The family of low-density lipoprotein (LDL) receptors has captured the interest of Alzheimer’s researchers, because several of these transmembrane sorting proteins help cells crank out Aβ. In the July 27 Journal of Neuroscience, researchers led by Dudley Strickland at the University of Maryland, Baltimore, introduce a novel family member, christened low-density lipoprotein receptor class A domain containing 3 (LRAD3). Like many of its siblings, the new kid can interact with amyloid precursor protein (APP), and it increases Aβ levels in cultured cells. LRAD3 seems to bind different protein partners than its kin do, suggesting it could act through distinct signaling pathways. Although much work remains to be done to show that this protein plays a role in AD in vivo, the discovery could potentially open up new routes for modulating Aβ production.

Other members of the LDL receptor family, as well as other related sorting proteins, have been shown to take a hand in increasing or decreasing Aβ generation by moving APP into intracellular compartments, where it is either snipped to release Aβ or digested. For example, LDL receptor-related protein 1 (LRP1) helps pump up Aβ levels (see, e.g., Kounnas et al., 1995; Ulery et al., 2000; Pietrzik et al., 2002; Pietrzik et al., 2004; and Waldron et al., 2008), as does the apolipoprotein E receptor 2 (see Fuentealba et al., 2007). The sorting protein SorCS1, by contrast, reduces Aβ generation (see ARF related news story on Lane et al., 2010). “We seem to have moved from the ‘secretase generation’ to the ‘sortase generation,’” Sam Gandy at Mount Sinai Medical Center wrote to ARF (see full comment below).

To discover whether the LDL receptor family held hidden members, first author Sripriya Ranganathan screened a database of human expressed sequence tags and turned up LRAD3. LRAD3’s sequence shows several differences compared to other LDL receptor family members, suggesting the new protein may bind distinct extracellular ligands and/or intracellular partners. In support of this, LRAD3 failed to bind receptor-associated protein (RAP), a common ligand for the family, and did not interact with the intracellular adaptor protein Fe65, which binds LRP1 and helps it interact with APP. Fe65 is implicated in APP signaling (see ARF related news story).

Analysis of human RNA showed that LRAD3 is expressed in many tissues, including total brain extract. Staining of mouse brain sections clarified the picture, revealing the presence of LRAD3 in hippocampus and cortex, among other regions. In cell culture experiments, LRAD3 sat in the cell membrane and was able to bring ligands into the cell through endocytosis, although more slowly than LRP1 does. Co-immunoprecipitations showed that LRAD3 interacts with APP, although it is not known if the interaction is direct or if it occurs through an adaptor protein. In cell culture, LRAD3 alters APP handling in a similar way as LRP1 does, pushing processing toward the amyloidogenic pathway.

As might be expected from a new find, numerous questions remain to be answered. Strickland said they are working on developing an LRAD3 knockout mouse, which they plan to cross with an AD mouse model to look for in-vivo effects on AD pathology. The authors are also actively searching for ligands and adaptor proteins that bind LRAD3, and hope to elucidate those pathways soon.

“This is solid work,” said Guojun Bu at the Mayo Clinic in Jacksonville, Florida. He suggested that one intriguing direction for future studies will be to look for mutations in LRAD3, or changes in its gene expression that associate with human AD. If these exist, it would strengthen the case for LRAD3 playing a significant role in disease pathology. Strickland said this is something he hopes to do.

Joachim Herz at UT Southwestern, Dallas, Texas, is most interested in what connection LRAD3 might have with ApoE, the most important risk factor for sporadic AD. “What makes the LDL receptor gene family special is that they can physically interact with ApoE,” he noted, and, therefore, these receptors forge a direct link between ApoE and amyloid generation. Herz believes the key question is whether isoforms of ApoE can modulate LRAD3 activity. If they can, it would suggest LRAD3 might play a dynamic role in AD pathology. If LRAD3 activity is not easily modified, on the other hand, the receptor could be just a housekeeping gene that regulates baseline trafficking of APP, Herz suggested. Strickland told ARF he is planning to investigate this question in collaboration with ApoE expert Karl Wiesgraber at the Gladstone Institute for Neurodegenerative Diseases, San Francisco, California.—Madolyn Bowman Rogers.

Reference:
Ranganathan S, Noyes NC, Migliorini M, Winkles JA, Battey FD, Hyman BT, Smith E, Yepes M, Mikhailenko I, Strickland DK. LRAD3, a novel low-density lipoprotein receptor family member that modulates amyloid precursor protein trafficking. J Neurosci. 2011 Jul 27;31(30):10836-46. Abstract

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  1. This work from Dudley Strickland's lab describes a new protein related to the LDL receptor family, termed LRAD3. This protein shares structural similarities to other members of the family, including domains for the internalization and sorting of (as yet) unidentified ligands. It also shares another interesting characteristic: It interacts with APP and affects its processing. Interactions with APP have already been demonstrated for several members of the LDL receptor family, including LRP1 (Kounnas et al., 1995; Waldron et al., 2008), ApoER2 (Fuentealba et al., 2007), and sorLA (Andersen et al., 2005; Spoelgen et al., 2006). Both families share common adaptor proteins (Fe65, X11, Dab1), and the structure of LRAD3 suggests that it may interact with some of these adaptor proteins as well. These observations are even more interesting, since the functions of members of the APP and LDL receptor families may be related: Evidence shows that both families affect neuron migration during development, and both affect synapse structure in mature neurons.

    This study provides further data demonstrating that our understanding of APP trafficking, metabolism, and function will require our understanding of members of the LDL receptor family, including LRAD3.

    View all comments by G. William Rebeck
  2. This work looks very solid and very interesting. We seem to have moved from the “secretase generation” to the “sortase generation,” what with LRP, vps35, SORL1, and SORCS1 all playing key roles in sorting APP and its C-terminal fragments in and out of Aβ-generating and Aβ-lytic compartments.

    This is all well and good, but what we really need is a clinical success. We now know how to lower Aβ with drugs or vaccines, but we don’t yet know whether pre-symptomatic Aβ lowering will prevent dementia, nor do we know how early is early enough. Based on the Dominantly Inherited Alzheimer Network (DIAN) and published data, and the desire not to start too soon the first time, it looks to me as if we have to begin treating people with presenilin-1 mutations at least 20 years before age at onset. Maybe 25 years is early enough.

    While waiting for those DIAN data, we need to focus on potential prophylactic interventions with more impeccable safety profiles than we might accept for therapeutics, because we face the prospect of treating clinically normal, pre-symptomatic subjects for at least as long (i.e., 25 years) in the first approximation.

    The cell biology of the new Strickland paper provides yet another potentially druggable strategy for Aβ lowering. The data are clear and compelling, but it would be a whole lot easier to whip up enthusiasm if we were getting some runs on the board with Aβ-lowering interventions in the clinic.

    View all comments by Samuel Gandy
  3. There is no question that the complex processing of amyloid-β precursor protein (APP), via at least two intensely studied pathways (α- and β-secretase cleavage) that are relevant to Alzheimer’s disease (AD), occurs during APP’s still insufficiently understood journey within neurons along the secretory, endocytic, and degradative routes. It follows that the mechanisms and proteins that regulate intracellular trafficking of APP are most likely to affect processing of the precursor by altering its targeting into, or out of, different compartments. Based on what is currently known, proteins that regulate APP processing are potential targets for drugs against AD. Ranganathan et al. now add another protein to the plethora of those that bind (directly or indirectly) APP and regulate production of amyloid-β (Aβ). What is important is that the newly identified protein, LRAD3, an LDL receptor family member, participates in endocytic processes. Based on this role, and on the fact that LRAD3 affects APP proteolytic cleavage—an event that mostly occurs along APP’s endocytic route—the authors propose that LRAD3 modulates APP trafficking, and in this way, also modulates APP processing. The study actually does not directly address the issue of APP trafficking, and it still remains to be demonstrated that the proposed mechanism indeed constitutes the basis for the increased amyloidogenic cleavage of APP, and generation of Aβ by overexpressed LRAD3. It is also possible—although less likely—that increased expression of LRAD3 disrupts the interaction of APP with other endogenous APP binding proteins that themselves could play roles in APP processing.

    There is no doubt that this study is relevant to AD. The increased levels of secreted Aβ, detected in this study in the conditioned medium upon overexpression of LRAD3 in cultured cells, point to a potential source of the toxic extracellular Aβ in the AD brain. It would be also interesting to know if, under these conditions, the neurons also contain increased levels of Aβ. This is especially important considering the current debate on the role of intraneuronal Aβ in the pathogenic process of AD, as discussed in the ARF Webinar: Intraneuronal Aβ: Was It APP All Along?. Finally, we would like to point to the possibility that, by modulating the metabolism of APP, and the generation of APP-derived polypeptides, LRAD3 could very well regulate the function of full-length APP and its fragments, whatever those functions are (1). One cannot disregard the possibility that dysregulated APP or its fragments (other than Aβ) (2), in addition to potential detrimental effects on neuronal physiology (3,4), might contribute to AD pathology. While Aβ is part of the mechanisms leading to AD, it certainly is not the only player (5,6).

    View all comments by Virgil Muresan

References

News Citations

  1. APP Sorting Protein May Link Alzheimer’s and Diabetes
  2. Long-elusive Function for APP Cleavage Product Comes into View: It's Gene Transcription

Paper Citations

  1. . LDL receptor-related protein, a multifunctional ApoE receptor, binds secreted beta-amyloid precursor protein and mediates its degradation. Cell. 1995 Jul 28;82(2):331-40. PubMed.
  2. . Modulation of beta-amyloid precursor protein processing by the low density lipoprotein receptor-related protein (LRP). Evidence that LRP contributes to the pathogenesis of Alzheimer's disease. J Biol Chem. 2000 Mar 10;275(10):7410-5. PubMed.
  3. . The cytoplasmic domain of the LDL receptor-related protein regulates multiple steps in APP processing. EMBO J. 2002 Nov 1;21(21):5691-700. PubMed.
  4. . FE65 constitutes the functional link between the low-density lipoprotein receptor-related protein and the amyloid precursor protein. J Neurosci. 2004 Apr 28;24(17):4259-65. PubMed.
  5. . LRP1 modulates APP trafficking along early compartments of the secretory pathway. Neurobiol Dis. 2008 Aug;31(2):188-97. PubMed.
  6. . ApoER2 expression increases Abeta production while decreasing Amyloid Precursor Protein (APP) endocytosis: Possible role in the partitioning of APP into lipid rafts and in the regulation of gamma-secretase activity. Mol Neurodegener. 2007;2:14. PubMed.
  7. . Diabetes-associated SorCS1 regulates Alzheimer's amyloid-beta metabolism: evidence for involvement of SorL1 and the retromer complex. J Neurosci. 2010 Sep 29;30(39):13110-5. PubMed.
  8. . LRAD3, a novel low-density lipoprotein receptor family member that modulates amyloid precursor protein trafficking. J Neurosci. 2011 Jul 27;31(30):10836-46. PubMed.

Further Reading

Papers

  1. . LRAD3, a novel low-density lipoprotein receptor family member that modulates amyloid precursor protein trafficking. J Neurosci. 2011 Jul 27;31(30):10836-46. PubMed.

News

  1. APP Sorting Protein May Link Alzheimer’s and Diabetes
  2. Long-elusive Function for APP Cleavage Product Comes into View: It's Gene Transcription

Primary Papers

  1. . LRAD3, a novel low-density lipoprotein receptor family member that modulates amyloid precursor protein trafficking. J Neurosci. 2011 Jul 27;31(30):10836-46. PubMed.