By David N. Leff
Assisted suicide is much in the news these days, as it affects people. But it's also making news at the cellular level.
When a cell ends it all, its demise is ascribed to the phenomenon of apoptosis -- programmed cell death. After a cell gets the command to commit apoptosis, it fragments into constituent proteins and other components, which phagocytic cells then scavenge and digest.
What triggers this apoptotic process is less voluntarily self-imposed than the term suicide suggests.
In Parkinson's disease (PD), for example, neurons that contain the neurotransmitter dopamine are thought to die off by apoptosis. A research paper in the current Proceedings of the National Academy of Sciences (PNAS), dated July 8, 1997, casts experimental light on the neuronal self-immolation mechanism. Its title: "Nuclear translocation of NF-kappaB is increased in dopaminergic neurons of patients with Parkinson's disease."
NF-kappaB -- nuclear factor kappa B -- is a high-strung, pit-bull protein manufactured by neurons in their cytoplasm. Forever straining at the leash to go after genes in the cell's nucleus, NF-kappaB is kept in check by another inhibitory protein, I-kappaB. (See BioWorld Today, Oct. 16, 1995, p. 1.)
"NF-kappaB's function," explained molecular pharmacologist Étienne Hirsch, senior author of the PNAS paper, "is to activate certain nuclear genes when it translocates from the cytoplasm to the nucleus. These genes are involved in inducing apoptosis in those neurons. But their targets have not yet been identified."
PD results from the progressive and massive die-off -- from unknown causes -- of neuronal cells that use dopamine as their neurotransmitter. Hence, therapies to halt or slow its neurodegenerative clinical course include levodopa, a pharmaceutical precursor of dopamine, and human fetal dopaminergic tissue implanted into human Parkinsonian brains.
Levodopa provides only a delaying action, with age-related diminishing returns; fetal tissue is fraught with both neurological and ethical drawbacks.
Opening A Window On PD's Etiology
"The cause of PD," Hirsch told BioWorld Today, "is not known yet. So a way to approach the molecular mechanism by which dopaminergic neurons degenerate in the disease, and why they degenerate, is to try to analyze the cascade of events that will lead to this degeneration." Hirsch is research director of the unit on Mechanisms and Consequences of Neuronal Death at INSERM, the French National Institute of Health and Medical Research, in Paris.
In earlier research, he and his co-authors have issued bills of indictment against two alleged perpetrators of PD nerve cell death -- apoptosis and oxidative stress.
"What we have done in this PNAS paper," he said, "is try to find the biochemical link between oxidative stress and the apoptotic death of dopaminergic neurons.
"Oxidative stress produces very reactive oxygen free radicals, in excessive amounts," Hirsch pointed out, "which can attack the polyunsaturated fatty acids of lipids in cell membranes and activate NF-kB in the cytoplasm."
Beginning with in vitro experiments, he and his team took neurons from the rat mesencephalon, a major brain structure rich in dopaminergic cells that succumb to PD. In these neurons they turned on the NF-kappaB transduction pathway, releasing that transcription factor to head for the nucleus.
"When we activated this pathway," Hirsch recalled, "we found that the cell degenerates by means of apoptosis. Also, the translocation of NF-kappaB was preceded by the production of oxygen free radicals. And when we blocked production of those free radicals, we also blocked induction of apoptosis."
One activator of that pathway, he pointed out, is tumor necrosis factor-alpha (TNF-a). This cytokine is also implicated in the production of apoptosis-inducing free radicals.
When they turned from cultured rodent neurons in vitro to fresh postmortem human brain tissue from five PD patients and seven matched control subjects, results were harder to come by.
"It is not possible to show the formation of free radicals in postmortem brain," Hirsch observed. "You cannot have an idea of the events with regard to time course, because you get the brain postmortem at a single time. So you just have to compare what you observed in vivo with what you observed in the postmortem."
In the human mesencephalon's substantia nigra -- a hotbed of dwindling PD neurons -- they detected 71 times as much NF-kappaB in the neuronal nuclei of PD patients as in controls.
In both, the French investigators found the same translocation of NF-kB, and the same degeneration of dopaminergic neurons by apoptosis.
These novel pieces of evidence, Hirsch suggested, have two bottom-line implications:
"First of all," he said, "I think it is important for the understanding of apoptosis as playing a role in PD. The second thing," he added, "is that the identification of this pathway may call for some pharmacological manipulation of the system, by trying to prevent the translocation of NF-kappaB from the cytoplasm to the nucleus."
He cautioned that as regards such potential pharmaceutical applications, "This is only a very theoretical outcome, and we have to be extremely cautious about not giving too much hope to the patients."
His group now is "trying to find the factor that may induce the translocation of NF-kappaB and, more generally, the factors that may activate this pathway. But it is too early to have results to release."
As for the possibility that this same neuronal mechanism may be at work in other neurodegenerative afflictions, such as Alzheimer's disease, Hirsch observed, "I definitely don't know if, in other neurodegenerative diseases, NF-kappaB also translocates from cytoplasm to nucleus." *