Advanced Molecular Cell Biology - Lecture 15

Cell polarity generates a wide variety of forms (i.e. shapes and sizes), allowing?

a diverse array of functions

give examples of different types of polarity:

• migrating fibroblast

• vesicles moving along a microtubule network

• yeast cell forming hyphae, the proteins important for hyphae formation moving to a new environment, like relocation to the tip of the hyphae to sample and push through the environment

• planar cell polarity (in hair cells in hearing or wings in drosophila), the way different cells are organised to allow different behaviours; this is dependent on different protein concentrations in different parts of the cell

the ___________ is key to cell polarity

cytoskeleton

why is the cytoskeleton important in cell polarity?

Actin cortex, microtubules, and intermediate filaments maintain the polarised state of the cell. They form a meshwork of the cytoskeleton beneath the plasma membrane, to which the actin is attached and allow movement; microtubules allow movement of vesicles forwards and backwards in the cell

what can help us visualise apical,m basolateral domains and junctional complexes?

immunofluorescence

in mammalian epithelial cells what do tight junctions do?

Block movement through the epithelial layer (they are located above the adherence complexes)

What does adherence junction allow?

allow adherence proteins to stick tightly together in the process of adhesion

cell polarity requires organisation of proteins at the plasma membrane and?

inside cells

regions of the cell have distinct ________ compositions, this allows different capabilities, __________ and functions

protein, morphologies

why is polarity important is animals?

• asymmetric cell division

• tissue patterning

• directional transport

• polarised growth

• directional movement

why is polarity important in plants:

• asymmetric cell division

• tissue patterning

• directional transport

• polarised growth

• environmental responses

What did Whitman find in 1878 that contributed to studies of polarity in development and early cell fate determination and laid the foundations of modern developmental genetics and biology?

distinct cytoplasmic domains and differentially partitioned to leech descendants and that these differences were reflected in different cell lineages

What did Conklin find in 1905 that contributed to studies of polarity in development and early cell fate determination, and that laid the foundations of modern developmental genetics and biology?

Identified 5 different cytoplasm types in the ascidian oocyte that were differentially inherited to determine tissue types

what are the 2 main routes to generate diversity?

intrinsic and extrinsic

describe intrinsic polarity:

Polar mother cells divide → daughters inherit different components

via localised determinants or an asymmetric division plane

describe extrinsic polarity:

daughters are equal at ‘birth’ but → exposure to different environmental signals

Signals can come from other cells or between daughter cells

what are model organisms to study cell fate?

C.elegans and drosophila

C.elegans has how many founder cells that give rise to all the different tissues via asymmetric cell division?

6

P0 gives rise to?

What is a key player in asymmetric division, and what did the discovery of this gene lead to?

• par genes (partitioning defective)

• In mutants, the size and fate difference between the daughter cells AB and P1 are less pronounced and in extreme cases the 2 are identical

• The par genes encode the Par proteins Par1-6, and the seventh member of the group is atypical protein kinase C (aPKC).

• Only Par2 is not conserved in other metazoans (multicellular animals)

Symmetry at P0 is broken when?

following fertilisation

what defines the posterior pole and the axis of polarity of the embryo?

sperm entry point

What does sperm deliver to the embryo?

a microtubule organizing centre (MTOC)

how is polarity established in the embryo?

There is antagonism which prevents certain Par proteins from accumulating at the anterior or posterior pole; this is mediated by phosphorylation, as one of the proteins is a kinase (therefore, cell division causes very different daughter cells)

describe the hierarchy of events in the establishment of polarity:

• microtubules recruit Par1 and Par2 to the posterior cortex

• antagonises anterior Par proteins, which accumulate at the anterior cortical domain

• distinct localisations of the 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

• interactions between microtubules and the cortex result in pulling forces which act on the mitotic spindle, which causes the spindle to be displaced toward the posterior end

• redistribution of the Par proteins and cell fate determinants requires a directional and actin-myosin-based process

in epithelial cells, where do we find specific Par complexes which maintain polarity in an adult epithelial cell?

apical and basolateral domains as well as junctional contacts

Why is maintaining epithelial sheets important?

so the epithelium faces the environment, so pathogens can’t penetrate into the body

When tissues need to change shape, i.e. the basolateral domain is extended, what happens to the size of the apical domain?

reduces

define EMT

When epithelial cells migrate out of the epithelium and migrate to become more mesenchymal (important in neural tube formation)

regulation of endocytosis and exocytosis is key for?

membrane polarity (i.e. if material is delivered to the apical instead of basolateral domain it will be re-endocytosed and retargeted to the correct domain)

Why are cadherins important for the maintenance of the epithelium?

They are adhesive molecules which hold the cells together. In mammalian cells, they are often found along basolateral connections between cells

Par proteins form the core of a cell polarity network in many animals and in many developmental contexts. The ________ of the network is one of mutual _________ with the establishment of opposing and ___________ membrane domains that define a cell’s axis of polarity. The network was first identified in _________.

output, antagonism, complementary, C. elegans

_____________ transport of glucose is an example of the need for cell polarity

transcellular

How many asymmetrically distributed transporters do you need to transport glucose across the stomach lining?

3

how does glucose travel against it’s concentration gradient into the cell?

via the glucose sodium symporter

how does glucose leave an area of high glucose and enter the bloodstream?

through a carrier, passive transport, in the membrane

How is excess sodium removed from the epithelial cell to ensure the gradient is maintained?

by the sodium-potassium ATPase pumping sodium out of the cell

how is polarity established in cells?

• Vesicle trafficking contributes to the establishment of polarity

• Delivery of material to the correct domain in the process of secretion

• endocytosis and recycling

why is polarity required in bacteria?

for plasmid segregation

what are plasmids?

Extra DNA separates from circular DNA, they code for antibiotic resistance, virulence factors or metabolic advantages

how does plasmid segragation work?

• recruitment of the cytoskeleton for plasmids to be segregated into daughter cells

• Cytoskeletal protein ParM, an actin cytoskeleton-related protein to mammalian actin, can assemble into a long fibre. This pushes the plasmids to either end of the cell and ensures that the cells have the required components

apical ______ cell polarity is essential for functional epithelia. Maintenance of cell polarity is highly _______. Interference with membrane traffic disrupts _______.

basal, dynamic, polarity

majority of human cancers are?

epithelial in origin

loss of polarity leads to?

malignancy

cell polarity is lost in?

advanced tumours, particularly with invasive and malignant properties

The amoTL family of proteins is a scaffolding protein with binding domains for ZO-1 and other junctional proteins. They interact with actin and are postulated to integrate apical polarity, junctional formation and the actin cytoskeleton. Why is there a loss of polarity when Amotl2 is overexpressed?

High levels of AmotL2 cause Par3 and Crb3 proteins to be in the wrong subcellular location