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L2 Molecular Cell Biology > Cell polarity > Flashcards

Cell polarity Flashcards

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1
Q

What is cell polarity?

A

The organisation of proteins inside and at the surface of cells, so that regions of the cell have distinct protein compositions and the cell can have different capabilities, morphologies and functions

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2
Q

What are the key functional requirements in cell polarisation? (4)

A
  • Marking the site
  • Decoding the site (signals)
  • Establishing the site (protein/machinery organisation)
  • Maintaining the site (feedback loops)
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3
Q

Why is budding yeast a good model organism for cell polarity? (3)

A
  • Yeast undergoes significant morphological changes in response to internal/external signals
  • Genetically tractable
  • Entire genome sequence is known
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4
Q

What stain is used to follow budding events in yeast? (2)

A
  • Calcofluor fluorescent dye which binds to chitin in the yeast cell wall
  • Marks the birth scar as a bright ring which shows the site of previous cell separations
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5
Q

What determines the budding pattern in yeast cells?

A

Whether the cell is haploid or diploid

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6
Q

How do haploid cells bud? (2)

A
  • Axial pattern
  • Each bud forms next to the site of where a previous bud formed
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7
Q

What are the 2 mating types of haploid cells? (2)

A
  • MATa
  • MATα
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8
Q

Why do haploid cells bud in an axial pattern? (2)

A
  • Haploid cells want to be diploid
  • Budding next to each other increases the chance of finding another cell to mate with
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9
Q

How do diploid cells bud? (2)

A
  • Bipolar pattern
  • Bud from the ends of the cell
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10
Q

Why do diploid cells bud in a bipolar pattern? (2)

A
  • Diploid cells have no interest in mating
  • By budding from either end, the cells can move away and explore the environment for nutrition
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11
Q

How were genes identified that are required for marking the site for the axial budding pattern? (3)

A
  • Genetic screens of mutants
  • Proteins were recognised that when mutated, caused the yeast to do a different budding pattern
  • These mutations didn’t cause defects in the budding pattern of diploid cells
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12
Q

How were genes identified that are required for marking the site for the bipolar budding pattern? (3)

A
  • Genetic screens of mutants
  • Proteins were recognised that when mutated, caused the yeast to do a different budding pattern
  • These mutations didn’t cause defects in the budding pattern of haploid cells
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13
Q

What is the role of Bud1, 2 and 5? (4)

A
  • Decoding the site
  • Bud1 is a small GTPase
  • Bud2 and Bud5 activate/inactivate Bud1
  • Bud1 in its GTP-bound state recruits the machinery for polarisation
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14
Q

Which genes were identified as being required for decoding the site in both axial and bipolar budding? (3)

A

Bud1, 2 and 5

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15
Q

Which protein family is involved in establishing the site in budding?

A

Rho GTPase family

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16
Q

Which is the most important protein for polarity establishment?

A

Cdc42

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17
Q

What kind of molecule is cdc42?

A

Rho-type small GTPase

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18
Q

What is isotropic growth?

A

Cell budding occurring all over the surface

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19
Q

How does cdc42 establish the site for cell polarisation? (5)

A
  • GTP-bound Bud1 recruits proteins including cdc42 at the membrane
  • GTP-bound Cdc42 recruits and activates proteins including bni1 and sec3
  • Bni1 recruits actin filaments to the site
  • Sec3 is part of the exocyst complex where vesicles can dock for growth
  • Kinases link the process of polarity establishment to the cell cycle
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20
Q

Why do yeast cells need to establish polarity? (2)

A
  • Budding
  • Mating
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21
Q

What happens when you add pheromones to haploid yeast cells?

A

Change from rounded shape to forming projections (‘Shmoo’)

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22
Q

What kind of response is the change in shape due to pheromones?

A

Chemotropic

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23
Q

What peptide pheromone is released by MATa haploid cells?

A

a-factor

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24
Q

What peptide pheromone is released by MATα haploid cells?

A

α-factor

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25
Q

How is mating initiated in yeast? (3)

A
  • Marked: G- protein coupled receptor recognises the peptide pheromone released by the opposite mating type (a/α)
  • Decoded: beta-gamma subunit recruits a MAPK signalling cascade resulting in cell cycle arrest
  • Established: far1 recruits proteins including cdc42 to establish polarity
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26
Q

How are yeast daughter cells not identical to their mothers? (2)

A
  • Myosin proteins can move along actin filaments carrying different mRNAs
  • Causes asymmetric inheritance of specific factors
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27
Q

What is the limitation of yeast studies?

A

Yeast are unicellular so can’t be used to study intercellular interactions/signals in tissues and multicellular organisms

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28
Q

How can yeast cause disease? (2)

A
  • Candida Albicans is usually benign but can become invasive
  • Can be life threatening when it leaves the bloodstream and penetrates tissues
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29
Q

What yeast is used as a model organism?

A

Saccharomyces Cerevisiae (budding yeast)

30
Q

What feature of Candida Albicans underpins its pathogenicity? (3)

A
  • Can switch between the rounded yeast and hyphal form
  • Hyphae are more adherent to mammalian cells so can penetrate tissue
  • Can then revert to the yeast form in the blood and be carried by circulation causing systemic infection and invasion of more tissues
31
Q

What are the 2 main routes to diversity from cell division?

A
  • Mother cells could be polarised so they divide creating daughters which inherit different components
  • Daughters could be equal at ‘birth’ but become different from exposure to different environmental signals
32
Q

What are the important steps in generating polarity for cell fate decisions? (3)

A
  • Establishment of an axis of polarity (marking a site. signalling and establishing)
  • Positioning of mitotic spindle
  • Cell fate determinants are often distributed differentially to daughters
33
Q

What are the par proteins? (2)

A
  • Par proteins form the core of a cell polarity network
  • The output is a network of mutual antagonism
34
Q

What happens in early development of C. elegans?

A

A series of asymmetric cell divisions

35
Q

How does the first cell division happen in C. elegans? (2)

A
  • Point of sperm entry into the oocyte determines the posterior end of the zygote (P0 cell)
  • P0 divides asymmetrically into larger anterior cell AB and smaller posterior cell P1
36
Q

What is the fate of the AB cell in C. elegans?

A

Becomes ectoderm

37
Q

What is the fate of the P1 cell in C. elegans?

A

Becomes mesoderm, endoderm and germline

38
Q

What was observed in genetic screens for par mutants?

A

The size and fate difference between cells AB and P1 was less pronounced

39
Q

Which par proteins were discovered in the genetic screen of the first division in C. elegans? (3)

A
  • 7 par proteins in total
  • Par1-6 and the 7th member is atypical protein kinase C (aPKC/PKC3 in C. elegans)
  • All apart from par2 are conserved in other metazoans
40
Q

How is polarity established in the C. elegans zygote after sperm entry? (7)

A
  • Sperm delivers a microtubule organising centre (MTOC) which defines the posterior pole
  • MTOC recruits Par1/2 to the posterior end with LGL
  • This antagonises Par3/6/aPKC which accumulate in the anterior pole with cdc42
  • Par4/5 define the boundary
  • Mitotic spindle is aligned towards the posterior end
  • LGL causes a greater pulling force towards the posterior end
  • Actin/myosin machinery reorganises cell fate determinants
41
Q

Which proteins accumulate in the anterior portion of the C. elegans P0 cell before the first division? (4)

A
  • Par3
  • Par6
  • aPkc
  • Cdc42
42
Q

Which proteins accumulate in the posterior portion of the C. elegans P0 cell before the first division? (3)

A
  • Par1
  • Par2
  • LGL
43
Q

What is the MTOC?

A

Microtubule organising centre which is delivered to the oocyte by sperm entry and defines the posterior pole of the zygote

44
Q

Which proteins define the boundary between anterior and posterior of the C. elegans P0 cell before the first division? (2)

A
  • Par4
  • Par5
45
Q

What are neuroblasts in drosophila?

A

Progenitor cells found in the ventral neuroectoderm which develop neurons

46
Q

What happens in drosophila neuroblast cell division? (4)

A
  • Neuroblasts delaminate and do asymmetric cell division
  • Results in a larger apical daughter and a small basal daughter cell called the ganglion mother cell (GMC)
  • GMC divides only once more to form a neuron and a glial cell
  • Apical daughter divides again to form another GMC
47
Q

What is delamination?

A

The process by which an individual epithelial cell detaches from the epithelium

48
Q

How does asymmetric cell division occur in drosophila neuroblasts? (4)

A
  • Polarity is set up while the neuroblast is delaminating
  • Cdc42, par3/6 (bazooka) localise in the apical region
  • Bazooka anchors Insc/Pins complex which orients the mitotic spindle with Scribble
  • Cell fate determinants are transported to the basal end to the GMC including Prospero and Staufen
49
Q

What is the equivalent of par6 in drosophila?

A

Bazooka

50
Q

What 3 steps are required for cell migration?

A
  • Protrusion
  • Attachment
  • Traction
51
Q

What is protrusion in cell migration?

A

Pushing out the plasma membrane in front of the cell i.e. lamellipodia/filopodia

52
Q

What is attachment in cell migration?

A

The actin cytoskeleton inside the cell attaches to membrane proteins e.g. integrins which attach to the ECM (focal adhesions)

53
Q

What is traction in cell migration?

A

The bulk of the cell is dragged forward by a contraction process involving stress fibres

54
Q

What are the 3 main actin structures in cell migration?

A
  • Filopodia
  • Lamellipodia
  • Stress fibres
55
Q

What are filopodia? (2)

A
  • ‘Microspikes’
  • Thin projections containing a core of bundled actin filaments
56
Q

What are lamellipodia?

A

Sheet-like broad structures containing short-branched, cross-linked actin

57
Q

What are stress fibres? (3)

A
  • Long bundles of actin filaments which go along the length of the polarity axis
  • Perform contraction to drag the cell along
  • Involved in disassembly of the previous adhesions
58
Q

Which actin structure is triggered by cdc42 activity?

A

Filopodia

59
Q

Which actin structure is triggered by rac activity?

A

Lamellipodia

60
Q

Which actin structure is triggered by rho activity?

A

Stress fibres

61
Q

What are the 3 main proteins involved in forming actin structures for cell migration?

A

Small Rho GTPases:
- Cdc42
- Rac
- Rho

62
Q

What is chemotaxis?

A

The movement of a cell towards/away from a signal such as a diffusible chemical

63
Q

What is an example of chemotaxis?

A

Movement of a neutrophil towards a site of bacterial infection

64
Q

How does chemotaxis occur in neutrophils? (3)

A
  • Bacterial peptides are detected by G-protein coupled receptors on neutrophils
  • Causes activation of Rac which causes production of lamellipodia
  • Rho also activated which causes production of stress fibres for contraction
65
Q

What does the apical side of an epithelial cell face?

A

The external environment/lumen of the tissue

66
Q

What does the basal side of an epithelial cell face?

A

The basement membrane

67
Q

How do epithelial cells adhere to eachother?

A

Homophilic adhesion molecules e.g. E-cadherin

68
Q

What are the features of the epithelial polarity programme? (3)

A
  • Polarised actin cytoskeleton allows the apical surface to constrict forming a curve
  • Mitotic spindle can be orientated for division in the plane of the sheet (growth) or perpendicular (generate different daughter cells)
  • Epithelial cells can lose and re-acquire the epithelial phenotype (EMT/MET)
69
Q

What is EMT/MET?

A

Epithelial-mesenchymal transition and the mesenchymal-epithelial transition

70
Q

How is epithelial polarity set up in drosophila? (4)

A
  • Nuclei within the syncytium line up around the periphery and undergo cellularisation forming an epithelium = cellular blastoderm
  • Involves par complex and cdc42
  • Proteins are recruited to the apical face which define it as apical, results in mutual antagonism
  • In vertebrates tight junctions are set up after the rest of the polarity machinery to separate apical and basal
71
Q

What happens to an epithelial cell during EMT? (3)

A
  • Epithelial apical/basal polarity is converted into a front/rear migration polarity
  • Triggered by signalling causing loss of E-cadherin
  • Asymmetric activation of Rho GTPases (Cdc42 and Rac at the front, Rho at the rear)
72
Q

What is involved in the maintenance of cell polarity?

A

Positive/negative feedback loops involving phosphorylation

L2 Molecular Cell Biology (13 decks)

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