Can genes modify stroke outcome and by what mechanisms?

Correction to: Neuroimmune Response in Ischemic Preconditioning

Ischemic preconditioning (IPC) is a robust neuroprotective phenomenon in which a brief period of cerebral ischemia confers transient tolerance to subsequent ischemic challenge. Research on IPC has implicated cellular, molecular, and systemic elements of the immune response in this phenomenon. Potent molecular mediators of IPC include innate immune signaling pathways such as Toll-like receptors and type 1 interferons. Brain ischemia results in release of pro- and anti-inflammatory cytokines and chemokines that orchestrate the neuroinflammatory response, resolution of inflammation, and transition to neurological recovery and regeneration. Cellular mediators of IPC include microglia, the resident central nervous system immune cells, astrocytes, and neurons. All of these cell types engage in cross-talk with each other using a multitude of signaling pathways that modulate activation/suppression of each of the other cell types in response to ischemia. As the postischemic neuroimmune response evolves over time there is a shift in function toward provision of trophic support and neuroprotection. Peripheral immune cells infiltrate the central nervous system en masse after stroke and are largely detrimental, with a few subtypes having beneficial, protective effects, though the role of these immune cells in IPC is largely unknown. The role of neural progenitor cells in IPC-mediated neuroprotection is another active area of investigation as is the role of microglial proliferation in this setting. A mechanistic understanding of these molecular and cellular mediators of IPC may not only facilitate more effective direct application of IPC to specific clinical scenarios, but also, more broadly, reveal novel targets for therapeutic intervention in stroke.

Early Rehabilitation After Stroke: a Narrative Review

PURPOSE OF REVIEW

Despite current rehabilitative strategies, stroke remains a leading cause of disability in the USA. There is a window of enhanced neuroplasticity early after stroke, during which the brain's dynamic response to injury is heightened and rehabilitation might be particularly effective. This review summarizes the evidence of the existence of this plastic window, and the evidence regarding safety and efficacy of early rehabilitative strategies for several stroke domain-specific deficits.

RECENT FINDINGS

Overall, trials of rehabilitation in the first 2 weeks after stroke are scarce. In the realm of very early mobilization, one large and one small trial found potential harm from mobilizing patients within the first 24 h after stroke, and only one small trial found benefit in doing so. For the upper extremity, constraint-induced movement therapy appears to have benefit when started within 2 weeks of stroke. Evidence for non-invasive brain stimulation in the acute period remains scant and inconclusive. For aphasia, the evidence is mixed, but intensive early therapy might be of benefit for patients with severe aphasia. Mirror therapy begun early after stroke shows promise for the alleviation of neglect. Novel approaches to treating dysphagia early after stroke appear promising, but the high rate of spontaneous improvement makes their benefit difficult to gauge. The optimal time to begin rehabilitation after a stroke remains unsettled, though the evidence is mounting that for at least some deficits, initiation of rehabilitative strategies within the first 2 weeks of stroke is beneficial. Commencing intensive therapy in the first 24 h may be harmful.

Neuroimmune Response in Ischemic Preconditioning

Ischemic preconditioning (IPC) is a robust neuroprotective phenomenon in which a brief period of cerebral ischemia confers transient tolerance to subsequent ischemic challenge. Research on IPC has implicated cellular, molecular, and systemic elements of the immune response in this phenomenon. Potent molecular mediators of IPC include innate immune signaling pathways such as Toll-like receptors and type 1 interferons. Brain ischemia results in release of pro- and anti-inflammatory cytokines and chemokines that orchestrate the neuroinflammtory response, resolution of inflammation, and transition to neurological recovery and regeneration. Cellular mediators of IPC include microglia, the resident central nervous system immune cells, astrocytes, and neurons. All of these cell types engage in cross-talk with each other using a multitude of signaling pathways that modulate activation/suppression of each of the other cell types in response to ischemia. As the postischemic neuroimmune response evolves over time there is a shift in function toward provision of trophic support and neuroprotection. Peripheral immune cells infiltrate the central nervous system en masse after stroke and are largely detrimental, with a few subtypes having beneficial, protective effects, though the role of these immune cells in IPC is largely unknown. The role of neural progenitor cells in IPC-mediated neuroprotection is another active area of investigation as is the role of microglial proliferation in this setting. A mechanistic understanding of these molecular and cellular mediators of IPC may not only facilitate more effective direct application of IPC to specific clinical scenarios, but also, more broadly, reveal novel targets for therapeutic intervention in stroke.

Plasticity of Sensorimotor Networks: Multiple Overlapping Mechanisms

Redundancy is an important feature of the motor system, as abundant degrees of freedom are prominent at every level of organization across the central and peripheral nervous systems, and musculoskeletal system. This basic feature results in a system that is both flexible and robust, and which can be sustainably adapted through plasticity mechanisms in response to intrinsic organismal changes and dynamic environments. While much early work of motor system organization has focused on synaptic-based plasticity processes that are driven via experience, recent investigations of neuron-glia interactions, epigenetic mechanisms and large-scale network dynamics have revealed a plethora of plasticity mechanisms that support motor system organization across multiple, overlapping spatial and temporal scales. Furthermore, an important role of these mechanisms is the regulation of intrinsic variability. Here, we review several of these mechanisms and discuss their potential role in neurorehabilitation.

Genetics as a molecular window into recovery, its treatment, and stress responses after stroke

Stroke remains a major source of adult disability in the USA and worldwide. Most patients show some recovery during the weeks to months following a stroke, but this is generally incomplete. An emerging branch of therapeutics targets the processes underlying this behavioral recovery from stroke toward the goal of reducing long-term disability. A key factor hampering these efforts is the very large degree of variability between stroke survivors. Available data suggest that genetic differences could explain an important fraction of the differences between subjects. The current review considers this from several angles, including genetic differences in relation to drugs that promote recovery. Genetic factors related to physiological and psychological stress responses may also be critically important to understanding recovery after stroke and its treatment. The studies reviewed provide insights into recovery and suggest directions for further research to improve clinical decision-making in this setting. Genetic differences between patients might be used to help clinical trials select specific patient subgroups, on a biological basis, in order to sharpen the precision with which new treatments are evaluated. Pharmacogenomic factors might also provide insights into inter-subject differences in treatment side effects for pharmacological prescriptions, and behavioral interventions, and others. These efforts must be conducted with the strictest ethical standards given the highly sensitive nature of genetic data. Understanding the effect of selected genetic measures could improve a clinician's ability to predict the risk and efficacy of a restorative therapy and to make maximally informed decisions, and in so doing, facilitate individual patient care.

Cerebral Ischemic Preconditioning: the Road So Far…

Cerebral preconditioning constitutes the brain's adaptation to lethal ischemia when first exposed to mild doses of a subtoxic stressor. The phenomenon of preconditioning has been largely studied in the heart, and data from in vivo and in vitro models from past 2-3 decades have provided sufficient evidence that similar machinery exists in the brain as well. Since preconditioning results in a transient protective phenotype labeled as ischemic tolerance, it can open many doors in the medical warfare against stroke, a debilitating cerebrovascular disorder that kills or cripples thousands of people worldwide every year. Preconditioning can be induced by a variety of stimuli from hypoxia to pharmacological anesthetics, and each, in turn, induces tolerance by activating a multitude of proteins, enzymes, receptors, transcription factors, and other biomolecules eventually leading to genomic reprogramming. The intracellular signaling pathways and molecular cascades behind preconditioning are extensively being investigated, and several first-rate papers have come out in the last few years centered on the topic of cerebral ischemic tolerance. However, translating the experimental knowledge into the clinical scaffold still evades practicality and faces several challenges. Of the various preconditioning strategies, remote ischemic preconditioning and pharmacological preconditioning appears to be more clinically relevant for the management of ischemic stroke. In this review, we discuss current developments in the field of cerebral preconditioning and then examine the potential of various preconditioning agents to confer neuroprotection in the brain.

Pharmacogenetics of neural injury recovery

Relatively few pharmacological agents are part of routine care for neural injury, although several are used or under consideration in acute stroke, chronic stroke, traumatic brain injury and secondary stroke prevention. Tissue plasminogen activator is approved for the treatment of acute ischemic stroke, and genetic variants may impact the efficacy and safety of this drug. In the chronic phase of stroke, several drugs such as L-dopa, fluoxetine and donepezil are under investigation for enhancing rehabilitation therapy, with varying levels of evidence. One potential reason for the mixed efficacy displayed by these drugs may be the influence of genetic factors that were not considered in prior studies. An understanding of the genetics impacting the efficacy of dopaminergic, serotonergic and cholinergic drugs may allow clinicians to target these potential therapies to those patients most likely to benefit. In the setting of stroke prevention, which is directly linked to neural injury recovery, the most highly studied pharmacogenomic interactions pertain to clopidogrel and warfarin. Incorporating pharmacogenomics into neural injury recovery has the potential to maximize the benefit of several current and potential pharmacological therapies and to refine the choice of pharmacological agent that may be used to enhance benefits from rehabilitation therapy.

Functional up-regulation of endopeptidase neurolysin during post-acute and early recovery phases of experimental stroke in mouse brain

In this study, we provide evidence for the first time that membrane-bound endopeptidase neurolysin is up-regulated in different parts of mouse brain affected by focal ischemia-reperfusion in a middle cerebral artery occlusion model of stroke. Radioligand binding, enzymatic and immunoblotting experiments in membrane preparations of frontoparietal cortex, striatum, and hippocampus isolated from the ischemic hemisphere of mouse brain 24 h after reperfusion revealed statistically significant increase (≥ twofold) in quantity and activity of neurolysin compared with sham-operated controls. Cerebellar membranes isolated from the ischemic hemisphere served as negative control supporting the observations that up-regulation of neurolysin occurs in post-ischemic brain regions. This study also documents sustained functional up-regulation of neurolysin in frontoparietal cortical membranes for at least 7 days after stroke, which appears not to be transcriptionally or translationally regulated, but rather depends on translocation of cytosolic neurolysin to the membranes and mitochondria. Considering diversity of endogenous neurolysin substrates (neurotensin, bradykinin, angiotensins I/II, substance P, hemopressin, dynorphin A(1-8), metorphamide, somatostatin) and the well-documented role of these peptidergic systems in pathogenesis of stroke, resistance to ischemic injury and/or post-stroke brain recovery, our findings suggest that neurolysin may play a role in processes modulating the brain's response to stroke and its recovery after stroke.

Ischemic preconditioning alters the epigenetic profile of the brain from ischemic intolerance to ischemic tolerance

Ischemic preconditioning is an innate neuroprotective mechanism in which a sub-injurious ischemic exposure increases the brain's ability to withstand a subsequent, normally injurious ischemic insult. Part of ischemic preconditioning neuroprotection stems from an epigenetic reprogramming of the brain to a phenotype of ischemic tolerance, which results in a gene expression profile different from that observed in the non-injured and ischemia-injured brains. Such neuroprotective reprograming, activated by ischemic preconditioning, requires specific changes in DNA accessibility coordinated with activation of transcriptional activator and repressor proteins, which allows for expression of specific neuroprotective proteins despite a general repression of gene expression. In this review we examine the effects of injurious ischemia and ischemic preconditioning on the regulation of DNA methylation, histone post-translational modifications, and non-coding RNA expression. There is increasing interest in the role of epigenetics in disease pathobiology, and whether and how pharmacological manipulation of epigenetic processes may allow for ischemic neuroprotection. Therefore, a better understanding of the epigenomic determinants underlying the modulation of gene expression that lead to ischemic tolerance or cell death offers the promise of novel neuroprotective therapies that target global reprograming of genomic activity versus individual cellular signaling pathways.

Epigenetics and the environment: in search of the "toleroasome" vital to execution of ischemic preconditioning

Activation and repression of gene expression are key features of ischemic tolerance. Converging lines of inquiry from several groups suggests that epigenetic proteins may transduce sublethal stresses, including bioenergetic or oxidative stress into durable (2-3 days) changes in gene expression that mediate ischemic tolerance. Here we discuss the potential mechanisms by which changes in cell state (e.g., ATP, NAD+, and oxygen) can modify specific targets including polycomb complexes, jumonji domain histone demethylases, and zinc and NAD-dependent histone decetylases and thus trigger an adaptive program. A major unanswered question is whether these proteins work in parallel or convergently as part of a "tolerosome" (tolero is the Latin word for tolerance), a multiprotein complex recruited to promoters or enhancers of specific genes, to mediate preconditioning. Whatever the case may be, epigenetic proteins are fertile targets for the treatment of stroke.

Spatial and temporal gene expression differences in core and periinfarct areas in experimental stroke: a microarray analysis

Background

A large number of genes are regulated to promote brain repair following stroke. The thorough analysis of this process can help identify new markers and develop therapeutic strategies. This study analyzes gene expression following experimental stroke.

Methodology/Principal Findings

A microarray study of gene expression in the core, periinfarct and contralateral cortex was performed in adult Sprague-Dawley rats (n = 60) after 24 hours (acute phase) or 3 days (delayed stage) of permanent middle cerebral artery (MCA) occlusion. Independent qRT-PCR validation (n = 12) was performed for 22 of the genes. Functional data were evaluated by Ingenuity Pathway Analysis. The number of genes differentially expressed was 2,612 (24 h) and 5,717 (3 d) in the core; and 3,505 (24 h) and 1,686 (3 d) in the periinfarct area (logFC>|1|; adjP<0.05). Expression of many neurovascular unit development genes was altered at 24 h and 3 d including HES2, OLIG2, LINGO1 and NOGO-A; chemokines like CXCL1 and CXCL12, stress-response genes like HIF-1A, and trophic factors like BDNF or BMP4. Nearly half of the detected genes (43%) had not been associated with stroke previously.

Conclusions

This comprehensive study of gene regulation in the core and periinfarct areas at different times following permanent MCA occlusion provides new data that can be helpful in translational research.

Hypoxic preconditioning induces stroke tolerance in mice via a cascading HIF, sphingosine kinase, and CCL2 signaling pathway

The induction of ischemic tolerance by preconditioning provides a platform to elucidate endogenous mechanisms of stroke protection. In these studies, we characterize the relationship between hypoxia-inducible factor (HIF), sphingosine kinase 2 (SphK2), and chemokine (C-C motif) ligand 2 (CCL2) in models of hypoxic or pharmacological preconditioning-induced ischemic tolerance. A genetics-based approach using SphK2- and CCL2-null mice showed both SphK2 and CCL2 to be necessary for the induction of ischemic tolerance following preconditioning with hypoxia, the hypoxia-mimetic cobalt chloride, or the sphingosine-1-phosphate (S1P) agonist FTY720. A pharmacological approach confirmed the necessity of HIF signaling for all three preconditioning stimuli, and showed that the SphK/S1P pathway transduces tolerance via the S1P(1) receptor. In addition, our data suggest significant cross-talk between HIF and SphK2-produced S1P signaling, which together act to up-regulate CCL2 expression. Overall, HIF, SphK, S1P, and CCL2 participate in a signaling cascade to induce the gene expression responsible for the stroke-tolerant phenotype established by hypoxic and FTY720 preconditioning. The identification of these common molecular mediators involved in signaling the genomic response to multiple preconditioning stimuli provides several targets for therapeutic manipulation.