Regulated exocytosis is a process in which the membranes of cytoplasmic organelles fuse with the plasma membrane in response to stimulation. In many cases (secretory exocytoses), the process functions to secrete specific products that are segregated in the organelle lumen (for example, neurotransmitters, hormones and enzymes) to the extracellular space. In other cases ('non-secretory exocytoses'), it functions to transfer the organelle membrane and its components to the cell surface. Here, the general properties of non-secretory exocytoses are discussed.
The adducin heterodimer is a protein affecting the assembly of the actin-based cytoskeleton. Point mutations in rat adducin ␣ ( F316Y ) and  ( Q529R ) subunits are involved in a form of rat primary hypertension (MHS) associated with faster kidney tubular ion transport. A role for adducin in human primary hypertension has also been suggested. By studying the interaction of actin with purified normal and mutated adducin in a cell-free system and the actin assembly in rat kidney epithelial cells (NRK-52E) transfected with mutated rat adducin cDNA, we show that the adducin isoforms differentially modulate: ( a ) actin assembly both in a cell-free system and within transfected cells; ( b ) topography of ␣ V integrin together with focal contact proteins; and ( c ) Na-K pump activity at V max (faster with the mutated isoforms, 1281 Ϯ 90 vs 841 Ϯ 30 nmol K/ h · mg pt., P Ͻ 0.0001). This co-modulation suggests a role for adducin in the constitutive capacity of the epithelia both to transport ions and to expose adhesion molecules. These findings may also lead to the understanding of the relation between adducin polymorphism and blood pressure and to the development of new approaches to the study of hypertension-associated organ damage. ( J. Clin. Invest. 1996. 97:2815-2822.)
Abstract. Synapsin I is a synaptic vesicle-associated protein which inhibits neurotransmitter release, an effect which is abolished upon its phosphorylation by Ca2÷/calmodulin-dependent protein kinase II (CaM kinase ]I). Based on indirect evidence, it was suggested that this effect on neurotransmitter release may be achieved by the reversible anchoring of synaptic vesicles to the actin cytoskeleton of the nerve terminal. Using video-enhanced microscopy, we have now obtained experimental evidence in support of this model: the presence of dephosphorylated synapsin I is necessary for synaptic vesicles to bind actin; synapsin I is able to promote actin polymerization and bundling of actin filaments in the presence of synaptic vesicles; the ability to cross-link synaptic vesicles and actin is specific for synapsin I and is not shared by other basic proteins; the cross-linking between synaptic vesicles and actin is specific for the membrane of synaptic vesicles and does not reflect either a non-specific binding of membranes to the highly surface active synapsin I molecule or trapping of vesicles within the thick bundles of actin filaments; the formation of the ternary complex is virtually abolished when synapsin I is phosphorylated by CaM kinase II. The data indicate that synapsin I markedly affects synaptic vesicle traffic and cytoskeleton assembly in the nerve terminal and provide a molecular basis for the ability of synapsin I to regulate the availability of synaptic vesicles for exocytosis and thereby the efficiency of neurotransmitter release.
The function of ␣-synuclein, a soluble protein abundant in the brain and concentrated at presynaptic terminals, is still undefined. Yet, ␣-synuclein overexpression and the expression of its A30P mutant are associated with familial Parkinson's disease. Working in cell-free conditions, in two cell lines as well as in primary neurons we demonstrate that ␣-synuclein and its A30P mutant have different effects on actin polymerization. Wild-type ␣-synuclein binds actin, slows down its polymerization and accelerates its depolymerization, probably by monomer sequestration; A30P mutant ␣-synuclein increases the rate of actin polymerization and disrupts the cytoskeleton during reassembly of actin filaments. Consequently, in cells expressing mutant ␣-synuclein, cytoskeleton-dependent processes, such as cell migration, are inhibited, while exo-and endocytic traffic is altered. In hippocampal neurons from mice carrying a deletion of the ␣-synuclein gene, electroporation of wild-type ␣-synuclein increases actin instability during remodeling, with growth of lamellipodia-like structures and apparent cell enlargement, whereas A30P ␣-synuclein induces discrete actin-rich foci during cytoskeleton reassembly. In conclusion, ␣-synuclein appears to play a major role in actin cytoskeletal dynamics and various aspects of microfilament function. Actin cytoskeletal disruption induced by the A30P mutant might alter various cellular processes and thereby play a role in the pathogenesis of neurodegeneration.
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