Mystery at the heart of life

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December 21, 2014

Cell biology, Intelligent Design, News

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Cell biology
Intelligent Design
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By Biologic Institute’s Ann Gauger, at Christianity Today’s Behemoth, the secret life of cells:

Our bodies are made up of some 100 trillion cells. We tend to think of cells as static, because that’s how they were presented to us in textbooks. In fact, the cell is like the most antic, madcap, crowded (yet fantastically efficient) city you can picture. And at its heart lies a mystery—or I should say, several mysteries—involving three special kinds of molecules: DNA, RNA, and proteins.

These molecules are assembled into long chains called polymers, and are uniquely suited for the roles they play. More importantly, life absolutely depends upon them. We have to have DNA, RNA, and protein all present and active at the same time for a living organism to live.

How they work together so optimally and efficiently is not merely amazing, but also a great enigma, a mystery that lies at the heart of life itself. More. Paywall soon after. May be worth it.

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Many aspects of the RC function during meiosis remain to be elucidated. [...] future studies are needed to address the role of SUMO proteases in RC disassembly. A remarkable feature of the RC is that within a 30-min period, it undergoes two cycles of assembly/disassembly linked to two waves of SUMO modification/deconjugation that are regulated with exquisite precision both temporally and spatially. [...] it remains to be shown what signal(s) regulate the balance between E3 and protease activities [...] [...] precise mechanisms that guarantee proper chromosome orientation, congression, and segregation might differ between meiosis and mitosis and also among species [...] [...] highly dynamic, coordinated, and spatially constrained sumoylation regulates chromosome congression during meiosis in C. elegans oocytes.
A SUMO-Dependent Protein Network Regulates Chromosome Congression during Oocyte Meiosis Federico Pelisch, Triin Tammsalu, Bin Wang, Ellis G. Jaffray, Anton Gartner, Ronald T. Hay DOI: http://dx.doi.org/10.1016/j.molcel.2016.11.001 Molecular Cell

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Meiosis is a specialized division in which a single round of DNA replication is followed by two consecutive segregation steps. Homologous chromosomes segregate in Meiosis I, while sister chromatids segregate in Meiosis II, giving rise to haploid gametes (Duro and Marston, 2015). In contrast to mitotic spindles, meiotic spindles in many animal species (including humans and nematodes) lack centrosomes (Dumont and Desai, 2012), and how these spindles are organized is poorly understood (Ohkura, 2015).
A SUMO-Dependent Protein Network Regulates Chromosome Congression during Oocyte Meiosis Federico Pelisch, Triin Tammsalu, Bin Wang, Ellis G. Jaffray, Anton Gartner, Ronald T. Hay DOI: http://dx.doi.org/10.1016/j.molcel.2016.11.001 Molecular Cell

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[...] acetylated segments observed far from MT ends in vivo most likely results from the dynamicity of MTs. [...] in vitro and cellular MTs may differ in their intrinsic properties. [...] the shaft of in vitro reconstituted MTs is permeable to ?TAT1 while the one of cellular MTs is not. [...] in vitro assembled MTs presents holes or defects along their shaft that allow lateral entry of ?TAT1 to the lumen. The nature of such holes and why in vitro MTs would display more holes as compare to cellular MTs is not clear. [...] in vitro reconstituted MTs do not recapitulate the properties of cellular MTs [...] [...] multiple contacts between MTs and these structures would enhance ?TAT1 acquisition by front-oriented MTs leading to the progressive acetylation of this MT subset from their open extremities. This selective, tip-oriented acetylation mechanism has important consequences since cell-front oriented acetylated MTs are instrumental in controlling directional cell migration.
?TAT1 controls longitudinal spreading of acetylation marks from open microtubules extremities. Ly N, Elkhatib N, Bresteau E, Piétrement O, Khaled M, Magiera MM, Janke C, Le Cam E, Rutenberg AD, Montagnac G Sci Rep. 6:35624. doi: 10.1038/srep35624.

Sometimes in-vitro results differ from in-vivo or ex-vivo experiments. Complex complexity.Dionisio_{December 31, 2016
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Acetylation of the lysine 40 of ?-tubulin (K40) is a post-translational modification occurring in the lumen of microtubules (MTs) and is controlled by the ?-tubulin acetyl-transferase ?TAT1. How ?TAT1 accesses the lumen and acetylates ?-tubulin there has been an open question. [...] ?TAT1 enters the lumen from open extremities and spreads K40 acetylation marks longitudinally along cellular MTs. This mode of tip-directed microtubule acetylation may allow for selective acetylation of subsets of microtubules.
?TAT1 controls longitudinal spreading of acetylation marks from open microtubules extremities. Ly N, Elkhatib N, Bresteau E, Piétrement O, Khaled M, Magiera MM, Janke C, Le Cam E, Rutenberg AD, Montagnac G Sci Rep. 6:35624. doi: 10.1038/srep35624.

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The regional control of lysosome movement is likely critical for cellular processes that require regulated encounters of lysosomes with other organelles in different parts of the cytoplasm. The relationship of PTMs to kinesin selectivity is likely more complex and dependent on combinations of multiple PTMs, MAPs, and cargos. [...] both kinesins are required to counteract the function of dynein in centripetal transport of lysosomes. Our findings are a striking example of cellular processes that depend on cooperation of multiple kinesins. Other such processes are mitosis and cytokinesis [...] intraflagellar particle transport [...] and hyphal growth in filamentous fungi [...] The results reported in the present study represent yet another mechanism in which two co-regulated kinesin types drive movement of the same cytoplasmic organelle in different regions of the cell.
BORC Functions Upstream of Kinesins 1 and 3 to Coordinate Regional Movement of Lysosomes along Different Microtubule Tracks. Guardia CM1, Farías GG1, Jia R1, Pu J1, Bonifacino JS Cell Rep. 17(8):1950-1961. doi: 10.1016/j.celrep.2016.10.062.

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[...] BORC and Arl8a/b function upstream of the kinesin-1 KIF5B and the kinesin-3 KIF1B? and KIF1A proteins to move lysosomes toward the cell periphery along different microtubule tracks and in different regions of the cell [...] In future studies, it will be of interest to determine whether BORC and Arl8 also regulate these kinesins, and, if so, by what mechanism. We were intrigued by the involvement of different kinesin types in lysosome movement toward the cell periphery. Various kinesins exhibit preferences for microtubule tracks that are characterized by specific tubulin PTMs or associated microtubule-associated proteins (MAPs). [...] the biochemical properties of different microtubule populations underlie the preferential recruitment of different kinesins and, in turn, the regional movement of lysosomes, even in non-polarized cells.
BORC Functions Upstream of Kinesins 1 and 3 to Coordinate Regional Movement of Lysosomes along Different Microtubule Tracks. Guardia CM1, Farías GG1, Jia R1, Pu J1, Bonifacino JS Cell Rep. 17(8):1950-1961. doi: 10.1016/j.celrep.2016.10.062.

How to get the right spatiotemporal combination of microtubules and kinesins properties for the correct regional movement of lysosomes? Complex complexity.Dionisio_{December 31, 2016
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Classical electron and light microscopy techniques, recently enhanced by the development of super-resolution microscopy, have produced a detailed view of the spatial organization of cytoplasmic organelles within eukaryotic cells. Live-cell imaging methodologies have further revealed that this organization is highly dynamic [...] Indeed, organelles move around the cytoplasm, change their size and shape, and establish transient contacts with one another, all under precise regulatory controls.
BORC Functions Upstream of Kinesins 1 and 3 to Coordinate Regional Movement of Lysosomes along Different Microtubule Tracks. Guardia CM1, Farías GG1, Jia R1, Pu J1, Bonifacino JS Cell Rep. 17(8):1950-1961. doi: 10.1016/j.celrep.2016.10.062.

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The multiple functions of lysosomes are critically dependent on their ability to undergo bidirectional movement along microtubules between the center and the periphery of the cell. Centrifugal and centripetal movement of lysosomes is mediated by kinesin and dynein motors, respectively. Common regulation by BORC enables coordinate control of lysosome movement in different regions of the cell.
BORC Functions Upstream of Kinesins 1 and 3 to Coordinate Regional Movement of Lysosomes along Different Microtubule Tracks. Guardia CM1, Farías GG1, Jia R1, Pu J1, Bonifacino JS Cell Rep. 17(8):1950-1961. doi: 10.1016/j.celrep.2016.10.062.

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The question of how katanin senses lumenal acetylation of MTs remains to be answered. [...] the role of acetylation on motor function [remains] unresolved. [...] physiological roles of ?-tubulin acetylation remain enigmatic. Combining these reagents with the TG2 knockout mouse [...] will further our understanding of tubulin polyamination. Tubulin O-linked glycosylation appears heterogeneous and cell specific, but our knowledge regarding its cellular functions is limited. Low levels of tubulin in membrane fractions are consistently observed [...], but poorly understood. [...] it is not known whether this plays a role in the action of vinblastine on MT polymerization. [...] these modifications play a role in the degradation of tubulin, but may have additional effects that are less well understood. Relating specific tubulin modifications to specific cellular functions remains a major challenge. [...] common themes are emerging and questions continue to arise (Outstanding questions box).
Posttranslational Modifications of Tubulin: Pathways to Functional Diversity of Microtubules Yuyu Song1 and Scott T. Brady Trends Cell Biol. 25(3): 125–136. doi: 10.1016/j.tcb.2014.10.004

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Tubulin PTMs are found in all cells with MTs [...] and they are particularly diverse in neurons [...], but many questions remain, such as the fraction of tubulins with a given modification, the distribution of modifications along a MT or between MTs, and the functional consequences of many modifications. Although ?2-tubulin may represent ?35% of tubulin in the brain [...], the functional role of ?2-MTs remains poorly understood. Additional acetylation sites on tubulin have also been identified in proteomic screens [...] and in studies of other acetyltransferases [...]. The prevalence and function of these sites are not well understood, but may affect MT polymerization [...]
Posttranslational Modifications of Tubulin: Pathways to Functional Diversity of Microtubules Yuyu Song1 and Scott T. Brady Trends Cell Biol. 25(3): 125–136. doi: 10.1016/j.tcb.2014.10.004

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Tubulin and microtubules are subject to a remarkable number of posttranslational modifications. Understanding the roles these modifications play in determining functions and properties of microtubules has presented a major challenge that is only now being met. Many of these modifications are found concurrently, leading to considerable diversity in cellular microtubules, which varies with development, differentiation, cell compartment and cell cycle. We now know that posttranslational modifications of tubulin affect not only the dynamics of the microtubules, but also their organization and interaction with other cellular components. Many early suggestions of how posttranslational modifications affect microtubules have been replaced with new ideas and even new modifications as our understanding of cellular microtubule diversity comes into focus.
Posttranslational Modifications of Tubulin: Pathways to Functional Diversity of Microtubules Yuyu Song1 and Scott T. Brady Trends Cell Biol. 25(3): 125–136. doi: 10.1016/j.tcb.2014.10.004

Many early suggestions [...] have been replaced with new ideas ? Seen that before, haven't we? :) What else is new? Complex complexity.Dionisio_{December 30, 2016
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DNA damage acquired during meiosis can lead to infertility and miscarriage. Hence, it should be important for an oocyte to be able to detect and respond to such events in order to make a healthy egg. The Spindle Assembly Checkpoint, which is a well-known mitotic pathway employed by somatic cells to monitor chromosome attachment to spindle microtubules, appears to be utilised by oocytes also to respond to DNA damage. [...] maturing oocytes are arrested at metaphase I due to an active Spindle Assembly Checkpoint. This is surprising given this checkpoint has been previously studied in oocytes and considered to be weak and ineffectual because of its poor ability to be activated in response to microtubule attachment errors. Therefore, the involvement of the Spindle Assembly Checkpoint in DNA damage responses of mature oocytes during meiosis I uncovers a novel second function for this ubiquitous cellular checkpoint.
DNA damage responses in mammalian oocytes. Collins JK, Jones KT Reproduction. 152(1):R15-22. doi: 10.1530/REP-16-0069. http://www.reproduction-online.org/content/152/1/R15

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[...] oocytes respond to DNA damage by arresting in meiosis I through activity of the Spindle Assembly Checkpoint (SAC) and DNA Damage Response (DDR) pathways. It is currently not known if DNA damage is the primary trigger for arrest, or if the pathway is sensitive to levels of DNA damage experienced physiologically. This study establishes a clinical relevance to the DDR induced SAC in oocytes. It helps explain how oocytes respond to a highly prevalent human disease and the reduced fertility associated with endometriosis. In the human ovary, oocytes would be exposed to follicular fluid at higher concentrations and for longer periods of time. Therefore the pathway is likely highly sensitive to diseases such as endometriosis, and possibly others that could elevate ROS. Encouragingly, although the pathway is sensitive it can also be reversed in-vitro by anti-oxidant treatment. Reducing oxidative stress in the oocyte may therefore be of clinical importance when treating sub-fertility in endometriosis either in-vivo or in-vitro.
The sensitivity of the DNA damage checkpoint prevents oocyte maturation in endometriosis Mukhri Hamdan,1,2 Keith T. Jones,3 Ying Cheong,1 and Simon I. R. Lanea Sci Rep. 6: 36994. doi: 10.1038/srep36994 http://www.nature.com/articles/srep36994

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Flavonoids are a large and diverse group of plant secondary metabolites that are mainly present as glycosides. They are often accumulated in response to abiotic stresses such as UV radiation, drought, cold and freezing. The most extensively studied function of flavonoids is their antioxidant activity although their importance as antioxidants in plants has been questioned. Changes in the vibration bands attributed to the phenolic ring structures of the flavonols in the presence of liposomes provided further evidence of interactions of these molecules in particular with the interfacial region of the bilayers.
Effects of flavonol glycosides on liposome stability during freezing and drying Antoaneta V. Popova, Dirk K. Hincha Biochimica et Biophysica Acta (BBA) - Biomembranes Volume 1858, Issue 12, Pages 3050–3060 DOI: http://dx.doi.org/10.1016/j.bbamem.2016.09.020

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More direct experimental evidence could be gained only by manipulating each of the two pathways at different times as the cell enters mitosis and forms the bipolar spindle. Unfortunately, this is currently technically not possible. [...] it will be of interest to investigate further the onset timing of the augmin pathway and its effects on spindle assembly. The importance for cell division of the sequential activation of the centrosomal and chromosome-dependent MT assembly pathways may be a general principle in most somatic animal cells [...] Bipolar spindle assembly and correct kinetochore–MT attachment are both critical events for faithful mitosis and may be interconnected. [...] we cannot rule out that other factors may also be limiting for spindle assembly. [...] optimal equilibrium between the level and the timing of centrosome maturation [...] the chromatin MT assembly pathway activity, and the timing of NEBD may be adapted to determine specific spindle assembly dynamics in every cell type [...] [...] a novel important mechanism determines the correct balance between the mitotic MT assembly pathways, ensuring correct bipolar spindle formation and cell division fidelity. This mechanism relies on the sequential activation of the MT assembly pathways defined by centrosome maturation and NEBD.
The sequential activation of the mitotic microtubule assembly pathways favors bipolar spindle formation. Cavazza T, Malgaretti P, Vernos I Mol Biol Cell. 27(19):2935-45. doi: 10.1091/mbc.E16-05-0322

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In most animal cells, the mitotic spindle assembles in the presence of two centrosomes. Before mitosis, during G2, a complex network of kinases and feedback loops drives centrosome maturation [...] This process promotes the recruitment of various proteins around the centrosome, increasing the amount of pericentriolar material (PCM) and centrosome microtubule (MT) nucleation activity [...] [...] in most animal cells, centrosome maturation defines the level of activity of the chromosome-dependent MT assembly pathway, thus establishing a balance that favors spindle bipolarity.
The sequential activation of the mitotic microtubule assembly pathways favors bipolar spindle formation. Cavazza T, Malgaretti P, Vernos I Mol Biol Cell. 27(19):2935-45. doi: 10.1091/mbc.E16-05-0322

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Centrosome maturation is the process by which the duplicated centrosomes recruit pericentriolar components and increase their microtubule nucleation activity before mitosis. The role of this process in cells entering mitosis has been mostly related to the separation of the duplicated centrosomes and thereby to the assembly of a bipolar spindle. However, spindles can form without centrosomes. In fact, all cells, whether they have centrosomes or not, rely on chromatin-driven microtubule assembly to form a spindle. Our data suggest a novel function for centrosome maturation that determines the contribution of the chromosomal microtubule assembly pathway and favors bipolar spindle formation in most animal cells in which tubulin is in limiting amounts.
The sequential activation of the mitotic microtubule assembly pathways favors bipolar spindle formation. Cavazza T, Malgaretti P, Vernos I Mol Biol Cell. 27(19):2935-45. doi: 10.1091/mbc.E16-05-0322

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Polarized epithelial cells exhibit a characteristic array of microtubules that are oriented along the apicobasal axis of the cells. The minus-ends of these microtubules face apically, and the plus-ends face toward the basal side. The mechanisms underlying this epithelial-specific microtubule assembly remain unresolved [...] [...] apically localized CAMSAP3 determines the proper orientation of microtubules, and in turn that of organelles, in mature mammalian epithelial cells.
CAMSAP3 orients the apical-to-basal polarity of microtubule arrays in epithelial cells. Toya M1, Kobayashi S1, Kawasaki M1, Shioi G2, Kaneko M3, Ishiuchi T1, Misaki K4, Meng W1, Takeichi M Proc Natl Acad Sci U S A. 113(2):332-7. doi: 10.1073/pnas.1520638113.

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Polarization is essential for epithelial cells to exert a variety of functions. Epithelial polarization includes characteristic microtubule array formation. The microtubules are oriented along the apicobasal axis with their minus ends facing apically. The molecules that regulate such epithelial-specific microtubule assembly remain unknown [...] Our findings facilitate our understanding of how epithelial cells acquire polarized structures, which are crucial for their physiological functions.
CAMSAP3 orients the apical-to-basal polarity of microtubule arrays in epithelial cells. Toya M1, Kobayashi S1, Kawasaki M1, Shioi G2, Kaneko M3, Ishiuchi T1, Misaki K4, Meng W1, Takeichi M Proc Natl Acad Sci U S A. 113(2):332-7. doi: 10.1073/pnas.1520638113.

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Cadherin-related 23 (CDH23) is an adhesive protein important for hearing and vision, while CAMSAP3/Marshalin is a microtubule (MT) minus-end binding protein that regulates MT networks. [...] CDH23-C is a CAMSAP3/Marshalin-binding protein that can modify MT networks indirectly through its interaction with CAMSAP3/Marshalin. CDH23 is an essential protein for auditory and visual signal transduction and is important for cell adhesion. Function of the cytoplasmic isoforms, however, is not fully understood [...] [...] onal studies are required to further define CDH23-C’s roles in various physiological contexts that may ultimately impact our understanding of the fundamental mechanisms associated with hearing and vision.
Cadherin 23-C Regulates Microtubule Networks by Modifying CAMSAP3's Function. Takahashi S, Mui VJ, Rosenberg SK, Homma K, Cheatham MA, Zheng J Sci Rep. 6: 28706. doi: 10.1038/srep28706

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Efficient use of seed nutrient reserves is crucial for germination and establishment of plant seedlings. Organogenesis in multicellular organ(ism)s involves a coordinated interplay of cell proliferation and differentiation. Control of size is a longstanding issue in developmental biology. [...] our understanding of compensation is limited to the triggering factors, but the link(s) between cell proliferation defects and enhanced post-mitotic cell expansion remain to be elucidated. Although our working model is reasonable, it has to be addressed experimentally in the future for its validation. Compensation is a heterogeneous phenomenon with different inputs and outputs that differ in each individual mutant that displays CCE [...] CCE suppressor screens for each individual mutant background must be conducted to elucidate the molecular mechanism(s) in such mutants.
Suppressor Screen and Phenotype Analyses Revealed an Emerging Role of the Monofunctional Peroxisomal Enoyl-CoA Hydratase 2 in Compensated Cell Enlargement Mana Katano,1,† Kazuki Takahashi,1,† Tomonari Hirano,2 Yusuke Kazama,3 Tomoko Abe,3 Hirokazu Tsukaya,4,5 and Ali Ferjani Front Plant Sci. 7: 132. doi: 10.3389/fpls.2016.00132

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The mechanism through which overexpression of LGO regulates gene activity requires further investigation [...] [...] endoreduplication alone is not a sufficient mechanism to explain all of the gene expression in LGOoe sepals, although it may explain some transcriptomic effects. Further investigation of possible transcriptional complexes upon which LGO might act directly remains for future research. Transcriptional responses to the environment can be highly cell-type specific [...]
Transcriptomic Effects of the Cell Cycle Regulator LGO in Arabidopsis Sepals Erich M. Schwarz and Adrienne H. K. Roeder Sepals. Front. Plant Sci. 7:1744. doi: 10.3389/fpls.2016.01744

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Little is known about how CKK domain activity is regulated [...] In conclusion, we have revealed an unexpected regulatory interaction between DAPK-1 kinase and the MT cytoskeleton in epidermal development and wound responses. Many questions remain to be explored, especially whether PTRN-1 or other MT-associated proteins are direct substrates of DAPK-1. More broadly, the mechanisms and roles of intracellular transport within the epidermis could be a model for intracellular transport in other syncytial tissues.
DAPK interacts with Patronin and the microtubule cytoskeleton in epidermal development and wound repair Marian Chuang, Tiffany I Hsiao, Amy Tong, Suhong Xu,† and Andrew D Chisholm eLife. 5: e15833. doi: 10.7554/eLife.15833

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[...] a reductionist approach by studying single factors in isolation seems insufficient to explain MT behaviours in cellular environments.
A conceptual view at microtubule plus end dynamics in neuronal axons André Voelzmann,a Ines Hahn,a Simon P. Pearce,a,b Natalia Sánchez-Soriano,c and Andreas Prokop Brain Res Bull. 126: 226–237. doi: 10.1016/j.brainresbull.2016.08.006

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Future work is needed to understand how particles containing bicoid mRNA are tethered at the front end of the egg cell and whether other mRNAs are also packaged in a similar manner. In future, it will be interesting to determine whether other localised RNAs are packaged into similar structures.
bicoid mRNA localises to the Drosophila oocyte anterior by random Dynein-mediated transport and anchoring Vítor Trovisco,1,2 Katsiaryna Belaya,1,2† Dmitry Nashchekin,1,2 Uwe Irion,1,2‡ George Sirinakis,1,2 Richard Butler,1 Jack J Lee,3 Elizabeth R Gavis,3 and Daniel St Johnston eLife. 5: e17537. doi: 10.7554/eLife.17537

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It will therefore be interesting to investigate whether the different modes of MT binding by Shot are mutually exclusive and how this is regulated. The combination of Patronin binding to the MT minus ends and Shot binding to the MT lattice may therefore provide a robust anchor to retain MTs at the apical cortex.
Patronin/Shot Cortical Foci Assemble the Noncentrosomal Microtubule Array that Specifies the Drosophila Anterior-Posterior Axis. Nashchekin D, Fernandes AR, St Johnston D Dev Cell. 38(1):61-72. doi: 10.1016/j.devcel.2016.06.010.

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[...] there are a number of studies focused on understanding the required molecular events. Surprisingly, this fascinating genus has been little studied to date. An area of great promise for future research in Naegleria is how the majority of differentiating Naegleria cells assemble exactly two basal bodies and two flagella. Why heat-shock temporarily alters flagellar number, as well as the nature of the normal control mechanism, remain interesting challenges for future investigation. [..] resolving the issues caused by heterologous antibodies as well as more precise colocalization studies are essential to understanding their results. We hope these issues can be resolved in the near future. While NaegleriaPlk1 might play the role of Plk4 in the amoeboflagellate, any role of polo-like kinases in this system remains a challenge for future research, particularly given the current lack of tools for gene manipulation in Naegleria cells. All that is needed is that researchers meet the challenge of learning to apply molecular genetics to this fascinating system.
Naegleria: a classic model for de novo basal body assembly Lillian K. Fritz-Laylin and Chandler Fulton Cilia. 5: 10. doi: 10.1186/s13630-016-0032-6

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The next challenge consists of studying the effect of more than one MAP on the generation of complexity as it occurs in the spindle. Ultimately, it will be necessary to explain how MTs and MAPs, which act at the nanometer scale, build the mitotic spindle, which is a factor of 1,000 larger, in a robust manner. Ever since the first descriptions of cell division 130 years ago, the means by which the mitotic spindle orchestrates cell division has been a mystery. Grasping how hundreds of proteins can self-assemble into the mitotic spindle and segregate chromosomes at biochemical and structural levels is a challenge that may be finally within reach during this century.
Mechanisms of Mitotic Spindle Assembly. Petry S Annu Rev Biochem. 85:659-83. doi: 10.1146/annurev-biochem-060815-014528

Work in progress... stay tuned. Complex complexity.Dionisio_{December 29, 2016
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It remains to be demonstrated whether the negative regulation of MT depolymerases is the only mechanism by which the CPC operates. [...] the exact role of MT nucleation from KTs remains to be determined. [...] it is not clear if both the Ran and CPC pathways are activated at the same time, and whether there is a time difference between MT generation from chromosome arms and centromeres/KTs. In the future, quantitative and mechanistic studies of each individual MT nucleation pathway should be performed to determine how MTs are assembled and organized to form the spindle. Although it is difficult enough to resolve individual MTs of any kind, it would be ideal if MT populations from different MTOCs could be separately detected. Independent of this challenge, it is necessary to understand how MTs of two or more MTOCs interdigitate toward creating the metaphase spindle.
Mechanisms of Mitotic Spindle Assembly. Petry S Annu Rev Biochem. 85:659-83. doi: 10.1146/annurev-biochem-060815-014528

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[...] information about the function, activity, and location of spindle MAPs needs to be combined with the directly measured parameters of MT organization to derive a complete molecular model of the metaphase spindle. [...] how this secondary phosphorylation gradient contributes to spindle assembly remains poorly understood. [...] the exact mechanism by which this RanGTP-mediated regulation occurs remains to be determined. It is currently unclear whether these SAFs all act in separate or in common MT nucleation pathways and how exactly they contribute to MT formation and organization toward subsequent spindle assembly. How these factors work together to induce this reaction and what the branch point looks like remain to be resolved.
Mechanisms of Mitotic Spindle Assembly. Petry S Annu Rev Biochem. 85:659-83. doi: 10.1146/annurev-biochem-060815-014528

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