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 comples 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. It is becoming clear that endocytosis involves both acidic and non-acidic compartments and that monensin inhibits those processes that normally occur in acidic compartments.

Thus, monensin, which is capable of collapsing Na⁺ and H⁺ gradients, has gained wide-spread acceptance as a tool for studying Golgi apparatus function and for localizing and identifying the molecular pathways of subcellular vesicular traffic involving acid compartments. Among its advantages are the low concentrations at which inhibitions are produced (0.01–1.0 μM), a minimum of troublesome side effects (e.g., little or no change of protein synthesis or ATP levels) and a reversible action. Because the affinity of monensin for Na⁺ is ten times that for K⁺, its nearest competitor, monensin mediates primarily a Na⁺-H⁺ exchange. Monensin has little tendency to bind calcium.

Not only is monensin of importance as an experimental tool, it is of great commercial value as a coccidiostat for poultry and to promote more efficient utilization of feed in cattle. The mechanisms by which monensin interact with coccidia and rumen microflora to achieved these benefits are reasonably well documented. However, the interactions between monensin and the tissues of the host animal are not well understood although the severe toxicological manifestations of monensin poisoning are well known. Equine species are particularly susceptible to monensin poisoning, and a common effect of monensin poisoning is vacuolization and/or swelling of mitochondria in striated muscle. Other pathological injuries to striated muscle, spleen, lung, liver and kidney also have been noted. A consistent observation is cardiac myocyte degeneration as well as vacuolization. Differences in cellular response resulting from exposure to monensin (i.e., Golgi apparatus swelling in cultured cells, isolated tissues, and plants vs.mitochondrial swelling in animals fed monensin) suggest that myocardial damage is due either to a monensin metabolite or is a secondary response to some other derivation. However, as pointed out by Bergen and Bates [26], the underlying mode of action of ionophores is on transmembrane ion fluxes which dissipate cation and proton gradients. Consequently, some or all of the observed monensin effects in vivo in animals could be secondary phenomena caused by disruption of normal membrane physiology resulting from altered ion fluxes.