Regulatory modules mediating the complex neural expression patterns of the homeobrain gene during Drosophila brain development

Kirsten Hildebrandt,Olga Hartwig,Christine Klöppel,Dieter Kolb,Uwe Walldorf,Jannic Federspiel,Petra Kaspar,India Findji,Fabienne Wittling

doi:10.1186/s41065-021-00218-5

Abstract

BackgroundThe homeobox gene homeobrain (hbn) is located in the 57B region together with two other homeobox genes, Drosophila Retinal homeobox (DRx) and orthopedia (otp). All three genes encode transcription factors with important functions in brain development. Hbn mutants are embryonic lethal and characterized by a reduction in the anterior protocerebrum, including the mushroom bodies, and a loss of the supraoesophageal brain commissure.ResultsIn this study we conducted a detailed expression analysis of Hbn in later developmental stages. In the larval brain, Hbn is expressed in all type II lineages and the optic lobes, including the medulla and lobula plug. The gene is expressed in the cortex of the medulla and the lobula rim in the adult brain. We generated a new hbnKOGal4 enhancer trap strain by reintegrating Gal4 in the hbn locus through gene targeting, which reflects the complete hbn expression during development. Eight different enhancer-Gal4 strains covering 12 kb upstream of hbn, the two large introns and 5 kb downstream of the gene, were established and hbn expression was investigated. We characterized several enhancers that drive expression in specific areas of the brain throughout development, from embryo to the adulthood. Finally, we generated deletions of four of these enhancer regions through gene targeting and analysed their effects on the expression and function of hbn.ConclusionThe complex expression of Hbn in the developing brain is regulated by several specific enhancers within the hbn locus. Each enhancer fragment drives hbn expression in several specific cell lineages, and with largely overlapping patterns, suggesting the presence of shadow enhancers and enhancer redundancy. Specific enhancer deletion strains generated by gene targeting display developmental defects in the brain. This analysis opens an avenue for a deeper analysis of hbn regulatory elements in the future.

Highlights

The homeobox gene homeobrain is located in the 57B region together with two other homeobox genes, Drosophila Retinal homeobox (DRx) and orthopedia
Our analysis shows that Hbn is expressed in all type II lineages, some type I lineages, the mushroom bodies, the medulla, and the lobula plug in the larval brain, and in the medulla and lobula rim of the adult brain
Hbn mutants are embryonic lethal and are characterized by severe defects in the protocerebrum, where the supraesophageal commissure and parts of the anterior protocerebrum, including the mushroom bodies, are lost through apoptosis [26]. These findings revealed an important function of Hbn in embryonic brain development

Summary

Introduction

The homeobox gene homeobrain (hbn) is located in the 57B region together with two other homeobox genes, Drosophila Retinal homeobox (DRx) and orthopedia (otp). The type I and type II lineages build the central brain region of the larval brain hemispheres and are flanked by the optic lobes, whose cells are derived from the optic placodes already present in the embryo and are closely associated with the brain. These cells grow during the larval stages [13] and through complex morphogenetic movements, they build the larval optic lobes with four neuropil structures: the medulla, the lamina and the lobula complex, consisting of the lobula and lobula plug ([14] for review). The central complex is of great interest as an integration centre for motor, sensory, learning and memory activities ([16] for review)

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