Alteration of intracellular traffic by monensin; mechanism, specificity and relationship to toxicity

Abstract

Monensin, a monovalent ion-selective ionophore, facilitates the transmembrane exchange of principally sodium ions for protons. The outer surface of the ionophore-ion complex is composed largely of nonpolar hydrocarbon, which imparts a high solubility to the complexes in nonpolar solvents. In biological systems, these complexes are freely soluble in the lipid components of membranes and, presumably, diffuse or shuttle through the membranes from one aqueous membrane interface to the other. The net effect for monensin is a trans-membrane exchange of sodium ions for protons. However, the interaction of an ionophore with biological membranes, and its ionophoric expression, is highly dependent on the biochemical configuration of the membrane itself. One apparent consequence of this exchange is the neutralization of acidic intracellular compartments such as the trans Golgi apparatus cisternae and associated elements, lysosomes, and certain endosomes. This is accompanied by a disruption of trans Golgi apparatus cisternae and of lysosome and acidic endosome function. At the same time, Golgi apparatus cisternae appear to swell, presumably due to osmotic uptake of water resulting from the inward movement of ions. Monensin effects on Golgi apparatus are observed in cells from a wide range of plant and animal species. The action of monensin is most often exerted on the trans half of the stacked cisternae, often near the point of exit of secretory vesicles at the trans face of the stacked cisternae, or, especially at low monensin concentrations or short exposure times, near the middle of the stacked cisternae. The effects of monensin are quite rapid in both animal and plant cells; i.e., changes in Golgi apparatus may be observed after only 2-5 min of exposure. It is implicit in these observations that the uptake of osmotically active cations is accompanied by a concomitant efflux of H+ and that a net influx of protons would be required to sustain the ionic exchange long enough to account for the swelling of cisternae observed in electron micrographs. In the Golgi apparatus, late processing events such as terminal glycosylation and proteolytic cleavages are most susceptible to inhibition by monensin. Yet, many incompletely processed molecules may still be secreted via yet poorly understood mechanisms that appear to bypass the Golgi apparatus. In endocytosis, monensin does not prevent internalization. However, intracellular degradation of internalized ligands may be prevented.(ABSTRACT TRUNCATED AT 400 WORDS)

Sphingomyelin is synthesized at the plasma membrane of oligodendrocytes and by purified myelin membranes: a study with fluorescent‐ and radio‐labelled ceramide analogues

“…Two inhibitors were used: (i) monensin, which inhibits transport of glycoproteins between medial-and trans-regions of the Golgi apparatus [24] and blocks the appearance of newly synthesized sphingolipids at the plasma membrane (see [23] for references) without inhibiting SM synthesis from a fluorescent ceramide precursor [25]; (ii) the fungal metabolite brefeldin A (BFA) that causes a reversible disassembly of the Golgi apparatus and inhibits vesicular protein transport along the exocytotic pathway towards the plasma membrane [26,27]. In addition, we carried out incubations at 10°C, a temperature that does not allow vesicular transport of proteins and also blocks sphingolipid transport [25,28].…”

Brefeldin A or monensin inhibits the 3D organizer in gastropod, polyplacophoran, and scaphopod molluscs

“…Scale bar=25 urn window of organizer specification and patterning had minimal or no discernable effects in all three equal-cleaving species. In addition, although monensin and BFA act through distinct molecular or subcellular processes (Dinter and Berger 1998;Mayer 2003;Mollenhauer et al 1990), they had seemingly identical effects on development in P VII/gala, suggesting that the observed phenotypes are not the result of a chemical-specific toxicity. Monensin and BFA radialization results in a similar loss of animal-vegetal extension in the three equal-cleaving species, indicating that radialization is similarly achieved through the inhibition of specification .…”

“…Although the 3D organizer has been demonstrated in gastropod, polyplacophoran, and scaphopod classes, direct comparisons are problematic due to differences in the experimental techniques of different studies . To comparatively characterize 3D in a more standardized fashion, we used two general inhibitors of protein processing and secretion, monensin and BFA (Dinter and Berger 1998;Mayer 2003;Mollenhauer et al 1990), to test for the presence and function of 3D patterning in the equalcleaving gastropods, P vulgata and L. stagnalis, the equal-cleaving chiton , Mopalia muscosa, and the polarlobe-forming scaphopod, Anta/is enta/is. We then documented the resulting radialized development in each , focusing in particular on M. muscosa.…”

“…Treatment with BFA and monensin BFA and monensin are small organic molecules used as pharmacological tools to provide a rapid and reversible inhibition of protein processing and secretion in eukaryotes (Dinter and Berger 1998;Mayer 2003;Mollenhauer et al 1990). Monensin inhibits 3D specification at specific timepoints in gastropod development (Kuhtreiber et al 1988), and because the target and/or function of each 107 chemical is highly conserved, both can be used to test for organizer presence and function in developmentally uncharacterized species, where genomic and molecular genetic tools are lacking.…”

Enhancement of immunotoxin activity using chemical and biological reagents

“…Moensin is an open-chain molecule that is capable of ion complexation through a cyclic form stabilized by hydrogen bonding between the carboxyl and hydroxyl groups (Mollenhauer et al, 1990). Carboxylic ionophores exchange monovalent cations across membranes.…”