Gamma-irradiation increased meiotic crossovers in mouse spermatocytes

Cornichon protein CNIH4 is not essential for mice gametogenesis and fertility

BACKGROUND

Cornichon is a functionally conserved transmembrane protein family that generally acts as a cargo-sorting receptor and cycles between the ER and the Golgi. Four Cornichon family members (CNIH1-4) have been identified. The key residues responsible for CNIH1-3 to bind to AMPA receptors are not conserved in CNIH4. Additionally, the function of CNIH1-3 in GPCR signaling is less established, while more established in case of CNIH4 protein that interact with GPCR and control their exportation. Many GPCRs are known for their essential roles in male and female gonad development. But whether CNIH4 plays a role in gametogenesis remains unknown.

DESIGN

Mice carrying the Cnih4 knockout allele (Cnih4^tm1a-/-) were generated by insertion of a LacZ reporter and a polyadenylation site after exon 1. Western blot, Immunofluorescence, computer-aided sperm analysis and other methods were used in the functional analysis.

RESULTS

We identified that both Cnih4^tm1a-/- male and female mice have normal fertility. Though, the sperm count, morphology, and motility of Cnih4^tm1a-/- mice were slightly impaired compared to those of wild-type mice, the testes to body weight ratio and testicular histology were similar to those in control mice. Histological examination of Cnih4^tm1a-/- ovaries detected follicles from primordial to antral stages and the numbers of follicles at each stage were also comparable to wild-type controls. Normal fertility was noticed after six-month fertility tests. That was likely due to the compensatory role of Chin3, which significantly upregulated in the Cnih4^tm1a-/- mice to preserve the fertility role.

CONCLUSION

Despite CNIH4 showing enriched expression in mouse germ cells, our genetic knockout studies demonstrated that CNIH4 is not essential for gametogenesis and fertility in mice although with a slight reduction in count, motility and morphology of sperm in male mice.

Flurochloridone induced abnormal spermatogenesis by damaging testicular Sertoli cells in mice

BACKGROUND

Flurochloridone (FLC), a selective herbicide used on a global scale, has been reported to have male reproductive toxicity whose evidence is limited, but its mechanism remains unclear. The present study was conducted to systematically explore the male reproductive toxicity of FLC, including sperm quality, spermatogenesis, toxicity targets, and potential mechanisms.

METHODS

Male C57BL/6 mice aged 6-7 weeks received gavage administration of FLC (365/730 mg/kg/day) for 28 consecutive days. Then, the tissue and sperm of mice were collected for analysis. We measured the gonadosomatic index and analyzed sperm concentration, motility, malformation rate, and mitochondrial membrane potential (MMP). Spermatocyte immunofluorescence staining was performed to analyze meiosis. We also performed pathological staining on the testis and epididymis tissue and TUNEL staining, immunohistochemical analysis, and ultrastructural observation on the testicular tissue.

RESULTS

Results showed that FLC caused testicular weight reduction, dysfunction, and architectural damage in mice, but no significant adverse effect was found in the epididymis. The exposure interfered with spermatogonial proliferation and meiosis, affecting sperm concentration, motility, kinematic parameters, morphology, and MMP, decreasing sperm quality. Furthermore, mitochondrial damage and apoptosis of testicular Sertoli cells were observed in mice treated with FLC.

CONCLUSION

We found that FLC has significant adverse effects on spermatogonial proliferation and meiosis. Meanwhile, apoptosis and mitochondrial damage may be the potential mechanism of Sertoli cell damage. Our study demonstrated that FLC could induce testicular Sertoli cell damage, leading to abnormal spermatogenesis, which decreased sperm quality. The data provided references for the toxicity risk and research methods of FLC application in the environment.

Chronic exposure to diesel exhaust particulate matter impairs meiotic progression during spermatogenesis in a mouse model

Exposure to ambient PM_2.5 may correlate with the decline of semen quality, and the underlying biological mechanism has not been fully understood. In the present study, mice were intratracheally instilled with diesel exhaust PM_2.5 (DEP), and its effects on the spermatogenic process as well as the alterations of testicular gene expression profile were assessed. Our results showed that chronic exposure to DEP impaired the fertility of male mice without influencing their libido. Compared with Vehicle-exposed group, the sperm count and motility from DEP-exposed mice were significantly decreased. In addition, immunohistological staining of γH2AX and DMC1, biomarkers for meiotic double strand breaks (DSBs), demonstrated that chronic exposure to DEP comprised the repair of meiotic DSBs, thus disrupting the spermatogenesis. Deep RNA sequencing test showed altered expressions of testicular genes including the GnRH signaling pathway. In summary, our research demonstrated that chronic exposure to DEP may disrupt spermatogenesis through targeting the meiotic recombination, providing a new perspective for the research on the male reproductive system damage caused by air pollution.

UHRF1-repressed 5'-hydroxymethylcytosine is essential for the male meiotic prophase I

5’-hydroxymethylcytosine (5hmC), an important 5’-cytosine modification, is altered highly in order in male meiotic prophase. However, the regulatory mechanism of this dynamic change and the function of 5hmC in meiosis remain largely unknown. Using a knockout mouse model, we showed that UHRF1 regulated male meiosis. UHRF1 deficiency led to failure of meiosis and male infertility. Mechanistically, the deficiency of UHRF1 altered significantly the meiotic gene profile of spermatocytes. Uhrf1 knockout induced an increase of the global 5hmC level. The enrichment of hyper-5hmC at transcriptional start sites (TSSs) was highly associated with gene downregulation. In addition, the elevated level of the TET1 enzyme might have contributed to the higher 5hmC level in the Uhrf1 knockout spermatocytes. Finally, we reported Uhrf1, a key gene in male meiosis, repressed hyper-5hmC by downregulating TET1. Furthermore, UHRF1 facilitated RNA polymerase II (RNA-pol2) loading to promote gene transcription. Thus our study demonstrated a potential regulatory mechanism of 5hmC dynamic change and its involvement in epigenetic regulation in male meiosis.

Transition from a meiotic to a somatic-like DNA damage response during the pachytene stage in mouse meiosis

Homologous recombination (HR) is the principal mechanism of DNA repair acting during meiosis and is fundamental for the segregation of chromosomes and the increase of genetic diversity. Nevertheless, non-homologous end joining (NHEJ) mechanisms can also act during meiosis, mainly in response to exogenously-induced DNA damage in late stages of first meiotic prophase. In order to better understand the relationship between these two repair pathways, we studied the response to DNA damage during male mouse meiosis after gamma radiation. We clearly discerned two types of responses immediately after treatment. From leptotene to early pachytene, exogenous damage triggered the massive presence of γH2AX throughout the nucleus, which was associated with DNA repair mediated by HR components (DMC1 and RAD51). This early pathway finished with the sequential removal of DMC1 and RAD51 and was no longer inducible at mid pachytene. However, from mid-pachytene to diplotene, γH2AX appeared as large discrete foci. This late repair pattern was mediated initially by NHEJ, involving Ku70 and XRCC4, which were constitutively present, and 53BP1, which appeared at sites of damage soon after irradiation. Nevertheless, 24 hours after irradiation, a HR pathway involving RAD51 but not DMC1 mostly replaced NHEJ. Additionally, we observed the occurrence of synaptonemal complex bridges between bivalents, most likely representing chromosome translocation events that may involve DMC1, RAD51 or 53BP1. Our results reinforce the idea that the early “meiotic” repair pathway that acts by default at the beginning of meiosis is replaced from mid-pachytene onwards by a “somatic-like” repair pattern. This shift might be important to resolve DNA damage (either endogenous or exogenous) that could not be repaired by the early meiotic mechanisms, for instance those in the sex chromosomes, which lack a homologous chromosome to repair with. This transition represents another layer of functional changes that occur in meiotic cells during mid pachytene, in addition to epigenetic reprograming, reactivation of transcription, changes in the gene expression profile and acquisition of competence to proceed to metaphase.

DNA repair is critical for both somatic and meiotic cells. During meiosis, hundreds of DNA double strand breaks (DSBs) are introduced endogenously. To repair this damage, meiotic cells use a specialized version of the homologous recombination (HR) pathway that uses specific meiotic recombinases, such as DMC1, to promote repair with the homologous chromosome instead of the sister chromatid. This process is important to ensure chromosome segregation during meiosis and, as a side consequence, increases the genetic diversity of offspring. Nevertheless, under specific circumstances, meiotic cells can use other DNA repair mechanisms such as non-homologous end joining (NHEJ), which is error-prone. We investigated the response of mouse spermatocytes to increased DNA damage caused by gamma radiation, which is commonly used in cancer therapy. We found that the excess of DSBs produced by irradiation is processed by the meiotic HR recombination pathway in spermatocytes at the early stages of first meiotic prophase. However, this response is not inducible from the mid-pachytene stage onwards. From this point on, spermatocytes rely on a response that shares many features with that of somatic cells. In this response, the NHEJ pathway is first used to repair DNA damage but is subsequently replaced by a HR mechanism that does not use DMC1. Instead, it relies only on RAD51, which is known to function in both somatic and meiosis cells and, contrary to DMC1, has a preference for the sister chromatid. This switch from a meiotic to a somatic-like response is accompanied by a conspicuous change in the epigenetic response to DNA damage, reinforcing the idea that a functional transition occurs in meiotic cells during the mid-pachytene stage.

The protective function of noncoding DNA in genome defense of eukaryotic male germ cells

Peripheral and abundant noncoding DNA has been hypothesized to protect the genome and the central protein-coding sequences against DNA damage in somatic genome. In the cytosol, invading exogenous nucleic acids may first be deactivated by small RNAs encoded by noncoding DNA via mechanisms similar to the prokaryotic CRISPR-Cas system. In the nucleus, the radicals generated by radiation in the cytosol, radiation energy and invading exogenous nucleic acids are absorbed, blocked and/or reduced by peripheral heterochromatin, and damaged DNA in heterochromatin is removed and excluded from the nucleus to the cytoplasm through nuclear pore complexes. To further strengthen the hypothesis, this review summarizes the experimental evidence supporting the protective function of noncoding DNA in the genome of male germ cells. Based on these data, this review provides evidence supporting the protective role of noncoding DNA in the genome defense of sperm genome through similar mechanisms to those of the somatic genome.

Ndrg3 gene regulates DSB repair during meiosis through modulation the ERK signal pathway in the male germ cells

The N-myc downstream regulated gene (NDRG) family consists of 4 members, NDRG-1, -2, -3, -4. Physiologically, we found Ndrg3, a critical gene which led to homologous lethality in the early embryo development, regulated the male meiosis in mouse. The expression of Ndrg3 was enhanced specifically in germ cells, and reached its peak level in the pachytene stage spermatocyte. Haplo-insufficiency of Ndrg3 gene led to sub-infertility during the male early maturation. In the Ndrg3^+/- germ cells, some meiosis events such as DSB repair and synaptonemal complex formation were impaired. Disturbances on meiotic prophase progression and spermatogenesis were observed. In mechanism, the attenuation of pERK1/2 signaling was detected in the heterozygous testis. With our primary spermatocyte culture system, we found that lactate promoted DSB repair via ERK1/2 signaling in the male mouse germ cells in vitro. Deficiency of Ndrg3 gene attenuated the activation of ERK which further led to the aberrancy of DSB repair in the male germ cells in mouse. Taken together, we reported that Ndrg3 gene modulated the lactate induced ERK pathway to facilitate DSB repair in male germ cells, which further regulated meiosis and subsequently fertility in male mouse.

Do Exogenous DNA Double-Strand Breaks Change Incomplete Synapsis and Chiasma Localization in the Grasshopper Stethophyma grossum?

Meiotic recombination occurs as a programmed event that initiates by the formation of DNA double-strand breaks (DSBs) that give rise to the formation of crossovers that are observed as chiasmata. Chiasmata are essential for the accurate chromosome segregation and the generation of new combinations of parental alleles. Some treatments that provoke exogenous DSBs also lead to alterations in the recombination pattern of some species in which full homologous synapsis is achieved at pachytene. We have carried out a similar approach in males of the grasshopper Stethophyma grossum, whose homologues show incomplete synapsis and proximal chiasma localization. After irradiating males with γ rays we have studied the distribution of both the histone variant γ-H2AX and the recombinase RAD51. These proteins are cytological markers of DSBs at early prophase I. We have inferred synaptonemal complex (SC) formation via identification of SMC3 and RAD 21 cohesin subunits. Whereas thick and thin SMC3 filaments would correspond to synapsed and unsynapsed regions, the presence of RAD21 is only restricted to synapsed regions. Results show that irradiated spermatocytes maintain restricted synapsis between homologues. However, the frequency and distribution of chiasmata in metaphase I bivalents is slightly changed and quadrivalents were also observed. These results could be related to the singular nuclear polarization displayed by the spermatocytes of this species.

Germline deletion of huntingtin causes male infertility and arrested spermiogenesis in mice

Human Huntingtin (HTT), a Huntington's disease gene, is highly expressed in the mammalian brain and testis. Simultaneous knockout of mouse Huntingtin (Htt) in brain and testis impairs male fertility, providing evidence for a link between Htt and spermatogenesis; however, the underlying mechanism remains unclear. To understand better the function of Htt in spermatogenesis, we restricted the genetic deletion specifically to the germ cells using the Cre/loxP site-specific recombination strategy and found that the resulting mice manifested smaller testes, azoospermia and complete male infertility. Meiotic chromosome spread experiments showed that the process of meiosis was normal in the absence of Htt. Notably, we found that Htt-deficient round spermatids did not progress beyond step 3 during the post-meiotic phase, when round spermatids differentiate into mature spermatozoa. Using an iTRAQ-based quantitative proteomic assay, we found that knockout of Htt significantly altered the testis protein profile. The differentially expressed proteins exhibited a remarkable enrichment for proteins involved in translation regulation and DNA packaging, suggesting that Htt might play a role in spermatogenesis by regulating translation and DNA packaging in the testis.

Specific deletion of Cdh2 in Sertoli cells leads to altered meiotic progression and subfertility of mice

CDH2 (cadherin 2, Neural-cadherin, or N-cadherin) is the predominant protein of testicular basal ectoplasmic specializations (basal ES; a testis-specific type of adhesion junction), one of the major cell junctions composing the blood-testis barrier (BTB). The BTB is found between adjacent Sertoli cells in seminiferous tubules, which divides the tubules into basal and adluminal compartments and prevents the deleterious exchange of macromolecules between blood and seminiferous tubules. However, the exact roles of basal ES protein CDH2 in BTB function and spermatogenesis is still unknown. We thus generated mice with Cdh2 specifically knocked out in Sertoli cells by crossing Cdh2 loxP mice with Amh-Cre mice. Cdh2 deletion in Sertoli cells did not affect Sertoli cell counts, but led to compromised BTB function, delayed meiotic progression from prophase to metaphase I in testes, increased germ cell apoptosis, sloughing of meiotic cells, and, subsequently, reduced sperm counts in epididymides and subfertility of mice. However, the testes with Cdh2-specific deletion in germ cells did not show any difference from the normal control testes, and phenotypes observed in Sertoli cell and germ cell Cdh2 double-knockout mice were indistinguishable from those in mice with Cdh2 specifically knocked out only in Sertoli cells. Taken together, our data demonstrate that the adhesion junction component, Cdh2, functions just in Sertoli cells, but not in germ cells during spermatogenesis, and is essential for the integrity of BTB function, its deletion in Sertoli cells would lead to the BTB damage and subsequently meiosis and spermatogenesis failure.

Defending the genome from the enemy within: mechanisms of retrotransposon suppression in the mouse germline

The viability of any species requires that the genome is kept stable as it is transmitted from generation to generation by the germ cells. One of the challenges to transgenerational genome stability is the potential mutagenic activity of transposable genetic elements, particularly retrotransposons. There are many different types of retrotransposon in mammalian genomes, and these target different points in germline development to amplify and integrate into new genomic locations. Germ cells, and their pluripotent developmental precursors, have evolved a variety of genome defence mechanisms that suppress retrotransposon activity and maintain genome stability across the generations. Here, we review recent advances in understanding how retrotransposon activity is suppressed in the mammalian germline, how genes involved in germline genome defence mechanisms are regulated, and the consequences of mutating these genome defence genes for the developing germline.