Cell biology. Fast forward to fusion

Neurotranmission systems as targets for toxicants: a review

Neurotransmitters are chemicals that transmit impulses from one nerve to another or from nerves to effector organs. Numerous neurotransmitters have been described in mammals, amongst them acetylcholine, amino acids, amines, peptides and gases. Toxicants may interact with various parts of neurotransmission systems, including synthetic and degradative enzymes, presynaptic vesicles and the specialized receptors that characterize neurotransmission systems. Important toxicants acting on the cholinergic system include the anticholinesterases (organophosphates and carbamates) and substances that act on receptors such as nicotine and the neonicotinoid insecticides, including imidacloprid. An important substance acting on the glutamatergic system is domoic acid, responsible for amnesic shellfish poisoning. 4-Aminobutyric acid (GABA) and glycine are inhibitory neurotransmitters and their antagonists, fipronil (an insecticide) and strychnine respectively, are excitatory. Abnormalities of dopamine neurotransmission occur in Parkinson's disease, and a number of substances that interfere with this system produce Parkinsonian symptoms and clinical signs, including notably 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, which is the precursor of 1-methyl-4-phenylpyridinium. Fewer substances are known that interfere with adrenergic, histaminergic or seroninergic neurotransmission, but there are some examples. Among peptide neurotransmission systems, agonists of opioids are the only well-known toxic compounds.

A distinct class of intracellular storage vesicles, identified by expression of the glucose transporter GLUT4

Some cell types have cytoplasmic storage vesicles whose fusion with the cell surface is triggered by an extracellular signal. To explore the relationship between different classes of storage vesicles, we expressed, in the neuro-endocrine cell line PC12, the facilitative glucose transporter GLUT4, which is stored in small cytoplasmic vesicles in fat and muscle cells and mobilized to the cell surface when insulin is present. PC12 cells have two known types of storage vesicles, secretory granules and synaptic vesicles, but GLUT4 is targeted to neither. It is recovered, however, in a class of small vesicles that sediment approximately twice as fast as synaptic vesicles. Immunoelectron microscopy confirmed the presence of such small vesicles in transfected PC12 cells. By velocity sedimentation analysis, GLUT4 vesicles efficiently exclude the synaptic vesicle markers synaptophysin, SV2, and synaptobrevin; the transferrin receptor, a marker of conventional endocytosis; and the polymeric immunoglobulin receptor, a marker of transcytosis. The exclusion of synaptophysin and the transferrin receptor from most of the GLUT4-containing structures was confirmed by confocal immunofluorescence microscopy. Like synaptic vesicles, therefore, GLUT4 vesicles of PC12 cells appear to be a unique type of organelle. A GLUT4-containing organelle of identical sedimentation properties was found in transfected fibroblast cell lines and in rat adipocytes. On stimulation of the adipocytes with insulin, GLUT4 was translocated from the peak of small vesicles to faster sedimenting membranes. We propose that the class of vesicles described here is present in a wide range of cell types and is involved in transient modification of the cell surface.