Axonal transport and distribution of synaptobrevin I and II in the rat peripheral nervous system

Abstract

Synaptobrevin, a membrane protein of synaptic vesicles that plays a key role in exocytosis, occurs in two closely related isoforms, synaptobrevin I and II. We have analyzed the axonal transport of both isoforms in sciatic nerve and spinal roots. When fast axonal transport was interrupted by crushing, the proteins accumulated continuously proximal to the crush. Accumulation also was observed distal to the crush, but to a lesser extent (47 and 63% of the proximal accumulation for synaptobrevin I and II, respectively). Immunoelectron microscopy revealed that, proximal to the crush, synaptobrevin I and II were associated with small clear vesicles reminiscent of typical synaptic vesicles. Distal to the crush, membranes positive for synaptobrevin I or II were more heterogeneous, including larger membrane profiles that may represent endosomes. In spinal cord, synaptobrevin I and II were colocalized in many terminals. However, labeling for synaptobrevin I was more intense whereas labeling for synaptobrevin II was stronger in dorsal than in ventral horn terminals. Motor endplates contained only synaptobrevin I. In the sciatic nerve, synaptobrevin I was present predominantly in large, myelinated axons, whereas synoptobrevin II was virtually absent but abundant in small- and medium-sized axons. Lumbar sympathectomy, ventral rhizotomy, and double-labeling studies confirmed that synaptobrevin I is present predominantly in motor neurons whereas synaptobrevin II is present in adrenergic and sensory neurons. We conclude that synaptobrevin I and II are transported bidirectionally by fast axonal transport and are expressed heterogeneously in different neurons in the peripheral nervous system of the adult rat, suggesting that these isoforms have special functional roles in different sets of neurons.

Isoform-Specific Exocytotic Protein mRNA Expression in Hypothalamic Magnocellular Neurons: Regulation after Osmotic Challenge

“…This phenomenon has previously been observed for VAMP-LI in perikarya of motor nuclei of the brainstem and spinal cord motoneurons, indicating that newly synthesized exocytotic proteins may be rapidly transported out of the cell bodies [2, 34, 40]. Even during conditions where VAMP-2 mRNA is increased such as after spinal axon injury, VAMP-2-LI is not detected in the cell bodies of spinal motoneurons [34].…”

Synaptic Localization and Presynaptic Function of Calcium Channel β4-Subunits in Cultured Hippocampal Neurons

“…Overexpression of a functional GFP-␤ 4 fusion protein in cultured hippocampal neurons revealed a punctate staining pattern similar to the distribution of synaptic proteins synapsin I, synaptophysin (25), Munc-13-1 (26), syntaphilin (18), and synaptobrevin (27,28). Colocalization of GFP-␤ 4 with synaptobrevin II, a highly enriched presynaptic vesicle protein involved in transmitter release (29), and the presynaptic P/Qtype Ca 2ϩ -channel ␣ 1A -subunit suggests that GFP-␤ 4 is also targeted to the presynaptic terminals.…”

Vesicular and conductive mechanisms of nucleotide release

“…The synaptic fusion complex is initiated by formation of a ternary (synaptobrevin, syntaxin, SNAP-25) complex, but additional proteins, including Rabs and Muncs, function at steps up-stream of the SNARE complex formation. During the late step of neurotransmitter release, syntaxin/SNAP 25 binds to the vesicle protein synaptotagmin, a Ca 2+ sensor, triggering fusion of the presynaptic vesicle with the plasma membrane [29][30][31][32][33].…”