CELL BIOLOGY: LIFE IS DICTATED BY CELLS, ORGANELLES, MOLECULES
Everything is regulated by molecules → input to the molecules, to the cells for normal behaviour
Order of magnitudes
- human approximately 2 metres
- thumb a few centimetres (1/2 cm in diameter) (eye, magnifying glass)
- finger prints close to 2 mm (eye, magnifying glass)
- cells at 0.2 mm (use light microscope)
- organelles at 20 μm (use light or electron microscope)
- cisternae in mitochondria at 2 μm (“)
- macromolecular complexes like ribosomes at 0.2 μm (“)
- protein structures (use electron microscope, X-ray diffraction, and nuclear magnetic resonance, and
also cryo electron microscopy) at 20 nm
- see the atomic structure of proteins: important for adding a small molecule (atomic level is 2 nm)
that can repair the defect or inhibit the effect of the defect, if there is something wrong in a protein
- beyond atoms no longer in cell biology (than we go to physics)
Antoni van Leeuwenhoek was a scientist that was using a microscope and studied life (dirty water,
smear from the nose, sperm cells)
Zernike won a nobel prize for improving the microscope
Most cell-cell contacts are identified with electron microscopes and their names is
also derived from their appearance in the electron microscope
Cell-cell junctions: discovered with electron microscope
- cell-cell junctions are multi-molecular complexes
- apical, basal + lateral (basolateral) sides
- cells are held together with all kind of cell-cell junctions
- hemidesmosomes: junction that adhere cells to the ECM
- tight junctions most apical (membranes are very tight, almost fused, hardly any
space in between the membranes)
- adherens junctions: provide strength in cell-cell adhesion
- more electrical dense structure (better to see in the EM than adherens junction): desmosome
- gap: gap junction
- if there are diseases that affect the cell-cell junctions and if we know what the underlying molecular
defect is, we might think of curing or preventing the disease
What are the two major underlying causes of disease? Very broadly 80% of the diseases maybe even
more you can easily classify in two defects
1. Genetics (mutations in DNA)
2. Pathogens (affect life), autoimmune diseases (responses), how the metabolism is
disturbed
Tight junctions
- picture (light microscope): approximately 20 cells
- antibodies to detect one protein (zona-occludens protein-1 is a tight junction
protein)
- cross-section in group of cells: very strong label only where the cells make contact
and where they are exposed to the environment (medium)
- only at the very apical side you see the tight junction protein
,- tight connection of the two membranes → prevents entry of molecules
from the outside into the human body (epithelial cell in gut)
- molecules cannot pass! → barrier
- tight junctions also provide a barrier in the membrane: membranes are
fused together by protein complexes → proteins cannot freely diffuse
from the apical plasmamembrane to basolateral plasmamembrane
- measure functions of tight junctions:
- if there is something wrong with the barrier function, you might
have fluorescent or other dies in the lumen and then see if there is
leakage along the cells → fluorescence/labeling
- electrical dense markers in the lumen and then see if they pass
the membrane → electrical resistance: in healthy situations the
electrical dense marker will stay outside (tight junction prevents
that the marker leaks between the cells
Complexes exist of proteins and might also be
regulated: sometimes the barrier might be more strict
and sometimes there is paracellular transport possible
Proteins of the tight junctions
- multiple strands of proteins
- junctional adhesion molecules (JAMs)
- occludins: cross the plasmamembrane 4 times,
interact with occludins from the other cell
- claudins: cross the membrane 4 times, interact with
claudins from the other cell
- function of occludins and claudins: strong protein
complexes that fuses together the two plasma membranes (still own identity, but really close next to
each other) (fusion together by cystine bridges, covalently bound) (glue between the cells) and they
form a barrier that is no longer permeable for e.g. molecules from outside to inside, but also not
permeable for transmembrane proteins that might want to diffuse to the apical side
- several linker proteins at the cytoplasmic side of the membrane: e.g. zona occludens proteins and
signalling proteins → ZO proteins connect tight junctions to the cytoskeleton (F-actin)
- main functions of tight junctions:
- fence function: regulating the difference between the basolateral membrane and the
apical membrane
- barrier function
- also important signalling function
- enough strands that glue the membranes together (if one of the strands is broken, no worries what
,so ever, because there are enough strands that the function is still intact)
- tight junctions might also be sensors and regulate nuclear transcription
Tight junctions linked to diseases: the claudins alone consist of a family of at least 23 proteins → the
difference is often the different tissue where they are being expressed → if they are expressed in a
certain tissue, also that tissue is affected
Adherens junctions (zona adherens)
- signal not only at the apical side, but also at the lateral side
- are made of transmembrane proteins, linked to cytoplasmic
linker/anker proteins and to the (actin) cytoskeleton
- function: cell-cell adhesion
cell strength: cytoskeleton anchoring
- cadherins: calcium dependent adherens junction molecules →
name of cadherin is linking to where it has been found (E-cadherin
(epithelia), N- (neurons, heart, skeletal muscle, lens, fibroblasts), P-
(placenta, epidermis, breast epithelium), VE- (endothelial cells))
- diseases/inactive → phenotype linked to where the cadherins are most
abundantly expressed
- non-calcium bound state: monomeric
- calcium bound state: cadherin dimer → one dimer will be able to bind
to a dimer on the adjacent cell → strong cell-cell contact (in extracellular
space)
- at the cytoplasmic site, cadherins bind to small cytoplasmic proteins
(catenins): p120, α-catenin, β-catenin (plakoglobin) → catenins link to
the cytoplasmic cytoskeleton (actin) → gives structure to the cell
- microtubules (cytoskeletal structure) might also end at adherens
junctions
- adherens junctions provide strength in (epithelial) cells
, Desmosomes
- specialized adherens junctions
- very dense, easy to recognize in electron microscope
- intermediate filaments are linked to the desmosomes
- intermediate filaments provide strength within the tissue
- intermediate filaments go from one desmosome to the next desmosome, and
from one cell to the other cell → defects (genetic, or immune attack) leads to
dysfunction and could lead to blistering diseases in the skin
- function: cell-cell adhesion between cells
ancher cytoskeleton
strength in tissue
- desmosomal cadherins: desmocolin and desmoglein
(homodimer when calcium is bound) → link to cytoplasmic
proteins that are very important to provide the link to the
intermediate filaments
- mutation in (lacking) desmoplakin (linker protein) → not
connected to filaments → strength of tissue is lost → blistering
diseases
- desmosomes have a higher concentration than the adherens
junctions (the intermediate filaments are also dense) → easy to
recognize
- pathology: pathogens, mutations, (auto)immune attack
There are also cell-matrix junctions (cells adhere to the matrix):
focal adhesion, hemidesmosomes
No particular order of the junctions (very complex in reallife biology)
- membranes in tight junctions almost fuse (glue in between), but desmocolin and
desmoglein molecules keep a certain distance between the membranes
(desmosomal cadherins molecules bigger than in tight junctions)
- junctions not only found in epithelial cells, but in most of the body (not always the
same junctions, but adherens junctions and desmosomes are in most of the cells)
- not only the cell-cell contact, but also the linkage to the cytoskeleton is very
important to get tissue strength
