Many a pathogenic microbe gets in its low blows by secreting abacterial toxin aimed at its victim's cells. Thus, for example,Bordetella pertussis goes for the lungs, and inflicts whooping cough.Cholera toxin makes for the gut, and wreaks life-threatening diarrhea.
"All these bacterial toxins," said pulmonary clinician and biochemistJoel Moss, "cause their effects on cells involved in the pathogenesisof disease by a post-translational reaction called ADP-ribosylation."
ADP-ribose stands for "adenosine diphosphate," a major player in acell's trafficking of energy. Ribosylation refers to ribose, the five-carbon sugar component of RNA (deoxyribonucleic acid).
"In the case of pertussis toxin," Moss told BioWorld Today, "ADP-ribosylation essentially inhibits the ability of a G protein to properlycouple to certain cell receptors." G proteins _ so-called because theybind guanine nucleotides _ are the switches that pick up signalsfrom hormones outside a cell and relay them in to the scene of theirintended action.
"When pertussis toxin ADP-ribosylates them, the G proteins are nolonger able to couple their receptors, so the receptors can't transmittheir message into the cell. The net effect of cholera toxin," Mosssaid, "is to stimulate a G protein, which creates the diarrhealsyndrome of the disease."
Moss heads the Pulmonary-Critical Care Medicine Branch of theNational Heart, Lung and Blood Institute (NHLBI), in Bethesda, Md.Some years ago, he and his laboratory asked: "Are bacterial toxins infact mimicking control reactions that mammalian cells normally carryout? In other words," he explained, "toxins don't do anything normalcells wouldn't do, but they are doing it for their own benefit. Andthat's why you get disease."
Mammalian Cells Do It Too
Their research disclosed the fact that "animal cells, like bacterialtoxins, have enzymes that transfer ADP-ribose." But unlike toxins,cells use these transferases to control and regulate their ownmetabolic pathways.
Eventually, his team discovered a second enzyme, hydrolase, whichtakes off ADP-ribose-ribose rather than put it on. "So the cells have atransferase/hydrolase ADP-ribose-ribosylation on-off cycle," Mosssaid.
He is a lead inventor on two NHLBI patent applications covering thetwo on/off enzymes, ADP-ribose transferase and ADP-ribosehydrolase. "Our objective is that we might want to regulate puttingADP-ribose on and taking it off."
Cloning these enzymes, he discovered that the identical molecules arepresent in T lymphocytes and in skeletal and cardiac muscle. "Theprotein it works on," he added, "is integrin, which is involved in celladhesion. Because it's on the outside of the cell, affecting itsmembrane surface, we think it probably will be a good drug target forregulating the immune system."
His institute's immediate target is to hook up with a qualified biotechor pharmaceutical company in a Cooperative Research andDevelopment Agreement (CRADA) to pursue therapeuticapplications of this new insight.
Delayed by the recent partial federal shut-down, the NHLBI is aboutto submit an announcement to the Federal Register seeking such aresearch and development partner "to further characterizelymphocyte ADP-ribosyltransferase as a potential target fortherapeutic intervention in diseases of the immune system."
Moss's main interests include lymphocytic alveolitis, "where incertain diseases, lymphocytes infiltrate into the lungs. What we'd liketo do, obviously, is regulate them."
Among such pulmonary disorders, Moss cites Pneumocystis cariniipneumonia, a leading cause of death in AIDS victims. "Some HIVpeople with P. carinii," he observed, "have lymphocytes in theirlungs. These cells are also present in hypersensitivity pneumonitis,where people get an allergic reaction to various agents in the air theybreathe."
Inhalation, Injection, Ingestion Therapies Foreseen
Moss cites recent research by others, "who found that incubatingcytotoxic lymphocytes with the ADP-ribose precursor inhibits thesekiller T cells, due to ADP-ribosylation." Hence, "lymphocytes mayplay a role in autoimmune diseases that affect the lung."
A CRADA ally, he suggests, could help develop "the use ofinhalation therapies that minimize systemic toxicity." Moss alsoforesees intravenous or oral administration of drugs eventuallydeveloped against those key ADP-ribose enzymes, to treat otherautoimmune diseases.
But he cautioned, "We're not sure yet how these enzymes couldfunction; the research is ongoing. We're not sure whether it'sbeneficial or not beneficial to have lymphocytes, whether we want tojazz them up or tone them down."
Editor's note: For CRADA information, consult Lili Portilla,technology transfer specialist, NHLBI, (301) 496-4653; for technicalquestions concerning this research, Joel Moss, Pulmonary-CriticalCare Medicine Branch, NHLBI, (301) 496-1597. n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.