Trophic factor therapy in the adult CNS: remodelling of injured basalo-cortical neurons

Gangliosides, NGF, brain aging and disease: a mini-review with personal reflections

In this mini-review I summarize our research efforts in ascertaining the possible neuro-reparative properties of the GM1 ganglioside and its cooperative effects with NGF in stroke-lesion models. We also review aspects of our NGF investigations which have recently led to the discovery that NGF is released in an activity-dependent manner in the form of its precursor molecule, proNGF. These studies support the notion that in the CNS NGF metabolism conversion and degradation occur in the extracellular milieu. We have also validated this pathway in vivo demonstrating that the pharmacological inhibition of the pro-to mature NGF conversion results in the brain accumulation of proNGF and loss and atrophy of cortical cholinergic synapses. Furthermore, we have gathered neurochemical evidence for a compromise of this newly discovered NGF metabolic pathway in Alzheimer's disease, explaining the vulnerability of NGF-dependent forebrain cholinergic neurons in this disease despite normal NGF synthesis and abundance of NGF precursor.

The effects of decompression and exogenous NGF on compressed cerebral cortex

Using a rat epidural bead implantation model, we found that compression alone could reduce the overall and individual layer thicknesses of cerebral cortex with no apparent cell death. The dendritic lengths and spine densities of layer II/III and V pyramidal neurons started to decrease within 3 days of compression. Decompression for 14 days resulted in near complete to partial recovery of the cortical thickness and of the dendritic lengths of layer II/III and V pyramidal neurons, depending on the duration of the preceding compression. The recoverability was better following short (3-day) than long (1- or 3-month) periods of compression. The loss of dendritic spines nevertheless persisted. An intraventricular infusion of NGF was performed after decompressing the lesions following 3 days of cortical compression, and this increased the recovery of the spines but not the dendritic length of the cortical pyramidal neurons, nor did it alter the recovery of the cortical thickness. NGF also promoted the increase of the dendritic spines, but not the dendritic length of the cortical pyramidal neurons of normal animals. In short, the data show that a few days of compression alone can cause permanent cortical damage. Exogenous NGF, if applied topically, may restore the dendritic spine density of cortical neurons subjected to compression.

Nerve growth factor signaling, neuroprotection, and neural repair

Nerve growth factor (NGF) was discovered 50 years ago as a molecule that promoted the survival and differentiation of sensory and sympathetic neurons. Its roles in neural development have been characterized extensively, but recent findings point to an unexpected diversity of NGF actions and indicate that developmental effects are only one aspect of the biology of NGF. This article considers expanded roles for NGF that are associated with the dynamically regulated production of NGF and its receptors that begins in development, extends throughout adult life and aging, and involves a surprising variety of neurons, glia, and nonneural cells. Particular attention is given to a growing body of evidence that suggests that among other roles, endogenous NGF signaling subserves neuroprotective and repair functions. The analysis points to many interesting unanswered questions and to the potential for continuing research on NGF to substantially enhance our understanding of the mechanisms and treatment of neurological disorders.

Phenobarbital administration directed against kindled seizures delays functional recovery following brain insult

Anti-convulsant drug administration or recurrent seizures can impact functional recovery following brain insult. The nature of that impact depends on a variety of factors, including timing of drug administration and drug mechanism of action, as well as seizure number, timing, and severity. The objective of this study was to determine the functional consequences of anti-convulsant administration directed against seizure activity in brain-damaged animals. To this end, phenobarbital was coupled with daily electrical kindling of the amygdala beginning 48 h after a unilateral anteromedial cortex lesion. Recovery from somatosensory deficits was assessed, as was regional atrophy and basic fibroblast growth factor (bFGF) expression. Animals receiving phenobarbital prior to daily kindling failed to recover within 2 months of testing. In contrast, animals receiving saline prior to kindling as well as phenobarbital-treated non-kindled animals recovered within 2 months after the lesion. Though the exact mechanisms underlying these behavioral phenomena remain uncertain, patterns of bFGF expression among the groups provide some insight. Taken together, results from the present study suggest that anti-convulsant drug administration directed against subclinical seizure activity can be more detrimental to functional recovery than seizures alone or anti-convulsant drug treatment after seizure activity has occurred.

The use of nonneuronal cells for gene delivery

The implantation of genetically engineered nonneuronal cells can provide an effective method for achieving localized delivery of discrete molecules to the CNS or for providing substrates for regrowth of neural structures. Most primary nonneuronal cells have the advantage of being easily obtainable from the prospective host for ex vivo retrovirus-mediated genetic manipulation (most will be mitotic in culture) and reimplantation as an autologous graft (circumventing the problem of immune rejection). As primary cells, they are unlikely to be tumorigenic. The most vexing problem for such systems remains the apparent loss of transgene expression from viral promoters after prolonged periods of engraftment. Much effort is currently being directed at optimizing sustained transgene expression by varying the promoters, by varying the cell types to be engineered, or by regulating expression by enhancing promoter function or substrate availability. While nonneuronal cells are excellent vehicles for achieving passive delivery of substances to the CNS, they lack the ability to incorporate into the host cytoarchitecture in a functional manner (e.g., make synaptic contacts). For this reason, not only may certain essential circuits not be re-formed, but the regulated release of certain substances through feedback loops may be missing. While apparently unimportant for some substances (e.g., ACh), for others (e.g., NGF), their unregulated, inappropriate, excessive, or ectopic release may actually be inimical to the host. Furthermore, the loss of foreign gene expression (the bane of gene therapy) may leave engineered nonneural cells incapacitated, whereas donor tissue originating from brain may intrinsically produce various CNS factors allowing correction to proceed despite inactivation of the introduced gene. In fact, CNS-derived tissue may provide as-yet-unrecognized endogenous neuralspecific substances which are equally as beneficial to the host as the gene in question. Thus, future developments in gene delivery to the brain for some conditions may emphasize using neurons or neural progenitors for ex vivo genetic manipulation (Fisher, 1997) and refining techniques for the direct injection of therapeutic genes into neurons in vivo (see Snyder and Fisher, 1996). For a wide variety of conditions, however, using nonneuronal cellular vehicles or even nonbiologic synthetic vehicles may be efficient, effective, and safe strategies for the passive delivery of therapeutic molecules to discrete regions of the CNS. In fact, this approach may come closer than any other to immediate human applications.

Nerve growth factor stimulates growth of cortical pyramidal neurons in young adult rats

The cytoarchitectonics of pyramidal neurons in the cerebral cortex of non-lesioned rats can be re-modeled by i.c.v. infusions of nerve growth factor (NGF). 4 months after the application of NGF, the pyramidal neurons in layers III and V of the motor cortex and layer V of the anterior cingulate cortex were analyzed and compared with pyramidal neurons from vehicle-treated rats. NGF-treated brains showed: (1) significant increase in dendritic branching in the basilar fields of the layer V, but not layer III, neurons; and (2) a significant increase in spine density in the terminal, but not proximal, dendritic branches. These findings indicated that, besides its known effects on forebrain cholinergic neurons, NGF produces a very generalized synaptic re-modeling involving the cells responsible for the major output of the cerebral cortex in the intact adult brain.