Loss of putzig in the germline impedes germ cell development by inducing cell death and new niche like microenvironments

Drosophila Protein Z4 Possesses ZAD Dimerization Domain

The transcription factor Z4 (putzig) is one of the key proteins that determine the chromatin structure in Drosophila. Z4 is found at the boundaries of bands on polytene chromosomes, and the bands are currently thought to correlate with chromatin domains. Z4 is a component of a protein complex that additionally includes Chromator and BEAF-32, and a conserved domain is necessary to occur at the N end of Z4 to ensure its interaction with the two proteins. In this study, a zinc finger-associated domain (ZAD) domain was identified in Z4. The capability of dimerization was confirmed for the domain by biochemical methods. A dimer model of the domain was obtained using AlphaFold2, and the model structure was confirmed using small-angle X-ray scattering (SAXS). The dimer structure shows a fold typical of ZAD domains.

Novel Genome-Engineered H Alleles Differentially Affect Lateral Inhibition and Cell Dichotomy Processes during Bristle Organ Development

Hairless (H) encodes the major antagonist in the Notch signaling pathway, which governs cellular differentiation of various tissues in Drosophila. By binding to the Notch signal transducer Suppressor of Hairless (Su(H)), H assembles repressor complexes onto Notch target genes. Using genome engineering, three new H alleles, HFA, HLLAA and HWA were generated and a phenotypic series was established by several parameters, reflecting the residual H-Su(H) binding capacity. Occasionally, homozygous HWA flies develop to adulthood. They were compared with the likewise semi-viable HNN allele affecting H-Su(H) nuclear entry. The H homozygotes were short-lived, sterile and flightless, yet showed largely normal expression of several mitochondrial genes. Typical for H mutants, both HWA and HNN homozygous alleles displayed strong defects in wing venation and mechano-sensory bristle development. Strikingly, however, HWA displayed only a loss of bristles, whereas bristle organs of HNN flies showed a complete shaft-to-socket transformation. Apparently, the impact of HWA is restricted to lateral inhibition, whereas that of HNN also affects the respective cell type specification. Notably, reduction in Su(H) gene dosage only suppressed the HNN bristle phenotype, but amplified that of HWA. We interpret these differences as to the role of H regarding Su(H) stability and availability.

Somatic chromosome pairing has a determinant impact on 3D chromatin organization

In the nucleus, chromatin is intricately structured into multiple layers of 3D organization important for genome activity. How distinct layers influence each other is not well understood. In particular, the contribution of chromosome pairing to 3D chromatin organization has been largely neglected. Here, we address this question in Drosophila, an organism that shows robust chromosome pairing in interphasic somatic cells. The extent of chromosome pairing depends on the balance between pairing and anti-pairing factors, with the anti-pairing activity of the CAP-H2 condensin II subunit being the best documented. Here, we identify the zinc-finger protein Z4 as a strong anti-pairer that interacts with and mediates the chromatin binding of CAP-H2. We also report that hyperosmotic cellular stress induces fast and reversible chromosome unpairing that depends on Z4/CAP-H2. And, most important, by combining Z4 depletion and osmostress, we show that chromosome pairing reinforces intrachromosomal 3D interactions. On the one hand, pairing facilitates RNAPII occupancy that correlates with enhanced intragenic gene-loop interactions. In addition, acting at a distance, pairing reinforces chromatin-loop interactions mediated by Polycomb (Pc). In contrast, chromosome pairing does not affect which genomic intervals segregate to active (A) and inactive (B) compartments, with only minimal effects on the strength of A-A compartmental interactions. Altogether, our results unveil the intimate interplay between inter-chromosomal and intra-chromosomal 3D interactions, unraveling the interwoven relationship between different layers of chromatin organization and the essential contribution of chromosome pairing.

The Membrane-Bound Notch Regulator Mnr Supports Notch Cleavage and Signaling Activity in Drosophila melanogaster

The Notch signaling pathway is pivotal to cellular differentiation. Activation of this pathway involves proteolysis of the Notch receptor and the release of the biologically active Notch intracellular domain, acting as a transcriptional co-activator of Notch target genes. While the regulation of Notch signaling dynamics at the level of ligand-receptor interaction, endocytosis, and transcriptional regulation has been well studied, little is known about factors influencing Notch cleavage. We identified EP555 as a suppressor of the Notch antagonist Hairless (H). EP555 drives expression of CG32521 encoding membrane-bound proteins, which we accordingly rename membrane-bound Notch regulator (mnr). Within the signal-receiving cell, upregulation of Mnr stimulates Notch receptor activation, whereas a knockdown reduces it, without apparent influence on ligand-receptor interaction. We provide evidence that Mnr plays a role in γ-secretase-mediated intramembrane cleavage of the Notch receptor. As revealed by a fly-eye-based reporter system, γ-secretase activity is stimulated by the overexpression of Mnr, and is inhibited by its knockdown. We conclude that Mnr proteins support Notch signaling activity by fostering the cleavage of the Notch receptor. With Mnr, we identified a membrane-bound factor directly augmenting Notch intra-membrane processing, thereby acting as a positive regulator of Notch signaling activity.

Phospho-Site Mutations in Transcription Factor Suppressor of Hairless Impact Notch Signaling Activity During Hematopoiesis in Drosophila

The highly conserved Notch signaling pathway controls a multitude of developmental processes including hematopoiesis. Here, we provide evidence for a novel mechanism of tissue-specific Notch regulation involving phosphorylation of CSL transcription factors within the DNA-binding domain. Earlier we found that a phospho-mimetic mutation of the Drosophila CSL ortholog Suppressor of Hairless [Su(H)] at Ser269 impedes DNA-binding. By genome-engineering, we now introduced phospho-specific Su(H) mutants at the endogenous Su(H) locus, encoding either a phospho-deficient [Su(H) S269A ] or a phospho-mimetic [Su(H) S269D ] isoform. Su(H) S269D mutants were defective of Notch activity in all analyzed tissues, consistent with impaired DNA-binding. In contrast, the phospho-deficient Su(H) S269A mutant did not generally augment Notch activity, but rather specifically in several aspects of blood cell development. Unexpectedly, this process was independent of the corepressor Hairless acting otherwise as a general Notch antagonist in Drosophila. This finding is in agreement with a novel mode of Notch regulation by posttranslational modification of Su(H) in the context of hematopoiesis. Importantly, our studies of the mammalian CSL ortholog (RBPJ/CBF1) emphasize a potential conservation of this regulatory mechanism: phospho-mimetic RBPJ S221D was dysfunctional in both the fly as well as two human cell culture models, whereas phospho-deficient RBPJ S221A rather gained activity during fly hematopoiesis. Thus, dynamic phosphorylation of CSL-proteins within the DNA-binding domain provides a novel means to fine-tune Notch signal transduction in a context-dependent manner.

An RBPJ-Drosophila Model Reveals Dependence of RBPJ Protein Stability on the Formation of Transcription-Regulator Complexes

Notch signaling activity governs widespread cellular differentiation in higher animals, including humans, and is involved in several congenital diseases and different forms of cancer. Notch signals are mediated by the transcriptional regulator RBPJ in a complex with activated Notch (NICD). Analysis of Notch pathway regulation in humans is hampered by a partial redundancy of the four Notch receptor copies, yet RBPJ is solitary, allowing its study in model systems. In Drosophila melanogaster, the RBPJ orthologue is encoded by Suppressor of Hairless [Su(H)]. Using genome engineering, we replaced Su(H) by murine RBPJ in order to study its function in the fly. In fact, RBPJ largely substitutes for Su(H)’s function, yet subtle phenotypes reflect increased Notch signaling activity. Accordingly, the binding of RBPJ to Hairless (H) protein, the general Notch antagonist in Drosophila, was considerably reduced compared to that of Su(H). An H-binding defective RBPJ^LLL mutant matched the respective Su(H)^LLL allele: homozygotes were lethal due to extensive Notch hyperactivity. Moreover, RBPJ^LLL protein accumulated at lower levels than wild type RBPJ, except in the presence of NICD. Apparently, RBPJ protein stability depends on protein complex formation with either H or NICD, similar to Su(H), demonstrating that the murine homologue underlies the same regulatory mechanisms as Su(H) in Drosophila. These results underscore the importance of regulating the availability of RBPJ protein to correctly mediate Notch signaling activity in the fly.