FLUOROMETRIC STUDY OF RABBIT SPERM HEAD MEMBRANE PHOSPHOLIPID ASYMMETRY DURING CAPACITATION AND ACROSOME REACTION USING ANNEXIN-V FITC

Flagging fusion: Phosphatidylserine signaling in cell-cell fusion

Formations of myofibers, osteoclasts, syncytiotrophoblasts, and fertilized zygotes share a common step, cell–cell fusion. Recent years have brought about considerable progress in identifying some of the proteins involved in these and other cell-fusion processes. However, even for the best-characterized cell fusions, we still do not know the mechanisms that regulate the timing of cell-fusion events. Are they fully controlled by the expression of fusogenic proteins or do they also depend on some triggering signal that activates these proteins? The latter scenario would be analogous to the mechanisms that control the timing of exocytosis initiated by Ca²⁺ influx and virus-cell fusion initiated by low pH- or receptor interaction. Diverse cell fusions are accompanied by the nonapoptotic exposure of phosphatidylserine at the surface of fusing cells. Here we review data on the dependence of membrane remodeling in cell fusion on phosphatidylserine and phosphatidylserine-recognizing proteins and discuss the hypothesis that cell surface phosphatidylserine serves as a conserved “fuse me” signal regulating the time and place of cell-fusion processes.

Anoctamins/TMEM16 Proteins: Chloride Channels Flirting with Lipids and Extracellular Vesicles

Anoctamin (ANO)/TMEM16 proteins exhibit diverse functions in cells throughout the body and are implicated in several human diseases. Although the founding members ANO1 (TMEM16A) and ANO2 (TMEM16B) are Ca²⁺-activated Cl^- channels, most ANO paralogs are Ca²⁺-dependent phospholipid scramblases that serve as channels facilitating the movement (scrambling) of phospholipids between leaflets of the membrane bilayer. Phospholipid scrambling significantly alters the physical properties of the membrane and its landscape and has vast downstream signaling consequences. In particular, phosphatidylserine exposed on the external leaflet of the plasma membrane functions as a ligand for receptors vital for cell-cell communication. A major consequence of Ca²⁺-dependent scrambling is the release of extracellular vesicles that function as intercellular messengers by delivering signaling proteins and noncoding RNAs to alter target cell function. We discuss the physiological implications of Ca²⁺-dependent phospholipid scrambling, the extracellular vesicles associated with this activity, and the roles of ANOs in these processes.

A Role of TMEM16E Carrying a Scrambling Domain in Sperm Motility

Transmembrane protein 16E (TMEM16E) belongs to the TMEM16 family of proteins that have 10 transmembrane regions and appears to localize intracellularly. Although TMEM16E mutations cause bone fragility and muscular dystrophy in humans, its biochemical function is unknown. In the TMEM16 family, TMEM16A and -16B serve as Ca(2+)-dependent Cl(-) channels, while TMEM16C, -16D, -16F, -16G, and -16J support Ca(2+)-dependent phospholipid scrambling. Here, we show that TMEM16E carries a segment composed of 35 amino acids homologous to the scrambling domain in TMEM16F. When the corresponding segment of TMEM16A was replaced by this 35-amino-acid segment of TMEM16E, the chimeric molecule localized to the plasma membrane and supported Ca(2+)-dependent scrambling. We next established TMEM16E-deficient mice, which appeared to have normal skeletal muscle. However, fertility was decreased in the males. We found that TMEM16E was expressed in germ cells in early spermatogenesis and thereafter and localized to sperm tail. TMEM16E(-/-) sperm showed no apparent defect in morphology, beating, mitochondrial function, capacitation, or binding to zona pellucida. However, they showed reduced motility and inefficient fertilization of cumulus-free but zona-intact eggs in vitro. Our results suggest that TMEM16E may function as a phospholipid scramblase at inner membranes and that its defect affects sperm motility.

Short- and long-term effects of lipopolysaccharide-induced inflammation on rabbit sperm quality

Infections and resulting inflammation are widely known to cause transient or permanent male infertility. The objectives of this study were (1) to provide a suitable animal model of a sub-acute inflammatory state by intraperitoneally inoculating bacterial lipopolysaccharides (LPS) and (2) to define the short- and long-term effects of this state on the sperm quality of rabbit bucks. Two series of experiments were performed to accomplish these objectives. In experiment 1, 15 healthy New Zealand White rabbit bucks were divided into five homogeneous groups, receiving 25, 50, 100 and 150 microg/kg body weight (b.w.) of E. coli LPS dissolved in 2ml of sterile saline or only saline (control), respectively. White blood cells (WBC), rectal temperature, feed intake and mating ability were observed for 3 consecutive days following inoculation. Inoculation of 50 microg/kg b.w. produces a reversible inflammation-like state that lasts for about 3 days, with minimal distress to the animals, and therefore it was used in our experiment. The major symptoms were fever and anorexia. Changes in WBC count and a moderate reduction in reproductive activity also occurred. In experiment 2, two groups of five rabbit bucks each were treated with 50 microg/kg b.w. E. coli LPS diluted in 2ml of saline or only saline (controls), respectively. Semen samples were collected weekly up to 56 days after inoculation and the changes in semen characteristics were examined. During the first 3 days, semen volume and concentration decreased in both experimental groups, probably due to the high collection frequency. Sperm membrane integrity and the number of necrotic sperm were seriously affected 30 days after the LPS challenge, reaching a maximum at the end of the spermatogenic cycle (56 days). These results suggest that a sub-acute inflammation may cause infertility by compromising sperm membrane integrity which decreases a month after LPS-treatment. In addition, the rabbit could be a useful LPS animal model for further study of the effects of inflammation and the underlying mechanisms on sperm characteristics.

Phosphatidylserine externalization in human sperm induced by calcium ionophore A23187: relationship with apoptosis, membrane scrambling and the acrosome reaction

BACKGROUND

Translocation of phosphatidylserine (PS) from the inner to the outer leaflet of the plasma membrane is a modification of the lipid architecture occurring in sperm. This is one of the earliest signs of apoptosis that can be monitored by the calcium-dependent binding of annexin V.

METHODS AND RESULTS

Flow cytometric analysis of annexin V binding was performed. Calcium ionophore A23187 led to a significant increase in the proportion of living sperm with PS exposure: 7.3 3.2% of cells in the untreated ejaculate versus 47.5 5.6% of cells after 1 h of incubation with A23187. Conversely, diminution of mitochondrial membrane potential [DiOC6(3)/propidium iodide (PI) assay], caspase activation [fluorescein isothiocyanate (FITC)-Val-Ala-Asp-fluoromethylketone (VAD-FMK)/PI assay], increased plasma membrane permeability (Yo-Pro-1/PI assay) and increased DNA fragmentation [TdT (terminal deoxynucleotidyl transferase)-mediated dUDP nick-end labelling assay], which are among the main signs of apoptosis, were not observed in sperm, even after 4 h of incubation with A23187. However, A23187 significantly increased the proportion of sperm with plasma membrane scrambling and with a reacted acrosome, as detected with the merocyanine 540 probe (M540) and the monoclonal anti-human CD46-PE antibody respectively.

CONCLUSIONS

Our results suggest that PS exposure in human sperm, as induced by A23187, is mainly related to the acrosome reaction rather than to apoptosis.