Research Article: Lhx8 interacts with a novel germ cell-specific nuclear factor containing an Nbl1 domain in rainbow trout (Oncorhynchus mykiss)

Date Published: February 2, 2017

Publisher: Public Library of Science

Author(s): Liyuan Fu, Prasanthi P. Koganti, Jian Wang, Lei Wang, Cheng-Lun Wang, Jianbo Yao, Patrick Prunet.


Lhx8 is an important transcription factor that is preferentially expressed in germ cells. Lhx8 null mice are infertile due to lack of oocytes and impairment of the transition from primordial follicles to primary follicles. Lhx8 deficiency also affects the expression of many important oocyte-specific genes. In this study, we report the characterization of rainbow trout lhx8 genes and identification of a novel germ cell-specific nuclear factor that interacts with Lhx8. Two lhx8 genes, lhx8a and lhx8b, were identified, encoding proteins of 344 and 361 amino acids, respectively. The two proteins share 83% sequence identity and both transcripts are specifically expressed in the ovary. Quantitative real time PCR analysis demonstrated that both genes are expressed highly in pre-vitellogenic ovaries as well as in early stage embryos. Using a yeast two-hybrid screening system, a novel protein (Borealin-2) interacting with Lhx8 was identified. The interaction between either Lhx8a or Lhx8b and Borealin-2 was further confirmed by a bimolecular fluorescence complementation (BiFC) assay. Borealin-2 is a protein of 255 amino acids containing an Nbl1 domain, and its mRNA expression is restricted to the ovary and testis. A GFP reporter assay revealed that Borealin-2 is a nuclear protein. Collectively, results indicate that both Lhx8a and Lhx8b function through interaction with Borealin-2, which may play an important role during oogenesis and early embryogenesis in rainbow trout.

Partial Text

Through gene knockout studies in mouse, the functional roles of many oocyte-specific factors during folliculogenesis and early embryonic development have been revealed [1]. A number of key oocyte-specific nuclear factors known to be vital in follicular development are transcription factors, which include factor in the germline alpha (Figla) [2], newborn ovary homeobox (Nobox) [3], and LIM homeobox 8 (Lhx8) [4]. However, our understanding of the regulatory mechanisms of these important transcription factors in oocyte and follicular development is far from complete, particularly in species like fish.

LIM homeodomain genes play vital roles in tissue patterning and differentiation [12]. Lhx8 represents the first LIM homeodomain family member that plays a crucial role in oogenesis [4]. In this study, we characterized the rainbow trout lhx8 genes and discovered a novel oocyte-specific nuclear protein that interacts with Lhx8 proteins.