類3號DNA甲基化酶於小鼠雄性生殖前驅細胞之恆定維持與分化調控的角色

Abstract

正確的建立與調整表觀基因組是產製具有完整功能之生殖細胞的要素。相關的研究指出，擁有留駐於靜止態的能力是前精原细胞進行完善分化與避免精原幹/前驅细胞提前進入早熟之耗盡的必要條件，但調控精原幹/前驅细胞穩定於靜止態的內在表觀基因之調節者仍然有待研究與發掘。類3號DNA甲基化酶 (DNMT3L)是一個詮釋染色質狀態與協助DNA 甲基化作用的表觀基因調節者，在此篇論文，我們探討類3號DNA甲基化酶於維持生殖細胞群的角色。我們的研究結果證實，在小鼠出生後的睪丸，類3號DNA甲基化酶表現於THY1+ 精原幹/前驅细胞族群。我們的研究指出，類3號DNA甲基化酶藉由調控Cdk2/CDK2的表現，進而影響 PLZF的蛋白質穩定度。在出生八天的THY1+ 精原幹/前驅细胞，類3號甲基酶的缺失會造成ETS1鍵結於已高度H3K4me3修飾的Cdk2 啟動子，以調節Cdk2/CDK2 的表現量。此外，我們亦觀察到CDK2會與PLZF鍵結，且類3號DNA甲基化酶的缺失會造成PLZF被過量泛素化。類3號DNA甲基化酶影響著PLZF的穩定性，減弱的PLZF進而釋放它的的束縛蛋白質(SALL4)，造成過量的下游ERK與AKT訊息傳遞鍊的表現，及導致過量的精原幹/前驅细胞增生。這些研究的結果呈現，類3號DNA甲基化酶在避免精原幹/前驅细胞的提前早熟、維持增生與靜止態的平衡，扮演著重要的角色。 因為類3號DNA甲基化酶的已知功能為抑制跳躍基因，且小片段RNA (piRNA)已知與跳躍基因靜默有著高度關聯。我們進一步探討小片段RNA (piRNA)之相關聯的成員的表現狀態，我們的研究結果指出，在調節整體跳躍基因靜默的胚胎時期(E18.5)，類3號DNA甲基化酶會影響著MAEL的表現位置。此外，我們的研究成果亦發現在出生後的THY1+ 精原幹/前驅细胞，類3號DNA甲基化酶的缺失會造成減量的細胞質MAEL表現。此低量的MAEL表現量與其錯位的分布可能是類3號DNA甲基化酶缺失之雄性生殖細胞呈現不正常的小片段RNA (piRNA)成分組成的其一原因 。 整合前述之研究成果，這些新發現提供了類3號DNA甲基化酶在雄性生殖細胞的新功能: 類3號DNA甲基化酶藉由調節精原幹/前驅细胞的增生與分化、影響小片段RNA (piRNA)的相關路徑，維持雄性生細胞之恆定。

The establishment and rearrangement of proper germline epigenome is crucial for generating functional germ cells. Accumulating results demonstrated that the ability of prosprematogonia and spermatogonial stem/progenitor cells (SPCs) to reside in a quiescent state is important for prospermatogonia differentiation and preventing premature exhaustion of the postnatal stem cell pool. However, the intrinsic epigenetic factors that regulate SPC quiescence are largely unknown. Here, we investigated how DNA methyltransferase 3-like (DNMT3L), an epigenetic regulator important for interpreting chromatin context and facilitating de novo DNA methylation, sustains the long-term male germ cell pool. We demonstrated that stem cell-enriched THY1+ SPCs constitute a DNMT3L-expressing population in the postnatal testes. DNMT3L influenced the stability of PLZF, potentially by downregulating Cdk2/CDK2 expression, which sequestered CDK2-mediated PLZF degradation. Increased level of ETS1 was found to bind the DNA hypomethylated and H3K4me3-over-accumulated Cdk2 promoter that extended Cdk2/CDK2 expression in 8 dpp Dnmt3l KO THY1+ cells. In addition, CDK2 interacted with PLZF in THY1+ cells and PLZF was excessively post-translational modification with ubiquitin in Dnmt3l KO THY1+ cells, suggesting that DNMT3L participates in the regulation of PLZF stability in postnatal germ cells. Reduced PLZF in Dnmt3l KO THY1+ cells released its antagonist, SALL4A, which overactivated ERK and AKT signaling cascades to stimulate SPC proliferation. These results indicate that DNMT3L is required for preventing premature SPC exhaustion and to delicately balance the cycling and quiescence of SPCs. Since a well-known function of DNMT3L is to repress transposable elements (TEs) in order to safeguard the integrity of the genome. PiRNAs are highly involved in TE silencing in male germ cells. Further analysis of piRNA-related components revealed that Dnmt3l KO embryonic germ cells display detachment of some cytoplasmic MAEL from P-body components in E18.5 when genome-wide transposon silencing is undergoing. Furthermore, we observed that postnatal Dnmt3l KO THY1+ germ cells exhibit less MAEL protein in the cytoplasmic fraction compared to wild-type THY1+ germ cells. The aberrant MAEL expression and subcellular distribution may account for the abnormal characteristics of piRNA compositions in Dnmt3l KO germ cells. Cumulatively, these findings provide novel roles for DNMT3L in mouse male germ cells. DNMT3L is important for germline maintenance via influencing SPC self-renewal/differentiation-associated factors and the piRNA-related pathway.