Lecture 26- Mechanobiology II Flashcards
What is an alternative to culturing on petri dishes where the stiffness can be altered and tailored?
Hydrogels (mix of bisacrylamide and acylamide)
The ratio will alter the stiffness. The more bisacryalmide, the stiffer the dish will be
What contributes to determining MSC tissue fate?
Partially determined by the stiffness of the surrounding environment
How can you culture MSC tissues to have different fates and how can they be indentified?
Culture on hydrogels with a soft matrix will form brain tissue/neuronal cells etc
Culture on hydrogels with stiff matrix will form bone cells
The type of tissues that forms can be identified using markers e.g neuro, myo or osteo markers
What can be concluded from studies where cells cultured at different matrix stiffness will form different tissues?
ECM stiffness regulates stem cell differentiation
Give 2 examples of diseases associated with an increase stiffness
- Liver fibrosis
2. Cancer tumours (e.g breast cancer)
How can an increase in ECM stiffness diagnose liver fibrosis?
The liver increases in stiffness in fibrosis
Fibrotic livers have an increase production and deposition of ECM stiffness which can be viewed and measured using an MRI/elastogram
How can knowledge of tissue stiffness assist intervention and therapeutic decisions in patients with liver disease?
Non-invasive elastogram procedures can help identify the stage of chronic liver disease
Allows some patients whose disease is less severe to avoid liver biopsies
How can tissue stiffness help diagnose cancer?
Palpitation of breast tissue can allow a lump to be discovered
A lump suggests the lump tissue is stiffer than the surrounding tissue
This understanding can aid the diagnosis and knowledge about the progression of cancer
How do mechanical forces promote tumour aggression?
- Tumour tissue increases the amount of secreted ECM. This leads to an increase in ECM deposition And an increase ECM stiffness around the tumour
- Crosslinking between collagen and upregulation of crosslinking enzymes increases ECM stiffness
- Fibroblasts also migrate to the tumour which differentiate into myofibroblasts which can contract and contribute to ECM stiffness surrounding the tissue
What 3 things does tumour stiffness effect?
- Proliferation
- Migration
- Metastasis
Mechanical control of tumour cell fate- what happens under high tension/ECM stiffness?
- ECM stiffness increases the number of focal adhesion kinase activation and increases Rho
- YAP/TAZ are then activated which moves into the nucleus and regulate transcription and leads to upregulation of collagen
- This leads to further increase in secretion of collagen and deposition
How does matrix crosslinking force tumour progression?
By enhancing integrin signalling
Give 5 techniques to measure cellular mechanical forces
- Atomic force microscopy
- Micropipette aspiration
- Optical tweezers
- Magnetic tweezers
- Uniaxial stretcher
Explain the technique of atomic force microscopy
- Allows a measure of indentation on top of the cell
- Centilever with a probe which touches the membrane and produces an indentation
- Indentation can be measured using laser deflection to calculate how much force was applied to produce the indentation
- Plot graph of indentation vs force applied
- Graph allows the stiffness of the cell to be calculated
Give 3 examples of mechanosensors
- Piezo ion channel
- Integrins
- Caveolae
Explain how piezo ion channels are involved in mechanotransduction
The translation of a mechanical signal (increase in membrane tension) into a chemical signal (ion influx through the channel)
What are the 2 proposed models for piezo ion channels?
Model 1: the channel can induce curvature of the membrane and the stretching of the membrane causes the ion channels to open
Model 2: force from lipids and actin filaments push the membranes apart to open the ion channels
Explain how integrins behave as mechanosensors
- The point where integrins interact with the ECM forms focal adhesions
- Talin interacts with the actin-cytoskeleton which are connected to stress fibres
- The stress fibres transduce a force from the PM to the nucleus
- This leads to the opening of nuclear pores or the rearrangement of chromatin and changes transcription
- Talin partially unfolds when force/stress is applied and allows the binding of Vinculin
- This leads to the stabilisation of the focal adhesions which mature into fibrillar adhesions by replacing Talin and Vinculin with Tensin
- These changes lead to downstream signalling/mechanotransduction cascades
How do integrin proteins change when there is no force and then force?
No force: Talin is folded and Vinculin cannot bind
Force: Talin unfolds and Vinculin binds
Therefore Vinculin is important for regulating the stiffness of the cells and responding to ECM stiffness
What are caveolae and how to they act as mechanosensors
- Caveolae are invaginations of the PM and flatten upon stress/increase in membrane tension
- This flattening leads to the release of the Cavin complex
- Cavin detaches from the caveolae and elicits downstream signal mechanotransduction cascades and alter transcription in the nucleus
