NEUR30080 L8 SF striated muscle: excitation-contraction coupling

learning goals

3 types of muscle

• skeletal: attached to bones and moves and supports skeleton

• smooth: surrounds hollow cavities and tubes

• cardiac: the muscle of the heart

skeletal muscle structure

• composed of cylindrical muscle fibres (cells)

• linked to bones by tendons at each end of the muscle

skeletal muscle structure image

the 3 types of muscle fibres

• skeletal muscle fibres have a repeating, striated pattern on light and dark bands (due to arrangement of thick and thin filaments within the myofibrils)

why does skeletal muscle have a pattern of light and dark bands?

due to arrangement of thick and thin filaments within the myofibrils

skeletal excitation-contraction coupling overview

1. excitation (AP generation)

2. propagation along T-tubules

3. voltage sensing and mechanical coupling

4. calcium release from SR

5. muscle contraction

6. relaxation (calcium reuptake)

Step 1. skeletal excitation-contraction coupling

• excitation (gen of AP)

Step 2. skeletal excitation-contraction coupling

propagation along T-tubules

Step 3. skeletal excitation-contraction coupling

voltage sensing and mechanical coupling

Step 4. skeletal excitation-contraction coupling

Ca release from SR

Step 5. skeletal excitation-contraction coupling

muscle contraction

Step 6. skeletal excitation-contraction coupling

relaxation (Ca reuptake)

skeletal excitation-contraction coupling overview: image

what is excitation-contraction coupling?

the process of coupling surface membrane depolarisation to Ca2+ release from the sarcoplasmic reticulum

sarcoplasmic reticulum (SR)

• endoplasmic reticulum of muscle cell

• stores and releases Ca2+

filaments in skeletal muscle fibre

• actin-containing thin filaments are anchored to the Z lines at each end of a sarcomere

• the free ends of thin filaments partially overlap the myosin-containing thick filaments in the A band at the centre of the sarcomere

filaments in skeletal muscle fibre image

actin-containing thin filaments = yellow

*myosin cross bridge IMPORTANT

filaments in skeletal muscle fibre: band overlap image

actin and myosin overlap at dark point

muscle fibre → myofibrils → sarcomeres

one muscle fibre has many myofibrils, each myofibril has multiple sarcomeres

thick and thin filaments in the sarcomere

black: Z line

yellow: actin thin filaments

red: myosin thick filaments

whole unti: sarcomere

Q:

50,000 sarcomeres

molecular mechanism of contraction

• when a skeletal muscle actively shortens, the thin filaments are propelled towards the centre of their sarcomere by movements of the myosin cross-bridge that bind to actin

• the two globular heads of each cross bridge contain a binding site for actin and an enzymatic site that splits ATP

globular head of cross bridge diagram

thin actin filament: troponin-tropomyosin

• two other proteins which assist in regulating the actin-myosin relationship

• tropomyosin protein coils around actin helically and attaches via troponin complex

troponin-tropomyosin complex image

each black dot is myosin binding site

which filament type is troponin-tropomyosin complex present in?

thin actin filament

Ca2+ conc role

• induces contraction

• myosin-binding site exposed

cross-bridge activation: summary

1. in a resting/relaxed muscle: attachment of myosin cross-bridges to actin is blocked by tropomyosin molecules that are in contact with the actin subunits of the thin filaments

2. contraction: initiated by an increase in cytosolic Ca2+ conc

3. the Ca2+ ions bind to troponin, producing a change in it’s shape that is transmitted via tropomyosin to uncover the myosin binding sites of actin, allowing the myosin cross bridges to bind to the thin filaments

contraction: 4 steps

1. cross-bridge formation: ADP + Pi → cross-bridge

2. power stroke: release of Pi, then ADP

3. detachment: ATP binding

4. ATP hydrolysis: ADP + Pi + energy (cocked position)

contraction: 4 steps image

actin-myosin interaction

by converting chemical bond energy into mechanical energy, myosin is able to move along the actin filament:

1. ATP binds to the myosin head: causes it to detach from actin

2. this ATP hydrolyses, removes from the myosin head and undergoes ATP → ADP + Pi rxn: releases energy, myosin now in high-energy state

3. myosin in it’s high energy state ‘pushes’ on the actin filament

4. ADP released and the process loops back around, except now the myosin is in a new position on the actin filament

actin-myosin interaction aka

actin filament sliding theory

filaments mechanism timing

filaments work out of sync with each other, preventing actin from slipping back to original position

actin-myosin interaction image

SR/transverse tubules/myofibrils

• forms branching web of memb-bound tubes and sacks that wrap around myosin fibrils

• SR: stores and releases Ca2+

• anytime there is a Ca2+ influx, there will be a shape change

SR/transverse tubules/myofibrils image

malignant hyperthermia

• excessive Ca2+ release from the SR

• causes uncontrolled muscle contraction and increases body temp

• Ca2+ channel blockers regulate Ca2+

contraction and Ca2+

• contraction depends on a rise in Ca2+ in cytosol

• this is triggered by an AP in plasma memb.

• the AP is propagated into the interior of the fibre along the transverse tubules to the region of the SR, where it produces a release of Ca2+ ions from the reticulum

• relaxation of a contracting muscle fibre occurs as a result of the active transport of cytosolic Ca2+ back into SR

release/uptake of Ca2+

1. muscle AP propagated

2. Ca2+ released from sac

3. Ca2+ binding to troponin removes blocking action of tropomyosin

4. myosin cross-bridge moves

5. Ca2+ takes back up via SERCA pumps

6. Ca2+ removal from troponin restores tropomyosin, blocking action

triad junction structure

• AP travels to transverse (T)-tubules

• results in SR Ca2+ release

• triad junction: repeating structural unit composed of 2 SR terminal cisternae facing a central T-tubule

triad junction structure image

cisternae

flattened sacks in the memb

electron MS of triads location

electron MS of triads: triads parallel to each other

transverse tubules (T-tubules)

• T-tubules: the central elements of the triad form a continuous network across the muscle fibres

• bring APs into interior of muscle fibre

• depolarisation: voltage-dependent activation & conformational change of the subunits of the L-type Ca2+ channel or DHRP (dihydropyridine receptor)

T-tubules image

mechanical transmission at triad junction

• in the early 1070s, there was much debate around the nature of transmission across the triad junction in skeletal muscle

• compelling arguments were made for each of the chemical transmission, electrical transmission and for a mechanical coupling process

• electrical transmission was eliminated largely because the memb. capacitance was too small to include the very large amount of SR membrane (surface area 10x larger than surface memb)

• chemical transmission was argued at this time because of lack of suitable candidate

*mechanical transmission, through the junctional feet, was favoured, with a model of a series of levers connecting a voltage sensor in the T-tubule membrane with a Ca2+ release pathway in the SR memb

mechanical transmission at triad junction image

excitation-contraction coupling at the triad junction

• focus on:

  • electrical impulse transmission (mechanical because directly acts with protein)

  • SERCA pump

  • calsequestrin (like a Ca sponge)

  • calmodulin (CaM) (modulates Ca)

  • DHPR

DHPR

• dihydropyridine receptor

• voltage sensor

• L-type Ca2+ channel in the membrane of T-tubules

• 4 subunits:

  • alpha1S (voltage-sensing and pore-forming)

  • alpha2-sigma

  • beta1a (EC coupling, essential for surface expression of DHPRs and tetrad formation)

  • upsilon

DHPR image

RYR1

• ryanodine receptor

• Ca2+ release channel

• ligand-gated (DHPR is a ligand itself & activates)

• forms a homotetramer

• largest known ion channel

• RYR1s structures highly clustered square structures, arranged in regular rows in which corners of adjacent channels contact each other

• mechanical coupling happens here

RYR1 structure image

the SR and ryanodine receptors

• SR initially develops as ER containing generic ER proteins

• as the cell differentiates, it becomes greatly enriched with SR-specific protiens

  • Ca ATPase

  • calsequestrin (CaSQ): low-affinity CA binding prot

  • RYR1

• triadin and junctin: interconnectors between CaSQ, RYR1 and SR

  • keeps CaSQ immobilised and in close proximity to RYR1

junctional feet at SR image

focus on triad & junctional feet

junctional feet at SR

• SR vesicles contain junctional feet on outer surface

• possible candidates for a role in communication between surface and internal membranes

• DHPRs (voltage sensor) are grouped into tetrads

• 4 DHPRs composing a tetrad linked to a subunit of an underlying foot

junctional feet at SR arial view image

SERCA: reuptake of Ca2+

SERCA = sarcoplasmic endoplasmic reticulum ATPase

• Ca ATPase pump (transmemb prot)

• transfers Ca2+ from cytosol to sarcoplasm

SERCA: main domains

3 major domains:

1. phosphorylation site

2. nucleotide binding site

3. the actuator

SERCA: regulation of process

• phospholamban (PLN): a 52-AA integral memb prot that regulates Ca2+ pump in skeletal muscles

• sarcolipin (SLN): a small regulatory transmemb peptide that binds to and modulates SERCA pumps, reducing their Ca affinity and promoting uncoupled Ca cycling for muscle heat prod

decreases the apparent affinity of SERCA for CA

SERCA image

cardiac vs. skeletal muscle

• main difference is the modulation of the extent of thin filament activation

• in cardiac muscle: force development must be controlled mainly at the cellular level because each cardiac cell is activated on each beat

  thus, each cell must be able to undergo the full dynamic range shown by the cardiac output

learning outcomes

1. understand the 6 steps

summary - 1

summary - 2

summary - 3