Adipose triglyceride lipase protects renal cell endocytosis in a Drosophila dietary model of chronic kidney disease.

Obesity-related renal lipotoxicity and chronic kidney disease (CKD) are prevalent pathologies with complex aetiologies. One hallmark of renal lipotoxicity is the ectopic accumulation of lipid droplets in kidney podocytes and in proximal tubule cells. Renal lipid droplets are observed in human CKD patients and in high-fat diet (HFD) rodent models, but their precise role remains unclear. Here, we establish a HFD model in Drosophila that recapitulates renal lipid droplets and several other aspects of mammalian CKD. Cell type-specific genetic manipulations show that lipid can overflow from adipose tissue and is taken up by renal cells called nephrocytes. A HFD drives nephrocyte lipid uptake via the multiligand receptor Cubilin (Cubn), leading to the ectopic accumulation of lipid droplets. These nephrocyte lipid droplets correlate with endoplasmic reticulum (ER) and mitochondrial deficits, as well as with impaired macromolecular endocytosis, a key conserved function of renal cells. Nephrocyte knockdown of diglyceride acyltransferase 1 (DGAT1), overexpression of adipose triglyceride lipase (ATGL), and epistasis tests together reveal that fatty acid flux through the lipid droplet triglyceride compartment protects the ER, mitochondria, and endocytosis of renal cells. Strikingly, boosting nephrocyte expression of the lipid droplet resident enzyme ATGL is sufficient to rescue HFD-induced defects in renal endocytosis. Moreover, endocytic rescue requires a conserved mitochondrial regulator, peroxisome proliferator-activated receptor-gamma coactivator 1α (PGC1α). This study demonstrates that lipid droplet lipolysis counteracts the harmful effects of a HFD via a mitochondrial pathway that protects renal endocytosis. It also provides a genetic strategy for determining whether lipid droplets in different biological contexts function primarily to release beneficial or to sequester toxic lipids.

Pax6 organizes the anterior eye segment by guiding two distinct neural crest waves.

Cranial neural crest (NC) contributes to the developing vertebrate eye. By multidimensional, quantitative imaging, we traced the origin of the ocular NC cells to two distinct NC populations that differ in the maintenance of sox10 expression, Wnt signalling, origin, route, mode and destination of migration. The first NC population migrates to the proximal and the second NC cell group populates the distal (anterior) part of the eye. By analysing zebrafish pax6a/b compound mutants presenting anterior segment dysgenesis, we demonstrate that Pax6a/b guide the two NC populations to distinct proximodistal locations. We further provide evidence that the lens whose formation is pax6a/b-dependent and lens-derived TGFß signals contribute to the building of the anterior segment. Taken together, our results reveal multiple roles of Pax6a/b in the control of NC cells during development of the anterior segment.

ATF4-Induced Warburg Metabolism Drives Over-Proliferation in Drosophila.

The mitochondrial electron transport chain (ETC) enables essential metabolic reactions; nonetheless, the cellular responses to defects in mitochondria and the modulation of signaling pathway outputs are not understood. We show that Notch signaling and ETC attenuation via knockdown of COX7a induces massive over-proliferation. The tumor-like growth is caused by a transcriptional response through the eIF2α-kinase PERK and ATF4, which activates the expression of metabolic enzymes, nutrient transporters, and mitochondrial chaperones. We find this stress adaptation to be beneficial for progenitor cell fitness, as it renders cells sensitive to proliferation induced by the Notch signaling pathway. Intriguingly, over-proliferation is not caused by transcriptional cooperation of Notch and ATF4, but it is mediated in part by pH changes resulting from the Warburg metabolism induced by ETC attenuation. Our results suggest that ETC function is monitored by the PERK-ATF4 pathway, which can be hijacked by growth-promoting signaling pathways, leading to oncogenic pathway activity.

Hox Function Is Required for the Development and Maintenance of the Drosophila Feeding Motor Unit.

Feeding is an evolutionarily conserved and integral behavior that depends on the rhythmic activity of feeding muscles stimulated by specific motoneurons. However, critical molecular determinants underlying the development of the neuromuscular feeding unit are largely unknown. Here, we identify the Hox transcription factor Deformed (Dfd) as essential for feeding unit formation, from initial specification to the establishment of active synapses, by controlling stage-specific sets of target genes. Importantly, we found Dfd to control the expression of functional components of synapses, such as Ankyrin2-XL, a protein known to be critical for synaptic stability and connectivity. Furthermore, we uncovered Dfd as a potential regulator of synaptic specificity, as it represses expression of the synaptic cell adhesion molecule Connectin (Con). These results demonstrate that Dfd is critical for the establishment and maintenance of the neuromuscular unit required for feeding behavior, which might be shared by other group 4 Hox genes.

The cis-regulatory code of Hox function in Drosophila.

Precise gene expression is a fundamental aspect of organismal function and depends on the combinatorial interplay of transcription factors (TFs) with cis-regulatory DNA elements. While much is known about TF function in general, our understanding of their cell type-specific activities is still poor. To address how widely expressed transcriptional regulators modulate downstream gene activity with high cellular specificity, we have identified binding regions for the Hox TF Deformed (Dfd) in the Drosophila genome. Our analysis of architectural features within Hox cis-regulatory response elements (HREs) shows that HRE structure is essential for cell type-specific gene expression. We also find that Dfd and Ultrabithorax (Ubx), another Hox TF specifying different morphological traits, interact with non-overlapping regions in vivo, despite their similar DNA binding preferences. While Dfd and Ubx HREs exhibit comparable design principles, their motif compositions and motif-pair associations are distinct, explaining the highly selective interaction of these Hox proteins with the regulatory environment. Thus, our results uncover the regulatory code imprinted in Hox enhancers and elucidate the mechanisms underlying functional specificity of TFs in vivo.

Antagonistic regulation of apoptosis and differentiation by the Cut transcription factor represents a tumor-suppressing mechanism in Drosophila.

Apoptosis is essential to prevent oncogenic transformation by triggering self-destruction of harmful cells, including those unable to differentiate. However, the mechanisms linking impaired cell differentiation and apoptosis during development and disease are not well understood. Here we report that the Drosophila transcription factor Cut coordinately controls differentiation and repression of apoptosis via direct regulation of the pro-apoptotic gene reaper. We also demonstrate that this regulatory circuit acts in diverse cell lineages to remove uncommitted precursor cells in status nascendi and thereby interferes with their potential to develop into cancer cells. Consistent with the role of Cut homologues in controlling cell death in vertebrates, we find repression of apoptosis regulators by Cux1 in human cancer cells. Finally, we present evidence that suggests that other lineage-restricted specification factors employ a similar mechanism to put the brakes on the oncogenic process.