Fertilization: Conserved sperm factors are no longer a bone of contention
Osteoclasts are multinucleated cells that break down bone for skeletal maintenance, repair, and remodeling. Experiments on mice have established that a gene called DC-stamp (dendritic cell-specific transmembrane protein) is involved in progenitor cells fusing to make osteoclasts (Kukita et al., 2004; Kodama and Kaito, 2020). Two related genes, Dcst1 and Dcst2 (DC-stamp domain containing 1 and 2), are expressed in the testes of mice, and likely share a common ancestor with a group of invertebrate genes required for fertilization (Mei and Singson, 2021), including snky in Drosophila (Wilson et al., 2006), and spe-49 and spe-42 in C. elegans (Kroft et al., 2005; Wilson et al., 2018). Together these sperm-specific genes span between 700 million and one billion years of evolutionarily conserved function.
Now, in eLife, Naokazu Inoue (Fukushima Medical University), Yoshihisa Hagihara (AIST) and Ikuo Wada (Fukushima Medical University) report that the DCST1 and DCST2 proteins are required for fertilization in mice (Inoue et al., 2021). After sperm have migrated to the egg, fertilization involves several stages: the spermatozoa must first interact with and penetrate the egg coat, and then adhere to the egg plasma membrane. Next, the plasma membrane of the sperm and egg must fuse to form a zygote. The sperm of male mice lacking the genes Dcst1 and Dcst2 can penetrate the egg coat, but they are unable to fuse: this indicates that these genes have a direct or indirect role in cell fusion that is reminiscent of the role of DC-stamp in osteoclast formation.
Comparing DCST1 and DCST2 to related invertebrate and human proteins, Inoue et al. found that mouse DCST1 was most closely related to human DCST1, nematode SPE-49 and fruit fly SNKY, whereas mouse DCST2 was closer to human DCST2, nematode SPE-42 and fruit fly DCST2. Single-gene knockouts of Dcst1 and Dcst2, as well as double-knockout mice, exhibited male-specific sterility, with mutant spermatozoa failing to fertilize eggs in vitro. The spermatozoa from the double knockouts could reach the egg and undergo the acrosome reaction to penetrate the egg coat, but then they accumulated in the region between the egg coat and the egg membrane (Figure 1).
These results indicate that DCST1 and 2 are not required for sperm migration to egg, the acrosome reaction, or penetration of the egg coat. In fact, the phenotype of the mouse double mutant is similar to that of mice lacking other key sperm molecules during fertilization, including the immunoglobulin superfamily proteins IZUMO1 and SPACA6. These two proteins are involved in sperm-egg recognition, adhesion or fusion (Bianchi and Wright, 2020). When spermatozoa mutant for both Dcst1 and Dcst2 contacted the egg in vitro, it appeared that IZUMO1, its egg-surface binding partner JUNO, and an egg-surface molecule called CD9, were all recruited normally to the interface between the sperm and the egg. This suggests that key molecules are recruited normally despite fusion failing.
Inoue et al. next investigated the presence of IZUMO1 and SPACA6 in sperm mutant for different molecules. IZUMO1 was present in sperm lacking Dcst1 and Dcst2, and also in sperm mutant for Spaca6. SPACA6, on the other hand, was lost in Izumo1, Dcst1, and Dcst2 mutant sperm. These results suggest that these proteins, which are all needed for sperm-egg fusion, likely assemble in a hierarchical fashion, with IZUMO1 being assembled independently of other molecules (Krauchunas et al., 2016). Further analyses of these proteins in various mutant backgrounds may provide new insights into how they assemble and interact during fertilization.
The groundbreaking work of Inoue et al. suggests important future questions. Why are two similar proteins both required non-redundantly for fertility in worms and mammals? Precise protein localization, domain swapping, and studies examining the relationship between structure and function could shed light on this question. Additionally, the biochemical role of these proteins is not clear. It is possible that they act as signaling molecules with an unknown ligand (Chiu et al., 2017). However, loss of function phenotypes appear consistent with some role in either membrane fusion (in mammals and worms) or in membrane breakdown (in flies; Figure 1).
DCST1 and DCST2 and related proteins could be better understood by investigating the molecules they interact with. For instance, it has been shown in C. elegans that SPE-42 binds to other sperm membrane proteins involved in spermatogenesis and fertilization (Marcello et al., 2018). Its interaction with the dysferlin FER-1 is particularly intriguing, since FER-1 regulates calcium-mediated membrane fusion during worm spermatogenesis (Washington and Ward, 2006). Mutations in a human dysferlin gene are associated with limb-girdle muscular dystrophy due to a loss of membrane repair in skeletal muscles (Bashir et al., 1998). This indicates that, in addition to a better understanding of fertilization, ongoing work on genes related to Dcst1 and Dcst2 may provide new insights into muscle and bone health.
In most species, relatively few gamete interaction molecules have been genetically defined (Mei and Singson, 2021), so the existence of conserved gamete interaction genes between invertebrates and mammals has been ‘a bone of contention’. A manuscript recently posted on bioRxiv confirms the role of Dcst1 and Dcst2 in male fertility described by Inoue and co-workers (Noda et al., 2021). This paper further demonstrates that zebrafish dcst1/2 are also required for fertilization. The characterization of genes related to sperm Dcst1 and Dcst2 in diverse species, including humans, should go a long way towards ending debates over deeply conserved gamete function genes. As the pace of fertility gene discovery increases in both vertebrate and invertebrate model systems, we fully expect that more fundamental molecular parallels and key features of the interaction between sperm and egg will be discovered.
References
-
Dendritic cell-specific transmembrane protein (DC-STAMP) regulates osteoclast differentiation via the Ca2+ /NFATc1 axisJournal of Cellular Physiology 232:2538–2549.https://doi.org/10.1002/jcp.25638
-
Osteoclast multinucleation: review of current literatureInternational Journal of Molecular Sciences 21:5685.https://doi.org/10.3390/ijms21165685
-
The molecular complexity of fertilization: introducing the concept of a fertilization synapseMolecular Reproduction and Development 83:376–386.https://doi.org/10.1002/mrd.22634
-
RANKL-induced DC-STAMP is essential for osteoclastogenesisJournal of Experimental Medicine 200:941–946.https://doi.org/10.1084/jem.20040518
-
Caenorhabditis elegans sperm membrane protein interactomeBiology of Reproduction 98:776–783.https://doi.org/10.1093/biolre/ioy055
-
FER-1 regulates Ca2+ -mediated membrane fusion during C. elegans spermatogenesisJournal of Cell Science 119:2552–2562.https://doi.org/10.1242/jcs.02980
-
The Caenorhabditis elegans spe-49 gene is required for fertilization and encodes a sperm-specific transmembrane protein homologous to SPE-42Molecular Reproduction and Development 85:563–578.https://doi.org/10.1002/mrd.22992
Article and author information
Author details
Publication history
Copyright
© 2021, Mei and Singson
This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.
Metrics
-
- 989
- views
-
- 94
- downloads
-
- 0
- citations
Views, downloads and citations are aggregated across all versions of this paper published by eLife.
Download links
Downloads (link to download the article as PDF)
Open citations (links to open the citations from this article in various online reference manager services)
Cite this article (links to download the citations from this article in formats compatible with various reference manager tools)
Further reading
-
- Developmental Biology
The morphogen FGF8 establishes graded positional cues imparting regional cellular responses via modulation of early target genes. The roles of FGF signaling and its effector genes remain poorly characterized in human experimental models mimicking early fetal telencephalic development. We used hiPSC-derived cerebral organoids as an in vitro platform to investigate the effect of FGF8 signaling on neural identity and differentiation. We found that FGF8 treatment increases cellular heterogeneity, leading to distinct telencephalic and mesencephalic-like domains that co-develop in multi-regional organoids. Within telencephalic regions, FGF8 affects the anteroposterior and dorsoventral identity of neural progenitors and the balance between GABAergic and glutamatergic neurons, thus impacting spontaneous neuronal network activity. Moreover, FGF8 efficiently modulates key regulators responsible for several human neurodevelopmental disorders. Overall, our results show that FGF8 signaling is directly involved in both regional patterning and cellular diversity in human cerebral organoids and in modulating genes associated with normal and pathological neural development.
-
- Developmental Biology
Wnt signaling plays crucial roles in embryonic patterning including the regulation of convergent extension (CE) during gastrulation, the establishment of the dorsal axis, and later, craniofacial morphogenesis. Further, Wnt signaling is a crucial regulator of craniofacial morphogenesis. The adapter proteins Dact1 and Dact2 modulate the Wnt signaling pathway through binding to Disheveled. However, the distinct relative functions of Dact1 and Dact2 during embryogenesis remain unclear. We found that dact1 and dact2 genes have dynamic spatiotemporal expression domains that are reciprocal to one another suggesting distinct functions during zebrafish embryogenesis. Both dact1 and dact2 contribute to axis extension, with compound mutants exhibiting a similar CE defect and craniofacial phenotype to the wnt11f2 mutant. Utilizing single-cell RNAseq and an established noncanonical Wnt pathway mutant with a shortened axis (gpc4), we identified dact1/2-specific roles during early development. Comparative whole transcriptome analysis between wildtype and gpc4 and wildtype and dact1/2 compound mutants revealed a novel role for dact1/2 in regulating the mRNA expression of the classical calpain capn8. Overexpression of capn8 phenocopies dact1/2 craniofacial dysmorphology. These results identify a previously unappreciated role of capn8 and calcium-dependent proteolysis during embryogenesis. Taken together, our findings highlight the distinct and overlapping roles of dact1 and dact2 in embryonic craniofacial development, providing new insights into the multifaceted regulation of Wnt signaling.