Cortical neurogenesis in adult rats after reversible photothrombotic stroke

Cell transplantation for stroke

Cell transplantation has emerged as an experimental approach to restore brain function after stroke. Various cell types including porcine fetal cells, stem cells, immortalized cell lines, and marrow stromal cells are under investigation in experimental and clinical stroke trials. This review discusses the unique advantages and limitations of the different graft sources and emphasizes the current, limited knowledge about their biology. The survival, integration, and efficacy of neural transplants in stroke patients will depend on the type, severity, chronicity, adequacy of circulation, and location of the stroke lesion.

Stem cell factor stimulates neurogenesis in vitro and in vivo

Cerebral ischemia stimulates neurogenesis in proliferative zones of the rodent forebrain. To identify the signaling factors involved, cerebral cortical cultures prepared from embryonic mouse brains were deprived of oxygen. Hypoxia increased bromodeoxyuridine (BrdU) incorporation into cells that expressed proliferation markers and immature neuronal markers and that lacked evidence of DNA damage or caspase-3 activation. Hypoxia-conditioned medium and stem cell factor (SCF), which was present in hypoxia-conditioned medium at increased levels, also stimulated BrdU incorporation into normoxic cultures. The SCF receptor, c-kit, was expressed in neuronal cultures and in neuroproliferative zones of the adult rat brain, and in vivo administration of SCF increased BrdU labeling of immature neurons in these regions. Cerebral hypoxia and ischemia may stimulate neurogenesis through trophic factors, including SCF.

BetaIII tubulin-expressing neurons reveal enhanced neurogenesis in hippocampal and cortical structures after a contusion trauma in rats

Neurogenesis is not only restricted to embryonic development, but also occurs in adult mammalian brains, including human. In this study, evidence is provided, that neurogenesis is involved in the repair of hippocampal and cortical structures after CNS injury. Cortical contusion was induced in 8-week-old Wistar rats. This trauma resulted in a primary cortical lesion and ipsilateral distant remote hippocampal damage, involving primarily CA3-pyramidal cells. The progression of injury was followed over a time course of 7 days, using Nissl-staining and a monoclonal antibody against betaIII tubulin-a specific marker for neurogenic cells. Nissl staining showed a partial recovery of damaged cortical and hippocampal cells at day 7. This recovery was accompanied by an increase of neurogenic cells in these structures, particularly in the dentate gyrus and the neocortical areas. Taken together, these findings provide evidence for the involvement of neurogenesis in the repair processes after traumatic brain injury.

Heparin-binding epidermal growth factor-like growth factor: hypoxia-inducible expression in vitro and stimulation of neurogenesis in vitro and in vivo

Heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) is found in cerebral neurons, and its expression is increased after hypoxic or ischemic injury, which also stimulates neurogenesis. To investigate the possible role of HB-EGF in hypoxic-ischemic induction of neurogenesis, we measured its expression, effects, and target receptors in embryonic murine cerebral cortical cultures and in adult rat brain. Hypoxia increased HB-EGF expression by approximately 50% in cortical cultures, where expression was associated with mature and immature neurons. HB-EGF (5-100 ng/ml) stimulated by approximately 80% the incorporation of bromodeoxyuridine (BrdU) into cultured cells that expressed the HB-EGF receptors epidermal growth factor receptor (EGFR)/avian erythroblastic leukemia viral oncogene homolog 1 (ErbB1) and N-arginine dibasic convertase (NRDc). Intracerebroventricular administration of HB-EGF in adult rats increased BrdU labeling in the subventricular zone and in the subgranular zone of dentate gyrus, where EGFR/ErbB1 and NRDc were also expressed and where ischemia-induced neurogenesis is observed. We conclude that HB-EGF stimulates neurogenesis in proliferative zones of the adult brain that are also affected in ischemia and that it does so by interacting with EGFR/ErbB1 and possibly NRDc. Therefore, HB-EGF may help to trigger proliferation of neuronal precursors in brain after hypoxic or ischemic injury.

Insulin-like growth factor-I and neurogenesis in the adult mammalian brain

In most brain regions of highly developed mammals, the majority of neurogenesis is terminated soon after birth. However, new neurons are continually generated throughout life in the subventricular zone and the dentate gyrus of the hippocampus. Insulin-like growth factor-I (IGF-I) is a polypeptide hormone that has demonstrated effects on these progenitor cells. IGF-I induces proliferation of isolated progenitors in culture, as well as affecting various aspects of neuronal induction and maturation. Moreover, systemic infusion of IGF-I increases both proliferation and neurogenesis in the adult rat hippocampus, and uptake of serum IGF-I by the brain parenchyma mediates the increase in neurogenesis induced by exercise. Neurogenesis in the adult brain is regulated by many factors including aging, chronic stress, depression and brain injury. Aging is associated with reductions in both hippocampal neurogenesis and IGF-I levels, and administration of IGF-I to old rats increases neurogenesis and reverses cognitive impairments. Similarly, stress and depression also inhibit neurogenesis, possibly via the associated reductions in serotonin or increases in circulating glucocorticoids. As both of these changes have the potential to down regulate IGF-I production by neural cells, stress may inhibit neurogenesis indirectly via downregulation of IGF-I. In contrast, brain injury stimulates neurogenesis, and is associated with upregulation of IGF-I in the brain. Thus, there is a tight correlation between IGF-I and neurogenesis in the adult brain under different conditions. Further studies are needed to clarify whether IGF-I does indeed mediate neurogenesis in these situations.

Is it all DNA repair? Methodological considerations for detecting neurogenesis in the adult brain

Since the early 1960s, in vivo observations have shown the generation of new neurons from dividing precursor cells. Nevertheless, these experiments suffered from skepticism, suggesting that the prevailing labeling method, which incorporates tagged thymidine analogs, such as [3H]-thymidine or bromodeoxyuridine (BrdU), may not be detecting a proliferative event, but could rather mark DNA repair in postmitotic neurons. Even today many scientists outside the field are still skeptical, because the question of specificity for thymidine labeling has not been sufficiently answered. This current paper aims at evaluating the arguments that are used by proponents and skeptics of this method by (i) presenting histological evidence of specificity of BrdU labeling for neural stem cell/progenitor activity in the adult brain; (ii) validating and comparing BrdU labeling with other histological methods; and (iii) combining BrdU and labeling methods for apoptosis to argue against DNA repair being a major contribution of BrdU-positive cells.

Induction of neuronal type-specific neurogenesis in the cerebral cortex of adult mice: manipulation of neural precursors in situ

Over the past 3 decades, research exploring potential neuronal replacement therapies have focused on replacing lost neurons by transplanting cells or grafting tissue into diseased regions of the brain [Nat. Neurosci. 3 (2000) 67-78]. Over most of the past century of modern neuroscience, it was thought that the adult brain was completely incapable of generating new neurons. However, in the last decade, the development of new techniques has resulted in an explosion of new research showing that neurogenesis, the birth of new neurons, normally occurs in two limited and specific regions of the adult mammalian brain, and that there are significant numbers of multipotent neural precursors in many parts of the adult mammalian brain [Mol. Cell. Neurosci. 19 (1999) 474-486]. Recent findings from our laboratory demonstrate that it is possible to induce neurogenesis de novo in the adult mammalian brain, particularly in the neocortex where it does not normally occur, and that it may become possible to manipulate endogenous multipotent precursors in situ to replace lost or damaged neurons [Nature 405 (2000) 951-955; Neuron 25 (2000) 481-492]. Recruitment of new neurons can be induced in a region-specific, layer-specific, and neuronal type-specific manner, and newly recruited neurons can form long-distance connections to appropriate targets. Elucidation of the relevant molecular controls may both allow control over transplanted precursor cells and potentially allow the development of neuronal replacement therapies for neurodegenerative disease and other central nervous system injuries that do not require transplantation of exogenous cells.

Expression changes of somatostatin receptor subtypes sst2A, sst2B, sst3 and sst4 after a cortical contusion trauma in rats

The neuropeptide somatostatin acts as a neuromodulator in the CNS in a predominantly inhibitory manner. In this study, an ipsilateral cortical and hippocampal damage in the brain of adult rats was induced by a cortical contusion trauma in order to examine subsequent changes of expression of different somatostatin receptor subtypes (sst). By using subtype specific antibodies we found a clear decline of expression level for sst2A, sst2B, sst3 and sst4 subtypes in the pyramidal cell layer of the ipsilateral hippocampus. Nissl staining revealed that this decline of expression level is due to cell death of sst expressing neurons within the first 48 h after trauma. Additionally we found a progressive infiltration of sst4 positive cells into regions of cortical and hippocampal damage. The number of these cells increases strikingly within the first 3 days after trauma and it seems that their morphology changes from a round to an astrocyte-like shape. Moreover, sst4 and sst2A positive cells accumulate in the ipsilateral ependym and pyramidal-like cells expressing sst4 were found beneath the damaged CA3 pyramidal layer. Taken together, after trauma we found deterioration of sst positive neurons and an additional activation of sst4 and sst2A expressing cells the final fate of which has to be elucidated further.

Increased proliferation of neural progenitor cells but reduced survival of newborn cells in the contralateral hippocampus after focal cerebral ischemia in rats

Recent studies demonstrated that neurogenesis in the adult hippocampus increased after transient global ischemia; however, the molecular mechanism underlying increased neurogenesis after ischemia remains unclear. The finding that proliferation of progenitor cells occurred at least a week after ischemic insult suggests that the stimulus was not an ischemic insult to progenitor cells. To clarify whether focal ischemia increases the rate of neurogenesis in the remote area, the authors examined the contralateral hemisphere in rats subjected to permanent occlusion of the middle cerebral artery. In the subgranular zone of the hippocampal dentate gyrus, the numbers of bromodeoxyuridine (BrdU)-positive cells increased approximately sixfold 7 days after ischemia. In double immunofluorescence staining, more than 80% of newborn cells expressed Musashi1, a marker of neural stem/progenitor cells, but only approximately 10% of BrdU-positive cells expressed glial fibrillary acidic protein (GFAP), a marker of astrocytes. The number of BrdU-positive cells markedly decreased 28 days after BrdU administration after ischemia, but it was still elevated compared with that of sham-operated rats. In double immunofluorescence staining, 80% of newborn cells expressed NeuN, a marker of differentiated neurons, and 10% of BrdU-positive cells expressed GFAP. However, in the other areas of the contralateral hemisphere including the rostral subventricular zone, the number of BrdU-positive cells remained unchanged. These results showed that focal ischemia stimulated the proliferation of neuronal progenitor cells, but did not support survival of newborn cells in the contralateral hippocampus.

Amyloid-associated neuron loss and gliogenesis in the neocortex of amyloid precursor protein transgenic mice

APP23 transgenic mice express mutant human amyloid precursor protein and develop amyloid plaques predominantly in neocortex and hippocampus progressively with age, similar to Alzheimer's disease. We have previously reported neuron loss in the hippocampal CA1 region of 14- to 18-month-old APP23 mice. In contrast, no neuron loss was found in neocortex. In the present study we have reinvestigated neocortical neuron numbers in adult and aged APP23 mice. Surprisingly, results revealed that 8-month-old APP23 mice have 13 and 14% more neocortical neurons compared with 8-month-old wild-type and 27-month-old APP23 mice, respectively. In 27-month-old APP23 mice we found an inverse correlation between amyloid load and neuron number. These results suggest that APP23 mice have more neurons until they develop amyloid plaques but then lose neurons in the process of cerebral amyloidogenesis. Supporting this notion, we found more neurons with a necrotic-apoptotic phenotype in the neocortex of 24-month-old APP23 mice compared with age-matched wild-type mice. Stimulated by recent reports that demonstrated neurogenesis after targeted neuron death in the mouse neocortex, we have also examined neurogenesis in APP23 mice. Strikingly, we found a fourfold to sixfold increase in newly produced cells in 24-month-old APP23 mice compared with both age-matched wild-type mice and young APP23 transgenic mice. However, subsequent cellular phenotyping revealed that none of the newly generated cells in neocortex had a neuronal phenotype. The majority were microglial and to a lesser extent astroglial cells. We conclude that cerebral amyloidosis in APP23 mice causes a modest neuron loss in neocortex and induces marked gliogenesis.

Mild cerebral ischemia induces loss of cyclin-dependent kinase inhibitors and activation of cell cycle machinery before delayed neuronal cell death

After mild ischemic insults, many neurons undergo delayed neuronal death. Aberrant activation of the cell cycle machinery is thought to contribute to apoptosis in various conditions including ischemia. We demonstrate that loss of endogenous cyclin-dependent kinase (Cdk) inhibitor p16(INK4a) is an early and reliable indicator of delayed neuronal death in striatal neurons after mild cerebral ischemia in vivo. Loss of p27(Kip1), another Cdk inhibitor, precedes cell death in neocortical neurons subjected to oxygen-glucose deprivation in vitro. The loss of Cdk inhibitors is followed by upregulation of cyclin D1, activation of Cdk2, and subsequent cytoskeletal disintegration. Most neurons undergo cell death before entering S-phase, albeit a small number ( approximately 1%) do progress to the S-phase before their death. Treatment with Cdk inhibitors significantly reduces cell death in vitro. These results show that alteration of cell cycle regulatory mechanisms is a prelude to delayed neuronal death in focal cerebral ischemia and that pharmacological interventions aimed at neuroprotection may be usefully directed at cell cycle regulatory mechanisms.

Expression of polysialylated neural cell adhesion molecule in rat brain after transient middle cerebral artery occlusion

The highly polysialylated form of neural cell adhesion molecule (PSA-NCAM) is important for neurite outgrowth. With this molecule as a marker of plastic change in neurons, we investigated its temporal expression in rat brain after transient middle cerebral artery (MCA) occlusion. In sham-control brain, only subependymal neurons showed a positive immunoreactivity for PSA-NCAM. After 90 min of transient MCA occlusion, neurons in the piriform cortex began to be positively stained at 1 h, while neurons in the cortex and caudate of the MCA territory became positive after 8 h. The stainings persisted for 1 and 3 days after reperfusion. The present results indicate that neurons in the cerebral cortex and caudate have the capability of plastic change in the adult brain, and that those in the piriform cortex rapidly undergo plastic change probably in response to transneuronal injury.

Neurogenesis and neuroprotection in the adult brain. A putative role for 5-lipoxygenase?

5-Lipoxygenase (5-LOX) and cyclooxygenase-2 (COX-2) are two enzymes that are critical for the synthesis of eicosanoids, the inflammatory metabolites of arachidonic acid. Both 5-LOX and COX-2 are expressed in the brain, including in CNS neurons. The physiologic role of these proteins in neuronal functioning is not clear. In non-neuronal tissues these two enzymes often assume similar roles: in addition to their function in inflammation, 5-LOX and COX-2 appear to be associated with cell proliferation, that is, with tumor growth. High 5-LOX expression has been noticed in the proliferating brain or pancreatic tumor cells; reduction in tumor cell proliferation and/or destruction of tumor cells was achieved with 5-LOX inhibitors. Proliferation of immature neurons/neuroblasts is an important component of mitotic neurogenesis. We investigated the role of 5-LOX in proliferation using cultures of human neuronal precursor cells, NT2. We found that these cells express 5-LOX mRNA and we used 3H-thymidine incorporation as a measure of cell proliferation; this was reduced by treating the cultures with 5-LOX inhibitor AA-861. We propose that the 5-LOX pathway plays a crucial role in mitotic neurogenesis. Additional studies should explore whether 5-LOX may participate in neurogenesis related pathologies and whether it should be considered a target for procedures aimed at altering neurogenesis for therapeutic purposes.

Cortical neurogenesis in adult rats after transient middle cerebral artery occlusion

BACKGROUND AND PURPOSE

This study explored the possible occurrence of newly generated nerve cells in the ischemic cortex of adult rats after middle cerebral artery occlusion and reperfusion.

METHODS

Nine- to 10-week-old male Wistar rats were subjected to 2 hours of middle cerebral artery occlusion by the monofilament method. Rats received repeated intraperitoneal injections of the cell proliferation-specific marker 5-bromodeoxyuridine (BrdU) after stroke induction. Brain sections were processed for immunohistochemistry with an avidin-biotin complex-alkaline phosphatase and/or -peroxidase method. Brain sections processed with double-immunofluorescent staining were further scanned by confocal microscopy.

RESULTS

Interspersed among the predominantly newly formed glial cells, some cells were double labeled by BrdU and 1 of the neuron-specific markers, Map-2, beta-tubulin III, and Neu N, at 30 and 60 days after stroke onset. These cells were randomly distributed throughout cortical layers II through VI, occurring with highest density in the ischemic boundary zone. Three-dimensional confocal analyses of BrdU and the neuron-specific marker Neu N confirmed their colocalization within the same cortical cells.

CONCLUSIONS

This study suggests that new neurons can be generated in the cerebral cortex of adult rats after transient focal cerebral ischemia. Cortical neurogenesis may be a potential pathway for brain repair after stroke.

Neurogenesis in dentate subgranular zone and rostral subventricular zone after focal cerebral ischemia in the rat

Because neurogenesis persists in the adult mammalian brain and can be regulated by physiological and pathological events, we investigated its possible involvement in the brain's response to focal cerebral ischemia. Ischemia was induced by occlusion of the middle cerebral artery in the rat for 90 min, and proliferating cells were labeled with 5-bromo-2'-deoxyuridine-5'-monophosphate (BrdUrd) over 2-day periods before sacrificing animals 1, 2 or 3 weeks after ischemia. Ischemia increased the incorporation of BrdUrd into cells in two neuroproliferative regions-the subgranular zone of the dentate gyrus and the rostral subventricular zone. Both effects were bilateral, but that in the subgranular zone was more prominent on the ischemic side. Cells labeled with BrdUrd coexpressed the immature neuronal markers doublecortin and proliferating cell nuclear antigen but did not express the more mature cell markers NeuN and Hu, suggesting that they were nascent neurons. These results support a role for ischemia-induced neurogenesis in what may be adaptive processes that contribute to recovery after stroke.

Progressive and reproducible focal cortical ischemia with or without late spontaneous reperfusion generated by a ring-shaped, laser-driven photothrombotic lesion in rats

Clinical stroke is mostly of thromboembolic origin, in which the magnitude of brain damage resulting from arterial occlusions depends on the degree and duration of the concomitant ischemia. To facilitate more controllable and reproducible study of stroke-related pathophysiological mechanisms, a photothrombotic ring stroke model was initially developed in adult rats. The ring interior zone comprises an anatomically well confined cortical region-at-risk which is gradually encroached by progressive hypoperfusion, thus mimicking the situation (albeit in inverse fashion) of an ischemic penumbra or stroke-in-evolution. Modification of this model using a thinner ring irradiation beam resulted in late spontaneous reperfusion in the cortical region-at-risk and a remarkable morphological tissue recovery in this ostensibly critically injured region. On the other hand, doubling the thin irradiating beam intensity facilitates a complementary situation in which lack of reperfusion in the region-at-risk after stroke induction leads to tissue pannecrosis. The dual photothrombotic ring stroke model, effectuated either with or without reperfusion and thereby tissue recovery or pannecrosis, may be well suited for the study of events related to postischemic survival or cell death in the penumbra region. To popularize the photothrombotic ring stroke model, we present a detailed protocol of how this model is induced in either version as well as protocols for transcardial carbon black perfusion and laser-Doppler flowmetry experiments.

Motor activity-mediated partial recovery in ischemic rats

Spontaneous partial recovery in motor and/or cognitive dysfunctions in stroke patients has been documented, but the factors that affect such functional improvement have not been well elucidated. The present study demonstrates that repeated behavioral testing (daily or once a week over a period of 4 weeks) promoted partial recovery from motor asymmetry in adult ischemic rats. In contrast, ischemic animals that were only tested once every 2 weeks or once after 4 weeks did not show such partial recovery. These results suggest that repeated behavioral testing (i.e., increased use of the ischemia-affected limbs and body parts) may contribute to partial recovery of motor deficits following an experimental stroke, even in the absence of pharmacological therapeutic intervention.