Functional sensory circuits built from neurons of two species

A central question for regenerative neuroscience is whether synthetic neural circuits, such as those built from two species, can function in an intact brain. Here, we apply blastocyst complementation to selectively build and test interspecies neural circuits. Despite approximately 10-20 million years of evolution, and prominent species differences in brain size, rat pluripotent stem cells injected into mouse blastocysts develop and persist throughout the mouse brain. Unexpectedly, the mouse niche reprograms the birth dates of rat neurons in the cortex and hippocampus, supporting rat-mouse synaptic activity. When mouse olfactory neurons are genetically silenced or killed, rat neurons restore information flow to odor processing circuits. Moreover, they rescue the primal behavior of food seeking, although less well than mouse neurons. By revealing that a mouse can sense the world using neurons from another species, we establish neural blastocyst complementation as a powerful tool to identify conserved mechanisms of brain development, plasticity, and repair.

Probucol treatment after traumatic brain injury activates BDNF/TrkB pathway, promotes neuroregeneration and ameliorates functional deficits in mice

BACKGROUND AND PURPOSE

Traumatic brain injury (TBI) is a major cause of mortality and morbidity worldwide, yet pharmacotherapies for TBI are currently lacking. Neuroregeneration is important in brain repair and functional recovery. In this study, probucol, a cholesterol-lowering drug with established safety profiles, was examined for its therapeutic effects and neuroregenerative actions in TBI.

EXPERIMENTAL APPROACH

Male mice were subjected to the controlled cortical impact model of TBI, followed by daily administration of probucol. Neurological and cognitive functions were evaluated. Histological analyses of the neocortex and hippocampus were performed to detect the lesion, dendritic degeneration (microtubule-associated protein 2), synaptic density (synaptophysin), neurogenesis (doublecortin), brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) activation. Involvement of BDNF/TrkB pathway in probucol-mediated effects was examined in primary cultures of cortical neurons.

KEY RESULTS

Probucol reduced brain lesion volume, enhanced the recovery of body symmetry, improved motor function and attenuated memory dysfunction after TBI. Meanwhile, probucol promoted post-injury dendritic growth and synaptogenesis and increased hippocampal proliferating neuronal progenitor cells, along with the formation as well as the survival of newborn neurons. Moreover, probucol enhances BDNF expression and TrkB activation. In vitro, probucol promoted neurite outgrowth, which was inhibited by a selective TrkB antagonist ANA-12.

CONCLUSIONS AND IMPLICATIONS

Probucol enhanced functional restoration and ameliorated cognitive impairment after TBI by promoting post-injury neuronal remodelling and neurogenesis. Increased activation of BDNF/TrkB pathway by probucol, at least in part, contributed to the neuroregenerative effects of probucol. Together, it may be promising to repurpose probucol for TBI.

X-ray micro-computed tomography of Xenopus tadpole reveals changes in brain ventricular morphology during telencephalon regeneration

Xenopus tadpoles serve as an exceptional model organism for studying post-embryonic development in vertebrates. During post-embryonic development, large-scale changes in tissue morphology, including organ regeneration and metamorphosis, occur at the organ level. However, understanding these processes in a three-dimensional manner remains challenging. In this study, the use of X-ray micro-computed tomography (microCT) for the three-dimensional observation of the soft tissues of Xenopus tadpoles was explored. The findings revealed that major organs, such as the brain, heart, and kidneys, could be visualized with high contrast by phosphotungstic acid staining following fixation with Bouin's solution. Then, the changes in brain shape during telencephalon regeneration were analyzed as the first example of utilizing microCT to study organ regeneration in Xenopus tadpoles, and it was found that the size of the amputated telencephalon recovered to >80% of its original length within approximately 1 week. It was also observed that the ventricles tended to shrink after amputation and maintained this state for at least 3 days. This shrinkage was transient, as the ventricles expanded to exceed their original size within the following week. Temporary shrinkage and expansion of the ventricles, which were also observed in transgenic or fluorescent dye-injected tadpoles with telencephalon amputation, may be significant in tissue homeostasis in response to massive brain injury and subsequent repair and regeneration. This established method will improve experimental analyses in developmental biology and medical science using Xenopus tadpoles.

Pluripotent stem cell strategies for rebuilding the human brain

Neurodegenerative disorders have been extremely challenging to treat with traditional drug-based approaches and curative therapies are lacking. Given continued progress in stem cell technologies, cell replacement strategies have emerged as concrete and potentially viable therapeutic options. In this review, we cover advances in methods used to differentiate human pluripotent stem cells into several highly specialized types of neurons, including cholinergic, dopaminergic, and motor neurons, and the potential clinical applications of stem cell-derived neurons for common neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease, ataxia, and amyotrophic lateral sclerosis. Additionally, we summarize cellular differentiation techniques for generating glial cell populations, including oligodendrocytes and microglia, and their conceivable translational roles in supporting neural function. Clinical trials of specific cell replacement therapies in the nervous system are already underway, and several attractive avenues in regenerative medicine warrant further investigation.

Innate Immune Pathways Promote Oligodendrocyte Progenitor Cell Recruitment to the Injury Site in Adult Zebrafish Brain

The oligodendrocyte progenitors (OPCs) are at the front of the glial reaction to the traumatic brain injury. However, regulatory pathways steering the OPC reaction as well as the role of reactive OPCs remain largely unknown. Here, we compared a long-lasting, exacerbated reaction of OPCs to the adult zebrafish brain injury with a timely restricted OPC activation to identify the specific molecular mechanisms regulating OPC reactivity and their contribution to regeneration. We demonstrated that the influx of the cerebrospinal fluid into the brain parenchyma after injury simultaneously activates the toll-like receptor 2 (Tlr2) and the chemokine receptor 3 (Cxcr3) innate immunity pathways, leading to increased OPC proliferation and thereby exacerbated glial reactivity. These pathways were critical for long-lasting OPC accumulation even after the ablation of microglia and infiltrating monocytes. Importantly, interference with the Tlr1/2 and Cxcr3 pathways after injury alleviated reactive gliosis, increased new neuron recruitment, and improved tissue restoration.

Regenerative Neurology and Regenerative Cardiology: Shared Hurdles and Achievements

From the first success in cultivation of cells in vitro, it became clear that developing cell and/or tissue specific cultures would open a myriad of new opportunities for medical research. Expertise in various in vitro models has been developing over decades, so nowadays we benefit from highly specific in vitro systems imitating every organ of the human body. Moreover, obtaining sufficient number of standardized cells allows for cell transplantation approach with the goal of improving the regeneration of injured/disease affected tissue. However, different cell types bring different needs and place various types of hurdles on the path of regenerative neurology and regenerative cardiology. In this review, written by European experts gathered in Cost European action dedicated to neurology and cardiology-Bioneca, we present the experience acquired by working on two rather different organs: the brain and the heart. When taken into account that diseases of these two organs, mostly ischemic in their nature (stroke and heart infarction), bring by far the largest burden of the medical systems around Europe, it is not surprising that in vitro models of nervous and heart muscle tissue were in the focus of biomedical research in the last decades. In this review we describe and discuss hurdles which still impair further progress of regenerative neurology and cardiology and we detect those ones which are common to both fields and some, which are field-specific. With the goal to elucidate strategies which might be shared between regenerative neurology and cardiology we discuss methodological solutions which can help each of the fields to accelerate their development.

Transcriptome Analyses Reveal IL6/Stat3 Signaling Involvement in Radial Glia Proliferation After Stab Wound Injury in the Adult Zebrafish Optic Tectum

Adult zebrafish have many neurogenic niches and a high capacity for central nervous system regeneration compared to mammals, including humans and rodents. The majority of radial glia (RG) in the zebrafish optic tectum are quiescent under physiological conditions; however, stab wound injury induces their proliferation and differentiation into newborn neurons. Although previous studies have functionally analyzed the molecular mechanisms of RG proliferation and differentiation and have performed single-cell transcriptomic analyses around the peak of RG proliferation, the cellular response and changes in global gene expression during the early stages of tectum regeneration remain poorly understood. In this study, we performed histological analyses which revealed an increase in isolectin B4+ macrophages prior to the induction of RG proliferation. Moreover, transcriptome and pathway analyses based on differentially expressed genes identified various enriched pathways, including apoptosis, the innate immune system, cell proliferation, cytokine signaling, p53 signaling, and IL6/Jak-Stat signaling. In particular, we found that Stat3 inhibition suppressed RG proliferation after stab wound injury and that IL6 administration into cerebroventricular fluid activates RG proliferation without causing injury. Together, the findings of these transcriptomic and functional analyses reveal that IL6/Stat3 signaling is an initial trigger of RG activation during optic tectum regeneration.

Proliferative Capacity of Adult Mouse Brain

We studied cell proliferation in the postnatal mouse brain between the ages of 2 and 30 months and identified four compartments with different densities of proliferating cells. The first identified compartment corresponds to the postnatal pallial neurogenic (PPN) zone in the telencephalon; the second to the subpallial postnatal neurogenic (SPPN) zone in the telencephalon; the third to the white matter bundles in the telencephalon; and the fourth to all brain parts outside of the other three compartments. We estimated that about 3.4 million new cells, including 0.8 million in the subgranular zone (SGZ) in the hippocampus, are produced in the PPN zone. About 21 million new cells, including 10 million in the subependymal zone (SEZ) in the lateral walls of the lateral ventricle and 2.7 million in the rostral migratory stream (RMS), are produced in the SPPN zone. The third and fourth compartments together produced about 31 million new cells. The analysis of cell proliferation in neurogenic zones shows that postnatal neurogenesis is the direct continuation of developmental neurogenesis in the telencephalon and that adult neurogenesis has characteristics of the late developmental process. As a developmental process, adult neurogenesis supports only compensatory regeneration, which is very inefficient.

Insight Into the Mechanisms and the Challenges on Stem Cell-Based Therapies for Cerebral Ischemic Stroke

Strokes are the most common types of cerebrovascular disease and remain a major cause of death and disability worldwide. Cerebral ischemic stroke is caused by a reduction in blood flow to the brain. In this disease, two major zones of injury are identified: the lesion core, in which cells rapidly progress toward death, and the ischemic penumbra (surrounding the lesion core), which is defined as hypoperfusion tissue where cells may remain viable and can be repaired. Two methods that are approved by the Food and Drug Administration (FDA) include intravenous thrombolytic therapy and endovascular thrombectomy, however, the narrow therapeutic window poses a limitation, and therefore a low percentage of stroke patients actually receive these treatments. Developments in stem cell therapy have introduced renewed hope to patients with ischemic stroke due to its potential effect for reversing the neurological sequelae. Over the last few decades, animal tests and clinical trials have been used to treat ischemic stroke experimentally with various types of stem cells. However, several technical and ethical challenges must be overcome before stem cells can become a choice for the treatment of stroke. In this review, we summarize the mechanisms, processes, and challenges of using stem cells in stroke treatment. We also discuss new developing trends in this field.

Application of 18-fluorodeoxyglucose as an indicator of symptom severity in a rat model of tic disorder following treatment with traditional Chinese medicine

Quantitative evaluation of tic disorders (TDs) is challenging as there are few objective indicators that can be used for the assessment of treatment outcomes. 18F-Fluorodeoxyglucose (FDG) is a radioactive tracer that is able to cross the blood-brain barrier and can be detected by positron emission tomography/CT (PET/CT). In the present study, it was hypothesized that FDG PET/CT scan can be applied to reflect the severity of tic symptoms in a rat TD model, where signals detected in the brain striatum can be used to evaluate the efficacy of tic treatment with traditional Chinese medicine (TCM). A rat model of TD was established by treatment with iminodipropionitrile. Rats were divided into the following four groups (n=10 each): i) Control; ii) TCM; iii) haloperidol; and iv) model only. Observations of stereotypic behavior in rats were subsequently scored and micro-PET/CT was used to evaluate the rate of FDG uptake. Stereotypy scores were found to be significantly higher (P<0.05) in the TD rat model (P<0.05) compared with those in control rats. Both stereotypy scores (P<0.05) and standardized FDG uptake values (SUV; P<0.05) were revealed to be significantly reduced in the TD model rats after treatment with TCM or haloperidol. SUV correlated positively with stereotypy score (R=0.926; P<0.05) and the SUV scores were found to be significantly different among control group, TCM group, haloperidol group and model only group (P<0.05). These data suggest that the application of FDG in the striatum can be used to evaluate the effectiveness of TCM treatment for TDs.

Adult neuronal regeneration in the telencephalon of the killifish Aphaniops hormuzensis

The potential of central nervous system regeneration was evaluated for the first time in the injured brain of the old world killifish Aphaniops hormuzensis. The histomorphological organization in the regeneration procedure was evaluated using the hematoxylin and eosin (H&E) staining and the bromodeoxyuridine (BrdU) immunohistochemistry technique. The histological tissue sections were sampled daily for 10 days. Based on the H&E staining, a large gliosis reaction was detected along with vacuolization and telencephalon deformation on 1-day post-lesion (dpl). The vacuolated zone declined fast and the telencephalon hemisphere recovered on 3 dpl. The symptoms of injured telencephalon nervous tissue were resolved within 7 dpl in both genders. In the BrdU test of the control group, BrdU-labeled cells were observed in the ventricular zone (VZ), pallium (Pa), and lateral pallium (LPa). On 1 dpl, the BrdU⁺ cells accumulated in the VZ, Pa, and LPa (located near the injury area). From 3 dpl onwards, the BrdU⁺ cells were reduced in the telencephalic VZ, Pa, and LPa. Based on the BrdU⁺ results, the adult brain in A. hormuzensis possesses a remarkable capacity for neuronal regeneration. By taking into account the high neural regeneration potency of A. hormuzensis and its relatively short lifespan, it could be concluded that besides the currently known models, the members of aphaniid fishes could probably be valuable animals to study the regeneration phenomenon in the vertebrates.

The Wnt/Ca2+ signaling pathway is essential for the regeneration of GABAergic neurons in planarian Dugesia japonica

The growth and differentiation of neurons are critical events in the establishment of proper neuron connectivity and function. Planarians have a remarkable ability to completely regenerate a functional nervous system from a pluripotent stem cell population. Thus, planarians provide a powerful model to identify genes required for neuronal differentiation in vivo. The Wnt/Ca²⁺ signaling pathway is crucial for cancer development, arousing inflammatory responses, and neurodegeneration. We analyzed the expression patterns and RNAi phenotypes for members of the Wnt/Ca²⁺ signaling pathway in the planarian, Dugesia japonica. The expression of DjWnt5a, DjPLC-β, DjCamKII, and DjCaln during regeneration was surprisingly similar and revealing in the regenerated brain. RNAi knockdown of DjWnt5a, DjPLC-β, DjCamKII, and DjCaln led to defects in regenerated brains including brain partial deletions, incompact phenotypes at the posterior of the new brain, and lateral branches, which could not regenerate. Furthermore, the expressions of GAD and the number of GABAergic neurons decreased. Together, these results suggest that the Wnt/Ca²⁺ signaling pathway is required for GABAergic neuron regeneration.

Comparative Transcriptome Analysis of the Regenerating Zebrafish Telencephalon Unravels a Resource With Key Pathways During Two Early Stages and Activation of Wnt/β-Catenin Signaling at the Early Wound Healing Stage

Owing to its pronounced regenerative capacity in many tissues and organs, the zebrafish brain represents an ideal platform to understand the endogenous regeneration mechanisms that restore tissue integrity and function upon injury or disease. Although radial glial and neuronal cell populations have been characterized with respect to specific marker genes, comprehensive transcriptomic profiling of the regenerating telencephalon has not been conducted so far. Here, by processing the lesioned and unlesioned hemispheres of the telencephalon separately, we reveal the differentially expressed genes (DEGs) at the early wound healing and early proliferative stages of regeneration, i.e., 20 h post-lesion (hpl) and 3 days post-lesion (dpl), respectively. At 20 hpl, we detect a far higher number of DEGs in the lesioned hemisphere than in the unlesioned half and only 7% of all DEGs in both halves. However, this difference disappears at 3 dpl, where the lesioned and unlesioned hemispheres share 40% of all DEGs. By performing an extensive comparison of the gene expression profiles in these stages, we unravel that the lesioned hemispheres at 20 hpl and 3 dpl exhibit distinct transcriptional profiles. We further unveil a prominent activation of Wnt/β-catenin signaling at 20 hpl, returning to control level in the lesioned site at 3 dpl. Wnt/β-catenin signaling indeed appears to control a large number of genes associated primarily with the p53, apoptosis, forkhead box O (FoxO), mitogen-activated protein kinase (MAPK), and mammalian target of rapamycin (mTOR) signaling pathways specifically at 20 hpl. Based on these results, we propose that the lesioned and unlesioned hemispheres react to injury dynamically during telencephalon regeneration and that the activation of Wnt/β-catenin signaling at the early wound healing stage plays a key role in the regulation of cellular and molecular events.

Сlass II histone deacetylases in the post-stroke recovery period-expression, cellular, and subcellular localization-promising targets for neuroprotection

Histone deacetylases (HDAC) inhibitors can protect nerve cells after a stroke, but it is unclear which HDAC isoform is involved in this effect. We studied cellular and intracellular rearrangement of class II HDACs at late periods after photothrombotic infarct (PTI) in the mouse sensorimotor cortex in the tissue surrounding the ischemia core and in the corresponding region of the contralateral hemisphere. We observed a decrease in HDAC4 in cortical neurons and an increase in its nuclear translocation. HDAC6 expression in neurons was also increased. Moreover, HDAC6-positive cells had elevated apoptosis. Tubostatin A (Tub A)-induced decrease in the activity of HDAC6 restored acetylation of α-tubulin during the early poststroke recovery period and reduced apoptosis of nerve cells thus protecting the brain tissue. Selective inhibition of HDAC6 elevated expression of growth-associated protein-43 (GAP43), which remained high up to 14 days after stroke and promoted axogenesis and recovery from the PTI-induced neurological deficit. Selective HDAC6 inhibitor Tub A markedly reduced neuronal death and increased acetylation of α-tubulin and the level of GAP43. Thus, HDAC6 inhibition could be a promising strategy for modulation of brain recovery as it can increase the intensity and reduce the duration of reparation processes in the brain after stroke.

Perspectives of RAS and RHEB GTPase Signaling Pathways in Regenerating Brain Neurons

Cellular activation of RAS GTPases into the GTP-binding “ON” state is a key switch for regulating brain functions. Molecular protein structural elements of rat sarcoma (RAS) and RAS homolog protein enriched in brain (RHEB) GTPases involved in this switch are discussed including their subcellular membrane localization for triggering specific signaling pathways resulting in regulation of synaptic connectivity, axonal growth, differentiation, migration, cytoskeletal dynamics, neural protection, and apoptosis. A beneficial role of neuronal H-RAS activity is suggested from cellular and animal models of neurodegenerative diseases. Recent experiments on optogenetic regulation offer insights into the spatiotemporal aspects controlling RAS/mitogen activated protein kinase (MAPK) or phosphoinositide-3 kinase (PI3K) pathways. As optogenetic manipulation of cellular signaling in deep brain regions critically requires penetration of light through large distances of absorbing tissue, we discuss magnetic guidance of re-growing axons as a complementary approach. In Parkinson’s disease, dopaminergic neuronal cell bodies degenerate in the substantia nigra. Current human trials of stem cell-derived dopaminergic neurons must take into account the inability of neuronal axons navigating over a large distance from the grafted site into striatal target regions. Grafting dopaminergic precursor neurons directly into the degenerating substantia nigra is discussed as a novel concept aiming to guide axonal growth by activating GTPase signaling through protein-functionalized intracellular magnetic nanoparticles responding to external magnets.

Adult axolotls can regenerate original neuronal diversity in response to brain injury

The axolotl can regenerate multiple organs, including the brain. It remains, however, unclear whether neuronal diversity, intricate tissue architecture, and axonal connectivity can be regenerated; yet, this is critical for recovery of function and a central aim of cell replacement strategies in the mammalian central nervous system. Here, we demonstrate that, upon mechanical injury to the adult pallium, axolotls can regenerate several of the populations of neurons present before injury. Notably, regenerated neurons acquire functional electrophysiological traits and respond appropriately to afferent inputs. Despite the ability to regenerate specific, molecularly-defined neuronal subtypes, we also uncovered previously unappreciated limitations by showing that newborn neurons organize within altered tissue architecture and fail to re-establish the long-distance axonal tracts and circuit physiology present before injury. The data provide a direct demonstration that diverse, electrophysiologically functional neurons can be regenerated in axolotls, but challenge prior assumptions of functional brain repair in regenerative species.

DOI:http://dx.doi.org/10.7554/eLife.13998.001

Humans and other mammals have a very limited ability to regenerate new neurons in the brain to replace those that have been injured or damaged. In striking contrast, some animals like fish and salamanders are capable of filling in injured brain regions with new neurons. This is a complex task, as the brain is composed of many different types of neurons that are connected to each other in a highly organized manner across both short and long distances.

The extent to which even the most regenerative species can build new brain regions was not known. Understanding any limitations will help to set realistic expectations for the success of potential treatments that aim to replace neurons in mammals.

Amamoto et al. found that the brain of the axolotl, a species of salamander, could selectively regenerate the specific types of neurons that were damaged. This finding suggests that the brain is able to somehow sense which types of neurons are injured. The new neurons were able to mature into functional neurons, but they were limited in their ability to reconnect to their original, distant target neurons.

More research is now needed to investigate how the axolotl brain recognizes which types of neurons have been damaged. It will also be important to understand which cells respond to the injury to give rise to the new neurons that fill the injury site, and to uncover the molecules that are important for governing this regenerative process.

DOI:http://dx.doi.org/10.7554/eLife.13998.002

Classic Studies on the Potential of Stem Cell Neuroregeneration

The 1990s and 2000s were the beginning of an exciting time period for developmental neuroscience and neural stem cell research. By better understanding brain plasticity and the birth of new neurons in the adult brain, contrary to established dogma, hope for therapy from devastating neurological diseases was generated. The potential for stem cells to provide functional recovery in humans remains to be further tested and to further move into the clinical trial realm. The future certainly has great promise on stem cells to assist in alleviation of difficult-to-treat neurologic disorders. This article reviews classic studies of the 1990s and 2000s that paved the way for the advances of today, which can in turn lead to tomorrow's therapies.

Excitotoxic brain injury in adult zebrafish stimulates neurogenesis and long-distance neuronal integration

Zebrafish maintain a greater capacity than mammals for central nervous system repair after injury. Understanding differences in regenerative responses between different vertebrate species may shed light on mechanisms to improve repair in humans. Quinolinic acid is an excitotoxin that has been used to induce brain injury in rodents for modeling Huntington's disease and stroke. When injected into the adult rodent striatum, this toxin stimulates subventricular zone neurogenesis and neuroblast migration to injury. However, most new neurons fail to survive and lesion repair is minimal. We used quinolinic acid to lesion the adult zebrafish telencephalon to study reparative processes. We also used conditional transgenic lineage mapping of adult radial glial stem cells to explore survival and integration of neurons generated after injury. Telencephalic lesioning with quinolinic acid, and to a lesser extent vehicle injection, produced cell death, microglial infiltration, increased cell proliferation, and enhanced neurogenesis in the injured hemisphere. Lesion repair was more complete with quinolinic acid injection than after vehicle injection. Fate mapping of her4-expressing radial glia showed injury-induced expansion of radial glial stem cells that gave rise to neurons which migrated to injury, survived at least 8 weeks and formed long-distance projections that crossed the anterior commissure and synapsed in the contralateral hemisphere. These findings suggest that quinolinic acid lesioning of the zebrafish brain stimulates adult neural stem cells to produce robust regeneration with long-distance integration of new neurons. This model should prove useful for elucidating reparative mechanisms that can be applied to restorative therapies for mammalian brain injury.

Vascular regulation of adult neurogenesis under physiological and pathological conditions

Neural stem cells in the mammalian adult brain continuously produce new neurons throughout life. Accumulating evidence in rodents suggests that various aspects of adult neurogenesis, including the genesis, migration, and maturation of new neurons, are regulated by factors derived from blood vessels and their microenvironment. Brain injury enhances both neurogenesis and angiogenesis, thereby promoting the cooperative regeneration of neurons and blood vessels. In this paper, we briefly review the mechanisms for the vascular regulation of adult neurogenesis in the ventricular-subventricular zone under physiological and pathological conditions, and discuss their clinical potential for brain regeneration strategies.