Neural differentiation and synaptogenesis in retinal development

To investigate the pattern of neural differentiation and synaptogenesis in the mouse retina, immunolabeling, Brd U assay and transmission electron microscopy were used. We show that the neuroblastic cell layer is the germinal zone for neural differentiation and retinal lamination. Ganglion cells differentiated initially at embryonic day 13（E13）, and at E18 horizontal cells appeared in the neuroblastic cell layer. Neural stem cells in the outer neuroblastic cell layer differentiated into photoreceptor cells as early as postnatal day 0（P0）, and neural stem cells in the inner neuroblastic cell layer differentiated into bipolar cells at P7. Synapses in the retina were mainly located in the outer and inner plexiform layers. At P7, synaptophysin immunostaining appeared in presynaptic terminals in the outer and inner plexiform layers with button-like structures. After P14, presynaptic buttons were concentrated in outer and inner plexiform layers with strong staining. These data indicate that neural differentiation and synaptogenesis in the retina play important roles in the formation of retinal neural circuitry. Our study showed that the period before P14, especially between P0 and P14, represents a critical period during retinal development. Mouse eye opening occurs during that period, suggesting that cell differentiation and synaptic formation lead to the attainment of visual function.

Thrombospondins and synaptogenesis

Here, we review research on the mechanisms underlying the ability of thrombospondin to promote synaptogenesis and examine its role in central nervous system diseases and drug actions. Thrombospondin secreted by glial cells plays a critical role in synaptogenesis and maintains synapse stability. Thrombospondin regulates synaptogenesis through receptor a26-1 and neuroligin 1, and promotes the proliferation and differentiation of neural progenitor cells. It also participates in synaptic remodeling following injury and in the action of some nervous system drugs.

Time dependent integration of matrix metalloproteinases and their targeted substrates directs axonal sprouting and synaptogenesis following central nervous system injury

Over the past two decades, many investigators have reported how extracellular matrix molecules act to regulate neuroplasticity. The majority of these studies involve proteins which are targets of matrix metalloproteinases. Importantly, these enzyme/substrate interactions can regulate degenerative and regenerative phases of synaptic plasticity, directing axonal and dendritic reorganization after brain insult. The present review first summarizes literature support for the prominent role of matrix metalloproteinases during neuroregeneration, followed by a discussion of data contrasting adaptive and maladaptive neuroplasticity that reveals time-dependent metalloproteinase/substrate regulation of postinjury synaptic recovery. The potential for these enzymes to serve as therapeutic targets for enhanced neuroplasticity after brain injury is illustrated with experiments demonstrating that metalloproteinase inhibitors can alter adaptive and maladaptive outcome. Finally, the complexity of metalloproteinase role in reactive synaptogenesis is revealed in new studies showing how these enzymes interact with immune molecules to mediate cellular response in the local regenerative environment, and are regulated by novel binding partners in the brain extracellular matrix. Together, these different examples show the complexity with which metalloproteinases are integrated into the process of neuroregeneration, and point to a promising new angle for future studies exploring how to facilitate brain plasticity.

Differentiation and functional connectivity of fetal tectal transplants

Data from studies analyzing the differentiation and functional connectivity of embryo nic neural tissue grafted into the mammalian nervous system has led to the clinical testing of the fetal graft approach in patients with neurodegenerative disease.While some success has been achieved,ethical concerns have led to a search for alternative therapeutic strategies,mostly exploring the use of neural precursors or neurons derived from pluripotent stem cells to replace damaged host neurons and restore lost circuitries.These more recent studies address questions of graft viability,differentiation,and connectivity similar to those posed by researchers in earlier fetal transplant work,thus reviews of the fetal graft literature may inform and help guide ongoing research in the stem cell/organoid field.This brief review describes some key observations from research into the transplantation of neural tissue into the rat visual syste m,focusing on grafts of the fetal supe rior colliculus(tectal grafts) into neonatal or adult hosts.In neonate hosts,grafts quickly develop connections with the underlying host mid b rain and attain a morphology typical of mature grafts by about 2 weeks.G rafts consistently contain numerous localized regions which,based on neurofibrillar staining,neuronal morphology(Golgi),neurochemistry,receptor expression,and glial architecture,are homologous to the stratum griseum supe rficiale of normal superior colliculus.These localized "patches" are also seen after explant culture and when donor tectal tissue is dissociated and reaggregated prior to transplantation.In almost all circumstances,host retinal innervation is restricted to these localized patches,but only those that are located adjacent to the graft surfa ce.Synapses are formed and there is evidence of functional drive.The only exception occurs when Schwann cells are added to dissociated tecta prior to reaggregation.In these co-grafts,the peripheral glia appear to compete with local target fa ctors and host retinal ingrowth is more widespread.Other afferent systems(e.g.,host co rtex,serotonin) show different patterns of innervation.The host cortical input originates more from extrastriate regions and establishes functional excitato ry synapses with grafted neurons.Finally,when grafted into optic tra ct lesions in adult rat hosts,spontaneously regrowing host retinal axons retain the capacity to selectively innervate the localized patches in embryonic tectal grafts,showing that the specific affinities between adult retinal axons and their targets are not lost during regeneration.While the research described here provides some pertinent information about development and plasticity in visual pathways,a more general aim is to highlight how the review of the extensive fetal graft lite rature may aid in an appreciation of the positive(and negative) fa ctors that influence survival,differentiation,connectivity and functionality of engineered cells and organoids transplanted into the central nervous system.

C-X-C chemokine receptor type 7 antibody enhances neural plasticity after ischemic stroke

Stromal cell-derived factor-1 and its receptor C-X-C chemokine receptor 4(CXCR4) have been shown to regulate neural regeneration after stroke.Howeve r,whether stromal cell-derived factor-1 receptor CXCR7,which is widely distributed in the develo ping and adult central nervous system,participates in neural regeneration remains poorly unde rstood.In this study,we established rat models of focal cerebral ischemia by injecting endothelin-1 into the cerebral co rtex and striatum.Starting on day 7 after injury,CXCR7-neutralizing antibody was injected into the lateral ventricle using a micro drug delivery system for 6 consecutive days.Our results showed that CXCR7-neutralizing antibody increased the total length and number of sprouting co rticospinal tra ct fibers in rats with cerebral ischemia,increased the expression of vesicular glutamate transporter 1 and growth-related protein 43,marke rs of the denervated spinal cord synapses,and promoted the differentiation and maturation of oligodendrocyte progenitor cells in the striatum.In addition,CXCR7 antibody increased the expression of CXCR4 in the striatum,increased the protein expression of RAS and ERK1/2 associated with the RAS/ERK signaling pathway,and im proved rat motor function.These findings suggest that CXCR7 improved neural functional recovery after ischemic stroke by promoting axonal regeneration,synaptogenesis,and myelin regeneration,which may be achieved by activation of CXCR4 and the RAS/ERK1/2 signaling pathway.

Matrine promotes neural circuit remodeling to regulate motor function in a mouse model of chronic spinal cord injury

In chronic phase of spinal cord injury, functional recovery is more untreatable compared with early intervention in acute phase of spinal cord injury. In the last decade, several combination therapies successfully improved motor dysfunction in chronic spinal cord injury. However, their effectiveness is not sufficient. We previously found a new effective compound for spinal cord injury, matrine, which induced axonal growth and functional recovery in acute spinal cord injury mice via direct activation of extracellular heat shock protein 90. Although our previous study clarified that matrine was an activator of extracellular heat shock protein 90, the potential of matrine for spinal cord injury in chronic phase has not been sufficiently evaluated. Thus, this study aimed to investigate whether matrine ameliorates chronic spinal cord injury in mice. Once daily intragastric administration of matrine(100 μmol/kg per day) to spinal cord injury mice were starte at 28 days after injury, and continued for 154 days. Continuous mat rine treatment improved hindlimb motor function in chronic spinal cord injury mice. In injured spinal cords of the matrine-treated mice, the density of neurofilament-H-positive axons was increased. Moreover, matrine treatment increased the density of bassoon-positive presynapses in contact with choline acetyltransferase-positive motor neurons in the lumbar spinal cord. These findings suggest that matrine promotes remodeling and reconnection of neural circuits to regulate hindlimb movement. All protocols were approved by the Committee for Animal Care and Use of the Sugitani Campus of the University of Toyama(approval No. A2013 INM-1 and A2016 INM-3) on May 7, 2013 and May 17, 2016, respectively.

Synaptic aging disrupts synaptic morphology and function in cerebellar Purkinje cells

Synapses are key structures in neural networks,and are involved in learning and memory in the central nervous system.Investigating synaptogenesis and synaptic aging is important in understanding neural development and neural degeneration in diseases such as Alzheimer disease and Parkinson’s disease.Our previous study found that synaptogenesis and synaptic maturation were harmonized with brain development and maturation.However,synaptic damage and loss in the aging cerebellum are not well understood.This study was designed to investigate the occurrence of synaptic aging in the cerebellum by observing the ultrastructural changes of dendritic spines and synapses in cerebellar Purkinje cells of aging mice.Immunocytochemistry,Di I diolistic assays,and transmission electron microscopy were used to visualize the morphological characteristics of synaptic buttons,dendritic spines and synapses of Purkinje cells in mice at various ages.With synaptic aging in the cerebellum,dendritic spines and synaptic buttons were lost,and the synaptic ultrastructure was altered,including a reduction in the number of synaptic vesicles and mitochondria in presynaptic termini and smaller thin specialized zones in pre-and post-synaptic membranes.These findings confirm that synaptic morphology and function is disrupted in aging synapses,which may be an important pathological cause of neurodegenerative diseases.

Synaptic development of layer V pyramidal neurons in the prenatal human prefrontal neocortex: a Neurolucida-aided Golgi study

The prefrontal neocortex is involved in many high cognitive functions in humans.Deficits in neuronal and neurocircuitry development in this part of the cerebrum have been associated with various neuropsychiatric disorders in adolescents and adults.There are currently little available data regarding prenatal dendrite and spine formation on projecting neurons in the human prefrontal neocortex.Previous studies have demonstrated that Golgi silver staining can identify neurons in the frontal lobe and visual cortex in human embryos.In the present study,five fetal brains,at 19,20,26,35,and 38 gestational weeks,were obtained via the body donation program at Xiangya School of Medicine,Central South University,China.Golgi-stained pyramidal neurons in layer V of Brodmann area 46 in fetuses were quantitatively analyzed using the Neurolucida morphometry system.Results revealed that somal size,total dendritic length,and branching points of these neurons increased from 26 to 38 gestational weeks.There was also a large increase in dendritic spines from 35 to 38 gestational weeks.These findings indicate that,in the human prefrontal neocortex,dendritic growth in layer V pyramidal neurons occurs rapidly during the third trimester of gestation.The use of human fetal brain tissue was approved by the Animal Ethics Committee of Xiangya School of Medicine,Central South University,China(approval No.2011-045)on April 5,2011.

Contactins in the central nervous system: role in health and disease

Contactins are a group of cell adhesion molecules that are mainly expressed in the brain and play pivotal roles in the organization of axonal domains, axonal guidance, neuritogenesis, neuronal development, synapse formation and plasticity, axo-glia interactions and neural regeneration. Contactins comprise a family of six members. Their absence leads to malformed axons and impaired nerve conduction. Contactin mediated protein complex formation is critical for the organization of the axon in early central nervous system development. Mutations and differential expression of contactins have been identified in neuro-developmental or neurological disorders. Taken together, contactins are extensively studied in the context of nervous system development. This review summarizes the physiological roles of all six members of the Contactin family in neurodevelopment as well as their involvement in neurological/neurodevelopmental disorders.

Acute statin treatment improves recovery after experimental intracerebral hemorrhage

Background and Purpose: We have previously demonstrated that 2-week treatment of experimental intracerebral hemorrhage (ICH) with a daily dose of 2 mg/kg statin starting 24 hours post-injury exerts a neuroprotective effect. The present study extends our previous investigation and tests the effect of acute high-dose (within 24 hours) statin therapy on experimental ICH. Material and Methods: Fifty-six male wistar rats were subjected to ICHby stereotactic injection of 100 μl of autologous blood into the striatum. Rats were divided randomly into seven groups: saline control group (n = 8);10, 20 and 40 mg/kg simvastatin-treated groups (n = 8);and 10, 20 and 40 mg/kg atorvastatin-treated groups (n = 8). Simvastatin or atorvastatin were administered orally at 3 and 24 hours after ICH. Neurological functional outcome was evaluated using behavioral tests (mNSS and corner turn test) at multiple time points afterICH. Animals were sacrificed at 28 days after treatment, and histological studies were completed. Results: Acute treatment with simvastatin or atorvastatin at doses of 10 and 20 mg/kg, but not at 40 mg/kg, significantly enhanced recovery of neurological function starting from 2 weeks post-ICH and persisting for up to 4 weeks postICH. In addition, at doses of 10 mg/kg and 20 mg/kg, histological evaluations revealed that simvastatin or atorvastatin reduced tissue loss, increased cell proliferation in the subventricular zone and enhanced vascular density and synaptogenesis in the hematoma boundary zone when compared to salinetreated rats. Conclusions: Treatment with simvastatin or atorvastatin at doses of 10 and 20 mg/kg significantly improves neurological recovery after administration during the first 24 hours after ICH. Decreased tissue loss, increased cell proliferation and vascularity likely contribute to improved functional recovery in rats treated with statins after ICH.

Fine-tuning the response of growth cones to guidance cues: a perspective on the role of microRNAs

In the development and regeneration of the nervous system, neurons face the complex task of establishing and/or repairing neuronal connections and contacts. The formation of these neuronal circuits is largely coordinated by tightly regulated temporal and spatial changes in mRNA translation, which enables incredibly precise control over protein expression and localization (Jung and Holt, 2011). Local mRNA translation in specific cellular compartments appears to play a role in many processes that are important to nervous system development and regeneration, including: cell survival, migration, growth cone guidance, and synaptogenesis (Jung and Holt, 2011).

Cathepsins in neuronal plasticity

Proteases comprise a variety of enzymes defined by their ability to catalytically hydrolyze the peptide bonds of other proteins,resulting in protein lysis.Cathepsins,specifically,encompass a class of at least twenty proteases with potent endopeptidase activity.They are located subcellularly in lysosomes,organelles responsible for the cell’s degradative and autophagic processes,and are vital for normal lysosomal function.Although cathepsins are involved in a multitude of cell signaling activities,this chapter will focus on the role of cathepsins(with a special emphasis on Cathepsin B)in neuronal plasticity.We will broadly define what is known about regulation of cathepsins in the central nervous system and compare this with their dysregulation after injury or disease.Importantly,we will delineate what is currently known about the role of cathepsins in axon regeneration and plasticity after spinal cord injury.It is well established that normal cathepsin activity is integral to the function of lysosomes.Without normal lysosomal function,autophagy and other homeostatic cellular processes become dysregulated resulting in axon dystrophy.Furthermore,controlled activation of cathepsins at specialized neuronal structures such as axonal growth cones and dendritic spines have been positively implicated in their plasticity.This chapter will end with a perspective on the consequences of cathepsin dysregulation versus controlled,localized regulation to clarify how cathepsins can contribute to both neuronal plasticity and neurodegeneration.

Selective and constructive mechanisms contribute to neural circuit formation in the barrel cortex of the developing rat

The cellular strategy leading to formation of neuronal circuits in the rodent barrel cortex is still a matter of controversy. Both selective and constructive mechanisms have been proposed. The selective mechanism involves an overproduction of neuronal processes and synapses followed by activity dependent pruning. Conversely, a constructive mechanism would increase the number of axons, dendrites, and synapses during development to match functionality. In order to discern the contributions of these two mechanisms in establishing a neuronal circuit in the somatosensory cortex, morphometric analysis of dendritic and axonal arbor growth was performed. Also, the number of synapses was followed by electron microscopy during the first month of life. We observed that axonal and dendritic arbors retracted distal branches, and elongated proximal branches, resulting in increased arbor complexity. This neuronal remodeling was accompanied by the steady increase in the number of synapses within barrel hollows. Similarly, the content of molecular markers for dendrites, axons and synapses also increased during this period. Finally, cytochrome oxidase activity rose with age in barrels indicating that the arbors became more complex while synapse density and metabolic demands increased. Our results support the simultaneous use of both selective and constructive mechanisms in establishing the barrel cortex circuitry.

Early anesthetic exposure and long-term cognitive impairment

Several lines of evidence from clinical cohort studies and animal studies have shown that early exposure to anesthetics is a significant risk factor for later development of learning disabilities.However,the underlying molecular mechanism is unclear.Recent studies have indicated that hippocampal neurogenesis and synaptogenesis may be involved in the mechanisms by which early anesthetic exposure produces long-term cognitive impairment.It is possible that synaptic scaffolding protein postsynaptic density-95(PSD-95)PDZ(PSD 95/Discs large/Zona occludens-1)domain-mediated protein-protein interactions are involved in the regulation of neurogenesis and synaptogenesis in the central nervous system.PDZ domain-mediated protein-protein interactions are disrupted by clinically relevant concentrations of inhaled anesthetics.It will help us understand the molecular mechanism underlying anestheticinduced long-term cognitive dysfunction if we can demonstrate the role of synaptic PDZ interactions in early anesthetic exposure-produced long-term cognitive impairment.

Neurobiology of Neuronal Network Alteration in Intellectual Disability Related to Fetal Alcohol Spectrum Disorders

The molecular and cellular mechanisms by which alcohol produces its deleterious effects on neuronal networks are only now beginning to be understood. This review focused on alcohol-induced neurobiological alterations on neuronal network components underlying information processing, for further understanding of intellectual disability related to FASD. Abnormal neurodevelopmental events related to alcohol-damaged fetal brain included neurogenesis inhibition, aberrant migration, impaired differentiation, exacerbated apoptosis, impaired axon outgrowth and branching altering synaptogenesis and synaptic plasticity, abnormal GABAergic interneurons triggering synaptic inhibitory/excitatory imbalance, reduced myelinogenesis causing injured white matter in prefrontal lobe and atrophied corpus callosum compromising interhemispheric information transfer, the whole compromising neuronal network scaffolding which may lead to biased information processing with deficits in executive function. What added to these abnormalities are smaller gray matter and reduced hippocampus, resulting in cognition and memory failures. As a whole, these developmental disorders may underlie intellectual disability related to FASD. In rodents, these neuronal network components matured mainly during the second and third trimesters equivalents of human gestation. Transferability of results from animal to human was also discussed. It was hoped that the understanding of alcohol-induced neuronal networks failure mechanisms during the developing brain may lay a foundation for prospective new treatments and interventions.

Neuronal guidance genes in health and diseases

Neurons migrate from their birthplaces to the destinations,and extending axons navigate to their synaptic targets by sensing various extracellular cues in spatiotemporally controlled manners.These evolutionally conserved guidance cues and their receptors regulate multiple aspects of neural development to establish the highly complex nervous system by mediating both short-and long-range cell-cell communications.Neuronal guidance genes(encoding cues,receptors,or downstream signal transducers)are critical not only for development of the nervous system but also for synaptic maintenance,remodeling,and function in the adult brain.One emerging theme is the combinatorial and complementary functions of relatively limited classes of neuronal guidance genes in multiple processes,including neuronal migration,axonal guidance,synaptogenesis,and circuit formation.Importantly,neuronal guidance genes also regulate cell migration and cell-cell communications outside the nervous system.We are just beginning to understand how cells integrate multiple guidance and adhesion signaling inputs to determine overall cellular/subcellular behavior and how aberrant guidance signaling in various cell types contributes to diverse human diseases,ranging from developmental,neuropsychiatric,and neurodegenerative disorders to cancer metastasis.We review classic studies and recent advances in understanding signaling mechanisms of the guidance genes as well as their roles in human diseases.Furthermore,we discuss the remaining chalienges and therapeutic potentials of modulating neuronal guidance pathways in neural repair.

Interactions Between Astrocytes and Oligodendroglia in Myelin Development and Related Brain Diseases

Astrocytes(ASTs)and oligodendroglial lineage cells(OLGs)are major macroglial cells in the central nervous system.ASTs communicate with each other through connexin(Cx)and Cx-based network structures,both of which allow for quick transport of nutrients and signals.Moreover,ASTs interact with OLGs through connexin(Cx)-mediated networks to modulate various physiological processes in the brain.In this article,following a brief description of the infrastructural basis of the glial networks and exocrine factors by which ASTs and OLGs may crosstalk,we focus on recapitulating how the interactions between these two types of glial cells modulate myelination,and how the AST-OLG interactions are involved in protecting the integrity of the blood-brain barrier(BBB)and regulating synaptogenesis and neural activity.Recent studies further suggest that AST-OLG interactions are associated with myelin-related diseases,such as multiple sclerosis.A better understanding of the regulatory mechanisms underlying AST-OLG interactions may inspire the development of novel therapeutic strategies for related brain diseases.

Autophagy in synaptic development,function,and pathology

In the nervous system, neurons contact each other to form neuronal circuits and drive behavior, relying heavily on synaptic connections. The proper development and growth of synapses allows functional transmission of electrical information between neurons or between neurons and muscle fibers. Defects in synapse-formation or development lead to many diseases. Autophagy, a major determinant of protein turnover, is an essential process that takes place in developing synapses. During the induction of autophagy, proteins and cytoplasmic components are encapsulated in autophagosomes, which fuse with lysosomes to form autolysosomes. The cargoes are subsequently degraded and recycled. However, aberrant autophagic activity may lead to synaptic dysfunction, which is a common pathological characteristic in several disorders. Here, we review the current understanding of autophagy in regulating synaptic development and function. In addition, autophagy-related synaptic dysfunction in human diseases is also summarized.