Active Mechanical Processes Research Articles

Nonequilibrium Dynamics at Cellular Interfaces: Insights From Simulation and Theory

ABSTRACTActive matters, which consume energy to exert mechanical forces, include molecular motors, synthetic nanomachines, actively propelled bacteria, and viruses. A series of unique phenomena emerge when active matters interact with cellular interfaces. Activity changes the mechanism of nanoparticle intracellular delivery, while active mechanical processes generated in the cytoskeleton play a major role in membrane protein distribution and transport. This review provides a comprehensive overview of the theoretical and simulation models used to study these nonequilibrium phenomena, offering insights into how activity enhances cellular uptake, influences membrane deformation, and governs surface transport dynamics. Furthermore, we explore the impact of membrane properties, such as fluidity and viscosity, on transport efficiency and discuss the slippage dynamics and active rotation behaviors on the membrane surface. The interplay of active particles and membranes highlights the essential role of nonequilibrium dynamics in cellular transport processes, with potential applications in drug delivery and nanotechnology. Finally, we provide an outlook highlighting the significance of deeper theoretical and simulation‐based investigations to optimize active particles and understand their behavior in complex biological environments.

Preexisting tissue mechanical hypertension at adherens junctions disrupts apoptotic extrusion in epithelia.

Apical extrusion is a tissue-intrinsic process that allows epithelia to eliminate unfit or surplus cells. This is exemplified by the early extrusion of apoptotic cells, which is critical to maintain the epithelial barrier and prevent inflammation. Apoptotic extrusion is an active mechanical process, which involves mechanotransduction between apoptotic cells and their neighbors, as well as local changes in tissue mechanics. Here we report that the preexisting mechanical tension at adherens junctions (AJs) conditions the efficacy of apoptotic extrusion. Specifically, increasing baseline mechanical tension by overexpression of a phosphomimetic Myosin II regulatory light chain (MRLC) compromises apoptotic extrusion. This occurs when tension is increased in either the apoptotic cell or its surrounding epithelium. Further, we find that the proinflammatory cytokine, TNFα, stimulates Myosin II and increases baseline AJ tension to disrupt apical extrusion, causing apoptotic cells to be retained in monolayers. Importantly, reversal of mechanical tension with an inhibitory MRLC mutant or tropomyosin inhibitors is sufficient to restore apoptotic extrusion in TNFα-treated monolayers. Together, these findings demonstrate that baseline levels of tissue tension are important determinants of apoptotic extrusion, which can potentially be coopted by pathogenetic factors to disrupt the homeostatic response of epithelia to apoptosis.

A Model of Using the Asymmetric Polydopamine Thin Film for Mimicking Epithelial Folding In Vitro

AbstractThe basement membrane (BM) is a biointeractive ultrathin network with distinct composition and organization of its epithelial and stromal sides, which render BMs with asymmetric biofunctions and mechanical properties. There are difficulties in the recapitulation of the highly hierarchical structure and function of BM. Here, the interfacial assembly method for the generation of BM mimics is applied. Dopamine is the starting material for the polymerization and assembly of polydopamine (PDA) into asymmetric materials. Compared to the PDA coating formed at the solid/liquid interface (≈20 nm), the PDA film formed at the air/liquid interface displays a thickness of ≈100 nm. Moreover, it possesses an asymmetric surface topography and an apparent Young's modulus of ≈1.0 MPa, which is structurally and mechanically similar to natural BMs. Of interest, the airside and the waterside of the PDA film exhibit differences in their adhesion affinity to the human skin keratinocytes. With stronger active mechanical processes between living cells and the waterside of PDA film, epithelial folding could be mimicked. Together, the PDA film is able to recapitulate the structural and mechanical complexity of natural BMs, indicating the prospective future of using PDA films for in vitro modeling cell‐BM interaction and tissue formation.

Feedback between mechanosensitive signaling and active forces governs endothelial junction integrity

The formation and recovery of gaps in the vascular endothelium governs a wide range of physiological and pathological phenomena, from angiogenesis to tumor cell extravasation. However, the interplay between the mechanical and signaling processes that drive dynamic behavior in vascular endothelial cells is not well understood. In this study, we propose a chemo-mechanical model to investigate the regulation of endothelial junctions as dependent on the feedback between actomyosin contractility, VE-cadherin bond turnover, and actin polymerization, which mediate the forces exerted on the cell-cell interface. Simulations reveal that active cell tension can stabilize cadherin bonds, but excessive RhoA signaling can drive bond dissociation and junction failure. While actin polymerization aids gap closure, high levels of Rac1 can induce junction weakening. Combining the modeling framework with experiments, our model predicts the influence of pharmacological treatments on the junction state and identifies that a critical balance between RhoA and Rac1 expression is required to maintain junction stability. Our proposed framework can help guide the development of therapeutics that target the Rho family of GTPases and downstream active mechanical processes.

Inside Cover: Active Mechanical Threading by a Molecular Motor (Angew. Chem. Int. Ed. 19/2022)

A molecular motor setup allowing to harness the trajectory of directional motor rotation for an active mechanical threading process is presented by Henry Dube et al. in their Research Article (DOI: 10.1002/anie.202201882). The motor is embedded into a macrocycle and its indane-based rotor fragment then serves as “revolving door” in the light-driven–rotation. In this way a process akin to macroscopic threading of a needle is miniaturized to the molecular level offering a first entry point to mechanical molecular knitting or weaving processes.

Durotaxis of Passive Nanoparticles on Elastic Membranes.

The transport of macromolecules and nanoscopic particles to a target cellular site is a crucial aspect in many physiological processes. This directional motion is generally controlled via active mechanical and chemical processes. Here we show, by means of molecular dynamics simulations and an analytical theory, that completely passive nanoparticles can exhibit directional motion when embedded in nonuniform mechanical environments. Specifically, we study the motion of a passive nanoparticle adhering to a mechanically nonuniform elastic membrane. We observe a nonmonotonic affinity of the particle to the membrane as a function of the membrane's rigidity, which results in the particle transport. This transport can be both up or down the rigidity gradient, depending on the absolute values of the rigidities that the gradient spans across. We conclude that rigidity gradients can be used to direct average motion of passive macromolecules and nanoparticles on deformable membranes, resulting in the preferential accumulation of the macromolecules in regions of certain mechanical properties.

Advanced Actuator Materials Powered by Biomimetic Helical Fiber Topologies.

Helical constructs are ubiquitous in nature at all size domains, from molecular to macroscopic. The helical topology confers unique mechanical functions that activate certain phenomena, such as twining vines and vital cellular functions like the folding and packing of DNA into chromosomes. The understanding of active mechanical processes in plants, certain musculature in animals, and some biochemical processes in cells provides insight into the versatility of the helix. Most of these natural systems consist of helically oriented filaments embedded in a compliant matrix. In some cases, the matrix can change volume and in others the filaments can contract and the matrix is passive. In both cases, the helically arranged fibers determine the overall shape change with a great variety of responses involving length contraction/elongation, twisting, bending, and coiling. Synthetic actuator materials and systems that employ helical topologies have been described recently and demonstrate many fascinating and complex shape changes. However, significant new opportunities exist to mimic some of the most remarkable actions in nature, including the Vorticella's coiling stalk and DNA's supercoils, in the quest for superior artificial muscles.

Insect acoustic communication: The role of transmission channel and the sensory system and brain of receivers

Abstract For decades, acoustic insects have been used as model organisms for behavioural neurobiologists to understand mate choice or predator avoidance, because behaviour can easily and reliably be elicited in the laboratory, and behaviourally relevant, identified nerve cells be studied under these conditions. However, signalling often takes place in complex environments, in which the signal perceived by the receiver may differ greatly from the one broadcast due to the biotic and abiotic properties of the sound transmission channel. Thus, the key challenge is to transfer the insights of these laboratory‐oriented experiments to more natural settings. Signal detection, identification and discrimination, as well as localization, are complicated by the transmission channel in several ways. Here, I review the empirical evidence from outdoor studies, demonstrating how excess attenuation reduces the active space and the information of a signal at some distance from the sender. At the same time, these frequency‐dependent processes allow to maintain acoustic distances to neighbours in a population. Insects often communicate within choruses of signallers of the same and different species, giving rise to high levels of acoustic masking interference. I discuss the evidence found for temporal or spatial partitioning of species in multispecies assemblages, and I show that solutions to the masking problem are based on a combination of adaptations in the behaviour of signallers and in the sensory system of receivers. Whether or not the perceived signal elicits a behaviour in receivers will depend on the design of the sensory system and the brain. I give examples for active mechanical processes in insect sensory receptors that influence the responses to external stimuli. In addition, neuronal filters in the frequency, intensity or time domain, and even the memory of individual receivers, provide the basis for adaptive receiver decision‐making in mate choice scenarios. Finally, I describe the advantages of having access to the relatively simple nervous systems of insects and how this access, combined with the use of a variety of behavioural tests, allows new insights into acoustic communication and its evolution. A free Plain Language Summary can be found within the Supporting Information of this article.

Prestin and the cholinergic receptor of hair cells: Positively-selected proteins in mammals

The hair cells of the vertebrate inner ear posses active mechanical processes to amplify their inputs. The stereocilia bundle of various vertebrate animals can produce active movements. Though standard stereocilia-based mechanisms to promote amplification persist in mammals, an additional radically different mechanism evolved: the so-called somatic electromotility which refers to the elongation/contraction of the outer hair cells’ (OHC) cylindrical cell body in response to membrane voltage changes. Somatic electromotility in OHCs, as the basis for cochlear amplification, is a mammalian novelty and it is largely dependent upon the properties of the unique motor protein prestin. We review recent literature which has demonstrated that although the gene encoding prestin is present in all vertebrate species, mammalian prestin has been under positive selective pressure to acquire motor properties, probably rendering it fit to serve somatic motility in outer hair cells. Moreover, we discuss data which indicates that a modified α10 nicotinic cholinergic receptor subunit has co-evolved in mammals, most likely to give the auditory feedback system the capability to control somatic electromotility.

An Anion Antiporter Model of Prestin, the Outer Hair Cell Motor Protein

Cochlear amplification in mammalian hearing relies on an active mechanical feedback process generated by outer hair cells, driven by a protein, prestin (SLC26A5), in the lateral membrane. We have used kinetic models to understand the mechanism by which prestin might function. We show that the two previous hypotheses of prestin, which assume prestin cannot operate as a transporter, are insufficient to explain previously published data. We propose an alternative model of prestin as an electrogenic anion exchanger, exchanging one Cl − ion for one divalent or two monovalent anions. This model can reproduce the key aspects of previous experimental observations. The experimentally observed charge movements are produced by the translocation of one Cl − ion combined with intrinsic positively charged residues, while the transport of the counteranion is electroneutral. We tested the model with measurements of the Cl − dependence of charge movement, using S O 4 2 − to replace Cl −. The data was compatible with the predictions of the model, suggesting that prestin does indeed function as a transporter.

A generalization of the van-der-Pol oscillator underlies active signal amplification in Drosophila hearing

The antennal hearing organs of the fruit fly Drosophila melanogaster boost their sensitivity by an active mechanical process that, analogous to the cochlear amplifier of vertebrates, resides in the motility of mechanosensory cells. This process nonlinearly improves the sensitivity of hearing and occasionally gives rise to self-sustained oscillations in the absence of sound. Time series analysis of self-sustained oscillations now unveils that the underlying dynamical system is well described by a generalization of the van-der-Pol oscillator. From the dynamic equations, the underlying amplification dynamics can explicitly be derived. According to the model, oscillations emerge from a combination of negative damping, which reflects active amplification, and a nonlinear restoring force that dictates the amplitude of the oscillations. Hence, active amplification in fly hearing seems to rely on the negative damping mechanism initially proposed for the cochlear amplifier of vertebrates.

Long-Term Effects of Sectioning the Olivocochlear Bundle in Neonatal Cats

The olivocochlear bundle (OCB) was cut in neonatal cats to evaluate its role in the development of normal cochlear function. Approximately 1 year after deefferentation, acute auditory nerve fiber (ANF) recordings were made from lesioned animals, lesion shams, and normal controls. The degree of deefferentation was quantified via light microscopic evaluation of the density of OCB fascicles in the tunnel of Corti, and selected cases were analyzed via electron microscopy. In the most successful cases, the deefferentation was virtually complete. ANFs from successfully lesioned animals exhibited significant pathophysiology compared with normals and with other animals in which the surgery failed to interrupt the OCB. Thresholds at the characteristic frequency (CF), the frequency at which ANFs are most sensitive, were elevated across the CF range, with maximal effects for CFs in the 10 kHz region. Frequency threshold or tuning curves displayed reduction of tip-to-tail ratios (the difference between CF and low-frequency "tail" thresholds) and decreased sharpness of tuning. These pathological changes are generally associated with outer hair cell (OHC) damage. However, light microscopic histological analysis showed minimal hair cell loss and no significant differences between normal and deefferented groups. Spontaneous discharge rates (SRs) were lower than normal; however, those fibers with the highest SRs remained more sensitive than those with lower SRs. Findings suggest that the interaction between OC efferents and OHCs early in development may be critical for full expression of active mechanical processes.

Seismo‐acoustic remote sensing and inversion of sea ice fracture events

A new, layered seismo‐acoustic remote sensing concept was developed and applied during the SIMI field experiments in the Beaufort Sea in the Fall of 1993 and the Spring of 1994. A large‐aperture, 32‐element, horizontal hydrophone array was used to record the acoustic emission from ice events. Using real‐time array processing, maps of the seismic activity of the ice cover out to a range of 2 to 3 km were continuously generated. Once an active zone was detected, clusters of five 3‐axis geophones and a single hydrophone were deployed in the active zone for near‐field recording of the seismo‐acoustic emission. The data were transmitted back to the main camp via a wireless local area network, and recorded on tape. During the Spring experiment data were recorded continuously on the hydrophone array for 4 weeks, and several deployments of the geophone clusters were performed in the vicinity of active ice mechanical processes such as ridge building, finger rafting, and floe fracturing. In addition, the clusters w...

Cochlear basal and apical differences reflected in the effects of cooling on responses of single auditory nerve fibers

Responses of single auditory nerve fibers in the Mongolian gerbil were examined before and during rapid, moderate cooling of the cochlea. Reducing cochlear temperature from 35–39°C to 29–32°C led to stable, reversible changes in spontaneous firing rates (SRs), and responses to tonebursts, as characterized by frequency tuning curves and rate-versus-intensity curves. The nature and extent of effects of cooling were strongly linked to characteristic frequency (CF). Rate thresholds at the CF were increased by 0–15 dB for fibers with CFs below 8 kHz, and by 10–30 dB for higher CFs. Although SRs were generally reduced, the percent reduction in SR was strikingly CF dependent. For fibers with CFs below 4 kHz, the reduction did not exceed 50% of the initial SR. For higher CFs, the reduction was always greater than 50%. The effects of cooling on intensity curve shape differed qualitatively for fibers with CFs below and above 3–4 kHz. The slope of the curve was reduced by an average of 50% for lower CFs, but on average was unchanged for higher CFs. Cooling-related increases in CF threshold probably reflect impairment of active mechanical processes. The CF dependence of these increases suggests either that active mechanical processes are more impaired in the cochlear base relative to the apex, or that they play a more crucial role in determining sensitivity in the base. The CF-dependent changes in SR and in the shape of rate-intensity curves caused by cooling correspond to an enhancement of basal/apical differences seen at normal temperatures. They are best explained by longitudinal gradients in the properties of the inner hair cells and their afferent synapses. Basal and apical differences in the distribution of SRs and in supra-threshold response properties suggest that stimulus coding strategies differ between low and high frequency regions of the cochlea.

Visualization and functional testing of acetylcholine receptor-like molecules in cochlear outer hair cells

The efferent nerve endings at outer hair cells (OHCs) have been suggested to regulate active mechanical processes in the cochlea. The discovery of acetylcholine (ACh)-producing and -degrading enzymes in these synapses gave rise to the speculation that ACh might be one of the efferent transmitters. However, there has as yet been no identification and characterization of any corresponding receptor in OHCs which is required for further clarification of this question. In the present paper existence, location and first characterization of acetylcholine receptors (AChRs) in OHCs are reported. Using two anti-AChR monoclonal antibodies, AChR epitopes were found forming a cup at the basal end of the OHCs opposite to the efferent nerve endings. Furthermore, the studied molecules could be shown to extend through the cell membrane. In addition, the denervated OHC AChR-epitopes seem to move by lateral diffusion. Application of Carbachol and ACh to the basal pole of OHCs induced a weak, reversible cell contraction. Pharmacological controls revealed, that the motile responses were mediated by the AChRs.

Tinnitus associated spontaneous otoacoustic emissions:Active Outer Hair Cell Movements as Common Origin?

In the mammalian inner ear, active mechanical processes contribute to cochlear micromechanics. Direct evidence of these are spontaneous otoacoustic emissions (SOAE) which have been registered in the human and animal ear canal. In mammals, outer hair cells (OHC) seem to be the origin of this mechanical energy, because they exhibit motile responses to various stimuli. Here we report on bilateral tinnitus and bilateral SOAE with both acoustical phenomena located in the same frequency range. In a playback of the SOAEs their frequences were identified as the pitch of the tinnitus. An additional criterion favoring a common pathomechanism of SOAEs and tinnitus resulted from masking effects. Tinnitus and SOAEs from both sides were suppressed by the same tone, which indicates that the involved pathways are identical. Therefore we suggest that pathological long-term movements of only a small number of OHCs underlie both phenomena. From the SOAE emission spectra we calculated that not more than 60 OHCs might be involved.

Tinnitus Associated Spontaneous Otoacoustic Emissions: Active Outer Hair Cell Movements as Common Origin?

In the mammalian inner ear, active mechanical processes contribute to cochlear microme-chanics. Direct evidence of these are spontaneous otoacoustic emissions (SOAE) which have been registered in the human and animal ear canal. In mammals, outer hair cells (OHC) seem to be the origin of this mechanical energy, because they exhibit motile responses to various stimuli. Here we report on bilateral tinnitus and bilateral SOAE with both acoustical phenomena located in the same frequency range. In a playback of the SOAEs their frequences were identified as the pitch of the tinnitus. An additional criterion favoring a common pathomechanism of SOAEs and tinnitus resulted from masking effects. Tinnitus and SOAEs from both sides were suppressed by the same tone, which indicates that the involved pathways are identical. Therefore we suggest that pathological long-term movements of only a small number of OHCs underlie both phenomena. From the SOAE emission spectra we calculated that not more than 60 OHCs might be involved.

A subpopulation of outer hair cells possessing GABA receptors with tonotopic organization

The olivocochlear innervation has been postulated to regulate active mechanical processes in the mammalian cochlea. Histochemical studies led to the suggestion that a subpopulation of these efferent nerves, which predominantly terminate on outer hair cells (OHCs), are gamma-aminobutyric acid (GABA)-ergic. By means of two monoclonal antibodies, we were able to visualize GABAA-receptor immunoreactivity at the basal pole of isolated sensory cells. Both subunits of the GABAA receptor, the alpha- and beta-subunit, are known to form the transmembranous GABA/benzodiazepine-receptor complex and were present on OHCs. In addition, these inhibitory receptors were more numerous in the apical turns of the cochlea, indicating another criterion for distinguishing the apical from basal turns of the cochlea. These results support the concept that a subpopulation of axosomatic synapses at the basal pole of OHCs liberate the inhibitory neurotransmitter GABA into the synaptic cleft. Binding of the transmitter to these newly observed subsynaptic receptors is possibly followed by a change in OHC motility and a subsequent modulation of the movement of the basilar membrane.

Actin in cochlear outer hair cells: A structural basis for cell shape and motility

Threshold sensitivity and frequency selectivity in the cochlea depend on intact outer hair cells and models to explain these phenomena suggest that active mechanical processes, residing in outer hair cells, alter micromechanical properties of the basilar membrane. Auditory sensory hair cells isolated from the inner ear change their shape in response to stimulation in vitro. Three areas of the cell have been suggested to be involved in this form of cell motility: (1) the infracuticular network of actin filaments that is seen as a central core; (2) the cortical layer of actin filaments along the lateral wall; and (3) actin in the outer hair cell cytoplasm. The actin filaments in the stereocilia, the cuticular plate, and the infracuticular network are not involved, since mechanically disrupted cells lacking these structures still shorten. Actin filaments along the lateral wall are not involved in shortening since outer hair cell membrane ghosts do not shorten, however, they retain the ability to constrict so...

In vivo and in vitro studies of intracochlear electrical potential gradients

Currents of similar magnitude to those measured in vivo evoke rapid length changes of nearly 1 μm in isolated outer hair cells (OHCs) maintained in vitro. The ability of OHCs to follow electrical stimulation at greater than 8 kHz makes OHC electromotility a strong candidate for the active mechanical process assumed to contribute to the exquisite tuning and sensitivity of the intact cochlear partition. An attempt is made to identify the biophysical mechanism responsible for these rapid movements by measuring the length changes evoked by potential gradients produced with two tight seal whole cell electrodes. During these experiments, a second, considerably slower, type of electrically induced OHC shape change has been observed. The mechanism responsible for this slow, reversible, electrically induced volume change appears unlike that responsible for the fast electromotility. [Work supported by NIH (NS23567) and ONR (441k704).]