“Aβ42 has been our workhorse,” said Anne Fagan of Washington University in St. Louis, Missouri. “But that is not to say that there might not be more things that are even more informative,” added Fagan, who was not involved in either study.

As a biomarker, “The problem with Aβ itself is that it actually goes down; we think it is aggregating in the brain so Aβ42 levels in the CSF go down,” said Michael Wolfe of Harvard University. He explained that Aβ42 exists in two states—insoluble in the brain and soluble in the CSF, making it impossible to measure total Aβ42 with a spinal tap. But a marker that is always soluble would be fully accessible from CSF, perhaps yielding more accurate results. Wolfe was also not an author in the current works.

First authors Adnan Halim and Gunnar Brinkmalm, along with senior authors Göran Larson and Jonas Nilsson, led one study at the University of Gothenburg, Sweden. As they write in this week’s Proceedings of the National Academy of Sciences USA online, the group happened upon a rare, small, glycosylated, amino-terminal fragment of Aβ. The researchers were surprised to discover that the sugar group had set down not on a threonine or serine amino acid—as sugar groups generally do—but on a tyrosine. Studying this unexpected peptide in CSF samples, the researchers discovered that it was more abundant in fluid from people who had Alzheimer’s than in those who were cognitively normal.

Robert Perneczky of Technical University in Munich, Germany, led a study published in the June 22 Neurology online. He and his colleagues examined another potential biomarker, soluble APP (sAPP). Cleavage of APP, by α- or β-secretase, produces sAPP from the amino end of the precursor. Although other researchers have explored sAPP as a potential biomarker (Lewczuk et al., 2010), Perneczky’s work is the first to take a longitudinal approach, with an average follow-up time of three years. According to his statistical analysis, Perneczky said, sAPP was a better predictor of progression to AD than the standard Aβ42.

What Is That Sugar Doing There?
Nilsson and Larson recently developed a method to capture and identify glycoproteins (Nilsson et al., 2009). Halim was experimenting with the method in a CSF sample when he discovered a set of short, glycosylated Aβ fragments, 15-20 amino acids in length. To acquire enough of these bits to study in detail, the researchers used an antibody to pull down all forms of Aβ from CSF. They saw dozens of different-sized fragments, both glycosylated and not, but the team chose to focus on the small, glycosylated peptides because they were novel. “They are short and most likely not aggregation-prone,” said coauthor Henrik Zetterberg. These particular peptides made up less than 1 percent of the total Aβ species in the sample.

Using mass spectrometry, the researchers discovered that their fragments of interest came in six different lengths, always starting with amino acid 1 in the Aβ sequence and ending with any of amino acids 15-20. Since sugar groups normally attach to serines and threonines, they suspected serine-8 would be glycosylated. Instead, analysis showed it was on the tyrosine-10.

“Many people did not think such glycosylations existed,” Zetterberg said. There is no enzyme known to glycosylate tyrosine, and only a couple of examples of proteins glycosylated at this amino acid (Smythe et al., 1988; Aon and Curtino, 1985; Zarschler et al., 2010).

The authors were unable to determine the exact structure of the sugar group, which could be based on glucose, galactose, or mannitol, they write. Another unanswered question, Zetterberg said, is whether this glycosylation of Aβ is rare or commonplace, and whether it occurs only in the nervous system or in other cell types as well.

The scientists did not find Aβ42 glycosylated on tyrosine-10, suggesting to Zetterberg that the modification takes place before APP has been processed by γ-secretase. “Most likely the glycosylation occurs before the cleavage,” Zetterberg said, because glycosylation is an intercellular event that would have to occur before APP reaches the plasma membrane, where the first cleavage occurs.

Zetterberg hypothesizes that APP, once glycosylated at the cell surface, could be prevented from reentering the cell and encountering the γ-secretase that would normally cleave it. Alternatively, the glycosylation could conceivably alter the protein’s shape, so that after it reenters the cell, γ-secretase cuts it not at amino acid 42, but in the teens. Glycosylation has been shown to regulate proteolysis in a handful of systems (Schjoldager et al., 2010; Semenov et al., 2010; May et al., 2003; Maryon et al., 2007). Thus, the glycosylation could block or alter the pathway leading to Aβ42, explaining why those were never found glycosylated.

The authors wondered if the glycosylated peptides could correlate with Alzheimer’s disease. Using samples they had on hand, they measured the amounts of the fragments in CSF from six people with AD and seven healthy controls. They discovered that the relative abundance of the tyrosine-10 glycosylated fragments was increased by a factor of 1.1-2.5, in the AD samples than in the control fluids.

“I think this is a really critical modification of the protein,” Larson said. But he does not know if the glycosylation is pathogenic or compensatory.

Wolfe speculated that some of the small fragments could be products of both α- and β-secretase cleavage together. Normally, only one or the other cleaves APP, although Zetterberg and Kaj Blennow, another coauthor of the PNAS study, recently found that they could cleave the same protein (see ARF related news story on Portelius et al., 2010).

If the glycosylation does prevent Aβ42 production, Zetterberg acknowledges, then one might predict the glycosylated fragments would be downregulated in AD in favor of the larger, classical amyloid-β fragment. In fact, the scientists found the opposite. More studies, he hopes, will clarify the situation as well as help determine if glycosylated Aβ fragments could serve as a biomarker for AD. To use this fragment as a diagnostic or prognostic test, Zetterberg noted, doctors would need a less “cumbersome” method than antibody enrichment and mass spectrometry.

Begin at the Beginning
Aβ42 and tau levels in CSF are a commonly used biomarker for AD. But in looking at Aβ42, researchers are examining the very last product of APP cleavage. Instead, Perneczky suggests in his work, it could be more predictive to examine the very first step, the production of sAPP (which can be sAPPα or sAPPβ, depending on which secretase cleaves it from APP). Perneczky found similar trends for both sAPPα and sAPP-β, but the latter was more predictive. Cleavage at the α position prevents Aβ generation because it destroys the Aβ sequence.

The Munich team examined a small cohort of 58 people who had mild cognitive impairment (MCI). They followed them for an average of three years, assessing whether they progressed to Alzheimer’s, remained at the MCI stage, or returned to normal cognition.

Of those who went on to full-blown AD, their CSF samples had more sAPP-β—approximately 1,200 ng/ml—than their counterparts who had did not (900 ng/ml). “The biomarker, in combination with tau, was superior to the established combination of tau plus Aβ42,” Perneczky said. Considering sAPPβ, tau, and age, the researchers predicted who would or would not progress to AD with 80 percent sensitivity and 81 percent specificity. The team is now examining whether blood sAPP-β levels correlate with AD.

Looking at sAPP is a “worthwhile” pursuit, Larson said. However, Zetterberg noted that the researchers included no subjects who were cognitively healthy to start with. The rise and fall of sAPP could be due to many reasons, he suggested. Perneczky, now working to replicate the results, is including a cognitively normal group, he said.—Amber Dance.

References:
Halim A, Brinkmalm G, Rüetschi U, Westman-Brinkmalm, Portelius E, Zetterberg H, Blennow K, Larson G, Nilsson J. Site-specific characterization of threonine, serine, and tyrosine glycosylations of amyloid precursor protein/amyloid beta-peptides in human cerebrospinal fluid. PNAS. 2011 Jun 29. Abstract

Perneczky R, Tsolakidou A, Arnold A, Diehl-Schmid J, Grimmer T, Förstl H, Kurz A, Alexopoulos P. CSF soluble amyloid precursor proteins in the diagnosis of incipient Alzheimer disease. Neurology. 2011 Jun 22. Abstract