Peculiar properties of mycoplasmas: The smallest self-replicating prokaryotes

Mycoplasmas are the smallest and simplest prokaryotes capable of self-replication, with information provided by a genome which may be as small as 600 kb, estimated to carry less than 500 genes. Keeping the number of structural elements, metabolic pathways and components of the protein synthesizing machinery to an essential minimum places mycoplasmas closest to the concept of 'minimum cells'. Mycoplasmas are, therefore, most adequate candidates for the complete deciphering of the machinery of a self-replicating organism, and studies towards this goal are already underway. Living as 'minimum cells' was made possible by adopting a parasitic mode of life, securing from the host the many nutrients which cannot be synthesized by the mycoplasmas themselves. When pathogenic, infections by mycoplasmas usually follow a chronic course, with host immune reactions playing an important role in symptom production. Recent studies on the possible association of mycoplasmas with rheumatoid arthritis and AIDS are reviewed.     

Viable mouse thymocytes as a model system for studying the onset of hormone-induced cellular refractoriness

Mouse thymocytes are characterized as a model cellular system for studying the onset of hormone-induced cellular refractoriness (desensitization). This system has the following combination of useful features. (a) The cells can be isolated without the use of digestive enzymes, avoiding possible damage to surface receptors or to other exposed membranal constituents. (b) They can be kept viable for several hours, a period during which both stimulation and desensitization get well under way. (c) They can be stimulated by a variety of hormones which function via cAMP (β-agonists, prostaglandin E₁ and specific thymic humoral factors). (d) Their desensitization is receptor-specific. (e) They can be readily ruptured under mild conditions so as to allow a physiologically relevant biochemical analysis of hormonal stimulation and desensitization. (f) The hormonal response of these cells can be monitored simultaneously by the activation of adenylate cyclase, by the intracellular level of cAMP, and by the activation of cAMP-dependent protein kinase (which functions as a metabolic sensor for cAMP). In this cellular system, desensitization does not involve processes such as the efflux of cAMP, the activation of cAMP-phosphodiesterase or the synthesis of a protein mediator. On the other hand, desensitization can be accounted for by a hormone-triggered inactivation of the adenylate cyclase system. The immediate desensitization of thymocytes is reversible and occurs without apparent loss of functional receptors. Continuous presence of hormone is shown to be required not only for triggering the chain of events which leads to the readily reversible desensitization, but also for the process which transfers the cells to the subsequent, ‘locked’ desensitized state.     

A Novel Solution for the High Defibrillation Threshold in Patients with a DF-4 Lead: Adding a High Voltage Adaptor/Splitter

A high defibrillation threshold occurs in approximately 6% of implants. The defibrillation threshold can be improved by addition of a defibrillation lead. However, the DF-4 high energy ICD header precludes the addition of a defibrillation lead. Here we report on use of a new high voltage adaptor/splitter that enables the addition of an extra defibrillation lead.     

Generation of selective microenvironment at the cell periphery through bulk-phase hydrophilic polymers

In order to understand the previously demonstrated effect of poly(ethylene glycol) on the stimulation of lymphocyte responses to syngeneic tumor cells (Ben-Sasson, S.A. and Henkart, P.A. (1977) J. Immunol. 119, 227–231), we have investigated the effects of addition of poly(ethylene glycol) to the medium in a number of cellular systems. The binding of trimeric IgG to tumor-lymphocyte Fc receptors was greatly enhanced by poly(ethylene glycol); a substantial increase in binding of trimeric IgG to non-Fc-receptor-bearing tumor cells was also observed. Similarly, the binding of labeled bovine serum albumin to lymphocyte surfaces was increased by poly(ethylene glycol), implying that nonspecific binding of proteins to cells was generally enhanced. The dose-response curve of concanavalin A mitogenesis was shifted to the right, as would be expected from a local increase in concanavalin A concentration. Antibody binding to erythrocytes as detected by complement lysis was similarly increased. It was found that in aqueous two-phase mixtures created by poly(ethylene glycol) and dextran, erythrocytes partition into the dextran phase through exclusion into dextran-rich microdroplets. It is proposed that addition of poly(ethylene glycol) to cell culture media creates a similar separate phase around the cell surface in which the local concentration of proteins is greater than that in the bulk medium. This concept explains many of the diverse effects of addition of poly(ethylene glycol) to the medium. It also can partially explain the requirement for serum to observe the poly(ethylene glycol) effect on the lymphocyte response to syngeneic tumor cells.