Biology subjects

Hahn, D.

Publications and source records attributed to Hahn, D..

18 recordsLinked to original sources

bioRxiv2026

3D ultrasound fascicle tractography for objective muscle architecture analysis.

Muscle architecture shapes muscle function and changes with age, growth, training and disease, yet quantifying three-dimensional (3D) muscle architecture in vivo remains challenging. We introduce a hybrid fascicle tractography approach for freehand 3D ultrasound data that accurately reconstructs 3D muscle fascicles with respect to an objective, anatomically relevant coordinate system defined by the muscle's central aponeurosis. The hybrid approach combines Hessian-based fascicle detection with wavelet-based refinement to generate volumetric fascicle orientations. In a synthetic dataset with known ground truth, fascicle orientations and lengths were estimated with errors of [≤]2{degrees} and ~1.5%, respectively. In vivo, the approach detected physiologically plausible fascicle lengthening in the human tibialis anterior following a passive plantar flexion rotation, whereas diffusion tensor imaging of the same muscle did not. The proposed method enables anatomically relevant, objective and non-invasive quantification of 3D muscle architecture in vivo, providing a practical framework for applications in clinical and applied muscle physiology.

bioengineering↗

bioRxiv2026

Stretch versus shortening contractions subsequently decrease versus increase neural drive to the human tibialis anterior

EMG-based muscle force predictions are often inaccurate following active muscle stretch or shortening because of residual force enhancement (rFE) or depression (rFD), respectively, which can alter the neural drive to a muscle. However, the extent of neural drive modulation due to rFE or rFD remains unknown, making it difficult to correct EMG-based force predictions. Therefore, seventeen participants performed dorsiflexion contractions at 20 and 40% of maximum voluntary torque (MVT) in three conditions: stretch-hold, shortening-hold, and fixed-end reference (REF) conditions. The ankle dorsiflexion torques and angles were matched using dynamometry to the REF condition over a 10-s steady state following a 1-s 25{degrees} stretch or shortening, during which we recorded and decomposed tibialis anterior individual motor unit action potentials from high-density surface EMG recordings to gain insights into neural drive. Normalized EMG amplitudes were 2% lower following stretch and 1 or 3% higher following shortening relative to REF at 20 versus 40% MVT (p[≤].008), respectively. Discharge rates (DRs) from 19 matched motor units per person on average obtained via DEMUSE and MUedit were similar (p=.871). Following stretch and shortening, DRs were [~]1 Hz lower (p[≤].004) and 0 (p=.966) to 1 Hz higher relative to REF (p=.003), respectively. More unique motor units were also detected following shortening versus REF and in REF versus following stretch. These findings indicate that to account for rFE or rFD, neural drive is respectively decreased or increased via reduced or additional motor unit recruitment and DR modulation, with a contraction-intensity specific discharge rate modulation following active shortening.

physiology↗

bioRxiv2025

A passive upper-limb exoskeleton effectively reduces shoulder muscle activity over a large shoulder workspace

Industrial upper-limb exoskeletons are used to offload the upper limb during overhead tasks with the aim of preventing musculoskeletal disorders to the shoulder. Although numerous studies showed reduced shoulder muscle activity during upper-limb exoskeleton use for overhead postures, it remains unknown whether and how upper-limb exoskeletons provide support over a large shoulder workspace beyond overhead work. Therefore, this study evaluated the Ottobock Paexo Shoulder over a large shoulder workspace from overhead to hip height with shoulder abduction and adduction. Upper body kinematics, muscle activity, and subjective user feedback were obtained by three-dimensional motion capture, bipolar surface EMG and questionnaires, respectively, and captured while participants performed static and dynamic work tasks with an electric screwdriver. Participants completed these tasks while 1) not wearing the exoskeleton, 2) wearing a disengaged exoskeleton, with 3) moderate exoskeleton support, and with 4) high exoskeleton support. Exoskeleton support reduced deltoid muscle activity (-9 to -24s%, p[≤].001) in postures with an abducted shoulder or extended elbow, including non-overhead postures. Exoskeleton support modestly decreased shoulder flexion (-3 to -5{degrees}, p[≤].001) and increased shoulder abduction (2 to 5{degrees}, p[≤].032), but the movement patterns during the dynamic task were unaffected. Additionally, exoskeleton-related effects increased with increasing support, but the subjective perception of change also increased, and perceived comfort decreased. Our results indicate that the tested exoskeleton provides support beyond overhead work and that there is a trade-off between exoskeleton support and subjective perception. Accordingly, further optimization of user-exoskeleton interaction is warranted for long-term prevention of musculoskeletal disorders in overhead workers.

physiology↗

bioRxiv2024

Fascicle stretch during partial muscle deactivation does not enhance the human tibialis anterior's subsequent neuromechanical output

We were interested in whether stretching the muscle via elastic tissue recoil during partial muscle deactivation could trigger stretch-induced mechanisms that subsequently enhance the muscles steady-state neuromechanical output. Two torque-controlled experiments were conducted to test this aim. In Experiment 1, fifteen participants performed fixed-end dorsiflexion contractions to a moderate or high-then-moderate level with torque-drop rates of 0, 10, 20, 40, or 80% MVC{middle dot}s-1 from 60-40% MVC, while net ankle joint torque, tibialis anterior (TA) muscle activity level, and TA ultrasound images were recorded. The same measurements were performed in Experiment 2, which tested twelve different participants who performed fixed-end contractions to three reference levels or to a higher-then-lower level (with torque-drop amplitudes of 85-45, 85-30, and 85-15% MVC at 20% MVC{middle dot}s-1). Increased fascicle shortening amplitudes (Experiment 1: 1-2 mm, p[≤].049; Experiment 2: 5-10 mm, p<.001) to initially higher joint torques and different fascicle stretch rates (0.6 to 4.3 mm{middle dot}s-1, p[≤].041) and amplitudes (4-9 mm; p[≤].001) did not significantly affect TAs subsequent muscle activity level relative to the reference contractions at similar joint torques (0-1% MVC, p[≥].659 and p[≥].626). However, the torque steadiness relative to the reference conditions was significantly reduced after the 85-15% MVC (p=.036) and 85-30% MVC (p=.036) torque drops. Consequently, the history of force production affected the control of muscle force more than TAs steady-state neuromuscular output. These findings indicate that assessing fascicle kinematics concurrently with motor unit behavior during fixed-end contractions with large torque drops might provide unique insights into the neuromechanical contributors to impaired torque control. NEW & NOTEWORTHYThe history of force production affected the subsequent control of dorsiflexion torque more than tibialis anteriors (TAs) neuromuscular output following larger amounts of fascicle stretch during fixed-end contractions. Larger fascicle stretches also reduced the median frequency of TAs EMG signal, which tentatively indicates that the voluntary control of muscle force was altered in the steady state. Together, these findings indicate that voluntary force control might be impaired by stretch-induced and/or activity-level-dependent mechanisms during fixed-end contractions.

physiology↗

bioRxiv2024

The stretch-shortening cycle effect is not associated with cortical or spinal excitability modulations

It is unclear whether cortical and spinal excitability modulations contribute to enhanced SSC performance. Therefore, this study investigated cortical and spinal excitability modulations during and following shortening of stretch-shortening cycle (SSC) contractions compared with pure shortening (SHO) contractions. Participants (N = 18) performed submaximal voluntary plantar flexion contractions while prone on the dynamometer bench. The right foot was strapped onto the dynamometers footplate attachment and the resultant ankle joint torque and crank arm angle were recorded. Cortical and spinal excitability modulations of the soleus muscle were analyzed by eliciting compound muscle actional potentials via electrical nerve stimulation, cervicomedullary motor-evoked potentials (CMEPs) via electrical stimulation of the spinal cord, and motor-evoked potentials (MEPs) via magnetic stimulation of the motor cortex. Mean torque following stretch was significantly increased by 7{+/-}3% (p=0.029) compared with the fixed-end reference (REF) contraction and mean torque during shortening of SSC compared with SHO was significantly increased by 12{+/-}24% (p=0.046). Mean steady-state torque was significantly lower by 13{+/-}3% (p=0.006) and 9{+/-}12% (p=0.011) following SSC compared with REF and SHO, respectively. Mean steady-state torque was not significantly lower following SHO compared with REF (7{+/-}8%, p=0.456). CMEPs and MEPs were also not significantly different during shortening of SSC compared with SHO (p[≥]0.885) or during the steady state of SSC, SHO, and REF (p[≥]0.727). Therefore, our results indicate that SSC performance was not associated with cortical or spinal excitability modulations during or after shortening, but rather driven by mechanical mechanisms triggered during active stretch. Key pointsO_LIA stretch-shortening cycle (SSC) effect of 12% was observed during EMG-matched submaximal voluntary contractions of the human plantar flexors C_LIO_LIThe SSC effect was neither associated with cortical or spinal excitability modulations nor with stretch-reflex activity C_LIO_LIThe SSC effect was likely driven by mechanical mechanisms related to active muscle stretch, which have long-lasting effects during shortening C_LIO_LIResidual force depression following SSC was not attenuated by the long-lasting mechanical mechanisms triggered during active muscle stretch C_LIO_LISteady-state torques were lower following shortening of SSCs versus pure shortening and fixed-end contractions at the same final ankle joint angle, but the torque differences were not correlated with cortical or spinal excitability modulations C_LI

neuroscience↗

bioRxiv2024

UltraTimTrack: a Kalman-filter-based algorithm to track muscle fascicles in ultrasound image sequences

BackgroundBrightness-mode (B-mode) ultrasound is a valuable tool to non-invasively image skeletal muscle architectural changes during movement, but automatically estimating architectural features such as fascicle length remains a major challenge. Existing fascicle tracking algorithms either require time-consuming drift corrections or yield noisy estimates that require post-processing. We therefore aimed to develop an algorithm that tracks fascicles without drift and noise across a range of experimental conditions and image acquisition settings. MethodsWe applied a Kalman filter to combine fascicle length and fascicle angle estimates from existing and openly available UltraTrack and TimTrack algorithms into a hybrid algorithm called UltraTimTrack. We applied the hybrid algorithm to ultrasound image sequences collected from the human medial gastrocnemius of healthy individuals (N=8, 4 women), who performed cyclical submaximal plantar flexion contractions or remained at rest during passive ankle joint rotations at given frequencies and amplitudes whilst seated in a dynamometer chair. We quantified the algorithms tracking accuracy, noise, and drift as the respective mean, cycle-to-cycle, and accumulated between-contraction variability in fascicle length and fascicle angle. We expected UltraTimTracks estimates to be less noisy and to drift less across experimental conditions and image acquisition settings, compared with estimates from its parent algorithms. ResultsThe proposed algorithm had low-noise estimates like UltraTrack and was drift-free like TimTrack across the broad range of conditions we tested. Estimated fascicle length and fascicle angle deviations accumulated to 2.1 {+/-} 1.3 mm (mean {+/-} s.d.) and 0.8 {+/-} 0.7 deg, respectively, over 120 cyclical contractions. Average cycle-to-cycle variability was 1.4 {+/-} 0.4 mm and 0.6 {+/-} 0.3 deg, respectively. In comparison, UltraTrack had similar cycle-to-cycle variability (1.1 {+/-} 0.3 mm, 0.5 {+/-} 0.1 deg) but greater cumulative deviation (67.0 {+/-} 59.3 mm, 9.3 {+/-} 8.6 deg), whereas TimTrack had similar cumulative deviation (1.9 {+/-} 2.2 mm, 0.9 {+/-} 1.0 deg) but greater variability (3.5 {+/-} 1.0 mm, 1.4 {+/-} 0.5 deg). UltraTimTrack was significantly less affected by experimental conditions and image acquisition settings than its parent algorithms. It also performed well on a previously published image sequence from the human tibialis anterior, yielding a smaller root-mean-square deviation from manual tracking (fascicle length: 2.7 mm, fascicle angle: 0.7 deg) than a recently proposed hybrid algorithm (fascicle length: 4.5 mm, fascicle angle: 0.8 deg) and a machine-learning (DL_Track) algorithm (fascicle length: 8.2 mm, fascicle angle: 4.8 deg). ConclusionWe developed a Kalman-filter-based method to improve fascicle tracking from B-mode ultrasound image sequences. The proposed algorithm provides low-noise, drift-free estimates of muscle architectural changes that may better inform muscle function interpretations.

physiology↗

bioRxiv2023

Eccentric exercise is not equal to eccentric contraction

Whether eccentric exercise involves active fascicle stretch is unclear due to muscle-tendon unit (MTU) series elasticity. Therefore, this study investigated the impact of changing the activation timing and level (i.e., pre-activation) on muscle fascicle kinematics and kinetics of the human tibialis anterior during dynamometer-controlled maximal voluntary MTU-stretch-hold contractions. B-mode ultrasound and surface electromyography were employed to assess muscle fascicle kinematics and muscle activity levels, respectively. While joint kinematics were similar for MTU-stretch-hold contractions, increasing pre-activation increased fascicle shortening and stretch amplitudes (9.9 - 23.2 mm, p [≤] 0.015). This led to increasing positive and negative fascicle work with increasing pre-activation. Despite significantly different fascicle kinematics, similar peak fascicle forces during stretch occurred at similar fascicle lengths and joint angles regardless of pre-activation. Similarly, residual force enhancement (rFE) following MTU stretch was not significantly affected (6.5-7.6 %, p = 0.559) by pre-activation, but rFE was strongly correlated with peak fascicle force during stretch (rrm = 0.62, p = 0.003). These findings highlight that apparent eccentric exercise causes shortening-stretch contractions at the fascicle level rather than isolated eccentric contractions. The constant rFE despite different fascicle kinematics and kinetics suggests that a passive element was engaged at a common muscle length among conditions (e.g., optimal fascicle length). Although it remains unclear whether different fascicle mechanics trigger different adaptations to eccentric exercise, this study emphasizes the need to consider MTU series elasticity to better understand the mechanical drivers of adaptation to exercise. New & NoteworthyApparent eccentric exercises do not result in isolated eccentric contractions, but shortening-stretch contractions at the fascicle level. The amount of fascicle shortening and stretch depend on the pre-activation during the exercise and cannot be estimated from the muscle-tendon unit or joint kinematics. As different fascicle mechanics might trigger different adaptations to eccentric exercise, muscle-tendon unit series elasticity and muscle pre-activation need to be considered when eccentric exercise protocols and designed and evaluated.

physiology↗

bioRxiv2023

Residual force depression is not related to positive muscle fascicle work during submaximal voluntary dorsiflexion contractions in humans

Residual force depression (rFD) following active muscle shortening is commonly assumed to strongly and linearly increase with increasing muscle work, but this has not been systematically tested during voluntary contractions in humans. Using dynamometry, we compared steady-state ankle joint torques (N=16) following tibialis anterior (TA) muscle-tendon unit (MTU) lengthening and shortening to the torque during submaximal voluntary fixed-end dorsiflexion reference contractions (REF) at a matched MTU length and EMG amplitude. B-mode ultrasound revealed that TA fascicle shortening amplitudes were significantly reduced (p<0.001) during MTU lengthening with no preload over small (LENsmall) and medium (LENmedium) amplitudes, respectively, relative to REF. MTU lengthening with a preload over a large (LENlargeP) amplitude significantly (p<0.001) increased fascicle shortening relative to REF, as well as stretch amplitudes relative to LENsmall and LENmedium (p[≤]0.001), but the significant (p[≤]0.028) steady-state fascicle force enhancement relative to REF was similar to LENsmall and LENmedium (3-5%). MTU shortening with and without a preload over small (SHOsmallP/SHOsmall) and large (SHOlargeP/SHOlarge) amplitudes significantly (p<0.001) increased positive fascicle and MTU work relative to REF, but significant (p[≤]0.006) rFD was observed in SHOsmallP and SHOlargeP (7-10%) only. rFD was linearly related to positive MTU work (rrm(47)=0.48, p<0.001), but not positive fascicle work (rrm(47)=0.16, p=0.277). Our findings indicate that MTU lengthening without substantial fascicle stretch enhances steady-state force output, which might be due to less shortening-induced rFD. Our findings also indicate that different amounts of positive fascicle and MTU work induce similar rFD, which cautions against using work to predict rFD during submaximal voluntary contractions.

physiology↗

bioRxiv2022

Skeletal muscle Nox4 knockout prevents and Nox2 knockout blunts loss of maximal diaphragm force in mice with heart failure with reduced ejection fraction

Patients with heart failure with reduced ejection fraction (HFrEF) experience diaphragm weakness that contributes to the primary disease symptoms of fatigue, dyspnea, and exercise intolerance. Weakness in the diaphragm is related to excessive production of reactive oxygen species (ROS), but the exact source of ROS remains unknown. NAD(P)H Oxidases (Nox), particularly the Nox2 and 4 isoforms, are important sources of ROS within skeletal muscle that contribute to optimal cell function. There are reports of increased Nox activity in the diaphragm of patients and animal models of HFrEF, implicating these complexes as possible sources of diaphragm dysfunction in HFrEF. To investigate the role of these proteins on diaphragm weakness in HFrEF, we generated inducible skeletal muscle specific knockouts of Nox2 or Nox4 using the Cre-Lox system and assessed diaphragm function in a mouse model of HFrEF induced by myocardial infarction. Diaphragm maximal specific force measured in vitro was depressed by ~20% with HFrEF. Knockout of Nox4 provided full protection against the loss of maximal force (p < 0.01), while the knockout of Nox2 provided partial protection (7% depression, p < 0.01). Mitochondrial respiration measured in permeabilized diaphragm muscle bundles increased with HFrEF or the knockout of Nox4 from skeletal muscle fibers (p < 0.05). Knockout of Nox2 from skeletal myofibers improved survival from 50 to 80% following myocardial infarction (p = 0.026). Our findings show an important role for skeletal muscle NAD(P)H Oxidases contributing to loss of diaphragm maximal force in HFrEF, along with systemic pathophysiological responses following myocardial infarction.

physiology↗

bioRxiv2022

Residual force enhancement within the human quadriceps is greatest during submaximal stretch-hold contractions at a very long muscle length

Little is known about how muscle length affects residual force enhancement (rFE) in humans. We therefore investigated rFE at short, long, and very long muscle lengths within the human quadriceps and patellar tendon (PT) using conventional dynamometry with motion capture (rFETQ) and a new, non-invasive shear-wave tensiometry technique (rFEWS). Eleven healthy male participants performed submaximal (50% max.) EMG-matched fixed-end reference and stretch-hold contractions across these muscle lengths while muscle fascicle length changes of the vastus lateralis (VL) were captured using B-mode ultrasound. We found significant rFETQ at long (7{+/-}5%) and very long (12{+/-}8%) but not short (2{+/-}5%) muscle lengths, whereas rFEWS was only significant at the very long (38{+/-}27%), but not short (8{+/-}12%) or long (6{+/-}10%) muscle lengths. We also found significant relationships between VL fascicle length and rFETQ (r=0.63, p=.001) and rFEWS (r=0.52, p=.017), but relationships were not significant between VL fascicle stretch amplitude and rFETQ (r=0.33, p=.126) or rFEWS (r=0.29, p=.201). PT shear-wave speed-angle relationships did not agree with estimated quadriceps muscle force-angle relationships, which indicates that estimating PT loads from shear-wave tensiometry might be inaccurate. We conclude that increasing muscle length rather than stretch amplitude contributes more to rFE during submaximal voluntary contractions of the human quadriceps.

physiology↗

bioRxiv2021

Connecting neuromodulation, brain development, and life history timing associated with ecological speciation in Rhagoletis pomonella

Host shifts are considered a key generator of insect biodiversity. For insects, adaptation to new host plants often requires changes in larval/pupal development and behavioural preference towards new hosts. Neurochemicals play key roles in both development and behaviour, and therefore provide a potential source for such synchronization. Here, we correlated life history timing, brain development, and corresponding levels of 14 neurochemicals in Rhagoletis pomonella (Diptera: Tephritidae), a species undergoing ecological speciation through an ongoing host shift from hawthorn to apple fruit. These races exhibit differences in pupal diapause timing as well as adult behavioural preference with respect to their hosts. This difference in behavioral preference is coupled with differences in neurophysiological response to host volatiles. We found that apple race pupae exhibited adult brain morphogenesis three weeks faster after an identical simulated winter than the hawthorn race, which correlated with significantly lower titres of several neurochemicals. In some cases, particularly biogenic amines, differences in titres were reflected in the mature adult stage, when host preference is exhibited. In summary, life history timing, neurochemical titre, and brain development can be coupled in this speciating system, providing new hypotheses for the origins of new species through host shifts.

ecology↗

bioRxiv2021

Cardiac and respiratory muscle responses to dietary N-acetylcysteine in rats consuming a high-saturated fat, high-sucrose diet

BACKGROUNDExertional dyspnea is a significant clinical concern in individuals with overweight or obesity. The pathophysiology of dyspnea is multifactorial and complex. Previous data suggest that diaphragm and cardiac abnormalities should be considered as likely contributors to obesity-related exertional dyspnea. Additionally, oxidative stress is a causative factor in the general etiology of obesity as well as skeletal and cardiac muscle pathology. Thus, this preclinical study aimed to define diaphragm and cardiac morphological and functional alterations following an obesogenic diet in rats and the therapeutic potential of an antioxidant supplement, N-acetylcysteine (NAC). METHODSMale Wistar rats ([~]7 weeks old) consumed ad libitum either lean (20% protein, 70% carbohydrate, 10% fat) or high-saturated fat, high-sucrose (HFHS, 20% protein, 35% carbohydrate, 45% fat) diets for [~]22 weeks. Rats receiving HFHS diet were randomized to drink control water or water with NAC (2 mg/ml) for the last eight weeks of the dietary intervention: Lean, HFHS, and HFHS+NAC (n = 8 per group). We evaluated diaphragm bundles (in vitro function and histology) and hearts (weights and echocardiography) for all groups. RESULTSFinal body weights of HFHS rats, but not HFHS+NAC rats, were significantly higher than Lean controls. Neither HFHS diet nor NAC supplementation affected diaphragm specific force (N/cm2), peak power (W/kg), or morphology. In cardiac muscle, right and left ventricle weights (normalized to tibia length) of HFHS rats were greater than those of Lean controls and HFHS+NAC rats. Cardiac functional abnormalities were also present in HFHS rats, with left ventricular fractional shortening (%) and posterior wall maximal shortening velocity (cm/s) increasing compared to Lean controls, but HFHS+NAC rats did not demonstrate these markers of hypercontractility. HFHS rats showed an elevated deceleration rate of early transmitral diastolic velocity (E/DT) consistent with diastolic dysfunction, but NAC eliminated this effect. CONCLUSIONOur data suggest that an HFHS diet does not compromise diaphragm muscle morphology or in vitro function, suggesting other possible contributors to breathing abnormalities in obesity (e.g., neuromuscular transmission abnormalities). However, an HFHS diet resulted in cardiac hypertrophy, hypercontractility, and diastolic dysfunction. Supplementation with NAC did not affect diaphragm morphology or function but attenuated cardiac abnormalities in the HFHS diet. Our findings support future studies testing NAC supplementation in clinical trials of humans with obesity.

physiology↗

bioRxiv2020

Corticospinal drive does not contribute to increased torque production in the presence of residual force enhancement

Following active muscle stretch, muscle force is enhanced, which is known as residual force enhancement (rFE). As earlier studies found apparent corticospinal excitability modulations in the presence of rFE, this study aimed to test whether corticospinal excitability modulations contribute to rFE. Fourteen participants performed submaximal plantar flexion stretch-hold and fixed-end contractions at 30% of their maximal voluntary soleus muscle activity in a dynamometer. During the steady state of the contractions, participants either received subthreshold or suprathreshold transcranial magnetic stimulation (TMS) of their motor cortex, while triceps surae muscle responses to stimulation were obtained via electromyography (EMG), and net ankle joint torque was recorded. B-mode ultrasound imaging was used to confirm muscle fascicle stretch during stretch-hold contractions in a subset of participants. Following stretch of the plantar flexors, an average rFE of 7% and 11% was observed for contractions with subthreshold and suprathreshold TMS, respectively. 41-46 ms following subthreshold TMS, triceps surae muscle activity was suppressed by 19-25%, but suppression was not significantly different between stretch-hold and fixed-end contractions. Similarly, the reduction in plantar flexion torque following subthreshold TMS was not significantly different between contraction conditions. Motor evoked potentials, silent periods and superimposed twitches following suprathreshold TMS were also not significantly different between contraction conditions. As TMS of the motor cortex did not result in any differences between stretch-hold and fixed-end contractions, we conclude that rFE is not linked to changes in corticospinal excitability, making rFE a muscle mechanical property rather than a combined neuro-muscular property.

neuroscience↗

bioRxiv2020

Short-term heritable variation cancels hundreds of generations of mutational variance for metabolic traits in Caenorhabditis elegans

Metabolic disorders have a large heritable component, and have increased over the past few generations. Genome-wide association studies of metabolic traits typically find a substantial unexplained fraction of total heritability, suggesting an important role of spontaneous mutation. An alternative explanation is that epigenetic effects contribute significantly to the heritable variation. Here we report a study designed to quantify the cumulative effects of spontaneous mutation on adenosine metabolism in the nematode Caenorhabditis elegans, including both the activity and concentration of two metabolic enzymes and the standing pools of their associated metabolites. The only prior studies on the effects of mutation on metabolic enzyme activity, in Drosophila melanogaster, found that total enzyme activity presents a mutational target similar to that of morphological and life-history traits. However, those studies were not designed to account for short-term heritable effects. We find that the short-term heritable variance for most traits is of similar magnitude as the variance among MA lines. This result suggests that the potential heritable effects of epigenetic variation in metabolic disease warrant additional scrutiny.

evolutionary biology↗

bioRxiv2019

Triceps surae torque-length relationships relevant for walking activity levels with and without an ankle exoskeleton

AbstractAnkle exoskeletons have been developed to assist walking by offloading the plantar flexors work requirements, which reduces muscle activity level. However, reduced muscle activity alters plantar flexor muscle-tendon unit dynamics in a way that is poorly understood. We therefore evaluated torque-fascicle length properties of the soleus and lateral gastrocnemius during voluntary contractions at simulated activity levels typical during late stance with and without an ankle exoskeleton. Soleus activity levels (100, 30, and 22% maximal voluntary activity) were produced by participants via visual electromyography feedback at ankle angles ranging from -10{degrees} plantar flexion to 35{degrees} dorsiflexion. Using dynamometry and ultrasound imaging, torque-fascicle length data of the soleus and lateral gastrocnemius were produced. The results indicate that muscle activity reductions observed with an exoskeleton shift the torque-angle and torque-fascicle length curves to more dorsiflexed ankle angles and longer fascicle lengths where no descending limb is physiologically possible. This shift is in line with previous simulations that predicted a similar increase in the operating fascicle range when wearing an exoskeleton. These data suggest that a small reduction in muscle activity causes changes to torque-fascicle length properties, which has implications for the design and testing of future ankle exoskeletons for assisted walking.\n\nSignificance StatementAssistive lower-limb exoskeletons reduce the metabolic cost of walking by reducing the positive work requirements of the plantar flexor muscles. However, if the exoskeleton reduces plantar flexor muscle activity too much, then the metabolic benefit is lost. The biological reasons for this are unclear and hinder further exoskeleton development. This research study is the first to directly evaluate if a reduction in plantar flexor muscle activity similar to that caused by wearing an exoskeleton affects muscle function. We found that reduced muscle activity changes the torque-length properties of two plantar flexors, which could explain why reducing muscle activity too much can increase metabolic cost.

physiology↗

bioRxiv2019

3D microenvironment stiffness regulates tumor spheroid growth and mechanics via p21 and ROCK

Mechanical properties of cancer cells and their microenvironment contribute to breast cancer progression. While mechanosensing has been extensively studied using two-dimensional (2D) substrates, much less is known about it in a physiologically more relevant 3D context. Here we demonstrate that breast cancer tumor spheroids, growing in 3D polyethylene glycol-heparin hydrogels, are sensitive to their environment stiffness. During tumor spheroid growth, compressive stresses of up to 2 kPa built up, as quantitated using elastic polymer beads as stress sensors. Atomic force microscopy (AFM) revealed that tumor spheroid stiffness increased with hydrogel stiffness. Also, constituent cell stiffness increased in a ROCK- and F-actin-dependent manner. Increased hydrogel stiffness correlated with attenuated tumor spheroid growth, a higher proportion of cells in G0/G1 phase and elevated levels of the cyclin-dependent kinase inhibitor p21. Drug-mediated ROCK inhibition reversed not only cell stiffening upon culture in stiff hydrogels but also increased tumor spheroid growth. Taken together, we reveal here a mechanism by which the growth of a tumor spheroid can be regulated via cytoskeleton rearrangements in response to its mechanoenvironment. Thus, our findings contribute to a better understanding of how cancer cells react to compressive stress when growing under confinement in stiff environments and provide the basis for a more in-depth exploration of the underlying mechanosensory response.

bioengineering↗

bioRxiv2018

Genetic characterization of chytrids isolated from larval amphibians collected in central and east Texas

Chytridiomycosis, an emerging infectious disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), has caused amphibian population declines worldwide. Bd was first described in the 1990s and there are still geographic gaps in the genetic analysis of this globally distributed pathogen. Relatively few genetic studies have focused on regions where Bd exhibits low virulence, potentially creating a bias in our current knowledge of the pathogens genetic diversity. Disease-associated declines have not been recorded in Texas (USA), yet Bd has been detected on amphibians in the state. These strains have not been isolated and characterized genetically; therefore, we isolated, cultured, and genotyped Bd from central Texas and compared isolates to a panel of previously genotyped strains distributed across the Western Hemisphere. We also isolated other chytrids not known to infect amphibians from east Texas. To identify larval amphibian hosts, we sequenced part of the COI gene. Among 37 Bd isolates from Texas, we detected 19 unique multi-locus genotypes, but found no genetic structure associated with host species, Texas localities, or across North America. Isolates from central Texas exhibit high diversity and genetically cluster with Bd-GPL isolates from the western U.S. that have caused amphibian population declines. This study genetically characterizes isolates of Bd from the south central U.S. and adds to the global knowledge of Bd genotypes.

evolutionary biology↗

bioRxiv2018

An ABCA4 loss-of-function mutation causes a canine form of Stargardt disease

Autosomal recessive retinal degenerative diseases cause visual impairment and blindness in humans and dogs. Currently, no standard treatment is available but pioneering gene therapy-based canine models have been instrumental for clinical trials in humans. To study a novel form of retinal degeneration in Labrador retriever dogs with clinical signs indicating cone and rod degeneration, we used whole-genome sequencing of an affected sib-pair and their unaffected parents. A frameshift insertion in the ATP binding cassette subfamily A member 4 (ABCA4) gene (c.4176insC), leading to a premature stop codon in exon 28 (p.F1393Lfs1395) was identified. In contrast to unaffected dogs, no full-length ABCA4 protein was detected in the retina of an affected dog. The ABCA4 gene encodes a membrane transporter protein localized in the outer segments of rod and cone photoreceptors. In humans, the ABCA4 gene is associated with Stargardt disease (STGD), an autosomal recessive retinal degeneration leading to central visual impairment. A hallmark of STGD is the accumulation of lipofuscin deposits in the retinal pigment epithelium. The discovery of a canine homozygous ABCA4 loss-of-function mutation may advance the development of dog as a large animal model for human STGD.\n\nAuthor summaryStargardt disease (STGD) is the most common inherited retinal disease causing visual impairment and blindness in children and young adults, affecting 1 in 8-10 thousand people. For other inherited retinal diseases, the dog has become an established comparative animal model, both for identifying the underlying genetic causes and for developing new treatment methods.\n\nTo date, there is no standard treatment for STGD and the mouse model is the only available animal model to study the disease. As a nocturnal animal, the morphology of the mouse eye differs from humans and therefore the mouse model is not ideal for developing methods for treatment. We have studied a novel form of retinal degeneration in Labrador retrievers showing clinical signs similar to human STGD. To investigate the genetic cause of the disease, we used whole-genome sequencing of a family quartet including two affected offspring and their unaffected parents. This led to the identification of a loss-of-function mutation in the ABCA4 gene. The findings of this study may enable the development of a canine model for human STGD.

genetics↗