A role for ephrin-A5 in axonal sprouting, recovery, and activity-dependent plasticity after stroke

Implantation of Miniosmotic Pumps and Delivery of Tract Tracers to Study Brain Reorganization in Pathophysiological Conditions

Pharmacological treatment in animal models of cerebral disease imposes the problem of repeated injection protocols that may induce stress in animals and result in impermanent tissue levels of the drug. Additionally, drug delivery to the brain is delicate due to the blood brain barrier (BBB), thus significantly reducing intracerebral concentrations of selective drugs after systemic administration. Therefore, a system that allows both constant drug delivery without peak levels and circumvention of the BBB is in order to achieve sufficiently high intracerebral concentrations of drugs that are impermeable to the BBB. In this context, miniosmotic pumps represent an ideal system for constant drug delivery at a fixed known rate that eludes the problem of daily injection stress in animals and that may also be used for direct brain delivery of drugs. Here, we describe a method for miniosmotic pump implantation and post operatory care that should be given to animals in order to successfully apply this technique. We embed the aforementioned experimental paradigm in standard procedures that are used for studying neuroplasticity within the brain of C57BL6 mice. Thus, we exposed animals to 30 min brain infarct and implanted with miniosmotic pumps connected to the skull via a cannula in order to deliver a pro-plasticity drug. Behavioral testing was done during 30 days of treatment. After removal the animals received injections of anterograde tract tracers to analyze neuronal plasticity in the chronic phase of recovery. Results indicated that neuroprotection by the delivered drug was accompanied with increase in motor fibers crossing the midline of the brain at target structures. The results affirm the value of these techniques for drug administration and brain plasticity studies in modern neuroscience.

GDF10 is a signal for axonal sprouting and functional recovery after stroke

Stroke produces a limited process of neural repair. Axonal sprouting in cortex adjacent to the infarct is part of this recovery process, but the signal that initiates axonal sprouting is not known. Growth and differentiation factor 10 (GDF10) is induced in peri-infarct neurons in mice, non-human primates and humans. GDF10 promotes axonal outgrowth in vitro in mouse, rat and human neurons through TGFβRI and TGFβRII signaling. Using pharmacogenetic gain- and loss-of-function studies, we found that GDF10 produced axonal sprouting and enhanced functional recovery after stroke; knocking down GDF10 blocked axonal sprouting and reduced recovery. RNA sequencing from peri-infarct cortical neurons revealed that GDF10 downregulated PTEN, upregulated PI3 kinase signaling and induced specific axonal guidance molecules. Using unsupervised genome-wide association analysis of the GDF10 transcriptome, we found that it was not related to neurodevelopment, but may partially overlap with other CNS injury patterns. Thus, GDF10 is a stroke-induced signal for axonal sprouting and functional recovery.

Restoration of skilled locomotion by sprouting corticospinal axons induced by co-deletion of PTEN and SOCS3

The limited rewiring of the corticospinal tract (CST) only partially compensates the lost functions after stroke, brain trauma and spinal cord injury. Therefore it is important to develop new therapies to enhance the compensatory circuitry mediated by spared CST axons. Here by using a unilateral pyramidotomy model, we find that deletion of cortical suppressor of cytokine signaling 3 (SOCS3), a negative regulator of cytokine-activated pathway, promotes sprouting of uninjured CST axons to the denervated spinal cord. A likely trigger of such sprouting is ciliary neurotrophic factor (CNTF) expressed in local spinal neurons. Such sprouting can be further enhanced by deletion of phosphatase and tensin homolog (PTEN), a mechanistic target of rapamycin (mTOR) negative regulator, resulting in significant recovery of skilled locomotion. Ablation of the corticospinal neurons with sprouting axons abolishes the improved behavioural performance. Furthermore, by optogenetics-based specific CST stimulation, we show a direct limb motor control by sprouting CST axons, providing direct evidence for the reformation of a functional circuit.

A key pathological alteration after brain and spinal cord injury is the disruption of the corticospinal tract (CST) axons that control the voluntary movements. Here the authors show that activating the intrinsic regenerative ability by inhibiting PTEN and SOCS3 expression promotes robust sprouting growth and recovery of skilled locomotion after injury.

Rehabilitation and plasticity following stroke: Insights from rodent models

Ischemic injuries within the motor cortex result in functional deficits that may profoundly impact activities of daily living in patients. Current rehabilitation protocols achieve only limited recovery of motor abilities. The brain reorganizes spontaneously after injury, and it is believed that appropriately boosting these neuroplastic processes may restore function via recruitment of spared areas and pathways. Here I review studies on circuit reorganization, neuronal and glial plasticity and axonal sprouting following ischemic damage to the forelimb motor cortex, with a particular focus on rodent models. I discuss evidence pointing to compensatory take-over of lost functions by adjacent peri-lesional areas and the role of the contralesional hemisphere in recovery. One key issue is the need to distinguish "true" recovery (i.e. re-establishment of original movement patterns) from compensation in the assessment of post-stroke functional gains. I also consider the effects of physical rehabilitation, including robot-assisted therapy, and the potential mechanisms by which motor training induces recovery. Finally, I describe experimental approaches in which training is coupled with delivery of plasticizing drugs that render the remaining, undamaged pathways more sensitive to experience-dependent modifications. These combinatorial strategies hold promise for the definition of more effective rehabilitation paradigms that can be translated into clinical practice.

Astrocytes, therapeutic targets for neuroprotection and neurorestoration in ischemic stroke

Astrocytes are the most abundant cell type within the central nervous system. They play essential roles in maintaining normal brain function, as they are a critical structural and functional part of the tripartite synapses and the neurovascular unit, and communicate with neurons, oligodendrocytes and endothelial cells. After an ischemic stroke, astrocytes perform multiple functions both detrimental and beneficial, for neuronal survival during the acute phase. Aspects of the astrocytic inflammatory response to stroke may aggravate the ischemic lesion, but astrocytes also provide benefit for neuroprotection, by limiting lesion extension via anti-excitotoxicity effects and releasing neurotrophins. Similarly, during the late recovery phase after stroke, the glial scar may obstruct axonal regeneration and subsequently reduce the functional outcome; however, astrocytes also contribute to angiogenesis, neurogenesis, synaptogenesis, and axonal remodeling, and thereby promote neurological recovery. Thus, the pivotal involvement of astrocytes in normal brain function and responses to an ischemic lesion designates them as excellent therapeutic targets to improve functional outcome following stroke. In this review, we will focus on functions of astrocytes and astrocyte-mediated events during stroke and recovery. We will provide an overview of approaches on how to reduce the detrimental effects and amplify the beneficial effects of astrocytes on neuroprotection and on neurorestoration post stroke, which may lead to novel and clinically relevant therapies for stroke.

The Specific Requirements of Neural Repair Trials for Stroke

Novel molecular, cellular, and pharmacological therapies to stimulate repair of sensorimotor circuits after stroke are entering clinical trials. Compared with acute neuroprotection and thrombolysis studies, clinical trials for repair in subacute and chronic hemiplegic participants have a different time course for delivery of an intervention, different mechanisms of action within the milieu of the injury, distinct relationships to the amount of physical activity and skills practice of participants, and need to include more refined outcome measures. This review examines the biological interaction of targeted rehabilitation with neural repair strategies to optimize outcomes. We suggest practical guidelines for the incorporation of inexpensive skills training and exercise at home. In addition, we describe some novel outcome measurement tools, including wearable sensors, to obtain the more detailed outcomes that may identify at least some minimal level of success from cellular and regeneration interventions. Thus, proceeding in the shadow of acute stroke trial designs may unnecessarily limit the mechanisms of action of new repair strategies, reduce their impact on participants, and risk missing important behavioral outcomes.

Motor System Reorganization After Stroke: Stimulating and Training Toward Perfection

Stroke instigates regenerative responses that reorganize connectivity patterns among surviving neurons. The new connectivity patterns can be suboptimal for behavioral function. This review summarizes current knowledge on post-stroke motor system reorganization and emerging strategies for shaping it with manipulations of behavior and cortical activity to improve functional outcome.

Combined ampakine and BDNF treatments enhance poststroke functional recovery in aged mice via AKT-CREB signaling

Cerebral ischemia results in damage to neuronal circuits and lasting impairment in function. We have previously reported that stimulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors with the ampakine, CX1837, increases brain-derived neurotrophic factor (BDNF) levels and affords significant motor recovery after stroke in young mice. Here, we investigated whether administration of CX1837 in aged (24 months old) mice was equally effective. In a model of focal ischemia, administration of CX1837 from 5 days after stroke resulted in a small gain of motor function by week 6 after stroke. Mice that received a local delivery of BDNF via hydrogel implanted into the stroke cavity also showed a small gain of function from 4 to 6 weeks after stroke. Combining both treatments, however, resulted in a marked improvement in motor function from 2 weeks after insult. Assessment of peri-infarct tissue 2 weeks after stroke revealed a significant increase in p-AKT and p-CREB after the combined drug treatment. Using the pan-AKT inhibitor, GSK-690693, or deletion of CREB from forebrain neurons using the CREB-flox/CAMKii-cre mice, we were able to block the recovery of motor function. These data suggest that combined CX1837 and local delivery of BDNF are required to achieve maximal functional recovery after stroke in aged mice, and is occurring via the AKT-GSK3-CREB signaling pathway.

Targeted inhibition of the Shroom3-Rho kinase protein-protein interaction circumvents Nogo66 to promote axon outgrowth

BACKGROUND

Inhibitory molecules in the adult central nervous system, including NogoA, impede neural repair by blocking axon outgrowth. The actin-myosin regulatory protein Shroom3 directly interacts with Rho kinase and conveys axon outgrowth inhibitory signals from Nogo66, a C-terminal inhibitory domain of NogoA. The purpose of this study was to identify small molecules that block the Shroom3-Rho kinase protein-protein interaction as a means to modulate NogoA signaling and, in the longer term, enhance axon outgrowth during neural repair.

RESULTS

A high throughput screen for inhibitors of the Shroom3-Rho kinase protein-protein interaction identified CCG-17444 (Chem ID: 2816053). CCG-17444 inhibits the Shroom3-Rho kinase interaction in vitro with micromolar potency. This compound acts through an irreversible, covalent mechanism of action, targeting Shroom3 Cys1816 to inhibit the Shroom3-Rho kinase protein-protein interaction. Inhibition of the Shroom3-Rho kinase protein-protein interaction with CCG-17444 counteracts the inhibitory action of Nogo66 and enhances neurite outgrowth.

CONCLUSIONS

This study identifies a small molecule inhibitor of the Shroom3-Rho kinase protein-protein interaction that circumvents the inhibitory action of Nogo66 in neurons. Identification of a small molecule compound that blocks the Shroom3-Rho kinase protein-protein interaction provides a first step towards a potential new strategy for enhancing neural repair.

Effects of Postinfarct Myelin-Associated Glycoprotein Antibody Treatment on Motor Recovery and Motor Map Plasticity in Squirrel Monkeys

Background and Purpose—

New insights into the brain’s ability to reorganize after injury are beginning to suggest novel restorative therapy targets. Potential therapies include pharmacological agents designed to promote axonal growth. The purpose of this study was to test the efficacy of one such drug, GSK249320, a monoclonal antibody that blocks the axon outgrowth inhibition molecule, myelin-associated glycoprotein, to facilitate recovery of motor skills in a nonhuman primate model of ischemic cortical damage.

Methods—

Using a between-groups repeated-measures design, squirrel monkeys were randomized to 1 of 2 groups: an experimental group received intravenous GSK249320 beginning 24 hours after an ischemic infarct in motor cortex with repeated dosages given at 1-week intervals for 6 weeks and a control group received only the vehicle at matched time periods. The primary end point was a motor performance index based on a distal forelimb reach-and-retrieval task. Neurophysiological mapping techniques were used to determine changes in spared motor representations.

Results—

All monkeys recovered to baseline motor performance levels by postinfarct day 16. Functional recovery in the experimental group was significantly facilitated on the primary end point, albeit using slower movements. At 7 weeks post infarct, motor maps in the spared ventral premotor cortex in the experimental group decreased in area compared with the control group.

Conclusions—

GSK249320, initiated 24 hours after a focal cortical ischemic infarct, facilitated functional recovery. Together with the neurophysiological data, these results suggest that GSK249320 has a substantial biological effect on spared cortical tissue. However, its mechanisms of action may be widespread and not strictly limited to peri-infarct cortex and nearby premotor areas.

Neurorehabilitation: motor recovery after stroke as an example

The field of neurorehabilitation aims to translate neuroscience research toward the goal of maximizing functional recovery after neurological injury. A growing body of research indicates that the fundamental principles of neurological rehabilitation are applicable to a broad range of congenital, degenerative, and acquired neurological disorders. In this perspective, we will focus on motor recovery after acquired brain injuries such as stroke. Over the past few decades, a large body of basic and clinical research has created an experimental and theoretical foundation for approaches to neurorehabilitation. Recent randomized clinical trials all emphasize the requirement for intense progressive rehabilitation programs to optimally enhance recovery. Moreover, advances in multimodal assessment of patients with neuroimaging and neurophysiological tools suggest the possibility of individualized treatment plans based on recovery potential. There are also promising indications for medical as well as noninvasive brain stimulation paradigms to facilitate recovery. Ongoing or planned clinical studies should provide more definitive evidence. We also highlight unmet needs and potential areas of research. Continued research built upon a robust experimental and theoretical foundation should help to develop novel treatments to improve recovery after neurological injury.

Robust Axonal Regeneration Occurs in the Injured CAST/Ei Mouse CNS

Axon regeneration in the CNS requires reactivating injured neurons' intrinsic growth state and enabling growth in an inhibitory environment. Using an inbred mouse neuronal phenotypic screen, we find that CAST/Ei mouse adult dorsal root ganglion neurons extend axons more on CNS myelin than the other eight strains tested, especially when pre-injured. Injury-primed CAST/Ei neurons also regenerate markedly in the spinal cord and optic nerve more than those from C57BL/6 mice and show greater sprouting following ischemic stroke. Heritability estimates indicate that extended growth in CAST/Ei neurons on myelin is genetically determined, and two whole-genome expression screens yield the Activin transcript Inhba as most correlated with this ability. Inhibition of Activin signaling in CAST/Ei mice diminishes their CNS regenerative capacity, whereas its activation in C57BL/6 animals boosts regeneration. This screen demonstrates that mammalian CNS regeneration can occur and reveals a molecular pathway that contributes to this ability.

The synthetic NCAM mimetic peptide FGL mobilizes neural stem cells in vitro and in vivo

The neural cell adhesion molecule (NCAM) plays a role in neurite outgrowth, synaptogenesis, and neuronal differentiation. The NCAM mimetic peptide FG Loop (FGL) promotes neuronal survival in vitro and enhances spatial learning and memory in rats. We here investigated the effects of FGL on neural stem cells (NSC) in vitro and in vivo. In vitro, cell proliferation of primary NSC was assessed after exposure to various concentrations of NCAM or FGL. The differentiation potential of NCAM- or FGL-treated cells was assessed immunocytochemically. To investigate its influence on endogenous NSC in vivo, FGL was injected subcutaneously into adult rats. The effects on NSC mobilization were studied both via non-invasive positron emission tomography (PET) imaging using the tracer [(18)F]-fluoro-L-thymidine ([(18)F]FLT), as well as with immunohistochemistry. Only FGL significantly enhanced NSC proliferation in vitro, with a maximal effect at 10 μg/ml. During differentiation, NCAM promoted neurogenesis, while FGL induced an oligodendroglial phenotype; astrocytic differentiation was neither affected by NCAM or FGL. Those differential effects of NCAM and FGL on differentiation were mediated through different receptors. After FGL-injection in vivo, proliferative activity of NSC in the subventricular zone (SVZ) was increased (compared to placebo-treated animals). Moreover, non-invasive imaging of cell proliferation using [(18)F]FLT-PET supported an FGL-induced mobilization of NSC from both the SVZ and the hippocampus. We conclude that FGL robustly induces NSC mobilization in vitro and in vivo, and supports oligodendroglial differentiation. This capacity renders FGL a promising agent to facilitate remyelinization, which may eventually make FGL a drug candidate for demyelinating neurological disorders.

Modulating pathological oscillations by rhythmic non-invasive brain stimulation-a therapeutic concept?

A large amount of studies of the last decades revealed an association between human behavior and oscillatory activity in the human brain. Alike, abnormalities of oscillatory activity were related with pathological behavior in many neuropsychiatric disorders, such as in Parkinson’s disease (PD) or in schizophrenia (SCZ). As a therapeutic tool, non-invasive brain stimulation (NIBS) has demonstrated the potential to improve behavioral performance in patients suffering from neuropsychiatric disorders. Since evidence accumulates that NIBS might be able to modulate oscillatory activity and related behavior in a scientific setting, this review focuses on discussing potential interventional strategies to target abnormalities in oscillatory activity in neuropsychiatric disorders. In particular, we will review oscillatory changes described in patients after stroke, with PD or suffering from SCZ. Potential ways of targeting interventionally the underlying pathological oscillations to improve related pathological behavior will be further discussed.

Optogenetic mapping after stroke reveals network-wide scaling of functional connections and heterogeneous recovery of the peri-infarct

We used arbitrary point channelrhodopsin-2 (ChR2) stimulation and wide-scale voltage sensitive dye (VSD) imaging in mice to map altered cortical connectivity at 1 and 8 weeks after a targeted cortical stroke. Network analysis based on optogenetic stimulation revealed a symmetrical sham network with distinct sensorimotor and association groupings. This symmetry was disrupted after stroke: at 1 week after stroke, we observed a widespread depression of optogenetically evoked activity that extended to the non-injured hemisphere; by 8 weeks, significant recovery was observed. When we considered the network as a whole, scaling the ChR2-evoked VSD responses from the stroke groups to match the sham group mean resulted in a relative distribution of responses that was indistinguishable from the sham group, suggesting network-wide down-scaling and connectional diaschisis after stroke. Closer inspection revealed that connections that had little connectivity with the peri-infarct, such as contralateral visual areas, tended to escape damage, whereas some connections near the peri-infarct were more severely affected. When connections within the peri-infarct were isolated, we did not observe equal down-scaling of responses after stroke. Peri-infarct sites that had weak connection strength in the sham condition tended to have the greatest relative post-stroke recovery. Our findings suggest that, during recovery, most cortical areas undergo homeostatic upscaling, resulting in a relative distribution of responses that is similar to the pre-stroke (sham) network, albeit still depressed. However, recovery within the peri-infarct zone is heterogeneous and these cortical points do not follow the recovery scaling factor expected for the entire network.

Stroke and the connectome: how connectivity guides therapeutic intervention

Connections between neurons are affected within 3 min of stroke onset by massive ischemic depolarization and then delayed cell death. Some connections can recover with prompt reperfusion; others associated with the dying infarct do not. Disruption in functional connectivity is due to direct tissue loss and indirect disconnections of remote areas known as diaschisis. Stroke is devastating, yet given the brain's redundant design, collateral surviving networks and their connections are well-positioned to compensate. Our perspective is that new treatments for stroke may involve a rational functional and structural connections-based approach. Surviving, affected, and at-risk networks can be identified and targeted with scenario-specific treatments. Strategies for recovery may include functional inhibition of the intact hemisphere, rerouting of connections, or setpoint-mediated network plasticity. These approaches may be guided by brain imaging and enabled by patient- and injury-specific brain stimulation, rehabilitation, and potential molecule-based strategies to enable new connections.

The Ephrin-A5/EphA4 Interaction Modulates Neurogenesis and Angiogenesis by the p-Akt and p-ERK Pathways in a Mouse Model of TLE

Studies have shown that neurogenesis and angiogenesis do exist in temporal lobe epilepsy (TLE). The ephrin ligands and Eph receptors are the largest members of receptor tyrosine kinases, and their interaction via cell-cell contact participates in cell proliferation, differentiation, migration, and tissue remodeling. However, there is little information about the function of the ephrin-A5/EphA4 complex in TLE. In the current study, we found that ephrin-A5 was expressed in astrocytes, while EphA4 existed in endothelial cells in the hippocampus in a mouse model of TLE. Furthermore, the messenger RNA (mRNA) and protein levels of both ephrin-A5 and EphA4 and the binding capacity of ephrin-A5/EphA4 showed gradual increase in spatiotemporal course. When ephrin-A5-Fc was injected into the hippocampus at 3 days post-status epilepticus (SE) for 7 days, the spontaneous recurrent seizure (SRS) frequency and intensity of the mice attenuated in the following 2 weeks. Furthermore, doublecortin-positive neuronal progenitor cells were reduced in the subgranular zone, and the density of microvessels decreased in the hilus. The molecular mechanism was attributed to ephrin-A5-Fc-induced inhibition of phosphorylated ERK (p-ERK) and phosphorylated Akt (p-Akt), and also EphA4 and VEGF reduction. In summary, interaction between ephrin-A5 and EphA4 could mediate the ERK and Akt signaling pathways in pilocarpine-induced epilepsy, and intervention of the ephrin/Eph interaction may play an essential role in the suppression of newborn neuron generation, microvessel remodeling, and SRS in a mouse model of TLE. The ephrin-A5/EphA4 communication may provide a potential therapy for the treatment of TLE.

Astrocyte reactivity and reactive astrogliosis: costs and benefits

Astrocytes are the most abundant cells in the central nervous system (CNS) that provide nutrients, recycle neurotransmitters, as well as fulfill a wide range of other homeostasis maintaining functions. During the past two decades, astrocytes emerged also as increasingly important regulators of neuronal functions including the generation of new nerve cells and structural as well as functional synapse remodeling. Reactive gliosis or reactive astrogliosis is a term coined for the morphological and functional changes seen in astroglial cells/astrocytes responding to CNS injury and other neurological diseases. Whereas this defensive reaction of astrocytes is conceivably aimed at handling the acute stress, limiting tissue damage, and restoring homeostasis, it may also inhibit adaptive neural plasticity mechanisms underlying recovery of function. Understanding the multifaceted roles of astrocytes in the healthy and diseased CNS will undoubtedly contribute to the development of treatment strategies that will, in a context-dependent manner and at appropriate time points, modulate reactive astrogliosis to promote brain repair and reduce the neurological impairment.

Delivery of iPS-NPCs to the Stroke Cavity within a Hyaluronic Acid Matrix Promotes the Differentiation of Transplanted Cells

Stroke is the leading cause of adult disability with ~80% being ischemic. Stem cell transplantation has been shown to improve functional recovery. However, the overall survival and differentiation of these cells is still low. The infarct cavity is an ideal location for transplantation as it is directly adjacent to the highly plastic peri-infarct region. Direct transplantation of cells near the infarct cavity has resulted in low cell viability. Here we deliver neural progenitor cells derived from induce pluripotent stem cells (iPS-NPC) to the infarct cavity of stroked mice encapsulated in a hyaluronic acid hydrogel matrix to protect the cells. To improve the overall viability of transplanted cells, each step of the transplantation process was optimized. Hydrogel mechanics and cell injection parameters were investigated to determine their effects on the inflammatory response of the brain and cell viability, respectively. Using parameters that balanced the desire to keep surgery invasiveness minimal and cell viability high, iPS-NPCs were transplanted to the stroke cavity of mice encapsulated in buffer or the hydrogel. While the hydrogel did not promote stem cell survival one week post-transplantation, it did promote differentiation of the neural progenitor cells to neuroblasts.

Ophthalmic Uses of a Thiol-Modified Hyaluronan-Based Hydrogel

Significance: Hyaluronic acid (HA, or hyaluronan) is a ubiquitous naturally occurring polysaccharide that plays a role in virtually all tissues in vertebrate organisms. HA-based hydrogels have wound-healing properties, support cell delivery, and can deliver drugs locally. Recent Advances: A few HA hydrogels can be customized for composition, physical form, and biomechanical properties. No clinically approved HA hydrogel allows for in vivo crosslinking on administration, has a tunable gelation time to meet wound-healing needs, or enables drug delivery. Recently, a thiolated carboxymethyl HA (CMHA-S) was developed to produce crosslinked hydrogels, sponges, and thin films. CMHA-S can be crosslinked with a thiol-reactive crosslinker or by oxidative disulfide bond formation to form hydrogels. By controlled crosslinking, the shape and form of this material can be manipulated. These hydrogels can be subsequently lyophilized to form sponges or air-dried to form thin films. CMHA-S films, liquids, and gels have been shown to be effective in vivo for treating various injuries and wounds in the eye in veterinary use, and are in clinical development for human use. Critical Issues: Better clinical therapies are needed to treat ophthalmic injuries. Corneal wounds can be treated using this HA-based crosslinked hydrogel. CMHA-S biomaterials can help heal ocular surface defects, can be formed into a film to deliver drugs for local ocular drug delivery, and could deliver autologous limbal stem cells to treat extreme ocular surface damage associated with limbal stem cell deficiencies. Future Directions: This CMHA-S hydrogel increases the options that could be available for improved ocular wound care, healing, and regenerative medicine.

Eph receptors and ephrins: therapeutic opportunities

The erythropoietin-producing hepatocellular carcinoma (Eph) receptor tyrosine kinase family plays important roles in developmental processes, adult tissue homeostasis, and various diseases. Interaction with Eph receptor-interacting protein (ephrin) ligands on the surface of neighboring cells triggers Eph receptor kinase-dependent signaling. The ephrins can also transmit signals, leading to bidirectional cell contact-dependent communication. Moreover, Eph receptors and ephrins can function independently of each other through interplay with other signaling systems. Given their involvement in many pathological conditions ranging from neurological disorders to cancer and viral infections, Eph receptors and ephrins are increasingly recognized as attractive therapeutic targets, and various strategies are being explored to modulate their expression and function. Eph receptor/ephrin upregulation in cancer cells, the angiogenic vasculature, and injured or diseased tissues also offer opportunities for Eph/ephrin-based targeted drug delivery and imaging. Thus, despite the challenges presented by the complex biology of the Eph receptor/ephrin system, exciting possibilities exist for therapies exploiting these molecules.

Mouse intracerebral hemorrhage models produce different degrees of initial and delayed damage, axonal sprouting, and recovery

The mechanisms of delayed damage and recovery after intracerebral hemorrhage (ICH) remain poorly defined. Two rodent models of ICH are commonly used: injection of the enzyme collagenase (cICH) and injection of autologous blood (bICH). In mice, we compared the effects of these two models on initial and delayed tissue damage, motor system connections, and behavioral recovery. There is no difference in lesion size between models. Injection of autologous blood causes greater mass effect and early mortality. However, cICH produces greater edema, inflammation, and cell death. Injection of the enzyme collagenase causes greater loss of cortical connections and secondary shrinkage of the striatum. Intracerebral hemorrhage occurs within the motor system connections of the striatum. Mapping of the projections of the forelimb motor area shows a significant sprouting in motor cortex projections only in cICH. Both models of ICH produce deficits in forelimb motor control. Behavioral recovery occurs by 5 weeks in cICH and 9 weeks in bICH. In summary, cICH and bICH differ in almost every facet of initial and delayed stroke pathophysiology, with cICH producing greater initial and secondary tissue damage and greater motor system axonal sprouting than bICH. Motor recovery occurs in both models, suggesting that motor system axonal sprouting in cICH is not causally associated with recovery.

Gene expression profiles of patients with cerebral hematoma following spontaneous intracerebral hemorrhage

The present study aimed to investigate the gene functions and expression profiles in perihematomal (PH) brain regions following spontaneous intracerebral hemorrhage. The gene expression profiles were downloaded from the Gene Expression Omnibus database under accession number GSE24265, which includes 11 brain samples from different regions, including four samples from PH areas, four from contralateral grey matter (CG) and three from contralateral white matter (CW). The gene expression profiles were pre-processed and the differentially expressed genes (DEGs) between PH and CG tissue, and PH and CW tissue were identified using R packages. The expression of genes in different tissues was analyzed by hierarchical clustering. Then, the interaction network between the DEGs was constructed using String software. Finally, Gene Ontology was performed and pathway analysis was conducted using FuncAssociate and Expression Analysis Systematic Explorer to identify the gene function. As a result, 399 DEGs were obtained between PH and CG, and 756 DEGs were identified between PH and CW. There were 35 common DEGs between the two groups. These DEGs may be involved in PH edema by regulating the calcium signaling pathway [calcium channel, voltage-dependent, T-type, α1I subunit, Ca²⁺/calmodulin-dependent protein kinase II α (CAMK2A), ryanodine receptor 2 (RYR2) and inositol 1,4,5-trisphosphate receptor, type 1 (ITPR1)], cell proliferation (sphingosine kinase 1), neuron differentiation (Ephrin-A5) or extracellular matrix-receptor interaction [collagen, type I, α 2, laminin B1 (LAMB1), syndecan 2, fibronectin 1 and integrin α5 (ITGA5)]. A number of genes may cooperate to participate in the same pathway, such as ITPR1-RYR2, CAMK2A-RYR2 and ITGA5-LAMB1 interaction pairs. The present study provides several potential targets to decrease hematoma expansion and alleviate neuronal cell death following spontaneous intracerebral hemorrhage.

Protein tyrosine phosphorylation in the ischemic brain

Cerebral ischemia, a pathological condition in which brain tissue experiences a shortage of cerebral blood flow, is associated with cerebrovascular disease, brain trauma, epilepsy, and cardiac arrest. A reduction in blood flow leaves the brain tissue unsupplied with oxygen and glucose, thus leading to cell death in the ischemic core as well as subsequent peripheral injury in the penumbra. Neurons in the penumbra, where reperfusion occurs, are functionally inactive but still viable. Many biochemical changes, which may lead to neuronal cell death, thereby induce dysfunction of the central nervous system. However, the mechanisms responsible for ischemic stroke-induced cell damage remain to be determined. Protein phosphorylation has been implicated in the regulation of diverse cellular responses in the brain. Initially, tyrosine phosphorylation was considered to be involved in the regulation of cell growth and development. In addition, a variety of synaptic and cellular functions mediated by tyrosine phosphorylation in the brain were found to be associated with relatively high levels of protein tyrosine kinase activity. However, the involvement of this protein tyrosine kinase activity in ischemic cell death is still not fully understood. This review summarizes recent advances dealing with the possible implications of protein tyrosine phosphorylation in the ischemic brain.

The interaction between training and plasticity in the poststroke brain

PURPOSE OF REVIEW

Recovery after stroke can occur either via reductions in impairment or through compensation. Studies in humans and nonhuman animal models show that most recovery from impairment occurs in the first 1-3 months after stroke as a result of both spontaneous reorganization and increased responsiveness to enriched environments and training. Improvement from impairment is attributable to a short-lived sensitive period of postischemic plasticity defined by unique genetic, molecular, physiological, and structural events. In contrast, compensation can occur at any time after stroke. Here, we address both the biology of the brain's postischemic sensitive period and the difficult question of what kind of training (task-specific vs. a stimulating environment for self-initiated exploration of various natural behaviors) best exploits this period.

RECENT FINDINGS

Data suggest that three important variables determine the degree of motor recovery from impairment: the timing, intensity, and approach to training with respect to stroke onset; the unique postischemic plasticity milieu; and the extent of cortical reorganization.

SUMMARY

Future work will need to further characterize the unique interaction between types of training and postischemic plasticity, and find ways to augment and prolong the sensitive period using pharmacological agents or noninvasive brain stimulation.

Use it and/or lose it-experience effects on brain remodeling across time after stroke

The process of brain remodeling after stroke is time- and neural activity-dependent, and the latter makes it inherently sensitive to behavioral experiences. This generally supports targeting early dynamic periods of post-stroke neural remodeling with rehabilitative training (RT). However, the specific neural events that optimize RT effects are unclear and, as such, cannot be precisely targeted. Here we review evidence for, potential mechanisms of, and ongoing knowledge gaps surrounding time-sensitivities in RT efficacy, with a focus on findings from animal models of upper extremity RT. The reorganization of neural connectivity after stroke is a complex multiphasic process interacting with glial and vascular changes. Behavioral manipulations can impact numerous elements of this process to affect function. RT efficacy varies both with onset time and its timing relative to the development of compensatory strategies with the less-affected (nonparetic) hand. Earlier RT may not only capitalize on a dynamic period of brain remodeling but also counter a tendency for compensatory strategies to stamp-in suboptimal reorganization patterns. However, there is considerable variability across injuries and individuals in brain remodeling responses, and some early behavioral manipulations worsen function. The optimal timing of RT may remain unpredictable without clarification of the cellular events underlying time-sensitivities in its effects.

Reactive gliosis and the multicellular response to CNS damage and disease

The CNS is prone to heterogeneous insults of diverse etiologies that elicit multifaceted responses. Acute and focal injuries trigger wound repair with tissue replacement. Diffuse and chronic diseases provoke gradually escalating tissue changes. The responses to CNS insults involve complex interactions among cells of numerous lineages and functions, including CNS intrinsic neural cells, CNS intrinsic nonneural cells, and CNS extrinsic cells that enter from the circulation. The contributions of diverse nonneuronal cell types to outcome after acute injury, or to the progression of chronic disease, are of increasing interest as the push toward understanding and ameliorating CNS afflictions accelerates. In some cases, considerable information is available, in others, comparatively little, as examined and reviewed here.

Is integration and survival of newborn neurons the bottleneck for effective neural repair by endogenous neural precursor cells?

After two decades of research the existence of adult neural precursor cells and the phenomenon of adult neurogenesis is well established. However, there has been little or no effective harnessing of these endogenous cells to promote functional neuronal replacement following neural injury or disease. Neural precursor cells can respond to neural damage by proliferating, migrating to the site of injury, and differentiating into neuronal or glial lineages. However, after a month or so, very few or no newborn neurons can be detected, suggesting that even though neuroblasts are generated, they generally fail to survive as mature neurons and contribute to the local circuitry. Is this lack of survival and integration one of the major bottlenecks that inhibits effective neuronal replacement and subsequent repair of the nervous system following injury or disease? In this perspective article the possibility that this bottleneck can be targeted to enhance the integration and subsequent survival of newborn neurons will be explored and will suggest some possible mechanisms that may need to be modulated for this to occur.

The dual role of astrocyte activation and reactive gliosis

Astrocyte activation and reactive gliosis accompany most of the pathologies in the brain, spinal cord, and retina. Reactive gliosis has been described as constitutive, graded, multi-stage, and evolutionary conserved defensive astroglial reaction [Verkhratsky and Butt (2013) In: Glial Physiology and Pathophysiology]. A well- known feature of astrocyte activation and reactive gliosis are the increased production of intermediate filament proteins (also known as nanofilament proteins) and remodeling of the intermediate filament system of astrocytes. Activation of astrocytes is associated with changes in the expression of many genes and characteristic morphological hallmarks, and has important functional consequences in situations such as stroke, trauma, epilepsy, Alzheimer's disease (AD), and other neurodegenerative diseases. The impact of astrocyte activation and reactive gliosis on the pathogenesis of different neurological disorders is not yet fully understood but the available experimental evidence points to many beneficial aspects of astrocyte activation and reactive gliosis that range from isolation and sequestration of the affected region of the central nervous system (CNS) from the neighboring tissue that limits the lesion size to active neuroprotection and regulation of the CNS homeostasis in times of acute ischemic, osmotic, or other kinds of stress. The available experimental data from selected CNS pathologies suggest that if not resolved in time, reactive gliosis can exert inhibitory effects on several aspects of neuroplasticity and CNS regeneration and thus might become a target for future therapeutic interventions.

Thiolated hyaluronan-based hydrogels crosslinked using oxidized glutathione: an injectable matrix designed for ophthalmic applications

Future ophthalmic therapeutics will require the sustained delivery of bioactive proteins and nucleic acid-based macromolecules and/or provide a suitable microenvironment for the localization and sustenance of reparative progenitor cells after transplantation into or onto the eye. Water-rich hydrogels are ideal vehicles for such cargo, but few have all the qualities desired for novel ophthalmic use, namely in situ gelation speed, cytocompatibility, biocompatibility and capacity to functionalize. We describe here the development of an ophthalmic-compatible crosslinking system using oxidized glutathione (GSSG), a physiologically relevant molecule with a history of safe use in humans. When GSSG is used in conjunction with an existing hyaluronate-based, in situ crosslinkable hydrogel platform, gels form in less than 5 min using the thiol-disulfide exchange reaction. This GSSG hydrogel supports the 3-D culture of adipose-derived stem cells in vitro and shows biocompatibility in preliminary intracutaneous and subconjunctival experiments in vivo. In addition, the thiol-disulfide exchange reaction can also be used in conjunction with other thiol-compatible chemistries to covalently link peptides for more complex formulations. These data suggest that this hydrogel could be well suited for local ocular delivery, focusing initially on front of the eye therapies. Subsequent uses of the hydrogel include delivery of back of the eye treatments and eventually into other soft, hyaluronan-rich tissues such as those from the liver and brain.

Activity-dependent neural plasticity from bench to bedside

Much progress has been made in understanding how behavioral experience and neural activity can modify the structure and function of neural circuits during development and in the adult brain. Studies of physiological and molecular mechanisms underlying activity-dependent plasticity in animal models have suggested potential therapeutic approaches for a wide range of brain disorders in humans. Physiological and electrical stimulations as well as plasticity-modifying molecular agents may facilitate functional recovery by selectively enhancing existing neural circuits or promoting the formation of new functional circuits. Here, we review the advances in basic studies of neural plasticity mechanisms in developing and adult nervous systems and current clinical treatments that harness neural plasticity, and we offer perspectives on future development of plasticity-based therapy.

Astrocytic therapies for neuronal repair in stroke

Stroke is a leading cause of disability and death worldwide. Much of the work on improving stroke recovery has focused on preventing neuronal loss; however, these approaches have repeatedly failed in clinical trials. Conversely, relatively little is known about the mechanisms of repair and recovery after stroke. Stroke causes an initial process of local scar formation that confines the damage, and a later and limited process of tissue repair that involves the formation of new connections and new blood vessels. Astrocytes are central to both scar formation and to tissue repair after stroke. Astrocytes regulate the synapses and blood vessels within their cellular projections, or domain, and both respond to and release neuroimmune molecules in response to damage. Despite this central role in brain function, astrocytes have been largely neglected in the pursuit of effective stroke therapeutics. Here, we will review the changes astrocytes undergo in response to stroke, both beneficial and detrimental, and discuss possible points of intervention to promote recovery.

FTY720 treatment in the convalescence period improves functional recovery and reduces reactive astrogliosis in photothrombotic stroke

BACKGROUND

The Sphingosine-1-phosphate (S1P) signaling pathway is known to influence pathophysiological processes within the brain and the synthetic S1P analog FTY720 has been shown to provide neuroprotection in experimental models of acute stroke. However, the effects of a manipulation of S1P signaling at later time points after experimental stroke have not yet been investigated. We examined whether a relatively late initiation of a FTY720 treatment has a positive effect on long-term neurological outcome with a focus on reactive astrogliosis, synapses and neurotrophic factors.

METHODS

We induced photothrombotic stroke (PT) in adult C57BL/6J mice and allowed them to recover for three days. Starting on post-stroke day 3, mice were treated with FTY720 (1 mg/kg b.i.d.) for 5 days. Behavioral outcome was observed until day 31 after photothrombosis and periinfarct cortical tissue was analyzed using tandem mass-spectrometry, TaqMan®analysis and immunofluorescence.

RESULTS

FTY720 treatment results in a significantly better functional outcome persisting up to day 31 after PT. This is accompanied by a significant decrease in reactive astrogliosis and larger post-synaptic densities as well as changes in the expression of vascular endothelial growth factor α (VEGF α). Within the periinfarct cortex, S1P is significantly increased compared to healthy brain tissue.

CONCLUSION

Besides its known neuroprotective effects in the acute phase of experimental stroke, the initiation of FTY720 treatment in the convalescence period has a positive impact on long-term functional outcome, probably mediated through reduced astrogliosis, a modulation in synaptic morphology and an increased expression of neurotrophic factors.

Age-dependent exacerbation of white matter stroke outcomes: a role for oxidative damage and inflammatory mediators

BACKGROUND AND PURPOSE

Subcortical white matter stroke (WMS) constitutes up to 30% of all stroke subtypes. Mechanisms of oligodendrocyte and axon injury and repair play a central role in the damage and recovery after this type of stroke, and a comprehensive study of these processes requires a specialized experimental model that is different from common large artery, gray matter stroke models. Diminished recovery from stroke in aged patients implies that damage and repair processes are affected by advanced age, but such effects have not been studied in WMS.

METHODS

WMS was produced with focal microinjection of the vasoconstrictor N5-(1-iminoethyl)-L-ornithine into the subcortical white matter ventral to the mouse forelimb motor cortex in young adult (2 months), middle-aged (15 months), and aged mice (24 months).

RESULTS

WMS produced localized oligodendrocyte cell death with higher numbers of apoptotic cells and greater oxidative damage in aged brains than in young-adult brains. Increased expression of monocyte chemotactic protein-1 and tumor necrosis factor-α in motor cortex neurons correlated with a more distributed microglial activation in aged brains 7 days after WMS. At 2 months, aged mice displayed increased white matter atrophy and greater loss of corticostriatal connections compared with young-adult mice. Behavioral testing revealed an age-dependent exacerbation of forelimb motor deficits caused by the stroke, with decreased long-term functional recovery in aged animals.

CONCLUSIONS

Age has a profound effect on the outcome of WMS, with more prolonged cell death and oxidative damage, increased inflammation, greater secondary white matter atrophy, and a worse behavioral effect in aged versus young-adult mice.

Management of patients with stroke: is it time to expand treatment options?

Approximately 700,000 people in the United States have an ischemic stroke annually. Substantial research has tested therapies for the very early treatment of ischemic stroke but, to date, only intravenous thrombolysis and intra-arterial measures to restore perfusion have shown success. Despite a 15-year effort to increase the use of these therapies, only approximately 5% of patients with stroke are currently being treated. Although most patients with stroke have some neurological recovery, more than half of stroke survivors have residual impairments that lead to disability or long-term institutionalized care. Laboratory research has demonstrated several mechanisms that help the brain to recover after a stroke. New pharmacological and cell-based approaches that are known to promote brain plasticity are emerging from laboratory studies and may soon expand the window for stroke treatment to restore function. It is time to build on this knowledge and to translate the understanding of recovery after stroke into the clinical setting. Measures that might augment recovery should become a major focus of clinical research in stroke in the 21st century.

Noninvasive strategies to promote functional recovery after stroke

Stroke is a common and disabling global health-care problem, which is the third most common cause of death and one of the main causes of acquired adult disability in many countries. Rehabilitation interventions are a major component of patient care. In the last few years, brain stimulation, mirror therapy, action observation, or mental practice with motor imagery has emerged as interesting options as add-on interventions to standard physical therapies. The neural bases for poststroke recovery rely on the concept of plasticity, namely, the ability of central nervous system cells to modify their structure and function in response to external stimuli. In this review, we will discuss recent noninvasive strategies employed to enhance functional recovery in stroke patients and we will provide an overview of neural plastic events associated with rehabilitation in preclinical models of stroke.

Plasticity in the Injured Brain

Changes in brain circuits occur within specific paradigms of action in the adult brain. These paradigms include changes in behavioral activity patterns, alterations in environmental experience, and direct brain injury. Each of these paradigms can produce axonal sprouting, dendritic morphology changes, and alterations in synaptic connectivity. Activity-, experience-, and injury-dependent plasticity alter neuronal network function and behavioral output, and in the case of brain injury, may produce neurological recovery. The molecular substrate for adult neuronal plasticity overlaps in these three paradigms in key signaling pathways. These common pathways for adult plasticity suggest common mechanisms for activity-, experience-, and injury-dependent plasticity. These common pathways may also interact to enhance or impede each other during adult recovery of function after injury. This review focuses on common molecular changes evoked during the process of adult neuronal plasticity, with a focus on neural repair in stroke.

Motor system plasticity in stroke models: intrinsically use-dependent, unreliably useful

Background and Purpose

The natural response to disability in one limb is to learn new ways of using the other limb. This compensatory behavioral strategy after stroke has long been thought to contribute to persistent dysfunction in the paretic limb by encouraging its disuse. Our recent findings suggest that it goes beyond the encouragement of disuse to disrupt neural substrates of paretic limb functional improvements.

Methods

We overview recent findings from rodent models of chronic upper extremity impairments in which precise control and manipulation of forelimb experiences were used to understand bilateral and interhemispheric contributions to motor functional outcome.

Results

Skill learning with the less-affected (nonparetic) forelimb promotes neural plasticity in the contralesional motor cortex that subserves its function. At the same time, it exacerbates dysfunction and limits the efficacy of rehabilitative training in the paretic limb. The maladaptive effects of skill learning with the nonparetic forelimb are dependent on callosal connections and contralesional motor cortex, and linked with reduced neural activation of peri-infarct motor cortex during rehabilitative training.

Conclusions

These findings suggest that learning to rely on the nonparetic body side has the capacity to disrupt functionality in a region of the injured hemisphere that contributes to outcome of the paretic limb. Whether this effect generalizes across injury loci and functional modalities remains to be tested.

Multimodal examination of structural and functional remapping in the mouse photothrombotic stroke model

Recent studies show a limited capacity for neural repair after stroke, which includes remapping of sensorimotor functions and sprouting of new connections. However, physiologic and connectional plasticity of sensory maps during long-term functional recovery in the mouse have not been determined. Using a photothrombotic stroke model, we targeted the motor cortex, which we show results in lasting behavioral deficits on the grid-walking and in the cylinder tasks out to 8 weeks after stroke. Mice recovered performance in a skilled reaching task, showing no deficit from week 2 after stroke. Long-term optical intrinsic signal imaging revealed functional reorganization of sensory cortical maps for both forelimb and hindlimb, with more diffuse sensory physiologic maps. There was a small but significant increase in motor neuron projections within the areas of functional cortical reorganization as assessed using the neuroanatomic tracer biotinylated dextran amine. These findings show that the sensorimotor cortex undergoes remapping of cortical functions and axonal sprouting within the same regions during recovery after stroke. This suggests a linked structural and physiologic plasticity underlying recovery. Combined long-term structural and functional mapping after stroke in the mouse is practical and provides a rich data set for mechanistic analysis of stroke recovery.

Not just a rush of blood to the head

Angiogenesis is a key feature of central nervous system injury. A neovessel-derived signal mediated by prostacyclin triggers axonal sprouting and functional recovery in a mouse model of inflammatory spinal cord injury (pages 1658-1664). Are such angiocrine signals relevant to neurovascular remodeling and recovery in other neurological contexts?

Modifying expression of EphA4 and its downstream targets improves functional recovery after stroke

Functional recovery after stroke varies greatly between patients, potentially due to differences in gene expression. Several processes like angiogenesis, neurogenesis, axonal reorganization and synaptic plasticity act in concert to restore neurological functions. The ephrin family has known roles in all these processes. EphA4 is the most abundant ephrin receptor in the nervous system. Therefore, we investigated whether EphA4 affects functional recovery from stroke, and evaluated the potential of this receptor as a therapeutic target. Motor recovery after photothrombotic stroke was studied in transgenic mice in which expression of EphA4 was reduced. Furthermore, blocking a downstream target of EphA4, ROCK (Rho-associated kinase), by two different compounds was evaluated in the same model. Motor recovery after photothrombotic stroke was markedly enhanced in transgenic mice with reduced levels of EphA4, whereas infarct sizes were similar compared with non-transgenic controls. Pharmacological inhibition of the EphA4 signaling cascade using two ROCK inhibitors,Y-27632 and fasudil, improved motor function of mice after stroke. Infarct size was comparable in all groups studied, suggesting that the benefit obtained by EphA4 inhibition is not neuroprotective in nature but due to an effect on the mechanisms underlying recovery. Our findings show that reduction of EphA4 improves motor function after experimental stroke and demonstrate that ROCK inhibition is a promising therapeutic strategy to enhance recovery after ischemic stroke.

Neuronal restoration following ischemic stroke: influences, barriers, and therapeutic potential

Neurogenesis, the birth of new neurons, occurs throughout life in the subventricular zone and produces immature neurons that migrate tangentially through the rostral migratory stream to the olfactory bulb. This migration is tightly regulated by both structural and chemical influences. Interestingly, brain insults such as ischemic stroke increase neurogenesis and redirect neuroblast migration to the injury site. This injury-redirected neurogenesis and migration is coupled with angiogenic vasculature and is influenced by many of the factors that positively and negatively affect migration under developmental or normal adult conditions. Additionally, cytokines and chemokines such as stromal cell-derived factor-1 strongly influence neuronal migration poststroke. However, neuronal repopulation or brain regeneration is extremely limited. This limitation may potentially be due to the hostile poststroke microenvironment including the formation of the physical and chemical barriers of glial scar. Furthermore, interspecies differences in poststroke neurogenesis between rodents and humans complicate the translation of experimental results to humans. Despite these challenges, many drugs and other potential therapies have recently been evaluated for potential neurogenic properties poststroke. Improved understanding of poststroke neurorepair may lead to new and more effective neurorestorative therapies.

Regulation of endogenous neural stem/progenitor cells for neural repair-factors that promote neurogenesis and gliogenesis in the normal and damaged brain

Neural stem/precursor cells in the adult brain reside in the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) of the dentate gyrus in the hippocampus. These cells primarily generate neuroblasts that normally migrate to the olfactory bulb (OB) and the dentate granule cell layer respectively. Following brain damage, such as traumatic brain injury, ischemic stroke or in degenerative disease models, neural precursor cells from the SVZ in particular, can migrate from their normal route along the rostral migratory stream (RMS) to the site of neural damage. This neural precursor cell response to neural damage is mediated by release of endogenous factors, including cytokines and chemokines produced by the inflammatory response at the injury site, and by the production of growth and neurotrophic factors. Endogenous hippocampal neurogenesis is frequently also directly or indirectly affected by neural damage. Administration of a variety of factors that regulate different aspects of neural stem/precursor biology often leads to improved functional motor and/or behavioral outcomes. Such factors can target neural stem/precursor proliferation, survival, migration and differentiation into appropriate neuronal or glial lineages. Newborn cells also need to subsequently survive and functionally integrate into extant neural circuitry, which may be the major bottleneck to the current therapeutic potential of neural stem/precursor cells. This review will cover the effects of a range of intrinsic and extrinsic factors that regulate neural stem/precursor cell functions. In particular it focuses on factors that may be harnessed to enhance the endogenous neural stem/precursor cell response to neural damage, highlighting those that have already shown evidence of preclinical effectiveness and discussing others that warrant further preclinical investigation.

Remodeling of the axon initial segment after focal cortical and white matter stroke

BACKGROUND AND PURPOSE

Recovery from stroke requires neuroplasticity within surviving adjacent cortex. The axon initial segment (AIS) is the site of action potential initiation and a focal point for tuning of neuronal excitability. Remodeling of the AIS may be important to neuroplasticity after stroke.

METHODS

Focal cortical stroke in forelimb motor cortex was induced by photothrombosis and compared with sham controls. White matter stroke was produced through stereotactic injection of a vasoconstrictor together with biotinylated dextran amine to retrogradely label injured cortical neurons. AIS length, morphology and number were measured using immunofluorescence and confocal microscopy 2 weeks after stroke.

RESULTS

Within the peri-infarct cortex and after white matter stroke, AIS length decreases. This shortening is accompanied by altered AIS morphology. In peri-infarct cortex, the decrease in AIS length after stroke occurs from the distal end of the AIS, resulting in a Nav1.6. γ-aminobutyric acid type A receptor-α2 subunit staining at axoaxonic synapses along the AIS is significantly decreased. In addition, a significant increase in small, immature initial segments is present in layers 2/3 of peri-infarct cortex, reflecting maturation of axonal sprouting and new initial segments from surviving neurons.

CONCLUSIONS

Stroke alters the compartmental morphology of surviving adjacent neurons in peri-infarct cortex and in neurons whose distal axons are injured by white matter stroke. With a key role in modulation of neuronal excitability, these changes at the AIS may contribute to altered neuronal excitability after injury and prove crucial to increasing neuroplasticity in surviving tissue affected by stroke.

Phosphorylation and assembly of glutamate receptors after brain ischemia

BACKGROUND AND PURPOSE

Overassembly of synaptic glutamate receptors leads to excitotoxicity. The goal of this study is to investigate phosphorylation and assembly of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and N-methyl-D-aspartate receptors after brain ischemia with reperfusion (I/R).

METHODS

Rats were subjected to 15 minutes of global ischemia followed by 0.5, 4, and 24 hours of reperfusion. Phosphotyrosine peptides of glutamate receptors in synaptosomal fraction after I/R were identified and quantified by state-of-the-art immuno-affinity purification of phosphotyrosine peptides followed by liquid chromatography/mass spectrometry/mass spectrometry analysis (immunoaffinity purification-coupled liquid chromatography/mass spectrometry/mass spectrometry). Glutamate receptor phosphorylation and synaptic assembly after I/R were studied by biochemical methods.

RESULTS

Numerous phosphotyrosine-sites of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and N-methyl-D-aspartate were upregulated by approximately 2- to 37-fold after I/R. A core glutamate receptor kinase, Src kinase, was significantly activated. GluR2/3 and NR2A/B were rapidly clustered from extrasynaptic to synaptic membrane fractions after I/R. GluR2/3 was then translocated into the intracellular pool, whereas NR2A/B remained in the synaptic fraction for as long as 24 hours. Consistently, trafficking-related phosphorylation of GluR2/3-S880 was significantly but transiently upregulated, whereas NR2A/B-Y1246 and NR2A/B-Y1472 were significantly and persistently upregulated after I/R.

CONCLUSIONS

Phosphorylation of glutamate receptors at synapses may lead to overassembly of glutamate receptors, probably via activation of Src family kinases, after I/R. This study provides global proteomic information about glutamate receptor tyrosine phosphorylation after brain ischemia.