A new study offers up yet more evidence that the endosome may be ground zero for Alzheimer’s disease pathogenesis. In Proceedings of the National Academy of Sciences on January 18, scientists led by Olav Andersen of Aarhus University in Denmark and Gregory Petsko of Brigham and Women’s Hospital in Boston report that SORL1—an endosomal recycling protein with strong genetic ties to AD—forms dimers within the endosome. This molecular liaison, which can happen via either of two domains at opposite ends of the protein, attracts the retromer, a multi-subunit complex that ushers various proteins out of endosomal compartments. Together, SORL1 and the retromer then whisk APP out of the endosome, quelling production of amyloidogenic Aβ peptides. That SORL1 may simultaneously dimerize at opposite ends hints that it forms polymers within the endosomal membrane, stabilizing retromer complexes that facilitate protein recycling, the authors propose. The findings could also explain why risk variants are scattered across the SORL1 gene.
- SORL1 forms dimers on the endosomal membrane.
- These enhance retromer function, APP recycling.
- Modeling predicts SORL1 forms polymers, stabilizing retromer complexes.
“This study provides new and important information about how SORL1 can form dimeric structures important for recruiting APP into membrane tubules for endosomal transport,” wrote Brett Collins of the University of Queensland in Brisbane, Australia.
“These new insights could pave the way for designing either molecular or genetic tools that target these pathogenic pathways in Alzheimer’s disease.”
“With perturbed retromer function already closely associated with onset of AD, this thought-provoking study will be of significant interest to the AD community and, more broadly, those interested in the field of retromer biology and endosomal cargo sorting,” wrote Peter Cullen of the University of Bristol, U.K.
When paired with the retromer complex, sortilin-related receptor 1 (SORL1), aka SORLA, binds and transports proteins from the endosome to other cellular locales such as the trans-Golgi network or plasma membrane (Jan 2022 news). The gene is an AD hotspot, housing variants believed to cause autosomal dominant AD as well as those that up risk for the disease (Jan 2007 news; Jun 2018 news). These harmful variants are distributed throughout the gene. In the protein’s large lollipop-like ectodomain, VPS10p and YWTD domains top a stalk of 11 complement-type receptor domains and six fibronectin type III (3Fn) domains. A small endodomain sits in the cytosol (image below). While the CR region is known to bind cargo including APP, the diminutive cytoplasmic tail is tasked with snagging the retromer (Jun 2012 news). The functions of the VPS10p and 3Fn domains are unclear, although in other proteins, these domains can dimerize. Recent studies suggest that SORL1 can dimerize as well (Zhang et al., 2022). Might these domains play a part in that?
To investigate, first author Anne Jensen and colleagues launched a barrage of molecular and biochemical techniques, including the construction of a SORL1 “mini-receptor” comprising the 3Fn, transmembrane, and cytoplasmic tail domains. They found that the mini-receptor could dimerize, and that the 3Fn domains facilitated this pairing, both in vitro and within cells. Next, using a proximity ligation assay, which can detect close contacts between tagged proteins via fluorescence, the researchers found that SORL1 only forms dimers when located on the endosomal membrane. Importantly, these dimers co-localized with the retromer complex, which gathered on the cytoplasmic face of endosomes wherever SORL1 pairs were spotted. Indeed, they found that SORL1 dimers in the endosome existed almost entirely in complex with retromer.
How might SORL1 dimerization influence the recycling and processing of APP? To find out, the researchers overexpressed APP and either full-length SORL1 or the 3Fn-containing mini-receptor, in human neuroblastoma cells. Full-length SORL1 led to a dip in both amyloidogenic and non-amyloidogenic processing of APP. The scientists believe this stems from removal of APP from the endosomal compartment into the trans Golgi network, where it becomes waylaid or routed to the lysosomes for degradation. Transport to the TGN also keeps APP away from amyloidogenic, β-secretase cleavage in the endosomes, and from non-amyloidogenic, α-secretase cleavage in the plasma membrane. Surprisingly, overexpression of the mini-receptor, which can dimerize with itself and also with bind endogenous SORL1, increased non-amyloidogenic processing, but decreased amyloidogenic processing, thus reducing levels of Aβ42. The effects of the mini-receptor are reminiscent of those of previously reported retromer chaperones that route APP back to the plasma membrane (Apr 2014 news; Jan 2020 news).
“These results have multiple therapeutic implications,” wrote Samuel Gandy of Icahn School of Medicine at Mount Sinai in New York (comment below). “In addition to the chemical retromer stabilizers that this group have reported, it is now possible to consider an alternative strategy in which gene therapy might be used to deliver the SorL1 mini-receptor to the brains of AD patients.”
Strikingly, Jensen and colleagues found that in addition to linking up via its 3Fn domains, SORL1 dimerizes via VPS10p, a domain reported to bind Aβ peptides (Feb 2014 news). Using computational modeling, she found that SORL1 could theoretically use both of these domains to form polymers on the endosomal membrane (image below). When coupled with retromers, such SORL1 polymers could drive the formation of endosomal tubules—projections of the membrane formed by networks of retromer complexes that ultimately pinch off, delivering endosomal cargo to other compartments. Andersen emphasized that they have yet to obtain direct physical evidence of SORL1 polymers.
Polymeric Possibilities. By simultaneously dimerizing via its VPS10p (red asterisk) and its 3Fn (blue asterisk) domains, SORL1 could theoretically form polymers on the endosomal membrane, facilitating polymerization of retromer complexes on the cytoplasmic face, which in turn form endosomal tubules. [Courtesy of Jensen et al., PNAS, 2023.]
Still, he interprets the findings as clear evidence of the intimate collaboration between SORL1 and retromer in driving endosomal recycling, and suggests that SORL1 dimerization is a critical mechanistic step of this pathway. Moreover, Andersen emphasized that SORL1 cargo proteins, including APP, glutamate receptors, and TREKB, also form homodimers, suggesting that dimerization could be a critical regulatory feature of endosomal recycling.
Jessica Young of the University of Washington in Seattle noted that both the 3Fn and VPS10p domains harbor pathogenic variants in SORL1. “This paper is strongly suggestive that these variants may cause AD by disrupting endosomal recycling,” she wrote. “This work paves the way for new targets for AD that focus on the SORL1-retromer interaction as a tractable unit of the overall pathway for therapeutic design.” Young said that future studies should focus on how this unit works to recycle other proteins that that are implicated in AD-related neurodegeneration, including glutamate receptors and neurotrophin receptors.
Taking a broad perspective, Ralph Nixon of New York University in Orangeburg noted that defects in two reciprocal forces—entry and exit of proteins into endosomes—contribute to the bulging of these compartments observed in the early stages of AD pathogenesis (Cataldo et al., 2000). Nixon reported that accumulation of β-CTF, a product of β-secretase cleavage of APP that occurs within the endosome, activates Rab5, a GTPase that accelerates endocytosis (Jul 2015 news; Jan 2021 news). To his mind, the current study suggests that defects in SORL1 function not only slow endosomal egress, but also accelerate endosomal entry by way of increasing amyloidogenic processing of APP, and thus stoking levels of β-CTF and Rab5 activation. “The emerging biology integrates a recycling deficit, as shown here, with a cargo entry acceleration mediated by β-CTF/Rab5,” Nixon said.—Jessica Shugart
- Neuron-Specific Retromer Identified—Does It Stave Off Alzheimer’s?
- SORLA Soars—Large Study Links Gene to Late-onset AD
- Gaining Notoriety, SORL1 Claims Spot Among Top Alzheimer’s Genes
- Coming Into Vogue? Retromer in APP Processing, AD Pathogenesis
- Could Bolstering the Retromer Thwart Alzheimer’s?
- Chaperone Stabilizer Fends Off Amyloid, Memory Loss in Mice
- SORLA Serves Up Aβ for Destruction
- Partners in Crime: APP Fragment and Endosomal Protein Impair Endocytosis
- Doubling Rab5 in Mice Leads to Neurodegeneration—Without Plaques
- Zhang X, Wu C, Song Z, Sun D, Zhai L, Liu C. Cryo-EM structures reveal distinct apo conformations of sortilin-related receptor SORLA. Biochem Biophys Res Commun. 2022 Apr 16;600:75-79. Epub 2022 Feb 12 PubMed.
- Cataldo AM, Peterhoff CM, Troncoso JC, Gomez-Isla T, Hyman BT, Nixon RA. Endocytic pathway abnormalities precede amyloid beta deposition in sporadic Alzheimer's disease and Down syndrome: differential effects of APOE genotype and presenilin mutations. Am J Pathol. 2000 Jul;157(1):277-86. PubMed.
No Available Further Reading
- Jensen AM, Kitago Y, Fazeli E, Vægter CB, Small SA, Petsko GA, Andersen OM. Dimerization of the Alzheimer's disease pathogenic receptor SORLA regulates its association with retromer. Proc Natl Acad Sci U S A. 2023 Jan 24;120(4):e2212180120. Epub 2023 Jan 18 PubMed.