Microtubule based cell motility Flashcards
Why are protists useful?
Can help understand how cilia and flagella are put together
Describe paramecium
Free living protozoan. Hundreds of cilia beat. Causes organism to twist and helps protist tumble and swim through the water
Describe parasites
Lots of parastitic protozoa that also move around with cilia and flagella. E.g. trypanosoma brucei
Single flagellum that is attached to the cell body for most of its length
Why are parasites good models?
The structure of eukaryotic cilium/flagellum is evolutionary conserved from these organisms to humans. Many of the proteins found in human cilia and flagella are also found in unicellular organisms, so good models for human development
Describe chlamydomonas
Non-pathogenic, so better model organism. Photosynthetic. Bi-flagellate
Why do cells move?
Cell migration is a central process in the development and maintenance of multicellular organisms (embryos, wound healing, immune response etc.)
Cells often migrate in response to specific external signals – Chemical and mechanical
Why is understanding why cells move important?
Errors during this process have serious consequences
An understanding of the mechanism by which cells migrate may lead to the development of novel therapeutic strategies for controlling, for example, invasive tumour cells.
Describe flagella structure
Slide 17 for image
Cell body
PM: plasma membrane
CW: Cell wall
Flagellum:
Has to built from something….
BB: Basal body – template for building the flagellum.
TZ: Transition zone, structurally different to BB
Axoneme:
Microtubules of the axoneme proper, which contains all molecular motors that are necessary for motility to happen.
The plasma membrane is continuous wth the flagellar membrane
Structural analysis of the axoneme of motile flagella
Triplet microtubules of basal body,
Transition zone in pairs,
Then central pair (9 + 2 axoneme)
Describe the basal body
a microtubule organising centre that extends a cilium or flagellum. Usually has the same structure as a centriole, with added accessory structures that help it stick to the membrane so it can stably extend a cilium or flagellum.
Describe a centriole
Non extending basal body
a microtubule-based barrel-shaped structure generally composed of 9 triplet microtubules. Found in many eukaryotic cells.
Describe centrosomes
found in metazoa (not protists) and made up of two centrioles surrounded by pericentriolar material. Forms the major microtubule organising centre of many animal cells.
Describe the spindle pole body
a microtubule organising centre in yeast and fungi that acts as the equivalent of a centriole, but it’s always embedded inside the nuclear envelope and it never forms a cilium or a flagellum.
Describe the 9 + 2 axoneme structure
Slide 22
Describe dynein arms
Outer and inner dynein arms exist
Attached to the A tubule. Molecular motors. Each doublet MTs has a single outer dynein arm and a single inner.
Coordinated beating in highly organised fashion in cilia and flagella is because of the activation of dynein molecular motor activity in an organised manner
Describe the structure of dynein arms
Huge protein complex.
Tail binds to A tubule of one outer doublet. Heads bind B microtubules of adjacent doublet in ATP-dependent manner.
When heads hydrolyse ATP, they move towards the minus end of the B microtuble. Creates sliding of one microtubule doublet relative to another
Describe nexin links
Between outer doublets, prevents microtubule sliding. Allows them to bend rather than slice
What is the issue with ATP and Cilia/flagella
Cilia and flagella generally perform high levels of energy demanding activity. Small width, so mitochondria cant fit. ATP diffusion from the cytoplasm is too slow.
Therefore: Ability to metabolize nutrients that they incorporate from their surroundings, such as glucose, fructose, mannose, or other carbohydrates, break down glycogen, or mobilize ATP produced by the nearest mitochondria by means of ATP-shuttles
Issue with building cilia and flagella?
Cilia and flagella are assembled from the tip, not from the basal body.
There are no ribosomes in cilia or flagella – all ciliary/flagellar proteins are made in the cell body
Describe intraflagellar transport
Bi-directional movement of particles along the doublet microtubules of the flagellar axoneme, btween the axoneme and the plasma membrane
- Specialised transport system
- When moving towards + end, use kinesin molecular motor
- If cargo needs to move down, a similar dynein is used
- IFT particles are located between the doublet microtubules and the flagellar membrane. Can also carry membrane proteins
Summarise IFT
IFT particles are transported towards the flagellum + end by kinesin-II and towards the – end by dynein 1b (also called cytoplasmic dynein 2).
They carry both individual proteins and complexes e.g. The radial spokes are partially assembled in the cell prior to IFT (flat-packed!)
IFT particles can also carry membrane proteins because they are linked to the flagellar membrane
Describe anterograde and retrograde IFT
- IFT delivers proteins to the growing ciliary tip, recyling turnover products and selectively transporting signalling molecules
Critical roles in cilia biogenesis, quality control and signal transduction - IFT involves long polymeritic arrays, IFT trains, which move to and from the ciliary tip under the power of microtubule-based motor proteins
Anterograde IFT mutants and characteristic phenotypes
Slide 36, mice still form the node, but with absent or short ciliar. Cilia do not form Kinesin II in -/- mice
Describe retrograde mutants and characteristic phenotypes
Slide 37
In Chlamydomonas, dhc1b deletion mutants build very short flagella with “blobs”.
Summarise mutants
Anterograde IFT mutants = absent or very short cilia/flagella
Retrograde IFT mutants = short cilia/flagella with a blob at the top
Describe mutants that build too many basal bodies
Have too many flagella
- Centrin is a structural protein found at the basal body
- It is important for controlling basal body duplication (controls number of basal bodies)
- Centrin KO’s or mutants = uncoupling of centriole duplication from cell duplication
- So completely random number of basal bodies, and if they are next to a plasma membrane, you get the wrong number of flagella too.
- Lots of Chlamydomonas mutants used to study this type of mutant – “variable flagella number” mutants
Describe Uni1
Transition zone mutant
- TZWT - has mix of doublets and triplets (F), and doublets by (J)
- TZUni1 - transition to doublet microtubules doesn’t happen
- Uni1 functions in the transition from triplet to doublet microtubules
- So, mutants have an unusual, elongated basal body structure rather than a flagellum.
Describe primary ciliary dyskinesia
Mutations in the dynein arms.
Both absent, no motile
Absent outer: most common
PCD is the second most common inherited respiratory condition after cystic fibrosis (CF has nothing to do with cilia)
All the genes linked to PCD are linked to cilia and always ciliary motility
Decreased ciliary movements specifically at the motile cilia (they are built okay)
Infected individuals can’t clear the mucus containing bacteria, viruses, pollutants
Respiratory tract of affected individuals have predisposition to chronic respiratory infection
Major problems with fertility (sperm immobility and ectopic pregnancies)
Describe katagener’s syndrome
Kartagener syndrome is a type of PCD that is also characterized by situs inversus totalis (mirror-image reversal of internal organs).
The signs and symptoms vary but may include neonatal respiratory distress; frequent lung, sinus and middle ear infections beginning in early childhood; and infertility.
People with Kartagener’s syndrome don’t have the internal organ positioning set up correctly.
Instead, you have complete randomisation of the left right axis within the body.
Describe how body axes are specified
During early embryogenesis.
Three basic polarity axes are set up, each at right angles to each other:
- Anteroposterior, dorsoventral, left-right
All comes back to the node. Cilia are critical for organ placement. Left-right axis is set up by the node
The mouse node is present from pre-somite to early somite stages. Unlike the rest of the embryo, the cells of the node are ciliated. The node looks like a pit, and also contains vesicles. Nodal cilia rotate, rather than beating in a sine wave.
Describe situs inversus
Failure in nodal signalling
Rotational beating directs a leftward movement of latex beads added to the node surface – evidence for leftward flow
Cilia are angled, not straight up = sweeping motion (can see by dropping latex beads)
Morphogens are moved to the LHS of the node pit
This allows asymmetric localisation of major transcription factors that regulate L-R organism polarity e.g. Pitx2
Sets up left-right polarity inside developing embryo
