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From CellBiology

Lab Attendance

--z3217345 10:45, 10 March 2011 (EST)

--z3217345 09:08, 17 March 2011 (EST)

--z3217345 09:11, 24 March 2011 (EST)

--z3217345 10:44, 31 March 2011 (EST)

--z3217345 08:53, 7 April 2011 (EST)

Individual Assessments

Lab 1

1. What are the key cell biology journals?

2. Which journals allow reuse of their published content?

Lab 2

1. Which chromosomes contribute to the nucleolus?

The chromosomes that contribute to the make up of the nucleolus are 13, 14, 15, 21, and 22.

The Cell: A Molecular Approach. 2nd edition.

2. Identify and add a link to your page of a recent biology article using confocal microscopy.

Nature Cell Biology Article: Reducing background fluorescence reveals adhesions in 3D matrices

Lab 3

Work Area

x1

Lab 1

Here is some bold text.

Here is some italic text.

2011_Lab_1

[1]

Lab 2

http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed

PubMed


INTRODUCTION TO THE STRUCTURE OF DESMOSOMES

Desmosome originates from the greek terms desmo meaning bond and soma meaning body. [1] This etymology reveals the main function of desmosomes which is to bind cells to one another. [2] Desmosomes, also known as macula adhaerens, have a number of other roles including strengthening the internal structure of a cell, sensing environmental cues, regulating tissue homeostasis and assisting in tissue morphogenesis. [2] By indirectly connecting intermediate filaments of adjoining cells, they produce a strong adhesive force which can resist great shear forces. [1] Hence, they are found in such tissues as the myocardium, the bladder, the gastrointestinal mucosa and many epithelia. [3]


STRUCTURE ON A CELLULAR LEVEL INSERT PHOTO OF DESMOSOME & DIAGRAM OF DESMOSOME - NON-DETAILED

The desmosome, 300nm in diameter, consists of two distinct domains - the extracellular core domain, ~30-35nm in diameter, and the dense cytoplasmic plaque, ~30-40nm in diameter. [3] The extracellular core domain, also known as the desmoglea, is made up of the transmembrane proteins - cadherins. [2] There are two types of desmosomal cadherins: desmogleins (DSGs) and desmocollins (DSCs). They extend from the plaque to the extracellular core where they connect to the adjacent cell's cadherins. The plaque, situated parallel to the inner leaflet of the cell membrane, is composed of an outer dense plaque and an inner dense plaque separated by a space of ~8nm. [3] The outer dense plaque is located closer to the plasma membrane than the inner dense plaque and links cadherin tails to desmosomal anchor proteins. [3] The linker proteins involved are plakoglobin (PG), plakophilin (PKP) and desmoplakin (DP). Extending out from the outer dense plaque are the desmosomal anchor proteins desmoplakins. These form the majority of the inner dense plaque. On the other side of the inner dense plaque is where the intermediate filament network of the cytoskeleton attaches. [4]


STRUCTURE ON A MOLECULAR LEVEL INSERT DIAGRAM OF DESMOSOME - DETAILED


EXTRACELLULAR DOMAIN

As outlined above, the extracellular domain consists of desmosomal cadherins: desmogleins (DSGs) and desmocollins (DSCs). Cadherins are arranged along the mid-line in ~7nm intervals, with alternating cis (interaction between same cell surface) and trans (interaction between different cell surfaces) dimers causing a tightly packed zipper formation. They are each made up of four extracellular immunoglobulin domains (EC1-4), an immunoglobin extracellular anchor (EA), a transmembrane domain (TM), an intracellular anchor (IA) and additional cytoplasmic domains, which distinguish the cadherin isoforms. [3] Each immunoglobin domain is connected end to end by three calcium ions, forming a curved structure of about 100 degrees. Along the domains are disulfide bonds, O-linked sugars and N-linked sugars. [1]


The cadherin isoforms are composed of different additional cytoplasmic domains as follows [3]:

DSCa - ICS

DSCb - short carboxy-terminal domain

DSG1 - ICS, IPL, 5 RUDs, DTD

DSG2 - ICS, IPL, 6 RUDs, DTD

DSG3 - ICS, IPL, 2 RUDs, DTD

DSG4 - ICS, IPL, 3 RUDs, DTD


Note:

There are three isoforms of DSC (1-3), however they splice alternatively into two transcript variants named DSCa and DSCb.

ICS is an intracellular cadherin-like sequence, which binds to plakoglobin.

IPL is an intracellular proline-rich linker.

RUD is repeat unit domains.

DTD is a desmoglein terminal domain

INSERT DIAGRAM OF DESMOSOMAL CADHERINS


Tyrosinase-related protein 2 (Trp2) mediates the cis and trans interactions and is most probably responsible for the binding of cadherins to the plaque. [1] However it is currently unknown whether heterophilic cis and trans or homophobic cis and trans interactions are the main cause of cell adhesion. [5]

The extracellular domain mediates adhesion and whilst the mechanical structure is extremely strong, it is still dynamic since the attachment of both types of cadherins are reliant on Ca2+ levels. During embryonic development of the desmosome, the link between the two cells remains weak. Eventually the link strengths and the cell becomes hyperadhesive. This latter stage is calcium independent, as once protein kinase C is activated by Ca2+ it is able to remain activated in the long-term. Hence this hyperadhesive state will not be affected by changes in Ca2+ concentration. However the desmosome is able to revert to the earlier state when responding to certain environmental signals to allow for such things as cell migration if regeneration of the epithelia is necessary. [1] [2] The primary morphological attribute which indicates calcium independence is a distinct electron-dense midline that bisects adjoining cells' extracellular core domains. [6] Whereas when desmosomes are calcium dependent the midline is lost and the diameter of the intercellular space decreases by 10%. Potentially this is caused by Ca2+ ions becoming trapped to the cadherins and securing their position into an ordered structure. [6]


OUTER DENSE PLAQUE

The dense plaque is made up of two electron-dense zones: an outer dense plaque and an inner dense plaque. The outer dense plaque being lower in density that the inner dense plaque. They are separated by a electron-lucent zone, ~8nm in diameter. The outer dense plaque is located in the cytoplasm, parallel to the plasma membrane. [6] It is ~15-20nm in diameter and characterised by two zones: a ~4nm zone located ~10nm from the plasma membrane and a ~8nm zone located ~20nm from the plasma membrane. The major plaque proteins are plakoglobin (PG), plakophilin (PKP) and desmoplakin (DP).


The structure of the major plaque proteins is as follows [3]:

PG - Head, 12 arm repeats, Tail

PKP1 - Head, 9 arm repeats, Tail

PKP2 - Head, 9 arm repeats, Tail

DP1 - Head, Rod, C tail, GSR

DP2 - Head, Rod, C tail, GSR


Note:

Head is an amino-globular head domain with a N terminus.

Tail is a caboxy-terminal tail with a C terminus. In desmoplakin, the tail constits of three plakin repeat domains.

GSR is a glycerine-serine-arginine rich domain.

The isoforms of plakophilin, PKP1 and PKP2, differ in their arm repeat three and arm repeat four. In arm repeat three PKP1 has an additional 21 amino acids, and in arm repeat four PKP2 has an additional 44 amino acids. Hence PKP2 is longer than PKP1.

The isoforms of desmoplakin, DP1 and DP2, are characterised by the length of their rod domain - the DP1 rod being ~2/3 longer than the DP2 rod.

INSERT DIAGRAM OF MAJOR PLAQUE PROTEINS


Plakoglobin and desmoplakin complexes form an alternating arrangement which acts as a base for the desmosomal cadherins to attach. The cadherin tails bind to the N terminus of desmoplakin before they connecting to the adjoining cells' desmosomal cadherins. Formation of this initial structure is mediated by plakoglobin and plakophilin. For reinforcement the remaining space between the plaque and the plasma membrane is filled by plakophilins. 21464301


INNER DENSE PLAQUE The inner dense plaque, alike the outer dense plaque, is ~15-20nm in diameter. [6]

It is primarily comprised of the junctional complexes, desmoplakins, which tether to the intermediate filaments.

  1. 1.0 1.1 1.2 1.3 1.4 <pubmed>20672011</pubmed>
  2. 2.0 2.1 2.2 2.3 Molecular Biology of the Cell Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter New York and London: Garland Science; c2002
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 <pubmed>20066089</pubmed>
  4. <pubmed>20671997</pubmed>
  5. <pubmed>21098030</pubmed>
  6. 6.0 6.1 6.2 6.3 <pubmed>20626351</pubmed>