An Integrated View of Neural Repair After Stroke
Stroke triggers physiological and structural changes in neuronal circuits adjacent to the infarct. These changes affect stroke recovery and can be manipulated to lead to neural repair. Stroke neural repair can be classified into three distinct processes:
Enhancing neuronal connectivity/coherence
Altering tissue barrier formation, glial signaling and fibrotic scar
Building Brain
Enhancing neuronal connectivity/coherence. Stroke alters the neurons adjacent to and connected to the stroke site. These neurons survive the injury but have reduced functional connectivity: they have a reduction responding to their normal neuronal inputs (sensory inputs, upstream motor commands) and they have a reduction in correlated neuronal firing networks. This can be seen in voltage sensitive dye imaging of neuronal activity in mouse brain to somatosensory inputs after a stroke or in the excitability of human motor cortex after stroke in aggregate studies or single site studies. Recovery after damage to the main output of the motor cortex, the corticospinal tract, precedes with enhanced functional cortical networks in motor and premotor areas in humans and also in this study. Functional MRI mapping of motor cortex during recovery or older PET/MRI studies of recovery after stroke demonstrate that reduced brain activity after stroke and recover during functional recovery of the patient to take back over cortex in which responsiveness is lost, or new areas of brain. This same process of recovering responsivity of peri-infarct and connected cortical areas to the stroke as a process in recovery is seen in rodent models.
The biology of enhancing recovery after stroke involves enhancing the functional and structural connectivity of these adjacent and connected neuronal groups. In functional connectivity, measures the increase neurons firing in co-active networks enhances recovery (review). This can be done by increasing activity of the transcription factor CREB in direct ways with viral expression, in indirect ways by increasing signaling through a pathway connected to CREB, or in parallel pathways by blocking the chemokine signaling receptor, CCR5. When neuronal networks are directly visualized in the living mouse over time, motor and premotor functional connectivity is reduced by stroke, and this connectivity is enhanced by treatments that induce CREB, such as with isoform-specific phosphodiesterase inhibitors. This has an apparent effect in maps of the functional connectivity of neurons after stroke:
Recovery after stroke can also be enhanced by increasing the connections of adjacent cortical regions in a structural manner. This is distinct from functional connectivity, in that a treatment or molecular process increases measurable connections, often between motor-premotor or motor-somatosensory areas. Stroke causes changes in the pattern of the expressed genes in adjacent and connected brain regions which, we quantified, identifies key proteins that are altered that influence axonal sprouting and the formation of new connections. The Carmichael lab has published on many of these systems, and this includes growth factors (GDF10), epigenetic modifiers (ATRX), growth inhibitory systems, glial inhibitory molecules, and pro-growth extracellular matrix molecules and neurodevelopmental transcription factors (unpublished).
It is likely that this reduced functional and structural connectivity in neuronal networks after stroke extends to the downstream networks after stroke. Recent studies of direct spinal cord stimulation after stroke may function to enhance functional connectivity of descending motor networks in humans and enhance recovery, or reduce spasticity and lead to greater motor control.
Altering tissue barrier formation, glial signaling and fibrotic scar. Stroke alters the cell-cell signaling in tissue microenvironments of the brain. The core tissue microenvironment of the brain is the neurovascular niche.
The signaling in a cell niche responds to stress and transmits signals for further injury and secondary degeneration or for repair. This was classically described in the bone marrow in a stem cell niche. It exists in the brain and spinal cord, and has a role in tissue repair. In white matter injury, and particularly the progressive ischemia that underlies white matter stroke and vascular dementia, the neurovascular niche is central to lesion expansion and possible repair. Astrocytes in stroke form tissue barriers that wall off CNS from non-CNS, such as brain from infiltrating macrophages. Astrocytes also play a role in synaptogenesis, angiogenesis and blood brain barrier re-formation. Pericytes and perivascular fibroblasts appear to play a main role in tissue fibrosis, which forms scar (not astrocytes) and is a target for studies of neural repair after stroke. OPCs are a cell that possibly engage after injury with neuronal contact and interaction, phagocytosis, inflammation and through their pluripotent possibilities, glial responses. The neurovascular niche in white matter stroke/vascular dementia and “grey matter stroke” is a potent target in studies of neural repair.
Building Brain. This is a somewhat casual term that applies to some of the goals and findings of biomaterials studies in neural repair. Implantable matrices and injectable hydrogels induce growth of adjacent brain and spinal cord, and in so doing recruit and construct new cellular connections and circuits. Studies in collaboration with Dr. Tatiana Segura (Duke University) have shown that biomaterial hydrogels can be injected into the stroke cavity, self-assemble and induce the in-growth of blood vessels, pericytes and axons—essentially a partial neurovascular niche. This produces stroke recovery. A particularly powerful approach in neuroengineering is the ability display different protein motifs and growth factors, to tune this regenerative process. In one notable example, the display of immobilized VEGF alters downstream VEGF signaling, and in conjunction with a hyaluronan hydroge and heparin nanoparticles, is a strongly regenerative therapy. In schematic view, a biomaterial approach to “building brain” looks something like this:
This approach has the clinical translational applicability of injecting a biomaterial in a minimally invasive approach, into the infarct cavity (which is an area of pan-necrosis and a cavity) and adjacent to the region of greatest tissues reorganization and endogenous repair in stroke. Future studies might modify injectable hydrogels with displayed and diffusible proteins and in the porous structure of the hydrogel.
Neuroplasticity after stroke includes the formation of new connections in cortex adjacent to the stroke site (termed axonal sprouting), the formation of new neurons and their migration to areas of injury (termed post-stroke neurogenesis), the recruitment and differentiation of immature forms of glial cells (oligodendrocyte precursor cells, OPCs) and physiological changes in the responses of cortical circuits (post-stroke hypoexcitability). The Carmichael Laboratory has active projects in all four of these areas, and in determining how stem cell transplants in stroke interface with the normal, or endogenous, neuroplasticity to promote recovery.
A key aspect of neural repair after stroke is that none of these cellular and molecular events are occurring in isolation. Axonal sprouting after stroke means that neurons are induced into a growth state, but encounter growth inhibitory molecules. This is sort of a yin/yang of in axonal sprouting, and evidence is emerging that a focus on just one aspect, such as blocking growth inhibitors, is not enough to induce functional recovery. In post-stroke neurogenesis, migrating immature neurons preferentially associate with angiogenic blood vessels, forming a neurovascular niche. Neurogenesis and angiogenesis are integrated tissue reorganization processes after stroke.
Neural repair after stroke also occurs within a time spectrum, from the initial damage of the stroke itself and extending to later phases of recovery. Events that promote repair and recovery later in stroke might exacerbate the initial stroke damage early in this spectrum. This is another example of a yin/yang in stroke: cellular processes that initially protect the brain from further injury actually impair recovery if they persist for too long. We have shown this to be the case with tonic GABA inhibitory signaling.
Finally, the events that underlie neural repair after stroke are occurring within an injured brain that exhibits altered behavioral and neuronal activity patterns. The stroke itself causes alterations in movement, sensation, language and cognition, and impairments in these functions alter the normal neuronal activity patterns in these brain areas. If these areas are also undergoing axonal sprouting or neurogenesis, then the impaired activity patterns after stroke may impact these two neural repair processes. Neurorehabilitation after stroke imposes still other patterns of behavioral activity on the injured and reorganizing brain. Physical therapists work with stroke patients to increase walking and train the gait cycle; occupational therapists stimulate repetitive arm movements. These behavioral activity patterns directly influence brain maps, and alter, for example, angiogenesis and axonal sprouting.
The important aspect of all these interactions is that neural repair after stroke is an integrated process of activity, physiological plasticity, molecular growth and inhibitory programs, vascular remodeling and glial responses. A human therapy that repairs the brain after stroke will need to be developed with an integrated view of neural repair.