Actin and myosin inhibitors block elongation of kinetochore fibre stubs in metaphase crane-fly spermatocytes

We used an ultraviolet microbeam to cut individual kinetochore spindle fibres in metaphase crane-fly spermatocytes. We then followed the growth of the "kinetochore stubs", the remnants of kinetochore fibres that remain attached to kinetochores. Kinetochore stubs elongate with constant velocity by adding tubulin subunits at the kinetochore, and thus elongation is related to tubulin flux in the kinetochore microtubules. Stub elongation was blocked by cytochalasin D and latrunculin A, actin inhibitors, and by butanedione monoxime, a myosin inhibitor. We conclude that actin and myosin are involved in generating elongation and thus in producing tubulin flux in kinetochore microtubules. We suggest that actin and myosin act in concert with a spindle matrix to propel kinetochore fibres poleward, thereby causing stub elongation and generating anaphase chromosome movement in nonirradiated cells.

Cutaneous leishmaniasis in red kangaroos: isolation and characterisation of the causative organisms

This is the first report of cutaneous leishmaniasis in kangaroos where infection was acquired within Australia. The diagnosis is based on the clinical criteria used for humans, the lesion histopathology, the detection and isolation of parasites from the lesions, and the analysis of the small subunit ribosomal RNA genes using the polymerase chain reaction. Despite a clear indication that the parasites belong to the genus Leishmania, no assignation to a known Leishmania species could be made using these or other less conserved genetic loci such as the non-transcribed spacer of the mini-exon repeat. As is the case in humans, some but not all animals harbouring lesions had antibodies to the isolated parasites or to several other Leishmania species. The isolated parasites displayed two well characterised Leishmania glycoconjugates, the lipophosphoglycan and proteophosphoglycan. They were infectious for mouse macrophages in vitro and established long-term infection at 33 degrees C but not at 37 degrees C. Our findings raise the possibility of transmission to humans, which may be unrecognised and suggest the possibility that imported species of Leishmania could become endemic in Australia.

Traction fibre: toward a "tensegral" model of the spindle

Most current hypotheses of mitotic mechanisms are based on the "PAC-MAN" paradigm in which chromosome movement is generated and powered by disassembly of kinetochore microtubules (k-MTs) by the kinetochore. Recent experiments demonstrate that this model cannot explain force generation for anaphase chromosome movement [Pickett-Heaps et al., 1996: Protoplasma 192:1-10]. Another such experiment is described here: a UV-microbeam cut several kinetochore fibres (k-fibres) in newt epithelial cells at metaphase and the half-spindle immediately shortened: in several cells, the remaining intact spindle fibres bowed outwards as they came under increased compression. Thus, severing of k-MTs can lead to increased tension between chromosomes and poles. This observation cannot be explained by models in which force is produced by motor molecules at the kinetochore actively disassembling k-MTs. Rather, we argue that tensile forces act along the whole k-fibre, which, therefore, can be considered as a classic "traction fibre." We suggest that anaphase polewards force is generated by MTs interacting with the spindle matrix and when k-MTs are severed, polewards force continues to act on the remaining kMT-stub; spindle MTs act as rigid struts concurrently resisting and being controlled by these forces. We suggest that the principles of "cellular tensegrity" [Ingber, 1993: J. Cell Sci. 104:613-627] derived from the behaviour and organization of the interphase cell apply to the spindle. In an evolutionary context, this argument further suggests that the spindle might originally have evolved as the mechanism by which a single tensegral unit (cytoplast) is divided into two cytoplasts; use of the spindle for segregating chromosomes might represent a secondary, more recent development of this primary function. If valid, this concept has implications for the way the spindle functions and for the spindle's relationship to cytokinesis.

Ultraviolet microbeam irradiations of epithelial and spermatocyte spindles suggest that forces act on the kinetochore fibre and are not generated by its disassembly

Ultraviolet (UV) microbeam irradiations of crane-fly spermatocyte and newt epithelial spindles severed kinetochore fibres (KT-fibres), creating areas of reduced birefringence (ARBs): the remnant KT-fibre consists of two "stubs," a pole-stub attached to the pole and a KT-stub attached to the kinetochore. KT-stubs remained visible but pole-stubs soon became undetectable [Forer et al., 1996]. At metaphase, in both cell types the KT-stub often changed orientation immediately after irradiation and its tip steadily moved poleward. In spermatocytes, the chromosome attached to the KT-stub remained at the equator as the KT-stub elongated. In epithelial cells, the KT-stub sometimes elongated as the associated chromosome remained at the equator; other times the associated chromosome moved poleward together with the KT-stub, albeit only a short distance toward the pole. When an ARB was generated at anaphase, chromosome(s) with a KT-stub often continued to move poleward. In spermatocytes, this movement was accompanied by steady elongation of the KT-stub. In epithelial cells, chromosomes accelerated polewards after irradiation until the KT-stubs reached the pole, after which chromosome movement returned to normal speeds. In some epithelial cells fine birefringent fibres by chance were present along one edge of ARBs; these remnant fibres buckled and broke as the KT-stub and chromosome moved polewards. Similarly, KT-stubs that moved into pole stubs (or astral fibres) caused the pole stubs (or astral fibres) to bend sharply from the point of impact. Our results contradict models of chromosome movement that postulate that force is generated by the kinetochore disassembling the KT-fibre. Instead, these results suggest that poleward directed forces act on the KT-fibre and the KT-stub and suggest that continuity of microtubules between kinetochore and pole is not obligatory for achieving anaphase motion to the pole.

The amyloid precursor protein of Alzheimer's disease is found on the surface of static but not activity motile portions of neurites

We have previously found that the amyloid precursor protein (APP) of Alzheimer's disease is present on the surface of rat cortical neurons in culture, in a segmental pattern which first becomes evident after 24 hours and is fully developed by five days. As APP has previously been reported to have a short half-life in neuronal cell lines, and has been shown to contain binding sites for various extracellular matrix components within its extracellular domain, we hypothesized that APP would be associated with portions of neurites undergoing rapid structural change, such as growth cones. To test this hypothesis, we observed selected neurons by video time-lapse differential interference microscopy on 24-hour-old primary rat neuronal cultures for up to 45 minutes, followed by fixation and immunocytochemistry to ascertain surface APP distribution on those same neurons. In contrast to our predictions, surface APP was not found on active portions of neurites, even if the activity produced no net translational movement. This result indicates that surface APP is actually associated with stable portions of neurites, a conclusion that tallies with other recent results showing that neuronal surface APP has a longer half-life than general cellular APP, and is associated with markers of adhesion patches, which themselves are relatively stable structures.