The function and regulation of the bHLH gene, cato, in Drosophila neurogenesis
Lage and Jarman BMC Developmental Biology
The function and regulation of the bHLH gene, cato, in Drosophila neurogenesis
Petra I zur Lage 0
Andrew P Jarman 0
0 Centre for Integrative Physiology, School of Biomedical Sciences, University of Edinburgh , Edinburgh EH8 9XD , UK
Background: bHLH transcription factors play many roles in neural development. cousin of atonal (cato) encodes one such factor that is expressed widely in the developing sensory nervous system of Drosophila. However, nothing definitive was known of its function owing to the lack of specific mutations. Results: We characterised the expression pattern of cato in detail using newly raised antibodies and GFP reporter gene constructs. Expression is predominantly in sensory lineages that depend on the atonal and amos proneural genes. In lineages that depend on the scute proneural gene, cato is expressed later and seems to be particularly associated with the type II neurons. Consistent with this, we find evidence that cato is a direct target gene of Atonal and Amos, but not of Scute. We generated two specific mutations of cato. Mutant embryos show several defects in chordotonal sensory lineages, most notably the duplication of the sensory neuron, which appears to be caused by an extra cell division. In addition, we show that cato is required to form the single chordotonal organ that persists in atonal mutant embryos. Conclusions: We conclude that although widely expressed in the developing PNS, cato is expressed and regulated very differently in different sensory lineages. Mutant phenotypes correlate with cato's major expression in the chordotonal sensory lineage. In these cells, we propose that it plays roles in sense organ precursor maintenance and/or identity, and in controlling the number of cell divisions in the neuronal branch of the lineage arising from these precursors.
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Background
Basic-helix-loop-helix (bHLH) transcription factors are
central to neurogenesis in metazoans [1]. The most well
known role for such factors in neurogenesis is the
socalled proneural function. This function underlies the
commitment of neuroectodermal cells to a neural fate,
and the term comes originally from the study of
proneural genes in Drosophila. In this organism, proneural
genes include atonal (ato), amos, scute (sc), and achaete
(ac) which are required for the specification of sense
organ precursors (SOPs) of the peripheral nervous
system [2]. In mutations of these genes, specific subsets of
SOPs fail to be formed. For instance, ato is required for
the formation of SOPs of chordotonal (Ch)
proprioceptive sensory organs [3].
Other members of the bHLH protein family are
expressed after neural commitment and play a variety of
roles in neural cells leading up to neural differentiation.
This is particularly apparent in vertebrates, where for
instance the factors NeuroM and NeuroD are required
for neuronal migration and differentiation respectively
[4,5]. In Drosophila, such downstream neural bHLH
factors are represented by asense (ase), cousin of atonal
(cato), deadpan (dpn) and target of poxn (tap). These
genes are related to sc, ato, hairy/E(spl) and neurogenin
respectively. ase, cato and dpn are widely expressed in
developing neurons [6-8], whereas tap expression is
confined to a small subset of sensory neurons [9]. The
functions of these genes are less well known compared
with proneural genes. ase is expressed in all neural
precursors of both the CNS and PNS [6]. Mutations of ase
result in reduced viability but mutant embryos exhibit
only subtle PNS defects [10]. In the larval optic lobes,
ase participates in the control of mitotic activity in
neural precursors [11]. In this process, ase limits
proliferation by antagonising dpn. In turn, dpn antagonises
dacapo (dap) [12-14]. dap encodes a p21
cyclindependent kinase (CDK) inhibitor that is expressed
transiently in cells prior to their terminal cell division in
order to prevent further divisions [12-14].
Unlike ase and dpn, the expression of cato is confined
to the developing PNS, where it was reported to be
expressed in all SOPs and their progeny [7]. The
function of cato is poorly known. Examination of embryos
bearing large deficiencies of the cato region suggested a
role in sensory neuron differentiation [7]. We report
here the generation and analysis of specific cato
mutations. Flies homozygous for cato loss-of-function
mutations are viable. Mutant embryos show no gross
neuronal differentiation defects, but have a defect in cell
proliferation within the Ch sensory lineages.
Combination of cato mutation with those of ato and ase reveals a
second role for cato in the maintenance of Ch SOP fate
or survival.
Results
Expression of Cato differs in Ato/Amos and Sc lineages
It was previously reported that cato mRNA was initially
activated in Ch SOPs, and subsequently it appeared to
be expressed generally in all cells of the sensory PNS
(both ato-dependent Ch cells and sc-dependent External
Sensory (ES (...truncated)