A dual-function palindromic sequence regulates testis-specific transcription of the mouse lactate dehydrogenase c gene in vitro

Cancer-testis antigen lactate dehydrogenase C4 as a novel biomarker of male infertility and cancer

A unique lactate dehydrogenase (LDH) isoenzyme designated as lactate dehydrogenase C4 (LDH-C4) is found in mammalian mature testis and spermatozoa. Thus far, LDH-C4 has been well studied with regard to its gene and amino acid sequences, structure, biological properties, and peptide synthesis. Accumulating evidence has shown that LDH-C4 is closely related to spermatic energy metabolism and plays a critical role in sperm motility, capacitation, and fertilization. Defects in the catalytic activity of LDH-C4 are key to pathophysiological abnormalities underlying infertility. LDH-C4 was originally thought to be present only in mature testis and spermatozoa; however, recent studies have implicated LDH-C4 as a cancer-testis antigen (CTA), owing to its aberrant transcription in a broad spectrum of human neoplasms. This review highlights the recent findings on LDH-C4 with particular emphasis on its role in male infertility and tumors.

Regulation of murine lactate dehydrogenase C (Ldhc) gene expression

Expression of Ldhc begins with the onset of meiosis in male germ cells and continues throughout spermatogenesis. Transcriptional regulatory mechanisms, especially in primary spermatocytes, are poorly described because of the lack of a reliable cell culture system. We constructed mouse transgenics and transfected germ cells in situ to study expression of the testis-specific isozyme of lactate dehydrogenase (LDH). From previous work, we determined that a 100-bp Ldhc core promoter contained potential cis regulatory elements, including a palindrome (-21 to +10), GC box (-70 to -65), and cAMP-responsive element (CRE) sites (-53 to -49, -39 to -35). We provide here the demonstration of a functional role for these sequences by expression of mutated transgenes in vivo. Our results reveal for the first time that mutation of the GC box does not abolish promoter activity, which remains testis-specific. Mutation of GC box or CRE sites resulted in a 73% and 74% reduction in promoter activity, respectively, in a transient transfection of germ cells in vivo by electroporation; the combination of GC box and CRE site mutations eliminates promoter activity. Therefore, we conclude that simultaneous occupancy of the GC box and CRE sites in the core promoter is necessary for full expression of Ldhc in the testis.

Testis-specific transcriptional control

In the testis, tissue-specific transcription is essential for proper expression of the genes that are required for the reproduction of the organism. Many testis-specific genes are required for mitotic proliferation of spermatogonia, spermatocytes undergoing genetic recombination and meiotic divisions, and differentiation of haploid spermatids. In this article we describe some of the genes that are transcribed in male germinal cells and in non-germinal testis cells. Because significant progress has been made in examination of promoter elements and their cognate transcription factors that are involved in controlling transcription of the testis-specific linker histone H1t gene in primary spermatocytes, this work will be reviewed in greater detail. The gene is transcriptionally active in spermatocytes and repressed in all other germinal and non-germinal cell types and, therefore, it serves as a model for study of regulatory mechanisms involved in testis-specific transcription.

A transgenic analysis of mouse lactate dehydrogenase C promoter activity in the testis

Transcription of the mouse testis-specific lactate dehydrogenase c (mldhc) gene is limited to cells of the germinal epithelium. Cloning and analysis of the mldhc promoter revealed that a 100-bp core promoter was able to regulate testis-specific transcription in vitro and in transgenic mice. Surprisingly, expression of the reporter in transgenic testes was limited to pachytene spermatocytes, whereas native LDH-C(4) was detected in pachytene and all later germ cells. To locate additional regulatory sequence that could recapitulate the native LDH-C(4) distribution pattern, we investigated the contribution of 5' and 3' flanking sequences to the regulation of LDH-C(4) expression. We found that transcription factor YY1 binds to the mldhc promoter, that the mldhc 3' untranslated sequence does not permit a postmeiotic expression of a beta-galactosidase reporter in transgenic mice, and that native mldhc mRNA is predominately meiotic, with only a low level of postmeiotic distribution. Our results suggest that the high level of LDH-C(4) in postmeiotic cells results from mRNA and protein stability.

Methylation of CpG dinucleotides alters binding and silences testis-specific transcription directed by the mouse lactate dehydrogenase C promoter

The mouse lactate dehydrogenase c gene (mldhc) is transcribed only in cells of the germinal epithelium. Cloning and analysis of the mldhc promoter revealed that a 100-base pair fragment was able to drive testis-specific transcription in vitro and in transgenic mice. Several testis-specific genes are believed to be regulated at least in part through differential methylation of CpG dinucleotides. We investigated the possibility that transcriptional repression of the mldhc gene is mediated in somatic tissues by hypermethylation of CpG dinucleotides. The CpG dinucleotides within a fragment of the mldhc promoter containing a GC box and tandem activating transcription factor/cAMP-responsive element binding sites are hypermethylated in somatic tissues and hypomethylated in testis. Methylation of the activating transcription factor/cAMP-responsive elements altered the protein binding pattern observed in electrophoretic mobility shift assays using mouse liver but not testis nuclear extract. Furthermore, methylation of an extended mldhc promoter fragment driving lac Z silenced transcription from the promoter in a transient transfection assay. These data suggest that tissue-specific differential methylation plays a role in mldhc silencing in somatic tissues.

ldhc expression in non-germ cell nuclei is repressed by NF-I binding

Developmental and testis-specific expression of the mouse lactate dehydrogenase C (mldhc) gene requires mechanisms for activation in germ cells and repression in somatic cells. Promoter activity restricted to the testis has been demonstrated using in vitro transcription assays with a 60-base pair promoter sequence upstream of the transcription initiation site. This promoter fragment has a TATA box and an overlapping 31-base pair palindromic sequence. Here we have explored the role of the palindrome as a silencer of the ldhc gene in somatic tissues. A gel retardation assay detected two sites within the palindrome that were important for protein binding. A member of the NF-I/CTF family was identified as the protein binding to one of the sites. In transiently transfected mouse L cells, a promoter fragment in which the NF-I site was mutated showed a 4-fold greater activity as compared with the wild-type sequence. Overexpression of the four NF-I proteins, NF-IA, -B, -C, or -X, in mouse L cells transiently transfected with an ldhc promoter-reporter construct resulted in a 20-50% decrease in activity of the wild-type promoter but had no effect when the NF-I binding element in the palindrome was mutated. These results indicate a role for the NF-I proteins in regulation of the mldhc gene.

Detection of PACH1, a nuclear factor implicated in the transcriptional regulation of meiotic and early haploid stages of spermatogenesis

Spermatogenesis occurs in a series of well-defined stages and serves as an excellent model for lineage-specific cell development. Yet, little is known regarding the transcriptional mechanisms responsible for cell- and stage-dependent gene regulation in the male germ line. The rat and mouse proenkephalin genes are expressed from an alternative, spermatogenic cell-specific promoter specifically in meiotically-active pachytene spermatocytes and early post-meiotic spermatids. This promoter thus serves as an excellent model for defining transcriptional regulators involved in germ line-specific gene expression in meiotic cells. Previous transgenic studies identified a proximal, 51 bp 5'-flanking sequence containing two direct repeat elements that are absolutely required for in vivo proenkephalin promoter activity in spermatocytes and spermatids. Here, footprinting analyses were used to further delineate the specific interactions of a spermatogenic cell nuclear factor with the repeat elements within the proximal promoter region. This repeat-binding factor was also shown to be developmentally upregulated specifically in pachytene spermatocytes. Using Southwestern analysis, we have identified a unique nuclear protein enriched in pachytene spermatocytes that specifically recognizes the repeat elements within the proximal 5'-flanking sequence. We propose that this DNA binding factor, termed PACH1, is a key transcriptional regulator of the proenkephalin and potentially other gene promoters, uniquely expressed during meiosis in the male germ line.

A novel 14-base-pair regulatory element is essential for in vivo expression of murine beta4-galactosyltransferase-I in late pachytene spermatocytes and round spermatids

During murine spermatogenesis, beginning in late pachytene spermatocytes, the beta4-galactosyltransferase-I (beta4GalT-I) gene is transcribed from a male germ cell-specific start site. We had shown previously that a 796-bp genomic fragment that flanks the germ cell start site and contains two putative CRE (cyclic AMP-responsive element)-like motifs directs correct male germ cell expression of the beta-galactosidase reporter gene in late pachytene spermatocytes and round spermatids of transgenic mice (N. L. Shaper, A. Harduin-Lepers, and J. H. Shaper, J. Biol. Chem. 269:25165-25171, 1994). We now report that in vivo expression of beta4GalT-I in developing male germ cells requires an essential and previously undescribed 14-bp regulatory element (5'-GCCGGTTTCCTAGA-3') that is distinct from the two CRE-like sequences. This cis element is located 16 bp upstream of the germ cell-specific start site and binds a male germ cell protein that we have termed TASS-1 (transcriptional activator in late pachytene spermatocytes and round spermatids 1). The presence of the Ets signature binding motif 5'-GGAA-3' on the bottom strand of the TASS-1 sequence (underlined sequence) suggests that TASS-1 is a novel member of the Ets family of transcription factors. Additional transgenic analyses established that an 87-bp genomic fragment containing the TASS-1 regulatory element was sufficient for correct germ cell-specific expression of the beta-galactosidase reporter gene. Furthermore, when the TASS-1 motif was mutated by transversion, within the context of the original 796-bp fragment, transgene expression was reduced 12- to 35-fold in vivo.

DNA-protein interactions in the CCAAT box region of the murine lactate dehydrogenase C promoter

Electrophoretic Mobility (EMSA), using oligonucleotides containing CCAAT box sequences from the murine Idhc promoter show the presence of CCAAT binding proteins in nuclear extracts from liver and testis. In a liver extract, a single shifted band is seen. However, in the testis extract, two shifts are observed, one of which may be due to a testis specific isoform of CCAAT binding factor (CBF). Southwestern analysis with an oligonucleotide probe containing these sequences reveals the presence of a protein of approximately 120 kD in the testis extract. In the liver extract, a 70-kD protein binds the probe. An antibody against HeLa CBF causes a supershift in testis nuclear extract.

Transgenic mice demonstrate a testis-specific promoter for lactate dehydrogenase, LDHC

The mammalian genome encodes a family of lactate dehydrogenase (LDH) isozymes. Two of these, ldha and ldhb, are expressed ubiquitously. The ldhc gene is active only in the germinal epithelium during spermatogenesis. In our analysis of ldhc gene regulation, we found that a 60-base pair promoter sequence was sufficient for testis-specific expression in an in vitro transcription assay. To confirm these findings, a genomic fragment containing 100 base pairs overlapping the transcription start site was isolated and linked to the Escherichia coli lacZ gene. We report that this genomic fragment drives testis-specific expression in transgenic mice. We conclude that transcription of the transgene and possibly of the endogenous ldhc gene is restricted to leptotene/pachytene primary spermatocytes.

The mouse ADP-ribosylation factor-like 4 gene: two separate promoters direct specific transcription in tissues and testicular germ cell

ADP-ribosylation factor-like protein 4 (ARL4) is a Ras-related GTPase that has been cloned from the 3T3-L1 preadipocyte cell line as an adipocyte-specific cDNA [Schürmann, Breiner, Becker, Huppertz, Kainulainen, Kentrup and Joost (1994) J. Biol. Chem. 269, 15683-15688]. The Arl4 gene maps to the proximal region of mouse chromosome 12 linked to Lamb1-1, Hfhbf1 and Sos2. Compared with all other known genes of Ras-related GTPases, the genomic organization of Arl4 is unusual in that its entire coding region, the 3' untranslated region (UTR) and most of the 5' UTR are located on a single exon. This structure suggests that Arl4 has evolved by retroposition of an Arf (ADP-ribosylation factor) or Arf-like gene. Isolation of the 5' UTR by rapid amplification of cDNA ends (RACE)-PCR revealed heterogeneous transcription initiation sites in alternative exons 1. Both 5'-flanking regions exhibited promoter activity when expressed in COS-7 cells, indicating that the expression of Arl4 is directed by two separate promoters. mRNA transcribed under the control of the downstream promoter was isolated by RACE-PCR from all investigated tissues. In contrast, the upstream promoter seems to drive specifically the expression of Arl4 in adult testis. Hybridization of rat testis in situ indicated that Arl4 is expressed in germ cells of puberal and adult testis, but not in prepuberal testis, suggesting that Arl4 is involved in sperm production.

Novel repeat elements direct rat proenkephalin transcription during spermatogenesis

The developmental program controlling sperm formation occurs in multiple stages that sequentially involve mitosis, meiosis, and spermiogenesis. The transcriptional mechanisms regulating these distinct phases are poorly understood. In particular, while a required role for the germ cell transcription factor cyclic AMP response element modulator-tau during spermiogenesis has recently been demonstrated, the transcriptional mechanisms leading to early haploid cell formation are unknown. The rat and mouse proenkephalin genes are selectively expressed from an alternate, germ cell-specific promoter in meiotic and early haploid cells. In this study, the minimal rat proenkephalin germ line promoter was localized to a 116-bp region encompassing the transcriptional start site region. Further, a proximal 51-bp sequence located in the 5'-flanking region is absolutely required for germ line promoter activity. This 51 bp sequence corresponds to a previously characterized binding element (GCP1) that forms cell-specific complexes with rat spermatogenic cell nuclear factors distinct from cyclic AMP response element binding proteins. Further, GCP1 contains novel direct repeat sequences required for factor binding and transgene expression in spermatogenic cells. These repeat elements are highly similar to sequences within the active regions of other male germ line promoters expressed during meiosis. GCP1 may therefore contain transcriptional elements that participate more generally during meiosis in the differentiation of spermatocytes and early haploid spermatids.