18 May 2005. ApoE figures large in Alzheimer disease as the only known genetic risk factor for sporadic AD and an important regulator of amyloid-β (Aβ) metabolism and deposition (see ARF related news story). But for all the attention focused on this little protein, little is known about what normally controls the levels of ApoE in brain. In a paper published May 11 in the Journal of Biological Chemistry online, David Holtzman of Washington University in St. Louis and his colleagues show that the low-density lipoprotein receptor (LDLR) is the major receptor responsible for the for the uptake and degradation of ApoE-containing lipoproteins in murine brain. The researchers report that LDLR knockout mice had modestly increased extracellular ApoE in their brains and CSF, but the knockout did not affect amyloid plaque deposition when the mice were crossed with APP-transgenic mice. Using human ApoE knock-in mice, the researchers showed that LDLR also regulated CNS levels of the ApoE3 and 4 isoforms, but the possible impact of this increase on Aβ levels was not investigated.
To pinpoint which of several lipoprotein receptors are responsible for trafficking ApoE in the brain, first author John Fryer and his colleagues tested embryonic fibroblast cells from knockout mice for their ability to take up astrocyte-secreted ApoE-containing lipoprotein particles. Cells from LDLR knockout mice were unable to take up the lipoproteins, while fibroblasts from the low density lipoprotein receptor-associated receptor (LRP) knockout or wild-type mice took up the complexes avidly. CHO cells overexpressing LDLR, but none of four other candidate receptors, also took up and degraded astrocyte-derived ApoE, indicating that LDLR is the major, and perhaps only, receptor involved in clearing the types of ApoE-containing particles found in the brain.
Consistent with a lack of uptake of ApoE, the LDLR knockout mice had 50 percent more ApoE protein in the CNS and extracellular compartments in the brain. To see if this increase would be associated with higher Aβ, the researchers crossed the LDLR knockout mice with PDAPP mice. They saw no significant increases in either soluble or insoluble Aβ40 or Aβ42 in 3-month-old mice. In older animals (10 months), increases in plaque load and Aβ42 did not reach statistical significance. The mice had an increase in plasma cholesterol, but no changes in brain or CSF cholesterol, or APP levels or processing.
These results contrast with another knockout model where low expression of a different ApoE receptor, LRP, resulted in increased amyloid deposition in mice transgenic for human Aβ precursor protein (see ARF related news story). Fryer et al. speculate that perhaps the modest increase in ApoE seen in the LDLR knockouts is not sufficient to affect Aβ levels at the young ages or the early stages of deposition that were examined in this study. Or, LRP could be important for other, ApoE-dependent pathways that regulate Aβ metabolism or clearance.
The paper ends with a teaser. The authors present experiments that suggest the regulation of human ApoE protein levels differs significantly from that seen in mice. When they crossed mice carrying knocked-in human ApoE alleles with LDLR knockouts, offspring had two- to threefold higher levels of human ApoE3 and 4 in the CSF. It seems reasonable to ask whether this increase is of sufficient magnitude to enhance plaque deposition; the answer to that will require more experiments. However, to further muddy the waters, the authors report elsewhere (Fryer et al., 2005) that mating the human ApoE-expressing mice with APPsw mice yields animals with delayed Aβ deposition (see ARF related news story). Apparently, sorting out the role of the LDLR in regulating ApoE levels and in amyloid processing is going to take a few more crosses.—Pat McCaffrey.
Fryer JD, Demattos RB, McCormick LM, O'dell MA, Spinner ML, Bales KR, Paul SM, Sullivan PM, Parsadanian M, Bu G, Holtzman DM. The low-density lipoprotein receptor regulates the level of CNS human and murine apolipoprotein E but does not modify amyloid plaque pathology in PDAPP mice. J Biol Chem. 2005 May 11; [Epub ahead of print]