Regulation of 7B2 mRNA translation: dissecting the role of its 5'-untranslated region.

7B2 is a chaperone for the prohormone/proneuropeptide convertase PC2. Its mRNA is readily detectable in most neuronal and endocrine cells; the protein, in contrast, is often found at relatively low levels, suggesting that translation of the corresponding mRNA may be repressed. Because the 5' untranslated region (5'-UTR) of this mRNA is relatively long and burdened with multiple AUGs, it has been speculated that it contributes to this repression. In this report, the influence of this region was assessed using in vitro and ex vivo approaches. The results showed that, in a cell-free system, full-length 7B2 mRNA was a poor template for translation. Its translatability dramatically improved when its 5'-UTR was truncated or when it was replaced with the 5'-UTR of carboxypeptidase E mRNA. These observations were confirmed in transfected mouse insulinoma MIN6 cells and human embryonic kidney HEK293 cells. Acute exposure of MIN6 cells to high glucose increased endogenous 7B2 biosynthesis without affecting the levels of its mRNA, suggesting that translation repression of this mRNA can be relieved by physiological stimuli.

Sperm from mice genetically deficient for the PCSK4 proteinase exhibit accelerated capacitation, precocious acrosome reaction, reduced binding to egg zona pellucida, and impaired fertilizing ability.

The gene for proprotein convertase subtilisin/kexin-like 4 (PCSK4, previously known as PC4) is primarily transcribed in testicular spermatogenic cells. Its inactivation in mouse causes severe male subfertility. To better understand the reproductive function of PCSK4, we examined its subcellular localization in the testicular epithelium via immunohistochemistry, and on intact sperm via indirect immunofluorescence and immunoelectron microscopy. PCSK4 was detected in the acrosomal granules of round spermatids, in the acrosomal ridges of elongated spermatids, and on the sperm plasma membrane overlying the acrosome. We also investigated PCSK4 relevance for sperm acquisition of fertilizing ability by comparing wild-type and PCSK4-null sperm for their abilities in capacitation, acrosome reaction, and egg binding in vitro. PCSK4-null sperm underwent capacitation at a faster rate; they were induced to acrosome react by lower concentrations of zona pellucida; and their egg-binding ability was only half that of wild-type sperm. These sperm physiologic anomalies likely contribute to the severe subfertility of PCSK4-deficient male mice.

Genetic mapping of the gene for SKI-1/S1P protease (locus symbol Mbtps1) to mouse chromosome 8.

Subtilisin/kexin isozyme-1 (SKI-1), otherwise known as Site-1 protease (S1P), is a Golgi proteinase mediating the proteolytic activation of the precursor to sterol-regulated element-binding proteins (SREBPs) 1 and 2, two transcriptional factors that regulate expression of a variety of genes involved in cholesterol and lipid metabolism. Using PCR and RFLP analysis on a panel of genomic DNA from a mouse intersubspecific back-cross, we have mapped the SK1-/S1P gene (locus symbol: Mbtps1) to the distal part of mouse chromosome 8, in a region that exhibits synteny homology to the human chromosome 16q24 region where its orthologue had been previously mapped.