In the previous article, we described how the regulatory mechanism of the YBX2 gene influences development of cancer. In mouse models, what did the researchers find? How does Ybx2 knockout affect mouse phenotype? Please find the answers below, in our Weekly Model research review.
Ybx2 Knockout Mice
Firstly, Juxiang Yang et al. constructed Ybx2 knockout mouse; they found that Ybx2 represents one of a small number of germ-cell-specific genes whose deletion leads to the disruption of both spermatogenesis and oogenesis.
Targeting Construct and Generation of Mice
The Ybx2 –/– targeting construct (Fig. 1A) was produced by using a 129S6/SvEv mouse genomic library that is isogenic with the AB2.2 ES cells used for electroporation. ES cell clones were electroporated, selected, and screened by Southern blotting. As shown in Figure 1A, eight of 87 (9.2%) ES cell clones analyzed were correctly targeted on both the 5′ and 3′ sides by using the noted probes. Germ-line transmission was achieved from ES cell clone Ybx2-188-A6. Mice were genotyped by Southern blotting, and the mutation was maintained on a 129S5/SvEvBrd hybrid genetic background.
Figure 1. Strategy for targeted disruption of the Ybx2 gene 
Spermatogenesis Arrests in Late Spermiogenesis in Ybx2 –/– Males
To determine the cause of the infertility in the Ybx2 –/– males, testes were initially analyzed at 6 weeks of age. Testes of the Ybx2 –/– males (63.95 ± 3.52 mg; n = 6) were statistically smaller (P < 0.001) than testes of the wild-type males (88.68 ± 3.77 mg; n = 6). The absence of Ybx2 results in a block late in spermatogenesis, likely secondary to the altered regulation of key spermatogenic genes that are expressed after the meiotic division.
Figure 2. Histology of testes and epididymis from 6- and 7-week-old wild-type and Ybx2-null (knockout) mice. (A and B) Ybx2 –/– testis. (C) Ybx2 +/+ testis. (D) Ybx2 –/– testis. (E) Ybx2 +/+ epididymis. (F) Ybx2 –/– epididymis. (Scale bars: A, E, and F, 20 μm; B–D, 80 μm.)
An Early Loss of Oocytes Occurs in Ybx2 –/– Females
The researchers analyzed the ovaries at various time points to study infertility in Ybx2-/- females. By 21 days of age, they found there were fewer follicles in the Ybx2 –/– female ovaries (Fig. 5C ), and in many of those present, the granulosa cells appeared to be disorganized. This finding is in contrast to the age-matched littermate control ovaries that had multiple secondary and earlier follicles (Fig. 5 A and B). By 8 weeks of age, wild-type ovaries had a full range of follicles and corpora lutea (Fig. 5D), whereas Ybx2 –/– ovaries had fewer follicles and occasional hemorrhagic cysts (Fig. 5E). By 20 weeks, there was a range of phenotypes in the Ybx2 –/– ovaries that likely depended on the number of oocytes lost earlier in life: some ovaries had cysts, oocytes lacking cumulus cells (Fig. 5F), and significant oocyte death that included an interstitium full of zona pellucida remnants and zonae pellucidae being “invaded” by granulosa cells (Fig. 5G). Other ovaries had mostly preantral follicles, with an occasional follicle containing two oocytes and oocytes that appeared larger than normal (data not shown). By 8 months of age, Ybx2 –/– ovaries continued to vary in size and follicle number. Three of nine Ybx2 –/– mice had ovaries that were small and typically devoid of oocytes and follicles (Fig. 5 H and I).
Figure 4. Histology of ovaries of Ybx2-null and control mice. (A–C) Three-week-old wild-type (A), Ybx2 +/– (B), and Ybx2 –/– (C) ovaries captured at the same magnification. (D) Wild-type 8-week-old ovary. (E–I) Ybx2 –/– ovaries at 8 weeks (E), 20 weeks (F and G), and 8 months (H and I). Note the numerous follicles in A and B compared with C. Note the numerous corpora lutea (CL) and follicles at various stages of development in D and the absence of corpora lutea, reduced number of follicles, hemorrhagic cyst (HC), and increased interstitial tissue in E. (Scale bars: 100 μm.)
New Research Progress
Recent years have seen an upsurge of interest in brown adipose tissue (BAT) to combat the epidemic of obesity
and diabetes. Dan Xu et. al. found Ybx2 is a novel regulator that controls BAT activation by regulating mRNA stability.
Ybx2 Is an Essential Regulator of Brown Adipocyte Differentiation In Vitro
Figure 5. Ybx2 is an essential regulator of brown adipocyte differentiation in vitro. A: Primary brown preadipocytes were infected by retroviral shRNAs targeting RBPs, Ybx2, and Akap1, followed by induction of differentiation for 5 days. Oil Red O staining was used to assess lipid accumulation. B–D: Real-time PCR was used to measure the knockdown efficiency (left), pan-adipogenic marker expression (right), and BAT-selective marker expression (bottom) in cultured primary brown adipocytes (day 5) infected by retroviral shRNAs targeting Ybx2 (B), Akap1 (C), and Rbpms2 (D) (n = 3). *P , 0.05 by one-way ANOVA. F: Representative metabolic flux curves from cultured brown adipocytes (day 5) infected by retroviral shRNA targeting Ybx2. Cells were sequentially treated with oligomycin, FCCP, and rotenone. OCRs are normalized by protein concentration (n = 5). *P , 0.05 by Student t test. F: Western blot was used to examine the protein levels of Ybx2 during primary brown and white adipocyte differentiation in culture. The error bars in the graphs are mean 6 SEM. Sh-1, short hairpin RNA-1; Sh-2, short hairpin RNA-2; Sh-3, short hairpin RNA-3. 
These observations indicate that Ybx2 can promote brown adipogenesis in white adipocyte culture and also accelerate brown adipogenesis in brown adipocyte culture. Additional in vivo research, such as involving the use of mouse models, would provide further evidence of the mechanism of action (MOA) of Ybx2 in brown adipogenesis to evaluate potential therapeutic approaches.
The YBX2 gene is a germ cell-specific gene, and the phenotype of knockout mice is mainly focused on germ cells. These results indicate that suitable knockout mice are helping to study how the formation of germ cells are influenced by Ybx2. In addition, the Ybx2 gene has a lot to do with the stability of mRNA. Researchers can refer to the results of Ybx2 in vitro experiments to see whether the gene will produce a similar phenomenon in mice and whether there is a compensatory pathway.
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