Protamine expression in somatic cells condenses chromatin and disrupts transcription without altering DNA methylation

Protamines play a crucial role in nuclear condensation during spermiogenesis, a process associated with significant chromatin remodeling and replacement of histones. While much research has focused on the function of protamines in sperm development and fertility, their effects in non-sperm cells remain largely unexplored. Protamines are increasingly used in the clinical setting, and understanding better, the role of protamines in somatic cells remains a critical unmet need.

In this study, we investigated the impact of overexpressing murine and human protamine 1 and 2 (PRM1 and PRM2) on nuclear architecture, histone eviction, DNA methylation, and transcription in HEK293T cells and mesenchymal stromal cells (MSCs). Overexpression of protamines resulted in nuclear condensation; particularly PRM1 showed notable enrichment in nucleoli, and cells exhibited cell cycle abnormalities. Immunofluorescence staining indicated a significant reduction in specific histone modifications (H3K9me3, H3K4me1, and H3K27Ac) in response to protamine expression, especially in MSCs. Interestingly, despite these changes in nuclear organization, the methylome remained largely stable. However, expression of protamines significantly diminished transcription, particularly of the ribosomal genes, upon PRM1 expression.

Our studies indicate that PRM1 and PRM2 may bind to and condense distinct genomic regions in somatic cells, resulting in widespread silencing of gene expression, while retaining a largely stable DNA methylome.