15 June 2012. Poor traffic control causes nasty pile-ups—not only for cars but for proteins, too. This realization fuels the study of intracellular pathways that regulate the comings and goings of amyloid-β precursor protein (APP). Three recent papers solidify the role of the retromer, a protein complex mediating endosome-to-Golgi transport, in APP processing and in Alzheimer’s disease. They help explain how the Aβ production engine runs hot when APP gets stuck in endosomal traffic jams.
In one study, described in the June 4 Neurobiology of Aging online, researchers led by Lindsay Farrer of Boston University School of Medicine, Massachusetts, and Matthew Seaman of the University of Cambridge, U.K., identified AD-associated variants in several genes that regulate retromer function. Another study found that the neuronal retromer regulates APP transport primarily in dendrites and axons, and that retromer deficiency leads to APP buildup and Aβ overproduction. That work, published online April 6 in Neurobiology of Disease, was led by Scott Small and Gilbert Di Paolo of Columbia University in New York. And in the January 25 Journal of Neuroscience, Olav Andersen of Aarhus University, Denmark, and colleagues identify a motif in the neuronal sorting receptor SorLA that binds retromer and regulates APP processing. These reports “consolidate the idea that abnormal intracellular trafficking of APP by SorLA is an underlying cause of amyloidogenic processing,” noted Thomas Willnow of Max Delbrück Center for Molecular Medicine, Berlin, Germany, in an e-mail to Alzforum. Willnow is a coauthor on the Journal of Neuroscience paper.
Farrer and Seaman build on prior work that identified SorLA (aka SORL1) as a risk gene for late-onset AD (see ARF related news story on Rogaeva et al., 2007) and that suggested the receptor influences AD pathology by directing APP compartmentalization (see ARF related news story on Schmidt et al., 2007). Seaman's lab identified new retromer or retromer-associated genes via an siRNA library screen. First author Badri Vardarajan and colleagues in Farrer’s group then tested 15 of the top hits for association with AD in the large Caucasian dataset (8,309 AD cases and 7,366 normal elderly) provided by the Alzheimer’s Disease Genetics Consortium (ADGC), which identified new AD risk genes last year (see ARF related news story on Naj et al., 2011). Four of the new retromer genes associated with AD—RAB7A, KIAA1033, SNX1, and SNX3. The KIAA1033 association replicated in a smaller African-American cohort, as did SNX3 variants in an Israeli-Arab dataset. The findings emphasize the importance of protein trafficking to AD pathology.
Seaman’s lab further characterized SNX3, the gene showing the most robust association with AD, and RAB7A, a GTPase that helps recruit the retromer into endosomes (see Seaman et al., 2009). Using yeast two-hybrid assays and transfected HeLa cells, the researchers demonstrated that RAB7A and SNX3 interact with the retromer through independent mechanisms to regulate how the complex interacts with cell membranes. This paper “achieves two things,” said Small. “First, it adds to previous genetics findings showing that retromer dysfunction plays a pathogenic role in AD. Second, it uses cell biological techniques to expand our understanding of retromer biology.”
A previous microarray analysis of brain tissue samples by Small and colleagues implicated the retromer complex in AD by showing reduced levels of retromer proteins Vps35 and Vps26 in the entorhinal cortex of AD patients (Small et al., 2005). These brain areas are especially vulnerable to the disease. The researchers predicted that SorLA mediates retromer trafficking of APP, and that retromer dysfunction would cause APP to cluster inappropriately in endosomes, where levels of β-secretase are high. That could drive production of Aβ. “Our new findings confirm the second prediction,” Small told Alzforum.
As they reported in Neurobiology of Disease, Small, Di Paolo, and colleagues clarified what role the retromer plays in neurons. Most retromer cell biology studies to date were based on non-neuronal cells, Small said. Imaging mouse hippocampal neurons, first author Akhil Bhalla and colleagues detected Vps35 in endosomes and the Golgi, where the retromer typically functions. They also found Vps35 in dendrites and axons, suggesting it might be involved in long-range transport, including, perhaps, trafficking of APP. Sure enough, when they silenced Vps35 in hippocampal neurons using lentiviral shRNA, APP accumulated in early endosomes in processes, but not in the soma. Furthermore, APP colocalized more with BACE1, and Aβ levels rose. “The real novelty of this paper is that it shows that the retromer's involvement in APP trafficking occurs mostly in distal processes,” Small said. “Retromer dysfunction appears to ‘load up’ APP in endosomes of dendrites and axons—sites where APP processing is most likely to proceed.”
Olav Andersen’s work reported in the Journal of Neuroscience paper fleshes out the retromer’s role in APP processing even further by showing how the protein complex interacts directly with SorLA. Using biochemistry, cell biology, and electron microscopy approaches, first author Anja Fjorback and colleagues show that the retromer component that binds to SorLA is Vps26. More specifically, Vps26 binds a hexapeptide motif (FANSHY) in the cytoplasmic tail of the sorting receptor. SorLA mutants lacking this motif still bound APP, but failed to direct the precursor to the Golgi, leaving it more susceptible to amyloidogenic processing. Again, this work reveals the vital role played by the retromer and by SorLA to control Aβ production.
The retromer might play a similar role in other diseases as well. Recently, research linked Vps35 mutations to late-onset Parkinson’s disease (see ARF related news story). Based on previous work, it was assumed that Vps35, not Vps26, bound SorLA, Small said. The new data are important because “they might explain how a single complex, the retromer, could be involved in different diseases,” said Small. He speculated that maybe Vps35 is linked to receptors that are important in PD, whereas by virtue of its binding SorLA, Vps26 is more linked to APP and AD.—Esther Landhuis.
Vardarajan BN, Bruesegem SY, Harbour ME, St. George-Hyslop P, Seaman MN, Farrer LA. Identification of Alzheimer disease-associated variants in genes that regulate retromer function. Neurobiol Aging. 2012 June 4. Abstract
Bhalla A, Vetanovetz CP, Morel E, Chamoun Z, Di Paolo G, Small SA. The location and trafficking routes of the neuronal retromer and its role in amyloid precursor protein transport. Neurobiol Dis. 2012 Jul;47(1):126-134. Epub 2012 Apr 6. Abstract
Fjorback AW, Seaman M, Gustafsen C, Mehmedbasic A, Gokool S, Wu C, Militz D, Schmidt V, Madsen P, Nyengaard JR, Willnow TE, Christensen EI, Mobley WB, Nykjaer A, Andersen OM. Retromer Binds the FANSHY Sorting Motif in SorLA to Regulate Amyloid Precursor Protein Sorting and Processing. J Neurosci. 25 Jan 2012;32(4):1467-1480. Abstract