cell polarity Flashcards
what is cell polarity
the organization of proteins inside, and at the surface of cells, such that regions of the cell have distinct protein compositions and the cell can thereby have different capabilities, morphologies and functions
why is cell polarity necessary
for cells to generate a wide variety of forms to perform a diverse array of functions.
stages of cell polarity development
- site is marked for binding
- what proteins/signals are required
- key proteins arrive at site to act on signals e.g making protein/cytoskeleton
- how does site continue to function and stay at site and hoe does it all come apart?
benefits of budding yeast as cell polarity model
- Yeast undergoes morphological changes in response to both internal and external signals.
- Yeast is genetically tractable, the entire genome sequence is known and annotated.
- It has been used to understand fundamental aspects of many key cell processes including the cell cycle, secretion and cell polarity.
internal signals yeast
in response to growth and division signals eg growth of a bud and cytokinesis
external signals yeast
in response to pheromones (for mating) and nutritional signals (cells can elongate
what does position of new bud in budding cells depend on
position of the new bud, which will grow to form a new daughter cell depends on the cell type, depends on whether the cell is haploid or diploid
budding patterns in haploid budding cells
AXIAL pattern in which both mother and
daughter cells are constrained to form buds immediately adjacent to the previous site of cell separation because they can find a cell to mate with
budding patterns in diploid budding cells
- Diploid cells bud in a BIPOLAR manner in which mother and daughter
cells bud at the poles of their ellipsoidal cells.- focus is on survival → find nutrition
genes identified for axial pattern
BUD10, BUD3, BUD4 and the septins
Products from these genes are involved in marking the mother bud neck during one cycle as a site for budding in the next cycle.
default cell polarity pattern
without specific proteins to recognise, cell defaults from axial to bipolar pattern
genes required for bipolar budding pattern
BUD8, BUD9, RAX2 and components of the actin cytoskeleton are involved
Products from these genes mark the ends of diploid cells
genes required for bipolar and axial budding pattern
BUD1, BUD2, BUD5.
Proteins encoded by these genes decode the axial and bipolar marks and signal to the machinery involved in generating the polarity axis.
Mutations in these genes cause a random budding pattern in both haploid and diploid cells.
what is responsible for polarisation of the cell cytoskeleton and other cell
components.
polarity establishment machinery
what proteins are involved in polarity establishment
family of Rho-GTPases
role of Cdc42
Cdc42 is a small GTPase of the Rho family, that is regulated through cycles of activation and inactivation by its binding partners Cdc24 (a GEF) and several GAPs.
how is polarity site to become established in yeast uding Cdc42 and Cdc24
The GEF (Guanine nucleotide exchange factor) for Cdc42 (Cdc24) binds to the active form of Bud1 at sites marked for budding. Cdc24 then binds Bud1 and can then activate Cdc42 to allow the polarity site to become established.
what is Cdc24
The GEF (Guanine nucleotide exchange factor) for Cdc42
purpose of polarisation
a polarised yeast cell with machinery in place for inheritance of genetic material and for movement of cytoplasmic organelles and other material from mother to daughter cell
steps for mating polarity
- marking site
- decoding site
- establishing site
- maintaining site
receptors that detect and bind pheremones
- members of the conserved G-protein coupled receptor family
- interact with a heterotrimeric G-protein that orchestrates the downstream cell response
why can cells only mate during G1
cells only mate during G1 as once mate nuclei fuse, during replication cell cant survive as can not do both at once = cell cycle arrest during mating
how do daughter cells have different properties from mother cells
- because it can inherit different RNAs
- cell cycles start at different times
- certain myosin filaments travel along actin and carry RNAs
candida albicans fungus in disease
- benign member of the mucosal flora
- commonly causes mucosal disease. If it becomes invasive there is substantial morbidity and in vulnerable patients it causes life-threatening bloodstream infections. 30-50% fatal
properties of candida that allow it to cause disease
- Studies have shown that the ability to switch between the yeast and hyphal form of Candida are central to the virulence of this organism
- Hyphal formation is stimulated at 37°C by serum or neutral pH.
- Hyphae are more adherent to mammalian cells and are important for tissue penetration.
- Yeast cells are carried more effectively in the bloodstream promoting fungal dissemination in the body.
what causes diversity in daughter cells
- Polarised mother cells could 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
steps in generating polarity and cell fate decisions
- Establishment of an axis of polarity (this involves marking a site, signalling and establishing – as discussed in last lecture)
- Mitotic spindle is positioned along the axis
- Cell fate determinants are often distributed differentially to daughter cells
role of Par proteins in cell polarity networks
- Par proteins form the core of a cell polarity network in many animal cells and in many developmental contexts
- if proteins go into wrong parts, it is recognised and changed to go back to where it came from
asymmetric cell division in Caenorhabditis elegans (nematode)
- The position of sperm entry defines the posterior end of the zygote.
- The zygote then divides asymmetrically along the anterior-posterior axis.
- produces a larger anterior cell (AB) and a smaller posterior cell (P1). The daughters are different in size and are committed to different cell fates.
- P1 → mesoderm, endoderm germline
- AB → ectoderm
when is symmetry of daughter cells AB and P1 broken
→ Symmetry is broken on fertilization when the sperm delivers a microtubule organizing centre (MTOC).
→ This site becomes the posterior pole and so defines the axis of polarity.
how do microtubules organise par proteins
- microtubules generated recruit Par 1 and Par2 and this antagonises anterior Par proteins and they accumulate at anterior cortical domain.
- This results in distinct localizations of the par proteins.
organisation of par proteins
- Par3/Par6/aPkc localise to the anterior cortex; Par1 and Par2 are at the posterior cortex and Par5 maintains the boundary.
- Phosphorylation is key in the feedback loops that allow the poles to be defined.
whitman 1878 research
studied leeches and showed that distinct cytoplasmic domains are differentially partitioned to descendants and that these differences were reflected in different cell lineages.
conklin 1905 research
identified 5 different cytoplasm types in the ascidina oocyte that were differentially inherited to determine tissue types.
main reasons why sister cells have different fates = diversity
- Polarised mother cells could 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
steps in generating polarity and cell fate decisions
- Establishment of an axis of polarity (this involves marking a site, signalling and establishing – as discussed in last lecture)
- Mitotic spindle is positioned along the axis
- Cell fate determinants are often distributed differentially to daughter cells
in many animal cells what forms the core of cell polarity
Par proteins
how does asymmetric cell division occur in Caenorhabditis elegans
- Early development in C.elegans is a series of asymmetric cell divisions
- Polarisation starts with entry of sperm into the oocyte where the position of entry defines the posterior end of zygote
- zygote then divides asymmetrically along the anterior-posterior axis
- produces larger anterior cell and smaller posterior cell = committed to different cell fates
P1 = mesoderm, endoderm germline
AB -> ectoderm
what defines posterior end of zygote in nematodes
sperm entry site
what to AB and P1 cells become
P1 = mesoderm, endoderm germline
AB -> ectoderm
what do par genes encode
encode par proteins 1-6 and the seventh member of the group is atypical protein kinase C (aPKC also known as PKC3 in C.elegans).
when is symmetry broken
on fertilization when the sperm delivers a microtubule organizing centre (MTOC)
role of microtubules in defining poles
- The microtubules generated recruit Par 1 and Par2 and this antagonises anterior Par proteins and they accumulate at anterior cortical domain.
- results in distinct localizations of the par proteins
where do different par proteins localise in the cell
Par3/Par6/aPkc localise to the anterior cortex; Par1 and Par2 are at the posterior cortex and Par5 maintains the boundary.
role of phosphorylation in defining cell boundaries
Phosphorylation is key in the feedback loops that allow the poles to be defined.
what sets up asymmetric plane of cells
Interactions between microtubules and the cortex results in pulling forces which act on the mitotic spindle which causes the spindle to be displaced TOWARD the posterior end.
how does redistribution of the par proteins and cell fate determinants occur
requires a directional (→ apical) and actin-myosin based process
where are CNS progenitor cells (neuroblasts) found in drosophila
found within a specific region of an epithelial monolayer called the ventral neuroectoderm
function of ventral neuroectoderm
develops neurones for system
how does drosophila neuroblast cell division occur
cells delaminate (cell moves out) and undergo repeated rounds of asymmetric cell division. Each division gives rise to a small basal daughter cell (called a ganglion mother cell; GMC) and a larger apical daughter cell. The GMC divides once more to give rise to a neuron and a glia cell, while the apical daughter continues to divide asymmetrically.
how many times does the ganglion mother cell of drosophila divide
4 times
small daughter, large daughter, neurone, glia
how is asymmetric division established
- Establishment of an axis of polarity
- Mitotic spindle is positioned
- Cell fate determinants are distributed
what layer is cell in when polarity is established
Polarity is established when the cell is still in the neuroectoderm layer and stalk region is still in ectoderm
what proteins are found in a stalk that extends to epithelium when neuroblasts delaminate
Cdc24, Par3, Par6
how is the mitotic spindle oriented in neuroblast development
- Baz anchors another complex (Insc/Pins) at the membrane in order to orient the mitotic spindle.
- ## A complex called Scribble helps in spindle alignment.
role of cell determinants Prospero and Staufen
regulate expression of specific genes in the GMC
Following cell division the GMC has a different fate because of asymmetric inheritance of these determinants.
what is cell migration dependent on
actin-rich cortex beneath the plasma membrane
3 main activities required for movement
- Protrusion – the pushing out of the plasma membrane in front of the cell
- Attachment – the actin cytoskeleton inside the cell is attached via interacting proteins across the plasma membrane to the substratum (eg extracellular matrix)
- Traction – the bulk of the cell body is drawn forward through a process of contraction
different types of protrusion and their properties
filopodia and lamellipodia
- filled with filaments of actin
- Filopodia or microspikes are a dense core of bundled actin filaments while lamellipodia are sheet-like broad structures.
function of stress fibres in cells
bundles of actin filaments that are involved in the contractility required to move the body of the cell forward.
role of small GTPases and proteins in migration
- Signals triggering cell migration converge on Rho small GTPases.
- proteins switch between active GTP and inactive GDP form
- different GTPases are responsible for generating different actin structures in cells because each of the proteins have different downstream activators
what is Rac protein responsible for
forming broader branched actin filaments
what is Rho protein responsible for
helps provide traction via stress fibres
what is chemotaxis
This is the movement of cells towards or away from a signal such as a diffusible chemical
mechanism of chemotaxis in neutrophils -> bacteria
- Receptors on the surface of the neutrophils detect very low levels of bacterial peptides.
- peptides can bind to GPCRs and this triggers intracellular activation of a heterotrimeric G-protein.
- leads to activation of the Rac GTPase leading to lamellipodial protrusion in the direction of the peptide gradient.
- Another pathway is switched on to activate Rho which enhances myosin based contractility causing movement of the cell body.
key properties of epithelium
- The apical side faces the external environment or lumen of the tissue; the basal side faces the basement membrane
- Lateral sides of epithelial cells adhere to each other through homophilic (bind to eachother) adhesion molecules, such as E-cadherin
- polarised actin cytoskeleton = forms curve and eventually tube
- orient mitotic spindle in preferable planes
- rapidly lose their phenotype and reacquire it = metastasis
how are epithelial membranes established and maintained in drosophila
- De novo apicobasal polarity is set up during Drosophila embryo cellularization
- nuclear division → nuclei line up round edge of cell → membrane comes down between each nuclei forming complete epithelium round cell (forms cellular blastoderm)
how are cell junctions established and maintained
- complexes that interact physically and genetically with the PAR complex/Cdc42
- Crumbs or CRB complex: CRB, Stardust (PALS in vertebrates)
- Scribbled or SCRIB complex: Disks large homologue (DLG), lethal giant larva (LGL) and SCRIB
what is epithelial-mesenchymal transition (EMT)
- EMT is a process during development and is also associated with cancer metastasis
- ## conversion of epithelial apical-basal polarity axis into a migration axis with front-rear polarity = apical and basal swap
