- 1 The Golgi Apparatus
- 2 Introduction
- 3 History
- 4 Structure
- 5 Function
- 6 Morphology and Molecular Mechannisms
- 7 Current Model for Behaviour during Mitosis
- 8 Limitations of Current Models
- 9 Areas of Future Research
- 10 External Links
- 11 Glossary
- 12 References
The Golgi Apparatus
This page will specifically address the processes of mitosis in regards to the Golgi Apparatus as well as provide a well rounded explanation of structure, function and other major processes. It will explore past, current and future research prospects and provide experimental evidence which either supports or dis-regards the presented research.
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. 
|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 term 'Golgi apparatus' was coined in 1910, and its first appearance in scientific literature was in 1913. 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'. 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
|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. |
|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. These studies suggest that the Golgi apparatus, along with the nuclear envelope are reabsorbed into the ER, and hence disappears during mitosis.|
The Golgi apparatus is a relatively large, membrane-bound organelle and thus one of the easiest cell structures to study in detail . 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 . 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 .
Overall, the Golgi apparatus is made of 4-8 flattened, membrane-bound sacs that are stacked upon one another . 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 . 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 .
The Golgi apparatus is considered to be an essential membrane-bound organelle in eukaryotic cells. . As mentioned in the previous section it's physical structure is a composition of dozens of flattened cristernae. These cristernae have been brought together and flattened to produce fenestrated rims. The Golgi is is involved in several cellular processes however its primary function in to Modify and package proteins and lipids.
Incoming proteins and lipids are sorted into several transport carriers which allow them to be sent to their proper locations. More specifically the Golgi coordinates the Sorting of cytosolic/secreted proteins, the Glycosylation of secreted proteins, the modification of carbohydrates and trimming of side chains. (REFERENCE LECTURE NOTES)
For example, proteins such as secretory or trans-membrane proteins are delivered from the endoplasmic reticulum (ER) to the cis-Golgi network. The Golgi acts as a series of transportation canals where cargo molecules travel via different Golgi cristernae. In these pathways residential enzymes modify and post-transitionally process them. Different modification involve different processes such as proteolysis, phosphorylation, glycosylation and sulfation .Different cargo molecules move in different manner in the Golgi stack . 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 functions by coating the vesicle transporting proteins from the cis end of the endoplasmic reticulum (ER)and Golgi compartments. 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   .
In conjunction, COPI is a useful 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  . Cargo molecules may flow through the transient tubular connections between adjacent cisternae  or by quickly partitioning between different lipid domains . The Golgi body also has a region where cargo molecules are actively segregated, sorted and transported to their appropriate destination. This region is called the 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 contributes to the process of protein trafficking which is essential in cell polarity  and the process of controlling the cell cycle . Studies suggested that there is a strong link between cell trafficking, its effect on cell growth and the regulation of cell cycle.
The main mechanism for these processes are endocytosis and exocytosis.
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.
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.
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:
Morphology and Molecular Mechannisms
Prior to Cell Division
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. 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. 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 
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 . 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) . 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 . However, this can be a delicate process with the presence of MEK1 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 . Similarly, GRASP65 serve the role in severing the ribbon and connects the bridge in progression from G2 into M-phase . 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) . If BARS is non functional because of any defect or inhibition, it would prevent the golgi's ability to severe the ribbon and consequently affecting the G2/M transition during interphase .
During Cell Division
Here, fragmentation of the Golgi stacks continues 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. These rudimentary building blocks are organised around the templates of spindle poles and a microtubule network. CDC2 kinase or Cyclin-dependent 2 Kinase is a phosphoprotein that functions as a subunit of the maturation-promoting factor.Research indicates that peaking levels of CDC2 kinase activity triggers much of the structural changes seen in the disassembly process. 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  . The process occurs instantly and synchronously during vesiculation in prophase stage. Consequently, this gives an effect of fast transformation of Golgi into vesicular and tubular membranes. Vesiculation occurs due to the imbalance of membrane budding and fusion in mitosis  . Interactive proteins in Golgi apparatus are responsible for trafficking of proteins by sorting their cargo concentration. 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 . P115 serves as a bridge that links giantin and GM130 on the Golgi cristernae  regulated and maintained by small GTPase Rab1 . The link between the three form a complex called (Soluble NSF Attachment Protein) Receptor (SNARE) complexes which is directly formed by P115 that consequently fusing two membranes  . ADP-ribosylation factor 1 (Arf-1) is involved in protein trafficking among different compartments. It modulates vesicle budding and uncoating within the Golgi complex. In this case, Arf-1 continues to be active and the synthesis of COPI also remains active .
On the other hand, the vesicles are unable to fuse with the membranes that they are targeting because there is a disruption of vesicle restraint complex . Cyclin-dependent kinase 1 also known as CDK1 is considered to be a key player in cell cycle regulation. CDK1 prevents P115 binding via phosphorylation of GM130 in the early mitosis stage. It is done to block the vesicle from forming a restraint and rapid as well as repetitive fusion  . In addition, CDK1 also phosphorylates Rab1 which serves the same function when P115 is inhibited . This process continues and does not stop the COPI vesicles from budding. However, the persistent fusion gradually consumes the cisternae due to COPI-dependent vesiculation. If this process is done in an efficient way in an early mitosis via cristernae unstacking, it greatly enhances COPI formation. The phosphorylation of GRASP65 and 55 are essential in Golgi unstacking. These two Golgi peripheral membrane proteins form dimers that would interact together to form trans-oligomers which are essential because they hold the golgi cristernae together which is required in cell division . The process of mitotic phosphorylation using GRASP proteins lead to GRASP deoligomerization that affects the Golgi to unstack.
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.
This occurs during metaphase where the Golgi is partitioned into two daughter cells. After the disassembling process, the Golgi membrane is now present in two pools. In partitioning, the vesicles are now evenly dispersed between each pools in the cytoplasm . In addition, there is a presence of concentrated portion of Golgi membranes around the spindle poles and react with astral microtubules (see the Figure 3 and 4) . The reaction would then continue in the clusters of tubulo-vesicular structures to form a spindle-associated membrane. These tubulo-vesicular structures are in relatively constant number. They are also polarized because they contain cis-Golgi proteins compared to trans-Golgi stacks present during interphase, however both have similar pattern . The spindle reaction and association made the idea about their relationship and thus Golgi partitioning is regulated by the spindle . Consequently, spindle plays a vital role in maintaining a successful partitioning of an intact Golgi ribbon .
This process occurs in the late anaphase to early telophase and cytokinesis which is part of postmitotic reassembling. In this process, there are two successfully divided Golgi membranes being reassembled on opposite side of each nucleus. Two distinct Golgi ribbons are also found: a smaller goldi ribbon which sits adjacent to the midbody while the larger golgi ribbon is typically found in the pericentriolar region on the side which is directly away from the cleavage site . Knowing the function of the smaller ribbon is still being observed but knowing that it is located in the midbody, it has been theorized that it may function to assist the facilitation of efficient polarized delivery membranes to the cleavage site to completely seal the plasma membrane during abscission. To support this theory, study shows that Golgi-derived vesicles are sent from each daughter cells directly to the cleavage site where membrane fusion takes place . After the abscission, the smaller golgi ribbon would them travel all the way to the opposite side of the nucleus to merge with the larger Golgi ribbon.
Biochemical and mechanical processes would then take place at the end of mitosis for the reformation of Golgi by undergoing into two interrelated processes. These processes are the formation of flattened cristernae by fusing the membrane of the two golgi ribbon and stacking of cisternae. N-ethylmaleimide-sensitive factor (NSF) and the valosin-containing protein (VCP)/p97 are both component of cellular machinery in the transfer of membrane vesicles and are involved in membrane fusion. Membrane fusion is done using these enzymes and being mediated by AAA-ATPase (ATPases Associated with diverse cellular Activities-Adenosinetriphosphatase) which involves degradation of ATP for NSF and VCP/p97 to be biochemically processed, thus Golgi fusion occurs . Another process involves stacking of golgi cristernae where factor p115 is needed simultaneously that results in linking adjacent membranes . Stacking of golgi cisternae also uses GRASP proteins that holds the cisternae into stacks .
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.
Current Model for Behaviour during Mitosis
During mitosis, the Golgi can undergoes one of two courses, depending on whether 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).
Continued Presence via Mitotic Fragmentation of the Golgi Ribbon
It was once theorized 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 . 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 . 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. However, experimental evidence presented by David T. Shima et al highlights several behaviors exhibited by the mitotic Golgi contradicted the notion of random distribution facilitated by fragmentation. Specifically, the process was determined as being highly regulated and be segregating into defined time intervals. For example, one mechanism would be activated and terminated before the next event in partitioning could occurs.
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. 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.
Disappearance via The Golgi-ER Transport Model
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. This theory suggests that during the dis-assembly process the Golgi fragments are recycled into the ER. As the ER and GA are very closely associated in cell activity it is fair to hypothesis that there is a connection throughout cell division. To accurately test this hypothesis David T. Shima et al developed strategy using mSar1p (dominant interfering mutant of GTPase) micro-injection to determine if there is an accumulation of mitotic Golgi fragments in the ER. Their results yielding evidence that did not support their expectations. The mSar1p did not produce an accumulation at the ER. This could be explained by the use of an alternate pathway, one that is mSar1p-independent. This theory does not substantially explain the mechanism for dis-assembly of the Golgi.
Limitations of Current Models
The contradictory aspect of these two models (continued presence of Golgi apparatus as fragments versus reapsorption of Golgi enzymes in the ER) are argued to be the result of methodological problems
Continued Presence Model
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 fragments dispersed throughout the cytoplasm.
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.
Presence of Golgi proteins or enzymes detected in the ER during mitosis could be caused by the fact that protein folding in ER is continued during mitosis which generates a pool of proteins that could be detected as Golgi proteins .
Also, in the presence of cycloheximide (used to inhibit protein synthesis in eukaryotic cells), the presence of Golgi enzymes in the ER was sixfold reduced, indicating that presence of enzymes in the ER that previously was assumed to indicate recycling was actually mostly due to new synthesis and not recycling from the Golgi apparatus. However, the presence of the Golgi proteins in the ER, even at low levels raises the possibility that microscopy methods used are not always able to detect low concentrations of tagged proteins and recycling could have been missed
In summary, the recycling of Golgi enzymes to the ER observed during interphase could take place to a certain extent during mitosis, but more sensitive electron microscopy methods need to be developed to detect the tagged proteins at low levels.  Presence of enzymes in the ER can indicate partitioning in an ER-dependent manner while other Golgi components such as structural Golgi proteins could be partitioned as fragments.
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.
- 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 . 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. 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.
External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name.
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
kD - kilo-dalton refers to the measurement that is used to meansure the molecular weight of different proteins on an atomic or molecular scale.
Microtubules - are the largest component of the cytoskeleton found in eukaryotic cells. They are composed of tubulin and are involved in several vital cell processes involving intracellular mobility.
Cis - bottom of stack closest to endoplasmic reticulum, receives transport vesicles from ER medial
Medial - middle of stack, processing of proteins, modification of side chains
trans - top of stack closest to plasma membrane, buds off secretory vesicles
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- Dr Mark Hill 2013, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G