Mapping mouse gamete interaction forces reveal several oocyte membrane regions with different mechanical and adhesive properties

Membrane Tubulation with a Biomembrane Force Probe

Tubulation is a common cellular process involving the formation of membrane tubes ranging from 50 nm to 1 µm in diameter. These tubes facilitate intercompartmental connections, material transport within cells and content exchange between cells. The high curvature of these tubes makes them specific targets for proteins that sense local geometry. In vitro, similar tubes have been created by pulling on the membranes of giant unilamellar vesicles. Optical tweezers and micromanipulation are typically used in these experiments, involving the manipulation of a GUV with a micropipette and a streptavidin-coated bead trapped in optical tweezers. The interaction forms streptavidin/biotin bonds, leading to tube formation. Here, we propose a cost-effective alternative using only micromanipulation techniques, replacing optical tweezers with a Biomembrane Force Probe (BFP). The BFP, employing a biotinylated erythrocyte as a nanospring, allows for the controlled measurement of forces ranging from 1 pN to 1 nN. The BFP has been widely used to study molecular interactions in cellular processes, extending beyond its original purpose. We outline the experimental setup, tube formation and characterization of tube dimensions and energetics, and discuss the advantages and limitations of this approach in studying membrane tubulation.

Binding of sperm protein Izumo1 and its egg receptor Juno drives Cd9 accumulation in the intercellular contact area prior to fusion during mammalian fertilization

Little is known about the molecular mechanisms that induce gamete fusion during mammalian fertilization. After initial contact, adhesion between gametes only leads to fusion in the presence of three membrane proteins that are necessary, but insufficient, for fusion: Izumo1 on sperm, its receptor Juno on egg and Cd9 on egg. What happens during this adhesion phase is a crucial issue. Here, we demonstrate that the intercellular adhesion that Izumo1 creates with Juno is conserved in mouse and human eggs. We show that, along with Izumo1, egg Cd9 concomitantly accumulates in the adhesion area. Without egg Cd9, the recruitment kinetics of Izumo1 are accelerated. Our results suggest that this process is conserved across species, as the adhesion partners, Izumo1 and its receptor, are interchangeable between mouse and human. Our findings suggest that Cd9 is a partner of Juno, and these discoveries allow us to propose a new model of the molecular mechanisms leading to gamete fusion, in which the adhesion-induced membrane organization assembles all key players of the fusion machinery.

In situ quantification of living cell adhesion forces: single cell force spectroscopy with a nanotweezer

A novel method is presented for in situ quantification of living cell adhesion forces using a homemade nanorobotic system provided with two independently actuated probes that form a dual-probe nanotweezer capable of pick-and-place manipulation of a single living cell in an aqueous environment. Compared with single-cell force spectroscopy (SCFS) based on traditional atomic force microscopy (AFM), cell immobilization via chemical trapping is unnecessary and the test cell can be efficiently released using the nanotweezer to significantly enhance production of the SCFS. Benefiting from the accurate force sensing capability of AFM, the nanotweezer allows reliable force measurement ranging from picoNewtons to microNewtons and is sufficiently sensitive to characterize short- and long-term adhesion of cell-cell and cell-substrate adhesions. Capabilities of the nanotweezer have been validated through experimental qualification of cell-substrate and cell-cell adhesion events of C2C12 cells (mouse myoblast adherent) with different contact times.

Three functions of cadherins in cell adhesion

Cadherins are transmembrane proteins that mediate cell-cell adhesion in animals. By regulating contact formation and stability, cadherins play a crucial role in tissue morphogenesis and homeostasis. Here, we review the three major functions of cadherins in cell-cell contact formation and stability. Two of those functions lead to a decrease in interfacial tension at the forming cell-cell contact, thereby promoting contact expansion--first, by providing adhesion tension that lowers interfacial tension at the cell-cell contact, and second, by signaling to the actomyosin cytoskeleton in order to reduce cortex tension and thus interfacial tension at the contact. The third function of cadherins in cell-cell contact formation is to stabilize the contact by resisting mechanical forces that pull on the contact.

Polarized Cdc42 activation promotes polar body protrusion and asymmetric division in mouse oocytes

Asymmetric meiotic divisions in mammalian oocytes rely on the eccentric positioning of the spindle and the remodeling of the overlying cortex, resulting in the formation of small polar bodies. The mechanism of this cortical polarization, exemplified by the formation of a thick F-actin cap, is poorly understood. Cdc42 is a major player in cell polarization in many systems; however, the spatio-temporal dynamics of Cdc42 activation during oocyte meiosis, and its contribution to mammalian oocyte polarization, have remained elusive. In this study, we investigated Cdc42 activation (Cdc42–GTP), dynamics and role during mouse oocyte meiotic divisions. We show that Cdc42–GTP accumulates in restricted cortical regions overlying meiotic chromosomes or chromatids, in a Ran–GTP-dependent manner. This polarized activation of Cdc42 is required for the recruitment of N-WASP and the formation of F-actin-rich protrusions during polar body formation. Cdc42 inhibition in MII oocytes resulted in the release of N-WASP into the cytosol, a loss of the polarized F-actin cap, and a failure to protrude the second polar body. Cdc42 inhibition also resulted in central spindle defects in activated MII oocytes. In contrast, emission of the first polar body during oocyte maturation could occur in the absence of a functional Cdc42/N-WASP pathway. Therefore, Cdc42 is a new protagonist in chromatin-induced cortical polarization in mammalian oocytes, with an essential role in meiosis II completion, through the recruitment and activation of N-WASP, downstream of the chromatin-centered Ran–GTP gradient.

Aggregation on a membrane of particles undergoing active exchange with a reservoir

We investigate the dynamics of clusters made of aggregating particles on a membrane which exchanges particles with a reservoir. Exchanges are driven by chemical reactions which supply energy to the system, leading to the establishment of a non-equilibrium steady state. We predict the distribution of cluster size at steady state. We show in particular that in a regime, that cannot exist at equilibrium, the distribution is bimodal: the membrane is mainly populated of single particles and finite-size clusters. This work is motivated by the observations that have revealed the existence of submicrometric clusters of proteins in biological membranes.

The role of adhesion energy in controlling cell-cell contacts

Recent advances in microscopy techniques and biophysical measurements have provided novel insight into the molecular, cellular and biophysical basis of cell adhesion. However, comparably little is known about a core element of cell–cell adhesion—the energy of adhesion at the cell–cell contact. In this review, we discuss approaches to understand the nature and regulation of adhesion energy, and propose strategies to determine adhesion energy between cells in vitro and in vivo.

CD9 tetraspanin generates fusion competent sites on the egg membrane for mammalian fertilization

CD9 tetraspanin is the only egg membrane protein known to be essential for fertilization. To investigate its role, we have measured, on a unique acrosome reacted sperm brought in contact with an egg, the adhesion probability and strength with a sensitivity of a single molecule attachment. Probing the binding events at different locations of wild-type egg we described different modes of interaction. Here, we show that more gamete adhesion events occur on Cd9 null eggs but that the strongest interaction mode disappears. We propose that sperm-egg fusion is a direct consequence of CD9 controlled sperm-egg adhesion properties. CD9 generates adhesion sites responsible for the strongest of the observed gamete interaction. These strong adhesion sites impose, during the whole interaction lifetime, a tight proximity of the gamete membranes, which is a requirement for fusion to take place. The CD9-induced adhesion sites would be the actual location where fusion occurs.

A nonlinear characteristic regime of biomembrane force probe

A linear relation between stiffness and aspiration pressure is the basis for biomembrane force probe (BFP), a widely used technique to measure minuscule forces. Here we perform finite element simulations and semi-analytical modeling of the BFP operation to show that, at low aspiration pressures, there exists a characteristic regime in which the relation between stiffness and aspiration pressure is actually nonlinear. We find that this nonlinear characteristic regime arises from a transition in configuration of a partially aspirated biomembrane force probe under increasing aspiration pressure. We discuss both the conditions for the transition and the characteristics of the nonlinear characteristic regime, as well as its potential applications.

Impact of marine drugs on cytoskeleton-mediated reproductive events

Marine organisms represent an important source of novel bioactive compounds, often showing unique modes of action. Such drugs may be useful tools to study complex processes such as reproduction; which is characterized by many crucial steps that start at gamete maturation and activation and virtually end at the first developmental stages. During these processes cytoskeletal elements such as microfilaments and microtubules play a key-role. In this review we describe: (i) the involvement of such structures in both cellular and in vitro processes; (ii) the toxins that target the cytoskeletal elements and dynamics; (iii) the main steps of reproduction and the marine drugs that interfere with these cytoskeleton-mediated processes. We show that marine drugs, acting on microfilaments and microtubules, exert a wide range of impacts on reproductive events including sperm maturation and motility, oocyte maturation, fertilization, and early embryo development.