Associations between serum cholesterol levels and cerebral amyloidosis

JAMA Neurol. 2014

CONCLUSIONS AND RELEVANCE: Elevated cerebral AB level was associated with cholesterol fractions in a pattern analogous to that found in coronary artery disease. This finding, in living humans, is consistent with prior autopsy reports, epidemiologic findings, and animal and in vitro work, suggesting an important role for cholesterol in AB processing. Because cholesterol levels are modifiable, understanding their link to AB deposition could potentially and eventually have an effect on retarding the pathologic cascade of AD. These findings suggest that understanding the mechanisms through which serum lipids modulate AB could offer new approaches to slowing AB deposition and thus to reducing the incidence of AD.

http://www.ncbi.nlm.nih.gov/pubmed/24378418

I would like to get your thoughts and comments on the role of cholesterol in Alzheimer's disease. The members of this message board always have great information and advice to share, and I really appriciate that.

Over the last few months I have found an abundance of research linking cholesterol, especially LDL, to Alzheimer's. The evidence reveals not only that higher LDL levels are associated with Alzheimer's, but the research points to specific mechanisms within the cell that tie cholesterol to the disease. I will be happy to post more. Does anyone else have information, personal thoughts or hypotheses to share on this?

The cholesterol/LDL problem is most likely linked to the activation of g protein-coupled receptors and this is probably especially true for people with the Apoe4 gene because Apoe4 binds to LDL and increases the activation of LDL receptors:

Functional interaction between APOE4 and LDL receptor isoforms in Alzheimer's disease.

CONCLUSION:

These results imply a functional interaction between ApoE and LDL receptor proteins that determines risk for Alzheimer's disease.

We present evidence of a link between low-density lipoprotein (LDL) receptor binding and activation of a platelet G-coupled protein. LDL stimulation induced cytosolic [Ca2+]i mobilization, increase in inositol 1,4,5-triphosphate (IP3) formation and a rapid cytosol-to-membrane translocation of protein kinase C (PKC) enzymatic activity.

The release of intracellular calcium is what leads to the formation of amyloid oligomers. The activation of protein kinase C is what leads to peroxynitrite and the formation of the amyloid precursor protein (via caspase-3) and to amyloid plaques (via nitration).  Amyloid oligomers also activate g protein-coupled receptors, but amyloid plaques apparently do not. In light of this, the following conclusion is quite important.

Malinow’s team found that when mice are missing the PKC alpha gene, neurons functioned normally, even when amyloid beta was present. Then, when they restored PKC alpha, amyloid beta once again impaired neuronal function. In other words, amyloid beta doesn’t inhibit brain function unless PKC alpha is active.

If a person has high levels of LDL with the Apoe4 gene and protein kinase C is inhibited early on, that person should not develop Alzheimer's disease or if they have Alzheimer's disease and use an effective peroxynitrite scavenger they should be able to treat the disease.  The same is likely also true for almost every other trigger for Alzheimer's disease.

The article you posted doesn't say that ApoE4 increases LDL receptor (LDLr) activation, only that there is an association between ApoE4 and variations in the genotype for the LDLr. Do you have any studies showing increased LDLr activity involved with AD?

Here is the only article I can find. It is done with mice in 2004, and basically says that increased LDLr activity traps ApoE4 leaving a deficiency of ApoE4 to handle cholesterol elsewhere.

Harmful Effects of Increased LDLR Expression in Mice With Human APOE*4 But Not APOE*3

 The adverse effects of increased LDLR suggest a possibility that the receptor can trap apoE4, reducing its availability for the transfer to nascent lipoproteins needed for their rapid clearance, thereby increasing the production of apoE-poor remnants that are slowly cleared. The lower affinity for the LDLR of apoE3 compared with apoE4 could then explain why increased receptor expression had no adverse effects with apoE3.

Conclusions— Our results emphasize the occurrence of important and unexpected interactions between APOE genotype, LDLR expression, and diet.

Good observation.  Yes, the evidence is that ApoE4 is the ApoE isoform that most strongly binds to low density lipid receptors, not that it necessarily activates these receptors. However, directly or indirectly the interaction between ApoE4 and low density lipid receptors appears to trigger the pathway (g protein activation, phospholipase C, protein kinase C, NMDA receptor activation, peroxynitrite, caspase-3) that leads to the death of neurons in Alzheimer's disease.

Neuronal Apoptosis by Apolipoprotein E4 through Low-Density Lipoprotein Receptor-Related Protein and Heterotrimeric GTPases

The e4 genotype of apolipoprotein E (apoE4) is the most established predisposing factor in Alzheimer’s disease (AD); however, it remains unclear how apoE4 contributes to the pathophysiology. Here, we report that the apoE4 protein (ApoE4) evokes apoptosis in neuronal cells through the low-density lipoprotein receptor-related protein (LRP) and heterotrimeric GTPases. We examined neuron/neuroblastoma hybrid F11 cells and found that these cells were killed by 30 mg/ml ApoE4, but not by 30 mg/ml ApoE3.

Neurobiol Dis. 2014 Apr;64:150-62. doi: 10.1016/j.nbd.2013.12.016. Epub 2014 Jan 9.

Apolipoprotein E-low density lipoprotein receptor interaction affects spatial memory retention and brain ApoE levels in an isoform-dependent manner.

Our results show that plasma and brain apoE levels, cortical cholesterol, and spatial memory are all regulated by isoform-dependent interactions between apoE and LDLR. Conversely, both anxiety-like behavior and cued associative memory are strongly influenced by APOE genotype, but these processes appear to occur via an LDLR-independent mechanism. Both the lack of LDLR and the interaction between E4 and the LDLR were associated with significant impairments in the retention of long term spatial memory. Finally, levels of hippocampal apoE correlate with long term spatial memory retention in mice with human LDLR. In summary, we demonstrate that the apoE-LDLR interaction affects regional brain apoE levels, brain cholesterol, and cognitive function in an apoE isoform-dependent manner.

Lane, your above article was very enlightening:

Apolipoprotein E-low density lipoprotein receptor interaction affects spatial memory retention and brain ApoE levels in an isoform-dependent manner.

http://www.ncbi.nlm.nih.gov/pubmed/24412220#

“The low-density lipoprotein receptor (LDLR) has a high affinity for apoE, and is the only member of its receptor family to demonstrate an apoE isoform specific binding affinity (E4>E3>>E2).” “Both the lack of LDLR and the interaction between E4 and the LDLR were associated with significant impairments in the retention of long term spatial memory.”

They are indicating that LDLR doesn’t work when bound to ApoE4 LDL cholesterol saying it’s the same as not having LDL receptors at all. And, that LDLR prefers ApoE4 to its other ligands (activators) ApoE3 and ApoB. Can we assume that LDLR is malfunctioning when bound to ApoE4? Is this why reducing LDL cholesterol is beneficial?

As for LDL receptor-related protein (LRP) which is the subject of your other article in the previous post, I have lots of evidence for the benefits it provides. It seems to be necessary for many essential functions.

When you block the association between ApoE4 and low density lipid receptors, ApoE4 and the receptors don't appear to cause the death of neurons.  It appears that it is this interaction which triggers a g protein-coupled receptor that leads to the death of neurons. The buildup of cholesterol due to the dysfunction of the low density lipid receptor (caused by ApoE4 binding?) may accentuate this by further increasing phospholipase C activity (I just found this very interesting article).

Increasing Membrane Cholesterol Level Increases the Amyloidogenic Peptide by Enhancing the Expression of Phospholipase C

http://www.hindawi.com/journals/jnd/2013/407903/

The ApoE4 gene also appears to increase the risk of neuropsychiatric problems in Alzheimer's disease, perhaps also because of the relationship between ApoE4 and phospholipase C.

http://www.hindawi.com/journals/ijad/2011/721457/

I will try to pick up on this in a week or so.

Thanks, that’s a good article about membrane cholesterol. I too have found abundant evidence that membrane-associated cholesterol and lipid rafts in the membrane affect Amyloid production in cells. But how? What I’d like to discover is which mechanisms are involved and how cholesterol plays a part in it. The article above points to an interesting clue in PIP2 (phosphatidylinositol bisphosphate). Maybe this molecule (and not cholesterol) in the cell membrane, is the key factor. Check this out:

Phosphatidylinositol 4,5-Bisphosphate Functions as a Second Messenger that Regulates Cytoskeleton–Plasma Membrane Adhesion

“Binding interactions between the plasma membrane and the cytoskeleton define cell functions such as cell shape, formation of cell processes, cell movement, and endocytosis. Our study suggests that plasma membrane PIP2 controls dynamic membrane functions and cell shape by locally increasing and decreasing the adhesion between the actin-based cortical cytoskeleton and the plasma membrane.” “…Particularly, cytoskeletal–membrane interactions drive the formation and retraction of filopodia, lamellipodia, neurites, and other membrane processes in response to chemoattractants and other stimuli. Cytoskeletal–membrane interactions are also thought to regulate the rates of exocytosis and endocytosis and to control signaling processes.”

http://www.sciencedirect.com/science/article/pii/S0092867400815603

Remember, tau is involved with the “actin-based cytoskeleton” mentioned above. And, PIP2 is affected by the activation of a G-coupled protein. Lane’s above article post said: “We present evidence of a link between low-density lipoprotein (LDL) receptor binding and activation of a platelet G-coupled protein.” These alterations in the cell membrane of neurons are very interesting since they could connect ApoE and cholesterol to the processing of APP in the cell membrane.

From Lane’s posted article: “We present evidence of a link between low-density lipoprotein (LDL) receptor binding and activation of a platelet G-coupled protein. LDL stimulation induced cytosolic [Ca2+]i mobilization, increase in inositol 1,4,5-triphosphate (IP3) formation and a rapid cytosol-to-membrane translocation of protein kinase C (PKC) enzymatic activity.”

So my interpretation goes something like this; LDL cholesterol attaches to the LDL receptor, activates G-coupled protein which leads to modification of PIP2 which affects many processes in the cell membrane including APP processing and the cytoskeleton, and increases PKC and release of Ca+…

Low-density lipoprotein (LDL) binds to a G-protein coupled receptor in human platelets. Evidence that the proaggregatory effect induced by LDL is modulated by down-regulation of binding sites and desensitization of its mediated signaling. (2001)

“We present evidence of a link between low-density lipoprotein (LDL) receptor binding and activation of a platelet G-coupled protein. LDL stimulation induced cytosolic [Ca2+]i mobilization, increase in inositol 1,4,5-triphosphate (IP3) formation and a rapid cytosol-to-membrane translocation of protein kinase C (PKC) enzymatic activity.”

http://www.ncbi.nlm.nih.gov/pubmed/11223431

This LDL receptor is getting very interesting. If this is true that LDL cholesterol attachment to the LDL receptor on the cell surface also activates G-protein coupled receptors, then here we have a mechanism which can affect many cell processes including Amyloid beta production not previously connected to the LDL receptor in a very specific way. So, LDL cholesterol may do two things at the neuron cell membrane, it gets absorbed by the cell (endocytosis), which is well known, but also it activates a G-coupled protein. This second effect brings into play molecules like PIP2, PLC, DAG, IP3, Ca2+ and PKC (as Lane has mentioned before). 

I'm still learning the biology. So, I've discovered that the LDL receptor is not a G-protein coupled receptor, but belongs to a different family of receptors. The "G-protein" must be connected to the "G-protein coupled receptor" which is a transmembrane receptor and different from the LDL receptor. So the missing piece is how the LDL receptor may be connected to or activate the G-protein coupled receptor which then activates the G-protein. There is not a lot of info on the LDL receptor's connection to all the G-protein initiated events mentioned above. But, I am thinking that this possible connection between LDL cholesterol and the activation of the G-protein (specifically G-protein q) may be critical to understanding the pathology of Alzheimer's.

If anyone is interested there are some good youtube videos that really helped me. Here's a simple one:

https://www.youtube.com/watch?v=V_0EcUr_txk

 Of course G-protein activates phospholipase C (PLC). As it turns out Lane had a lot to say about PLC way back in 2012. I just found the post, “A Pathway to Alzheimer’s Disease” Lots of good info, thanks Lane!

Presenilin mutations linked to familial Alzheimer's disease cause an imbalance in phosphatidylinositol 4,5-bisphosphate (PIP2) metabolism (National Academy of Sciences, 2006)

“Phosphatidylinositol 4,5-bisphosphate (PIP2) is an important cellular effector whose functions include the regulation of ion channels and membrane trafficking. Aberrant PIP2 metabolism has also been implicated in a variety of human disease states, e.g., cancer and diabetes. Here we report that familial Alzheimer's disease (FAD)-associated presenilin mutations cause an imbalance in PIP2 metabolism. Our data suggest that PIP2 imbalance may contribute to Alzheimer's disease pathogenesis by affecting multiple cellular pathways, such as the generation of toxic AB42 as well as the activity of the MIC/TRPM7 channel, which has been linked to other neurodegenerative conditions. Thus, our study suggests that brain-specific modulation of PIP2 may offer a therapeutic approach in Alzheimer's disease.”

 http://www.pnas.org/content/103/51/19524.short

A simple way to think of it might be that PIP2 (a phospholipid) is an abundant component in the cell membrane (a phospholipid bilayer) and when too much of it gets broken down, bad things happen. I like simple explanations.

Much of this fits together.  I have been trying to figure out how ApoE4 increases the risk for Alzheimer's disease for a long time.  Recently, the question has been how does ApoE4 binding to the low density lipid receptor increase g protein-coupled receptor activity?  This may be part of the answer.  ApoE4 binding to the low density lipid receptor inhibits the canonical Wnt pathway and likely increases a non-canonical Wnt pathway--the former is neuroprotective and the latter via phospholipase C is likely neurodegenerative.

http://onlinelibrary.wiley.com/doi/10.1002/iub.559/pdf (figure on page 916).

I had also troubles posting articles and I found out that it's a problem with unicode.

Currently message boards don't handle for example Greek letters "alpha", "beta" (often used to describe amyloid). I had to replace those letters.

Yes, the Greek symbol must be replaced for it to post. Looking forward to your article Lane.

In the diagram below you can see that it is beneficial to the cell, except in two circumstances: 1) when is inhibited by Dkk, 2) when it is activated independently of the LRP receptor. Very interesting…

https://resources.rndsystems.com/images/site/bb_summer07_17wide_2360.png

A little more on this Dkk molecule:

Increased Dickkopf-1 expression in transgenic mouse models of neurodegenerative disease (2010)

“To investigate the role of the Wnt inhibitor Dickkopf-1 (DKK-1) in the pathophysiology of neurodegenerative diseases, we analysed DKK-1 expression and localization in transgenic mouse models expressing familial Alzheimer’s disease mutations and a frontotemporal dementia mutation. A significant increase of DKK-1 expression was found in the diseased brain areas of all transgenic lines, where it co-localized with hyperphosphorylated tau-bearing neurons. In TgCRND8 mice, DKK-1 immunoreactivity was detected in neurons surrounding amyloid deposits and within the choline acetyltransferase-positive neurons of the basal forebrain. Active glycogen synthase kinase-3 (GSK-3) was found to co-localize with DKK-1 and phospho-tau staining. Downstream to GSK-3, a significant reduction in B-catenin translocation to the nucleus, indicative of impaired Wnt signaling functions, was found as well. Cumulatively, our findings indicate that DKK-1 expression is associated with events that lead to neuronal death in neurodegenerative diseases and support a role for DKK-1 as a key mediator of neurodegeneration with therapeutic potential.”

http://onlinelibrary.wiley.com/doi/10.1111/j.1471-4159.2009.06566.x/full

I need to remember about the symbols preventing the post.  Thank you for this and for all the valuable information, Serenoa.

I just came across DKK-1 a few days ago.  Here are some more clues.

Dkk1 reduced the distribution of LRP6 in the lipid raft fraction where caveolin is associated. These results indicate that Wnt3a and Dkk1 shunt LRP6 to distinct internalization pathways in order to activate and inhibit the beta-catenin signaling, respectively.

Induction of Dickkopf-1, a Negative Modulator of the Wnt Pathway, Is Associated with Neuronal Degeneration in Alzheimer's Brain

 It is particularly interesting that the DKK1 receptor LRP5 is also one of the putative receptors for apolipoprotein E (ApoE), and that the genotype 4/4 is an established risk factor for late onset AD (Saunders et al., 2000; Rocchi et al., 2003). Whether ApoE4 binds with high affinity to LRP5 and mimics the action of DKK1 on the Wnt pathway is worthy of investigation. Finally, our findings encourage the search for DKK1 antagonist molecules to be tested as selective neuroprotective agents in experimental models of AD. 

My guess is that DKK1 by reducing LRP6 distribution and Apoe4  by binding to LRP5 inhibit canonical Wnt signalling and increases non-canonical Wnt signalling via frizzled receptors. Another guess is that DKK1 and Apoe4 increase lipid rafts and phospholipase C expression.  All of this would increase the risk for Alzheimer's disease.

Amyloid oligomers also activate frizzled receptors, which partially explains their neurotoxicity.

In conclusion, we present the first biochemical evidence of binding of Abeta members of the Fz family, suggesting that the Abeta40-binding site in Fz co-localizes or is in close proximity to the Wnt-binding site. 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2431018/

But there are many other g protein-coupled receptors and other types of receptors that lead to the production of peroxynitrite through phospholipase C, protein kinase C, and NMDA receptors.  This is why removing amyloid oligomers usually only leads to a slight improvement early in Alzheimer's disease.

 This explains the Wnt pathways very well, especially the diagram. Now I see how the noncanonical Wnt/Ca+ pathway leads to the breakdown of PIP2 and the production of PKC just like with the LDL receptor (LDLr). Then I see that the canonical Wnt pathway inhibits PKC, preserves beta-catenin (which is good), and is associated with a different LDL receptor, LRP (which is also good).

I think Lane’s quote from above nails it:

 “I have been trying to figure out how ApoE4 increases the risk for Alzheimer's disease for a long time.  Recently, the question has been how does ApoE4 binding to the low density lipid receptor increase g protein-coupled receptor activity?  This may be part of the answer.  ApoE4 binding to the low density lipid receptor inhibits the canonical Wnt pathway and likely increases a non-canonical Wnt pathway--the former is neuroprotective and the latter via phospholipase C is likely neurodegenerative.”  

So I am interpreting that ApoE4 is affecting these two receptors, LDLr and LRP, in different ways. We know from above that LDLr has an affinity for ApoE4 over ApoE3/2 or ApoB, which could lead to over activation of the noncanonical Wnt pathway. But, is there evidence that ApoE4 is not activating, or is inhibiting, the LRP receptors (LRP1, LRP5, LRP6) which would prevent the activation of the canonical Wnt pathway?

Most certainly, ApoE4 inhibits the canonical (and beneficial) Wnt pathway, but exactly how is dogging me.  This may be part of the explanation.

Low-density lipoprotein receptor-related protein-5 binds to Axin and regulates the canonical Wnt signaling pathway.

The trimeric G protein Go inflicts a double impact on axin in the Wnt/frizzled signaling pathway.

Additionally, we show that the betagamma-component of Go can directly bind and recruit Dishevelled from cytoplasm to the plasma membrane, where activated Dishevelled can act on the DIX domain of Axin. Thus, the two components of the trimeric Go protein mediate a double-direct and indirect-impact on different regions of Axin, which likely serves to ensure a robust inhibition of this protein and transduction of the Wnt signal.

Frizzled Proteins are bona fide G Protein-Coupled Receptors

Frizzled-related protein (a Wnt antagonist) prevents Go activation, as does pretreatment of Go with pertussis toxin. These experiments provide a biochemical proof of the GPCR activities of Frizzled receptors and establish an in vitro assay to monitor Frizzled activation by Wnt ligands, applicable for the high-throughput agonist/antagonist screening.

ApoE4 may inhibit canonical Wnt signalling by preferentially activating g proteins via frizzled receptors.  And perhaps they do so by interfering with the interaction between Wnt and low density lipid receptor related proteins.

Here are two more important clues.

Interactions between the APP C-terminal domain and G-proteins mediate calcium dysregulation and Abeta toxicity in Alzheimer disease

The results presented here support a role for APP in Abeta-induced G-protein activation and suggest a mechanism by which basal APP binding to Go is reduced under pathological loads of Abeta, liberating Go and activating the G-protein system which may in turn result in downstream effects including calcium dysregulation. These results also suggest that specific antagonists of G-protein activity may have a therapeutic relevance in AD.

C-terminal fragment of amyloid precursor protein induces astrocytosis.

One of the pathophysiological features of Alzheimer's disease is astrocytosis around senile plaques. Reactive astrocytes may produce proinflammatory mediators, nitric oxide, and subsequent reactive oxygen intermediates such as peroxynitrites. In the present study, we investigated the possible role of the C-terminal fragment of amyloid precursor protein (CT-APP), which is another constituent of amyloid senile plaque and an abnormal product of APP metabolism, as an inducer of astrocytosis. We report that 100 nM recombinant C-terminal 105 amino acid fragment (CT105) of APP induced astrocytosis morphologically and immunologically. CT105 exposure resulted in activation of mitogen-activated protein kinase (MAPK) pathways as well as transcription factor NF-kappaB. Pretreatment with PD098059 and/or SB203580 decreased nitric oxide (NO) production and nuclear factor-kappa b (NF-kappaB) activation. But inhibitors of NF-kappaB activation did not affect MAPKs activation whereas they abolished NO production and attenuated astrocytosis. Furthermore, conditioned media derived from CT105-treated astrocytes enhanced neurotoxicity and pretreatment with NO and peroxynitrite scavengers attenuated its toxicity. These suggest that CT-APP may participate in Alzheimer's pathogenesis through MAPKs- and NF-kappaB-dependent astrocytosis and iNOS induction.

I am slowly making some connections that I have missed in the past.  This one concerns the Go protein.

These results suggest that the extracellular signals that induce the dissociation of G(o) or Gi, the heterotrimeric G proteins abundant in brain, should enhance the hydrolysis of PtdIns 4,5-P2 in brain primarily through activation of PLC-beta 3 (PLC-beta 2 is not detectable in brain).

http://www.jbc.org/content/268/7/4573.short

We find that the alpha subunit of Go is strongly expressed in the presynaptic cell, and that under- or overactivation of this G protein leads to neurotransmission and behavioral defects...

Upon dissociation of the heterotrimeric Go protein by activated GPCRs such as Fz2, the liberated G-alpha-o subunit can signal to its downstream targets both in the GTP- and GDP-bound state.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4199127/

Via activation of the g protein-coupled receptor (the Wnt frizzled receptor), ApoE4, the c-terminal fragment of the amyloid precursor protein (caused by beta secretase), and amyloid oligomers increase the risk of Alzheimer's disease via the increased formation of peroxynitrite and caspase-3 (the so-called death enzyme that also increases beta secretase activity).

So much good info. Currently experiencing overload. Must digest...

I will say that I have made much progress in understanding all these complex concepts and potential pathologies. I have been reviewing these posts and looking at related studies but don't have much to add yet. I'm feeling the need to find more definitive potential corrective measures related to these above mentioned concepts.

However, one thought did occur to me. Maybe dysfunction is a normal part of cell biology and it is only when the natural corrective mechanism is lacking or inhibited that disease occurs. Instead of looking for a pathological dysfunction causing disease, we should look for the lack of a natural corrective mechanism as the cause.

Ok, so we agree that LRP activation is good (no G protein involved, but activates canonical Wnt), and LDLr overactivation is bad (G protein is overactivated, as well as non-cannonical Wnt). And, the overactivation of both G protein o, and G protein i is bad?. Is that what you are finding Lane? 

Apolipoprotein E-Containing Lipoproteins Protect Neurons from Apoptosis via a Signaling Pathway Involving Low-Density Lipoprotein Receptor-Related Protein-1

“Apolipoprotein E (apoE)-containing lipoproteins (LPs) are secreted by glia and play important roles in lipid homeostasis in the CNS… Here, we provide evidence that glia-derived LPs protect CNS neurons from apoptosis by a receptor-mediated signaling pathway. The protective effect was greater for apolipoprotein E3 than for apolipoprotein E4, the expression of which is a risk factor for Alzheimer's disease. The anti-apoptotic effect of LPs required the association of apolipoprotein E with lipids but did not require cholesterol. Apoptosis was not prevented by lipids alone or by apoA1- or apoJ-containing lipoproteins. The prevention of neuronal apoptosis was initiated after the binding of LPs to the low-density lipoprotein receptor-related protein (LRP), a multifunctional receptor of the low-density lipoprotein receptor family. We showed that inhibition of LRP activation.…reduced the protective effect of LPs. Furthermore, another LRP ligand, alpha2-macroglobulin, also protected the neurons from apoptosis. After binding to LRP, LPs initiate a signaling pathway that involves activation of protein kinase C…and inactivation of glycogen synthase kinase-3beta. These findings indicate the potential for using glial lipoproteins or an activator of the LRP signaling pathway for treatment for neurodegenerative disorders such as Alzheimer's disease.”
 

http://www.jneurosci.org/content/27/8/1933.short  

I think this is all correct, Serenoa. 

The Apoe4 inhibition of the canonical Wnt signalling pathway via inhibition of LRPs may be one of the reasons why it is a risk factor for Alzheimer's disease.

If demonstrated in neurons, inhibition of the canonical Wnt signaling pathway by ApoE4 may be relevant for the pathophysiology of AD. The presence of the e4 allele encoding for ApoE4 is a major risk factor for sporadic AD, and reduces the age of onset for familial AD (Corder et al. 1998 ). The underlying mechanisms are still debated. ApoE4 may influence the extracellular deposition and/or clearance of b-amyloid peptide (Strittmatter et al. 1993; LaDu et al. 1994; Ma et al. 1994; Wisniewski et al. 1994; Bales et al. 1999; Holtzman et al. 2000; Irizarry et al. 2000) or may be toxic to neurons by promoting oxidative stress (Miyata and Smith 1996), alterations of cytoskeleton dynamics (Nathan et al. 1994, 1995), tau hyperphosphorylation, and formation of neurofibrillary tangles (Strittmatter et al. 1994;. Tesseur et al. 2000; Huang et al. 2001; Ljungberg et al. 2002). Inhibition of Wnt signaling by ApoE4 might contribute to the pathological cascade leading to neuronal degeneration in AD because an impairment of Wnt signaling has been associated with this disorder (De Ferrari and Inestrosa 2000; Inestrosa et al. 2002). Expression of Dkk-1 is causally related to tau protein hyperphosphorylation in cultured neurons challenged with b-amyloid, and Dkk-1 colocalizes with neurofibrillary tangles in degenerating neurons of the AD brain (Caricasole et al. 2004). An additive inhibition of Wnt by ApoE4 and Dkk-1 (as shown by our data in PC12 cells) might severely impair the canonical Wnt signaling pathway with ensuing GSK3b activation, b-catenin degradation, tau protein hyperphosphorylation, and neuronal death.

http://onlinelibrary.wiley.com/doi/10.1111/j.1471-4159.2006.03867.x/pdf 

 1) LDL does the same thing as mutations in Familial AD. PLC activation associated with LDLr and G-proteins promotes hydrolysis of PIP2 and inhibits TRPM7 ion channel just like Familial AD. 

The TRPM7 channel is inactivated by PIP(2) hydrolysis

“TRPM7 (ChaK1, TRP-PLIK, LTRPC7) is a ubiquitous, calcium-permeant ion channel that is unique in being both an ion channel and a serine/threonine kinase. The kinase domain of TRPM7 directly associates with the C2 domain of phospholipase C (PLC). Here, we show that in native cardiac cells and heterologous expression systems, G alpha q-linked receptors or tyrosine kinase receptors that activate PLC potently inhibit channel activity. Numerous experimental approaches demonstrated that phosphatidylinositol 4,5-bisphosphate (PIP(2)), the substrate of PLC, is a key regulator of TRPM7. We conclude that receptor-mediated activation of PLC results in the hydrolysis of localized PIP(2), leading to inactivation of the TRPM7 channel.”

2) PIP2 levels are decreased by high LDL levels through the activity of PLC hydrolysis of PIP2 into DAG and IP3. Overactivation of this pathway is associated with G-proteins and non-cannonical Wnt activation. 

 Effect of lipoprotein on phosphatidylinositol -4,5-bisphosphate and phosphatidic acid in platelet

“Low-density lipoprotein (LDL) and high-density lipoprotein (HDL) were separated from normal human serum and incubated with 32P labelled human platelets. Studies were designed to explore the effects of thrombin, LDL, HDL plus LDL on the changes of the important metabolites: phosphatidylinositol-4,5-bisphosphate (PIP2) and phosphatidic acid (PA) of phosphatidylinositol cycle (PI). The results indicated that both thrombin and LDL caused a significant decrease of PIP2 within 15 seconds. Then, they gradually returned to the control level. This was accompanied simultaneously by a significant increase of PA level for 60 seconds in time-dependent manner. The effects of thrombin or LDL on PIP2 decrease and PA increase were also both dose-dependent. In addition, HDL significantly antagonized the decrease of PIP2 and increase of PA induced by LDL. It is suggested that PI cycle is closely related to atherosclerosis.”

3) PIP2 in the membrane is necessary for the beneficial action of PI3k/Akt pathway which is a known cell survival mechanism. PI3k acts on PIP2 converting it to PIP3 instead of DAG IP3 as happens when PLC acts of PIP2. 

 PIP2 and PIP3: Complex Roles at the Cell Surface

“PIP3 is the effector of multiple downstream targets of the phosphoinositide 3 kinase (PI3K) pathway (Rameh and Cantley, 1999); PIP2 is the precursor of the mediators diacylglycerol and inositol(1,4,5)P3 following its hydrolysis by hormone-sensitive phospholipase C (PLC) enzymes. New experiments are now revealing yet another signaling mode controlled by PIP2 (19, 8 and 17). This novel cascade depends on intact PIP2 rather than products of its hydrolysis. Recent work demonstrates not only new signaling functions of PIP2 but also intricate regulation of membrane phospholipids. New insights on these events highlight the cell surface membrane as a major site of action of both PIP2 and PIP3 and reveal unexpected cross-talk between these polyphosphoinositides.”

This PIP2 molecule in the cell membrane appears to be central to Alzheimer’s pathology, and so far as I have been able to determine seems to play a part in all the various known pathological aspects of the disease. The important thing here is the connection to LDL cholesterol which which is a modifiable factor that we can all do something about right now.   

I have never thought about this before.  Low density lipids by increasing phospholipase C activity reduces the amount of PIP2 (phosphatidylinositol 4,5 biphosphate) that can be converted into PIP3 (phosphatidyinositol 3,4,5 triphosphate). This problem is magnified in a person with Apoe4 as Apoe4 increases activation of the g protein coupled Wnt/frizzled receptor.  This results in even greater phospholipase C activity.  Reducing low density lipid levels in helpful in reducing the risk of Alzheimer's disease, but it is especially helpful in people with copies of the Apoe4 gene.

The presenilin 1 gene mutation prevents the conversion of PIP2 into PIP3 via inhibition of the phosphatidylinositol 3-kinase.

Familial Alzheimer disease (FAD) mutations suppress the ability of PS1 to promote PI3K/AKT signaling, prevent phosphorylation/inactivation of GSK-3 and promote activation of caspase-3. These mutation effects are reversed upon coexpression of constitutively active Akt. Together, our data indicate that the neuroprotective role of PS1 depends on its ability to activate the PI3K/Akt signaling pathway and that PS1 FAD mutations increase GSK-3 activity and promote neuronal apoptosis by inhibiting the function of PS1 in this pathway. These observations suggest that stimulation of PI3K/Akt signaling may be beneficial to FAD patients.

 http://www.jneurosci.org/content/28/2/483.full.pdf

In the case of late Alzheimer's disease, peroxynitrite does the same thing by mediating the nitration of the phosphatidylinositol 3-kinase.

Phosphatidylinositol 3-kinase is a target for protein tyrosine nitration.

http://www.ncbi.nlm.nih.gov/pubmed/9826526

 Peroxynitrite induces inactivation of the Akt pathway.

http://www.ncbi.nlm.nih.gov/pubmed/16410804

Thanks to your posts Serenoa and those of Tom(ek), we are likely much closer to understanding the risk factors and possible treatments for Alzheimer's disease.

Thank you Lane for all your great insights and research. And, I would add that PI3k/Akt pathway is also inhibited with insulin resistance (Type II diabetes).  

Now, let me pile on a bit here regarding this PIP2 pathway. I was researching Larrytherunner’s post on Montelukast, an inhibitor for the Leukotriene receptor. Yes, this inflammatory leukotriene molecule seems to tie in to the PIP2 pathway by activating a G-protein that may lead to the breakdown of PIP2 to DAG and IP3 (although I could only find a few articles indicating this). So where does leukotriene come from? Dietary omega-6 fats (which are high in American diet) are converted into leukotriene. More evidence that eating omega-3 fats avoids inflammation and maybe Alzheimer’s. 

The role of leukotrienes in allergic diseases

“Leukotrienes (LTs), both LTB4 and the cysteinyl LTs (CysLTs) LTC4, LTD4 and LTE4, are implicated in a wide variety of inflammatory disorders. These lipid mediators are generated from arachidonic acid (an omega 6 fatty acid) via multistep enzymatic reactions through which arachidonic acid is liberated from membrane phospholipids through the action of phospholipase A2. LTB4 and CysLTs exert their biological effects by binding to cognate receptors, which belong to the G protein-coupled receptor superfamily.”

http://www.sciencedirect.com/science/article/pii/S132389301400015X

I like this type of piling on. 

Here is some more evidence that omega 6 fatty acids could increase the risk of Alzheimer's disease by initiating the formation of leukotrienes (figure 3 speaks to Serenoa's discovery).

Alzheimer’s disease (AD) is the most common, and, arguably, one of the most-well studied, neurodegenerative conditions. Several decades of investigation have revealed that amyloid-beta and tau proteins are critical pathological players in this condition. Genetic analyses have revealed specific mutations in the cellular machinery that produces amyloid-beta, but these mutations are found in only a small fraction of patients with the early-onset variant of AD. In addition to development of amyloid-beta and tau pathology, oxidative damage and inflammation are consistently found in the brains of these patients. The 5-lipoxygenase protein enzyme (5LO) and its downstream leukotriene metabolites have long been known to be important modulators of oxidation and inflammation in other disease states. Recent in vivo evidence using murine knock-out models has implicated the 5LO pathway, which also requires the 5LO activating protein (FLAP), in the molecular pathology of AD, including the metabolism of amyloid-beta and tau. In this manuscript, we will provide an overview of 5LO and FLAP, discussing their involvement in biochemical pathways relevant to AD pathogenesis. We will also discuss how the 5LO pathway contributes to the molecular and behavioral insults seen in AD and provide an assessment of how targeting these proteins could lead to therapeutics relevant not only for AD, but also other related neurodegenerative conditions.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4294160/

This may relate to why gm-csf (the drug Leukine) seemed to be helpful with my mother. Even though gm-csf is part of the inflammatory response it seems to be more tissue repair and anti-inflammatory. Also, it may indicate that drug intervention further downstream (at LT receptor level instead of 5-LOX level) might be better. The takeaway being that maybe we should be looking at why the body is not naturally resolving inflammation.

The Resolution Code of Acute Inflammation: Novel Pro-Resolving Lipid Mediators in Resolution

“Studies into the mechanisms in resolution of self-limited inflammation and acute reperfusion injury have uncovered a new genus of pro-resolving lipid mediators coined specialized pro-resolving mediators (SPM) including lipoxins, resolvins, protectins and maresins that are each temporally produced by resolving-exudates with distinct actions for return to homeostasis. SPM evoke potent anti-inflammatory and novel pro-resolving mechanisms as well as enhance microbial clearance. While born in inflammation-resolution, SPM are conserved structures with functions discovered in microbial defense, pain, organ protection and tissue regeneration, wound healing, cancer, reproduction, and neurobiology-cognition. This review covers these SPM mechanisms and other new omega-3 PUFA pathways that open their path for functions in resolution physiology.”

Lipid Mediators in the Resolution of Inflammation

“Mounting of the acute inflammatory response is crucial for host defense and pivotal to the development of chronic inflammation, fibrosis, or abscess formation versus the protective response and the need of the host tissues to return to homeostasis. Within self-limited acute inflammatory exudates, novel families of lipid mediators are identified, named resolvins (Rv), protectins, and maresins, which actively stimulate cardinal signs of resolution, namely, cessation of leukocytic infiltration, counterregulation of proinflammatory mediators, and the uptake of apoptotic neutrophils and cellular debris. The biosynthesis of these resolution-phase mediators in sensu stricto is initiated during lipid-mediator class switching, in which the classic initiators of acute inflammation, prostaglandins and leukotrienes (LTs), switch to produce specialized proresolving mediators (SPMs). In this work, we review recent evidence on the structure and functional roles of these novel lipid mediators of resolution. Together, these show that leukocyte trafficking and temporal spatial signals govern the resolution of self-limited inflammation and stimulate homeostasis.”

The body it seems has mechanisms to counter inflammatory processes.  Another one is the oxidation of g protein-coupled receptors (both leukotriene receptors and prostaglandin receptors).   This may explain why non-steroidal anti-inflammatory drugs may help reduce the risk of Alzheimer's disease, but don't help treat the disease.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3033490/

Some of the specialized pre-resolving mechanisms appear to act upon endothelial nitric oxide which is reduced in Alzheimer's disease, so may be that limits their effectiveness in countering inflammation.

http://www.ncbi.nlm.nih.gov/pubmed/23745722

Here is another possible explanation for the downregulation of pre-resolving mechanisms.

In Alzheimer's disease, soluble amyloid precursor protein-alpha, which stimulates  in vitro proliferation of neural embryonic stem cells, activates neuroprotectin D1 biosynthesis. The hippocampal cornu ammonis region 1, but not the thalamus or occipital lobes, has decreased levels of DHA and neuroprotectin D169. The hippocampus of patients with Alzheimer's disease shows decreased expression of phospholipase A2 and 15-lipoxygenase — key enzymes in neuroprotectin D1 biosynthesis in this tissue hence they produces less of this endogenous protective mediator than healthy tissues66, 69. Neuroprotectin D1 reduces the expression of pro-inflammatory genes and upregulates the expression of anti-apoptotic genes69, 70. These suggest that neuroprotectin D1 promotes brain cell survival by inducing anti-apoptotic and neuroprotective genes69. 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2744593/

In Alzheimer's disease, the body may partially cut off inflammation via oxidation of g protein-coupled receptors but that may also inhibit some other anti-inflammatory mechanisms.

http://www.nature.com/aps/journal/v33/n3/full/aps2011200a.html

Unfortunately, some of the other g protein-coupled receptors oxidized in Alzheimer's disease affect short-term memory, smell, mood, sleep, social recognition, and alertness.

With some more clarity (regarding my previous post):

In recent years, evidence has prompted a paradigm shift whereby the resolution of acute inflammation is a biochemically active process regulated in part by endogenous PUFA (polyunsaturated fatty acid)-derived autacoids. Among these are a novel genus of SPMs (specialized proresolving mediators) that comprise novel families of mediators including lipoxins, resolvins, protectins and maresins. SPMs have distinct structures and act via specific G-protein seven transmembrane receptors that signal intracellular events on selective cellular targets activating proresolving programmes while countering pro-inflammatory signals.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3133883/

Thank you for the articles and the discussion.  Is there a target LDL level?

Iris L.

This is a good question, Iris.  I have a feeling that the target level depends on whether a person has the ApoE4 gene or not.  You would want to maintain lower LDL levels if you had one or especially two copies of the gene (although it is probably a good idea for all people to maintain relatively low levels of LDL).

I found this weekend a more precise mechanism by which ApoE4 increases the risk for Alzheimer's disease via low density lipid receptor-related proteins.  The main culprit appears to be the low density lipid receptor-related protein 1 (which is also a receptor for beta amyloid).  This receptor prevents the interaction between frizzled receptors and low density lipid receptor-related proteins 5/6.  The upshot of this is that the neuroprotective canonical Wnt pathway is repressed and the neurodestructive non-canonical Wnt pathway is upregulated. 

This may be the most important conclusion regarding the problem posed by the ApoE4 gene.

We thus conclude that one of the neurotoxic mechanisms triggered by ApoE4 is to activate a cell type-specific apoptogenic program involving LRP [LRP1 ; my addition] and the Gi class of GTPases and that the apoE4 gene may play a direct role in the pathogenesis of AD and other forms of dementia.

http://www.jneurosci.org/content/20/22/8401.full

Thanks Iris. Your question goes right to the practical reason for all this complex research. What should we do to improve, treat, avoid disease? I started this posting to prove that cholesterol (especially LDL) is a critical link to AD pathology. Because when we understand how important a lifestyle factor is in the pathology of disease, we will be much more motivated to change. I think the above posts are overwhelming evidence for the involvment of cholesterol and all its associated metabolic interactions as primary causal factors in AD. I also agree with what Lane says above regarding levels of LDL.

Interesting. I was watching a rerun of an Alzheimer's program this evening. A doctor stated that Vascular Dementia is the second most common dementia and is preventable.

When I found a neurologist in NYC he wanted me also to be seen by an internist. Good move. He was very proactive in watching my heart.  It was good.