2013 Group 3 Project

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The Golgi Apparatus

A) Electron micrograph of typical metazoan cell. The Golgi is a stack of cisternae arranged from cis to trans (light to dark green) .

B) A Golgi from the unicellular green alga Ostreococcus tauri imaged by electron cryotomography.


The Golgi Apparatus discovered in 1897 by physician Camillo Golgi is a vital cellular organelle which is found in almost all eukaryotic cells. It is found in the cytoplasm and facilitates the formation and direction of membrane bound vesicles, mainly formed from proteins directed from the rough endoplasmic reticulum before they reach the plasma membrane. The Golgi is capable of regulating cellular transport and secretion depending on the volume and density of the vesicles and their contents. Structurally, it consists of a series of stacked components of cisternae and has two identified faces- a cis face and a trans face. [1]


Structure

Diagram of the Golgi Apparatus

The Golgi apparatus is a relatively large, membrane-bound organelle and thus one of the easiest cell structures to study in detail [2]. The organelle is located nearby the cell nucleus and is closely associated with the endoplasmic reticulum. By observing via metallic impregnation, it can be seen through phase contrast microscopy that the Golgi has a convoluted, dense and “ill-formed” morphology [3]. Initial studies have shown that the organelle has great variance in its form dependent on the type of cell it is in as well as the state of activity that the cell is in. There are roughly around 40-100 Golgi apparatus ‘stacks’ within a mammalian cell [4].

Overall, the Golgi apparatus is made of 4-8 flattened, membrane-bound sacs that are stacked upon one another [5]. These are known as cisternae. The Golgi also includes associated nearby vesicles. Each cisterna primarily contains products from the endoplasmic reticulum, which enter the Golgi at the cis face – the end that is closest to the ER and accepts incoming vesicles [6]. The cis face is where new cisternae are formed. The products eventually pass through two more functional regions (medial Golgi and endo Golgi networks) and then are exported via outgoing vesicles at the trans face of the organelle. The trans face is where formed proteins are sent off and is the face furthest away from the ER. There is a constant and relatively consistent distance kept between cisternae of the Golgi apparatus [5].

Function

Golgi apparatus is considered to be essential membrane-bound organelle in eukaryotic cells that sums all plant, animal and fungi life [7]. It's physical structure is a composition of dozens of flattened cristernae that has been brought together and flattened with fenestrated rims. Golgi has a primary function of modifying and packaging proteins and lipids into several transport carriers to be able to send them to their proper locations. Proteins such as secretory or trans-membrane proteins are delivered from the endoplasmic reticulum (ER) to the cis-Golgi network. Consequently, the Golgi act as a series of transportation canal where cargo molecules travel via different Golgi cristernae where residential enzymes modify and post-transitionally process them.


History

Time Discovery
1898 The organelle was first described by Camillo Golgi. It was observed via treatment of tissue with potassium dichromate and osmic acid followed by silver nitrate. He termed the organelle 'appareil reticulaire interne' (internal reticular apparatus) and documented his findings in 'Arhives Italliennes de Biol 1898'.
Up to 1950s The Golgi was beginning to be observed in many animal cells and from this, SR Cajal concluded that it was a common component in all living cells. However, some French cytologists denied the presence of a Golgi apparatus, claiming it to be an artefact.
1950s With electron microscopy becoming more and more accessible, Dalton and Felix were able to demonstrate that certain structures observed under the electron microscope matched Golgi and Cajal’s description of the 'appareil reticulaire interne'. Vacuoles and cisternae could now also be observed.
1961-1981 G Farquhar and G Palade recognised 3 key features to the Golgi apparatus through 20 years of research (published 1981 Journal of Cell Biology 91)

• It is divided into compartments
• It is required for the transport of secretory proteins and formation of secretory granules
• It houses proteins responsible for the modification of cell products.
G Palade also theorised a vesicular transport hypothesis – Communication between Golgi components is irregular and probably caused by the budding-fission-fusion of vesicles.


1984-1996 Studies conducted by J Rothmann et. al. (published as J Rothmann, 1996, Protein Sci, 5) supported the vesicular transport hypothesis. This was done via viral proteins marking vesicle membranes.


Behavior of the Golgi during Mitosis

During mitosis, the Golgi can undergoes one of two courses, depending on wether it is from an animal cell or a plant or yeast cell. In animal cells, it completely disintegrates and separates. The cisternae stacks come apart and move away from one another. However in plant or yeast cells, the Golgi remains together. It is just before cytokinesis (during telophase) that the parts of the Golgi reassemble and from an intact organelle (in plant cell mitosis).

The relationship between the Golgi and microtubules

Despite being one of the earliest organelles to be identified and studied, there are still grey areas concerning the division of the Golgi Apparatus. Some existing models of division for the Golgi suggest that it interacts with the endoplasmic reticulum during several points throughout cell division. This is due to their structural similarities during the interphase stage of mitosis in mammalian cells. Both consist of interconnect membrane networks and studies have found that both Golgi and ER proteins can be found together in bound vesicles. An experiment conducted by Jesch SA et. al. compares the distribution of the two organelles during interphase using innumnoflourecence microscopy, velocity gradient fractionation and density gradient fractionation. They concluded that the Golgi and the ER do not combine and furthermore that mitotic cells are unable to facilitate the fusion of the two.[8]


It was once theorised that the Golgi apparatus could be formed de novo within daughter and mitotic cells. It has since been discovered that in animal cells, the organelle cannot be synthesised de novo and thus, must divide when a cell divides [9]. Through cytochemical and immunofluorescence light microscopy, it has been shown that the Golgi would undergo breakdown early on in mitosis, giving rise to many Golgi fragments [10]. These fragments (in the form of small tubules and vesicles) would be dispersed throughout the cytoplasm at random. The partitioned Golgi would segregate into even groups and end up in the daughter cells to serve as a template to new Golgi apparatuses[11].


In both animal and plant cells, cytokinesis is the final stage of mitosis, which describes the division of the cell membrane into two separate and independent new cells. It is known that part of the driving mechanism of this process are vesicles formed by the Golgi apparatus[12]. These formed vesicles travel along microtubules of the cytoplasm to the centre of the cell and assist in the cleaving of the cell membrane. Within plant cells, the vesicles travel along the phragmoplast, which forms at the beginning of mitosis and begins the creation of a cell plate.


Morphology of GA prior to cell division

Interphase

flattened membrane-bound structures approxi- mately 1 m m long, also termed Golgi cisternae. Between two and five cisternae are aligned in a parallel manner to form a Golgi stack.

The Golgi stacks are polarized in a cis / trans fashion

morphology during each part of cell divisions

prophase

prometaphase

metaphase

anaphase

limitations of current models

A key problem with this line of reasoning is that micro- injection of the identical Sar1p mutant protein (GDP- or GTP- bound form) in interphase cells leads to the fragmentation of the Golgi apparatus into vesicles and tubules very similar to those comprising the MGCs.

This suggests that in itself cessation of transport from the ER is a detrimental factor for the maintenance of the Golgi apparatus and is independent of the localization of Golgi enzymes. One possibility is the requirement for a factor that needs to be transported to a downstream location to regulate maintenance/reassembly of the Golgi apparatus is accumulated in the ER. It is perhaps therefore not possible for normal Golgi reassembly to occur at the end of the mitosis in cells micro-injected with Sar1p DN .

The recycling of Golgi enzymes to the ER observed during interphase could take place to a certain extent during mitosis. More sensitive electron microscopy methods need to be developed to detect endogenous and transfected and epitope-tagged proteins that are expressed at low levels. These enzymes, when present in the ER, could indeed be partitioned in an ER-dependent manner, while others Golgi components, the structural Golgi proteins (see the Introduction), could be partitioned with the MGCs.



References

  1. <pubmed>PMC2106267</pubmed>
  2. <pubmed>9865849</pubmed>
  3. Tixier-Vidal A - The History of the Golgi apparatus. From structure to concepts. Biology of the Cell, Volume 90, Number 1, January 1998 , pp. 106-107(2)8
  4. <pubmed>18385516</pubmed>
  5. 5.0 5.1 <pubmed>19866649</pubmed>
  6. Krieger M, Scott MP, Matsudaira PT, Lodish HF, Darnell JE. Lawrence Z, Kaiser C, Arnold B. Molecular cell biology (5th edn ed.). 2004 New York: W.H. Freeman and CO
  7. <pubmed>21646379</pubmed>
  8. <pubmed>PMC25291</pubmed>
  9. <pubmed>519753</pubmed>
  10. John M.Lucocq and Graham Warren. Fragmentation and partitioning of the Golgi apparatus during mitosis in HeLa cells. The EMBO Journal vol.6 no. 11 pp. 3239 -3246, 1987
  11. <pubmed>12851069</pubmed>
  12. <pubmed>15695096</pubmed>


Links

Golgi Apparatus Maintains Its Organization Independent of the Endoplasmic Reticulum

Partitioning of the Golgi Apparatus during Mitosis in Living HeLa Cells

Golgi biogenesis



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Dr Mark Hill 2013, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G