Two molecularly distinct intracellular pathways to oligodendrocyte differentiation: role of a p53 family protein

H3K27M mutant glioma: Disease definition and biological underpinnings

High-grade glioma (HGG) is the most common cause of cancer death in children and the most common primary central nervous system tumor in adults. While pediatric HGG was once thought to be biologically similar to the adult form of disease, research has shown these malignancies to be significantly molecularly distinct, necessitating distinct approaches to their clinical management. However, emerging data have shown shared molecular events in pediatric and adult HGG including the histone H3K27M mutation. This somatic missense mutation occurs in genes encoding one of two isoforms of the Histone H3 protein, H3F3A (H3.3), or HIST1H3B (H3.1), and is detected in up to 80% of pediatric diffuse midline gliomas and in up to 60% of adult diffuse gliomas. Importantly, the H3K27M mutation is associated with poorer overall survival and response to therapy compared to patients with H3 wild-type tumors. Here, we review the clinical features and biological underpinnings of pediatric and adult H3K27M mutant glioma, offering a groundwork for understanding current research and clinical approaches for the care of patients suffering with this challenging disease.

Sobetirome rescues α-synuclein-mediated demyelination in an in vitro model of multiple system atrophy

Multiple system atrophy (MSA) is a rare and rapidly progressive atypical parkinsonian disorder characterized by oligodendroglial cytoplasmic inclusions containing α-synuclein (α-syn), demyelination, inflammation and neuronal loss. To date, no disease-modifying therapy is available. Targeting α-syn-driven oligodendroglial dysfunction and demyelination presents a potential therapeutic approach for restricting axonal dysfunction, neuronal loss and disease progression. The present study investigated the promyelinogenic potential of sobetirome, a blood-brain barrier permeable and central nervous system selective thyromimetic in the context of an in vitro MSA model. Oligodendrocyte precursor cells (OPCs) were obtained from transgenic mice overexpressing human α-syn specifically in oligodendrocytes (MBP29 mouse line), a well-described MSA model, and non-transgenic littermates. mRNA and protein expression analyses revealed a substantial rescue effect of sobetirome on myelin-specific proteins in control and α-syn overexpressing oligodendrocytes. Furthermore, myelination analysis using nanofibres confirmed that sobetirome increases both the length and number of myelinated segments per oligodendrocyte in primary murine α-syn overexpressing oligodendrocytes and their respective control. These results suggest that sobetirome may be a promising thyromimetic compound targeting an important neuropathological hallmark of MSA.

Influence of hormones in multiple sclerosis: focus on the most important hormones

Hormonal imbalance may be an important factor in the severity of multiple sclerosis (MS) disease. In this context, hormone therapy has been shown to have immunoregulatory potential in various experimental approaches. There is increasing evidence of potentially beneficial effects of thyroid, melatonin, and sex hormones in MS models. These hormones may ameliorate the neurological impairment through immunoregulatory and neuroprotective effects, as well as by reducing oxidative stress. Expanding our knowledge of hormone therapy may be an effective step toward identifying additional molecular/cellular pathways in MS disease. In this review, we discuss the role of several important hormones in MS pathogenesis in terms of their effects on immunoregulatory aspects and neuroprotection.

Promoting remyelination: A case study in regenerative medicine

Therapeutics that modulate regenerative mechanisms by targeting the activity of endogenous (adult) stem cell populations have the potential to revolutionize medicine. In many human disease states, capacity to repair damaged tissue underlies progressive decline and disease progression. Recent insights derived from efforts aimed at promoting remyelination for the treatment of multiple sclerosis (MS) highlight the importance of considering the limiting factors and underlying mechanisms associated with all aspects of disease onset, progression and recovery, during both the discovery and clinical stages of developing a regenerative medicine. This perspective presents general considerations for the development of regenerative therapies, using remyelination as a case study.

Derivation of preoligodendrocytes from human-induced pluripotent stem cells through overexpression of microRNA 338

MicroRNAs (miRNAs) control gene expression at the posttranscriptional level and have a critical role in many biological processes such as oligodendrocyte differentiation. Recent studies have shown that microRNA 338 (miR-338) is overexpressed during the oligodendrocyte development process in the central nervous system; this finding indicates a potentially important role for miR-338 in oligodendrocyte development. To evaluate this assumption, we studied the effect of miR-338 overexpression on promoting the differentiation of oligodendrocyte progenitor cells (OPCs), derived from human-induced pluripotent stem cells (hiPSC), into preoligodendrocyte. hiPSCs were differentiated into OPCs after treating for 16 days with basic fibroblast growth factor (BFGF), epidermal growth factor (FGF), and platelet-derived growth factor (PDGF)-AA. Bipolar OPCs appeared and the expression of OPC-related markers, including Nestin, Olig2, Sox10, PDGFRα, and A2B5 was confirmed by real-time polymerase chain reaction (PCR) and immunofluorescence. Then, OPCs were transduced by miR-338 expressing lentivirus or were treated with triiodothyronine (T3) for 6 days. Data obtained from real-time PCR and immunofluorescence experiment indicated that preoligodendrocyte markers such as Sox10, O4, and MBP were expressed at higher levels in transduced cells with miR-338 in comparison with the T3 group. So, the overexpression of miR-338 in iPSC-derived OPCs can promote their differentiation into preoligodendrocyte which can be used in cell therapy of myelin-related diseases.

PRMT5-mediated regulation of developmental myelination

Oligodendrocytes (OLs) are the myelin-forming cells of the central nervous system. They are derived from differentiation of oligodendrocyte progenitors through a process requiring cell cycle exit and histone modifications. Here we identify the histone arginine methyl-transferase PRMT5, a molecule catalyzing symmetric methylation of histone H4R3, as critical for developmental myelination. PRMT5 pharmacological inhibition, CRISPR/cas9 targeting, or genetic ablation decrease p53-dependent survival and impair differentiation without affecting proliferation. Conditional ablation of Prmt5 in progenitors results in hypomyelination, reduced survival and differentiation. Decreased histone H4R3 symmetric methylation is followed by increased nuclear acetylation of H4K5, and is rescued by pharmacological inhibition of histone acetyltransferases. Data obtained using purified histones further validate the results obtained in mice and in cultured oligodendrocyte progenitors. Together, these results identify PRMT5 as critical for oligodendrocyte differentiation and developmental myelination by modulating the cross-talk between histone arginine methylation and lysine acetylation.

Myelin-forming cells derive from oligodendrocyte progenitors. Here the authors identify histone arginine methyl-transferase PRMT5 as critical for developmental myelination by modulating the cross-talk between histone arginine methylation and lysine acetylation, to favor differentiation.

Encapsulated oligodendrocyte precursor cell fate is dependent on PDGF-AA release kinetics in a 3D microparticle-hydrogel drug delivery system

Biomaterial drug delivery systems (DDS) can be used to regulate growth factor release and combat the limited intrinsic regeneration capabilities of central nervous system (CNS) tissue following injury and disease. Of particular interest are systems that aid in oligodendrocyte regeneration, as oligodendrocytes generate myelin which surrounds neuronal axons and helps transmit signals throughout the CNS. Oligodendrocyte precursor cells (OPCs) are found in small numbers in the adult CNS, but are unable to effectively differentiate following CNS injury. Delivery of signaling molecules can initiate a favorable OPC response, such as proliferation or differentiation. Here, we investigate the delivery of one such molecule, platelet derived growth factor-AA (PDGF-AA), from poly(lactic-co-glycolic) acid microparticles to OPCs in a 3D polyethylene glycol-based hydrogel. The goal of this DDS was to better understand the relationship between PDGF-AA release kinetics and OPC fate. The system approximates native brain tissue stiffness, while incorporating PDGF-AA under seven different delivery scenarios. Within this DDS, supply of PDGF-AA followed by PDGF-AA withdrawal caused OPCs to upregulate gene expression of myelin basic protein (MBP) by factors of 1.6-9.2, whereas continuous supply of PDGF-AA caused OPCs to remain proliferative. At the protein expression level, we observed an upregulation in O1, a marker for mature oligodendrocytes. Together, these results show that burst release followed by withdrawal of PDGF-AA from a hydrogel DDS stimulates survival, proliferation, and differentiation of OPCs in vitro. Our results could inform the development of improved neural regeneration strategies that incorporate delivery of PDGF-AA to the injured CNS. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A:2402-2411, 2018.

The Healthy and Diseased Microenvironments Regulate Oligodendrocyte Properties: Implications for Regenerative Medicine

White matter disorders are characterized by deficient myelin or myelin loss, lead to a range of neurologic dysfunctions, and can result in early death. Oligodendrocytes, which are responsible for white matter formation, are the first targets for treatment. However, many studies indicate that failure of white matter repair goes beyond the intrinsic incapacity of oligodendrocytes to (re)generate myelin and that failed interactions with neighboring cells or factors in the diseased microenvironment can underlie white matter defects. Moreover, most of the white matter disorders show specific white matter pathology caused by different disease mechanisms. Herein, we review the factors within the cellular and the extracellular microenvironment regulating oligodendrocyte properties and discuss stem cell tools to identify microenvironmental factors of importance to the development of improved regenerative medicine for patients with white matter disorders.

Quiescence of adult oligodendrocyte precursor cells requires thyroid hormone and hypoxia to activate Runx1

The adult mammalian central nervous system (CNS) contains a population of slowly dividing oligodendrocyte precursor cells (OPCs), i.e., adult OPCs, which supply new oligodendrocytes throughout the life of animal. While adult OPCs develop from rapidly dividing perinatal OPCs, the mechanisms underlying their quiescence remain unknown. Here, we show that perinatal rodent OPCs cultured with thyroid hormone (TH) under hypoxia become quiescent and acquire adult OPCs-like characteristics. The cyclin-dependent kinase inhibitor p15/INK4b plays crucial roles in the TH-dependent cell cycle deceleration in OPCs under hypoxia. Klf9 is a direct target of TH-dependent signaling. Under hypoxic conditions, hypoxia-inducible factors mediates runt-related transcription factor 1 activity to induce G1 arrest in OPCs through enhancing TH-dependent p15/INK4b expression. As adult OPCs display phenotypes of adult somatic stem cells in the CNS, the current results shed light on environmental requirements for the quiescence of adult somatic stem cells during their development from actively proliferating stem/progenitor cells.

Cellular and molecular introduction to brain development

Advances in the study of brain development over the last decades, especially recent findings regarding the evolutionary expansion of the human neocortex, and large-scale analyses of the proteome/transcriptome in the human brain, have offered novel insights into the molecular mechanisms guiding neural maturation, and the pathophysiology of multiple forms of neurological disorders. As a preamble to reviews of this issue, we provide an overview of the cellular, molecular and genetic bases of brain development with an emphasis on the major mechanisms associated with landmarks of normal neural development in the embryonic stage and early postnatal life, including neural stem/progenitor cell proliferation, cortical neuronal migration, evolution and folding of the cerebral cortex, synaptogenesis and neural circuit development, gliogenesis and myelination. We will only briefly depict developmental disorders that result from perturbations of these cellular or molecular mechanisms, and the most common perinatal brain injuries that could disturb normal brain development.

Thyroid Hormone Signaling in Oligodendrocytes: from Extracellular Transport to Intracellular Signal

Thyroid hormone plays an important role in central nervous system (CNS) development, including the myelination of variable axonal calibers. It is well-established that thyroid hormone is required for the terminal differentiation of oligodendrocyte precursor cells (OPCs) into myelinating oligodendrocytes by inducing rapid cell-cycle arrest and constant transcription of pro-differentiation genes. This is well supported by the hypomyelinating phenotypes exhibited by patients with congenital hypothyroidism, cretinism. During development, myelinating oligodendrocytes only appear after the formation of neural circuits, indicating that the timing of oligodendrocyte differentiation is important. Since fetal and post-natal serum thyroid hormone levels peak at the stage of active myelination, it is suspected that the timing of oligodendrocyte development is finely controlled by thyroid hormone. The essential machinery for thyroid hormone signaling such as deiodinase activity (utilized by cells to auto-regulate the level of thyroid hormone), and nuclear thyroid hormone receptors (for gene transcription) are expressed on oligodendrocytes. In this review, we discuss the known and potential thyroid hormone signaling pathways that may regulate oligodendrocyte development and CNS myelination. Moreover, we evaluate the potential of targeting thyroid hormone signaling for white matter injury or disease.

Unveiling alterative splice diversity from human oligodendrocyte proteome data

Oligodendrocytes produce and maintain the myelin sheath of axons in the central nervous system. Because misassembled myelin sheaths have been associated with brain disorders such as multiple sclerosis and schizophrenia, recent advances have been made towards the description of the oligodendrocyte proteome. The identification of splice variants represented in the proteome is as important as determining the level of oligodendrocyte-associated proteins. Here, we used an oligodendrocyte proteome dataset deposited in ProteomeXchange to search against a customized protein sequence file containing computationally predicted splice variants. Our approach resulted in the identification of 39 splice variants, including one variant from the GTPase KRAS gene and another from the human glutaminase gene family. We also detected the mRNA expression of five selected splice variants and demonstrated that a fraction of these have their canonical proteins participating in direct protein-protein interactions. In conclusion, we believe our findings contribute to the molecular characterization of oligodendrocytes and may encourage other research groups working with central nervous system disorders to investigate the biological significance of these splice variants. The splice variants identified in this study may encode proteins that could be targeted in novel treatment strategies and diagnostic methods.

SIGNIFICANCE

Several disorders of the central nervous system (CNS) are associated with misassembled myelin sheaths, which are produced and maintained by oligodendrocytes (OL). Recently, the OL proteome has been explored to identify key proteins and molecular functions associated with CNS disorders. We developed an innovative approach to select, with a higher level of confidence, a relevant list of splice variants from a proteome dataset and detected the mRNA expression of five selected variants: EEF1D, KRAS, MFF, SDR39U1, and SUGT1. We also described splice variants extracted from OL proteome data. Among the splice variants identified, some are from genes previously linked to CNS and related disorders. Our findings may contribute to oligodendrocyte characterization and encourage other research groups to investigate the biological role of splice variants and to improve current treatments and diagnostic methods for CNS disorders.

Tamoxifen promotes differentiation of oligodendrocyte progenitors in vitro

The most promising therapeutic approach to finding the cure for devastating demyelinating conditions is the identification of clinically safe pharmacological agents that can promote differentiation of endogenous oligodendrocyte precursor cells (OPCs). Here we show that the breast cancer medication tamoxifen (TMX), with well-documented clinical safety and confirmed beneficial effects in various models of demyelinating conditions, stimulates differentiation of rat glial progenitors to mature oligodendrocytes in vitro. Clinically applicable doses of TMX significantly increased both the number of CNPase-positive oligodendrocytes and protein levels of myelin basic protein, measured with Western blots. Furthermore, we also found that OPC differentiation was stimulated, not only by the pro-drug TMX-citrate (TMXC), but also by two main TMX metabolites, 4-hydroxy-TMX and endoxifen. Differentiating effects of TMXC and its metabolites were completely abolished in the presence of estrogen receptor (ER) antagonist, ICI182780. In contrast to TMXC and 4-hydroxy-TMX, endoxifen also induced astrogliogenesis, but independent of the ER activation. In sum, we showed that the TMX prodrug and its two main metabolites (4-hydroxy-TMX and endoxifen) promote ER-dependent oligodendrogenesis in vitro, not reported before. Given that differentiating effects of TMX were achieved with clinically safe doses, TMX is likely one of the most promising FDA-approved drugs for the possible treatment of demyelinating diseases.

Oligodendrocyte Development and Plasticity

Oligodendrocyte precursor cells (OPCs) originate in the ventricular zones (VZs) of the brain and spinal cord and migrate throughout the developing central nervous system (CNS) before differentiating into myelinating oligodendrocytes (OLs). It is not known whether OPCs or OLs from different parts of the VZ are functionally distinct. OPCs persist in the postnatal CNS, where they continue to divide and generate myelinating OLs at a decreasing rate throughout adult life in rodents. Adult OPCs respond to injury or disease by accelerating their cell cycle and increasing production of OLs to replace lost myelin. They also form synapses with unmyelinated axons and respond to electrical activity in those axons by generating more OLs and myelin locally. This experience-dependent "adaptive" myelination is important in some forms of plasticity and learning, for example, motor learning. We review the control of OL lineage development, including OL population dynamics and adaptive myelination in the adult CNS.

Thyroid hormone alleviates demyelination induced by cuprizone through its role in remyelination during the remission period

Multiple sclerosis (MS) is a disease induced by demyelination in the central nervous system, and the remission period of MS is crucial for remyelination. In addition, abnormal levels of thyroid hormone (TH) have been identified in MS. However, in the clinic, insufficient attention has been paid to the role of TH in the remission period. Indeed, TH not only functions in the development of the brain but also affects myelination. Therefore, it is necessary to observe the effect of TH on remyelination during this period. A model of demyelination induced by cuprizone (CPZ) was used to observe the function of TH in remyelination during the remission period of MS. Through weighing and behavioral tests, we found that TH improved the physical symptoms of mice impaired by CPZ. Supplementation of TH led to the repair of myelin as detected by immunohistochemistry and western blot. In addition, a sufficient TH supply resulted in an increase in myelinated axons without affecting myelin thickness and g ratio in the corpus callosum, as detected by electron microscopy. Double immunostaining with myelin basic protein and neurofilament 200 (NF200) showed that the CPZ-induced impairment of axons was alleviated by TH. Conversely, insufficient TH induced by 6-propyl-2-thiouracil resulted in the enlargement of mitochondria. Furthermore, we found that an adequate supply of TH promoted the proliferation and differentiation of oligodendrocyte lineage cells by immunofluorescence, which was beneficial to remyelination. Further, we found that TH reduced the number of astrocytes without affecting microglia. Conclusively, it was shown that TH alleviated demyelination induced by CPZ by promoting the development of oligodendrocyte lineage cells and remyelination. The critical time for remyelination is the remission period of MS. TH plays a significant role in alleviating demyelination during the remission period in the clinical treatment of MS.

Overexpression of cyclin dependent kinase inhibitor P27/Kip1 increases oligodendrocyte differentiation from induced pluripotent stem cells

Cell transplantation therapy with oligodendrocyte precursor cells (OPCs) is a promising and effective treatment for diseases involving demyelination in the central nervous system (CNS). In previous studies, we succeeded in producing O4(+) oligodendrocytes (OLs) from mouse- and human-induced pluripotent stem cells (iPSCs) in vitro; however, the efficiency of differentiation into OLs was lower for iPSCs than that for embryonic stem cells (ESCs). To clarify the cause of this difference, we compared the expression of proteins that contribute to OL differentiation in mouse iPSC-derived cells and in mouse ESC-derived cells. The results showed that the expression levels of cyclin dependent kinase inhibitor P27/Kip1, mitogen-activated protein kinase (MAPK) JNK3, and transcription factor Mash1 were lower in iPSC-derived cells. In contrast, the expression levels of MAPK P38α, P38γ, and thyroid hormone receptor β1 were higher in iPSC-derived cells. We attempted to compensate for the expression changes in P27/Kip1 protein and Mash1 protein in iPSC-derived cells through retrovirus vector-mediated gene expression. Although the overexpression of Mash1 had no effect, the overexpression of P27/Kip1 increased the differentiation efficiency of iPSC-derived cells into O4(+) OLs.

From precursors to myelinating oligodendrocytes: contribution of intrinsic and extrinsic factors to white matter plasticity in the adult brain

Oligodendrocyte precursor cells (OPC) are glial cells that metamorphose into myelinating oligodendrocytes during embryogenesis and early stages of post-natal life. OPCs continue to divide throughout adulthood and some eventually differentiate into oligodendrocytes in response to demyelinating lesions. There is growing evidence that OPCs are also involved in activity-driven de novo myelination of previously unmyelinated axons and myelin remodeling in adulthood. In this review, we summarize the interwoven factors and cascades that promote the activation, recruitment and differentiation of OPCs into myelinating oligodendrocytes in the adult brain based mostly on results found in the study of demyelinating diseases. The goal of the review was to draw a complete picture of the transformation of OPCs into mature oligodendrocytes to facilitate the study of this transformation in both the normal and diseased adult brain.

A regenerative approach to the treatment of multiple sclerosis

Progressive phases of multiple sclerosis are associated with inhibited differentiation of the progenitor cell population that generates the mature oligodendrocytes required for remyelination and disease remission. To identify selective inducers of oligodendrocyte differentiation, we performed an image-based screen for myelin basic protein (MBP) expression using primary rat optic-nerve-derived progenitor cells. Here we show that among the most effective compounds identifed was benztropine, which significantly decreases clinical severity in the experimental autoimmune encephalomyelitis (EAE) model of relapsing-remitting multiple sclerosis when administered alone or in combination with approved immunosuppressive treatments for multiple sclerosis. Evidence from a cuprizone-induced model of demyelination, in vitro and in vivo T-cell assays and EAE adoptive transfer experiments indicated that the observed efficacy of this drug results directly from an enhancement of remyelination rather than immune suppression. Pharmacological studies indicate that benztropine functions by a mechanism that involves direct antagonism of M1 and/or M3 muscarinic receptors. These studies should facilitate the development of effective new therapies for the treatment of multiple sclerosis that complement established immunosuppressive approaches.

Purification and culture of oligodendrocyte lineage cells

Oligodendrocytes are the cells of the vertebrate central nervous system responsible for forming myelin sheaths, which are essential for the rapid propagation of action potential. The formation of oligodendrocytes and myelin sheaths is tightly regulated, both temporally and spatially, by a number of extracellular and intracellular factors. For example, notch ligands, thyroid hormones, and mitogens such as platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) can all interact with oligodendrocyte precursor cell-expressed receptors to impact proliferation, differentiation, and myelin gene expression. To facilitate oligodendrocyte biology research, we have developed a technique using immunopanning to isolate different stages of the oligodendrocyte lineage, oligodendrocyte precursor cells and/or postmitotic oligodendrocytes, from postnatal rat or mouse brains. These cells can be cultured in defined, serum-free media in conditions that either promote differentiation into mature oligodendrocytes or continued proliferation as immature oligodendrocyte precursors. These cells represent an ideal system in which to study the regulation of oligodendrocyte proliferation, migration, differentiation, myelin gene expression, or other fundamental aspects of oligodendrocyte biology.

Thyroid hormone receptors, cell growth and differentiation

BACKGROUND

Tissue homeostasis depends on the balance between cell proliferation and differentiation. Thyroid hormones (THs), through binding to their nuclear receptors, can regulate the expression of many genes involved in cell cycle control and cellular differentiation. This can occur by direct transcriptional regulation or by modulation of the activity of different signaling pathways.

SCOPE OF REVIEW

In this review we will summarize the role of the different receptor isoforms in growth and maturation of selected tissues and organs. We will focus on mammalian tissues, and therefore we will not address the fundamental role of the THs during amphibian metamorphosis.

MAJOR CONCLUSIONS

The actions of THs are highly pleiotropic, affecting many tissues at different developmental stages. As a consequence, their effects on proliferation and differentiation are highly heterogeneous depending on the cell type, the cellular context, and the developmental or transformation status. Both during development and in the adult, stem cells are essential for proper organ formation, maintenance and regeneration. Recent evidence suggests that some of the actions of the thyroid hormone receptors could be secondary to regulation of stem/progenitor cell function. Here we will also include the latest knowledge on the role of these receptors in proliferation and differentiation of embryonic and adult stem cells.

GENERAL SIGNIFICANCE

The thyroid hormone receptors are potent regulators of proliferation and differentiation of many cell types. This can explain the important role of the thyroid hormones and their receptors in key processes such as growth, development, tissue homeostasis or cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

The T3-induced gene KLF9 regulates oligodendrocyte differentiation and myelin regeneration

Hypothyroidism is a well-described cause of hypomyelination. In addition, thyroid hormone (T3) has recently been shown to enhance remyelination in various animal models of CNS demyelination. What are the ways in which T3 promotes the development and regeneration of healthy myelin? To begin to understand the mechanisms by which T3 drives myelination, we have identified genes regulated specifically by T3 in purified oligodendrocyte precursor cells (OPCs). Among the genes identified by genomic expression analyses were four transcription factors, Kruppel-like factor 9 (KLF9), basic helix-loop-helix family member e22 (BHLHe22), Hairless (Hr), and Albumin D box-binding protein (DBP), all of which were induced in OPCs by both brief and long term exposure to T3. To begin to investigate the role of these genes in myelination, we focused on the most rapidly and robustly induced of these, KLF9, and found it is both necessary and sufficient to promote oligodendrocyte differentiation in vitro. Surprisingly, we found that loss of KLF9 in vivo negligibly affects the formation of CNS myelin during development, but does significantly delay remyelination in cuprizone-induced demyelinated lesions. These experiments indicate that KLF9 is likely a novel integral component of the T3-driven signaling cascade that promotes the regeneration of lost myelin. Future analyses of the roles of KLF9 and other identified T3-induced genes in myelination may lead to novel insights into how to enhance the regeneration of myelin in demyelinating diseases such as multiple sclerosis.

9-cis retinoic acid promotes lymphangiogenesis and enhances lymphatic vessel regeneration: therapeutic implications of 9-cis retinoic acid for secondary lymphedema

BACKGROUND

The lymphatic system plays a key role in tissue fluid homeostasis and lymphatic dysfunction caused by genetic defects, or lymphatic vessel obstruction can cause lymphedema, disfiguring tissue swelling often associated with fibrosis and recurrent infections with no available cures to date. In this study, retinoic acids (RAs) were determined to be a potent therapeutic agent that is immediately applicable to reduce secondary lymphedema.

METHODS AND RESULTS

We report that RAs promote proliferation, migration, and tube formation of cultured lymphatic endothelial cells by activating fibroblast growth factor receptor signaling. Moreover, RAs control the expression of cell-cycle checkpoint regulators such as p27(Kip1), p57(Kip2), and the aurora kinases through both an Akt-mediated nongenomic action and a transcription-dependent genomic action that is mediated by Prox1, a master regulator of lymphatic development. Moreover, 9-cisRA was found to activate in vivo lymphangiogenesis in animals in mouse trachea, Matrigel plug, and cornea pocket assays. Finally, we demonstrate that 9-cisRA can provide a strong therapeutic efficacy in ameliorating experimental mouse tail lymphedema by enhancing lymphatic vessel regeneration.

CONCLUSION

These in vitro and animal studies demonstrate that 9-cisRA potently activates lymphangiogenesis and promotes lymphatic regeneration in an experimental lymphedema model, presenting it as a promising novel therapeutic agent to treat human lymphedema patients.

Development and maturation of the spinal cord: implications of molecular and genetic defects

The human central nervous system (CNS) may be the most complex structure in the universe. Its development and appropriate specification into phenotypically and spatially distinct neural subpopulations involves a precisely orchestrated response, with thousands of transcriptional regulators combining with epigenetic controls and specific temporal cues in perfect synchrony. Understandably, our insight into the sophisticated molecular mechanisms which underlie spinal cord development are as yet limited. Even less is known about abnormalities of this process - putative genetic and molecular causes of well-described defects have only begun to emerge in recent years. Nonetheless, modern scientific techniques are beginning to demonstrate common patterns and principles amid the tremendous complexity of spinal cord development and maldevelopment. These advances are important, given that developmental anomalies of the spinal cord are an important cause of mortality and morbidity (Sadler, 2000); it is hoped that research advances will lead to better methods to detect, treat, and prevent these lesions.

Early developmental actions of endocrine disruptors on the hypothalamus, hippocampus, and cerebral cortex

Sex steroids and thyroid hormones play a key role in the development of the central nervous system. The critical role of these hormonal systems may explain the sensitivity of the hypothalamus, the cerebral cortex, and the hippocampus to endocrine-disrupting chemicals (EDC). This review examines the evidence for endocrine disruption of glial-neuronal functions in the hypothalamus, hippocampus, and cerebral cortex. Focus was placed on two well-studied EDC, the insecticide dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyls (PCB). DDT is involved in neuroendocrine disruption of the reproductive axis, whereas polychlorinated biphenyls (PCB) interact with both the thyroid hormone- and sex steroid-dependent systems and disturb the neuroendocrine control of reproduction and development of hippocampus and cortex. These results highlight the impact of EDC on the developing nervous system and the need for more research in this area.

Mathematical and experimental approaches to identify and predict the effects of chemotherapy on neuroglial precursors

The adverse effects of chemotherapy on normal cells of the body create substantial clinical problems for many cancer patients. However, relatively little is known about the effects, other than promotion of cell death, of such agents on the function of normal precursor cells critical in tissue homeostasis and repair. We have combined mathematical and experimental analyses to identify the effects of sublethal doses of chemotherapy on glial precursor cells of the central nervous system. We modeled the temporal development of a population of precursor and terminally differentiated cells exposed to sublethal doses of carmustine (BCNU), a classic alkylating chemotherapeutic agent used in treatment of gliomas and non-Hodgkin's lymphomas, as a multitype age-dependent branching process. We fitted our model to data from in vitro clonal experiments using the method of pseudo-likelihood. This approach identifies several novel drug effects, including modification of the cell cycle length, the time between division and differentiation, and alteration in the probability of undergoing self-renewal division in precursor cells. These changes of precursor cell function in the chemotherapy-exposed brain may have profound clinic implications.

MAJOR FINDINGS

We applied our computational approach to analyze the effects of BCNU on clonal cultures of oligodendrocyte progenitor cells-one of the best-characterized neural progenitor cells in the mammalian brain. Our analysis reveals that transient exposures to BCNU increased the cell cycle length of progenitor cells and decreased their time to differentiation, while also decreasing the likelihood that they will undergo self-renewing divisions. By investigating the behavior of our mathematical model, we demonstrate that precursor cell populations should recover spontaneously from transient modifications of the timing of division and of differentiation, but such recovery will not happen after alteration of cell fate. These studies identify means by which precursor cell function can be critically compromised by transient exposure to chemotherapy with long-term consequences on the progenitor cell pool even in the absence of drug-induced apoptosis. These analyses also provide novel tools that apply broadly to identify effects of chemotherapeutic agents and other physiological stressors.

Dicer1 and miR-219 Are required for normal oligodendrocyte differentiation and myelination

To investigate the role of microRNAs in regulating oligodendrocyte (OL) differentiation and myelination, we utilized transgenic mice in which microRNA processing was disrupted in OL precursor cells (OPCs) and OLs by targeted deletion of Dicer1. We found that inhibition of OPC-OL miRNA processing disrupts normal CNS myelination and that OPCs lacking mature miRNAs fail to differentiate normally in vitro. We identified three miRNAs (miR-219, miR-138, and miR-338) that are induced 10-100x during OL differentiation; the most strongly induced of these, miR-219, is necessary and sufficient to promote OL differentiation, and partially rescues OL differentiation defects caused by total miRNA loss. miR-219 directly represses the expression of PDGFRalpha, Sox6, FoxJ3, and ZFP238 proteins, all of which normally help to promote OPC proliferation. Together, these findings show that miR-219 plays a critical role in coupling differentiation to proliferation arrest in the OL lineage, enabling the rapid transition from proliferating OPCs to myelinating OLs.

Tapping into the glial reservoir: cells committed to remaining uncommitted

The development and maturation of the oligodendrocyte requires a series of highly orchestrated events that coordinate the proliferation and differentiation of the oligodendrocyte precursor cell (OPC) as well as the spatiotemporal regulation of myelination. In recent years, widespread interest has been devoted to the therapeutic potential of adult OPCs scattered throughout the central nervous system (CNS). In this review, we highlight molecular mechanisms controlling OPC differentiation during development and the implication of these mechanisms on adult OPCs for remyelination. Cell-autonomous regulators of differentiation and the heterogeneous microenvironment of the developing and the adult CNS may provide coordinated inhibitory cues that ultimately maintain a reservoir of uncommitted glia.

Comparison of efficiency of terminal differentiation of oligodendrocytes from induced pluripotent stem cells versus embryonic stem cells in vitro

Oligodendrocytes are the myelinating cells of the central nervous system (CNS), and defects in these cells can result in the loss of CNS functions. Although oligodendrocyte progenitor cells transplantation therapy is an effective cure for such symptoms, there is no readily available source of these cells. Recent studies have described the generation of induced pluripotent stem cells (iPS cells) from somatic cells, leading to anticipation of this technique as a novel therapeutic tool in regenerative medicine. In this study, we evaluated the ability of iPS cells derived from mouse embryonic fibroblasts to differentiate into oligodendrocytes and compared this with the differential ability of mouse embryonic stem cells (ES cells). Experiments using an in vitro oligodendrocyte differentiation protocol that was optimized to ES cells demonstrated that 2.3% of iPS cells differentiated into O4(+) oligodendrocytes compared with 24.0% of ES cells. However, the rate of induction of A2B5(+) oligodendrocyte precursor cell (OPC) was similar for both iPS-derived cells and ES-derived cells (14.1% and 12.6%, respectively). These findings suggest that some intracellular factors in iPS cells inhibit the terminal differentiation of oligodendrocytes from the OPC stage.

p57kip2 is dynamically regulated in experimental autoimmune encephalomyelitis and interferes with oligodendroglial maturation

The mechanisms preventing efficient remyelination in the adult mammalian central nervous system after demyelinating inflammatory diseases, such as multiple sclerosis, are largely unknown. Partial remyelination occurs in early disease stages, but repair capacity diminishes over time and with disease progression. We describe a potent candidate for the negative regulation of oligodendroglial differentiation that may underlie failure to remyelinate. The p57kip2 gene is dynamically regulated in the spinal cord during MOG-induced experimental autoimmune encephalomyelitis. Transient down-regulation indicated that it is a negative regulator of post-mitotic oligodendroglial differentiation. We then applied short hairpin RNA-mediated gene suppression to cultured oligodendroglial precursor cells and demonstrated that down-regulation of p57kip2 accelerates morphological maturation and promotes myelin expression. We also provide evidence that p57kip2 interacts with LIMK-1, implying that p57kip2 affects cytoskeletal dynamics during oligodendroglial maturation. These data suggest that sustained down-regulation of p57kip2 is important for oligodendroglial maturation and open perspectives for future therapeutic approaches to overcome the endogenous remyelination blockade in multiple sclerosis.

A crucial role for p57(Kip2) in the intracellular timer that controls oligodendrocyte differentiation

The intracellular molecular mechanism that controls the timing of oligodendrocyte differentiation remains unknown. Temple and Raff (1986) previously showed that an oligodendrocyte precursor cell (OPC) can divide a maximum of approximately eight times before its daughter cells simultaneously cease proliferating and differentiate into oligodendrocytes. They postulated that over time the level of an intracellular molecule might synchronously change in each daughter cell, ultimately reaching a level that prohibited additional proliferation. Here, we report the discovery of such a molecule, the cyclin-dependent kinase inhibitor p57(Kip2) (Cdkn1c). We show in vitro that all daughters of a clone of OPCs express similar levels of p57(Kip2), that p57(Kip2) levels increase over time in proliferating OPCs, and that p57(Kip2) levels regulate how many times an OPC can divide before differentiating. These findings reveal a novel part of the mechanism by which OPCs measure time and are likely to extend to similar timers in many other precursor cell types.

The mystery of intracellular developmental programmes and timers

There has been a revolution in understanding animal development in the last 25 years or so, but there is at least one area of development that has been relatively neglected and therefore remains largely mysterious. This is the intracellular programmes and timers that run in developing precursor cells and change the cells over time. The molecular mechanisms underlying these programmes are largely unknown. My colleagues and I have studied such programmes in two types of rodent neural precursor cells: those that give rise to oligodendrocytes, which make myelin in the CNS (central nervous system), and those that give rise to the various cell types in the retina.

The Yin and Yang of cell cycle progression and differentiation in the oligodendroglial lineage

In white matter disorders such as leukodystrophies (LD), periventricular leucomalacia (PVL), or multiple sclerosis (MS), the hypomyelination or the remyelination failure by oligodendrocyte progenitor cells involves errors in the sequence of events that normally occur during development when progenitors proliferate, migrate through the white matter, contact the axon, and differentiate into myelin-forming oligodendrocytes. Multiple mechanisms underlie the eventual progressive deterioration that typifies the natural history of developmental demyelination in LD and PVL and of adult-onset demyelination in MS. Over the past few years, pathophysiological studies have mostly focused on seeking abnormalities that impede oligodendroglial maturation at the level of migration, myelination, and survival. In contrast, there has been a strikingly lower interest for early proliferative and differentiation events that are likely to be equally critical for white matter development and myelin repair. This review highlights the Yin and Yang principles of interactions between intrinsic factors that coordinately regulate progenitor cell division and the onset of differentiation, i.e. the initial steps of oligodendrocyte lineage progression that are obviously crucial in health and diseases.

Identification of Sox17 as a transcription factor that regulates oligodendrocyte development

Microarray analysis of oligodendrocyte lineage cells purified by fluorescence-activated cell sorting (FACS) from 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP)-enhanced green fluorescent protein (EGFP) transgenic mice revealed Sox17 (SRY-box containing gene 17) gene expression to be coordinately regulated with that of four myelin genes during postnatal development. In CNP-EGFP-positive (CNP-EGFP+) cells, Sox17 mRNA and protein levels transiently increased between postnatal days 2 and 15, with white matter O4+ preoligodendrocytes expressing greater Sox17 levels than Nkx2.2+ (NK2 transcription factor related, locus 2) NG2+, or GalC+ (galactocerebroside) cells. In spinal cord, Sox17 protein expression was undetectable in the primary motor neuron domain between embryonic days 12.5 and 15.5 but was evident in Nkx2.2+ and CC1+ cells. In cultured oligodendrocyte progenitor cells (OPCs), Sox17 levels were maximal in O4+ cells and peaked during the phenotypic conversion from bipolar to multipolar. Parallel increases in Sox17 and p27 occurred before MBP protein expression, and Sox17 upregulation was prevented by conditions inhibiting differentiation. Sox17 downregulation with small interfering RNAs increased OPC proliferation and decreased lineage progression after mitogen withdrawal, whereas Sox17 overexpression in the presence of mitogen had opposite effects. Sox17 overexpression enhanced myelin gene expression in OPCs and directly stimulated MBP gene promoter activity. These findings support important roles for Sox17 in controlling both oligodendrocyte progenitor cell cycle exit and differentiation.

Apotransferrin decreases the response of oligodendrocyte progenitors to PDGF and inhibits the progression of the cell cycle

In the CNS, transferrin (Tf) is expressed by the oligodendroglial cells (OLGcs) and is essential for their development. We have previously shown that apotransferrin (aTf) accelerates maturation of OLGcs in vivo as well as in vitro. The mechanisms involved in this action appear to be complex and have not been completely elucidated. The aim of this study was to investigate if Tf participates in the regulation of the cell cycle of oligodendroglial progenitor cells (OPcs). Primary cultures of OPcs were treated with aTf and/or with different combinations of mitogenic factors. Cell cycle progression was studied by BrdU incorporation, flow cytometry and by the expression of cell cycle regulatory proteins. Apotransferrin decreased the number of BrdU+ cells, increasing the cell cycle time and decreasing the number of cells in S phase. The cell cycle inhibitors p27kip1, p21cip1 and p53 were increased, and in agreement with these results, the activity of the complexes involved in G1-S progression (cyclin D/CDK4, cyclin E/CDK2), was dramatically decreased. Apotransferrin also inhibited the mitogenic effects of PDGF and PDGF/IGF on OPcs, but did not affect their proliferation rate in the presence of bFGF, bFGF/PDGF or bFGF/IGF. Our results indicate that inhibition of the progression of the cell cycle of OPcs by aTf, even in the presence of PDGF, leads to an early beginning of the differentiation program, evaluated by different maturation markers (O4, GC and MBP) and by morphological criteria. The modulation by aTf of the response of OPcs to PDGF supports the idea that this glycoprotein might act as a key regulator of the OLGc lineage progression.

Activated microglia stimulate transcriptional changes in primary oligodendrocytes via IL-1beta

No therapy currently exists to repair demyelinated lesions in multiple sclerosis. However, the use of IgM antibodies may provide a valuable therapeutic avenue for evoking such repair. Unfortunately, the mechanism of immunoglobulin action in CNS repair is currently unknown but may depend upon complex interactions between multiple cell types rather than upon direct activation of a single cell type. Using rat mixed glial cultures containing oligodendrocytes, microglia, and astrocytes, we found that the Fc portion of human IgM shifts microglia to an activated phenotype, reduces glial proliferation, upregulates a variety of immediate early genes, including JunB, Egr-1, and c-Fos, and stimulates microglial production and release of IL-1beta. Microglia-derived IL-1beta consequently triggers transcriptional upregulation of immediate early genes such as c-Jun, Egr-1, and c-Fos in the mixed glial cultures, and stimulates the upregulation of late response genes such as lipocalin in purified oligodendrocytes. Treatment with an IL-1beta receptor antagonist abrogates the effects of Fcmu on glial proliferation and prevents the upregulation of lipocalin in response to Fcmu, but does not prevent Fcmu-mediated upregulation of IL-1beta, suggesting that IL-1beta mediates at least some of the downstream effects of Fcmu in mixed glial cultures. We hypothesize that Fcmu-stimulated IL-1beta-induced upregulation of immediate early and late response genes in oligodendrocytes may promote CNS repair.

A screen for mutations in zebrafish that affect myelin gene expression in Schwann cells and oligodendrocytes

Myelin is the multi-layered glial sheath around axons in the vertebrate nervous system. Myelinating glia develop and function in intimate association with neurons and neuron-glial interactions control much of the life history of these cells. However, many of the factors that regulate key aspects of myelin development and maintenance remain unknown. To discover new molecules that are important for glial development and myelination, we undertook a screen of zebrafish mutants with previously characterized neural defects. We screened for myelin basic protein (mbp) mRNA by in situ hybridization and identified four mutants (neckless, motionless, iguana and doc) that lacked mbp expression in parts of the peripheral and central nervous systems (PNS or CNS), despite the presence of axons. In all four mutants electron microscopy revealed that myelin-forming glia were present and had formed loose wraps around axons but did not form compact myelin. We found that addition of exogenous retinoic acid (RA) rescued mbp expression in neckless mutant embryos, which lack endogenous RA synthesis. Timed application of the RA synthesis inhibitor DEAB to wild type embryos showed that RA signalling is required at least 48 h before the onset of myelin protein synthesis in both CNS and PNS.

p53 is required for nerve growth factor-mediated differentiation of PC12 cells via regulation of TrkA levels

p53 is necessary for the elimination of neural cells inappropriately differentiated or in response to stimuli. However, the role of p53 in neuronal differentiation is not certain. Here, we showed that nerve growth factor (NGF)-mediated differentiation in PC12 cells is enhanced by overexpression of wild-type p53 but inhibited by mutant p53 or knockdown of endogenous wild-type p53, the latter of which can be rescued by expression of exogenous wild-type p53. Interestingly, p53 knockdown or overexpression of mutant p53 attenuates NGF-mediated activation of TrkA, the high-affinity receptor for NGF and a tyrosine kinase, and activation of the mitogen-activated protein kinase pathway. In addition, p53 knockdown reduces the constitutive levels of TrkA, which renders PC12 cells inert to NGF. And finally, we showed that both constitutive and stimuli-induced expressions of TrkA are regulated by p53 and that induction of TrkA by activated endogenous p53 enhances NGF-mediated differentiation. Taken together, our data demonstrate that p53 plays a critical role in NGF-mediated neuronal differentiation in PC12 cells at least in part via regulation of TrkA levels.

Environmental perchlorate: why it matters

The only known mechanism of toxicity for perchlorate is interference with iodide uptake at the sodium-iodide symporter (NIS). The NIS translocates iodide across basolateral membranes to the thyroid gland so it can be used to form thyroid hormones (TH). NIS is also expressed in the mammary gland during lactation, so that iodide can be transferred from a mother to her child. Without adequate iodide, an infant cannot produce sufficient TH to meet its developmental needs. Effects expected from perchlorate are those that would be seen in conditions of hypothyroidism or hypothyroxinemia. The probability of a permanent adverse effect is greatest during early life, as successful neurodevelopment is TH-dependent. Study of perchlorate risk is complicated by a number of factors including thyroid status of the mother during gestation, thyroid status of the fetus, maternal and infant iodine intake, and exposure of each to other TH-disrupting chemicals. Perhaps the greatest standing issue, and the issue most relevant to the field of analytical chemistry, is the simple fact that human exposure has not been quantified. This review will summarize perchlorate's potential to adversely affect neurodevelopment. Whether current environmental exposures to perchlorate contribute to neuro-impairment is unknown. Risks posed by perchlorate must be considered in conjunction with iodine intake.

The proliferative status of thyrotropes is dependent on modulation of specific cell cycle regulators by thyroid hormone

In this report we have examined changes in cell growth parameters, cell cycle effectors, and signaling pathways that accompany thyrotrope growth arrest by thyroid hormone (TH) and growth resumption after its withdrawal. Flow cytometry and immunohistochemistry of proliferation markers demonstrated that TH treatment of thyrotrope tumors resulted in a reduction in the fraction of cells in S-phase that is restored upon TH withdrawal. This is accompanied by dephosphorylation and rephosphorylation of retinoblastoma (Rb) protein. The expression levels of cyclin-dependent kinase 2 and cyclin A, as well as cyclin-dependent kinase 1 and cyclin B, were decreased by TH, and after withdrawal not only did these regulators of Rb phosphorylation and mitosis increase in their expression but so too did the D1 and D3 cyclins. We also noted a rapid induction and subsequent disappearance of the type 5 receptor for the growth inhibitor somatostatin with TH treatment and withdrawal, respectively. Because somatostatin can arrest growth by activating MAPK pathways, we examined these pathways in TtT-97 tumors and found that the ERK pathway and several of its upstream and downstream effectors, including cAMP response element binding protein, were activated with TH treatment and deactivated after its withdrawal. This led to the hypothesis that TH, acting through increased type 5 somatostatin receptor, could activate the ERK pathway leading to cAMP response element binding protein-dependent decreased expression of critical cell cycle proteins, specifically cyclin A, resulting in hypophosphorylation of Rb and its subsequent arrest of S-phase progression. These processes are reversed when TH is withdrawn, resulting in an increase in the fraction of S-phase cells.

pH is an intracellular effector controlling differentiation of oligodendrocyte precursors in culture via activation of the ERK1/2 pathway

We reported previously that onset of oligodendrocyte precursor cell (OPC) differentiation is accompanied by an increase in intracellular pH (pH(i)). We show that OPC differentiation is dependent primarily on a permissive pH(i) value. The highest differentiation levels were observed for pH(i) values around 7.15 and inhibition of differentiation was observed at slightly more acidic or alkaline values. Clamping the pH(i) of OPCs at 7.15 caused a transient activation of ERK1/2 that was not observed at more acidic or alkaline values. Furthermore, inhibition of ERK activation with the UO126 compound totally prevented OPC differentiation in response to pH(i) shift. These results indicate that pH(i), acting through the ERK1/2 pathway, is a key determinant for oligodendrocyte differentiation. We also show that this pH(i) pathway is involved in the process of retinoic acid-induced OPC differentiation.

Interferon-gamma inhibits cell cycle exit in differentiating oligodendrocyte progenitor cells

The developmental processes of the oligodendrocyte progenitor cell (OPC) lineage that are targeted by interferon-gamma (IFN-gamma) were studied in primary rat OPC cultures. Under conditions of thyroid hormone-mediated oligodendrocyte differentiation, IFN-gamma produced a dose-dependent apoptotic response in OPCs. The lowest dose tested (15 ng/ml or 75 U/ml) was nonapoptotic, but activated detectable STAT1 DNA-binding. At this dose, IFN-gamma reduced the percentage of mature O1+ cells and increased the percentage of immature A2B5+ OPCs. This was observed without significant change in total cell number and cytotoxicity, and was accompanied by an increase in BrdU-labeled A2B5+ and O4+ cells. FACS analysis confirmed a lack of apoptotic sub-G1 cells and revealed a greater percentage of S- and G2/M-phase OPCs with IFN-gamma treatment. Dual immunostaining with Ki-67 and Olig2 showed a smaller percentage of Olig2+ cells in G0 phase in IFN-gamma-treated OPCs, indicating loss of G1 control. Instead, increased levels and phosphorylation of the checkpoint protein p34cdc2 by IFN- suggested increased partial arrest in G2. IFN-gamma not only sustained expression of PCNA and the G1-S regulators retinoblastoma protein, cyclin D1, cyclin E, and cdk2, but also decreased p27 levels. In addition to changes in cell proliferation and differentiation, IFN-gamma attenuated myelin basic protein (MBP) expression significantly, which was associated with decreased expression of both MBP and Sox10 RNAs. These findings indicate that IFN-gamma not only maintains cell cycle activity that could predispose OPCs to apoptosis, but also overrides G1-G0 signals leading to thyroid hormone-mediated terminal differentiation and myelin gene expression.

Macrocephaly and the control of brain growth in autistic disorders

Autism is a childhood-onset neuropsychiatric disorder characterized by marked impairments in social interactions and communication, with restricted stereotypic and repetitive patterns of behavior, interests, and activities. Genetic epidemiology studies indicate that a strong genetic component exists to this disease, but these same studies also implicate significant environmental influence. The disorder also displays symptomatologic heterogeneity, with broad individual differences and severity on a graded continuum. In the search for phenotypes to resolve heterogeneity and better grasp autism's underlying biology, investigators have noted a statistical overrepresentation of macrocephaly, an indicator of enlarged brain volume. This feature is one of the most widely replicated biological findings in autism. What then does brain enlargement signify? One hypothesis invoked for the origin of macrocephaly is a reduction in neuronal pruning and consolidation of synapses during development resulting in an overabundance of neurites. An increase in generation of cells is an additional mechanism for macrocephaly, though it is less frequently discussed in the literature. Here, we review neurodevelopmental mechanisms regulating brain growth and highlight one underconsidered potential causal mechanism for autism and macrocephaly--an increase in neurogenesis and/or gliogenesis. We review factors known to control these processes with an emphasis on nuclear receptor activation as one signaling control that may be abnormal and contribute to increased brain volume in autistic disorders.

Redox regulation of precursor cell function: insights and paradoxes

Studies on oligodendrocytes, the myelin-forming cells of the central nervous system, and on the progenitor cells from which they are derived, have provided several novel insights into the role of intracellular redox state in cell function. This review discusses our findings indicating that intracellular redox state is utilized by the organism as a means of regulating the balance between progenitor cell division and differentiation. This regulation is achieved in part through cell-intrinsic differences that modify the response of cells to extracellular signaling molecules, such that cells that are slightly more reduced are more responsive to inducers of cell survival and division and less responsive to inducers of differentiation or cell death. Cells that are slightly more oxidized, in contrast, show a greater response to inducers of differentiation or cell death, but less response to inducers of proliferation or survival. Regulation is also achieved by the ability of exogenous signaling molecules to modify intracellular redox state in a highly predictable manner, such that signaling molecules that promote self-renewal make progenitor cells more reduced and those that promote differentiation make cells more oxidized. In both cases, the redox changes induced by exposure to exogenous signaling molecules are a necessary component of their mode of action. Paradoxically, the results obtained through studies on the oligodendrocyte lineage are precisely the opposite of what might be predicted from a large number of studies demonstrating the ability of reactive oxidative species to enhance the effects of signaling through receptor tyrosine kinase receptors and to promote cell proliferation. Taken in sum, available data demonstrate clearly the existence of two distinct programs of cellular responses to changes in oxidative status. In one of these, becoming even slightly more oxidized is sufficient to inhibit proliferation and induce differentiation. In the second program, similar changes enhance proliferation. It is not yet clear how cells can interpret putatively identical signals in such opposite manners, but it does already seem clear that resolving this paradox will provide insights of considerable relevance to the understanding of normal development, tissue repair, and tumorigenesis.

Sensitivity to DNA damage is a common component of hormone-based strategies for protection of the mammary gland

An early full-term pregnancy significantly reduces the risk of getting breast cancer in women. In animals, this protection can be mimicked by a short-term exposure to physiologic doses of estrogen plus progesterone. Sensitization of p53 and up-regulation of transforming growth factor beta are believed to be important aspects of the mechanism by which protection is imparted. Little is known, however, about the use of this pathway in response to other chemopreventive agents. In this article, we investigated the ability of retinoids, such as 9-cis retinoic acid, all-trans retinoic acid, and N-4-hydroxyphenylretinamide (4-HPR), to sensitize the ductal epithelial cells of virgin mammary glands to DNA damage responses. Using a whole-organ culture system, we observed enhanced cell death in response to gamma-irradiation in the virgin tissues treated with retinoids for 72 hours. These retinoids were partially dependent on p53 and transforming growth factor beta to exert their radiosensitizing effects. However, 4-HPR seemed to sensitize other cells or activate these pathways in a different manner as costimulation with ovarian hormones and 4-HPR was additive, whereas coculture of ovarian hormones and the natural retinoids did not increase amount of death. Taken together, these data suggest that sensitization of the mammary epithelium to p53-dependent apoptosis is a common pathway, which is engaged by retinoids as well as ovarian hormones.

S-phase entry of oligodendrocyte lineage cells is associated with increased levels of p21Cip1

The mechanisms regulating the number of myelinating cells in the central nervous system are crucial for both normal development and repair in pathological conditions. Among relevant growth factors involved in this process, fibroblast growth factor-2 (FGF2) induces oligodendrocyte progenitors (OLPs) to proliferate and stimulates mature oligodendrocytes (OLs) to reenter the S-phase of the cell cycle. S-phase entry is modulated by the formation of complexes between cyclins and cyclin-dependent kinases (CDKs), on one hand, and by their interactions with cell cycle inhibitors (e.g., p18INK, p27Kip1, p21Cip1), on the other. Although the roles of cyclin E/CDK2 complexes and the inhibitor p27Kip1 have been extensively investigated relative to proliferation and differentiation in the OL lineage, less is known about the regulation of the formation of cyclin D1/CDK4 complexes and the role of p21Cip1 in these events. In this study, we show that the FGF2-mediated increase in bromodeoxyuridine (BrdU) incorporation into OL progenitors and mature OLs occurs concomitantly with increase in the levels of p21Cip1 and the formation of p21Cip1/cyclin D1/CDK4 ternary complexes. These complexes are functionally active is indicated by the ensuing FGF2-dependent hyperphosphorylation of the downstream target Rb. In untreated mature OLs that do not incorporate BrdU, the levels of p21Cip1 are low, and the level of the inhibitor p18INK is high. Furthermore, p18INK sequesters CDK2 into binary complexes, precluding the formation of p21Cip1/cyclin D1/CDK4 ternary complexes in these cells. Therefore, we propose that p21Cip1 is acting as a positive regulator, rather than an inhibitor, of cell cycle entry by favoring the assembly of active cyclin D1/CDK4 complexes.