Anthony Robertson

Control of Aggregation in Dictyostelium discoideum by an External Periodic Pulse of Cyclic Adenosine Monophosphate.

We have induced and controlled normal aggregation of Dictyostelium discoideum amoebas by electrophoretic release of pulses of cyclic adenosine monophosphate from a microelectrode. This has yielded information about the sequence of development of aggregation competences during interphase. We believe that modifications of the technique will have wide application in investigations of other developing systems.

Wave propagation in the early stages of aggregation of cellular slime molds

Abstract A detailed theory of the velocity of propagation of the acrasin pulse responsible for the aggregation of some of the cellular slime molds is presented. The ingredients are diffusion of acrasin, triggering after a threshold concentration is reached, an intracellular delay between the reaching of threshold and subsequent release of acrasin, the destruction of acrasin by acrasinase, and an intracellular refractory period. It is suggested that the rate limiting factor in the velocity of propagation is probably the intracellular delay and not intercellular diffusion. It is shown that there is a critical density of amebae below which the waves cannot propagate. The number of neighboring amebae triggered simultaneously by the propagating wave, and therefore the spatial width of the pulse tends to increase with density. Rough estimates are given for the number of acrasin molecules per pulse and for its threshold concentration.

Chemotaxis and the early stages of aggregation in cellular slime molds

Abstract We consider here the effect of aggregative movement on organizing wave propagation in the cellular slime molds. We start by formulating chemotactic response in a way that is susceptible to quantitative treatment. Next we report a series of quantitative results on chemotactic aggregative movements obtained from observations and subsequent theoretical analysis. Perhaps the most important of these are that the chemotactic signal is impulsive, i.e. short compared to the duration of the response, that the response is of the all-or-none variety, and that differences in period between Dictyostelium discoideum and Polysphondylium violaceum are compensated by differences of chemotactic response to give comparable aggregation territories. An explicit formula is derived for the density changes produced by the chemotactic response to the periodic signals and their effect on the wave propagation discussed. The most startling effect is an appreciable Doppler shift of the apparent frequency of the signal as seen by those amebae having a chemotactic response different from the average. The shift may be important for understanding the efficiency of entrainment by the center of all amebae in its territory.

Effect of low-dose bromocriptine in treatment of psychosis: The dopamine autoreceptor-stimulation strategy

Bromocriptine (0.5–6.0 mg/day) was administered to seven unmedicated chronic schizophrenic and two schizoaffective patients. Transient slight improvement was noted in four patients and marked improvement in one other. Clinical improvement was associated with nausea and drowsiness. These doses of bromocriptine stimulated serum growth hormone and inhibited serum prolactin levels in some subjects. These results suggest that bromocriptine may stimulate dopamine autoreceptors and, through this mechanism, attenuate symptoms in a small proportion of psychiatric patients.

Stimulation of late interphase Dictyostelium discoideum amoebae with an external cyclic AMP signal

The microelectrode system described in the accompanying paper was used to investigate properties of fields of Dictyostelium discoideum amoebae in late interphase. Cells in the fields were competent to respond chemotactically to, and to relay, a c-AMP signal, but not to produce an aggregative signal autonomously. The experimental results are generally consistent with c-AMP being the sole compound required for chemotaxis and signal relaying. A periodic signal from the microelectrode can initiate and control aggregation and can complete with spontaneously arising aggregates. The electrode was used to measure the refractory period for relaying which decreases from 9 min or more to between 2 and 3 min with increasing developmental age, and to measure thresholds for chemotaxis and signal relaying. The results are discussed in relation to models for the control of aggregation in D. discoideum.

Periodic movements of Dictyostelium discoideum sorocarps

We report periodic movements during erection of Dictyostelium discoideum (Dd) sorocarps. Our observations lead to the working hypothesis that Dd sorocarp erection occurs by two superimposed processes: one periodic, with a modal period of 6 1/2 min, and one continuous. We tentatively identify the periodic process with cell movement into the apex of the Dd stalk, and the continuous process with cell vacuolation, together with stalk sheath extension.

Iontophoresis of cyclic AMP.

The design, calibration, and operation of a source of controlled amounts of cyclic AMP (c-AMP) are described. Typically, 1.5 s pulses containing 10(10)-10(-12) molecules of c-AMP can be delivered to a region about 10 mum in diameter on an agar plate. The resulting concentration profiles are given as functions of distance and time. The diffusion coefficient of c-AMP in agar was measured to be 0.97 times 10(-5) cm2-s-1 at 21 degrees C.

A system for interactive film analysis

Abstract An interactive, minicomputer system has been constructed for analyzing dynamic phenomena recorded on movie film in a developmental biology laboratory. The minicomputer interfaces a stop-motion, variable speed projector, a digitizing pen, and real-time graphics display equipment. An analyst uses the pen to digitize features in a film, e.g. by following a cell. A computer-generated animation portraying all data entered is superimposed on the film image and synchronized with it. Noteworthy system features include: image overlays on a large screen, data entry with the projector running, large data capacity, computer control of the projector, and convenient data entry tools.

The cAMP signal from Dictyostelium discoideum amoebae.

Abstract Dictyostelium discoideum cells were allowed to differentiate on agar for 600 min at room temperature. All of the cells were then competent to relay or amplify a cAMP signal, but none to produce a cAMP signal autonomously. The cells were stimulated with cAMP concentrations ranging from 10−9 to 3.5 × 10−7 M. Populations of 106 cells could amplify an initial cAMP concentration of 2.5 × 10−9 M with a low probability, while an initial cAMP concentration of 5 × 10−8 M always induced a response. An initial cAMP concentration of 1.2 × 10−7 M induced the maximum cellular release of cAMP observed; this corresponded to 3 × 107 molecules per cell. No cellular release of cAMP was detected for initial cAMP concentrations of 3 × 10−7 M or more. The amplification of a 10−7 M cAMP stimulus was complete within 8 sec, indicating the pulsatile nature of the cellular release of cAMP. The phosphodiesterase (PDE) activities of D. discoideum cells were measured over a wide range of cell densities. At densities above 7.5 × 104 cells/cm2, both cell-bound and extracellular (ePDE) activities declined, per cell, as cell density increased. These results are compared to ePDE activities derived from critical density measurements. We found that PDE activities were in the range of 10−13–10−14 moles of cAMP converted/cell/min under culture conditions consistent with normal aggregation.

The role of cyclic AMP in slime mold development.

Summary Cyclic AMP acts as an aggregative attractant in Dictyostelium discoideum and possibly in other cellular slime molds. There is good evidence that extra-cellular cyclic AMP is also involved in the control of morphogenetic movement and of differentiation throughout the D. discoideum life cycle. In this paper we review what is known of the cyclic AMP signal, of the responses to it and of cyclic AMP metabolism in D. discoideum.