Immunocytochemical localization of synaptophysin on the smooth-surfaced tubular membranes present in nerve terminal and preterminal areas in the rat cerebellar cortex

Synaptosomes: new vesicles for neuronal mitochondrial transplantation

Background

Mitochondrial dysfunction is a critical factor in the onset and progression of neurodegenerative diseases. Recently, mitochondrial transplantation has been advised as an innovative and attractive strategy to transfer and replace damaged mitochondria. Here we propose, for the first time, to use rat brain extracted synaptosomes, a subcellular fraction of isolated synaptic terminal that contains mitochondria, as mitochondrial delivery systems.

Results

Synaptosome preparation was validated by the presence of Synaptophysin and PSD95. Synaptosomes were characterized in terms of dimension, zeta potential, polydispersity index and number of particles/ml. Nile Red or CTX-FITCH labeled synaptosomes were internalized in LAN5 recipient cells by a mechanism involving specific protein–protein interaction, as demonstrated by loss of fusion ability after trypsin treatment and using different cell lines. The loading and release ability of the synaptosomes was proved by the presence of curcumin both into synaptosomes and LAN5 cells. The vitality of mitochondria transferred by Synaptosomes was demonstrated by the presence of Opa1, Fis1 and TOM40 mitochondrial proteins and JC-1 measurements. Further, synaptosomes deliver vital mitochondria into the cytoplasm of neuronal cells as demonstrated by microscopic images, increase of TOM 40, cytochrome c, Hexokinase II mitochondrial proteins, and presence of rat mitochondrial DNA. Finally, by using synaptosomes as a vehicle, healthy mitochondria restored mitochondrial function in cells containing rotenone or CCCp damaged mitochondria.

Conclusions

Taken together these results suggest that synaptosomes can be a natural vehicle for the delivery of molecules and organelles to neuronal cells. Further, the replacement of affected mitochondria with healthy ones could be a potential therapy for treating neuronal mitochondrial dysfunction-related diseases.

A subset of thalamocortical projections to the retrosplenial cortex possesses two vesicular glutamate transporter isoforms, VGluT1 and VGluT2, in axon terminals and somata

Vesicular glutamate transporter isoforms, VGluT1-VGluT3, accumulate glutamate into synaptic vesicles and are considered to be important molecules in glutamatergic transmission. Among them, VGluT2 mRNA is expressed predominantly throughout the dorsal thalamus, whereas VGluT1 mRNA is expressed in a few thalamic nuclei. In the thalamic nuclei that project to the retrosplenial cortex (RSC), VGluT1 mRNA is expressed strongly in the anterodorsal thalamic nucleus (AD), is expressed moderately in the anteroventral and laterodorsal thalamic nuclei, and is not expressed in the anteromedial thalamic nucleus. Thus, it has been strongly suggested that a subset of thalamocortical projections to RSC possesses both VGluT1 and VGluT2. In this study, double-labeled neuronal somata showing both VGluT1 and VGluT2 immunolabelings were found exclusively in the ventral region of AD (vAD). Many double-labeled axon terminals were also found in two major targets of vAD, the rostral part of the reticular thalamic nucleus and layers Ia and III-IV of the retrosplenial granular b cortex (RSGb). Some were also found in layer Ia of the retrosplenial granular a cortex (RSGa). These axon terminals contain significant amounts of both VGluTs. Because the subset of thalamocortical projections to RSC has a unique molecular basis in the glutamatergic transmission system, it might play an important role in the higher cognitive functions processed in the RSC. Furthermore, double-labeled axon terminals of a different type were distributed in RSGb and RSGa. Because they are small and the immunoreactivity of VGluT2 is significantly weaker than that of VGluT1, they seemed to be a subset of corticocortical terminals.

Clostridium perfringens epsilon toxin targets granule cells in the mouse cerebellum and stimulates glutamate release

Epsilon toxin (ET) produced by C. perfringens types B and D is a highly potent pore-forming toxin. ET-intoxicated animals express severe neurological disorders that are thought to result from the formation of vasogenic brain edemas and indirect neuronal excitotoxicity. The cerebellum is a predilection site for ET damage. ET has been proposed to bind to glial cells such as astrocytes and oligodendrocytes. However, the possibility that ET binds and attacks the neurons remains an open question. Using specific anti-ET mouse polyclonal antibodies and mouse brain slices preincubated with ET, we found that several brain structures were labeled, the cerebellum being a prominent one. In cerebellar slices, we analyzed the co-staining of ET with specific cell markers, and found that ET binds to the cell body of granule cells, oligodendrocytes, but not astrocytes or nerve endings. Identification of granule cells as neuronal ET targets was confirmed by the observation that ET induced intracellular Ca(2+) rises and glutamate release in primary cultures of granule cells. In cultured cerebellar slices, whole cell patch-clamp recordings of synaptic currents in Purkinje cells revealed that ET greatly stimulates both spontaneous excitatory and inhibitory activities. However, pharmacological dissection of these effects indicated that they were only a result of an increased granule cell firing activity and did not involve a direct action of the toxin on glutamatergic nerve terminals or inhibitory interneurons. Patch-clamp recordings of granule cell somata showed that ET causes a decrease in neuronal membrane resistance associated with pore-opening and depolarization of the neuronal membrane, which subsequently lead to the firing of the neuronal network and stimulation of glutamate release. This work demonstrates that a subset of neurons can be directly targeted by ET, suggesting that part of ET-induced neuronal damage observed in neuronal tissue is due to a direct effect of ET on neurons.

Selective reduction in synaptic proteins involved in vesicle docking and signalling at synapses in the ataxic mutant mouse stargazer

The spontaneous recessive mutant mouse stargazer has a specific and pronounced deficit in brain-derived neurotrophic factor (BDNF) mRNA expression in the cerebellum. Cerebellar granule cells, in particular, show a selective and near-total loss of BDNF. The mutation involves a defect in the calcium channel subunit Cacng2. This severely reduces expression of stargazin. A stargazin-induced failure in BDNF expression is thought to underlie the cerebellar ataxia with which the mutant presents. BDNF is known to regulate plasticity at cerebellar synapses. However, relatively little is known about the mechanism involved. We previously demonstrated that the stargazer mutation affects the phenotype of cerebellar glutamatergic neurons. Stargazer neurons have less glutamate and proportionally fewer docked vesicles at presynaptic sites than controls. In the current study, we investigate the mechanism underlying BDNF-induced synaptic changes by analyzing alterations in synaptic signalling proteins in the stargazer cerebellum. Expression levels of synaptic proteins were evaluated by measuring relative density of immunogold label over granule cell terminals in ultrathin sections from ataxic stargazer mutants compared with matched nonataxic littermates. We show that there is a selective and marked depletion in the levels of vesicle-associated proteins (synaptobrevin, synaptophysin, synaptotagmin, and Rab3a) but not of plasma membrane-associated protein (SNAP-25) in the terminals of the BDNF-deficient granule cells. Changes are restricted to the cerebellum; levels in the hippocampus are unaltered. These data suggest that the BDNF deficits in the cerebellum of stargazer affect synaptic vesicle docking by selectively altering synaptic-protein distribution and abundance.

Overexpression of heat shock proteins in pallido-nigral axonal spheroids of nonhuman aged primates

The occurrence of spheroids has been described in the globus pallidus (GP) and substantia nigra pars reticulata (SNr) of aged rhesus monkeys. Opinions vary as to the origin of spheroids. Ultrastructural and immunohistochemical analysis suggested that spheroids originate from degenerating axons or astroglia. In the present study, we have investigated the GP and SNr of aged monkeys (Macaca fascicularis and Macaca mulatta). Although immunoreactive for microtubule-associated protein (MAP) 1A, tau, amyloid precursor protein, synaptophysin and phosphorylated neurofilament, spheroids were not immunoreactive for MAP1B and MAP2. We confirmed the axonal nature of pallido-nigral spheroids in aged rhesus monkeys. Pallido-nigral spheroids have been reported to overexpress stress proteins, such as ubiquitin, alphaB-crystallin, and heat shock protein (Hsp) 27. We further evaluated the expression of Hsps in pallido-nigral spheroids. As well as being intensely immunoreactive for ubiquitin, alphaB-crystallin, Hsp27, and Hsp70, spheroids were immunoreactive for Hsp32 (heme oxygenase-1), Hsp40, Hsp60, and Hsp90. On the basis of these findings, we speculate that Hsp32-immunoreactive spheroids might be expressed as an oxidative stress response. Induction of other Hsps might play a role in protection of axons from the aggregation of neurofilament, MAPs and other proteins, and failure to protect degenerating axons might result in their proteolysis by the ubiquitin-proteasome system.

Ultrastructural evidence for a separate, small synaptic vesicle (SSV) pathway in ligated bovine splenic nerves, incubated in vitro

In sympathetic nerves the tubules of the axonal reticulum make up the immature elements of the neurosecretory apparatus. The formation of the mature large dense granules occurs via a less dense tubular intermediate, representing the maturing part. At a terminal small synaptophysin-positive vesicles are found intermingled with the dense granules. The biogenesis of these clear, small synaptic vesicles and their relationship with dense granules remains to be determined. In search for the small synaptic vesicles we undertook a careful ultrastructural examination of the axons proximal to a ligation in bovine splenic nerve incubated in vitro for 3 h. The distended axons were crowded with tubules, granulo-tubular elements and dense granules. Occasionally homogeneous clusters of small, uniform vesicles were detected. They were shown to be positive for synaptophysin and were negative for dopamine-beta-hydroxylase, a marker for the granular pathway. The clusters of small vesicles could be found in close spatial relationship with the maturing and mature elements of granular secretion. Our findings argue for the presence of two separate neurosecretory pathways in sympathetic nerves and favour the idea that both small synaptic vesicles and dense granules are a differentiation product of the axonal reticulum. This configuration can explain the biogenesis of small synaptic vesicles and dense granules both in the cell body and at the nerve terminal.

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.

Rapid recovery of structure and function of the cholinergic synapses in the cat superior cervical ganglion in vivo following stimulation-induced exhaustion

Cat superior cervical ganglia (SCG) were tetanically stimulated in vivo at 30-100 Hz until neural transmission was exhausted, and then were allowed to rest and recover. Changes in their cholinergic synapses were examined electrophysiologically and morphologically during the time of tetanic stimulation and during recovery. For morphometric analysis the presynaptic terminal was subdivided into two areas: an area directly over the active zone, termed zone-I, (bounded by a hemicircle with a diameter equivalent to the active zone length), and the remaining preterminal area, termed zone-II. In control ganglia before stimulation synaptic vesicle density in zone-I (SVD-I) averaged 90 microns-2 and the number of vesicles actually attached to the active zone (SVA) averaged about 2.5 per single profile of nerve terminal. Upon stimulation, the postganglionic potential immediately began to decline in amplitude and disappeared after 1 min of stimulation. Simultaneously, SVD-I declined to less than 35 microns-2 and SVA declined to less than 1 per section. Thereafter, stimulation was terminated and the ganglion was allowed to rest. Recovery of the postganglionic potential was monitored by stimulation at 1 Hz. The postganglionic potential reached control levels after only 1 min of rest. Likewise, the structural parameters, SVD-I and SVA, also rapidly recovered, reaching control levels after only 30 sec of rest, slightly faster than the postganglionic potential. This illustrates that stimulation-induced fatigue of transmitter output and depletion of synaptic vesicles recover to the control level at a high rate in synapses of the cat SCG with a normal supply of blood. In fact, morphological recovery may be slightly faster than electrophysiological recovery. Mechanisms of vesicle formation and migration to the presynaptic area are discussed in light of these observations.