Connecting cis-elements and trans-factors with mechanisms of developmental regulation of mRNA translation in meiotic and haploid mammalian spermatogenic cells

Abstract

mRNA-specific regulation of translational activity plays major roles in directing the development of meiotic and haploid spermatogenic cells in mammals. Although many RNA-binding proteins (RBPs) have been implicated in normal translational control and sperm development, little is known about the keystone of the mechanisms: the interactions of RBPs and microRNAs with cis-elements in mRNA targets. The problems in connecting factors and elements with translational control originate in the enormous complexity of posttranscriptional regulation in mammalian cells. This creates confusion as to whether factors have direct or indirect and large or small effects on the translation of specific mRNAs. This review argues that gene knockouts, heterologous systems, and overexpression of factors cannot provide convincing answers to these questions. As a result, the mechanisms involving well-studied mRNAs (Ddx4/Mvh, Prm1, Prm2, and Sycp3) and factors (DICER1, CPEB1, DAZL, DDX4/MVH, DDX25/GRTH, translin, and ELAV1/HuR) are incompletely understood. By comparison, mutations in elements can be used to define the importance of specific pathways in regulating individual mRNAs. However, few elements have been studied, because the only reliable system to analyze mutations in elements, transgenic mice, is considered impractical. This review describes advances that may facilitate identification of the direct targets of RBPs and analysis of mutations in cis-elements. The importance of upstream reading frames in the developmental regulation of mRNA translation in spermatogenic cells is also documented.

Linking transcriptomics and proteomics in spermatogenesis

“…This substitution allows the progressive condensation of chromatin and thus leads to the complete cessation of transcription from mid-spermiogenesis onwards (Kierszenbaum & Tres 1975). As a consequence, many genes that are required for the development and/or functioning of spermatozoa are transcribed much earlier during germ cell differentiation, then translationally repressed, and finally translated several days after the mRNA production, thanks to a complex interplay of RNA-binding proteins and non-coding RNA (for review, see Kleene (2013)). In this context, it is interesting to note the existence of the chromatoid body, a germ cell-specific RNA processing center suggested to be involved in the sequestration and translation repression of several mRNAs during spermiogenesis (Kotaja & Sassone-Corsi 2007) and whose RNA and protein content was recently analyzed (Meikar et al 2014).…”

Mechanisms of translational repression of the Smcp mRNA in round spermatids

“…The developmental regulation of translational of specific mRNAs is important in controlling protein expression during the development of post-meiotic, haploid spermatogenic cells, spermatids, because transcription in late spermatids ceases due to chromatin remodeling (reviewed in Meistrich et al (2003) and Kleene (2013)). This phenomenon is illustrated by the protamine 1 (Prm1) and sperm-mitochondria cysteine-rich protein (Smcp) mRNAs, which are transcribed in round spermatids stored as translationally inactive messenger ribonucleoprotein particles (free-mRNPs) for 3 and 7 days, respectively, before translation begins in transcriptionally compromised elongating and elongated spermatids (Kleene 1989(Kleene , 2013.…”

“…This phenomenon is illustrated by the protamine 1 (Prm1) and sperm-mitochondria cysteine-rich protein (Smcp) mRNAs, which are transcribed in round spermatids stored as translationally inactive messenger ribonucleoprotein particles (free-mRNPs) for 3 and 7 days, respectively, before translation begins in transcriptionally compromised elongating and elongated spermatids (Kleene 1989(Kleene , 2013. The initial block to mRNA translation avoids deleterious effects of premature PRM1 expression on male fertility (Lee et al 1995).…”

“…About 20 RBPs and several miRNAs have been reported to be implicated in translational control in spermatids by studies of knockout mice and overexpression of RBPs (Kleene 2013, Kotaja 2014. However, RBPs and miRNAs often interact with huge numbers of mRNA targets and are expressed for prolonged periods in spermatogenic cells.…”

“…However, RBPs and miRNAs often interact with huge numbers of mRNA targets and are expressed for prolonged periods in spermatogenic cells. This complexity creates uncertainties whether the effects of individual factors on target mRNAs are direct or indirect and whether the factor produces major or minor changes in translational activity (Kleene 2013). The functions of factors in translational regulation can be accurately defined by analyzing mutations in cis-elements that abrogate binding of factors.…”