Myelin basic protein mRNA levels affect myelin sheath dimensions, architecture, plasticity, and density of resident glial cells.

Myelin Basic Protein (MBP) is essential for both elaboration and maintenance of CNS myelin, and its reduced accumulation results in hypomyelination. How different Mbp mRNA levels affect myelin dimensions across the lifespan and how resident glial cells may respond to such changes are unknown. Here, to investigate these questions, we used enhancer-edited mouse lines that accumulate Mbp mRNA levels ranging from 8% to 160% of wild type. In young mice, reduced Mbp mRNA levels resulted in corresponding decreases in Mbp protein accumulation and myelin sheath thickness, confirming the previously demonstrated rate-limiting role of Mbp transcription in the control of initial myelin synthesis. However, despite maintaining lower line specific Mbp mRNA levels into old age, both MBP protein levels and myelin thickness improved or fully normalized at rates defined by the relative Mbp mRNA level. Sheath length, in contrast, was affected only when mRNA levels were very low, demonstrating that sheath thickness and length are not equally coupled to Mbp mRNA level. Striking abnormalities in sheath structure also emerged with reduced mRNA levels. Unexpectedly, an increase in the density of all glial cell types arose in response to reduced Mbp mRNA levels. This investigation extends understanding of the role MBP plays in myelin sheath elaboration, architecture, and plasticity across the mouse lifespan and illuminates a novel axis of glial cell crosstalk.

Transcriptional regulators of the Golli/myelin basic protein locus integrate additive and stealth activities.

Myelin is composed of plasma membrane spirally wrapped around axons and compacted into dense sheaths by myelin-associated proteins. Myelin is elaborated by neuroepithelial derived oligodendrocytes in the central nervous system (CNS) and by neural crest derived Schwann cells in the peripheral nervous system (PNS). While some myelin proteins accumulate in only one lineage, myelin basic protein (Mbp) is expressed in both. Overlapping the Mbp gene is Golli, a transcriptional unit that is expressed widely both within and beyond the nervous system. A super-enhancer domain within the Golli/Mbp locus contains multiple enhancers shown previously to drive reporter construct expression specifically in oligodendrocytes or Schwann cells. In order to determine the contribution of each enhancer to the Golli/Mbp expression program, and to reveal if functional interactions occur among them, we derived mouse lines in which they were deleted, either singly or in different combinations, and relative mRNA accumulation was measured at key stages of early development and at maturity. Although super-enhancers have been shown previously to facilitate interaction among their component enhancers, the enhancers investigated here demonstrated largely additive relationships. However, enhancers demonstrating autonomous activity strictly in one lineage, when missing, were found to significantly reduce output in the other, thus revealing cryptic "stealth" activity. Further, in the absence of a key oligodendrocyte enhancer, Golli accumulation was markedly and uniformly attenuated in all cell types investigated. Our observations suggest a model in which enhancer-mediated DNA-looping and potential super-enhancer properties underlie Golli/Mbp regulatory organization.

TIE: A Method to Electroporate Long DNA Templates into Preimplantation Embryos for CRISPR-Cas9 Gene Editing.

Precise genome editing using CRISPR typically requires delivery of guide RNAs, Cas9 endonuclease, and DNA repair templates. Both microinjection and electroporation effectively deliver these components into mouse zygotes provided the DNA template is an oligonucleotide of only a few hundred base pairs. However, electroporation completely fails with longer double-stranded DNAs leaving microinjection as the only delivery option. Here, we overcome this limitation by first injecting all CRISPR components, including long plasmid-sized DNA templates, into the sub-zona pellucida space. There they are retained, supporting subsequent electroporation. We show that this simple and well-tolerated method achieves intracellular reagent concentrations sufficient to effect precise gene edits.

Fortilin potentiates the peroxidase activity of Peroxiredoxin-1 and protects against alcohol-induced liver damage in mice.

Fortilin, a pro-survival molecule, inhibits p53-induced apoptosis by binding to the sequence-specific DNA-binding domain of the tumor suppressor protein and preventing it from transcriptionally activating Bax. Intriguingly, fortilin protects cells against ROS-induced cell death, independent of p53. The signaling pathway through which fortilin protects cells against ROS-induced cell death, however, is unknown. Here we report that fortilin physically interacts with the antioxidant enzyme peroxiredoxin-1 (PRX1), protects it from proteasome-mediated degradation, and keeps it enzymatically active by blocking its deactivating phosphorylation by Mst1, a serine/threonine kinase. At the whole animal level, the liver-specific overexpression of fortilin reduced PRX1 phosphorylation in the liver, enhanced PRX1 activity, and protected the transgenic animals against alcohol-induced, ROS-mediated, liver damage. These data suggest the presence of a novel oxidative-stress-handling pathway where the anti-p53 molecule fortilin augments the peroxidase PRX1 by protecting it against degradation and inactivation of the enzyme. Fortilin-PRX1 interaction in the liver could be clinically exploited further to prevent acute alcohol-induced liver damage in humans.

Pathogenic inflammation in the CNS of mice carrying human PLP1 mutations.

Progressive forms of multiple sclerosis lead to chronic disability, substantial decline in quality of life and reduced longevity. It is often suggested that they occur independently of inflammation. Here we investigated the disease progression in mouse models carrying PLP1 point mutations previously found in patients displaying clinical features of multiple sclerosis. These mouse models show loss-of-function of PLP1 associated with neuroinflammation; the latter leading to clinically relevant axonal degeneration, neuronal loss and brain atrophy as demonstrated by inactivation of the recombination activating gene 1. Moreover, these pathological hallmarks were substantially amplified when we attenuated immune regulation by inactivation of the programmed cell death-1 gene. Our observations support the view that primary oligodendroglial abnormalities can evoke pathogenically relevant neuroinflammation that drives neurodegeneration, as observed in some forms of multiple sclerosis but also in other, genetically-mediated neurodegenerative disorders of the human nervous system. As many potent immunomodulatory drugs have emerged during the last years, it is tempting to consider immunomodulation as a treatment option not only for multiple sclerosis, but also for so far non-treatable, genetically-mediated disorders of the nervous system accompanied by pathogenic neuroinflammation.

Functional organization of an Mbp enhancer exposes striking transcriptional regulatory diversity within myelinating glia.

In mammals, large caliber axons are ensheathed by myelin, a glial specialization supporting axon integrity and conferring accelerated and energy-efficient action potential conduction. Myelin basic protein (MBP) is required for normal myelin elaboration with maximal mbp transcription in oligodendrocytes requiring the upstream M3 enhancer. To further characterize the mechanism regulating mbp transcription, we defined M3 structure/function relationships by evaluating its evolutionary conservation, DNA footprints and the developmental programing conferred in mice by M3 derivatives. Multiple M3 regulatory element combinations were found to drive expression in oligodendrocytes and Schwann cells with a minimal 129 bp sequence conferring expression in oligodendrocytes throughout myelin elaboration, maintenance and repair. Unexpectedly, M3 derivatives conferred markedly different spatial and temporal expression programs thus illuminating striking transcriptional heterogeneity within post-mitotic oligodendrocytes. Finally, one M3 derivative engaged only during primary myelination, not during adult remyelination, demonstrating that transcriptional regulation in the two states is not equivalent.

XIAP protects oligodendrocytes against cell death in vitro but has no functional role in toxic demyelination.

Oligodendroglial damage and loss are typical characteristics of demyelinating diseases such as multiple sclerosis (MS) and the leukodystrophies. Axonal loss is the underlying cause of permanent neurological deficits in MS and it is thought to arise from a combination of immune-mediated axonal damage and the loss of trophic support to axons from myelin sheaths after demyelination. Prevention of oligodendroglial damage or death and demyelination are therefore attractive neuroprotective treatment strategies. However, a better understanding of mechanisms leading to oligodendroglial damage and demyelination is a prerequisite for the development of such treatment options. Here, we demonstrate that X-linked inhibitor of apoptosis (XIAP), the most potent member of the inhibitor of apoptosis proteins (IAP) family is expressed in oligodendrocytes in vivo and in vitro. Increased expression of XIAP is associated with protection against selected cell death pathways, whereas decreased expression increases oligodendroglial cell death in vitro. However, lack of XIAP does not modulate oligodendroglial cell death in toxic demyelination in vivo.

Targeted insertion of two Mthfr promoters in mice reveals temporal- and tissue-specific regulation.

Methylenetetrahydrofolate reductase (MTHFR), a key enzyme in folate metabolism, synthesizes 5-methyltetrahydrofolate, the main circulatory form of folate which is required for maintaining nontoxic levels of homocysteine and providing one-carbon units for methylation. A common 677C â†’ T variant in MTHFR confers mild MTHFR deficiency and has been associated with a number of human disorders, including neural tube defects and vascular disease. Two promoters of Mthfr, designated as upstream and downstream promoters, are located upstream of a transcription start site cluster and have previously demonstrated cell-specific activities. In this study we used a unique approach for targeted, single-copy transgene insertion to generate transgenic mice carrying a Mthfr upstream or Mthfr downstream promoter-reporter construct located 5' to the endogenous Hprt (hypoxanthine-guanine phosphoribosyltransferase) locus. The Mthfr downstream promoter demonstrated activity in the neural tube, neural crest cells, dorsal root ganglia, heart, and endothelial cells of blood vessels in 10.5-days post coitum embryos and placentas. Upstream promoter activity was absent at this developmental stage. Postnatally, both promoters demonstrated activity in the brain stem, hippocampus, and thalamus of 1-week-old brain that became stronger in the adult. The Mthfr upstream promoter also showed activity in the cerebellum and cerebral cortex. Both promoters were active in male reproductive tissues, including 1-week-old epididymides, and there was upstream promoter-specific activity in the adult testis. Our investigation of Mthfr regulation in an in vivo mouse model revealed temporal- and tissue-specific regulation that supports important roles for MTHFR in the developing embryo, and in postnatal brain and male reproductive tissues.

Towards resolving the transcription factor network controlling myelin gene expression.

In the central nervous system (CNS), myelin is produced from spirally-wrapped oligodendrocyte plasma membrane and, as exemplified by the debilitating effects of inherited or acquired myelin abnormalities in diseases such as multiple sclerosis, it plays a critical role in nervous system function. Myelin sheath production coincides with rapid up-regulation of numerous genes. The complexity of their subsequent expression patterns, along with recently recognized heterogeneity within the oligodendrocyte lineage, suggest that the regulatory networks controlling such genes drive multiple context-specific transcriptional programs. Conferring this nuanced level of control likely involves a large repertoire of interacting transcription factors (TFs). Here, we combined novel strategies of computational sequence analyses with in vivo functional analysis to establish a TF network model of coordinate myelin-associated gene transcription. Notably, the network model captures regulatory DNA elements and TFs known to regulate oligodendrocyte myelin gene transcription and/or oligodendrocyte development, thereby validating our approach. Further, it links to numerous TFs with previously unsuspected roles in CNS myelination and suggests collaborative relationships amongst both known and novel TFs, thus providing deeper insight into the myelin gene transcriptional network.

Regulatory modules function in a non-autonomous manner to control transcription of the mbp gene.

Multiple regulatory modules contribute to the complex expression programs realized by many loci. Although long thought of as isolated components, recent studies demonstrate that such regulatory sequences can physically associate with promoters and with each other and may localize to specific sub-nuclear transcription factories. These associations provide a substrate for putative interactions and have led to the suggested existence of a transcriptional interactome. Here, using a controlled strategy of transgenesis, we analyzed the functional consequences of regulatory sequence interaction within the myelin basic protein (mbp) locus. Interactions were revealed through comparisons of the qualitative and quantitative expression programs conferred by an allelic series of 11 different enhancer/inter-enhancer combinations ligated to a common promoter/reporter gene. In a developmentally contextual manner, the regulatory output of all modules changed markedly in the presence of other sequences. Predicted by transgene expression programs, deletion of one such module from the endogenous locus reduced oligodendrocyte expression levels but unexpectedly, also attenuated expression of the overlapping golli transcriptional unit. These observations support a regulatory architecture that extends beyond a combinatorial model to include frequent interactions capable of significantly modulating the functions conferred through regulatory modules in isolation.

Integrin-mediated axoglial interactions initiate myelination in the central nervous system.

All but the smallest-diameter axons in the central nervous system are myelinated, but the signals that initiate myelination are unknown. Our prior work has shown that integrin signaling forms part of the cell-cell interactions that ensure only those oligodendrocytes contacting axons survive. Here, therefore, we have asked whether integrins regulate the interactions that lead to myelination. Using homologous recombination to insert a single-copy transgene into the hypoxanthine phosphoribosyl transferase (hprt) locus, we find that mice expressing a dominant-negative beta1 integrin in myelinating oligodendrocytes require a larger axon diameter to initiate timely myelination. Mice with a conditional deletion of focal adhesion kinase (a signaling molecule activated by integrins) exhibit a similar phenotype. Conversely, transgenic mice expressing dominant-negative beta3 integrin in oligodendrocytes display no myelination abnormalities. We conclude that beta1 integrin plays a key role in the axoglial interactions that sense axon size and initiate myelination, such that loss of integrin signaling leads to a delay in myelination of small-diameter axons.

Separate proteolipid protein/DM20 enhancers serve different lineages and stages of development.

The gene encoding DM20 emerged in cartilaginous fish, descending from a bilaterian ancestor of the M6 proteolipid gene family. Its proteolipid protein (PLP) isoform appeared in amphibians, contains an additional 35 amino acids, and, in the mammalian CNS, is the dominant myelin protein in which it confers an essential neuroprotective function. During development, the DM20 isoform is prominent in a number of tissues, and plp/DM20 transcripts are detected in multiple progenitor populations, including those that continue to express plp/DM20 as they differentiate into myelinating oligodendrocytes. The locus also encodes isoforms with extended leader sequences that accumulate in the cell bodies of several types of neurons. Here, to locate and characterize regulatory sequences controlling the complex plp/DM20 transcription program, putative regulatory sequences, suggested by interspecies conservation, were ligated individually to a minimally promoted eGFPlacZ reporter gene. These constructs were inserted in single copy at a common site adjacent to the hypoxanthine-guanine phosphoribosyltransferase locus in embryonic stem cells and their in vivo expression programs were compared in transgenic mice. Most expressed developmental and cell-specific subprograms accommodated within the known expression phenotype of the endogenous plp/DM20 locus, thus defining multiple components of the combinatorial mechanism controlling its normal temporal and cell-specific program. Along with previously characterized nervous system enhancers, those described here should help expose the content and configuration of elements that are operational in multiple glial and neuronal lineages. The transgenic lines derived here also provide effective markers for multiple stages of glial and neuronal lineage progression.

Altered expression of methylenetetrahydrofolate reductase modifies response to methotrexate in mice.

OBJECTIVE: Folates provide one-carbon units for nucleotide synthesis and methylation reactions. A common polymorphism (677C-->T) in methylenetetrahydrofolate reductase (MTHFR) encodes an enzyme with reduced activity. Response to the antifolate methotrexate (MTX) may be modified in 677TT individuals because MTHFR converts nonmethylated folates, used for thymidine and purine synthesis, to 5-methyltetrahydrofolate, used in homocysteine remethylation to methionine. To study potential interactions between MTHFR activity and MTX, we examined the impact of decreased and increased MTHFR expression on MTX response in mice. METHODS: Mthfr-deficient (Mthfr and Mthfr) and wild-type (Mthfr) mice were injected with MTX or saline and assessed for hematological parameters (hematocrit, hemoglobin, red, and white blood cell numbers), plasma homocysteine, nephrotoxicity, hepatotoxicity, and splenic 2'-deoxyuridine 5'-triphosphate/2'-deoxythymidine 5'-triphosphate ratios. MTHFR-overexpressing transgenic mice (MTHFR-Tg) were generated, metabolites and folate distributions were measured, and response to MTX was assessed. RESULTS: MTX-treated Mthfr and Mthfr mice displayed hyperhomocysteinemia and decreased hematocrit, hemoglobin, and red blood cell numbers compared with wild-type animals. Mthfr mice also showed increased nephrotoxicity and hepatotoxicity. MTHFR-Tg mice were generated and confirmed to have increased levels of MTHFR with altered distributions of folate and thiols in a tissue-specific manner. After MTX treatment, MTHFR-Tg mice exhibited the same decreases in hematological parameters as Mthfr-deficient mice, and significantly decreased thymidine synthesis (higher 2'-deoxyuridine 5'-triphosphate/2'-deoxythymidine 5'-triphosphate ratios) compared with wild-type mice, but they were protected from MTX-induced hyperhomocysteinemia. CONCLUSION: Underexpression and overexpression of Mthfr/MTHFR increase MTX-induced myelosuppression but have distinct effects on plasma homocysteine and nephrotoxicity. Pharmacogenetic analysis of polymorphisms in folate-dependent enzymes may be useful in optimization of MTX therapy.

Temporally controlled prostate epithelium-specific gene alterations.

Employing the Hprt locus as the site for targeted transgenesis we have developed mice expressing the tamoxifen-inducible Cre-ER(T2) fusion protein under the control of the ARR2-rat probasin promoter. This system enables external control over the timing of prostate epithelial cell-specific gene alterations. Using both the ROSA26-lacZ and ROSA26-EYFP reporter strains to monitor recombinase activity, Cre-ER(T2) was found to be specifically expressed in the prostatic epithelium and was strictly tamoxifen dependent. This strain thus allows precise control over the timing of gene alterations in the mouse prostate, enabling analyses of the phenotypic consequences of gene alterations in mice of any age. It also provides an ideal platform to study the impact of environmental, hormonal, and age-related factors on prostate tumorigenesis. This latter feature will be of particular value given the paucity of murine models that accurately mimic the late onset and prolonged natural history of human prostate cancer.

Functional organization of a Schwann cell enhancer.

Myelin basic protein (MBP) gene expression is conferred in oligodendrocytes and Schwann cells by different upstream enhancers. In Schwann cells, expression is controlled by a 422 bp enhancer lying -9 kb from the gene. We show here that it contains 22 mammalian conserved motifs > or =6 bp. To investigate their functional significance, different combinations of wild-type or mutated motifs were introduced into reporter constructs that were inserted in single copy at a common hypoxanthine phosphoribosyltransferase docking site in embryonic stem cells. Lines of transgenic mice were derived, and the subsequent qualitative and quantitative expression phenotypes were compared at different stages of maturation. In the enhancer core, seven contiguous motifs cooperate to confer Schwann cell specificity while different combinations of flanking motifs engage, at different stages of Schwann cell maturation, to modulate expression level. Mutation of a Krox-20 binding site reduces the level of reporter expression, whereas mutation of a potential Sox element silences reporter expression. This potential Sox motif was also found conserved in other Schwann cell enhancers, suggesting that it contributes widely to regulatory function. These results demonstrate a close relationship between phylogenetic footprints and regulatory function and suggest a general model of enhancer organization. Finally, this investigation demonstrates that in vivo functional analysis, supported by controlled transgenesis, can be a robust complement to molecular and bioinformatics approaches to regulatory mechanisms.

A combinatorial network of evolutionarily conserved myelin basic protein regulatory sequences confers distinct glial-specific phenotypes.

Myelin basic protein (MBP) is required for normal myelin compaction and is implicated in both experimental and human demyelinating diseases. In this study, as an initial step in defining the regulatory network controlling MBP transcription, we located and characterized the function of evolutionarily conserved regulatory sequences. Long-range human-mouse sequence comparison revealed over 1 kb of conserved noncoding MBP 5' flanking sequence distributed into four widely spaced modules ranging from 0.1 to 0.4 kb. We demonstrate first that a controlled strategy of transgenesis provides an effective means to assign and compare qualitative and quantitative in vivo regulatory programs. Using this strategy, single-copy reporter constructs, designed to evaluate the regulatory significance of modular and intermodular sequences, were introduced by homologous recombination into the mouse hprt (hypoxanthine-guanine phosphoribosyltransferase) locus. The proximal modules M1 and M2 confer comparatively low-level oligodendrocyte expression primarily limited to early postnatal development, whereas the upstream M3 confers high-level oligodendrocyte expression extending throughout maturity. Furthermore, constructs devoid of M3 fail to target expression to newly myelinating oligodendrocytes in the mature CNS. Mutation of putative Nkx6.2/Gtx sites within M3, although not eliminating oligodendrocyte targeting, significantly decreases transgene expression levels. High-level and continuous expression is conferred to myelinating or remyelinating Schwann cells by M4. In addition, when isolated from surrounding MBP sequences, M3 confers transient expression to Schwann cells elaborating myelin. These observations define the in vivo regulatory roles played by conserved noncoding MBP sequences and lead to a combinatorial model in which different regulatory modules are engaged during primary myelination, myelin maintenance, and remyelination.