Comparison of spindle elongation in vivo and in vitro in Stephanopyxis turris

Electron tomography reveals aspects of spindle structure important for mechanical stability at metaphase

Metaphase spindles exert pole-directed forces on still-connected sister kinetochores. The spindle must counter these forces with extensive forces to prevent spindle collapse. In small spindles, kinetochore microtubules (KMTs) connect directly with the poles, and countering forces are supplied either by interdigitating MTs that form interpolar bundles or by astral MTs connected to the cell cortex. In bigger spindles, particularly those without structured poles, the origin of extensive forces is less obvious. We have used electron tomography of well-preserved metaphase cells to obtain structural evidence about interactions among different classes of MTs in metaphase spindles from Chlamydomonas rheinhardti and two strains of cultured mammalian cells. In all these spindles, KMTs approach close to and cross-bridge with the minus ends of non-KMTs, which form a framework that interdigitates near the spindle equator. Although this structure is not pole-connected, its organization suggests that it can support kinetochore tension. Analogous arrangements of MTs have been seen in even bigger spindles, such as metaphase spindles in Haemanthus endosperm and frog egg extracts. We present and discuss a hypothesis that rationalizes changes in spindle design with spindle size based on the negative exponential distribution of MT lengths in dynamically unstable populations of tubulin polymers.

Biophysics of mitosis

Mitosis is the process by which eukaryotic cells organize and segregate their chromosomes in preparation for cell division. It is accomplished by a cellular machine composed largely of microtubules (MTs) and their associated proteins. This article reviews literature on mitosis from a biophysical point of view, drawing attention to the assembly and motility processes required to do this complex job with precision. Work from both the recent and the older literature is integrated into a description of relevant biological events and the experiments that probe their mechanisms. Theoretical work on specific subprocesses is also reviewed. Our goal is to provide a document that will expose biophysicists to the fascination of this quite amazing process and provide them with a good background from which they can pursue their own research interests in the subject.

Unusual centrosome cycle in Dictyostelium: correlation of dynamic behavior and structural changes

Centrosome duplication and separation are of central importance for cell division. Here we provide a detailed account of this dynamic process in Dictyostelium. Centrosome behavior was monitored in living cells using a gamma-tubulin-green fluorescent protein construct and correlated with morphological changes at the ultrastructural level. All aspects of the duplication and separation process of this centrosome are unusual when compared with, e.g., vertebrate cells. In interphase the Dictyostelium centrosome is a box-shaped structure comprised of three major layers, surrounded by an amorphous corona from which microtubules emerge. Structural duplication takes place during prophase, as opposed to G1/S in vertebrate cells. The three layers of the box-shaped core structure increase in size. The surrounding corona is lost, an event accompanied by a decrease in signal intensity of gamma-tubulin-green fluorescent protein at the centrosome and the breakdown of the interphase microtubule system. At the prophase/prometaphase transition the separation into two mitotic centrosomes takes place via an intriguing lengthwise splitting process where the two outer layers of the prophase centrosome peel away from each other and become the mitotic centrosomes. Spindle microtubules are now nucleated from surfaces that previously were buried inside the interphase centrosome. Finally, at the end of telophase, the mitotic centrosomes fold in such a way that the microtubule-nucleating surface remains on the outside of the organelle. Thus in each cell cycle the centrosome undergoes an apparent inside-out/outside-in reversal of its layered structure.

DSK1, a novel kinesin-related protein from the diatom Cylindrotheca fusiformis that is involved in anaphase spindle elongation

We have identified an 80-kD protein that is involved in mitotic spindle elongation in the diatom Cylindrotheca fusiformis. DSK1 (Diatom Spindle Kinesin 1) was isolated using a peptide antibody raised against a conserved region in the motor domain of the kinesin superfamily. By sequence homology, DSK1 belongs to the central motor family of kinesin-related proteins. Immunoblots using an antibody raised against a non-conserved region of DSK1 show that DSK1 is greatly enriched in mitotic spindle preparations. Anti-DSK1 stains in diatom central spindle with a bias toward the midzone, and staining is retained in the spindle midzone during spindle elongation in vitro. Furthermore, preincubation with anti-DSK1 blocks function in an in vitro spindle elongation assay. This inhibition of spindle elongation can be rescued by preincubating concurrently with the fusion protein against which anti-DSK1 was raised. We conclude that DSK1 is involved in spindle elongation and is likely to be responsible for pushing hal-spindles apart in the spindle midzone.

Identification of dynamin, a novel mechanochemical enzyme that mediates interactions between microtubules

We report that calf brain microtubules prepared without nucleotide contain, in addition to kinesin and dynein, a polypeptide of 100 kd that could be dissociated by nucleotide. The protein was selectively extracted from microtubules using a combination of GTP and AMP-PNP. The extract contained microtubule-stimulated (6-fold) MgATPase activity that partitioned into two components upon further purification: the 100 kd polypeptide and a soluble activating fraction. The 100 kd protein induced microtubules to form hexagonally packed bundles containing periodic cross bridges spaced 13 nm apart. In the presence of ATP and the activating fraction, bundles fragmented, elongated, and exhibited other behavior indicative of sliding between microtubules. These findings indicate that the 100 kd protein is part of a novel mechanochemical enzyme, which we term "dynamin", that may mediate microtubule sliding in vivo.

Distribution of phosphorylated spindle-associated proteins in the diatom Stephanopyxis turris

Mitotic spindles isolated from the diatom Stephanopyxis turris become thiophosphorylated in the presence of ATP gamma S at specific locations within the mitotic apparatus, resulting in a stimulation of ATP-dependent spindle elongation in vitro. Here, using indirect immunofluorescence, we compare the staining pattern of an antibody against thiophosphorylated proteins to that of MPM-2, an antibody against mitosis-specific phosphoproteins, in isolated spindles. Both antibodies label spindle poles, kinetochores, and the midzone. Neither antibody exhibits reduced labeling in salt-extracted spindles, although prior salt extraction inhibits thiophosphorylation in ATP gamma S. Furthermore, both antibodies recognize a 205 kd band on immunoblots of spindle extracts. Microtubule-organizing centers and mitotic spindles label brightly with the MPM-2 antibody in intact cells. These results show that functional mitotic spindles isolated from S. turris are phosphorylated both in vivo and in vitro. We discuss the possible role of phosphorylated cytoskeletal proteins in the control of mitotic spindle function.

The mechanism of anaphase spindle elongation: uncoupling of tubulin incorporation and microtubule sliding during in vitro spindle reactivation

To study tubulin polymerization and microtubule sliding during spindle elongation in vitro, we developed a method of uncoupling the two processes. When isolated diatom spindles were incubated with biotinylated tubulin (biot-tb) without ATP, biot-tb was incorporated into two regions flanking the zone of microtubule overlap, but the spindles did not elongate. After biot-tb was removed, spindle elongation was initiated by addition of ATP. The incorporated biot-tb was found in the midzone between the original half-spindles. The extent and rate of elongation were increased by preincubation in biot-tb. Serial section reconstruction of spindles elongating in tubulin and ATP showed that the average length of half-spindle microtubules increased due to growth of microtubules from the ends of native microtubules. The characteristic packing pattern between antiparallel microtubules was retained even in the "new" overlap region. Our results suggest that the forces required for spindle elongation are generated by enzymes in the overlap zone that mediate the sliding apart of antiparallel microtubules, and that tubulin polymerization does not contribute to force generation. Changes in the extent of microtubule overlap during spindle elongation were affected by tubulin and ATP concentration in the incubation medium. Spindles continued to elongate even after the overlap zone was composed entirely of newly polymerized microtubules, suggesting that the enzyme responsible for microtubule translocation either is bound to a matrix in the spindle midzone, or else can move on one microtubule toward the spindle midzone and push another microtubule of opposite polarity toward the pole.

Direct observation of mitotic spindle elongation in vitro

Successful reactivation in vitro of anaphase B has recently been achieved with mitotic spindles isolated from the diatom Stephanopyxis turris. When a population of isolated spindles was studied indirectly by using immunofluorescence, nearly all of them were found to have elongated; however, when studied directly by using video microscopy, only a small proportion of spindles elongated. We report here conditions that allow nearly all of the spindles to elongate when observed directly with video microscopy. These direct observations validate previous ones made using indirect immunofluorescence. In addition, we find that the isolated spindles elongate with a linear rate, that the elongation is unchanged after the chromatin surrounding the spindles is digested with DNase I, and that during elongation a phase-dense matrix may accumulate in the spindle midzone.

Reactivation of spindle elongation in vitro is correlated with the phosphorylation of a 205 kd spindle-associated protein

Mitotic spindles isolated from the diatom Stephanopyxis turris consist of two half-spindles of closely interdigitating microtubules that slide relative to one another in the presence of ATP, reinitiating spindle elongation (anaphase B) in vitro. Purified spindles that have been exposed to ATP-gamma-S undergo ATP-dependent reactivation more readily than do control spindles. Thiophosphorylated proteins in such spindles are located in the spindle midzone, kinetochores, and a portion of the pole complex. One major thiophosphorylated peptide of 205 kd is detected in extracts prepared from spindles labeled with [35S]ATP-gamma-S, and is also localized in the spindle midzone by using an antibody that recognizes thiophosphorylated proteins. It is likely that this 205 kd peptide is either a positive regulator or mechanochemical transducer of microtubule sliding when it is in a phosphorylated state.

The role of tubulin polymerization during spindle elongation in vitro

We describe the effect of exogenous tubulin on reactivation of anaphase spindle elongation in isolated diatom spindles. In the absence of tubulin, spindle elongation is limited to the equivalent of the microtubule overlap zone, but in the presence of tubulin spindle elongation is several times the length of the overlap zone. Biotinylated neurotubulin is incorporated into the overlap zone and around the poles. Before spindles have elongated by the equivalent of the overlap zone, there are two regions of incorporated tubulin flanking this zone. After further elongation, there is one broad zone of incorporated tubulin in the spindle midzone. Spindle elongation and the pattern of tubulin incorporation into the midzone, but not the poles, are ATP-dependent and vanadate-sensitive. These results suggest that tubulin adds onto the ends of microtubules in the overlap zone, which then slide through the midzone as the spindle elongates.