Introduction to tissue mechanics, mechanotransduction & fibrosis

the order of soft tissue elasticity

• brain

• fat

• muscle

• cartilage

• precalcified bone

the young modulus (E)

• measure of stiffness or elasticity 

• E = stress/strain

what is the most common protein in the body

• ECM proteins such as collagen

how is decellularisation of liver achieved

soap solution is perfused throughout, cells are washed out, leaving extracellular matrix behind

cell signalling to matrix general cycle

• extracellular signalling - mechano signalling

• cellular responses to signals

• new matrix deposition; matrix degradation and modification

which cell behaviours can be controlled by stiffness 

Cell morphology (e.g., spreading and shape)

Contractility (how hard cells pull on their surroundings)

Propagation rate and apoptosis

Cell movement

Differentiation (commitment to lineage)

hydrogel

• synthetic polymer where cells can be cultured

• stiffness depends on density of crosslinks

• stiff gel causes cells to spread

• soft hydrogel causes cells to be more compact

how can cells sense stiffness 

• deforming their surroundings

• cells pull harder on stiffer substrates 

how do cells act on stiffer substances - proliferation and apoptosis

• cells grow faster on stiffer substrates

• apoptosis is lower

durotaxis

• cells move in response to gradients in the mechanical stiffness of their surroundings

• typically towards a stiffer substrate

how does soft and stiff substrates affect differentiation

• soft substrates drive differentiation to soft tissue types eg fats

• stiff substrates drive differentiation to stiff tissue types eg bone

mechanotransduction

• conversion of mechanical input into biochemical signal

integrins

membrane proteins that form focal adhesion complexes that tether the cytoskeleton to the matrix

actin

polymeric filaments; major component of the cytoskeleton; growth of filaments drives cell spreading

myosins

‘molecular motors’ pull against actin filaments, causing contractility

talin

a protein that deforms when pulled on, activating a signaling cascade (conversion into biochemical signal)

retrograde flow 

actin is polymerised at the edge of the cell and pulled by myosin II, pushing and pulling

what happens if cell is attached to stiff substrate

• talin will be deformed

• activates MAPK and RhoA pathway

• increased expression of actin and integrins to form contractility

LINC complex

• nesprins protiens and SUN proteins

• sun proteins of LINC complex binds to the nuclear lamina and lamin proteins

• tether the cytoskeleton across the nuclear membrane to chromatin

YAP1

• transcription factor

• nuclear localisation of YAP1 drives osteogenic differentiation by activating genetic programmes

fibrosis

• misregulation of feedback and loss of haemostasis causes cells to deposit too much matrix

• makes tissues stiffer

• mechanical properties are no longer matched to function

myofibroblasts 

• fibroblasts move to site of injury 

• activated to make myofibroblasts 

  • more contractile and secrete more ECM

2 fibrotic diseases

• severe atherosclerosis

• COPD

idiopathic pulmonary fibrosis

• area of fibrosis in lung alternate with normal lung

• clustered cystic air spaces - honeycomb

• dec gas exchange, lead to lung failure

symptoms of idiopathic pulmonary fibrosis

shortness of breath, finger clubbing, chronic dry cough

risk factors for idiopathic pulmonary fibrosis 

smoking, chronic viral infections, abnormal acid reflux, family history

what does lung slide show for idiopathic pulmonary fibrosis

myofibroblasts at leading edge of the disease region produce too much matrix

tight junctions

protein complexes anchored to the actomyosin skeleton

gap junctions

connexon channels that allow ion exchange

desmosomes

link intermediate filaments through adhesion plaques

nesprins

• tether the nucleus with the actomyosin cytoskeleton

focal adhesion 

large protein complexes that anchor cells to the extracellular matrix, connect actin to external

how are traction forces generated and where are they transmitted to

head of myosin ii pulls on actin filaments to generate traction forces, then transmitted to focal adhesions to deform the ECM

when are ecm bound growth factors activated and released

mechanical forces which trigger cellular signaling

which protein domains are unfolded with force 

• talin, paxilin

• this leads to stabilisation of nascent adhesions 

adherens junctions

link cytoskeletons of adjacent cells via clusters of cadherins