The germ cell specific Deleted in Azoospermia (DAZ) family of proteins is composed of three members; the primate and catarrhine lineage specific protein DAZ, located within the long arm of the Y chromosome (Reijo et al. 1995); the autosomal and family ancestor Boule, with homologs described from flies and worms to mice and humans (Karashima et al. 2000); and the vertebrate specific DAZ-like (DAZL) (Bielawski and Yang 2001). This family of proteins owes its name to the phenotype of patients were it was first identified. In a genetic screen of azoospermic men, 14 % of cases had deletions within the area containing the DAZ coding region (Reijo et al. 1995). The other members of the DAZ family have also been associated with male infertility as reported in several SNPs studies (Chen et al. 2010).

All members of the DAZ family are required during spermatogenesis. DAZ and Boule are necessary for spermatid development, with Boule-null mice showing a developmental arrest at step 6 of spermatids (VanGompel and Xu 2010). DAZL phenotype can be detected much earlier compared to its orthologues, with a spermatogenic arrest at prophase of meiosis I (Ruggiu et al. 1997; Saunders et al. 2003). This phenotype becomes more penetrant in a pure BL6/C57 background, with apoptosis observed at embryonic day 15.5 of mouse development (Lin and Page 2005). Genetic studies have associated this phenotype with an impairment of a canonical PGC feature, sex differentiation.

The nuclear and cytoplasmic localization of DAZL (Xu et al. 2001), the ability to bind mRNA (Tsui et al. 2000b; Venables et al. 2001), the association with polyribosomes (Tsui et al. 2000b) and the interaction with several known RBPs (Brook et al. 2009) suggest a role in mRNA processing. Several molecular functions have been associated with DAZL, ranging from mRNA repression (Urano et al. 2005; Padmanabhan and Richter 2006) mRNA transport (Lee et al. 2006; Kim et al. 2012), miRNA-mediated repression protection (Takeda et al. 2009), but to date the best characterized role for DAZL is in translation activation (Collier et al. 2005; Reynolds et al. 2005, 2007; Chen et al. 2011a). In vitro studies have demonstrated that DAZ family members all share a common function as translation activators. Tethering assays, where DAZL-RNA binding activity is circumvented, showed that this family upregulates translation of artificially bound luciferase reporters (Collier et al. 2005). In vivo experiments have since demonstrated translation of endogenous transcripts (Reynolds et al. 2005, 2007; Chen et al. 2011a). Insight into the mechanism of DAZ-family translation activation demonstrates that these proteins act at the step of translation initiation. A direct interaction with PABP1 (but none of the other canonical translation initiation factors) was shown to be necessary to promote translation in a reconstitution system. The observation that DAZL was able to stimulate translation of non-adenylated reporter mRNAs indicated that this can work distinctly from the canonical CPEB translation activator (Pique et al. 2008) by promoting PABP recruitment in a poly(A) independent manner (Collier et al. 2005).

DAZL interacts with transcripts with a U-rich 3′UTR (Tsui et al. 2000b). A variety of assays identified the trinucleotide GUU has the minimal binding motif of DAZL (Tsui et al. 2000b; Ruggiu and Cooke 2000; Venables et al. 2001). The crystal structure of DAZL RNA recognition motif (RRM) demonstrated that this presents the highest affinity towards GUU[U/C] sequences (Jenkins et al. 2011). The relatively short sequence of this element greatly challenges the identification of DAZL targets. In vitro evidence for the requirement of multiple DAZL binding sites for a maximal translation activation can in the future help to overcome this challenge (Collier et al. 2005; Chen et al. 2011a). Several studies have identified DAZL targets (Brook et al. 2009). In spermatogenesis, pull down experiments have demonstrated an interaction with messages involved in sperm development (Tpx1, Grsf-1, Trf-2, TssK proteins, etc.) (Jiao et al. 2002; Zeng et al. 2008). Nonetheless, to date the only two bona fide spermatogenic targets for DAZL are Sycp3 and Mvh (Reynolds et al. 2005, 2007).

Whereas the role of Puf proteins in model organisms have been extensively investigated, comparable little information is available on the role of Pumilio proteins, Pum1 and Pum2, in spermatogenesis and oogenesis in mammals. A gene trap disruption of Pum2 was reported to produce a decrease in testis weight and some disruption in spermatogenesis but the exact steps affected were not investigated (Xu et al. 2007). Pum1 knockout mice have decreased fertility and decreased sperm count and 25–30 % reduction in testis weight. It has been proposed that this is due to deregulation of the apoptotic pathway in the Pum1^−/− mice during spermatogenesis (Chen et al. 2012). Pum1-dependent repression of Map3k1 is required to maintain P53 inactive. Since Pum1 targets a large number of mRNAs in cell lines and in the testis it is most likely that Pum1 functions go well beyond P53 regulation. In model organism, Pumilios play a critical role in maintaining the germ cell fate and prevent germ cell differentiation. It may be possible that accelerated differentiation of spermatogenic cells in the Pum1 knockout model leads to unbalanced ratio/germ cell supporting cells in the testis, a condition known to trigger apoptosis and spermatogenic failure.

DAZAP1 is a member of the heterogeneous nuclear ribonuclear protein family (hnRNP). This protein was initially identified in a study looking at DAZL interacting proteins (Tsui et al. 2000a). But despite this interaction, both proteins differ in their protein expression pattern and were described to be associated with different subsets of target mRNAs, indicating that both proteins can have different roles (Kurihara et al. 2004).

DAZAP1 is ubiquitously expressed but highly enriched in testes (Dai et al. 2001). Whereas it is predominantly found in the nucleus of somatic cells (Lin and Yen 2006), in testes, DAZAP1 shows a dynamic distribution. In germ cells DAZAP1 is found in the nucleus from mid-pachytene spermatocytes to round spermatids, and relocates to the cytoplasm in elongating spermatids (Vera et al. 2002).

As with other hnRNPs, DAZAP1 binds newly synthesized transcripts and, through processes of splicing, export and translation, controls expression of specific messages (Martinez-Contreras et al. 2007). Recent evidence further elucidated the mechanism employed by DAZAP1 in splicing control based on extracellular cues. Through interaction with hnRNPs governed by 3′UTR cis-elements of target transcripts, DAZAP1 promotes splicing of weak exons. Phosphorylation of the C-terminal proline rich domain of DAZAP1 by MEK/Erk pathway alters the ability of DAZAP1 to maintain protein interactions with splicing factors and induces cytoplasmic translocation of DAZAP1 (Choudhury et al. 2014).

The molecular mechanisms of translational modulation of DAZAP1 are still not fully understood. It binds RNA at a consensus sequence AAAUAG and GU_[1–3]AG (Yang et al. 2009), and the function is dependent on the protein partners to which it binds. Protein–protein interactions have been described with DAZL, KH-type splicing regulatory protein (KHSRP), hnRNPs, DEAD box polypeptide 20 (DDX20), let 7 and many other RBPs (Yang et al. 2009). The function as a translation activator has been studied in vitro models where DAZAP1 proteins were tethered to luciferase reporters. These show that both mouse and human DAZAP, as observed for X. laevis, stimulates translation in a cap-independent manner and it shows preferential translation towards nonadenylated messages (Smith et al. 2011).

Interestingly, DAZAP1 is itself a target for translation control. Although initially present at mid-pachytene spermatocytes, in situ analysis of DAZAP1 detects high levels of this transcript in spermatogonia and early spermatocytes (Vera et al. 2002). Through alternative usage of cleavage and polyadenylation sites, translation of each isoform is regulated by different mechanisms. At day 12 post-partum association with polyribosomes shows that both isoforms are actively translated. Upon reaching puberty the isoform with the longest 3′UTR relocates to the translational inactive mRNP fraction with a concomitant shortening of the poly(A) tail. The smaller isoform is also recruited to the mRNP fraction but this shows no changes in poly(A) length. The germ cell specific DAZL was shown to preferentially bind in vivo to the shorter isoform of DAZAP1, and in vitro assays also described a preferential DAZL promoted translation of a reporter with the 3′UTR of the shorter isoform (Yang and Yen 2013).

Ddx25 gained its common name, GRTH, from the fact that translation of this protein is dependent on hormonal cues. Three consensus Kozak sequence s can be identified in frame with the coding region of GRTH. Whilst the second AUG is used by Leydig cells, germ cells preferentially use the first and third start codons. Although, treatment of rats with hCG, resulted in a shift towards the second AUG by round spermatids, but not spermatocytes (Sheng et al. 2003). This translational control is believed to result from induced paracrine factors that regulate the internal ribosomal entry site mechanisms.

GRTH promotes survival of spermatocytes by regulating apoptotic pathways. In Grth-null models, spermatogenesis arrests at step 8 spermatids due to increased apoptosis in spermatocytes at stage XII (Tsai-Morris et al. 2008). Despite not totally understood how GRTH regulates translation, an initial mechanism was directly linked to the function of RNA helicases that control RNA unwinding and nuclear transport (Tsai-Morris et al. 2010). An association with chromatoid bodies and polyribosomes further supported a role of this protein in translation (Tang et al. 1999; Sheng et al. 2006; Tsai-Morris et al. 2004b). Animal models provided the direct link to translation control by providing evidence that decrease levels of GRTH in germ cells resulted in decreased levels of specific proteins (TNPs and tACE) with no differences observed in mRNA levels (Tsai-Morris et al. 2004a). GRTH is present in the nucleus and cytoplasm, and is coupled with nuclear export, dependent on its phosphorylation status. Depletion of GRTH resulting in smaller chromatoid bodies and a deficit in the cytoplasmic ratios of proteins associated with nuclear histone-protamine (Tsai-Morris et al. 2004b; Sheng et al. 2006).

In humans, a mutation on GRTH is associated with non-obstructive azoospermia. This mutation results in a nonphosphorilated form of GRTH, and was proposed to impair RNA-binding and protein–protein interactions due to the location of the substituted residue in a hydrophobic pocket (Tsai-Morris et al. 2007).

Our current understanding of translational regulation during oocyte growth and maturation is mainly based on findings in the X. laevis oocyte model. In the frog system, it has been proposed that a combinatorial code of cis-acting elements in the 3′UTR of the messages regulates protein synthesis according to the temporal requirement of meiosis arrest and progression, as well as early embryo development.

One of the best characterized mechanisms of translation regulation in frog oocyte requires the cis-acting element CPE and the cognate binding proteins CPEB . By controlling the length of the cytoplasmic poly(A) tail, Cpeb1 represses translation in immature oocytes and, when phosphorylated upon cell cycle reentry, activates translation (McGrew et al. 1989). A mammalian isoform of CPEB, with 80 % identity with the frog Cpeb, was first described in mouse oocytes as a protein bound to the c-Mos mRNA (Gebauer and Richter 1996). Early studies of the tissue-type plasminogen activator (tPA) mRNA provided initial evidence that maternal mRNA repression and translation in mammals are regulated similarly to frogs (Huarte et al. 1992; Stutz et al. 1997). For instance, tPA is stored in a silenced form with a short poly(A) tail (30–40 nt) in GV stage oocytes. Upon meiotic re-entry, the poly(A) tail is elongated (to about 200 nt) and tPA becomes translated (Huarte et al. 1992). The role of a CPE cis-acting element was soon recognized as an important player in this regulation. By inserting a CPE or deleting an existing one, the translation is respectively repressed or activated, together with shortening or lengthening of the poly(A) tail (Huarte et al. 1992; Gebauer and Richter 1996).

How CPEB functions in repressing poly(A) elongation and translation of maternal transcripts in mammals is still not fully understood. At least three different models have been proposed in frog oocytes, depending on the composition of the repressor complex, as reviewed in (Villalba et al. 2011). One possibility is that Cpeb simultaneously recruits the poly(A)-ribonuclease Parn and the poly(A)-polymerase Gld-2. The competition between the two enzymatic activities maintains a short poly(A) tail, hence promoting repression (Kim and Richter 2006). Alternatively a repressor protein, identified as Maskin , may bind to Cpeb and eIF4E, blocking the formation of the cap-binding complex on the mRNA, resulting in translational inhibition (Stebbins-Boaz et al. 1999). Similarly, a third model predicts that Cpeb bind to Clast 4 (also known as 4E-transporter (4E-T)), which in turn recruits isoforms of eIF4E, like eIF4E1b, that have low affinity for the cap structure (Minshall et al. 2007).

Rather than being mutually exclusive, these models may be integrated in the control of translational repression of different mRNA species or during different steps of oogenesis. Moreover extra layers of complexity are achieved through multiple regulatory cis and trans-acting factors. For instance additional regulation can be mediated by Pumilio binding elements (PBEs) that are frequently present in mRNAs that possess CPEs (Ota et al. 2011). As previously discussed in model organisms, Pumilio proteins (PUM1, PUM2) are mainly repressor proteins, but they can also stabilize CPEB binding and promote poly(A) tail elongation in some context (Padmanabhan and Richter 2006; Pique et al. 2008). However their role in mammalian oogenesis has not been elucidated yet. Interestingly, it has been reported that Pum1 protein in zebrafish oocytes is required to localize cyclin B1 to ribonuclear particles and this co-localization was confirmed in mouse oocytes (Kotani et al. 2013).

Another cis-acting element known to repress translation in immature oocytes is the translational control sequence (TCS). The corresponding trans-acting factor has been recently discovered in X. leavis and it is represented by zygote arrest 1 (Zar1) and Zar2. Zar1 and Zar2 bind to Mos and Wee1 mRNA 3′UTRs via a zinc finger. The TCS-mediated repression of translation is exerted during oocyte growth and initial phases of meiotic resumption until the metaphase I (MI) stage (Charlesworth et al. 2012; Yamamoto et al. 2013). It is not clear if the translational activation of Zar2 target mRNAs occurs following partial degradation of Zar2 and release of the repressive state, or if the control requires more than one trans-acting factor, as proposed for CPE. Based on the observation that mice embryos that are null for Zar1 arrest at the 1-cell stage due to failure in EGA, mammalian ZAR family proteins were initially proposed to be transcriptional regulators (Wu et al. 2003). However, a role for ZAR proteins in translational control in mammalian oocytes cannot be excluded. In this view, protein product(s) of ZAR-target mRNAs would be responsible of EGA, and the absence of ZAR would prevent their timely translation with subsequent failure in embryonic transcription and development.

In X. laevis oocytes CPE-induced translation is responsible for activating synthesis of RBPs, like the zinc finger protein C3H-4, that are required in the later stages of maturation for deadenylation of transcripts carrying A-U rich elements (AREs) (Belloc and Mendez 2008). In mammals, the embryonic lethal abnormal vision like 2 (ELAVL2) protein regulates ARE -mediated mRNA repression during oocyte growth (Chalupnikova et al. 2014). ELAVL2 decreased in abundance as the oocyte reaches a fully grown, meiotically competent stage of development, which is characterized by the transition from the non-surrounded nucleolus (NSN) chromatin configuration into the transcriptionally quiescent surrounded nucleolus (SN) configuration. ELAVL2 is absent in MII oocytes and zygotes. Mammalian proteins that function analogous to the frog C3H-4 protein to mediate ARE dependent deadenylation and repression during oocyte maturation have not been identified.

A regulatory feedback involving CPE-mediated translation to recruit decapping mRNA1 and 2 (Dcp1a and Dpc2) has been recently described in mice (Ma et al. 2013). Decapping , i.e. the removal of the 5′ 7-methyl-guanosine cap, exposes mRNAs to exonucleases and represents a critical step in the 5′→3′ transcript degradation. Both the regulatory and catalytic subunits, DCP1A and DCP2, are synthesized between MI and MII and the inhibition of their accumulation interferes with the proper EGA execution. It is not clear at this point whether and how the DCP1A/DCP2-mediated degradation could be selective. One of the possibilities is that their involvement is the downstream event in a multi-step process that first requires mRNAs to be destabilized and tagged for degradation by sequence specific regulatory factors.