Differential Signaling Mediated by ApoE2, ApoE3, and ApoE4 in Human Neurons Parallels Alzheimer’s Disease Risk

Abstract

Apolipoprotein E (ApoE) mediates clearance of circulating lipoproteins from blood by binding to ApoE receptors. Humans express three ApoE genetic variants, ApoE2, ApoE3, and ApoE4, that exhibit distinct ApoE receptor binding properties. However, ApoE is also abundantly produced in brain by activated astrocytes and microglia. Here, the three variants differentially affect Alzheimer's disease (AD), such that ApoE2 protects against, and ApoE4 predisposes to AD. A role for ApoE4 in driving microglial dysregulation in AD and impeding Ab clearance is well documented, but the possible direct effects of ApoE4 on neurons are poorly understood.Extending previous studies, we here demonstrate that ApoE variants differentially activate multiple neuronal signaling pathways by binding to ApoE receptors on human neurons.Specifically, using human neurons cultured in the absence of glia to exclude indirect glial mechanisms, we show that ApoE broadly stimulates multiple signal transduction cascades. These cascades among others enhance synapse formation with an ApoE4>ApoE3>ApoE2 potency rank order, paralleling the relative risk for AD conferred by these three variants. ApoE-induced synaptogenesis required MAP-kinase activation similar to induction of APP transcription, but involved CREB activation instead of cFos activation. We thus propose that in brain, ApoE acts as a paracrine signal that is secreted by astrocytes and microglia in response to neuroinflammation, and broadly activates neuronal signaling pathways in what may represent a protective response, with the differential potency of ApoE variants causing distinct levels of chronic signaling that may contribute to AD pathogenesis. 239 words 3

Machine learning methods applied to genotyping data capture interactions between single nucleotide variants in late onset Alzheimer’s disease

“…The 6 SNVs were located in a chromosome19 hot-spot region comprising TOMM40, APOE and APOC1 genes. rs429358 is the most well-characterized LOAD genetic determinant 27 10 , rs7412 is known to be protective against AD 28 , and the two SNVs define the distinct apolipoprotein E (ApoE) isoforms 29 . rs4420638 is in strong LD with rs429358 9 and for this reason, its link with AD is attributed to rs429358.…”

“…In this context, the selection of genetic determinants for the follow-up in laboratory and clinical studies remains a challenge, and most of the mechanisms in which the discovered predisposing and protective genetic alterations contribute to AD, are still unknown 7 . In the case of LOAD, heritability is estimated to be around 56-79% 8 , and APOE polymorphic alleles are the major genetic determinants of susceptibility discovered until now 9 10 . Nevertheless, there are more candidate genes such as TOMM40, PVRL2, ABCA7, ADAM10, BIN1, CLU and CR1 among others 5 11 , and nowadays, there is a growing consensus considering LOAD a polygenic risk disease 12 .…”

A simple Ca2+-imaging approach to neural network analysis in cultured neurons

“…We used a co-culture system of induced human neurons (iN cells) with mouse glia, which provides a supporting layer and factors required for neuronal survival, dendritic arborization, and synapse formation 15–17 ( Fig. 1A ).…”

Regulation of autophagy, lipid metabolism, and neurodegenerative pathology by heparan sulfate proteoglycans

“…It is also important to note the signaling functions of ApoE, including activation of Erk1/2. The disease associated variant, ApoE4 shows the greatest capacity to trigger phosphorylation and activation of Erk (Huang et al, 2019) and it is possible that a co-receptor function of HSPGs with ApoE is a determinant of their functional interplay in AD.…”