Retrograde Signaling Another Way

The reports of two research groups interested in retrograde signaling caught the attention of investigators at the recent Society for Neuroscience (SFN)meeting; the teams used similar methods but arrived at two distinct conclusions. One team says that there is only one way to send nerve growth factor (NGF) signals from the axon to the cell body. The other group thinks that this retrograde signaling occurs in another way. Previously, scientists thought that when NGF bound to TrkA receptors at the axon tip, the complex was internalized into a vesicle and shipped back up the length of the axon to the cell body, where the complex conveys a signal for survival and growth. But Bronwyn MacInnis and Bob Campenot, Department of Cell Biology, University of Alberta, have caused a minor earthquake in the field of retrograde signaling with their results.1 Many scientists, including David Kaplan of the Research Institute at The Hospital for Sick Children, Toronto, see merit in the new thought process. "Bob's work has changed the paradigm." FINDING THE SECOND METHOD Working in a compartmentalized culturing system that enables researchers to treat the cell bodies with one type of media and the axonal projections with another, MacInnis and Campenot found that when they tied radioactively labeled NGF to beads too large to be internalized by the axon, the survival signal still reached the cell body. That, they say, means a vesicle-independent system must exist for NGF signaling. Furthermore, they found that by adding K252a, a specific inhibitor of TrkA, to the cell body chamber, TrkA activation was blocked but the cells still survived. However, when they added K252a to the axonal compartment, the neurons died. This further supports their theory that localized TrkA signaling at the axons is sufficient to prevent cell death and that there is no requirement for activated TrkA to reach the cell body. Although Campenot emphasizes that his team is by no means suggesting that vesicle-based trafficking does not occur in retrograde NGF signaling, he says the data is consistent with a second method that is sufficient to convey the signal. He speculates that this other, vesicle-independent signal may be a wave of calcium release from the endoplasmic reticulum. VESICLE-BASED TRANSPORT By contrast, David Ginty, Department of Neuroscience, Johns Hopkins University, and a Howard Hughes Medical investigator, says that the work in his lab belies a vesicle-independent mode of signaling. During his SFN meeting presentation, Ginty said his five experiments are only consistent with vesicle-based transport. When he added K252a to a proximal axon chamber, he did not see cell death, which is in direct contrast to Campenot's results and inconsistent with a domino or wave-propagation model. Ginty declined to talk about specifics because his work is unpublished. "We have data that says that under some conditions, vesicle-based signaling occurs and can support survival." While an undisputed point, it does not answer whether another signaling mode also exists. Campenot says that he thinks much of the difference comes down to small but significant variations in culture conditions. In his cultures, the axons grow at a rate of 1-1.5 mm per day, which he says is similar to growth rates in vivo. But Ginty's cultures, he contrasts, grow at one-tenth that rate. "The neurons are sick to begin with," Campenot says of Ginty's cultures, "and go into tail spin if you remove NGF." "My overall take on this is that there are probably multiple mechanisms operating," says Campenot. "Some survival mechanisms probably work when you bring NGF back to the cell body; some of them, like the one we found, don't have to transport NGF back." Ginty's neurons, he says, are depressed. "They have very little surface area generating in the distal compartments so that they need to have more than one signal working to keep them alive because they are near death to begin with." As to replicating each other's culturing conditions, Campenot says that he only recently became aware of the difference and does not rule out such an approach, but would prefer not to work with neurons he feels are unhealthy. Ginty plans to use a combination of genetics and biochemistry to study the problem in vivo. Like Campenot, Kaplan thinks many of the dissimilarities between the two labs can be ascribed to subtle but important protocol differences. William Mobley, Department of Pediatrics, Stanford University, Calif., says that he perceives no flaws in the methods. Instead, he suggests, it is just more complex than previously thought. "The truth is gnarly. It is a California oak, not a slender simple birch; it is gnarled and twisted and that is what we are going to find with signaling." Rabiya Tuma (rabiya@nasw.org) is a freelance writer in Brooklyn, New York. 1. B.L. MacInnis, R.B. Campenot, "Retrograde support of neuronal survival without retrograde transport of nerve growth factor," Science, 295:1536-9, 2002.

Retrograde Signaling Another Way

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