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Research Highlights

 

 
 
           

Research Highlight #2

Lazy or Old: Mitochondria decline in vivo in human muscle similarly with inactivity and age.

   

Research Highlight #3

Compensation for mitochondrial dysfunction preserves energy homeostasis in muscle of individuals with pre-manifest HD

   

 

 

 
Research highlight - oxidative stress
           

Research Highlight #5

Mitochondria co-opt exercise adaptations in defense against oxidative stress in vivo.

   
   
 

 


       
 
 

Research Highlight #1

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Energetics of positive and negative work in vivo: an innovative apparatus for human muscle studies.

Here we present an innovative apparatus that provides the rigor of in vitro muscle mechanical measurements for in vivo measurement of the cost of contraction in human muscle. A simple lever linked to a pulley system 1) eliminates the release of elastic energy, 2) permits generating both positive and negative work and 3) sits in an MRI for simultaneous determination of muscle work and ATP flux.  Our measurements reject the notion that elastic recoil is responsible for the high economy of eccentric contractions.  Instead, they reveal that reduced cost underlies the greater economy of eccentric vs. concentric contractions.

Frank E. Nelson, University of Washington, Dept. of Radiology
Justus O. Ortega, University of Washington, Dept. of Radiology
Stan L. Lindstedt, University of Washington, Dept. of Radiology
Sharon A. Jubrias, University of Washington, Dept. of Radiology
Kevin E. Conley, University of Washington, Depts. of Radiology, Physiology & Biophysics, and Bioengineering

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Research Highlight #2

 

Lazy or Old: Mitochondria decline in vivo in human muscle similarly with inactivity and age.

Is inactivity or age responsible for the mitochondrial decline found in elderly humans?  We tested the role of lifestyle vs. age using innovative noninvasive tools to measure key mitochondrial functional properties in vivo.  Mitochondrial coupling (ATP/O2 or P/O), and phosphorylation capacity (ATPmax) were assessed in human vastus lateralis muscle (VL). Oxidative capacity was biochemically assessed in vitro from biopsies taken from the same site as the in vivo measures.  The chronic activity level was characterized in all subjects using a physical activity questionnaire and a VO2max test (active >40 ml O2 kg-1 min-1).  The assessed level was verified in a subset of subjects using a triaxial accelerometer.  Subjects were screened to be healthy and disease free.  The active (n=9) and sedentary (n=18) groups had a mean age of 30 yrs as compared to a mean age of 68 yrs for elderly subjects (n=30).  Mitochondrial coupling in active controls (2.3+0.20, mean+SE) corresponded with that in well-coupled mitochondria (2.3-2.5) but both sedentary (P/O=1.41+0.09) and elderly groups (P/O=1.69+0.11) were substantially uncoupled.  Similarly, active subjects had a higher ATPmax (1.11+0.08 mM ATP sec-1) than either sedentary (0.67+0.03) or aged groups (0.61+0.04).  Oxidative capacity was reduced in both groups by 25% relative to active adults.  These results demonstrate that mitochondrial properties in vivo drop substantially in sedentary adults to the level found in elderly subjects.  Thus inactivity as much as age accounts for the mitochondrial decline found in the elderly.  Supported by NIH R01 AG 030226, R01 AR 41928, 1RC2AG036606.

Kevin Conley, Sharon Jubrias, Sudip Bajpeyi, Magdalena Pasarica, Cedric Moro, Olga Sereda, David H. Burk, Alok Gupta, Lise Kjems, Kori Murray, Lori Arakaki, and Steven Smith

University of Washington Medical Center, Seattle, WA and Pennington Biomedical Research Center, Baton Rouge, LA

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Research Highlight #3

 

Compensation for mitochondrial dysfunction preserves energy homeostasis in muscle of individuals with pre-manifest HD

Huntington Disease is a genetic disorder that manifests brain atrophy and loss of neural cell metabolites well before the onset of clinical symptoms.  Here we show using new non-invasive tools profound mitochondrial changes in muscle that trade-off to maintain cell energy homeostasis in pre-manifest individuals as compared to age and gender matched controls.  Innovative optical and magnetic resonance spectroscopic methods for measuring ATP synthesis and O2 uptake in vivo revealed a reduction in mitochondrial coupling (ATP/O2) in proportion to early motor dysfunction as measured by the Unified Huntington’s Disease Rating Score (UHDRS).  They also revealed unchanged mitochondrial phosphorylation capacity (ATPmax) and stable levels of energetic metabolites (PCr and ATP). This stability of energy state and ATPmax is consistent with oxidative stress not only causing mitochondrial uncoupling but also compensatory mitochondrial biogenesis.  In contrast, atrophy of the caudate and loss of brain metabolites (NAA and Glutamine) per caudate volume in proportion to the UHDRS was apparent in these individuals.  These results reveal that mutant huntingtin has a profound impact on both brain and muscle tissue well before the onset of clinical symptoms.  They also show that the muscle compensates for this impact to maintain mitochondrial capacity, energy homeostasis and cell metabolite levels while atrophy and depletion of metabolites proceeds in the brain.  This compensation for the impact of mutant huntington in muscle may hold the key to effective interventions for halting progression of this chronic disease in brain.

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Research Highlight #4

Research highlight - oxidative stress

In vivo coupling of oxidative phosphorylation is inversely proportional to oxidative stress.

These results are the first in vivo support of the “Uncoupling to Survive,” which suggests that oxidative stress induces reduced coupling of oxidative phosphorylation (P/O), which then reduces reactive oxygen species production by the mitochondria.  This study sought to test whether oxidative stress leads to mitochondrial uncoupling in skeletal muscle in vivo.  We subjected mice to two different doses of Paraquat (PARA), a redox cycling agent known to stimulate production of reactive oxygen species (ROS) in mitochondria.  After two weeks of treatment, we used magnetic resonance spectroscopy to measure ATP flux and optical spectroscopy to measure oxygen uptake to determine the P/O value in the distal hindlimb.  PARA treatment led to a significant dose dependent decrease in mitochondrial P/O values in vivo (P<0.05), supporting the hypothesis the oxidative stress leads to mitochondrial uncoupling. We are currently investigating the mechanism of this reduced coupling using a UCP3 knockout mouse. Early results indicate no change in the coupling of resting skeletal muscle in the absence of UCP3. Studies are currently underway to determine if the presence of UCP3 is necessary for oxidative stress-induced uncoupling in vivo and whether this adaptive response changes with age.  This work was supported by NIH grants AG022385 and AG028455 and the Ellison Medical Foundation.

Michael Siegel, University of Washington, Bioengineering Department
Gary Knowles, University of Washington, Biochemistry Department
Mary-Ellen Harper, University of Ottawa, Department of Biochemistry, Microbiology and Immunology, Faculty of Medicine
David Marcinek, University of Washington, Bioengineering and Radiology Departments

 

Related Abstract

Siegel M, Harper M-E, Marcinek DJ. The absence of UCP3 does not alter coupling of oxidative phosphorylation in resting skeletal muscle. Experimental Biology Conference, April 2009. New Orleans, LA.

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Research Highlight #5

Research Highlight 2

Mitochondria co-opt exercise adaptations in defense against oxidative stress in vivo

Kevin Conley, Sharon Jubrias, Mike Siegel, Karen Syrjala, Holly Van Remmen, Elizabeth Aylward and David Marcinek

University of Washington, Fred Hutchinson Cancer Research Center, Seattle Children’s Research Institute, Seattle, WA and University of Texas, San Antonio, TX.

Reactive oxygen species (ROS) are implicated in oxidative damage but at low levels act as signals, especially in exercise.  Here we show that the mitochondrial defense against ROS emulates the impact of exercise.  Innovative spectroscopic tools applied to muscles in vivo found two key cell changes in a mouse model of the knockout of a key antioxidant defense, Cu,Zn-superoxide dismutase (SOD1 -/-).  These changes were paralleled in two human conditions with oxidative stress but prior to development of oxidative damage (pre-symptomatic Huntington’s disease and cancer survivors).  First is an increase in mitochondrial uncoupling (reduced ATP per O2) that raises respiration.  Dissipating the mitochondrial membrane potential causes uncoupling and is implicated as a negative feedback mechanism that counteracts elevated ROS but comes at the cost of greater O2 uptake.  Second is a mobilization of metabolism. Uncoupling not only elevates respiration but also cellular AMP level, which increases substrate supply and activates mitochondrial biogenesis to raise oxidative capacity of muscle. Thus mitochondria are the first line of defense in response to elevated ROS with a series of compensatory cellular changes that ameliorate the impact of oxidative stress and activate cellular metabolism in a manner similar to the effect of exercise.  Supported by AG028455, AR041928, AR036606, CA103728 and the CHDI Foundation.

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Research Highlight #6

Fig1_sample_config_BW.tif

Quantitative 19F Imaging Using Inductively Coupled Reference Signal Injection

Authors: Donghoon Lee, Kenneth Marro, Eric Shankland and Mark Mathis
Magnetic Resonance in Medicine (in press).

This report describes recent efforts on our continuous development of a synthetic signal injection method for quantification of metabolite content in MR spectroscopy and imaging. Previous work showed that conversion of spectral peaks to quantitative units of metabolite content could be achieved with a calibrated synthetic free induction decay (FID) generated by an inductively coupled injection coil. This work demonstrates that calibrated synthetic voxels, injected in the same manner, can be used to quantify metabolite content in real 19F image voxels. Images of vials containing different concentrations of sodium fluoride (NaF) were converted to units of moles by reference to precalibrated synthetically-injected voxels. Additional images of vials containing variable sodium chloride (NaCl) demonstrate that the quantification process is robust and immune to changes in coil loading conditions.


The eight pseudo-voxels appear at the top of the image (on the right) of the seven NaF vials. The configuration of the NaF vials and the loading syringe is illustrated in the middle diagram. The table indicates the NaF concentrations in each of the vials. The signals from vials 6 and 7, containing the two lowest NaF concentrations, were not detectable above the image noise so no attempt was made to calculate these concentrations. Salt concentrations in the loading syringe, sample 0, were varied (0, 0.2 and 2 M) to induce different coil loading conditions.

Fig3_abs-rel2.tif

(a) The graph shows the correlation between known concentrations and the concentrations calculated using the proposed method. In comparison, the relative concentrations (b) reflect a substantial drop in signal as the loading of the coil increased. The relative amplitudes acquired with no salt in the loading syringe were about 25% higher than they were at the highest salt concentration. Relative concentrations generate the slopes of 0.99 ± 0.02, 0.94 ± 0.01 and 0.76 ± 0.02 for the loading samples of 0, 0.2 and 2 M NaCl, respectively.

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