Oligodendroglial modulation of fast axonal transport in a mouse model of hereditary spastic paraplegia

Oligodendrocytes are critical for the development of the plasma membrane and cytoskeleton of the axon. In this paper, we show that fast axonal transport is also dependent on the oligodendrocyte. Using a mouse model of hereditary spastic paraplegia type 2 due to a null mutation of the myelin Plp gene, we find a progressive impairment in fast retrograde and anterograde transport. Increased levels of retrograde motor protein subunits are associated with accumulation of membranous organelles distal to nodal complexes. Using cell transplantation, we show categorically that the axonal phenotype is related to the presence of the overlying Plp null myelin. Our data demonstrate a novel role for oligodendrocytes in the local regulation of axonal function and have implications for the axonal loss associated with secondary progressive multiple sclerosis.     

Regulation of mitochondrial proteostasis by the proton gradient

Mitochondria adapt to different energetic demands reshaping their proteome. Mitochondrial proteases are emerging as key regulators of these adaptive processes. Here, we use a multiproteomic approach to demonstrate the regulation of the m‐AAA protease AFG3L2 by the mitochondrial proton gradient, coupling mitochondrial protein turnover to the energetic status of mitochondria. We identify TMBIM5 (previously also known as GHITM or MICS1) as a Ca²⁺/H⁺ exchanger in the mitochondrial inner membrane, which binds to and inhibits the m‐AAA protease. TMBIM5 ensures cell survival and respiration, allowing Ca²⁺ efflux from mitochondria and limiting mitochondrial hyperpolarization. Persistent hyperpolarization, however, triggers degradation of TMBIM5 and activation of the m‐AAA protease. The m‐AAA protease broadly remodels the mitochondrial proteome and mediates the proteolytic breakdown of respiratory complex I to confine ROS production and oxidative damage in hyperpolarized mitochondria. TMBIM5 thus integrates mitochondrial Ca²⁺ signaling and the energetic status of mitochondria with protein turnover rates to reshape the mitochondrial proteome and adjust the cellular metabolism.     

Perturbations of model membranes induced by pathogenic dynorphin A mutants causing neurodegeneration in human brain

Several effects of the endogenous opioid peptide dynorphin A (Dyn A) are not mediated through the opioid receptors. These effects are generally excitatory, and result in cell loss and induction of chronic pain and paralysis. The mechanism(s) is not well defined but may involve formation of pores in cellular membranes. In the 17-amino acid peptide Dyn A we have recently identified L5S, R6W, and R9C mutations that cause the dominantly inherited neurodegenerative disorder Spinocerebellar ataxia type 23. To gain further insight into non-opioid neurodegenerative mechanism(s), we studied the perturbation effects on lipid bilayers of wild type Dyn A and its mutants in large unilamellar phospholipid vesicles encapsulating the fluorescent dye calcein. The peptides were found to induce calcein leakage from uncharged and negatively charged vesicles to different degrees, thus reflecting different membrane perturbation effects. The mutant Dyn A R6W was the most potent in producing leakage with negatively charged vesicles whereas Dyn A L5S was virtually inactive. The overall correlation between membrane perturbation and neurotoxic response [3] suggests that pathogenic Dyn A actions may be mediated through transient pore formation in lipid domains of the plasma membrane.     

Uncoupling the roles of the SUV3 helicase in maintenance of mitochondrial genome stability and RNA degradation

Yeast SUV3 is a nuclear encoded mitochondrial RNA helicase that complexes with an exoribonuclease, DSS1, to function as an RNA degradosome. Inactivation of SUV3 leads to mitochondrial dysfunctions, such as respiratory deficiency; accumulation of aberrant RNA species, including excised group I introns; and loss of mitochondrial DNA (mtDNA). Although intron toxicity has long been speculated to be the major reason for the observed phenotypes, direct evidence to support or refute this theory is lacking. Moreover, it remains unknown whether SUV3 plays a direct role in mtDNA maintenance independently of its degradosome activity. In this paper, we address these questions by employing an inducible knockdown system in Saccharomyces cerevisiae with either normal or intronless mtDNA background. Expressing mutants defective in ATPase (K245A) or RNA binding activities (V272L or ΔCC, which carries an 8-amino acid deletion at the C-terminal conserved region) resulted in not only respiratory deficiencies but also loss of mtDNA under normal mtDNA background. Surprisingly, V272L, but not other mutants, can rescue the said deficiencies under intronless background. These results provide genetic evidence supporting the notion that the functional requirements of SUV3 for degradosome activity and maintenance of mtDNA stability are separable. Furthermore, V272L mutants and wild-type SUV3 associated with an active mtDNA replication origin and facilitated mtDNA replication, whereas K245A and ΔCC failed to support mtDNA replication. These results indicate a direct role of SUV3 in maintaining mitochondrial genome stability that is independent of intron turnover but requires the intact ATPase activity and the CC conserved region.     

Identification of nuclear localisation sequences in spastin (SPG4) using a novel Tetra-GFP reporter system

Mutations in the human spastin gene (SPG4) cause the most prevalent form of autosomal dominant hereditary spastic paraplegia (HSP), a neurodegenerative disorder characterised by progressive weakness and spasticity of the lower limbs. We address the question of intracellular localisation of spastin. Using polyclonal antibodies against N-terminal spastin sequences, we find that the native protein is localised in both the perinuclear cytoplasm and the nucleus. To identify structural motifs within the protein that can explain entry into the nucleus, we developed a reporter system to test nuclear localisation sequence (NLS)-functionality based on four in-frame fused copies of green fluorescent protein. Using this novel tool we demonstrate that spastin carries two NLSs located in exons 1 and 6. Both are independently functional in mediating nuclear entry.     

Calcium-free calmodulin is a substrate of proteases from human immunodeficiency viruses 1 and 2

Calcium-free calmodulin-(CaM) is rapidly hydrolyzed by proteases from both human immunodeficiency viruses (HIV) 1 and 2. Kinetic analysis reveals a sequential order of cleavage by both proteases which initiates in regions of the molecule known from X-ray crystallographic analysis of Ca2+/CaM to be associated with calcium binding. Although HIV-1 and HIV-2 proteases hydrolyze two bonds in common, the initial site of cleavage required for subsequent events differs in each case. The first bond hydrolyzed by the HIV-1 protease is the Asn-Tyr linkage in the sequence, -N-I-D-G-D-G-Q-V-N-Y-E-E-, found in the fourth calcium binding loop. In contrast, it is an Ala-Ala bond in the third calcium loop, -D-K-D-G-N-G-Y-I-S-A-A-E-, that is first hydrolyzed by the HIV-2 enzyme, followed in short order by cleavage of the same Asn-Tyr linkage described above. Thereafter, both enzymes proceed to hydrolyze additional peptide bonds, some in common, some not. Considerable evidence exists that inhibitors are bound to the protease in an extended conformation and yet all of the cleavages we observed occur within, or at the beginning of helices in Ca2+/CaM, regions that also appear to be insufficiently exposed for protease binding. Molecular modeling studies indicate that CaM in solution must adopt a conformation in which the first cleavage site observed for each enzyme is unshielded and extended, and that subsequent cleavages involve further unwinding of helices.(ABSTRACT TRUNCATED AT 250 WORDS)     

Human NLRP1 is a sensor of pathogenic coronavirus 3CL proteases in lung epithelial cells

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Emerging roles of Orai3 in pathophysiology

Calcium (Ca²⁺) is a ubiquitous second messenger that regulates a plethora of physiological functions. Deregulation of calcium homeostasis has been reported in a wide variety of pathological conditions including cardiovascular disorders, cancer and neurodegenerative diseases. One of the most ubiquitous pathways involved in regulated Ca²⁺ influx into cells is the store-operated Ca²⁺ entry (SOCE) pathway. In 2006, Orai1 was identified as the channel protein that mediates SOCE in immune cells. Orai1 has two mammalian homologs, Orai2 and Orai3. Although Orai1 has been the most widely studied Orai isoform, Orai3 has recently received significant attention. Under native conditions, Orai3 was demonstrated to be an important component of store-independent arachidonate-regulated Ca²⁺ (ARC) entry in HEK293 cells, and more recently of a store-independent leukotrieneC₄-regulated Ca²⁺ (LRC) entry pathway in vascular smooth muscle cells. Recent studies have shown upregulation of Orai3 in estrogen receptor-expressing breast cancers and a critical role for Orai3 in breast cancer development in immune-compromised mice. Orai3 upregulation was also shown to contribute to vascular smooth muscle remodeling and neointimal hyperplasia caused by vascular injury. Furthermore, Orai3 has been shown to contribute to proliferation of effector T-lymphocytes under oxidative stress. In this review, we will discuss the role of Orai3 in reported pathophysiological conditions and will contribute ideas on the potential role of Orai3 in native Ca²⁺ signaling pathways and human disease.     

Emerging roles of PHLPP phosphatases in metabolism

Over the last decades, research has focused on the role of pleckstrin homology (PH) domain leucine-rich repeat protein phosphatases (PHLPPs) in regulating cellular signaling via PI3K/Akt inhibition. The PKB/Akt signaling imbalances are associated with a variety of illnesses, including various types of cancer, inflammatory response, insulin resistance, and diabetes, demonstrating the relevance of PHLPPs in the prevention of diseases. Furthermore, identification of novel substrates of PHLPPs unveils their role as a critical mediator in various cellular processes. Recently, researchers have explored the increasing complexity of signaling networks involving PHLPPs whereby relevant information of PHLPPs in metabolic diseases was obtained. In this review, we discuss the current knowledge of PHLPPs on the well-known substrates and metabolic regulation, especially in liver, pancreatic beta cell, adipose tissue, and skeletal muscle in relation with the stated diseases. Understanding the context-dependent functions of PHLPPs can lead to a promising treatment strategy for several kinds of metabolic diseases.     

Alsin and the Molecular Pathways of Amyotrophic Lateral Sclerosis

Autosomal recessive mutations in the ALS2 gene lead to a clinical spectrum of motor dysfunction including juvenile onset amyotrophic lateral sclerosis (ALS2), primary lateral sclerosis, and hereditary spastic paraplegia. The 184-kDa alsin protein, encoded by the full-length ALS2 gene, contains three different guanine-nucleotide-exchange factor-like domains, which may play a role in the etiology of the disease. Multiple in vitro biochemical and cell biology assays suggest that alsin dysfunction affects endosome trafficking through a Rab5 small GTPase family-mediated mechanism. Four ALS2-deficient mouse models have been generated by different groups and used to study the behavioral and pathological impact of alsin deficiency. These mouse models largely fail to recapitulate hallmarks of motor neuron disease, but the subtle deficits that are observed in behavior and pathology have aided in our understanding of the relationship between alsin and motor dysfunction. In this review, we summarize recent clinical and molecular reports regarding alsin and attempt to place these results within the larger context of motor neuron disease.     

Cyclophilin D-dependent mitochondrial permeability transition is not involved in neurodegeneration in mnd2 mutant mice

Parkinson’s disease (PD) is a common neurodegenerative disorder. The motor neuron degeneration 2 mutant (mnd2) mouse exhibits loss of striatal neurons, muscle wasting, weight loss, and death within 40 days of birth, and is considered to be a useful animal model of PD. mnd2 was identified as an autosomal recessive mutation in the HtrA2/Omi gene, which encodes a mitochondrial serine protease. Omi-deficient mitochondria are more sensitive to mitochondrial permeability transition (mPT), which raises the possibility that mPT plays a role in motor neurodegeneration in mnd2 mice. Given that cyclophilin D (CypD)-deficient mitochondria are resistant to mPT, we examined whether CypD-dependent mPT is involved in the pathogenesis of neurodegenerative disorders in mnd2 mice by generating CypD-deficient mnd2 mice. Brain mitochondria isolated from CypD-deficient mnd2 mice were more resistant to Ca²⁺-induced mPT than those of mnd2 mice. However, both mnd2 mice and CypD-deficient mnd2 mice showed similar survival periods and phenotypes, including the lack of weight gain, muscle wasting, and resting tremor. Our data suggest that CypD-dependent mPT does not play a major role in neurodegeneration in mnd2 mice.     

Oligodendrocytes enhance axonal energy metabolism by deacetylation of mitochondrial proteins through transcellular delivery of SIRT2

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Interdomain interactions in aspartic proteases of higher organisms and their analogs in retroviral enzymes

On recent evidence, the efficiency of catalysis and the specificity of aspartic proteases depend considerably on the dynamic properties of particular molecular regions and their correlations. Analysis of the three-dimensional structures of these enzymes showed the presence of a continuous chain of hydrogen-bonded groups, which connects regions with highly correlated dynamic parameters and provides for a “cross-hand” interaction between domains. This chain includes the inner oxygens of the active carboxyls and the conserved internal water molecules. The so-called “fireman grip” interdomain hydrogen bonding is part of this chain. Such networks are abortive in retroviral proteases. The role of these interactions in the functions of aspartic proteases is discussed.     

Emerging roles of lipids in BCL-2 family-regulated apoptosis

Apoptosis is an intricately regulated process required for the health and homeostasis of living systems. The mitochondrial apoptotic pathway depends on the BCL-2 family of pro- and anti-apoptotic members whose interactions form a complex network of checks and balances in regulating cell fate. A diverse set of signals recruits distinct BH3-domain only BCL-2 proteins to trigger activation of the executioner proteins BAX and BAK. In addition to protein components of the apoptotic machinery, literature of the past several decades supports crucial functions for lipids in apoptosis and cooperation between lipid metabolism and BCL-2 proteins. In this review we present the two key examples of ceramide and cardiolipin in apoptosis, focusing particularly on BCL-2 family-regulated pathways at the mitochondrial level. This article is part of a Special Issue entitled Lipid Metabolism in Cancer.     

Involvement of protein kinase C delta in the alteration of mitochondrial mass in human cells under oxidative stress

Alteration of mitochondrial mass of human 143B osteosarcoma cells upon exposure to hydrogen peroxide (H₂O₂) was investigated. We found that mitochondrial mass and the intracellular level of H₂O₂ were increased by exogenous H₂O₂, which was accompanied with up-regulation of functional PKCδ. To investigate the role of PKCδ in H₂O₂-induced increase of mitochondrial mass, we treated 143B cells with PKCδ activator, bistratene A, and PKCδ inhibitor, rottlerin, respectively. The results show that bistratene A caused an increase of mitochondrial mass and that the H₂O₂-induced increase of mitochondrial mass was completely suppressed by rottlerin. Furthermore, we found that activation of PKCδ by bistratene A increased the intracellular levels of H₂O₂ and MnSOD protein expression. By contrast, suppression of PKCδ by rottlerin decreased the intracellular levels of H₂O₂ and MnSOD protein expression. Moreover, we noted that MnSOD expression was highly correlated with the expression of p53, which was controlled by PKCδ. Finally, we demonstrated that PKCδ was overexpressed in skin fibroblasts of patients with MERRF syndrome. Taken together, we conclude that PKCδ is involved in the regulation of mitochondrial mass and intracellular H₂O₂ in human cells and may play a key role in the overproliferation of mitochondria in the affected tissues of patients with mitochondrial diseases such as MERRF syndrome.