Distribution of Rab3a in rat nervous system: comparison with other synaptic vesicle proteins and neuropeptides

The differences in the anatomy of the thoracolumbar and sacral autonomic outflow are quantitative

PURPOSE

We have re-evaluated the anatomical arguments that underlie the division of the spinal visceral outflow into sympathetic and parasympathetic divisions.

METHODOLOGY

Using a systematic literature search, we mapped the location of catecholaminergic neurons throughout the mammalian peripheral nervous system. Subsequently, a narrative method was employed to characterize segment-dependent differences in the location of preganglionic cell bodies and the composition of white and gray rami communicantes.

RESULTS AND CONCLUSION

One hundred seventy studies were included in the systematic review, providing information on 389 anatomical structures. Catecholaminergic nerve fibers are present in most spinal and all cranial nerves and ganglia, including those that are known for their parasympathetic function. Along the entire spinal autonomic outflow pathways, proximal and distal catecholaminergic cell bodies are common in the head, thoracic, and abdominal and pelvic region, which invalidates the "short-versus-long preganglionic neuron" argument. Contrary to the classically confined outflow levels T1-L2 and S2-S4, preganglionic neurons have been found in the resulting lumbar gap. Preganglionic cell bodies that are located in the intermediolateral zone of the thoracolumbar spinal cord gradually nest more ventrally within the ventral motor nuclei at the lumbar and sacral levels, and their fibers bypass the white ramus communicans and sympathetic trunk to emerge directly from the spinal roots. Bypassing the sympathetic trunk, therefore, is not exclusive for the sacral outflow. We conclude that the autonomic outflow displays a conserved architecture along the entire spinal axis, and that the perceived differences in the anatomy of the autonomic thoracolumbar and sacral outflow are quantitative.

Coordinated bi-directional trafficking of synaptic vesicle and active zone proteins in peripheral nerves

Synaptic transmission is mediated by neurotransmitters that are stored in synaptic vesicles (SV) and released at the synaptic active zone (AZ). While in recent years major progress has been made in unraveling the molecular machinery responsible for SV docking, fusion and exocytosis, the mechanisms governing AZ protein and SV trafficking through axons still remain unclear. Here, we performed stop-flow nerve ligation to examine axonal trafficking of endogenous AZ and SV proteins. Rat sciatic nerves were collected 1 h, 3 h and 8 h post ligation and processed for immunohistochemistry and electron microscopy. First, we followed the transport of an integral synaptic vesicle protein, SV2A and a SV-associated protein involved in SV trafficking, Rab3a, and observed that while SV2A accumulated on both sides of ligation, Rab3a was only noticeable in the proximal segment of the ligated nerve indicating that only SV trans-membrane protein SV2A displayed a bi-directional axonal transport. We then demonstrate that multiple AZ proteins accumulate rapidly on either side of the ligation with a timescale similar to that of SV2A. Overall, our data uncovers an unexpected robust bi-directional, coordinated -trafficking of SV and AZ proteins in peripheral nerves. This implies that pathological disruption of axonal trafficking will not only impair trafficking of newly synthesized proteins to the synapse but will also affect retrograde transport, leading to neuronal dysfunction and likely neurodegeneration.

Impaired slow axonal transport in diabetic peripheral nerve is independent of RAGE

Diabetic peripheral nerve dysfunction is a common complication occurring in 30-50% of long-term diabetic patients. The pathogenesis of this dysfunction remains unclear but growing evidence suggests that it might be attributed, in part, to alteration in axonal transport. Our previous studies demonstrated that RAGE (Receptor for Advanced Glycation Endproducts) contributes to the pathogenesis of diabetic peripheral neuropathy and impairs nerve regeneration consequent to sciatic nerve crush, particularly in diabetes. We hypothesize that RAGE plays a role in axonal transport impairment via the interaction of its cytoplasmic domain with mammalian Diaphanous 1 (mDia1) - actin interacting molecule. Studies showed that mDia1-RAGE interaction is necessary for RAGE-ligand-dependent cellular migration, AKT phosphorylation, macrophage inflammatory response and smooth muscle migration. Here, we studied RAGE, mDia1 and markers of axonal transport rates in the peripheral nerves of wild-type C57BL/6 and RAGE null control and streptozotocin-injected diabetic mice at 1, 3 and 6 h after sciatic nerve crush. The results show that in both control and diabetic nerves, the amount of RAGE accumulated at the proximal and distal side of the crush area is similar, indicating that the recycling rate for RAGE is very high and that it is evenly transported from and towards the neuronal cell body. Furthermore, we show that slow axonal transport of proteins such as Neurofilament is affected by diabetes in a RAGE-independent manner. Finally, our study demonstrates that mDia1 axonal transport is impaired in diabetes, suggesting that diabetes-related changes affecting actin binding proteins occur early in the course of the disease.

APP anterograde transport requires Rab3A GTPase activity for assembly of the transport vesicle

The amyloid precursor protein (APP) is anterogradely transported by conventional kinesin in a distinct transport vesicle, but both the biochemical composition of such a vesicle and the specific kinesin-1 motor responsible for transport are poorly defined. APP may be sequentially cleaved by beta- and gamma-secretases leading to accumulation of beta-amyloid (Abeta) peptides in brains of Alzheimer's disease patients, whereas cleavage of APP by alpha-secretases prevents Abeta generation. Here, we demonstrate by time-lapse analysis and immunoisolations that APP is a cargo of a vesicle containing the kinesin heavy chain isoform kinesin-1C, the small GTPase Rab3A, and a specific subset of presynaptic protein components. Moreover, we report that assembly of kinesin-1C and APP in this vesicle type requires Rab3A GTPase activity. Finally, we show cleavage of APP in transport vesicles by alpha-secretase activity, likely mediated by ADAM10. Together, these data indicate that maturation of APP transport vesicles, including recruitment of conventional kinesin, requires Rab3 GTPase activity.

Protein sorting in the synaptic vesicle life cycle

At early stages of differentiation neurons already contain many of the components necessary for synaptic transmission. However, in order to establish fully functional synapses, both the pre- and postsynaptic partners must undergo a process of maturation. At the presynaptic level, synaptic vesicles (SVs) must acquire the highly specialized complement of proteins, which make them competent for efficient neurotransmitter release. Although several of these proteins have been characterized and linked to precise functions in the regulation of the SV life cycle, a systematic and unifying view of the mechanisms underlying selective protein sorting during SV biogenesis remains elusive. Since SV components do not share common sorting motifs, their targeting to SVs likely relies on a complex network of protein-protein and protein-lipid interactions, as well as on post-translational modifications. Pleiomorphic carriers containing SV proteins travel and recycle along the axon in developing neurons. Nevertheless, SV components appear to eventually undertake separate trafficking routes including recycling through the neuronal endomembrane system and the plasmalemma. Importantly, SV biogenesis does not appear to be limited to a precise stage during neuronal differentiation, but it rather continues throughout the entire neuronal lifespan and within synapses. At nerve terminals, remodeling of the SV membrane results from the use of alternative exocytotic pathways and possible passage through as yet poorly characterized vacuolar/endosomal compartments. As a result of both processes, SVs with heterogeneous molecular make-up, and hence displaying variable competence for exocytosis, may be generated and coexist within the same nerve terminal.

Morphological evidence for functional capsaicin receptor expression and calcitonin gene-related peptide exocytosis in isolated peripheral nerve axons of the mouse

Rat sciatic nerve axons express capsaicin, proton and heat sensitivity and respond to stimulation with a Ca2+-dependent and graded calcitonin gene-related peptide (CGRP) release. In this study we demonstrate that similar functions, including capsaicin-induced CGRP release, are to be found in the desheathed sciatic nerve of the mouse. We have morphologically investigated the mechanisms of this axonal release in regions away from the active zones of synapses. Capsaicin receptor 1 (TRPV1) and CGRP immunostaining was performed using electron microscopic visualization. TRPV1 was identified in the axoplasm and inside vesicles--presumably on axonal transport--as well as in considerable quantity in the axonal plasma membrane of unmyelinated nerve fibers. Most of the unmyelinated axons were immunopositive for CGRP and in unstimulated nerves CGRP-containing vesicles almost entirely filled the axoplasm. After capsaicin stimulation (10(-6) M for 5 min), the fibers appeared depleted of CGRP with only few vesicles remaining as well as some residual staining of the axoplasm. In addition a large number of vesicles were fused with the axonal membrane, forming classical exocytotic figures--the omega structures--lined with CGRP immunoreactive product. These results present morphological evidence for the distribution of TRPV1 along unmyelinated axons in peripheral nerve and also provide the first demonstration of vesicular neuropeptide exocytosis along unmyelinated axons in peripheral nerve.

Distribution and intraneuronal trafficking of a novel member of the chromogranin family, NESP55, in the rat peripheral nervous system

NESP55 (neuroendocrine secretory protein of M(r) 55000) is a novel member of the chromogranin family. In the present study, we have investigated the distribution, axonal transport and proteolytic processing of NESP55 in the peripheral nervous system. The amount of NESP55 immunoreactivity in adrenal gland was more than 240 times higher than that in the vas deferens. Double or triple immunostaining demonstrated that NESP55 immunoreactivity was highly co-localized with tyrosine hydroxylase immunoreactivity in bundles of thin axons and postganglionic sympathetic neurons; that NESP55 immunoreactivity also co-existed with vesicular acetylcholine transporter immunoreactivity in large-sized axons in sciatic nerves, and that NESP55 immunoreactivity overlapped with calcitonin gene-related peptide immunoreactivity in some large-sized axons, but NESP55 immunoreactivity was not detected in sensory neurons. Strong NESP55 immunoreactivity was found in cell bodies and axons, but it was not detectable in any terminal region by immunohistochemistry. In crush-operated sciatic nerves, NESP55 immunoreactivity could be found as early as 1 h after operation, and accumulated amounts increased substantially with time. However, NESP55 immunoreactivity was only observed in axons proximal to the crush, but none or very little distal to the crush, which was consistent with the data from radioimmunoassay. Finally, extracts of the normal and crushed sciatic nerve and vas deferens were subjected to high-performance liquid chromatography followed by radioimmunoassay. The results indicate that NESP55 is processed slowly to small peptides (GAIPIRRH) during axonal transport. NESP55 immunoreactivity was only detected in axons proximal to the crush. The data in the present study indicate that NESP55 immunoreactivity is widely distributed in adrenergic, cholinergic, and peptidergic neurons, but not in sensory neurons, and that this peptide is anterogradely, but not retrogradely, transported with fast axonal transport and slowly processed to smaller peptides during axonal transport in the peripheral nervous system.

Fast axonal transport of the vesicular acetylcholine transporter (VAChT) in cholinergic neurons in the rat sciatic nerve

The sciatic nerve, as a part of the peripheral nervous system (PNS), has been used to study axonal transport for decades. It contains motor, sensory as well as autonomic axons. The present study has concentrated on the axonal transport of the synaptic vesicle acetylcholine transporter (VAChT), using the "stop-flow/nerve crush" method. After blocking fast axonal transport by means of a crush, distinct accumulations of various synaptic vesicle proteins, including VAChT, and peptides developed during the first hour after crush-operation and marked increases were observed up to 8 h post-operative. Semiquantitative analysis, using cytofluorimetric scanning (CFS) of immuno-incubated sections, revealed a rapid rate of accumulation proximal to the crush, and that the ratio between distal accumulations (organelles in retrograde transport) and proximal accumulations (organelles in anterograde transport) was about 40%. Most synaptic vesicle proteins were colocalized in the axons proximal to the crush. VAChT-immunoreactive axons were also immunoreactive for choline acetyltransferase (ChAT). Autonomic axons with VAChT also contained VIP-LI. The results demonstrate (1) that VAChT, as well as other synaptic vesicle proteins, is transported with fast axonal transport in motor axons as well as in autonomic post-ganglionic neurons in this nerve, (2) VAChT colocalized in motor axons with SV2 as well as with synaptophysin, indicating storage in the same axonal particle, (3) in the autonomic postganglionic sympathetic cholinergic fibres, VAChT colocalized with VIP, but VIP-LI was present in rather large granular structures while VAChT-LI was present mostly as small granular elements, (4) in motor as well as in autonomic axons ChAT-LI was present in VAChT-positive axons, and (5) the ratio of recycling (retrogradely accumulated) VAChT-IR was about 40%, in contrast to the recycling fraction of synaptophysin that was about 70%.

Rab3A immunolocalization in the mammalian vestibular end-organs during development and comparison with synaptophysin expression

Rab proteins are essential for membrane vesicle docking and fusion and for transport vesicle formation at the presynaptic membrane, a step in the release of neurotransmitters. The vestibular sensory epithelia contain three types of synapses: afferent terminals, efferent endings and possible synaptic contacts between the apex of the afferent nerve calyces and the sensory cells. We report an immunocytochemical codetection of rab3A and synaptophysin in the vestibular end-organs of mouse, between fetal day 14 and adult, and of rat during the postnatal development. During mouse fetal development, rab3A appeared in afferent neurites on F16, and in sensory cells on F19. This was respectively two and five days later than the appearance of synaptophysin-IR in the same compartments. During the late postnatal development and in the adult sensory epithelia, rab3A and synaptophysin were strongly detected in nerve terminals of efferent and possibly afferent nature and in the upper part of the nerve calyces. The presence of rab3A in the nerve calyces is consistent with the putative secretory function of the calyx. In addition, rab3A immunostaining was also present in the sensory cells together with a faint synaptophysin-IR, that had not been described in previous reports [Scarfone, E., Demêmes, D. and Sans, A. J. Neurosci., 11 (1991) 1173-1181.]. The presence of these two proteins in the sensory cells supports the existence of a synaptic vesicle cycle in these cells.

Axonal transport of synaptic vesicle proteins in the rat optic nerve

The optic nerve, as a part of the central nervous system (CNS), has been used to study axonal transport for decades. The present study has concentrated on the axonal transport of synaptic vesicle proteins in the optic nerve, using the "stop-flow/nerve crush" method. After blocking fast axonal transport, distinct accumulations of synaptic vesicle proteins developed during the first hour after crush-operation and marked increases were observed up to 8 h postoperative. Semiquantitative analysis, using cytofluorimetric scanning (CFS) of immunoincubated sections, revealed that the ratio between distal accumulations (organelles in retrograde transport) and proximal accumulations (organelles in anterograde transport) was much higher (up to 80-90%) for the transmembrane proteins than that for surface adsorbed proteins (only 10-20%). The pattern of axonal transport in the optic nerve was comparable to that in the sciatic nerve. However, clathrin and Rab3a immunoreactivities were accumulated in much lower amounts than that in the sciatic nerve. Most synaptic vesicle proteins were colocalized in the axons proximal to the crush. A differential distribution of synaptobrevin I and II, however, was observed in the optic nerve axons; synaptobrevin I was present in large-sized axons, while synaptobrevin II immunoreactivity was present in most axons, including the large ones. The two isoforms were, thus, partially colocalized. The results demonstrate that (1) cytofluorimetric scanning techniques could be successfully used to study axonal transport not only in peripheral nerves, but also in the CNS; (2) synaptic vesicles are transported with fast axonal transport in this nerve; and (3) some differences were noted compared with the sciatic nerve, especially for Rab3a and clathrin.

Synaptic vesicle proteins in cells of the sympathoadrenal lineage

The cells of sympathoadrenal lineage display different characteristics after differentiation, although they stem from the same neural crest precursor during embryonic development. In the present study we compared the distribution patterns of several synaptic vesicle proteins in the superior cervical ganglion (SCG) and the adrenal medulla. Using indirect immunofluorescence combined with confocal laser scanning microscopy, it was observed that antisera against integral synaptic vesicle membrane proteins (SV2, synaptotagmin I, synaptobrevin II and synaptophysin) induced strong immunoreactivities in these cells, but anti-synaptobrevin I caused only a faint fluorescence. Immunoreactivities of the synaptic vesicle-associated proteins Rab3a and SNAP25 were also observed in the cells. Synapsin-Ia-reactive material appeared absent from chromaffin cells but present in small amounts in sympathetic neurons in the SCG and iris terminals. On the other hand, synapsin IIa immunoreactive material was strong in most SCG neurons and in adrenergic iris terminals. The neural specific clatrin light chain was detected in the SCG cells and in ganglion cells of the adrenal, but only weak traces could be observed in chromaffin cells. One of the vesicular monoamine transmitter transporters, VMAT2, which is expressed in catecholamine neurons in the brain stem, was observed in most cells in the SCG and also in groups of cells in the adrenal medulla, where the VMAT2-positive small chromaffin cells were PNMT-negative. SIF cells in the SCG contained most of the synaptic vesicle proteins investigated. The results show that after differentiation, sympathetic neurons, SIF cells and adrenal chromaffin cells still share many vesicle proteins even though their physiology is different.