Particle segregation in chromaffin granule membranes by forced physical contact

Ultrastructural and cytochemical characterization of adrenal medullary plasma membrane vesicles and their interaction with chromaffin granules

Plasma membrane vesicles obtained by density gradient centrifugation of bovine adrenal medullary homogenates were analyzed by electron microscopic methods, including negative staining, ultrathin sections and freeze-fracture replicas. Rapid freezing showed the intramembrane structure of plasma membrane vesicles to be distinct from that of other organelle membranes, such as chromaffin granules. Cytochemical demonstration of acetylcholinesterase (EC 3.1.1.7) activity on most membrane profiles confirmed that plasma membrane vesicles are derived predominantly from plasma membranes. About half of the plasma membrane vesicles were smaller than 0.15 micron and almost none larger than 0.55 micron. Practically all were composed of single shells. Most vesicles were impermeable to cytochemical markers of the size of Ruthenium red (Mr 800) and none were permeable to markers larger than 40 kDa. Surface charge probes, concanavalin A binding and endogenous actin decoration with heavy meromyosin indicated that the major fraction of plasma membrane vesicles is oriented right-side-out. A minor population with opposite orientation could also be detected. Isotonic ionic media caused vesicle aggregation in suspensions of plasma membrane vesicles and chromaffin granules. Freeze-fracturing always revealed clusters of membrane-intercalated particles at the sites of contact between aggregated membranes.

The chromaffin granule as a model for membrane fusion: implications for exocytosis

Rapid freeze/freeze-fracture and thin section electron micrographic studies of the Ca2+-promoted aggregation and fusion of isolated bovine adrenal medullary chromaffin granule membranes show that the granules undergo a series of morphological changes. The contact region becomes quite extensive and the membrane curvature changes radically at the edge of the contact site. The core material retracts away from the contact site leaving an electron lucent "stripe"; however, it remains adjacent to the membrane in the non-contact areas. The pentalaminar double membrane of the contact region often shows breaks. Close examination reveals that the two granule membranes have fused and become one continuous membrane. Rearrangement of large membrane associated particles (MAPs) can be seen by freeze fracture after Ca2+-promoted granule-granule contact. The broken pentalaminar septum becomes smaller and may break down into globular structures. These observations suggest a series of reactions in which the granules first form an encounter complex, then a stable complex. The membranes within the contact region undergo lateral displacement of the proteins and phase separations of the lipids, and then fuse. Analysis of the kinetics of turbidity and fluorescence changes during granule aggregation and fusion support the main postulates of the model. The initial events of aggregation are facilitated by putative recognition proteins and K+ will promote all activities except fusion. Recent observations that several soluble proteins (synexin, and albumin) will act as fusogens are discussed in terms of the relevance to exocytosis in vivo.

Lipid domain structures in biological membranes

Phase separation represents a possibility for segregation of lipidic membrane components into structurally distinct domains. Freeze-fracture electronmicroscopy is a useful method for detection of lipid domains. Indications of a possible domain-nature of structures are a regular pattern within a separated area, a regular outline of such an area and a local modulation of curvature (evagination or invagination). Candidates for domain structures in biological membranes are smooth particle-free areas and arrays of regularly arranged particles. The interpretation of the particle-free areas is more reliable than that of the arrays with regularly arranged particles. Phase separation in biological membranes can be induced experimentally by lowering the temperature, but physiologically the isothermically induced domains are more important. Factors in control of isothermic domain formation are divalent cations, proteins, cholesterol etc. Suggestions on the biological relevance of domain formation concern mainly their role in the mechanism of membrane fusion, but domains in form of transient or stable membrane structures seem to occur also otherwise and disturbances in domain formation or artificially induced domains can be suitable for pathological alterations.

Calcium-promoted resonance energy transfer between fluorescently labeled proteins during aggregation of chromaffin granule membranes

Proteins of the chromaffin granule membrane were covalently labeled in situ with sulfhydryl-specific fluorophores. Using MIANS (maleimide iodoaminonaphthyl sulfonate) as the donor and fluorescein mercury acetate or fluorescein-5-maleimide as the acceptor. Förster fluorescence resonance energy transfer (FRET) could be employed to measure the degree of inter-membrane and intra-membrane protein-protein contact upon Ca2+-induced aggregation of the membranes. The four major findings were: (1) Raising the Ca2+ concentration to approx. 500 microM causes the proteins to aggregate in the plane of the membrane. This is demonstrated by Ca2+-induced increases in the fluorescence resonance energy transfer in double labeled membranes. This effect is not protein-concentration dependent and occurs at calcium concentrations too low for granule aggregation, implying intra-membrane protein clustering or patching. To our knowledge this is the first direct demonstration of the fluid mosaic nature of subcellular organelles. (2) If two sets of granules are labeled separately, Ca2+-induced aggregation brings at least 74% of the labeled proteins into close transmembrane proximity. This effect is also observed at 10-100-fold slower rates in the absence of calcium and can be greatly reduced by depleting the granule membrane of labeled peripheral proteins. It is enhanced if the granules are aggregated by Ca2+ or K+. We conclude that (some) peripheral proteins can transfer from one membrane surface to another. (3) Aggregation of separately labeled sets of membranes by Ca2+ also produces transmembrane energy transfer since: (a) the Km for Ca2+-induced quantum transfer is in the same range as the Km for aggregation; (b) the reaction is protein-concentration dependent; (c) reversal of aggregation also (partially) reverses donor quenching. (4) A kinetic analysis of the transmembrane effect shows it to be 5-10-fold slower than aggregation itself, supporting earlier suggestions (Haynes, D.H., Kolber, M. and Morris, S.J., (1979) J. Theor. Biol. 81, 713-743) that lipid and protein rearrangements are secondary to granule membrane aggregation.

Understanding the artefact problem in freeze-fracture replication: a review

Freeze-fracture and freeze-etching techniques do not provide artefact-free images of native in vivo or in vitro cells and tissues. Each preparation stage can produce specific artefacts which must be recognized and understood if these methods are to contribute meaningful information to cell biology, This paper reviews the latest information available on artefacts in freeze-fracture replication (and etching) methods and points to possibilities for avoiding some of them. Different specimens show different sensitivity to artefactual changes and the final images must be interpreted carefully with regard to the multi-event process that has led to their production.

The molluscan cardioactive neuropeptide FMRFamide: subcellular localization in bivalve ganglia

Ganglia of the mollusk Macrocallista nimbosa were pooled, homogenized, and subjected to differential centrifugation. The neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) was concentrated in the microsomal pellet. When the medium-speed supernatant was centrifuged in a discontinuous sucrose gradient, three separate peaks of activity were detected and identified as acetylcholine, 5-hydroxytryptamine, and FMRFamide. The relative concentration of FMRFamide in each fraction was determined by bioassay and by radioimmunoassay (RIA). Both determinations revealed a peak of peptide in the middle of the sucrose gradient. Electron micrographs of each of the gradient interfaces were analyzed. The interface containing the peak of biological FMRFamide activity was enriched in two- to fivefold in neurosecretory granules with a mean diameter of 104 nm and various electron densities. Morphologically similar vesicles were also seen in intact ganglia. These findings support the notion that FMRFamide is a neurosecretory product. But the physiological function of the peptide in bivalve ganglia remains unknown.

Membrane interactions between secretion granules and plasmalemma in three exocrine glands

Three types of membrane interactions were studied in three exocrine systems (the acinar cells of the rat parotid, rat lacrimal gland, and guinea pig pancrease) by freeze- fracture and thin-section electron microscopy: exocytosis, induced in vivo by specific pharmacological stimulations; the mutual apposition of secretory granule membranes in the intact cell; membrane appositions induced in vitro by centrifugation of the isolated granules. In all three glandular cells, the distribution of intramembrane particles (IMP) on the fracture faces of the luminal plasmagranule membrane particles (IMP) on the fracture faces of the lumenal plasmalemma appeared random before stimulation. However, after injection of secretagogues, IMP were rapidly clearly from the areas of granule- plasmalemma apposition in the parotid cells and, especially, in lacrimocytes. In the latter, the cleared areas appeared as large bulges toward the lumen, whereas in the parotid they were less pronounced. Exocytotic openings were usually large and the fracture faces of their rims were covered with IMP. In contrast, in stimulated pancreatic acinar cells, the IMP distribution remained apparently random after stimulation. Exocytoses were established through the formation of narrown necks, and no images which might correspond to early stages of membrane fusion were revealed. Within the cytoplasm of parotid and lacrimal cells (but not in the pancreas), both at rest and after stimulation, secretion granules were often closely apposed by means of flat, circular areas, also devoid of IMP. In thin sections, the images corresponding to IMP-free areas were close granule-granule and granule-plasmalemma appositions, sometimes with focal merging of the membrane outer layers to yield pentalaminar structures. Isolated secretion granules were forced together in vitro by centrifugation. Under these conditions, increasing the centrifugal force from 1,600 to 50,000 g for 10 min resulted in a progressive, statistically significant increase of the frequency of IMP-free flat appositions between parotid granules. In contrast, no such areas were seen between freeze-fractured pancreatic granules, although some focal pentalaminar appositions appeared in section after centrifugation at 50 and 100,000 g for 10 min. On the basis of the observation that, in secretory cells, IMP clearing always develops in deformed membrane areas (bulges, depressions, flat areas), it is suggested that it might result from the forced mechanical apposition of the interacting membranes. This might be a preliminary process not sufficient to initiate fusion. In the pancreas, IMP clearing could occur over surface areas too small to be detected. In stimulated parotid and lacrimal glands they were exceptional. These structures were either attached at the sites of continuity between granule and plasma membranes, or free in the acinar lumen, with a preferential location within exocytotic pockets or in their proximity. Experiments designed to investigate the nature of these blisters and vesicles revealed that they probably arise artifactually during glutaraldehyde fixation. In fact, (a) they were large and numerous in poorly fixed samples but were never observed in thin sections of specimens fixed in one step with glutaraldehyde and OsO(4); and (b) no increase in concentration of phospholipids was observed in the parotid saliva and pancreatic juice after stimulation of protein discharge, as was to be expected if release of membrane material were occurring after exocytosis.