GABA promotes survival and axonal regeneration in identifiable descending neurons after spinal cord injury in larval lampreys

Baclofen modulates the immune response after spinal cord injury with locomotor benefits

BACKGROUND AND PURPOSE

Spinal cord injury (SCI) is a neurological condition that affects motor and sensory functions below the injury site. The consequences of SCI are devastating for the patients, and although significant efforts have been done in the last years, there is no effective therapy. Baclofen has emerged in the last few years as an interesting drug in the SCI field. Already used in the SCI clinical setting to control spasticity, baclofen has shown important impact on SCI recovery in animal models, such as lampreys and mice.

EXPERIMENTAL APPROACH AND KEY RESULTS

Herein, we proposed to go deeper into baclofen's mechanism of action and to study its role on the modulation of the immune response after SCI, a major process associated with the severeness of the lesion. Using a SCI compression mice model, we confirmed that baclofen leads to higher locomotor performance, but only at 1 mg·kg-1 and not in higher concentrations, as 5 mg·kg-1. Moreover, we found that baclofen at 1 mg·kg-1 can strongly modulate the immune response after SCI at local, systemic and peripheric levels. This is interesting and intriguingly at the same time, since now, additional studies should be performed to understand if the modulation of the immune response is the responsible for the locomotor outcomes observed on Baclofen treated animals.

CONCLUSION AND IMPLICATIONS

Our findings showed, for the first time, that baclofen can modulate the immune response after SCI, becoming a relevant drug in the field of the immunomodulators.

GABAB receptors regulate the neural stem cell potential of Pkd2l1+ cerebrospinal fluid-contacting neurons via the PI3K/Akt signaling pathway

Cerebrospinal fluid-contacting neurons (CSF-cNs) exhibit neural stem cell (NSC) properties both in vitro and in vivo, and they may play a critical role in recovery after spinal cord injury (SCI). GABAB receptors (GABABRs) are expressed in Pkd2l1+ CSF-cNs. However, their role in Pkd2l1+ CSF-cNs still needs to be discovered. In this study, we observed a significant reduction in GABABR expression in a murine model 7 d after SCI. We further discovered that GABABR activation enhanced the proliferation of Pkd2l1+ CSF-cNs while inhibiting apoptosis. Additionally, this activation mitigated vacuole loss and neuronal damage in the pericentral canal region of the spinal cord, attenuated myelin and axonal loss within the spinal cord, and facilitated motor function recovery in SCI model mice. Mechanistically, GABABR primed quiescent Pkd2l1+ CSF-cNs for cell cycle reentry through the activation of PI3K/Akt signaling. Our findings suggest that GABABR activation enhances the NSC potential of Pkd2l1+ CSF-cNs, ultimately enabling post-SCI recovery in murine models.

Preferential Synaptic Type of GABA-A Receptor Ligands Enhancing Neuronal Survival and Facilitating Functional Recovery After Ischemic Stroke

Selective enhancement of synaptic GABA signaling mediated by GABA-A receptors has been previously reported to promote functional recovery after ischemic stroke, while tonic GABA signaling has been detrimental. To identify agents that enhance synaptic signaling, we synthesized GABA-A ligands based on three chemotypes with affinity values pKi= 6.44-8.32. Representative compounds showed a preference in functional responses toward synaptic type of GABA-A receptors, compared to the extrasynaptic ones. In a cellular ischemia model (OGD), selected compounds showed the potential to improve neuronal recovery. The selected lead, compound 4, demonstrated the ability to reduce mitochondrial dysfunction, regulate intracellular calcium levels, decrease caspase 3 levels, and promote neurite outgrowth in in vitro assays. In an animal model, compound 4 enhanced motor recovery and showed neuroprotective activity by reducing infarct volume and decreasing poststroke acidosis. These findings underscore the value of selective ligands modulating synaptic GABA-A receptors in promoting recovery from ischemic stroke.

Cognitive Impairment Mechanisms in High-Altitude Exposure: Proteomic and Metabolomic Insights

High-altitude exposure can adversely affect neurocognitive functions; however, the underlying mechanisms remain elusive. Why and how does high-altitude exposure impair neurocognitive functions, particularly sleep? This study seeks to identify the molecular markers and mechanisms involved, with the goal of forming prevention and mitigation strategies for altitude sickness. Using serum proteomics and metabolomics, we analyzed blood samples from 23 Han Chinese plain dwellers before and after six months of high-altitude work in Tibet. The correlation analysis revealed biomarkers associated with cognitive alterations. Six months of high-altitude exposure significantly compromised cognitive function, notably, sleep quality. The key biomarkers implicated include SEPTIN5, PCBP1, STIM1, UBE2L3/I/N, amino acids (l/d-aspartic acid and l-glutamic acid), arachidonic acid, and S1P. Immune and neural signaling were suppressed, with sex-specific differences observed. This study innovatively identified GABA, arachidonic acid, l-glutamic acid, 2-arachidonoyl glycerol, and d-aspartic acid as biomarkers and elucidated the underlying mechanisms contributing to high-altitude-induced neurocognitive decline with a particular focus on sleep disruption. These findings pave the way for developing preventive measures and enhancing adaptation strategies. This study underscores the physiological significance of high-altitude adaptation, raising new questions about sex-specific responses and long-term consequences. It sets the stage for future research exploring individual variability and intervention efficacy.

Spontaneous regeneration of cholecystokinergic reticulospinal axons after a complete spinal cord injury in sea lampreys

In contrast to humans, lampreys spontaneously recover their swimming capacity after a complete spinal cord injury (SCI). This recovery process involves the regeneration of descending axons. Spontaneous axon regeneration in lampreys has been mainly studied in giant descending neurons. However, the regeneration of neurochemically distinct descending neuronal populations with small-caliber axons, as those found in mammals, has been less studied. Cholecystokinin (CCK) is a regulatory neuropeptide found in the brain and spinal cord that modulates several processes such as satiety, or locomotion. CCK shows high evolutionary conservation and is present in all vertebrate species. Work in lampreys has shown that all CCKergic spinal cord axons originate in a single neuronal population located in the caudal rhombencephalon. Here, we investigate the spontaneous regeneration of CCKergic descending axons in larval lampreys following a complete SCI. Using anti-CCK-8 immunofluorescence, confocal microscopy and lightning adaptive deconvolution, we demonstrate the partial regeneration of CCKergic axons (81% of the number of axonal profiles seen in controls) 10 weeks after the injury. Our data also revealed a preference for regeneration of CCKergic axons in lateral spinal cord regions. Regenerated CCKergic axons exhibit colocalization with synaptic vesicle marker SV2, indicative of functional synaptic connections. We also extracted swimming dynamics in injured animals by using DeepLabCut. Interestingly, the degree of CCKergic reinnervation correlated with improved swimming performance in injured animals, suggesting a potential role in locomotor recovery. These findings open avenues for further exploration into the role of specific neuropeptidergic systems in post-SCI spinal locomotor networks.

Molecular mechanisms of spinal cord injury repair across vertebrates: A comparative review

In humans and other adult mammals, axon regeneration is difficult in axotomized neurons. Therefore, spinal cord injury (SCI) is a devastating event that can lead to permanent loss of locomotor and sensory functions. Moreover, the molecular mechanisms of axon regeneration in vertebrates are not very well understood, and currently, no effective treatment is available for SCI. In striking contrast to adult mammals, many nonmammalian vertebrates such as reptiles, amphibians, bony fishes and lampreys can spontaneously resume locomotion even after complete SCI. In recent years, rapid progress in the development of next-generation sequencing technologies has offered valuable information on SCI. In this review, we aimed to provide a comparison of axon regeneration process across classical model organisms, focusing on crucial genes and signalling pathways that play significant roles in the regeneration of individually identifiable descending neurons after SCI. Considering the special evolutionary location and powerful regenerative ability of lamprey and zebrafish, they will be the key model organisms for ongoing studies on spinal cord regeneration. Detailed study of SCI in these model organisms will help in the elucidation of molecular mechanisms of neuron regeneration across species.

Histone methyltransferase Ezh2 coordinates mammalian axon regeneration via regulation of key regenerative pathways

Current treatments for neurodegenerative diseases and neural injuries face major challenges, primarily due to the diminished regenerative capacity of neurons in the mammalian CNS as they mature. Here, we investigated the role of Ezh2, a histone methyltransferase, in regulating mammalian axon regeneration. We found that Ezh2 declined in the mouse nervous system during maturation but was upregulated in adult dorsal root ganglion neurons following peripheral nerve injury to facilitate spontaneous axon regeneration. In addition, overexpression of Ezh2 in retinal ganglion cells in the CNS promoted optic nerve regeneration via both histone methylation-dependent and -independent mechanisms. Further investigation revealed that Ezh2 fostered axon regeneration by orchestrating the transcriptional silencing of genes governing synaptic function and those inhibiting axon regeneration, while concurrently activating various factors that support axon regeneration. Notably, we demonstrated that GABA transporter 2, encoded by Slc6a13, acted downstream of Ezh2 to control axon regeneration. Overall, our study underscores the potential of modulating chromatin accessibility as a promising strategy for promoting CNS axon regeneration.

Mouse intestinal microbiome modulation by oral administration of a GABA-producing Bifidobacterium adolescentis strain

IMPORTANCE

The gut microbiome-brain communication signaling has emerged in recent years as a novel target for intervention with the potential to ameliorate some conditions associated with the central nervous system. Hence, probiotics with capacity to produce neurotransmitters, for instance, have come up as appealing alternatives to treat disorders associated with disbalanced neurotransmitters. Herein, we further deep into the effects of administering a gamma-aminobutyric acid (GABA)-producing Bifidobacterium strain, previously demonstrated to contribute to reduce serum glutamate levels, in the gut microbiome composition and metabolic activity in a mouse model. Our results demonstrate that the GABA-producing strain administration results in a specific pattern of gut microbiota modulation, different from the one observed in animals receiving non-GABA-producing strains. This opens new avenues to delineate the specific mechanisms by which IPLA60004 administration contributes to reducing serum glutamate levels and to ascertain whether this effect could exert health benefits in patients of diseases associated with high-glutamate serum concentrations.

The role of neurotransmitters in mediating the relationship between brain alterations and depressive symptoms in patients with inflammatory bowel disease

A growing body of evidence from neuroimaging studies suggests that inflammatory bowel disease (IBD) is associated with functional and structural alterations in the central nervous system and that it has a potential link to emotional symptoms, such as anxiety and depression. However, the neurochemical underpinnings of depression symptoms in IBD remain unclear. We hypothesized that changes in cortical gamma-aminobutyric acid (GABA+) and glutamine (Glx) concentrations are related to cortical thickness and resting-state functional connectivity in IBD as compared to healthy controls. To test this, we measured whole-brain cortical thickness and functional connectivity within the medial prefrontal cortex (mPFC), as well as the concentrations of neurotransmitters in the same brain region. We used the edited magnetic resonance spectroscopy (MRS) with the MEGA-PRESS sequence at a 3 T scanner to quantitate the neurotransmitter levels in the mPFC. Subjects with IBD (N = 37) and healthy control subjects (N = 32) were enrolled in the study. Compared with healthy controls, there were significantly decreased GABA+ and Glx concentrations in the mPFC of patients with IBD. The cortical thickness of patients with IBD was thin in two clusters that included the right medial orbitofrontal cortex and the right posterior cingulate cortex. A seed-based functional connectivity analysis indicated that there was higher connectivity of the mPFC with the left precuneus cortex (PC) and the posterior cingulate cortex, and conversely, lower connectivity in the left frontal pole was observed. The functional connectivity between the mPFC and the left PC was negatively correlated with the IBD questionnaire score (r = -0.388, p = 0.018). GABA+ concentrations had a negative correlation with the Hamilton Depression Scale (HAMD) score (r = -0.497, p = 0.002). Glx concentration was negatively correlated with the HAMD score (r = -0.496, p = 0.002) and positively correlated with the Short-Form McGill Pain Questionnaire score (r = 0.330, p = 0.046, uncorrected). There was a significant positive correlation between the ratio of Glx to GABA+ and the HAMD score (r = 0.428, p = 0.008). Mediation analysis revealed that GABA+ significantly mediated the main effect of the relationship between the structural and functional alterations and the severity of depression in patients with IBD. Our study provides initial evidence of neurochemistry that can be used to identify potential mechanisms underlying the modulatory effects of GABA+ on the development of depression in patients with IBD.

Mechanosensory Feedback in Lamprey Swimming Models and Applications in the Field of Spinal Cord Regeneration

The central pattern generator (CPG) in anguilliform swimming has served as a model for examining the neural basis of locomotion. This system has been particularly valuable for the development of mathematical models. As our biological understanding of the neural basis of locomotion has expanded, so too have these models. Recently, there have been significant advancements in our understanding of the critical role that mechanosensory feedback plays in robust locomotion. This work has led to a push in the field of mathematical modeling to incorporate mechanosensory feedback into CPG models. In this perspective piece, we review advances in the development of these models and discuss how newer complex models can support biological investigation. We highlight lamprey spinal cord regeneration as an area that can both inform these models and benefit from them.

Lampreys and spinal cord regeneration: "a very special claim on the interest of zoologists," 1830s-present

Employing history of science methods, including analyses of the scientific literature, archival documents, and interviews with scientists, this paper presents a history of lampreys in neurobiology from the 1830s to the present. We emphasize the lamprey's roles in helping to elucidate spinal cord regeneration mechanisms. Two attributes have long perpetuated studies of lampreys in neurobiology. First, they possess large neurons, including multiple classes of stereotypically located, 'identified' giant neurons in the brain, which project their large axons into the spinal cord. These giant neurons and their axonal fibers have facilitated electrophysiological recordings and imaging across biological scales, ranging from molecular to circuit-level analyses of nervous system structures and functions and including their roles in behavioral output. Second, lampreys have long been considered amongst the most basal extant vertebrates on the planet, so they have facilitated comparative studies pointing to conserved and derived characteristics of vertebrate nervous systems. These features attracted neurologists and zoologists to studies of lampreys between the 1830s and 1930s. But, the same two attributes also facilitated the rise of the lamprey in neural regeneration research after 1959, when biologists first wrote about the spontaneous, robust regeneration of some identified CNS axons in larvae after spinal cord injuries, coupled with recovery of normal swimming. Not only did large neurons promote fresh insights in the field, enabling studies incorporating multiple scales with existing and new technologies. But investigators also were able to attach a broad scope of relevance to their studies, interpreting them as suggesting conserved features of successful, and sometimes even unsuccessful, CNS regeneration. Lamprey research demonstrated that functional recovery takes place without the reformation of the original neuronal connections, for instance, by way of imperfect axonal regrowth and compensatory plasticity. Moreover, research performed in the lamprey model revealed that factors intrinsic to neurons are integral in promoting or hindering regeneration. As this work has helped illuminate why basal vertebrates accomplish CNS regeneration so well, whereas mammals do it so poorly, this history presents a case study in how biological and medical value have been, and could continue to be, gleaned from a non-traditional model organism for which molecular tools have been developed only relatively recently.

Swimming and L-arginine loaded chitosan nanoparticles ameliorates aging-induced neuron atrophy, autophagy marker LC3, GABA and BDNF-TrkB pathway in the spinal cord of rats

Aging is associated with muscle atrophy, and erosion and destruction of neuronal pathways in the spinal cord. The study aim was to assess the effect of swimming training (Sw) and L-arginine loaded chitosan nanoparticles (LA-CNPs) on the sensory and motor neuron population, autophagy marker LC3, total oxidant status/total antioxidant capacity, behavioural test, GABA and BDNF-TrkB pathway in the spinal cord of aging rats. The rats were randomized to five groups: young (8-weeks) control (n = 7), old control (n = 7), old Sw (n = 7), old LA-CNPs (n = 7) and old Sw + LA-CNPs (n = 7). Groups under LA-CNPs supplementation received 500 mg/kg/day. Sw groups performed a swimming exercise programme 5 days per week for 6 weeks. Upon the completion of the interventions the rats were euthanized and the spinal cord was fixed and frozen for histological assessment, IHC, and gene expression analysis. The old group had more atrophy in the spinal cord with higher changes in LC3 as an indicator of autophagy in the spinal cord compared to the young group (p < 0.0001). The old Sw + LA-CNPs group increased (improved) spinal cord GABA (p = 0.0187), BDNF (p = 0.0003), TrkB (p < 0.0001) gene expression, decreased autophagy marker LC3 protein (p < 0.0001), nerve atrophy and jumping/licking latency (p < 0.0001), improved sciatic functional index score and total oxidant status/total antioxidant capacity compared to the old group (p < 0.0001). In conclusion, swimming and LA-CNPs seems to ameliorate aging-induced neuron atrophy, autophagy marker LC3, oxidant-antioxidant status, functional restoration, GABA and BDNF-TrkB pathway in the spinal cord of aging rats. Our study provides experimental evidence for a possible positive role of swimming and L-arginine loaded chitosan nanoparticles to decrease complications of aging.

Acute baclofen administration promotes functional recovery after spinal cord injury

BACKGROUND CONTEXT

Traumatic spinal cord injury (SCI) leads to severe motor and sensory functional impairments that affect personal and social behaviors. Medical advancements have improved supportive therapeutic measures for SCI patients, but no effective neuroregenerative therapeutic options exist to date. Deficits in motor function are the most visible consequence of SCI. However, other complications, as spasticity, produce a significant impact on SCI patient's welfare. Baclofen, a GABA agonist, is the most effective drug for spasticity treatment. Interestingly, emerging data reveals that baclofen can also play a role on neuroprotection and regeneration after SCI.

PURPOSE

The goal of this study was to understand the potential of baclofen as a treatment to promote recovery after SCI.

STUDY DESIGN

We used a pre-clinical SCI mouse model with the administration of baclofen 1 mg/Kg at different time-points after injury.

METHODS

Behavior analysis (locomotor and bladder function) were performed during nine weeks of the in vivo experiment. Afterwards, spinal cords were collected and processed for histological and molecular analysis.

RESULTS

Our data showed that baclofen leads to locomotor improvements in mice when its administered acutely after SCI. Moreover, baclofen administration also led to improved bladder function control in all experimental groups. Interestingly, acute baclofen administration modulates microglia activation state and levels of circulating chemokines and cytokines, suggesting a putative role of baclofen in the modulation of the immune response.

CONCLUSIONS

Although further studies must be performed to understand the mechanisms that underlie the functional improvements produced by baclofen, our data shed light into the pharmacological potential of baclofen to promote recovery after SCI.

CLINICAL RELEVANCE

Our outcomes revealed that baclofen, a well-known drug used for spasticity management, improves the motor performance after SCI in a pre-clinical animal model. Our data opens new avenues for pharmacological strategies design to promote SCI recovery.

Spinal cord RNA-seq data after a baclofen treatment in mice with a spinal cord injury

Spinal cord injury (SCI) leads to severe functional deficits. Currently, there are no available pharmacological treatments to promote neurological recovery in SCI patients. Recent work from our group has shown that a baclofen treatment can promote functional recovery after a compression SCI in mice [1]. Here, we provide transcriptomic (RNA-seq) data from adult mouse spinal cords collected 7 days after a compression SCI and baclofen (vs vehicle) administration. The Illumina NovaSeq 6000 platform was used to generate the raw transcriptomic data. In addition, we also present bioinformatic analyses including differential gene expression analysis, enrichment analyses for various functional annotations (gene ontology, KEGG and BioCarta pathways or InterPro domains) and transcription factor targets. The raw RNA-seq data has been uploaded to the NCBI Sequence Read Archive (SRA) database (Bioproject ID PRJNA886048). The data generated from the bioinformatic analyses is contained within the article.

Full regeneration of descending corticotropin-releasing hormone axons after a complete spinal cord injury in lampreys

Sea lampreys are a vertebrate model of interest for the study of spontaneous axon regeneration after spinal cord injury (SCI). Axon regeneration research in lampreys has focused on the study of giant descending neurons, but less so on neurochemically-distinct descending neuronal populations with small caliber axons. Corticotropin-releasing hormone (CRH) is a neuropeptide that regulates the stress response or locomotion. CRH is also a neuropeptide of interest in the SCI context because descending CRHergic projections from the Barrington's nucleus control micturition behavior in mammals. Recent work from our group revealed that in sea lampreys the CRHergic innervation of the spinal cord is only of descending origin. Thus, the lack of intrinsic CRH spinal cord neurons provides the opportunity to analyze the regeneration of this descending system by using immunofluorescence methods. Here, we used an antibody against the sea lamprey mature CRH peptide, confocal microscopy, lightning adaptive deconvolution, and ImageJ to analyze the regenerative capacity of the descending CRH-immunoreactive (-ir) axons of larval sea lampreys after a complete SCI at the level of the fifth gill. At 10 weeks post-lesion, when behavioral analyses showed that injured animals had recovered normal appearing locomotion, our results revealed a full recovery of the number of CRH-ir profiles (axons) at the level of the sixth gill. Thus, the CRH descending axons of lampreys fully regenerate after a complete SCI. Our study provides a new model to study spontaneous and successful axonal regeneration in a specific neuronal type with small caliber axons by using simple immunohistochemical methods.

Novel adult cortical neuron processing and screening method illustrates sex- and age-dependent effects of pharmaceutical compounds

Neurodegenerative diseases and neurotraumatic injuries are typically age-associated disorders that can reduce neuron survival, neurite outgrowth, and synaptic plasticity leading to loss of cognitive capacity, executive function, and motor control. In pursuit of reducing the loss of said neurological functions, novel compounds are sought that promote neuron viability, neuritogenesis, and/or synaptic plasticity. Current high content in vitro screenings typically use cells that are iPSC-derived, embryonic, or originate from post-natal tissues; however, most patients suffering from neurodegenerative diseases and neurotrauma are of middle-age and older. The chasm in maturity between the neurons used in drug screens and those in a target population is a barrier for translational success of in vitro results. It has been historically challenging to culture adult neurons let alone conduct screenings; therefore, age-appropriate drug screenings have previously not been plausible. We have modified Miltenyi's protocol to increase neuronal yield, neuron purity, and neural viability at a reduced cost to expand our capacity to screen compounds directly in primary adult neurons. To our knowledge, we developed the first morphology-based screening system using adult cortical neurons and the first to incorporate age and sex as biological variables in a screen using adult cortical neurons. By using primary adult cortical neurons from mice that were 4 to 48 weeks old for screening pharmaceutical agents, we have demonstrated age- and sex-dependent effects on neuritogenesis and neuron survival in vitro. Utilizing age- and sex-appropriate in vitro models to find novel compounds increasing neuron survival and neurite outgrowth, made possible by our modified adult neuron processing method, will greatly increase the relevance of in vitro screening for finding neuroprotective compounds.

The Time Sequence of Gene Expression Changes after Spinal Cord Injury

Gene expression changes following spinal cord injury (SCI) are time-dependent, and an accurate understanding of these changes can be crucial in determining time-based treatment options in a clinical setting. We performed RNA sequencing of the contused spinal cord of rats at five different time points from the very acute to chronic stages (1 hour, 1 day, 1 week, 1 month, and 3 months) following SCI. We identified differentially expressed genes (DEGs) and Gene Ontology (GO) terms at each time point, and 14,257 genes were commonly expressed at all time points. The biological process of the inflammatory response was increased at 1 hour and 1 day, and the cellular component of the integral component of the synaptic membrane was increased at 1 day. DEGs associated with cell activation and the innate immune response were highly enriched at 1 week and 1 month, respectively. A total of 2841 DEGs were differentially expressed at any of the five time points, and 18 genes (17 upregulated and 1 downregulated) showed common expression differences at all time points. We found that interleukin signaling, neutrophil degranulation, eukaryotic translation, collagen degradation, LGI–ADAM interactions, GABA receptor, and L1CAM-ankyrin interactions were prominent after SCI depending on the time post injury. We also performed gene–drug network analysis and found several potential antagonists and agonists which can be used to treat SCI. We expect to discover effective treatments in the clinical field through further studies revealing the efficacy and safety of potential drugs.

mTOR activity is essential for retinal pigment epithelium regeneration in zebrafish

The retinal pigment epithelium (RPE) plays numerous critical roles in maintaining vision and this is underscored by the prevalence of degenerative blinding diseases like age-related macular degeneration (AMD), in which visual impairment is caused by progressive loss of RPE cells. In contrast to mammals, zebrafish possess the ability to intrinsically regenerate a functional RPE layer after severe injury. The molecular underpinnings of this regenerative process remain largely unknown yet hold tremendous potential for developing treatment strategies to stimulate endogenous regeneration in the human eye. In this study, we demonstrate that the mTOR pathway is activated in RPE cells post-genetic ablation. Pharmacological and genetic inhibition of mTOR activity impaired RPE regeneration, while mTOR activation enhanced RPE recovery post-injury, demonstrating that mTOR activity is essential for RPE regeneration in zebrafish. RNA-seq of RPE isolated from mTOR-inhibited larvae identified a number of genes and pathways dependent on mTOR activity at early and late stages of regeneration; amongst these were components of the immune system, which is emerging as a key regulator of regenerative responses across various tissue and model systems. Our results identify crosstalk between macrophages/microglia and the RPE, wherein mTOR activity is required for recruitment of macrophages/microglia to the RPE injury site. Macrophages/microglia then reinforce mTOR activity in regenerating RPE cells. Interestingly, the function of macrophages/microglia in maintaining mTOR activity in the RPE appeared to be inflammation-independent. Taken together, these data identify mTOR activity as a key regulator of RPE regeneration and link the mTOR pathway to immune responses in facilitating RPE regeneration.

Synaptic or Non-synaptic? Different Intercellular Interactions with Retinal Ganglion Cells in Optic Nerve Regeneration

Axons of adult neurons in the mammalian central nervous system generally fail to regenerate by themselves, and few if any therapeutic options exist to reverse this situation. Due to a weak intrinsic potential for axon growth and the presence of strong extrinsic inhibitors, retinal ganglion cells (RGCs) cannot regenerate their axons spontaneously after optic nerve injury and eventually undergo apoptosis, resulting in permanent visual dysfunction. Regarding the extracellular environment, research to date has generally focused on glial cells and inflammatory cells, while few studies have discussed the potentially significant role of interneurons that make direct connections with RGCs as part of the complex retinal circuitry. In this study, we provide a novel angle to summarize these extracellular influences following optic nerve injury as "intercellular interactions" with RGCs and classify these interactions as synaptic and non-synaptic. By discussing current knowledge of non-synaptic (glial cells and inflammatory cells) and synaptic (mostly amacrine cells and bipolar cells) interactions, we hope to accentuate the previously neglected but significant effects of pre-synaptic interneurons and bring unique insights into future pursuit of optic nerve regeneration and visual function recovery.

Dose optimization of intrathecal administration of human umbilical cord mesenchymal stem cells for the treatment of subacute incomplete spinal cord injury

Human umbilical cord mesenchymal stem cells (hUC-MSCs) are a promising candidate for spinal cord injury (SCI) repair owing to their advantages of low immunogenicity and easy accessibility over other MSC sources. However, modest clinical efficacy hampered the progression of these cells to clinical translation. This discrepancy may be due to many variables, such as cell source, timing of implantation, route of administration, and relevant efficacious cell dose, which are critical factors that affect the efficacy of treatment of patients with SCI. Previously, we have evaluated the safety and efficacy of 4 × 10⁶ hUC-MSCs/kg in the treatment of subacute SCI by intrathecal implantation in rat models. To search for a more accurate dose range for clinical translation, we compared the effects of three different doses of hUC-MSCs – low (0.25 × 10⁶ cells/kg), medium (1 × 10⁶ cells/kg) and high (4 × 10⁶ cells/kg) – on subacute SCI repair through an elaborate combination of behavioral analyses, anatomical analyses, magnetic resonance imaging-diffusion tensor imaging (MRI-DTI), biotinylated dextran amine (BDA) tracing, electrophysiology, and quantification of mRNA levels of ion channels and neurotransmitter receptors. Our study demonstrated that the medium dose, but not the low dose, is as efficient as the high dose in producing the desired therapeutic outcomes. Furthermore, partial restoration of the γ-aminobutyric acid type A (GABA_A) receptor expression by the effective doses indicates that GABA_A receptors are possible candidates for therapeutic targeting of dormant relay pathways in injured spinal cord. Overall, this study revealed that intrathecal implantation of 1 × 10⁶ hUC-MSCs/kg is an alternative approach for treating subacute SCI.

Importance of brain alterations in spinal cord injury

Spinal cord injury (SCI) destroys the sensorimotor pathway and blocks the information flow between the peripheral nerve and the brain, resulting in autonomic function loss. Numerous studies have explored the effects of obstructed information flow on brain structure and function and proved the extensive plasticity of the brain after SCI. Great progress has also been achieved in therapeutic strategies for SCI to restore the "re-innervation" of the cerebral cortex to the limbs to some extent. Although no thorough research has been conducted, the changes of brain structure and function caused by "re-domination" have been reported. This article is a review of the recent research progress on local structure, functional changes, and circuit reorganization of the cerebral cortex after SCI. Alterations of structure and electrical activity characteristics of brain neurons, features of brain functional reorganization, and regulation of brain functions by reconfigured information flow were also explored. The integration of brain function is the basis for the human body to exercise complex/fine movements and is intricately and widely regulated by information flow. Hence, its changes after SCI and treatments should be considered.

Data on the Quantification of Aspartate, GABA and Glutamine Levels in the Spinal Cord of Larval Sea Lampreys after a Complete Spinal Cord Injury

We used high-performance liquid chromatography (HPLC) methods to quantify aspartate, GABA, and glutamine levels in the spinal cord of larval sea lampreys following a complete spinal cord injury. Mature larval sea lampreys recover spontaneously from a complete spinal cord transection and the changes in neurotransmitter systems after spinal cord injury might be related to their amazing regenerative capabilities. The data presented here show the concentration of the aminoacidergic neurotransmitters GABA (and its precursor glutamine) and aspartate in the spinal cord of control (non-injured) and 2-, 4-, and 10-week post-lesion animals. Statistical analyses showed that GABA and aspartate levels significantly increase in the spinal cord four weeks after a complete spinal cord injury and that glutamine levels decrease 10 weeks after injury as compared to controls. These data might be of interest to those studying the role of neurotransmitters and neuromodulators in recovery from spinal cord injury in vertebrates.

GABAA receptor-mediated inhibition of Dahlgren cells electrical activity in the olive flounder, Paralichthys olivaceus

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the central nervous system. We investigated its potential role as a neurotransmitter in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) of the flounder Paralichthys olivaceus. The application of GABA in vitro resulted in a decrease in electrical activity of Dahlgren cells, followed by an increase of the number of silent cells, together with a decreased firing frequency of all three activity patterns (tonic, phasic, bursting). GABA_A receptor agonist etomidate decreased Dahlgren cell firing activity, in a similar way to GABA. The response to GABA was blocked by the GABA_A receptor antagonist bicuculline. GABA_A receptor gamma2 subunit (Gabrg2) and chloride channel (Clcn2) mRNA expression were significantly upregulated in the CNSS after GABA superfusion. These data suggest that GABA may modulate CNSS activity in vivo mediated by GABA_A receptors.

Neurochemical changes in the primate lateral geniculate nucleus following lesions of striate cortex in infancy and adulthood: implications for residual vision and blindsight

Following lesions of the primary visual cortex (V1), the lateral geniculate nucleus (LGN) undergoes substantial cell loss due to retrograde degeneration. However, visually responsive neurons remain in the degenerated sector of LGN, and these have been implicated in mediation of residual visual capacities that remain within the affected sectors of the visual field. Using immunohistochemistry, we compared the neurochemical characteristics of LGN neurons in V1-lesioned marmoset monkeys (Callithrix jacchus) with those of non-lesioned control animals. We found that GABAergic neurons form approximately 6.5% of the neuronal population in the normal LGN, where most of these cells express the calcium-binding protein parvalbumin. Following long-term V1 lesions in adult monkeys, we observed a marked increase (~ sevenfold) in the proportion of GABA-expressing neurons in the degenerated sector of the LGN, indicating that GABAergic cells are less affected by retrograde degeneration in comparison with magno- and parvocellular projection neurons. In addition, following early postnatal V1 lesions and survival into adulthood, we found widespread expression of GABA in putative projection neurons, even outside the degenerated sectors (lesion projection zones). Our findings show that changes in the ratio of GABAergic neurons in LGN need to be taken into account in the interpretation of the mechanisms of visual abilities that survive V1 lesions in primates.

Role of Baclofen in Modulating Spasticity and Neuroprotection in Spinal Cord Injury

Spinal cord injury (SCI) affects an estimated three million persons worldwide, with ∼180,000 new cases reported each year leading to severe motor and sensory functional impairments that affect personal and social behaviors. To date, no effective treatment has been made available to promote neurological recovery after SCI. Deficits in motor function is the most visible consequence of SCI; however, other secondary complications produce a significant impact on the welfare of patients with SCI. Spasticity is a neurological impairment that affects the control of muscle tone as a consequence of an insult, trauma, or injury to the central nervous system, such as SCI. The management of spasticity can be achieved through the combination of both nonpharmacological and pharmacological approaches. Baclofen is the most effective drug for spasticity treatment, and it can be administered both orally and intrathecally, depending on spasticity location and severity. Interestingly, recent data are revealing that baclofen can also play a role in neuroprotection after SCI. This new function of baclofen in the SCI scope is promising for the prospect of developing new pharmacological strategies to promote functional recovery in patients with SCI.

Recovery of Burrowing Behavior After Spinal Cord Injury in the Larval Sea Lamprey

AbstractFollowing traumatic spinal cord injury, most mammalian species are unable to achieve substantial neuronal regeneration and often experience loss of locomotor function. In contrast, larval sea lampreys (Petromyzon marinus) spontaneously recover normal swimming behaviors by 10-12 weeks post-injury, which is supported by robust regeneration of spinal axons. While recovery of swimming behavior is well established, the lamprey's ability to recover more complex behaviors, such as burrowing, is unknown. Here we evaluated the lamprey's ability to burrow into a sand substrate over the typical time course of functional recovery (1-11 weeks post-injury). Compared to uninjured control lampreys, which burrow rapidly and completely, spinal-transected animals did not attempt burrowing until 2 weeks post-injury; and they often did not succeed in fully covering their entire body in the sand. Burrowing behavior gradually improved over post-injury time, with most animals burrowing partially or completely by 9-11 weeks post-injury. Burrowing behavior has two components: the initial component that resembles swimming with propagated body undulations and the final component that pulls the tail under the sand. While the duration of the initial component did not differ between control and spinal-transected animals across the entire recovery period, the duration of the final component in spinal-transected animals was significantly longer at all time points measured. These data indicate that, after spinal cord injury, lampreys are able to recover burrowing behaviors, though some deficits persist.

A data-driven polynomial approach to reproduce the scar tissue outgrowth around neural implants

Despite the huge complexity of the foreign body reaction, a quantitative assessment over time of the scar tissue thickness around implanted materials is needed to figure out the evolution of neural implants for long times. A data-driven approach, based on phenomenological polynomial functions, is able to reproduce experimental data. Nevertheless, a misuse of this strategy may lead to unsatisfactory results, even if standard indexes are optimized. In this work, an effective in silico procedure was presented to reproduce the scar tissue dynamics around implanted synthetic devices and to predict the capsule thickness for times before and after experimental detections.

Inhibition of Gamma-Secretase Promotes Axon Regeneration After a Complete Spinal Cord Injury

In a recent study, we showed that GABA and baclofen (a GABAB receptor agonist) inhibit caspase activation and promote axon regeneration in descending neurons of the sea lamprey brainstem after a complete spinal cord injury (Romaus-Sanjurjo et al., 2018a). Now, we repeated these treatments and performed 2 independent Illumina RNA-Sequencing studies in the brainstems of control and GABA or baclofen treated animals. GABA treated larval sea lampreys with their controls were analyzed 29 days after a complete spinal cord injury and baclofen treated larvae with their controls 9 days after the injury. One of the most significantly downregulated genes after both treatments was a HES gene (HESB). HES proteins are transcription factors that are key mediators of the Notch signaling pathway and gamma-secretase activity is crucial for the activation of this pathway. So, based on the RNA-Seq results we subsequently treated spinal cord injured larval sea lampreys with a novel gamma-secretase inhibitor (PF-3804014). This treatment also reduced the expression of HESB in the brainstem and significantly enhanced the regeneration of individually identifiable descending neurons after a complete spinal cord injury. Our results show that gamma-secretase could be a novel target to promote axon regeneration after nervous system injuries.

Modulation of extrasynaptic GABAergic receptor activity influences glutamate release and neuronal survival following excitotoxic damage to mouse spinal cord neurons

Excitotoxic levels of released glutamate trigger a cascade of deleterious cellular events leading to delayed neuronal death. This phenomenon implies extensive dysregulation in the balance between network excitation and inhibition. Our hypothesis was that enhancing network inhibition should prevent excitotoxicity and provide neuroprotection. To test this notion, we used mouse organotypic spinal slice cultures and explored if excitotoxicity caused by the potent glutamate analogue kainate was blocked by pharmacological increase in GABA_A receptor activity. To this end we monitored (with a biosensor) real-time glutamate release following 1 h kainate application and quantified neuronal survival 24 h later. Glutamate release evoked by kainate was strongly decreased by the allosteric GABA_A modulator midazolam (10 nM) or the GABA agonist THIP (10 μM), leading to neuroprotection. On the contrary, much higher glutamate release was induced by the GABA antagonist bicuculline (20 μM) that inhibits synaptic and extrasynaptic GABA_A receptors. Gabazine (20 μM), an antagonist of synaptic GABA_A receptors, had no effect on glutamate release or neuroprotection. No effect was observed with the glycine antagonist strychnine or the glycine agonist L-alanine. These findings indicate that enhancement of GABA receptor activity was an effective tool to counteract excitotoxic death in spinal networks. In view of the potent activity by THIP, preferentially acting on extrasynaptic GABA_A receptors, the present data imply a significant role for extrasynaptic GABA_A receptors in sparing spinal cord neurons from injury.

Serotonin inhibits axonal regeneration of identifiable descending neurons after a complete spinal cord injury in lampreys

Classical neurotransmitters are mainly known for their roles as neuromodulators, but they also play important roles in the control of developmental and regenerative processes. Here, we used the lamprey model of spinal cord injury to study the effect of serotonin in axon regeneration at the level of individually identifiable descending neurons. Pharmacological and genetic manipulations after a complete spinal cord injury showed that endogenous serotonin inhibits axonal regeneration in identifiable descending neurons through the activation of serotonin 1A receptors and a subsequent decrease in cyclic adenosine monophosphate (cAMP) levels. RNA sequencing revealed that changes in the expression of genes that control axonal guidance could be a key factor determining the serotonin effects during regeneration. This study provides new targets of interest for research in non-regenerating mammalian models of traumatic central nervous system injuries and extends the known roles of serotonin signalling during neuronal regeneration.

This article has an associated First Person interview with the first author of the paper.

Summary: Pharmacological and genetic manipulations show that endogenous serotonin inhibits axonal regeneration of individually identifiable descending neurons of lampreys after a complete spinal cord injury.

Regenerative capacity in the lamprey spinal cord is not altered after a repeated transection

The resilience of regeneration in vertebrates is not very well understood. Yet understanding if tissues can regenerate after repeated insults, and identifying limitations, is important for elucidating the underlying mechanisms of tissue plasticity. This is particularly challenging in tissues, such as the nervous system, which possess a large number of terminally differentiated cells and often exhibit limited regeneration in the first place. However, unlike mammals, which exhibit very limited regeneration of spinal cord tissues, many non-mammalian vertebrates, including lampreys, bony fishes, amphibians, and reptiles, regenerate their spinal cords and functionally recover even after a complete spinal cord transection. It is well established that lampreys undergo full functional recovery of swimming behaviors after a single spinal cord transection, which is accompanied by tissue repair at the lesion site, as well as axon and synapse regeneration. Here we begin to explore the resilience of spinal cord regeneration in lampreys after a second spinal transection (re-transection). We report that by all functional and anatomical measures tested, lampreys regenerate after spinal re-transection just as robustly as after single transections. Recovery of swimming, synapse and cytoskeletal distributions, axon regeneration, and neuronal survival were nearly identical after spinal transection or re-transection. Only minor differences in tissue repair at the lesion site were observed in re-transected spinal cords. Thus, regenerative potential in the lamprey spinal cord is largely unaffected by spinal re-transection, indicating a greater persistent regenerative potential than exists in some other highly regenerative models. These findings establish a new path for uncovering pro-regenerative targets that could be deployed in non-regenerative conditions.

Role of axon resealing in retrograde neuronal death and regeneration after spinal cord injury

Spinal cord injury leads to persistent behavioral deficits because mammalian central nervous system axons fail to regenerate. A neuron's response to axon injury results from a complex interplay of neuron-intrinsic and environmental factors. The contribution of axotomy to the death of neurons in spinal cord injury is controversial because very remote axotomy is unlikely to result in neuronal death, whereas death of neurons near an injury may reflect environmental factors such as ischemia and inflammation. In lampreys, axotomy due to spinal cord injury results in delayed apoptosis of spinal-projecting neurons in the brain, beyond the extent of these environmental factors. This retrograde apoptosis correlates with delayed resealing of the axon, and can be reversed by inducing rapid membrane resealing with polyethylene glycol. Studies in mammals also suggest that polyethylene glycol may be neuroprotective, although the mechanism(s) remain unclear. This review examines the early, mechanical, responses to axon injury in both mammals and lampreys, and the potential of polyethylene glycol to reduce injury-induced pathology. Identifying the mechanisms underlying a neuron's response to axotomy will potentially reveal new therapeutic targets to enhance regeneration and functional recovery in humans with spinal cord injury.

Data on the effect of a muscimol treatment in caspase activation in descending neurons of lampreys after a complete spinal cord injury

In this article, caspase activation in identifiable reticulospinal neurons of lampreys was inhibited after a complete spinal cord injury using a specific agonist of the GABAA receptor (muscimol). The data presented in this article are quantifications of fluorescent labelling of identifiable descending neurons of larval lampreys after a complete spinal cord injury using fluorochrome-labelled inhibitors of caspases (FLICA) and the corresponding statistical analysis. A single dose of muscimol decreased the intensity of FLICA labelling in giant identifiable reticulospinal neurons following spinal cord injury in lampreys.