lecture 6 - introduction to tissue mechanics

tissue microenvironments are physically diverse

• measure stiffness through elasticity

• calcified bone is very stiff

• brain is soft

• heart is stiff

• bone is rigid

young’s modulus(E)

• it is a measure of stiffness or elasticity

• measures how much force it takes to deform something

• the higher it is on the scale, more force is needed to deform it

mechanical properties are matched to function of the tissue

• bones need to be very stiff to carry weight

• brain needs to be soft to remodel, during development

• skull is stiff and protective, it allows the brain to be soft

extracellular matrix

ECM is 3d network of extracellular macromolecules found in multicellular organisms

ECM proteins such as collagen are the most common proteins in our bodies

• ECM defines the mechanical properties of our tissues

• mostly made of water, then ECM, most of the ECM is collagen

homeostatic systems are actively regulated to maintain a steady state

• cells are in constant contact with ECM

• signals are biochemical and cells can feel their surroundings, feel mechanical properties of their environment

• secrete new proteins and remodel proteins

• cells are reading signals from the environment

cell morphology - cells spread more on stiffer substrates

• can be controlled by stiffness

• place cells on substrate, control mechanical properties of that substrate

• on stiff gel, cells are bigger and spread out

• cells get bigger on stiffer substrate, nuclei also becomes bigger

contractility - cells pull harder(more contractile) on stiffer substrates

• cells can only feel stiffness by deforming their surroundings

• able to know mechanical properties by interacting with it

• experiment- cells cultured on a spring, the stiffer the spring, the harder the cell pulls against it

• it reaches a plateau, a cell can only exert so much force, reached maximum

proliferation

cells grow faster on stiffer substrates

apoptosis - apoptosis is lower on stiffer substrates

• cell on stiffer environment are less likely to undergo apoptosis

• there is faster proliferation, so apoptosis is slower on a stiff substrate

movement(durotaxis) - cells migrate towards stiffer regions

• cells move from soft to stiff environment

• durotaxis is the movement across a gradient o stiffness

differentiation - stiffness can direct cell fate

• soft substances drive differentiation to soft tissue types (e.g.fat)

• stiff substrates drive differentiation to stiff tissue types (e.g.bone)

• stem cells differentiate like bone on stiff environment

• form adipose when on soft tissue

mechanotransduction is the conversion of a mechanical input into a biochemical signal

• mechanical input converted to biochemical signalling pathway

cells can only feel stiffness by deforming their surroundings

• needs feedback mechanism to know if surrounding is soft or stiff

• a biochemical signal leads to a pathway which can modify the protein

cells need mechanisms of

• force generation - acto-myosin contraction

• force transmission- cytoskeleton

• mechanosensing - conversion into biochemical signals

focal adhesion complex and cytoskeletal tension (i)

• 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 signalling cascade(conversion into biochemical signal)

• formation of filaments drives cell spreading

• actin pushes forwards, filaments are dissolved and pulled back- retrograde flow

focal adhesion complex and cytoskeletal tension (ii)

• cells pull on their surroundings

• actin is polymerised at the edge of the cell and pulled by myosin-II

• if surrounding is soft, soft surroundings will be deformed

protein unfolding releases new domains and interactions

• this activates downstream signalling pathways

• if stiff, proteins in the cell are deformed

• binding sites are revealed, heads up signalling cascade

• if substrate is stiff, signalling proteins are activated

• MAPK- mitogen activated protein kinase

• RhoA- transforming protein RhoA

• MAPK and RhoA cause the cell to make more myosin and actin, so the cell pulls harder

• increased expression of myosin leads to increased contraction

mechanosensitive ion channels

• form pores

• TRPV4- transient receptor potential vanilloid 4

• opening of pore allows ions to enter and leave which can lead to a signalling cascade

transmission of force to the nucleus

• transferred into chromatin

• different sites will be active

disruption of LINC complex blocks mechanotransmission to nucleus

• ties cytoskeleton and cytosol to the nucleus

• inside of nucleus tethers to lamina, leads to robustness to nucleus

• organisation of chromatin can regulate its activity

mechanosensitive translocation of transcription factors

• YAP1 has roles in development and cancer

• regulated by whether it’s in the nucleus

• if protein is out of the nucleus, it becomes inactive because it can’t interact with DNA

• moving transcription factor can be regulated mechanically

• stiff environment, YAP moved in nucleus

• YAP drives cell differentiation

• mechanical regulation of transcription factors allows control of specific genetic programmes

fibrosis is dysregulation of extracellular matrix

• misregulation of feedback and loss of homeostasis cause cells to deposit too much matrix - firbrosis

• when out of balance, too much matrix is produced

fibrotic diseases

• fibrosis makes tissues stiffer

• mechanical properties are no longer matched to function

• tissue mechanics are matched to function

• fibrotic system has too much matrix which pushes cells out of healthy position, moves on different position on the scale

role of fibroblasts

• fibroblasts are the cells responsible to synthesising extracellular matrix

• fibroblasts can move towards sites of injury(e.g. durotaxis)

• they are necessary for wound healing

• can be cultured in 2D

• can undergo durotaxis

• fibroblasts can be activated at sites of injury to make myofibroblasts

• myofibroblasts are more contractile and secrete more ECM

fibrotic diseases

• too much matrix can compromise gas exchange or lead to too little blood

• some diseases are sever atherosclerosis and COPD(chronic obstructive pulmonary disorder)

tissue repair and fibrosis

• scarring can be healthy

• rapidly prevents further damage

• scarring is healthy response to tissue injury

• too much scar tissue can lead to loss of function

• excess scar tissue can restrict movement and prevent healthy function

• further damage exacerbates the injury

case study- idiopathic pulmonary fibrosis (i)

• air sacs in thin membrane allows gas exchange

• IPF is fibrosis of the alveoli, prevents gas exchange, can’t get rid of CO2

• affects 14-43 per 100,000 people

• seen more commonly in men

• common symptoms -

  • shortness of breath

  • chronic, dry cough

  • finger clubbing due to growth factor signalling

• occasional symptoms

  • fatigue

  • weakness

  • weight loss

• cause is unknown but risk factors- smoking, environmental exposure, chronic viral infections, abnormal acid reflux, family history of the disease

• average time to diagnosis is 1-2 years after onset of sytmptoms

• 50% die within 2-3 years after diagnossi

• aging is a risk factor

case study- idiopathic pulmonary fibrosis (ii)

• build up collagen I causes stiffness and blockage of gas exchange

• change in tissue mechanics

• can lead to organ failure

• positive feedback loop