The Gene Regulatory Cascade Linking Proneural Specification with Differentiation in Drosophila Sensory Neurons

Abstract

Temporal expression profiling of sensory precursor cells reveals how the atonal proneural transcription factor regulates a specialized neuronal differentiation pathway.

Abstract

In neurogenesis, neural cell fate specification is generally triggered by proneural transcription factors. Whilst the role of proneural factors in fate specification is well studied, the link between neural specification and the cellular pathways that ultimately must be activated to construct specialised neurons is usually obscure. High-resolution temporal profiling of gene expression reveals the events downstream of atonal proneural gene function during the development of Drosophila chordotonal (mechanosensory) neurons. Among other findings, this reveals the onset of expression of genes required for construction of the ciliary dendrite, a key specialisation of mechanosensory neurons. We determine that atonal activates this cellular differentiation pathway in several ways. Firstly, atonal directly regulates Rfx, a well-known highly conserved ciliogenesis transcriptional regulator. Unexpectedly, differences in Rfx regulation by proneural factors may underlie variations in ciliary dendrite specialisation in different sensory neuronal lineages. In contrast, fd3F encodes a novel forkhead family transcription factor that is exclusively expressed in differentiating chordotonal neurons. fd3F regulates genes required for specialized aspects of chordotonal dendrite physiology. In addition to these intermediate transcriptional regulators, we show that atonal directly regulates a novel gene, dilatory, that is directly associated with ciliogenesis during neuronal differentiation. Our analysis demonstrates how early cell fate specification factors can regulate structural and physiological differentiation of neuronal cell types. It also suggests a model for how subtype differentiation in different neuronal lineages may be regulated by different proneural factors. In addition, it provides a paradigm for how transcriptional regulation may modulate the ciliogenesis pathway to give rise to structurally and functionally specialised ciliary dendrites.

Author Summary

Early during development, cells differentiate and take on specialized forms and functions. This requires the activation of specific genes for different cellular pathways. Our study addresses how this activation is regulated in the developing Drosophila nervous system. In this model, it is well known that proneural transcription factors are involved in directing cells to differentiate into various types of neurons. However, the mechanism by which they choreograph the activation of genes for neuronal differentiation is not clear. In this study, we focused on events leading to differentiation of mechanosensory neurons, which have specialized dendritic processes that mediate sensory perception. In these developing neurons we profiled the time course of gene expression that is triggered by the proneural factor atonal. Our analysis revealed the activation of genes required for the formation of these specialized dendrites, called cilia. We then identified several ways in which atonal regulated these genes. First, it activates intermediate transcription factors that regulate different subsets of differentiation genes. Second, in at least one case, atonal activates a differentiation gene directly, one that is involved in the formation of cilia (ciliogenesis). These findings offer new insight into how proneural factors regulate specialized neuronal differentiation pathways.