Inverse regulation of target genes at the brink of the BMP morphogen activity gradient.

BMP-dependent patterning in the Drosophila melanogaster wing imaginal disc serves as a paradigm to understand how morphogens specify cell fates. The observed profile of the transcriptional response to the graded signal of BMP relies upon two counter-active gradients of pMad and Brinker (Brk). This patterning model is inadequate to explain the expression of target genes, like vestigial and spalt, in lateral regions of the wing disc where BMP signals decline and Brk levels peak. Here, we show that in contrast to the reciprocal repressor gradient mechanism, where Brk represses BMP targets in medial regions, target expression in lateral regions is downregulated by BMP signalling and activated by Brk. Brk induces lateral expression indirectly, apparently through repression of a negative regulator. Our findings provide a model explaining how the expression of an established BMP target is differentially and inversely regulated along the anterior-posterior axis of the wing disc.

Recognition and killing of human and murine pancreatic beta cells by the NK receptor NKp46.

Type 1 diabetes is an incurable disease that is currently treated by insulin injections or in rare cases by islet transplantation. We have recently shown that NKp46, a major killer receptor expressed by NK cells, recognizes an unknown ligand expressed by ß cells and that in the absence of NKp46, or when its activity is blocked, diabetes development is inhibited. In this study, we investigate whether NKp46 is involved in the killing of human ß cells that are intended to be used for transplantation, and we also thoroughly characterize the interaction between NKp46 and its human and mouse ß cell ligands. We show that human ß cells express an unknown ligand for NKp46 and are killed in an NKp46-dependent manner. We further demonstrate that the expression of the NKp46 ligand is detected on human ß cells already at the embryonic stage and that it appears on murine ß cells only following birth. Because the NKp46 ligand is detected on healthy ß cells, we wondered why type 1 diabetes does not develop in all individuals and show that NK cells are absent from the vicinity of islets of healthy mice and are detected in situ in proximity with ß cells in NOD mice. We also investigate the molecular mechanisms controlling NKp46 interactions with its ß cell ligand and demonstrate that the recognition is confined to the membrane proximal domain and stalk region of NKp46 and that two glycosylated residues of NKp46, Thr(125) and Asn(216), are critical for this recognition.

Hrp48 attenuates Sxl expression to allow for proper notch expression and signaling in wing development.

Different signaling pathways are deployed in specific developmental contexts to generate sexually dimorphic traits. Recently, Sex-lethal (Sxl), the female determinant in Drosophila melanogaster, was shown to down-regulate Notch (N) signaling to accomplish sex-specific patterning. Paradoxically, however, both Sxl and N are ubiquitously expressed in all of the female cells. This raises a key question as to how, during monomorphic female development, N signaling escapes the negative impact of Sxl. Here, we uncover a regulatory loop involving Hrp48, an abundant Drosophila hnRNP, Sxl and N. Phenotypic consequences of the partial loss of hrp48 resemble that of N but are more pronounced in females than in males. Likewise, N levels are drastically diminished only in females. Interestingly, monomorphic female tissues including wing, eye and antennal discs display considerable increase in Sxl amounts. Finally, female-specific attenuation of N signaling is rescued upon simultaneous removal of Sxl. Thus, our data demonstrate that in monomorphic contexts, Hrp48 functions as a moderator of Sxl expression to achieve adequate levels of N receptor production and signaling. We propose that it is critical to modulate the activities of the master determinant underling sexual dimorphism, to ensure that it does not function inappropriately in monomorphic tissues and disrupt their development.

The NAB-Brk signal bifurcates at JNK to independently induce apoptosis and compensatory proliferation.

Apoptosis operates to eliminate damaged or potentially dangerous cells. This loss is often compensated by extra proliferation of neighboring cells. Studies in Drosophila imaginal discs suggest that the signal for the additional growth emanates from the dying cells. In particular, it was suggested that the initiator caspase Dronc mediates compensatory proliferation (CP) through Dp53 in wing discs. However, the exact mechanism that governs this CP remained poorly understood. We have previously shown that elimination of misspecified cells due to reduced Dpp signaling is achieved by the interaction of the co-repressor NAB with the transcriptional repressor Brk, which in turn induces Jun N-terminal kinase-dependent apoptosis. Here, we performed a systematic in vivo loss- and gain-of-function analysis to study NAB-induced death and CP. Our findings indicate that the NAB primary signal activates JNK, which in turn transmits two independent signals. One triggers apoptosis through the pro-apoptotic proteins Reaper and Hid, which in turn promote activation of caspases by the apoptosome components Ark and Dronc. The other signal induces CP in a manner that is independent of the death signal, Dronc, or Dp53. Once induced, the apoptotic pathway further activates a CP response. Our data suggest that JNK is the candidate factor that differentiates between apoptosis that involves CP and apoptosis that does not.

Gastrin: a distinct fate of neurogenin3 positive progenitor cells in the embryonic pancreas.

Neurogenin3(+) (Ngn3(+)) progenitor cells in the developing pancreas give rise to five endocrine cell types secreting insulin, glucagon, somatostatin, pancreatic polypeptide and ghrelin. Gastrin is a hormone produced primarily by G-cells in the stomach, where it functions to stimulate acid secretion by gastric parietal cells. Gastrin is expressed in the embryonic pancreas and is common in islet cell tumors, but the lineage and regulators of pancreatic gastrin(+) cells are not known. We report that gastrin is abundantly expressed in the embryonic pancreas and disappears soon after birth. Some gastrin(+) cells in the developing pancreas co-express glucagon, ghrelin or pancreatic polypeptide, but many gastrin(+) cells do not express any other islet hormone. Pancreatic gastrin(+) cells express the transcription factors Nkx6.1, Nkx2.2 and low levels of Pdx1, and derive from Ngn3(+) endocrine progenitor cells as shown by genetic lineage tracing. Using mice deficient for key transcription factors we show that gastrin expression depends on Ngn3, Nkx2.2, NeuroD1 and Arx, but not Pax4 or Pax6. Finally, gastrin expression is induced upon differentiation of human embryonic stem cells to pancreatic endocrine cells expressing insulin. Thus, gastrin(+) cells are a distinct endocrine cell type in the pancreas and an alternative fate of Ngn3+ cells.

The expression of the beta cell-derived autoimmune ligand for the killer receptor nkp46 is attenuated in type 2 diabetes.

NK cells rapidly kill tumor cells, virus infected cells and even self cells. This is mediated via killer receptors, among which NKp46 (NCR1 in mice) is prominent. We have recently demonstrated that in type 1 diabetes (T1D) NK cells accumulate in the diseased pancreas and that they manifest a hyporesponsive phenotype. In addition, we found that NKp46 recognizes an unknown ligand expressed by beta cells derived from humans and mice and that blocking of NKp46 activity prevented diabetes development. Here we investigated the properties of the unknown NKp46 ligand. We show that the NKp46 ligand is mainly located in insulin granules and that it is constitutively secreted. Following glucose stimulation the NKp46 ligand translocates to the cell membrane and its secretion decreases. We further demonstrate by using several modalities that the unknown NKp46 ligand is not insulin. Finally, we studied the expression of the NKp46 ligand in type 2 diabetes (T2D) using 3 different in vivo models and 2 species; mice and gerbils. We demonstrate that the expression of the NKp46 ligand is decreased in all models of T2D studied, suggesting that NKp46 is not involved in T2D.

Males and females: creating differences while maintaining the similarities.

During metazoan development, a small number of signaling pathways are iteratively used to orchestrate diverse processes such as cell division, cell fate specification and survival. Temporal and spatial regulation of these pathways underlies the final cellular makeup, size and shape of organs. In Drosophila melanogaster, the master switch gene Sex-lethal (Sxl) orchestrates all aspects of female development and behavior by modulating gene expression. Many of the sex-specific differences in gene expression and morphology are controlled through a gene activity cascade that involves Sxl→tra→dsx-fru. However, various aspects of somatic sexual development appear to be independent of this cascade. Consistent with this idea, Sxl protein, on its own, was recently implicated in the regulation of both Hh and Notch signaling to shape some of the sexually dimorphic traits. Paradoxically, however, Sxl activity is essential in every female cell to prevent the activation of the male-specific dosage compensation system and thus to ensure the proper level of X-linked gene expression. This raises a key question as to how the sex-specific effects of Sxl on major signaling pathways are prevented in monomorphic tissues during female development. We have elucidated a novel mechanism where Hrp48, an abundant essential hnRNP functions to restrict Sxl expression in monomorphic tissues and thus allow for proper development. Our findings bring into focus the critical role played by general homeostatic factors in specification of diverse cell fates and morphogenesis.

The co-regulator dNAB interacts with Brinker to eliminate cells with reduced Dpp signaling.

The proper development of tissues requires morphogen activity that dictates the appropriate growth and differentiation of each cell according to its position within a developing field. Elimination of underperforming cells that are less efficient in receiving/transducing the morphogenetic signal is thought to provide a general fail-safe mechanism to avoid developmental misspecification. In the developing Drosophila wing, the morphogen Dpp provides cells with growth and survival cues. Much of the regulation of transcriptional output by Dpp is mediated through repression of the transcriptional repressor Brinker (Brk), and thus through the activation of target genes. Mutant cells impaired for Dpp reception or transduction are lost from the wing epithelium. At the molecular level, reduced Dpp signaling results in Brk upregulation that triggers apoptosis through activation of the JNK pathway. Here we show that the transcriptional co-regulator dNAB is a Dpp target in the developing wing that interacts with Brk to eliminate cells with reduced Dpp signaling through the JNK pathway. We further show that both dNAB and Brk are required for cell elimination induced by differential dMyc expression, a process that depends on reduced Dpp transduction in outcompeted cells. We propose a novel mechanism whereby the morphogen Dpp regulates the responsiveness to its own survival signal by inversely controlling the expression of a repressor, Brk, and its co-repressor, dNAB.