During which phase do the centromeres split and sister chromatids move to the poles?

Mol Biol Cell. 1998 Nov; 9[11]: 3211–3225.

J. Richard McIntosh, Monitoring Editor

Abstract

In higher eukaryotic cells, the spindle forms along with chromosome condensation in mitotic prophase. In metaphase, chromosomes are aligned on the spindle with sister kinetochores facing toward the opposite poles. In anaphase A, sister chromatids separate from each other without spindle extension, whereas spindle elongation takes place during anaphase B. We have critically examined whether such mitotic stages also occur in a lower eukaryote, Schizosaccharomyces pombe. Using the green fluorescent protein tagging technique, early mitotic to late anaphase events were observed in living fission yeast cells. S. pombe has three phases in spindle dynamics, spindle formation [phase 1], constant spindle length [phase 2], and spindle extension [phase 3]. Sister centromere separation [anaphase A] rapidly occurred at the end of phase 2. The centromere showed dynamic movements throughout phase 2 as it moved back and forth and was transiently split in two before its separation, suggesting that the centromere was positioned in a bioriented manner toward the poles at metaphase. Microtubule-associating Dis1 was required for the occurrence of constant spindle length and centromere movement in phase 2. Normal transition from phase 2 to 3 needed DNA topoisomerase II and Cut1 but not Cut14. The duration of each phase was highly dependent on temperature.

INTRODUCTION

The fission yeast Schizosaccharomyces pombe is an excellent model organism in which to study mitosis, because many genes required for mitosis have been identified, and their products have been characterized by cellular and molecular biological methods [e.g., Yanagida, 1995

In this study, mitotic events in living S. pombe cells were investigated by using green fluorescent protein [GFP]-tagged spindle pole body [SPB] protein and also centromeric DNA. The GFP tagging technique was successfully introduced in S. pombe to visualize the spindle using a GFP–Dis1 construct [Nabeshima et al., 1995

The entry into and the exit from M phase of S. pombe are characterized, respectively, by the rise and the fall of Cdc2–Cdc13 [mitotic cyclin] kinase activity [e.g., Yamano et al., 1996

Mitosis in larger eukaryotic cells can be divided into four stages, prophase, prometaphase, metaphase, and anaphase. It is unknown, however, whether a stage identical to prophase exists in fission yeast cells, because nuclear envelope breakdown, a prophase event, does not take place in yeast cells. Therefore, the period of spindle formation was designated as a prophase-like stage. In the in situ hybridization study of fission yeast [Funabiki et al., 1993

The kinase activity of Cdc2–Cdc13 complexes is thought to be high from prophase to metaphase. Critical mitotic proteins are degraded by anaphase-promoting complex [cyclosome]-mediated ubiquitination and subsequent proteasome-dependent proteolysis. The destruction of mitotic cyclin [Cdc13] leads to mitotic exit, and the destruction of Cut2 induces sister chromatid separation [Funabiki et al., 1996

MATERIALS AND METHODS

Preparation of Specimens

HM123 [h− leu1] was used as a wild-type strain of S. pombe. Wild-type cells carrying plasmid with the GFP-tagged sad1+ gene were exponentially grown to 3 × 106/ml in 20 ml minimal EMM2 medium. One or 2 ml of the culture were centrifuged at 10,000 rpm for 1 min and resuspended in 80–100 μl Edinburgh minimal medium 2 [EMM2]. Twenty to 30 μl of resuspended culture were placed on a glass-bottom culture dish [P35G-0-10-C, MatTeck, Ashland, MA]. The technical details were described by Nabeshima et al. [1997]

Plasmids and Mutant Strains

The fission yeast sad1+ gene codes for an SPB component [Hagan and Yanagida, 1995

For visualizing the centromeric DNA–GFP and Sad1–GFP simultaneously, the S. pombe strain MKY7A-4 was transformed with pSD8. The temperature- and cold-sensitive mutant strains used in the present study were dis1 null [Nabeshima et al., 1995

RESULTS

Visualization of SPB Movements in Mitosis

To observe the SPB in living cells, GFP-tagged Sad1 [designated hereafter Sad1–GFP] was expressed and found to be bound to the SPB throughout the cell cycle [Figure ​1A], identical to immunolocalization data [Hagan and Yanagida, 1995

Visualization of GFP-tagged Sad1, an SPB protein, in living cells [A and B] Wild-type cells transformed with plasmid carrying the Sad1–GFP fusion gene were observed at 33°C. Green fluorescence was found at the SPB positions and also at the nuclear envelope, as seen in cells carrying an elevated gene dosage of sad1+ [Hagan and Yanagida, 1995

Images of one living cell at 33°C [taken by a cooled charged-coupled device camera attached to a microscope in a temperature-controlled room] are shown in Figure ​1C. The cell initially contained a single SPB, demonstrating that it was in G2 [at 0 s]. Separation of the SPB [e.g., spindle formation], which marks entry into mitosis, occurred between 400 and 500 s. One of the duplicated GFP–SPB signals was weak, because it was not in the focal plane. The spindle length reached 2.5 μm and was roughly constant from 700 to 900 s. Spindle length began to increase again at 900 s and continued to 1500 s, reaching 15 μm, the maximal length. The spindle axis was initially oblique to the cell axis but rotated toward the direction parallel to the cell axis during elongation. Simultaneous focusing of the two SPBs was thus often difficult when the spindle length was short. The presence of two SPBs, however, could be seen by through-focusing at each time point. After full spindle elongation, the daughter nuclei made backward movement toward the center of daughter cells [1600 s], which is a microtubule-dependent process [Hagan and Yanagida, 1997

Three Distinct Phases of SPB Movements

Distances between the separated SPBs were measured in living wild-type cells [strain HM123] cultured at different temperatures [20, 26, 33, and 36°C; Figure ​2]. Quantitative measurements revealed three phases in the movements of the SPBs [Figure ​2A]. Phase 1 corresponded to the period of spindle formation [e.g., a prophase-like stage], in which the spindle length increased from 0 to 2.5 μm; phase 2 represented the period of constant spindle length [e.g., metaphase–anaphase A]; and phase 3 was the period of spindle extension from 2.5 to 12–15 μm [anaphase B]. These three distinct phases were found in all the wild-type cells examined, although the duration of each phase was strongly dependent on the temperature used [Figure ​2, B–E].

Time course changes of the pole-to-pole distances during mitosis; 1, 2, and 3 represent the periods of phases 1, 2, and 3. [A] Time course changes for the distances between the mitotic SPBs [in micrometers] of two wild-type cells cultured at 33 or 20°C are shown for comparison. [B–E] Data of the distances between the SPBs obtained from cells cultured at 36°C [B], 33°C [C], 26°C [D], and 20°C [E] are plotted.

The average time duration of the three phases at 20, 26, and 36°C is shown in Table ​1. The time from the initiation of spindle formation to the completion of late anaphase spindle extension was ∼12 min at 36°C and 3.75-fold longer [45 min] at 20°C. Note that the generation time at 20°C is 2.5-fold longer than that at 36°C [Table ​1], so that the relative duration of mitosis became longer at a lower temperature. Phase 2 was particularly temperature dependent; it was only 4 min at 36°C but 19 min [4.8-fold increase] at 20°C. It may be noteworthy that a number of spindle-defective mutants were isolated as cold sensitive [e.g., Hiraoka et al., 1984

Table 1

Duration of three phases at different temperature

Interestingly, the rates at which the SPBs separated from each other in phases 1 and 3 were similar [1.3 ± 0.3 and 1.4 ± 0.2 μm/min at 36°C, respectively] and temperature dependent [0.47 ± 0.1 and 0.38 ± 0.1 μm/min at 20°C, respectively]. In phase 2, the spindle elongated very slowly; the rates of SPB separation were 0.08 and 0.2 μm/min at 20 and 36°C, respectively.

Visualization of the Centromeric DNA in Mitosis

Previous studies [Funabiki et al., 1993

Visualization of the centromeric DNA in living cells. [A] Schematic representation for construction of an S. pombe strain expressing GFP–LacI–NLS, which can enter the nucleus and associates with the LacO array integrated near cen1. [B] Time-lapse GFP images of a single cell expressing the GFP–LacI–NLS located in the nucleus and associated with the cen1-linked DNA. At 10.5 min, two closely situated dots were observed, and these were further separated at 11 min; this represented sister centromere separation followed by nuclear elongation [12 min] and division [15 min]. The distance between the separated signals increased continuously up to 20 min. Anaphase B occurred from 11 to 20 min. Bar, 10 μm. [C] The distances between the cen1 signals are plotted vs. time.

Fast Centromere Separation

Cells containing the cen1-linked LacO array and GFP–LacI–NLS contained one or two fluorescent dots in the nucleus [Figure ​3B] against a background of fainter homogenous nuclear fluorescence. When GFP–LacI–NLS was expressed in control wild-type cells that did not contain the integrated LacO repeats, the fluorescent dot was absent, and only homogenous nuclear fluorescence was seen [our unpublished result]. The GFP–LacI–NLS signal thus detected the integrated LacO repeats. Time-lapse microscopy of GFP–LacI–NLS in single cells taken at 33°C indicated that separation of a single dot into two occurred abruptly. Separation of the signal occurred within 1 min [starting at ∼10 min and completed at 11 min, as seen in Figure ​3, B and C]. The average time required for this separation was 0.5–1.0 min, and the rate of separation was thus 3–5 μm/min. The separated signals moved further apart and reached the ends of the cell at ∼20 min. The rate of this latter movement was slower than that at separation and identical to that of anaphase B spindle extension.

Occurrence of Centromere Separation at the End of Phase 2

To precisely determine the timing of centromere separation with regard to spindle dynamics, we constructed the S. pombe strain expressing both Sad1–GFP and GFP–LacI–NLS with the integrated centromeric LacO repeats. The SPB and the cen1-adjacent DNA could thus be observed in the same cells. A series of images for three example cells [cultured at 26°C] are shown in Figure ​4 [the numbers indicate minutes]. In Figure ​4A, the SPB signal was separated at 1.5–2.0 min [the left signal was weak because the SPB was out of focus]. At 3.5 min, another signal, representing the cen1 DNA, dissociated from the SPB and was visible between the SPBs. This cen1 signal moved back and forth between the two SPBs in phase 2. The movement was fast and continued until 7.5 min. The centromere signal [indicated by the arrowhead] split into two at 8 min, and the separated signals moved swiftly toward the opposite poles. The duration of sister centromere separation [anaphase A] was thus a small part of the whole period of constant spindle length. This is clearly seen in the time course plot of the position of a pair of sister centromeres relative to the SPBs [Figure ​5A]. The distances between the cen1 signals [cen1–cen1′], and between the cen1 and either one of the two SPBs [SPB1–cen1, cen1′–SPB2] were measured. These results established that phase 2 contained not only metaphase but also a brief period of anaphase A at its end.

GFP images of the SPB and the cen1 DNA in the same cells. Three example cells are shown in A–C. The numbers indicate minutes. Bars, 10 μm. In A and B, the SPB and cen1 DNA signals initially reside at the same positions, whereas in C the SPB was already separated. In A, splitting of the signal into two takes place at 1.5–2.0 min, representing separation of the SPBs. The weak signal in one of the separated SPBs is due to defocusing. The third signal clearly seen at 3.5 min represents the cen1 DNA dissociated from the SPB. This cen1 DNA signal moves back and forth from 3.5 to 7.5 min and is separated into two at 8 min [indicated by arrowheads] and nearly completely separated at 9 min. Nuclear division takes place in the following 3 min. Anaphase B [spindle extension] continues until 16 min; the thin channel structure connecting the daughter nuclei contains the spindle [Tanaka and Kanbe, 1986

Measurements of the distances between the SPBs and between the SPB and the sister centromeres. The distance between the SPBs is divided into three segments, the segments between the cen1 and the SPBs [cen1–SPB1, cen1′–SPB2] and between the sister cen1 DNA [cen1–cen1′]. The distances of these three segments are plotted vs. time as indicated. Sister centromeres were transiently separated in [B]. Sister centromere separation abruptly took place at the end of phase 2.

The second set of images [Figure ​4B] showed that the cen1 DNA [indicated by the arrowhead] made fluctuating movements around the middle of the spindle but transiently split into two a short distance away before separation [also see enlarged [2×] inset for the centromere signals]. This temporal splitting [two arrowheads] occurred multiple times before final centromere separation. The timing and extent of this transient centromere separation are shown in Figure ​5B. In this cell, the period of constant spindle length was ∼10 min, somewhat longer than the average [7 min]. In the third set of time-lapse images [Figure ​4C], this splitting also occurred. The transient centromere separation suggested that sister centromeres might be pulled and briefly separated by the spindle force toward the opposite directions during the period of constant spindle length. Biorientation of sister centromeres toward the spindle ends was thus established in phase 2 [at least for the centromere visualized]. The timing of this transient splitting indicated that biorientation could be established at an early stage of phase 2. It was unlikely that the split centromeres could rotate so that the signals might become single when the direction of the split centromeres was parallel to the light axis. The signals of the SPBs and the sister centromeres appeared to exist in the same focal plane. In addition, the direction of the split centromeres was always parallel to the spindle axis. Note, however, that splitting could not be detected if it occurred in the distances below light microscopic resolution.

Phase 2 Is Absent in dis1 Mutant

We then addressed the question of whether any mitotic mutations could alter the duration of phases 1, 2, and 3. Four mutants, dis1, top2, cut1, and cut14, were examined. These mutants show different types of defects in sister chromatid separation, although the spindle was made and at least partly elongated in all of them [Uemura and Yanagida, 1986

Time-lapse images of Sad1–GFP [Figure ​6A] showed that phase 2 was clearly lacking in dis1 mutant cells at the restrictive temperature [20°C]. The spindle length measured in three dis1 mutant cells [Figure ​6B] continuously increased and did not pause at the spindle length of 2.5 μm. The increase rate of the SPB distance [0.3 μm/min at 20°C] was similar to that of wild-type phases 1 and 3 at 20°C, suggesting that the machinery for spindle formation and elongation might be functioning. At the permissive temperature [33°C], phase 2 was clearly present in dis1, although the duration was longer [12 min] than in wild-type cells at 33°C [our unpublished result]. Functional Dis1 was thus required for establishing phase 2 or completing phase 1, possibly by restraining spindle extension. Dis1 might be implicated in establishing the normal linkage between the kinetochores and the SPBs via the kinetochore microtubules. The linkage is necessary for generating the opposing force that would balance the spindle extension force. The constant spindle period may be maintained by balancing the two forces.

Phase 2 is absent in dis1 mutant. [A] dis1 deletion mutant cell expressing Sad1–GFP at the restrictive temperature is shown. Separated SPBs continuously increased the distance between them. [B] Time-course increase of the distance between the Sad1–GFP signals is shown for three examples of dis1 mutant cells.

To also observe movement of the cen1 DNA, GFP–LacI–NLS was expressed in the dis1 mutant integrated at cen1 with the tandem LacO repeats. Cells observed at 20°C revealed two frequent types of cen1 behavior. In one type of cells, the cen1 signal was not separated but moved toward one end of the cell while the spindle extended [Figure ​7A]. In the other type of cells, the cen1 signal was situated in the middle of the cell but was split into two with a small separation for significant time length [∼15 min] and then reassociated [Figure ​7B]. The sister centromeres appeared to be separate from each other but not properly pulled during this period of transient splitting.

Behavior of the cen1 DNA signal in dis1 mutant cells. [A] The cen1 DNA signal remains associated in dis1 deletion cell at the restrictive temperature, although the nucleus has been elongated by spindle extension. After 60 min the cell was stained by Hoechst 33342. [B] Another dis1 deletion cell revealing GFP–LacI–NLS. Two closely situated cen1 signals are seen for a considerable length of time without further separation. [C] Simultaneous observation of the cen1 DNA [indicated by arrowheads] and the GFP–Sad1 signals in dis1 deletion. Two closely situated cen1 signals are seen between two SPBs. Bars, 10 μm. [D] Kinetic data for the distances between the centromeres [cen1–cen1′] and between the sister centromeres and the SPBs [cen1–SPB1, cen1′–SPB2]. Data collected from the cell in C are shown. The distance between the SPBs increased continuously.

We looked at the relationship between centromere splitting and spindle dynamics by constructing and observing a strain that could express both GFP–Sad1 and GFP–LacI–NLS. The split cen1 signals were seen while the spindle length was long and continuing to elongate [Figure ​7C, arrowheads]. Kinetic data obtained by measurements of the distances between the sister centromeres [cen1–cen1′] and between the SPBs and the sister centromeres [SPB1–cen1, cen1′–SPB2] are shown in Figure ​7D. A striking feature in dis1 mutant cells was that the back-and-forth cen1 DNA movements seen in phase 2 of wild-type cells were entirely absent. After spindle elongation [the SPB distance, ∼8 μm], the cen1 signals were fused again and moved to one of the SPBs. Such prolonged centromere splitting while the spindle was elongating was never seen in wild-type or any of the other mutant cells examined so far.

Transition to Phase 3 Was Abnormal in top2 and cut1 Mutants

Type II topoisomerase activity is required to allow complete separation of sister chromatids in fission yeast and several other eukaryotes. Three sets of time-lapse series for Sad1–GFP were taken from top2–191 mutant cells, which lack type II topoisomerase activity at the restrictive temperature [36°C]. The average rate of SPB separation in phase 3 was much slower [0.35 μm/min] than that of wild-type cells [Figure ​8A, WT]. There was no clear transition from phase 2 to phase 3 in top2 [Figure ​8A]. Spindle elongation in phase 3 appeared to be strongly inhibited, probably by the inability to fully separate the entangled sister chromatid DNAs formed in top2 mutant cells. At the permissive temperature [26°C], the three phases were clearly present in top2 mutant [our unpublished result].

Spindle dynamics in top2, cut1, and cut14 mutants. The distances between the SPBs were measured for top2–191 [A], cut1–206 [B], and cut14–208 [C] mutant cells that expressed Sad1–GFP. In A, the wild-type control is shown by a dark blue line, and three mutant cells that exhibited aberrant initiation of phase 3 are also shown. In B, phase 3 only partially took place.

Cut1 is a protein that is required for sister chromatid separation and is activated when its regulator, Cut2, is destroyed by anaphase-promoting complex [cyclosome]- and ubiquitin-mediated proteolysis. Seven sets of time-lapse images for Sad1–GFP were taken from cut1–206 cultured at 36°C [Figure ​8B]. Phase 2 occurred, but spindle elongation in phase 3 was only partial, and spindle length increase was arrested at ∼6–7 μm. The terminal archery bow phenotype of the cut1 mutant [Hirano et al., 1986

Cut14 is a condensin subunit that plays a role in mitotic chromosome condensation [Hirano et al., 1997

DISCUSSION

We examined the movements of the SPBs and the centromere DNA in living fission yeast cells. Images of wild-type cells and mitotic mutants were analyzed to determine how spindle and chromosome dynamics were spatially and temporally regulated during mitosis. We showed that the normal spindle dynamics consisted of three distinct periods, phase 1 [spindle formation], phase 2 [constant spindle length], and phase 3 [spindle elongation]. Phase 1, corresponding to a prophase-like stage, probably occurred after Cdc2 kinase was activated [Hagan and Yanagida, 1992

A striking feature revealed in this study was that the centromeric DNA moved along the spindle throughout phase 2. Such movements were observed neither in interphase nor in other mitotic stages, indicating that the movements might correspond to prometaphase oscillation in higher eukaryotic mitosis. S. pombe thus appears to have a stage equivalent to prometaphase in which the sister centromeres moved together in the same directions, whereas in anaphase A, they moved in the opposite directions, leading to sister chromatid separation. The maximal rates of movements in metaphase and anaphase A were 2 and 4 μm/min [at 33°C], respectively, much faster than that of anaphase B spindle extension. The direct cause of these fast centromere movements in a prometaphase-like stage was unclear. Certain mitotic motors or factors affecting the properties of mitotic microtubules might be implicated. The rates of these centromere movements in phase 2 are faster than the rate of poleward microtubule flux in vertebrate tissue culture cells [Mitchison, 1989

Another feature in phase 2 was that the centromere signal was often transiently split into two. The separation distance was