G-Protein Linked Signal Transduction Pathways in Development:Dictyostetium as an Experimental System

It has been 22 years since cAMP was identified as the acrasin, i.e., the chemotactic substance mediating aggregation in Dictyostelium discoideum. cAMP is also known to control gene expression throughout development via cell surface cAMP receptors. Over the last few years, substantial progress has been made in understanding these pathways at a biochemical and molecular level. In this article, we review our present understanding of these mechanisms and compare this system with those controlling similar processes in other eukaryotes

Cortical Factor Feedback Model for Cellular Locomotion and Cytofission

Eukaryotic cells can move spontaneously without being guided by external cues. For such spontaneous movements, a variety of different modes have been observed, including the amoeboid-like locomotion with protrusion of multiple pseudopods, the keratocyte-like locomotion with a widely spread lamellipodium, cell division with two daughter cells crawling in opposite directions, and fragmentations of a cell to multiple pieces. Mutagenesis studies have revealed that cells exhibit these modes depending on which genes are deficient, suggesting that seemingly different modes are the manifestation of a common mechanism to regulate cell motion. In this paper, we propose a hypothesis that the positive feedback mechanism working through the inhomogeneous distribution of regulatory proteins underlies this variety of cell locomotion and cytofission. In this hypothesis, a set of regulatory proteins, which we call cortical factors, suppress actin polymerization. These suppressing factors are diluted at the extending front and accumulated at the retracting rear of cell, which establishes a cellular polarity and enhances the cell motility, leading to the further accumulation of cortical factors at the rear. Stochastic simulation of cell movement shows that the positive feedback mechanism of cortical factors stabilizes or destabilizes modes of movement and determines the cell migration pattern. The model predicts that the pattern is selected by changing the rate of formation of the actin-filament network or the threshold to initiate the network formation

Pattern Formation of the Attraction-Repulsion Keller-Segel System

In this paper, the pattern formation of the attraction-repulsion Keller-Segel (ARKS) system is studied analytically and numerically. By the Hopf bifurcation theorem as well as the local and global bifurcation theorem, we rigorously establish the existence of time-periodic patterns and steady state patterns for the ARKS model in the full parameter regimes, which are identified by a linear stability analysis. We also show that when the chemotactic attraction is strong, a spiky steady state pattern can develop. Explicit time-periodic rippling wave patterns and spiky steady state patterns are obtained numerically by carefully selecting parameter values based on our theoretical results. The study in the paper asserts that chemotactic competitive interaction between attraction and repulsion can produce periodic patterns which are impossible for the chemotaxis model with a single chemical (either chemo-attractant or chemo-repellent)

An Oscillatory Contractile Pole-Force Component Dominates the Traction Forces Exerted by Migrating Amoeboid Cells

We used principal component analysis to dissect the mechanics of chemotaxis of amoeboid cells into a reduced set of dominant components of cellular traction forces and shape changes. The dominant traction force component in wild-type cells accounted for ~40% of the mechanical work performed by these cells, and consisted of the cell attaching at front and back contracting the substrate towards its centroid (pole-force). The time evolution of this pole-force component was responsible for the periodic variations of cell length and strain energy that the cells underwent during migration. We identified four additional canonical components, reproducible from cell to cell, overall accounting for an additional ~20% of mechanical work, and associated with events such as lateral protrusion of pseudopodia. We analyzed mutant strains with contractility defects to quantify the role that non-muscle Myosin II (MyoII) plays in amoeboid motility. In MyoII essential light chain null cells the polar-force component remained dominant. On the other hand, MyoII heavy chain null cells exhibited a different dominant traction force component, with a marked increase in lateral contractile forces, suggesting that cortical contractility and/or enhanced lateral adhesions are important for motility in this cell line. By compressing the mechanics of chemotaxing cells into a reduced set of temporally-resolved degrees of freedom, the present study may contribute to refined models of cell migration that incorporate cell-substrate interactions

Intervening sequences in a Dictyostelium gene that encodes a low abundance class mRNA.

Using S1 nuclease protection experiments and DNA sequencing, we have identified two intervening sequences (introns) within a Dictyostelium gene that codes for a low abundance class mRNA. The two introns are located within the protein coding region of this gene. Both are small (approximately 100 bp) and extremely (approximately 95%) A + T rich. The splice junction sequences are similar to the splice sites in other eukaryotes. Finally, we have shown that these introns are transcribed as part of a higher molecular weight nuclear precursor

Identification and characterization of multiple A/T-rich cis-acting elements that control expression from Dictyostelium actin promoters: the Dictyostelium actin upstream activating sequence confers growth phase expression and has enhancer-like properties.

The promoter elements in the Dictyostelium actin 15 and actin 6 genes required for full growth phase expression were identified by assaying promoter/luciferase reporter constructs. We find that these promoters contain common cis-acting elements, an actin upstream activating sequence (UAS) and sequences proximal to the transcription start site that overlap with a poly(dT) region. The actin 15 promoter has two additional cis-acting elements not present in the actin 6 promoter that may account for the higher level of expression from the actin 15 promoter. All of the identified promoter elements are unusual for Dictyostelium in that they are all A/T-rich. Two cis-acting elements, the actin UAS and the poly(dT) domain were studied in greater detail. The actin UAS was tested on a heterologous promoter from the prespore-specific gene SP60 and shown to have the ability to confer growth phase expression. The actin UAS also exhibited the ability to function in a distance- and orientation-independent manner and activate expression synergistically when present in two copies. The poly(dT) domain of the actin 15 promoter was studied in greater detail by using a genetic selection scheme to define parameters that effect the strength of this element. This element is comprised of 45 consecutive dT residues immediately upstream of the putative TATA box. We show that the length of the homopolymer dT region correlates with the expression level of the promoter. The poly(dT) element is also shown to function to promote wild-type levels of expression with small deviations in the sequence, indicating that the element is not required to be homopolymeric to function