Final Report Summary - SPERMDESTRUCT (Cellular Destruction Mechanisms that Create New Lives)
A key discovery in the field of programmed cell death (PCD), made in the late ‘70s and early ‘80s of the previous century, is the realization that apoptosis, the most abundant form of PCD in metazoa, is executed by an evolutionary conserved family of proteases called caspases. Since then, the combined efforts of many research groups have led to a deep understanding of how caspases are activated and regulated during apoptosis. However, a constantly growing body of research indicates that caspase activation may not necessarily lead to cell death. To date, dozens of caspase-dependent non-lethal cellular processes (CDPs) have been described in a variety of tissues and organisms. On the other hand, there is emerging evidence suggesting that cell death in metazoa can sometimes proceed in the absence of caspases by triggering alternative cell death (ACD) pathways. The Drosophila sperm system is an excellent paradigm to study both CDPs and ACDs. Furthermore, after fertilization, the paternal mitochondria is selectively eliminated by mechanisms originated in the egg, thus also constituting a paradigm to study organelle-specific destruction.
The main goal of this ERC-CoG was to delineate the molecular mechanisms, pathways and components underlying three distinct, evolutionarily conserved, cellular destruction processes, all associated with the Drosophila sperm life cycle, and to screen samples from fertility clinics for genetic mutations associated with human male infertility. The first process, termed germ cell death (GCD), eliminates about 25% of the pre-meiotic germ cells in the adult testis through an ACD pathway. In the second process, the bulk cytoplasmic contents of the terminally differentiating spermatids are removed in a process called ‘individualization’, which involves active caspases, and results in the stereotypical streamlined shape of the mature sperm cells. The third process, termed paternal mitochondria destruction (PMD), occurs immediately after fertilization to selectively eliminate the sperm mitochondria, a process that appears to be highly conserved in evolution, albeit its purpose remains largely vague.
We have made substantial progress with all the four original aims. We demonstrated that the somatic cells that encapsulate the developing germ cells trigger GCD, and uncovered the underlying signaling pathway. We discovered a non-canonical role of a Krebs cycle enzyme subunit in spatially restricting caspase activation during spermatid individualization, thus preventing excessive caspase activity and unwanted cell death. We also demonstrated that egg-derived vacuoles, reminiscent of multivesicular bodies (MVBs), associate with the penetrating sperm plasma membrane immediately after fertilization, and trigger a stereotypical set of cellular processes, eventually leading to the elimination of the sperm mitochondria. Finally, we established a fruitful collaboration with a fertility clinic in Israel and initiated a screen to identify new genetic mutations associated with human male infertility. This screen already yielded several new genetic aberrations associated with male infertility, suggesting that complete screening of the affected genes might be beneficial for clinical evaluation of infertile male patients and make better decisions about the most appropriate treatments.
Not only have we made much progress with the original specific aims, our studies have also been extended to additional derivative paths. We uncovered another ACD pathway, distinct from GCD, thorough which excessive primordial germ cells (PGCs) are eliminated during embryonic development. We also discovered another non-lethal role of caspases in maintaining epithelial tissue integrity by inhibiting unwanted cell migration and invasion. We developed a protocol for isolation of the MVBs from fertilized eggs and identifying their protein contents, which recently identified several proteins with intriguing implications to the idea that the egg “sees” the sperm as a foreign intruder.
Collectively, the period under the ERC grant has been extremely fruitful, allowing us to extend our research to new in vivo paradigms, and address several major unresolved questions in the field. We are grateful for this opportunity.
The main goal of this ERC-CoG was to delineate the molecular mechanisms, pathways and components underlying three distinct, evolutionarily conserved, cellular destruction processes, all associated with the Drosophila sperm life cycle, and to screen samples from fertility clinics for genetic mutations associated with human male infertility. The first process, termed germ cell death (GCD), eliminates about 25% of the pre-meiotic germ cells in the adult testis through an ACD pathway. In the second process, the bulk cytoplasmic contents of the terminally differentiating spermatids are removed in a process called ‘individualization’, which involves active caspases, and results in the stereotypical streamlined shape of the mature sperm cells. The third process, termed paternal mitochondria destruction (PMD), occurs immediately after fertilization to selectively eliminate the sperm mitochondria, a process that appears to be highly conserved in evolution, albeit its purpose remains largely vague.
We have made substantial progress with all the four original aims. We demonstrated that the somatic cells that encapsulate the developing germ cells trigger GCD, and uncovered the underlying signaling pathway. We discovered a non-canonical role of a Krebs cycle enzyme subunit in spatially restricting caspase activation during spermatid individualization, thus preventing excessive caspase activity and unwanted cell death. We also demonstrated that egg-derived vacuoles, reminiscent of multivesicular bodies (MVBs), associate with the penetrating sperm plasma membrane immediately after fertilization, and trigger a stereotypical set of cellular processes, eventually leading to the elimination of the sperm mitochondria. Finally, we established a fruitful collaboration with a fertility clinic in Israel and initiated a screen to identify new genetic mutations associated with human male infertility. This screen already yielded several new genetic aberrations associated with male infertility, suggesting that complete screening of the affected genes might be beneficial for clinical evaluation of infertile male patients and make better decisions about the most appropriate treatments.
Not only have we made much progress with the original specific aims, our studies have also been extended to additional derivative paths. We uncovered another ACD pathway, distinct from GCD, thorough which excessive primordial germ cells (PGCs) are eliminated during embryonic development. We also discovered another non-lethal role of caspases in maintaining epithelial tissue integrity by inhibiting unwanted cell migration and invasion. We developed a protocol for isolation of the MVBs from fertilized eggs and identifying their protein contents, which recently identified several proteins with intriguing implications to the idea that the egg “sees” the sperm as a foreign intruder.
Collectively, the period under the ERC grant has been extremely fruitful, allowing us to extend our research to new in vivo paradigms, and address several major unresolved questions in the field. We are grateful for this opportunity.