G-proteins and the role of second messengers in the regulation of the human neutrophil.

The ideas discussed above clearly point to increasingly complex and interactive transduction mechanisms for the regulation of the neutrophil. The central challenges to be met include the following: 1. Better assays are needed for the study of physiological parameters such as adherence, aggregation, shape change, and cytoskeletal rearrangements, assays which are not prohibitively complex and expensive while still allowing for more detailed physiological observations. 2. The neutrophil receptors need to be characterized in greater detail at the molecular level. Protein purification, sequencing, and cloning approaches are needed. Given the inherent shortcomings of working with the neutrophil system due to the presence of proteases and the problems of obtaining sufficient amounts of plasma membranes as source material for receptor purification, this is a difficult task. Advances in micropurification and sequencing may alleviate some of the difficulties here. 3. The size and complexity of the G-protein family continue to expand. However, as pointed out earlier, stimulus-responsive enzymes without G-protein-associated regulation, and G-proteins without clearly identified targets, remain. A better definition and description of the G-protein family will be required if cellular regulation is to be understood at the molecular level. In terms of second messengers and their role in cellular regulation, the main questions which remain to be answered concern identification of the precise pathways which are important to cellular regulation. In order to understand the complex cascades of arachidonate metabolism, phospholipid turnover, and calcium homeostasis, it is all the more important that the manner in which second messengers may regulate particular cell functions be better understood. An omission in this review is the role of kinases in cellular regulation. Activation of kinase C (through calcium and diacylglycerol) and kinase A (through cAMP) has been demonstrated. The substrates for these kinases have been described by various investigators. However, relating phosphorylation changes in a particular protein to the activity of the protein, and assignment of activity to particular physiological roles, has not been satisfactorily accomplished and remains a challenge for the future.