Serotonin organelles of rabbit platelets contain synaptophysin

Abstract

Synaptophysin, an integral membrane protein of synaptic vesicles in nerve terminals and a class of small translucent vesicles in neuroendocrine cells, was detected in intact rabbit platelets by immunoblotting, immunofluorescence staining and immuno-electron microscopy. In a highly purified preparation of serotonin organelles isolated from rabbit platelets, synaptophysin was enriched approximately 10 -15-fold over platelet homogenate. About 80% of total platelet synaptophysin was present in this purified fraction. The apparent molecular mass ( x 38 kDa) and the extent of glycosylation of platelet-derived synaptophysin was more similar to the neuronal than to the neuroendocrine form of the protein. Immunofluorescence microscopy revealed that synaptophysin was compartmentalized in intact rabbit platelets and immuno-electron microscopy of subcellular fractions showed that it was localized exclusively to the membrane surface of serotonin organelles. No synaptophysin-like immunoreactivity was detected in platelets from other species such as human, guinea pig and rat. Another integral membrane protein of synaptic vesicles, p65, and a family of synaptic vesicle-associated phosphoproteins, the synapsins, were not detected in platelets of any species tested. These results provide evidence that serotonin organelles from rabbit platelets share a subset of protein components with synaptic vesicles from neurons. Synaptophysin in serotonin organelles from rabbit platelets, as suggested for small synaptic vesicles in neurons, might play a role in the formation of protein channels for the exocytotic release of serotonin.Synaptic vesicles are secretory organelles (40 -60 nm in diameter), which are clustered in presynaptic nerve terminals, take up classical neurotransmitter molecules, store them and release their content via exocytosis upon stimulation. Several proteins specific for these organelles, have been identified and characterized, e. g. synaptophysin, p65 and the synapsin protein family [I -51. Synaptophysin and p65 have also been shown to be expressed in neuroendocrine cells [3, 61, in which synaptophysin was found to be localized to a newly discovered class of clear small vesicles apparently concentrated in the Golgi area [6]. Other reports, however, suggest that synaptophysin and p65 might not be exclusively localized on these structures, but that they are also present on secretory dense core organelles of larger diameter ( 2 60 nM), which store peptides [7, 81. Due to the presence of similar biochemical systems for the uptake, storage and metabolism of the neurotransmitter serotonin (5-hydroxy-tryptamine), blood platelets have often been used as a model for the study of serotoninergic neurons of the central nervous system [9, 101. Platelets contain at least two secretory organelles, namely the a-granules, with a diameter of about 300 nm, in which several peptides are stored, and the serotonin organelles (dense bodies, diameter 150 nm) which store serotonin and other monoamines together with a non-metabolic pool of ...

Synaptophysin: A Novel Marker for Human and Rat Hepatic Stellate Cells

“…32,33 To the best of our knowledge, only neural and neuroendocrine cell types have been shown to express SYN, 34 with the exception of rabbit thrombocytes. 35 In the present study we demonstrated expression of SYN in quiescent as well as activated human and rat hepatic stellate cells.…”

Prostasomes are neuroendocrine-like vesicles in human semen

“…Other organelles of a similar structure as that of Interestingly, Sy has also been recovered in the serotonin-containing vesicles in platelets [56]. Further studies indicated that this synaptophysin-like protein was slightly different from the neurone-derived molecular form and it was also detected in granulocytes and hence the name granulophysin was introduced [14,57].…”

Neuroregulation of the neuroendocrine compartment of the liver

“…Immunohistochemical detection of SYN is commonly used in conjunction with chromogranin‐A, neuron‐specific enolase, and Leu‐7 immunohistochemistry to determine neuroendocrine origin of or differentiation in tissues and tumors throughout the body (Schurmann et al, 1990; Poola and Graziano, 1998). To the best of our knowledge, only neural and neuroendocrine cell types have been demonstrated to express SYN (Bargou and Leube, 1991), with the exception of rabbit thrombocytes (Bahler et al, 1990).…”