Fission and Fusion of the Neuronal Endoplasmic Reticulum

Endoplasmic Reticulum in Metaplasticity: From Information Processing to Synaptic Proteostasis

The ER (endoplasmic reticulum) is a Ca²⁺ reservoir and the unique protein-synthesizing machinery which is distributed throughout the neuron and composed of multiple different structural domains. One such domain is called EMC (endoplasmic reticulum membrane protein complex), pleiotropic nature in cellular functions. The ER/EMC position inside the neurons unmasks its contribution to synaptic plasticity via regulating various cellular processes from protein synthesis to Ca²⁺ signaling. Since presynaptic Ca²⁺ channels and postsynaptic ionotropic receptors are organized into the nanodomains, thus ER can be a crucial player in establishing TMNCs (transsynaptic molecular nanocolumns) to shape efficient neural communications. This review hypothesized that ER is not only involved in stress-mediated neurodegeneration but also axon regrowth, remyelination, neurotransmitter switching, information processing, and regulation of pre- and post-synaptic functions. Thus ER might not only be a protein-synthesizing and quality control machinery but also orchestrates plasticity of plasticity (metaplasticity) within the neuron to execute higher-order brain functions and neural repair.

Zn2+-Depletion Enhances Lysosome Fission in Cultured Rat Embryonic Cortical Neurons Revealed by a Modified Epifluorescence Microscopic Technique

Lysosomes are integration hubs for several signaling pathways, such as autophagy and endocytosis, and also crucial stores of ions, including Zn2+. Lysosomal dysfunction caused by changes in their morphology by fusion and fission processes can result in several pathological disorders. However, the role of Zn2+ in modulating the morphology of lysosomes is unclear. The resolution of conventional epifluorescence microscopy restricts accurate observation of morphological changes of subcellular fluorescence punctum. In this study, we used a modified epifluorescence microscopy to identify the center of a punctum from a series of z-stack images and calculate the morphological changes. We stained primary cultured rat embryonic cortical neurons with FluoZin3, a Zn2+-sensitive fluorescent dye, and Lysotracker, a lysosome-specific marker, to visualize the distribution of Zn2+-enriched vesicles and lysosomes, respectively. Our results revealed that treating neurons with N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine, a cell-permeable Zn2+ chelator, shrank Zn2+-enriched vesicles and lysosomes by up to 25% in an hour. Pretreating the neurons with YM201636, a blocker of lysosome fission, could suppress this shrinkage. These results demonstrate the usefulness of the modified epifluorescence microscopy for investigating the homeostasis of intracellular organelles and related disorders.

Influence of a 30-day spaceflight on the structure of motoneurons of the trochlear nerve nucleus in mice

During spaceflight and immediately after it, adaptive neuroplastic changes occur in the sensorimotor structures of the central nervous system, which are associated with changes of mainly vestibular and visual signals. It is known that the movement of the eyeball in the vertical direction is carried out by muscles that are innervated by the trochlear nerve (CN IV) and the oculomotor nerve (CN III). To elucidate the cellular processes underlying the atypical vertical nystagmus that occurs under microgravity conditions, it seems necessary to study the state of these nuclei in animals in more detail after prolonged space flights. We carried out a qualitative and quantitative light-optical and ultrastructural analysis of the nuclei of the trochlear nerve in mice after a 30-day flight on the Bion-M1 biosatellite. As a result, it was shown that the dendrites of motoneurons in the nucleus of the trochlear nerve significantly reorganized their geometry and orientation under microgravity conditions. The number of dendritic branches was increased, possibly in order to amplify the reduced signal flow. To ensure such plastic changes, the number and size of mitochondria in the soma of motoneurons and in axons coming from the vestibular structures increased. Thus, the main role in the adaptation of the trochlear nucleus to microgravity conditions, apparently, belongs to the dendrites of motoneurons, which rearrange their structure and function to enhance the flow of sensory information. These results complement our knowledge of the causes of atypical nystagmus in microgravity.

Peroxiredoxin 4 ameliorates amyloid beta oligomer-mediated apoptosis by inhibiting ER-stress in HT-22 hippocampal neuron cells

Alzheimer's disease (AD) is a neurodegenerative disorder caused by amyloid beta oligomers (AβO), which induce cell death by triggering oxidative stress and endoplasmic reticulum (ER) stress. Oxidative stress is regulated by antioxidant enzymes, including peroxiredoxins. Peroxiredoxins (Prx) are classified into six subtypes, based on their localization and cysteine residues, and protect cells by scavenging hydrogen peroxide (H₂O₂). Peroxiredoxin 4 (Prx4) is unique in being localized to the ER; however, whether Prx4 protects neuronal cells from AβO-induced toxicity remains unclear, although Prx4 expression is upregulated in AβO-induced oxidative stress and ER stress. In this study, we established HT-22 cells in which Prx4 was either overexpressed or silenced to investigate its role in AβO-induced toxicity. AβO-stimulation of HT-22 cells with overexpressed Prx4 caused decreases in both AβO-induced ROS and ER stress (followed by ER expansion). In contrast, AβO stimulation caused increases in both ROS and ER stress that were notably higher in HT-22 cells with silenced Prx4 expression than in HT-22 cells. Consequently, Prx4 overexpression decreased apoptotic cell death and ameliorated the AβO-induced increase in intracellular Ca²⁺. Therefore, we conclude that Prx4 has a protective effect against AβO-mediated oxidative stress, ER stress, and neuronal cell death. Furthermore, these results suggest that Prx4 may be a target for preventing AβO toxicity in AD. Graphical abstract .

Chromatolysis: Do injured axons regenerate poorly when ribonucleases attack rough endoplasmic reticulum, ribosomes and RNA?

After axonal injury, chromatolysis (fragmentation of Nissl substance) can occur in the soma. Electron microscopy shows that chromatolysis involves fission of the rough endoplasmic reticulum. In CNS neurons (which do not regenerate axons back to their original targets) or in motor neurons or dorsal root ganglion neurons denied axon regeneration (e.g., by transection and ligation), chromatolysis is often accompanied by degranulation (loss of ribosomes from rough endoplasmic reticulum), disaggregation of polyribosomes and degradation of monoribosomes into dust‐like particles. Ribosomes and rough endoplasmic reticulum may also be degraded in autophagic vacuoles by ribophagy and reticulophagy, respectively. In other words, chromatolysis is disruption of parts of the protein synthesis infrastructure. Whereas some neurons may show transient or no chromatolysis, severely injured neurons can remain chromatolytic and never again synthesize normal levels of protein; some may atrophy or die. Ribonuclease(s) might cause the following features of chromatolysis: fragmentation and degranulation of rough endoplasmic reticulum, disaggregation of polyribosomes and degradation of monoribosomes. For example, ribonucleases in the EndoU/PP11 family can modify rough endoplasmic reticulum; many ribonucleases can degrade mRNA causing polyribosomes to unchain and disperse, and they can disassemble monoribosomes; Ribonuclease 5 can control rRNA synthesis and degrade tRNA; Ribonuclease T2 can degrade ribosomes, endoplasmic reticulum and RNA within autophagic vacuoles; and Ribonuclease IRE1α acts as a stress sensor within the endoplasmic reticulum. Regeneration might be improved after axonal injury by protecting the protein synthesis machinery from catabolism; targeting ribonucleases using inhibitors can enhance neurite outgrowth and could be a profitable strategy in vivo. © 2018 Wiley Periodicals, Inc. Develop Neurobiol, 2018

Structural Plasticity of Synaptopodin in the Axon Initial Segment during Visual Cortex Development

The axon initial segment (AIS) is essential for action potential generation. Recently, the AIS was identified as a site of neuronal plasticity. A subpopulation of AIS in cortical principal neurons contains stacks of endoplasmic reticulum (ER) forming the cisternal organelle (CO). The function of this organelle is poorly understood, but roles in local Ca2+-trafficking and AIS plasticity are discussed. To investigate whether the presence and/or the size of COs are linked to the development and maturation of AIS of cortical neurons, we analyzed the relationship between COs and the AIS during visual cortex development under control and visual deprivation conditions. In wildtype mice, immunolabeling for synaptopodin, ankyrin-G, and ßIV-spectrin were employed to label COs and the AIS, respectively. Dark rearing resulted in an increase in synaptopodin cluster sizes, suggesting a homeostatic function of the CO in this cellular compartment. In line with this observation, synaptopodin-deficient mice lacking the CO showed AIS shortening in the dark. Collectively, these data demonstrate that the CO is an essential part of the AIS machinery required for AIS plasticity during a critical developmental period of the visual cortex.

Effects of a Sublethal and Transient Stress of the Endoplasmic Reticulum on the Mitochondrial Population

Endoplasmic reticulum (ER) and mitochondria are not discrete intracellular organelles but establish close physical and functional interactions involved in several biological processes including mitochondrial bioenergetics, calcium homeostasis, lipid synthesis, and the regulation of apoptotic cell death pathways. As many cell types might face a transient and sublethal ER stress during their lifetime, it is thus likely that the adaptive UPR response might affect the mitochondrial population. The aim of this work was to study the putative effects of a non-lethal and transient endoplasmic reticulum stress on the mitochondrial population in HepG2 cells. The results show that thapsigargin and brefeldin A, used to induce a transient and sublethal ER stress, rapidly lead to the fragmentation of the mitochondrial network associated with a decrease in mitochondrial membrane potential, O2 (•-) production and less efficient respiration. These changes in mitochondrial function are transient and preceded by the phosphorylation of JNK. Inhibition of JNK activation by SP600125 prevents the decrease in O2 (•-) production and the mitochondrial network fragmentation observed in cells exposed to the ER stress but has no impact on the reduction of the mitochondrial membrane potential. In conclusion, our data show that a non-lethal and transient ER stress triggers a rapid activation of JNK without inducing apoptosis, leading to the fragmentation of the mitochondrial network and a reduction of O2 (•-) production. J. Cell. Physiol. 231: 1913-1931, 2016. © 2015 Wiley Periodicals, Inc.

Novel computer vision algorithm for the reliable analysis of organelle morphology in whole cell 3D images--A pilot study for the quantitative evaluation of mitochondrial fragmentation in amyotrophic lateral sclerosis

The function of intact organelles, whether mitochondria, Golgi apparatus or endoplasmic reticulum (ER), relies on their proper morphological organization. It is recognized that disturbances of organelle morphology are early events in disease manifestation, but reliable and quantitative detection of organelle morphology is difficult and time-consuming. Here we present a novel computer vision algorithm for the assessment of organelle morphology in whole cell 3D images. The algorithm allows the numerical and quantitative description of organelle structures, including total number and length of segments, cell and nucleus area/volume as well as novel texture parameters like lacunarity and fractal dimension. Applying the algorithm we performed a pilot study in cultured motor neurons from transgenic G93A hSOD1 mice, a model of human familial amyotrophic lateral sclerosis. In the presence of the mutated SOD1 and upon excitotoxic treatment with kainate we demonstrate a clear fragmentation of the mitochondrial network, with an increase in the number of mitochondrial segments and a reduction in the length of mitochondria. Histogram analyses show a reduced number of tubular mitochondria and an increased number of small mitochondrial segments. The computer vision algorithm for the evaluation of organelle morphology allows an objective assessment of disease-related organelle phenotypes with greatly reduced examiner bias and will aid the evaluation of novel therapeutic strategies on a cellular level.