Calcium-Handling Defects and Neurodegenerative Disease

Acrylamide-induced damage to postsynaptic plasticity is CYP2E1 dependent in an SH-SY5Y co-culture system

Acrylamide (ACR), a neurotoxic substance, is characterized by a range of industrial and population exposures. The effects of ACR on synapses have been examined, but the regulation and molecular mechanism of key proteins related to ACR and its metabolite glycidamide (GA) have not been elucidated. In this study, we constructed two co-culture systems to mimic neurons that do not express and overexpress CYP2E1. In these co-cultures, we observed the effects and relative influence of ACR and GA on cell survival as well as synaptic structural and functional plasticity. Next, we investigated the relationship between ACR-induced nerve damage and key proteins in the postsynaptic membrane. After ACR exposure, cell death and synaptic damage were significantly worse in CYP2E1-overexpressing co-culture systems, suggesting that ACR-induced neurotoxicity may be related to metabolic efficiency (including CYP2E1 activity). Moreover, with increasing doses of ACR, the key postsynaptic membrane proteins PSD-95 expression was reduced and CaMKII and NMDAR-2B phosphorylation was increased. ACR exposure also triggered a rapid dose- and time-dependent increase in intracellular Ca²⁺, whose changes can affect the expression of the above-mentioned key proteins. In summary, we clarified the relationship between ACR exposure, neuronal damage and postsynaptic plasticity and proposed an ACR-CYP2E1-GA: Ca²⁺-PSD-95-NMDAR-Ca²⁺-CaMKII effect chain. This information will further improve the development of an alternative pathway strategy for investigating the risk posed by ACR.

Mitochondrial behavior when things go wrong in the axon

Axonal homeostasis is maintained by processes that include cytoskeletal regulation, cargo transport, synaptic activity, ionic balance, and energy supply. Several of these processes involve mitochondria to varying degrees. As a transportable powerplant, the mitochondria deliver ATP and Ca2+-buffering capabilities and require fusion/fission to maintain proper functioning. Taking into consideration the long distances that need to be covered by mitochondria in the axons, their transport, distribution, fusion/fission, and health are of cardinal importance. However, axonal homeostasis is disrupted in several disorders of the nervous system, or by traumatic brain injury (TBI), where the external insult is translated into physical forces that damage nervous tissue including axons. The degree of damage varies and can disconnect the axon into two segments and/or generate axonal swellings in addition to cytoskeletal changes, membrane leakage, and changes in ionic composition. Cytoskeletal changes and increased intra-axonal Ca2+ levels are the main factors that challenge mitochondrial homeostasis. On the other hand, a proper function and distribution of mitochondria can determine the recovery or regeneration of the axonal physiological state. Here, we discuss the current knowledge regarding mitochondrial transport, fusion/fission, and Ca2+ regulation under axonal physiological or pathological conditions.

Store-operated calcium entry is reduced in spastin-linked hereditary spastic paraplegia

Pathogenic variants in SPAST, the gene coding for spastin, are the single most common cause of hereditary spastic paraplegia, a progressive motor neuron disease. Spastin regulates key cellular functions, including microtubule-severing and endoplasmic reticulum-morphogenesis. However, it remains unclear how alterations in these cellular functions due to SPAST pathogenic variants result in motor neuron dysfunction. Since spastin influences both microtubule network and endoplasmic reticulum structure, we hypothesized that spastin is necessary for the regulation of Ca2+ homeostasis via store-operated calcium entry. Here, we show that the lack of spastin enlarges the endoplasmic reticulum and reduces store-operated calcium entry. In addition, elevated levels of different spastin variants induced clustering of STIM1 within the endoplasmic reticulum, altered the transport of STIM1 to the plasma membrane and reduced store-operated calcium entry, which could be rescued by exogenous expression of STIM1. Importantly, store-operated calcium entry was strongly reduced in induced pluripotent stem cell-derived neurons from hereditary spastic paraplegia patients with pathogenic variants in SPAST resulting in spastin haploinsufficiency. These neurons developed axonal swellings in response to lack of spastin. We were able to rescue both store-operated calcium entry and axonal swellings in SPAST patient neurons by restoring spastin levels, using CRISPR/Cas9 to correct the pathogenic variants in SPAST. These findings demonstrate that proper amounts of spastin are a key regulatory component for store-operated calcium entry mediated Ca2+ homeostasis and suggest store-operated calcium entry as a disease relevant mechanism of spastin-linked motor neuron disease.

Dysregulated Ca2+ Homeostasis as a Central Theme in Neurodegeneration: Lessons from Alzheimer's Disease and Wolfram Syndrome

Calcium ions (Ca²⁺) operate as important messengers in the cell, indispensable for signaling the underlying numerous cellular processes in all of the cell types in the human body. In neurons, Ca²⁺ signaling is crucial for regulating synaptic transmission and for the processes of learning and memory formation. Hence, the dysregulation of intracellular Ca²⁺ homeostasis results in a broad range of disorders, including cancer and neurodegeneration. A major source for intracellular Ca²⁺ is the endoplasmic reticulum (ER), which has close contacts with other organelles, including mitochondria. In this review, we focus on the emerging role of Ca²⁺ signaling at the ER–mitochondrial interface in two different neurodegenerative diseases, namely Alzheimer’s disease and Wolfram syndrome. Both of these diseases share some common hallmarks in the early stages, including alterations in the ER and mitochondrial Ca²⁺ handling, mitochondrial dysfunction and increased Reactive oxygen species (ROS) production. This indicates that similar mechanisms may underly these two disease pathologies and suggests that both research topics might benefit from complementary research.

Neuroprotective effect of CTK 01512-2 recombinant toxin at the spinal cord in a model of Huntington's disease

NEW FINDINGS

What is the central question of this study? We investigated the action of intrathecal administration of a novel toxin (CTK01512-2) in a mouse model for Huntington´s disease (HD). We asked if spinal cord neurons can represent a therapeutic target, as the spinal cord seems to be involved in HD motor-symptoms. Pharmacological approaches focusing on the spinal cord and skeletal muscles might represent a more feasible strategy. What is the main finding and its importance? We provided evidence of a novel, local, neuroprotector effect of CTK01512-2, paving a path for the development of approaches to treat HD-motor symptoms beyond the brain.

ABSTRACT

Phα1β is a neurotoxin from the venom of the Phoneutria nigriventer spider, available as CTK01512-2, a recombinant peptide. Due to its antinociceptive and analgesic properties, CTK01512-2 has been described to alleviate neuroinflammatory responses. Despite the diverse CTK01512-2 actions on the nervous system, little is known regarding its neuroprotective effect, especially in neurodegenerative conditions such as Huntington's disease (HD), a genetic movement disorder without cure. Here, we investigated whether CTK01512-2 has a neuroprotector effect in a mouse model of HD. We hypothesized that spinal cord neurons might represent a therapeutic target, as the spinal cord seems to be involved in the motor-symptoms of HD mice (BACHD). Then, we treated BACHD mice with CTK01512-2 by intrathecal injection, and performed in vivo motor behavior and morphological analyses in the central nervous system (brain and spinal cord) and muscles. Our data showed that intrathecal injection of CTK01512-2 significantly improves motor-performance in the Open-field task. CTK01512-2 protects neurons in the spinal cord (but not in the brain) from death, suggesting a local effect. CTK01512-2 exerts its neuroprotective effect by inhibiting BACHD-neuronal apoptosis, as revealed by a reduction in caspase-3 in the spinal cord. CTK01512-2 was also able to revert BACHD muscle atrophy. In conclusion, our data provide a novel role for CTK01512-2 acting directly in the spinal cord, ameliorating morphofunctional aspects of spinal cord neurons, and muscles, and improving BACHD mice performance in motor-behavioral tests. Since HD shares similar symptoms to many neurodegenerative conditions, the findings presented herein may also be applicable to other disorders. This article is protected by copyright. All rights reserved.

Impact of β-Amyloids Induced Disruption of Ca2+ Homeostasis in a Simple Model of Neuronal Activity

Alzheimer’s disease is characterized by a marked dysregulation of intracellular Ca²⁺ homeostasis. In particular, toxic β-amyloids (Aβ) perturb the activities of numerous Ca²⁺ transporters or channels. Because of the tight coupling between Ca²⁺ dynamics and the membrane electrical activity, such perturbations are also expected to affect neuronal excitability. We used mathematical modeling to systematically investigate the effects of changing the activities of the various targets of Aβ peptides reported in the literature on calcium dynamics and neuronal excitability. We found that the evolution of Ca²⁺ concentration just below the plasma membrane is regulated by the exchanges with the extracellular medium, and is practically independent from the Ca²⁺ exchanges with the endoplasmic reticulum. Thus, disruptions of Ca²⁺ homeostasis interfering with signaling do not affect the electrical properties of the neurons at the single cell level. In contrast, the model predicts that by affecting the activities of L-type Ca²⁺ channels or Ca²⁺-activated K⁺ channels, Aβ peptides promote neuronal hyperexcitability. On the contrary, they induce hypo-excitability when acting on the plasma membrane Ca²⁺ ATPases. Finally, the presence of pores of amyloids in the plasma membrane can induce hypo- or hyperexcitability, depending on the conditions. These modeling conclusions should help with analyzing experimental observations in which Aβ peptides interfere at several levels with Ca²⁺ signaling and neuronal activity.

Pathogenesis of sporadic Alzheimer's disease by deficiency of NMDA receptor subunit GluN3A

The Ca2+ hypothesis for Alzheimer's disease (AD) conceives Ca2+ dyshomeostasis as a common mechanism of AD; the cause of Ca2+ dysregulation, however, is obscure. Meanwhile, hyperactivities of N-Methyl-D-aspartate receptors (NMDARs), the primary mediator of Ca2+ influx, are reported in AD. GluN3A (NR3A) is an NMDAR inhibitory subunit. We hypothesize that GluN3A is critical for sustained Ca2+ homeostasis and its deficiency is pathogenic for AD. Cellular, molecular, and functional changes were examined in adult/aging GluN3A knockout (KO) mice. The GluN3A KO mouse brain displayed age-dependent moderate but persistent neuronal hyperactivity, elevated intracellular Ca2+ , neuroinflammation, impaired synaptic integrity/plasticity, and neuronal loss. GluN3A KO mice developed olfactory dysfunction followed by psychological/cognitive deficits prior to amyloid-β/tau pathology. Memantine at preclinical stage prevented/attenuated AD syndromes. AD patients' brains show reduced GluN3A expression. We propose that chronic "degenerative excitotoxicity" leads to sporadic AD, while GluN3A represents a primary pathogenic factor, an early biomarker, and an amyloid-independent therapeutic target.

Deficiency of ER Ca2+ sensor STIM1 in AgRP neurons confers protection against dietary obesity

Store-operated calcium entry (SOCE) is pivotal in maintaining intracellular Ca2+ level and cell function; however, its role in obesity development remains largely unknown. Here, we show that the stromal interaction molecule 1 (Stim1), an endoplasmic reticulum (ER) Ca2+ sensor for SOCE, is critically involved in obesity development. Pharmacological blockade of SOCE in the brain, or disruption of Stim1 in hypothalamic agouti-related peptide (AgRP)-producing neurons (ASKO), significantly ameliorates dietary obesity and its associated metabolic disorders. Conversely, constitutive activation of Stim1 in AgRP neurons leads to an obesity-like phenotype. We show that the blockade of SOCE suppresses general translation in neuronal cells via the 2',5'-oligoadenylate synthetase 3 (Oas3)-RNase L signaling. While Oas3 overexpression in AgRP neurons protects mice against dietary obesity, deactivation of RNase L in these neurons significantly abolishes the effect of ASKO. These findings highlight an important role of Stim1 and SOCE in the development of obesity.

Lighting Up Ca2+ Dynamics in Animal Models

Calcium (Ca2+) signaling coordinates are crucial processes in brain physiology. Particularly, fundamental aspects of neuronal function such as synaptic transmission and neuronal plasticity are regulated by Ca2+, and neuronal survival itself relies on Ca2+-dependent cascades. Indeed, impaired Ca2+ homeostasis has been reported in aging as well as in the onset and progression of neurodegeneration. Understanding the physiology of brain function and the key processes leading to its derangement is a core challenge for neuroscience. In this context, Ca2+ imaging represents a powerful tool, effectively fostered by the continuous amelioration of Ca2+ sensors in parallel with the improvement of imaging instrumentation. In this review, we explore the potentiality of the most used animal models employed for Ca2+ imaging, highlighting their application in brain research to explore the pathogenesis of neurodegenerative diseases.

Calcium channels and their role in regenerative medicine

Stem cells hold indefinite self-renewable capability that can be differentiated into all desired cell types. Based on their plasticity potential, they are divided into totipotent (morula stage cells), pluripotent (embryonic stem cells), multipotent (hematopoietic stem cells, multipotent adult progenitor stem cells, and mesenchymal stem cells [MSCs]), and unipotent (progenitor cells that differentiate into a single lineage) cells. Though bone marrow is the primary source of multipotent stem cells in adults, other tissues such as adipose tissues, placenta, amniotic fluid, umbilical cord blood, periodontal ligament, and dental pulp also harbor stem cells that can be used for regenerative therapy. In addition, induced pluripotent stem cells also exhibit fundamental properties of self-renewal and differentiation into specialized cells, and thus could be another source for regenerative medicine. Several diseases including neurodegenerative diseases, cardiovascular diseases, autoimmune diseases, virus infection (also coronavirus disease 2019) have limited success with conventional medicine, and stem cell transplantation is assumed to be the best therapy to treat these disorders. Importantly, MSCs, are by far the best for regenerative medicine due to their limited immune modulation and adequate tissue repair. Moreover, MSCs have the potential to migrate towards the damaged area, which is regulated by various factors and signaling processes. Recent studies have shown that extracellular calcium (Ca²⁺) promotes the proliferation of MSCs, and thus can assist in transplantation therapy. Ca²⁺ signaling is a highly adaptable intracellular signal that contains several components such as cell-surface receptors, Ca²⁺ channels/pumps/exchangers, Ca²⁺ buffers, and Ca²⁺ sensors, which together are essential for the appropriate functioning of stem cells and thus modulate their proliferative and regenerative capacity, which will be discussed in this review.

Normalization of Calcium Balance in Striatal Neurons in Huntington's Disease: Sigma 1 Receptor as a Potential Target for Therapy

Huntington's disease (HD) is a neurodegenerative, dominantly inherited genetic disease caused by expansion of the polyglutamine tract in the huntingtin gene. At the cellular level, HD is characterized by the accumulation of mutant huntingtin protein in brain cells, resulting in the development of the HD phenotype, which includes mental disorders, decreased cognitive abilities, and progressive motor impairments in the form of chorea. Despite numerous studies, no unambigous connection between the accumulation of mutant protein and selective death of striatal neurons has yet been established. Recent studies have shown impairments in the calcium homeostasis in striatal neurons in HD. These cells are extremely sensitive to changes in the cytoplasmic concentration of calcium and its excessive increase leads to their death. One of the possible ways to normalize the balance of calcium in striatal neurons is through the sigma 1 receptor (S1R), which act as a calcium sensor that also exhibits modulating chaperone activity upon the cell stress observed during the development of many neurodegenerative diseases. The fact that S1R is a ligand-operated protein makes it a new promising molecular target for the development of drug therapy of HD based on the agonists of this receptor.

Sustained Hippocampal Synaptic Pathophysiology Following Single and Repeated Closed-Head Concussive Impacts

Traumatic brain injury (TBI), and related diseases such as chronic traumatic encephalopathy (CTE) and Alzheimer's (AD), are of increasing concern in part due to enhanced awareness of their long-term neurological effects on memory and behavior. Repeated concussions, vs. single concussions, have been shown to result in worsened and sustained symptoms including impaired cognition and histopathology. To assess and compare the persistent effects of single or repeated concussive impacts on mediators of memory encoding such as synaptic transmission, plasticity, and cellular Ca2+ signaling, a closed-head controlled cortical impact (CCI) approach was used which closely replicates the mode of injury in clinical cases. Adult male rats received a sham procedure, a single impact, or three successive impacts at 48-hour intervals. After 30 days, hippocampal slices were prepared for electrophysiological recordings and 2-photon Ca2+ imaging, or fixed and immunostained for pathogenic phospho-tau species. In both concussion groups, hippocampal circuits showed hyper-excitable synaptic responsivity upon Schaffer collateral stimulation compared to sham animals, indicating sustained defects in hippocampal circuitry. This was not accompanied by sustained LTP deficits, but resting Ca2+ levels and voltage-gated Ca2+ signals were elevated in both concussion groups, while ryanodine receptor-evoked Ca2+ responses decreased with repeat concussions. Furthermore, pathogenic phospho-tau staining was progressively elevated in both concussion groups, with spreading beyond the hemisphere of injury, consistent with CTE. Thus, single and repeated concussions lead to a persistent upregulation of excitatory hippocampal synapses, possibly through changes in postsynaptic Ca2+ signaling/regulation, which may contribute to histopathology and detrimental long-term cognitive symptoms.

Selective linkage of mitochondrial enzymes to intracellular calcium stores differs between human-induced pluripotent stem cells, neural stem cells, and neurons

Mitochondria and releasable endoplasmic reticulum (ER) calcium modulate neuronal calcium signaling, and both change in Alzheimer's disease (AD). The releasable calcium stores in the ER are exaggerated in fibroblasts from AD patients and in multiple models of AD. The activity of the alpha-ketoglutarate dehydrogenase complex (KGDHC), a key mitochondrial enzyme complex, is diminished in brains from AD patients, and can be plausibly linked to plaques and tangles. Our previous studies in cell lines and mouse neurons demonstrate that reductions in KGDHC increase the ER releasable calcium stores. The goal of these studies was to test whether the relationship was true in human iPSC-derived neurons. Inhibition of KGDHC for one or 24 hr increased the ER releasable calcium store in human neurons by 69% and 144%, respectively. The effect was mitochondrial enzyme specific because inhibiting the pyruvate dehydrogenase complex, another key mitochondrial enzyme complex, diminished the ER releasable calcium stores. The link of KGDHC to ER releasable calcium stores was cell type specific as the interaction was not present in iPSC or neural stem cells. Thus, these studies in human neurons verify a link between KGDHC and releasable ER calcium stores, and support the use of human neurons to examine mechanisms and potential therapies for AD.

Sodium-Calcium Exchangers of the SLC8 Family in Oligodendrocytes: Functional Properties in Health and Disease

The solute carrier 8 (SLC8) family of sodium-calcium exchangers (NCXs) functions as an essential regulatory system that couples opposite fluxes of sodium and calcium ions across plasmalemmal membranes. NCXs, thereby, play key roles in maintaining an ion homeostasis that preserves cellular integrity. Hence, alterations in NCX expression and regulation have been found to lead to ionic imbalances that are often associated with intracellular calcium overload and cell death. On the other hand, intracellular calcium has been identified as a key driver for a multitude of downstream signaling events that are crucial for proper functioning of biological systems, thus highlighting the need for a tightly controlled balance. In the CNS, NCXs have been primarily characterized in the context of synaptic transmission and ischemic brain damage. However, a much broader picture is emerging. NCXs are expressed by virtually all cells of the CNS including oligodendrocytes (OLGs), the cells that generate the myelin sheath. With a growing appreciation of dynamic calcium signals in OLGs, NCXs are becoming increasingly recognized for their crucial roles in shaping OLG function under both physiological and pathophysiological conditions. In order to provide a current update, this review focuses on the importance of NCXs in cells of the OLG lineage. More specifically, it provides a brief introduction into plasmalemmal NCXs and their modes of activity, and it discusses the roles of OLG expressed NCXs in regulating CNS myelination and in contributing to CNS pathologies associated with detrimental effects on OLG lineage cells.