Oligodendroglial pathology in the development of myelin breakdown in the dmy mutant rat

The many "Neurofaces" of Prohibitins 1 and 2: Crucial for the healthy brain, dysregulated in numerous brain disorders

Prohibitin 1 (PHB1) and prohibitin 2 (PHB2) are proteins that are nearly ubiquitously expressed. They are localized in mitochondria, cytosol and cell nuclei. In the healthy CNS, they occur in neurons and non-neuronal cells (oligodendrocytes, astrocytes, microglia, and endothelial cells) and fulfill pivotal functions in brain development and aging, the regulation of brain metabolism, maintenance of structural integrity, synapse formation, aminoacidergic neurotransmission and, probably, regulation of brain action of certain hypothalamic-pituitary hormones.With regard to the diseased brain there is increasing evidence that prohibitins are prominently involved in numerous major diseases of the CNS, which are summarized and discussed in the present review (brain tumors, neurotropic viruses, Alzheimer disease, Down syndrome, Fronto-temporal and vascular dementia, dementia with Lewy bodies, Parkinson disease, Huntington disease, Multiple sclerosis, Amyotrophic lateral sclerosis, stroke, alcohol use disorder, schizophrenia and autism). Unfortunately, there is no PHB-targeted therapy available for any of these diseases.

Prohibitins: A Key Link between Mitochondria and Nervous System Diseases

Prohibitins (PHBs) are conserved proteins in eukaryotic cells, which are mainly located in the inner mitochondrial membrane (IMM), cell nucleus, and cell membrane. PHBs play crucial roles in various cellular functions, including the cell cycle regulation, tumor suppression, immunoglobulin M receptor binding, and aging. In addition, recent in vitro and in vivo studies have revealed that PHBs are important in nervous system diseases. PHBs can prevent apoptosis, inflammation, mitochondrial dysfunction, and autophagy in neurological disorders through different molecules and pathways, such as OPA-1, PINK1/Parkin, IL6/STAT3, Tau, NO, LC3, and TDP43. Therefore, PHBs show great promise in the protection of neurological disorders. This review summarizes the relevant studies on the relationship between PHBs and neurological disorders and provides an update on the molecular mechanisms of PHBs in nervous system diseases.

Magnesium Is a Key Player in Neuronal Maturation and Neuropathology

Magnesium (Mg) is the second most abundant cation in mammalian cells, and it is essential for numerous cellular processes including enzymatic reactions, ion channel functions, metabolic cycles, cellular signaling, and DNA/RNA stabilities. Because of the versatile and universal nature of Mg²⁺, the homeostasis of intracellular Mg²⁺ is physiologically linked to growth, proliferation, differentiation, energy metabolism, and death of cells. On the cellular and tissue levels, maintaining Mg²⁺ within optimal levels according to the biological context, such as cell types, developmental stages, extracellular environments, and pathophysiological conditions, is crucial for development, normal functions, and diseases. Hence, Mg²⁺ is pathologically involved in cancers, diabetes, and neurodegenerative diseases, such as Parkinson's disease, Alzheimer's disease, and demyelination. In the research field regarding the roles and mechanisms of Mg²⁺ regulation, numerous controversies caused by its versatility and complexity still exist. As Mg²⁺, at least, plays critical roles in neuronal development, healthy normal functions, and diseases, appropriate Mg²⁺ supplementation exhibits neurotrophic effects in a majority of cases. Hence, the control of Mg²⁺ homeostasis can be a candidate for therapeutic targets in neuronal diseases. In this review, recent results regarding the roles of intracellular Mg²⁺ and its regulatory system in determining the cell phenotype, fate, and diseases in the nervous system are summarized, and an overview of the comprehensive roles of Mg²⁺ is provided.

Downregulation of aspartoacylase during the progression of myelin breakdown in the dmy mutant rat with mitochondrial magnesium channel MRS2 defect

OBJECTIVE

The dmy rat is an autosomal recessive mutant that exhibits severe rapid myelin breakdown throughout the central nervous system at 7-8 weeks of age. The dmy rat has a point mutation in Mrs2 gene, which encodes an essential component of the major electrophoretic Mg²⁺ influx system in the mitochondria. However, it remains unknown how mitochondrial dysfunction leads to the myelin breakdown.

METHODS

We focused on the aspartoacylase (ASPA) and mitochondrion-related metabolites to clarify the mechanism of myelin pathology in dmy rats. Aspa mRNA was significantly decreased in both the gray matter and the ventral white matter of spinal cord in the dmy rats from 4 to 8 weeks of age. Very faint immunohistochemical expression for ASPA was noted in the gray and white matter of the affected dmy rats at 8 weeks. Liquid chromatography mass spectrometry revealed no different amount of N-acetylaspartate (NAA), which is synthesized from aspartate and acetyl-coenzyme A (CoA) in neurons, in the brain and spinal cord between the dmy and control rats.

CONCLUSION

Our results indicated that the pyruvate dehydrogenase activity might be reduced due to the loss of Mg²⁺ transport activity in the mitochondria of the dmy rats, suggesting acetyl CoA production might be reduced. The number of oligodendrocytes was well preserved until 7 weeks. It is intriguing that prior to the myelin destruction at 7-8 weeks, disrupted expression of Aspa mRNA and ASPA protein undergoes from early stage of myelinogenesis. These data indicate that ASPA expression would be a useful index to evaluate a function of oligodendrocyte in the dmy rat.

Magnesium Extravaganza: A Critical Compendium of Current Research into Cellular Mg2+ Transporters Other than TRPM6/7

Magnesium research has boomed within the last 20 years. The real breakthrough came at the start of the new millennium with the discovery of a plethora of possible Mg homeostatic factors that, in particular, included putative Mg²⁺ transporters. Until that point, Mg research was limited to biochemical and physiological work, as no target molecular entities were known that could be used to explore the molecular biology of Mg homeostasis at the level of the cell, tissue, organ, or organism and to translate such knowledge into the field of clinical medicine and pharmacology. Because of the aforementioned, Mg²⁺ and Mg homeostasis, both of which had been heavily marginalized within the biomedical field in the twentieth century, have become overnight a focal point of many studies ranging from primary biomedical research to translational medicine.The amount of literature concerning cellular Mg²⁺ transport and cellular Mg homeostasis is increasing, together with a certain amount of confusion, especially about the function(s) of the newly discovered and, in the majority of instances, still only putative Mg²⁺ transporters/Mg²⁺ homeostatic factors. Newcomers to the field of Mg research will thus find it particularly difficult to orient themselves.Here, we briefly but critically summarize the status quo of the current understanding of the molecular entities behind cellular Mg²⁺ homeostasis in mammalian/human cells other than TRPM6/7 chanzymes, which have been universally accepted as being unspecific cation channel kinases allowing the flux of Mg²⁺ while constituting the major gateway for Mg²⁺ to enter the cell.

Enhanced Expression of Trib3 during the Development of Myelin Breakdown in dmy Myelin Mutant Rats

The demyelination (dmy) rat exhibits hind limb ataxia and severe myelin breakdown in the central nervous system. The causative gene of dmy rats is the MRS2 magnesium transporter gene. Tribbles homolog 3 (Trib3) is a pseudokinase molecule that modifies certain signal pathways, and its expression is increased in response to various stresses. Here we sought to clarify the mechanism of myelin breakdown by focusing Trib3, which is remarkably up-regulated in dmy rats. The expression of Trib3 mRNA was significantly increased at 4, 5, 6, 7 and 8 weeks of age in the dmy rats, prior to the prominent myelin breakdown between 7 and 10 weeks of age. The expression level of Trib3 was increased concurrently with the progression of the clinical and pathological conditions in the dmy rats. Double immunofluorescence demonstrated that TRIB3 was mainly expressed in neurons and oligodendrocytes and localized in the Golgi apparatus. Our findings indicate that Trib3 may be associated with the pathogenic mechanism of dmy rats.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

The structure and regulation of magnesium selective ion channels

The magnesium ion (Mg(2+)) is the most abundant divalent cation within cells. In man, Mg(2+)-deficiency is associated with diseases affecting the heart, muscle, bone, immune, and nervous systems. Despite its impact on human health, little is known about the molecular mechanisms that regulate magnesium transport and storage. Complete structural information on eukaryotic Mg(2+)-transport proteins is currently lacking due to associated technical challenges. The prokaryotic MgtE and CorA magnesium transport systems have recently succumbed to structure determination by X-ray crystallography, providing first views of these ubiquitous and essential Mg(2+)-channels. MgtE and CorA are unique among known membrane protein structures, each revealing a novel protein fold containing distinct arrangements of ten transmembrane-spanning α-helices. Structural and functional analyses have established that Mg(2+)-selectivity in MgtE and CorA occurs through distinct mechanisms. Conserved acidic side-chains appear to form the selectivity filter in MgtE, whereas conserved asparagines coordinate hydrated Mg(2+)-ions within the selectivity filter of CorA. Common structural themes have also emerged whereby MgtE and CorA sense and respond to physiologically relevant, intracellular Mg(2+)-levels through dedicated regulatory domains. Within these domains, multiple primary and secondary Mg(2+)-binding sites serve to staple these ion channels into their respective closed conformations, implying that Mg(2+)-transport is well guarded and very tightly regulated. The MgtE and CorA proteins represent valuable structural templates to better understand the related eukaryotic SLC41 and Mrs2-Alr1 magnesium channels. Herein, we review the structure, function and regulation of MgtE and CorA and consider these unique proteins within the expanding universe of ion channel and transporter structural biology.

Increased density of prohibitin-immunoreactive oligodendrocytes in the dorsolateral prefrontal white matter of subjects with schizophrenia suggests extraneuronal roles for the protein in the disease

Prohibitin has previously been implicated in the synaptic pathology of schizophrenia. The recently discovered abundant expression of prohibitin in human prefrontal oligodendrocytes raises the issue, whether this protein might also be part of the well-known white matter abnormalities in schizophrenia. Hence, post-mortem brains of ten patients with schizophrenia and ten matched control cases were investigated. Using a direct, 3D-counting technique we morphometrically analyzed the number and density of prohibitin-immunoreactive oligodendroglial cells in the left and right dorsolateral, anterior cingulate, and orbitofrontal cortex white matter. Additionally, we studied the prohibitin expression in different neuronal and non-neuronal cell populations in rat cell cultures. We could confirm the strong expression of prohibitin in oligodendrocytes. Intracellularly, the protein was localized to mitochondria and some cell nuclei. In schizophrenia, the numerical density of prohibitin-expressing oligodendrocytes was significantly increased in the right dorsolateral white matter area. Taking into consideration the dual intracellular localization of prohibitin in oligodendrocyte mitochondria and cell nuclei, one may suggest an involvement of the protein in mitochondrial dysfunction and/or cycle abnormalities in schizophrenia.

Prohibitin reduces mitochondrial free radical production and protects brain cells from different injury modalities

Prohibitin is an essential mitochondrial protein that has been implicated in a wide variety of functions in many cell types, but its role in neurons remains unclear. In a proteomic screen of rat brains in which ischemic tolerance was induced by electrical stimulation of the cerebellar fastigial nucleus, we found that prohibitin is upregulated in mitochondria. This observation prompted us to investigate the role of prohibitin in neuronal death and survival. We found that prohibitin is upregulated also in the ischemic tolerance induced by transient ischemia in vivo, or oxygen-glucose deprivation in neuronal cultures. Cell fractionation and electron-microscopic immunolabeling studies demonstrated that prohibitin is localized to neuronal mitochondria. Upregulation of prohibitin in neuronal cultures or hippocampal slices was markedly neuroprotective, whereas prohibitin gene silencing increased neuronal vulnerability, an effect associated with loss of mitochondrial membrane potential and increased mitochondrial production of reactive oxygen species. Prohibitin upregulation was associated with reduced production of reactive oxygen species in mitochondria exposed to the complex I inhibitor rotenone. In addition, prohibitin protected complex I activity from the inhibitory effects of rotenone. These observations, collectively, establish prohibitin as an endogenous neuroprotective protein involved in ischemic tolerance. Prohibitin exerts beneficial effects on neurons by reducing mitochondrial free radical production. The data with complex I activity suggest that prohibitin may stabilize the function of complex I. The protective effect of prohibitin has potential translational relevance in diseases of the nervous system associated with mitochondrial dysfunction and oxidative stress.