Exogenous Expression of Human Protamine 1 (hPrm1) Remodels Fibroblast Nuclei into Spermatid-like Structures

Abstract

Protamines confer a compact structure to the genome of male gametes. Here, we find that somatic cells can be remodeled by transient expression of protamine 1 (Prm1). Ectopically expressed Prm1 forms scattered foci in the nuclei of fibroblasts, which coalescence into spermatid-like structures, concomitant with a loss of histones and a reprogramming barrier, H3 lysine 9 methylation. Protaminized nuclei injected into enucleated oocytes efficiently underwent protamine to maternal histone TH2B exchange and developed into normal blastocyst stage embryos in vitro. Altogether, our findings present a model to study male-specific chromatin remodeling, which can be exploited for the improvement of somatic cell nuclear transfer.

Graphical abstract

Figure 1

Ectopic Prm1 Binds to Somatic DNA, Replaces the Histones, and Changes Completely Nuclear Morphology (A–F) Nuclear incorporation of Prm1 in somatic chromatin. This shows tracking the RFP tag in cells transfected with pTag-RFP (A–C) and pPrm1-RFP (D–F); (A and D) Nuclei. (B) RFP. (E) Prm1-RFP localizations. (C and F) Merge. Scale bar represents 10 μm. (G–I) Elongated nucleus in pPrm1-RFP-transfected cells. Scale bar represents 10 μm. (J) ChIP-seq profiles in genomic regions of different chromosomes for pPrm1-RFP in transfected cells (after peak calling the represented genomic regions from pPrm1-RFP sample are enriched than input samples). (K) Histones/Prm exchange in H2B-GFP fibroblasts: nuclei (blue); H2A.B-GFP (green); Prm1-RFP (red); Merge H2A.B+Prm1. (Left to right) The gradual incorporation (0-, 16-, 24-, 48-hr post-transfection) of Prm1 into somatic nuclei. H3K9m3/Prm exchange (L) was as follows: H3K9me3 (green), Prm1-RFP (red), and Merge H3K9me3+ Prm1.

Figure 2

The Gradual Prm1 Incorporation Leads to Nuclear Compaction Similar to that of Elongated Spermatids without Causing DNA Brakes (A–C; E–G) Timing of incorporation of Prm in somatic nuclei. This is a representation of post-transfection incorporation of Prm in somatic nuclei. pPrm1-RFP-Prm plasmide is tagged with RFP. (A) Nuclei (Hoechst, blue) of fibroblasts before the transcription of Prm1. (B) Incorporation of Prm1 in nucleus 16- to 20-hr post-transfection, visible as red spots. (C) Complete incorporation of Prm1 in nucleus 40- to 48-hr post-transfection. Prm1 is red, and nuclei are blue. (E–G) TEM analysis of adult sheep fibroblasts transfected with pPrm1-RFP. (E) Nucleus of fibroblasts before the transcription of Prm1. (F) Nucleus of fibroblast after 16- to 20-hr post-transfection. Visible partial compaction of chromatin is seen. (G) Nucleus of fibroblasts after 40- to 48-hr post-transfection. Visible complete chromatin compaction is seen. Bars represent 8 μm (A) and 2 μm (F). (D and H) Nucleus of spermatozoa stained with Hoechst (D) and analyzed by TEM (H). Scale bar represents 500 nm. (I) Representation of nucleus of control (CTR) and Prm-transfected cells (Prm1-RFP) from frontal and lateral view. Nucleus of spermatozoa has been use as a control (SPZ). (J) DNA double-strand breaks in sheep fibroblasts transfected with pPrm1-RFP visualized by γH2A.X immunostaining. (a, e, and i) Control adult fibroblasts. (b, f, and j) Positive control: adult fibroblasts irradiated with UV light. (c, g, and k) Adult fibroblasts 20-hr post-transfection with pPrm1-RFP (red). (d, h, l) Adult fibroblasts treated with TSA and transfected with pPrm1-RFP. Stained with antibody anti-γH2A.X. Scale bar represents 10 μm.

Figure 3

Protaminized Somatic Nuclei Re-acquire a Nucleosome Organization after Nuclear Transfer (A) Displacement Prm1 during pronucleus formation after nuclear transfer (NT). Scale bar represents 20 μm. Prm1 is red, and nucleus is blue. (B) Incorporation TH2B in pronucleus (PN) after NT of protaminized nuclei: pronucleus (PN, blue), TH2B (green), Merge (TH2B/PN). Scale bar represents 20 μm. (C) SCNT of fibroblast transfected with pPrm1-RFP. (a) Picture represents Prm1 (red)-positive fibroblasts used as donors for SCNT. (a1) Protaminized nucleus in injected capillary before nuclear transfer into enucleated MII sheep oocyte. (a2) In vitro development of nuclear transfer embryos. (D) Table 2. Quality of blastocysts is as follows: IVF, in vitro fertilized; NT Prm1 cells. Total cell number is mean ± SD. Cellular composition is the number of inner cell mass/trophectoderm cell number. Karyotype composition is the cell with euploid chromosome number divided by total cells (%).