ABSTRACT In multicellular organisms, endocrine factors such as hormones and cytokines regulate development and homoeostasis through communication between different organs. For understanding such interorgan communications through endocrine factors, the fruit fly Drosophila melanogaster serves as an excellent model system due to conservation of essential endocrine systems between flies and mammals and availability of powerful genetic tools. In Drosophila and other insects, functions of neuropeptides or peptide hormones from the central nervous system have been extensively studied. However, a series of recent studies conducted in Drosophila revealed that peptide hormones derived from peripheral tissues also play critical roles in regulating multiple biological processes, including growth, metabolism, reproduction, and behaviour. Here, we summarise recent advances in understanding target organs/tissues and functions of peripherally derived peptide hormones in Drosophila and describe how these hormones contribute to various biological events through interorgan communications.

Interorgan communication through peripherally derived peptide hormones in Drosophila

The Notch signaling pathway regulates cell invasion, adhesion, proliferation, apoptosis, and differentiation via cell-to-cell interactions and plays important physiological roles in the ovary. This review summarizes current knowledge about the Notch signaling pathway in relation to ovarian functions and reveals the potential underlying mechanisms. We conducted an in-depth review of relevant literature to determine the current status of research into the Notch signaling pathway in relation to ovarian functioning and reveal potential underlying mechanisms. The activation of different Notch receptors promotes the formation of primordial follicles and proliferation of granulosa cells and inhibits steroid secretion. Abnormal regulation of the Notch signaling pathway or direct mutations might lead to over-activation or under-activation of the receptors, resulting in Notch upregulation or downregulation. It can also disrupt the normal physiological functions of the ovary. The lncRNA HOTAIR and growth hormones improved premature ovarian failure (POF) and promoted follicle maturation in a mouse model of POF by upregulating Notch1 expression. They also stimulated the Notch1 signaling pathway, increased the level of plasma estradiol, and decreased the level of plasma follicle-stimulating hormone. Thus, Notch1 could serve as a novel therapeutic target for POF. Several studies have reported multiple roles of Notch in regulating female primordial follicle formation and follicle maturation. Direct mutations in Notch-related molecules or abnormal gene regulation in the signaling pathway can lead to ovarian dysfunction. However, the underlying mechanisms are not fully understood.

Roles of the Notch Signaling Pathway in Ovarian Functioning

Zebrafish (Danio rerio) has been used as a promising animal model to study gonadal development and gametogenesis. Although previous studies have identified critical molecules participating in zebrafish gonad differentiation, a landscape view of the biological processes involved in this process is still lacking. Here we isolated intact zebrafish differentiating gonads, at 25 days post-fertilization (dpf) and 30 dpf, and conducted RNA-seq analysis between the juvenile gonads that tended to develop into ovaries or testes. Our study demonstrates that the juvenile ovary and testis at 25 dpf and 30 dpf are different at the level of biological process. During ovary differentiation, the biological processes related to metabolic activities in production of energy and maternal substances, RNA degradation, and DNA repair were enriched. During testis differentiation, the biological processes related to cell proliferation, differentiation, and morphogenesis were enriched, with a total of 15 signaling pathways. Notably, we reveal that the immune-related processes are extensively involved in the regulation of testis development. Overall, this study provides a landscape of differentiated biological processes and novel insights into the initiation of sex differentiation in zebrafish.

A landscape of differentiated biological processes involved in the initiation of sex differentiation in zebrafish

Males have the ability to compete for fertilizations through both precopulatory and postcopulatory intrasexual competition. Precopulatory competition has selected for large weapons and other adaptations to maximize access to females and mating opportunities, while postcopulatory competition has resulted in ejaculate adaptations to maximize fertilization success. Negative associations between these strategies support the hypothesis that there is a trade-off between success at pre- and postcopulatory mating success. Recently, this trade-off has been demonstrated with experimental manipulation. Males of the leaf-footed cactus bug Narniafemorata use hind limbs as the primary weapon in male-male competition. However, males can drop a hind limb to avoid entrapment. When this autotomy occurs during development, they invest instead in large testes. While evolutionary outcomes of the trade-offs between pre- and postcopulatory strategies have been identified, less work has been done to identify proximate mechanisms by which the trade-off might occur, perhaps because the systems in which the trade-offs have been investigated are not ones that have the molecular tools required for exploring mechanism. Here, we applied knowledge from a related model species for which we have developmental knowledge and molecular tools, the milkweed bug Oncopeltus fasciatus, to investigate the proximate mechanism by which autotomized N. femorata males developed larger testes. Autotomized males had evidence of a higher rate of transit amplification divisions in the spermatogonia, which would result more spermatocytes and thus in greater sperm numbers. Identification of mechanisms underlying a trade-off can help our understanding of the direction and constraints on evolutionary trajectories and thus the evolutionary potential under multiple forms of selection.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/ece3.7686

The trade-off between investment in weapons and fertility is mediated through spermatogenesis in the leaf-footed cactus bug Narnia femorata.

Phagoptosis is a frequently occurring nonautonomous cell death pathway in which phagocytes eliminate viable cells. While it is thought that phosphatidylserine (PS) “eat-me” signals on target cells initiate the process, the precise sequence of events is largely unknown. Here, we show that in Drosophila testes, progenitor germ cells are spontaneously removed by neighboring cyst cells through phagoptosis. Using live imaging with multiple markers, we demonstrate that cyst cell–derived early/late endosomes and lysosomes fused around live progenitors to acidify them, before DNA fragmentation and substantial PS exposure on the germ cell surface. Furthermore, the phagocytic receptor Draper is expressed on cyst cell membranes and is necessary for phagoptosis. Significantly, germ cell death is blocked by knockdown of either the endosomal component Rab5 or the lysosomal associated protein Lamp1, within the cyst cells. These data ascribe an active role for phagocytic cyst cells in removal of live germ cell progenitors.

The phagocytic cyst cells in Drosophila testis eliminate germ cell progenitors via phagoptosis

In all metazoan species, sperm is produced from germline stem cells. These self-renew and produce daughter cells that amplify and differentiate dependent on interactions with somatic support cells. In the male gonad of Drosophila melanogaster, the germline and somatic cyst cells co-differentiate as cysts, an arrangement in which the germline is completely enclosed by cytoplasmic extensions from the cyst cells. Notch is a developmentally relevant receptor in a pathway requiring immediate proximity with the signal sending cell. Here, we show that Notch is expressed in the cyst cells of wild-type testes. Notch becomes activated in the transition zone, an apical area of the testes in which the cyst cells express stage-specific transcription factors and the enclosed germline finalizes transit-amplifying divisions. Reducing the ligand Delta from the germline cells via RNA-Interference or reducing the receptor Notch from the cyst cells via CRISPR resulted in cell death concomitant with loss of germline cells from the transition zone. This shows that Notch signaling is essential for the survival of the germline stem cell lineage.

/pdf/notch-and-delta-are-required-for-survival-of-the-germline-1fcew39z1a.pdf

Notch and Delta are required for survival of the germline stem cell lineage in testes of Drosophila melanogaster.

Adult stem cells divide to renew the stem cell pool and replenish specialized cells that are lost due to death or usage. However, little is known about the mechanisms regulating how stem cells adjust to a demand for specialized cells. A failure of the stem cells to respond to this demand can have serious consequences, such as tissue loss, or prolonged recovery post injury. Here, we challenge the male germline stem cells (GSCs) of Drosophila melanogaster for the production of specialized cells, sperm cells, using mating experiments. We show that repeated mating reduced the sperm pool and increased the percentage of GSCs in M- and S-phase of the cell cycle. The increase in dividing GSCs depended on the activity of the highly conserved G-proteins. Germline expression of RNA-Interference (RNA-i) constructs against G-proteins, or a dominant negative G-protein eliminated the increase in GSC division frequency in mated males. Consistent with a role for the G-proteins in regulating GSC division frequency, RNA-i against seven out of 35 G-protein coupled receptors (GPCRs) within the germline cells also eliminated the capability of males to increase the numbers of dividing GSCs in response to mating.

/pdf/g-protein-signaling-is-required-for-increasing-germline-stem-4kzp5vl9ta.pdf

G-protein signaling is required for increasing germline stem cell division frequency in response to mating in Drosophila males

Adult stem cells divide to renew the stem cell pool and replenish specialized cells that are lost due to death or usage. However, little is known about the mechanisms regulating how stem cells adjust to a demand for specialized cells. A failure of the stem cells to respond to this demand can have serious consequences, such as tissue loss, or prolonged recovery post injury. 
Here, we challenge the male germline stem cells (GSCs) of Drosophila melanogaster for the production of specialized cells using mating experiments. We show that repeated mating reduced the sperm pool and accelerated germline stem cell (GSC) divisions. The increase in GSC divisions depended on the activity of the highly conserved G-proteins. Germline expression of RNA-Interference (RNA-i) constructs against G-proteins or a dominant negative G-protein eliminated the increase in GSC divisions in mated males. Consistent with a role for the G-proteins in the regulation of GSC divisions, RNA-i against seven out of 35 G-protein coupled receptors (GPCRs) within the germline cells also eliminated the capability of males to accelerate their GSC divisions in response to mating. Our data show that GSCs are receptive to GPCR stimulus, potentially through a network of interactions among multiple signaling pathways.

/pdf/g-protein-signaling-accelerates-stem-cell-divisions-in-10mx834s4p.pdf

Yue Qian

Papers

Notch and Delta are required for survival of the germline stem cell lineage in testes of Drosophila melanogaster.

G-protein signaling is required for increasing germline stem cell division frequency in response to mating in Drosophila males

G-Protein signaling accelerates stem cell divisions in Drosophila males