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Cellular and Molecular > Lecture 23- Cell polarity > Flashcards

Lecture 23- Cell polarity Flashcards

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 allowing the cell to have different capabilities, morphologies and functions

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2
Q

Why is cell polarity important?

A
  1. Required for asymmetric cell division

2. As the cytoplasm is dense, polarity allows cells to bring components together and organise them

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3
Q

What did Whitman discover about polarity fields in 1878?

A

Studied leeches and showed that distinct cytoplasmic domains are differentially partitioned to descendants and that these differences were reflected in different cell lineages

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4
Q

What did Conklin discover about polarity field in 1905?

A
  • Identified 5 different cytoplasm types that were differently inherited to determine tissue types
  • Showed a region that looked different was ultimately confined to a subset of cells after a number of cell divisions
  • Showed the formation of muscle cells and mesoderm were patterned at the early stages of ooplasmic segregation
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5
Q

What are the key functional requirements to be able to polarise a cell?

A
  1. Internal/external cues: signal sent to mark the need for an organisational change
  2. Marking the site: normally at the PM
  3. Decoding the site: sending a signal inside the cell to indicate something needs to happen
  4. Establishing the site: when more complexes are recruited to the site to allow the changes to occur
  5. Maintaining the site: duration will vary. Depending on what polarity has been established and why, the site may need to be maintained
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6
Q

What does cell polarity lead to changes in?

A

Changes in cytoskeleton organisation and membrane trafficking

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7
Q

Give 3 examples of where mechanisms can lead to cell polarity

A
  1. Asymmetric cell division
  2. Epithelial cell polarity
  3. Cell migration
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8
Q

Explain the establishment and maintenance of diverse cell shapes using common protein complexes

A
  1. Protein complexes build signalling centres that act as scaffold for small Rho-GTPases on specific membranes
  2. This control shape by regulating acto-myosin cytoskeleton and directing protein/vesicular trafficking
  3. These complexes are deployed in different combinations to yield distinct polarity outcomes
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9
Q

Explain how Rho-GTP is converted to Rho-GDP and vice versa

A

GAP proteins convert active Rho-GTP to inactive Rho-GDP

GEF proteins convert inactive Rho-GDP into active Rho-GTP

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10
Q

How were common protein complexes that are required to establish and maintain diverse cell shapes first identified?

A

In genetic screens in yeast, drosophila and C.elegans

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11
Q

Wha Rho-GTPase is essential for yeast to establish polarity?

A

Cdc42

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12
Q

Why must yeast generate cell polarity?

A

In order to grow and divide asymmetrically

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13
Q

Outline how budding yeast generate cell polarity

A
  1. Marking the sites: cortical membrane protein marks where a new daughter cell will be generated
  2. Decoding the site: signalling complex indicates a change needs to happen
  3. Establishing the site: Rho-GTPase Cdc42 gets activated and organises cytoskeleton and trafficking pathways
  4. Maintaining the site: feedback loops ensure complexes remain localised to ensure growth continues in the same part of the cell
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14
Q

What type of proteins form the core of cell polarity networks?

A

PAR proteins

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15
Q

How is the output of the cell polarity network established?

A

Established by opposing and complementary membrane domains that define a cells axis of polarity

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16
Q

How are PAR proteins maintained?

A

Set of proteins anteriorly and posteriorly which antagonise each other to maintain themselves

17
Q

What was the cell polarity network first identified in?

A

C.elegans

18
Q

Where is polarity first established in C.elegans?

A

The first asymmetric division on the zygote

19
Q

How is asymmetric cell division and consequently cell polarity established in C.elegans?

A
  1. Polarisation starts with entry of sperm into the oocyte
  2. The position of sperm entry defines the posterior end of the zygote
  3. The zygote divides asymmetrically along the A/P axis
  4. This produces a larger anterior cell (AB) and a smaller posterior cell (P1)
  5. The daughters are different sizes and committed to different fates
20
Q

What tissue types do AB and P1 cells give rise to and why are they different?

A

AB = ectoderm

P1 = mesoderm, endoderm and germ line tissues

Different tissues are generated as there is different distribution of PAR proteins at the end of cell division

21
Q

How were PAR genes discovered?

A
  1. A genetic screen

2. In PAR mutants, the size and fate difference between daughter cells AB and P1 was less pronounced/identical

22
Q

What do PAR genes encode and which protein is atypical and which is not conserved?

A

Par genes encode the par proteins PAR1-6

The 7th par protein is an atypical kinase called aPKC/PKC3

Par2 is not conserved

23
Q

How is symmetry broken on fertilisation?

A

The sperm delivers a MTOC

24
Q

How is the axis of polarity created post fertilisation?

A

Symmetry is broken on fertilisation when the sperm delivers the MTOC and defines the posterior pole

  1. The microtubules generated through the process of fertilisation recruit Par1/2 (posterior par proteins)
  2. This antagonises anterior Par proteins and they accumulate at the anterior cortical domain
  3. This results in the distinct localisation of par proteins
  4. Interactions between microtubules and the cortex results in pulling forces which act on the spindle causing the spindle to be de isolated towards the posterior end
  5. This causes the cleavage furrow to be generated asymmetrically giving rise to the size difference in AB and P1
  6. AB/P1 are different sizes and the cell fate determinants are unequally distributed at the point of cell division
25
Q

How are the par proteins localised during cell division?

A

Anteriorly localised = Par3/6/aPKC

Posteriorly localised = Par1/2

Maintained at the boundary = Par5

26
Q

How are par proteins and cell fate determinants redistributed after cell division?

A

Requires a directional and actin-myosin based process

27
Q

What is the significance of Par5 binding to phosphorylated proteins?

A

Phosphorylation is associated with proteins not binding to membranes so explains why Par5 is maintained at the boundary

28
Q

Outline the mechanism of neuroblast cell division in drosophila

A
  1. Neuro blasts delaminate from the ventral neuroectoderm and undergo repeated asymmetric divisions
  2. Each division gives rise to a small basal daughter cell (GMC) and a larger apical daughter cell
  3. The GMC divides once more to give rise to a neuron and a glia cell
  4. The apical daughter continues to divide asymmetrically
29
Q

How is cell polarity established in drosophila neuroblast cell division?

A

Polarity is established when the cells are still in the neuroectoderm layer

  1. When the neuroblast delaminate, Par3/6 are found in a stalk that continues to extend into the epithelium
  2. After delamination, they continue to localise to the apical region
  3. Par3/Baz anchors another complex (Insc/Pins) at the membrane to orient the mitotic spindle
  4. A scribble complex helps in spindle alignment
30
Q

What are similarities in asymmetric cell division/cell polarity in yeast, c.elegans and drosophila?

A
  1. All require asymmetric mitotic spindle alignment when setting up an axis
  2. All require polarity of key cortical determinants
  3. The cytoskeleton and membrane trafficking are important to allow the growth to occur
31
Q

What are some key properties of epithelial cells?

A
  1. They have polarised actin cytoskeleton which allows apical surface to construct
  2. Can orient their mitotic spindle to allow division in the plane of the epithelial sheet to increase their number OR perpendicular to the sheet to generate different daughter cells
  3. Rapidly loose their phenotype (via EMT) and re-acquire it (via MET) which allows the cells to delaminate and move sites
32
Q

What complexes interact with Par proteins and are important for establishing epithelial polarity?

A
  1. CRB complex apically

2. SCRIB complex basally

33
Q

What is EMT?

A
  1. Critical process during development which involves the conversion of epithelial apical-basal polarity into a migration axis with front-rear polarity
  2. Triggered by signals that lead to a loss of E-cadherin
  3. There is also asymmetric activation of Cdc42, Rac1 and RhoA
34
Q

How do epithelial cells arrive at there junctional organisation?

A
  1. Transport of Baz/Par3 to adheren junctions
  2. Diffusion of Baz onto basolateral membrane is restricted by Par1 kinase phosphorylation (and subsequent binding 14-3-3)
  3. Invertebrates/vertebrates have adheren junctions which contain E-cadherin which regulates a switch from the epithelial state
35
Q

How are epithelial cells established in drosophila?

A
  1. Following fertilisation, the embryo undergoes 13 rounds of nuclear division
  2. This produced a syncytial blastoderm of around 6000 nuclei underlying the outer membrane
  3. In cellularisation, the PM simultaneously encapsulates all nuclei forming the embryonic epithelium
  4. As cellularisation progresses, specialised junctions form on lateral membranes just below the apical surface
  5. Vertebrate epithelial form tight adheren junctions and desmosomes. Invertebrates form septate junctions directly below adheren junctions to prohibit the movement of small molecules/ions

Cellular and Molecular (24 decks)

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