Stroke research at a crossroad: asking the brain for directions

Abstract

Ischemic stroke remains a vexing public health problem. Although progress has been made in prevention and supportive care, efforts to protect the brain from ischemic cell death have failed. Thus, no new treatment has made it from bench to bedside since tissue plasminogen activator was introduced in 1996. The brain has a remarkable capacity for self-preservation, illustrated by the protective responses induced by ischemia, preconditioning and exercise. Here we describe the mechanisms underlying brain self-protection, with the goal of identifying features that could provide insight into stroke therapy. Unlike traditional therapeutic approaches based on counteracting selected pathways of the ischemic cascade, endogenous neuroprotection relies on coordinated neurovascular programs that support cerebral perfusion, mitigate the harmful effects of cerebral ischemia and promote tissue restoration. Learning how the brain triggers and implements these protective measures may advance our quest to treat stroke.

Figure 1. Protective pathways activated by cerebral ischemia

Cerebral ischemia, while activating damaging processes, also triggers a coordinated response that attempts to counteract tissue damage. The reduction in blood flow produced by the arterial occlusion is opposed by an increase in blood pressure, by the production of vasoactive mediators in the ischemic brain and by the activation of eNOS, which increase perfusion pressure and reduce vascular resistance in collateral vessels supplying the ischemic territory. Hypoxia activates HIF1 leading to a transcriptional response that promotes oxygen and glucose delivery to the tissue. The energy deficit associated with ischemia is countered by suppression of protein synthesis and neuronal activity (spike arrest and channel closure), which reduce energy expenditures. Post-ischemic oxidative stress triggers an antioxidant response via the transcription factor Nrf2, while inhibitory neurotransmitters and glutamate transporters (GLT1/EAAT2) counterbalance the excitotoxicity associated with glutamate receptor activation. The deleterious effects of post-ischemic apoptosis are antagonized by expression of anti-apoptotic factors (Bcl2, IAP), HSP and activation of the protective kinase Akt. Inflammation is mitigated by production of anti-inflammatory cytokines and neurotransmitters, as well as an influx of lymphocytes with anti-inflammatory properties (T_reg, B_reg). Systemic immunosuppression limits the development of adaptive and innate immune responses that may induce tissue damage. Ischemia is also associated with expression of CREB-dependent prosurvival genes, including growth factors, and with proliferation of neural and vascular progenitor cells that participate in tissue repair. These endogenous protective pathways limit the extent of ischemic brain injury as shown by studies in which their inhibition enhances the damage, e.g.^,.

Figure 2. Intracellular events leading to ischemic tolerance

PC triggers act through G-coupled receptors dependent phospholipase C (PLC) activation leading to hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) and generation of diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3), which acts on smooth endoplasmic reticulum (ER) Ca²⁺ channels to mobilize intracellular Ca²⁺ stores leading to PKC activation. PIP2 is also phosphorylated by phosphatidylinositol 3-kinase (PI3K) resulting in phosphatidylinositol 3-phosphate (PIP3) generation and Akt activation. Ca²⁺ influx through glutamate receptors activates NOS. NO increases guanylate cyclase activity (GC) resulting in protein kinase G (PKG) activation. Together these early mediators enhance the activity of mK_ATP channels and inhibit pro-apoptotic signaling and opening of the mitochondrial permeability transition pore (mPTP). At the same time, transcription factors activated by these signaling cascades as well as reduced oxygen levels, reactive oxygen species (ROS), and ATP deficit lead to the expression of pro-survival genes, like the anti-apoptotic factor Bcl2, HSP, and the antioxidant enzymes MnSOD and HO-1. Genes are also expressed that help the tissue operate under reduced oxygen and glucose availability, like the glucose transporter GLUT-1, the pro-angiogenic growth factor VEGF, and the hematopoietic and cytoprotective factor EPO. DAMPs released from stressed cells activate Toll-like receptors (TLR) leading to NF-κB and type-I interferon response. Epigenetic factors are also likely to contribute to the reprogramming of post-ischemic gene expression and may include the epigenetic modifiers Polycomb Group proteins (PcG) and Sirtuin class histone deacetyalses (SIRT).

Figure 3. Local and remote mechanisms of endogenous neuroprotection

In brain, there are protective interactions among neurons, astrocytes, microglia and cerebral blood vessels. These are mediated by cell-cell contact, by the uptake of excessive glutamate, and by the release of growth factors and cytokines. These interactions are directed at preserving tissue homeostasis by maintaining CBF, suppressing excitotoxicity, reducing energy use, dampening inflammation and apoptosis, and boosting repair mechanisms. Central signals (red arrows) through neurohumoral pathways act on peripheral organs to support the cardiovascular system, release growth factors and cytokines, and mobilize protective cells, such as T_reg and B_reg lymphocytes and EPC. Peripheral signals (blue arrows) generated by the systemic response, in turn, may feed back on the brain and exert protective effects.