Neurologists are pinning their faith, investors their dollars, and brain-impaired patients their devout hopes on a family of neuron-nourishing proteins, the recombinant neurotrophins.

An increasing number of these nerve growth factors are in preclinicalstudies and clinical trials for Alzheimer's and Parkinson's diseases,and for ALS _ amyotrophic lateral sclerosis (Lou Gehrig's disease),as well as stroke.

Alkermes Inc. (Cambridge, Mass.), Amgen Inc. (Thousand Oaks,Calif.), CytoTherapeutics Inc. (Providence, R.I.), Genentech Inc.(South San Francisco), Regeneron Pharmaceuticals Inc. (Tarrytown,N.Y.) and Synergen Inc. (Boulder, Colo.) are the most frequentlycited U.S. biotech companies pursuing neurotrophic therapies, withthe usual mix of optimism and disappointment. (See BioWorldToday, Jan. 27, 1995, p. 1; June 24, 1994, p. 1.)

Now comes a call for caution. Neurotrophins, it turns out, can betwo-edged swords, smiting with necrosis neurons they save fromapoptosis. This caveat is baldly stated by the title of a paper in thecurrent Science, dated April 28: "Potentiated necrosis of culturedcortical neurons by neurotrophins."

The article's first author is post-doctoral neurology resident Jae-Young Koh at Washington University School of Medicine in St.Louis. Koh told BioWorld Today: "This finding strikes a cautionarynote because a lot of people are trying to use neurotrophins againstbrain disorders."

As his experiments have been performed so far only in cell culture,Koh pointed out, "We don't know if this can be extrapolated to an invivo situation. Anyway, it's the first time this potentiation of celldeath was seen _ ever." He added, "I think that's very important,actually."

But What About In Vivo?

Koh observed that "a lot of in vivo studies show that theseneurotrophins are protective against stroke models, for example. Thequestion is whether those protective facts are somehow masked. Inother words, both types of cell death, apoptosis and necrosis, happenconcurrently after the stroke."

Hence, he suggested, "people have reported modest protective effectsof these neurotrophins, which could be due to their double effect _blocking one type of neuronal death and potentiating the other type."Curbing that necrosis "by some other measures," the authorproposed, "could maybe maximize the protective effect ofneurotrophins."

These two forms of cellular demise in the central nervous system,Koh explained, can be easily told apart: "In necrotic death, there iscell swelling, lack of DNA fragmentation and other signs. That'scaused by a maximum calcium influx, due to glutamate receptoractivation; in other words, cytotoxicity."

Apoptosis (cell suicide) is a quite different story: "The cell shrinks,its membrane blisters, its DNA fragments into uniform ladder-likepieces, hallmark of apoptotic death. This can be blocked by proteinsynthesis inhibitors, such as cycloheximide."

To test his hypothesis that neurotrophins wear two hats - forfendingapoptosis but succumbing to necrosis - Koh began by considering theproteins' function in health, namely, "to promote survival,differentiation, and neurite extension in mammalian central neurons."By this token, he surmised, they also bring about programmed celldeath as required by a growing organism, with the help of undefined"death proteins."

To induce apoptosis of murine neuronal cell cultures, he marinatedthem in serum-free media. This resulted in "widespread neuronaldegeneration over 24 hours." BDNF, neurotrophin-3 (NT-3) or NT4/5 "markedly reduced" these death throes.

To bring on necrosis, he exposed mixed neuron-glia cultures to a fullhour of oxygen-glucose deprivation. This starvation diet, whichmimicked the ischemia of cerebral stroke, "produced acute swellingof neuronal cell bodies, followed by widespread neuronal death."

But when the cytotoxic treatment lasted only 40 minutes, fully halfof the cells treated with BDNF, NT-3 or NT-4/5 died. Over 80percent of treated control cells suffered little neuronal injury.

The same neurotrophins that protected cultured cortical neurons fromdeath by suicide "can potentiate excitotoxic necrosis in the sameneurons," Koh emphasized." Hence his warning to researchersattempting to use these neurotrophins clinically.

Koh concluded that "Pharmacologically, if we use a glutamatereceptor antagonist, such as the MK-801 blocker of ischemic celldeath, "we can reverse this potentiating effect on neurotrophins ofoxygen-glucose deprivation."

He and his co-authors at Washington University's Center for Study ofNervous System Injury, are now seeking to elucidate the mechanismbehind this dichotomy of neuroptrophin's effects. Principal authorDennis Choi, who heads the institute, says, "Unmasking a potentiallydeleterious side of these molecules might allow us to interfere withthat side, and make that net effect better."

CytoTherapeutics Inc., of Providence, R.I., is a front-runner in the delivery of nerve growth factors across the blood-brainbarrier into cerebral tissues. (See BioWorld Today, March 14, 1994,p. 2.) The company's director of research, E. Edward Baetge, toldBioWorld Today that he finds the Koh/Choi paper "excellent; verywell done."

But he questions the validity of their in vitro dose concentrations ofthe neurotrophins with regard to real-life in vivo levels.

"Their concentration of growth factors is artificially high," Baetgesaid. Most of their studies are done at 100 nanograms per milliliter.We deliver very low concentrations of growth factors into the brain."

He added, "They actually show in their paper that at low doses youdon't see that effect. I would put the question back in their camp, andask whether or not in an in vivo situation, lower doses may beprotective against apoptotic death and/or necrotic death, withoutgiving you the toxic side effects. n

-- David N. Leff Science Editor

(c) 1997 American Health Consultants. All rights reserved.