Difference between revisions of "2013 Group 4 Project"

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== Function ==
 
== Function ==
  
Mitotic spindle is made up of spindle microtubules which anchor at the end of both centrosomes and kinetochores. When mechanical force is generated, chromsomes will be separated and pulled to opposite poles towards the centromeres by the spindle apparatus.
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Mitotic spindle is made up of spindle microtubules which anchor at the end of both centrosomes and kinetochores. When mechanical force is generated, chromsomes will be separated and pulled to opposite poles towards the centromeres by the spindle apparatus. In the spindle, kinetochore microtubules have their plus ends embedded in the kinetochores of the sister chromatids and their minus ends at the spindle pole.
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Kinesins are important to maintain spindle bipolarity. <ref><pubmed>14681690</pubmed></ref>
  
 
== Current research ==
 
== Current research ==

Revision as of 18:25, 10 April 2013

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

Spindle Apparatus

Introduction

Historical Research

Brief Timeline of Historical Research Regarding Spindle Formation

Time Discovery
'Early 1880s Walther Flemming used the term ‘mitosis’, which is a Greek word meaning ‘thread’, to represent the shape of the chromosomes seen during mitosis. [1]


1838


The Cell theory was formulated by Theodor Schwann and Matthias Jakob Schleiden: All living things are created from cells. The cell is the smallest unit of life. All cells arise from pre-existing cells.

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Most Groundbreaking discoveries on spindles: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3514024/


  • More text to be added soon *

Structure

As reviewed in Glotzer (2009), the spindle apparatus is made from a combination of microtubules, motors and microtubule associated proteins (MAPs). [2] Microtubules that make up spindles are cylindrical polymers that are assembled from dimers of alpha-tubulin and beta-tubulin. They are polar filaments that have a fast-growing plus end and a slow-growing minus end that is often capped by the gamma-tubulin ring complex, a ring-shaped microtubule nucleator.

Diagrammatic representation of mitosis, the mitotic apparatus, and different types of kinetochore attachments

In most animal cells microtubules are nucleated at the centrosomes found at the spindle poles. However, it has been observed that spindles can still form in cells lacking centrosomes. The results show that non-centrosomal microtubules contribute to to spindle formation even in cells with centrosomes. These cells expressed GFP-alpha-tubulin. [3]

During metaphase, the mitotic spindle is comprised of kinetochore fibres, astral microtubules and interpolar microtubules. The fusiform shape of the spindle is the result of the microtubule minus ends focusing at the poles and by cross-linking interpolar microtubules in an overlapping region situated in the midzone.

Spindles showing fusiform shape of microtubules (green) in spindle assembly

In the spindle, kinetochore microtubules have their plus ends embedded in the kinetochores of the sister chromatids and their minus ends at the spindle pole. Kinesins are important to maintain spindle bipolarity. [4] The simulataneous KinI induced disassembly at both the plus and minus ends may result in the poleward driving forces. GTPase Ran and its exchange factor RCC1 have been shown to induce microtubule polymerization around chromosomes and allows for chromatin induced mitotic spindle formation.[5]

At the beginning of anaphase, the kinetchore fibres shorten ( delivering sister chromatids to the poles) and astral microtubules elongate . The region between the two poles is called the spindle midzone and the microtubules that populate this region are called midzone microtubules. The term central spindle refers to the structure at the centre of the midzone, where the plus ends of the microtubules interdigitate. The microtubules of the central spindle eventually lose their interaction with the spindle poles. As the formation of the cleavage furrow progresses, the central spindle becomes compacted dense structure known as a the midbody.[6]

The kinetochore forms bivalent attachments with the spindle microtubules and the kinteochores position themselves correctly with respect to the division plane of the cell during anaphase. Bivalent attachment of the sister chromatids to the spindle is achieved when the plus ends of the microtubules emanating from each pole interacts with the kinetochores of each sister pair and then becomes embedded. It is well established that CLIP-170/Tip1 localizes to the kinetochore.The plus-end microtubule binding proteins ( +TIP) play a significant role in the regulation of microtubule stability and cell polarity during interphase. Research suggests that the +TIP protein Tip1 affects directly or indirectly the movement of the chromosomes towards to the poles during anaphase [7] .

Partially reducing XMAP215 or knocking out its homologues, ZYG-9 and Msps, disrupts spindle assembly or causes the formation of very small spindles. [8] [9] Furthermore, Stathmin/ Op18 is family of destabilising proteins, and when the concentration of nonphosphorylatable Stathmin/ Op18 is increased, shorter spindles are formed. This provides evidence to suggest that microtubule catastrophe frequency and growth rate can determine spindle length. [10] Studies also show that spindles formed in the presence of a total depletion of XMAP230/XMAP4 had fewer or no interpolar microtubules and were of decreased width. [11]

Mechanism of Formation

The spindle itself is defined by microtubule nucleation occurring mainly at the two centrosomes. The microtubules aid in organising the spindle as well as its function in segregating chromosomes into two daughter cells. When a spindle is properly assembled, the sets of chromosomes are aligned along the central equator of the cell. Microtubules align perpendicular and kinetochores attach to its plus ends with the minus-ends embedded at the cell poles. It is crucial that the alignment is correct so that the cell division occurs correctly. Up to this point, there has been a development of two generally accepted spindle assembly models which explain the mechanism of formation of the spindle apparatus: Search and capture, and Microtubule Self-Organisation. [12]

The Search and Capture model involves microtubules nucleating from centrosomes, chromosomes and kinetochores simply by chance and become stabilised to form a spindle. The Microtubule Self-Organisation model on the other hand depicts randomly orientated microtubules that undergo nucleation with the absence of centrosomes being organised into bipolar arrays by MOPs. [13]

Search and Capture model

Diagram explaining Search and Capture model of spindle assembly along with example in yeast and complex organisms

Due to the centrosomes, microtubules end up nucleated and form symmetrical asters (star shapes). They are relative unstable and due to this condition, will "search" the cytoplasm by alternating between growing and shrinking phases.[14] The "capture" part occurs when a kinetochore attaches to the searching microtubule. The kinetochore attachment turns to an end by microtubule bundle association. The unattached end is then attached to a microtubule from the opposite pole. This leads to the chromosome moving towards the congression (or the metaphase plate). The chromosome eventually will oscillate there and cell division can begin.[15]

Microtubule Self-Organisation model

Diagrammatic representation of difference between two of the predominant spindle assembly models

This model, also known as "self-assembly", states that after the breakdown of the nuclear envelope of the cell, microtubules have the ability to grow from various sites around the now condensed chromatin and "self-organise" into a spindle apparatus. This is regardless of the absence of centrosomes or microtubule-organizing centres (MOPs).[16] It also suggests that it is the chromosomes themselves that can nucleate microtubules (not centrosomes as the search and capture model suggests) and organise them into bipolar arrays.[17] This is done by stimulating the signalling cascades of the Ran-GTP and the CPC (chromosomal passenger complex) pathway.[18]

The Ran-GTP cascade (mediated by the nuclear transport factor importin β) releases spindle assembly factors (also known as SAFs) and promotes the formation of the spindle by organizing microtubules in the vicinity of chromosomes.[19] The CPC pathway generates an Aurora B signal which stimulates microtubule assembly located around the chromosomes and does so by locally inhibiting factors that decrease the chances of microtubule catastrophes.[20]

Function

Mitotic spindle is made up of spindle microtubules which anchor at the end of both centrosomes and kinetochores. When mechanical force is generated, chromsomes will be separated and pulled to opposite poles towards the centromeres by the spindle apparatus. In the spindle, kinetochore microtubules have their plus ends embedded in the kinetochores of the sister chromatids and their minus ends at the spindle pole.

Kinesins are important to maintain spindle bipolarity. [21]

Current research

Knockdown of Myo10 leads to mitotic spindle defects

One of the article in recent year searches the roles of actin filaments (F-actin) and F-actin-based motors (myosins) which are required components of mitotic spindles. In their research, they found out that myosin-10 (Myo10) is important for assembly of meiotic spindles. In more detail, Myo10 set themselves to mitotic spindle poles and is very important for proper spindle anchoring, normal spindle length, spindle pole integrity as well as progression through metaphase. They also found out the antagonistic relationship between F-actin and Myo10 in maintenance of spindle length and that they work independently.[22] Actin filaments (F-actin) and F-actin-based motors (myosins) are essential components in the proper functioning of spindle apparatus. They are required for correct positioning of the spindle towards the anchor point.

Furthermore, some recent research found out the function of the long-tailed class-1 myosin myosin-1C from Dictyostelium discoideum during mitosis. They use the data obtained as back up, suggested that myosin-1C binds to microtubules and play parts in maintenance of spindle stability during chromosome separation and that the association of myosin-1C with microtubules is mediated through the tail domain. Further data has leaded to another suggestion that myosin-1C tail can inhibit kinesin motor activity, strengthen the stability of microtubules as well as forming crosslinks between microtubules and F-actin. [23] Myosin-1C motor and tail-domain-mediated MT-F-actin are required for the relocalization of certain protein from the cell periphery to the spindle. Therefore, both contribute to the formation and stability of spindle apparatus in considerable amount.

By combine the use of force-calibrated needles, high-resolution microscopy, and biochemical perturbations, recent researchers analyze the vertebrate metaphase spindle and found that spindle viscosity is dependent on microtubule density and cross-linking. Spindle elasticity are said to be relating to kinetochore and non-kinetochore microtubule rigidity, and also to spindle pole organization by kinesin-5 and dynein. [24] The data obtain in their research provides micromechanics modal insight of this cytoskeletal architecture and provide insight into how structural and functional stability is maintained for proper control of spindle function.

Glossary

Images


Useful Links

References

  1. <pubmed>11146645</pubmed>
  2. <pubmed>19197328</pubmed>
  3. <pubmed>14588246</pubmed>
  4. <pubmed>14681690</pubmed>
  5. <pubmed>10408446</pubmed>
  6. <pubmed>19197328</pubmed>
  7. <pubmed>20498706</pubmed>
  8. <pubmed>10620801</pubmed>
  9. <pubmed>9606208</pubmed>
  10. <pubmed>8895574</pubmed>
  11. <pubmed>10564651</pubmed>
  12. <pubmed>18275887</pubmed>
  13. <pubmed>15380094</pubmed>
  14. <pubmed>9281583</pubmed>
  15. <pubmed>18275887</pubmed>
  16. <pubmed>9811565</pubmed>
  17. <pubmed>22090343</pubmed>
  18. <pubmed>20627073</pubmed>
  19. <pubmed>11163243</pubmed>
  20. <pubmed>15260989</pubmed>
  21. <pubmed>14681690</pubmed>
  22. <pubmed>18606852</pubmed>
  23. <pubmed>21712373</pubmed>
  24. <pubmed>21703450</pubmed>







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

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