Chromatolysis of dorsal root ganglion cells studied by cryofixation

Schwann Cell-Specific Ablation of Beclin 1 Impairs Myelination and Leads to Motor and Sensory Neuropathy in Mice

The core component of the class III phosphatidylinositol 3-kinase complex, Beclin 1, takes part in different protein networks, thus switching its role from inducing autophagy to regulating autophagosomal maturation and endosomal trafficking. While assessed in neurons, astrocytes, and microglia, its role is far less investigated in myelinating glia, including Schwann cells (SCs), responsible for peripheral nerve myelination. Remarkably, the dysregulation in endosomal trafficking is emerging as a pathophysiological mechanism underlying peripheral neuropathies, such as demyelinating Charcot-Marie-Tooth (CMT) diseases. By knocking out Beclin 1 in SCs here a novel mouse model (Becn1 cKO) is generated, developing a severe and progressive neuropathy, accompanied by involuntary tremors, body weight loss, and premature death. Ultrastructural analysis revealed abated myelination and SCs displaying enlarged cytoplasm with progressive accumulation of intracellular vesicles. Transcriptomic and histological analysis from sciatic nerves of 10-day and 2-month-old mice revealed pro-mitotic gene deregulation and increased SCs proliferation at both stages with axonal loss and increased immune infiltration in adults, well reflecting the progressive motor and sensory functional impairment that characterizes Becn1 cKO mice, compared to controls. The study establishes a further step in understanding key mechanisms in SC development and points to Beclin 1 and its regulated pathways as targets for demyelinating CMT forms.

miR-30c-5p Gain and Loss of Function Modulate Sciatic Nerve Injury-Induced Nucleolar Stress Response in Dorsal Root Ganglia Neurons

Neuropathic pain is a prevalent and debilitating chronic syndrome that is often resistant to treatment. It frequently arises as a consequence of damage to first-order nociceptive neurons in the lumbar dorsal root ganglia (DRG), with chromatolysis being the primary neuropathological response following sciatic nerve injury (SNI). Nevertheless, the function of miRNAs in modulating this chromatolytic response in the context of neuropathic pain remains unexplored. Our previous research demonstrated that the intracisternal administration of a miR-30c mimic accelerates the development of neuropathic pain, whereas the inhibition of miR-30c prevents pain onset and reverses established allodynia. In the present study, we sought to elucidate the role of miR-30c-5p in the pathogenesis of neuropathic pain, with a particular focus on its impact on DRG neurons following SNI. The organisation and ultrastructural changes in DRG neurons, particularly in the protein synthesis machinery, nucleolus, and Cajal bodies (CBs), were analysed. The results demonstrated that the administration of a miR-30c-5p mimic exacerbates chromatolytic damage and nucleolar stress and induces CB depletion in DRG neurons following SNI, whereas the administration of a miR-30c-5p inhibitor alleviates these effects. We proposed that three essential cellular responses-nucleolar stress, CB depletion, and chromatolysis-are the pathological mechanisms in stressed DRG neurons underlying neuropathic pain. Moreover, miR-30c-5p inhibition has a neuroprotective effect by reducing the stress response in DRG neurons, which supports its potential as a therapeutic target for neuropathic pain management. This study emphasises the importance of miR-30c-5p in neuropathic pain pathogenesis and supports further exploration of miRNA-based treatments.

Long-term incubation with mifepristone (MLTI) increases the spine density in developing Purkinje cells: new insights into progesterone receptor mechanisms

Cerebellar Purkinje cells (PC) physiologically reveal an age-dependent expression of progesterone with high endogenous concentrations during the neonatal period. Even if progesterone has been previously shown to induce spinogenesis, dendritogenesis and synaptogenesis in immature PC, data about the effects of progesterone on mature PC are missing, even though they could be of significant therapeutic interest. The current study demonstrates for the first time a progesterone effect, depending on the developmental age of PC. Comparable with the physiological course of the progesterone concentration, experimental treatment with progesterone for 24 h achieves the highest effects on the dendritic tree during the early neonate, inducing an highly significant increase in dendritic length, spine number and spine area, while spine density in mature PC could not be further stimulated by progesterone incubation. Observed progesterone effects are certainly mediated by classical progesterone receptors, as spine area and number were comparable to controls when progesterone incubation was combined with mifepristone (incubation for 24 h), an antagonist of progesterone receptors A and B (PR-A/PR-B). In contrast, an increase in the spine number and area of both immature and mature PC was detected when slice cultures were incubated with mifepristone for more than 72 h (mifepristone long-time incubation, MLTI). By including time-lapse microscopy, electron microscopic techniques, PCR, western blot, and MALDI IMS receptor analysis, as well as specific antagonists like trilostane and AG 205, we were able to detect the underlying mechanism of this diverging mifepristone effect. Thus, our results provide new insights into the function and signaling mechanisms of the recently described progesterone receptor membrane component 1 (PGRMC1) in PC. It is highly suitable that progesterone does not just induce effects by the well-known genomic mechanisms of the classical progesterone receptors but also acts through PGRMC1 mediated non-genomic mechanisms. Thus, our results provide first proofs for a previously discussed progesterone-dependent induction of neurosteroidogenesis in PC by interaction with PGRMC1. But while genomic progesterone effects mediated through classical PR-A and PR-B seem to be restricted to the neonatal period of PC, PGRMC1 also transmits signals by non-genomic mechanisms like regulation of the neurosteroidogenesis in mature PC. Thus, PGRMC1 might be an interesting target for future clinical studies and therapeutic interventions.

Decreased gap junctional communication in neurobiotin microinjected lens epithelial cells after taxol treatment

The aim of the study was to examine gap-junction-mediated intercellular communication after experimentally induced aggregations of microtubules in cultured bovine lens epithelial cells. Intercellular communication between lens cells appears to be crucial for normal lens homeostasis. However, investigations on the maintenance of direct ion and metabolite exchange via gap junctions and its quantified dependency of cytoskeletal microtubules have not been available under conditions leading to bundling of microtubules. Thus, metabolic coupling of neighboring lens epithelial cells was quantified following microinjections of neurobiotin into single cells under various conditions. In controls, intensive gap-junction-mediated intercellular communication could be documented by dye-spreading of microinjected neurobiotin. In contrast, taxol treatment for 1-3 days impaired, but did not completely block gap-junction-mediated intercellular communication. After depletion of taxol, a complete recovery of intercellular communication was achieved. In addition, confocal laser scanning microscopy and rapid-freeze deep-etch electron microscopy revealed a displacement of actin-filaments from the perinuclear cytoplasm, accompanied by an abnormal aggregation of microtubules after taxol treatment, including impeded translocation of connexin 43 from the cytoplasm into the plasma membrane. Incubation of cells with nocodazole destroyed the microtubule network, accompanied by a clear reduction of plasma-membrane-integrated connexin 43 and significant impairment of dye spreading. Thus, in lens epithelial cells intercellular communication at gap junctions made by connexin 43 depends on the integrity of the microtubule network through the translocation of connexins to the plasma membrane.

Satellite cell reactions to axon injury of sensory ganglion neurons: increase in number of gap junctions and formation of bridges connecting previously separate perineuronal sheaths

This study investigated satellite cell changes in mouse L4 and L5 spinal ganglia 14 days after unilateral transection of sciatic and saphenous nerves. The ganglia were studied under the electron microscope in single and serial sections, and by dye injection. Satellite cell responses to axon injury of the neurons with which they are associated included the formation of bridges connecting previously separate perineuronal sheaths and the formation of new gap junctions, resulting in more extensive cell coupling. Some possible consequences of these satellite cell reactions are briefly discussed.

Changes in protein synthesis and calcium homeostasis in the thalamus of spontaneously hypertensive rats with focal cerebral ischemia

The thalamus has been shown to undergo secondary degeneration after cerebrocortical ischemia. However, little is known about the time course of the retrograde thalamic degeneration. The present study was designed to investigate time-dependent changes in the morphology, protein synthesis and calcium metabolism of thalamic neurons in middle cerebral artery (MCA)-occluded spontaneously hypertensive stroke-prone rats that showed primary focal ischemia in the temporoparietal cortex after permanent occlusion of the left distal MCA. In the histologic study by light and electron microscopy, swelling of the nucleus and shrinkage of the perikarya were seen in some neurons of the ventroposterior (VP) thalamic nucleus on the lesioned side at 5 days after ischemia. At the same time, the incorporation of radiolabeled leucine in VP thalamic neurons began to decrease significantly with concomitant a decrease in the number of polyribosomes in the neurons. Conspicuous 45Ca accumulation was noted at 3 days after ischemia and persisted up to 1 month in the VP thalamic nucleus on the lesioned side. These findings suggest that the secondary thalamic degeneration after cortical infarction starts with disruption of calcium homeostasis in situ at the third day after MCA occlusion, followed by a decrease in polyribosomes but not by disaggregation of polyribosomes as seen in hippocampal CA1 neurons subjected to transient forebrain ischemia.

Expression of superoxide dismutase following axotomy

Oxidative injury has been implicated in the pathophysiology of neuronal injury and neurodegenerative disease. Antioxidant proteins provide an endogenous defense against such oxidative injury and may yield important clues to mechanisms of cytoprotection and neuronal recovery. Axotomy is the simplest model of neuronal injury and lesioning the sciatic nerve allows concurrent study of both motor (spinal cord) and sensory (dorsal root ganglia, DRG) neurons affected by the same injury. This study was designed to examine the expression of superoxide dismutase (SOD), an essential antioxidant protein, in motor and sensory neurons following complete axotomy of peripheral nerve. Immunocytochemical, quantitative immunoblot, and enzymatic activity assay techniques are used. By 12 days after axotomy, immunocytochemical expression of Mn-SOD is markedly increased in affected DRG and spinal cord. A similar increase in Cu/Zn-SOD is not seen in DRG or spinal cord. This immunocytochemical staining is associated with a significant increase in specific activity and Mn-SOD protein content as measured on quantitative immunoblots. This report suggests, for the first time, that Mn-SOD and not Cu/Zn-SOD increases in sensory neurons of the DRG and motor neurons of the spinal cord following distal axotomy of the sciatic nerve. Quantitative measurements of Mn-SOD following axotomy reveals that the increase in immunocytochemical reactivity is associated with an approximately 30% increase in specific activity when comparing lesioned and contralateral spinal cord samples. These data suggest that Mn-SOD may have a more significant role in the pathophysiology of neuronal injury than Cu/Zn-SOD.

Sciatic nerve injury in the adult rat: comparison of effects on oligosaccharide, CGRP and GAP43 immunoreactivity in primary afferents following two types of trauma

Using immunocytochemical and morphometric techniques, the localisation of three neuronal oligosaccharide antigens (two lactoseries and one globoseries oligosaccharide) were studied in the spinal cord and dorsal root ganglia of adult rats following unilateral crushing or transection of the sciatic nerve. The expression of CGRP and GAP43 was also studied for comparison. We found that following transection of the nerve the expression of lactoseries oligosaccharides and CGRP was permanently depressed, whilst that of the globoseries antigen (SSEA4) was unaffected. However following crush trauma and subsequent regeneration after 2 months, only the expression of one lactoseries antigen, LA4 remained significantly depressed. Our results suggest that different subsets of sensory neurons vary in the rate of reaction to injury and that one subset of neurons expressing a lactoseries oligosaccharide antigen is particularly susceptible to axotomy-induced changes. Furthermore neurons expressing the globoseries oligosaccharide antigen SSEA4 appear to be relatively unaffected by peripheral axotomy.

Cryo-electron microscopy of nervous tissue. A review

The present work attempts to demonstrate that cryofixation is a valuable method for the study of the nervous tissue. The use of the newly developed methods of cryofixation and freeze-etching without fixatives or cryoprotectants allows new exciting perspectives for the electron microscopical observation of cellular components, emphasizing their three-dimensional morphological structures. Significant contributions have been made on the fine structure of the cytoskeleton, cell membranes and cell organelles. The components of the cytoskeleton are distributed in different composition through the perikarya, dendrites and axon. The ubiquitous presence of the cytoskeleton suggests a crucial role in the functional activities of the neurons, especially in relation to the intracellular communication and to developmental and regeneration processes. Vitrified cellular membranes of myelin sheaths and rod outer segments have been observed in hydrated state by using cryofixation and cryotransfer techniques. These procedures allow new insights into the supramolecular structure and an approximation of morphological data to the present biophysical membrane model including a critical comparison with the current descriptions gained by conventional electron microscopy.

Ultrastructure of an identified molluscan neuron in organ culture and cell culture following axotomy

We examined the ultrastructure of neuron 5 from the buccal ganglion of the mollusc Helisoma trivolvis after axotomy and organ culture, and after isolation of the same neuron in culture. Buccal ganglia containing axotomized neurons 5 were cultured either in host snails or in Leibovitz medium conditioned with ganglia. In addition, some neurons 5 were isolated from buccal ganglia by micro-dissection and plated into culture. Neuron 5 and its processes were identified in both whole mounts and plastic sections of buccal ganglia after intracellular injection with Lucifer Yellow or horseradish peroxidase. Five days after axotomy of neuron 5, thick sections of buccal ganglia stained with toluidine blue revealed that densely staining basophilic bodies (Nissl bodies) within the cytoplasm had dispersed, i.e., they had undergone chromatolysis. Coincident with chromatolysis was an overall increase in diffuse basophilic staining within the cytoplasm of neuron 5 when maintained in organ culture. The dispersion of Nissl bodies viewed by light microscopy correlated with a more freely arranged rough endoplasmic reticulum and associated polysomes within neuron 5 as seen by electron microscopy. Isolated neurons 5 did not possess densely staining Nissl bodies when examined after 2 days in vitro, thus indicating that chromatolysis occurred earlier in isolated neurons. Furthermore, no increase in diffuse cytoplasmic basophilia was observed within isolated neurons 5 cultured in vitro. However, isolated neurons 5 exhibited a marked increase in the number of lipid-like bodies (0.5-1.5 micron in diameter) that were particularly evident in scanning electron micrographs. Scanning and transmission electron micrographs revealed that the isolated neurons were free of associated glia, but non-neuronal cells (hemocytes) would attach themselves to the somata and neurites. Glia surrounding neuron 5 within buccal ganglia exhibited a marked hypertrophy following axotomy and organ culture. Hypertrophy of glia was absent, however, if ganglia were axotomized and left within the animal or axotomized ganglia were implanted into host animals and examined 5 days later by electron microscopy. These observations indicate that, following axotomy, a molluscan neuron may exhibit different morphological features depending on its microenvironment. In addition, the hypertrophy of glia surrounding neurons in Helisoma was not associated with axotomy per se, but with organ culture.