(269) Tracking Chromatin Dynamics During Sperm Nuclear Condensation

Masashi Hada¹; Yuko Fukuda¹; Chizuko Koga¹; Erina Inoue¹; Yasuhiro Fujiwara¹; Yuki Okada¹

 1. Institute for Quantitative Biosciences (IQB), the University of Tokyo, Tokyo, Japan

Abstract Text:

Upon the completion of meiosis, spermatids undergo a critical maturation process to transform into functional sperm. This process involves nuclear condensation, during which chromatin components, specifically histones, are replaced by protamines. This replacement is essential for sperm motility and the structural stability of the paternal genome. However, due to the technical limitation in isolating spermatids at specific stages of nuclear condensation, the precise chromatin dynamics and properties during this process remain poorly understood. 
 To address this question, we recently developed a method for stage-specific isolation of spermatids at each differentiation step by combining a reporter mouse expressing fluorescently labeled histones with fluorescence-activated cell sorting (FACS) (Fujiwara, Hada, et al., Cytometry A, 2023). Using this method, we analyzed the chromatin status of spermatids during nuclear condensation through ATAC-seq by which open chromatin regions can be visualized based on Tn5 accessibility. We found that spermatid chromatin undergoes a transient hyper-relaxed state across the genome prior to the condensation process. To investigate the functional significance of this hyper-relaxed state, we examined the genomic localization of protamines. Interestingly, we found that protamines were preferentially incorporated into relaxed chromatin regions, particularly gene-rich regions. This finding suggests that the hyper-relaxed state is required for efficient protamine incorporation. Furthermore, a comparative analysis of human sperm demonstrated that, similar to mouse sperm, gene-rich regions are also highly condensed, indicating that these chromatin properties are evolutionarily conserved between mice and humans. While gene-rich regions typically exhibit lower chromatin condensation in somatic cells, our findings suggest that in sperm, the higher-order genomic structure is fundamentally altered by protamine incorporation. These insights provide a deeper understanding of the mechanisms underlying nuclear condensation and its implications for genome integrity.