Localization and physiological regulation of the exocytosis protein SNAP-25 in the brain and pituitary gland of Xenopus laevis.
In mammals, the synaptosomal-associated protein of 25 kDa, SNAP-25, is generally thought to play a role in synaptic exocytosis of neuronal messengers. Using a polyclonal antiserum against rat SNAP-25, we have shown the presence of a SNAP-25-like protein in the brain of the South-African clawed toad Xenopus laevis by Western blotting and immunocytochemistry. Xenopus SNAP-25 is ubiquitously present throughout the brain, where its distribution in various identified neuronal perikarya and axon tracts is described. Western blot analysis and immunocytochemistry also demonstrated the presence of SNAP-25 in the neural, intermediate and distal lobes of the pituitary gland. Intensity line plots of confocal laser scanning microscope images of isolated melanotropes indicated that SNAP-25 is produced and processed in the rough endoplasmatic reticulum and Golgi apparatus, and is associated with the plasma membrane. Immunoelectron microscopy substantiated the idea that SNAP-25 is present in the plasma membrane but also showed a close association of SNAP-25 with the bounding membrane of peptide-containing secretory granules in both the neurohemal axon terminals in the neural lobe and the endocrine melanotropes in the intermediate lobe. Quantitative Western blotting revealed that adapting Xenopus to a dark background has a clear stimulatory effect on the expression of SNAP-25 in the neural lobe and in the melanotrope cells. This background light intensity-dependent stimulation of SNAP-25 expression was confirmed by the demonstration of increased immunofluorescence recorded by confocal laser scanning microscopy of individual melanotropes of black background-adapted toads. On the basis of this study on Xenopus laevis, we conclude that SNAP-25 (i) plays a substantial role in the secretion of a wide variety of neuronal messengers; (ii) functions in the central nervous system but also in neurohormonal and endocrine systems; (iii) acts at the plasma membrane but possibly also at the membrane of synaptic vesicles and peptide-containing secretory granules; (iv) acts not only locally (as in synapses), but at various sites of the plasma membrane (as in the endocrine melanotrope cell); and (v) can be upregulated in its expression by physiological stimuli that increase the extent of the molecular machinery involved in exocytosis.
Kolk
S M
SM
Department of Cellular Animal Physiology, Nijmegen Institute for Neurosciences, University of Nijmegen, The Netherlands. smkolk@sci.kun.nl
Nordquist
R
R
Tuinhof
R
R
Gagliardini
L
L
Thompson
B
B
Cools
A R
AR
Roubos
E W
EW
D000818:Animals; D015153:Blotting, Western; D001921:Brain / Q000378:metabolism*; D005089:Exocytosis / Q000502:physiology*; D007150:Immunohistochemistry; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D010902:Pituitary Gland / Q000166:cytology / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism; D050825:Synaptosomal-Associated Protein 25; D014018:Tissue Distribution; D014982:Xenopus laevis / Q000378:metabolism*
D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D050825:Synaptosomal-Associated Protein 25