Kinetic and photonic techniques to study chemotactic signaling in sea urchin sperm

Cold storage and cryopreservation methods for spermatozoa of the sea urchins Lytechinus pictus and Strongylocentrotus purpuratus

BACKGROUND

Sea urchins have contributed greatly to knowledge of fertilization, embryogenesis, and cell biology. However, until now, they have not been genetic model organisms because of their long generation times and lack of tools for husbandry and gene manipulation. We recently established the sea urchin Lytechinus pictus, as a multigenerational model Echinoderm, because of its relatively short generation time of 4-6 months and ease of laboratory culture. To take full advantage of this new multigenerational species, methods are needed to biobank and share genetically modified L. pictus sperm.

RESULTS

Here, we describe a method, based on sperm ion physiology that maintains L. pictus and Strongylocentrotus purpuratus sperm fertilizable for at least 5-10 weeks when stored at 0°C. We also describe a new method to cryopreserve sperm of both species. Sperm of both species can be frozen and thawed at least twice and still give rise to larvae that undergo metamorphosis.

CONCLUSIONS

The simple methods we describe work well for both species, achieving >90% embryo development and producing larvae that undergo metamorphosis to juvenile adults. We hope that these methods will be useful to others working on marine invertebrate sperm.

Static and Dynamic Biomaterial Engineering for Cell Modulation

In the biological microenvironment, cells are surrounded by an extracellular matrix (ECM), with which they dynamically interact during various biological processes. Specifically, the physical and chemical properties of the ECM work cooperatively to influence the behavior and fate of cells directly and indirectly, which invokes various physiological responses in the body. Hence, efficient strategies to modulate cellular responses for a specific purpose have become important for various scientific fields such as biology, pharmacy, and medicine. Among many approaches, the utilization of biomaterials has been studied the most because they can be meticulously engineered to mimic cellular modulatory behavior. For such careful engineering, studies on physical modulation (e.g., ECM topography, stiffness, and wettability) and chemical manipulation (e.g., composition and soluble and surface biosignals) have been actively conducted. At present, the scope of research is being shifted from static (considering only the initial environment and the effects of each element) to biomimetic dynamic (including the concepts of time and gradient) modulation in both physical and chemical manipulations. This review provides an overall perspective on how the static and dynamic biomaterials are actively engineered to modulate targeted cellular responses while highlighting the importance and advance from static modulation to biomimetic dynamic modulation for biomedical applications.

Simultaneous recording of multiple cellular signaling events by frequency- and spectrally-tuned multiplexing of fluorescent probes

Fluorescent probes that change their spectral properties upon binding to small biomolecules, ions, or changes in the membrane potential (Vm) are invaluable tools to study cellular signaling pathways. Here, we introduce a novel technique for simultaneous recording of multiple probes at millisecond time resolution: frequency- and spectrally-tuned multiplexing (FASTM). Different from present multiplexing approaches, FASTM uses phase-sensitive signal detection, which renders various combinations of common probes for Vm and ions accessible for multiplexing. Using kinetic stopped-flow fluorimetry, we show that FASTM allows simultaneous recording of rapid changes in Ca2+, pH, Na+, and Vm with high sensitivity and minimal crosstalk. FASTM is also suited for multiplexing using single-cell microscopy and genetically encoded FRET biosensors. Moreover, FASTM is compatible with optochemical tools to study signaling using light. Finally, we show that the exceptional time resolution of FASTM also allows resolving rapid chemical reactions. Altogether, FASTM opens new opportunities for interrogating cellular signaling.

CatSper channels in sea urchin sperm

Sea urchin sperm swimming is regulated by speract, a decapeptide released from egg jelly that induces chemotaxis and triggers membrane potential (Em) changes, intracellular increases in cyclic nucleotides (cGMP, cAMP), pH (pH_i) and calcium concentration ([Ca²⁺]_i). The identity of the ionic transporters associated with the [Ca²⁺]_i changes required for chemotaxis is not fully known. CatSper, a sperm exclusive Ca²⁺ channel has been detected by proteomic analysis and immunofluorescence in sea urchin sperm and there is evidence for its involvement in chemotaxis. This work presents an electrophysiological characterization of a CatSper channel in sea urchin sperm. By swelling sperm suspending them in 10-fold diluted artificial sea water (ASW) we achieve on-cell patch-clamp recordings that document a mildly voltage and pH_i dependent Na⁺ permeable channel (in absence of divalent ions in the pipette), sensitive to speract, and blocked by Mibefradil (Mibe), NNC55-0396 (NNC) and RU1968 (RU) resembling CatSper. We also recorded a voltage dependent Cl^- channel inhibited by Niflumic Acid and the TMEM16A blocker.

Reconstruction of the three-dimensional beat pattern underlying swimming behaviors of sperm

 The eukaryotic flagellum propels sperm cells and simultaneously detects physical and chemical cues that modulate the waveform of the flagellar beat. Most previous studies have characterized the flagellar beat and swimming trajectories in two space dimensions (2D) at a water/glass interface. Here, using refined holographic imaging methods, we report high-quality recordings of three-dimensional (3D) flagellar bending waves. As predicted by theory, we observed that an asymmetric and planar flagellar beat results in a circular swimming path, whereas a symmetric and non-planar flagellar beat results in a twisted-ribbon swimming path. During swimming in 3D, human sperm flagella exhibit torsion waves characterized by maxima at the low curvature regions of the flagellar wave. We suggest that these torsion waves are common in nature and that they are an intrinsic property of beating axonemes. We discuss how 3D beat patterns result in twisted-ribbon swimming paths. This study provides new insight into the axoneme dynamics, the 3D flagellar beat, and the resulting swimming behavior.

The Ca2+ channel CatSper is not activated by cAMP/PKA signaling but directly affected by chemicals used to probe the action of cAMP and PKA

The sperm-specific Ca²⁺ channel CatSper (cation channel of sperm) controls the influx of Ca²⁺ into the flagellum and, thereby, the swimming behavior of sperm. A hallmark of human CatSper is its polymodal activation by membrane voltage, intracellular pH, and oviductal hormones. Whether CatSper is also activated by signaling pathways involving an increase of cAMP and ensuing activation of PKA is, however, a matter of controversy. To shed light on this question, we used kinetic ion-sensitive fluorometry, patch-clamp recordings, and optochemistry to study transmembrane Ca²⁺ flux and membrane currents in human sperm from healthy donors and from patients that lack functional CatSper channels. We found that human CatSper is neither activated by intracellular cAMP directly nor indirectly by the cAMP/PKA-signaling pathway. Instead, we show that nonphysiological concentrations of cAMP and membrane-permeable cAMP analogs used to mimic the action of intracellular cAMP activate human CatSper from the outside via a hitherto-unknown extracellular binding site. Finally, we demonstrate that the effects of common PKA inhibitors on human CatSper rest predominantly, if not exclusively, on off-target drug actions on CatSper itself rather than on inhibition of PKA. We conclude that the concept of an intracellular cAMP/PKA-activation of CatSper is primarily based on unspecific effects of chemical probes used to interfere with cAMP signaling. Altogether, our findings solve several controversial issues and reveal a novel ligand-binding site controlling the activity of CatSper, which has important bearings on future studies of cAMP and Ca²⁺ signaling in sperm.

The 2020 motile active matter roadmap

Activity and autonomous motion are fundamental in living and engineering systems. This has stimulated the new field of 'active matter' in recent years, which focuses on the physical aspects of propulsion mechanisms, and on motility-induced emergent collective behavior of a larger number of identical agents. The scale of agents ranges from nanomotors and microswimmers, to cells, fish, birds, and people. Inspired by biological microswimmers, various designs of autonomous synthetic nano- and micromachines have been proposed. Such machines provide the basis for multifunctional, highly responsive, intelligent (artificial) active materials, which exhibit emergent behavior and the ability to perform tasks in response to external stimuli. A major challenge for understanding and designing active matter is their inherent nonequilibrium nature due to persistent energy consumption, which invalidates equilibrium concepts such as free energy, detailed balance, and time-reversal symmetry. Unraveling, predicting, and controlling the behavior of active matter is a truly interdisciplinary endeavor at the interface of biology, chemistry, ecology, engineering, mathematics, and physics. The vast complexity of phenomena and mechanisms involved in the self-organization and dynamics of motile active matter comprises a major challenge. Hence, to advance, and eventually reach a comprehensive understanding, this important research area requires a concerted, synergetic approach of the various disciplines. The 2020 motile active matter roadmap of Journal of Physics: Condensed Matter addresses the current state of the art of the field and provides guidance for both students as well as established scientists in their efforts to advance this fascinating area.

Absolute proteomic quantification reveals design principles of sperm flagellar chemosensation

Cilia serve as cellular antennae that translate sensory information into physiological responses. In the sperm flagellum, a single chemoattractant molecule can trigger a Ca2+ rise that controls motility. The mechanisms underlying such ultra-sensitivity are ill-defined. Here, we determine by mass spectrometry the copy number of nineteen chemosensory signaling proteins in sperm flagella from the sea urchin Arbacia punctulata. Proteins are up to 1,000-fold more abundant than the free cellular messengers cAMP, cGMP, H+ , and Ca2+ . Opto-chemical techniques show that high protein concentrations kinetically compartmentalize the flagellum: Within milliseconds, cGMP is relayed from the receptor guanylate cyclase to a cGMP-gated channel that serves as a perfect chemo-electrical transducer. cGMP is rapidly hydrolyzed, possibly via "substrate channeling" from the channel to the phosphodiesterase PDE5. The channel/PDE5 tandem encodes cGMP turnover rates rather than concentrations. The rate-detection mechanism allows continuous stimulus sampling over a wide dynamic range. The textbook notion of signal amplification-few enzyme molecules process many messenger molecules-does not hold for sperm flagella. Instead, high protein concentrations ascertain messenger detection. Similar mechanisms may occur in other small compartments like primary cilia or dendritic spines.

Electrophysiology, Unplugged: Imaging Membrane Potential with Fluorescent Indicators

Membrane potential is a fundamental biophysical property maintained by every cell on earth. In specialized cells like neurons, rapid changes in membrane potential drive the release of chemical neurotransmitters. Coordinated, rapid changes in neuronal membrane potential across large numbers of interconnected neurons form the basis for all of human cognition, sensory perception, and memory. Despite the importance of this highly orchestrated and distributed activity, the traditional method for recording membrane potential is through the use of highly invasive single-cell electrodes that offer only a small glimpse of the total activity within a system. Fluorescent dyes that change their optical properties in response to changes in biological voltage have the potential to provide a powerful complement to traditional electrode-based methods of inquiry. Voltage-sensitive fluorescent indicators would allow the direct observation of membrane potential changes, significantly expanding our ability to monitor membrane potential dynamics in living systems. Toward this end, we have initiated a program to design, synthesize, and apply voltage-sensitive fluorophores that report on membrane potential dynamics with high sensitivity and speed. The basis for this optical voltage sensing is membrane potential-dependent photoinduced electron transfer (PeT). Voltage-sensitive fluorophores, or VoltageFluors, possess a fluorophore, a conjugated molecular wire, and an aniline donor. At resting potentials, in which the cell has a hyperpolarized or negative potential relative to the outside of the cell, PeT from the aniline donor is enhanced and fluorescence is diminished. At depolarized potentials, the membrane potential decreases the rate of PeT, allowing an increase in fluorescence. We show that a number of different fluorophores, molecular wires, and aniline donors can be employed to generate fast and sensitive VoltageFluor dyes. Multiple lines of evidence point to a PeT-based mechanism for voltage sensing, delivering fast response kinetics (∼25 ns), good sensitivity (>60% ΔF/F), compatibility with two-photon illumination, excellent signal-to-noise, and the ability to detect neuronal and cardiac action potentials in single trials. In this Account, we provide an overview of the challenges facing the design of fluorescent voltage indicators. We trace the development of molecular wire-based fluorescent voltage indicators within our group, beginning from fluorescein-based VoltageFluor to long-wavelength indicators that use modern fluorophores like silicon rhodamine and carbofluorescein. We examine design principles for PeT-based voltage indicators, showcase the use of our recent indicators for two-photon voltage imaging in intact brains, and explore the development of hybrid indicators that can localize to genetically defined cells. Finally, we highlight outstanding challenges to and opportunities for voltage imaging.

Quantitative Intracellular pH Determinations in Single Live Mammalian Spermatozoa Using the Ratiometric Dye SNARF-5F

Intracellular pH (pH i ) plays a crucial role in mammalian sperm physiology. However, it is a challenging task to acquire quantitative single sperm pH i images due to their small size and beating flagella. In this study, we established a robust pH i imaging system using the dual-emission ratiometric pH indicator, SNARF-5F. Simultaneous good signal/noise ratio fluorescence signals were obtained exciting with a green high-power LED (532 nm) and acquiring with an EM-CCD camera through an image splitter with two band-pass filters (550-600 nm, channel 1; 630-650 nm, channel 2). After in vivo calibration, we established an imaging system that allows determination of absolute pH i values in spermatozoa, minimizing cell movement artifacts. Using this system, we determined that bicarbonate increases non-capacitated human pH i with slower kinetics than in mouse spermatozoa. This difference suggests that distinct ionic transporters might be involved in the bicarbonate influx into human and mouse spermatozoa. Alternatively, pH i regulation downstream bicarbonate influx into spermatozoa could be different between the two species.