Novel role of calcium in exocytosis: mechanism of nematocyst discharge as shown by x-ray microanalysis.

Mature nematocysts of the sea anemones Rhodactis rhodostoma and Anthopleura elegantissima contain a fluid that has a high concentration of solutes and is extraordinarily rich in calcium (ca. 500-600 mmol/kg wet weight); this contrasts with the surrounding cytoplasm which is rich in potassium but poor in calcium. The undischarged capsule is surrounded by a membrane that probably acts as a selective permeability barrier between the cytoplasm and the nematocyst fluid. During discharge the nematocyst moves to the surface of the nematocyte and comes into contact with the external sea water medium. Calcium, which may be bound to proteins in the undischarged state, is rapidly lost from the fluid; at the same time, sea water enters the capsule. In vitro experiments have already shown that calcium loss increases the osmotic pressure of the capsular fluid, causing an influx of water from the external medium; this influx appears to increase the hydrostatic pressure inside the capsule to the point that the thread everts explosively.

Removal of bound calcium from nematocyst contents causes discharge.

Nematocysts are the stinging organelles of jellyfish, sea anemones and other cnidarians. Each one consists of a closed capsule filled with fluid. In the resting state, part of the surface of the capsule is inverted, forming a tubular thread, which is everted explosively on excitation. The mechanism of explosion is not yet understood, it may be relevant to exocytosis in general, as the nematocyst is a specialized type of exocytotic secretion originating in the Golgi apparatus. Picken and Skaer observed that the capsular fluid showed a large depression of freezing point, suggesting that the osmotic pressure might be as high as 140 bar. They tentatively ascribed the explosion to a sudden increase in permeability of the capsule wall, allowing a rapid osmotic influx of water. However, there is evidence that the material of the thread itself may be capable of some degree of extension and it has been suggested that osmosis plays no part in discharge. We present here a new theory: that the capsule wall is permeable to water even in the undischarged state; discharge is initiated by an increase in the osmotic pressure of the capsular fluid which is brought about by removal of bound calcium ions.

Electrical activity following cellular recognition of self and non-self in a sea anemone.

The sea anemone Anthopleura elegantissima lives in clonal colonies and possesses a cellular recognition system of remarkable specificity, by which it can recognize members of its own clone; other anemones, including individuals of the same species which are not syngeneic, are attacked. Attack is initiated by contact with a foreign anthozoan and involves the inflation of specialized tentacle-like structures known as acrorhagi, which contain numerous stinging cells. These stinging cells only discharge when the tip of the acrorhagus is in physical contact with the surface of a foreign anthozoan; contact with syngeneic individuals, organisms other than anthozoans and inanimate objects does not elicit discharge. We show here that the recognition of allogeneic tissue is accompanied by a novel form of local electrical activity in the acrorhagus that is usually, but not invariably, followed by nematocyst discharge. This type of electrical activity was not found during contact with syngeneic tissue or inanimate objects and seemed to be a consequence of the recognition of allogeneic surface markers by cells at the tip of the acrorhagus.

Clone-specific cellular recognition in a sea anemone.

A highly specific cellular recognition system, capable of distinguishing between syngeneic and allogeneic tissue, exists in Anthopleura elegantissima, a sea anemone that lives in clonal colonies and attacks foreign clones. During the attack, specialized surface protrusions (acrorhagi) are used for stinging. The recognition process was studied by presenting various tissues to the surface of inflated acrorhagi and observing whether nematocyst discharge occurred. Nematocyte excitation required direct contact of th acrorhagus with foreign tissue and is presumably mediated by cell surface receptors. Most foreign anthozoans were excitatory, but intact syngeneic individuals, organisms other than anthozoans, and inanimate objects consistently failed to elicit discharge. When the intact surface of an excised tentacle from one anemone was presented to the acrorhagus of another, discharge occurred in 101 of 102 allogeneic combinations; more than 50 tests with tentacles from clone mates (i.e., syngeneic combinations) were all negative. No evidence for specific immunological memory was found. It is suggested that clonal recognition depends upon genetically determined chemical markers in the surface membrane of the epithelial cells; these are assumed to differ between clones although, in rare cases, allogeneic clones may have similar markers.