To the neuroscientists, it just didn't make sense.

Here they had a molecule, ciliary neurotrophic factor (CNTF),stock-piled in healthy, peripheral nerve cells, without mobilityor any apparent activity. This absence of secretion suggested tosome that CNTF is a "lesion factor," galvanized to act only whena nerve is injured. Even then, they won-dered, how can thedamage-control molecule come to the aid of a distressed cellbody -P the nerve's mission control center -P if the lesion is onan axon some distance away?

Peter DiStefano and Rory Curtis, neuroscientists at RegeneronPharmaceuticals, Inc., enlisted rats to solve this apparent nonsequitur. Their experiment, reported in last Thursday's Nature,tells the answer: "Retrograde axonal transport of ciliaryneurotrophic factor is in-creased by peripheral nerve injury."

That is, the paper explained, "CNTF can shuttle along nervefibers in the opposite direction from neuronal signals," fromaxon upstream to cell body.

The unexpected emergency traffic route the authors discoveredhas the potential to deliver exogenously administered nerve-regenerating neu-rotrophic factors through the blood-brainbarrier into the central nervous system, to treat trauma anddisease.

DiStefano, Curtis and their neurobiology group at Regeneron ofTarrytown, N.Y., exposed the sciatic nerve inside one thigh ofanesthetized laboratory rats. With fine jeweler's tweezers, theypinched and crushed a precise millimeter-wide section of thetwo-millimeter-thick nerve, which in rats extends some threeor four centimeters from point of injury to cell body. Sevendays later, they injected a large bolus dose -- 40 nanograms --of recombinant, radiolabeled CNTF into the nerve injury site.Over the next six hours they monitored a five-fold increase inits rate of retrograde transport to sensory neurons, and tomotor neurons in the ventral spinal cord.

CNTF is a 22-24 kiloDalton protein exactly 200 amino acidslong. It is synthesized in quantity by the peripheral nervoussystem's myelin-forming, axon-supporting Schwann cells.When the investigators observed a sharp drop in theseextremely high intracellular CNTF deposits soon after the crushinjury, DiStefano told BioWorld, they took this as evidence thatthe lesion had unleashed the endogenous neurotrophic factorand increased transport of the labeled, exogenous CNTF.

DiStefano explained, "At the crush site, the axons areundergoing re-generation, and what happens is that even withina day you get sprouting of regenerative axons, both sensoryand motor. And when you apply the CNTF, it presumably bindsto receptors on those sprouts. As we've shown, it then binds,internalizes and retrogradely transports back to the cell body.

"In normal circumstances," CNTF is apparently not releasedfrom the Schwann cells. It has no signal sequence, no secretorymechanism. This supported the hypothesis that CNTF acts as a'lesion factor,' only becoming released at the place of injury."

"We know for a fact," DiStefano said, "that CNTF does not crossthe blood-brain barrier, doesn't get into the central nervoussystem (CNS)...yet this factor is effective in ameliorating a lot ofthe motor neuron abnor-malities in some mice with geneticmotor neuron deficits, used as animal models for ALS(amyotrophic lateral sclerosis, or Lou Gehrig's disease).

"Motor neurons are unique, DiStefano continued, "in that theiraxons are in the peripheral nervous system, but their cellbodies lie within the CNS. So we asked: 'How does the CNTF acton these neurons within the CNS?' "

The Regeneron team offers one possible answer: Retrogradetransport.

Eugene Johnson, a professor of molecular biology andpharmacology at Washington University, is a pioneer in thefield of trophic factor biology.

Asked his view of the therapeutic future DiStefano foresees, hetold BioWorld:

"I think certainly the demonstration that CNTF is retrogradelytransported strengthens the notion that it could be aphysiologically important neurotrophic factor either duringdevelopment, after nerve injury or during neurodegeneration.

"This would indicate that there's no anatomical barrier to theprocesses and the receptors of that cell seeing the factor. Youwould almost think this would be a prerequisite for the factorto be able to exert an ameliorative effect. It doesn't prove thatit will, but it's one of the hoops that will have to be jumpedthrough in order for this to work in the real world after nerveinjury.

-- David N. Leff Science Editor

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