Axonal transport of synapsin I- and cholinergic synaptic vesicle-like material; further immunohistochemical evidence for transport of axonal cholinergic transmitter vesicles in motor neurons

Huntingtin-associated protein-1 is a synapsin I-binding protein regulating synaptic vesicle exocytosis and synapsin I trafficking

Huntingtin-associated protein-1 (HAP1) is involved in intracellular trafficking, vesicle transport, and membrane receptor endocytosis. However, despite such diverse functions, the role of HAP1 in the synaptic vesicle (SV) cycle in nerve terminals remains unclear. Here, we report that HAP1 functions in SV exocytosis, controls total SV turnover and the speed of vesicle fusion in nerve terminals and regulates glutamate release in cortical brain slices. We found that HAP1 interacts with synapsin I, an abundant neuronal phosphoprotein that associates with SVs during neurotransmitter release and regulates synaptic plasticity and neuronal development. The interaction between HAP1 with synapsin I was confirmed by reciprocal co-immunoprecipitation of the endogenous proteins. Furthermore, HAP1 co-localizes with synapsin I in cortical neurons as discrete puncta. Interestingly, we find that synapsin I localization is specifically altered in Hap1(-/-) cortical neurons without an effect on the localization of other SV proteins. This effect on synapsin I localization was not because of changes in the levels of synapsin I or its phosphorylation status in Hap1(-/-) brains. Furthermore, fluorescence recovery after photobleaching in transfected neurons expressing enhanced green fluorescent protein-synapsin Ia demonstrates that loss of HAP1 protein inhibits synapsin I transport. Thus, we demonstrate that HAP1 regulates SV exocytosis and may do so through binding to synapsin I. The Proposed mechanism of synapsin I transport mediated by HAP1 in neurons. HAP1 interacts with synapsin I, regulating the trafficking of synapsin I containing vesicles and/or transport packets, possibly through its engagement of microtubule motors. The absence of HAP1 reduces synapsin I transport and neuronal exocytosis. These findings provide insights into the processes of neuronal trafficking and synaptic signaling.

Mechanistic logic underlying the axonal transport of cytosolic proteins

Proteins vital to presynaptic function are synthesized in the neuronal perikarya and delivered into synapses via two modes of axonal transport. While membrane-anchoring proteins are conveyed in fast axonal transport via motor-driven vesicles, cytosolic proteins travel in slow axonal transport via mechanisms that are poorly understood. We found that in cultured axons, populations of cytosolic proteins tagged to photoactivatable GFP (PAGFP) move with a slow motor-dependent anterograde bias distinct from both vesicular trafficking and diffusion of untagged PAGFP. The overall bias is likely generated by an intricate particle kinetics involving transient assembly and short-range vectorial spurts. In vivo biochemical studies reveal that cytosolic proteins are organized into higher order structures within axon-enriched fractions that are largely segregated from vesicles. Data-driven biophysical modeling best predicts a scenario where soluble molecules dynamically assemble into mobile supramolecular structures. We propose a model where cytosolic proteins are transported by dynamically assembling into multiprotein complexes that are directly/indirectly conveyed by motors.

Congenital myasthenic syndromes: progress over the past decade

Congenital myasthenic syndromes (CMS) stem from defects in presynaptic, synaptic basal lamina, and postsynaptic proteins. The presynaptic CMS are associated with defects that curtail the evoked release of acetylcholine (ACh) quanta or ACh resynthesis. Defects in ACh resynthesis have now been traced to mutations in choline acetyltransferase. A basal lamina CMS is caused by mutations in the collagenic tail subunit (ColQ) of the endplate species of acetylcholinesterase that prevent the tail subunit from associating with catalytic subunits or from becoming inserted into the synaptic basal lamina. Most postsynaptic CMS are caused by mutations in subunits of the acetylcholine receptor (AChR) that alter the kinetic properties or decrease the expression of AChR. The kinetic mutations increase or decrease the synaptic response to ACh and result in slow- and fast-channel syndromes, respectively. Most low-expressor mutations reside in the AChR epsilon subunit and are partially compensated by residual expression of the fetal type gamma subunit. In a subset of CMS patients, endplate AChR deficiency is caused by mutations in rapsyn, a molecule that plays a critical role in concentrating AChR in the postsynaptic membrane.

The spectrum of congenital myasthenic syndromes

The past decade saw remarkable advances in defining the molecular and genetic basis of the congenital myasthenic syndromes. These advances would not have been possible without antecedent clinical observations, electrophysiologic analysis, and careful morphologic studies that pointed to candidate genes or proteins. For example, a kinetic abnormality of the acetylcholine receptor (AChR) detected at the single channel level pointed to a kinetic mutation in an AChR subunit; endplate AChR deficiency suggested mutations residing in an AChR subunit or in rapsyn; absence of acetylcholinesterase (AChE) from the endplate predicted mutations in the catalytic or collagen-tailed subunit of this enzyme; and a history of abrupt episodes of apnea associated with a stimulation dependent decrease of endplate potentials and currents implicated proteins concerned with ACh resynthesis or vesicular filling. Discovery of mutations in endplate-specific proteins also prompted expression studies that afforded proof of pathogenicity, provided clues for rational therapy, lead to precise structure function correlations, and highlighted functionally significant residues or molecular domains that previous systematic mutagenesis studies had failed to detect. An overview of the spectrum of the congenital myasthenic syndromes suggests that most are caused by mutations in AChR subunits, and particularly in the epsilon subunit. Future studies will likely uncover new types of CMS that reside in molecules governing quantal release, organization of the synaptic basal lamina, and expression and aggregation of AChR on the postsynaptic junctional folds.

Cytoplasmic dynein conversion at a crush injury in rat peripheral axons

Cytoplasmic dynein is a motor for retrograde axonal transport for movement of membranous organelles toward the neuronal cell body. However, cytoplasmic dynein is synthesized in the cell body and conveyed along the axon to nerve terminals. To characterize the axonal transport of cytoplasmic dynein in relation to synaptic vesicles and other membrane compartments, immunocytochemical and cytofluorimetric scanning analyses of crush-operated rat sciatic nerves were performed. Distal to the crush, the kinetics of dynein accumulation were consistent with its role in the retrograde transport of membranous organelles. During the initial 3 hr after crush, only small amounts of dynein-immunoreactive material accumulated proximal to the crush. This is consistent with metabolic labeling studies showing that most of the dynein moving in the anterograde direction is in the slow component of axonal transport. Thereafter, the rate of proximal accumulation of dynein increased, and by 8 hr postcrush a large amount of dynein immunoreactivity was observed. This accelerated accumulation may be due to recruitment of dynein from slow component b onto organelles proximal to the crush. Double labeling demonstrated that dynein immunoreactivity colocalized with synaptophysin, a transmembrane protein found in small, clear synaptic vesicles. In contrast, dynein immunoreactivity did not colocalize well with calcitonin gene-related peptide (CGRP), a peptide matrix marker for some large dense-cored vesicles. Finally, dynein immunoreactivity colocalized with the anterograde transport motor kinesin both proximal and distal to a crush, suggesting that kinesin may carry some dynein-containing membrane compartments during fast anterograde axonal transport.

Influence of the target on distribution and functioning of the varicosities of Helix pomatia metacerebral cell C1 in dissociated cell culture

The serotonergic metacerebral giant cell (C1) of Helix pomatia was isolated with its bifurcate axon and plated in culture under five conditions: (i) with no target; (ii) with the appropriate target B2 near the stump of the bigger branch (CBC); (iii) with B2 near the stump of the smaller branch (CC); (iv) with a wrong target (C3) near the stump of the CBC branch and (v) with B2 and C3 positioned near the CBC and CC stump, respectively. The counting of anti-serotonin antibody-labelled varicosities of the C1 neuron showed that the presence of the appropriate target in either axonal domain both down-regulated the number of varicosities of the contralateral neuritic field, and increased their average size, whereas the wrong target induced an overall reduction of the number of C1 neuron varicosities, and inhibited the evoked transmitter release. The action potential-evoked calcium concentration increase in the neuritic terminals of the C1 neuron cultured alone, or in presence of the appropriate target, reached a value significantly higher than that reached in presence of the wrong target. These results provide evidence that the postsynaptic neuron regulates both morphological and functional development of presynaptic terminals.

Changing distribution patterns of synaptophysin-immunoreactive structures in the human dorsal striatum of the fetal brain

Within the striatum two compartments, matrix and patches, can be distinguished by differences in the expression of neuroactive substances, afferent and efferent connections and time of neurogenesis. The present study was done to demonstrate the pattern of synaptophysin (SYN) expression which is indicative of synaptogenesis in the human fetal striatum (15th-32nd weeks of gestation) with special reference to developmental changes. From the 15th to the 22nd gestational weeks an intense diffuse SYN immunolabelling of striatal patches is observed. In the matrix SYN-immunoreactive fiber bundles are seen until the 20th week. Thereafter, the matrix is nearly devoid of SYN-immunoreactive structures. From the 28th week of gestation the matrix contains diffuse SYN immunoreactivity which gradually becomes as intense as that of the patches. The latter, thus, can no longer be delineated in the 30th week. The results show that fibrous SYN immunolabelling most probably indicating intra-axonal transport of synaptic vesicles can only be observed during the first half of gestation. Moreover, it becomes obvious that the patch compartment can selectively be visualized by anti-SYN until the 28th week. This pattern may correspond to the early dopaminergic innervation from the substantia nigra which is known to reach the developing patches. From the 28th week a transition from patchy to diffuse immunolabelling is seen. The increase in matrix labelling may be due to the occurrence of new neuronal contacts. The changeover from patchy to homogeneous SYN immunolabelling takes place distinctly earlier than changes in the distribution of other neuroactive substances described before.

Axonal transport of ribonucleoprotein particles (vaults)

RNA was previously shown to be transported into both dendritic and axonal compartments of nerve cells, presumably involving a ribonucleoprotein particle. In order to reveal potential mechanisms of transport we investigated the axonal transport of the major vault protein of the electric ray Torpedo marmorata. This protein is the major protein component of a ribonucleoprotein particle (vault) carrying a non-translatable RNA and has a wide distribution in the animal kingdom. It is highly enriched in the cholinergic electromotor neurons and similar in size to synaptic vesicles. The axonal transport of vaults was investigated by immunofluorescence, using the anti-vault protein antibody as marker, and cytofluorimetric scanning, and was compared to that of the synaptic vesicle membrane protein SV2 and of the beta-subunit of the F1-ATPase as a marker for mitochondria. Following a crush significant axonal accumulation of SV2 proximal to the crush could first be observed after 1 h, that of mitochondria after 3 h and that of vaults after 6 h, although weekly fluorescent traces of accumulations of vault protein were observed in the confocal microscope as early as 3 h. Within the time-period investigated (up to 72 h) the accumulation of all markers increased continuously. Retrograde accumulations also occurred, and the immunofluorescence for the retrograde component, indicating recycling, was weaker than that for the anterograde component, suggesting that more than half of the vaults are degraded within the nerve terminal. High resolution immunofluorescence revealed a granular structure-in accordance with the biochemical characteristics of vaults. Of interest was the observation that the increase of vault immunoreactivity proximal to the crush accelerated with time after crushing, while that of SV2-containing particles appeared to decelerate, indicating that the crush procedure with time may have induced perikaryal alterations in the production and subsequent export to the axon of synaptic vesicles and vault protein. Our data show that ribonucleoprotein-immunoreactive particles can be actively transported within axons in situ from the soma to the nerve terminal and back. The results suggest that the transport of vaults is driven by fast axonal transport motors like the SV2-containing vesicles and mitochondria. Vaults exhibit an anterograde and a retrograde transport component, similar to that observed for the vesicular organelles carrying SV2 and for mitochondria. Although the function of vaults is still unknown studies of the axonal transport of this organelle may reveal insights into the mechanisms of cellular transport of ribonucleoprotein particles in general.

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%.

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.

Accumulation of synaptic vesicle proteins and cytoskeletal specializations at the peripheral node of Ranvier

Nodes of Ranvier of peripheral nerve fibres represent repetitive physiological axon constrictions. The nodal attenuation of the axon cylinder is expected to facilitate eliciting axon potentials. But as revealed by immunocytochemical analysis of synaptic vesicle proteins such as SV2 and synaptophysin, nodes are also sites of accumulation of the synaptic vesicle membrane compartment. Results from our studies and other laboratories suggest that the local nodal retardation of the axonally transported synaptic vesicle membrane compartment serves membrane processing and/or turnover. Nodes of Ranvier as well as incisures of Schmidt-Lanterman are rich in filamentous actin and can easily be depicted by fluoresceinated phalloidin. At the node and paranode phalloidin fluorescence appears to be mainly associated with the Schwann cell compartment. Immunofluorescence demonstrates that this compartment also contains myosin and spectrin. The nodal contents in actin and myosin may be effective in actively constricting the axon cylinder at both the node of Ranvier and the Schmidt-Lanterman incisures. This hypothesis is discussed in the light of the nodal cytoskeletal specializations of the axon cylinder and the ensheathing Schwann cell.

Localization of synapsin I in normal fibers and regenerating axonal sprouts of the rat sciatic nerve

The localization of synapsin I, a synaptic vesicle-associated protein, was investigated immunocyto-chemically in normal nerve fibers and regenerating axonal sprouts following crush-injuries to the rat sciatic nerve. In normal myelinated axons, weak synapsin I immunoreactivity was found in the axoplasmic/smooth endoplasmic domains, but not in the cytoskeletal domains comprising neurofilaments and microtubules. In non-myelinated axons without dense cytoskeletal structures, moderate immunoreactivity was distributed diffusely throughout the axoplasm. In the crush-injured nerves, intense synapsin I immunoreactivity was demonstrated by light microscopy in early regenerating sprouts emerging from nodes of Ranvier. These nodal sprouts subsequently elongated as regenerating axons through the space between the basal lamina and the myelin sheath (or Schwann cell plasma membrane). Intense synapsin I immunoreactivity was also found in the growth cones of such long regenerating axons. Electron microscopy revealed that synapsin I immunoreactivity was associated mainly with vesicular organelles in the nodal sprouts and growth cones of regenerating axons. Long regenerating axons exhibited no synapsin I immunoreactivity in the shaft, which contained an abundance of neurofilaments. However, vesicle accumulations remaining in the periphery of the shaft still exhibited intense synapsin I immunoreactivity. Thus, it can be concluded that synapsin I is localized at especially high density in the domains comprising vesicular organelles, which are characteristic of early nodal sprouts, as well as in growth cones of regenerating axons. These findings, together with the proposed functions of synapsin I investigated in other studies, suggest that synapsin I may play important roles in vesicular dynamics including the translocation of vesicles to the plasma membrane in sprouts and growth cones of regenerating axons.

Fast and slow axonal transport-different methodological approaches give complementary information: contributions of the stop-flow/crush approach

This 'minireview' describes experiments in short term crush operated rat nerves, to study endogenous substances in anterograde and retrograde fast axonal transport. Immunofluorescence was used to recognize transported antigens, and cytofluorimetric scanning was employed to quantitate different antigens which had accumulated proximal and distal to the crushes. Vesicle membrane components p38 (synaptophysin) and SV2 accumulated on both sides of a crush. This was expected from a number of studies from different laboratories. Surface associated molecules, however, like synapsins and rab3a, have been studied by other groups with biochemical methods, and suggested to be transported with slow transport. The crush method, however, revealed that a considerable fraction of these two substances are transported with the fast transport system, and, thus, associated with fast transported organelles in the living neuron. Evidently, more than one technique is required to give a more complete picture of intraneuronal transport related events.

GAP 43-like immunoreactivity in normal adult rat sciatic nerve, spinal cord, and motoneurons: axonal transport and effect of spinal cord transection

Using immunofluorescence and cytofluorimetric scanning techniques in the rat, the fast anterograde and retrograde axonal transport of growth-associated protein-43-like immunoreactivity in normal sciatic nerves, and after spinal cord transection in the lower thoracic region, were investigated. Spinal roots and motor endplates in the peroneal muscles were also studied. For comparison, anti-synaptophysin (p38) was used. In intact adult animals, the amounts of immunoreactive growth-associated protein-43 increased linearly, both proximally and distally to the crush site, between 1 and 24 h after crushing the sciatic nerve. The accumulations were present in thick as well as in thin axons. Distal accumulations in the sciatic nerve were about 40-60% of the proximal amounts, indicating a recycling of organelles with growth-associated protein-43-like immunoreactivity. During the week after spinal cord transection, no clear changes were observed; the anterograde transport of growth-associated protein-43-like immunoreactivity showed a tendency to decrease at day 1 and then a tendency to increase, reaching 120% of control at seven days (not significant). Transported p38-like immunoreactivity showed similar but smaller changes. In the lumbar spinal cord gray matter many nerve terminals with growth-associated protein-43-like immunoreactivity were seen in intact animals. After spinal transection, these terminals gradually decreased, suggesting that they belonged to descending pathways. However, p38-positive terminals were not obviously decreased. After crushing ventral and dorsal roots, accumulations of pf growth-associated protein-43-like immunoreactivity were present in thick axons in the ventral roots and in thin to medium-sized axons in the dorsal roots. In peroneal muscles, growth-associated protein-43-like immunoreactivity was present in some (but not all) motor endplates in all groups. These results indicate that: (i) growth-associated protein-43 is normally present in nerve terminals of many descending projections of the spinal cord; (ii) growth-associated protein-43-like immunoreactivity is expressed and bidirectionally transported in neurons (motor as well as sensory) of normal sciatic nerves; (iii) growth-associated protein-43-like immunoreactivity is present in some adult motor endplates; and (iv) inhibited supraspinal input causes minor, if any, alterations--paralleled by p38--in axonal transport of growth-associated protein-43-like immunoreactivity.

Distribution of GAP-43 in relation to CGRP and synaptic vesicle markers in rat skeletal muscles during development

GAP 43 in nerve terminal structures of rat skeletal muscles, was investigated during postnatal development using immunofluorescence and confocal laser scanning microscopy. Comparison with synaptophysin, synapsin, SV2, CGRP, SP and NF was done in double immunoincubation studies. GAP 43-like immunoreactivity (LI) was demonstrated in preterminal axons and motor endplates in all age groups (from E18 to adult), although the intensity of immunofluorescence was considerably higher in the younger rats. The outgrowing nerve sprouts in E18 muscles were strongly GAP 43-positive. The intensity decreased with increasing age, but even in adult animals GAP 43-LI was present in some p38- or SV2-positive endplates. GAP 43-LI was also present in muscle spindles and preterminal nerve branches, and likewise decreased with age. Perivascular nerve terminals (around arteries mainly) were, however, strong in GAP 43-LI during both development and adulthood. GAP 43-LI was strong, and present in both small and large granules. SP-LI was observed in a few thin, presumably sensory, axons around vessels, which also contained a few GAP 43-positive large granules. Most of the strongly GAP 43-positive terminals around vessels were probably autonomic postganglionic terminals. The results suggest that GAP 43, in addition to development and regeneration, may play a significant role also in normal adult rats, especially in perivascular nerve terminals, possibly connected with a high potential for plasticity in this kind of nerve terminals.

Axonal transport of dopamine-containing vesicles labelled in vivo with [3H]reserpine

Axonal transport of the vesicular monoamine transporter was assayed in the rat brain by in vivo binding of the specific ligand [3H]reserpine. Because of the marked localization of reserpine binding sites in dopaminergic cell bodies and nerve terminals, the dopaminergic nigrostriatal pathway was chosen for the study of the axonal transport of the monoamine carrier present in the membrane of synaptic vesicles. When labelled reserpine was injected into the substantia nigra, a delayed accumulation of radioactivity in the ipsilateral striatum was observed approximately 4 h after the injection. Similarly, injection into the right striatum was followed by a substantial accumulation of radioactivity in the ipsilateral substantia nigra, which was prevented by peripheral injection of unlabelled reserpine or tetrabenazine. This process was rapid and dependent on microtubules. In senescent rats, the amount of retrogradely transported [3H]reserpine was significantly reduced. These results demonstrate that labelled reserpine may be used to monitor in vivo fast axonal transport in central neurons.

Influence of spinal cord transection on the presence and axonal transport of CGRP-, chromogranin A-, VIP-, synapsin I-, and synaptophysin-like immunoreactivities in rat motor nerve

Using immunofluorescence and cytofluorimetric scanning (CFS), we investigated the short-term (1-7 days) influence of lower thoracic spinal cord transection on lumbar motor neurons. The content of calcitonin gene-related peptide- (CGRP) like immunoreactivity (LI), chromogranin A (Chr A)-LI, vasoactive intestinal polypeptide (VIP)-LI, Syn I-LI, and synaptophysin (p38)-LI in motor perikarya, and the anterograde and retrograde axonal transport of these substances in the sciatic nerve, were studied in nerve crush (6 h) experiments. During the week after transection, CGRP-LI in perikarya decreased, whereas Chr A-LI increased. VIP-LI, co-localized with Chr A-LI in motor perikarya, did not change after transection. The antero- and retrograde transport of CGRP-LI in the sciatic nerve, occurring in both motor and sensory axons, appeared unchanged in cytofluorimetric scanning (CFS) graphs, but the microscopical picture clearly showed that large motor axons had a decreased content of CGRP-LI at 3 and 7 days posttransection, whereas thinner axons were unchanged in fluorescence intensity. The anterograde transport of Chr A-LI, present in both motor and postganglionic adrenergic axons, was decreased 1 and 3 days after lesion, but returned to control by day 7. There was a marked decrease in anterograde transport of VIP-LI, present mainly in postganglionic sympathetic axons, at day 3, but at 7 days transport was normal. The amounts of transported p38, the synaptic vesicle marker, were in the normal range during the whole period. Syn I-LI accumulation anterogradely was somewhat decreased at 3 and 7 days posttransection, and at 1 day the retrograde accumulation was significantly increased. The results suggest that removal of supraspinal input to intact lower motor neurons causes alterations in metabolism and axonal transport of organelle-associated substances, partly probably related to the complex pattern of transmitter leakage from degenerating, descending nerve terminals. These alterations appear to take place also in postganglionic sympathetic neurons in the sciatic nerve, that originate in the lumbar sympathetic chain.

Organelles in fast axonal transport. What molecules do they carry in anterograde vs retrograde directions, as observed in mammalian systems?

The present minireview describes experiments carried out, in short-term crush-operated rat nerves, using immunofluorescence and cytofluorimetric scanning techniques to study endogenous substances in anterograde and retrograde fast axonal transport. Vesicle membrane components p38 (synaptophysin) and SV2 are accumulating on both sides of a crush, but a larger proportion of p38 (about 3/4) than of SV2 (about 1/2) is recycling toward the cell body, compared to the amount carried with anterograde transport. Matrix peptides, such as CGRP, ChRA, VIP, and DBH are recycling to a minor degree, although only 10-20% of surface-associated molecules, such as synapsins and kinesin, appear to recycle. The described methodological approach to study the composition of organelles in fast axonal transport, anterograde as compared to retrograde, is shown to be useful for investigating neurobiological processes. We make use of the "in vivo chromatography" process that the fast axonal transport system constitutes. Only substances that are in some way either stored in, or associated with, transported organelles can be clearly observed to accumulate relative to the crush region. Emphasis in this paper was given to the synapsins, because of diverging results published concerning the degree of affiliation with various neuronal organelles. Our previously published results have indicated that in the living axons the SYN I is affiliated with mainly anterogradely fast transported organelles. Therefore, some preliminary, previously unpublished results on the accumulations of the four different synapsins (SYN Ia, SYN Ib, SYN IIa, and SYN IIb), using antisera specific for each of the four members of the synapsin family, are described. It was found that SYN Ib clearly has a stronger affiliation to anterogradely transported organelles than SYN Ia, and that both SYN IIa and SYN IIb are bound to some degree to transported organelles.

Simultaneous purification of the neuroproteins synapsin I and synaptophysin

A procedure for the simultaneous purification of synapsin I and synaptophysin from calf brain was developed. Demyelinated membranes were extracted with 2% Triton X-100 and 2 M KCl. The extracted proteins were separated by weak cation-exchange chromatography on carboxymethyl-Sepharose Fast Flow. Synaptophysin was finally purified by preparative sodium dodecyl sulphate-polyacrylamine gel electrophoresis and synapsin I by affinity chromatography using a calmodulin-Sepharose column. The recovery obtained was 40 micrograms/g in brain for synaptophysin and 25 micrograms/g in brain for synapsin I.

The synapsins and the regulation of synaptic function

Synapsin I and II are a family of synaptic vesicle-associated phosphoproteins involved in the short-term regulation of neurotransmitter release. In this review, we discuss a working model for the molecular mechanisms by which the synapsins act. We propose that synapsin I links synaptic vesicles to actin filaments in the presynaptic nerve terminal and that these interactions are modulated by the reversible phosphorylation of synapsin I through various signal transduction pathways. The high degree of homology between the synapsins suggests that some of the functional properties of synapsin I are also shared by synapsin II.

Quantification of axonally transported material using cytofluorimetric scanning

This paper describes in detail a cytofluorimetric scanning technique used for studying amounts of material axonally transported in antero- and retrograde direction in peripheral nerves. Operating procedures, preparation of tissues and instrumental set-up are described. The basis for quantification of material in a nerve section treated for immunofluorescence is discussed. The reliability of the method has been tested by comparing results with biochemical data. There are several advantages of the technique. (1) Many different substances can be studied in one single nerve segment, thus reducing biological variation and costs. (2) Both morphological data and quantitative figures can be obtained; following scanning the section can be photographed. (3) The method can also be used on studies in the central nervous system and on tissue cultures, since it is possible to scan on single axons or bundles of fibres.

Alignment and intracytoplasmic disintegration of synaptic vesicles in the brain cortex

Perfusion fixation with highly concentrated aldehydes suggests that the synaptic vesicles undergo disintegration within the presynaptic ending upon touching the presynaptic membrane rather than being released by exocytosis into the intersynaptic cleft. Three factors have been explored in order to inquire further into the possible significance of the findings: (a) fixative concentration; (b) physiological activity; (c) cell depolarization. The transformation of the vesicles into amorphous, electron-dense material was observed in all experiments in all synapses, including those fixed with the lowest concentration of aldehydes. Besides, after acute ischemia and perfusion of excitatory and depolarizing pharmacological agents, the synaptic vesicles were seen to conflue upon the intersynaptic cleft in well-aligned rows. It was also found that the vesicles flow post mortem towards the intersynaptic cleft with absolute specificity.

Calcitonin gene-related peptide and chromogranin A: presence and intra-axonal transport in lumbar motor neurons in the rat, a comparison with synaptic vesicle antigens in immunohistochemical studies

The presence and intra-axonal transport of calcitonin gene-related peptide and chromogranin A were investigated in motor neurons belonging to the rat sciatic nerve. Their co-localization with markers of cholinergic organelles (SV2, p38, and synapsin I) was also investigated, using immunofluorescence techniques, including double labelling experiments. It was found that motor perikarya in the lumbar spinal cord contained calcitonin gene-related peptide-like immunoreactivity and chromogranin A-like immunoreactivity, and probably also caligulin-like immunoreactivity, located in the Nissl substance of the cytoplasm. Also, some SV2 (detected by the monoclonal antibody 10H) was present in some motor neuron perikarya, but most often these were devoid of SV2 and p38, as well as of synapsin I-like immunoreactivity. These three antigens were, on the other hand, concentrated in nerve terminals in the entire gray substance of the spinal cord. In the ventral root, after crushing, calcitonin gene-related peptide, chromogranin A, synapsin I, SV2, p38 and caligulin-like immunoreactivity accumulated in thick and medium-sized axons proximal to the crush, while only antisera against SV2 and p38 labelled accumulated material distal to the crush. In the sciatic nerve, the same essential picture was observed as in the ventral root, but here two other nervous components were also present in the normal sciatic nerve, i.e. peripheral branches of the sensory system and axons of the sympathetic system. By various denervation procedures, it was demonstrated that most calcitonin gene-related peptide-like immunoreactivity and almost all chromogranin A-like immunoreactivity, accumulating in thick axons proximally, emanated from the ventral root. Thin and medium-sized axons originated from the sensory and sympathetic systems and contributed to accumulations both proximally and distally to the crush. Synapsin I-like immunoreactive material accumulated only proximal to the crush, while SV2 and p38-like material accumulated bidirectionally in axons of all sizes. In motor endplates of the rat diaphragm and gastrocnemic muscle, no calcitonin gene-related peptide-like material was observed. However, some chromogranin A-like immunoreactivity was present, in addition to large amounts of synapsin I-like, p38-like and SV2-like material, which had a finely granular appearance and was concentrated near the presynaptic membrane of the nerve terminal endfeet, where synaptic vesicles are known to be located.

Axonal transport of the synaptic vesicle monoamine carrier identified by [3H]dihydrotetrabenazine binding in rat sciatic nerve

The monoamine carrier of noradrenergic synaptic vesicles was assayed in the rat sciatic nerve by in vitro binding of [3H]dihydrotetrabenazine. The number of binding sites increased with time on both sides of a ligature, indicating fast anterograde and retrograde axoplasmic transports of comparable amplitude. Anterogradely and retrogradely transported binding sites had similar pharmacological properties, which were characteristic of binding to the monoamine carrier embedded within a lipid bilayer. Contrary to the secreted synaptic vesicle matrix constituents, the intrinsic monoamine carrier assayed by [3H]dihydrotetrabenazine binding is a marker adapted to retrograde transport studies.

Translocations of fodrin and its binding proteins

Fodrin, a protein related to erythrocyte spectrin, redistributes within the cell in certain situations. We compare such movements of fodrin and several fodrin binding proteins during the processes of axonal transport in neurons, and capping of surface proteins in lymphocytes. In neurons, three different populations of newly synthesized fodrin appear to be transported down the axons at different velocities corresponding to those of groups of transported proteins designated II, IV, and V. Actin, which can interact with fodrin, is transported at the velocity of group IV. Synapsin, a component of synaptic vesicles, is also reported to bind to fodrin. One population of synapsin is transported more rapidly than fodrin, at the velocity of group I: two additional populations of transported synapsin may overlap fodrin in groups II and IV. We consider possible functional associations of these different populations of fodrin and fodrin binding proteins. We note that the transport of group IV proteins resembles in certain respects the process of capping in lymphocytes, suggesting the possibility of a common mechanism. We outline one of several possible mechanisms.