3 October 2007. Both amyloid-β precursor protein (APP) and apolipoprotein E (ApoE) are key players in the pathophysiology of Alzheimer disease (AD). But are the two directly connected? In tomorrow’s Neuron, researchers led by Guojun Bu at Washington University School of Medicine, St. Louis, report that APP plays a key role in suppressing lipoprotein receptor 1 (LRP1) expression. LRP1 is one of two major brain lipoprotein receptors; it contributes to ApoE uptake and intracellular degradation. “The finding extends the function of APP to brain lipoprotein metabolism and it also links the two major genetic determinants for early- and late-onset AD,” said Bu in an interview with ARF.
There are already numerous lines of evidence linking Aβ with ApoE and cholesterol (see ARF related news story). “The interesting thing about this new APP/ApoE connection is that it is completely independent of Aβ,” said Bu. His team discovered that it is the APP intracellular domain, or AICD, that suppresses LRP1 expression. As such, the research not only links APP and ApoE, but also proposes an answer to the contested question of what physiological role, if any, AICD fulfills.
Bu and colleagues made the AICD/LRP1 connection while trying to determine if APP processing is connected with ApoE/cholesterol metabolic pathways. The scientists found that in APP knockout and APP/APLP2 double knockout mice, brain lysates contained less ApoE and more cholesterol than in wild-type. The same was true in mouse embryonic fibroblasts derived from the same knockout strains, suggesting that APP and/or its homolog APLP2 play a role in lipid and lipoprotein regulation.
The researchers wondered if these changes might be due to faster intracellular degradation of ApoE. Since LRP1 and the low-density lipoprotein receptor (LDLR) are both involved in ApoE uptake into cells, the researchers next looked at whether knocking out APP or APLP2 altered lipoprotein receptor levels, as well. They found that while LDLR expression was unchanged, LRP1 levels were significantly higher in APP or APP/APLP2 double knockout (KO) mouse embryonic fibroblasts. There also seemed to be a dose response; deleting APP increased LRP1 about eightfold, deleting both genes about 16-fold.
How might APP suppress LRP1 expression? Given the various links between Aβ and ApoE/cholesterol metabolism, the amyloid peptide might seem a likely nexus, yet the researchers found that LRP1 levels stayed stable in BACE1 knockouts. This made the Aβ connection less likely, because BACE1 is the major catalyst for APP β-cleavage. Moreover, the researchers found that mouse embryonic fibroblast cells expressing APP with the Swedish mutation also had normal LRP1 levels despite increased Aβ production.
With Aβ out of contention, the researchers next looked to see if LRP1 suppression required γ-secretase activity. Indeed, presenilin 1/presenilin 2 double KO mouse embryonic fibroblasts had a ninefold increase in LRP1. This increase appears to be directly related to loss of γ-secretase activity, since it also showed up in nicastrin knockouts and in cells treated with a γ-secretase inhibitor.
AICD is one product of γ-secretase activity, and there is some evidence that the intracellular domain might regulate transcription upon binding to other factors such as Fe65 and Tip60 (see ARF related news story and ARF news story). To test if AICD might regulate LRP1 expression, the researchers transiently expressed AICD in U87 cells, where it reduced LRP1 by about 30 percent but did not affect LDLR. Fe65 alone also slightly reduced LRP1, and AICD and Fe65 expression combined cut LRP1 levels in half.
Together, this and further experiments suggest that APP processing, particularly production of AICD, influences lipoprotein and cholesterol metabolism. “I would not go as far as to say that AICD is a transcription factor, because our experiments were all cellular with many components present. But what this work does show is that the presence or absence of AICD makes a big difference to lipid metabolism,” said Bu.
How this relates to AD pathology is not clear, but links between APP and lipid metabolism seem to be growing by leaps and bounds. Researchers led by Tobias Hartmann, now at the University of the Saarland, Homburg, Germany, have also reported that presenilin knockout leads to increased cholesterol in mouse embryonic fibroblasts, though they attributed this finding to loss of Aβ’s proposed inhibitory effect on cholesterol synthesis (see ARF related news story). “It is possible that Aβ affects cholesterol synthesis while AICD affects its metabolism,” said Bu. That suggests that both the head and tail of APP are involved in cholesterol metabolism.
Whether AICD’s suppression of LRP might be a cause or effect of AD pathology is uncertain, as well. Given that mature neurons cannot make their own cholesterol—a necessary component of synapses—and typically import it via LRP1, the authors speculate that loss of LRP1 may contribute to synaptic dysfunction. There is some independent evidence that LRP1 is reduced in AD (see Kang et al., 2000). Bu hopes that his latest work will generate interest in APP beyond Aβ and spur research into the role of synaptic cholesterol in the aging brain and AD.
On the broader lipid research front, European researchers recently concluded that the ApoE4 genotype, besides being the strongest genetic risk factor for late-onset AD, also puts people at risk for coronary heart disease, albeit less strongly. In the September 19 Lancet, John Danesh, University of Cambridge, England, and colleagues describe their meta-analysis of 82 lipid level studies and 121 coronary outcome studies. It found that people carrying two ApoE4 alleles face a slightly higher risk of coronary disease (odds ratio 1.06), while E2/E2 carriers have about a 20 percent reduced risk compared to the E3/E3 genotype. Previous reviews of ApoE and coronary disease are susceptible to bias because they were dominated by small studies. To overcome this weakness, first author Anna Bennet and colleagues focused on lipid studies that had at least 1,000 participants and coronary studies that had at least 500 participants. The analysis further predicts a linear relationship between ApoE genotype and serum low density lipoprotein cholesterol (LDL-C) —the bad kind. People with E2/E2 genotype had about 30 percent less serum LDL-C than those homozygous for ApoE4. The difference is about as much as statin medication achieves, write the authors.—Tom Fagan.
Liu Q, Zerbinatti CV, Zhang J, Hoe H-S, Wang B, Cole SL, Herz J, Muglia L, Bu G. Amyloid precursor protein regulates brain apolipoprotein E and cholesterol metabolism through lipoprotein receptor LRP1. Neuron. 2007 Oct 4;56:1-13. Abstract
Bennet AM, Di Angelantonio E, Ye Z, Wensley F, Dahlin A, Ahlbom A, Keavney B, Collins R, Wiman B, de Faire U, Danesh J. Association of apolipoprotein E genotypes with lipid levels and coronary risk. JAMA. 2007 Sep 19;298:1300-1311. Abstract