Blimp-1 Mediates Tracheal Lumen Maturation in Drosophila melanogaster

Domain analysis of Drosophila Blimp-1 reveals the importance of its repression function and instability in determining pupation timing

The PRDM family transcription repressor Blimp-1 is present in almost all multicellular organisms and plays important roles in various developmental processes. This factor has several conserved motifs among different species, but the function of each motif is unclear. Drosophila Blimp-1 plays an important role in determining pupation timing by acting as an unstable transcriptional repressor of the βftz-f1 gene. Thus, Drosophila provides a good system for analyzing the molecular and biological functions of each region in Blimp-1. Various Blimp-1 mutants carrying deletions at the conserved motifs were induced under the control of the heat shock promoter in prepupae, and the expression patterns of βFTZ-F1 and Blimp-1 and pupation timing were observed. The results showed that the regions with strong and weak repressor functions exist within the proline-rich middle section of the factor and near the N-terminal conserved motif, respectively. Rapid degradation was supported by multiple regions that were mainly located in a large proline-rich region. Results revealed that pupation timing was affected by the repression ability and stability of Blimp-1. This suggests that both the repression function and instability of Blimp-1 are indispensable for the precise determination of pupation timing.

Blimp-1/PRDM1 and Hr3/RORβ specify the blue-sensitive photoreceptor subtype in Drosophila by repressing the hippo pathway

During terminal differentiation of the mammalian retina, transcription factors control binary cell fate decisions that generate functionally distinct subtypes of photoreceptor neurons. For instance, Otx2 and RORβ activate the expression of the transcriptional repressor Blimp-1/PRDM1 that represses bipolar interneuron fate and promotes rod photoreceptor fate. Moreover, Otx2 and Crx promote expression of the nuclear receptor Nrl that promotes rod photoreceptor fate and represses cone photoreceptor fate. Mutations in these four transcription factors cause severe eye diseases such as retinitis pigmentosa. Here, we show that a post-mitotic binary fate decision in Drosophila color photoreceptor subtype specification requires ecdysone signaling and involves orthologs of these transcription factors: Drosophila Blimp-1/PRDM1 and Hr3/RORβ promote blue-sensitive (Rh5) photoreceptor fate and repress green-sensitive (Rh6) photoreceptor fate through the transcriptional repression of warts/LATS, the nexus of the phylogenetically conserved Hippo tumor suppressor pathway. Moreover, we identify a novel interaction between Blimp-1 and warts, whereby Blimp-1 represses a warts intronic enhancer in blue-sensitive photoreceptors and thereby gives rise to specific expression of warts in green-sensitive photoreceptors. Together, these results reveal that conserved transcriptional regulators play key roles in terminal cell fate decisions in both the Drosophila and the mammalian retina, and the mechanistic insights further deepen our understanding of how Hippo pathway signaling is repurposed to control photoreceptor fates for Drosophila color vision.

The Blimp-1 transcription factor acts in non-neuronal cells to regulate terminal differentiation of the Drosophila eye

The formation of a functional organ such as the eye requires specification of the correct cell types and their terminal differentiation into cells with the appropriate morphologies and functions. Here, we show that the zinc-finger transcription factor Blimp-1 acts in secondary and tertiary pigment cells in the Drosophila retina to promote the formation of a bi-convex corneal lens with normal refractive power, and in cone cells to enable complete extension of the photoreceptor rhabdomeres. Blimp-1 expression depends on the hormone ecdysone, and loss of ecdysone signaling causes similar differentiation defects. Timely termination of Blimp-1 expression is also important, as its overexpression in the eye has deleterious effects. Our transcriptomic analysis revealed that Blimp-1 regulates the expression of many structural and secreted proteins in the retina. Blimp-1 may function in part by repressing another transcription factor; Slow border cells is highly upregulated in the absence of Blimp-1, and its overexpression reproduces many of the effects of removing Blimp-1. This work provides insight into the transcriptional networks and cellular interactions that produce the structures necessary for visual function.

Crosstalk between basal extracellular matrix adhesion and building of apical architecture during morphogenesis

Tissues build complex structures like lumens and microvilli to carry out their functions. Most of the mechanisms used to build these structures rely on cells remodelling their apical plasma membranes, which ultimately constitute the specialised compartments. In addition to apical remodelling, these shape changes also depend on the proper attachment of the basal plasma membrane to the extracellular matrix (ECM). The ECM provides cues to establish apicobasal polarity, and it also transduces forces that allow apical remodelling. However, physical crosstalk mechanisms between basal ECM attachment and the apical plasma membrane remain understudied, and the ones described so far are very diverse, which highlights the importance of identifying the general principles. Here, we review apicobasal crosstalk of two well-established models of membrane remodelling taking place during Drosophila melanogaster embryogenesis: amnioserosa cell shape oscillations during dorsal closure and subcellular tube formation in tracheal cells. We discuss how anchoring to the basal ECM affects apical architecture and the mechanisms that mediate these interactions. We analyse this knowledge under the scope of other morphogenetic processes and discuss what aspects of apicobasal crosstalk may represent widespread phenomena and which ones are used to build subsets of specialised compartments.

polished rice mediates ecdysone-dependent control of Drosophila embryonic organogenesis

In many animals, progression of developmental stages is temporally controlled by steroid hormones. In Drosophila, the level of ecdysone titer oscillates and developmental stage transitions, such as larval molting and metamorphosis, are induced at each of ecdysone peaks. Ecdysone titer also peaks at the stage of mid-embryogenesis and the embryonic ecdysone is necessary for morphogenesis of several organs, although the regulatory mechanisms of embryonic organogenesis dependent on ecdysone signaling are still open questions. In this study, we find that absence or interruption of embryonic ecdysone signaling caused multiple defects in the tracheal system, including decrease in luminal protein deposition, uneven dilation of the dorsal trunk and loss of terminal branches. We also reveal that an ecdysone-inducible gene polished rice (pri) is essential for tip cell fate decision in dorsal branches. As over-expression of pri can restore the defects caused by disturbance of ecdysone biosynthesis, pri functions as one of the major mediators of embryonic ecdysone signal in tracheogenesis. These results demonstrate that ecdysone and its downstream target pri play essential roles in tracheal development by modulating cell fate decision.

Form and function of the apical extracellular matrix: new insights from Caenorhabditis elegans, Drosophila melanogaster, and the vertebrate inner ear

Apical extracellular matrices (aECMs) are the extracellular layers on the apical sides of epithelia. aECMs form the outer layer of the skin in most animals and line the luminal surface of internal tubular epithelia. Compared to the more conserved basal ECMs (basement membranes), aECMs are highly diverse between tissues and between organisms and have been more challenging to understand at mechanistic levels. Studies in several genetic model organisms are revealing new insights into aECM composition, biogenesis, and function and have begun to illuminate common principles and themes of aECM organization. There is emerging evidence that, in addition to mechanical or structural roles, aECMs can participate in reciprocal signaling with associated epithelia and other cell types. Studies are also revealing mechanisms underlying the intricate nanopatterns exhibited by many aECMs. In this review, we highlight recent findings from well-studied model systems, including the external cuticle and ductal aECMs of Caenorhabditis elegans, Drosophila melanogaster, and other insects and the internal aECMs of the vertebrate inner ear.