Lessons on transplant survival from a successful model system

Studies on the snailMelampusreveal that connectivity is crucial to the survival of transplanted ganglia. Transplanted CNS ganglia can innervate targets or induce supernumerary structures. Neuron survival is optimized by the neural incorporation that occurs when a transplanted ganglion is substituted for an excised ganglion. Better provision for the trophic requirements of neurons will improve the success of mammalian fetal transplants.

Repairing the brain: Trophic factor or transplant?

Three experiments on neural grafting with adult rat hosts are described. Working memory impairments were produced by lesioning the hippocampus or severing its connections with the septum by ablating the fimbria-fornix. The results suggest that the survival and growth of a neural graft, whether an autograft or a xenograft, is not a necessary condition for functional recovery on a task tapping working memory.

Will brain tissue grafts become an important therapy to restore visual function in cerebrally blind patients?

Grafting embryonic brain tissue into the brain of patients with visual field loss due to cerebral lesions may become a method to restore visual function. This method is not without risk, however, and will only be considered in cases of complete blindness after bilateral occipital lesions, when other, risk-free neuropsychological methods fail.

Difficulties inherent in the restoration of dynamically reactive brain systems

The responses displayed by an injured or diseased nervous system are complex. Some of the responses may effect a functional reorganization of the affected neural circuitry. Strategies aimed at the restoration of function, whether or not these involve transplantation, need to recognize the innate reactive capacity of the nervous system to damage. More successful strategies will probably incorporate, rather than ignore, the adaptive responses of the compromised neural systems.

Elegant studies of transplant-derived repair of cognitive performance

Cholinergic-rich grafts have been shown to be effective in restoring maze-learning deficits in rats with lesions of the forebrain cholinergic projection system. However, the relevance of those studies to developing novel therapies for Alzheimer's disease is questioned.

Neural transplants are grey matters

The lesion and transplantation data cited by Sinden et al., when considered in tandem, seem to harbor an internal inconsistency, raising questions of false localization of function. The extrapolation of such data to cognitive impairment and potential treatment strategies in Alzheimer's disease is problematic. Patients with focal basal forebrain lesions (e.g., anterior communicating artery aneurysm rupture) might be a more appropriate target population.

Immunobiology of neural transplants and functional incorporation of grafted dopamine neurons

In contrast to the views put forth by Stein & Glasier, we support the use of inbred strains of rodents in studies of the immunobiology of neural transplants. Inbred strains demonstrate homology of the major histocompatibility complex (MHC). Virtually all experimental work in transplantation immunology is performed using inbred strains, yet very few published studies of immune rejection in intracerebral grafts have used inbred animals.

Local and global gene therapy in the central nervous system

For focal neurodegenerative diseases or brain tumors, localized delivery of protein or genetic vectors may be sufficient to alleviate symptoms, halt disease progression, or even cure the disease. One may circumvent the limitation imposed by the blood-brain barrier by transplantation of genetically altered cell grafts or focal inoculation of virus or protein. However, permanent gene replacement therapy for diseases affecting the entire brain will require global delivery of genetic vectors. The neurotoxicity of currently available viral vectors and the transient nature of transgene expression invivomust be overcome before their use in human gene therapy becomes clinically applicable.

Neural grafting in human disease versus animal models: Cautionary notes

Over the past two decades, research on neural transplantation in animal models of neurodegeneration has provided provocative in sights into the therapeutic use of grafted tissue for various neurological diseases. Although great strides have been made and functional benefits gained in these animal models, much information is still needed with regard to transplantation in human patients. Several factors are unique to human disease, for example, age of the recipient, duration of disease, and drug interaction with grafted cells; these need to be explored before grafting can be considered a safe and effective therapeutic tool.

Building a rational foundation for neural transplantation

The neural transplantation research described by Sinden and colleagues provides part of the rationale for the clinical application of neural transplantation. The authors are asked to clarify their view of the role of the cholinergic system in cognition, to address extrahippocampal damage caused by transient forebrain ischemia, and to consider the effects of delayed neural degeneration in their structure-function analysis.

Intraretrosplenial grafts of cholinergic neurons and spatial memory function

The transplantation of cholinergic neurons into the hippocampal formation has been well characterized. We describe our studies on the effects of cholinergic transplants in the retrosplenial cortex. These transplants were capable of ameliorating spatial navigation deficits in rats with septohippocampal lesions. In addition, we provide evidence for the modulation of transplanted neurons by the host brain.

Gene therapy and neural grafting: Keeping the message switched on

A major problem in developing an effective gene therapy for the nervous system lies in understanding the principles that maintain or turn off the expression of genes following their transfer into the CNS.

Therapeutic neural transplantation: Boon or boondoggle?

Despite reports of recovery of function after neural transplantation, the biological interactions between transplanted neurons and the host brain that are necessary to mediate recovery are unclear at present. One source of confusion is in the variety of models and protocols used in these studies. It is suggested that multisite experimentation using standard protocols, models, and recovery criteria would be helpful in moving neural transplantation from the laboratory to the clinic.

The ethics of fetal tissue grafting should be considered along with the science

In addition to the scientific and medical issues surrounding the use of fetal tissue transplants, the ethical implications should be considered. Two major ethical issues are relevant. The first of these is whether this experimental procedure can be justified on the basis of potential benefit to the patient. The second is whether the use of tissue obtained from intentionally aborted fetuses can be justified in the context of historical and existing guidelines for the protection of human subjects. The separation of ethical decisions from medical practice and scientific research is necessary to prevent the exploitation of innocent human life.

Gene therapy for neurodegenerative disorders and malignant brain tumors

Gene therapy approaches have great promise in the treatment of neurodegenerative disorders and malignant brain tumors. Neuwelt et al. review available viral-mediated gene therapy methods and their blood-brain-barrier (BBB) disruption delivery technique, briefly mentioning nonviral mediated gene therapy methods. This commentary discussed the BBB disruption delivery technique, viral and nonviral mediated gene therapy approaches to Parkinson's disease, and the potential use of antisense oligo to suppress malignant brain tumors.

Behavioral effects of neural grafts: Action still in search of a mechanism

This commentary reviews data supporting circuitry reconstruction, replacement neurotransmitters, and trophic action as mechanisms whereby transplants promote recovery of function. Issue is taken with the thesis of Sinden et al. that adequate data exist to indicate that reconstruction of hippocampal circuitry damaged by hypoxia with CA1 transplants is a confirmed mechanism whereby these transplants produce recovery. Sinden et al.'s and Stein & Glasier's proposal that there is definitive evidence showing that all transplants produce trophic effects is also questioned.

Neural transplantation, cognitive aging and speech

Research on neural transplantation has great potential societal importance in part because of the expanding proportion of the population that is elderly. Transplantation studies can benefit from the guidance of research on cognitive aging, especially in connection with the assessment of behavioral outcomes. Speech for example, might be explored using avian models.

Pathway rewiring with neural transplantation

A lesion to the brain is not necessary for a successful neural transplantation. Embryonic Purkinje cells placed on the surface of an uninjured adult cerebellum can develop and migrate into the host molecular layer. Both the Purkinje cells that migrated into the host cerebellum and those that remained in the graft were innervated by collateral sprouting of adult intact climbing fibers.

Multiple potential mechanisms of graft action is not a new idea

It is well established that neural grafts can exert functional effects on the host animal by a multiplicity of different mechanisms – by diffuse release of trophic molecules, neurohormones, and deficient neurotransmitters, as well as by growth and reformation of neural circuits. Our challenge is to understand how these different mechanisms complement each other.

Grafts and the art of mind's reconstruction

The use of neural transplantation to alleviate cognitive deficits is still in its infancy. We have an inadequate understanding of the deficits induced by different types of brain damage and their homologies in animal models against which to assess graft-induced recovery, and of the ways in which graft growth and function are influenced by factors within the host brain and the environment in which the host is operating. Further, use of fetal tissue may only be a transitory phase in the search for appropriate donor sources. Nevertheless, findings from our laboratory and elsewhere have made aprima faciecase for successful cognitive reconstruction by graft methods.

Studying restoration of brain function with fetal tissue grafts: Optimal models

We concur that basic research on the use of CNS grafts is needed. Two important model systems for functional studies of grafts are ignored by Stein & Glasier. In the first, reproductive function is restored in hypogonadal mice by transplantation of GnRH-synthesizing neurons. In the second, circadian rhythmicity is restored by transplantation of the suprachiasmatic nucleus.

Gene replacement therapy in the CNS: A view from the retina

Gene replacement therapy holds great promise in the treatment of many genetic CNS disorders. This commentary discusses the feasibility of gene replacement therapy in the unique context of the retina, with regard to: (1) the genetics of retinal neoplasia and degeneration, (2) available gene transfer technology, and (3) potential gene delivery vehicles.

The limitations of central nervous systemdirected gene transfer

Complementation and correction of a genetic defect with CNS manifestations lags behind gene therapy for inherited disorders affecting other organ systems because of shortcomings in delivery vehicles and access to the CNS. The effects of improvements in viral and nonviral vectors, coupled with the development of delivery strategies designed to transfer genetic material thoughout the CNS are being investigated by a number of laboratories in efforts to overcome these problems.

CNS transplant utility may surive even their hasty clinical application

Neural cell transplants have been introduced in clinical practice during the last decade with mixed results, encouraged by success with simple animal models. This commentary is a reminder that although the ideas and techniques of transplantation appear simple, the variables involved in host-transplant integration still require further study. The field may benefit from a concerted, multidisciplinary approach.

Are fetal brain tissue grafts necessary for the treatment of brain damage?

Despite some clinical promise, using fetal transplants for degenerative and traumatic brain injury remains controversial and a number of issues need further attention. This response reexamines a number of questions. Issues addressed include: temporal factors relating to neural grafting, the role of behavioral experience in graft outcome, and the relationship of rebuilding of neural circuitry to functional recovery. Also discussed are organization and type of transplanted tissue, the “trophic hypothesis” of transplant viability, and whether transplants are really needed to obtain functional recovery after brain damage.

Transplantation, plasticity, and the aging host

Neural transplantation as a recovery strategy for neuro-degenerative diseases in humans has used mainly grafting following acute denervation strategies in young adult hosts. Our work in aged mice and rats demonstrates an age-related increase in susceptibility to oxidative damage from neurotoxins, a remarkably poor recovery of C57BL/6 mice from MPTP insult with transplantation and growth factors, even at 12 months of age, and diminished plasticity of host neurons. We believe that extrapolation of data from young adult animal models to aged humans without thorough investigation of transplantation and host response inagedrecipients is scientifically and ethically inappropriate.

Preservation of acetylcholine muscarinic M2 receptor G-protein interactions in the neocortex of patients with Alzheimer's disease

The efficacy of acetylcholine muscarinic M2 receptor-G protein coupling was investigated in Alzheimer's disease and control neocortical membranes by measuring the effects of MgCl2 and 5'-guanylylimidodiphosphate (Gpp[NH]p) on high-affinity [3H]oxotremorine-M ([3H]OXO-M) binding. MgCl2 gave similar enhancements of [3H]OXO-M binding in Alzheimer's disease and control occipital cortex. In contrast, MgCl2 enhanced [3H]OXO-M binding was significantly higher in Alzheimer's disease superior temporal cortex, compared to controls. MgCl2 enhanced [3H]OXO-M binding in both the occipital and temporal cortices of the Alzheimer's disease cases was reversed to control levels by Gpp[NH]p. It is concluded that the number of high-affinity muscarinic M2 sites is increased in Alzheimer's disease superior temporal, but not occipital, cortex and that M2 sites in both regions maintain an efficient G-protein coupling.

beta-Amyloid precursor protein isoforms show correlations with neurones but not with glia of demented subjects

Post-mortem cerebral cortex from 15 demented patients was specially collected to minimise autolysis and two membrane fractions and one soluble fraction were quantitatively examined for the major species of beta-amyloid precursor protein (APP) of high apparent molecular mass (> or = 80 kDa) together with the major mRNA species encoding APP isoforms. The number of pyramidal neurones and astrocytes, putative biochemical indices of interneurones and pyramidal neurones, and choline acetyl transferase activity were also determined. Multiple regression analysis has been used to investigate intercorrelations of APP species with biochemical and morphometric measures, free of any effects of confounding demographic variables. Subjects with Alzheimer's disease showed a loss of cholinergic activity and D-aspartate uptake compared with patients with other causes of dementia. The major finding of the study is that measures of neurones rather than astrocytes most closely correlate with the concentration of APP. Pyramidal cell numbers were positively correlated with mRNA for APP695. APP in the soluble fraction showed a negative correlation with pyramidal cell numbers and cholinergic activity. These results indicate that neurones within the cerebral cortex are the major source of APP, and that secretion of APP is dependent upon cortical pyramidal neuronal activity and cholinergic activity.

Selective increase in lipid peroxidation in the inferior temporal cortex in Alzheimer's disease

The concentration of a product of lipid peroxidation (malondialdehyde) was determined in six areas of neocortex of 8 subjects with Alzheimer's disease and 8 control subjects. Malondialdehyde concentration was significantly increased by incubation with iron and ascorbate in all samples. Both basal and iron/ascorbate-stimulated malondialdehyde concentration were higher in the inferior temporal cortex of Alzheimer subjects than corresponding controls; other regions were unaffected. Basal concentrations of malondialdehyde correlated with age in both the inferior parietal lobule and the sensory/motor cortex.

Precis of executive dyscontrol as a cause of problem behavior in dementia

Frontal lobe executive control functions (ECF) are proposed as a source of problem behavior in dementia. The behavior and personality changes that follow frontal lobe brain damage overlap with those seen in dementia, and frontal lobe structural and metabolic lesions can be demonstrated across a variety of dementing illnesses. ECF help explain the importance of social and environmental cues in the production of disruptive behavior and suggest interpersonal strategies for their control. This model has considerable face validity and leads to several testable hypotheses.

Disrupted beta 1-adrenoceptor-G protein coupling in the temporal cortex of patients with Alzheimer's disease

The efficacy of beta 1-adrenoceptor-G protein coupling was studied in postmortem temporal cortex synaptic membranes from a series of control and Alzheimer's disease subjects. For the control cases, the non-hydrolysable GTP analogue 5'-guanylylimidodiphosphate (Gpp[NH]p) gave a significant reduction in the affinity of the agonist isoprenaline to displace binding of the radiolabelled antagonist (+/)-4-(3-t-butylamino-2-hydroxypropoxy)[5,7-3H]benzimidazol-2-one ([3H]CGP-12177). This effect was attributed to the conversion of high agonist-affinity sites to a lower-affinity state and was not found for the Alzheimer's disease cases. These data indicate that a disruption of beta 1-adrenoceptor-G protein coupling occurs in the temporal cortex of Alzheimer's disease patients.

Brain S-adenosylmethionine decarboxylase activity is increased in Alzheimer's disease

We measured the activity of S-adenosylmethionine decarboxylase (SAMDC), a key regulatory enzyme of polyamine biosynthesis, in autopsied brain from 13 patients with Alzheimer's Disease (AD). As compared with the controls, mean enzyme activity was increased by 37-96% in all seven examined brain regions with statistically significant increases in temporal cortex (+96%), frontal cortex (+69%) and hippocampus (+90%). The elevated SAMDC may have occurred as part of a generalized polyamine response to brain injury, which has been previously described in experimental animal conditions. Above-normal SAMDC activity implies increased levels/metabolism of spermidine and spermine, two polyamines which are involved in neuronal regeneration, growth factor production, and activation of excitatory N-methyl-D-aspartate preferring glutamate receptors. Our data suggest the involvement of the polyamine system in the brain reparative and/or pathogenetic mechanisms of AD.

Primate nucleus basalis of Meynert p75NGFR-containing cholinergic neurons are protected from retrograde degeneration by the ganglioside GM1

The effects of unilateral devascularizing lesions of the neocortex in primates (Cercopithecus aethiops) on the immunoreactivity of choline acetyltransferase and the low-affinity nerve growth factor receptor (p75NGFR) were investigated in cell bodies of the nucleus basalis of Meynert. Choline acetyltransferase enzymatic activity was measured in the dissected ipsi- and contralateral nucleus basalis of Meynert as well as in the remaining cortex adjacent to the lesion. Cortically lesioned animals displayed a shrinkage of p75NGFR-immunoreactive cholinergic cell bodies in only the intermediate portion of the nucleus basalis of Meynert as well as a depletion of choline acetyltransferase activity in this cellular complex. In contrast, cortically lesioned monkeys treated with monosialoganglioside did not reveal a significant loss of choline acetyltransferase activity or shrinkage of nucleus basalis of Meynert cholinergic neurons, but rather a modest hypertrophy. These results are discussed in relation to a possible use of putative trophic agents in the repair of the damaged central nervous system.

Amygdala cell loss and atrophy in Alzheimer's disease

The amygdala and its subnuclei undergo severe volumetric atrophy in Alzheimer's disease (AD). To determine whether this atrophy is due to loss of neuropil, specific neuronal populations, or both, we evaluated the number, size, and packing density of neurons and glia in the cortical and magnocellular basal amygdaloid subregions. The neuropil fraction did not change with AD in either region. Despite a mean 35% increase in cell packing density in the AD amygdala, total numbers of neurons and glia within tissue sections were reduced significantly; medium and large neurons were preferentially affected. The total number of small neurons was stable in the AD sample despite sharp reductions in nuclear size, suggesting that AD also results in pronounced amygdaloid neuronal shrinkage. Differences in the degree of cell loss between the two nuclei as well as changes in glial cell numbers are discussed in relation to characteristic AD neuropathology and relevant anatomical connectivity.

Preservation of Gi-protein inhibited adenylyl cyclase activity in the brains of patients with Alzheimer's disease

The coupling of inhibitory guanine nucleotide binding (Gi) proteins to the adenylyl cyclase signal transduction complex was compared in 4 brain regions from a series of Alzheimer's disease and matched control subjects by measuring the inhibition of membrane enzyme activities in response to guanosine 5'-[beta gamma-imido]diphosphate (Gpp[NH]p) and aluminium fluoride (AlF4-). Basal adenylyl cyclase activities were significantly lower in preparations of angular gyrus and frontal and temporal cortices, but not cerebellum, from the Alzheimer's disease cases compared to controls. Gpp[NH]p and AlF4- gave significant inhibitions of adenylyl cyclase activity in all brain regions. The magnitude of these inhibitions, when corrected for altered basal activities, were similar for the Alzheimer's disease and control cases. These results indicate that there is no impairment of Gi-protein mediated inhibition of adenylyl cyclase activity in Alzheimer's disease brain.

An 'anatomical cascade hypothesis' for Alzheimer's disease

The molecular mechanisms of the cytopathology of Alzheimer's disease are very rapidly being elucidated. However, the factors that restrict the effects of this disease to specific neuroanatomical systems are less well understood. In this brief article a possible hypothesis is outlined to explain this apparent specific localization.

Alzheimer's disease and metal-containing glia

Considerable evidence suggests that in Alzheimer's disease, olfactory bulb damage may be a primary factor, causing degeneration and neurofibrillary tangles primarily in neurons connected with this brain area. Also, deposits of amyloid may involve an improper regulation of the cleavage of a precursor protein by glia. Finally, toxic effects of aluminium may be an etiological factor. This review proposes that all these seemingly unrelated aspects of Alzheimer's disease could be related to a disturbed function of metal-containing glia. Such a disturbance, initiated by or aggravating toxic effects of aluminum, may underlie initial damage in the olfactory bulb and/or other brain areas with a weakened blood-brain barrier and may be responsible for amyloid deposition.

Adenylyl cyclase activity in postmortem human brain: evidence of altered G protein mediation in Alzheimer's disease

The effects of agonal status, postmortem delay, and age on human brain adenylyl cyclase activity were determined in membrane preparations of frontal cortex from a series of 18 nondemented subjects who had died with no history of neurological or psychiatric disease. Basal and guanosine 5'-O-(3-thiotriphosphate)-, aluminum fluoride-, and forskolin-stimulated enzyme activities were not significantly reduced over an interval from death to postmortem of between 3 and 37 h and were also not significantly different between individuals dying with a long terminal phase of an illness and those dying suddenly. Basal and aluminum fluoride-stimulated enzyme activities showed a negative correlation with increasing age of the individual. In subsequent experiments, basal and guanosine 5'-O-(3-thiotriphosphate)-, aluminum fluoride-, and forskolin-stimulated enzyme activities were compared in five brain regions from a series of eight Alzheimer's disease and seven matched nondemented control subjects. No significant differences were observed between the groups for either basal activity or activities in response to forskolin stimulation of the catalytic subunit of the enzyme. In contrast, enzyme activities in response to stimulation with guanosine 5'-O-(3-thiotriphosphate) and aluminum fluoride were significantly reduced in preparations of neocortex and cerebellum from the Alzheimer's disease cases compared with the nondemented controls. Lower guanosine 5'-O-(3-thiotriphosphate)-, but not aluminum fluoride-, stimulated activity was also observed in preparations of frontal cortex from a group of four disease controls compared with nondemented control values. The disease control group, which contained Parkinson's disease and progressive supranuclear palsy patients, showed increased forskolin-stimulated activity compared with both the nondemented control and the Alzheimer's disease groups. These findings indicate a widespread impairment of G protein-stimulated adenylyl cyclase activity in Alzheimer's disease brain, which occurs in the absence of altered enzyme catalytic activity and which is unlikely to be the result of non-disease-related factors associated with the nature of terminal illness of individuals.

Potential role of viruses in neurodegeneration

Viruses have the capacity to induce alterations and degenerations of neurons by different direct and indirect mechanisms. In the review, we have focused on some examples that may provide new avenues for treatment or altering the course of infections, i.e., antibodies to fusogenic virus membrane proteins, drugs that interfere with lipid metabolism, calcium channel blockers, immunoregulatory molecules, and, and inhibitors of excitotoxic amino acids. Owing to their selectivity in attack on regions of nervous tissue, governed by viral factors and by routes of invasion, viral receptors or metabolic machineries of infected cells, certain viral infections show similarities in distribution of their resulting lesions in the nervous system to that of the common human neurodegenerative diseases (namely, motor neurons disease, Parkinson's disease, and Alzheimer's disease). However, it should be emphasized that no infectious agent has as yet provided a complete animal model for any of these diseases, nor has any infectious agent been linked to them from observations on clinical or postmortem materials.