High fat diet-induced obesity prolongs critical stages of the spermatogenic cycle in a Ldlr-/-.Leiden mouse model

Obesity can disturb spermatogenesis and subsequently affect male fertility and reproduction. In our study, we aim to elucidate at which cellular level of adult spermatogenesis the detrimental effects of obesity manifest. We induced high fat diet (HFD) obesity in low-density lipoprotein receptor knock-out Leiden (Ldlr^−/−.Leiden) mice, and studied the morphological structure of the testes and histologically examined the proportion of Sertoli cells, spermatocytes and spermatids in the seminiferous tubules. We examined sperm DNA damage and chromatin condensation and measured plasma levels of leptin, testosterone, cholesterol and triglycerides. HFD-induced obesity caused high plasma leptin and abnormal testosterone levels and induced an aberrant intra-tubular organisation (ITO) which is associated with an altered spermatids/spermatocytes ratio (2:1 instead of 3:1). Mice fed a HFD had a higher level of tubules in stages VII + VIII in the spermatogenic cycle. The stages VII + VII indicate crucial processes in spermatogenic development like initiation of meiosis, initiation of spermatid elongation, and release of fully matured spermatids. In conclusion, HFD-induced obese Ldlr^−/−.Leiden mice develop an aberrant ITO and alterations in the spermatogenic cycle in crucial stages (stages VII and VII). Thereby, our findings stress the importance of lifestyle guidelines in infertility treatments.

A de novo paradigm for male infertility

De novo mutations are known to play a prominent role in sporadic disorders with reduced fitness. We hypothesize that de novo mutations play an important role in severe male infertility and explain a portion of the genetic causes of this understudied disorder. To test this hypothesis, we utilize trio-based exome sequencing in a cohort of 185 infertile males and their unaffected parents. Following a systematic analysis, 29 of 145 rare (MAF < 0.1%) protein-altering de novo mutations are classified as possibly causative of the male infertility phenotype. We observed a significant enrichment of loss-of-function de novo mutations in loss-of-function-intolerant genes (p-value = 1.00 × 10^-5) in infertile men compared to controls. Additionally, we detected a significant increase in predicted pathogenic de novo missense mutations affecting missense-intolerant genes (p-value = 5.01 × 10^-4) in contrast to predicted benign de novo mutations. One gene we identify, RBM5, is an essential regulator of male germ cell pre-mRNA splicing and has been previously implicated in male infertility in mice. In a follow-up study, 6 rare pathogenic missense mutations affecting this gene are observed in a cohort of 2,506 infertile patients, whilst we find no such mutations in a cohort of 5,784 fertile men (p-value = 0.03). Our results provide evidence for the role of de novo mutations in severe male infertility and point to new candidate genes affecting fertility.

In vivo repair of DNA damage induced by X-rays in the early stages of mouse fertilization, and the influence of maternal PARP1 ablation

The early pronucleus stage of the mouse zygote has been characterised in vitro as radiosensitive, due to a high rate of induction of chromosome-type chromosome abnormalities (CA). We have investigated the repair of irradiation induced double strand DNA breaks in vivo by γH2AX foci and first cleavage metaphase analysis. Breaks were induced in sperm and in the early zygote stages comprising sperm chromatin remodelling and early pronucleus expansion. Moreover, the role of PARP1 in the formation and repair of spontaneous and radiation-induced double strand breaks in the zygote was evaluated by comparing observations in C57BL/6J and PARP1 genetically ablated females. The results confirmed in vivo that the rate of chromosome aberration induction by X-rays was approximately 3-fold higher in the zygote than in mouse lymphocytes. This finding was related to a diminished efficiency of double strand break signalling, as shown by a lower rate of γH2AX radiation-induced foci compared to that measured in most other somatic cell types. The spontaneous frequency of CA in PARP1 depleted zygotes was slightly but significantly higher than in wild type zygotes. Also, these zygotes showed some impairment of the radiation-induced DNA Damage Response when exposed closer to the start of S-phase, revealed by a higher number of γH2AX foci and a longer cell cycle delay. The rate of chromosome aberrations, however, was not elevated over that of wild type zygotes, possibly thanks to backup repair pathways and/or selection mechanisms against damaged cells. When comparing with the literature data on irradiation induced CA in mouse zygotes in vitro, the levels of induction were strikingly similar as was the frequency of misrepair of double strand breaks (γH2AX foci). This result can be reassuring for in vitro human gamete and embryo handling, because it shows that culture conditions do not significantly affect double strand DNA break repair.

Parental origin of chromatin in human monopronuclear zygotes revealed by asymmetric histone methylation patterns, differs between IVF and ICSI

In mouse zygotes, many post-translational histone modifications are asymmetrically present in male and female pronuclei. We investigated whether this principle could be used to determine the genetic composition of monopronuclear human zygotes in conventional IVF and ICSI. First we determined whether male female asymmetry is conserved from mouse to human by staining polypronuclear zygotes with antibodies against a subset of histone N-tail post-translational modifications. To analyze human monopronuclear zygotes, a modification, H3K9me3, was selected that is present in the maternal chromatin. After IVF a total of 45 monopronuclear zygotes were obtained. In 39 (87%) of zygotes a nonuniform staining pattern was observed, proof of a bi-parental origin and assumed to result into a diploid conception. Two zygotes showed no staining for the modification, indicating that the single pronucleus was of paternal origin. Four zygotes contained only maternally derived chromatin. ICSI-derived monopronuclear zygotes (n = 33) could also be divided into three groups based on the staining pattern of their chromatin: (1) of maternal origin (n = 15), (2) of paternal origin (n = 8) or (3) consisting of two chromatin domains as dominating in IVF (n = 10). Our data show that monopronuclear zygotes originating from IVF generally arise through fusion of parental chromatin after sperm penetration. Monopronuclear zygotes derived from ICSI in most cases contain uni-parental chromatin. The fact that chromatin was of paternal origin in 24% of ICSI and in 4% of the IVF zygotes confirms earlier results obtained by FISH on cleavage stages. Our findings are of clinical importance in IVF and ICSI practice.

Incomplete nuclear transformation of human spermatozoa in oligo-astheno-teratospermia: characterization by indirect immunofluorescence of chromatin and thiol status

BACKGROUND Sperm heterogeneity in the human, as observed in oligo-astheno-teratozoospermia (OAT), is associated with hypospermatogenesis. METHODS The chromatin of sperm from OAT and normospermic males was characterized with antibodies specific for nucleosomes, the histone H3.1/H3.2 isoform, histone TH2B, apoptosis-associated H4 acetylation (KM-2) and protamines. Subsequently, sperm samples were stained with the thiol-specific fluorochrome monobromobimane (mBBr) before and after reduction with dithiotreitol (DTT) as most thiol groups reside in the cysteine-rich protamines. We also used fluorescence-activated cell sorter (FACS) for sperm histograms and sorting for high or low free and total thiol levels. These fractions were further analysed for DNA damage with the TdT-UTP nick end-labelling (TUNEL) assay. RESULTS OAT sperm nuclei stained higher for nucleosomes and KM2-epitopes, and lower for TH2B. For free, and total, thiol groups, OAT sperm were characterized by biphasic distributions, reflecting incomplete thiol oxidation as well as overoxidation, and possibly reduced protamine contents. The TUNEL assay on sperm subfractions, for both control and OAT sperm, revealed that a lower level of free thiol groups is associated with a higher TUNEL incidence, and this relationship was also found for total thiol levels. Hence, both overoxidation and a low total number of thiol groups predestine for sperm apoptosis. CONCLUSIONS Chromatin characteristics reflecting an incomplete nucleosome to protamine remodelling were found in subfertile males. Sperm apoptosis is related to both incomplete remodelling and protamine overoxidation.

Maternal effects of the scid mutation on radiation-induced transgenerational instability in mice

The results of a number of recent studies show that mutation rates in the offspring of irradiated parents are substantially elevated, however, the effect of parental genotype on transgenerational instability remains poorly understood. Here, we have analysed the mutation frequency at an expanded simple tandem repeat (ESTR) locus in the germline and bone marrow of the first-generation male offspring of control and irradiated male mice. The frequency of ESTR mutation was studied in the offspring of two reciprocal matings male symbol scid x female symbol BALB/c and male symbol BALB/c x female symbol scid, which were compared with that in BALB/c mice. In the offspring of the BALB/c x BALB/c and male symbol scid x female symbol BALB/c matings, which were conceived after paternal sperm irradiation, the frequency of ESTR mutation was significantly elevated in both tissues. In contrast, ESTR mutation frequency was only slightly elevated in the offspring of male symbol BALB/c x female symbol scid mating conceived after paternal irradiation. The results of this study suggest that the oocytes of scid females are unable to fully support the repair of double-strand breaks induced in paternal sperm which may in turn result in the elimination of cells/embryos containing high levels of DNA damage, thus partially preventing the manifestation of genomic instability.

Transmission of modified nucleosomes from the mouse male germline to the zygote and subsequent remodeling of paternal chromatin

Rapidly after gamete fusion, the sperm nucleus loses its specific chromatin conformation and the DNA is repopulated with maternally derived nucleosomes. We evaluated the nature of paternally derived nucleosomes and the dynamics of sperm chromatin remodeling in the zygote directly after gamete fusion. We observed histone H4 acetylated at K8 or K12 already prior to full decondensation of the sperm nucleus, suggesting that these marks are transmitted by the spermatozoon. Tracking down the origin of H4K8ac and H4K12ac during spermiogenesis revealed the retention of nucleosomes with these modifications in the chromocenter of elongating spermatids. We show that sperm constitutive heterochromatin is enriched for nucleosomes carrying specific histone modifications which are transmitted to the zygote. Our results suggest an epigenetic mechanism for inheritance of chromosomal architecture. Furthermore, up to pronucleus formation, histone acetylation and phosphorylation build up in a cascade-like fashion in the paternal chromatin. After formation of the pronucleus, a subset of these marks is removed from the heterochromatin, which suggests a reestablishment of the euchromatin-heterochromatin partition.

Reduced oocyte activation and first cleavage rate after ICSI with spermatozoa from a sterile mouse chromosome mutant

BACKGROUND

Male mice, heterozygous for two semi-identical reciprocal translocations T(1;13)70H and T(1;13)1Wa are usually sterile. We have investigated this oligoasthenoteratozoospermic mouse model using ICSI.

METHODS

B6D2F1 oocytes were injected with epididymal or testicular sperm from fertile or sterile translocation carriers and from chromosomally normal fertile controls. ICSI efficiency was determined by pronucleus formation and first cleavage rates. For arrested zygotes, cell cycle progression was evaluated by BrdU incorporation and incubation with okadaic acid.

RESULTS

Epididymal sperm from infertile translocation carriers showed a slightly lower fertilization rate (70% vs. 92%, 95% and 95% for fertile translocation carriers and two groups of normal fertile control males, respectively) and a severely reduced cleavage rate (33% vs. 87%, 96% and 89% for the same control groups). However, the use of testicular sperm significantly improved the cleavage rate (62% vs. 83% for normal fertile controls). Development of arrested zygotes was delayed or blocked during S- and G2-phase.

CONCLUSIONS

Whereas control testicular and epididymal sperm performed equally well, the use of testicular sperm from oligospermic T/T' males significantly increased first cleavage rates when compared to the low rates with epididymal sperm. Epididymal storage in oligospermics may negatively influence zygote division.

Xylem hydraulic conductivity related to conduit dimensions along chrysanthemum stems

The stem xylem conduit dimensions and hydraulic conductivity of chrysanthemum plants (Dendranthema x grandiflorum Tzvelev cv. Cassa) were analysed and quantified. Simple exponential relations describe conduit length distribution, height dependency of conduit length distribution, and height dependency of stem hydraulic conductivity. These mathematical descriptions can be used to model the xylem water transport system. Within a chrysanthemum stem of 1.0 m, the conduit half-length (the length within which 50% of the conduits have their end) was 0.029 m at soil surface and decreased by half at a height of 0.6 m. With each 0.34 m increase in height up the stem, the hydraulic conductivity decreased by 50%. The resistance calculated from conduit lumen characteristics was 70% of the measured resistance. The remaining unexplained part of the hydraulic resistance is at least partly caused by inter-conduit connections.