By David N. Leff
On the map of Italy, down south in the instep of its "boot," sits the little city of Nicastro. There, some 40 years ago, a large, multigenerational family made medical history. Its members were afflicted with what became known as familial, early-onset Alzheimer's disease (AD).
Usually, this neurodegenerative senile dementia strikes its victims in their 70s. The Nicastrin family members showed AD's signal symptoms - memory loss, confusion, disorientation - while in their 30s or 40s.
In 1995, Canadian scientists at the Center for Research in Neurodegenerative Diseases (CRND) in Toronto identified and cloned the genes for two key proteins - presenilin-1 and presenilin-2 - which have to answer for the most severe forms of familial, early-onset AD.
Now, in today's issue of Nature, dated Sept. 7, 2000, the same CRND team announces it isolated a long-sought protein, which binds to both presenilins, and influences the pathway that leads to senile neuritic amyloid plaques - the toxic hallmarks of AD neurons.
They named their new protein "nicastrin," explained one of the paper's two co-senior authors, biochemist Paul Fraser, "as a tribute to the Italian families of Nicastro, who provided one of the key steps in our identification of the presenilins."
The Nature paper bears the title: "Nicastrin modulates presenilin-mediated notch/glp-1 signal transduction and bAPP processing." The article's other senior author is pioneer AD investigator Peter St. George-Hyslop, director of CRND.
The beta-amyloid precursor protein (bAPP) is where the pathogenic pathway en route to plaque formation begins. First, an enzyme called beta-secretase, parked in the neuron's outer membrane - as is bAPP - cuts that lengthy protein in two. The longer portion gets lost; the shorter, stubby amino-acid sequence, Fraser told BioWorld Today, "is the residual portion of the APP that is left over once it's cleaved, and the large section is secreted into the extracellular space.
A Nicastrin-Plaque Connection?
"It seems," Fraser observed, "that this particular little stub of APP is associated with nicastrin, and possibly the presenilins. Both are likely involved in subsequent processing of this stub to produce the amyloid peptide. It polymerizes to form fibrils and plaques, which get deposited in the brain. Plaques are considered to be one of the principal contributing factors of nerve cell death and eventually dementia. (See BioWorld Today, June 17, 2000, p. 1.)
"The nicastrin gene resides on human chromosome 1," Fraser noted. "Our team has cloned it," he added, "from human, mouse, Caenorhabditis elegans, and Drosophila melanogaster. But as for the function of nicastrin, the protein it expresses," Fraser allowed, "that's the puzzle right now. What its precise role is in terms of presenilins is still a bit of a mystery. Solving that is one of our major goals at the moment. Another of our objectives at present," he went on, "is to find some additional members of this presenilin complex, if at all possible. We don't know how many there might be, but certainly nicastrin isn't the last new AD protein."
In an in vivo experiment to probe nicastrin's function, the co-authors knocked its gene out of C. elegans embryos. "When we removed nicastrin from them," he recounted, "we got a major developmental problem - loss of the embryonic pharynx. This is similar to a phenotype related to notch signaling and also presenilin. It links the presenilins directly - functionally - with nicastrin.
"Notch is a key developmental protein," Fraser explained. "It resides on the cell surface, and when cells or tissues are undergoing differentiation and development, they get certain extracellular signals from adjacent cells. So these interact with notch, which has a small, transmembrane intracellular tail. This little bit," he continued, "gets cleaved off and is translocated to the nucleus where it activates transcription, and switches on a whole bunch of developmental genes."
When the co-authors sought to find a mutant nicastrin protein in the genomes of AD families, they struck a snag. "Obviously," Fraser observed, "one of the possibilities is that other familial forms of AD are associated with mutations in nicastrin. So we searched in the 19 sets of families that we have that are linked to familial AD, but not to presenilins. Unfortunately the families that we checked didn't show any mutations - but there certainly is that possibility, which we intend to pursue. It's just a question of collecting samples from individuals who have presenilin or APP-related sequences - of getting the right data sets."
While chasing down such unanswered questions, the co-authors are also following up on the therapeutic potentials exhibited by their new nicastrin find. As St. George-Hyslop stated in a news release, "This opens the way for the development of drugs that will target the new protein to manipulate the process that leads to the disease."
In The Cards: Amyloid Wipeout
As alluded to in the Nature paper, Fraser recounted, "We took one short, highly conserved domain of the nicastrin protein's 709 amino acids. Those dozen residues of the DYIGS [aspartate-tyrosine-isoleucine-glycine-serine] domain are present in everything from humans to fruit flies. When we took the human, mouse, C. elegans and fruit fly sequences and matched them up to see which regions have been conserved over evolution, we found that there were a couple. Once we removed the DYIGS segment of nicastrin, with this particular region, we could abolish amyloid production.
"So this suggests," he pointed out, "that maybe this particular region has more potential in terms of deactivating this AD amyloid pathway.
"If we could," he suggested, "design or find or screen for molecules from either rational drug design or just by screening through a chemical library, for compounds that would bind to this particular region and produce the same amyloid-abrogating effect, then we might be able to affect the course of the disease.
"There are a number of different drug-discovery ways," Fraser said. "We could take a large pharmaceutical chemical library and see - in a model system - if it would bind to nicastrin, and produce this same physiological response regarding amyloid. Or the other thing would be to take this particular critical region and solve its molecular structure. Once we know the general shape of the region, we could start looking for lock-and-key compounds."
Fraser concluded: "We have a major collaboration - a longstanding working relationship - with Schering-Plough Corp., both in Canada and the U.S." n