Lecture 15 - establishment of apical basolateral polarity in health & disease Flashcards
What does cell polarity generate?
a wide variety of forms, allowing a diverse array of function
What is cell polarity?
- organisation of proteins at the plasma membrane and inside cells
- regions of the cell have distinct protein compositions
- allows different capabilities, morphologies & functions
What are the 2 main routes to generate diversity through polarity & cell fate decisions?
- Polar mother cells divide to generate daughters that have inherited different components
- Daughters could be equal at ‘birth’ but then become different by exposure to different environmental signals
What is a historical perspective into polarity in development and early cell fate determination?
Whitman (1878) - distinct cytoplasmic domains are are differently partitioned to leech descendants and that these differences were reflected in different cell lineages
Conklin (1905) - identified 5 different cytoplasm types in the ascidina (sea squid) oocyte that were differentially inherited to determine tissue types
These studies laid the foundations of modern developmental genetics & biology
What model organisms are widely used to understand cell fate decision?
C.elegans & Drosophila
What model organism is used to understand polarisation?
C.elegans (type of roundworm)
What arises after particular asymmetric divisions?
There are particular lineages that arise after particular asymmetric divisions, which leads to different tissues.
How is polarity established in the C.elegans embryo?
- P0
- AB & P1
P1: EMS & P2
EMS: MS & E
P2: C & P3
P3: P4 & D
What is the molecular machinery which establishes apical basolateral polarity?
- a genetic screen to identify key players in this asymmetric division led to the discovery of the PAR genes (for partitioning defective)
- in PAR mutants, the size & fate differences between cells AB & P1 are less pronounced and in extreme cases the 2 are identical
- The par genes encode the par proteins Par1-6 and the 7th member of the group is atypical protein kinase C (aPKC). Only Par2 is not conserved in other metazoans
- 7 members in PAR family and highly conserved
When is symmetry broken?
Following fertilisation
How is symmetry broken following fertilisation?
- Sperm entry point defines the posterior pole and the axis of polarity
- Sperm delivered a microtubule organizing centre (MTOC)
What does the sperm entry point define?
Defines the posterior pole and the axis of polarity
What does sperm deliver?
microtubule organizing centre (MTOC)
How is polarity established in the embryo?
PAR protein complexes mediate polarity through a negative feedback loop
What are features of PAR proteins and cell polarity networks?
- PAR proteins form the core of a cell polarity in many animal cells and in many developmental contexts
- the output of the network is one of mutual antagonism with the establishment of opposing and complementary membrane domains that define a cell’s axis of polarity
- The network was first identified in C.elegans, however useful, as well conserved throughout evolution
What maintains polarity?
Mutual antagonism
Summarise PAR proteins and embryo
- PAR proteins reliant on cytoskeleton to maintain polarity
- Microtubules recruit Par1 & Par2 to the posterior cortex
- Antagonises anterior Par proteins which accumulate at anterior cortical domain
- Distinct localization of the Par proteins, Par3/Par6/aPKC localize to the anterior cortex, Par1 & Par2 are at the posterior cortex and Par5 maintains the boundary
- phosphorylation is key in the feedback loops that allows poles to be defined
- Interactions between microtubules and the cortex results in pulling forces, which act on the mitotic spindle which causes the spindle to be displaced TOWARDS the posterior end.
- redistribution of the Par proteins and cell fate determinants requires a directional and actin-myosin based process
What are PAR proteins reliant on?
PAR proteins are reliant on cytoskeleton to maintain polarity
What is the key to cell polarity?
The cytoskeleton
What does actin do in microvilli?
Enhance the ability of gut to absorb components.
What do microtubules allow in microvilli?
Microtubules allow transportation
What do intermediate filaments contribute to microvilli?
Intermediate filaments contribute to the structural support of the cell.
How does polarity in bacteria occur?
- plasmid segregation
- ParM ensures that plasmid segregation occurs in the correct formation
What do junctions in epithelial cells allow?
Junctions in epithelial cells establish polarity through interaction with the cytoskeleton
Junctions are key in establishment of epithelial cells. Apical domain faces the environment - e.g. skin, nasal passages, gut - tight epithelium. Basolateral domain - faces inwards & connected to basal laminae, which is a thin layer of extracellular matrix. Architecture of cell based on junctions.
What are the 4 different types of junctions?
- Tight junctions
- Adherens junctions
- Desmosomes
- Gap junctions
What are tight junctions?
Tight barrier between epithelial cells, preventing movement of many molecules across the barrier. Some can move through paracellular transport e.g. ions
What are adherens junctions?
Connects actin filament bundle in one cell with that in the next cell
What is a desmosome?
connects intermediate filaments in one cell to those in the next cell
What are gap junctions?
allows the passage of small water-soluble molecules from cell to cell
What are the links from the cytoskeleton to the extracellular matrix?
These junctions bring the membrane together and decide how permeable the junction is
What are junctions important for?
Junctions are important for the barrier function of epithelia
What is a good experiment to show how good epithelia is at acting as a permeability barrier?
Placing a tracer molecule on one side of the membrane and seeing if it flows past the junction (or not).
What are cadherins important for?
Tissue organisation & the formation of junctional complexes
What does a xenopus embryo develop into?
Develop into neural plate, mesoderm and epithelial cells
What do cadherins have a role in?
key role in tissue organization
What is the affinity between cadherins?
Interactions between cadherins have a low affinity - so junctional complexes have low affinity, however there are a lot of interactions leading to overall high affinity
What does immunofluorescence allow?
It allows us to visualise apical & basolateral domains and junctional complexes
What does transcellular transport of glucose rely on?
Cell proliferation
- Composition of membranes have to be different to perform tasks like uptake of glucose in gut. Symporter needs to be in apical membrane
What membrane is the symporter found in, for uptake of glucose to occur?
Apical membrane
What is a symporter?
A symporter is a type of membrane transport protein that moves two or more molecules across a cell membrane in the same direction
What does the maintenance of polarity rely on?
Endocytosis, secretion & recycling
- Vesicle trafficking contributes to the establishment of polarity
- Delivery of material to the correct domain by secretion, endocytosis & recycling
Describe key features to development and tissue remodeling
- apical basal cell polarity is essential for functional epithelial
- cell polarity is lost in advanced tumours - invasive & malignant properties
- Metastasis is the largest killer in cancer
- Maintenance of cell polarity is highly dynamic - interference of human cancers is epithelial in origin
What does a loss of polarity lead to?
Malignancy
