Migration and interactions of mammalian gametes occur in deep body tissues after mating, rendering difficult any in situ noninvasive evaluation of their performances with current methods. In our effort to develop an effective and real-time in vivo imaging approach, we have successfully labelled porcine gametes with self-illuminating bioluminescent and red-shifted quantum dot nanoparticles (QD) in our previous studies (Feugang et al. 2012 J. Nanobiotechnol. 10, 45; Feugang et al. 2015, J. Nanobiotechnol. 13, 38). The present effort aimed at investigating whether QD could be incorporated into spermatozoa through induced in vitro capacitation, which increases sperm plasma membrane fluidity. Fresh extended boar semen was placed on top of a Percoll gradient and centrifuged. Purified motile spermatozoa were collected and washed with pre-warmed PBS. Pelleted spermatozoa were resuspended in the modified Tris-buffered medium with BSA fraction-V (1 mg mL–1; modified Tween medium B with milk powder and BSA). Sperm aliquots (108) were supplemented or not (control) with QD only (QD+; 1 nM), QD+caffeine (2 mM), or QD+heparin (10 µg mL–1); with caffeine and heparin being used as routine capacitant agents in fertilization media. All aliquots were incubated at 38.5°C, under 5% CO2 for 0.5, 1, or 3 h. Spermatozoa were then analysed for motility characteristics and imaged for confirmation of QD-sperm interactions (bioluminescence emission) and localization (transmission electron microscope; TEM). Motility data of 5 replicates were analysed with ANOVA-2, and P < 0.05 was set as threshold of significance. Total sperm motility (TSM) significantly improved with the presence of either or both QDs and capacitant agents after 0.5 and 1 h incubations. With exception of the QD+heparin, all other groups had significantly decreased TSM after 3 h of incubation, when compared with TSM at 0.5 and 1 h. Higher proportions of progressive and rapid (≥45 µm s–1) spermatozoa were observed in the presence of both capacitant agents (P < 0.05), and only QD+heparin maintained greater proportions after 3 h. Sperm straight-line velocity significantly increased in the QD+caffeine at 0.5 h and in both QD+caffeine and QD+heparin thereafter. Sperm straightness data were increased by both caffeine and heparin during incubations. Strong bioluminescence signals were observed in spermatozoa incubated with QDs compared to the background signal seen in the control group. The TEM images revealed consistent surface membrane attachment of QDs in all QD+ groups, whereas transmembrane and intra-spermatic localizations were visible in both QD+caffeine and QD+heparin groups. We concluded that supplementations of medium containing QDs with caffeine or heparin allow the crossing of sperm plasma membrane by QD. No toxic effect of QD on sperm motility was observed, which confirmed our previous report using a similar ratio of QDs over spermatozoa. Exploration of efficient incorporation of QD into spermatozoa as a promising approach for noninvasive molecular imaging is still ongoing, as well as further sperm viability assessments.
Supported by the NIH grant #5T35OD010432 and USDA-ARS Biophotonics Initiative grant #58–6402–3-0120.
Maturation of human oocytes in vitro and their developmental competence
