CSM News Electronic Edition Volume 4, number 13 April 7, 1995 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to CSM-News@worms.cmsbio.nwu.edu. Back issues of CSM-News, the CSM Reference database and other useful information is available by anonymous ftp from worms.cmsbio.nwu.edu [165.124.233.50], via Gopher at the same address, or by World Wide Web at the URL "http://worms.cmsbio.nwu.edu/dicty.html" =========== Abstracts =========== A novel type of unorthodox mitosis in amoebae of the cellular slime mold (mycetozoan) Acrasis rosea. Urs-Peter Roos and Bruno Guhl, Institut fur Pflanzenbiologie, Universitat Zurich, Zurich, Switzerland. Eur. J. Protozool, in press SUMMARY We investigated mitosis in trophic amoebae of Acrasis rosea (Olive and Stoianovitch) by video microscopy of live cells, by immunofluorescence with an antibody against tubulin, and by transmission electron microscopy. As interphase cells contain neither microtubules (MTs) nor microtubule- organizing centers (MTOCs) the mitotic spindle forms entirely de novo from presumptive diffuse intranuclear MTOCs. Mitosis is entirely closed and devoid of any visible pole organelles during all its stages. The spindle axis is probably established by parallel alignment of MTs due to spatial constraints or lateral interaction. Chromosomes condense only when spindle formation is well advanced; their kinetochores presumably acquire MTs (kMTs) by capture. The two main elements of the nucleolus, viz. granular body and dense granules, disperse, but do not disintegrate. They later arrange in the center of the spindle along its axis and eventually segregate as two granular bodies and two groups of granules. Reconstruction of the nucleolus during telophase involves the compaction of the granular bodies and co-aggregation of the dense granules. At metaphase the chromosomes are aligned at the equator of a spindle that tapers towards the blunt spindle poles. Chromosomes are small, but they have distinct layered kinetochores with two MTs each that terminate in their outer layer. Cytokinesis is accomplished ca. eight minutes after the first signs of prophase. Chromosome segregation during anaphase is effectuated almost solely by spindle elongation, which begins when chromosomes are still undivided and aligned at the equator. The velocity of spindle elongation and chromosome segregation was 6 um/min. Chromosome congression and segregation are most likely driven by interactions, static or dynamic, between kMTs and non-kinetochore MTs (nkMTs), whereas the mechanism of spindle elongation is probably based on movements between staggeredly overlapping nkMTs. At the end of telophase the closure of the nuclear envelope around the daughter nuclei pinches off a membrane tube containing remnant nkMTs. Mitosis in A. rosea differs markedly from that in other cellular slime molds and has much in common with that in several protozoa. ---------------------------------------------------------------------------- Serpentine cAMP receptors may act through a G-protein-independent pathway to induce post-aggregative development in Dictyostelium Gavin R. Schnitzler, Celia Briscoe, Jason M. Brown, and Richard A. Firtel Cell, in press. Summary The transcription factor GBF is required for the developmental switch between aggregation and post-aggregative gene expression, cell-type differentiation, and morphogenesis (Schnitzler et al., 1994). We show that constitutive expression of GBF allows ectopic expression of post-aggregative genes, but only in response to exogenous cAMP. GBF activation requires the serpentine cAMP receptors required for aggregation but not the coupled Ga2 or only known Gb subunit, suggesting a novel signaling pathway. In response to high cAMP, ga2 null cells can bypass the aggregation stage, expressing cell-type-specific genes and forming fruiting bodies. Our results demonstrate that the same receptors regupost-aggregative aggregation and cell-type differentiation but via distinct pathways depending upon whether the receptor perceives a pulsatile or sustained signal. --------------------------------------------------------------------------- Three-dimensional scroll waves of cAMP could direct cell movement and gene expression in Dictyostelium slugs Till Bretschneider, Florian Siegert, and Cornelis J. Weijer Zoologisches Institut, Ludwig-Maximilians Universitaet Muenchen, Luisenstr. 14, 80333 Muenchen, Germany PNAS, in press. ABSTRACT Complex three dimensional waves of excitation can explain the observed cell movement pattern in Dictyostelium slugs. Here we show that these three- dimen- sional waves can be produced by a realistic model for the cAMP relay system (Martiel, J.L. & Goldbeter, A. (1987) Biophys J.52, 807-828). The conversion of scroll waves in the prestalk zone of the slug into planar wave fronts in the prespore zone can result from a smaller fraction of relaying cells in the prespore zone. Further, we show that the cAMP concentrations to which cells in a slug are exposed over time display a simple pattern, despite the complex spatial geometry of the waves. This cAMP distribution agrees well with observed patterns of cAMP regulated cell type specific gene expression. The core of the spiral, which is a region of low cAMP concentration, might direct expression of stalk specific genes during culmination. -----------------------------------------------------------------------