Faculty Bibliography

The PC12 clone is a line of rat pheochromocytoma cells that undergoes neuronal differentiation in the presence of NGF protein. In the absence of NGF, PC12 cells are electrically inexcitable, while after several weeks of NGF treatment they develope Na+ action potentials. Past estimates made by measuring binding of 3H-saxitoxin (STX) indicate that NGF treatment brings about a large increase in Na channel density that is of sufficient magnitude to account for the induction of excitability. We have now used 22Na uptake to measure the Na permeability of PC12 cells before and after long-term NGF treatment. Treatment with NGF does not change the resting Na+ permeability. The alkaloid toxins veratridine and batrachotoxin (BTX) and scorpion toxin were used to activate Na channels. Such studies demonstrate that these toxins induce TTX-sensitive Na uptake in both NGF-treated and untreated cells and reveal differences in functional Na channel numbers per cell and per unit of membrane area that are similar to those found in the STX binding studies. On the other hand, affinities for drugs that activate these channels are not affected by NGF treatment. We also find that NGF-treated PC12 cells contain a population of Na channels with low affinity for TTX. These channels account for 5-20% of total BTX or veratridine-stimulated flux. Thus, NGF has 2 effects regarding the Na channels of PC12 cells: it increases the number of functional Na channels that otherwise behave similarly to those present before NGF treatment, and it induces the presence of TTX-resistant Na channels. These findings indicate that the PC12 model system may serve to study the developmental regulation of Na channel expression and properties

The physico-chemical and biological properties of cytolytic peptides derived from diverse living entities have been discussed. The principal sources of these agents are bacteria, higher fungi, cnidarians (coelenterates) and the venoms of snakes, insects and other arthropods. Attention has been directed to instances in which cytolytic peptides obtained from phylogenetically remote as well as from related sources show similarities in nature and/or mode of action (congeneric lysins). The manner in which cytolytic peptides interact with plasma membranes of eukaryotic cells, particularly the membranes of erythrocytes, has been discussed with emphasis on melittin, thiolactivated lysins and staphylococcal alpha-toxin. These and other lytic peptides are characterized in Table III. They can be broadly categorized into: (a) those which alter permeability to allow passage of ions, this process eventuating in colloid osmotic lysis, signs of which are a pre-lytic induction or latent period, pre-lytic leakage of potassium ions, cell swelling and inhibition of lysis by sucrose. Examples of lysins in which this mechanism is involved are staphylococcal alpha-toxin, streptolysin S and aerolysin; (b) phospholipases causing enzymic degradation of bilayer phospholipids as exemplified by phospholipases C of Cl. perfringens and certain other bacteria; (c) channel-forming agents such as helianthin, gramicidin and (probably) staphylococcal delta-toxin in which toxin molecules are thought to embed themselves in the membrane to form oligomeric transmembrane channels