Shar-pei Mediates Cell Proliferation Arrest During Imaginal Disc Growth in Drosophila

During animal development, organ size is determined primarily by the amount of cell proliferation, which must be tightly regulated to ensure the generation of properly proportioned organs. However, little is known about the molecular pathways that direct cells to stop proliferating when an organ has attained its proper size. We have identified mutations in a novel gene, shar-pei, that is required for proper termination of cell proliferation during Drosophila imaginal disc development. Clones of shar-pei mutant cells in imaginal discs produce enlarged tissues containing more cells of normal size. We show that this phenotype is the result of both increased cell proliferation and reduced apoptosis. Hence,shar-pei restricts cell proliferation and promotes apoptosis. By contrast, shar-pei is not required for cell differentiation and pattern formation of adult tissue. Shar-pei is also not required for cell cycle exit during terminal differentiation, indicating that the mechanisms directing cell proliferation arrest during organ growth are distinct from those directing cell cycle exit during terminal differentiation. shar-pei encodes a WW-domain-containing protein that has homologs in worms, mice and humans, suggesting that mechanisms of organ growth control are evolutionarily conserved

The bantam microRNA is a target of the hippo tumor-suppressor pathway

The Hippo tumor-suppressor pathway has emerged as a key signaling pathway that controls tissue size in Drosophila. Hippo signaling restricts tissue size by promoting apoptosis and cell-cycle arrest, and animals carrying clones of cells mutant for hippo develop severely overgrown adult structures. The Hippo pathway is thought to exert its effects by modulating gene expression through the phosphorylation of the transcriptional coactivator Yorkie. However, how Yorkie regulates growth, and thus the identities of downstream target genes that mediate the effects of Hippo signaling, are largely unknown.status: publishe

Hippo promotes proliferation arrest and apoptosis in the Salvador/Warts pathway

Proliferation and apoptosis must be precisely regulated to form organs with appropriate cell numbers and to avoid tumour growth. Here we show that Hippo (Hpo), the Drosophila homologue of the mammalian Ste20-like kinases, MST1/2, promotes proper termination of cell proliferation and stimulates apoptosis during development. hpo mutant tissues are larger than normal because mutant cells continue to proliferate beyond normal tissue size and are resistant to apoptotic stimuli that usually eliminate extra cells. Hpo negatively regulates expression of Cyclin E to restrict cell proliferation, downregulates the Drosophila inhibitor of apoptosis protein DIAP1, and induces the proapoptotic gene head involution defective (hid) to promote apoptosis. The mutant phenotypes of hpo are similar to those of warts (wts), which encodes a serine/threonine kinase of the myotonic dystrophy protein kinase family, and salvador (sav), which encodes a WW domain protein that binds to Wts. We find that Sav binds to a regulatory domain of Hpo that is essential for its function, indicating that Hpo acts together with Sav and Wts in a signalling module that coordinately regulates cell proliferation and apoptosis.status: publishe

The Tumour-suppressor Genes NF2/Merlin and Expanded Act Through Hippo Signalling to Regulate Cell Proliferation and Apoptosis

Merlin, the protein product of the Neurofibromatosis type-2 gene, acts as a tumour suppressor in mice and humans. Merlin is an adaptor protein with a FERM domain and it is thought to transduce a growth-regulatory signal. However, the pathway through which Merlin acts as a tumour suppressor is poorly understood. Merlin, and its function as a negative regulator of growth, is conserved in Drosophila, where it functions with Expanded, a related FERM domain protein. Here, we show that Drosophila Merlin and Expanded are components of the Hippo signalling pathway, an emerging tumour-suppressor pathway. We find that Merlin and Expanded, similar to other components of the Hippo pathway, are required for proliferation arrest and apoptosis in developing imaginal discs. Our genetic and biochemical data place Merlin and Expanded upstream of Hippo and identify a pathway through which they act as tumour-suppressor genes

Senseless acts as a binary switch during sensory organ precursor selection

During sensory organ precursor (SOP) specification, a single cell is selected from a proneural cluster of cells. Here, we present evidence that Senseless (Sens), a zinc-finger transcription factor, plays an important role in this process. We show that Sens is directly activated by proneural proteins in the presumptive SOPs and a few cells surrounding the SOP in most tissues. In the cells that express low levels of Sens, it acts in a DNA-binding-dependent manner to repress transcription of proneural genes. In the presumptive SOPs that express high levels of Sens, it acts as a transcriptional activator and synergizes with proneural proteins. We therefore propose that Sens acts as a binary switch that is fundamental to SOP selection

The tumour-suppressor genes NF2/Merlin and Expanded act through Hippo signalling to regulate cell proliferation and apoptosis

Merlin, the protein product of the Neurofibromatosis type-2 gene, acts as a tumour suppressor in mice and humans. Merlin is an adaptor protein with a FERM domain and it is thought to transduce a growth-regulatory signal. However, the pathway through which Merlin acts as a tumour suppressor is poorly understood. Merlin, and its function as a negative regulator of growth, is conserved in Drosophila, where it functions with Expanded, a related FERM domain protein. Here, we show that Drosophila Merlin and Expanded are components of the Hippo signalling pathway, an emerging tumour-suppressor pathway. We find that Merlin and Expanded, similar to other components of the Hippo pathway, are required for proliferation arrest and apoptosis in developing imaginal discs. Our genetic and biochemical data place Merlin and Expanded upstream of Hippo and identify a pathway through which they act as tumour-suppressor genes.status: publishe

Shar-pei mediates cell proliferation arrest during imaginal disc growth in Drosophila

During animal development, organ size is determined primarily by the amount of cell proliferation, which must be tightly regulated to ensure the generation of properly proportioned organs. However, little is known about the molecular pathways that direct cells to stop proliferating when an organ has attained its proper size. We have identified mutations in a novel gene, shar-pei, that is required for proper termination of cell proliferation during Drosophila imaginal disc development. Clones of shar-pei mutant cells in imaginal discs produce enlarged tissues containing more cells of normal size. We show that this phenotype is the result of both increased cell proliferation and reduced apoptosis. Hence, shar-pei restricts cell proliferation and promotes apoptosis. By contrast, shar-pei is not required for cell differentiation and pattern formation of adult tissue. Shar-pei is also not required for cell cycle exit during terminal differentiation, indicating that the mechanisms directing cell proliferation arrest during organ growth are distinct from those directing cell cycle exit during terminal differentiation. shar-pei encodes a WW-domain-containing protein that has homologs in worms, mice and humans, suggesting that mechanisms of organ growth control are evolutionarily conserved.status: publishe

Notch activation inhibits AML growth and survival: a potential therapeutic approach

Although aberrant Notch activation contributes to leukemogenesis in T cells, its role in acute myelogenous leukemia (AML) remains unclear. Here, we report that human AML samples have robust expression of Notch receptors; however, Notch receptor activation and expression of downstream Notch targets are remarkably low, suggesting that Notch is present but not constitutively activated in human AML. The functional role of these Notch receptors in AML is not known. Induced activation through any of the Notch receptors (Notch1–4), or through the Notch target Hairy/Enhancer of Split 1 (HES1), consistently leads to AML growth arrest and caspase-dependent apoptosis, which are associated with B cell lymphoma 2 (BCL2) loss and enhanced p53/p21 expression. These effects were dependent on the HES1 repressor domain and were rescued through reexpression of BCL2. Importantly, activated Notch1, Notch2, and HES1 all led to inhibited AML growth in vivo, and Notch inhibition via dnMAML enhanced proliferation in vivo, thus revealing the physiological inhibition of AML growth in vivo in response to Notch signaling. As a novel therapeutic approach, we used a Notch agonist peptide that led to significant apoptosis in AML patient samples. In conclusion, we report consistent Notch-mediated growth arrest and apoptosis in human AML, and propose the development of Notch agonists as a potential therapeutic approach in AML

The tumour-suppressor genes NF2/Merlin and Expanded act through Hippo signalling to regulate cell proliferation and apoptosis

Merlin, the protein product of the Neurofibromatosis type-2 gene, acts as a tumour suppressor in mice and humans. Merlin is an adaptor protein with a FERM domain and it is thought to transduce a growth-regulatory signal. However, the pathway through which Merlin acts as a tumour suppressor is poorly understood. Merlin, and its function as a negative regulator of growth, is conserved in Drosophila, where it functions with Expanded, a related FERM domain protein. Here, we show that Drosophila Merlin and Expanded are components of the Hippo signalling pathway, an emerging tumour-suppressor pathway. We find that Merlin and Expanded, similar to other components of the Hippo pathway, are required for proliferation arrest and apoptosis in developing imaginal discs. Our genetic and biochemical data place Merlin and Expanded upstream of Hippo and identify a pathway through which they act as tumour-suppressor genes