Clinical translation of stem cell transplantation in Parkinson's disease.

In Parkinson's disease (PD), the main pathology underlying the motor symptoms is a loss of nigrostriatal dopaminergic neurons. Clinical trials of intrastriatal transplantation of human foetal mesencephalic tissue have shown that the grafted dopaminergic neurons re-innervate the striatum, restore striatal dopamine release and, in some cases, induce major, long-lasting improvement of motor function. However, nonmotor symptoms originating from degeneration outside the striatum or in nondopaminergic systems are not alleviated by intrastriatal implantation of dopaminergic neurons. Stem cells and reprogrammed cells could potentially be used to produce dopaminergic neurons for transplantation in patients with PD. Recent studies demonstrate that standardized preparations of dopaminergic neurons of the correct substantia nigra phenotype can be generated from human embryonic stem cells in large numbers, and they will soon be available for patient application. In addition, dopaminergic neurons derived from human induced pluripotent stem cells are being considered for clinical translation. Important challenges include the demonstration of potency (growth capacity and functional efficacy) and safety of the generated dopaminergic neurons in preclinical animal models. The dopaminergic neurons should subsequently be tested, using optimal patient selection and cell preparation and transplantation procedures, in controlled clinical studies.

Proximity of brain infarcts to regions of endogenous neurogenesis and involvement of striatum in ischaemic stroke.

BACKGROUND AND PURPOSE: Clinical stroke trials with stem cell-based approaches aiming for trophic actions, modulation of inflammation and neuroprotection are ongoing. However, experimental studies also suggest that neuronal replacement by grafted neural stem cells (NSCs) and possibly by endogenous NSCs from the subventricular zone (SVZ) may restore function in the stroke-damaged striatum. To evaluate the potential clinical impact of these findings, we analyzed the spatial relationship of infarcts to the SVZ and the proportion of individuals with striatal lesions in a consecutive series of ischaemic stroke patients. METHODS: Patients aged 20-75 years with first-ever ischaemic stroke underwent DW-MRI of the brain within 4 days after stroke onset. We analyzed location, size, number of acute focal ischaemic abnormalities and their spatial relationship to the SVZ. Stroke severity was assessed using NIH Stroke Scale (NIHSS). RESULTS: Of 108 included patients, the distance from the nearest margin of the infarct(s) to the SVZ was ≤2 mm in 51/102 patients with visible ischaemic lesions on DW-MRI. Twenty-four patients had involvement of striatum. Eight of these had predominantly striatal lesions, that is >50% of the total ischaemic lesion volume was located in caudate nucleus and/or putamen. These 8 patients had a median NIHSS of 3. CONCLUSIONS: Many stroke patients have infarcts located close to the SVZ, providing some supportive evidence that optimized endogenous neurogenesis may have therapeutic potential. However, predominantly striatal infarcts are rare and tend to give mild neurological deficits, indicating that striatum should not be the primary target for neuronal replacement efforts in humans.

Brain inflammation and adult neurogenesis: the dual role of microglia.

In the adult mammalian brain, neurogenesis from neural stem/progenitor cells continues in two regions: the subgranular zone in the dentate gyrus and the subventricular zone lining the lateral ventricles. The generated neuroblasts migrate to their appropriate location and differentiate to mature granule cells and olfactory bulb interneurons, respectively. Following injury such as stroke, neuroblasts generated in the subventricular zone migrate also into areas which are not normally neurogenic, e.g. striatum and cerebral cortex. In the initial studies in rodents, brain inflammation and microglia activation were found to be detrimental for the survival of the new hippocampal neurons early after they had been born. The role of inflammation for adult neurogenesis has, however, turned out to be much more complex. Recent experimental evidence indicates that microglia under certain circumstances can be beneficial and support the different steps in neurogenesis, progenitor proliferation, survival, migration, and differentiation. Here we summarize the current knowledge on the role of inflammation and in particular of microglia in adult neurogenesis in the intact and injured mammalian brain. We conclude that microglia activation, as an indicator of inflammation, is not pro- or antineurogenic per se but the net outcome is dependent on the balance between secreted molecules with pro- and antiinflammatory action.

Grafts of fetal dopamine neurons survive and improve motor function in Parkinson's disease.

Neural transplantation can restore striatal dopaminergic neurotransmission in animal models of Parkinson's disease. It has now been shown that mesencephalic dopamine neurons, obtained from human fetuses of 8 to 9 weeks gestational age, can survive in the human brain and produce marked and sustained symptomatic relief in a patient severely affected with idiopathic Parkinson's disease. The grafts, which were implanted unilaterally into the putamen by stereotactic surgery, restored dopamine synthesis and storage in the grafted area, as assessed by positron emission tomography with 6-L-[18F]fluorodopa. This neurochemical change was accompanied by a therapeutically significant reduction in the patient's severe rigidity and bradykinesia and a marked diminuation of the fluctuations in the patient's condition during optimum medication (the "on-off" phenomenon). The clinical improvement was most marked on the side contralateral to the transplant.

Increased levels of messenger RNAs for neurotrophic factors in the brain during kindling epileptogenesis.

Kindling, induced by repeated subconvulsive electrical or chemical stimulations leads to progressive and permanent amplification of seizure activity, culminating in generalized seizures. We report that kindling induced by electrical stimulation in the ventral hippocampus leads to a marked and transient increase in mRNA for NGF and BDNF in the dentate gyrus, the parietal cortex, and the piriform cortex. BDNF mRNA increased also in the pyramidal layer of hippocampus and in the amygdaloid complex. No change was seen in the level of HDNF/NT-3 mRNA. The increased expression of NGF and BDNF mRNAs was not influenced by pretreatment with the NMDA receptor antagonist MK801, but was partially blocked by the quisqualate, AMPA receptor antagonist NBQX. The presumed subsequent increase of the trophic factors themselves may be important for kindling-associated plasticity in specific neuronal systems in the hippocampus, which could promote hyperexcitability and contribute to the development of epileptic syndromes.

Increased production of the TrkB protein tyrosine kinase receptor after brain insults.

The protein-tyrosine kinases Trk, TrkB, and TrkC are signal-transducing receptors for a family of neurotrophic factors known as the neurotrophins. Here we show that seizures induced by hippocampal kindling lead to a rapid, transient increase of trkB mRNA and protein in the hippocampus. TrkB is a component of a high affinity receptor for brain-derived neurotrophic factor (BDNF). No change was detected in mRNAs for Trk or TrkC, components of the high affinity nerve growth factor or neurotrophin-3 receptors, respectively. trkB mRNA was also transiently increased in the dentate gyrus following cerebral ischemia and hypoglycemic coma; these treatments had no effect on trk and trkC mRNAs. The increase in trkB mRNA and protein showed the same time course and distribution as the increase in BDNF mRNA. These data suggest that BDNF and its receptor may play a local role within the hippocampus in kindling-associated neural plasticity and in neuronal protection following epileptic, ischemic, and hypoglycemic insults.

Cell replacement therapies for central nervous system disorders.

In animal models, immature neural precursors can replace lost neurons, restore function and promote brain self-repair. Clinical trials in Parkinson's disease suggest that similar approaches may also work in the diseased human brain. But how realistic is it that cell replacement can be developed into effective clinical therapy?

Dopamine release from nigral transplants visualized in vivo in a Parkinson's patient.

Synaptic dopamine release from embryonic nigral transplants has been monitored in the striatum of a patient with Parkinson's disease using [11C]-raclopride positron emission tomography to measure dopamine D2 receptor occupancy by the endogenous transmitter. In this patient, who had received a transplant in the right putamen 10 years earlier, grafts had restored both basal and drug-induced dopamine release to normal levels. This was associated with sustained, marked clinical benefit and normalized levels of dopamine storage in the grafted putamen. Despite an ongoing disease process, grafted neurons can thus continue for a decade to store and release dopamine and give rise to substantial symptomatic relief.

Prospects of transplantation in human neurodegenerative diseases.

Over the past decade experimental data obtained from animals have suggested that restoration or preservation of function through cell transplantation into the CNS might be developed into a useful therapeutic approach in human neurodegenerative disorders. Clinical trials in patients with Parkinson's disease have provided evidence that grafts of fetal dopaminergic neurons can survive and induce functional effects in the human brain, but no treatment based on transplantation is available yet. Initiation of studies of patients with striatal neural grafts in Huntington's disease is supported by findings in animal models, and is motivated by the lack of therapy and the severity of the symptoms in this disorder. Application of cell transplantation to other neurodegenerative disorders such as Alzheimer's disease, amyotrophic lateral sclerosis, and hereditary ataxia is definitely premature. Further progress can be made only by systematic studies in animals of the scientific issues that can now be defined, but will also require clinical trials in a few well-monitored patients.

Neurotrophins and brain insults.

Epileptic, hypoglycaemic, ischaemic and traumatic insults to the brain induce marked changes of gene expression for the neurotrophins, nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3, and their high-affinity receptors, TrkB and TrkC, in cortical and hippocampal neurones. Release of glutamate and influx of Ca2+ are the most important triggering factors. The major hypotheses for the functional effects of the insult-induced neurotrophin changes are protection against neuronal damage and stimulation of sprouting and synaptic reorganization. More insight into the regulation and role of the neurotrophins after brain insults should increase our understanding of pathophysiological mechanisms in, for example, epileptogenesis and cell death, and could lead to new therapeutic strategies.

Clinical application of cell transplantation and neurotrophic factors in CNS disorders.

Cell transplantation and administration of neurotrophic factors are now being explored as new therapeutic strategies to restore and preserve function in the diseased human central nervous system. Neural grafts show long-term survival and restore function in patients with Parkinson's disease, but the symptomatic relief needs to be increased. Cell transplantation also seems justified in patients with Huntington's disease and, possibly, in demyelinating disorders. Clinical trials with neurotrophic factors have been initiated in amyotrophic lateral sclerosis, dementia and Huntington's disease, and may later be started in Parkinson's disease and after acute brain insults. However, it remains to be shown if neurotrophic factors can rescue damaged cells in the brain and spinal cord of patients with these disorders.

Transplantation strategies in the treatment of Parkinson's disease: experimental basis and clinical trials.

Neural grafting has over the last decade emerged as a possible tool for the substitution of damaged neurons in the central nervous system and for the promotion of symptomatic recovery after brain damage. Transplantation studies in the 6-hydroxydopamine lesion rat model of Parkinson's disease were initiated in the late seventies. The first studies were based on the neuronal replacement paradigm, using developing dopamine brain cells obtained from the substantia nigra region of embryonic cadavers. When implanted into the striatum such grafts were found to reinnervate part of the previously denervated striatum and restore dopamine turnover and release to near-normal levels. In both rats and monkeys the nigral grafts have been shown to normalize some, but not all, Parkinson-like symptoms in the dopamine deficient recipients. Grafting of adrenal medullary tissue was introduced in the early eighties as an alternative to the use of embryonic cadaver tissue. The adrenal medullary grafts have, however, so far shown poor long-term survival in both rats and monkeys, and consistent with this no sustained dopamine release have been observed in the brain of long-term grafted animals. Likewise, no long-lasting effects of adrenal medullary grafts on spontaneous motor or sensori-motor behavior have so far been documented in either the rat or the monkey model. The results so far reported from trials using adrenal medullary grafts in patients with Parkinson's disease appear to conform to the available animal experimental data at least in two important respects: significant long-term graft survival has not been possible to document, and any clear-cut functional effects consistent with sustained graft-induced dopamine release have not been demonstrated. Initial results from ongoing trials using grafts of fetal nigral tissue are presented and discussed.

Increased neurogenesis in a model of electroconvulsive therapy.

BACKGROUND: Electroconvulsive therapy (ECT) is a widely used and efficient treatment modality in psychiatry, although the basis for its therapeutic effect is still unknown. Past research has shown seizure activity to be a regulator of neurogenesis in the adult brain. This study examines the effect of a single and multiple electroconvulsive seizures on neurogenesis in the rat dentate gyrus. METHODS: Rats were given either a single or a series of 10 electroconvulsive seizures. At different times after the seizures, a marker of proliferating cells, Bromodeoxyuridine (BrdU), was administered to the animals. Subsequently, newborn cells positive for BrdU were counted in the dentate gyrus. Double staining with a neuron-specific marker indicated that the newborn cells displayed a neuronal phenotype. RESULTS: A single electroconvulsive seizure significantly increased the number of new born cells in the dentate gyrus. These cells survived for at least 3 months. A series of seizures further increased neurogenesis, indicating a dose-dependent mechanism. CONCLUSIONS: We propose that generation of new neurons in the hippocampus may be an important neurobiologic element underlying the clinical effects of electroconvulsive seizures.

Delayed postischemic hypoperfusion: evidence against involvement of the noradrenergic locus ceruleus system.

This study explores the possibility that the delayed hypoperfusion observed after an ischemic insult might be due to vasoconstriction induced by the release of noradrenaline from nerves originating in the locus ceruleus. Bilateral 6-hydroxydopamine lesions of the ascending bundles from the locus ceruleus were carried out in the caudal mesencephalon of rats. Local CBF was measured with an autoradiographic technique 60 min following the start of recirculation after incomplete forebrain ischemia. No significant differences in CBF between nonoperated, sham-operated, and noradrenaline-depleted animals were observed in any structure of the forebrain. It is concluded that the noradrenergic locus ceruleus system does not contribute to the development of delayed postischemic hypoperfusion.

Evidence for neuroprotective effects of endogenous brain-derived neurotrophic factor after global forebrain ischemia in rats.

The levels of brain-derived neurotrophic factor (BDNF) vary between different forebrain areas and show region-specific changes after cerebral ischemia. The present study explores the possibility that the levels of endogenous BDNF determine the susceptibility to ischemic neuronal death. To block BDNF activity the authors used the TrkB-Fc fusion protein, which was infused intraventricularly in rats during 1 week before and 1 week after 5 or 30 minutes of global forebrain ischemia. Ischemic damage was quantified in the striatum and hippocampal formation after 1 week of reperfusion using immunocytochemistry and stereological procedures. After the 30-minute insult, there was a significantly lower number of surviving CA4 pyramidal neurons, neuropeptide Y-immunoreactive dentate hilar neurons, and choline acetyltransferase- and TrkA-positive, cholinergic striatal interneurons in the TrkB-Fc-infused rats as compared to controls. In contrast, the TrkB-Fc treatment did not influence survival of CA1 or CA3 pyramidal neurons or striatal projection neurons. Also, after the mild ischemic insult (5 minutes), neuronal death in the CA1 region was similar in the TrkB-Fc-treated and control groups. These results indicate that endogenous BDNF can protect certain neuronal populations against ischemic damage. It is conceivable, though, that efficient neuroprotection after brain insults is dependent not only on this factor but on the concerted action of a large number of neurotrophic molecules.

Hyperglycemia and hypercapnia suppress BDNF gene expression in vulnerable regions after transient forebrain ischemia in the rat.

Preischemic hyperglycemia or superimposed hypercapnia exaggerates brain damage caused by transient forebrain ischemia. Because high regional levels of brain-derived neurotrophic factor (BDNF) protein correlate with resistance to ischemic damage, we studied the expression of BDNF mRNA using in situ hybridization in rats subjected to 10 minutes of forebrain ischemia under normoglycemic, hyperglycemic, or hypercapnic conditions. Compared with normoglycemic animals, the increase of BDNF mRNA using in situ hybridization in rats subjected to 10 minutes of forebrain ischemia under normoglycemic, or hypercapnic conditions. Compared with normoglycemic animals, the increase of BDNF mRNA in dentate granule cells was attenuated and that in CA3 pyramidal neurons completely prevented in hyperglycemic rats. No ischemia-induced increases of BDNF mRNA levels in the hippocampal formation were detected in hypercapnic animals. Hyperglycemic and hypercapnic rats showed transiently decreased expression of BDNF mRNA levels in the cingulate cortex, which was not observed in normoglycemic animals. The results suggest that suppression of the BDNF gene might contribute to the increased vulnerability of the CA3 region and cingulate cortex in hyperglycemic and hypercapnic animals.

Long distance directed axonal growth from human dopaminergic mesencephalic neuroblasts implanted along the nigrostriatal pathway in 6-hydroxydopamine lesioned adult rats.

Dissociated ventral mesencephalon of 6 to 8-week-old human embryos were implanted by stereotaxic injection at different sites along the nigrostriatal pathway in adult rats, previously subjected to a 6-hydroxydopamine lesion of the intrinsic mesotelencephalic dopamine pathways. The recipients were immunosuppressed by daily injections of cyclosporin A to prevent rejection. At 13-20 weeks after transplantation, the implanted human neurons and their associated fiber outgrowths were visualized with a species-specific antibody recognizing human, but not rat, intermediary neurofilaments (HNF). From implants placed in the host rostral mesencephalic region, HNF-positive axonal projections were seen to extend in large numbers rostrally along the medial forebrain bundle and the internal capsule, and ramify within the caudate putamen, the ventral striatum and the amygdaloid nuclei (a distance of about 5-6 mm), and more sparsely in the frontal cortex and the olfactory bulb (a distance of about 10 mm). From implants placed in the internal capsule, abundant HNF-positive axons extended in the rostral, but not caudal, direction along the myelinated fiber bundles into the caudate putamen and the ventral striatum. Tyrosine hydroxylase (TH) immunohistochemistry revealed that the vast majority of the rostrally projecting HNF-positive axons were also TH-positive, and that the graft-derived axons gave rise to dense TH-positive terminal networks, above all in large areas of the previously denervated caudate putamen. From control implants of cortical neuroblasts, axonal projections were seen along the medial forebrain bundle and the internal capsule, but the axons were TH-negative and showed only sparse projections to the striatal areas. Instead, dense projections were seen, e.g., in the frontal cortex. The results demonstrate a remarkable ability of human mesencephalic neuroblasts to extend axons along the trajectories of the nigrostriatal and mesolimbocortical pathways to reach and innervate the principal striatal and limbic target areas in the forebrain. This shows that the basic requirements for the formation of long axonal pathways may be present in the adult mammalian central nervous system (CNS) at least for certain types of projection neurons. Furthermore, it implies that the developing human neuroblasts can escape the inhibitory features known to be present along myelinated growth trajectories in the adult mammalian brain. In addition, the present approach may offer new possibilities for functional neural grafting in the rat Parkinson model, since transplanted nigral neurons placed in their natural position within the rostral mesencephalon could provide an anatomically and functionally more integrated system than the standard model with ectopically placed intrastriatal nigral grafts.

Converging projections from the mediodorsal thalamic nucleus and mesencephalic dopaminergic neurons to the neocortex in three species.

Previous studies in the rat have shown that the neocortical dopaminergic afferents, originating in the mesencephalon, terminate in those areas of the frontal lobe which receive projections from the mediodorsal thalamic nucleus i.e., the prefrontal cortex. In order to clarify whether this overlap is accidental for the rat or a consistent feature of several species we have compared the projection areas of the ventral tegmental area and the mediodorsal thalamic nucleus in three species, rat, opossum and tree shrew, using HRP injections in combination with glyoxylic acid histofluorescence method. The results have shown, first, that the area innervated by the mediodorsal nucleus of the thalamus is localized in a different part of the frontal lobe in each species: dorsolateral in the opossum, anteromedial, polar and suprarhinal in the rat and frontopolar in the tree shrew. Secondly, this area alone in each species receives projections from the ventral tegmental area. Thirdly, this area alone receives a dense innervation in the deep cortical layers by fluorescent fibres probably containing dopamine. The neighbouring neocortical areas receive afferents neither from the mediodorsal nucleus of the thalamus nor from the ventral mesencephalic tegmentum; their catecholamine innervation is mainly confined to the superficial layers and appears to be of noradrenergic nature. Although the techniques used did not allow a precise determination of the borders of the two projection areas and, therefore, the exact degree of overlap, it appears that mesencephalic dopaminergic innervation is a characteristic feature of the prefrontal cortex in the mammalian brain.

Human fetal dopamine neurons grafted into the striatum in two patients with severe Parkinson's disease. A detailed account of methodology and a 6-month follow-up.

By using stereotaxic surgical techniques, ventral mesencephalic tissues from aborted human fetuses of 8 to 10 weeks' gestational age were implanted unilaterally into the striata in two patients with advanced Parkinson's disease. The patients were treated with a cyclosporine, azathioprine, and steroid regimen to minimize the risk for graft rejection. They were examined for 6 months preoperatively and 6 months postoperatively and continued to receive the same doses of antiparkinsonian medication. There were no significant postoperative complications. No major therapeutic effect from the operation was observed. However, in the clinical tests, both patients showed small but significant increases of movement speed for repeated pronation-supination, fist clenching, and foot lifting. The rate of walking also increased in the one patient tested. For both patients, there was an initial worsening postoperatively, followed by improvement vs preoperative performance at 1 to 3 months. Both patients also showed significant improvement in the magnitude of response to a single dose of levodopa (L-dopa), but there was no increase in the duration of drug action. The motor readiness potential increased in both patients postoperatively, primarily over the operated hemisphere. Neurophysiological measurements also showed a more rapid performance of simple and complex arm and hand movements on the side contralateral to transplantation in one patient at 5 months postoperatively. Positron emission tomography demonstrated no increased uptake of 6-L-(18F)-fluorodopa in the transplanted striatum at 5 and 6 months. Taken together, these results suggest that the fetal nigral implants may have provided a modest improvement in motor function, consistent with the presence of small surviving grafts. Although our results support further scientific experimentation with transplantation in Parkinson's disease, widespread clinical trials with this procedure are probably not warranted at this time.