Can molecular properties be inferred from experiments on whole muscle cells

1
Can molecular properties be inferred from
experiments on whole muscle cells?
muscle cell (mN, mm)
molecule (pN, nm)

• Sharon Bullimore
• Meakins-Christie Labs, McGill University,
  Montreal, Quebec, Canada

2
Motor Proteins
cargo binding domain
myosin V

• chemical energy ?
• mechanical work

• Force pN
• Motion nm

Vale and Milligan (2000) Science 288 88-95.
actin filament
3
Motor Proteins in Muscle
2 µm
16 nm
1 µm
sarcomeres
molecule (myosin II)
myofibril
muscle
M. Ferenczi, Imperial College London
4
Motor Proteins in Muscle
16 nm
molecule (myosin II)
Is it really possible to infer molecular
properties from cellular mechanics?

• Force pN
• Motion nm

• Force mN
• Motion mm

• Back of the envelope calculation
• 1 cell contains
• 107 sarcomeres
• 1012 pairs of myosin molecules

5
Inhomogeneous Behaviour Within Muscle Cells
Force
Edman and Reggiani (1984) J Physiol 351 169-198.
6
Huxley and Simmons (1971)

• Current understanding of muscle contraction
• two types of filaments actin and myosin
• muscle shortening occurs by filament sliding
• sliding caused by projections from the myosin
  filaments
• Problem models could not simultaneously fit
  steady-state and transient behaviour

Nature 233 533-538.
7
Huxley and Simmons (1971)
Nature 233 533-538.
8
Huxley and Simmons (1971)
myosin filament
S2 (elastic)
myosin head
actin filament
q
Nature 233 533-538.
9
Huxley and Simmons (1971)

• Approach has been used to determine
• myosin stiffness
• myosin strain in active muscle
• size of working stroke, load dependence and
  temperature dependence
• effect of temperature on free energy drops
  between attached states
• rate constant for myosin rotation, load
  dependence and temperature dependence
• (Piazzesi et al., 2003 Decostre et al., 2005).

Nature 233 533-538.
10
Can molecular properties be inferred from
experiments on whole muscle cells?

• Possible approaches
• complexity theory
• multisarcomere model

11
Huxley and Simmons (1971)
Nature 233 533-538.
12
Huxley and Simmons (1971)
Nature 233 533-538.
13
Calculations

• sarcomere volume radius 0.5 µm, length 2 µm
• ? volume 1.57 µm3
• fibre volume radius 40 µm, length 5000 µm
• ? volume 25,000,000 µm3
• if fibre is 70 myofibrils (refs 2 and 3)
• ? sarcomeres (25,000,0000.7) / 1.57
  11,000,000
• sarcomere cross-sectional area 0.79 µm2
• approx 500 thick filaments per µm2 (ref 1)
• ? 395 thick filaments per sarcomere
• 39511,000,000 4,345,000,000 thick filaments
  per fibre
• 294 prs of myosin molecules per thick filament
  (ref 2)
• ? 1.281012 myosin pairs per fibre