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Signaling factors related to atrophy and hypertrophy in denervated skeletal muscle
Linnaeus University, Faculty of Health and Life Sciences, Department of Chemistry and Biomedical Sciences.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The human body consists of about 40 % skeletal muscles which control the body’s movement, ability to stand up, force generation, locomotion, heat production and are also the body’s protein reservoir. Muscle mass is controlled by the relationship between protein synthesis and protein degradation. Atrophy, a decrease in muscle mass, can be trigged by disuse, immobilization, inflammation and cancer. Hypertrophy, an increase in muscle mass, can occur after increased mechanical load, high usage and/or anabolic stimulation. The aim of this thesis was to investigate changes in expression and post translational modifications of some factors involved in the regulation of protein synthesis and protein degradation in 6-days denervated atrophic hind-limb muscles (anterior tibial and pooled gastrocnemius and soleus muscles) and in 6-days denervated hypertrophic hemidiaphragm muscle in mice. Protein expression and post translational modifications were studied semi-quantitatively using Western blots with whole muscle homogenates and separated nuclear and cytosolic fractions from both innervated and denervated muscles.  An increase in protein synthesis after denervation in both atrophic and hypertrophic muscles was suggested after studies of factors downstream of mTOR (paper I).  Other results suggest that FoxO1 and MuRF1 (paper II) participate in the tissue remodeling that occurs after denervation. A differential response of MK2 phosphorylation in denervated hypertrophic and atrophic muscles was confirmed (paper III). An increase in phosphorylation of the MK2 substrate Hsp 25 in all denervated muscles studied (paper III) indicates that other factors than MK2 are involved in regulating this phosphorylation. eIF4G phosphorylation at S1108 was investigated (paper IV) and a decrease was observed in atrophic muscle but an increase in hypertrophic muscle. The results in this thesis suggest that there are several factors that control protein degradation and protein synthesis in denervated atrophic and hypertrophic skeletal muscles. This is an intricate labyrinth with many different cell signaling factors, the function of which are still far from fully understood.

Place, publisher, year, edition, pages
Växjö: Linnaeus University Press, 2014. , p. 96
Series
Linnaeus University Dissertations ; 198/2014
Keywords [en]
Atrophy, Hypertrophy, Skeletal muscle, Denervation, Protein synthesis, Protein degradation
National Category
Natural Sciences
Research subject
Natural Science, Biomedical Sciences
Identifiers
URN: urn:nbn:se:lnu:diva-37670ISBN: 978-91-87925-26-9 (print)OAI: oai:DiVA.org:lnu-37670DiVA, id: diva2:759757
Public defence
2014-11-21, N2007, Västergård, Smålandsgatan 26E, Kalmar, 09:00 (Swedish)
Opponent
Supervisors
Available from: 2014-11-03 Created: 2014-10-15 Last updated: 2015-01-14Bibliographically approved
List of papers
1. Akt (protein kinase B) isoform phosphorylation and signaling downstream of mTOR (mammalian target of rapamycin) in denervated atrophic and hypertrophic mouse skeletal muscle.
Open this publication in new window or tab >>Akt (protein kinase B) isoform phosphorylation and signaling downstream of mTOR (mammalian target of rapamycin) in denervated atrophic and hypertrophic mouse skeletal muscle.
Show others...
2012 (English)In: Journal of Molecular Signaling, ISSN 1750-2187, E-ISSN 1750-2187, Vol. 7, no June, p. Article ID: 7-Article in journal (Refereed) Published
Abstract [en]

ABSTRACT: BACKGROUND: The present study examines the hypothesis that Akt (protein kinase B)/mTOR (mammalian target of rapamycin) signaling is increased in hypertrophic and decreased in atrophic denervated muscle. Protein expression and phosphorylation of Akt1, Akt2, glycogen synthase kinase-3beta (GSK-3beta), eukaryotic initiation factor 4E binding protein 1 (4EBP1), 70 kD ribosomal protein S6 kinase (p70S6K1) and ribosomal protein S6 (rpS6) were examined in six-days denervated mouse anterior tibial (atrophic) and hemidiaphragm (hypertrophic) muscles. RESULTS: In denervated hypertrophic muscle expression of total Akt1, Akt2, GSK-3beta, p70S6K1 and rpS6 proteins increased 2-10 fold whereas total 4EBP1 protein remained unaltered. In denervated atrophic muscle Akt1 and Akt2 total protein increased 2-16 fold. A small increase in expression of total rpS6 protein was also observed with no apparent changes in levels of total GSK-3beta, 4EBP1 or p70S6K1 proteins. The level of phosphorylated proteins increased 3-13 fold for all the proteins in hypertrophic denervated muscle. No significant changes in phosphorylated Akt1 or GSK-3beta were detected in atrophic denervated muscle. The phosphorylation levels of Akt2, 4EBP1, p70S6K1 and rpS6 were increased 2-18 fold in atrophic denervated muscle. CONCLUSIONS: The results are consistent with increased Akt/mTOR signaling in hypertrophic skeletal muscle. Decreased levels of phosphorylated Akt (S473/S474) were not observed in denervated atrophic muscle and results downstream of mTOR indicate increased protein synthesis in denervated atrophic anterior tibial muscle as well as in denervated hypertrophic hemidiaphragm muscle. Increased protein degradation, rather than decreased protein synthesis, is likely to be responsible for the loss of muscle mass in denervated atrophic muscles.

National Category
Cell and Molecular Biology Cell Biology Neurosciences
Research subject
Natural Science, Biomedical Sciences
Identifiers
urn:nbn:se:lnu:diva-20341 (URN)10.1186/1750-2187-7-7 (DOI)22657251 (PubMedID)2-s2.0-84861707979 (Scopus ID)
Available from: 2012-06-21 Created: 2012-06-21 Last updated: 2018-01-12Bibliographically approved
2. Expression and phosphorylation of eukaryotic translation initiation factor 4-gamma (eIF4G) in denervated atrophic and hypertrophic mouse skeletal muscle
Open this publication in new window or tab >>Expression and phosphorylation of eukaryotic translation initiation factor 4-gamma (eIF4G) in denervated atrophic and hypertrophic mouse skeletal muscle
2015 (English)In: Cell Biology International, ISSN 1065-6995, E-ISSN 1095-8355, Vol. 39, no 4, p. 496-501Article in journal (Refereed) Published
Abstract [en]

The eukaryotic translation initiation factor 4-gamma (eIF4G) is important for the initiation of protein synthesis and phosphorylation on S1108 regulates this function of eIF4G. Thus, increased phosphorylation has been reported in conditions associated with increased protein synthesis such as meal feeding and insulin/IGF-1 treatment whereas decreased phosphorylation occurs following starvation, dexamethasone treatment, in sepsis and in atrophic denervated hind-limb muscle. The aim of the present study was to test the hypothesis that S1108 phosphorylation of eIF4G is differentially affected in denervated atrophic hind-limb muscles and denervated hypertrophic hemidiaphragm muscle. Protein expression and phosphorylation in innervated and 6-days denervated atrophic hind-limb muscles (pooled gastrocnemius and soleus) and hypertrophic hemidiaphragms were studied semi-quantitatively using Western blots. Total expression of eIF4G did not change in denervated hind-limb muscles but increased about 77% in denervated hemidiaphragm. S1108 phosphorylated eIF4G decreased about 64% in denervated hind-limb muscles but increased about 1.3-fold in denervated hemidiaphragm. The ratio of S1108 phosphorylated eIF4G to total eIF4G decreased about 60% in denervated hind-limb muscles but no statistically significant change was observed in denervated hemidiaphragm. The differential effect of denervation on eIF4G expression and S1108 phosphorylation in hemidiaphragm (hypertrophic) and hind-limb muscle (atrophic) may represent a regulatory mechanism that helps clarify the differential response of these muscles following denervation.

National Category
Biochemistry and Molecular Biology
Research subject
Natural Science, Biomedical Sciences
Identifiers
urn:nbn:se:lnu:diva-37958 (URN)10.1002/cbin.10402 (DOI)000351678000017 ()25623635 (PubMedID)2-s2.0-84925292774 (Scopus ID)
Available from: 2014-10-31 Created: 2014-10-31 Last updated: 2017-12-05Bibliographically approved
3. Forkhead box O1 and muscle RING finger 1protein expression in atrophic and hypertrophicdenervated mouse skeletal muscle
Open this publication in new window or tab >>Forkhead box O1 and muscle RING finger 1protein expression in atrophic and hypertrophicdenervated mouse skeletal muscle
2014 (English)In: Journal of Molecular Signaling, ISSN 1750-2187, E-ISSN 1750-2187, Vol. 9, article id 9Article in journal (Refereed) Published
Abstract [en]

Background: Forkhead box O (FoxO) transcription factors and E3 ubiquitin ligases such as Muscle RING finger 1 (MuRF1) are believed to participate in the regulation of skeletal muscle mass. The function of FoxO transcription factors is regulated by post-translational modifications such as phosphorylation and acetylation. In the present study FoxO1 protein expression, phosphorylation and acetylation as well as MuRF1 protein expression, were examined in atrophic and hypertrophic denervated skeletal muscle. Methods: Protein expression, phosphorylation and acetylation were studied semi-quantitatively using Western blots. Muscles studied were 6-days denervated mouse hind-limb muscles (anterior tibial as well as pooled gastrocnemius and soleus muscles, all atrophic), 6-days denervated mouse hemidiaphragm muscles (hypertrophic) and innervated control muscles. Total muscle homogenates were used as well as separated nuclear and cytosolic fractions of innervated and 6-days denervated anterior tibial and hemidiaphragm muscles. Results: Expression of FoxO1 and MuRF1 proteins increased 0.3-3.7-fold in all 6-days denervated muscles studied, atrophic as well as hypertrophic. Phosphorylation of FoxO1 at S256 increased about 0.8-1-fold after denervation in pooled gastrocnemius and soleus muscles and in hemidiaphragm but not in unfractionated anterior tibial muscle. A small (0.2-fold) but statistically significant increase in FoxO1 phosphorylation was, however, observed in cytosolic fractions of denervated anterior tibial muscle. A statistically significant increase in FoxO1 acetylation (0.8-fold) was observed only in denervated anterior tibial muscle. Increases in total FoxO1 and in phosphorylated FoxO1 were only seen in cytosolic fractions of denervated atrophic anterior tibial muscle whereas in denervated hypertrophic hemidiaphragm both total FoxO1 and phosphorylated FoxO1 increased in cytosolic as well as in nuclear fractions. MuRF1 protein expression increased in cytosolic as well as in nuclear fractions of both denervated atrophic anterior tibial muscle and denervated hypertrophic hemidiaphragm muscle. Conclusions: Increased expression of FoxO1 and MuRF1 in denervated muscles (atrophic as well as hypertrophic) suggests that these proteins participate in the tissue remodelling occurring after denervation. The effect of denervation on the level of phosphorylated and acetylated FoxO1 differed in the muscles studied and may be related to differences in fiber type composition of the muscles.

Place, publisher, year, edition, pages
BioMed Central, 2014
Keywords
Acetylation, Atrophy, Denervation, Cytosolic fraction, Forkhead box O, Hypertrophy, MuRF1, Nuclear fraction, Phosphorylation, Skeletal muscle
National Category
Biochemistry and Molecular Biology
Research subject
Natural Science, Biomedical Sciences
Identifiers
urn:nbn:se:lnu:diva-37669 (URN)10.1186/1750-2187-9-9 (DOI)2-s2.0-84907392088 (Scopus ID)
Available from: 2014-10-15 Created: 2014-10-15 Last updated: 2017-12-05Bibliographically approved
4. p38 mitogen-activated protein kinase and mitogen-activated protein kinase-activated protein kinase 2 (MK2) signaling in atrophic and hypertrophic denervated mouse skeletal muscle
Open this publication in new window or tab >>p38 mitogen-activated protein kinase and mitogen-activated protein kinase-activated protein kinase 2 (MK2) signaling in atrophic and hypertrophic denervated mouse skeletal muscle
2014 (English)In: Journal of Molecular Signaling, ISSN 1750-2187, E-ISSN 1750-2187, Vol. 9, no 2Article in journal (Refereed) Published
Abstract [en]

BACKGROUND: p38 mitogen-activated protein kinase has been implicated in both skeletal muscle atrophy and hypertrophy. T317 phosphorylation of the p38 substrate mitogen-activated protein kinase-activated protein kinase 2 (MK2) correlates with muscle weight in atrophic and hypertrophic denervated muscle and may influence the nuclear and cytoplasmic distribution of p38 and/or MK2. The present study investigates expression and phosphorylation of p38, MK2 and related proteins in cytosolic and nuclear fractions from atrophic and hypertrophic 6-days denervated skeletal muscles compared to innervated controls.

METHODS: Expression and phosphorylation of p38, MK2, Hsp25 (heat shock protein25rodent/27human, Hsp25/27) and Hsp70 protein expression were studied semi-quantitatively using Western blots with separated nuclear and cytosolic fractions from innervated and denervated hypertrophic hemidiaphragm and atrophic anterior tibial muscles. Unfractionated innervated and denervated atrophic pooled gastrocnemius and soleus muscles were also studied.

RESULTS: No support was obtained for a differential nuclear/cytosolic localization of p38 or MK2 in denervated hypertrophic and atrophic muscle. The differential effect of denervation on T317 phosphorylation of MK2 in denervated hypertrophic and atrophic muscle was not reflected in p38 phosphorylation nor in the phosphorylation of the MK2 substrate Hsp25. Hsp25 phosphorylation increased 3-30-fold in all denervated muscles studied. The expression of Hsp70 increased 3-5-fold only in denervated hypertrophic muscles.

CONCLUSIONS: The study confirms a differential response of MK2 T317 phosphorylation in denervated hypertrophic and atrophic muscles and suggests that Hsp70 may be important for this. Increased Hsp25 phosphorylation in all denervated muscles studied indicates a role for factors other than MK2 in the regulation of this phosphorylation.

Place, publisher, year, edition, pages
BioMed Central, 2014
National Category
Cell Biology
Identifiers
urn:nbn:se:lnu:diva-37957 (URN)10.1186/1750-2187-9-2 (DOI)24629011 (PubMedID)2-s2.0-84899065573 (Scopus ID)
Available from: 2014-10-31 Created: 2014-10-31 Last updated: 2017-12-05Bibliographically approved

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