Mutations within the Ddc promoter alter its neuron-specific pattern of expression

Dopa decarboxylase: a model gene-enzyme system for studying development, behavior, and systematics

Throughout its long evolutionary history, the Dopa decarboxylase gene (Ddc) has acquired a variety of functions in insects. The enzyme (DDC) catalyzes the production of the neural transmitters dopamine and serotonin. Not surprisingly, evidence of the enzyme's involvement in the behavior of insects is beginning to accumulate. In addition, DDC plays a role in wound healing, parasite defense, pigmentation, and cuticle hardening. A high degree of sequence conservation has allowed comparisons of the Ddc-coding regions from various insects, facilitating a number of recent studies on insect systematics. This review outlines the diverse functions of Ddc and illustrates how studies of this model system address many questions on insect neurobiology, developmental biology, and systematics.

The bHLH protein Dimmed controls neuroendocrine cell differentiation in Drosophila

Neuroendocrine cells are specialized to produce, maintain and release large stores of secretory peptides. We show that the Drosophila dimmed/Mist1 bHLH gene confers such a pro-secretory phenotype on neuroendocrine cells. dimmed is expressed selectively in central and peripheral neuroendocrine cells. In dimmed mutants, these cells survive, and adopt normal cell fates and morphology. However, they display greatly diminished levels of secretory peptide mRNAs, and of diverse peptides and proteins destined for regulated secretion. Secretory peptide levels are lowered even in the presence of artificially high secretory peptide mRNA levels. In addition, overexpression of dimmed in a wild-type background produces a complimentary phenotype: an increase in secretory peptide levels by neuroendocrine cells, and an increase in the number of cells displaying a neuroendocrine phenotype. We propose that dimmed encodes an integral component of a novel mechanism by which diverse neuroendocrine lineages differentiate and maintain the pro-secretory state.

PHM is required for normal developmental transitions and for biosynthesis of secretory peptides in Drosophila

To understand the roles of secretory peptides in developmental signaling, we have studied Drosophila mutant for the gene peptidylglycine alpha-hydroxylating monooxygenase (PHM). PHM is the rate-limiting enzyme for C-terminal alpha-amidation, a specific and necessary modification of secretory peptides. In insects, more than 90% of known or predicted neuropeptides are amidated. PHM mutants lack PHM protein and enzyme activity; most null animals die as late embryos with few morphological defects. Natural and synthetic PHM hypomorphs revealed phenotypes that resembled those of animals with mutations in genes of the ecdysone-inducible regulatory circuit. Animals bearing a strong hypomorphic allele contain no detectable PHM enzymatic activity or protein; approximately 50% hatch and initially display normal behavior, then die as young larvae, often while attempting to molt. PHM mutants were rescued with daily induction of a PHM transgene and complete rescue was seen with induction limited to the first 4 days after egg-laying. The rescued mutant adults produced progeny which survived to various stages up through metamorphosis (synthetic hypomorphs) and displayed prepupal and pupal phenotypes resembling those of ecdysone-response gene mutations. Examination of neuropeptide biosynthesis in PHM mutants revealed specific disruptions: Amidated peptides were largely absent in strong hypomorphs, but peptide precursors, a nonamidated neuropeptide, nonpeptide transmitters, and other peptide biosynthetic enzymes were readily detected. Mutant adults that were produced by a minimal rescue schedule had lowered PHM enzyme levels and reproducibly altered patterns of amidated neuropeptides in the CNS. These deficits were partially reversed within 24 h by a single PHM induction in the adult stage. These genetic results support the hypothesis that secretory peptide signaling is critical for transitions between developmental stages, without strongly affecting morphogenetic events within a stage. Further, they show that PHM is required for peptide alpha-amidating activity throughout the life of Drosophila. Finally, they define novel methods to study neural and endocrine peptide biosynthesis and functions in vivo.

The differentiation of the serotonergic neurons in the Drosophila ventral nerve cord depends on the combined function of the zinc finger proteins Eagle and Huckebein

The Drosophila ventral nerve cord (vNC) derives from a stereotyped population of neural stem cells, neuroblasts (NBs), each of which gives rise to a characteristic cell lineage. The mechanisms leading to the specification and differentiation of these lineages are largely unknown. Here we analyse mechanisms leading to cell differentiation within the NB 7–3 lineage. Analogous to the grasshopper, NB 7–3 is the progenitor of the Drosophila vNC serotonergic neurons. The zinc finger protein Eagle (Eg) is expressed in NB 7–3 just after delamination and is present in all NB 7–3 progeny until late stage 17. DiI cell lineage tracing and immunocytochemistry reveal that eg is required for normal pathfinding of interneuronal projections and for restricting the cell number in the thoracic NB 7–3 lineage. Moreover, eg is required for serotonin expression. Ectopic expression of Eg protein forces specific additional CNS cells to enter the serotonergic differentiation pathway. Like NB 7–3, the progenitor(s) of these ectopic cells express Huckebein (Hkb), another zinc finger protein. However, their progenitors do not express engrailed (en) as opposed to the NB 7–3 lineage, where en acts upstream of eg. We conclude that eg and hkb act in concert to determine serotonergic cell fate, while en is more distantly involved in this process by activating eg expression. Thus, we provide the first functional evidence for a combinatorial code of transcription factors acting early but downstream of segment polarity genes to specify a unique neuronal cell fate.

Aromatic L-amino acid decarboxylase: a neglected and misunderstood enzyme

Classically, aromatic L-amino acid decarboxylase (AADC) has been regarded as an unregulated, rather uninteresting enzyme. In this review, we describe advances made during the past 10 years, demonstrating that AADC is regulated both pre- and post-translation. The significance of such regulatory mechanisms is poorly understood at present, but the presence of tissue specific control of expression raises the real possibility of AADC being involved in processes other than neuro-transmitter synthesis. We further discuss clinical and physiological situations in which such regulatory mechanisms may be important, including the intriguing possibility of AADC gene regulation being linked to that of factors thought to have a role in apoptosis and its prevention.

Immunocytochemical study of choline acetyltransferase in Drosophila melanogaster: an analysis of cis-regulatory regions controlling expression in the brain of cDNA-transformed flies

We have analyzed the cis-regulatory regions in the 5' flanking DNA of the Drosophila melanogaster choline acetyltransferase (ChAT; E.C. 2.3.1.6) gene by using germline transformants. These transformants are carrying wild-type ChAT cDNA fused to different lengths of 5' flanking sequence of the ChAT gene. Appropriate genetic crosses were used to introduce the transgene into animals with a presumptive null genetic background for endogenous ChAT. Expression of ChAT protein could thus be attributed exclusively to the transgene. Using a monoclonal antibody against Drosophila ChAT, we have investigated the spatial distribution of transgenic ChAT and compared it to the normal distribution of ChAT protein in wild-type animals. The brains of 7.4 kb cDNA transformants showed a ChAT expression pattern similar to that of wild-type animals in the first- and second-order sensory neuropil but reduced expression in other highly ordered neuropil. Several lines that were transformed with 1.2 kb or 0.8 kb of 5' flanking DNA demonstrated relatively normal expression in sensory neuropil. In addition, these lines also showed ectopic expression in higher order neuropil. In the optic lobe, the expression pattern directed by 7.4 kb of 5' flanking DNA was very similar to that of wild-type ChAT expression. In contrast, 1.2 kb or 0.8 kb transformants showed reduced levels of expression and a more limited pattern of distribution in the optic lobe. Our results suggest that the 5' flanking DNA of the ChAT gene can be divided into several separable positive and negative regulatory regions, which define various subsets of cholinergic neurons in the nervous system.

Cell type-specific transcriptional regulation of the Drosophila FMRFamide neuropeptide gene

We have used lacZ reporter gene constructs to study the promoter/enhancer regions of the Drosophila FMRFamide neuropeptide gene in germ line transformants. FMRFamide is normally expressed in approximately 60 diverse neurons of the larval CNS that represent approximately 15 distinct cell types. An 8 kb FMRFamide DNA fragment (including 5 kb of 5' upstream sequence) was sufficient to direct a pattern of lacZ expression that mimicked nearly all spatial aspects of the normal pattern. This result indicates that the cell-specific regulation of FMRFamide expression is largely generated by transcriptional mechanisms. Reporter gene expression was lost from selected cell types when smaller fragments were tested, suggesting that multiple control regions are included in the FMRFamide promoter. One region (a 300 bp fragment from -476 to -162) acted as an enhancer for 1 of the approximately 15 FMRFamide-positive cell types, the OL2 neurons. These results suggest that, in the mature nervous system, the complex pattern of FMRFamide neuropeptide gene expression derives from the activity of discrete, cell type-specific enhancers that are independently regulated.

Repression and activation of the Drosophila dopa decarboxylase gene in glia

Glial expression of the Drosophila dopa decarboxylase gene (Ddc) is repressed by a regulatory region located approximately 1 kb upstream of the transcriptional start site. We have used in vitro mutagenesis and germ line transformation to determine which elements within the Ddc promoter mediate repression. Our evidence suggests that the hypodermal cell activator elements IIA and IIB play a major role in the transcriptional regulation of Ddc in glial cells. A variety of mutations demonstrate that element IIA is a strong glial activator element and that element IIB is necessary for glial repression. Although these two regulatory elements are nearly identical in sequence, our data suggest that they are not redundant. Altering the wild-type number and spacing of elements IIA and IIB indicates that the wild-type arrangement of this repeat is critical for repression. We conclude that these key elements of the Ddc promoter regulate both activation and repression in glia.

Repression and activation of the Drosophila dopa decarboxylase gene in glia

Glial expression of the Drosophila dopa decarboxylase gene (Ddc) is repressed by a regulatory region located approximately 1 kb upstream of the transcriptional start site. We have used in vitro mutagenesis and germ line transformation to determine which elements within the Ddc promoter mediate repression. Our evidence suggests that the hypodermal cell activator elements IIA and IIB play a major role in the transcriptional regulation of Ddc in glial cells. A variety of mutations demonstrate that element IIA is a strong glial activator element and that element IIB is necessary for glial repression. Although these two regulatory elements are nearly identical in sequence, our data suggest that they are not redundant. Altering the wild-type number and spacing of elements IIA and IIB indicates that the wild-type arrangement of this repeat is critical for repression. We conclude that these key elements of the Ddc promoter regulate both activation and repression in glia.