Regulation of self-renewal and differentiation in adult stem cell lineages: lessons from the Drosophila male germ line.

The ability to identify stem cells and trace their descendants in vivo has yielded insights into how self-renewal, proliferation, and differentiation are regulated in adult stem cell lineages. Analysis of male germ-line stem cells in Drosophila has revealed the importance of local signals from the microenvironment, the stem cell niche, in controlling stem cell behavior. Germ-line stem cells physically attach to the niche via localized adherens junctions that provide a polarity cue for orientation of centrosomes in interphase and the spindle in mitosis. As a result, stem cells divide asymmetrically: One daughter inherits attachment to the niche and remains within its embrace, whereas the other is displaced away and initiates differentiation. Strikingly, much as leukemia inhibitory factor (LIF) and transforming growth factor-beta (TGF-beta) signaling maintain mouse embryonic stem (ES) cells, maintenance of stem cell state in the Drosophila male germ line is regulated by cytokine-like signals from hub cells that activate the transcription factor STAT (signal transducer and activator of transcription) and TGF-beta class signals from surrounding support cells that repress expression of a key differentiation factor. Surprisingly, transit-amplifying cells can revert to the stem cell state if they reoccupy the niche. Upon cessation of mitosis and the switch to terminal differentiation, germ cells express cell-type- and stage-specific transcription machinery components that drive expression of terminal differentiation genes, in part by removing Polycomb transcriptional silencing machinery.

Stem cell self-renewal specified by JAK-STAT activation in response to a support cell cue.

Stem cells generate many differentiated, short-lived cell types, such as blood, skin, and sperm, throughout adult life. Stem cells maintain a long-term capacity to divide, producing daughter cells that either self-renew or initiate differentiation. Although the surrounding microenvironment or "niche" influences stem cell fate decisions, few signals that emanate from the niche to specify stem cell self-renewal have been identified. Here we demonstrate that the apical hub cells in the Drosophila testis act as a cellular niche that supports stem cell self-renewal. Hub cells express the ligand Unpaired (Upd), which activates the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway in adjacent germ cells to specify self-renewal and continual maintenance of the germ line stem cell population.

Developmental regulation of transcription by a tissue-specific TAF homolog.

Alternate forms of the general transcription machinery have been described in several tissues or cell types. However, the role of tissue-specific TBP-associated factors (TAF(II)s) and other tissue-specific transcription components in regulating differential gene expression during development was not clear. Here we show that the cannonball gene of Drosophila encodes a cell type-specific homolog of a more ubiquitously expressed component of the general transcription factor TFIID. cannonball is required in vivo for high level transcription of a set of stage- and tissue-specific target genes during male gametogenesis. Regulation of transcription by cannonball is absolutely required for spermatogenesis, as null mutations block meiotic cell cycle progression and result in a complete failure of spermatid differentiation. Our results demonstrate that cell type-specific TAF(II)s play an important role in developmental regulation of gene expression.

Control of mitochondrial morphology by a human mitofusin.

Although changes in mitochondrial size and arrangement accompany both cellular differentiation and human disease, the mechanisms that mediate mitochondrial fusion, fission and morphogenesis in mammalian cells are not understood. We have identified two human genes encoding potential mediators of mitochondrial fusion. The mitofusins (Mfn1 and Mfn2) are homologs of the Drosophila protein fuzzy onion (Fzo) that associate with mitochondria and alter mitochondrial morphology when expressed by transient transfection in tissue culture cells. An internal region including a predicted bipartite transmembrane domain (TM) is sufficient to target Mfn2 to mitochondria and requires hydrophobic residues within the TM. Co-expression of Mfn2 with a dominant interfering mutant dynamin-related protein (Drp1(K38A)) proposed to block mitochondrial fission resulted in long mitochondrial filaments and networks. Formation of mitochondrial filaments and networks required a wild-type Mfn2 GTPase domain, suggesting that the Mfn2 GTPase regulates or mediates mitochondrial fusion and that mitofusins and dynamin related GTPases play opposing roles in mitochondrial fusion and fission in mammals, as in yeast.

Genetic analysis of dPsa, the Drosophila orthologue of puromycin-sensitive aminopeptidase, suggests redundancy of aminopeptidases.

Abstract. The Drosophila genome contains a single orthologue of mammalian puromycin-sensitive aminopeptidases, dPsa. Even though dPsa was expressed in many tissues during development, animals lacking dPsa activity were viable. Ubiquitous overexpression of dPsa during embryonic or larval development resulted in lethality and overexpression in isolated tissues during development resulted in localized lesions. These results suggest that even though dPsa function was not essential for viability, dPsa expression must be tightly regulated for normal development. By screening the Drosophila genome we found 43 predicted aminopeptidases and generated a phylogenetic tree of aminopeptidases related to dPsa by sequence. We discuss possible functions of dPsa and the idea that other Drosophila aminopeptidases might perform redundant functions with dPsa for regulating protein turnover.

Transcription of meiotic cell cycle and terminal differentiation genes depends on a conserved chromatin associated protein, whose nuclear localisation is regulated.

The Drosophila always early (aly) gene coordinately regulates meiotic cell cycle progression and terminal differentiation during male gametogenesis. aly is required for transcription of key G2-M cell cycle control genes and of spermatid differentiation genes, and for maintenance of normal chromatin structure in primary spermatocytes. We show that aly encodes a homologue of the Caenorhabditis elegans gene lin-9, a negative regulator of vulval development that acts in the same SynMuvB genetic pathway as the LIN-35 Rb-like protein. The aly gene family is conserved from plants to humans. Aly protein is both cytoplasmic and nuclear in early primary spermatocytes, then resolves to a chromatin-associated pattern. It remains cytoplasmic in a loss-of-function missense allele, suggesting that nuclear localisation is critical for Aly function, and that other factors may alter Aly activity by controlling its subcellular localisation. MAPK activation occurs normally in aly mutant testes. Therefore aly, and by inference lin-9, act in parallel to, or downstream of, activation of MAPK by the RTK-Ras signalling pathway. We favour a model where aly may regulate cell cycle progression and terminal differentiation during male gametogenesis by regulating chromatin conformation in primary spermatocytes.

Somatic support cells restrict germline stem cell self-renewal and promote differentiation.

Stem cells maintain populations of highly differentiated, short-lived cell-types, including blood, skin and sperm, throughout adult life. Understanding the mechanisms that regulate stem cell behaviour is crucial for realizing their potential in regenerative medicine. A fundamental characteristic of stem cells is their capacity for asymmetric division: daughter cells either retain stem cell identity or initiate differentiation. However, stem cells are also capable of symmetric division where both daughters remain stem cells, indicating that mechanisms must exist to balance self-renewal capacity with differentiation. Here we present evidence that support cells surrounding the stem cells restrict self-renewal and control stem cell number by ensuring asymmetric division. Loss of function of the Drosophila Epidermal growth factor receptor in somatic cells disrupted the balance of self-renewal versus differentiation in the male germline, increasing the number of germline stem cells. We propose that activation of this receptor specifies normal behaviour of somatic support cells; in turn, the somatic cells play a guardian role, providing information that prevents self-renewal of stem cell identity by the germ cell they enclose.

A phospholipid kinase regulates actin organization and intercellular bridge formation during germline cytokinesis.

The endgame of cytokinesis can follow one of two pathways depending on developmental context: resolution into separate cells or formation of a stable intercellular bridge. Here we show that the four wheel drive (fwd) gene of Drosophila melanogaster is required for intercellular bridge formation during cytokinesis in male meiosis. In fwd mutant males, contractile rings form and constrict in dividing spermatocytes, but cleavage furrows are unstable and daughter cells fuse together, producing multinucleate spermatids. fwd is shown to encode a phosphatidylinositol 4-kinase (PI 4-kinase), a member of a family of proteins that perform the first step in the synthesis of the key regulatory membrane phospholipid PIP2. Wild-type activity of the fwd PI 4-kinase is required for tyrosine phosphorylation in the cleavage furrow and for normal organization of actin filaments in the constricting contractile ring. Our results suggest a critical role for PI 4-kinases and phosphatidylinositol derivatives during the final stages of cytokinesis.

Developmental genetics of the essential Drosophila nucleoporin nup154: allelic differences due to an outward-directed promoter in the P-element 3' end.

Drosophila nup154 encodes a predicted nucleoporin homologous to yeast Nup170p, Nup157p, and vertebrate Nup155, all of which are major components of the nuclear pore complex (NPC). Unlike its yeast homologs, nup154 is essential for viability. Animals with strong loss-of-function nup154 mutations caused by P-element insertion in the 5'-UTR of the gene died as larvae with small discs, brains, and testes. nup154 mRNA expression appeared developmentally regulated in tissues of wild-type embryos, larvae, and adults, suggesting that new nup154 synthesis is required when assembly of new NPCs is required, as in proliferating or growing tissues. Two additional nup154 alleles also associated with different P-element inserts in the 5'-UTR were viable but had strong loss-of-function sterile phenotypes, including failure to maintain spermatogenic stem cells and failure to progress into vitellogenic stages of oogenesis. Lethality vs. viability correlated with orientation of the P-element inserts in the different alleles. Transcript analysis by 5'-RACE suggested a mechanism for allelic differences: an outward-directed promoter internal to the P-element 3' end able to drive sufficient expression of the nup154 transcript for viability but not for fertility.

Molecular characterization of mutant alleles of the DNA repair/basal transcription factor haywire/ERCC3 in Drosophila.

The haywire gene of Drosophila encodes a putative helicase essential for transcription and nucleotide excision repair. A haywire allele encoding a dominant acting poison product, lethal alleles, and viable but UV-sensitive alleles isolated as revertants of the dominant acting poison allele were molecularly characterized. Sequence analysis of lethal haywire alleles revealed the importance of the nucleotide-binding domain, suggesting an essential role for ATPase activity. The viable haync2 allele, which encodes a poison product, has a single amino acid change in conserved helicase domain VI. This mutation results in accumulation of a 68-kD polypeptide that is much more abundant than the wild-type haywire protein.

Genetic control of cell proliferation and differentiation in Drosophila spermatogenesis.

Outlines of the genetic circuitry regulating male gametogenesis in Drosophila have begun to appear. Cessation of mitotic proliferation and onset of the meiotic program is regulated by the bam and bgcn genes acting within male germ cells and a TGF-beta class signaling cascade in surrounding somatic cells. Onset of spermatid differentiation is regulated by a stage- and tissue-specific transcriptional program controlled by the aly, can, mia and sa genes. A cross-regulatory mechanism might act, in part by controlling expression of the twine cell cycle phosphatase, to delay the G2/M transition of meiosis I until genes required for spermatid differentiation have been transcribed.

Mitochondrial fusion in yeast requires the transmembrane GTPase Fzo1p.

Membrane fusion is required to establish the morphology and cellular distribution of the mitochondrial compartment. In Drosophila, mutations in the fuzzy onions (fzo) GTPase block a developmentally regulated mitochondrial fusion event during spermatogenesis. Here we report that the yeast orthologue of fuzzy onions, Fzo1p, plays a direct and conserved role in mitochondrial fusion. A conditional fzo1 mutation causes the mitochondrial reticulum to fragment and blocks mitochondrial fusion during yeast mating. Fzo1p is a mitochondrial integral membrane protein with its GTPase domain exposed to the cytoplasm. Point mutations that alter conserved residues in the GTPase domain do not affect Fzo1p localization but disrupt mitochondrial fusion. Suborganellar fractionation suggests that Fzo1p spans the outer and is tightly associated with the inner mitochondrial membrane. This topology may be required to coordinate the behavior of the two mitochondrial membranes during the fusion reaction. We propose that the fuzzy onions family of transmembrane GTPases act as molecular switches to regulate a key step in mitochondrial membrane docking and/or fusion.

The DUG gene of Drosophila melanogaster encodes a structural and functional homolog of the S. cerevisiae SUG1 predicted ATPase associated with the 26S proteasome.

The DUG gene of Drosophila encodes a putative ATPase that is a structural and functional homolog of the yeast SUG1 product. When introduced into S. cerevisiae, the Drosophila DUG gene rescued the lethality associated with a SUG1 mutant. Anti-DUG antibodies recognized a protein that migrated in high molecular weight complexes, along with components of the 26S proteasome, and also immunoprecipitated components of the 26S proteasome from embryonic extracts. Proteins recognized by the affinity-purified antibody raised against DUG were localized in either a punctate cytoplasmic distribution or in the nucleus, depending on the cell type, consistent with the subcellular localization of the 26S proteasome in various cell types.

Transcriptional and post-transcriptional control mechanisms coordinate the onset of spermatid differentiation with meiosis I in Drosophila.

The aly, can, mia and sa genes of Drosophila are essential in males both for the G2-meiosis I transition and for onset of spermatid differentiation. Function of all four genes is required for transcription in primary spermatocytes of a suite of spermatid differentiation genes. aly is also required for transcription of the cell cycle control genes cyclin B and twine in primary spermatocytes. In contrast can, mia and sa are required for accumulation of twine protein but not twine transcript. We propose that the can, mia and sa gene products act together or in a pathway to turn on transcription of spermatid differentiation genes, and that aly acts upstream of can, mia and sa to regulate spermatid differentiation. We also propose that control of translation or protein stability regulates entry into the first meiotic division. We suggest that a gene or genes transcribed under the control of can, mia and sa allow(s) accumulation of twine protein, thus coordinating meiotic division with onset of spermatid differentiation.

Developmentally regulated mitochondrial fusion mediated by a conserved, novel, predicted GTPase.

The Drosophila melanogaster fuzzy onions (fzo) gene encodes the first known protein mediator of mitochondrial fusion. During Drosophila spermatogenesis, mitochondria in early postmeiotic spermatids aggregate, fuse, and elongate beside the growing flagellar axoneme. fzo mutant males are defective in this developmentally regulated mitochondrial fusion and are sterile. fzo encodes a large, novel, predicted transmembrane GTPase that becomes detectable on spermatid mitochondria late in meiosis II, just prior to fusion, and disappears soon after fusion is complete. Missense mutations that alter conserved residues required for GTP binding in other GTPases inhibit the fusogenic activity of Fzo in vivo but do not affect its localization. Fzo has homologs of unknown function in mammals, nematodes, and yeast.

Differential expression of two gamma-tubulin isoforms during gametogenesis and development in Drosophila.

Previous work identified a gamma-tubulin gene, gamma Tub23C, in Drosophila (Zheng et al., 1991). We now report identification of a second gamma-tubulin gene, gamma Tub37CD. Immunoblot analysis and immunolocalization show that gamma Tub37CD and gamma Tub23C are differentially expressed during gametogenesis and development. During oogenesis, gamma Tub23C was detected at centrosomes and in the cytoplasm of mitotic germ cells, but was not detected in germ cells following completion of mitosis. Conversely, gamma Tub37CD was not detected in proliferating germ cells, but appeared to accumulate in germ cells during egg chamber development. Neither gamma-tubulin isoform was detected at the anterior or posterior poles of developing oocytes. During spermatogenesis, only gamma Tub23C was detected at centrosomes, where it showed cell cycle- and differentiation-dependent organization. During the transition into the first meiotic division, gamma Tub23C became organized as a corpuscular focus at centrioles until completion of meiosis II. During postmeiotic spermatid differentiation, gamma Tub23C was detected first as a rod and then as a collar-like structure near the juncture of the nucleus and the elongating flagellum, but was not detected in bundles of mature sperm. The germline-specific CDC25 encoded by twine is required for organization of gamma Tub23C into corpuscular focus in spermatocytes, but not for separation of centriole pairs in M-phase or postmeiotic organization of gamma Tub23C at centrioles. Following reconstitution of a canonical centrosome at fertilization, only gamma Tub37CD was detected at centrosomes in syncytial embryos, but both gamma Tub37CD and gamma Tub23C were detected at centrosomes in cellularized embryos. Colocalization of these two isoforms suggests that gamma Tub23C and gamma Tub37CD both contain structural features of gamma-tubulins essential for localization to centrosomes.

Monastral bipolar spindles: implications for dynamic centrosome organization.

Implicit to all models for mitotic spindle assembly is the view that centrosomes are essentially permanent structures. Yet, immunofluorescence revealed that spindles in larval brains of urchin mutants in Drosophila were frequently monastral but bipolar; the astral pole contained a centrosome while the opposing anastral pole showed neither gamma tubulin nor a radial array of astral microtubules. Thus, mutations in the urchin gene seem to uncouple centrosome organization and spindle bipolarity in mitotic cells. Hypomorphic mutants showed a high frequency of monastral bipolar spindles but low frequencies of polyploidy, suggesting that monastral bipolar spindles might be functional. To test this hypothesis, we performed pedigree analysis of centrosome distribution and spindle structure in the four mitotic divisions of gonial cells. Prophase gonial cells showed two centrosomes, suggesting cells entered mitosis with the normal number of centrosomes and that centrosomes separated during prophase. Despite a high frequency of monastral bipolar spindles, the end products of the four mitotic divisions were equivalent in size and chromatin content. These results indicate that monastral bipolar spindles are functional and that the daughter cell derived from the anastral pole can assemble a functional bipolar spindle in the subsequent cell cycle. Cell proliferation despite high frequencies of monastral bipolar spindles can be explained if centrosome structure in mitotic cells is dynamic, allowing transient and benign disorganization of pericentriolar components. Since urchin proved to be allelic to KLP61F which encodes a kinesin related motor protein (Heck et al. (1993) J. Cell Biol. 123, 665-671), our results suggest that motors influence the dynamic organization of centrosomes.

A chromatin-associated kinesin-related protein required for normal mitotic chromosome segregation in Drosophila.

The tiovivo (tio) gene of Drosophila encodes a kinesin-related protein, KLP38B, that colocalizes with condensed chromatin during cell division. Wild-type function of the tio gene product KLP38B is required for normal chromosome segregation during mitosis. Mitotic cells in tio larval brains displayed circular mitotic figures, increased ploidy, and abnormal anaphase figures. KLP38B mRNA is maternally provided and expressed in cells about to undergo division. We propose that KLP38B, perhaps redundantly with other chromosome-associated microtubule motor proteins, contributes to interactions between chromosome arms and microtubules important for establishing bipolar attachment of chromosomes and assembly of stable bipolar spindles.

Assembly of ring canals in the male germ line from structural components of the contractile ring.

Stable intercellular bridges called ring canals form following incomplete cytokinesis, and interconnect mitotically or meiotically related germ cells. We show that ring canals in Drosophila melanogaster males are surprisingly different from those previously described in females. Mature ring canal walls in males lack actin and appear to derive directly from structural proteins associated with the contractile ring. Ring canal assembly in males, as in females, initiates during cytokinesis with the appearance of a ring of phosphotyrosine epitopes at the site of the contractile ring. Following constriction, actin and myosin II disappear. However, at least four proteins present at the contractile ring remain: the three septins (Pnut, Sep1 and Sep2) and anillin. In sharp contrast, in ovarian ring canals, septins have not been detected, anillin is lost from mature ring canals and filamentous actin is a major component. In both males and females, a highly branched vesicular structure, termed the fusome, interconnects developing germ cells via the ring canals and is thought to coordinate mitotic germ cell divisions. We show that, in males, unlike females, the fusome persists and enlarges following cessation of the mitotic divisions, developing additional branches during meiosis. During differentiation, the fusome and its associated ring canals localize to the distal tip of the elongating spermatids.

Coordinate developmental control of the meiotic cell cycle and spermatid differentiation in Drosophila males.

Wild-type function of four Drosophila genes, spermatocyte arrest, cannonball, always early and meiosis I arrest, is required both for cell-cycle progression through the G2/M transition of meiosis I in males and for onset of spermatid differentiation. In males mutant for any one of these meiotic arrest genes, mature primary spermatocytes with partially condensed chromosomes accumulate and postmeiotic cells are lacking. The arrest in cell-cycle progression occurs prior to degradation of cyclin A protein. The block in spermatogenesis in these mutants is not simply a secondary consequence of meiotic cell-cycle arrest, as spermatid differentiation proceeds in males mutant for the cell cycle activating phosphatase twine. Instead, the arrest of both meiosis and spermiogenesis suggests a control point that may serve to coordinate the male meiotic cell cycle with the spermatid differentiation program. The phenotype of the Drosophila meiotic arrest mutants is strikingly similar to the histopathological features of meiosis I maturation arrest infertility in human males, suggesting that the control point may be conserved from flies to man.