Caspase-3 activation in oligodendrocytes from the myelin-deficient rat

A Comprehensive Insight into Apoptosis: Molecular Mechanisms, Signaling Pathways, and Modulating Therapeutics

Apoptosis, or programmed cell death, is a fundamental biological process essential for maintaining tissue homeostasis. Dysregulation of apoptosis is implicated in a variety of diseases, including cancer, neurodegenerative disorders, and autoimmune conditions. This review provides an in-depth insight into the molecular mechanisms and signaling pathways that regulate apoptosis, highlighting both intrinsic and extrinsic pathways. Additionally, the review explains the tumor microenvironment's influence on apoptosis and its implications for cancer therapy resistance. Understanding the complex interplay between apoptotic signaling and cellular responses is crucial for developing targeted therapies that can effectively manage diseases associated with apoptosis dysregulation. The effects of conventional therapeutics and alternative substances with natural sources such as herbal compounds, alongside vitamins, minerals, and trace elements on cellular homeostasis and disease pathogenesis have been thoroughly investigated. Moreover, recent advances in therapeutic strategies aimed at modulating apoptosis are discussed, with a focus on novel interventions such as nutrition bio shield dietary supplement. These emerging approaches offer potential benefits beyond conventional treatments by selectively targeting apoptotic pathways to inhibit cancer progression and metastasis. By integrating insights from recent studies, this review aims to enhance our understanding of apoptosis and guide future research in developing innovative therapeutic approaches.

Insertion of proteolipid protein into oligodendrocyte mitochondria regulates extracellular pH and adenosine triphosphate

Proteolipid protein (PLP) and DM20, the most abundant myelin proteins, are coded by the human PLP1 and non-human Plp1 PLP gene. Mutations in the PLP1 gene cause Pelizaeus-Merzbacher disease (PMD) with duplications of the native PLP1 gene accounting for 70% of PLP1 mutations. Humans with PLP1 duplications and mice with extra Plp1 copies have extensive neuronal degeneration. The mechanism that causes neuronal degeneration is unknown. We show that native PLP traffics to mitochondria when the gene is duplicated in mice and in humans. This report is the first demonstration of a specific cellular defect in brains of PMD patients; it validates rodent models as ideal models to study PMD. Insertion of nuclear-encoded mitochondrial proteins requires specific import pathways; we show that specific cysteine motifs, part of the Mia40/Erv1 mitochondrial import pathway, are present in PLP and are required for its insertion into mitochondria. Insertion of native PLP into mitochondria of transfected cells acidifies media, partially due to increased lactate; it also increases adenosine triphosphate (ATP) in the media. The same abnormalities are found in the extracellular space of mouse brains with extra copies of Plp1. These physiological abnormalities are preventable by mutations in PLP cysteine motifs, a hallmark of the Mia40/Erv1 pathway. Increased extracellular ATP and acidosis lead to neuronal degeneration. Our findings may be the mechanism by which microglia are activated and proinflammatory molecules are upregulated in Plp1 transgenic mice (Tatar et al. (2010) ASN Neuro 2:art:e00043). Manipulation of this metabolic pathway may restore normal metabolism and provide therapy for PMD patients.

Maturation-dependent sensitivity of oligodendrocyte lineage cells to apoptosis: implications for normal development and disease

Apoptosis plays a crucial role in brain development by ensuring that only appropriately growing, migrating, and synapse-forming neurons and their associated glial cells survive. This process involves an intimate relationship between cell-cell interactions and developmental cues and is further impacted by environmental stress during neurogenesis and disease. Oligodendrocytes (OLs), the major myelin-forming cells in the central nervous system, largely form after this wave of neurogenesis but also show a selective vulnerability to cell death stimuli depending on their stage of development. This can affect not only embryonic and early postnatal brain formation but also the response to demyelinating pathologies. In the present review, we discuss the stage-specific sensitivity of OL lineage cells to damage-induced death and how this might impact myelin survival and regeneration during injury or disease.

Minimal role for caspase 12 in the unfolded protein response in oligodendrocytes in vivo

The unfolded protein response (UPR) is implicated in many neurodegenerative disorders including Alzheimer, Parkinson and prion diseases, and the leukodystrophy, Pelizaeus-Merzbacher disease (PMD). Critical features of degeneration in several of these diseases involve activation of cell death pathways in various neural cell populations, and the initiator caspase 12 has been proposed to play a central role. Accordingly, pharmacological strategies to inhibit caspase 12 activity have received remarkable attention in anticipation of effecting disease amelioration. Our investigation in animal models of PMD demonstrates that caspase 12 is activated following accumulation of mutant proteins in oligodendrocytes; however, eliminating caspase 12 activity does not alter pathophysiology with respect to levels of apoptosis, oligodendrocyte function, disease severity or life span. We conclude that caspase 12 activation by UPR signaling is an epiphenomenon that plays little discernable role in the loss of oligodendrocytes in vivo and may portend the inconsequence of caspase 12 to the pathophysiology of other protein conformational diseases.

Iron contributes to dopamine-induced toxicity in oligodendrocyte progenitors

Iron is potentially toxic to oligodendrocyte progenitors due to its high intracellular levels and its ability to catalyse oxidant-producing reactions. Oxidative stress resulting from a hypoxic-ischaemic insult has been implicated in death of oligodendrocyte progenitors that occurs in the hypomyelinating disorder periventricular leucomalacia. Ischaemic insults induce the release of various neurotransmitters, including dopamine (DA), and we previously showed that DA is toxic to cultured oligodendrocytes, by inducing oxidative stress and apoptosis. Therefore, we investigated the role of iron in DA-induced cell death in oligodendrocyte progenitors. Intracellular iron levels were altered using an iron chelator, deferoxamine (DFO), and supplementation with ferrous sulphate (FeSO(4)). Addition of FeSO(4) to cultures increased DA-induced toxicity as assessed by mitochondrial dehydrogenase activity and cellular release of lactate dehydrogenase. Furthermore, FeSO(4) increased expression of the stress protein heme oxygenase-1 (HO-1), nuclear condensation and caspase-3 activation. In contrast, preincubation with DFO reduced these events as well as cleavage of alpha-spectrin, a caspase-3 substrate. In addition, FeSO(4) reversed the protective effect of DFO on DA-induced cytotoxicity, HO-1 expression and caspase-3 activation. These results indicate that elevated levels of free iron contribute to DA-induced toxicity in oligodendrocyte progenitors.

Bcl-2-related protein family gene expression during oligodendroglial differentiation

Oligodendroglial lineage cells (OLC) vary in susceptibility to both necrosis and apoptosis depending on their developmental stages, which might be regulated by differential expression of Bcl-2-related genes. As an initial step to test this hypothesis, we examined the expression of 19 Bcl-2-related genes in purified cultures of rat oligodendroglial progenitors, immature and mature oligodendrocytes. All 'multidomain' anti-apoptotic members (Bcl-x, Bcl-2, Mcl-1, Bcl-w and Bcl2l10/Diva/Boo) except Bcl2a1/A1 are expressed in OLC. Semiquantitative and real-time RT-PCR revealed that Bcl-xL and Mcl-1 mRNAs are the dominant anti-apoptotic members and increase four- and twofold, respectively, with maturation. Bcl-2 mRNA is less abundant than Bcl-xL mRNA in progenitors and falls an additional 10-fold during differentiation. Bcl-w mRNA also increases, with significant changes in its splicing pattern, as OLC mature. Transfection studies demonstrated that Bcl-xL overexpression protects against kainate-induced excitotoxicity, whereas Bcl-2 overexpression does not. As for 'multidomain' pro-apoptotic members (Bax, Bad and Bok/Mtd), Bax and Bak are highly expressed throughout differentiation. Among 'BH3 domain-only' members examined (Bim, Biklk, DP5/Hrk, Bad, Bid, Noxa, Puma/Bbc3, Bmf, BNip3 and BNip3L), BNip3 and Bmf mRNAs increase markedly during differentiation. These results provide basic information to guide further studies on the roles for Bcl-2-related family proteins in OLC death.

Differential lipid peroxidation, Mn superoxide, and bcl-2 expression contribute to the maturation-dependent vulnerability of oligodendrocytes to oxidative stress

To understand the basis of oligodendrocyte (OL) susceptibility to oxidative injury, purified rat OL cultures at different stages of maturation were exposed to nitric oxide (NO) donors with fast or slow kinetics of release and to tert-butyl-hydroperoxide, a membrane-permeant organic hydroperoxide. OL precursors (pre-OL) displayed the highest vulnerability to both oxygen or nitrogen reactive species, whereas mature OLs were uniquely vulnerable to long-lasting levels of NO. Cell death occurred by necrosis as well as apoptosis associated with increased caspase-3 activity and, only in the case of pre-OLs, with a decreased expression of the anti-apoptotic protein bcl-2. Pre-OLs were also more susceptible than mature OLs to lipid peroxidation, as measured by F2-isoprostane content in culture media. Finally, pre-OLs, but not mature OLs, expressed high levels of the mitochondrial scavenging enzyme Mn superoxide dismutase, suggesting that pre-OLs may efficiently convert anion superoxide into hydrogen peroxide and, paradoxically, be more predisposed than mature OLs to a toxic imbalance between hydrogen peroxide production and detoxification processes. These data suggest that susceptibility to lipid peroxidation, expression of the scavenging enzyme Mn superoxide dismutase and of the anti-apoptotic protein bcl-2, may contribute to the maturation-dependent vulnerability of OLs to oxidant injury.

AMPA receptor-mediated toxicity in oligodendrocyte progenitors involves free radical generation and activation of JNK, calpain and caspase 3

The molecular mechanisms underlying AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate) receptor-mediated excitotoxicity were characterized in rat oligodendrocyte progenitor cultures. Activation of AMPA receptors, in the presence of cyclothiazide to selectively block desensitization, produced a massive Ca(2+) influx and cytotoxicity which were blocked by the antagonists CNQX and GYKI 52466. A role for free radical generation in oligodendrocyte progenitor cell death was deduced from three observations: (i) treatment with AMPA agonists decreased intracellular glutathione; (ii) depletion of intracellular glutathione with buthionine sulfoximine potentiated cell death; and (iii) the antioxidant N -acetylcysteine replenished intracellular glutathione and protected cultures from AMPA receptor-mediated toxicity. Cell death displayed some characteristics of apoptosis, including DNA fragmentation, chromatin condensation and activation of caspase-3 and c-Jun N-terminal kinase (JNK). A substrate of calpain and caspase-3, alpha-spectrin, was cleaved into characteristic products following treatment with AMPA agonists. In contrast, inhibition of either caspase-3 by DEVD-CHO or calpain by PD 150606 protected cells from excitotoxicity. Our results indicate that overactivation of AMPA receptors causes apoptosis in oligodendrocyte progenitors through mechanisms involving Ca(2+) influx, depletion of glutathione, and activation of JNK, calpain, and caspase-3.