Multiple isoforms of the high molecular weight microtubule associated protein XMAP215 are expressed during development in Xenopus

Use of Xenopus cell-free extracts to study size regulation of subcellular structures

Striking size variations are prominent throughout biology, at the organismal, cellular, and subcellular levels. Important fundamental questions concern organelle size regulation and how organelle size is regulated relative to cell size, also known as scaling. Uncovering mechanisms of organelle size regulation will inform the functional significance of size as well as the implications of misregulated size, for instance in the case of nuclear enlargement in cancer. Xenopus egg and embryo extracts are powerful cell-free systems that have been utilized extensively for mechanistic and functional studies of various organelles and subcellular structures. The open biochemical nature of the extract permits facile manipulation of its composition, and in recent years extract approaches have illuminated mechanisms of organelle size regulation. This review largely focuses on in vitro Xenopus studies that have identified regulators of nuclear and spindle size. We also discuss potential relationships between size scaling of the nucleus and spindle, size regulation of other subcellular structures, and extract experiments that have clarified developmental timing mechanisms. We conclude by offering some future prospects, notably the integration of Xenopus extract with microfluidic technology.

Growth cone-specific functions of XMAP215 in restricting microtubule dynamics and promoting axonal outgrowth

Background

Microtubule (MT) regulators play essential roles in multiple aspects of neural development. In vitro reconstitution assays have established that the XMAP215/Dis1/TOG family of MT regulators function as MT ‘plus-end-tracking proteins’ (+TIPs) that act as processive polymerases to drive MT growth in all eukaryotes, but few studies have examined their functions in vivo. In this study, we use quantitative analysis of high-resolution live imaging to examine the function of XMAP215 in embryonic Xenopus laevis neurons.

Results

Here, we show that XMAP215 is required for persistent axon outgrowth in vivo and ex vivo by preventing actomyosin-mediated axon retraction. Moreover, we discover that the effect of XMAP215 function on MT behavior depends on cell type and context. While partial knockdown leads to slower MT plus-end velocities in most cell types, it results in a surprising increase in MT plus-end velocities selective to growth cones. We investigate this further by using MT speckle microscopy to determine that differences in overall MT translocation are a major contributor of the velocity change within the growth cone. We also find that growth cone MT trajectories in the XMAP215 knockdown (KD) lack the constrained co-linearity that normally results from MT-F-actin interactions.

Conclusions

Collectively, our findings reveal unexpected functions for XMAP215 in axon outgrowth and growth cone MT dynamics. Not only does XMAP215 balance actomyosin-mediated axon retraction, but it also affects growth cone MT translocation rates and MT trajectory colinearity, all of which depend on regulated linkages to F-actin. Thus, our analysis suggests that XMAP215 functions as more than a simple MT polymerase, and that in both axon and growth cone, XMAP215 contributes to the coupling between MTs and F-actin. This indicates that the function and regulation of XMAP215 may be significantly more complicated than previously appreciated, and points to the importance of future investigations of XMAP215 function during MT and F-actin interactions.

Soluble tubulin complexes, γ-tubulin, and their changing distribution in the zebrafish (Danio rerio) ovary, oocyte and embryo

Tubulin dynamics, i.e., the interchange of polymeric and soluble forms, is important for microtubule (MTs) cellular functions, and thus plays essential roles in zebrafish oogenesis and embryogenesis. A novel finding in this study revealed that there were soluble pools of tubulins in zebrafish oocytes that were sequestered and maintained in a temporary "oligomeric" state, which retained assembling and disassembling potential (suggested by undetected acetylated tubulin, marker of stable tubulin), but lacked abilities to assemble into MTs spontaneously in vivo. Using differential centrifugation, gel chromatography and DM1A-probed western blot, soluble alpha-tubulin was found to be associated with large molecular weight complexes (MW range to over 2 MDa) which were reduced in amount by the blastula stage, especially in some batches of embryos, with a concomitant decrease in soluble tubulin. Complexes (MW range less than 2 MDa) then increased in the gastrula with an increase in soluble alpha-tubulin. Two different anti-gamma-tubulin monoclonal antibodies, GTU 88 and TU 30, revealed the existence of soluble gamma-tubulin in both zebrafish oocytes and embryos, which also decreased by the blastula stage and increased in the gastrula stage. Soluble alpha-tubulin and gamma-tubulin extracted from zebrafish ovaries, oocytes and embryos co-localized in fractions on three different columns: S-200 Sephacryl, DEAE and Superose-6b. The soluble tubulin complexes were competent to assemble into MTs in vitro induced by taxol, and gamma-tubulin was co-localized with assembled MTs. These soluble tubulin complexes were stable during freeze-thaw cycles and resisted high ionic interaction (up to 1.5 M NaCl). Furthermore, some ovarian soluble alpha-tubulin could be co-immunoprecipitated with gamma-tubulin, and vice versa. Two antibodies specific for Xenopus gamma-tubulin ring complex proteins (Xgrip 109 and Xgrip 195) detected single bands from ovarian extracts in western blots, suggesting the existence of Xgrip 109 and Xgrip 195 homologues in zebrafish. These findings, together with recent work on gamma-tubulin ring complexes in oocytes, eggs and embryos of other species, suggest that soluble gamma-tubulin-associated protein complexes may be involved in regulating tubulin dynamics during zebrafish oogenesis and embryogenesis.

The microtubule-associated protein tumor overexpressed gene binds to the RNA trafficking protein heterogeneous nuclear ribonucleoprotein A2

In neural cells, such as oligodendrocytes and neurons, transport of certain RNAs along microtubules is mediated by the cis-acting heterogeneous nuclear ribonucleoprotein A2 response element (A2RE) trafficking element and the cognate trans-acting heterogeneous nuclear ribonucleoprotein (hnRNP) A2 trafficking factor. Using a yeast two-hybrid screen, we have identified a microtubule-associated protein, tumor overexpressed gene (TOG)2, as an hnRNP A2 binding partner. The C-terminal third of TOG2 is sufficient for hnRNP A2 binding. TOG2, the large protein isoform of TOG, is the only isoform detected in oligodendrocytes in culture. TOG coimmunoprecipitates with hnRNP A2 present in the cytoskeleton (CSK) fraction of neural cells, and both coprecipitate with microtubule stabilized pellets. Staining with anti-TOG reveals puncta that are localized in proximity to microtubules, often at the plus ends. TOG is colocalized with hnRNP A2 and A2RE-mRNA in trafficking granules that remain associated with CSK-insoluble tissue. These data suggest that TOG mediates the association of hnRNP A2-positive granules with microtubules during transport and/or localization.

MAPping the Eukaryotic Tree of Life: Structure, Function, and Evolution of the MAP215⧸Dis1 Family of Microtubule-Associated Proteins

The MAP215/Dis1 family of proteins is an evolutionarily ancient family of microtubule-associated proteins, with characterized members in all major kingdoms of eukaryotes, including fungi (Stu2 in S. cerevisiae, Dis1 and Alp14 in S. pombe), Dictyostelium (DdCP224), plants (Mor1 in A. thaliana and TMBP200 in N. tabaccum), and animals (Zyg9 in C. elegans, Msps in Drosophila, XMAP215 in Xenopus, and ch-TOG in humans). All MAP215/Dis1 proteins (with the exception of those in plants) localize to microtubule-organizing centers (MTOCs), including spindle pole bodies in yeast and centrosomes in animals, and all bind to microtubules in vitro and?or in vivo. Diverse roles in regulating microtubule assembly and organization have been proposed for individual family members, and a substantial body of evidence suggests that MAP215/Dis1-related proteins play critical roles in the assembly and function of the meiotic/mitotic spindles and/or cell division. An extensive search of public databases (including both EST and genome databases) identified partial sequences predicted to encode more than three dozen new members of the MAP215/Dis1 family, including putative MAP215/Dis1-related proteins in Giardia lamblia and four other protists, sixteen additional species of fungi, six plants, and twelve animals. The structure and function of MAP215/Dis1 proteins are discussed in relation to the evolution of this ancient family of microtubule-associated proteins.

XMAP215, XKCM1, NuMA, and cytoplasmic dynein are required for the assembly and organization of the transient microtubule array during the maturation of Xenopus oocytes

During the maturation of Xenopus oocytes, a transient microtubule array (TMA) is nucleated from a novel MTOC near the base of the germinal vesicle. The MTOC-TMA transports the meiotic chromosomes to the animal cortex, where it serves as the precursor to the first meiotic spindle. To understand more fully the assembly of the MTOC-TMA, we used confocal immunofluorescence microscopy to examine the localization and function of XMAP215, XKCM1, NuMA, and cytoplasmic dynein during oocyte maturation. XMAP215, XKCM1, and NuMA were all localized to the base of the MTOC-TMA and the meiotic spindle. Microinjection of anti-XMAP215 inhibited microtubule (MT) assembly during oocyte maturation, disrupting assembly of the MTOC-TMA and subsequent assembly of the first meiotic spindle. In contrast, microinjection of anti-XKCM1 promoted MT assembly throughout the cytoplasm, disrupting organization of the MTOC-TMA and meiotic spindle. Finally, microinjection of anti-dynein or anti-NuMA disrupted the organization of the MTOC-TMA and subsequent assembly of the meiotic spindles. These results suggest that XMAP215 and XKCM1 act antagonistically to regulate MT assembly and organization during maturation of Xenopus oocytes, and that dynein and NuMA are required for organization of the MTOC-TMA.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Microtubule organization in the green kingdom: chaos or self-order?

Plant microtubule arrays differ fundamentally from their animal, fungal and protistan counterparts. These differences largely reflect the requirements of plant composite polymer cell walls and probably also relate to the acquisition of chloroplasts. Plant microtubules are usually dispersed and lack conspicuous organizing centres. The key to understanding this dispersed nature is the identification of proteins that interact with and regulate the spatial and dynamic properties of microtubules. Over the past decade, a number of these proteins have been uncovered, including numerous kinesin-related proteins and a 65 kDa class of structural microtubule-associated proteins that appear to be unique to plants. Mutational analysis has identified MOR1, a probable stabilizer of microtubules that is a homologue of the TOGp-XMAP215 class of high-molecular-weight microtubule-associated proteins, and a katanin p60 subunit homologue implicated in the severing of microtubules. The identification of these two proteins provides new insights into the mechanisms controlling microtubule assembly and dynamics, particularly in the dispersed cortical array found in highly polarized plant cells.