Biochimica et Biophysica Acta (BBA) - General Subjects
ReviewFluid-phase uptake and transit in axenic Dictyostelium cells
Section snippets
Introducing major steps
The dramatic transition of Dictyostelium from a largely unknown soil amoeba to a well-respected model system came with its introduction into the laboratory and subsequent axenic cultivation in liquid media containing sugar, peptone and yeast extract [1]. Since then, its dramatic efficiency to endocytose particles and fluid can be studied and described separately (see the article by Rupper and Cardelli, pp. 205–216, this issue and this article).
The present state of knowledge about the pathway of
The internalization stage
The dynamic movement of pseudopodia is the most obvious similarity between phagocytosis and macropinocytosis: A circular, funnel-shaped, broad membrane lamella is protruded into the surrounding medium, as if guided by the shape of an invisible particle, and fusion of the distal membrane edges results in an aliquot of fluid entrapped in a macropinosome (Fig. 1a–d). Knowledge of the membrane movements allows surface images obtained from scanning electron microscopy to be ordered into a similar
Forward traffic in the endocytic pathway
Endocytic transit of fluid-phase marker begins after the cytoskeletal coat has dissociated from the macropinosome, about 1 min after its fission from the plasma membrane is completed [6]. At the same time the lumen of the macropinosome is acidified (Fig. 1i–m), and after a few minutes lysosomal enzymes identified by the Man-6-SO4 or the GlcNAc-1-P modification accumulate to detectable levels [50]. During this early phase the endosomes participate in multiple fusion and fission events, and
Membrane recycling
The observation that Dictyostelium cells secrete lysosomal enzymes provides the simplest possible explanation for membrane homeostasis during endocytic transit: endocytosed membrane is just re-integrated into the plasma membrane after transit is complete. As often, the truth is rather complicated than elegant. A number of observations point to the existence of additional membrane recycling pathways linked to endocytic transit: Firstly, endosomal acidification requires that the vacuolar H+ATPase
Regulation and specificity of vesicle traffic
Rab proteins represent the paradigm of regulators of vesicle traffic. For the Dictyostelium Rab7 homolog, the phenotype observed in constitutively active or dominant negative mutants is consistent with a role of Rab7 in the regulation of marker progression from acidic to neutral endosomes and the concomitant retention of lysosomal proteins and the vacuolar H+ATPase [59]. In addition to Rab7, Dictyostelium homologs of NSF, SNAPs, and syntaxin have been identified and assayed for their
Macropinocytosis in mammalian cells
Review articles specifically dealing with macropinocytosis in mammalian cells are relatively rare [73] and only recently possible parallels to the Dictyostelium system have been included in the discussion [74]. To avoid a repetitive treatment of the latter issue, only the main points are raised here. Thus, cited literature rather provides examples than a complete listing and is discussed from the standpoint of a lower eukaryote.
Among the many mammalian cell types only a few are thought to be
Conclusions
Macropinocytosis is a cellular activity highly similar to phagocytosis. Both processes rely on a cytoskeletal machinery which is common to a great number of cell types. However, only in professional phagocytes are macropinosomes processed in a way similar to phagosomes. In other cells, macropinocytosis superficially appears to be a futile activity, but may in fact be linked to cell motility. Vesicle trafficking steps and cytoskeletal activities are interdependent and subject to substantial
Acknowledgments
I wish to thank Drs. Thierry Soldati and Jim Cardelli for comments on the manuscript, Harald Rühling for confocal microscopy on the Leica TCS SP, help with the illustrations in Fig. 1 and measurements in Fig. 2. Photographs e–h in Fig. 1 are reproduced with permission of The Company of Biologists Ltd.
References (100)
- et al.
Biochim. Biophys. Acta
(1994) Methods Enzymol.
(1983)- et al.
Biochem. Biophys. Res. Commun.
(1986) - et al.
J. Biol. Chem.
(1993) - et al.
Cell
(1992) - et al.
Biophys. J.
(1999) - et al.
Trends Cell Biol.
(1996) - et al.
J. Biol. Chem.
(1995) - et al.
Curr. Biol.
(2000) - et al.
Curr. Biol.
(1997)