MUSCLE BIOLOGY OVERVIEW OF THE

Ted Huiatt, Ph.D. Associate Professor, Department of Animal Science and Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University Richard M. Robson 1941-2013

• Long time member of AMSA • Received the AMSA Distinguished Research Award, 1984 • Served on many AMSA Committees • Driving force for study of inter- mediate filaments and cytoskeletal attachments in the Muscle Biology Group at Iowa State University Skeletal Muscle

The most beautiful cells in the animal kingdom Why? What structures are responsible for maintaining the structural integrity of these very highly organized cells? The Skeletal Muscle Cytoskeleton

Extracellular Matrix Integrins

Intermediate Filaments Z-disk The Attachments Inside • = the sarcolemmal cytoskeleton • Elements: – – – Z-disk – M-line

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie Titin Z-line M-line Z-line

• Largest known (3,700 kDa) • Extends over ½ of (M-line to Z-disk) • Bound to thick filament in A-band • Elastic region in I band • Extends into Z-line • Rapidly degraded postmortem

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie Possible Functions of Titin • Passive tension • Restoration of sarcomere length during relaxation • Maintains thick filaments in center of sarcomere during contraction • Molecular ruler to regulate thick filament length • Assembly of myofibrils during • Molecular blueprint for sarcomere • Signaling of mechanical stretch/strain; protein turnover • Mechanical integrity of entire • Other functions ??

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie Nebulin • Firmly attached to thin filament • Molecular ruler for F-/thin filament length • Stabilizes thin filament • Functions in attachment of thin filament to Z-line • Nemaline – Nebulin – Disrupts Z-iline structure • Knockout in mice – alters contractility, disrupts Z-line

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie Z-disk

• Transmits contractile force from actin to rest of cell and rest of muscle • Complex structure • Actin filaments overlap in Z-disk – No TM or TN in Z-disk, but nebulin continues into Z • CapZ – Binds barbed end of F-actin (caps filament) – Binds to region of nebulin located at edge of Z-disk - could link to thin filament from opposing sarcomere • Titin associates with actin at edge of Z-disk, extends into Z – # of Z-disk repeats correlate with Z-disk width in different muscles

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• α- crosslinks actin from opposing – also binds titin – # of α-actinin crosslinks correlates with # of Z-disk titin domains • Titin crosslinked by • Many other associated with Z-disk - myotilin - myopallidin - archvillin - cypher/ZASP - MURF-1 - muscle LIM protein • Loss of Z-disks postmorten due to titin & nebulin degradation?

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie M-line

• Links thick filaments • Proteins common to all vertebrate skeletal muscle M-lines: – – Titin – Myomesin – /Obscurin-like protein 1 (obsl1) • Obscurin at periphery, obsl1 integral to M-line – Creatine – M-protein only in fast, not considered essential to all M-lines • Attachments from complex of titin, myomesin and obsl1 • Obscurin involved in SR attachment with isoform (sAnk1)

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie The Skeletal Muscle Cytoskeleton

Extracellular Matrix Sarcolemma Integrins

Intermediate Filaments Z-disk Attachment of Myofibrils to Each Other and to the Sarcolemma • Intermediate filaments (IFs) primarily responsible • major IF protein in muscle • Desmin localized at periphery of Z-disk • IFs surround Z-disks and run to attachment sites on sarcolemma • Other IF proteins in muscle: , , (paranemin),

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie How are IFs attached to Z-disks? Synemin – Robson Group • Originally identified as an IF-associated protein (Granger & Lazarides, 1980) • Contains characteristic IF α-helical domain • Large IF protein: – α-synemin, 173 kDa; β-synemin, 140 kDa (compare to 55 kDa desmin – Long C-terminal tail • Does not form IFs alone, but forms heteropolymers with desmin • Tail binds α-actinin • Colocalized with desmin at periphery of Z-disks • Highly susceptible to proteolysis

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie • IF Attachment to Z-disks, continued

• – the universal cytoskeletal crosslinker (G Wiche and coworkers) – Large protein, ~500 kDa, several isoforms – Binds IFs (desmin, , …); also F-actin and – Colocalized with desmin at Z-disk periphery – Knockouts result in IF aggregation, disruption of MF organization, – Mechanism for attachment to Z-disk not characterized • Z-disk region of nebulin interacts with α-helical region of desmin (Bang et al., 2002)

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie : Sarcolemmal attachments

• First identified by immunofluorescence localization of , an attachment site protein (Pardo et al., 1983). • Located on inside of sarcolemma, around cell (“rib-like”), level of myofibril Z-lines • Contain 3 types of cytoskeletal-sarcolemma-extracellular matrix attachments – Focal adhesion – -associated protein complex (DAPC) – -ankyrin attachments

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie Cytoskeletal-Membrane-ECM Attachments Potential Mechanisms for Myofibril-Sarcolemmal Attachment • Focal Adhesions – α-synemin tail interacts with both vinculin and – attach heteropolymeric desmin/synemin IFs to focal adhesions – Linkage of actin in membrane skeleton – linkage to Z-line? • DAPC – Tail of α-synemin can bind dystrophin – Rod region of synemin can bind dystrophin and α- – Plectin binds β- in DAPC and dystrophin. – Unusual IF protein syncoilin binds desmin and α-dystrobrevin • Ankyrin-spectrin interactions with the membrane skeleton

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie Conclusions • Cytoskeletal attachment sites are of vital importance in maintaining the integrity of skeletal muscle cells • Each of the attachment sites is complex, and each appears to utilize more than one mechanism to carry out their function • All of these attachments have roles in muscle disease • Postmortem degradion of these cytoskeletal attachments has important consequences for meat quality

American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Science Association AMSA American Meat Scie Acknowledgements

Contributors to the work from ISU Collaborators for the work on described in this presentation. synemin were: • Bruno Becker • Kevin P. Campbell • Stephen Bilak • Yassemi Capetanaki • Robert Bellin • David R. Critchley • Rahul Bhosle • Denise Paulin • Susan Sernett • Wallace Ip • Ning Sun

This research was supported in part by: • American Heart Association • Iowa Agriculture Experiment Station • Association • National Live Stock and Meat Board • USDA National Research Initiative Competitive Grants Program

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