Chemotactic Receptors of Dictyostelium discoideum

Abstract

For over 30 years, the cellular slime molds, and in particular the species Dictyostelium discoideum, have been viewed as a eukaryotic model system for the study of a variety of regulatory and developmental processes [l]. In particular, the transition from the free-living amoeboid stage to the multicellular slug stage has been the focus of a great deal of attention by those investigators studying the regulation of development [ 1-31. The first morphological evidence that differentiation of a starved population of cells is proceeding on schedule is the appearance of ripples or streams in a lawn of cells resting on a moist surface [l-31. This prelude to the readily identified tight aggregate stage indicates the functioning of the cells' endogenous chemosensory system. In 1969, Bonner and coworkers [4] identified the endogenously produced chemoattractant or acrasin of these cells as cyclic AMP (CAMP). While cAMP is widely encountered as an intracellular regulatory agent or second messenger, the Dictyostelia provide one of the few well documented examples of cAMP as an extracellular messenger molecule.The Polysphondylia such as P violaceum and P pallidum appear to employ a nonnucleotide molecule as their acrasin during starvation induced chemotaxis. This compound has been studied by Bonner and coworkers [S] and has been tentatively identified as a peptide with a blocked amino terminus, although further characterization is required for unambiguous identification. Bonner's group has also been responsible for the identification of folic acid and pterin compounds as chemoattractants of vegetative Dictyostelium amoebae [6-81. This identification of a nonacrasin chemoattractant raises the possibility of studying the transition from a vegetative to a developmental mode of chemotaxis. DEVELOPMENTAL CHEMOTAXIS IN D discoideumThe phenomenology of the CAMP chemotactic system in D discoideum has been well established [9,10]. It is convenient for descriptive purposes to imagine

Effects of pteridines on the filopodia of Dictyotelium discoideum vegetative amoebae

“…Different binding sites for pterin and for folic acid have been found in the amoebae of both axenic [Wurster and Butz, 19801 and non-axenic [VanHaastert et al, 19821 strains of D. discoideum. Furthermore, there are a variety of folate receptors in both axenic and non-axenic cells, and some of these receptors will accept methotrexate [VanDriel, 1981;Frazier et al, 1982;DeWit and VanHaastert, 19851. Continued studies are needed to clarify this complex picture of the various receptors that bind folates and pterins and their relationship to different but specific cytological effects [DeWit et al, 19851.…”

Association of the cyclic AMP chemotaxis receptor with the detergent-insoluble cytoskeleton of Dictyostelium discoideum.

“…At a concentration of 1 AM, only 3':5' CAMP was able to substantially inhibit [3Hl-cAMP binding to the detergent-insoluble binding site. Thus this binding site is neither the "nonspecific" nor the cGMP specific phosphodiesterase (21,22), both of which are inhibited by cGMP. The intracellular regulatory subunit of the cAMP-dependent protein kinase of Dictyostelium does not have as high a selectivity for cAMP and would be significantly inhibited by 1 AM cGMP and even more so by dibutyryl cAMP (34).…”

“…Conditions of pH and ionic composition that lead to disruption of the cytoskeleton upon detergent treatment also result in the loss of CAMP binding . During differentiation, the detergent-insoluble CAMP binding increases in parallel with cell surface CAMP receptors and chernotaxis to CAMP.Both human leukocytes (45,57,72,74) and cellular slime molds (16,21,22), particularly the species Dictyostelium discoideum, have been studied extensively as paradigms for eucaryotic chemotaxis. Recently, a great deal of progress has been made in the study ofchemotaxis receptors on leukocytes and slime molds .…”

“…function of the adenylate cyclase in D. discoideum (16,21,22,66) . However, in the slime mold, responses of the cells related solely to chemotaxis can be identified by comparing the intracellular events in vegetative cells in response to folate, the chemoattractant of vegetative amebae (43,51,52,71), with those occurring in differentiated cells in response to cAMP (6,21,22) . It is based on reasoning of this sort that a rapid, transient accumulation ofintracellular cyclic guanosine monophosphate has been proposed to be a link in the chemotactic transduction system in slime mold (22,39,71) .…”

“…A detergent-insoluble residue is obtained from D . discoideum cells (23,63) that is ofsufficient density that it can be centrifuged through a layer of silicone/ mineral oil to separate bound and free cAMP rapidly enough to measure binding to the chemotactic receptor (21,22) as is done with intact cells. The residue contains a cAMP-binding site with many of the properties of the cell surface cAMP chemotactic receptor that clearly distinguish it from all other known cAMP-binding proteins.…”