2013 Group 3 Project

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THE GOLGI APPARATUS

A) Electron micrograph of typical metazoan cell.
B) A Golgi from unicellular green alga 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. Different modification involve different processes such as proteolysis, phosphorylation, glycosylation and sulfation [8]. Different cargo molecules move in different manner in the Golgi stack [9]. One such manner is through cisternal maturation where the cargo remains steady inside the cisternae, eventually forming a new functional cisternae at the cis-side. Coat Protein (COPI) is a complex protein that function by coating the vesicles transporting proteins from the cis end of the endoplasmic reticulum (ER)and golgi compartmets. COPI in the Golgi apparatus will be responsible for the process of delivering the Golgi enzymes necessary for maturation of the newly synthesized cisternae that contains the cargo [10] [11] [12].


As an alternative, COPI is a usueful protein that facilitates transportation of different proteins between cisternae however it does not affect the resident enzymes since they reside in a relatively stable cisternal compartments [13] [14]. Cargo molecules may flow through the transient tubular connections between adjacent cisternae [15] or by fast partitioning between different lipid domains [16]. Golgi body also have a region where cargo molecules are being segregated and sorted out and transported to their appropriate destination, such region is called trans-Golgi network. It transports cargo proteins into the wider area destinations of the cell (in and out of cells)such as the plasma membrane and endosomal-lysosomal systems.It also contribute to the process of protein trafficking which is essentialin cell polarity [17] and the process of controlling the cell cycle [18]. Studies suggested that there is a strong link between cell trafficking and its effect to cell growth and the homeostasis of cell cycle.

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[19]. He termed the organelle 'appareil reticulaire interne' (internal reticular apparatus) and documented his findings in 'Arhives Italliennes de Biol 1898'.
Up to 1950s The term 'Golgi apparatus' was coined in 1910, and its first appearance in scientific literature was in 1913[2]. The organelle 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'[20]. 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) [21]

• 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.


Up to 1990s It was believed that all organelles, including the Golgi Apparatus never disappeared during mitosis but remained intact or in a fragmented state which would then be distributed between the two daughter cells. Thus, the two models of organelle partitioning were proposed. [22]
After the 1990s The theory that organelle identity remained (via either models of division) were challenged due to new imaging techniques allowing in vivo studies of intracellular compartments during cell division[23]. These studies suggest that the Golgi apparatus, along with the nuclear envelope are reabsorbed into the ER, and hence disappears during mitosis[24].

Models of Division

Within the category of animal cells, there are several methods of division used by organelles to ensure the proper partitioning of cellular content into daughter cells.

1.) The Stochastic Strategy, determined by the law of probabilities is adopted by organelles which are dispersed and numerous. This accuracy of this method of separation relies of the equal dispersion of organelles throughout the cytoplasm.[25]

2.) An alternate strategy for cell division is the Ordered Partitioning Strategy. Unlike the former this method is highly regulated and organised. It is structured around the theory of mitotic spindles. This method ensures a high degree of accuracy particularly for low number organelles. In general, most membrane bound organelles use both methods through the process of cell division, however some organelles depend on one method more heavily.[26]

There is still much to be uncovered regarding the mitotic division of the Golgi. There are several steps in the process which remain obscure and controversial. Researchers have proposing various models for cell division. However, a general consensus can be seen in three major stages in its partitioning:

  • Disassembly/Fragmentation
  • Division
  • Re-assembly

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 [27]. 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.[28]


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 [29]. 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 [30]. 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[31].


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[32]. 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 and Molecular Mechannisms[33]

Morphology of the GA Throughout Cell Division

Interphase

The Golgi apparatus in mammalian interphase cells is composed of flattened, membrane-bound structures approximately 1µm long, named Golgi cisternae. Between two and five cisternae align in a parallel fashion forming a Golgi stack[34]. At the onset of mitosis, the Golgi stacks take a polarized position around the cell nucleus and centrosome in a cis-trans fashion. The cisternae of same polarity belonging to two adjacent stacks are connected by thin tububules, forming the Golgi Ribbons [35]

Towards the end on interphase at G2/M of the cell cycle the Golgi ribbons begin to disassemble and assume a peri-nuclear arrangement around the nucleus. Micro-tubules are known to assist in this structural organization[36].

Unlinking the Golgi ribbon

This process emerge from Interphase to early G2 (prophase). It unlinks the golgi ribbon by detaching the cell's tubular connections between the cell's stacks [36]. In this process the ribbon may be converted into stacks depending on the protein enzymes such as MAP(Mitogen-activated protein) kinase and MEK1 (MAP ERK [Extracellular signal-regulated kinases] kinase 1) [37]. Study shows that the presence of MEK1 in cell division causes ribbon unlinking by reacting with ERK2 that also reacts with GRASP55 (Golgi reassembly stacking protein of 55kD) via the process of mitotic phosphorylation [38]. However, this can be a delicate process because if it reacted with the nonphosphorylatable GRASP55 which is a mutant form of GRASP55, it could interfere the ribbon unlinking especially in the late G2 [39]. Similarly, GRASP65 serve the role in severing the ribbon and connects the bridge in progression from G2 into M-phase [40]. There are also other proteins besides these kinases that serve an essential role in ribbon unlinking like the membrane fission protein CtBP1/BARS (C-Terminal Binding Protein 1/beta-adrenergic receptors) [41]. If BARS is non functional because of any defection or inhibition, it would prevent the golgi's ability to detach severe the ribbon and consequently affecting the G2/M transition during interphase [42].

Cell division that shows two disassembling processes, partitioning, reassembling and merging

During Cell Division

Prophase

Here, fragmentation of the Golgi stacks continue until hundreds of Golgi stacks and vesicles are formed.At this stage the Golgi is a heterogeneous collection of tubular networks, short tubules, and vesicles, termed mitotic clusters.[43] These rudimentary building blocks are organised around the templates of spindle poles and a microtubule network. Research indicates that peaking levels of cdc2 kinase activity triggers much of the structural changes seen in the disassembly process.[44] This indicates that there is a possible correlation between kinase activity and structural fragmentation/assembly.

Vesiculation and Unstacking

This process involves the cisternae. After the successful conversion of the Golgi ribbon into stacks via protein kinases and membrane fission protein, the cisternae will undergo into the process of vesiculation which is made possible by the COPI-dependent vesicle formation [45] [46]. The process occurs instantly and synchronously in early mitosis. Consequently, this gives an effect of fast transformation of Golgi into vesicular and tubular membranes. Such process occurs due to the imbalance of membrane budding and fusion in mitosis [47] [48]. During interphase, the cis-cisternae contains the COPI vesicles that has been captured and secured by giantin,p115 (protein115) interactive proteins and GM130 (Golgi matrix protein) via giantin-p115-GM130 tethering complex before the membrane fusion [49].

Molecular mechanisms of Golgi body during cell division

Metaphase & Anaphase

The clusters formed in prophase adopt a ring-like formation around each centrosome, which are partitioned according to associated spindle poles and a sister chromatids. The division of the Golgi clusters are balanced due to the arrangement of spindle poles and structural support of the microtubules. This method of division should ultimately allow for the accurate partitioning of the Golgi into the resulting daughter cells.


Telophase & Cytokinises

In the final steps of cell division the Golgi stacks begin to combine and re-position themselves at either side of the mid body and at each daughter cell centrosome. Ultimately they slowly begin to merge and reform into Golgi Apparatus.

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.

Areas of Future Research

  • Mechanisms for the redistribution of Golgi clusters in structural dis-assembly during mitosis
  • Microtubules are known to play a large role in the partitioning in the Golgi, however there many obscure areas regarding the specific relationship between the two. Studies have shown that there is a hindrance of structural organisation of the Golgi occurs once microtubule disrupting drugs such as nocodazole are introduced into the cells. Further research could clearly identify the role of microtubules and determine whether the relationship is direct or secondary element of regulation.[50]
  • Research is currently delving into the involvement of the Golgi apparatus in cell death and thus, the use of its pathways in the treatment of cancer [51]. Inducing cell death within neoplastic tissue is imperative in cancer therapy as both a treatment as well as an indication of the progression of tumor growth and efficacy of treatments.

In current times, a number of pathways and cell processes have been identified and targeted as a way of treating cancer. However, recent studies have revealed the involvement of the ER-Golgi compartment in the apoptosis of cells[52]. Proteins that are involved in the homeostasis of the Golgi and ER have been noted and may be used in the development of therapeutic drugs. Further studies are required to implement drugs that may activate signalling pathways of the Golgi apparatus to create suicide programming of neoplastic tissue.

Useful Links

Golgi Apparatus Maintains Its Organization Independent of the Endoplasmic Reticulum

Partitioning of the Golgi Apparatus during Mitosis in Living HeLa Cells

Golgi biogenesis

Apparatus - an animatic

Protein trafficking

Glossary

GA - Golgi Apparatus

ER - Endoplasmic Reticulum, an organelle closely situated with the nucleus and Golgi apparatus, responsible for protein synthesis.

Endocytosis - The process of cell absorption of molecules via engulfment

Exocytosis - The secretion of internally synthesised molecules from of a cell to the external environment via vesicles

Microtubules -

Spindles -

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2013 Projects: Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6 | Group 7

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