Demyelination-Remyelination of the Rat Caudal Cerebellar Peduncle Evaluated with Magnetic Resonance Imaging

N-butyl-β-carboline-3-carboxylate (β-CCB) systemic administration promotes remyelination in the cuprizone demyelinating model in mice

Demyelination is generated in several nervous system illnesses. Developing strategies for effective clinical treatments requires the discovery of promyelinating drugs. Increased GABAergic signaling through γ-aminobutyric acid type A receptor (GABAAR) activation in oligodendrocytes has been proposed as a promyelinating condition. GABAAR expressed in oligodendroglia is strongly potentiated by n-butyl-β-carboline-3-carboxylate (β-CCB) compared to that in neurons. Here, mice were subjected to 0.3% cuprizone (CPZ) added in the food to induce central nervous system demyelination, a well-known model for multiple sclerosis. Then β-CCB (1 mg/Kg) was systemically administered to analyze the remyelination status in white and gray matter areas. Myelin content was evaluated using Black-Gold II (BGII) staining, immunofluorescence (IF), and magnetic resonance imaging (MRI). Evidence indicates that β-CCB treatment of CPZ-demyelinated animals promoted remyelination in several white matter structures, such as the fimbria, corpus callosum, internal capsule, and cerebellar peduncles. Moreover, using IF, it was observed that CPZ intake induced an increase in NG2+ and a decrease in CC1+ cell populations, alterations that were importantly retrieved by β-CCB treatment. Thus, the promyelinating character of β-CCB was confirmed in a generalized demyelination model, strengthening the idea that it has clinical potential as a therapeutic drug.

β-carbolines that enhance GABAA receptor response expressed in oligodendrocytes promote remyelination in an in vivo rat model of focal demyelination

Demyelination is typically followed by a remyelination process through mature oligodendrocytes (OLs) differentiated from precursor cells (OPCs) recruited into the lesioned areas, however, this event usually results in uncompleted myelination. Potentiation of the remyelination process is an important target for designing effective therapeutic strategies against white matter loss. Here, it was evaluated the remyelinating effect of different β-carbolines that present differential allosteric modulation on the GABAA receptor expressed in OLs. For this, we used a focalized demyelination model in the inferior cerebellar peduncle (i.c.p.) of rats (DRICP model), in which, demyelination by ethidium bromide (0.05%) stereotaxic injection was confirmed histologically by staining with Black-Gold II (BGII) and toluidine blue. In addition, a longitudinal analysis with diffusion-weighted magnetic resonance imaging (dMRI) was made by computing fractional anisotropy (FA), apparent diffusion coefficient (ADC) and diffusivity parameters to infer i.c.p. microstructural changes. First, dMRI analysis revealed FA decreases together with ADC and radial diffusivity (RD) increases after demyelination, which correlates with histological BGII observations. Then, we evaluated the effect produced by three allosteric GABAA receptor modulators, the N-butyl-β-carboline-3-carboxylate (β-CCB), ethyl 9H-pyrido [3,4-b]indole-3-carboxylate (β-CCE), and 4-ethyl-6,7-dimethoxy-9H-pyrido [3,4-b]indole-3-carboxylic acid methyl ester (DMCM). The results indicated that daily systemic β-CCB (1 mg/Kg) or β-CCE (1 mg/Kg) administration for 2 weeks, but not DMCM (0.35 mg/Kg), in lesioned animals increased FA and decreased ADC or RD, suggesting myelination improvement. This was supported by BGII staining analysis that showed a recovery of myelin content. Also, it was quantified by immunohistochemistry both NG2+ and CC1+ cellular population in the different experimental sceneries. Data indicated that either β-CCB or β-CCE, but not DMCM, produced an increase in the population of CC1+ cells in the lesioned area. Finally, it was also calculated the g-ratio of myelinated axons and observed a similar value in those lesioned animals treated with β-CCB or β-CCE compared to controls. Thus, using the DRICP model, it was observed that either β-CCB or β-CCE, positive modulators of the GABAA receptor in OLs, had a potent promyelinating effect.

Transcriptional and bioinformatic analysis of GABAA receptors expressed in oligodendrocyte progenitor cells from the human brain

Introduction

Oligodendrocyte progenitor cells (OPCs) are vital for neuronal myelination and remyelination in the central nervous system. While the molecular mechanisms involved in OPCs' differentiation and maturation are not completely understood, GABA is known to positively influence these processes through the activation of GABAA receptors (GABAARs). The molecular identity of GABAARs expressed in human OPCs remains unknown, which restricts their specific pharmacological modulation to directly assess their role in oligodendrocytes' maturation and remyelination.

Methods

In this study, we conducted a transcriptomic analysis to investigate the molecular stoichiometry of GABAARs in OPCs from the human brain. Using eight available transcriptomic datasets from the human brain cortex of control individuals, we analyzed the mRNA expression of all 19 known GABAARs subunit genes in OPCs, with variations observed across different ages.

Results

Our analysis indicated that the most expressed subunits in OPCs are α1-3, β1-3, γ1-3, and ε. Moreover, we determined that the combination of any α with β2 and γ2 is likely to form heteropentameric GABAARs in OPCs. Importantly, we also found a strong correlation between GABAAR subunits and transcripts for postsynaptic scaffold proteins, suggesting the potential postsynaptic clustering of GABAARs in OPCs.

Discussion

This study presents the first transcriptional-level identification of GABAAR subunits expressed in human OPCs, providing potential receptor combinations. Understanding the molecular composition of GABAARs in OPCs not only enhances our knowledge of the underlying mechanisms in oligodendrocyte maturation but also opens avenues for targeted pharmacological interventions aimed at modulating these receptors to promote remyelination in neurological disorders.

Inhibition of CXCR2 enhances CNS remyelination via modulating PDE10A/cAMP signaling pathway

CXC chemokine receptor 2 (CXCR2) plays an important role in demyelinating diseases, but the detailed mechanisms were not yet clarified. In the present study, we mainly investigated the critical function and the potential molecular mechanisms of CXCR2 on oligodendrocyte precursor cell (OPC) differentiation and remyelination. The present study demonstrated that inhibiting CXCR2 significantly enhanced OPC differentiation and remyelination in primary cultured OPCs and ethidium bromide (EB)-intoxicated rats by facilitating the formation of myelin proteins, including PDGFRα, MBP, MAG, MOG, and Caspr. Further investigation identified phosphodiesterase 10A (PDE10A) as a main downstream protein of CXCR2, interacting with the receptor to regulate OPC differentiation, in that inhibition of CXCR2 reduced PDE10A expression while suppression of PDE10A did not affect CXCR2. Furthermore, inhibition of PDE10A promoted OPC differentiation, whereas overexpression of PDE10A down-regulated OPC differentiation. Our data also revealed that inhibition of CXCR2/PDE10A activated the cAMP/ERK1/2 signaling pathway, and up-regulated the expression of key transcription factors, including SOX10, OLIG2, MYRF, and ZFP24, that ultimately promoted remyelination and myelin protein biosynthesis. In conclusion, our findings suggested that inhibition of CXCR2 promoted OPC differentiation and enhanced remyelination by regulating PDE10A/cAMP/ERK1/2 signaling pathway. The present data also highlighted that CXCR2 may serve as a potential target for the treatment of demyelination diseases.

GABAB receptor agonist baclofen promotes central nervous system remyelination

Promoting remyelination is considered as a potential neurorepair strategy to prevent/limit the development of permanent neurological disability in patients with multiple sclerosis (MS). To this end, a number of clinical trials are investigating the potential of existing drugs to enhance oligodendrocyte progenitor cell (OPC) differentiation, a process that fails in chronic MS lesions. We previously reported that oligodendroglia express GABA_B receptors (GABA_B Rs) both in vitro and in vivo, and that GABA_B R-mediated signaling enhances OPC differentiation and myelin protein expression in vitro. Our goal here was to evaluate the pro-remyelinating potential of GABA_B R agonist baclofen (Bac), a clinically approved drug to treat spasticity in patients with MS. We first demonstrated that Bac increases myelin protein production in lysolecithin (LPC)-treated cerebellar slices. Importantly, Bac administration to adult mice following induction of demyelination by LPC injection in the spinal cord resulted in enhanced OPC differentiation and remyelination. Thus, our results suggest that Bac repurposing should be considered as a potential therapeutic strategy to stimulate remyelination in patients with MS.

Sea anemone Bartholomea annulata venom inhibits voltage-gated Na+ channels and activates GABAA receptors from mammals

Toxin production in nematocysts by Cnidaria phylum represents an important source of bioactive compounds. Using electrophysiology and, heterologous expression of mammalian ion channels in the Xenopus oocyte membrane, we identified two main effects produced by the sea anemone Bartholomea annulata venom. Nematocysts isolation and controlled discharge of their content, revealed that venom had potent effects on both voltage-dependent Na⁺ (Na_v) channels and GABA type A channel receptors (GABA_AR), two essential proteins in central nervous system signaling. Unlike many others sea anemone toxins, which slow the inactivation rate of Na_v channels, B. annulata venom potently inhibited the neuronal action potential and the Na⁺ currents generated by distinct Na_v channels opening, including human TTX-sensitive (hNa_v1.6) and TTX-insensitive Na_v channels (hNa_v1.5). A second effect of B. annulata venom was an agonistic action on GABA_AR that activated distinct receptors conformed by either α1β2γ2, α3β2γ1 or, ρ1 homomeric receptors. Since GABA was detected in venom samples by ELISA assay at low nanomolar range, it was excluded that GABA from nematocysts directly activated the GABA_ARs. This revealed that substances in B. annulata nematocysts generated at least two potent and novel effects on mammalian ion channels that are crucial for nervous system signaling.

Ion Channels as New Attractive Targets to Improve Re-Myelination Processes in the Brain

Multiple sclerosis (MS) is the most demyelinating disease of the central nervous system (CNS) characterized by neuroinflammation. Oligodendrocyte progenitor cells (OPCs) are cycling cells in the developing and adult CNS that, under demyelinating conditions, migrate to the site of lesions and differentiate into mature oligodendrocytes to remyelinate damaged axons. However, this process fails during disease chronicization due to impaired OPC differentiation. Moreover, OPCs are crucial players in neuro-glial communication as they receive synaptic inputs from neurons and express ion channels and neurotransmitter/neuromodulator receptors that control their maturation. Ion channels are recognized as attractive therapeutic targets, and indeed ligand-gated and voltage-gated channels can both be found among the top five pharmaceutical target groups of FDA-approved agents. Their modulation ameliorates some of the symptoms of MS and improves the outcome of related animal models. However, the exact mechanism of action of ion-channel targeting compounds is often still unclear due to the wide expression of these channels on neurons, glia, and infiltrating immune cells. The present review summarizes recent findings in the field to get further insights into physio-pathophysiological processes and possible therapeutic mechanisms of drug actions.

Therapeutic Potential of GABAergic Signaling in Myelin Plasticity and Repair

Oligodendrocytes (OLs) produce myelin to insulate axons. This accelerates action potential propagation, allowing nerve impulse information to synchronize within complex neuronal ensembles and promoting brain connectivity. Brain plasticity includes myelination, a process that starts early after birth and continues throughout life. Myelin repair, followed by injury or disease, requires new OLs differentiated from a population derived from oligodendrocyte precursor cells (OPCs) that continue to proliferate, migrate and differentiate to preserve and remodel myelin in the adult central nervous system. OPCs represent the largest proliferative neural cell population outside the adult neurogenic niches in the brain. OPCs receive synaptic inputs from glutamatergic and GABAergic neurons throughout neurodevelopment, a unique feature among glial cells. Neuron-glia communication through GABA signaling in OPCs has been shown to play a role in myelin plasticity and repair. In this review we will focus on the molecular and functional properties of GABA_A receptors (GABA_ARs) expressed by OPCs and their potential role in remyelination.

GABAA Receptors Expressed in Oligodendrocytes Cultured from the Neonatal Rat Contain α3 and γ1 Subunits and Present Differential Functional and Pharmacological Properties

Oligodendrocytes (OLs) express functional GABA_A receptors (GABA_ARs) that are activated by GABA released at synaptic contacts with axons or by ambient GABA in extrasynaptic domains. In both instances, the receptors' molecular identity has not been fully defined. Furthermore, data on their structural diversity in different brain regions and information on age-dependent changes in their molecular composition are scant. This lack of knowledge has delayed access to a better understanding of the role of GABAergic signaling between neurons and OLs. Here, we used functional, and pharmacological analyses, as well as gene and protein expression of GABA_AR subunits, to explore the subunit combination that could explain the receptor functional profile expressed in OLs from the neonate rat. We found that GABA_AR composed of α3β2γ1 subunits mimicked the characteristics of the endogenous receptor when expressed heterologously in Xenopus laevis oocytes. Either α3 or γ1 subunit silencing by small interfering RNA transfection changed the GABA-response characteristics in oligodendrocyte precursor cells, indicating their participation in the endogenous receptor conformation. Thus, α3 subunit silencing shifted the mean EC₅₀ for GABA from 75.1 to 46.6 µM, whereas γ1 silencing reduced the current amplitude response by 55%. We also observed that β-carbolines differentially enhance GABA responses in oligodendroglia as compared with those in neurons. These results contribute to defining the molecular and pharmacological properties of GABA_ARs in OLs. Additionally, the identification of β-carbolines as selective enhancers of GABA_ARs in OLs may help to study the role of GABAergic signaling during myelination. SIGNIFICANCE STATEMENT: GABAergic signaling through GABA_A receptors (GABA_ARs) expressed in the oligodendroglial lineage contributes to the myelination control. Determining the molecular identity and the pharmacology of these receptors is essential to define their specific roles in myelination. Using GABA_AR subunit expression and silencing, we identified that the GABA_AR subunit combination α3β2γ1 conforms the bulk of GABA_ARs in oligodendrocytes from rat neonates. Furthermore, we found that these receptors have differential pharmacological properties that allow specific positive modulation by β-carbolines.

Expression and Function of GABA Receptors in Myelinating Cells

Myelin facilitates the fast transmission of nerve impulses and provides metabolic support to axons. Differentiation of oligodendrocyte progenitor cells (OPCs) and Schwann cell (SC) precursors is critical for myelination during development and myelin repair in demyelinating disorders. Myelination is tightly controlled by neuron-glia communication and requires the participation of a wide repertoire of signals, including neurotransmitters such as glutamate, ATP, adenosine, or γ-aminobutyric acid (GABA). GABA is the main inhibitory neurotransmitter in the central nervous system (CNS) and it is also present in the peripheral nervous system (PNS). The composition and function of GABA receptors (GABARs) are well studied in neurons, while their nature and role in glial cells are still incipient. Recent studies demonstrate that GABA-mediated signaling mechanisms play relevant roles in OPC and SC precursor development and function, and stand out the implication of GABARs in oligodendrocyte (OL) and SC maturation and myelination. In this review, we highlight the evidence supporting the novel role of GABA with an emphasis on the molecular identity of the receptors expressed in these glial cells and the possible signaling pathways involved in their actions. GABAergic signaling in myelinating cells may have potential implications for developing novel reparative therapies in demyelinating diseases.

Thalamocortical structural connectivity abnormalities in drug-resistant generalized epilepsy: A diffusion tensor imaging study

BACKGROUND AND PURPOSE

Epilepsy is one of the most common diseases of the nervous system. Approximately one-third of epilepsy cases are drug-resistant, among which generalized-onset seizures are very common. The present study aimed to analyze abnormalities of the thalamocortical fiber pathways in each hemisphere of the brains of patients with drug-resistant generalized epilepsy.

MATERIALS AND METHODS

The thalamocortical structural pathways were identified by diffusion tensor imaging (DTI) in 15 patients with drug-resistant generalized epilepsy and 16 gender/age-matched controls. The thalami of both groups were parcellated into subregions according to the local thalamocortical connectivity pattern. DTI measures of thalamocortical connections were compared between the two groups.

RESULTS

Probabilistic tractography analyses showed that fractional anisotropy of thalamocortical pathways in patients with epilepsy decreased significantly, and the radial diffusivity of the left thalamus pathways with homolateral motor and parietal-occipital cortical regions in the drug-resistant epilepsy group increased significantly. In addition to the right thalamus pathway and prefrontal cortical region, fractional anisotropy of all other pathways was inversely correlated with disease duration.

CONCLUSION

The results provide evidence indicating widespread bilateral abnormalities in the thalamocortical pathways in epilepsy patients and imply that the degree of abnormality in the pathway increases with the disease duration.