Regulatory Roles of Fluctuation-Driven Mechanotransduction in Cell Function

Fabrication of fully aligned self-assembled cell-laden collagen filaments for tissue engineering via a hybrid bioprinting process

Cell-laden structures play a pivotal role in various tissue engineering applications, particularly in tissue restoration. Interactions between cells within bioprinted structures are crucial for successful tissue development and regulation of stem cell fate through intricate cell-to-cell signaling pathways. In this study, we developed a new technique that combines polyethylene glycol (PEG)-infused submerged bioprinting with a stretching procedure. This approach facilitated the generation of fully aligned collagen structures consisting of myoblasts and a low concentration (2 wt%) of collagen to efficiently encourage muscle tissue regeneration. By adjusting several processing parameters, we obtained biologically safe and mechanically stable cell-laden collagen filaments with uniaxial alignment. Notably, the cell filaments exhibited markedly elevated cellular activities compared to those exhibited by conventional bioprinted filaments, even at similar cell densities. Moreover, when we implanted structures containing adipose stem cells into mice, we observed a significantly increased level of myogenesis compared to that in normally bioprinted struts. Thus, this promising approach has the potential to revolutionize tissue engineering by fostering enhanced cellular interactions and promoting improved outcomes in regenerative medicine.

Platelets drive fibronectin fibrillogenesis using integrin αIIbβ3

Platelets interact with multiple adhesion proteins during thrombogenesis, yet little is known about their ability to assemble fibronectin matrix. In vitro three-dimensional superresolution microscopy complemented by biophysical and biochemical methods revealed fundamental insights into how platelet contractility drives fibronectin fibrillogenesis. Platelets adhering to thrombus proteins (fibronectin and fibrin) versus basement membrane components (laminin and collagen IV) pull fibronectin fibrils along their apical membrane versus underneath their basal membrane, respectively. In contrast to other cell types, platelets assemble fibronectin nanofibrils using αIIbβ3 rather than α5β1 integrins. Apical fibrillogenesis correlated with a stronger activation of integrin-linked kinase, higher platelet traction forces, and a larger tension in fibrillar-like adhesions compared to basal fibrillogenesis. Our findings have potential implications for how mechanical thrombus integrity might be maintained during remodeling and vascular repair.

Stabilizing breathing pattern using local mechanical vibrations: comparison of deterministic and stochastic stimulations in rodent models of apnea of prematurity

Mechanical stimulation has been shown to reduce apnea of prematurity (AOP), a major concern in preterm infants. Previous work suggested that the underlying mechanism is stochastic resonance, amplification of a subthreshold signal by stochastic stimulation. We hypothesized that the mechanism behind the reduction of apnea length may not be a solely stochastic phenomenon, and suggest that a purely deterministic, non-random mechanical stimulation could be equally as effective. Mice and rats were anesthetized, tracheostomized, and mechanically ventilated to halt spontaneous breathing. Two miniature motors controlled by a microcontroller were attached around the abdomen. Ventilation was paused, stimulations were applied, and the time to the rodent's first spontaneous breath (T) was measured. Six spectrally different signals were compared to one another and the no-stimulation control in mice. The most successful deterministic stimulation (D) at reducing apnea was then compared to a pseudo-random noise (PRN) signal of comparable amplitude and frequency. CO₂%, CO₂ stabilization time (T_s), O₂ saturation (SpO₂%), and T were also measured. D significantly reduced T compared to no stimulation for medium and high amplitudes. PRN also reduced T, without  a difference between D and PRN. Furthermore, both stimulations significantly reduced T_s with no significant differences between the respective stimulations. However, there was no effect of D or PRN on SpO₂%. The lack of differences between D and PRN led to an additional series of experiment comparing the same D to a band-limited white noise (WN) signal in young rats. Both D and WN were shown to significantly reduce T, with D showing statistical superiority in reduction of apnea. We further speculate that both deterministic and stochastic mechanical stimulations induce some form of mechanotransduction which is responsible for their efficacy, and our findings suggest that mechanical stimulation may be effective in treating AOP.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13534-021-00203-x.

Precision Surface Microtopography Regulates Cell Fate via Changes to Actomyosin Contractility and Nuclear Architecture

Cells are able to perceive complex mechanical cues from their microenvironment, which in turn influences their development. Although the understanding of these intricate mechanotransductive signals is evolving, the precise roles of substrate microtopography in directing cell fate is still poorly understood. Here, UV nanoimprint lithography is used to generate micropillar arrays ranging from 1 to 10 µm in height, width, and spacing to investigate the impact of microtopography on mechanotransduction. Using mesenchymal stem cells (MSCs) as a model, stark pattern-specific changes in nuclear architecture, lamin A/C accumulation, chromatin positioning, and DNA methyltransferase expression, are demonstrated. MSC osteogenesis is also enhanced specifically on micropillars with 5 µm width/spacing and 5 µm height. Intriguingly, the highest degree of osteogenesis correlates with patterns that stimulated maximal nuclear deformation which is shown to be dependent on myosin-II-generated tension. The outcomes determine new insights into nuclear mechanotransduction by demonstrating that force transmission across the nuclear envelope can be modulated by substrate topography, and that this can alter chromatin organisation and impact upon cell fate. These findings have potential to inform the development of microstructured cell culture substrates that can direct cell mechanotransduction and fate for therapeutic applications in both research and clinical sectors.

Quantification of Translaminar Pressure Gradient (TLPG) With Continuous Wireless Telemetry in Nonhuman Primates (NHPs)

Purpose

Recent retrospective clinical and animal studies suggest that cerebrospinal fluid pressure (CSFP) is important in glaucoma pathogenesis. Intraocular pressure (IOP) and CSFP are the driving components of translaminar pressure (TLP = IOP - CSFP), which acts across the lamina cribrosa (LC) thickness to create the translaminar pressure gradient (TLPG = TLP/LC thickness).

Methods

We developed an implantable wireless telemetry system based on a small piezoelectric sensor with low temporal drift. IOP, measured in the anterior chamber, and intracranial pressure (ICP), measured in the brain parenchyma (as a surrogate for CSFP) were measured at 200 Hz in three male rhesus macaques (nonhuman primates, NHPs) on a 10% duty cycle (15 seconds of every 150-second period). Three-dimensional LC thickness was autosegmented as the mean thickness of the visible hyperreflective band in 48 radial spectral-domain optical coherence tomography b-scans centered on the optic nerve head.

Results

Results indicated the rank order of IOP, ICP, TLP, and TLPG for waking, sleeping, and 24-hour periods averaged across all days. NHP 150110 had the highest IOP and ICP in all periods; however, it had the lowest TLPG in all periods due to its relatively thick LC. The other two NHPs showed similar shifts in the rank order of possible glaucoma risk factors.

Conclusions

IOP is the only modifiable and readily measurable pressure-based risk factor for glaucoma. However, other potential risk factors such as ICP, TLP, and TLPG, as well as their rank-order patterns, differed compared to IOP across subjects, demonstrating that a comprehensive view of relevant risk factors is warranted.

Translational Relevance

Future studies should consider including CSFP, TLP, and TLPG in addition to IOP as potential risk factors when assessing eye-specific glaucoma susceptibility.

Diurnal Cycle of Translaminar Pressure in Nonhuman Primates Quantified With Continuous Wireless Telemetry

Purpose

Recent retrospective clinical studies and animal experiments have suggested that cerebrospinal fluid pressure (CSFP) is important in glaucoma pathogenesis. Intraocular pressure (IOP) and CSFP are the driving components of the translaminar pressure (TLP), which directly effects the optic nerve head. This study measured the diurnal cycle of TLP using continuous wireless telemetry in nonhuman primates (NHPs), a common animal model of glaucoma.

Methods

We have developed an implantable wireless telemetry system based on a small piezoelectric pressure transducer with low drift. Unilateral IOP was measured in the anterior chamber of the eye, and intracranial pressure (ICP, a surrogate measure of CSFP) was measured in the brain parenchyma in four awake, behaving NHPs for periods of 22 to 281 days. IOP and ICP telemetry transducers were calibrated with direct pressure measurements in the eye (every 2 weeks) and brain (monthly). TLP was quantified in real time as IOP-ICP, and hourly means of IOP, ICP, and TLP were analyzed.

Results

Results show that mean ICP is significantly higher by an average of 4.8 ± 0.8 mmHg during sleeping hours in NHPs (P < 0.01). IOP showed a small but significant nocturnal elevation in two of four animals despite NHPs sleeping upright (P < 0.05). TLP was significantly lower during sleep (7.1 ± 0.6 mmHg; P < 0.01) than when the animals were awake and active (11.0 ± 0.9 mmHg), driven primarily by the large increase in ICP during sleep.

Conclusions

The 56% increase in TLP during waking hours in NHPs matches the increase in TLP due to postural change from supine to upright reported previously in humans.

Tuning mitochondrial structure and function to criticality by fluctuation-driven mechanotransduction

Cells in vascular walls are exposed to blood pressure variability (BPV)-induced cycle-by-cycle fluctuations in mechanical forces which vary considerably with pathology. For example, BPV is elevated in hypertension but reduced under anesthesia. We hypothesized that the extent of mechanical fluctuations applied to vascular smooth muscle cells (VSMCs) regulates mitochondrial network structure near the percolation transition, which also influences ATP and reactive oxygen species (ROS) production. We stretched VSMCs in culture with cycle-by-cycle variability in area strain ranging from no variability (0%), as in standard laboratory conditions, through abnormally small (6%) and physiological (25%) to pathologically high (50%) variability mimicking hypertension, superimposed on 0.1 mean area strain. To explore how oxidative stress and ATP-dependent metabolism affect mitochondria, experiments were repeated in the presence of hydrogen peroxide and AMP-PNP, an ATP analog and competitive inhibitor of ATPases. Physiological 25% variability maintained activated mitochondrial cluster structure at percolation with a power law distribution and exponent matching the theoretical value in 2 dimensions. The 25% variability also maximized ATP and minimized cellular and mitochondrial ROS production via selective control of fission and fusion proteins (mitofusins, OPA1 and DRP1) as well as through stretch-sensitive regulation of the ATP synthase and VDAC1, the channel that releases ATP into the cytosol. Furthermore, pathologically low or high variability moved mitochondria away from percolation which reduced the effectiveness of the electron transport chain by lowering ATP and increasing ROS productions. We conclude that normal BPV is required for maintaining optimal mitochondrial structure and function in VSMCs.

Transient Intraocular Pressure Fluctuations: Source, Magnitude, Frequency, and Associated Mechanical Energy

Purpose

To characterize intraocular pressure (IOP) dynamics by identifying the sources of transient IOP fluctuations and quantifying the frequency, magnitude, associated cumulative IOP-related mechanical energy, and temporal distribution.

Methods

IOP was monitored at 500 Hz for periods of 16 to 451 days in nine normal eyes of six conscious, unrestrained nonhuman primates using a validated, fully implanted wireless telemetry system. IOP transducers were calibrated every two weeks via anterior chamber cannulation manometry. Analysis of time-synchronized, high-definition video was used to identify the sources of transient IOP fluctuations.

Results

The distribution of IOP in individual eyes is broad, and changes at multiple timescales, from second-to-second to day-to-day. Transient IOP fluctuations arise from blinks, saccades, and ocular pulse amplitude and were as high as 14 mm Hg (>100%) above momentary baseline. Transient IOP fluctuations occur ∼10,000 times per waking hour, with ∼2000 to 5000 fluctuations per hour greater than 5 mm Hg (∼40%) above baseline. Transient IOP fluctuations account for up to 17% (mean of 12%) of the total cumulative IOP-related mechanical energy that the eye must withstand during waking hours.

Conclusions

Transient IOP fluctuations occur frequently and comprise a large and significant portion of the total IOP loading in the eye and should, therefore, be considered in future studies of cell mechanotransduction, ocular biomechanics, and/or clinical outcomes where transient IOP fluctuations may be important. If IOP dynamics are similar in humans, clinical snapshot IOP measurements are insufficient to capture true IOP.

Blood pressure-induced physiological strain variability modulates wall structure and function in aorta rings

Vascular smooth muscle cells respond to mechanical stretch by reorganizing their cytoskeletal and contractile elements. Recently, we showed that contractile forces in rat aorta rings were maintained when the rings were exposed to 4 h of physiological variability in cycle-by-cycle strain, called variable stretch (VS), mimicking beat-to-beat blood pressure variability. Contractility, however, was reduced when the aorta was exposed to monotonous stretch (MS) with an amplitude equal to the mean peak strain of VS.

OBJECTIVE

Here we reanalyzed the data to obtain wall stiffness as well as added new histologic and inhibitor studies to test the effects of VS on the extracellular matrix.

MAIN RESULTS

The results demonstrate that while the stiffness of the aorta did not change during 4 h MS or VS, nonlinearity in mechanical behavior was slightly stronger following MS. The inhibitor studies also showed that mitochondrial energy production and cytoskeletal organization were involved in this fluctuation-driven mechanotransduction. Reorganization of β-actin in the smooth muscle layer quantified from immunohistochemically labeled images correlated with contractile forces during contraction. Histologic analysis of wall structure provided evidence of reorganization of elastin and collagen fibers following MS but less so following VS. The results suggested that the loss of muscle contraction in MS was compensated by reorganization of fiber structure leading to similar wall stiffness as in VS.

SIGNIFICANCE

We conclude that muscle tone modulated by variability in stretch plays a role in maintaining aortic wall structural and mechanical homeostasis with implications for vascular conditions characterized by a loss or an increase in blood pressure variability.

Regulation of Mitochondrial Structure and Dynamics by the Cytoskeleton and Mechanical Factors

Mitochondria supply cells with energy in the form of ATP, guide apoptosis, and contribute to calcium buffering and reactive oxygen species production. To support these diverse functions, mitochondria form an extensive network with smaller clusters that are able to move along microtubules aided by motor proteins. Mitochondria are also associated with the actin network, which is involved in cellular responses to various mechanical factors. In this review, we discuss mitochondrial structure and function in relation to the cytoskeleton and various mechanical factors influencing cell functions. We first summarize the morphological features of mitochondria with an emphasis on fission and fusion as well as how network properties govern function. We then review the relationship between the mitochondria and the cytoskeletal structures, including mechanical interactions. We also discuss how stretch and its dynamic pattern affect mitochondrial structure and function. Finally, we present preliminary data on how extracellular matrix stiffness influences mitochondrial morphology and ATP generation. We conclude by discussing the more general role that mitochondria may play in mechanobiology and how the mechanosensitivity of mitochondria may contribute to the development of several diseases and aging.

Optimization of Variable Ventilation for Physiology, Immune Response and Surfactant Enhancement in Preterm Lambs

Preterm infants often require mechanical ventilation due to lung immaturity including reduced or abnormal surfactant. Since cyclic stretch with cycle-by-cycle variability is known to augment surfactant release by epithelial cells, we hypothesized that such in vivo mechanotransduction improves surfactant maturation and hence lung physiology in preterm subjects. We thus tested whether breath-by-breath variability in tidal volume (V_T) in variable ventilation (VV) can be tuned for optimal performance in a preterm lamb model. Preterm lambs were ventilated for 3 h with conventional ventilation (CV) or two variants of VV that used a maximum V_T of 1.5 (VV1) or 2.25 (VV2) times the mean V_T. V_T was adjusted during ventilation to a permissive pCO₂ target range. Respiratory mechanics were monitored continuously using the forced oscillation technique, followed by postmortem bronchoalveolar lavage and tissue collection. Both VVs outperformed CV in blood gas parameters (pH, SaO₂, cerebral O₂ saturation). However, only VV2 lowered PaCO₂ and had a higher specific respiratory compliance than CV. VV2 also increased surfactant protein (SP)-B release compared to VV1 and stimulated its production compared to CV. The production and release of proSP-C however, was increased with CV compared to both VVs. There was more SP-A in both VVs than CV in the lung, but VV2 downregulated SP-A in the lavage, whereas SP-D significantly increased in CV in both the lavage and lung. Compared to CV, the cytokines IL-1β, and TNFα decreased with both VVs with less inflammation during VV2. Additionally, VV2 lungs showed the most homogeneous alveolar structure and least inflammatory cell infiltration assessed by histology. CV lungs exhibited over-distension mixed with collapsed and interstitial edematous regions with occasional hemorrhage. Following VV1, some lambs had normal alveolar structure while others were similar to CV. The IgG serum proteins in the lavage, a marker of leakage, were the highest in CV. An overall combined index of performance that included physiological, biochemical and histological markers was the best in VV2 followed by VV1. Thus, VV2 outperformed VV1 by enhancing SP-B metabolism resulting in open alveolar airspaces, less leakage and inflammation and hence better respiratory mechanics.

Lamina cribrosa in glaucoma

PURPOSE OF REVIEW

This article presents, summarizes, and interprets the most recent advances in the study and understanding of the lamina cribrosa in glaucoma, in the context of previous work.

RECENT FINDINGS

The lamina is an active living structure that responds to strain by changing morphology at the micro-scale and macro-scale in glaucoma. Changes in lamina cribrosa morphology in glaucoma include posteriorization of the laminar insertion into the sclera, increased cupping or depth of the lamina cribrosa, and the development of focal lamina cribrosa defects. These lamina cribrosa changes are associated with disk hemorrhages and visual field damage, and are detectable with clinical imaging techniques such as optical coherence tomography. Glaucomatous changes in the lamina cribrosa are thought to be driven by cellular processes mediated by focal cyclical mechanical strain. Strain is eye specific and mediated by intraocular pressure, cerebrospinal fluid pressure, scleral and lamina cribrosa morphology, and structural stiffness; deleterious lamina cribrosa strains can occur at all levels of mean intraocular pressure.

SUMMARY

Laminar morphology is ever changing in health and disease, and recent studies have identified several promising morphological changes that are indicative of glaucoma susceptibility, onset, and progression.