Sperm motility, the capacity of sperm cells to navigate through the female reproductive tract towards the oocyte, is fundamental for successful fertilization. Despite decades of research, the intricate protein networks orchestrating this motility remain only partially mapped. The axoneme, a highly conserved microtubule-based structure forming the skeleton of eukaryotic cilia and flagella, houses a myriad of proteins critical to flagellar beating and energy transduction. CFAP91—Cilia and Flagella Associated Protein 91—has been identified previously as a scaffolding component within the axoneme, but its interactome and functional partners in sperm remained largely unknown.

Leveraging a proximity-dependent biotin identification technique—proximity labeling—researchers were able to go beyond traditional protein interaction studies. This method allows for the tagging of proteins that lie in the immediate vicinity of a bait protein under physiological conditions, providing a refined snapshot of protein neighborhoods within complex structures like the axoneme. CFAP91 was used as the bait, and by fusing it with an engineered enzyme that biotinylates nearby proteins, the team successfully captured a dynamic interactome within sperm flagella.

Among the proteins identified, EFCAB5 (EF-hand Calcium Binding domain 5) stood out due to its unique domain architecture and its potential calcium regulation capability. Calcium ions have long been implicated in modulating flagellar beating patterns, yet how specific calcium-binding proteins integrate into the axonemal framework has remained speculative. EFCAB5’s designation suggested a pivotal role in sensing or transducing calcium signals necessary for motility regulation.

Subsequent functional characterization using gene-editing techniques in murine sperm elucidated the essentiality of EFCAB5 for proper flagellar function. Loss of EFCAB5 led to severely impaired sperm motility characterized by erratic and non-progressive movement patterns. High-speed video microscopy and subsequent kinematic analyses revealed that the absence of EFCAB5 disrupted the fine-tuned coordination of axonemal components that produce symmetrical and well-tempered flagellar waves. This deficient motility phenotype closely mirrors clinical characteristics observed in certain cases of idiopathic male infertility.

The mechanistic investigation extended to decipher how EFCAB5 interacts with CFAP91 and other axonemal proteins. Co-immunoprecipitation assays, combined with in situ proximity ligation, confirmed a physical and functional coupling between EFCAB5 and CFAP91 within the radial spoke complex—a critical regulator of flagellar beat frequency and amplitude. This suggests that EFCAB5 functions as a crucial calcium sensor or modulator that influences axonemal mechanics via CFAP91, thus directly impacting the physical forces driving sperm progressivity.

Notably, calcium regulation in flagella is a rapidly reversible and highly localized process, critical for enabling sperm to adjust their motility patterns in response to chemical and mechanical cues encountered during their journey in the female reproductive tract. EFCAB5’s ability to bind calcium via its EF-hand domains likely positions it as a molecular switch modulating conformational changes in the axoneme, thereby translating transient calcium fluxes into mechanical output. Such nuanced regulation extends our conceptual framework of how local signaling cascades integrate with cytoskeletal dynamics to generate motility.

From a methodological perspective, the success of proximity labeling in dissecting the axonemal proteome underscores the power of innovative biochemical tools to map organelle-specific interactomes. Traditional co-immunoprecipitation approaches often fall short due to the insolubility, low abundance, or transient interactions of key axonemal proteins. Proximity labeling overcomes these limitations, facilitating a more comprehensive and physiologically relevant portrait of protein-protein networks, especially in structurally intricate systems like sperm flagella.

This discovery also holds clinical implications. Male infertility affects approximately 7% of men worldwide, and in nearly half of these cases, sperm motility defects are the underlying cause. Identification of EFCAB5 as a crucial regulator invites exploration of its genetic variants among infertile men. Screening for mutations or dysregulation of EFCAB5 could complement current diagnostic paradigms, offering a molecular target for personalized assessment and possibly therapeutic modulation.

Moreover, understanding how EFCAB5 orchestrates calcium-mediated signaling to govern flagellar dynamics may inspire novel fertility treatments aimed at restoring or enhancing sperm motility. Pharmacological agents or gene therapies designed to modulate EFCAB5’s function could become a reality, presenting alternatives to assisted reproductive technologies that bypass—but do not address—the motility deficits.

Expanding the scope, this study also contributes to the broader field of cilia and flagella biology. Given that ciliary dysfunction underpins a host of human diseases collectively termed ciliopathies—with manifestations spanning respiratory, renal, and neurological deficits—the principles learned from EFCAB5’s role in sperm flagella may have wider relevance. Similar calcium-dependent regulatory mechanisms might exist in other motile cilia, revealing broader biological paradigms and therapeutic opportunities.

Structurally, EF-hand motifs exemplify versatile calcium-binding domains found across various proteins involved in signal transduction and cytoskeletal regulation. EFCAB5’s localization within the highly specialized confines of the radial spoke complex suggests a finely adapted function integrating calcium sensing with mechanical modulation. This attests to the evolutionary refinement of motility regulation in cells where precision and responsiveness are paramount.

Future lines of investigation will likely explore the dynamic interplay between EFCAB5 and other calcium-binding proteins within the axoneme, elucidating how combinatorial interactions and post-translational modifications influence sperm motility patterns under physiological and pathological conditions. Integrating high-resolution cryo-electron microscopy with live-cell imaging and proteomics could provide unprecedented insights into these processes.

In conclusion, the identification of EFCAB5 as a key regulatory protein for sperm motility marks a seminal advance in reproductive biology. By harnessing proximity labeling centered on CFAP91, the research team unlocked new dimensions in our understanding of the molecular machinery driving sperm function. Beyond male infertility, these findings ripple across the fields of cell motility, calcium signaling, and organelle biology, reinforcing the centrality of precise protein networks in cellular function.

The impact of this work extends into translational realms, offering hope for novel diagnostics and treatments tailored to combat male infertility. Furthermore, the methodological innovations exemplified herein stand as a beacon for future studies investigating complex protein-interaction landscapes within challenging cellular environments. As we venture deeper into the nanoscale choreography of sperm flagella, insights such as those revealed by EFCAB5 promise to illuminate fundamental principles sculpting life’s earliest moments.

Subject of Research: Sperm motility regulation via axonemal protein interactions and calcium signaling.

Article Title: Proximity labeling of axonemal protein CFAP91 identifies EFCAB5 that regulates sperm motility.

Article References:
Wang, H., Shimada, K., Pham, A.H. et al. Proximity labeling of axonemal protein CFAP91 identifies EFCAB5 that regulates sperm motility. Nat Commun 16, 8238 (2025). https://doi.org/10.1038/s41467-025-63705-7

Image Credits: AI Generated

Spermatozoa propulsion, essential for fertilization success, relies on the whip-like action of their flagella—complex organelles whose performance dictates the sperm’s capacity to traverse the female reproductive tract. At the core of this mechanism lies a sophisticated structural arrangement involving radial spokes, protein complexes pivotal in modulating the flagellum’s beating patterns. Disruptions in this architecture often underlie male infertility, a condition affecting millions worldwide but still not fully understood at the molecular level.

Central to this study is the protein CFAP91, a component of the radial spoke apparatus whose precise role had remained elusive despite its noted association with human infertility. The research team utilized advanced genetic engineering techniques to create murine models lacking CFAP91 expression, observing significant aberrations in sperm morphology and impaired motility that culminated in complete male infertility. These phenotypic manifestations underscore CFAP91’s indispensable role in maintaining the structural integrity and dynamic function of sperm flagella.

To deepen their understanding, researchers employed a strategic approach to reintroduce CFAP91 in these knockout models, enabling detailed interrogation of its protein-protein interactions during sperm development. Employing proximity labeling—a cutting-edge biochemical method allowing for the identification of proteins in close physical proximity within complex cellular milieus—they discovered that CFAP91 physically associates with other established radial spoke constituents. This revelation positions CFAP91 not merely as a structural component but as a critical scaffold necessary for the assembly of the radial spoke complex.

Perhaps most intriguingly, the proximity labeling technique identified EFCAB5 as a novel CFAP91-adjacent protein implicated in the regulation of sperm motility. EFCAB5 appears to function as a calcium-binding adaptor, modulating the responsiveness of flagellar movement to intracellular signaling cues. This highlights a previously uncharacterized layer of regulatory control over how sperm generate the precise locomotive forces needed for navigation and fertilization.

The comprehensive analysis elucidates that CFAP91’s absence disrupts the radial spoke’s assembly, precipitating malformed flagella incapable of effective propulsion. Consequently, sperm lacking CFAP91 exhibit erratic or diminished motility, rendering them unable to fulfill their reproductive role. By contrast, the re-expression of CFAP91 restores the structural formation of the radial spokes and reinstates proper motility patterns, firmly establishing the protein’s centrality in sperm function.

Beyond the immediate implications for understanding infertility, these findings provide a compelling model for dissecting the molecular machinery underlying ciliary and flagellar motion—biological phenomena present across diverse cellular systems. Insights gleaned from CFAP91 and its associated proteins could thus inform broader biomedical inquiries into diseases caused by ciliary dysfunction, including respiratory illnesses and developmental disorders.

The Osaka team’s work also underscores the power of combining genetic manipulation with novel proteomic approaches to unravel complex cellular architectures. The deployment of proximity labeling in fully differentiated sperm cells allowed for unprecedented mapping of molecular interactions in situ, a methodological advance that promises to accelerate discoveries in cell biology and reproductive medicine.

From a translational perspective, identifying CFAP91 and EFCAB5 as crucial players opens promising avenues for diagnostic and therapeutic innovation. Molecular assays targeting these proteins or their functional pathways could enhance the precision of male infertility diagnoses, moving beyond gross morphological analysis to detailed molecular profiling. Furthermore, interventions aimed at correcting or compensating for the dysfunction of these proteins may one day underpin novel fertility treatments.

Importantly, this research spotlights the intricate modularity of the sperm flagellum, a marvel of evolutionary bioengineering. The radial spoke complex, with CFAP91 at its core, represents a finely tuned regulatory hub where structural proteins and signaling molecules converge to dictate motility patterns essential for successful reproduction. Such complexity conveys the biological necessity of maintaining fertility and the vulnerability of this system to disruption by molecular defects.

This deeper molecular understanding also lends insight into why certain forms of male infertility have remained refractory to treatment—if the underlying issue stems from molecular scaffolding deficits rather than hormonal or environmental factors, therapeutic strategies must be fundamentally reimagined. The elucidation of CFAP91’s role thus marks a paradigm shift in the approach to male reproductive health.

Moreover, the identification of EFCAB5 as a calcium-regulated modulator accentuates the importance of intracellular signaling dynamics in sperm motility. Given calcium’s central role in cellular function regulation, dissecting how EFCAB5 integrates into motility control pathways may reveal new targets for modulating sperm behavior in both clinical and contraceptive contexts.

In conclusion, this groundbreaking study from The University of Osaka not only deepens the scientific community’s grasp of the molecular determinants of sperm motility and male fertility but also paves the way for translating these insights into clinical practice. As infertility continues to be a global health concern, unraveling the molecular scripts that choreograph sperm function offers a beacon of hope for affected couples worldwide.

Subject of Research: Animals

Article Title: Proximity Labeling of Axonemal Protein CFAP91 Identifies EFCAB5 that Regulates Sperm Motility

News Publication Date: 10-Sep-2025

Web References: https://doi.org/10.1038/s41467-025-63705-7

Image Credits: Masahito Ikawa

Keywords: Cell biology, Spermatogenesis, Sperm, Flagella, Human fertilization, Human reproduction, Sexual reproduction

Spermiogenesis consists of numerous cellular transformations, including the drastic alteration of cell morphology, in which the sperm cell undergoes substantial changes, especially in its head structure. It has long been observed that any disruption in this finely tuned process can lead to the production of nonfunctional sperm, directly contributing to male infertility. The implications of unraveling the molecular interactions involved in spermatogenesis are profound, not only for understanding male reproductive health but also for developing potential contraceptive methods.

Lead author Yuki Kaneda explains, “The process of sperm development, known as spermiogenesis, is governed by an array of proteins and genes that guide the structural evolution of the sperm cell." His team’s investigation was sparked by the previously noted significance of TEX38, a protein primarily found in the testes. In earlier research, it was observed that mice deficient in TEX38 produced sperm with abnormal morphology, particularly heads that were deformed and bent backward at a staggering 180 degrees.

To delve deeper into the underlying mechanisms behind this deformity, the Osaka team employed a targeted genetic disruption of TEX38 in murine models. The outcomes were telling—the resulting sperm displayed consistent patterns of deformation. This led the researchers to postulate that the absence of TEX38 triggers a cascade effect that not only affects the structure of sperm heads but also diminishes overall fertility in male subjects.

Analysing the functional repercussions of TEX38 deficiency, the team sought out the proteins that interact with it and consequently unravelled a significant link with ZDHHC19, a protein known for its role in a biochemical process called S-palmitoylation. This process is critical for the addition of lipid groups to proteins, influencing their localization and function within the cell. Fascinatingly, the researchers discovered that the absence of either TEX38 or ZDHHC19 would lead to similar sperm deformations, suggesting that these proteins work in concert within a complex regulatory network.

Masahito Ikawa, a senior contributor to the study, observed, “The discovery of the interaction between TEX38 and ZDHHC19 sheds light on the precise molecular pathways that facilitate proper sperm head morphogenesis." Particularly, ZDHHC19 was found to perform S-palmitoylation on ARRDC5, a protein implicated in sperm development. When the enzymatic activity of ZDHHC19 was interfered with, sperm cells exhibited the same structural defects as those produced from TEX38-deficiency, showing that proper lipid modifications are of utmost importance for normal sperm morphology.

The research presents a significant leap toward deciphering the biological complexities of male fertility. Since the architecture of sperm is closely aligned with its functional abilities, understanding the intricacies of these protein dynamics can lead to innovative approaches in the realm of reproductive health. This could pave the way for the development of male contraceptives aimed at inhibiting the lipid modifications necessary for functional sperm generation, thus potentially offering new avenues for managing fertility.

This groundbreaking research demonstrates that the TEX38-ZDHHC19 partnership is a fundamental component of the sperm development process, providing essential insights at a molecular level. In today’s context where male infertility is a growing concern, findings from this research not only address a critical gap in our understanding but also hold potential for impactful applications in reproductive technologies.

The findings emphasize the necessity of further exploration into the molecular genetics of sperm formation. With innovative approaches such as genome editing and comparative proteomics, researchers could delve deeper into the exact functions of proteins involved in sperm morphology. These investigative avenues will undoubtedly aid in discerning the complexities of spermatogenesis, ultimately contributing to enhanced fertility outcomes.

Moreover, the study underscores the importance of interdisciplinary research approaches, integrating molecular biology, genetics, and reproductive health. Collaborations across various scientific realms can amplify discoveries and lead to novel interventions for addressing male infertility. As we continue to investigate and understand the molecular underpinnings of human reproduction, the potential for therapeutic advancements and improved fertility solutions appears brighter than ever.

In summary, the interplay between TEX38 and ZDHHC19 reveals a sophisticated regulatory network essential for normal sperm head formation. The implications of these findings rest not only in understanding male infertility but also in paving the way for innovative contraceptive methods based on these molecular interactions. As we stand on the threshold of new discoveries in reproductive health, this research serves as a vital stepping stone for future explorations aimed at enhancing both human fertility and our understanding of developmental biology.

Subject of Research: Proteins involved in sperm development and morphology
Article Title: TEX38 localizes ZDHHC19 to the plasma membrane and regulates sperm head morphogenesis in mice
News Publication Date: 3-Mar-2025
Web References: Proceedings of the National Academy of Sciences
References: None available
Image Credits: Masahito Ikawa

Keywords: Sperm development, TEX38, ZDHHC19, S-palmitoylation, male infertility, spermatogenesis, reproductive health, lipid modification.