The QKI-PLP pathway controls SIRT2 abundance in CNS myelin

Alternative splicing of HDAC7 regulates its interaction with 14-3-3 proteins to alter histone marks and target gene expression

Chromatin regulation and alternative splicing are both critical mechanisms guiding gene expression. Studies have demonstrated that histone modifications can influence alternative splicing decisions, but less is known about how alternative splicing may impact chromatin. Here, we demonstrate that several genes encoding histone-modifying enzymes are alternatively spliced downstream of T cell signaling pathways, including HDAC7, a gene previously implicated in controlling gene expression and differentiation in T cells. Using CRISPR-Cas9 gene editing and cDNA expression, we show that differential inclusion of HDAC7 exon 9 controls the interaction of HDAC7 with protein chaperones, resulting in changes to histone modifications and gene expression. Notably, the long isoform, which is induced by the RNA-binding protein CELF2, promotes expression of several critical T cell surface proteins including CD3, CD28, and CD69. Thus, we demonstrate that alternative splicing of HDAC7 has a global impact on histone modification and gene expression that contributes to T cell development.

Multiple Roles of SIRT2 in Regulating Physiological and Pathological Signal Transduction

Sirtuin 2 (SIRT2), as a member of the sirtuin family, has representative features of evolutionarily highly conserved nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase activity. In addition, SIRT2, as the only sirtuin protein colocalized with tubulin in the cytoplasm, has its own functions and characteristics. In recent years, studies have increasingly shown that SIRT2 can participate in the regulation of gene expression and regulate signal transduction in the metabolic pathway mainly through its post-translational modification of target genes; thus, SIRT2 has become a key centre in the metabolic pathway and participates in the pathological process of metabolic disorder-related diseases. In this paper, it is discussed that SIRT2 can regulate all aspects of gene expression, including epigenetic modification, replication, transcription and translation, and post-translational modification, which enables SIRT2 to participate in energy metabolism in life activities, and it is clarified that SIRT2 is involved in metabolic process-specific signal transduction mechanisms. Therefore, SIRT2 can be involved in metabolic disorder-related inflammation and oxidative stress, thereby triggering the occurrence of metabolic disorder-related diseases, such as neurodegenerative diseases, tumours, diabetes, and cardiovascular diseases. Currently, although the role of SIRT2 in some diseases is still controversial, given the multiple roles of SIRT2 in regulating physiological and pathological signal transduction, SIRT2 has become a key target for disease treatment. It is believed that with increasing research, the clinical application of SIRT2 will be promoted.

Sirt2 promotes white matter oligodendrogenesis during development and in models of neonatal hypoxia

Delayed oligodendrocyte (OL) maturation caused by hypoxia (Hx)-induced neonatal brain injury results in hypomyelination and leads to neurological disabilities. Previously, we characterized Sirt1 as a crucial regulator of OL progenitor cell (OPC) proliferation in response to Hx. We now identify Sirt2 as a critical promoter of OL differentiation during both normal white matter development and in a mouse model of Hx. Importantly, we find that Hx reduces Sirt2 expression in mature OLs and that Sirt2 overexpression in OPCs restores mature OL populations. Reduced numbers of Sirt2+ OLs were also observed in the white matter of preterm human infants. We show that Sirt2 interacts with p27Kip1/FoxO1, p21Cip1/Cdk4, and Cdk5 pathways, and that these interactions are altered by Hx. Furthermore, Hx induces nuclear translocation of Sirt2 in OPCs where it binds several genomic targets. Overall, these results indicate that a balance of Sirt1 and Sirt2 activity is required for developmental oligodendrogenesis, and that these proteins represent potential targets for promoting repair following white matter injury.

The Quaking RNA-binding proteins as regulators of cell differentiation

The RNA-binding protein Quaking (QKI) has emerged as a potent regulator of cellular differentiation in developmental and pathological processes. The QKI gene is itself alternatively spliced to produce three major isoforms, QKI-5, QKI-6, and QKI-7, that possess very distinct functions. Here, we highlight roles of the different QKI isoforms in neuronal, vascular, muscle, and monocyte cell differentiation, and during epithelial-mesenchymal transition in cancer progression. QKI isoforms control cell differentiation through regulating alternative splicing, mRNA stability and translation, with activities in gene transcription now also becoming evident. These diverse functions of the QKI isoforms contribute to their broad influences on RNA metabolism and cellular differentiation. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Processing > Splicing Regulation/Alternative Splicing RNA in Disease and Development > RNA in Development.

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

Neurons require mechanisms to maintain ATP homeostasis in axons, which are highly vulnerable to bioenergetic failure. Here, we elucidate a transcellular signaling mechanism by which oligodendrocytes support axonal energy metabolism via transcellular delivery of NAD-dependent deacetylase SIRT2. SIRT2 is undetectable in neurons but enriched in oligodendrocytes and released within exosomes. By deleting sirt2, knocking down SIRT2, or blocking exosome release, we demonstrate that transcellular delivery of SIRT2 is critical for axonal energy enhancement. Mass spectrometry and acetylation analyses indicate that neurons treated with oligodendrocyte-conditioned media from WT, but not sirt2-knockout, mice exhibit strong deacetylation of mitochondrial adenine nucleotide translocases 1 and 2 (ANT1/2). In vivo delivery of SIRT2-filled exosomes into myelinated axons rescues mitochondrial integrity in sirt2-knockout mouse spinal cords. Thus, our study reveals an oligodendrocyte-to-axon delivery of SIRT2, which enhances ATP production by deacetylating mitochondrial proteins, providing a target for boosting axonal bioenergetic metabolism in neurological disorders.

Qki activates Srebp2-mediated cholesterol biosynthesis for maintenance of eye lens transparency

Defective cholesterol biosynthesis in eye lens cells is often associated with cataracts; however, how genes involved in cholesterol biosynthesis are regulated in lens cells remains unclear. Here, we show that Quaking (Qki) is required for the transcriptional activation of genes involved in cholesterol biosynthesis in the eye lens. At the transcriptome level, lens-specific Qki-deficient mice present downregulation of genes associated with the cholesterol biosynthesis pathway, resulting in a significant reduction of total cholesterol level in the eye lens. Mice with Qki depletion in lens epithelium display progressive accumulation of protein aggregates, eventually leading to cataracts. Notably, these defects are attenuated by topical sterol administration. Mechanistically, we demonstrate that Qki enhances cholesterol biosynthesis by recruiting Srebp2 and Pol II in the promoter regions of cholesterol biosynthesis genes. Supporting its function as a transcription co-activator, we show that Qki directly interacts with single-stranded DNA. In conclusion, we propose that Qki-Srebp2-mediated cholesterol biosynthesis is essential for maintaining the cholesterol level that protects lens from cataract development.

Epigenetic regulation of oligodendrocyte myelination in developmental disorders and neurodegenerative diseases

Oligodendrocytes are the critical cell types giving rise to the myelin nerve sheath enabling efficient nerve transmission in the central nervous system (CNS). Oligodendrocyte precursor cells differentiate into mature oligodendrocytes and are maintained throughout life. Deficits in the generation, proliferation, or differentiation of these cells or their maintenance have been linked to neurological disorders ranging from developmental disorders to neurodegenerative diseases and limit repair after CNS injury. Understanding the regulation of these processes is critical for achieving proper myelination during development, preventing disease, or recovering from injury. Many of the key factors underlying these processes are epigenetic regulators that enable the fine tuning or reprogramming of gene expression during development and regeneration in response to changes in the local microenvironment. These include chromatin remodelers, histone-modifying enzymes, covalent modifiers of DNA methylation, and RNA modification-mediated mechanisms. In this review, we will discuss the key components in each of these classes which are responsible for generating and maintaining oligodendrocyte myelination as well as potential targeted approaches to stimulate the regenerative program in developmental disorders and neurodegenerative diseases.

Microtubule-Associated Proteins with Regulatory Functions by Day and Pathological Potency at Night

The sensing, integrating, and coordinating features of the eukaryotic cells are achieved by the complex ultrastructural arrays and multifarious functions of the cytoskeleton, including the microtubule network. Microtubules play crucial roles achieved by their decoration with proteins/enzymes as well as by posttranslational modifications. This review focuses on the Tubulin Polymerization Promoting Protein (TPPP/p25), a new microtubule associated protein, on its "regulatory functions by day and pathological functions at night". Physiologically, the moonlighting TPPP/p25 modulates the dynamics and stability of the microtubule network by bundling microtubules and enhancing the tubulin acetylation due to the inhibition of tubulin deacetylases. The optimal endogenous TPPP/p25 level is crucial for its physiological functions, to the differentiation of oligodendrocytes, which are the major constituents of the myelin sheath. Pathologically, TPPP/p25 forms toxic oligomers/aggregates with α-synuclein in neurons and oligodendrocytes in Parkinson's disease and Multiple System Atrophy, respectively; and their complex is a potential therapeutic drug target. TPPP/p25-derived microtubule hyperacetylation counteracts uncontrolled cell division. All these issues reveal the anti-mitotic and α-synuclein aggregation-promoting potency of TPPP/p25, consistent with the finding that Parkinson's disease patients have reduced risk for certain cancers.

Chromatin modification and epigenetic control in functional nerve regeneration

The repair and functional recovery of the nervous system is a highly regulated process that requires the coordination of many different components including the proper myelination of regenerated axons. Dysmyelination and remyelination failures after injury result in defective nerve conduction, impairing normal nervous system functions. There are many convergent regulatory networks and signaling mechanisms between development and regeneration. For instance, the regulatory mechanisms required for oligodendrocyte lineage progression could potentially play fundamental roles in myelin repair. In recent years, epigenetic chromatin modifications have been implicated in CNS myelination and functional nerve restoration. The pro-regenerative transcriptional program is likely silenced or repressed in adult neural cells including neurons and myelinating cells in the central and peripheral nervous systems limiting the capacity for repair after injury. In this review, we will discuss the roles of epigenetic mechanisms, including histone modifications, chromatin remodeling, and DNA methylation, in the maintenance and establishment of the myelination program during normal oligodendrocyte development and regeneration. We also discuss how these epigenetic processes impact myelination and axonal regeneration, and facilitate the improvement of current preclinical therapeutics for functional nerve regeneration in neurodegenerative disorders or after injury.

Inhibition of SIRT2 by Targeting GSK3β-Mediated Phosphorylation Alleviates SIRT2 Toxicity in SH-SY5Y Cells

Sirtuin 2 (SIRT2) is thought to be important in the pathogenesis of Parkinson’s disease (PD), and the inhibition of SIRT2 rescues α-synuclein toxicity in a cellular model of PD. Recent studies have focused on identifying inhibitors of SIRT2, but little is known about the processes that directly regulate its function. GSK3β is a serine/threonine protein kinase that affects a wide range of biological functions, and it is localized in Lewy bodies (LBs). Therefore, we investigated whether SIRT2 is regulated by GSK3β and enhances cell death in PD. In the present study, Western blot showed that total SIRT2 levels did not change noticeably in a cellular model of PD but that SIRT2 phosphorylation was increased, and GSK3β activity was elevated. In addition, mass spectrometry (MS) studies indicated that SIRT2 was phosphorylated by GSK3β at three specific sites. Phospho- or dephospho-mimicking studies demonstrated that this postmodification (phosphorylation) increased SIRT2 toxicity in SH-SY5Y cells. Collectively, our findings identify a posttranslational mechanism that controls SIRT2 function in PD and provide evidence for a novel regulatory pathway involving GSK3β, SIRT2, and α-synuclein.

SIRT2 knockout exacerbates insulin resistance in high fat-fed mice

The NAD+-dependent deacetylase SIRT2 is unique amongst sirtuins as it is effective in the cytosol, as well as the mitochondria. Defining the role of cytosolic acetylation state in specific tissues is difficult since even physiological effects at the whole body level are unknown. We hypothesized that genetic SIRT2 knockout (KO) would lead to impaired insulin action, and that this impairment would be worsened in HF fed mice. Insulin sensitivity was tested using the hyperinsulinemic-euglycemic clamp in SIRT2 KO mice and WT littermates. SIRT2 KO mice exhibited reduced skeletal muscle insulin-induced glucose uptake compared to lean WT mice, and this impairment was exacerbated in HF SIRT2 KO mice. Liver insulin sensitivity was unaffected in lean SIRT2 KO mice. However, the insulin resistance that accompanies HF-feeding was worsened in SIRT2 KO mice. It was notable that the effects of SIRT2 KO were largely disassociated from cytosolic acetylation state, but were closely linked to acetylation state in the mitochondria. SIRT2 KO led to an increase in body weight that was due to increased food intake in HF fed mice. In summary, SIRT2 deletion in vivo reduces muscle insulin sensitivity and contributes to liver insulin resistance by a mechanism that is unrelated to cytosolic acetylation state. Mitochondrial acetylation state and changes in feeding behavior that result in increased body weight correspond to the deleterious effects of SIRT2 KO on insulin action.

Conditional depletion of GSK3b protects oligodendrocytes from apoptosis and lessens demyelination in the acute cuprizone model

Apoptosis is recognized as the main mechanism of oligodendrocyte loss in Multiple Sclerosis caused either by immune mediated injury (Barnett & Prineas, ) or a direct degenerative process (oligodendrogliapathy; Lucchinetti et al., ). Cuprizone induced demyelination is the result of non-immune mediated apoptosis of oligodendrocytes (OL) and represents a model of oligodendrogliapathy (Simmons, Pierson, Lee, & Goverman, ). Glycogen Synthase Kinase (GSK) 3b has been shown to be pro-apoptotic for cells other than OL. Here, we sought to investigate whether GSK3b plays a role in cuprizone-induced apoptosis of OL by using a novel inducible conditional knockout (cKO) of GSK3b in mature OL. While depletion of GSK3b has no effect on survival of uninjured OL, it increases survival of mature OL exposed to cuprizone. We show that GSK3b-deficient OLs are protected against caspase-dependent, but not against caspase-independent apoptosis. Active GSK3b is present in the nuclei of OL at peak of caspase-dependent apoptosis. Significant preservation of myelinated axons is associated with GSK3b depletion and glial cell activation is markedly reduced. Collectively, the data show that GSK3b is pro-apoptotic for caspase-dependent cell death, likely through activation of nuclear GSK3b and its depletion promotes survival of oligodendrocytes and attenuates myelin loss.

Myelin injury in the central nervous system and Alzheimer's disease

Myelin is a membrane wrapped around the axon of the nerve cell, which is composed of the mature oligodendrocytes. The role of myelin is to insulate and prevent the nerve electrical impulses from the axon of the neurons to the axons of the other neurons, which is essential for the proper functioning of the nervous system. Minor changes in myelin thickness could lead to substantial changes in conduction speed and may thus alter neural circuit function. Demyelination is the myelin damage, which characterized by the loss of nerve sheath and the relative fatigue of the neuronal sheath and axon. Studies have shown that myelin injury may be closely related to neurodegenerative diseases and may be an early diagnostic criteria and therapeutic target. Thus this review summarizes the recent result of pathologic effect and signal pathways of myelin injury in neurodegenerative diseases, especially the Alzheimer's disease to provide new and effective therapeutic targets.

Modulation Of Microtubule Acetylation By The Interplay Of TPPP/p25, SIRT2 And New Anticancer Agents With Anti-SIRT2 Potency

The microtubule network exerts multifarious functions controlled by its decoration with various proteins and post-translational modifications. The disordered microtubule associated Tubulin Polymerization Promoting Protein (TPPP/p25) and the NAD⁺-dependent tubulin deacetylase sirtuin-2 (SIRT2) play key roles in oligodendrocyte differentiation by acting as dominant factors in the organization of myelin proteome. Herein, we show that SIRT2 impedes the TPPP/p25-promoted microtubule assembly independently of NAD⁺; however, the TPPP/p25-assembled tubulin ultrastructures were resistant against SIRT2 activity. TPPP/p25 counteracts the SIRT2-derived tubulin deacetylation producing enhanced microtubule acetylation. The inhibition of the SIRT2 deacetylase activity by TPPP/p25 is evolved by the assembly of these tubulin binding proteins into a ternary complex, the concentration-dependent formation of which was quantified by experimental-based mathematical modelling. Co-localization of the SIRT2-TPPP/p25 complex on the microtubule network was visualized in HeLa cells by immunofluorescence microscopy using Bimolecular Fluorescence Complementation. We also revealed that a new potent SIRT2 inhibitor (MZ242) and its proteolysis targeting chimera (SH1) acting together with TPPP/p25 provoke microtubule hyperacetylation, which is coupled with process elongation only in the case of the degrader SH1. Both the structural and the functional effects manifesting themselves by this deacetylase proteome could lead to the fine-tuning of the regulation of microtubule dynamics and stability.

Loss of SIRT2 leads to axonal degeneration and locomotor disability associated with redox and energy imbalance

Sirtuin 2 (SIRT2) is a member of a family of NAD⁺‐dependent histone deacetylases (HDAC) that play diverse roles in cellular metabolism and especially for aging process. SIRT2 is located in the nucleus, cytoplasm, and mitochondria, is highly expressed in the central nervous system (CNS), and has been reported to regulate a variety of processes including oxidative stress, genome integrity, and myelination. However, little is known about the role of SIRT2 in the nervous system specifically during aging. Here, we show that middle‐aged, 13‐month‐old mice lacking SIRT2 exhibit locomotor dysfunction due to axonal degeneration, which was not present in young SIRT2 mice. In addition, these Sirt2^−/− mice exhibit mitochondrial depletion resulting in energy failure, and redox dyshomeostasis. Our results provide a novel link between SIRT2 and physiological aging impacting the axonal compartment of the central nervous system, while supporting a major role for SIRT2 in orchestrating its metabolic regulation. This underscores the value of SIRT2 as a therapeutic target in the most prevalent neurodegenerative diseases that undergo with axonal degeneration associated with redox and energetic dyshomeostasis.

Ulk4 deficiency leads to hypomyelination in mice

Brain nerve fibers are insulated by myelin which is produced by oligodendrocytes. Defects in myelination are increasingly recognized as a common pathology underlying neuropsychiatric and neurodevelopmental disorders, which are associated with deletions of the Unc-51-like kinase 4 (ULK4) gene. Key transcription factors have been identified for oligodendrogenesis, but little is known about their associated regulators. Here we report that Ulk4 acts as a key regulator of myelination. Myelination is reduced by half in the Ulk4^tm1a/tm1a hypomorph brain, whereas expression of axonal marker genes Tubb3, Nefh, Nefl and Nefm remains unaltered. Transcriptome analyses reveal that 8 (Gfap, Mbp, Mobp, Plp1, Slc1a2, Ttr, Cnp, Scd2) of the 10 most significantly altered genes in the Ulk4^tm1a/tm1a brain are myelination-related. Ulk4 is co-expressed in Olig2⁺ (pan-oligodendrocyte marker) and CC1⁺ (mature myelinated oligodendrocyte marker) cells during postnatal development. Major oligodendrogeneic transcription factors, including Olig2, Olig1, Myrf, Sox10, Sox8, Sox6, Sox17, Nkx2-2, Nkx6-2 and Carhsp1, are significantly downregulated in the mutants. mRNA transcripts enriched in oligodendrocyte progenitor cells (OPCs), the newly formed oligodendrocytes (NFOs) and myelinating oligodendrocytes (MOs), are significantly attenuated. Expression of stage-specific oligodendrocyte factors including Cspg4, Sox17, Nfasc, Enpp6, Sirt2, Cnp, Plp1, Mbp, Ugt8, Mag and Mog are markedly decreased. Indirect effects of axon caliber and neuroinflammation may also contribute to the hypomyelination, as Ulk4 mutants display smaller axons and increased neuroinflammation. This is the first evidence demonstrating that ULK4 is a crucial regulator of myelination, and ULK4 may therefore become a novel therapeutic target for hypomyelination diseases.

Does Sirt2 Regulate Cholesterol Biosynthesis During Oligodendroglial Differentiation In Vitro and In Vivo?

Sirtuin2 (SIRT2) is a deacetylase enzyme predominantly expressed in myelinating glia of the central nervous system (CNS). We have previously demonstrated that Sirt2 expression enhances oligodendrocyte (OL) differentiation and arborization in vitro, but the molecular targets of SIRT2 in OLs remain speculative. SIRT2 has been implicated in cholesterol biosynthesis by promoting the nuclear translocation of sterol regulatory element binding protein (SREBP)-2. We investigated this further in CNS myelination by examining the role of Sirt2 in cholesterol biosynthesis in vivo and in vitro employing Sirt2 ^-/- mice, primary OL cells and CG4-OL cells. Our results demonstrate that expression of cholesterol biosynthetic genes in the CNS white matter or cholesterol content in purified myelin fractions did not differ between Sirt2 ^-/- and age-matched wild-type mice. Cholesterol biosynthetic gene expression profiles and total cholesterol content were not altered in primary OLs from Sirt2 ^-/- mice and in CG4-OLs when Sirt2 was either down-regulated with RNAi or overexpressed. In addition, Sirt2 knockdown or overexpression in CG4-OLs had no effect on SREBP-2 nuclear translocation. Our results indicate that Sirt2 does not impact the expression of genes encoding enzymes involved in cholesterol biosynthesis, total cholesterol content, or nuclear translocation of SREBP-2 during OL differentiation and myelination.

The Effects of Prenatal Iron Deficiency and Risperidone Treatment on the Rat Frontal Cortex: A Proteomic Analysis

Prenatal iron deficiency (pID) has been described to increase the risk for neurodevelopmental disorders such as autism and schizophrenia; however, the precise molecular mechanisms are still unknown. Here, we utilized high-throughput MS to examine the proteomic effects of pID in adulthood on the rat frontal cortex area (FCA). In addition, the FCA proteome was examined in adulthood following risperidone treatment in adolescence to see if these effects could be prevented. We identified 1501 proteins of which 100 were significantly differentially expressed in the FCA at postnatal day 90. Pathway analysis of proteins affected by pID revealed changes in metabolic processes, including the tricyclic acid cycle, mitochondrial dysfunction, and P13K/Akt signaling. Interestingly, most of these protein changes were not present in the adult pID offspring who received risperidone in adolescence. Considering the link between pID and several neurodevelopmental disorders such as autism and schizophrenia these presented results bring new perspectives to understand the role of iron in metabolic pathways and provide novel biomarkers for future studies of pID.

Sirtuin-2 Protects Neural Cells from Oxidative Stress and Is Elevated in Neurodegeneration

Sirtuins are highly conserved lysine deacetylases involved in ageing, energy production, and lifespan extension. The mammalian SIRT2 has been implicated in Parkinson's disease (PD) where studies suggest SIRT2 promotes neurodegeneration. We therefore evaluated the effects of SIRT2 manipulation in toxin treated SH-SY5Y cells and determined the expression and activity of SIRT2 in postmortem brain tissue from patients with PD. SH-SY5Y viability in response to oxidative stress induced by diquat or rotenone was measured following SIRT2 overexpression or inhibition of deacetylase activity, along with α-synuclein aggregation. SIRT2 in human tissues was evaluated using Western blotting, immunohistochemistry, and fluorometric activity assays. In SH-SY5Y cells, elevated SIRT2 protected cells from rotenone or diquat induced cell death and enzymatic inhibition of SIRT2 enhanced cell death. SIRT2 protection was mediated, in part, through elevated SOD2 expression. SIRT2 reduced the formation of α-synuclein aggregates but showed minimal colocalisation with α-synuclein. In postmortem PD brain tissue, SIRT2 activity was elevated compared to controls but also elevated in other neurodegenerative disorders. Results from both in vitro work and brain tissue suggest that SIRT2 is necessary for protection against oxidative stress and higher SIRT2 activity in PD brain may be a compensatory mechanism to combat neuronal stress.

RNA-binding Protein Quaking Stabilizes Sirt2 mRNA during Oligodendroglial Differentiation

Myelination is controlled by timely expression of genes involved in the differentiation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes (OLs). Sirtuin 2 (SIRT2), a NAD⁺-dependent deacetylase, plays a critical role in OL differentiation by promoting both arborization and downstream expression of myelin-specific genes. However, the mechanisms involved in regulating SIRT2 expression during OL development are largely unknown. The RNA-binding protein quaking (QKI) plays an important role in myelination by post-transcriptionally regulating the expression of several myelin specific genes. In quaking viable (qk^v/qk^v ) mutant mice, SIRT2 protein is severely reduced; however, it is not known whether these genes interact to regulate OL differentiation. Here, we report for the first time that QKI directly binds to Sirt2 mRNA via a common quaking response element (QRE) located in the 3' untranslated region (UTR) to control SIRT2 expression in OL lineage cells. This interaction is associated with increased stability and longer half-lives of Sirt2.1 and Sirt2.2 transcripts leading to increased accumulation of Sirt2 transcripts. Consistent with this, overexpression of qkI promoted the expression of Sirt2 mRNA and protein. However, overexpression of the nuclear isoform qkI-5 promoted the expression of Sirt2 mRNA, but not SIRT2 protein, and delayed OL differentiation. These results suggest that the balance in the subcellular distribution and temporal expression of QKI isoforms control the availability of Sirt2 mRNA for translation. Collectively, our study demonstrates that QKI directly plays a crucial role in the post-transcriptional regulation and expression of Sirt2 to facilitate OL differentiation.

Adolescent Alcohol Exposure Persistently Impacts Adult Neurobiology and Behavior

Adolescence is a developmental period when physical and cognitive abilities are optimized, when social skills are consolidated, and when sexuality, adolescent behaviors, and frontal cortical functions mature to adult levels. Adolescents also have unique responses to alcohol compared with adults, being less sensitive to ethanol sedative-motor responses that most likely contribute to binge drinking and blackouts. Population studies find that an early age of drinking onset correlates with increased lifetime risks for the development of alcohol dependence, violence, and injuries. Brain synapses, myelination, and neural circuits mature in adolescence to adult levels in parallel with increased reflection on the consequence of actions and reduced impulsivity and thrill seeking. Alcohol binge drinking could alter human development, but variations in genetics, peer groups, family structure, early life experiences, and the emergence of psychopathology in humans confound studies. As adolescence is common to mammalian species, preclinical models of binge drinking provide insight into the direct impact of alcohol on adolescent development. This review relates human findings to basic science studies, particularly the preclinical studies of the Neurobiology of Adolescent Drinking in Adulthood (NADIA) Consortium. These studies focus on persistent adult changes in neurobiology and behavior following adolescent intermittent ethanol (AIE), a model of underage drinking. NADIA studies and others find that AIE results in the following: increases in adult alcohol drinking, disinhibition, and social anxiety; altered adult synapses, cognition, and sleep; reduced adult neurogenesis, cholinergic, and serotonergic neurons; and increased neuroimmune gene expression and epigenetic modifiers of gene expression. Many of these effects are specific to adolescents and not found in parallel adult studies. AIE can cause a persistence of adolescent-like synaptic physiology, behavior, and sensitivity to alcohol into adulthood. Together, these findings support the hypothesis that adolescent binge drinking leads to long-lasting changes in the adult brain that increase risks of adult psychopathology, particularly for alcohol dependence.

Quaking Regulates Neurofascin 155 Expression for Myelin and Axoglial Junction Maintenance

RNA binding proteins required for the maintenance of myelin and axoglial junctions are unknown. Herein, we report that deletion of the Quaking (QKI) RNA binding proteins in oligodendrocytes (OLs) using Olig2-Cre results in mice displaying rapid tremors at postnatal day 10, followed by death at postnatal week 3. Extensive CNS hypomyelination was observed as a result of OL differentiation defects during development. The QKI proteins were also required for adult myelin maintenance, because their ablation using PLP-CreERT resulted in hindlimb paralysis with immobility at ∼30 d after 4-hydroxytamoxifen injection. Moreover, deterioration of axoglial junctions of the spinal cord was observed and is consistent with a loss of Neurofascin 155 (Nfasc155) isoform that we confirmed as an alternative splice target of the QKI proteins. Our findings define roles for the QKI RNA binding proteins in myelin development and maintenance, as well as in the generation of Nfasc155 to maintain healthy axoglial junctions.

SIGNIFICANCE STATEMENT

Neurofascin 155 is responsible for axoglial junction formation and maintenance. Using a genetic mouse model to delete Quaking (QKI) RNA-binding proteins in oligodendrocytes, we identify QKI as the long-sought regulator of Neurofascin alternative splicing, further establishing the role of QKI in oligodendrocyte development and myelination. We establish a new role for QKI in myelin and axoglial junction maintenance using an inducible genetic mouse model that deletes QKI in mature oligodendrocytes. Loss of QKI in adult oligodendrocytes leads to phenotypes reminiscent of the experimental autoimmune encephalomyelitis mouse model with complete hindlimb paralysis and death by 30 d after induction of QKI deletion.

Msh2 deficiency leads to dysmyelination of the corpus callosum, impaired locomotion, and altered sensory function in mice

A feature in patients with constitutional DNA-mismatch repair deficiency is agenesis of the corpus callosum, the cause of which has not been established. Here we report a previously unrecognized consequence of deficiency in MSH2, a protein known primarily for its function in correcting nucleotide mismatches or insertions and deletions in duplex DNA caused by errors in DNA replication or recombination. We documented that Msh2 deficiency causes dysmyelination of the axonal projections in the corpus callosum. Evoked action potentials in the myelinated corpus callosum projections of Msh2-null mice were smaller than wild-type mice, whereas unmyelinated axons showed no difference. Msh2-null mice were also impaired in locomotive activity and had an abnormal response to heat. These findings reveal a novel pathogenic consequence of MSH2 deficiency, providing a new mechanistic hint to previously recognized neurological disorders in patients with inherited DNA-mismatch repair deficiency.

SIRTain regulators of premature senescence and accelerated aging

The sirtuin proteins constitute class III histone deacetylases (HDACs). These evolutionarily conserved NAD(+)-dependent enzymes form an important component in a variety of cellular and biological processes with highly divergent as well as convergent roles in maintaining metabolic homeostasis, safeguarding genomic integrity, regulating cancer metabolism and also inflammatory responses. Amongst the seven known mammalian sirtuin proteins, SIRT1 has gained much attention due to its widely acknowledged roles in promoting longevity and ameliorating age-associated pathologies. The contributions of other sirtuins in the field of aging are also gradually emerging. Here, we summarize some of the recent discoveries in sirtuins biology which clearly implicate the functions of sirtuin proteins in the regulation of premature cellular senescence and accelerated aging. The roles of sirtuins in various cellular processes have been extrapolated to draw inter-linkage with anti-aging mechanisms. Also, the latest findings on sirtuins which might have potential effects in the process of aging have been reviewed.

Gender influence on schizophrenia-relevant abnormalities in a cuprizone demyelination model

The aim of this study was to determine whether early demyelination can impact behavior in young adulthood. For this purpose, albino Wistar rats of either sex were exposed to cuprizone (CPZ) in two different intoxication protocols: one group was intoxicated before weaning (CPZ-BW), from postnatal day 7 (P7) to P21, through maternal milk, whereas the other group was intoxicated after weaning (CPZ-AW), from P21 to P35. After treatment, rats were returned to a normal diet until P90 when behavioral studies were performed. Both treatments produced marked demyelination in the corpus callosum and retraction of cortical myelin fibers. The subsequent normal diet allowed for effective remyelination at P90. Interestingly, CPZ-AW correlated with significant behavioral and neurochemical changes in a gender-dependent manner. CPZ-AW treatment altered both the number of social activities and the latency to the first social interaction in males, while also highly compromising recognition-related activities. In addition, only P90 males treated AW showed a hyperdopaminergic striatum, confirmed by an increase in tyrosine hydroxylase expression and in dopamine levels. Our results suggest that the timing of demyelination significantly influences the development of altered behavior, particularly in adult males.

A cytoplasmic quaking I isoform regulates the hnRNP F/H-dependent alternative splicing pathway in myelinating glia

The selective RNA-binding protein quaking I (QKI) plays important roles in controlling alternative splicing (AS). Three QKI isoforms are broadly expressed, which display distinct nuclear-cytoplasmic distribution. However, molecular mechanisms by which QKI isoforms control AS, especially in distinct cell types, still remain elusive. The quakingviable (qk^v) mutant mice carry deficiencies of all QKI isoforms in oligodendrocytes (OLs) and Schwann cells (SWCs), the myelinating glia of central and peripheral nervous system (CNS and PNS), respectively, resulting in severe dysregulation of AS. We found that the cytoplasmic isoform QKI-6 regulates AS of polyguanine (G-run)-containing transcripts in OLs and rescues aberrant AS in the qk^v mutant by repressing expression of two canonical splicing factors, heterologous nuclear ribonucleoproteins (hnRNPs) F and H. Moreover, we identified a broad spectrum of in vivo functional hnRNP F/H targets in OLs that contain conserved exons flanked by G-runs, many of which are dysregulated in the qk^v mutant. Interestingly, AS targets of the QKI-6-hnRNP F/H pathway in OLs are differentially affected in SWCs, suggesting that additional cell-type-specific factors modulate AS during CNS and PNS myelination. Together, our studies provide the first evidence that cytoplasmic QKI-6 acts upstream of hnRNP F/H, which forms a novel pathway to control AS in myelinating glia.

Regulation of G6PD acetylation by SIRT2 and KAT9 modulates NADPH homeostasis and cell survival during oxidative stress

Glucose-6-phosphate dehydrogenase (G6PD) is a key enzyme in the pentose phosphate pathway (PPP) and plays an essential role in the oxidative stress response by producing NADPH, the main intracellular reductant. G6PD deficiency is the most common human enzyme defect, affecting more than 400 million people worldwide. Here, we show that G6PD is negatively regulated by acetylation on lysine 403 (K403), an evolutionarily conserved residue. The K403 acetylated G6PD is incapable of forming active dimers and displays a complete loss of activity. Knockdown of G6PD sensitizes cells to oxidative stress, and re-expression of wild-type G6PD, but not the K403 acetylation mimetic mutant, rescues cells from oxidative injury. Moreover, we show that cells sense extracellular oxidative stimuli to decrease G6PD acetylation in a SIRT2-dependent manner. The SIRT2-mediated deacetylation and activation of G6PD stimulates PPP to supply cytosolic NADPH to counteract oxidative damage and protect mouse erythrocytes. We also identified KAT9/ELP3 as a potential acetyltransferase of G6PD. Our study uncovers a previously unknown mechanism by which acetylation negatively regulates G6PD activity to maintain cellular NADPH homeostasis during oxidative stress.

Clinically relevant intronic splicing enhancer mutation in myelin proteolipid protein leads to progressive microglia and astrocyte activation in white and gray matter regions of the brain

INTRODUCTION

Mutations in proteolipid protein (PLP), the most abundant myelin protein in the CNS, cause the X-linked dysmyelinating leukodystrophies, Pelizaeus-Merzbacher disease (PMD) and spastic paraplegia type 2 (SPG2). Point mutations, deletion, and duplication of the PLP1 gene cause PMD/SPG2 with varying clinical presentation. Deletion of an intronic splicing enhancer (ISEdel) within intron 3 of the PLP1 gene is associated with a mild form of PMD. Clinical and preclinical studies have indicated that mutations in myelin proteins, including PLP, can induce neuroinflammation, but the temporal and spatial onset of the reactive glia response in a clinically relevant mild form of PMD has not been defined.

METHODS

A PLP-ISEdel knockin mouse was used to examine the behavioral and neuroinflammatory consequences of a deletion within intron 3 of the PLP gene, at two time points (two and four months old) early in the pathological progression. Mice were characterized functionally using the open field task, elevated plus maze, and nesting behavior. Quantitative neuropathological analysis was for markers of astrocytes (GFAP), microglia (IBA1, CD68, MHCII) and axons (APP). The Aperio ScanScope was used to generate a digital, high magnification photomicrograph of entire brain sections. These digital slides were used to quantify the immunohistochemical staining in ten different brain regions to assess the regional heterogeneity in the reactive astrocyte and microglial response.

RESULTS

The PLP-ISEdel mice exhibited behavioral deficits in the open field and nesting behavior at two months, which did not worsen by four months of age. A marker of axonal injury (APP) increased from two months to four months of age. Striking was the robust reactive astrocyte and microglia response which was also progressive. In the two-month-old mice, the astrocyte and microglia reactivity was most apparent in white matter rich regions of the brain. By four months of age the gliosis had become widespread and included both white as well as gray matter regions of the brain.

CONCLUSIONS

Our results indicate, along with other preclinical models of PMD, that an early reactive glia response occurs following mutations in the PLP gene, which may represent a potentially clinically relevant, oligodendrocyte-independent therapeutic target for PMD.

Constitutive nuclear localization of an alternatively spliced sirtuin-2 isoform

Sirtuin-2 (SIRT2), the cytoplasmic member of the sirtuin family, has been implicated in the deacetylation of nuclear proteins. Although the enzyme has been reported to be located to the nucleus during G2/M phase, its spectrum of targets suggests functions in the nucleus throughout the cell cycle. While a nucleocytoplasmic shuttling mechanism has been proposed for SIRT2, recent studies have indicated the presence of a constitutively nuclear isoform. Here we report the identification of a novel splice variant (isoform 5) of SIRT2 that lacks a nuclear export signal and encodes a predominantly nuclear isoform. This novel isoform 5 fails to show deacetylase activity using several assays, both in vitro and in vivo, and we are led to conclude that this isoform is catalytically inactive. Nevertheless, it retains the ability to interact with p300, a known interaction partner. Moreover, changes in intrinsic tryptophan fluorescence upon denaturation indicate that the protein is properly folded. These data, together with computational analyses, confirm the structural integrity of the catalytic domain. Our results suggest an activity-independent nuclear function of the novel isoform.

p39, the primary activator for cyclin-dependent kinase 5 (Cdk5) in oligodendroglia, is essential for oligodendroglia differentiation and myelin repair

Cyclin-dependent kinase 5 (Cdk5) plays key roles in normal brain development and function. Dysregulation of Cdk5 may cause neurodegeneration and cognitive impairment. Besides the well demonstrated role of Cdk5 in neurons, emerging evidence suggests the functional requirement of Cdk5 in oligodendroglia (OL) and CNS myelin development. However, whether neurons and OLs employ similar or distinct mechanisms to regulate Cdk5 activity remains elusive. We report here that in contrast to neurons that harbor high levels of two Cdk5 activators, p35 and p39, OLs express abundant p39 but negligible p35. In addition, p39 is selectively up-regulated in OLs during differentiation along with elevated Cdk5 activity, whereas p35 expression remains unaltered. Specific knockdown of p39 by siRNA significantly attenuates Cdk5 activity and OL differentiation without affecting p35. Finally, expression of p39, but not p35, is increased during myelin repair, and remyelination is impaired in p39(-/-) mice. Together, these results reveal that neurons and OLs harbor distinct preference of Cdk5 activators and demonstrate important functions of p39-dependent Cdk5 activation in OL differentiation during de novo myelin development and myelin repair.

Oligodendroglia and neurotrophic factors in neurodegeneration

Myelination by oligodendroglial cells (OLs) enables the propagation of action potentials along neuronal axons, which is essential for rapid information flow in the central nervous system. Besides saltatory conduction, the myelin sheath also protects axons against inflammatory and oxidative insults. Loss of myelin results in axonal damage and ultimately neuronal loss in demyelinating disorders. However, accumulating evidence indicates that OLs also provide support to neurons via mechanisms beyond the insulating function of myelin. More importantly, an increasing volume of reports indicates defects of OLs in numerous neurodegenerative diseases, sometimes even preceding neuronal loss in pre-symptomatic episodes, suggesting that OL pathology may be an important mechanism contributing to the initiation and/or progression of neurodegeneration. This review focuses on the emerging picture of neuronal support by OLs in the pathogenesis of neurodegenerative disorders through diverse molecular and cellular mechanisms, including direct neuron-myelin interaction, metabolic support by OLs, and neurotrophic factors produced by and/or acting on OLs.

Improving myelin/oligodendrocyte-related dysfunction: a new mechanism of antipsychotics in the treatment of schizophrenia?

Schizophrenia is a severe psychiatric disorder with complex clinical manifestations and its aetiological factors remain unclear. During the past decade, the oligodendrocyte-related myelin dysfunction was proposed as a hypothesis for schizophrenia, supported initially by a series of neuroimaging studies and genetic evidence. Recently, the effects of antipsychotics on myelination and oligodendroglial lineage development and their underlying molecular mechanisms were evaluated. Data from those studies suggest that the antipsychotics-resulting improvement in myelin/oligodendrocyte-related dysfunction may contribute, at least in part, to their therapeutic effect on schizophrenia. Importantly, these findings may provide the basis for a new insight into the therapeutic strategy by targeting the oligodendroglia lineage cells against schizophrenia.

Coordinated control of oligodendrocyte development by extrinsic and intrinsic signaling cues

Oligodendrocytes, the myelin-forming cells for axon ensheathment in the central nervous system, are critical for maximizing and maintaining the conduction velocity of nerve impulses and proper brain function. Demyelination caused by injury or disease together with failure of myelin regeneration disrupts the rapid propagation of action potentials along nerve fibers, and is associated with acquired and inherited disorders, including devastating multiple sclerosis and leukodystrophies. The molecular mechanisms of oligodendrocyte myelination and remyelination remain poorly understood. Recently, a series of signaling pathways including Shh, Notch, BMP and Wnt signaling and their intracellular effectors such as Olig1/2, Hes1/5, Smads and TCFs, have been shown to play important roles in regulating oligodendrocyte development and myelination. In this review, we summarize our recent understanding of how these signaling pathways modulate the progression of oligodendrocyte specification and differentiation in a spatiotemporally-specific manner. A better understanding of the complex but coordinated function of extracellular signals and intracellular determinants during oligodendrocyte development will help to devise effective strategies to promote myelin repair for patients with demyelinating diseases.

Seven sirtuins for seven deadly diseases of aging

Sirtuins are a class of NAD(+)-dependent deacetylases having beneficial health effects. This extensive review describes the numerous intracellular actions of the seven mammalian sirtuins, their protein targets, intracellular localization, the pathways they modulate, and their role in common diseases of aging. Selective pharmacological targeting of sirtuins is of current interest in helping to alleviate global disease burden. Since all sirtuins are activated by NAD(+), strategies that boost NAD(+) in cells are of interest. While most is known about SIRT1, the functions of the six other sirtuins are now emerging. Best known is the involvement of sirtuins in helping cells adapt energy output to match energy requirements. SIRT1 and some of the other sirtuins enhance fat metabolism and modulate mitochondrial respiration to optimize energy harvesting. The AMP kinase/SIRT1-PGC-1α-PPAR axis and mitochondrial sirtuins appear pivotal to maintaining mitochondrial function. Downregulation with aging explains much of the pathophysiology that accumulates with aging. Posttranslational modifications of sirtuins and their substrates affect specificity. Although SIRT1 activation seems not to affect life span, activation of some of the other sirtuins might. Since sirtuins are crucial to pathways that counter the decline in health that accompanies aging, pharmacological agents that boost sirtuin activity have clinical potential in treatment of diabetes, cardiovascular disease, dementia, osteoporosis, arthritis, and other conditions. In cancer, however, SIRT1 inhibitors could have therapeutic value. Nutraceuticals such as resveratrol have a multiplicity of actions besides sirtuin activation. Their net health benefit and relative safety may have originated from the ability of animals to survive environmental changes by utilizing these stress resistance chemicals in the diet during evolution. Each sirtuin forms a key hub to the intracellular pathways affected.

Polarization and myelination in myelinating glia

Myelinating glia, oligodendrocytes in central nervous system and Schwann cells in peripheral nervous system, form myelin sheath, a multilayered membrane system around axons enabling salutatory nerve impulse conduction and maintaining axonal integrity. Myelin sheath is a polarized structure localized in the axonal side and therefore is supposed to be formed based on the preceding polarization of myelinating glia. Thus, myelination process is closely associated with polarization of myelinating glia. However, cell polarization has been less extensively studied in myelinating glia than other cell types such as epithelial cells. The ultimate goal of this paper is to provide insights for the field of myelination research by applying the information obtained in polarity study in other cell types, especially epithelial cells, to cell polarization of myelinating glia. Thus, in this paper, the main aspects of cell polarization study in general are summarized. Then, they will be compared with polarization in oligodendrocytes. Finally, the achievements obtained in polarization study for epithelial cells, oligodendrocytes, and other types of cells will be translated into polarization/myelination process by Schwann cells. Then, based on this model, the perspectives in the study of Schwann cell polarization/myelination will be discussed.

Global profiling of alternative splicing events and gene expression regulated by hnRNPH/F

In this study, we have investigated the global impact of heterogeneous nuclear Ribonuclear Protein (hnRNP) H/F-mediated regulation of splicing events and gene expression in oligodendrocytes. We have performed a genome-wide transcriptomic analysis at the gene and exon levels in Oli-neu cells treated with siRNA that targets hnRNPH/F compared to untreated cells using Affymetrix Exon Array. Gene expression levels and regulated exons were identified with the GenoSplice EASANA algorithm. Bioinformatics analyses were performed to determine the structural properties of G tracts that correlate with the function of hnRNPH/F as enhancers vs. repressors of exon inclusion. Different types of alternatively spliced events are regulated by hnRNPH/F. Intronic G tracts density, length and proximity to the 5′ splice site correlate with the hnRNPH/F enhancer function. Additionally, 6% of genes are differently expressed upon knock down of hnRNPH/F. Genes that regulate the transition of oligodendrocyte progenitor cells to oligodendrocytes are differentially expressed in hnRNPH/F depleted Oli-neu cells, resulting in a decrease of negative regulators and an increase of differentiation-inducing regulators. The changes were confirmed in developing oligodendrocytes in vivo. This is the first genome wide analysis of splicing events and gene expression regulated by hnRNPH/F in oligodendrocytes and the first report that hnRNPH/F regulate genes that are involved in the transition from oligodendrocyte progenitor cells to oligodendrocytes.

SIRT2 as a Therapeutic Target for Age-Related Disorders

Sirtuin proteins are conserved regulators of aging that have recently emerged as important modifiers of several diseases which commonly occur later in life such as cancer, diabetes, cardiovascular, and neurodegenerative diseases. In mammals, there are seven sirtuins (SIRT1-7), which display diversity in subcellular localization and function. SIRT1 has received much of attention due to its possible impact on longevity, while important biological and therapeutic roles of other sirtuins have been underestimated and just recently recognized. Here we focus on SIRT2, a member of the sirtuin family, and discuss its role in cellular and tissue-specific functions. This review summarizes the main scientific advances on SIRT2 protein biology and explores its potential as a therapeutic target for treatment of age-related disorders.

MicroRNA expression in mouse oligodendrocytes and regulation of proteolipid protein gene expression

Overexpression of the major myelin proteolipid protein (PLP) is detrimental to brain development and function and is the most common cause of Pelizaeus-Merzbacher disease. microRNA (miRNA), small, noncoding RNAs, have been shown to play critical roles in oligodendrocyte lineage. In this study, we sought to investigate whether miRNAs control PLP abundance. To identify candidate miRNAs involved in this regulation, we have examined differentiation-induced changes in the expression of miRNAs in the oligodendroglial cell line Oli-neu and in enhanced green fluorescent protein positive oligodendrocytes ex vivo. We have identified 145 miRNAs that are expressed in oligodendrocyte cell lineage progression. Dicer1 expression decreases in differentiated oligodendrocytes, and knock down of Dicer1 results in changes in miRNAs similar to those associated with differentiation. To identify miRNAs that control the PLP expression, we have selected miRNAs whose expression is lower in differentiated vs. undifferentiated Oli-neu cells and that have one or more binding site(s) in the PLP 3'-untranslated region (3'UTR). The PLP 3'UTR fused to the luciferase gene reduces the activity of the reporter, suggesting that it negatively regulates message stability or translation. Such suppression is relieved by knock down of miR-20a. Overexpression of miR-20a decreases expression of the endogenous PLP in primary oligodendrocytes and of the reporter gene. Deletion or mutation of the putative binding site for miR-20a in the PLP 3'UTR abrogated such effects. Our data indicate that miRNA expression is regulated by Dicer1 levels in differentiated oligodendrocytes and that miR-20a, a component of the cluster that controls oligodendrocyte cell number, regulates PLP gene expression through its 3'UTR.