Chapter 6 Cellular Movement and Muscles

I.     Overview

1.     Eukaryotic cells possess a cytoskeleton composed of microtubules (24 nm), microfilaments (7 nm), and intermediate filaments (10 nm). Both microtubules and microfilaments have important roles in cellular movement.

2.     Microtubules work in conjunction with the motor proteins tubulin, kinesin, and dynein.

3.     Microfilaments work in conjunction with the motor proteins actin and myosin.

4.     There are 4 ways that cells use these elements to conduct movement:

(1)  Polymerization (amoeboid movement): cells can move themselves by adding to cytoskeleton and pushing the cell membrane outward. This type of movement is common in many motile cells.

(2)  Mobile cytoskeleton: the motor protein is anchored in the membrane and the cytoskeleton can be moved.

(3)  Mobile motor: the cytoskeleton is stationary, and motor proteins are free to move.

(4)  Mobile motor and cytoskeleton: the cytoskeleton and motor proteins are arranged in complex arrays.

II.   Cytoskeleton and Motor Proteins

1.     Microtubules

(1)  Microtubules contain hollow assembled tubulin and are intracellular rod-like cytoskeletal fibers that radiate throughout the cell, performing many functions.

(2)  Microtubule-organizing center (MTOC) is a multiprotein complex near the center of the cell from which microtubules grow.

(3)  Cells use their microtubule network to control the movement of subcellular components, e.g., vesicles, organelles, and pigment granules.

(4)  Microtubules are composed of long strings of tubulin, itself a heterodimer of two closely related proteins: α-tubulin and β-tubulin.

(5)  Microtubule assembly is regulated by microtubule-associated proteins (MAPs), which bind to the surface of microtubules, stabilizing or destabilizing the microtubule structure.

(6)  Kinesin and dynein move along microtubules. Vesicle traffic depends on the polarity of the microtubules. Kinesin carries vesicles of neurotransmitters to the synapse, whereas dynein carries empty vesicles back to the MTOC.

(7)  Cilia and flagella are composed of microtubules. Axoneme is a microtubule-based structure wrapped in an extension of the plasma membrane that underlies cilia and flagella. The core structure of the flagella is composed of 9 doublets of microtubules, connected by the liker protein nexin at outer ring, and a central pair of microtubule singlets. Dynein inner and outer arms extend from doublets.

2.     Microfilaments

(1)  Microfilaments are the other types of cytoskeletal fibers used in movement. Like microtubules, microfilaments play important roles in the transport of vesicles throughout cells. In addition, microfilament-based movement allow cells to change shape and move from place to place.

(2)  Microfilaments are composed of long strings of actin. Actin monomer is called G-actin for globular structure, when G-actin assembles are called F-actin for filaments.

(3)  Myosin used actin as a motor protein. Myosin cotains 3 parts: myosin heavy chain (MHC), essential light chain (ELC), and regulatory light chain (RLC). MHC possesses a head, neck, and a tail. Myosin light chain has two different types: an ELC and an RLC.

(4)  The sliding filament model describes actin-myosin activity. Proposed by Hugh Huxley in 1969, the sliding filament theory described the interaction between actin and myosin during cross-bridge cycling.

1)    When ATP bind to myosin head, myosin detaches from the contact with actin and extends toward the attached end of actin (Z disk).

2)    ATP is hydrolyzed to ADP and Pi that are remain bound to myosin head.

3)    A cross-bridge is formed between myosin head and actin.

4)    Energy release of phosphate (Pi) bends myosin from 45 to 90 degree and promotes a power stroke to move actin 36 nm toward the free end of actin (M line).

5)    ADP is released from myosin head.

III. Muscle

1.     A muscle is composed of many types of muscle cells (myocytes; myofibers). A single muscle cell contains lots of myofibrils. Two main types of muscle are smooth muscle and striated muscle (including skeletal muscle and cardiac muscle).

2.     General features of striated muscles

(1)  Muscle cells possess thick and thin filaments.

(2)  The thick filament is a superstructure brought together by polymers of myosin (including myosin heavy chains (MHCs) and myosin light chains ELCs and RLCs). The thin filament is a dynamic actin network (including actins and regulatory proteins troponins and tropomyosins).

3.     Striated muscle thick and thin filaments are arranged into sarcomeres.

(1)  A sarcomere is the contractile unit of a myofibril of striated muscle, and it is bounded by two Z disks.

(2)  A band is a dark region of a sarcomere where thick (and thin) filaments occur.

(3)  I band is a bright region that spans a Z disk, and includes the portion of the thin filaments without overlapping with thick filaments.

(4)  M line is the central region of the sarcomere.

(5)  H zone is a more bright region in the A band, and includes the portion of the thick filaments without overlapping with thin filaments.

4.     Actin-myosin activity is activated by Ca²⁺.

(1)  The Ca²⁺ signal is transmitted to the thin filament regulatory proteins troponin and tropomyosin.

(2)  Troponin (Tn) is a trimeric regulatory protein bound to tropomyosin. The troponin component is composed of 3 subunits: TnC, TnI, and TnT.

1)    TnC (the C stands for calcium) subunit contains four Ca²⁺ binding sits and is the Ca²⁺ sensor.

2)    TnI (the I for inhibitory) subunit contains an inhibitory activity for actin-myosin ATPase.

3)    TnT (the T for tropomyosin) subunit contains an elongated portion to bind tropomyosin.

(3)  Tropomyosin is also a regulatory protein that stretches across 7 actin monomers per helical pitch in a thin filament, blocking myosin's access to its binding site on actin. Tropomyosin contains a dimer of two parallel α-helical polypeptide chains.

(4)  When [Ca²⁺] is low (typically below 200 nM), the tropomyosin is in a position that blocks actin's binding site for myosin.

(5)  When [Ca²⁺] rises (about 100-fold, >20 μM), Ca²⁺ binds to TnC and this binding causes a conformational change in troponin. Troponin pulls tropomyosin to roll out the way and allow myosin to bind to actin to initiate the cross-bridge cycle.

5.     Excitation in vertebrate skeletal and cardiac muscles

(1)  Striated muscles differ in the time course of the action potential. The action potential in cardiac muscle is prolonged.

(2)  Transverse (T) tubules enhance action potential penetration deep into the myocyte. T tubule is an extension of the plasma membrane  (sarcolemma; SL) of a muscle cell that serves to improve the conduction of the action potential deep into the myofiber.

6.     Excitation-contraction (EC) coupling in striated muscles

(1)  Depolarization leads to an increase in cytoplasmic [Ca²⁺].

1)    The main storage site for Ca²⁺ is the Ca²⁺ binding protein calsequestrin, located in terminal cisternae where is enlargements of sarcoplasmic reticulum (SR; muscle endoplasmic reticulum, ER) within the emuscle cell.

2)    Dihydropyridine receptor (DHPR) is a main voltage-sensitive Ca²⁺ ion channel of the sarcolemma that opens with depolarization.

3)    Ryanodine receptor (RyR) is the Ca²⁺ ion channel of the SR. The opening changes in DHPR trigger the opening of RyR that permits the release of Ca²⁺ from the SR stores into the sarcoplasm.

4)    Both DHPR and RyR interact physically in skeletal muscles.

(2)  Repolarization leads to an decrease in cytoplasmic [Ca²⁺].

1)    The Ca²⁺ ATPase is an active transporter in the sarcolemma that expels Ca²⁺ outside the cell during relaxation.

2)    The sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) is a Ca²⁺ ATPase in the SR (and ER) to pump Ca²⁺ from the sarcoplasm into the SR.

3) The sodium-calcium exchanger (NaCaX) is a reversible transporter of the sarcolemma that exchanges Na⁺ for Ca²⁺. NaCaX can work to allow Ca²⁺ into a cell or expel Ca²⁺ from the cell, depending on the electrochemical gradients for each ion and the membrane potential.

7.     Mechanism of excitation-contraction (EC) coupling

(1)  Excitation: depolarization of the sarcolemma opens DHPR, while Ca²⁺ enters into the sarcoplasm. The opening of DHPR then triggers the opening of RyR.

(2)  Calcium release: RyR opening allows Ca²⁺ to escape from the SR. The elevated cytoplasmic [Ca²⁺] triggers actin-myosin ATPase.

(3)  Relaxation: after depolarization, ion pumps (Ca²⁺ ATPase, NaCaX, and SERCA) begin returning Ca²⁺ to resting locations, outside the cell and in the SR.

IV. Diversity in Muscle Structure and Function

1.     Smooth muscle lacks organized sarcomeres, although smooth muscle contains contractile microfilaments (thick and thin filaments).

2.     Smooth muscle lacks T tubules, terminal cisternae, and troponin. (Smooth muscle has tropomyosin.) Instead of troponin, smooth muscle has calmodulin for Ca²⁺ binding. Calmodulin functionally replaces TnC.

3.     The effects of Ca²⁺ are mediated via another regulatory protein caldesmon in smooth muscle.

(1)  Caldesmon is an actin-binding protein that binds to the thin filament and prevents myosin from binding to actin.

(2)  Caldesmon functionally replaces TnI. Caldesmon moves out of this inhibitory position in response to Ca²⁺, but caldesmon does not directly bind Ca²⁺.

(3)  Caldesmon is a calmodulin binding protein.

4.     When the [Ca²⁺] increases, calmodulin binds to Ca²⁺, then calmodulin binds to caldesmon. The Ca²⁺-calmodulin-caldesmon complex dissociates from actin and allows the formation of a cross-bridge between myosin and actin.

5.     When the [Ca²⁺] level falls, the Ca²⁺-calmodulin-caldesmon complex dissociates and caldesmon returns to its inhibitory position on actin.