Cellular and molecular characterization of multipolar Map5-expressing cells: a subset of newly generated, stage-specific parenchymal cells in the mammalian central nervous system

Identification of Tau protein as a novel marker for maturation and pathological changes of oligodendrocytes

Microtubule-associated protein Tau is primarily expressed in axons of neurons, but also in Olig2-positive oligodendrocytes in adult rodent and monkey brains. In this study, we sought to determine at what cell stage Tau becomes expressed in the oligodendrocyte lineage. We performed immunostaining of adult mouse brain sections using well-known markers of oligodendrocyte lineage and found that Tau is expressed in mature oligodendrocytes, but not in oligodendrocyte progenitors and immature pre-oligodendrocytes. We also investigated Tau expression in developing mouse brain. Surprisingly, Tau expression occurred after the peak of myelination and even exceeded GSTπ expression, which has been considered as a marker of myelinating oligodendrocytes. These results suggest Tau as a novel marker of oligodendrocyte maturation. We then investigated whether Tau is important for oligodendrocyte development and/or myelination and how Tau changes in demyelination. First, we found no changes in myelination and oligodendrocyte markers in Tau knockout mice, suggesting that Tau is dispensable. Next, we analyzed the proteolipid protein 1 transgenic model of Pelizaeus-Merzbacher disease, which is a rare leukodystrophy. In hemizygous transgenic mice, the number of Tau-positive cells were significantly increased as compared with wild type mice. These cells were also positive for Olig2, CC1, and GSTπ, but not PDGFRα and GPR17. In stark contrast, the expression level of Tau, as well as GSTπ, was dramatically decreased in the cuprizone-induced model of multiple sclerosis. Taken together, we propose Tau as a new marker of oligodendrocyte lineage and for investigating demyelination lesions.

Functional Heterodimerization between the G Protein-Coupled Receptor GPR17 and the Chemokine Receptors 2 and 4: New Evidence

GPR17, a G protein-coupled receptor, is a pivotal regulator of myelination. Its endogenous ligands trigger receptor desensitization and downregulation allowing oligodendrocyte terminal maturation. In addition to its endogenous agonists, GPR17 could be promiscuously activated by pro-inflammatory oxysterols and chemokines released at demyelinating lesions. Herein, the chemokine receptors CXCR2 and CXCR4 were selected to perform both in silico modelling and in vitro experiments to establish their structural and functional interactions with GPR17. The relative propensity of GPR17 and CXCR2 or CXCR4 to form homo- and hetero-dimers was assessed by homology modelling and molecular dynamics (MD) simulations, and co-immunoprecipitation and immunoenzymatic assay. The interaction between chemokine receptors and GPR17 was investigated by determining receptor-mediated modulation of intracellular cyclic adenosine monophosphate (cAMP). Our data show the GPR17 association with CXCR2 or CXCR4 and the negative regulation of these interactions by CXCR agonists or antagonists. Moreover, GPR17 and CXCR2 heterodimers can functionally influence each other. In contrast, CXCR4 can influence GPR17 functionality, but not vice versa. According to MD simulations, all the dimers reached conformational stability and negative formation energy, confirming the experimental observations. The cross-talk between these receptors could play a role in the development of the neuroinflammatory milieu associated with demyelinating events.

c-Jun N-terminal kinase 1 (JNK1) modulates oligodendrocyte progenitor cell architecture, proliferation and myelination

During Central Nervous System ontogenesis, myelinating oligodendrocytes (OLs) arise from highly ramified and proliferative precursors called oligodendrocyte progenitor cells (OPCs). OPC architecture, proliferation and oligodendro-/myelino-genesis are finely regulated by the interplay of cell-intrinsic and extrinsic factors. A variety of extrinsic cues converge on the extracellular signal-regulated kinase/mitogen activated protein kinase (ERK/MAPK) pathway. Here we found that the germinal ablation of the MAPK c-Jun N-Terminal Kinase isoform 1 (JNK1) results in a significant reduction of myelin in the cerebral cortex and corpus callosum at both postnatal and adult stages. Myelin alterations are accompanied by higher OPC density and proliferation during the first weeks of life, consistent with a transient alteration of mechanisms regulating OPC self-renewal and differentiation. JNK1 KO OPCs also show smaller occupancy territories and a less complex branching architecture in vivo. Notably, these latter phenotypes are recapitulated in pure cultures of JNK1 KO OPCs and of WT OPCs treated with the JNK inhibitor D-JNKI-1. Moreover, JNK1 KO and WT D-JNKI-1 treated OLs, while not showing overt alterations of differentiation in vitro, display a reduced surface compared to controls. Our results unveil a novel player in the complex regulation of OPC biology, on the one hand showing that JNK1 ablation cell-autonomously determines alterations of OPC proliferation and branching architecture and, on the other hand, suggesting that JNK1 signaling in OLs participates in myelination in vivo.

Regulation and signaling of the GPR17 receptor in oligodendroglial cells

Remyelination, namely, the formation of new myelin sheaths around denuded axons, counteracts axonal degeneration and restores neuronal function. Considerable advances have been made in understanding this regenerative process that often fails in diseases like multiple sclerosis, leaving axons demyelinated and vulnerable to damage, thus contributing to disease progression. The identification of the membrane receptor GPR17 on a subset of oligodendrocyte precursor cells (OPCs), which mediate remyelination in the adult central nervous system (CNS), has led to a huge amount of evidence that validated this receptor as a new attractive target for remyelinating therapies. Here, we summarize the role of GPR17 in OPC function, myelination and remyelination, describing its atypical pharmacology, its downstream signaling, and the genetic and epigenetic factors modulating its activity. We also highlight crucial insights into GPR17 pathophysiology coming from the demonstration that oligodendrocyte injury, associated with inflammation in chronic neurodegenerative conditions, is invariably characterized by abnormal and persistent GPR17 upregulation, which, in turn, is accompanied by a block of OPCs at immature premyelinating stages. Finally, we discuss the current literature in light of the potential exploitment of GPR17 as a therapeutic target to promote remyelination.

Pharmacological Properties and Biological Functions of the GPR17 Receptor, a Potential Target for Neuro-Regenerative Medicine

In 2006, cells heterologously expressing the "orphan" receptor GPR17 were shown to acquire responses to both uracil nucleotides and cysteinyl-leukotrienes, two families of signaling molecules accumulating in brain or heart as a result of hypoxic/traumatic injuries. In subsequent years, evidence of GPR17 key role in oligodendrogenesis and myelination has highlighted it as a "model receptor" for new therapies in demyelinating and neurodegenerative diseases. The apparently contrasting evidence in the literature about the role of GPR17 in promoting or inhibiting myelination can be due to its transient expression in the intermediate stages of differentiation, exerting a pro-differentiating function in early oligodendrocyte precursor cells (OPCs), and an inhibitory role in late stage maturing cells. Meanwhile, several papers extended the initial data on GPR17 pharmacology, highlighting a "promiscuous" behavior of this receptor; indeed, GPR17 is able to respond to other emergency signals like oxysterols or the pro-inflammatory cytokine SDF-1, underlying GPR17 ability to adapt its responses to changes of the surrounding extracellular milieu, including damage conditions. Here, we analyze the available literature on GPR17, in an attempt to summarize its emerging biological roles and pharmacological properties.

Neuro-psychopharmacological perspective of Orphan receptors of Rhodopsin (class A) family of G protein-coupled receptors

BACKGROUND

In the central nervous system (CNS), G protein-coupled receptors (GPCRs) are the most fruitful targets for neuropsychopharmacological drug development. Rhodopsin (class A) is the most studied class of GPCR and includes orphan receptors for which the endogenous ligand is not known or is unclear. Characterization of orphan GPCRs has proven to be challenging, and the production pace of GPCR-based drugs has been incredibly slow.

OBJECTIVE

Determination of the functions of these receptors may provide unexpected insight into physiological and neuropathological processes. Advances in various methods and techniques to investigate orphan receptors including in situ hybridization and knockdown/knockout (KD/KO) showed extensive expression of these receptors in the mammalian brain and unmasked their physiological and neuropathological roles. Due to these rapid progress and development, orphan GPCRs are rising as a new and promising class of drug targets for neurodegenerative diseases and psychiatric disorders.

CONCLUSION

This review presents a neuropsychopharmacological perspective of 26 orphan receptors of rhodopsin (class A) family, namely GPR3, GPR6, GPR12, GPR17, GPR26, GPR35, GPR39, GPR48, GPR49, GPR50, GPR52, GPR55, GPR61, GPR62, GPR63, GPR68, GPR75, GPR78, GPR83, GPR84, GPR85, GPR88, GPR153, GPR162, GPR171, and TAAR6. We discussed the expression of these receptors in mammalian brain and their physiological roles. Furthermore, we have briefly highlighted their roles in neurodegenerative diseases and psychiatric disorders including Alzheimer's disease, Parkinson's disease, neuroinflammation, inflammatory pain, bipolar and schizophrenic disorders, epilepsy, anxiety, and depression.

MiR-125a-3p timely inhibits oligodendroglial maturation and is pathologically up-regulated in human multiple sclerosis

In the mature central nervous system (CNS), oligodendrocytes provide support and insulation to axons thanks to the production of a myelin sheath. During their maturation to myelinating cells, oligodendroglial precursors (OPCs) follow a very precise differentiation program, which is finely orchestrated by transcription factors, epigenetic factors and microRNAs (miRNAs), a class of small non-coding RNAs involved in post-transcriptional regulation. Any alterations in this program can potentially contribute to dysregulated myelination, impaired remyelination and neurodegenerative conditions, as it happens in multiple sclerosis (MS). Here, we identify miR-125a-3p, a developmentally regulated miRNA, as a new actor of oligodendroglial maturation, that, in the mammalian CNS regulates the expression of myelin genes by simultaneously acting on several of its already validated targets. In cultured OPCs, over-expression of miR-125a-3p by mimic treatment impairs while its inhibition with an antago-miR stimulates oligodendroglial maturation. Moreover, we show that miR-125a-3p levels are abnormally high in the cerebrospinal fluid of MS patients bearing active demyelinating lesions, suggesting that its pathological upregulation may contribute to MS development, at least in part by blockade of OPC differentiation leading to impaired repair of demyelinated lesions.

Species differences in behavior and cell proliferation/survival in the adult brains of female meadow and prairie voles

Microtine rodents display diverse patterns of social organization and behaviors, and thus provide a useful model for studying the effects of the social environment on physiology and behavior. The current study compared the species differences and the effects of oxytocin (OT) on anxiety-like, social affiliation, and social recognition behaviors in female meadow voles (Microtus pennsylvanicus) and prairie voles (Microtus ochrogaster). Furthermore, cell proliferation and survival in the brains of adult female meadow and prairie voles were compared. We found that female meadow voles displayed a higher level of anxiety-like behavior but lower levels of social affiliation and social recognition compared to female prairie voles. In addition, meadow voles showed lower levels of cell proliferation (measured by Ki67 staining) and cell survival (measured by BrdU staining) in the ventromedial hypothalamus (VMH) and amygdala (AMY), but not the dentate gyrus of the hippocampus (DG), than prairie voles. Interestingly, the numbers of new cells in the VMH and AMY, but not DG, also correlated with anxiety-like, social affiliation, and social recognition behaviors in a brain region-specific manner. Finally, central OT treatment (200 ng/kg, icv) did not lead to changes in behavior or cell proliferation/survival in the brain. Together, these data indicate a potential role of cell proliferation/survival in selected brain areas on different behaviors between vole species with distinct life strategies.

CNS remyelination as a novel reparative approach to neurodegenerative diseases: The roles of purinergic signaling and the P2Y-like receptor GPR17

Oligodendrocytes are the myelin-forming cells in the CNS. They enwrap axons, thus permitting fast impulse transmission and exerting trophic actions on neurons. Demyelination accompanied by neurological deficit is a rather frequent condition that is not only associated with multiple sclerosis but has been also recognized in several other neurodegenerative diseases, including brain trauma and stroke, Alzheimer's disease and amyotrophic lateral sclerosis. Recently, alterations of myelin function have been also reported in neuropsychiatric diseases, like depression and autism. Highly relevant for therapeutic purposes, oligodendrocyte precursor cells (OPCs) still persist in the adult brain and spinal cord. These cells are normally rather quiescent, but under specific circumstances, they can be stimulated to undergo differentiation and generate mature myelinating oligodendrocytes. Thus, approaches aimed at restoring myelin integrity and at fostering a correct oligodendrocyte function are now viewed as novel therapeutic opportunities for both neurodegenerative and neuropsychiatric diseases. Both OPCs and mature oligodendrocytes express purinergic receptors. For some of these receptors, expression is restricted at specific differentiation stages, suggesting key roles in OPCs maturation and myelination. Some of these receptors are altered under demyelinating conditions, suggesting that their dysregulation may contribute to disease development and could represent adequate new targets for remyelinating therapies. Here, we shall describe the current literature available on all these receptors, with special emphasis on the P2Y-like GPR17 receptor, that represents one of the most studied receptor subtypes in these cells. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Noncanonical Sites of Adult Neurogenesis in the Mammalian Brain

Two decades after the discovery that neural stem cells (NSCs) populate some regions of the mammalian central nervous system (CNS), deep knowledge has been accumulated on their capacity to generate new neurons in the adult brain. This constitutive adult neurogenesis occurs throughout life primarily within remnants of the embryonic germinal layers known as "neurogenic sites." Nevertheless, some processes of neurogliogenesis also occur in the CNS parenchyma commonly considered as "nonneurogenic." This "noncanonical" cell genesis has been the object of many claims, some of which turned out to be not true. Indeed, it is often an "incomplete" process as to its final outcome, heterogeneous by several measures, including regional location, progenitor identity, and fate of the progeny. These aspects also strictly depend on the animal species, suggesting that persistent neurogenic processes have uniquely adapted to the brain anatomy of different mammals. Whereas some examples of noncanonical neurogenesis are strictly parenchymal, others also show stem cell niche-like features and a strong link with the ventricular cavities. This work will review results obtained in a research field that expanded from classic neurogenesis studies involving a variety of areas of the CNS outside of the subventricular zone (SVZ) and subgranular zone (SGZ). It will be highlighted how knowledge concerning noncanonical neurogenic areas is still incomplete owing to its regional and species-specific heterogeneity, and to objective difficulties still hampering its full identification and characterization.