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Low-Density Lipoprotein Receptor-Related Protein 8 (LRP8) Is Upregulated in Granulosa Cells of Bovine Dominant Follicle: Molecular Characterization and Spatio-Temporal Expression Studies¹

Centre de Recherche en Reproduction Animale,³ Faculté de Médecine Vétérinaire, Université de Montréal, St-Hyacinthe, Québec, Canada J2S 7C6 Max F. Perutz Laboratories,⁴ Department of Medical Biochemistry, Medical University of Vienna, A-1030 Vienna, Austria

ABSTRACT

The low-density lipoprotein (LDL) receptor-related protein 8 (LRP8) is a member of the LDL receptor family that participates in endocytosis and signal transduction. We cloned the full-length bovine LRP8 cDNA in granulosa cells (GC) of the dominant follicle (DF) as well as several LRP8 mRNA splicing variants, including a variant that contains a proline-rich cytoplasmic insert (A⁷⁵⁹-K⁸¹⁷) that is involved in intracellular signaling. Expression of the A⁷⁵⁹-K⁸¹⁷ variant was analyzed in the GC of follicles at different developmental stages: the small follicle (SF; 2–4 mm), the DF at Day 5 (D5) of the estrus cycle, ovulatory follicles (OF) 24 h after hCG injection, and corpora lutea (CL) at D5. RT-PCR analysis showed that expression was predominant in the GC of DF compared to other follicles and CL (P < 0.0001), whereas the expression of other related receptors, such as LDLR and VLDLR, did not show differences. Temporal analyses of follicular walls from the OF following hCG treatment revealed a decrease in LRP8 mRNA expression starting 12 h post-hCG treatment (P < 0.0001). LRP8 protein was exclusively localized to the GC, with higher levels in the DF than in the SF (P < 0.05). RELN mRNA, which encodes an LRP8 ligand, was highly expressed in the theca of the DF as compared to the OF (P < 0.004), whereas MAPK8IP1 mRNA, which encodes an LRP8 intracellular interacting partner, is expressed in the GC of the DF. These results demonstrate the differential expression patterns of LRP8, RELN, and MAPK8IP1 mRNAs during final follicular growth and ovulation, and suggest that a RELN/LRP8/MAPK8IP1 paracrine interaction regulates follicular growth.

follicle, follicular development, granulosa cells, ovulation

¹Supported by a Discovery Grant to J.G.L. from the Natural Sciences and Engineering Research Council of Canada (NSERC), and a fellowship to T.F. from the Fonds Québecois de la Recherche sur la Nature et les Technologies (FQRNT). This work was presented in part at the 38th Annual Meeting of the Society for the Study of Reproduction, July 24–27, 2005, Quebec City, Quebec, Canada. The nucleotide sequence reported in this paper has been submitted to GenBank under the accession number AY364441.

Correspondence: ²Jacques G. Lussier, Faculté de Médecine Vétérinaire, Université de Montréal, P.O. Box 5000, St-Hyacinthe, Québec, Canada J2S 7C6. FAX: 450 778 8103; e-mail: jacques.lussier{at}umontreal.ca