Long non-coding RNAs (lncRNAs) in spermatogenesis and male infertility

Abstract

Background: Long non-coding RNAs (lncRNAs) have a size of more than 200 bp and are known to regulate a host of crucial cellular processes like proliferation, differentiation and apoptosis by regulating gene expression. While small noncoding RNAs (ncRNAs) such as miRNAs, siRNAs, Piwi-interacting RNAs have been extensively studied in male germ cell development, the role of lncRNAs in spermatogenesis remains largely unknown.

Objective: In this article, we have reviewed the biology and role of lncRNAs in spermatogenesis along with the tools available for data analysis.

Results and conclusions: Till date, three microarray and four RNA-seq studies have been undertaken to identify lncRNAs in mouse testes or germ cells. These studies were done on pre-natal, post-natal, adult testis, and different germ cells to identify lncRNAs regulating spermatogenesis. In case of humans, five RNA-seq studies on different germ cell populations, including two on sperm, were undertaken. We compared three studies on human germ cells to identify common lncRNAs and found 15 lncRNAs (LINC00635, LINC00521, LINC00174, LINC00654, LINC00710, LINC00226, LINC00326, LINC00494, LINC00535, LINC00616, LINC00662, LINC00668, LINC00467, LINC00608, and LINC00658) to show consistent differential expression across these studies. Some of the targets of these lncRNAs included CENPB, FAM98B, GOLGA6 family, RPGR, TPM2, GNB5, KCNQ10T1, TAZ, LIN28A, CDKN2B, CDKN2A, CDKN1A, CDKN1B, CDKN1C, EZH2, SUZ12, VEGFA genes. A lone study on human male infertility identified 9879 differentially expressed lncRNAs with three (lnc32058, lnc09522, and lnc98497) of them showing specific and high expression in immotile sperm in comparison to normal motile sperm. A few lncRNAs (Mrhl, Drm, Spga-lncRNAs, NLC1-C, HongrES2, Tsx, LncRNA-tcam1, Tug1, Tesra, AK015322, Gm2044, and LncRNA033862) have been functionally validated for their roles in spermatogenesis. Apart from rodents and humans, studies on sheep and bull have also identified lncRNAs potentially important for spermatogenesis. A number of these non-coding RNAs are strong candidates for further research on their roles in spermatogenesis.

Keywords: Long non-coding RNA; Male infertility; Spermatogenesis; Transcriptome; lncRNA; lncRNA databases; lncRNA regulation.

Fig. 1

Classification of non-coding RNAs. Based on size non-coding RNAs can be divided into small non-coding RNAs and long non-coding RNAs

Fig. 2

Genomic contexts of lncRNAs. LncRNAs (blue) may be stand-alone transcription units, intronic, antisense to other genes, or transcribed from promoters or enhancers

Fig. 3

Gene expression is regulated by lncRNAs in three ways. Transcriptional: lncRNA interacts with methylation and de-acetylation complex to affect gene expression. Post-transcriptional: lncRNA interacts with miRNA to inhibit its action (a), lncRNA is degraded by miRNA (b), competitive binding of lncRNA and miRNA to the target site on mRNA (c). Affecting alternative splicing: lncRNA facilitates alternative splicing by binding to the target site on pre-mRNA. Translational regulation: lnc recruits translational repressor to block translation (a), lnc recruits polysomes to promote translation (b)

Fig. 4

Functionally characterized lncRNAs expressed in different stages of spermatogenesis