Publications by authors named "Christopher Sundberg"

21 Publications

  • Page 1 of 1

Ca dependency of limb muscle fiber contractile mechanics in young and older adults.

Am J Physiol Cell Physiol 2020 06 29;318(6):C1238-C1251. Epub 2020 Apr 29.

Department of Biological Sciences, Marquette University, Milwaukee, Wisconsin.

Age-induced declines in skeletal muscle contractile function have been attributed to multiple cellular factors, including lower peak force (P), decreased Ca sensitivity, and reduced shortening velocity (V). However, changes in these cellular properties with aging remain unresolved, especially in older women, and the effect of submaximal Ca on contractile function is unknown. Thus, we compared contractile properties of muscle fibers from 19 young (24 ± 3 yr; 8 women) and 21 older adults (77 ± 7 yr; 7 women) under maximal and submaximal Ca and assessed the abundance of three proteins thought to influence Ca sensitivity. Fast fiber cross-sectional area was ~44% larger in young (6,479 ± 2,487 µm) compared with older adults (4,503 ± 2,071 µm, < 0.001), which corresponded with a greater absolute P (young = 1.12 ± 0.43 mN; old = 0.79 ± 0.33 mN, < 0.001). There were no differences in fast fiber size-specific P, indicating the age-related decline in force was explained by differences in fiber size. Except for fast fiber size and absolute P, no age or sex differences were observed in Ca sensitivity, rate of force development (k), or V in either slow or fast fibers. Submaximal Ca depressed k and V, but the effects were not altered by age in either sex. Contrary to rodent studies, regulatory light chain (RLC) and myosin binding protein-C abundance and RLC phosphorylation were unaltered by age or sex. These data suggest the age-associated reductions in contractile function are primarily due to the atrophy of fast fibers and that caution is warranted when extending results from rodent studies to humans.
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http://dx.doi.org/10.1152/ajpcell.00575.2019DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7311743PMC
June 2020

Age-related Deficits in Voluntary Activation: A Systematic Review and Meta-analysis.

Med Sci Sports Exerc 2020 03;52(3):549-560

Alliance for Research in Exercise, Nutrition and Activity, School of Health Sciences, University of South Australia, Adelaide, SA, AUSTRALIA.

Whether there are age-related differences in neural drive during maximal effort contractions is not clear. This review determined the effect of age on voluntary activation during maximal voluntary isometric contractions. The literature was systematically reviewed for studies reporting voluntary activation quantified with the interpolated twitch technique (ITT) or central activation ratio (CAR) during isometric contractions in young (18-35 yr) and old adults (>60 yr; mean, ≥65 yr). Of the 2697 articles identified, 54 were eligible for inclusion in the meta-analysis. Voluntary activation was assessed with electrical stimulation and transcranial magnetic stimulation on five different muscle groups. Random-effects meta-analysis revealed lower activation in old compared with young adults (d = -0.45; 95% confidence interval, -0.62 to -0.29; P < 0.001), with moderate heterogeneity (52.4%). To uncover the sources of heterogeneity, subgroup analyses were conducted for muscle group, calculation method (ITT or CAR), and stimulation type (electrical stimulation or transcranial magnetic stimulation) and number (single, paired, or train stimulations). The age-related reduction in voluntary activation occurred for all muscle groups investigated except the ankle dorsiflexors. Both ITT and CAR demonstrated an age-related reduction in voluntary activation of the elbow flexors, knee extensors, and plantar flexors. ITT performed with paired and train stimulations showed lower activation for old than young adults, with no age difference for the single electrical stimulation. Together, the meta-analysis revealed that healthy older adults have a reduced capacity to activate some upper and lower limb muscles during maximal voluntary isometric contractions; however, the effect was modest and best assessed with at least paired stimulations to detect the difference.
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http://dx.doi.org/10.1249/MSS.0000000000002179DOI Listing
March 2020

Bioenergetic basis of skeletal muscle fatigue.

Curr Opin Physiol 2019 Aug 10;10:118-127. Epub 2019 May 10.

Department of Biological Sciences, Marquette University, Milwaukee, WI.

Energetic demand from high-intensity exercise can easily exceed ATP synthesis rates of mitochondria leading to a reliance on anaerobic metabolism. The reliance on anaerobic metabolism results in the accumulation of intracellular metabolites, namely inorganic phosphate (P) and hydrogen (H), that are closely associated with exercise-induced reductions in power. Cellular and molecular studies have revealed several steps where these metabolites impair contractile function demonstrating a causal role in fatigue. Elevated P or H directly inhibits force and power of the cross-bridge and decreases myofibrillar Ca sensitivity, whereas P also inhibits Ca release from the sarcoplasmic reticulum (SR). When both metabolites are elevated, they act synergistically to cause marked reductions in power, indicating that fatigue during high-intensity exercise has a bioenergetic basis.
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http://dx.doi.org/10.1016/j.cophys.2019.05.004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6656370PMC
August 2019

Differential effects of aging and physical activity on corticospinal excitability of upper and lower limb muscles.

J Neurophysiol 2019 07 15;122(1):241-250. Epub 2019 May 15.

Exercise Science Program, Department of Physical Therapy, Marquette University , Milwaukee, Wisconsin.

Corticospinal tract excitability can be altered by age, physical activity (PA), and possibly sex, but whether these effects differ between upper and lower limb muscles is unknown. We determined the influence of age, PA, and sex on corticospinal excitability of an upper limb and a lower limb muscle during submaximal contractions by comparing stimulus-response curves of motor evoked potentials (MEPs). Transcranial magnetic stimulation (TMS) was used to evoke stimulus-response curves in active muscles by incrementally increasing the stimulator intensity from below the active motor threshold (AMT) until a plateau in MEP amplitudes was achieved. Stimulus-response curves were analyzed from the first dorsal interosseous (FDI) of 30 young (23.9 ± 3.8 yr) and 33 older (72.6 ± 5.6 yr) men and women and the vastus lateralis (VL) of 13 young (23.2 ± 2.2 yr) and 25 older (72.7 ± 5.5 yr) men and women. Corticospinal excitability was determined by fitting the curves with a four-parameter sigmoidal curve and calculating the maximal slope (slope). PA was assessed with triaxial accelerometry, and participants were dichotomized into high-PA (>10,000 steps/day, = 15) or low-PA (<10,000 steps/day, = 43) groups. Young adults had larger FDI MEP amplitudes (% maximum amplitude of compound muscle action potential) at higher TMS intensities (120-150% AMT) and greater slope than older adults ( < 0.05), with no differences between high- and low-PA groups ( > 0.05). VL MEP amplitudes and slope, however, were lower in the high-PA than low-PA participants, with no age or sex differences. These data suggest that aging and PA, but not sex, differentially influence the excitability of the corticospinal tracts projecting to muscles of the upper compared with the lower limb. Excitability of the corticospinal tract projecting to the first dorsal interosseous assessed with transcranial magnetic stimulation was reduced with age but independent of regular physical activity (steps/day) and sex of the individual. In contrast, corticospinal excitability of the vastus lateralis was not affected by age but was reduced in individuals achieving more than the physical activity recommendations of 10,000 steps/day. Aging and activity differentially affect corticospinal excitability of upper and lower limb muscles.
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http://dx.doi.org/10.1152/jn.00077.2019DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6689774PMC
July 2019

A CRISPR/Cas9 Whole-Genome Screen Identifies Genes Required for Aryl Hydrocarbon Receptor-Dependent Induction of Functional CYP1A1.

Toxicol Sci 2019 08;170(2):310-319

Department of Pathology and Laboratory Medicine, and Molecular Toxicology Program, University of California Los Angeles, Los Angeles, California 90095.

Environmental pollutants including halogenated aromatic hydrocarbons and polycyclic aromatic hydrocarbons, including benzo[a]pyrene, exert their deleterious effects through the activation of the aryl hydrocarbon receptor (AHR) and by the resulting transcription of genes not yet fully identified. Ligand-bound AHR translocates from cytoplasm to nucleus, where it dimerizes with the aryl hydrocarbon receptor nuclear translocator (ARNT) protein. The AHR/ARNT dimer binds to enhancer regions of responsive genes to activate transcription. AHR also mediates carcinogenesis caused by PAHs, likely via CYP1A1, CYP1A2, and CYP1B1, which are massively induced by activated AHR in many tissues and generate carcinogenic electrophilic derivatives of PAHs. In the current study, we have used the mouse GeCKOv2 genome-wide CRISPR/Cas9 library to identify novel genes in the AHR pathway by taking advantage of a B[a]P selection assay that we previously used to identify core AHR pathway genes in Hepa-1c1c7 murine hepatoma cells. Besides Ahr, Arnt, and Cyp1a1, we report the identification of multiple additional putative AHR pathway genes including several that we validated. These include cytochrome P450 reductase (Por), which mediates redox regeneration of cytochromes P450, and 5 genes of the heme biosynthesis pathway: delta-aminolevulinate synthase 1 (Alas1), porphobilinogen deaminase (Hmbs), uroporphyrinogen decarboxylase (Urod), coproporphyrinogen oxidase (Cpox), and ferrochelatase (Fech): heme being an essential prosthetic group of cytochrome P450 proteins. Notably, several of these genes were identified by GeCKO screening, despite not being identifiable by reverse genetics approaches. This indicates the power of high-sensitivity genome-wide genetic screening for identifying genes in the AHR pathway.
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http://dx.doi.org/10.1093/toxsci/kfz111DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6657581PMC
August 2019

Bioenergetic basis for the increased fatigability with ageing.

J Physiol 2019 10 20;597(19):4943-4957. Epub 2019 May 20.

Department of Physical Therapy, Marquette University, Milwaukee, WI, USA.

Key Points: The mechanisms for the age-related increase in fatigability during dynamic exercise remain elusive. We tested whether age-related impairments in muscle oxidative capacity would result in a greater accumulation of fatigue causing metabolites, inorganic phosphate (P ), hydrogen (H ) and diprotonated phosphate (H PO ), in the muscle of old compared to young adults during a dynamic knee extension exercise. The age-related increase in fatigability (reduction in mechanical power) of the knee extensors was closely associated with a greater accumulation of metabolites within the working muscle but could not be explained by age-related differences in muscle oxidative capacity. These data suggest that the increased fatigability in old adults during dynamic exercise is primarily determined by age-related impairments in skeletal muscle bioenergetics that result in a greater accumulation of metabolites.

Abstract: The present study aimed to determine whether the increased fatigability in old adults during dynamic exercise is associated with age-related differences in skeletal muscle bioenergetics. Phosphorus nuclear magnetic resonance spectroscopy was used to quantify concentrations of high-energy phosphates and pH in the knee extensors of seven young (22.7 ± 1.2 years; six women) and eight old adults (76.4 ± 6.0 years; seven women). Muscle oxidative capacity was measured from the phosphocreatine (PCr) recovery kinetics following a 24 s maximal voluntary isometric contraction. The fatiguing exercise consisted of 120 maximal velocity contractions (one contraction per 2 s) against a load equivalent to 20% of the maximal voluntary isometric contraction. The PCr recovery kinetics did not differ between young and old adults (0.023 ± 0.007 s vs. 0.019 ± 0.004 s , respectively). Fatigability (reductions in mechanical power) of the knee extensors was ∼1.8-fold greater with age and was accompanied by a greater decrease in pH (young = 6.73 ± 0.09, old = 6.61 ± 0.04) and increases in concentrations of inorganic phosphate, [P ], (young = 22.7 ± 4.8 mm, old = 32.3 ± 3.6 mm) and diprotonated phosphate, [H PO ], (young = 11.7 ± 3.6 mm, old = 18.6 ± 2.1 mm) at the end of the exercise in old compared to young adults. The age-related increase in power loss during the fatiguing exercise was strongly associated with intracellular pH (r = -0.837), [P ] (r = 0.917) and [H PO ] (r = 0.930) at the end of the exercise. These data suggest that the age-related increase in fatigability during dynamic exercise has a bioenergetic basis and is explained by an increased accumulation of metabolites within the muscle.
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http://dx.doi.org/10.1113/JP277803DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7205544PMC
October 2019

Effects of elevated H and P on the contractile mechanics of skeletal muscle fibres from young and old men: implications for muscle fatigue in humans.

J Physiol 2018 09 1;596(17):3993-4015. Epub 2018 Jul 1.

Department of Biological Sciences, Marquette University, Milwaukee, WI, USA.

Key Points: The mechanisms responsible for the loss in muscle power and increased fatigability with ageing are unresolved. We show that the contractile mechanics of fibres from the vastus lateralis of old men were well-preserved compared to those of young men, but the selective loss of fast myosin heavy chain II muscle was strongly associated with age-related decrements in whole-muscle strength and power. We reveal that the combination of acidosis (H ) and inorganic phosphate (P ) is an important mediator of muscle fatigue in humans by inhibiting the low- to high-force state of the cross-bridge cycle and peak power, but the depressive effects of these ions on cross-bridge function were similar in fibres from young and old men. These findings suggest that the age-related loss in muscle power is primarily determined by the atrophy of fast fibres, but the age-related increased fatigability cannot be explained by an increased sensitivity of the cross-bridge to H and P .

Abstract: The present study aimed to identify the mechanisms responsible for the loss in muscle power and increased fatigability with ageing by integrating measures of whole-muscle function with single fibre contractile mechanics. After adjusting for the 22% smaller muscle mass in old (73-89 years, n = 6) compared to young men (20-29 years, n = 6), isometric torque and power output of the knee extensors were, respectively, 38% and 53% lower with age. Fatigability was ∼2.7-fold greater with age and strongly associated with reductions in the electrically-evoked contractile properties. To test whether cross-bridge mechanisms could explain age-related decrements in knee extensor function, we exposed myofibres (n = 254) from the vastus lateralis to conditions mimicking quiescent muscle and fatiguing levels of acidosis (H ) (pH 6.2) and inorganic phosphate (P ) (30 mm). The fatigue-mimicking condition caused marked reductions in force, shortening velocity and power and inhibited the low- to high-force state of the cross-bridge cycle, confirming findings from non-human studies that these ions act synergistically to impair cross-bridge function. Other than severe age-related atrophy of fast fibres (-55%), contractile function and the depressive effects of the fatigue-mimicking condition did not differ in fibres from young and old men. The selective loss of fast myosin heavy chain II muscle was strongly associated with the age-related decrease in isometric torque (r = 0.785) and power (r = 0.861). These data suggest that the age-related loss in muscle strength and power are primarily determined by the atrophy of fast fibres, but the age-related increased fatigability cannot be explained by an increased sensitivity of the cross-bridge to H and P .
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http://dx.doi.org/10.1113/JP276018DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6117549PMC
September 2018

Mechanisms for the age-related increase in fatigability of the knee extensors in old and very old adults.

J Appl Physiol (1985) 2018 07 1;125(1):146-158. Epub 2018 Mar 1.

Exercise Science Program, Department of Physical Therapy, Marquette University , Milwaukee, Wisconsin.

The mechanisms for the age-related increase in fatigability during high-velocity contractions in old and very old adults (≥80 yr) are unresolved. Moreover, whether the increased fatigability with advancing age and the underlying mechanisms differ between men and women is not known. The purpose of this study was to quantify the fatigability of knee extensor muscles and identify the mechanisms of fatigue in 30 young (22.6 ± 0.4 yr; 15 men), 62 old (70.5 ± 0.7 yr; 33 men), and 12 very old (86.0 ± 1.3 yr; 6 men) men and women elicited by high-velocity concentric contractions. Participants performed 80 maximal velocity contractions (1 contraction per 3 s) with a load equivalent to 20% of the maximum voluntary isometric contraction. Voluntary activation and contractile properties were quantified before and immediately following exercise (<10 s) using transcranial magnetic stimulation and electrical stimulation. Absolute mechanical power output was 97 and 217% higher in the young compared with old and very old adults, respectively. Fatigability (reductions in power) progressively increased across age groups, with a power loss of 17% in young, 31% in old, and 44% in very old adults. There were no sex differences in fatigability among any of the age groups. The age-related increase in power loss was strongly associated with changes in the involuntary twitch amplitude ( r = 0.75, P < 0.001). These data suggest that the age-related increased power loss during high-velocity fatiguing exercise is unaffected by biological sex and determined primarily by mechanisms that disrupt excitation contraction coupling and/or cross-bridge function. NEW & NOTEWORTHY We show that aging of the neuromuscular system results in an increase in fatigability of the knee extensors during high-velocity exercise that is more pronounced in very old adults (≥80 yr) and occurs similarly in men and women. Importantly, the age-related increase in power loss was strongly associated with the changes in the electrically evoked contractile properties suggesting that the increased fatigability with aging is determined primarily by mechanisms within the muscle for both sexes.
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http://dx.doi.org/10.1152/japplphysiol.01141.2017DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6086970PMC
July 2018

Predicting human ageing with Masters athletics: 'one size doesn't fit all'.

J Physiol 2017 11 15;595(22):6813-6815. Epub 2017 Oct 15.

Exercise Science Program, Marquette University, Milwaukee, WI, USA.

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http://dx.doi.org/10.1113/JP274864DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5685818PMC
November 2017

Physical activity modulates corticospinal excitability of the lower limb in young and old adults.

J Appl Physiol (1985) 2017 Aug 11;123(2):364-374. Epub 2017 May 11.

Exercise Science Program, Department of Physical Therapy, Marquette University, Milwaukee, Wisconsin;

Aging is associated with reduced neuromuscular function, which may be due in part to altered corticospinal excitability. Regular physical activity (PA) may ameliorate these age-related declines, but the influence of PA on corticospinal excitability is unknown. The purpose of this study was to determine the influence of age, sex, and PA on corticospinal excitability by comparing the stimulus-response curves of motor evoked potentials (MEP) in 28 young (22.4 ± 2.2 yr; 14 women and 14 men) and 50 old adults (70.2 ± 6.1 yr; 22 women and 28 men) who varied in activity levels. Transcranial magnetic stimulation was used to elicit MEPs in the active vastus lateralis muscle (10% maximal voluntary contraction) with 5% increments in stimulator intensity until the maximum MEP amplitude. Stimulus-response curves of MEP amplitudes were fit with a four-parameter sigmoidal curve and the maximal slope calculated (slope). Habitual PA was assessed with tri-axial accelerometry and participants categorized into either those meeting the recommended PA guidelines for optimal health benefits (>10,000 steps/day, high-PA; = 21) or those not meeting the guidelines (<10,000 steps/day, low-PA; = 41). The MEP amplitudes and slope were greater in the low-PA compared with the high-PA group ( < 0.05). Neither age nor sex influenced the stimulus-response curve parameters ( > 0.05), suggesting that habitual PA influenced the excitability of the corticospinal tract projecting to the lower limb similarly in both young and old adults. These findings provide evidence that achieving the recommended PA guidelines for optimal health may mediate its effects on the nervous system by decreasing corticospinal excitability. Transcranial magnetic stimulation was used to determine whether achieving the recommended 10,000 steps/day for optimal health influenced the excitability of the corticospinal tract projecting to the knee extensor muscles. Irrespective of age and sex, individuals who achieved >10,000 steps/day had lower corticospinal excitability than those who performed <10,000 steps/day, possibly representing greater control of inhibitory and excitatory networks. Physical activity involving >10,000 steps/day may mediate its effects on the nervous system by decreasing corticospinal excitability.
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http://dx.doi.org/10.1152/japplphysiol.01078.2016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5614787PMC
August 2017

VISUAL FEEDBACK ARRAY TO ACHIEVE REPRODUCIBLE LIMB DISPLACEMENTS AND VELOCITIES IN HUMANS.

Biomed Sci Instrum 2017 Mar-Apr;53:100-105

Department of Physical Therapy, Marquette University, Milwaukee, WI 53233.

Precise and reproducible feedback is important for studies on motor control, the adaptive responses to exercise training and the limits of human neuromuscular performance. For this purpose, a custom instrumentation array was previously developed to provide human subjects with visual feedback on their contractile durations and limb velocities during knee extension exercise. The array consisted of two columns, each with 14 high-visibility light emitting diodes. One array column provided the subject with the target cadence of position change while the complementary array provided an indication of the actual rate of displacement achieved by the subject. The array design has been improved to include an onboard microcontroller-based signal generator to generate the desired protocol cadence and also the ability to easily customize and 'lock in' desired protocol parameters. The array allows for custom settings to provide feedback on the concentric and eccentric activation periods during exercise. It can also be used in clinical settings to track the active or passive ranges of motion during rehabilitation.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6086354PMC
August 2018

Rates of performance loss and neuromuscular activity in men and women during cycling: evidence for a common metabolic basis of muscle fatigue.

J Appl Physiol (1985) 2017 01 17;122(1):130-141. Epub 2016 Nov 17.

Biomechanics Laboratory, Departments of Health and Human Performance and Organismal Biology and Ecology, University of Montana, Missoula, Montana; and.

The durations that muscular force and power outputs can be sustained until failure fall predictably on an exponential decline between an individual's 3-s burst maximum to the maximum performance they can sustain aerobically. The exponential time constants describing these rates of performance loss are similar across individuals, suggesting that a common metabolically based mechanism governs muscle fatigue; however, these conclusions come from studies mainly on men. To test whether the same physiological understanding can be applied to women, we compared the performance-duration relationships and neuromuscular activity between seven men [23.3 ± 1.9 (SD) yr] and seven women (21.7 ± 1.8 yr) from multiple exhaustive bouts of cycle ergometry. Each subject performed trials to obtain the peak 3-s power output (P), the mechanical power at the aerobic maximum (P), and 11-14 constant-load bouts eliciting failure between 3 and 300 s. Collectively, men and women performed 180 exhaustive bouts spanning an ~6-fold range of power outputs (118-1116 W) and an ~35-fold range of trial durations (8-283 s). Men generated 66% greater P (956 ± 109 W vs. 632 ± 74 W) and 68% greater P (310 ± 47 W vs. 212 ± 15 W) than women. However, the metabolically based time constants describing the time course of performance loss were similar between men (0.020 ± 0.003/s) and women (0.021 ± 0.003/s). Additionally, the fatigue-induced increases in neuromuscular activity did not differ between the sexes when compared relative to the pedal forces at P These data suggest that muscle fatigue during short-duration dynamic exercise has a common metabolically based mechanism determined by the extent that ATP is resynthesized by anaerobic metabolism.

New & Noteworthy: Although men and women differed considerably in their absolute cycling performances, there was no sex difference in the metabolically based exponential time constant that described the performance-duration relationship. Similarly, the fatigue-induced increases in neuromuscular activity were not different between the sexes when compared from a metabolic perspective. These data suggest that men and women have similar rate-limiting mechanisms for short-duration dynamic exercise that are determined by the extent the exercise is supported by anaerobic metabolism.
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http://dx.doi.org/10.1152/japplphysiol.00468.2016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5504436PMC
January 2017

Muscle Fatigue from the Perspective of a Single Crossbridge.

Med Sci Sports Exerc 2016 11;48(11):2270-2280

1Department of Kinesiology, University of Massachusetts, Amherst, MA; 2Department of Biological Sciences, Marquette University, Milwaukee, WI; 3Department of Physical Therapy, Marquette University, Milwaukee, WI; and 4Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH.

The repeated intense stimulation of skeletal muscle rapidly decreases its force- and motion-generating capacity. This type of fatigue can be temporally correlated with the accumulation of metabolic by-products, including phosphate (Pi) and protons (H). Experiments on skinned single muscle fibers demonstrate that elevated concentrations of these ions can reduce maximal isometric force, unloaded shortening velocity, and peak power, providing strong evidence for a causative role in the fatigue process. This seems to be due, in part, to their direct effect on muscle's molecular motor, myosin, because in assays using isolated proteins, these ions directly inhibit myosin's ability to move actin. Indeed, recent work using a single molecule laser trap assay has revealed the specific steps in the crossbridge cycle affected by these ions. In addition to their direct effects, these ions also indirectly affect myosin by decreasing the sensitivity of the myofilaments to calcium, primarily by altering the ability of the muscle regulatory proteins, troponin and tropomyosin, to govern myosin binding to actin. This effect seems to be partially due to fatigue-dependent alterations in the structure and function of specific subunits of troponin. Parallel efforts to understand the molecular basis of muscle contraction are providing new technological approaches that will allow us to gain unprecedented molecular detail of the fatigue process. This will be crucial to fully understand this ubiquitous phenomenon and develop appropriately targeted therapies to attenuate the debilitating effects of fatigue in clinical populations.
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http://dx.doi.org/10.1249/MSS.0000000000001047DOI Listing
November 2016

Allosteric heat shock protein 70 inhibitors block hepatitis C virus assembly.

Int J Antimicrob Agents 2016 Apr 2;47(4):289-96. Epub 2016 Mar 2.

Department of Pathology and Laboratory Medicine, David Geffen School of Medicine at University of California, Los Angeles (UCLA), Los Angeles, CA, USA; Jonsson Comprehensive Cancer Center, David Geffen School of Medicine at University of California, Los Angeles (UCLA), Los Angeles, CA, USA; UCLA AIDS Institute, David Geffen School of Medicine at University of California, Los Angeles (UCLA), Los Angeles, CA, USA. Electronic address:

The human molecular chaperones heat shock protein 70 (Hsp70) and heat shock cognate protein 70 (Hsc70) bind to the hepatitis C viral nonstructural protein 5A (NS5A) and regulate its activity. Specifically, Hsp70 is involved in NS5A-augmented internal ribosomal entry site (IRES)-mediated translation of the viral genome, whilst Hsc70 appears to be primarily important for intracellular infectious virion assembly. To better understand the importance of these two chaperones in the viral life cycle, infected human cells were treated with allosteric Hsp70/Hsc70 inhibitors (AHIs). Treatment with AHIs significantly reduced the production of intracellular virus at concentrations that were non-toxic to human hepatoma Huh7.5 cells. The supernatant of treated cultures was then used to infect naïve cells, revealing that AHIs also lowered levels of secreted virus. In contrast to their effects on virion assembly, AHIs did not impact the stability of NS5A or viral protein translation in IRES assays. These results suggest that Hsc70 plays a particularly important and sensitive role in virion assembly. Indeed, it was found that combination of AHIs with a peptide-based viral translation inhibitor exhibited additive antiviral activity. Together these results suggest that the host Hsc70 is a new antiviral target and that its inhibitors utilise a new mechanism of action.
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http://dx.doi.org/10.1016/j.ijantimicag.2016.01.012DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4833571PMC
April 2016

Influence of duty cycle on the time course of muscle fatigue and the onset of neuromuscular compensation during exhaustive dynamic isolated limb exercise.

Am J Physiol Regul Integr Comp Physiol 2015 Jul 15;309(1):R51-61. Epub 2015 Apr 15.

Biomechanics Laboratory, Departments of Health and Human Performance and Organismal Biology and Ecology, University of Montana, Missoula, Montana

We investigated the influence of altered muscle duty cycle on the performance decrements and neuromuscular responses occurring during constant-load, fatiguing bouts of knee extension exercise. We experimentally altered the durations of the muscularly inactive portion of the limb movement cycle and hypothesized that greater relative durations of inactivity within the same movement task would 1) reduce the rates and extent of muscle performance loss and 2) increase the forces necessary to trigger muscle fatigue. In each condition (duty cycle = 0.6 and 0.3), male subjects [age = 25.9 ± 2.0 yr (SE); mass = 85.4 ± 2.6 kg], completed 9-11 exhaustive bouts of two-legged knee extension exercise, at force outputs that elicited failure between 4 and 290 s. The novel duty cycle manipulation produced two primary results; first, we observed twofold differences in both the extent of muscle performance lost (DC0.6 = 761 ± 35 N vs. DC0.3 = 366 ± 49 N) and the time course of performance loss. For example, exhaustive trials at the midpoint of these force ranges differed in duration by more than 30 s (t0.6 = 36 ± 2.6 vs. t0.3 = 67 ± 4.3 s). Second, both the minimum forces necessary to exceed the peak aerobic capacity and initiate a reliance on anaerobic metabolism, and the forces necessary to elicit compensatory increases in electromyogram activity were 300% greater in the lower vs. higher duty cycle condition. These results indicate that the fatigue-induced compensatory behavior to recruit additional motor units is triggered by a reliance on anaerobic metabolism for ATP resynthesis and is independent of the absolute level or fraction of the maximum force produced by the muscle.
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http://dx.doi.org/10.1152/ajpregu.00356.2014DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4491536PMC
July 2015

Structural characterization of the HSP70 interaction domain of the hepatitis C viral protein NS5A.

Virology 2015 Jan 25;475:46-55. Epub 2014 Nov 25.

Department of Pathology and Laboratory Medicine, David Geffen School of Medicine at University of California, Los Angeles, California, United States of America.

We previously identified the NS5A/HSP70 binding site to be a hairpin moiety at C-terminus of NS5A domain I and showed a corresponding cyclized polyarginine-tagged synthetic peptide (HCV4) significantly blocks virus production. Here, sequence comparison confirmed five residues to be conserved. Based on NS5A domain I crystal structure, Phe171, Val173, and Tyr178 were predicted to form the binding interface. Substitution of Phe171 and Val173 with more hydrophobic unusual amino acids improved peptide antiviral activity and HSP70 binding, while similar substitutions at Tyr178 had a negative effect. Substitution of non-conserved residues with arginines maintained antiviral activity and HSP70 binding and dispensed with polyarginine tag for cellular entry. Peptide cyclization improved antiviral activity and HSP70 binding. The cyclic retro-inverso analog displayed the best antiviral properties. FTIR spectroscopy confirmed a secondary structure consisting of an N-terminal beta-sheet followed by a turn and a C-terminal beta-sheet. These peptides constitute a new class of anti-HCV compounds.
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http://dx.doi.org/10.1016/j.virol.2014.10.011DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4284078PMC
January 2015

The NS5A-binding heat shock proteins HSC70 and HSP70 play distinct roles in the hepatitis C viral life cycle.

Virology 2014 Apr 28;454-455:118-27. Epub 2014 Feb 28.

Department of Pathology and Laboratory Medicine, David Geffen School of Medicine at University of California, Los Angeles, CA, United States of America; Jonsson Comprehensive Cancer Center, David Geffen School of Medicine at University of California, Los Angeles, CA, United States of America; UCLA AIDS Institute, David Geffen School of Medicine at University of California, Los Angeles, CA, United States of America. Electronic address:

We previously identified HSP70 and HSC70 in complex with NS5A in a proteomic screen. Here, coimmunoprecipitation studies confirmed NS5A/HSC70 complex formation during infection, and immunofluorescence studies showed NS5A and HSC70 to colocalize. Unlike HSP70, HSC70 knockdown did not decrease viral protein levels. Rather, intracellular infectious virion assembly was significantly impaired by HSC70 knockdown. We also discovered that both HSC70 nucleotide binding and substrate binding domains directly bind NS5A whereas only the HSP70 nucleotide binding domain does. Knockdown of both HSC70 and HSP70 demonstrated an additive reduction in virus production. This data suggests that HSC70 and HSP70 play discrete roles in the viral life cycle. Investigation of these different functions may facilitate developing of novel strategies that target host proteins to treat HCV infection.
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http://dx.doi.org/10.1016/j.virol.2014.02.016DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4011396PMC
April 2014

Structure and identification of a pterin dehydratase-like protein as a ribulose-bisphosphate carboxylase/oxygenase (RuBisCO) assembly factor in the α-carboxysome.

J Biol Chem 2014 Mar 23;289(11):7973-81. Epub 2014 Jan 23.

From the Molecular Biology Interdepartmental Ph.D. Program and.

Carboxysomes are proteinaceous bacterial microcompartments that increase the efficiency of the rate-limiting step in carbon fixation by sequestering reaction substrates. Typically, α-carboxysomes are genetically encoded as a single operon expressing the structural proteins and the encapsulated enzymes of the microcompartment. In addition, depending on phylogeny, as many as 13 other genes are found to co-occur near or within α-carboxysome operons. One of these genes codes for a protein with distant homology to pterin-4α-carbinolamine dehydratase (PCD) enzymes. It is present in all α-carboxysome containing bacteria and has homologs in algae and higher plants. Canonical PCDs play an important role in amino acid hydroxylation, a reaction not associated with carbon fixation. We determined the crystal structure of an α-carboxysome PCD-like protein from the chemoautotrophic bacterium Thiomonas intermedia K12, at 1.3-Å resolution. The protein retains a three-dimensional fold similar to canonical PCDs, although the prominent active site cleft present in PCD enzymes is disrupted in the α-carboxysome PCD-like protein. Using a cell-based complementation assay, we tested the PCD-like proteins from T. intermedia and two additional bacteria, and found no evidence for PCD enzymatic activity. However, we discovered that heterologous co-expression of the PCD-like protein from Halothiobacillus neapolitanus with RuBisCO and GroELS in Escherichia coli increased the amount of soluble, assembled RuBisCO recovered from cell lysates compared with co-expression of RuBisCO with GroELS alone. We conclude that this conserved PCD-like protein, renamed here α-carboxysome RuBisCO assembly factor (or acRAF), is a novel RuBisCO chaperone integral to α-carboxysome function.
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http://dx.doi.org/10.1074/jbc.M113.531236DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3953307PMC
March 2014

Gliopathic pain: when satellite glial cells go bad.

Neuroscientist 2009 Oct;15(5):450-63

Department of Anatomy, University of California, San Francisco, California 95143-0452, USA.

Neurons in sensory ganglia are surrounded by satellite glial cells (SGCs) that perform similar functions to the glia found in the CNS. When primary sensory neurons are injured, the surrounding SGCs undergo characteristic changes. There is good evidence that the SGCs are not just bystanders to the injury but play an active role in the initiation and maintenance of neuronal changes that underlie neuropathic pain. In this article the authors review the literature on the relationship between SGCs and nociception and present evidence that changes in SGC potassium ion buffering capacity and glutamate recycling can lead to neuropathic pain-like behavior in animal models. The role that SGCs play in the immune responses to injury is also considered. We propose the term gliopathic pain to describe those conditions in which central or peripheral glia are thought to be the principal generators of principal pain generators.
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http://dx.doi.org/10.1177/1073858409336094DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2852320PMC
October 2009

Adenovector GAD65 gene delivery into the rat trigeminal ganglion produces orofacial analgesia.

Mol Pain 2009 Aug 5;5:42. Epub 2009 Aug 5.

Department of Anatomy, University of California San Francisco, San Francisco, CA 94143, USA.

Background: Our goal is to use gene therapy to alleviate pain by targeting glial cells. In an animal model of facial pain we tested the effect of transfecting the glutamic acid decarboxylase (GAD) gene into satellite glial cells (SGCs) of the trigeminal ganglion by using a serotype 5 adenovector with high tropisms for glial cells. We postulated that GABA produced from the expression of GAD would reduce pain behavior by acting on GABA receptors on neurons within the ganglion.

Results: Injection of adenoviral vectors (AdGAD65) directly into the trigeminal ganglion leads to sustained expression of the GAD65 isoform over the 4 weeks observation period. Immunohistochemical analysis showed that adenovirus-mediated GAD65 expression and GABA synthesis were mainly in SGCs. GABAA and GABAB receptors were both seen in sensory neurons, yet only GABAA receptors decorated the neuronal surface. GABA receptors were not found on SGCs. Six days after injection of AdGAD65 into the trigeminal ganglion, there was a statistically significant decrease of pain behavior in the orofacial formalin test, a model of inflammatory pain. Rats injected with control virus (AdGFP or AdLacZ) had no reduction in their pain behavior. AdGAD65-dependent analgesia was blocked by bicuculline, a selective GABAA receptor antagonist, but not by CGP46381, a selective GABAB receptor antagonist.

Conclusion: Transfection of glial cells in the trigeminal ganglion with the GAD gene blocks pain behavior by acting on GABAA receptors on neuronal perikarya.
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http://dx.doi.org/10.1186/1744-8069-5-42DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2734545PMC
August 2009

Sustained loss of a neoplastic phenotype by brief inactivation of MYC.

Science 2002 Jul;297(5578):102-4

Division of Oncology, Departments of Medicine and Pathology, Stanford University, Stanford, CA 94305-5151, USA.

Pharmacological inactivation of oncogenes is being investigated as a possible therapeutic strategy for cancer. One potential drawback is that cessation of such therapy may allow reactivation of the oncogene and tumor regrowth. We used a conditional transgenic mouse model for MYC-induced tumorigenesis to demonstrate that brief inactivation of MYC results in the sustained regression of tumors and the differentiation of osteogenic sarcoma cells into mature osteocytes. Subsequent reactivation of MYC did not restore the cells' malignant properties but instead induced apoptosis. Thus, brief MYC inactivation appears to cause epigenetic changes in tumor cells that render them insensitive to MYC-induced tumorigenesis. These results raise the possibility that transient inactivation of MYC may be an effective therapy for certain cancers.
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http://dx.doi.org/10.1126/science.1071489DOI Listing
July 2002