Dynamic Interplay of Spectrosome and Centrosome Organelles in Asymmetric Stem Cell Divisions

Stem cells have remarkable self-renewal ability and differentiation potency, which are critical for tissue repair and tissue homeostasis. Recently it has been found, in many systems (e.g. gut, neurons, and hematopoietic stem cells), that the self-renewal and differentiation balance is maintained when the stem cells divide asymmetrically. Drosophila male germline stem cells (GSCs), one of the best characterized model systems with well-defined stem cell niches, were reported to divide asymmetrically, where centrosome plays an important role. Utilizing time-lapse live cell imaging, customized tracking, and image processing programs, we found that most acentrosomal GSCs have the spectrosomes reposition from the basal end (wild type) to the apical end close to hub-GSC interface (acentrosomal GSCs). In addition, these apically positioned spectrosomes were mostly stationary while the basally positioned spectrosomes were mobile. For acentrosomal GSCs, their mitotic spindles were still highly oriented and divided asymmetrically with longer mitosis duration, resulting in asymmetric divisions. Moreover, when the spectrosome was knocked out, the centrosomes velocity decreased and centrosomes located closer to hub-GSC interface. We propose that in male GSCs, the spectrosome recruited to the apical end plays a complimentary role in ensuring proper spindle orientation when centrosome function is compromised.     

Morphology and ultrastructure of the germarium in panoistic ovarioles of a basal “apterygotous” insect,Thermobia domestica

It has been shown that in Drosophila the germline stem cells (GSCs), similar to the germline and non-germline stem cells of other species, develop and function in specialized microenvironments formed by somatic cells, referred to as the niches. In the fruit fly ovaries, the female GSCs divide asymmetrically to produce new GSCs and the progenitor cells, the cystoblasts (Cbs). Each Cb then divides to generate the cyst composed of 16 interconnected sibling cells, the cystocytes. After cyst formation, specific molecules are transferred to one of the cystocytes which differentiates into the oocyte, whereas the other 15 cystocytes become the nurse cells. We have studied morphology and ultrastructure of the germaria in the ovarioles (ovaries) of a basal “apterygotous” insect, the firebrat (Thermobia domestica). Our analyses have revealed that in this insect, putative GSCs are present along the anterior apex of the germarium. These cells are separated from each other and from the basement lamina covering the ovariole by characteristic somatic cells, termed the apical somatic cells (ASCs), or their elongated processes. We believe that all the ASCs of a given ovariole constitute a “dispersed” niche in which putative GSCs are anchored. Our analyses have additionally shown that in Thermobia, both the Cbs and young (meiotic) oocytes are always individual and never form syncytial cysts. These findings indicate that in certain basal insects the syncytial phase of oogenesis has been eliminated during evolution. Finally, we show that in the early meiotic oocytes of Thermobia, during the so-called bouquet stage, prominent Balbiani bodies (Bbs) are formed. Analysis of serial micrographs indicates that the Bbs invariably arise next to the segment of the nuclear envelope to which the telomeres of the bouquet chromosomes are attached. We suggest, in the light of these data, that the localization of the Bb together with the polar attachment of the bouquet chromosomes play a crucial role in the early asymmetrization of Thermobia oocytes.     

Dynamics of the male germline stem cell population during aging of Drosophila melanogaster

Drosophila melanogaster has emerged as an important model system for the study of both stem cell biology and aging. Much is known about how molecular signals from the somatic niche regulate adult stem cells in the germline, and a variety of environmental factors as well as single point mutations have been shown to affect lifespan. Relatively little is known, however, about how aging affects specific populations of cells, particularly adult stem cells that may be susceptible to aging-related damage. Here we show that male germline stem cells (GSCs) are lost from the stem cell niche during aging, but are efficiently replaced to maintain overall stem cell number. We also find that the division rate of GSCs slows significantly during aging, and that this slowing correlates with a reduction in the number of somatic hub cells that contribute to the stem cell niche. Interestingly, slowing of stem cell division rate was not observed in long-lived methuselah mutant flies. We finally investigated whether two mechanisms that are thought to be used in other adult stem cell types to minimize the effects of aging were operative in this system. First, in many adult tissues stem cells exhibit markedly fewer cell cycles relative to transit-amplifying cells, presumably protecting the stem cell pool from replication-associated damage. Second, at any given time not all stem cells actively cycle, leading to 'clonal succession' from the reserve pool of initially quiescent stem cells. We find that neither of these mechanisms is used in Drosophila male GSCs.     

A novel mode of colony formation in a hydrozoan through fusion of sexually generated individuals

Coloniality, as displayed by most hydrozoans, is thought to confer a size advantage in substrate-limited benthic marine environments and affects nearly every aspect of a species' ecology and evolution. Hydrozoan colonies normally develop through asexual budding of polyps that remain interconnected by continuous epithelia. The clade Aplanulata is unique in that it comprises mostly solitary species, including the model organism Hydra, with only a few colonial species. We reconstruct a multigene phylogeny to trace the evolution of coloniality in Aplanulata, revealing that the ancestor of Aplanulata was solitary and that coloniality was regained in the genus Ectopleura. Examination of Ectopleura larynx development reveals a unique type of colony formation never before described in Hydrozoa, in that colonies form through sexual reproduction followed by epithelial fusion of offspring polyps to adults. We characterize the expression of manacle, a gene involved in foot development in Hydra, to determine polyp-colony boundaries. Our results suggest that stalks beneath the neck do not have polyp identity and instead are specialized structures that interconnect polyps. Epithelial fusion, brooding behavior, and the presence of a skeleton were all key factors behind the evolution of this novel pathway to coloniality in Ectopleura.     

A Krüppel-like factor is required for development and regeneration of germline and yolk cells from somatic stem cells in planarians

Sexually reproducing animals segregate their germline from their soma. In addition to gamete-producing gonads, planarian and parasitic flatworm reproduction relies on yolk cell–generating accessory reproductive organs (vitellaria) supporting development of yolkless oocytes. Despite the importance of vitellaria for flatworm reproduction (and parasite transmission), little is known about this unique evolutionary innovation. Here, we examine reproductive system development in the planarian Schmidtea mediterranea, in which pluripotent stem cells generate both somatic and germ cell lineages. We show that a homolog of the pluripotency factor Klf4 is expressed in primordial germ cells (PGCs), presumptive germline stem cells (GSCs), and yolk cell progenitors. Knockdown of this klf4-like (klf4l) gene results in animals that fail to specify or maintain germ cells; surprisingly, they also fail to maintain yolk cells. We find that yolk cells display germ cell–like attributes and that vitellaria are structurally analogous to gonads. In addition to identifying a new proliferative cell population in planarians (yolk cell progenitors) and defining its niche, our work provides evidence supporting the hypothesis that flatworm germ cells and yolk cells share a common evolutionary origin.     

Scratching the niche that controls Caenorhabditis elegans germline stem cells

The Caenorhabditis elegans gonad provides a well-defined model for a stem cell niche and its control of self-renewal and differentiation. The distal tip cell (DTC) forms a mesenchymal niche that controls germline stem cells (GSCs), both to generate the germline tissue during development and to maintain it during adulthood. The DTC uses GLP-1/Notch signaling to regulate GSCs; germ cells respond to Notch signaling with a network of RNA regulators to control the decision between self-renewal and entry into the meiotic cell cycle.     

A low-molecular weight factor from colonial hydroids affects body proportioning and cell differentiation

Treatment of metamorphosing planulae of Hydractinia echinata with proportion altering factor (PAF) causes oversizing in the hypostome of developing primary polyps. With increasing concentrations more specimens fail to acquire the appearance of a primary polyp but remain as cones lacking tentacles. Yet, such specimens are true primary polyps as they contain neurons of RF-amide-like immunoreactivity that are typical of the polyp stage. Numbers of RF-amide-positive cells are increased up to 2.5 times compared with the normal value. In intact Hydra vulgaris, PAF increases the number of neurons that differentiate from cycling precursor cells, or even from stem cells. The effect is dose dependent.     

The C. elegans adult male germline: Stem cells and sexual dimorphism

The hermaphrodite Caenorhabditis elegans germline has become a classic model for stem cell regulation, but the male C. elegans germline has been largely neglected. This work provides a cellular analysis of the adult C. elegans male germline, focusing on its predicted stem cell region in the distal gonad. The goals of this study were two-fold: to establish the C. elegans male germline as a stem cell model and to identify sex-specific traits of potential relevance to the sperm/oocyte decision. Our results support two major conclusions. First, adult males do indeed possess a population of germline stem cells (GSCs) with properties similar to those of hermaphrodite GSCs (lack of cell cycle quiescence and lack of reproducibly oriented divisions). Second, germ cells in the mitotic region, including those most distal within the niche, exhibit sex-specific behaviors (e.g. cell cycle length) and therefore have acquired sexual identity. Previous studies demonstrated that some germ cells are not committed to a sperm or oocyte cell fate, even in adults. We propose that germ cells can acquire sexual identity without being committed to a sperm or oocyte cell fate.     

Live imaging of the Drosophila spermatogonial stem cell niche reveals novel mechanisms regulating germline stem cell output

Adult stem cells modulate their output by varying between symmetric and asymmetric divisions, but have rarely been observed in living intact tissues. Germline stem cells (GSCs) in the Drosophila testis are anchored to somatic hub cells and were thought to exclusively undergo oriented asymmetric divisions, producing one stem cell that remains hub-anchored and one daughter cell displaced out of the stem cell-maintaining micro-environment (niche). We developed extended live imaging of the Drosophila testis niche, allowing us to track individual germline cells. Surprisingly, new wild-type GSCs are generated in the niche during steady-state tissue maintenance by a previously undetected event we term 'symmetric renewal', where interconnected GSC-daughter cell pairs swivel such that both cells contact the hub. We also captured GSCs undergoing direct differentiation by detaching from the hub. Following starvation-induced GSC loss, GSC numbers are restored by symmetric renewals. Furthermore, upon more severe (genetically induced) GSC loss, both symmetric renewal and de-differentiation (where interconnected spermatogonia fragment into pairs while moving towards then establishing contact with the hub) occur simultaneously to replenish the GSC pool. Thus, stereotypically oriented stem cell divisions are not always correlated with an asymmetric outcome in cell fate, and changes in stem cell output are governed by altered signals in response to tissue requirements.     

Clonal expansion of ovarian germline stem cells during niche formation in Drosophila

Stem cell niches are specific regulatory microenvironments formed by neighboring stromal cells. Owing to difficulties in identifying stem cells and their niches in many systems, mechanisms that control niche formation and stem cell recruitment remain elusive. In the Drosophila ovary, two or three germline stem cells (GSCs) have recently been shown to reside in a niche, in which terminal filaments (TFs) and cap cells are two major components. We report that signals from newly formed niches promote clonal expansion of GSCs during niche formation in the Drosophila ovary. After the formation of TFs and cap cells, anterior primordial germ cells (PGCs) adjacent to TFs/cap cells can develop into GSCs at the early pupal stage while the rest directly differentiate. The anterior PGCs are very mitotically active and exhibit two division patterns with respect to cap cells. One of these patterns generates two daughters that both contact cap cells and potentially become GSCs. Our lineage tracing study confirms that one PGC can generate two or three GSCs to occupy a whole niche ('clonal expansion'). decapentaplegic (dpp), the Drosophila homolog of human bone morphogenetic protein 2/4, is expressed in anterior somatic cells of the gonad, including TFs/cap cells. dpp overexpression promotes PGC proliferation and causes the accumulation of more PGCs in the gonad. A single PGC mutant for thick veins, encoding an essential dpp receptor, loses the ability to clonally populate a niche. Therefore, dpp is probably one of the mitotic signals that promote the clonal expansion of GSCs in a niche. This study also suggests that signals from newly formed niche cells are important for expanding stem cells and populating niches.     

Specific Tandem 3'UTR Patterns and Gene Expression Profiles in Mouse Thy1+ Germline Stem Cells

A recently developed strategy of sequencing alternative polyadenylation (APA) sites (SAPAS) with second-generation sequencing technology can be used to explore complete genome-wide patterns of tandem APA sites and global gene expression profiles. spermatogonial stem cells (SSCs) maintain long-term reproductive abilities in male mammals. The detailed mechanisms by which SSCs self-renew and generate mature spermatozoa are not clear. To understand the specific alternative polyadenylation pattern and global gene expression profile of male germline stem cells (GSCs, mainly referred to SSCs here), we isolated and purified mouse Thy1⁺ cells from testis by magnetic-activated cell sorting (MACS) and then used the SAPAS method for analysis, using pluripotent embryonic stem cells (ESCs) and differentiated mouse embryonic fibroblast cells (MEFs) as controls. As a result, we obtained 99,944 poly(A) sites, approximately 40% of which were newly detected in our experiments. These poly(A) sites originated from three mouse cell types and covered 17,499 genes, including 831 long non-coding RNA (lncRNA) genes. We observed that GSCs tend to have shorter 3''UTR lengths while MEFs tend towards longer 3''UTR lengths. We also identified 1337 genes that were highly expressed in GSCs, and these genes were highly consistent with the functional characteristics of GSCs. Our detailed bioinformatics analysis identified APA site-switching events at 3''UTRs and many new specifically expressed genes in GSCs, which we experimentally confirmed. Furthermore, qRT-PCR was performed to validate several events of the 334 genes with distal-to-proximal poly(A) switch in GSCs. Consistently APA reporter assay confirmed the total 3''UTR shortening in GSCs compared to MEFs. We also analyzed the cis elements around the proximal poly(A) site preferentially used in GSCs and found C-rich elements may contribute to this regulation. Overall, our results identified the expression level and polyadenylation site profiles and these data provide new insights into the processes potentially involved in the GSC life cycle and spermatogenesis.     

E-Cadherin Is Required for Centrosome and Spindle Orientation in Drosophila Male Germline Stem Cells

Many adult stem cells reside in a special microenvironment known as the niche, where they receive essential signals that specify stem cell identity. Cell-cell adhesion mediated by cadherin and integrin plays a crucial role in maintaining stem cells within the niche. In Drosophila melanogaster, male germline stem cells (GSCs) are attached to niche component cells (i.e., the hub) via adherens junctions. The GSC centrosomes and spindle are oriented toward the hub-GSC junction, where E-cadherin-based adherens junctions are highly concentrated. For this reason, adherens junctions are thought to provide a polarity cue for GSCs to enable proper orientation of centrosomes and spindles, a critical step toward asymmetric stem cell division. However, understanding the role of E-cadherin in GSC polarity has been challenging, since GSCs carrying E-cadherin mutations are not maintained in the niche. Here, we tested whether E-cadherin is required for GSC polarity by expressing a dominant-negative form of E-cadherin. We found that E-cadherin is indeed required for polarizing GSCs toward the hub cells, an effect that may be mediated by Apc2. We also demonstrated that E-cadherin is required for the GSC centrosome orientation checkpoint, which prevents mitosis when centrosomes are not correctly oriented. We propose that E-cadherin orchestrates multiple aspects of stem cell behavior, including polarization of stem cells toward the stem cell-niche interface and adhesion of stem cells to the niche supporting cells.     

From stem cell to embryo without centrioles

Centrosome asymmetry plays a key role in ensuring the asymmetric division of Drosophila neural stem cells (neuroblasts [NBs]) and male germline stem cells (GSCs) [1-3]. In both cases, one centrosome is anchored close to a specific cortical region during interphase, thus defining the orientation of the spindle during the ensuing mitosis. To test whether asymmetric centrosome behavior is a general feature of stem cells, we have studied female GSCs, which divide asymmetrically, producing another GSC and a cystoblast. The cystoblast then divides and matures into an oocyte, a process in which centrosomes exhibit a series of complex behaviors proposed to play a crucial role in oogenesis [4-6]. We show that the interphase centrosome does not define spindle orientation in female GSCs and that DSas-4 mutant GSCs [7], lacking centrioles and centrosomes, invariably divide asymmetrically to produce cystoblasts that proceed normally through oogenesis-remarkably, oocyte specification, microtubule organization, and mRNA localization are all unperturbed. Mature oocytes can be fertilized, but embryos that cannot support centriole replication arrest very early in development. Thus, centrosomes are dispensable for oogenesis but essential for early embryogenesis. These results reveal that asymmetric centrosome behavior is not an essential feature of stem cell divisions.     

Notch Signaling Mediates the Age-Associated Decrease in Adhesion of Germline Stem Cells to the Niche

Stem cells have an innate ability to occupy their stem cell niche, which in turn, is optimized to house stem cells. Organ aging is associated with reduced stem cell occupancy in the niche, but the mechanisms involved are poorly understood. Here, we report that Notch signaling is increased with age in Drosophila female germline stem cells (GSCs), and this results in their removal from the niche. Clonal analysis revealed that GSCs with low levels of Notch signaling exhibit increased adhesiveness to the niche, thereby out-competing their neighbors with higher levels of Notch; adhesiveness is altered through regulation of E-cadherin expression. Experimental enhancement of Notch signaling in GSCs hastens their age-dependent loss from the niche, and such loss is at least partially mediated by Sex lethal. However, disruption of Notch signaling in GSCs does not delay GSC loss during aging, and nor does it affect BMP signaling, which promotes self-renewal of GSCs. Finally, we show that in contrast to GSCs, Notch activation in the niche (which maintains niche integrity, and thus mediates GSC retention) is reduced with age, indicating that Notch signaling regulates GSC niche occupancy both intrinsically and extrinsically. Our findings expose a novel role of Notch signaling in controlling GSC-niche adhesion in response to aging, and are also of relevance to metastatic cancer cells, in which Notch signaling suppresses cell adhesion.     

A Highlights from MBoC Selection: Centrosome-dependent asymmetric inheritance of the midbody ring in Drosophila germline stem cell division

Many stem cells, including Drosophila germline stem cells (GSCs), divide asymmetrically, producing one stem cell and one differentiating daughter. Cytokinesis is often asymmetric, in that only one daughter cell inherits the midbody ring (MR) upon completion of abscission even in apparently symmetrically dividing cells. However, whether the asymmetry in cytokinesis correlates with cell fate or has functional relevance has been poorly explored. Here we show that the MR is asymmetrically segregated during GSC divisions in a centrosome age–dependent manner: male GSCs, which inherit the mother centrosome, exclude the MR, whereas female GSCs, which we here show inherit the daughter centrosome, inherit the MR. We further show that stem cell identity correlates with the mode of MR inheritance. Together our data suggest that the MR does not inherently dictate stem cell identity, although its stereotypical inheritance is under the control of stemness and potentially provides a platform for asymmetric segregation of certain factors.     

Aging shifts mitochondrial dynamics toward fission to promote germline stem cell loss

Changes in mitochondrial dynamics (fusion and fission) are known to occur during stem cell differentiation; however, the role of this phenomenon in tissue aging remains unclear. Here, we report that mitochondrial dynamics are shifted toward fission during aging of Drosophila ovarian germline stem cells (GSCs), and this shift contributes to aging‐related GSC loss. We found that as GSCs age, mitochondrial fragmentation and expression of the mitochondrial fission regulator, Dynamin‐related protein (Drp1), are both increased, while mitochondrial membrane potential is reduced. Moreover, preventing mitochondrial fusion in GSCs results in highly fragmented depolarized mitochondria, decreased BMP stemness signaling, impaired fatty acid metabolism, and GSC loss. Conversely, forcing mitochondrial elongation promotes GSC attachment to the niche. Importantly, maintenance of aging GSCs can be enhanced by suppressing Drp1 expression to prevent mitochondrial fission or treating with rapamycin, which is known to promote autophagy via TOR inhibition. Overall, our results show that mitochondrial dynamics are altered during physiological aging, affecting stem cell homeostasis via coordinated changes in stemness signaling, niche contact, and cellular metabolism. Such effects may also be highly relevant to other stem cell types and aging‐induced tissue degeneration.     

Abscission Is Regulated by the ESCRT-III Protein Shrub in Drosophila Germline Stem Cells

Abscission is the final event of cytokinesis that leads to the physical separation of the two daughter cells. Recent technical advances have allowed a better understanding of the cellular and molecular events leading to abscission in isolated yeast or mammalian cells. However, how abscission is regulated in different cell types or in a developing organism remains poorly understood. Here, we characterized the function of the ESCRT-III protein Shrub during cytokinesis in germ cells undergoing a series of complete and incomplete divisions. We found that Shrub is required for complete abscission, and that levels of Shrub are critical for proper timing of abscission. Loss or gain of Shrub delays abscission in germline stem cells (GSCs), and leads to the formation of stem-cysts, where daughter cells share the same cytoplasm as the mother stem cell and cannot differentiate. In addition, our results indicate a negative regulation of Shrub by the Aurora B kinase during GSC abscission. Finally, we found that Lethal giant discs (lgd), known to be required for Shrub function in the endosomal pathway, also regulates the duration of abscission in GSCs.     

Asymmetric division of cyst stem cells in Drosophila testis is ensured by anaphase spindle repositioning

Many stem cells divide asymmetrically to balance self-renewal and differentiation. In Drosophila testes, two stem cell populations, germline stem cells (GSCs) and somatic cyst stem cells (CySCs), cohere and regulate one another. Here, we report that CySCs divide asymmetrically through repositioning the mitotic spindle around anaphase. CySC spindle repositioning requires functional centrosomes, Dynein and the actin-membrane linker Moesin. Anaphase spindle repositioning is required to achieve high-fidelity asymmetric divisions in CySCs, thus maintaining both GSC and CySC numbers. We propose that dynamic spindle repositioning allows CySCs to divide asymmetrically while accommodating the structure of the GSCs they encapsulate.