Sequence determination and expression of the ovine doppel-encoding gene in transgenic mice

Role of the prion protein family in the gonads

The prion-gene family comprises four members named PRNP (PRP^c), PRND (Doppel), PRNT (PRT), and SPRN (Shadoo). According to species, PRND is located 16–52 kb downstream from the PRNP locus, whereas SPRN is located on another chromosome. The fourth prion-family gene, PRNT, belongs to the same genomic cluster as PRNP and PRND in humans and bovidae. PRNT and PRND possibly resulted from a duplication event of PRND and PRNP, respectively, that occurred early during eutherian species divergence. Although most of the studies concerning the prion-family has been done on PRP^c and its involvement in transmissible neurodegenerative disorders, different works report some potential roles of these proteins in the reproductive function of both sexes. Among them, a clear role of PRND, that encodes for the Doppel protein, in male fertility has been demonstrated through gene targeting studies in mice. In other species, Doppel seems to play a role in testis and ovary development but its cellular localization is variable according to the gonadal developmental stage and to the mammalian species considered. For the other three genes, their roles in reproductive function appear ill-defined and/or controversial. The present review aimed to synthesize all the available data on these prion-family members and their relations with reproductive processes, mainly in the gonad of both sexes.

Is prnt a pseudogene? Identification of ram Prt in testis and ejaculated spermatozoa

A hallmark of prion diseases or transmissible spongiform encephalopaties is the conversion of the cellular prion protein (PrP(C)), expressed by the prion gene (prnp), into an abnormally folded isoform (PrP(Sc)) with amyloid-like features that causes scrapie in sheep among other diseases. prnp together with prnd (which encodes a prion-like protein designated as Doppel), and prnt (that encodes the prion protein testis specific--Prt) with sprn (shadow of prion protein gene, that encodes Shadoo or Sho) genes, constitute the "prion gene complex". Whereas a role for prnd in the proper functioning of male reproductive system has been confirmed, the function of prnt, a recently discovered prion family gene, comprises a conundrum leading to the assumption that ruminant prnt is a pseudogene with no protein expression. The main objective of the present study was to identify Prt localization in the ram reproductive system and simultaneously to elucidate if ovine prnt gene is transcribed into protein-coding RNA. Moreover, as Prt is a prnp-related protein, the amyloid propensity was also tested for ovine and caprine Prt. Recombinant Prt was used to immunize BALB/c mice, and the anti-Prt polyclonal antibody (APPA) immune response was evaluated by ELISA and Western Blot. When tested by indirect immunofluorescence, APPA showed high avidity to the ram sperm head apical ridge subdomain, before and after induced capacitation, but did not show the same behavior against goat spermatozoa, suggesting high antibody specificity against ovine-Prt. Prt was also found in the testis when assayed by immunohistochemistry during ram spermatogenesis, where spermatogonia, spermatocytes, spermatids and spermatozoa, stained positive. These observations strongly suggest ovine prnt to be a translated protein-coding gene, pointing to a role for Prt protein in the ram reproductive physiology. Besides, caprine Prt appears to exhibit a higher amyloid propensity than ovine Prt, mostly associated with its phenylalanine residue.

Goat PRND expression pattern suggests its involvement in early sex differentiation

Expression of the goat prion protein gene locus was assessed by reverse transcriptase-polymerase chain reaction on testes and ovaries at various developmental stages. A weak and stochastic expression of the PRNP and PRNT genes was observed. For PRNT, it is consistent with the detected deletions of two single nucleotides within its open reading frame in ruminant genes. PRND was expressed in both tissues at all stages. Whereas its expression is constant in the ovaries, it increases in testes between 36 and 46 days postcoitum (dpc) and remains high thereafter. In testes, Doppel was found in the nucleus of germinal cells and in the cytoplasm of Leydig cells at 44 dpc. It was detected in the cytoplasm of Leydig cells and of some Sertoli and germinal cells at 62 dpc. In the ovaries, it was observed in the nucleus of germinal cells at 44 dpc and mainly in their cytoplasm at 62 dpc. This expression pattern was shown to parallel that of C-kit and suggests Doppel involvement in early testis differentiation.

The prion protein family: Diversity, rivalry, and dysfunction

The prion gene family currently consists of three members: Prnp which encodes PrP(C), the precursor to prion disease associated isoforms such as PrP(Sc); Prnd which encodes Doppel, a testis-specific protein involved in the male reproductive system; and Sprn which encodes the newest PrP-like protein, Shadoo, which is expressed in the CNS. Although the identification of numerous candidate binding partners for PrP(C) has hinted at possible cellular roles, molecular interpretations of PrP(C) activity remain obscure and no widely-accepted view as to PrP(C) function has emerged. Nonetheless, studies into the functional interrelationships of prion proteins have revealed an interesting phenomenon: Doppel is neurotoxic to cerebellar cells in a manner which can be blocked by either PrP(C) or Shadoo. Further examination of this paradigm may help to shed light on two prominent unanswered questions in prion biology: the functional role of PrP(C) and the neurotoxic pathways initiated by PrP(Sc) in prion disease.

Prion-like Doppel gene (PRND) in the goat: genomic structure, cDNA, and polymorphisms

The genomic structure of the caprine Doppel gene (PRND) was determined using the ovine sequence as a scaffold to generate PCR fragments that were aligned with a cDNA sequence obtained from testicular mRNA. The caprine gene contains two exons, 89 and >2291 bp long, separated by a 1689-bp intron. Two mRNA isoforms of 3.2 and 4.8 kb were identified in the testis, as well as the exact transcription start site by fluorescently labeled oligonucleotide extension (FLOE). Like in sheep and cattle, the open reading frame (ORF) (537 bp) lies within exon 2 and is very much conserved in sheep (99.3%) and cattle (97%). The intronic sequence is also highly conserved (95.3%) compared with sheep, with the only exception of a 47-bp insertion. The PRND ORF was sequenced in 47 healthy and 17 TSE-affected goats of the Italian Ionica breed. Seven nucleotide positions showed variation: T28C, C65T, A151G, G286A, C385G, T451C, and T528C. Five were commonly represented polymorphisms: T28C, T451C, and T528C are silent mutations at codons L10, L151, and I176, respectively, while A151G and C385G determine a T51A and L129V amino acid change, respectively. The two remaining variants, C65T and G286A, were rare, leading to the amino acid substitutions S22F and E96K, respectively. None of the polymorphisms was significantly relatable to the TSE status, and the same result was obtained by the analysis of the combined haplotypes at the five major polymorphic sites, namely, T28C, C65T, A151G, G286A, and C385G.