Regenerating the brain

A rat's whiskers point the way toward a novel stimulus-dependent, protective stroke therapy

Stroke is the fourth leading cause of death in the United States and the leading cause of long-term disability. Ischemic stroke, due to an interruption in blood supply, is particularly prevalent; 87% of all strokes are ischemic. Unfortunately, current options for acute treatment are extremely limited and there is a great need for new treatment strategies. This review will discuss evidence that mild sensory stimulation can completely protect the jeopardized brain from an impending stroke in a rodent model. When delivered within the first 2 hours following ischemic onset, this stimulation results in complete protection, including a full reestablishment of cortical function, sensorimotor capabilities, and blood flow. Identical stimulation, however, initiated 3 hours following ischemic onset, results in an increase in damage compared with untreated animals. The protective effect is not specific to a single sensory modality, anesthesia, or age, and increasing evoked cortical activity by increasing stimulation accelerates recovery. Taken together, these findings demonstrate that cortical activity is a critical factor for protection and suggest a new, exciting potential avenue for the development of acute stroke treatment strategies that may produce a noninvasive, drug-free, equipment-free, and side effect-free means of protecting from ischemic stroke.

Mild sensory stimulation reestablishes cortical function during the acute phase of ischemia

When delivered within 1 and in most cases 2 h of permanent middle cerebral artery occlusion (pMCAO), mild sensory stimulation (intermittent single whisker stimulation) was shown to be completely neuroprotective 24 h after pMCAO in a rodent model of ischemic stroke, according to assessment with multiple techniques (Lay et al., 2010). The acute effect of stimulation treatment on the ischemic cortex, however, has yet to be reported. Here we characterize cortical function and perfusion during the 120 min whisker stimulation period in four experimental groups with treatment initiated 0, 1, 2 (protected groups), or 3 h (unprotected group) post-pMCAO using multiple techniques. According to functional imaging, a gradual return of evoked whisker functional representation to baseline levels was initiated with treatment onset and completed within the treatment period. Evoked neuronal activity and reperfusion to the ischemic area also showed a gradual recovery in protected animals. Surprisingly, a similar recovery profile was observed in response to treatment in all protected animals, regardless of treatment onset time. Nonstimulated pMCAO control group data demonstrate that reperfusion is not spontaneous. This makes the complete protection observed in the majority of animals stimulated at 2 h post-pMCAO even more surprising, as these animals recovered despite having been in a severely ischemic state for two full hours. In summary, when delivered within a 2 h window post-pMCAO, whisker stimulation treatment initiated reperfusion and a gradual recovery of cortical function that was completed or nearly completed within the treatment period.

Perturbed cellular response to brain injury during aging

Old age is associated with an enhanced susceptibility to stroke and poor recovery from brain injury, but the cellular processes underlying these phenomena are only partly understood. Therefore, studying the basic mechanisms underlying structural and functional recovery after brain injury in aged subjects is of considerable clinical interest. Behavioral and cytological analyses of rodents that have undergone experimental injury show that: (a) behaviorally, aged rodents are more severely impaired by ischemia than are young animals, and older rodents also show diminished functional recovery; (b) compared to young animals, aged animals develop a larger infarct area, as well as a necrotic zone characterized by a higher rate of cellular degeneration and a larger number of apoptotic cells; (c) both astrocytes and macrophages are activated strongly and early following stroke in aged rodents; (d) in older animals, the premature, intense cytoproliferative activity following brain injury leads to the precipitous formation of growth-inhibiting scar tissue, a phenomenon amplified by the persistent expression of neurotoxic factors; (e) though the timing is altered, the regenerative capability of the brain is largely preserved in rats, at least into early old age. Whether endogenous neurogenesis contributes to spontaneous recovery after stroke has not yet been established. If neurogenesis from endogenous neuronal stem cells is to be used therapeutically, an individual approach will be required to assess the possible extent of neurogenic response as well as the possibilities to alter this response for functional improvement or prevention of further loss of brain function.

Stem cells in human neurodegenerative disorders--time for clinical translation?

Stem cell-based approaches have received much hype as potential treatments for neurodegenerative disorders. Indeed, transplantation of stem cells or their derivatives in animal models of neurodegenerative diseases can improve function by replacing the lost neurons and glial cells and by mediating remyelination, trophic actions, and modulation of inflammation. Endogenous neural stem cells are also potential therapeutic targets because they produce neurons and glial cells in response to injury and could be affected by the degenerative process. As we discuss here, however, significant hurdles remain before these findings can be responsibly translated to novel therapies. In particular, we need to better understand the mechanisms of action of stem cells after transplantation and learn how to control stem cell proliferation, survival, migration, and differentiation in the pathological environment.

Adult neurogenesis is functionally associated with AD-like neurodegeneration

Alzheimer's disease (AD) is predominantly characterized by progressive neuronal loss in the brain. It has been recently found that adult neurogenesis in the hippocampal dentate gyrus of AD patients is significantly enhanced, while its functional significance is still unknown. By using an AD-like neurodegeneration mouse model, we show here that neurogenesis in the dentate gyrus was neurodegenerative stage-dependent. At early stages of neurodegeneration, neurogenesis was significantly enhanced and newly generated neurons migrated into the local neuronal network. Up to late stages of neurodegeneration, however, the survival of newly generated neurons was impaired so that the enhanced neurogenesis could not be detected anymore. Most interestingly, these dynamic changes in neurogenesis were correlated with the severity of neuronal loss in the dentate gyrus, indicating that neurogenesis may work as a self-repairing mechanism to compensate for neurodegeneration. Therefore, to enhance endogenous neurogenesis at early stages of neurodegeneration may be a valuable strategy to delay neurodegenerative progress.

Neurogenic responses to amyloid-beta plaques in the brain of Alzheimer's disease-like transgenic (pPDGF-APPSw,Ind) mice

Formation and accumulation of amyloid-beta (A beta) plaques are associated with declined memory and other neurocognitive function in Alzheimer's disease (AD) patients. However, the effects of A beta plaques on neural progenitor cells (NPCs) and neurogenesis from NPCs remain largely unknown. The existing data on neurogenesis in AD patients and AD-like animal models remain controversial. For this reason, we utilized the nestin second-intron enhancer controlled LacZ (pNes-LacZ) reporter transgenic mice (pNes-Tg) and Bi-transgenic mice (Bi-Tg) containing both pPDGF-APPSw,Ind and pNes-LacZ transgenes to investigate the effects of A beta plaques on neurogenesis in the hippocampus and other brain regions of the AD-like mice. We chose transgenic mice at 2, 8 and 12 months of age, corresponding to the stages of A beta plaque free, plaque onset and plaque progression to analyze the effects of A beta plaques on the distribution and de novo neurogenesis of (from) NPCs. We demonstrated a slight increase in the number of NPCs in the hippocampal regions at the A beta plaque free stage, while a significant decrease in the number of NPCs at A beta plaque onset and progression stages. On the other hand, we showed that A beta plaques increase neurogenesis, but not gliogenesis from post-mitotic NPCs in the hippocampus of Bi-Tg mice compared with age-matched control pNes-Tg mice. The neurogenic responses of NPCs to A beta plaques suggest that experimental approaches to promote de novo neurogenesis may potentially improve neurocognitive function and provide an effective therapy for AD.