Stroke Pathway: Key Molecular and Cellular Interactions

A stroke occurs when blood flow to the brain is disrupted, depriving neurons of oxygen and nutrients. This triggers a cascade of molecular and cellular events that contribute to brain damage and influence recovery. Understanding these interactions is crucial for developing effective treatments and improving patient outcomes.

The biological response to stroke involves metabolic disturbances, excitotoxicity, oxidative stress, inflammation, and neurovascular dysfunction. These mechanisms drive initial injury and shape long-term repair and regeneration.

Neurovascular Unit Interactions

The neurovascular unit (NVU) is a network of neurons, endothelial cells, astrocytes, pericytes, and extracellular matrix components that regulate cerebral blood flow and maintain homeostasis. During a stroke, ischemia disrupts cellular communication and vascular integrity. Endothelial cells, which form the inner lining of cerebral blood vessels, experience hypoxic stress, leading to impaired autoregulation and increased permeability. This dysfunction compromises oxygen and glucose delivery, exacerbating energy deficits and accelerating neuronal injury.

Astrocytes, which support the blood-brain barrier (BBB) and modulate synaptic activity, respond to ischemic stress by undergoing reactive gliosis. This process involves hypertrophy and upregulation of glial fibrillary acidic protein (GFAP), altering interactions with endothelial cells and pericytes. While reactive astrocytes release neurotrophic factors to restore homeostasis, prolonged activation can destabilize vasculature. Pericytes, which regulate capillary tone and blood flow, suffer ischemic damage, leading to capillary constriction and the “no-reflow” effect, where microvascular perfusion remains impaired even after arterial recanalization.

Endothelial dysfunction disrupts the balance of vasoactive molecules such as nitric oxide (NO) and endothelin-1, shifting the vascular environment toward vasoconstriction and pro-thrombotic states. Reduced NO impairs vasodilation, while increased endothelin-1 exacerbates microvascular constriction. The loss of tight junction proteins, including occludin and claudin-5, weakens the BBB, allowing plasma proteins and other circulating factors to infiltrate the brain parenchyma. This breach alters ionic balance and promotes edema, amplifying neuronal stress.

Metabolic Disturbances

Ischemic stroke disrupts cerebral metabolism, depriving neurons of oxygen and glucose, the primary substrates for ATP production. With oxidative phosphorylation compromised, cells shift to anaerobic glycolysis, leading to lactate accumulation and intracellular acidification. This metabolic shift impairs enzymatic function and destabilizes ion homeostasis. As ATP levels decline, Na+/K+ ATPase pump failure results in membrane depolarization, triggering excessive ion influx and further energy depletion. The ensuing ionic imbalance drives cytotoxic edema, causing swelling and structural damage.

Mitochondrial function is further impaired by ischemia-induced oxidative damage and calcium overload. Dysfunctional mitochondria produce excess reactive oxygen species (ROS), damaging lipids, proteins, and DNA. The release of cytochrome c from compromised mitochondria activates apoptotic pathways, contributing to neuronal loss. Persistent mitochondrial failure correlates with worse neurological outcomes.

Glucose metabolism is also disrupted. While neurons rely on a steady glucose supply for ATP generation, ischemic conditions hinder glucose transport across the BBB. Compensatory mechanisms, such as glycogenolysis in astrocytes, provide temporary energy but are quickly depleted. Metabolic uncoupling between neurons and glial cells leads to inefficient energy utilization, increasing neuronal vulnerability. Hyperglycemia at stroke onset worsens these disturbances by promoting excessive lactate production and oxidative stress.

Key Molecular Pathways

Cellular damage following a stroke is driven by interconnected molecular mechanisms that exacerbate neuronal injury and influence recovery. Excitotoxicity, oxidative stress, and mitochondrial dysfunction play central roles in propagating ischemic damage.

Excitotoxicity

Excitotoxicity arises from excessive glutamate release and impaired reuptake due to energy failure. Under normal conditions, glutamate is regulated by astrocytic transporters such as excitatory amino acid transporter 2 (EAAT2). Ischemia disrupts this balance, leading to extracellular glutamate accumulation and overactivation of ionotropic glutamate receptors, particularly N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. This excessive stimulation results in massive calcium influx, triggering downstream pathways that activate proteases, lipases, and endonucleases, leading to cytoskeletal breakdown, lipid peroxidation, and DNA fragmentation.

Sustained calcium overload disrupts mitochondrial function, further depleting ATP and increasing oxidative stress. NMDA receptor antagonists, such as memantine, have shown potential in mitigating excitotoxic damage, though clinical trials for stroke treatment have yielded mixed results. Additionally, excessive glutamate signaling promotes neuronal hyperexcitability and spreading depolarization, extending tissue damage beyond the ischemic core.

Oxidative Stress

The ischemic brain experiences a surge in ROS due to mitochondrial dysfunction and enzymatic activity from sources such as NADPH oxidase and xanthine oxidase. Under normal conditions, antioxidant systems—including superoxide dismutase (SOD), catalase, and glutathione peroxidase—neutralize ROS. However, ischemia-reperfusion events overwhelm these defenses, leading to oxidative stress that damages lipids, proteins, and nucleic acids.

Lipid peroxidation compromises membrane integrity, leading to ion leakage and worsening excitotoxicity. Oxidative modifications to proteins impair enzymatic function and disrupt cellular signaling, while DNA damage activates poly(ADP-ribose) polymerase (PARP), depleting NAD+ and further reducing ATP availability. Antioxidants such as edaravone, a free radical scavenger approved in Japan for stroke treatment, have shown potential, though broader clinical efficacy remains under investigation.

Mitochondrial Dysfunction

Mitochondria regulate energy production, calcium homeostasis, and apoptotic signaling. During stroke, mitochondrial permeability transition pore (mPTP) opening is triggered by calcium overload and oxidative stress, leading to the release of pro-apoptotic factors such as cytochrome c and apoptosis-inducing factor (AIF). This cascade activates cell death pathways, accelerating neuronal loss.

Mitochondrial fragmentation and impaired biogenesis further compromise recovery. The balance between mitochondrial fission and fusion, regulated by proteins such as dynamin-related protein 1 (Drp1) and mitofusins, is disrupted, leading to dysfunctional organelles that fail to meet energy demands. Strategies targeting mitochondrial protection, such as cyclosporine A, which inhibits mPTP opening, have shown promise in preclinical models but have yet to translate into effective stroke therapies.

Inflammatory Cascades

After a stroke, the brain undergoes an inflammatory response that influences both acute injury and long-term recovery. This process involves resident immune cell activation, inflammatory mediator release, and peripheral immune cell recruitment. While inflammation helps clear cellular debris and promote repair, excessive activation exacerbates neuronal damage.

Microglial Activation

Microglia, the brain’s resident immune cells, shift from a resting surveillance state to an activated phenotype. Activated microglia release signaling molecules, including tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), and ROS, contributing to secondary neuronal injury.

Microglia exhibit both neurotoxic and neuroprotective effects. Early-phase pro-inflammatory microglia worsen tissue damage, while later anti-inflammatory microglia aid tissue repair by secreting neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and insulin-like growth factor-1 (IGF-1). Modulating microglial polarization is a potential therapeutic strategy, with agents like minocycline showing promise in reducing harmful activation and improving recovery.

Cytokine Signaling

Cytokines orchestrate the inflammatory response, mediating both detrimental and reparative processes. Pro-inflammatory cytokines such as IL-1β, TNF-α, and interleukin-6 (IL-6) are rapidly upregulated, promoting BBB disruption, leukocyte infiltration, and excitotoxicity. Anti-inflammatory cytokines like IL-10 and transforming growth factor-beta (TGF-β) counteract excessive inflammation and support tissue repair.

Leukocyte Infiltration

BBB breakdown allows peripheral immune cells, including neutrophils, monocytes, and lymphocytes, to infiltrate the brain. Neutrophils release proteolytic enzymes such as matrix metalloproteinases (MMPs) and myeloperoxidase (MPO), worsening BBB disruption. Monocytes and macrophages adopt either pro-inflammatory or reparative phenotypes, influencing injury progression.

Blood-Brain Barrier Disruption

Ischemia-induced BBB breakdown increases permeability, allowing plasma proteins, immune cells, and other circulating factors to infiltrate the brain parenchyma. This disruption facilitates neuroinflammation and promotes vasogenic edema, worsening tissue damage.

Neuroregeneration Processes

Despite extensive damage, the brain has a degree of regenerative capacity. Neural stem cells (NSCs) proliferate and migrate toward infarcted areas, attempting to replace lost neurons. Axonal remodeling and synaptic plasticity also contribute to functional recovery. Experimental approaches targeting endogenous repair mechanisms, including stem cell transplantation and gene therapy, are under investigation.