chx10 are required for specification of bipolar cells (Burmeister et al., 1996; Tomita et al., 2000; Hatakeyama et al., 2001). Interestingly, co-expression of Mash1 or Math3 with Chx10 efficiently induces bipolar cell genesis, whereas misexpression of either Mash1/Math3or Chx10alone cannot (Hatakeyama et al., 2001). These results suggest that combinations of bHLH and homeobox genes may be important for specification of cell types. These transcription factor codes for cell fate specification are, however, only beginning to be elucidated. Amacrine cells, interneurons present in the INL and GCL, make synapses onto bipolar cell terminals and ganglion cell endrites, and modulate the synaptic connection between bipolar and ganglion cells (Kolb, 1997). There are multiple morphologically and functionally distinct subtypes of amacrine ells. Some of them are located in the GCL (displaced amacrine cells), while others are in the inner region of the INL. It has been shown that the bHLH gene N uroDand the pairedtype homeobox gene Pax6 are expressed by differentiating amacrine cells (Jones et al., 1998; Morrow et al., 1999; Nishina et al., 1999). However, mutation for NeuroDor Pax6does not reduce amacrine cell genesis (Morrow et al., 1999; Marquardt et al., 2001) and, therefore, their functions are not yet known. We have found that the bHLH gene Math3is also transiently 831 Development 129, 831-842 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 DEV2780