小鼠卵母細胞特異性表現同源箱基因Ohx之分子選殖與功能分析

Abstract

生命開始於精子與卵的受精作用所產生的受精卵。以小鼠為例，卵巢中進行生長的卵母細胞會產生並儲存許多母源性之調節因子，提供作為卵母細胞減數分裂的完成、受精作用、以及小鼠胚發育至八細胞期之所需。小鼠胚發育至二細胞期被視為母源性基因轉換成胚源性基因之過渡時期。此等發育過程係由諸多轉錄因子共同協力調控之結果。目前已經有一些調節因子被確認，並針對其結構上特異性的單元加以分析，例如：同源箱蛋白質。Ohx基因 (卵母細胞特異性表現之同源箱基因) 即是利用電腦選殖的方式篩選獲得者，其僅在小鼠早期胚胎發育時期表現。在RT-PCR之試驗結果中證實，Ohx基因僅在小鼠早期胚胎發育之一至二細胞期可偵測到其表現情形；此外，在成熟小鼠的卵巢中亦可以偵測到其表現。有鑑於濾泡生成過程中，其成熟、排卵與黃體化等作用均是受到荷爾蒙的刺激所致，本試驗中針對尚未性成熟之母小鼠進行超級排卵處理，亦即施以PMSG與hCG腹腔注射。試驗結果發現，四週齡小鼠卵巢中，僅可偵測到微弱的Ohx訊號；但在注射hCG後9小時所取得之小鼠卵巢中，則可在卵巢濾泡中偵測到明顯的Ohx訊號。由上述結果可知Ohx在成熟卵巢中的卵母細胞有顯著的表現，而且其表現可能受到LH潮湧的調控。 爲謀了解Ohx基因的生物性功能，進一步藉由基因標定之方式產製Ohx突變小鼠。試驗結果顯示Ohx+/–小鼠與正常小鼠之間並無差異。然而基因型為Ohx–/–之突變小鼠在出生前即行死亡。此外，試驗中亦收集各個不同發育階段的小鼠胚進行基因型分析，俾確認同型合子突變小鼠之死亡時間。初步之資料顯示，此等突變小鼠胚可能在發育至囊胚時期前即已死亡。 在本論文中，利用shRNA之構築產製H1-Ohxi基因轉殖小鼠，試驗中所獲得之親代小鼠外觀是正常的，唯五隻基因轉殖小鼠中，其中兩隻母小鼠屆六月齡時即無法在重複配種後懷孕。將此等母小鼠於八月齡時犧牲，則可觀察到其卵巢大部分均被黃體細胞佔據。此外，試驗中亦產製Ohx過量表現之基因轉殖小鼠。由此產製之四隻基因轉殖親代公小鼠均可受孕、並能夠將外源基因遺傳至所產生的子代F1中。若令攜帶外源基因之F1子代互交，其所產生之F2子代中，可以發現母小鼠的卵巢中具有許多有腔濾泡。而RT-PCR的分析結果亦顯示在有腔濾泡特異性表現的基因P450 aromatase (cyp19) 也有明顯增加的情形。綜上所述，Ohx基因在早期胚胎發育與卵母細胞生成之過程中扮演很重要的角色。

Life begins when sperm fertilizes an egg to form a zygote. In the mouse ovaries, growing oocytes produce a group of maternal regulatory molecules sufficient to support completion of meiosis, fertilization, and development to the 8-cell stage. Formation of a 2-cell mouse embryo marks the transition from maternal gene to zygotic gene dependence. Such regulation is achieved by coordinated functional expression of a large repertoire of transcription factors to control these processes. A number of these factors have been characterized, and thereby found to possess specific modular structural motifs, such as homeodomains. Ohx (oocyte-specific homeobox gene) was one of the genes identified by in silico cloning involved in early embryo development. There were evidences showed that Ohx was preferentially expressed in one- and two-cell embryons, as well as in the oocytes of mature ovaries. Since the development of a single follicle is under the control of hormones to stimulate follicular maturation, ovulation and luteogenesis, we treated immature females with PMSG and hCG to study the hormones effects on the expression of Ohx gene. Weak signals of Ohx mRNA were detected in the follicles of the untreated ovaries of 4-week-old mice. The Ohx mRNA was clearly detected in the oocytes 9 h post-hCG. These results indicate that Ohx is predominantly expressed in oocytes of the mature ovary and may be under the regulation of an LH surge. To determine its biological function, we generated Ohx-null mouse lines by gene targeting approch. There was no remarkable difference between Ohx heterozygous and wild-type mice. However, prenatal lethality was found for Ohx homozygous mutants. In order to detect when the mutants died, the embryos were collected and studied at different embryonic stages. The preliminary data showed that the Ohx-null embryos probably died before blastocyst stage. A shRNA construct was also used to generate transgenic mice. The H1-Ohxi transgenic founders appeared normally, except that two of the five transgenic female did not produce any progeny after repeated mating at the age of 6 month. These two infertile founders were sacrificed at the age of 8 month and many luteal cells were found in their ovaries. To further investigate the roles of Ohx during oogenesis, we generated transgenic mice with Ohx gene over-expressed. Four male transgenic founders were produced, and appeared fertile. The transgene was later proven to be germline transmitted. We found many antrum follicles in the ovaries of the offspring (F2) produced by F1 intercrossed. The results of RT-PCR also demonstrated a remarkable increasing expression of antrum follicle-specific gene, P450 aromatase (Cyp19), in the ovaries of those F2 mice. In conclusion, Ohx plays several significant roles during oogenesis and early embryonic development in mice.