Lecture 24 - Mechanobiology II

Question 1

What is the lung on chip model?

Answer 1

Lung on chip - model to introduce organ on chip principal - facilitating in-vitro models of the future - PDMS based

Question 2

What does it mean that lung-on-chip is PDMS-based?

Answer 2

When we say that a lung-on-a-chip is PDMS-based, we mean that the chip is made primarily of polydimethylsiloxane (PDMS), which is a type of silicone polymer.

Question 3

Why is lung on chip PDMS-based (type of silicone polymer)?

Answer 3

• biocompatibility
• flexibility & transparency
• microfluidic properties

Question 4

What is stress measured in?

Answer 4

θ (theta) or N/m^2

θ (theta) = F/A

F = force
A = area

Question 5

What is strain measured in?

Answer 5

ε (epsilon) - unitless!

ε (epsilon) = ΔL/L₀

ΔL is the change in length,
L₀ is the original length of the material

Question 6

What is Stiffness measured in?

Answer 6

E, stress over strain, N/m^2 = Pa)

E = θ (theta)/ε (epsilon)

Question 7

At what rate does strain (length, volume) increase as stress (tension, pressure) increases?

Answer 7

Exponential

Question 8

How do different tissues have different stiffness?

Answer 8

SOFT TISSUES - brain, embryo and breast

INTERMEDIATE TISSUES - liver, kidney, fat, dermis, lung

STIFF TISSUES - muscle, cartilage, bone

Question 9

What does ECM (extracellular matrix) regulate?

Answer 9

stem cell differentiation

Question 10

What occurs to cells in an environment with low stiffness?

Answer 10

cells become positive for neuro marker

Question 11

What occurs to cells in an environment with medium stiffness?

Answer 11

cells become positive for myo marker (muscle cells)

Question 12

What occurs to cells in an environment with high stiffness?

Answer 12

cells become positive for osteo marker

Question 13

What is an example of how an increased stiffness can be used for diagnosis of diseased tissue?

Answer 13

Fibrotic liver displays shear stiffness of approximately 8 kPa, whereas a normal liver has a shear stiffness of approximately 2 kPa.

Question 14

How can increased tissue stiffness assist intervention and therapeutic decisions?

Answer 14

• e.g. biopsy & treatment
• assessing prognosis and candidacy for treatment in patients with chronic liver disease
• saves patients that don’t need the discomfort and risk of complications associated with liver biopsy

Question 15

What is one way a tumour can be identified?

Answer 15

• Tumour has a different stiffness form surrounding tissue, which allows a lump to be identified
• Surrounding tissues can become more stiff due to the expansion of tumour

Question 16

What does a stiffened extracellular matrix promote?

Answer 16

the epithelial to mesenchymal transition and invasion
- During EMT (Epithelial-to-mesenchymal transition), epithelial cells lose their tight cell-to-cell junctions (such as tight junctions and adherens junctions), lose their polarity (the organization of the cell structure), and acquire the ability to move.

Question 17

What can mechanical forces promote?

Answer 17

Tumour aggression

• increased epithelial to mesenchymal transition
• mechanical stress can disrupt cell adhesion (how cells stick together) -INVASION
• Shear stress (like fluid movement through blood vessels) can also increase the ability of tumor cells to invade through the ECM or surrounding tissue layers - INVASION

Question 18

Describe the mechanical control of tumour cell fate

Answer 18

• Tension activates integrins (cell surface receptors that connect the cell to the ECM) and other mechanical sensors on the cell membrane, triggering intracellular signaling pathways like the Rho GTPase pathway, which controls cell shape, movement, and survival.
  These signals can promote cell proliferation, survival, and invasion, allowing tumor cells to grow and spread.
• High mechanical tension can push tumor cells to undergo EMT, where they lose their epithelial characteristics (like strong cell-cell adhesion) and adopt more mesenchymal properties (like increased mobility and invasiveness).
  This transition is crucial for tumor cells to acquire the ability to migrate and invade surrounding tissues, which is a key step in metastasis.

Question 19

What are 3 types of mechano-sensors?

Answer 19

• Piezo channels
• Integrins
• Caveolae

Question 20

What are piezo-channels?

Answer 20

stretch-activated ion channels that have 38 transmembrane domains

Question 20

What actually activated Piezo?

Answer 20

Piezo is mechanically activated by membrane/cortical tension

Question 20

What role does actin play in piezo channels?

Answer 20

Lower forces needed to open the channel with actin, compared to without actin (almost double).

This means actin is supporting the force-channel connection.