Pac-man motility of kinetochores unleashed by laser microsurgery

Live-cell Imaging and Quantitative Analysis of Meiotic Divisions in Caenorhabditis elegans Males

Live-imaging of meiotic cell division has been performed in extracted spermatocytes of a number of species using phase-contrast microscopy. For the nematode Caenorhabditis elegans, removal of spermatocytes from gonads has damaging effects, as most of the extracted spermatocytes show a high variability in the timing of meiotic divisions or simply arrest during the experiment. Therefore, we developed a live-cell imaging approach for in situ filming of spermatocyte meiosis in whole immobilized C. elegans males, thus allowing an observation of male germ cells within an unperturbed environment. For this, we make use of strains with fluorescently labeled chromosomes and centrosomes. Here we describe how to immobilize male worms for live-imaging. Further, we describe the workflow for the acquisition and processing of data to obtain quantitative information about the dynamics of chromosome segregation in spermatocyte meiosis I and II. In addition, our newly developed approach allows us to re-orient filmed spindles in silico, regardless of the initial 3D orientation in the worm, and analyze spindle dynamics in living worms in a statistically robust manner. Our live-imaging approach is also applicable to C. elegans hermaphrodites and should be expandable to other fluorescently labelled nematodes or other fully transparent small model organisms.

Male meiotic spindle features that efficiently segregate paired and lagging chromosomes

Chromosome segregation during male meiosis is tailored to rapidly generate multitudes of sperm. Little is known about mechanisms that efficiently partition chromosomes to produce sperm. Using live imaging and tomographic reconstructions of spermatocyte meiotic spindles in Caenorhabditis elegans, we find the lagging X chromosome, a distinctive feature of anaphase I in C. elegans males, is due to lack of chromosome pairing. The unpaired chromosome remains tethered to centrosomes by lengthening kinetochore microtubules, which are under tension, suggesting that a ‘tug of war’ reliably resolves lagging. We find spermatocytes exhibit simultaneous pole-to-chromosome shortening (anaphase A) and pole-to-pole elongation (anaphase B). Electron tomography unexpectedly revealed spermatocyte anaphase A does not stem solely from kinetochore microtubule shortening. Instead, movement of autosomes is largely driven by distance change between chromosomes, microtubules, and centrosomes upon tension release during anaphase. Overall, we define novel features that segregate both lagging and paired chromosomes for optimal sperm production.

Anaphase A: Disassembling Microtubules Move Chromosomes toward Spindle Poles

The separation of sister chromatids during anaphase is the culmination of mitosis and one of the most strikingly beautiful examples of cellular movement. It consists of two distinct processes: Anaphase A, the movement of chromosomes toward spindle poles via shortening of the connecting fibers, and anaphase B, separation of the two poles from one another via spindle elongation. I focus here on anaphase A chromosome-to-pole movement. The chapter begins by summarizing classical observations of chromosome movements, which support the current understanding of anaphase mechanisms. Live cell fluorescence microscopy studies showed that poleward chromosome movement is associated with disassembly of the kinetochore-attached microtubule fibers that link chromosomes to poles. Microtubule-marking techniques established that kinetochore-fiber disassembly often occurs through loss of tubulin subunits from the kinetochore-attached plus ends. In addition, kinetochore-fiber disassembly in many cells occurs partly through 'flux', where the microtubules flow continuously toward the poles and tubulin subunits are lost from minus ends. Molecular mechanistic models for how load-bearing attachments are maintained to disassembling microtubule ends, and how the forces are generated to drive these disassembly-coupled movements, are discussed.

Manipulation of cells with laser microbeam scissors and optical tweezers: a review

The use of laser microbeams and optical tweezers in a wide field of biological applications from genomic to immunology is discussed. Microperforation is used to introduce a well-defined amount of molecules into cells for genetic engineering and optical imaging. The microwelding of two cells induced by a laser microbeam combines their genetic outfit. Microdissection allows specific regions of genomes to be isolated from a whole set of chromosomes. Handling the cells with optical tweezers supports investigation on the attack of immune systems against diseased or cancerous cells. With the help of laser microbeams, heart infarction can be simulated, and optical tweezers support studies on the heartbeat. Finally, laser microbeams are used to induce DNA damage in living cells for studies on cancer and ageing.

Using Photobleaching to Measure Spindle Microtubule Dynamics in Primary Cultures of Dividing Drosophila Meiotic Spermatocytes

In dividing animal cells, a microtubule (MT)-based bipolar spindle governs chromosome movement. Current models propose that the spindle facilitates and/or generates translocating forces by regionally depolymerizing the kinetochore fibers (k-fibers) that bind each chromosome. It is unclear how conserved these sites and the resultant chromosome-moving mechanisms are between different dividing cell types because of the technical challenges of quantitatively studying MTs in many specimens. In particular, our knowledge of MT kinetics during the sperm-producing male meiotic divisions remains in its infancy. In this study, I use an easy-to-implement photobleaching-based assay for measuring spindle MT dynamics in primary cultures of meiotic spermatocytes isolated from the fruit fly Drosophila melanogaster. By use of standard scanning confocal microscopy features, fiducial marks were photobleached on fluorescent protein (FP)-tagged MTs. These were followed by time-lapse imaging during different division stages, and their displacement rates were calculated using public domain software. I find that k-fibers continually shorten at their poles during metaphase and anaphase A through the process of MT flux. Anaphase chromosome movement is complemented by Pac-Man, the shortening of the k-fiber at its chromosomal interface. Thus, Drosophila spermatocytes share the sites of spindle dynamism and mechanisms of chromosome movement with mitotic cells. The data reveal the applicability of the photobleaching assay for measuring MT dynamics in primary cultures. This approach can be readily applied to other systems.

Back to the roots: segregation of univalent sex chromosomes in meiosis

In males of many taxa, univalent sex chromosomes normally segregate during the first meiotic division, and analysis of sex chromosome segregation was foundational for the chromosome theory of inheritance. Correct segregation of single or multiple univalent sex chromosomes occurs in a cellular environment where every other chromosome is a bivalent that is being partitioned into homologous chromosomes at anaphase I. The mechanics of univalent chromosome segregation vary among animal taxa. In some, univalents establish syntelic attachment of sister kinetochores to the spindle. In others, amphitelic attachment is established. Here, we review how this problem of segregation of unpaired chromosomes is solved in different animal systems. In addition, we give a short outlook of how mechanistic insights into this process could be gained by explicitly studying model organisms, such as Caenorhabditis elegans.

Chromosome interaction over a distance in meiosis

The challenge of cell division is to distribute partner chromosomes (pairs of homologues, pairs of sex chromosomes or pairs of sister chromatids) correctly, one into each daughter cell. In the 'standard' meiosis, this problem is solved by linking partners together via a chiasma and/or sister chromatid cohesion, and then separating the linked partners from one another in anaphase; thus, the partners are kept track of, and correctly distributed. Many organisms, however, properly separate chromosomes in the absence of any obvious physical connection, and movements of unconnected partner chromosomes are coordinated at a distance. Meiotic distance interactions happen in many different ways and in different types of organisms. In this review, we discuss several different known types of distance segregation and propose possible explanations for non-random segregation of distance-segregating chromosomes.

Direct kinetochore-spindle pole connections are not required for chromosome segregation

In the absence of continuous K-fiber attachment between each kinetochore and the spindle pole, one or more additional mechanisms dependent on dynein-mediated kinetochore transport exist to ensure proper chromosome segregation during mitosis.

Segregation of genetic material occurs when chromosomes move to opposite spindle poles during mitosis. This movement depends on K-fibers, specialized microtubule (MT) bundles attached to the chromosomes′ kinetochores. A long-standing assumption is that continuous K-fibers connect every kinetochore to a spindle pole and the force for chromosome movement is produced at the kinetochore and coupled with MT depolymerization. However, we found that chromosomes still maintained their position at the spindle equator during metaphase and segregated properly during anaphase when one of their K-fibers was severed near the kinetochore with a laser microbeam. We also found that, in normal fully assembled spindles, K-fibers of some chromosomes did not extend to the spindle pole. These K-fibers connected to adjacent K-fibers and/or nonkinetochore MTs. Poleward movement of chromosomes with short K-fibers was uncoupled from MT depolymerization at the kinetochore. Instead, these chromosomes moved by dynein-mediated transport of the entire K-fiber/kinetochore assembly. Thus, at least two distinct parallel mechanisms drive chromosome segregation in mammalian cells.

Kinetochore-driven outgrowth of microtubules is a central contributor to kinetochore fiber maturation in crane-fly spermatocytes

During meiosis I in crane-fly spermatocytes, the increase in microtubule density in K-fibers clearly originates at the kinetochore. The increased density first appears at the kinetochore, then elongates toward the pole, probably by addition of tubulin at microtubule plus ends anchored in the kinetochore.

We use liquid crystal polarized light imaging to record the life histories of single kinetochore (K-) fibers in living crane-fly spermatocytes, from their origins as nascent K-fibers in early prometaphase to their fully matured form at metaphase, just before anaphase onset. Increased image brightness due to increased retardance reveals where microtubules are added during K-fiber formation. Analysis of experimentally generated bipolar spindles with only one centrosome, as well as of regular, bicentrosomal spindles, reveals that microtubule addition occurs at the kinetochore-proximal ends of K-fibers, and added polymer expands poleward, giving rise to the robust K-fibers of metaphase cells. These results are not compatible with a model for K-fiber formation in which microtubules are added to nascent fibers solely by repetitive “search and capture” of centrosomal microtubule plus ends. Our interpretation is that capture of centrosomal microtubules—when deployed—is limited to early stages in establishment of nascent K-fibers, which then mature through kinetochore-driven outgrowth. When kinetochore capture of centrosomal microtubules is not used, the polar ends of K-fibers grow outward from their kinetochores and usually converge to make a centrosome-free pole.

Distance segregation of sex chromosomes in crane-fly spermatocytes studied using laser microbeam irradiations

Univalent sex chromosomes in crane-fly spermatocytes have kinetochore spindle fibres to each spindle pole (amphitelic orientation) from metaphase throughout anaphase. The univalents segregate in anaphase only after the autosomes approach the poles. As each univalent moves in anaphase, one spindle fibre shortens and the other spindle fibre elongates. To test whether the directionality of force production is fixed at anaphase, that is, whether one spindle fibre can only elongate and the other only shorten, we cut univalents in half with a laser microbeam, to create two chromatids. In both sex-chromosome metaphase and sex-chromosome anaphase, the two chromatids that were formed moved to opposite poles (to the poles to which their fibre was attached) at speeds about the same as autosomes, much faster than the usual speeds of univalent movements. Since the chromatids moved to the pole to which they were attached, independent of the direction to which the univalent as a whole was moving, the spindle fibre that normally elongates in anaphase still is able to shorten and produce force towards the pole when allowed (or caused) to do so.

Polarized light microscopy in reproductive and developmental biology

The polarized light microscope reveals orientational order in native molecular structures inside living cells, tissues, and whole organisms. It is a powerful tool used to monitor and analyze the early developmental stages of organisms that lend themselves to microscopic observations. In this article, we briefly discuss the components specific to a traditional polarizing microscope and some historically important observations on: chromosome packing in the sperm head, the first zygote division of the sea urchin, and differentiation initiated by the first asymmetric cell division in the sand dollar. We then introduce the LC-PolScope and describe its use for measuring birefringence and polarized fluorescence in living cells and tissues. Applications range from the enucleation of mouse oocytes to analyzing the polarized fluorescence of the water strider acrosome. We end with new results on the birefringence of the developing chick brain, which we analyzed between developmental stages of days 12-20.