Coordinated Plasticity of Synapses and Astrocytes Underlies Practice-Driven Functional Vicariation in Peri-Infarct Motor Cortex

In vivo calibration of genetically encoded metabolite biosensors must account for metabolite metabolism during calibration and cellular volume

Isotopic assays of brain glucose utilization rates have been used for more than four decades to establish relationships between energetics, functional activity, and neurotransmitter cycling. Limitations of these methods include the relatively long time (1-60 min) for the determination of labeled metabolite levels and the lack of cellular resolution. Identification and quantification of fuels for neurons and astrocytes that support activation and higher brain functions are a major, unresolved issues. Glycolysis is preferentially up-regulated during activation even though oxygen level and supply are adequate, causing lactate concentrations to quickly rise during alerting, sensory processing, cognitive tasks, and memory consolidation. However, the fate of lactate (rapid release from brain or cell-cell shuttling coupled with local oxidation) is long disputed. Genetically encoded biosensors can determine intracellular metabolite concentrations and report real-time lactate level responses to sensory, behavioral, and biochemical challenges at the cellular level. Kinetics and time courses of cellular lactate concentration changes are informative, but accurate biosensor calibration is required for quantitative comparisons of lactate levels in astrocytes and neurons. An in vivo calibration procedure for the Laconic lactate biosensor involves intracellular lactate depletion by intravenous pyruvate-mediated trans-acceleration of lactate efflux followed by sensor saturation by intravenous infusion of high doses of lactate plus ammonium chloride. In the present paper, the validity of this procedure is questioned because rapid lactate-pyruvate interconversion in blood, preferential neuronal oxidation of both monocarboxylates, on-going glycolytic metabolism, and cellular volumes were not taken into account. Calibration pitfalls for the Laconic lactate biosensor also apply to other metabolite biosensors that are standardized in vivo by infusion of substrates that can be metabolized in peripheral tissues. We discuss how technical shortcomings negate the conclusion that Laconic sensor calibrations support the existence of an in vivo astrocyte-neuron lactate concentration gradient linked to lactate shuttling from astrocytes to neurons to fuel neuronal activity.

Subventricular zone cytogenesis provides trophic support for neural repair in a mouse model of stroke

Stroke enhances proliferation of neural precursor cells within the subventricular zone (SVZ) and induces ectopic migration of newborn cells towards the site of injury. Here, we characterize the identity of cells arising from the SVZ after stroke and uncover a mechanism through which they facilitate neural repair and functional recovery. With genetic lineage tracing, we show that SVZ-derived cells that migrate towards cortical photothrombotic stroke in mice are predominantly undifferentiated precursors. We find that ablation of neural precursor cells or conditional knockout of VEGF impairs neuronal and vascular reparative responses and worsens recovery. Replacement of VEGF is sufficient to induce neural repair and recovery. We also provide evidence that CXCL12 from peri-infarct vasculature signals to CXCR4-expressing cells arising from the SVZ to direct their ectopic migration. These results support a model in which vasculature surrounding the site of injury attracts cells from the SVZ, and these cells subsequently provide trophic support that drives neural repair and recovery.

Spared Premotor Areas Undergo Rapid Nonlinear Changes in Functional Organization Following a Focal Ischemic Infarct in Primary Motor Cortex of Squirrel Monkeys

Recovery of motor function after stroke is accompanied by reorganization of movement representations in spared cortical motor regions. It is widely assumed that map reorganization parallels recovery, suggesting a causal relationship. We examined this assumption by measuring changes in motor representations in eight male and six female squirrel monkeys in the first few weeks after injury, a time when motor recovery is most rapid. Maps of movement representations were derived using intracortical microstimulation techniques in primary motor cortex (M1), ventral premotor cortex (PMv), and dorsal premotor cortex (PMd) in 14 adult squirrel monkeys before and after a focal infarct in the M1 distal forelimb area. Maps were derived at baseline and at either 2 (n = 7) or 3 weeks (n = 7) postinfarct. In PMv the forelimb maps remained unchanged at 2 weeks but contracted significantly (-42.4%) at 3 weeks. In PMd the forelimb maps expanded significantly (+110.6%) at 2 weeks but contracted significantly (-57.4%) at 3 weeks. Motor deficits were equivalent at both time points. These results highlight two features of plasticity after M1 lesions. First, significant contraction of distal forelimb motor maps in both PMv and PMd is evident by 3 weeks. Second, an unpredictable nonlinear pattern of reorganization occurs in the distal forelimb representation in PMd, first expanding at 2 weeks, and then contracting at 3 weeks postinjury. Together with previous results demonstrating reliable map expansions in PMv several weeks to months after M1 injury, the subacute time period may represent a critical window for the timing of therapeutic interventions.SIGNIFICANCE STATEMENT The relationship between motor recovery and motor map reorganization after cortical injury has rarely been examined in acute/subacute periods. In nonhuman primates, premotor maps were examined at 2 and 3 weeks after injury to primary motor cortex. Although maps are known to expand late after injury, the present study demonstrates early map expansion at 2 weeks (dorsal premotor cortex) followed by contraction at 3 weeks (dorsal and ventral premotor cortex). This nonlinear map reorganization during a time of gradual behavioral recovery suggests that the relationship between map plasticity and motor recovery is much more complex than previously thought. It also suggests that rehabilitative motor training may have its most potent effects during this early dynamic phase of map reorganization.

Rodent Stroke Models to Study Functional Recovery and Neural Repair

Rodent ischemic stroke models are essential research tools for studying this highly prevalent disease and represent a critical element in the translational pipeline for development of new therapies. The majority of ischemic stroke models have been developed to study the acute phase of the disease and neuroprotective strategies, but a subset of models is better suited for studying stroke recovery. Each model therefore has characteristics that lend itself to certain types of investigations and outcome measures, and it is important to consider both explicit and implicit details when designing experiments that utilize each model. The following chapter briefly summarizes the known aspects of the main rodent stroke models with emphasis on their clinical relevance and suitability for studying recovery and neural repair following stroke.

StrokeRehab: A Benchmark Dataset for Sub-second Action Identification

Automatic action identification from video and kinematic data is an important machine learning problem with applications ranging from robotics to smart health. Most existing works focus on identifying coarse actions such as running, climbing, or cutting vegetables, which have relatively long durations and a complex series of motions. This is an important limitation for applications that require identification of more elemental motions at high temporal resolution. For example, in the rehabilitation of arm impairment after stroke, quantifying the training dose (number of repetitions) requires differentiating motions with sub-second durations. Our goal is to bridge this gap. To this end, we introduce a large-scale, multimodal dataset, StrokeRehab, as a new action-recognition benchmark that includes elemental short-duration actions labeled at a high temporal resolution. StrokeRehab consists of high-quality inertial measurement unit sensor and video data of 51 stroke-impaired patients and 20 healthy subjects performing activities of daily living like feeding, brushing teeth, etc. Because it contains data from both healthy and impaired individuals, StrokeRehab can be used to study the influence of distribution shift in action-recognition tasks. When evaluated on StrokeRehab, current state-of-the-art models for action segmentation produce noisy predictions, which reduces their accuracy in identifying the corresponding sequence of actions. To address this, we propose a novel approach for high-resolution action identification, inspired by speech-recognition techniques, which is based on a sequence-to-sequence model that directly predicts the sequence of actions. This approach outperforms current state-of-the-art methods on StrokeRehab, as well as on the standard benchmark datasets 50Salads, Breakfast, and Jigsaws.

Motor Cortex Plasticity During Functional Recovery Following Brain Damage

Although brain damage causes functional impairment, it is often followed by partial or total recovery of function. Recovery is believed to occur primarily because of brain plasticity. Both human and animal studies have significantly contributed to uncovering the neuronal basis of plasticity. Recent advances in brain imaging technology have enabled the investigation of plastic changes in living human brains. In addition, animal experiments have revealed detailed changes at the neural and genetic levels. In this review, plasticity in motor-related areas of the cerebral cortex, which is one of the most well-studied areas of the neocortex in terms of plasticity, is reviewed. In addition, the potential of technological interventions to enhance plasticity and promote functional recovery following brain damage is discussed. Novel neurorehabilitation technologies are expected to be established based on the emerging research on plasticity from the last several decades.

Fear memory-associated synaptic and mitochondrial changes revealed by deep learning-based processing of electron microscopy data

Serial section electron microscopy (ssEM) can provide comprehensive 3D ultrastructural information of the brain with exceptional computational cost. Targeted reconstruction of subcellular structures from ssEM datasets is less computationally demanding but still highly informative. We thus developed a region-CNN-based deep learning method to identify, segment, and reconstruct synapses and mitochondria to explore the structural plasticity of synapses and mitochondria in the auditory cortex of mice subjected to fear conditioning. Upon reconstructing over 135,000 mitochondria and 160,000 synapses, we find that fear conditioning significantly increases the number of mitochondria but decreases their size and promotes formation of multi-contact synapses, comprising a single axonal bouton and multiple postsynaptic sites from different dendrites. Modeling indicates that such multi-contact configuration increases the information storage capacity of new synapses by over 50%. With high accuracy and speed in reconstruction, our method yields structural and functional insight into cellular plasticity associated with fear learning.

Present and future avenues of cell-based therapy for brain injury: The enteric nervous system as a potential cell source

Cell therapy is a promising strategy in the field of regenerative medicine; however, several concerns limit the effective clinical use, namely a valid cell source. The gastrointestinal tract, which contains a highly organized network of nerves called the enteric nervous system (ENS), is a valuable reservoir of nerve cells. Together with neurons and neuronal precursor cells, it contains glial cells with a well described neurotrophic potential and a newly identified neurogenic one. Recently, enteric glia is looked at as a candidate for cell therapy in intestinal neuropathies. Here, we present the therapeutic potential of the ENS as cell source for brain repair, too. The example of stroke is introduced as a brain injury where cell therapy appears promising. This disease is the first cause of handicap in adults. The therapies developed in recent years allow a partial response to the consequences of the disease. The only prospect of recovery in the chronic phase is currently based on rehabilitation. The urgency to offer other treatments is therefore tangible. In the first part of the review, some elements of stroke pathophysiology are presented. An update on the available therapeutic strategies is provided, focusing on cell‐ and biomaterial‐based approaches. Following, the ENS is presented with its anatomical and functional characteristics, focusing on glial cells. The properties of these cells are depicted, with particular attention to their neurotrophic and, recently identified, neurogenic properties. Finally, preliminary data on a possible therapeutic approach combining ENS‐derived cells and a biomaterial are presented.

PrimSeq: A deep learning-based pipeline to quantitate rehabilitation training

Stroke rehabilitation seeks to accelerate motor recovery by training functional activities, but may have minimal impact because of insufficient training doses. In animals, training hundreds of functional motions in the first weeks after stroke can substantially boost upper extremity recovery. The optimal quantity of functional motions to boost recovery in humans is currently unknown, however, because no practical tools exist to measure them during rehabilitation training. Here, we present PrimSeq, a pipeline to classify and count functional motions trained in stroke rehabilitation. Our approach integrates wearable sensors to capture upper-body motion, a deep learning model to predict motion sequences, and an algorithm to tally motions. The trained model accurately decomposes rehabilitation activities into elemental functional motions, outperforming competitive machine learning methods. PrimSeq furthermore quantifies these motions at a fraction of the time and labor costs of human experts. We demonstrate the capabilities of PrimSeq in previously unseen stroke patients with a range of upper extremity motor impairment. We expect that our methodological advances will support the rigorous measurement required for quantitative dosing trials in stroke rehabilitation.

Stroke commonly damages motor function in the upper extremity (UE), leading to long-term disability and loss of independence in a majority of individuals. Rehabilitation seeks to restore function by training daily activities, which deliver repeated UE functional motions. The optimal number of functional motions necessary to boost recovery is unknown. This gap stems from the lack of measurement tools to feasibly count functional motions. We thus developed the PrimSeq pipeline to enable the accurate and rapid counting of building-block functional motions, called primitives. PrimSeq uses wearable sensors to capture rich motion information from the upper body, and custom-built algorithms to detect and count functional primitives in this motion data. We showed that our deep learning algorithm precisely counts functional primitives performed by stroke patients and outperformed other benchmark algorithms. We also showed patients tolerated the wearable sensors and that the approach is 366 times faster at counting primitives than humans. PrimSeq thus provides a precise and practical means of quantifying functional primitives, which promises to advance stroke research and clinical care and to improve the outcomes of individuals with stroke.

A multicenter, randomized, double-blind, placebo-controlled trial to test efficacy and safety of transcranial direct current stimulation to the motor cortex after stroke (NETS): study protocol

Introduction

The WHO estimates that each year 5 million people are left permanently disabled after stroke. Adjuvant treatments to promote the effects of rehabilitation are urgently needed. Cortical excitability and neuroplasticity can be enhanced by non-invasive brain stimulation but evidence from sufficiently powered, randomized controlled multi-center clinical trials is absent.

Methods

Neuroregeneration enhanced by transcranial direct current stimulation (tDCS) in stroke (NETS) tested efficacy and safety of anodal tDCS to the primary motor cortex of the lesioned hemisphere in the subacute phase (day 5–45) after cerebral ischemia. Stimulation was combined with standardized rehabilitative training and repeatedly applied in 10 sessions over a period of 2 weeks in a planned sample of 120 patients. Primary outcome parameter was upper-extremity function at the end of the 2-weeks intervention period of active treatment or placebo (1:1 randomization), measured by the upper-extremity Fugl-Meyer assessment. Sustainability of the treatment effect was evaluated by additional follow-up visits after 30 and 90 days. Further secondary endpoints included metrics of arm and hand function, stroke impact scale, and the depression module of the patient health questionnaire.

Perspective

NETS was aimed at providing evidence for an effective and safe adjuvant treatment for patients after stroke.

Trial registration: ClinicalTrials.gov Identifier NCT00909714. Registered May 28, 2009.

Optogenetics stimulates nerve reorganization in the contralesional anterolateral primary motor cortex in a mouse model of ischemic stroke

The anterolateral motor cortex of rodents is an important motor auxiliary area, and its function is similar to that of the premotor area in humans. Activation and inhibition of the contralesional anterolateral motor cortex (cALM) have been shown to have direct effects on motor behavior. However, the significance of cALM activation and inhibition in the treatment of stroke remains unclear. This study investigated the role of optogenetic cALM stimulation in a mouse model of cerebral stroke. The results showed that 21-day optogenetic cALM inhibition, but not activation, improved neurological function. In addition, optogenetic cALM stimulation substantially altered dendritic structural reorganization and dendritic spine plasticity, as optogenetic cALM inhibition resulted in increased dendritic length, number of dendritic spines, and number of perforated synapses, whereas optogenetic activation led to an increase in the number of multiple synapse boutons and the number of dendritic intersections. Furthermore, RNA-seq analysis showed that multiple biological processes regulated by the cALM were upregulated immediately after optogenetic cALM inhibition, and that several immediate-early genes (including cFOS, Erg1, and Sema3f) were expressed at higher levels after optogenetic inhibition than after optogenetic activation. These results were confirmed by quantitative reverse transcription-polymerase chain reaction. Finally, immunofluorescence analysis showed that the c-FOS signal in layer V of the primary motor cortex in the ischemic hemisphere was higher after optogenetic cALM activation than it was after optogenetic cALM inhibition. Taken together, these findings suggest that optogenetic cALM stimulation promotes neural reorganization in the primary motor cortex of the ischemic hemisphere, and that optogenetic cALM inhibition and activation have different effects on neural plasticity. The study was approved by the Experimental Animal Ethics Committee of Fudan University (approval No. 201802173S) on March 3, 2018.

Longitudinal functional imaging of VIP interneurons reveals sup-population specific effects of stroke that are rescued with chemogenetic therapy

Stroke profoundly disrupts cortical excitability which impedes recovery, but how it affects the function of specific inhibitory interneurons, or subpopulations therein, is poorly understood. Interneurons expressing vasoactive intestinal peptide (VIP) represent an intriguing stroke target because they can regulate cortical excitability through disinhibition. Here we chemogenetically augmented VIP interneuron excitability in a murine model of photothrombotic stroke and show that it enhances somatosensory responses and improves recovery of paw function. Using longitudinal calcium imaging, we discovered that stroke primarily disrupts the fidelity (fraction of responsive trials) and predictability of sensory responses within a subset of highly active VIP neurons. Partial recovery of responses occurred largely within these active neurons and was not accompanied by the recruitment of minimally active neurons. Importantly, chemogenetic stimulation preserved sensory response fidelity and predictability in highly active neurons. These findings provide a new depth of understanding into how stroke and prospective therapies (chemogenetics), can influence subpopulations of inhibitory interneurons.

Developmental and Interventional Plasticity of Motor Maps after Perinatal Stroke

Within the perinatal stroke field, there is a need to establish preclinical models where putative biomarkers for motor function can be examined. In a mouse model of perinatal stroke, we evaluated motor map size and movement latency following optogenetic cortical stimulation against three factors of post-stroke biomarker utility: (1) correlation to chronic impairment on a behavioral test battery; (2) amenability to change using a skilled motor training paradigm; and (3) ability to distinguish individuals with potential to respond well to training. Thy1-ChR2-YFP mice received a photothrombotic stroke at postnatal day 7 and were evaluated on a battery of motor tests between days 59 and 70. Following a cranial window implant, mice underwent longitudinal optogenetic motor mapping both before and after 3 weeks of skilled forelimb training. Map size and movement latency of both hemispheres were positively correlated with impaired spontaneous forelimb use, whereas only ipsilesional hemisphere map size was correlated with performance in skilled reaching. Map size and movement latency did not show groupwise changes with training; however, mice with the smallest pretraining map sizes and worst impairments demonstrated the greatest expansion of map size in response to skilled forelimb training. Overall, motor map size showed utility as a potential biomarker for impairment and training-induced modulation in specific individuals. Future assessment of the predictive capacity of post-stroke motor representations for behavioral outcome in animal models opens the possibility of dissecting how plasticity mechanisms contribute to recovery following perinatal stroke.SIGNIFICANCE STATEMENT We investigated the utility of two cortical motor representation measures (motor map size and movement onset latency) as potential biomarkers for post-stroke motor recovery in a mouse model of perinatal stroke. Both motor map size and movement latency were associated with functional recovery after perinatal stroke, with map size showing an additional association between training responsiveness and severity of impairment. Overall, both motor map size and movement onset latency show potential as neurophysiological correlates of recovery. As such, future studies of perinatal stroke rehabilitation and neuromodulation should include these measures to help explain neurophysiological changes that might be occurring in response to treatment.

Reactive astrocytes facilitate vascular repair and remodeling after stroke

Brain injury causes astrocytes to assume a reactive state that is essential for early tissue protection, but how reactive astrocytes affect later reparative processes is incompletely understood. In this study, we show that reactive astrocytes are crucial for vascular repair and remodeling after ischemic stroke in mice. Analysis of astrocytic gene expression data reveals substantial activation of transcriptional programs related to vascular remodeling after stroke. In vivo two-photon imaging provides evidence of astrocytes contacting newly formed vessels in cortex surrounding photothrombotic infarcts. Chemogenetic ablation of a subset of reactive astrocytes after stroke dramatically impairs vascular and extracellular matrix remodeling. This disruption of vascular repair is accompanied by prolonged blood flow deficits, exacerbated vascular permeability, ongoing cell death, and worsened motor recovery. In contrast, vascular structure in the non-ischemic brain is unaffected by focal astrocyte ablation. These findings position reactive astrocytes as critical cellular mediators of functionally important vascular remodeling during neural repair.

Improved Functional Outcome After Peripheral Nerve Stimulation of the Impaired Forelimb Post-stroke

Lack of blood flow to the brain, i.e., ischemic stroke, results in loss of nerve cells and therefore loss of function in the effected brain regions. There is no effective treatment to improve lost function except restoring blood flow within the first several hours. Rehabilitation strategies are widely used with limited success. The purpose of this study was to examine the effect of electrical stimulation on the impaired upper extremity to improve functional recovery after stroke. We developed a rodent model using an electrode cuff implant onto a single peripheral nerve (median nerve) of the paretic forelimb and applied daily electrical stimulation. The skilled forelimb reaching test was used to evaluate functional outcome after stroke and electrical stimulation. Anterograde axonal tracing from layer V pyramidal neurons with biotinylated dextran amine was done to evaluate the formation of new neuronal connections from the contralesional cortex to the deafferented spinal cord. Rats receiving electrical stimulation on the median nerve showed significant improvement in the skilled forelimb reaching test in comparison with stroke only and stroke with sham stimulation. Rats that received electrical stimulation also exhibited significant improvement in the latency to initiate adhesive removal from the impaired forelimb, indicating better sensory recovery. Furthermore, axonal tracing analysis showed a significant higher midline fiber crossing index in the cervical spinal cord of rats receiving electrical stimulation. Our results indicate that direct peripheral nerve stimulation leads to improved sensorimotor recovery in the stroke-impaired forelimb, and may be a useful approach to improve post-stroke deficits in human patients.

Environmental Enrichment Enhances Cav 2.1 Channel-Mediated Presynaptic Plasticity in Hypoxic-Ischemic Encephalopathy

Hypoxic–ischemic encephalopathy (HIE) is a devastating neonatal brain condition caused by lack of oxygen and limited blood flow. Environmental enrichment (EE) is a classic paradigm with a complex stimulation of physical, cognitive, and social components. EE can exert neuroplasticity and neuroprotective effects in immature brains. However, the exact mechanism of EE on the chronic condition of HIE remains unclear. HIE was induced by a permanent ligation of the right carotid artery, followed by an 8% O₂ hypoxic condition for 1 h. At 6 weeks of age, HIE mice were randomly assigned to either standard cages or EE cages. In the behavioral assessments, EE mice showed significantly improved motor performances in rotarod tests, ladder walking tests, and hanging wire tests, compared with HIE control mice. EE mice also significantly enhanced cognitive performances in Y-maze tests. Particularly, EE mice showed a significant increase in Ca_v 2.1 (P/Q type) and presynaptic proteins by molecular assessments, and a significant increase of Ca_v 2.1 in histological assessments of the cerebral cortex and hippocampus. These results indicate that EE can upregulate the expression of the Ca_v 2.1 channel and presynaptic proteins related to the synaptic vesicle cycle and neurotransmitter release, which may be responsible for motor and cognitive improvements in HIE.

Engineered glycomaterial implants orchestrate large-scale functional repair of brain tissue chronically after severe traumatic brain injury

Severe traumatic brain injury (sTBI) survivors experience permanent functional disabilities due to significant volume loss and the brain's poor capacity to regenerate. Chondroitin sulfate glycosaminoglycans (CS-GAGs) are key regulators of growth factor signaling and neural stem cell homeostasis in the brain. However, the efficacy of engineered CS (eCS) matrices in mediating structural and functional recovery chronically after sTBI has not been investigated. We report that neurotrophic factor functionalized acellular eCS matrices implanted into the rat M1 region acutely after sTBI significantly enhanced cellular repair and gross motor function recovery when compared to controls 20 weeks after sTBI. Animals subjected to M2 region injuries followed by eCS matrix implantations demonstrated the significant recovery of "reach-to-grasp" function. This was attributed to enhanced volumetric vascularization, activity-regulated cytoskeleton (Arc) protein expression, and perilesional sensorimotor connectivity. These findings indicate that eCS matrices implanted acutely after sTBI can support complex cellular, vascular, and neuronal circuit repair chronically after sTBI.

Star power: the emerging role of astrocytes as neuronal partners during cortical plasticity

Plasticity is a fundamental property of neuronal circuits, allowing them to adapt to alterations in activation. Generally speaking, plasticity has been viewed from a 'neuron-centric' perspective, with changes in circuit function attributed to alterations in neuronal excitability, synaptic strength or neuronal connectivity. However, it is now clear that glial cells, in particular astrocytes, are key regulators of neuronal plasticity. This article reviews recent progress made in understanding astrocyte function and attempts to summarize these functions into a coherent framework that positions astrocytes as central players in the plasticity process.

Human placental mesenchymal stem cells improve stroke outcomes via extracellular vesicles-mediated preservation of cerebral blood flow

Background

Besides long-term trans-differentiation into neural cells, benefits of stem cell therapy (SCT) in ischemic stroke may include secretion of protective factors, which partly reflects extracellular vesicle (EVs) released by stem cell. However, the mechanism(s) by which stem cells/EVs limit stroke injury have yet to be fully defined.

Methods

We evaluated the protection effect of human placenta mesenchymal stem cells (hPMSC) as a potential form of SCT in experimental ischemic stroke ‘transient middle cerebral artery occusion (MCAO)/reperfusion’ mice model.

Findings

We found for the first time that intraperitoneal administration of hPMSCs or intravenous hPMSC-derived EVs, given at the time of reperfusion, significantly protected the ipsilateral hemisphere from ischemic injury. This protection was associated with significant restoration of normal blood flow to the post-MCAO brain. More importantly, EVs derived from hPMSC promote paracrine-based protection of SCT in the MCAO model in a cholesterol/lipid-dependent manner.

Interpretation

Together, our results demonstrated beneficial effects of hPMSC/EVs in experimental stroke models which could permit the rapid “translation” of these cells into clinical trials in the near-term.

Experience-dependent structural plasticity in the adult brain: How the learning brain grows

Volumetric magnetic resonance imaging studies have shown that intense learning can be associated with grey matter volume increases in the adult brain. The underlying mechanisms are poorly understood. Here we used monocular deprivation in rats to analyze the mechanisms underlying use-dependent grey matter increases. Optometry for quantification of visual acuity was combined with volumetric magnetic resonance imaging and microscopic techniques in longitudinal and cross-sectional studies. We found an increased spatial vision of the open eye which was associated with a transient increase in the volumes of the contralateral visual and lateral entorhinal cortex. In these brain areas dendrites of neurons elongated, and there was a strong increase in the number of spines, the targets of synapses, which was followed by spine maturation and partial pruning. Astrocytes displayed a transient pronounced swelling and underwent a reorganization of their processes. The use-dependent increase in grey matter corresponded predominantly to the swelling of the astrocytes. Experience-dependent increase in brain grey matter volume indicates a gain of structure plasticity with both synaptic and astrocyte remodeling.

A Window of Vascular Plasticity Coupled to Behavioral Recovery after Stroke

Stroke causes remodeling of vasculature surrounding the infarct, but whether and how vascular remodeling contributes to recovery are unclear. We established an approach to monitor and compare changes in vascular structure and blood flow with high spatiotemporal precision after photothrombotic infarcts in motor cortex using longitudinal 2-photon and multiexposure speckle imaging in mice of both sexes. A spatially graded pattern of vascular structural remodeling in peri-infarct cortex unfolded over the first 2 weeks after stroke, characterized by vessel loss and formation, and selective stabilization of a subset of new vessels. This vascular structural plasticity was coincident with transient activation of transcriptional programs relevant for vascular remodeling, reestablishment of peri-infarct blood flow, and large improvements in motor performance. Local vascular plasticity was strongly predictive of restoration of blood flow, which was in turn predictive of behavioral recovery. These findings reveal the spatiotemporal evolution of vascular remodeling after stroke and demonstrate that a window of heightened vascular plasticity is coupled to the reestablishment of blood flow and behavioral recovery. Our findings support that neovascularization contributes to behavioral recovery after stroke by restoring blood flow to peri-infarct regions. These findings may inform strategies for enhancing recovery from stroke and other types of brain injury.SIGNIFICANCE STATEMENT An improved understanding of neural repair could inform strategies for enhancing recovery from stroke and other types of brain injury. Stroke causes remodeling of vasculature surrounding the lesion, but whether and how the process of vascular remodeling contributes to recovery of behavioral function have been unclear. Here we used longitudinal in vivo imaging to track vascular structure and blood flow in residual peri-infarct cortex after ischemic stroke in mice. We found that stroke created a restricted period of heightened vascular plasticity that was associated with restoration of blood flow, which was in turn predictive of recovery of motor function. Therefore, our findings support that vascular remodeling facilitates behavioral recovery after stroke by restoring blood flow to peri-infarct cortex.

Reevaluation of Astrocyte-Neuron Energy Metabolism with Astrocyte Volume Fraction Correction: Impact on Cellular Glucose Oxidation Rates, Glutamate-Glutamine Cycle Energetics, Glycogen Levels and Utilization Rates vs. Exercising Muscle, and Na+/K+ Pumping Rates

Accurate quantification of cellular contributions to rates of substrate utilization in resting, activated, and diseased brain is essential for interpretation of data from studies using [¹⁸F]fluorodeoxyglucose-positron-emission tomography (FDG-PET) and [¹³C]glucose/magnetic resonance spectroscopy (MRS). A generally-accepted dogma is that neurons have the highest energy demands of all brain cells, and calculated neuronal rates of glucose oxidation in awake, resting brain accounts for 70-80%, with astrocytes 20-30%. However, these proportions do not take cell type volume fractions into account. To evaluate the conclusion that neuron-astrocyte glucose oxidation rates are similar when adjusted for astrocytic volume fraction (Hertz, Magn Reson Imaging 2011; 29, 1319), the present study analyzed data from 31 studies. On average, astrocytes occupy 6.1, 9.6, and 15% of tissue volume in hippocampus, cerebral cortex, and cerebellum, respectively, and regional astrocytic metabolic rates are adjusted for volume fraction by multiplying by 17.6, 11.4, and 6.8, respectively. After adjustment, astrocytic glucose oxidation rates in resting awake rat brain are 4-10 fold higher than neuronal oxidation rates. Volume-fraction adjustment also increases brain glycogen concentrations and utilization rates to be similar to or exceed exercising muscle. Ion flux calculations to evaluate sodium/potassium homeostasis during neurotransmission are not correct if astrocyte-neuron volume fractions are assumed to be equal. High rates of glucose and glycogen utilization after adjustment for volume fraction indicate that astrocytic energy demands are much greater than recognized, with most of the ATP being used for functions other than glutamate processing in the glutamate-glutamine cycle, challenging the notion that astrocytes 'feed hungry neurons'.

Training in a cooperative bimanual skilled reaching task, the popcorn retrieval task, improves unimanual function after motor cortical infarcts in rats

Disuse of the paretic hand after stroke is encouraged by compensatory reliance on the nonparetic hand, to exacerbate impairment and potentially constrain motor rehabilitation efficacy. Rodent stroke model findings support that learning new unimanual skills with the nonparetic forelimb diminishes functional improvements that can be driven by rehabilitative training of the paretic forelimb. The influence of learning new ways of skillfully using the two hands together on paretic side function is much less clear. To begin to explore this, we developed a new cooperative bimanual skilled reaching task for rats, the Popcorn Retrieval Task. After motor cortical infarcts impaired an established unimanual reaching skill in the paretic forelimb, rats underwent a 7 week period of de novo bimanual training (BiT) or no-training control procedures (Cont). Probes of paretic forelimb unimanual performance revealed significant improvements during and after the training period in BiT vs. Cont. We additionally observed a striking change in the bimanual task strategy over training days: a switch from the paretic to the nonparetic forelimb for initiating reach-to-grasp sequences. This motivated another study to test whether rats that established the bimanual skill prior to the infarcts would similarly switch handedness, which they did not, though paretic paw use for manipulative movements diminished. These results indicate that unimanual function of the paretic side can be improved by novel bimanual skill practice, even when it involves compensatory reliance on the nonparetic hand. They further support the suitability of the Popcorn Retrieval Task for studying bimanual skill learning effects in rats.

Rehabilitative Training Interacts with Ischemia-Instigated Spine Dynamics to Promote a Lasting Population of New Synapses in Peri-Infarct Motor Cortex

After subtotal infarcts of primary motor cortex (M1), motor rehabilitative training (RT) promotes improvements in paretic forelimb function that have been linked with its promotion of structural and functional reorganization of peri-infarct cortex, but how the reorganization unfolds is scantly understood. Cortical infarcts also instigate a prolonged period of dendritic spine turnover in peri-infarct cortex. Here we investigated the possibility that synaptic structural responses to RT in peri-infarct cortex reflect, in part, interactions with ischemia-instigated spine turnover. This was tested after artery-targeted photothrombotic M1 infarcts or Sham procedures in adult (4 months) C57BL/6 male and female GFP-M line (n = 24) and male yellow fluorescent protein-H line (n = 5) mice undergoing RT in skilled reaching or no-training control procedures. Regardless of training condition, spine turnover was increased out to 5 weeks postinfarct relative to Sham, as was the persistence of new spines formed within a week postinfarct. However, compared with no-training controls, new spines formed during postinfarct weeks 2-4 in mice undergoing RT persisted in much greater proportions to later time points, by a magnitude that predicted behavioral improvements in the RT group. These results indicate that RT interacts with ischemia-instigated spine turnover to promote preferential stabilization of newly formed spines, which is likely to yield a new population of mature synapses in peri-infarct cortex that could contribute to cortical functional reorganization and behavioral improvement. The findings newly implicate ischemia-instigated spine turnover as a mediator of cortical synaptic structural responses to RT and newly establish the experience dependency of new spine fates in the postischemic turnover context.SIGNIFICANCE STATEMENT Motor rehabilitation, the main treatment for motor impairments after stroke, is far from sufficient to normalize function. A better understanding of neural substrates of rehabilitation-induced behavioral improvements could be useful for understanding how to optimize it. Here, we investigated the nature and time course of synaptic responses to motor rehabilitative training in vivo Focal ischemia instigated a period of synapse turnover in peri-infarct motor cortex of mice. Rehabilitative training increased the stability of new synapses formed during the initial weeks after the infarct, the magnitude of which was correlated with improvements in skilled motor performance. Therefore, the maintenance of new synapses formed after ischemia could represent a structural mechanism of rehabilitative training efficacy.

Functions of subventricular zone neural precursor cells in stroke recovery

The proliferation and ectopic migration of neural precursor cells (NPCs) in response to ischemic brain injury was first reported two decades ago. Since then, studies of brain injury-induced subventricular zone cytogenesis, primarily in rodent models, have provided insight into the cellular and molecular determinants of this phenomenon and its modulation by various factors. However, despite considerable correlational evidence-and some direct evidence-to support contributions of NPCs to behavioral recovery after stroke, the causal mechanisms have not been identified. Here we discuss the subventricular zone cytogenic response and its possible roles in brain injury and disease, focusing on rodent models of stroke. Emerging evidence suggests that NPCs can modulate harmful responses and enhance reparative responses to neurologic diseases. We speculatively identify four broad functions of NPCs in the context of stroke: cell replacement, cytoprotection, remodeling of residual tissue, and immunomodulation. Thus, NPCs may have pleiotropic functions in supporting behavioral recovery after stroke.

Ischemic axonal injury up-regulates MARK4 in cortical neurons and primes tau phosphorylation and aggregation

Ischemic injury to white matter tracts is increasingly recognized to play a key role in age-related cognitive decline, vascular dementia, and Alzheimer’s disease. Knowledge of the effects of ischemic axonal injury on cortical neurons is limited yet critical to identifying molecular pathways that link neurodegeneration and ischemia. Using a mouse model of subcortical white matter ischemic injury coupled with retrograde neuronal tracing, we employed magnetic affinity cell sorting with fluorescence-activated cell sorting to capture layer-specific cortical neurons and performed RNA-sequencing. With this approach, we identified a role for microtubule reorganization within stroke-injured neurons acting through the regulation of tau. We find that subcortical stroke-injured Layer 5 cortical neurons up-regulate the microtubule affinity-regulating kinase, Mark4, in response to axonal injury. Stroke-induced up-regulation of Mark4 is associated with selective remodeling of the apical dendrite after stroke and the phosphorylation of tau in vivo. In a cell-based tau biosensor assay, Mark4 promotes the aggregation of human tau in vitro. Increased expression of Mark4 after ischemic axonal injury in deep layer cortical neurons provides new evidence for synergism between axonal and neurodegenerative pathologies by priming of tau phosphorylation and aggregation.

Motor Evoked Potentials as Potential Biomarkers of Early Atypical Corticospinal Tract Development in Infants with Perinatal Stroke

Diagnosis of cerebral palsy (CP) after perinatal stroke is often delayed beyond infancy, a period of rapid neuromotor development with heightened potential for rehabilitation. This study sought to assess whether the presence or absence of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) could be an early biomarker of atypical development within the first year of life. In 10 infants with perinatal stroke, motor outcome was assessed with a standardized movement assessment. Single-pulse TMS was utilized to assess presence of MEPs. Younger infants (3-6 months CA, n = 5, 4/5 (80%)) were more likely to present with an MEP from the more-affected hemisphere (MAH) compared to older infants (7-12 months CA, n = 5, 0/5, (0%)) (p = 0.048). Atypical movement was demonstrated in the majority of infants with an absent MEP from the MAH (5/6, 83%) compared to those with a present MEP (1/4, 25%) (p = 0.191). We found that age influences the ability to elicit an MEP from the MAH, and motor outcome may be related to MAH MEP absence. Assessment of MEPs in conjunction with current practice of neuroimaging and motor assessments could promote early detection and intervention in infants at risk of CP.

Potential therapeutic roles of stem cells in ischemia-reperfusion injury

Ischemia-reperfusion injury (I/RI), produced by an initial interruption of organ blood flow and its subsequent restoration, contributes significantly to the pathophysiologies of stroke, myocardial infarction, renal I/RI, intestinal I/RI and liver I/RI, which are major causes of disability (including transplant failure) and even mortality. While the restoration of blood flow is required to restore oxygen and nutrient requirements, reperfusion often triggers local and systemic inflammatory responses and subsequently elevate the ischemic insult where the duration of ischemia determines the magnitude of I/RI damage. I/RI increases vascular leakage, changes transcriptional and cell death programs, drives leukocyte entrapment and inflammation and oxidative stress in tissues. Therapeutic approaches which reduce complications associated with I/RI are desperately needed to address the clinical and economic burden created by I/RI. Stem cells (SC) represent ubiquitous and uncommitted cell populations with the ability to self-renew and differentiate into one or more developmental 'fates'. Like immune cells, stem cells can home to and penetrate I/R-injured tissues, where they can differentiate into target tissues and induce trophic paracrine signaling which suppress injury and maintain tissue functions perturbed by ischemia-reperfusion. This review article summarizes the present use and possible protective mechanisms underlying stem cell protection in diverse forms of ischemia-reperfusion.

Compensatory Relearning Following Stroke: Cellular and Plasticity Mechanisms in Rodents

von Monakow’s theory of diaschisis states the functional ‘standstill’ of intact brain regions that are remote from a damaged area, often implied in recovery of function. Accordingly, neural plasticity and activity patterns related to recovery are also occurring at the same regions. Recovery relies on plasticity in the periinfarct and homotopic contralesional regions and involves relearning to perform movements. Seeking evidence for a relearning mechanism following stroke, we found that rodents display many features that resemble classical learning and memory mechanisms. Compensatory relearning is likely to be accompanied by gradual shaping of these regions and pathways, with participating neurons progressively adapting cortico-striato-thalamic activity and synaptic strengths at different cortico-thalamic loops – adapting function relayed by the striatum. Motor cortex functional maps are progressively reinforced and shaped by these loops as the striatum searches for different functional actions. Several cortical and striatal cellular mechanisms that influence motor learning may also influence post-stroke compensatory relearning. Future research should focus on how different neuromodulatory systems could act before, during or after rehabilitation to improve stroke recovery.

Characterizing Spontaneous Motor Recovery Following Cortical and Subcortical Stroke in the Rat

BACKGROUND

Stroke is a leading cause of neurological disability, often resulting in long-term motor impairments due to damage to cortical or subcortical motor areas. Despite the high prevalence of subcortical strokes in the clinical population, preclinical research has primarily focused on investigating and treating cortical strokes. Moreover, while both humans and animals show spontaneous recovery following stroke, little is known about how injury location affects this process.

OBJECTIVE

To capture the heterogeneity of human stroke and examine how stroke location affects spontaneous motor recovery following damage to cortical, subcortical, or a combination of both areas.

METHODS

Endothelin-1 (ET-1), a potent vasoconstrictor, was used to produce focal infarcts in the forelimb motor cortex (FMC), the dorsolateral striatum (DLS) or both the FMC and DLS in male Sprague-Dawley rats. The spontaneous recovery profile of animals was followed over an 8-week period using a battery of behavioral tasks assessing motor function and limb preference.

RESULTS

All 3 groups showed significant impairments on the Montoya staircase, beam, and cylinder tests following stroke, with the combined group (FMC + DLS) having the largest and most persistent impairments. Importantly, spontaneous recovery was not simply dependent on lesion volume, but on location, and the behavioral test employed.

CONCLUSIONS

Stroke location markedly and differentially influences the level of spontaneous functional recovery, which is only captured by using multiple outcome measures. These results illustrate the need for preclinical stroke models to align with the heterogeneity of human stroke, especially with respect to lesion location, size, and outcome measures.

State-of-the-Art Techniques to Causally Link Neural Plasticity to Functional Recovery in Experimental Stroke Research

Current experimental stroke research faces the same challenge as neuroscience: to transform correlative findings in causative ones. Research of recent years has shown the tremendous potential of the central nervous system to react to noxious stimuli such as a stroke: Increased plastic changes leading to reorganization in form of neuronal rewiring, neurogenesis, and synaptogenesis, accompanied by transcriptional and translational turnover in the affected cells, have been described both clinically and in experimental stroke research. However, only minor attempts have been made to connect distinct plastic remodeling processes as causative features for specific behavioral phenotypes. Here, we review current state-of the art techniques for the examination of cortical reorganization and for the manipulation of neuronal circuits as well as techniques which combine anatomical changes with molecular profiling. We provide the principles of the techniques together with studies in experimental stroke research which have already applied the described methodology. The tools discussed are useful to close the loop from our understanding of stroke pathology to the behavioral outcome and may allow discovering new targets for therapeutic approaches. The here presented methods open up new possibilities to assess the efficiency of rehabilitative strategies by understanding their external influence for intrinsic repair mechanisms on a neurobiological basis.