Identifying tumor suppressors in genetic mosaics: the Drosophila lats gene encodes a putative protein kinase

Molecular cloning of a novel human protein kinase, kpm, that is homologous to warts/lats, a Drosophila tumor suppressor

A novel human protein kinase, designated kpm, was identified and molecularly cloned. The isolated cDNA clone had an open reading frame consisting of 1088 amino acid residues with a putative kinase domain located near the carboxy-terminus. Homology search revealed that kpm belongs to a subfamily of serine/threonine protein kinases including warts/lats, a Drosophila tumor suppressor. Among these, kpm is most homologous to, but distinct from, recently reported LATS1, a human homolog of Drosophila warts/lats. Northern blot analysis disclosed that kpm is expressed as a 6.0 kb transcript in most of the tissues examined and also as an additional shorter 4.0 kb transcript in testis. Western blotting using polyclonal rabbit anti-kpm antibody detected kpm protein as a band with an apparent Mr of 150 kD. Immune complex kinase assay of HA-tagged kpm showed that kpm had kinase activity and phosphorylated itself in vitro. Studies with synchronized HeLa cells indicated that kpm protein was expressed relatively constantly throughout the cell cycle and underwent significant phosphorylation at mitotic phase. These results suggest that kpm plays a role in cell cycle progression during mitosis and its deletion or dysfunction might be involved in certain types of human cancers.

Cellular distribution of COT1 kinase in Neurospora crassa

The Neurospora crassa cot-1 gene encodes a Ser/Thr protein kinase, which is involved in hyphal elongation. Many vacuoles, abnormally shaped mitochondria, and nuclei, along with differences in the structure of the cell wall and hyphal septa, were observed in hyphae of the cot-1 mutant shortly after a shift to the restrictive temperature. Immunolocalization experiments indicated that COT1 was associated with the cytoplasmic membrane; COT1 was also detected in the cytoplasm. The membrane-associated COT1 was absent from the cot-1 mutant when shifted to the restrictive temperature, as was a lower molecular weight isoform of COT1. We propose that COT1 may be involved in several cellular processes, and the spatial and temporal regulation of COT1 activity involves trafficking of the kinase within the fungal cell and its possible interaction with additional proteins.

Zyxin, a regulator of actin filament assembly, targets the mitotic apparatus by interacting with h-warts/LATS1 tumor suppressor

The mitotic apparatus plays a pivotal role in dividing cells to ensure each daughter cell receives a full set of chromosomes and complement of cytoplasm during mitosis. A human homologue of the Drosophila warts tumor suppressor, h-warts/LATS1, is an evolutionarily conserved serine/threonine kinase and a dynamic component of the mitotic apparatus. We have identified an interaction of h-warts/LATS1 with zyxin, a regulator of actin filament assembly. Zyxin is a component of focal adhesion, however, during mitosis a fraction of cytoplasmic-dispersed zyxin becomes associated with h-warts/LATS1 on the mitotic apparatus. We found that zyxin is phosphorylated specifically during mitosis, most likely by Cdc2 kinase, and that the phosphorylation regulates association with h-warts/LATS1. Furthermore, microinjection of truncated h-warts/LATS1 protein, including the zyxin-binding portion, interfered with localization of zyxin to mitotic apparatus, and the duration of mitosis of these injected cells was significantly longer than that of control cells. These findings suggest that h-warts/LATS1 and zyxin play a crucial role in controlling mitosis progression by forming a regulatory complex on mitotic apparatus.

The neurofibromatosis-2 homologue, Merlin, and the tumor suppressor expanded function together in Drosophila to regulate cell proliferation and differentiation

Neurofibromatosis-2 is an inherited disorder characterized by the development of benign schwannomas and other Schwann-cell-derived tumors associated with the central nervous system. The Neurofibromatosis-2 tumor suppressor gene encodes Merlin, a member of the Protein 4.1 superfamily most closely related to Ezrin, Radixin and Moesin. This discovery suggested a novel function for Protein 4.1 family members in the regulation of cell proliferation; proteins in this family were previously thought to function primarily to link transmembrane proteins to underlying cortical actin. To understand the basic cellular functions of Merlin, we are investigating a Drosophila Neurofibromatosis-2 homologue, Merlin. Loss of Merlin function in Drosophila results in hyperplasia of the affected tissue without significant disruptions in differentiation. Similar phenotypes have been observed for mutations in another Protein 4.1 superfamily member in Drosophila, expanded. Because of the phenotypic and structural similarities between Merlin and expanded, we asked whether Merlin and Expanded function together to regulate cell proliferation. In this study, we demonstrate that recessive loss of function of either Merlin or expanded can dominantly enhance the phenotypes associated with mutations in the other. Consistent with this genetic interaction, we determined that Merlin and Expanded colocalize in Drosophila tissues and cells, and physically interact through a conserved N-terminal region of Expanded, characteristic of the Protein 4.1 family, and the C-terminal domain of Merlin. Loss of function of both Merlin and expanded in clones revealed that these proteins function to regulate differentiation in addition to proliferation in Drosophila. Further genetic analyses suggest a role for Merlin and Expanded specifically in Decapentaplegic-mediated differentiation events. These results indicate that Merlin and Expanded function together to regulate proliferation and differentiation, and have implications for understanding the functions of other Protein 4.1 superfamily members.

The genetic control of organ growth: insights from Drosophila

The genetic control of growth ensures that animals grow to reproducible sizes and that tumourous growth is rare. This year, the regulation of organ growth has been studied extensively in Drosophila imaginal discs, and a signalling pathway that regulates organ growth and size has been identified. Furthermore, the role of Drosophila homologues to human tumour suppressor genes and oncogenes in imaginal disc growth has been investigated.

Structure, expression, and chromosome mapping of LATS2, a mammalian homologue of the Drosophila tumor suppressor gene lats/warts

We have cloned and characterized LATS2, a novel mammalian homologue of the Drosophila tumor suppressor gene lats/warts. Northern blot analysis showed ubiquitous expression of mouse LATS2 (MmLATS2) mRNA, whereas expression of human LATS2 (HsLATS2) mRNA was enhanced in skeletal muscle and heart. Immunoblotting analysis of fractionated cell lysates showed HsLats2 to be a nuclear protein. We mapped the MmLATS2 gene to mouse chromosome 14 by interspecific backcross analysis. We also mapped the HsLATS2 gene (by fluorescence in situ hybridization) to the 13q11-q12 region, in which a loss of heterozygosity has been frequently observed in many primary cancers and to which the tumor suppressor genes RB and BRCA2 have also been mapped.

Drosophila in cancer research. An expanding role

In recent years, Drosophila researchers have developed powerful genetic techniques that allow for the rapid identification and characterization of genes involved in tumor formation and development. The high level of gene and pathway conservation, the similarity of cellular processes and the emerging evidence of functional conservation of tumor suppressors between Drosophila and mammals, argue that studies of tumorigenesis in flies can directly contribute to the understanding of human cancer. In this review, we explore the historical and current roles of Drosophila in cancer research, as well as speculate on the future of Drosophila as a model to investigate cancer-related processes that are currently not well understood.

PTEN affects cell size, cell proliferation and apoptosis during Drosophila eye development

Mutations in the tumor suppressor gene PTEN (MMAC1/TEP1) are associated with a large number of human cancers and several autosomal-dominant disorders. Mice mutant for PTEN die at early embryonic stages and the mutant embryonic fibroblasts display decreased sensitivity to cell death. Overexpression of PTEN in different mammalian tissue culture cells affects various processes including cell proliferation, cell death and cell migration. We have characterized the Drosophila PTEN gene and present evidence that both inactivation and overexpression of PTEN affect cell size, while overexpression of PTEN also inhibits cell cycle progression at early mitosis and promotes cell death during eye development in a context-dependent manner. Furthermore, we have shown that PTEN acts in the insulin signaling pathway and all signals from the insulin receptor can be antagonized by either Drosophila or human PTEN, suggesting a potential means for alleviating symptoms associated with altered insulin signaling.

A mutation within the catalytic domain of COT1 kinase confers changes in the presence of two COT1 isoforms and in Ser/Thr protein kinase and phosphatase activities in Neurospora crassa

Neurospora crassa grows by forming spreading colonies. cot-1 belongs to a class of N. crassa colonial temperature-sensitive (cot) mutants and encodes a Ser/Thr protein kinase. We have mapped the cot-1 mutation to a single base change resulting in a His to Arg substitution at amino acid 351, which resides within the catalytic domain. Antibodies raised against COT1 detected and immunoprecipitated a predominant 73-kDa polypeptide in N. crassa extracts, whose abundance was constant under all growth conditions tested. An additional, lower MW COT1 isoform (67-kDa) present in the wild-type was not detected in cot-1 grown at the restrictive temperature. Similarly, this isoform was not detected in cot-3 or cot-5 strains, when grown at restrictive temperatures. Reduced levels of Ser/Thr kinase activity and an increase in type 1 and type 2B phosphatase (calcineurin) activities were measured in a cot-1 background. Apparent changes in the phosphorylation state of the p150(Glued) subunit of the dynactin cytoskeletal motor component (encoded by ro-3, a suppressor of cot-1) and evidence of in vitro physical interactions between COT1 and calcineurin indicate a functional linkage among COT1 kinase, type 2B phosphatase, and dynactin.

Serine/threonine protein kinases and phosphatases in filamentious fungi

Protein phosphorylation and dephosphorylation are one of the central currencies by which living cells perceive and respond to environmental cues. A number of fundamental processes in fungi such as the cell cycle, transcription, and mating have been shown to require protein phosphorylation. The analysis of protein kinases and phosphatases in filamentous fungi is in its infancy; however, it has already become clear that kinases and phosphatases are likely to be important mediators of fungal proliferation and development as well as signal transduction and infection-related morphogenesis. In this review, we describe, summarize, and consider the rapidly expanding field of protein phosphorylation/dephosphorylation in various aspects of filamentous fungal growth and development.

Human homologue of the Drosophila melanogaster lats tumour suppressor modulates CDC2 activity

We have previously used mosaic flies to screen for tumour suppressors or negative regulators of cell proliferation. The cellular composition of these flies resembles that of cancer patients who are chimaeric individuals carrying a small number of mutated somatic cells. One of the genes we identified is the large tumour suppressor gene, lats (also known as wts), which encodes a putative serine/threonine kinase. Somatic cells mutant for lats undergo extensive proliferation and form large tumours in many tissues in mosaic adults. Homozygous mutants for various lats alleles display a range of developmental defects including embryonic lethality. Although many tumour suppressors have been identified in Drosophila melanogaster, it is not clear whether these fly genes are directly relevant to tumorigenesis in mammals. Here, we have isolated mammalian homologues of Drosophila lats. Human LATS1 suppresses tumour growth and rescues all developmental defects, including embryonic lethality in flies. In mammalian cells, LATS1 is phosphorylated in a cell-cycle-dependent manner and complexes with CDC2 in early mitosis. LATS1-associated CDC2 has no mitotic cyclin partner and no kinase activity for histone H1. Furthermore, lats mutant cells in Drosophila abnormally accumulate cyclin A. These biochemical observations indicate that LATS is a novel negative regulator of CDC2/cyclin A, a finding supported by genetic data in Drosophila demonstrating that lats specifically interacts with cdc2 and cyclin A.

Mice deficient of Lats1 develop soft-tissue sarcomas, ovarian tumours and pituitary dysfunction

The lats gene has been identified as a tumour suppressor in Drosophila melanogaster using mosaic screens. Mosaic flies carrying somatic cells that are mutant for lats develop large tumours in many organs. The human LATS1 homologue rescues embryonic lethality and inhibits tumour growth in lats mutant flies, demonstrating the functional conservation of this gene. Biochemical and genetic analyses have revealed that LATS1 functions as a negative regulator of CDC2 (ref. 3). These data suggest that mammalian LATS1 may have a role in tumorigenesis. To elucidate the function of mammalian LATS1, we have generated Lats1-/- mice. Lats1-/- animals exhibit a lack of mammary gland development, infertility and growth retardation. Accompanying these defects are hyperplastic changes in the pituitary and decreased serum hormone levels. The reproductive hormone defects of Lats1-/- mice are reminiscent of isolated LH-hypogonadotropic hypogonadism and corpus luteum insufficiency in humans. Furthermore, Lats1-/- mice develop soft-tissue sarcomas and ovarian stromal cell tumours and are highly sensitive to carcinogenic treatments. Our data demonstrate a role for Lats1 in mammalian tumorigenesis and specific endocrine dysfunction.

slimb coordinates wg and dpp expression in the dorsal-ventral and anterior-posterior axes during limb development

In the Drosophila leg disc, wingless (wg) and decapentaplegic (dpp) are expressed in a ventral-anterior and dorsal-anterior stripe of cells, respectively. This pattern of expression is essential for proper limb development. While the Hedgehog (Hh) pathway regulates dpp and wg expression in the anterior-posterior (A/P) axis, mechanisms specifying their expression in the dorsal-ventral (D/V) axis are not well understood. We present evidence that slimb mutant clones in the disc deregulate wg and dpp expression in the D/V axis. This suggests for the first time that their expression in the D/V axis is actively regulated during imaginal disc development. Furthermore, slimb is unique in that it also deregulates wg and dpp in the A/P axis. The misexpression phenotypes of slimb- clones indicate that the regulation of wg and dpp expression is coordinated in both axes, and that slimb plays an essential role in integrating A/P and D/V signals for proper patterning during development. Our genetic analysis further reveals that slimb intersects the A/P pathway upstream of smoothened (smo).

Structural analysis of Drosophila merlin reveals functional domains important for growth control and subcellular localization

Merlin, the product of the Neurofibromatosis type 2 (NF2) tumor-suppressor gene, is a member of the protein 4.1 superfamily that is most closely related to ezrin, radixin, and moesin (ERM). NF2 is a dominantly inherited disease characterized by the formation of bilateral acoustic schwannomas and other benign tumors associated with the central nervous system. To understand its cellular functions, we are studying a Merlin homologue in Drosophila. As is the case for NF2 tumors, Drosophila cells lacking Merlin function overproliferate relative to their neighbors. Using in vitro mutagenesis, we define functional domains within Merlin required for proper subcellular localization and for genetic rescue of lethal Merlin alleles. Remarkably, the results of these experiments demonstrate that all essential genetic functions reside in the plasma membrane- associated NH2-terminal 350 amino acids of Merlin. Removal of a seven-amino acid conserved sequence within this domain results in a dominant-negative form of Merlin that is stably associated with the plasma membrane and causes overproliferation when expressed ectopically in the wing. In addition, we provide evidence that the COOH-terminal region of Merlin has a negative regulatory role, as has been shown for ERM proteins. These results provide insights into the functions and functional organization of a novel tumor suppressor gene.

Fission yeast orb6, a ser/thr protein kinase related to mammalian rho kinase and myotonic dystrophy kinase, is required for maintenance of cell polarity and coordinates cell morphogenesis with the cell cycle

The molecular mechanisms that coordinate cell morphogenesis with the cell cycle remain largely unknown. We have investigated this process in fission yeast where changes in polarized cell growth are coupled with cell cycle progression. The orb6 gene is required during interphase to maintain cell polarity and encodes a serine/threonine protein kinase, belonging to the myotonic dystrophy kinase/cot1/warts family. A decrease in Orb6 protein levels leads to loss of polarized cell shape and to mitotic advance, whereas an increase in Orb6 levels maintains polarized growth and delays mitosis by affecting the p34(cdc2) mitotic kinase. Thus the Orb6 protein kinase coordinates maintenance of cell polarity during interphase with the onset of mitosis. orb6 interacts genetically with orb2, which encodes the Pak1/Shk1 protein kinase, a component of the Ras1 and Cdc42-dependent signaling pathway. Our results suggest that Orb6 may act downstream of Pak1/Shk1, forming part of a pathway coordinating cell morphogenesis with progression through the cell cycle.

Photoregulation of cot-1, a kinase-encoding gene involved in hyphal growth in Neurospora crassa

Blue light plays a key role as an environmental signal in the regulation of growth and development of fungi and plants. Here we demonstrate that in Neurospora crassa hyphae branch more frequently in cultures grown in light. Previous studies had identified cot-1 as a gene that controls apical hyphal cell elongation. In the cot-1 mutant, cessation of elongation is accompanied by hyperbranching. Here we demonstrate that the cot-1 gene encodes two transcript species of about 2100 nt (cot-1 (s)) and about 2400 nt (cot-1 (l)) in length and that the ratio of both transcript species abundance is photoregulated. The origin of the difference between cot-1 (l) and cot-1 (s) was localized to the 5' end of the cot-1 transcripts, suggesting that two COT1 isoforms with different activities may be formed. Both light effects, on branching and on cot-1 expression, were dependent on functional wc-1 and wc-2 gene products. In addition to light, L-sorbose comprises another environmental cue that controls hyphal branching in N. crassa. In the presence of L-sorbose, photoregulation of cot-1 was blocked, suggesting the involvement of alternative and potentially interdependent signaling pathways for the regulation of hyphal elongation/branching.

The role of the hedgehog/patched signaling pathway in epithelial stem cell proliferation: from fly to human

The hedgehog-patched (hh-ptc) intercellular signaling pathway has recently been shown to control the proliferation of epithelial stem cells in both Drosophila and vertebrates. Mutant and ectopic expression analyses in Drosophila suggest that the HH protein diffuses from the signaling cells to promote the proliferation of nearby ovarian somatic stem cells by antagonizing the suppression of its receptor PTC towards the CI transcription factor in the stem cells. Consequently, the transcription of CI-dependent genes leads to stem cell proliferation. This regulatory pathway appears to function also in vertebrates, where defects in ptc cause basal cell carcinoma, tumors of epidermal stem cell origin. Basal cell carcinoma can also be induced by ectopic expression of Sonic hedgehog (shh) or Gli1, the vertebrate homolog of ci. These studies suggest the conservation of the hh signaling pathway in controlling epithelial stem cell divisions among different organisms.

Understanding human cancer in a fly?

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Developmental control of cell cycle regulators: a fly's perspective

During early development in many species, maternally supplied gene products permit the cell cycle to run at maximum velocity, subdividing the fertilized egg into smaller and smaller cells. As development proceeds, zygotic controls are activated that first limit divisions to defined spatial and temporal domains, coordinating them with morphogenesis, and then halt proliferation altogether, to allow cell differentiation. Analysis of the regulation of cyclin-dependent kinases (Cdks) in Drosophila has provided insights into how this embryonic program of cell proliferation is controlled at the molecular level and how it is linked to developmental cues. Recent studies have also begun to reveal how cell proliferation is controlled during the second phase of Drosophila development, which occurs in imaginal tissues. In contrast to their embryonic progenitors, imaginal cells proliferate with a cycle that requires cell growth and is linked to patterning processes controlled by secreted cell signaling molecules. The functions of these signaling molecules appear to be nearly as conserved between vertebrates and invertebrates as the cell cycle control apparatus itself, suggesting that the mechanisms that coordinate growth, patterning, and cell proliferation in developing tissues have ancient origins.

Micronuclear and macronuclear sequences of a Euplotes crassus gene encoding a putative nuclear protein kinase

The sequences of a 1.8-kbp macronuclear DNA molecule (V3), and the majority of its micronuclear counterpart, are reported. The macronuclear V3 DNA molecule contains an open reading frame that is interrupted by a single intron, while the micronuclear copy is interrupted by four internal eliminated sequences, one of which is located within the intron. The predicted protein product of the macronuclear V3 gene is a 471-amino acid polypeptide that is very similar to a group of protein-serine/threonine kinases from both plant and animal species, some of whose members appear to be involved in cell cycle or growth control.

Diversification of cell cycle controls in developing embryos

During embryogenesis, a genetic program coordinates cell proliferation with morphogenesis and cell differentiation. Recent studies using Drosophila have shown how, as development proceeds, this program directs different cell types to acquire unique modes of cell cycle regulation. As maternal cell cycle factors are exhausted and replaced by differentially expressed zygotic factors, an increasing repertoire of gene products become potential regulators of the cycle. Cyclin B, Cdc25, and Cyclin E each act as limiting regulators in Drosophila in specific cell types at particular developmental stages. The genes encoding these and many other candidate regulators have been cloned from mice, but their roles in vivo have yet to be understood.

Drosophila warts--tumor suppressor and member of the myotonic dystrophy protein kinase family

Tumor suppressor genes represent a broad class of genes that normally function in the negative regulation of cell proliferation. Loss-of-function mutations in these genes lead to unrestrained cell proliferation and tumor formation. A fundamental understanding of how tumor suppressor genes regulate cell proliferation and differentiation should reveal important aspects of signalling pathways and cell cycle control. A recent report describing the Drosophila tumor suppressor gene warts has implications in the study of the human myotonic dystrophy gene. These genes encode members of a cyclic AMP-dependent protein kinase subfamily that includes other plant and animal orthologues.