Download Cellular Biochemistry (BC4) – 21 Cell Polarity

Cellular Biochemistry (BC4) – 21
Cell Polarity
Naoko Mizuno – MPI of Biochemistry, January, 2017
What is a polarised cell?
Most cells of a complex organism are polarised. Even single-cellular organisms can be
polarised.
Definition of a polarised cell:
In a polarised cell, one part of the cell is structurally and functionally different
from an other part of the cell.
Examples of polarised cells
A muscle cell
(myofiber)
A ciliated cell
A migrating single cell
(Dictyostelium)
contraction - axis
fluid transport
cilia only at
one end.
A neuron
action potential direction (signal propagation)
cell has a “front” and a
”rear” to migrate.
Functions of cell polarity
Cell polarity equips cells with regional information. Polarised cells can have a “top” or
“bottom”, a “left” or “right”, a “front” or “rear”?
Cell polarity is essential for the correct function of the polarised cells.
Cell polarity is important for: - directed secretion and directed transport
- signal propagation in a defined direction
- contraction along an axis
- directed migration
and much more...
Directed transport within an axon of e.g. synaptic components;
pure diffusion would be far too slow.
Epithelia: a well studied paradigm of polarised cells/tissues
Epithelia come in different flavours:
Mono-layered epithelia:
Multi-layered epithelia:
Organisational principle of every epithelium
Free surface (air or gut lumen)
apical side
= at tip or top
tight junctions
basal side
Every epithelium has an apical side facing the surface of the tissue and a basal side that
mechanically anchors the epithelium to a basal lamina (also called basement membrane).
Apically located tight junctions seal the epithelium from the surface.
Epithelial polarity and the cytoskeleton
Localised transmembrane receptors connect the cytoskeleton of each cell with its
neighbours or the basal lamina (extracellular matrix) -> Mechanical stability.
The basal lamina
Scanning EM image of the basal lamina that supports the epithelium.
Tight junctions (I)
Tight junctions function as barriers to seal “outside” from “inside” of the animal.
They seal the extracellular space between cells.
Hence, they are located at the apical side of the epithelial cells.
Tight junctions (II)
Tight junctions allow a polarised distribution of membrane proteins, e.g.
transporters, because they seal/divide membrane into domains.
This is often essential for the cell’s function.
Polarised cells build polarised tissues
The skin of a mouse:
Protection against evaporation
The intestine wall (gut): Secretion
of enzymes
Resorption
of food
Tissues display a defined order of different cell types. The cell assembly is
stabilised by cell-cell junctions and by cell-extracellular matrix interactions.
Functional specialisations in the polarised gut epithelium
Absorptive cells have apical microvilli to resorb nutrients.
Goblet cells contain many mucous filled vesicles that will be secreted into the gut.
Summary I: Functions of polarised epithelia
- Resorption of nutrients (gut) or oxygen (lungs)
-  Secretion of digestive enzymes (gut and stomach)
or hormones, milk, tears, waste
- Barrier to protect against desiccation, salt loss and infections (skin) - Mechanical stability against ruptures (skin) - Movement of fluid over surface (respiratory tract) - Defects in function can lead to severe diseases
(eg. cystic fibrosis if mucous is defective)
How is polarity established?
Establishment and maintenance of polarity: the molecules
A Drosophila epithelial cell layer
integrins Crumbs complex sub-apical, Par3-Par6-aPKC at tight junction, scribble complex
lateral, integrins basal.
Integrins anchor epithelial cells to basal lamina
Integrins connect the keratin (and actin) filaments inside the cell to the extracellular
matrix, in particular to laminin and collagen IV, both located in the basal lamina.
They mechanically anchor (nail) epithelial cells to the basal lamina.
How were polar determinants identified?
Mutations in polar determinants: lessons from Drosophila
wt
stardust mutation (in Crumbs complex)
aPKC
aPKC
baz
aPKC or bazooka (Par3) mutation
Mutations in Crumbs, stardust (binds Crumbs), aPKC or bazooka (Par3) lead to Drosophila
embryos which have no skin (cuticle), only “crumbs” are left. Such embryos die at the end
of embryogenesis and no moving larvae can be found.
Such unbiased phenotypic searches (genetic screens) identified most polarity determinants.
Summary II: Important molecular features
of polarised epithelia
- Apical – basal polarity (top is molecularly different from bottom end)
-  Cell-cell and cell-matrix junctions at defined positions fulfill various functions
(mechanical stability, physical seal, functional subdivision of cell membrane...)
-  Molecular complexes (Crumbs complex, Par complex, Scribble, Integrin)
are important determinants of polarity
- All epithelia are supported by a basal lamina
Establishment of a polar epithelium:
Drosophila is a rather special case
Myosin in green, microtubules in red.
Both, the actin-myosin and the microtubule system are important to establish a polar
epithelium in Drosophila.
Establishment of polarity in neutrophils: chemotaxis
Neutrophils polarise upon an external signal, a chemokine (will be main topic of next
lecture).
Selective stabilisation of microtubules can polarise a cell
an “early”
developing
neuron:
Low amounts of
taxol stabilise
microtubules
Microtubules can promote axon versus dendrite outgrowth
Establishment of polarity in the C. elegans (worm) embryo Asymmetric cell division of the fertilised embryo.
Movie starts after fertilisation until first
asymmetric division (large anterior, small
posterior daughter cell).
Movie shows asymmetric segregation of
P-granules (green, mark future germ cells). Asymmetric cell divisions build and maintain the germ line
in the worm C. elegans
P-granules are always segregated only into one cell (usually the posterior cell). This
single cell will form the entire germline of the worm.
Establishment of asymmetry – Discovery of Par proteins
spindle
P-granules
Mutations in all of the Par (partition defective) proteins lead to symmetric divisions. The
embryo loses polarity, P-granules are in most of the daughter cells. All embryos die.
Establishment of asymmetry –
Par proteins localise to the cell cortex
Par2 (red) is anteriorly, Par6 (green) is
posteriorly concentrated at the cell cortex
Myosin is concentrated at the cortex with
already before the cell divides.
a concentration at the anterior. Myosin
This is somewhat similar to an epithelium.
mutants fail to segregate the Par proteins.
The complexes are asymmetrically segregated.
-> Actin-myosin network important to establish polarity. Summary III: Principles how to establish polarity
- Almost always requires actin-myosin and/or microtubule cytoskeleton of the cell.
-  Leads to the asymmetric localisation of molecules at the cell cortex (e.g. Par
complex, Crumbs complex), thus “marks” cortical domains.
-  Polarity can be induced from extracellular factors (chemokines, sperm entry).
- Polarity can be “inherited” from mother to daughter cells during cell division.
Asymmetric division – the concept of stem cells
Stem cells:
-  present in small numbers
-  not terminally differentiated, but competent
to produce cells with certain (often several
different) cell fates
- divide asymmetrically into a new stem
cell (self renewal) and a precursor cell
- precursor cell can divide several times to
produce a number of differentiating cells
Stem cells in skin and gut constantly produce new cells
Only basal cells divide. Some daughters move
apically and differentiate.
The rare stem cells are located at the
bottom of each crypt. Differentiated
cells are lost at the top of the crypt.
Too many divisions of the stem cells is bad: Cancer
Thousands of small and some large polyps
found in human gut with a mutation in the
tumor suppressor gene APC (adenomatous
polyposis coli).
APC is a negative regulator of Wnt
signalling. Active Wnt signalling induces
stem cell divisions in the gut.
Loss of polarity is often the first step towards cancer
EMT: epithelial to mesenchymal transition. Epithelial cells lose their polarity and start to
proliferate without control to form a primary tumor.
If these cells also become invasive, metastasis can form (--> Cancer).
Normal (polarised) breast epithelial cells –
and breast cancer cells.
Normal breast epithelial cells form polarised spheres with internal lumen (secrete milk) –
breast cancer cells do not form an epithelium, but are migratory, divide aggressively and form
clumps (tumors in the body).
Drosophila neuroblasts divide asymmetrically like stem cells
Polarity of the neuroblast is “inherited” from the epithelium.
Certain determinants are retained in the neuroblast (stem cell) others are specifically
inherited to the differentiating daughters.
A second asymmetric division determines the fate of neuron and glial cell.
Defects in asymmetric divisions can cause tumors
wild type
mutant
Defective asymmetric divisions in mutants of polarity genes can generate too many
neuroblasts and no neurons. All neuroblasts keep on dividing and generate a brain tumor.
Summary IV: Stem cells and asymmetric cell divisions
-  Division of stem cells is highly regulated depending on context (induced by
lesion, generally high in gut stem cells).
-  Stem cells are renewed at every asymmetric division.
-  Asymmetric divisions segregate (inherit) fate determinants into the daughter
cells to achieve different daughter fates.
- Loss of polarity can cause EMT, uncontrolled proliferation, migration and cancer.
-  Increased mitotic activity of stem cells can also generate tumors and may
cause cancer.
General summary of polarity
-  Almost all cells have some sign of polarity that makes certain regions of the
cell different to others.
-  Cell polarity is essential for the function of most cells (secretion, resorbtion,
communication, migration, mechanical stability, border etc.).
-  Epithelia are a great polarity model. Apical – basal polarity organises the
cytoskeleton within the cell as well as the cell-cell and cell-matrix interactions.
-  Establishment of polarity by external or internal signals that polarise the cell
cortex.
-  Polarity is important for stem cells and their asymmetric cell divisions.
Literature
Alberts et al., ‘Essential Cell Biology’, 3rd edition, 2010
Alberts et al., ‘Molecular Biology of the Cell’, 5th edition, 2008