EAAT expression by macrophages and microglia: still more questions than answers

Glutamate is the main excitatory amino acid, but its presence in the extracellular milieu has deleterious consequences. It may induce excitotoxicity and also compete with cystine for the use of the cystine–glutamate exchanger, blocking glutathione neosynthesis and inducing an oxidative stress-induced cell death. Both mechanisms are critical in the brain where up to 20% of total body oxygen consumption occurs. In normal conditions, the astrocytes ensure that extracellular concentration of glutamate is kept in the micromolar range, thanks to their coexpression of high-affinity glutamate transporters (EAATs) and glutamine synthetase (GS). Their protective function is nevertheless sensitive to situations such as oxidative stress or inflammatory processes. On the other hand, macrophages and microglia do not express EAATs and GS in physiological conditions and are the principal effector cells of brain inflammation. Since the late 1990s, a number of studies have now shown that both microglia and macrophages display inducible EAAT and GS expression, but the precise significance of this still remains poorly understood. Brain macrophages and microglia are sister cells but yet display differences. Both are highly sensitive to their microenvironment and can perform a variety of functions that may oppose each other. However, in the very particular environment of the healthy brain, they are maintained in a repressed state. The aim of this review is to present the current state of knowledge on brain macrophages and microglial cells activation, in order to help clarify their role in the regulation of glutamate under pathological conditions as well as its outcome.     

Coupling between phosphoinositide breakdown and early mitogenic events in fibroblasts. Studies with fluoroaluminate, vanadate, and pertussis toxin

In the preceding paper (Paris, S., and Pouysségur J. (1987) J. Biol. Chem. 262, 1970-1976), AlF4- and vanadate have been shown to induce inositol phosphate formation in resting hamster fibroblasts (CCL39). In this study, we show that these two phosphate analogs are good tools to explore the causal relationship between phosphoinositide breakdown and early mitogenic events. AlF4- can activate, very similarly to the mitogen alpha-thrombin: the amiloride-sensitive Na+/H+ antiport, the bumetanide-sensitive Na+/K+/Cl- co-transport, and the expression of c-myc mRNA. The link between phospholipase C activation and these early events of the mitogenic response is demonstrated by the similarity of all dose-response curves for NaF and AlCl3 and by the common sensitivity of the four events to pertussis toxin. Vanadate likewise stimulates the Na+/H+ antiport through a pertussis toxin-sensitive pathway. On longer incubations, both fluoride and vanadate were found to be toxic and failed to induce DNA synthesis. Therefore, we have used pertussis toxin to investigate the link between phospholipase C activation and commitment to DNA synthesis. We show that pertussis toxin strikingly inhibits thrombin-induced reinitiation of DNA synthesis but does not affect the stimulation by the epidermal or fibroblast growth factors, two mitogens that do not stimulate phosphoinositide breakdown in CCL39 cells. In conclusion, these studies demonstrate that activation of phospholipase C, if not an obligatory step in the action of all growth factors, plays a crucial role in the mitogenic signaling pathway of alpha-thrombin.     

Nasal allergy to fungi in guinea pigs

Sensitized guinea pigs produced specific IgG and IgE antibodies toward Cladosporium and Alternaria. In presence of fungal extracts, nasal mast cells degranulate. Ultrastructural modifications of the cells during degranulation have been established. The ciliary epithelium and the ciliary beating are not affected by fungal allergens.     

Kinetic Concepts for Measuring Microbial Rate Constants: Effects of Nutrients on Rate Constants

We investigated the effect of preincubation of environmental waters amended with inorganic nutrients (nitrogen, phosphorus, and traces of iron and magnesium) on the kinetics of the microbial transformation of phenol, propanil, propyl ester of (2,4-dichlorophenoxy)acetic acid, methyl parathion, Ronnel, and methoxychlor in pond and river waters. No effect on the second-order rate constants for these compounds was observed, although there was an increase in the bacterial populations and the pseudo-first-order rate constants. The use of nutrient-amended waters could be a useful tool for estimating second-order rate constants for an expanded number of compounds. This technique would provide a larger data base for predicting the behavior of xenobiotic compounds in the environment by using currently available mathematical models.     

Alpha-thrombin-induced inositol phosphate formation in G0-arrested and cycling hamster lung fibroblasts: evidence for a protein kinase C-mediated desensitization response

In resting Chinese hamster fibroblasts (CCL39) alpha-thrombin rapidly induces the breakdown of phosphoinositides. Accumulation of inositol phosphates (IP), measured in the presence of Li+, is detectable within 5s (seconds) of thrombin stimulation. Formation of inositol tris- and bisphosphates slightly precedes that of inositol monophosphate, indicating that thrombin activates primarily the phospholipase C-mediated generation of inositol trisphosphate from phosphatidylinositol 4,5-bisphosphate. Initial rates of IP production increase with thrombin concentration, with no apparent saturability over the range 10(-4)-10 U/ml. Thrombin-induced phosphoinositide hydrolysis rapidly desensitizes (t1/2 less than 5 min), but a residual activity, corresponding to about 10% of the initial stimulation is sustained for at least 9 h, in contrast with the undetectable activity of G0-arrested cells. This apparent desensitization may be due to a feedback regulation by protein kinase C, since pretreatment with the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) markedly inhibits (by up to 70%) subsequent thrombin-induced inositol phosphate formation. Conversely, growth factor deprivation of CCL39 cells results in a progressive increase of thrombin-induced phosphoinositide hydrolysis, from the very low level of exponentially growing cells to the maximal level of G0-arrested cells. This "up regulation" was found maximal in A51, a very well growth-arrested CCL39 derivative, and reduced or virtually abolished in two tumoral and growth factor-relaxed derivatives of CCL39. Although preliminary, this observation suggests that a persistent activation of phosphatidyl inositol breakdown might operate in variants selected for autonomous growth.