C H A P T E R
5 Cellular Movement and Muscles
PowerPoint® Lecture Slides prepared by Stephen Gehnrich, Salisbury University
Cytoskeleton and Motor Proteins
All physiological processes depend on movement
Intracellular transport
Changes in cell shape
Cell motility
Animal locomotion
Cytoskeleton and Motor Proteins
All movement is due to the same cellular “machinery” Cytoskeleton
Protein-based intracellular network
Motor proteins
Enzymes that use energy from ATP to move
Use of Cytoskeleton for Movement
Cytoskeleton elements Microtubules Microfilaments
Three ways to use the cytoskeleton for movement
Cytoskeleton “road” and motor protein carriers To reorganize the cytoskeletal network Motor proteins pull on the cytoskeletal “rope”
Cytoskeleton and Motor Protein Diversity
Structural and functional diversity
Multiple isoforms of cytoskeletal and motor proteins
Various ways of organizing cytoskeletal elements
Alteration of cytoskeletal and motor protein function
Microtubules
Microtubules
Are tubelike polymers of the protein tubulin
Similar protein in diverse animal groups
Multiple isoforms
Are anchored at both ends
Microtubule-organization center (MTOC) ( – ) near the nucleus Attached to integral proteins (+) in the plasma membrane
Microtubules
Function of Microtubules
Motor proteins can transport subcellular components along microtubules
Motor proteins kinesin and dynein
For example, rapid change in skin color
Movement of Pigment Granules
Microtubules: Composition and Formation
Microtubules are polymers of the protein tubulin
Tubulin is a dimer of a-tubulin and b-tubulin
Tubulin forms spontaneously
For example, does not require an enzyme
Polarity
The two ends of the microtubule are different
Minus ( – ) end
Plus (+) end
Microtubule Assembly
Activation of tubulin monomers by GTP
Monomers join to form tubulin dimer
Dimers form a single-stranded protofilament
Many protofilaments form a sheet
Sheet rolls up to form a tubule
Dimers can be added or removed from the ends of the tubule
Asymmetrical growth
Growth is faster at + end
Cell regulates rates of growth and shrinkage
Microtubule Assembly
Microtubule Growth and Shrinkage
Factors affecting growth/shrinkage are
Local concentrations of tubulin
High [tubulin] promotes growth
Dynamic instability
GTP hydrolysis on b-tubulin causes disassembly
Microtubule-associated proteins (MAPs)
Temperature
Low temperature causes disassembly
Chemicals that disrupt the dynamics
For example, plant poisons such as taxol and colchicine
Microtubule Dynamics
Regulation by MAPs
Pacific yew tree
Taxol
Movement Along Microtubules
Motor proteins move along microtubules Direction is determined by polarity and the type of motor protein
Kinesin move in (+) direction
Dynein moves in ( – ) direction
Movement is fueled by hydrolysis of ATP Rate of movement is determined by the ATPase domain of motor protein and regulatory proteins Dynein is larger than kinesin and moves five times faster
Vesicle Traffic in a Neuron
Cilia and Flagella
Cilia
Flagella
Numerous, wavelike motion Single or in pairs, whiplike movement
Composed of microtubules arranged into axoneme
Bundle of parallel microtubules Nine pairs of microtubules around a central pair
“Nine-plus-two”
Asymmetric activation of dynein causes movement
Cilia and Flagella
Cilia and Flagella
Microtubules and Physiology
Microfilaments
Microfilaments
Polymers composed of the protein actin
Found in all eukaryotic cells
Often use the motor protein myosin
Movement arises from
Actin polymerization
Sliding filaments using myosin
Microfilament Structure and Growth
G-actin monomers polymerize to form a polymer called F-actin Spontaneous growth
Treadmilling
6 – 10 times faster at + end Assembly and disassembly occur simultaneously and overall length is constant
Capping proteins
Increase length by stabilizing – end and slowing disassembly
Microfilament Structure and Growth
Microfilament (Actin) Arrangement
Arrangement of microfilaments in the cell
Tangled neworks
Bundles
Microfilaments linked by filamin protein Cross-linked by fascin protein
Networks and bundles of microfilaments are attached to cell membrane by dystrophin protein
Maintain cell shape
Can be used for movement
Microfilament (Actin) Arrangement
Movement by Actin Polymerization
Two types of amoeboid movement
Filapodia are rodlike extensions of cell membrane
Neural connections Microvilli of digestive epithelia
Lamellapodia are sheetlike extensions of cell membrane
Leukocytes
Macrophages
Movement by Actin Polymerization Filapodia
Movement by Actin Polymerization Filapodia
Movement by Actin Polymerization Lamellapodia
Actin Polymerization and Fertilization
Actin + Myosin = motor protein
Myosin
Most actin-based movements involve the motor protein myosin
Myosin is an ATPase
Sliding filament model Converts energy from ATP to mechanical energy
17 classes of myosin (I – XVII)
Multiple isoforms in each class
All isoforms have a similar structure
Head (ATPase activity)
Tail (can bind to subcellular components)
Neck (regulation of ATPase)
Myosin
Sliding Filament Model
Analogous to pulling yourself along a rope
Actin – the rope
Myosin – your arm
Alternating cycle of grasp, pull, and release
Your hand grasps the rope
Your muscle contracts to pull rope
Your hand releases, extends, and grabs further along the rope
Sliding Filament Model
Two processes
Chemical reaction
Structural change
Myosin bends ( power stroke)
Cross-bridge cycle
Myosin binds to actin (cross-bridge)
Formation of cross-bridge, power stroke, release, and extension
Need ATP to release and reattach to actin
Absence of ATP causes rigor mortis
Myosin cannot release actin
Sliding Filament Model
Actino-Myosin Activity Two factors affect movement
Unitary displacement
Distance myosin steps during each cross-bridge cycle
Depends on
Myosin neck length
Location of binding sites on actin
Helical structure of actin
Duty cycle
Cross-bridge time/cross-bridge cycle time
Typically ~0.5
Use of multiple myosin dimers to maintain contact
Myosin Activity
Actin and Myosin Function
Muscle Structure and Regulation of Contraction
Muscle Cells (Myocytes)
Myocytes (muscle cells)
Contractile cell unique to animals
Contractile elements within myocytes
Thick filaments
Polymers of myosin
~300 myosin II hexamers
Thin filaments
Polymers of a-actin
Ends capped by tropomodulin and CapZ to stabilize
Proteins troponin and tropomyosin on outer surface
Thick and Thin Filaments
Muscle Cells
Two main types of muscle cells are based on the arrangement of actin and myosin
Striated (striped appearance)
Skeletal and cardiac muscle
Actin and myosin arranged in parallel
Smooth (do not appear striped)
Actin and myosin are not arranged in any particular way
Striated and Smooth Muscle
Striated Muscle Types
Striated Muscle Cell Structure
Thick and thin filaments arranged into sarcomeres
Repeated in parallel and in series
Side-by-side across myocyte
Causes striated appearance
End-to-end along myocyte
Sarcomeres
Structural features of sarcomeres
Z-disk
Thin filaments are attached to the Z-disk and extend from it towards the middle of the sarcomere
A-band
Forms border of each sarcomere
Middle region of sarcomere occupied by thick filaments
I-band
Located on either side of Z-disk
Occupied by thin filament
Sarcomeres
Sarcomeres
Each thick filament is surrounded by six thin filaments Three-dimensional organization of thin and thick filaments is maintained by other proteins
Nebulin
Along length of thin filament
Titin
Keeps thick filament centered in sarcomere
Attaches thick filament to Z-disk
Three-Dimensional Structure of Sarcomere
Muscle Actinomyosin Activity is Unique
Myosin II cannot drift away from actin
Duty cycle of myosin II is 0.05 (not 0.5)
Structure of sarcomere Each head is attached for a short time Does not impede other myosins from pulling the thin filament
Unitary displacement is short
Small amount of filament sliding with each movement of the myosin head
Contractile Force
Contractile force depends on overlap of thick and thin filaments
More overlap allows for more force Amount of overlap depends on sarcomere length as measured by distance between Z-disks
Maximal force occurs at optimal length
Decreased force is generated at shorter or longer lengths
Length – Force Relationship
Myofibril
In muscle cells, sarcomeres are arranged into myofibrils
Single, linear continuous stretch of interconnected sarcomeres (i.e., in series)
Extends the length of the muscle cell
Have parallel arrangement in the cell
More myofibrils in parallel can generate more force
Myofibrils in Muscle Cells
Contraction and Relaxation in Vertebrate Striated Muscle
Regulation of Contraction Excitation-contraction coupling (EC coupling)
Depolarization of the muscle plasma membrane ( sarcolemma)
Elevation of intracellular Ca 2+
Contraction
Sliding filaments
Ca2+ Allows Myosin to Bind to Actin
At rest, cytoplasmic [Ca2+] is low
Troponin-tropomyosin cover myosin binding sites on actin
As cytoplasmic [Ca2+] increases
Ca2+ binds to TnC (calcium binding site on troponin) Troponin-tropomyosin moves, exposing myosin binding site on actin
Myosin binds to actin and cross-bridge cycle begins
Cycles continue as long as Ca 2+ is present
Cell relaxes when the sarcolemma repolarizes and intracellular Ca2+ returns to resting levels
Troponin and Tropomyosin
Regulation of Contraction by Ca 2+
Ionic Events in Muscle Contraction
Troponin – Tropomyosin Isoforms
Properties of isoforms affect contraction
For example, fTnC has a higher affinity for Ca2+ than s/cTnC
Muscle cells with the fTnC isoform respond to smaller increases in cytoplasmic [Ca2+]
Isoforms differ in the affect of temperature and pH
Myosin Isoforms
Properties of isoforms affect contraction
Multiple isoforms of myosin II in muscle
Isoforms can change over time
Excitation and EC coupling in Vertebrate Striated Muscle
Excitation of Vertebrate Striated Muscle
Skeletal muscle and cardiac muscle differ in mechanism of excitation and EC coupling Differences include
Initial cause of depolarization Time course of the change in membrane potential (action potential) Propagation of the action potential along the sarcolemma Cellular origins of Ca2+
Action Potentials
APs along sarcolemma signal contraction
Na+ enters cell when Na + channels open
Voltage-gated Ca2+ channel open
Increase in cytoplasmic [Ca2+]
Na+ channels close K + leave cell when K + channels open
Depolarization
Repolarization
Reestablishment of ion gradients by Na +/K + ATPase and Ca2+ ATPase
Time Course of Depolarization
Initial Cause of Depolarization
Myogenic (“beginning in the muscle”)
Spontaneous
For example, vertebrate heart
Pacemaker cells
Cells that depolarize fastest
Unstable resting membrane potential
Neurogenic (“beginning in the nerve”)
Excited by neurotransmitters from motor nerves
For example, vertebrate skeletal muscle
Can have multiple (tonic) or single (twitch) innervation sites
Neurogenic Muscle
T-Tubules and Sarcoplasmic Reticulum
Transverse tubules (T-tubules)
Invaginations of sarcolemma
Enhance penetration of action potential into myocyte
More developed in larger, faster twitching muscles
Less developed in cardiac muscle
Sarcoplasmic reticulum (SR)
Stores Ca2+ bound to protein sequestrin Terminal cisternae increase storage
T-tubules and terminal cisternae are adjacent to one another
T-Tubules and SR
Ca2+ Channels and Transporters
Channels allow Ca2+ to enter cytoplasm
Ca2+ channels in cell membrane
Ca2+ channels in the SR membrane
Dihydropyridine receptor (DHPR) Ryanodine receptor (RyR)
Transporters remove Ca2+ from cytoplasm
Ca2+ transporters in cell membrane
Ca2+ ATPase Na+/Ca2+ exchanger (NaCaX)
Ca2+ transporters in SR membrane
Ca2+ ATPase (SERCA)
Ca2+ Channels and Transporters
Induction of Ca2+ Release From SR
AP along sarcolemma conducted down T-tubules
Depolarization opens DHPR
Ca2+ enters cell from extracellular fluid
In heart, [Ca2+] causes RyR to open, allowing release of Ca2+ from SR
“Ca2+ induced Ca2+ release”
In skeletal muscle, change in DHPR shape causes RyR to open, allowing release of Ca2+ from SR
“Depolarization induced Ca2+ release”
Ca2+ Induced Ca2+ Release
Depolarization Induced Ca2+ Release
Relaxation
Repolarization of sarcolemma
Remove Ca2+ from cytoplasm
Ca2+ ATPase in sarcolemma and SR
Na+/Ca2+ exchanger (NaCaX) in sarcolemma
Parvalbumin
Cytosolic Ca2+ binding protein buffers Ca2+
Ca2+ dissociates from troponin
Tropomyosin blocks myosin binding sites
Myosin can no longer bind to actin
Relaxation
Summary of Striated Muscles
Smooth Muscle
Smooth Muscle
Slow, prolonged contractions
Often found in the wall of “tubes” in the body
Blood vessels, intestine, airway, etc.
Smooth Muscle
Key differences from skeletal muscle
No sarcomeres (no striations)
Thick and thin filaments are scattered in the cell
Attached to cell membrane at adhesion plaques
No T-tubules and minimal SR Often connected by gap junctions
Function as a single unit
Different mechanism of EC coupling
Smooth Muscle
Control of Smooth Muscle Contraction
Regulated by nerves, hormones, and physical conditions (e.g., stretch)
At rest, the protein caldesmon is bound to actin and blocks myosin binding
Smooth muscle does not have troponin
Stimulation of cell increases intracellular Ca 2+
Ca2+ binds to calmodulin
Calmodulin binds caldesmon and removes it from actin
Cross-bridges form and contraction occurs
Calmodulin also causes phosphorylation of myosin
Increase in myosin ATPase activity
Control of Smooth Muscle Contraction
Muscle Diversity in Vertebrates and Invertebrates
Diversity of Muscle Fibers
Different protein isoforms affect EC coupling
Ion channels
Ion pumps
Ca2+-binding proteins
Speed of myosin ATPase
Variation in other properties of muscle cells
Myoglobin content Number of mitochondria
Skeletal muscle cells can be classified as “fast,” “slow,” “white,” “red,” “oxidative,” “glycolytic”
Changing Fiber Types
Developmental (from embryo to adult)
Increased proportion of fast muscle isoforms
Physiological response
For example, exercise
Can change both cardiac and skeletal muscle
Changing Fiber Types
Changes due to hormonal and nonhormonal mechanisms
For example, thyroid hormones repress expression of b-myosin II gene and induce a-myosin II gene
a-myosin II exhibits the fastest actino-myosin ATPase rates
For example, direct stimulation of cell can alter gene expression
Nonhormonal Mechanisms
Trans-Differentiation of Muscle Cells
Trans-differentiation
Cells used for novel functions
For example, heater organs of billfish eye
Specialized muscle cells
Few myofibrils (little actin and myosin)
Abundant SR and mitochondria
Futile cycle of Ca2+ in and out of the SR
High rate of ATP synthesis and consumption
Electric organs
Heater Organ
Invertebrate Muscles
Variation in contraction force due to graded excitatory postsynaptic potentials (EPSP)
Innervation by multiple neurons EPSPs can summate to give stronger contraction Some nerve signals can be inhibitory
Asynchronous Insect Flight Muscles Wing beats: 250 – 1000 Hz
Fastest vertebrate contraction ≈ 100 Hz (toadfish)
Asynchronous Insect Flight Muscles Asynchronous muscle contractions
Contraction is not synchronized to nerve stimulation Stretch-activation
Sensitivity of the myofibril to Ca2+ changes during contraction/relaxation cycle Intracellular [Ca2+] remains high
Contracted muscle is Ca2+ insensitive Muscle relaxes Stretched muscle is Ca2+ sensitive Muscle contracts
Asynchronous Insect Flight Muscles
Mollusc (Bivalve) Catch Muscle
Muscle that holds shell closed Capable of long duration contractions with little energy consumption
Protein twitchin may stablilize actin-myosin cross bridges
Cross-bridges do not continue to cycle Phosphorylation/dephophorylation of twitchin regulates its function
