Continuous behavioural ‘switching’ in human spermatozoa and its regulation by Ca2+-mobilising stimuli

Abstract

Human sperm show a variety of different behaviours (types of motility) that have different functional roles. Previous reports suggest that sperm may reversibly switch between these behaviours. We have recorded and analysed the behaviour of individual human sperm (180 cells in total), each cell monitored continuously for 3–3.5 min either under control conditions or in the presence of Ca2+-mobilising stimuli. Switching between different behaviours was assessed visually (1 s bins using four behaviour categories), and was verified by fractal dimension analysis of sperm head tracks. In the absence of stimuli, ~90% of cells showed at least one behavioural transition (mean rate under control conditions = 6.4 ± 0.8 transitions.min−1). Type 1 behaviour (progressive, activated-like motility) was most common, but the majority of cells (>70%) displayed at least three behaviour types. Treatment of sperm with Ca2+-mobilising agonists had negligible effects on the rate of switching but increased the time spent in type 2 and type 3 (hyperactivation-like) behaviours (P < 2*10−8; chi-square). Treatment with 4-aminopyridine under alkaline conditions (pHo = 8.5), a highly-potent Ca2+-mobilising stimulus, was the most effective in increasing the proportion of type 3 behaviour, biasing switching away from type 1 (P < 0.005) and dramatically extending the duration of type 3 events (P < 10−16). Other stimuli, including 300 nM progesterone and 1% human follicular fluid, had qualitatively similar effects but were less potent. We conclude that human sperm observed in vitro constitutively display a range of behaviours and regulation of motility by [Ca2+]i, at the level of the single cell, is achieved not by causing cells to adopt a ‘new’ behaviour but by changing the relative contributions of those behaviours.

Long-term segmentation-free assessment of head–flagellum movement and intracellular calcium in swimming human sperm

“…However, this change is not immediate, it occurs after a significant time-delay (~75 s). In contrast, the control sperm depicted in red remains stable and only shows a large increase in beating frequency towards the end of the imaging period, demonstrating the potential for spontaneous self-switching, in agreement with earlier observations (Achikanu et al, 2019). Fig.…”

“…This latter mode of flagellar beating is named hyperactivation and is necessary for fertilization (Chang & Suarez, 2011). Its characteristics can vary among species and even within the same cell population (Drobnis et al, 1988;Ravaux et al, 2016), indicating possible functional switching between different sperm behaviors (Achikanu et al, 2019).…”

“…Recently, a mathematical image analysis development allowed to accurately characterize detailed kinematics of spermatozoa flagella in 3D but only for few beat cycles (Gadêlha et al, 2020). Achikanu et al (2019) showed that non-capacitated human spermatozoa are able to modify their swimming mode 6 times/min on average and at least once every 3.5 mins. This switching of the swimming gaits probably has evolved to contend with the changes occurring along the female genital tract.…”

Modelling Motility: The Mathematics of Spermatozoa

“…For example, though we might report the frequency of the flagellar beat as a single value, the actual frequency may be evolving with time. Of particular note, the recent study of Achikanu et al (2019) tracked sperm motion over a considerable time period, identifying sustained behavioural switching that is more significant than we have hinted at here, highlighting the significance of careful temporal considerations of the spermatozoan beat and its descriptors.…”

Tachykinins and Kisspeptins in the Regulation of Human Male Fertility

“…One study showed that sperm from NEP2 knockout mice had normal characteristics but lower fertilization capacity, and resulting embryos had a worse development [91]. Inhibition of NEP and NEP2 by phosphoramidon leads to an increase in linear and straight motility of sperm cells (sperm progressive motility), which is essential for adequate swimming through the female genital tract [92][93][94].…”