UC SanDiego News Center, 2014

http://ucsdnews.ucsd.edu/pressrelease/new_therapeutic_target_discovered_for_alzheimers_disease

 A team of scientists from the University of California, San Diego School of Medicine, the Medical University of South Carolina and San Diego-based American Life Science Pharmaceuticals, Inc., report that cathepsin B gene knockout or its reduction by an enzyme inhibitor blocks creation of key neurotoxic pGlu-Aβ peptides linked to Alzheimer’s disease (AD). Moreover, the candidate inhibitor drug has been shown to be safe in humans.

The findings, based on AD mouse models and published online in the Journal of Alzheimer’s Disease, support continued development of cysteine protease inhibitors as a new drug target class for AD. “No other therapeutic program is investigating cysteine protease inhibitors for treating AD,” said collaborator Vivian Hook, PhD, professor in the UC San Diego Skaggs School of Pharmacy and Pharmaceutical Sciences and in the UC San Diego School of Medicine.

Current AD drugs treat some symptoms of the devastating neurological disorder, but none actually slow its progress, prevent or cure it. No new AD drug has been approved in more than a decade.

The researchers focused on cathepsin B production of N-truncated pGlu-Aβ, a peptide or short chain of amino acids, and the blockade of cathepsin B by E64d, a compound shown to inhibit cysteine proteases, a type of enzyme. AD is characterized by accumulation of a variety of Aβ peptides as oligomers and amyloid plaques in the brain, factors involved in neuronal loss and memory deficits over time. These neurotoxic Aβ peptides are created when enzymes cleave a large protein called amyloid precursor protein (APP) into smaller Aβ peptides of varying toxicity. N-truncated pGlu-Aβ has been shown to be among the most neurotoxic of multiple forms of Aβ peptides.

Much AD research has focused on the APP-cutting enzyme BACE1 β-secretase, but its role in producing pGlu-Aβ was unknown. Cathepsin B is an alternative β-secretase which cleaves the wild-type β-secretase site of APP, which is expressed in the major sporadic and many familial forms of AD. Hook and colleagues looked at what happened after gene knockout of BACE1 or cathepsin B. They found that cathepsin B, but not BACE1, produced the highly toxic pGlu-Aβ.

Perhaps most interestingly, the scientists found that E64d, an enzyme inhibitor of cathepsin B, reduced production of pGlu-Aβ and other AD-associated Aβ peptides. Key was the finding that E64d and cathepsin B gene knock out resulted in improved memory deficits in a mouse model of AD.

“This is an exciting finding,” said Hook. “It addresses a new target – cathepsin B – and an effective, safe small molecule, E64d, to reduce the pGlu-Aβ that initiates development of the disease’s neurotoxicity. No other work in the field has addressed protease inhibition for reducing pGlu-Aβ of AD.”

Hook noted that E64d has already been shown to be safe in clinical trials of patients with muscular dystrophy and would, therefore, likely prove safe for treating AD as well. She hopes to launch Phase 1 human clinical trials in the near future with a modified version of the drug candidate.

Disturbed Ca2+ Homeostasis Increases Glutaminyl Cyclase Expression; Connecting Two Early Pathogenic Events in Alzheimer’s Disease In Vitro

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0044674

A major neuropathological hallmark of Alzheimer’s disease (AD) is the deposition of aggregated β amyloid (Aβ) peptide in the senile plaques. Aβ is a peptide of 38–43 amino acids and its accumulation and aggregation plays a key role early in the disease. A large fraction of β amyloid is N-terminally truncated rendering a glutamine that can subsequently be cyclized into pyroglutamate (pE). This makes the peptide more resistant to proteases, more prone to aggregation and increases its neurotoxicity. The enzyme glutaminyl cyclase (QC) catalyzes this conversion of glutamine to pE. In brains of AD patients, the expression of QC is increased in the earliest stages of pathology, which may be an important event in the pathogenesis. In this study we aimed to investigate the regulatory mechanism underlying the upregulation of QC expression in AD. Using differentiated SK-N-SH as a neuronal cell model, we found that neither the presence of Aβ peptides nor the unfolded protein response, two early events in AD, leads to increased QC levels. In contrast, we demonstrated increased QC mRNA levels and enzyme activity in response to another pathogenic factor in AD, perturbed intracellular Ca2+ homeostasis. The QC promoter contains a putative binding site for the Ca2+ dependent transcription factors c-fos and c-jun. C-fos and c-jun are induced by the same Ca2+-related stimuli as QC and their upregulation precedes QC expression. We show that in the human brain QC is predominantly expressed by neurons. Interestingly, the Ca2+- dependent regulation of both c-fos and QC is not observed in non-neuronal cells. Our results indicate that perturbed Ca2+ homeostasis results in upregulation of QC selectively in neuronal cells via Ca2+- dependent transcription factors. This suggests that disruption of Ca2+ homeostasis may contribute to the formation of the neurotoxic pE Aβ peptides in Alzheimer’s disease.

Ok, I'm intrigued. This seems like a solid lead to me.

When I first started researching Alzheimer's disease ten years ago, the calcium hypothesis was the first one that seemed to make sense.  So I started to try to trace the origins of intracellular calcium release and later what caused the influx of calcium into cells.   

Like BACE, Cathepsin B is the product of phospholipase C beta (via g proteins) and phospholipase C gamma (via tyrosine kinase receptors). 

http://www.ncbi.nlm.nih.gov/pubmed/1341708 

Less important, though, then the role of phospholipase C in the release of intracellular calcium (which leads to various forms of amyloid oligomers) is the role of phospholipase C in the production of peroxynitrites. 

I just found this interesting speculation as to the potential role of BACE and Cathepsin B in cutting the amyloid precursor protein. 

One reason why cathepsin B has been overshadowed by BACE could be that, as Hook and colleagues demonstrated several years ago, the enzyme prefers to cleave wild-type APP, and displays much lower activity toward the altered site created by the Swedish mutation. The mutant sequence is preferred by BACE, and pops up in many of the FAD animal models which have been used to study the role of BACE in vivo. For studying β-cleavage inhibitors, Hook and coworkers favor the guinea pig model, which has a wild-type APP sequence that matches the β-secretase cleavage site in humans.

Hook used that animal to look at the role of cathepsin B in APP processing in the CNS. In her talk, she showed that infusion of selective cathepsin B inhibitor CA-074Me into the brains of guinea pigs reduced Aβ40 and Aβ42 levels by approximately one-half. In a second talk, Hook showed that another cathepsin B inhibitor, loxistatin, had the same effect after direct intracerebroventricular infusion. She saw decreased production of Aβ and the C-terminal β-secretase fragment of APP in the brain and in isolated synaptosomes. These results suggest that cathepsin B does participate in Aβ production in vivo as a β-secretase, and that inhibitors may have some potential as Aβ-lowering agents. This work is now in press in Biological Chemistry, Hook said.

A listener was quick to ask how Hook could reconcile her results with the recent paper from Li Gan showing that AD mice with the cathepsin B gene knocked out have worse plaque pathology than mice with the enzyme (see ARF related news story). In that study, the researchers found no evidence that cathepsin B acted as a secretase, but in fact found that it was able to degrade Aβ fibrils.

Hook pointed out that the AD mouse model used in that paper (the hAPP J20 line) incorporated the Swedish mutation, which could explain why the researchers saw no evidence for APP processing by cathepsin B. In animals with wild-type human APP, there could be a different outcome. Clearly, there is a lot more work to be done, but the data so far suggest that where Aβ production is concerned, BACE may be just one part of the story.—Pat McCaffrey.

http://www.alzforum.org/news/conference-coverage/sfn-old-and-new-bace-and-cathepsin-b-share-v-secretase-stage 

p38 MAPK is involved both in the production of peroxynitrites and cathepsin B.

This strongly suggests that p38 MAPK and NF-kappaB are essential to the IL-6-induced activation of cathepsin B or uPA and that these two IL-6-activated pathways can act independently.

http://www.ncbi.nlm.nih.gov/pubmed/17849265

Our results suggest that activated p38 MAPK may serve as a potential signaling molecule in ONOO(-) generation through dual regulatory mechanisms, involving iNOS induction and NADPH oxidase activation.

http://www.ncbi.nlm.nih.gov/pubmed/18289732

So apparently p38 Mapk upregulates both cathepsin B and BACE1 (via peroxynitrites).  I am not sure if one produces a more toxic amyloid oligomer than the other.

This one is for autism, but has implications for Alzheimer's disease. 

Conclusions

The results suggest a self-enhancing pathological process in autism that is initiated by intraneuronal deposition of N-truncated Aβ in childhood. The cascade of events includes altered APP metabolism and abnormal intracellular accumulation of N-terminally truncated Aβ which is a source of reactive oxygen species, which in turn increase the formation of lipid peroxidation products. The latter enhance Aβ deposition and sustain the cascade of changes contributing to metabolic and functional impairments of neurons in autism of an unknown etiology and caused by chromosome 15q11.2-q13 duplication.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3893450/ 

I knew this was going to be a tough nut to crack.  In many studies, Cathepsin B has been linked to the production and clearance of n-truncated amyloid, but this clearance only occurs with the activation of Akt which does not happen in Alzheimer's disease.  So you are left with a n-truncated and c-truncated amyloid which can further be cut by the release of intracellular calcium.  So, so far I have read about three forms of amyloid: one a c-terminal fragment caused by the peroxynitrite and hydrogen peroxide mediated activation of the BACE1 enzyme, an N-truncated amyloid (apparently without calcium release), and an N-truncated amyloid with calcium release (pyroglutamate amyloid).  

N-truncated amyloid β (Aβ) 4-42 forms stable aggregates and induces acute and long-lasting behavioral deficits.

A list of plants that inhibit p38 Mapk signalling. 

Table 2: Plant extracts that inhibit the p38 signaling in macrophages.

Plant Action target of p38 Reference

Archidendron  clypearia Suppression of PGE2 production; amelioration of EtOH/HCl-induced gastritis [28] 

Scutellaria  baicalensis Inhibition of iNOS, COX-2, PGE2, IL-1, IL-2, IL-6, IL-12, and TNF-expression [118] 

Phaseolus  angularis Suppression of the release of PGE2 and NO; amelioration of EtOH/HCl-induced gastritis [119] 

Artemisia  vestita Inhibition of TNF- release; beneficial for the treatment of endotoxin shock or sepsis [141] 

Boswellia  serrata Inhibition of TNF-, IL-1, and IL-6 [142] 

Hibiscus  sabdariffa Suppression of nitrite, PGE2 release, and hepatic inflammation [143] 

Clinopodium  vulgare Suppression of NO production; MMP-9 activation [144]

Eriobotryae  folium Suppression of LPS-induced NO and PGE2 production [145]

Elaeocarpus  petiolatus Inhibition of the production of PGE2, TNF-, and IL-1 [146]

Polygonum  cuspidatum Inhibition of IL-6, TNF-, NO, and PGE2 [147] 

Ginkgo  biloba Inhibition of LPS-induced iNOS and COX-2 expression [148]

Lycium  chinense Inhibition of LPS-induced NO, PGE2, TNF-, and IL-6 production [149] 

Hopea  odorata Inhibition of NO, PGE2, and TNF- release; amelioration of gastritis and ear edema

And compounds that inhibit p38 Mapk signalling. 

Table 3: Naturally occurring compounds that inhibit p38 signaling in macrophages.

Compound Action target of p38 Reference

Sugiol Inhibition of IL-1, TNF-, and ROS production [151] 

Quercetin Inhibition of NO and TNF- [114] 

Ajoenes Inhibition of NO, PGE2, TNF-, IL-1, and IL-6 production [116] 

Ginsan Enhanced phagocytic activity; downregulation of TNF-, IL-1, IL-6, IFN-, and IL-18 [152] 

4-Methoxyhonokiol Inhibition of iNOS and COX-2 expression; inhibition of dye leakage and paw swelling [153] 

Schisandrin Suppression of NO production and PGE2 release [154] 

Rengyolone Inhibition of iNOS and COX-2 expression [155] 

Pseudocoptisine Inhibition of proinflammatory mediators such as iNOS, COX-2, TNF-, and IL-6 [156] 

Mycoepoxydiene Inhibition of LPS-induced proinflammatory mediators including TNF-, IL-1, IL-6, and NO [157] 

Britanin Suppression of NO, PGE2, TNF-, IL-1, and IL-6 [158] 

Hyperin Inhibition of NO production through suppression of iNOS expression [159] 

Carnosol Inhibition of LPS-stimulated NO production; antioxidative activity [160]

http://www.hindawi.com/journals/mi/2014/352371/

Some of these plants and compounds have shown promise in the treatment of Alzheimer's disease. 

Very good work, Serenoa.  There seems to be a different mix of amyloid oligomers in autism than there is in Alzheimer's disease, although maybe further study is needed to determine if this is correct.  I read once that autistic individuals have trouble breaking down phenols which may lead to increased behavioral problems, but except in the case of severe autism may provide some protection against cognitive impairment (as phenols scavenge peroxynitrites). 

http://www.tacanow.org/family-resources/phenols-salicylates-additives/ 

http://www.jneurosci.org/content/30/15/5346/reply 

Protein kinase C leads to the release of the soluble amyloid precursor proteins which apparently can be cleaved into either a c-terminal or n-truncated form, both of which can be toxic (likely through the enhanced production of hydrogen peroxide).  p38 Mapk seems to stimulate either cut.   

Found something very interesting while reviewing basic APP information on Wikipedia.

"APP knockout mice are viable and have relatively minor phenotypic effects including impaired long-term potentiation and memory loss without general neuron loss.[26] On the other hand, transgenic mice with upregulated APP expression have also been reported to show impaired long-term potentiation.[27]

The logical inference is that because Aβ accumulates excessively in Alzheimer's disease its precursor, APP, would be elevated as well. However, neuronal cell bodies contain less APP as a function of their proximity to amyloid plaques.[28] The data indicate that this deficit in APP results from a decline in production rather than an increase in catalysis. Loss of a neuron's APP may affect physiological deficits that contribute to dementia."

http://en.wikipedia.org/wiki/Amyloid_precursor_protein

The secreted form of the amyloid precursor protein is neuroprotective and is regulated by protein kinase C.  Protein kinase C activity decreases in Alzheimer's disease due to oxidation and nitration.  It is possible that the decrease in the secreted amyloid precursor protein contributes to memory impairment in Alzheimer's disease.

Secreted amyloid precursor protein-alpha (sAPPalpha) is a neuroprotective and neurotrophic protein derived from the parent APP molecule. 

http://www.ncbi.nlm.nih.gov/pubmed/19463893

Overexpression and altered metabolism of amyloid precursor protein (APP) resulting in increased 4 kDa amyloid β peptide (Aβ) production are believed to play a major role in Alzheimer’s disease (AD). Therefore, reducing Aβ production in the brain is a possible therapy for AD. Because AD pathology is fairly restricted to the CNS of humans, we have established human cerebral primary neuron cultures to investigate the metabolism of APP. In many cell lines and rodent primary neuron cultures, phorbol ester activation of protein kinase C (PKC) increases the release of the secreted large N-terminal fragment of amyloid precursor protein (sAPP) and decreases Aβ release (Buxbaum et al., 1993; Gadzuba et al., 1993; Hung et al., 1993). In contrast, we find that PKC activation in human primary neurons increases the rate of sAPP release and the production of APP C-terminal fragments and 4 kDa Aβ. Our results indicate species- and cell type-specific regulation of APP metabolism. Therefore, our results curtail the use of PKC activators in controlling human brain Aβ levels.

http://www.jneurosci.org/content/18/8/2907.long

http://www.ncbi.nlm.nih.gov/pubmed/9576922 (the mechanism might not be right on this one for the decline in protein kinase C alpha)

I think the positive role of leukine is through the activation of the phosphatidylinositol 3-kinase/Akt pathway (have not been able to find anything yet on whether it can increase the secreted amyloid precursor protein in Alzheimer's disesae).

On an interesting side note, the chromosome 15q duplication in autism appears to increase the activation of serotonin receptors (a g protein-coupled receptor).  This would lead to an increase in phospholipase C and intracellular calcium release.  

http://www.ncbi.nlm.nih.gov/pubmed/22431635

http://dmm.biologists.org/content/3/1-2/3.full

The n-truncated amyloid oligomer appears to occur first and this maybe why people with autism have this form and not other forms of amyloid oligomers in their brain.  I would not say that Alzheimer's disease is a more advanced form of autism but that might not be far off.

One of the main differences between autism and Alzheimer's disease is that peroxynitrite levels are probably higher and cause more damage in Alzheimer's disease than in autism (as people with autism likely have higher levels of polyphenols that scavenge peroxynitrites since they cannot breakdown polyphenols).  

This description of the causes and consequences of peroxynitrite production in autism, though, largely applies to Alzheimer's disease as well. 

Autism as a NO/ONOO- Cycle disease 

Martin L. Pall 

Washington State University, USA 

The NO/ONOO- cycle is a primarily local biochemical vicious cycle, such that depending where it is localized in the body, may be able to generate a variety of chronic inflammatory diseases. The elements of the cycle are: Elevated levels of oxidative  stress, peroxynitrite (ONOO-), nitric oxide (NO), superoxide, intracellular Ca2+, inflammatory cytokines and other inflammatory markers, NF-kappaB, excitotoxicity glutamatergic activity and NMDA activity and some of the TRP group of receptors, as well as mitochondrial dysfunction and lowered tetrahydrobiopterin (BH4) activity. With two exceptions, each of these have been found to occur in autism patients and most have been shown to play a causal role in the disease. One exception, the TRP group of receptors, there are few data on their possible role in autism. There may be a second possible partial exception: although excitotoxicity and elevated glutamatergic activity are well documented, there is some evidence that the NMDA activity may be low, rather than high - this will be discussed in the presentation. In general, then, there is an excellent agreement between the predictions of the NO/ONOO- cycle and the biochemical properties of autism patients. There is similarly a good agreement between the cycle and the properties of stressors reported to cause autism, when exposures occur in the perinatal period; some of the unique properties of autism may be due to the impact of this biochemistry/physiology on the developing brain, as has been the conventional wisdom about autism. The properties of such initiating stressors, including infections, toxic metals and organic toxicant exposures and how they may act to initiate the cycle will be discussed.   

http://omicsonline.org/2161-0460/2161-0460-S1.004-039.pdf 

Very clearly and importantly said.  I looked for the reason why the amyloid precursor protein may be neuroprotective and found this interesting article. 

Phosphatidylinositol-3-kinase-Akt kinase and p42/p44 mitogen-activated protein kinases mediate neurotrophic and excitoprotective actions of a secreted form of amyloid precursor protein.

When you put these two together, you likely have the answer to Alzheimer's disease. 

Two distinct signaling pathways regulate peroxynitrite-induced apoptosis in PC12 cells.

I ran across another reference to cell adhesion this morning and decided look at it's relationship to APP. Turns out that there are many connections, and this is another step toward understanding how APP is involved in AD. Cell adhesion is vital to all multi-cellular organisms and any disruption of it leads to problems. Cell adhesion seems to be one of the functions of APP.

Abnormal cleavage of APP impairs its functions in cell adhesion and migration

"Our results suggest that impaired cell adhesion and migration induced by abnormal cleavage of APP could contribute to the pathological effects in FAD brain."

http://www.researchgate.net/publication/247152651_Abnormal_cleavage_of_APP_impairs_its_functions_in_cell_adhesion_and_migration

Beta amyloid precursor protein mediates neuronal cell-cell and cell-surface adhesion.

"These data suggest that the APP, which is expressed primarily on differentiated neuronal cells, may play a role in the mediation of both cell-cell and cell-substrate adhesion."

http://www.ncbi.nlm.nih.gov/pubmed/1645774

Anything here that connects to PI3k/Atk pathway or peroxinitrites or other things?

Lane, more evidence here, and this is filled with factors you commonly refer to, like p38 MAPK, so I know you will be able to interpret it for us.

Adhesion of monocytes to type I collagen stimulates an APP-dependent proinflammatory signaling response and release of Aβ1-40

Results

Pull-down assays demonstrated that THP-1 adhesion to collagen stimulated a tyrosine kinase-associated signaling response which included subsequent phosphorylation of p38 MAP kinase and increased association of APP with α2β1 integrin, specifically. In addition, cell adhesion was dependent upon APP expression since APP siRNA knockdown attenuated THP-1 adhesion to collagen compared to mock transfected controls. One consequence of the tyrosine kinase-dependent signaling response was increased secretion of interleukin-1β (IL-1β) and Aβ1-40 but not the Aβ1-42 fragment of APP. Increased secretion of IL-1β was dependent upon p38 MAP kinase activity while Aβ1-40 secretion required Src family kinase activity since the specific p38 inhibitor, SB202190, and the Src family kinase inhibitor, PP2, attenuated IL-1β and Aβ1-40 secretion, respectively.

Conclusions

These data demonstrate that APP is involved in classic integrin-dependent tyrosine kinase-associated adhesion and activation of peripheral monocytic cells. Moreover, divergent APP-dependent signaling is required for increased secretion of both IL-1β and Aβ1-40 as a component of the adhesion-dependent change in phenotype. This suggests that APP may have a broad role in not only mediating cell-matrix adhesion but also in the function of peripheral immune cells.

http://link.springer.com/article/10.1186%2F1742-2094-7-22

Altered Metabolism of the Amyloid β Precursor Protein Is Associated with Mitochondrial Dysfunction in Down's Syndrome

Discussion

These experiments indicate that there is a marked alteration in AβPP processing and Aβ trafficking in cortical DS astrocytes and neurons that can be replicated in normal human astrocytes by inhibition of mitochondrial energy metabolism. Moreover, mitochondrial function is impaired in DS astrocytes, as indicated by reduced mitochondrial redox activity and membrane potential.

However, despite impaired mitochondrial function, DS astrocytes are completely viable in culture. We suggest, therefore, that impaired energy metabolism in DS cells gives rise to increased β-secretase cleavage of AβPP and altered Aβ trafficking resulting in intracellular accumulation of aggregated Aβ42. Similar patterns of AβPP processing were detected in the DS brain. These results raise the possibility that impaired mitochondrial energy metabolism in the DS brain may contribute to the pathogenesis of AD.

http://www.sciencedirect.com/science/article/pii/S0896627302006049

I had discounted the mitochondrial connection as being a critical causal factor, but I am reconsidering that in light of it's connections to APP processing and oxidative damage. Here is more evidence.

Changes in mitochondrial function are pivotal in neurodegenerative and psychiatric disorders: How important is BDNF?

Abstract

Cell adhesion does appear to be affected by the phosphatidylinositol-3 kinase/Akt pathway and in some cases peroxynitrites inhibit this pathway and cell adhesion (such as in Alzheimer's disease). 

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1219394/pdf/9531503.pdf 

The Src kinase can lead to the activation of phosphatidylinositol-3 kinase/Akt pathway but it can also increase the production of p38 Mapk and peroxynitrites. And peroxynitrites may inhibit the ability of the Src kinase to activate the phosphatidylinositol 3-kinase/Akt pathway. 

http://www.ncbi.nlm.nih.gov/pubmed/12842830 

With more certainty it can be said that peroxynitrites contribute to mitochondrial dysfunction.   

http://www.nature.com/cdd/journal/vaop/ncurrent/abs/cdd201472a.html 

I have been away from a computer for a few days (blissfully in a way), but will try to catch up as quickly as I can since so much good information has been posted in the last few days. 

I like the way you think, Serenoa.  There are some interesting connections between the amyloid precursor protein and oxidative stress.  Amyloid precursor protein mutations increase oxidative stress and oxidative stress increases the processing of the amyloid precursor protein into a c-terminal fragment (which may in turn increase oxidative stress).

http://www.ncbi.nlm.nih.gov/pubmed/16478525

http://www.ncbi.nlm.nih.gov/pubmed/21371311

http://www.ncbi.nlm.nih.gov/pubmed/11432978

Yes, that is good evidence that oxidative/peroxynitrite damage (or a mutation in the APP gene) is causing abnormal processing of the APP molecule. Excellent. And, it seems logical that for every APP that is abnormally processed, there is one less beneficially-processed APP molecule. Perhaps a double whammy.

 Very happy to see you are back Lane. Hope you had a nice vacation or if not a vacation, at least a break from the message boards.

This is essential it, Serenoa!  Very well analyzed.  Certain forms of amlyoid precursor proteins (such as found in various mutations) seem to increase oxidative stress in and of themselves, whereas other forms of amyloid precursor proteins occur concurrently with oxidative stress and that stress leads to a c terminal fragment that for awhile further increases oxidative stress.

I am still on my vacation visiting national parks in the West that my father either worked at or took us to as children.  

So many good posts in recent days that I have not been able to respond to or research further, but the cooperative work of so many people on this board to find answers to Alzheimer's disease always impresses and uplifts me.

Src kinases may play a critical intermediary role in the development of Alzheimer's disease.  Src kinases like phospholipase C gamma are activated by tyrosine receptor kinases (platelet derived growth factor receptors, epidermal growth factor receptors, insulin receptors, insulin growth factor receptors, etc.).  Platelet derived growth factor receptors which can be activated by high levels of glucose and angiotensin II (a risk factor for high blood pressure) appear to be the most important of these receptors in most cases of Alzheimer's disease.

Platelet-derived growth factor induces the beta-gamma-secretase-mediated cleavage of Alzheimer's amyloid precursor protein through a Src-Rac-dependent pathway.

Yes, welcome back! I think you have nailed it again Lane. Tyrosine kinases and phosphorylation of cellular proteins is the key.

I have been researching and reviewing, learning and brainstorming a lot in the last few weeks. The failures of the multitudes of experts to discover the true cause of this disease, and my own failure to put together the clues that will lead to a treatment has been haunting me. However, with the help of this forum and the availability of good scientific data, I think there is hope, and I am not giving up. My ideas lately, I think, are bringing me closer to understanding the possible real causes of AD, many of which Lane has been explaining for a long time.

I always appreciate your insights and input, Serenoa.  I think that you are on the right track.   

Here's a bit more on the Src kinase.  When it activates phosphatidylinositol-3 kinase/Akt pathway it helps sort out the amyloid precursor protein. 

When the phosphatidylinositol 3-kinase/Akt pathway is inhibited (by presenilin gene mutations and peroxynitrites, for instance) the src kinase increases the activation of p38 Mapk which results in the formation of more peroxynitrites and to the c-terminal fragment of the amyloid precursor protein. 

Defects in endosomal sorting have been implicated in Alzheimer's disease. Endosomal traffic is largely controlled by phosphatidylinositol-3-phosphate, a phosphoinositide synthesized primarily by lipid kinase Vps34 [a phosphatidylinositol 3-kinase isoform]. Here we show that phosphatidylinositol-3-phosphate is selectively deficient in brain tissue from humans with Alzheimer's disease and Alzheimer's disease mouse models. Silencing Vps34 causes an enlargement of neuronal endosomes, enhances the amyloidogenic processing of amyloid precursor protein in these organelles and reduces amyloid precursor protein sorting to intraluminal vesicles 

http://www.ncbi.nlm.nih.gov/pubmed/23907271 

Involvement of Protein Kinase C and Src Family Tyrosine Kinase in Gαq/11-induced Activation of c-Jun N-terminal Kinase and p38 Mitogen-activated Protein Kinase*

O.k. now the seeming contradictory studies regarding protein kinase C and Alzheimer's disease make sense.  Protein kinase C causes the secretion of the amyloid precursor protein and the sorting out of this protein via the two step activation of Src kinases and the phosphatidylinositol-3 kinase. However, when the phosphatidyinositol-3 kinase is inhibited by presenilin gene 1 mutations or by factors that lead to oxidative stress, the src kinases just leads to the production of more peroxynitrites and the c-terminal fragment of the amyloid precursor protein.  This probably means that src kinase activators such as diabetes drug metformin may help reduce the risk of Alzheimer's disease in those without the presenilin gene 1 mutation early on but not later on.  Also drugs that cross the blood brain barrier and inhibit the phosphatidylinositol 3-kinase should not be prescribed to those at risk for Alzheimer's disease.

Relatedly, mutated amyloid precursor proteins and the c-terminal fragment of the amyloid precursor protein stimulate g proteins that activate protein kinase C that lead to the formation of more peroxynitrites. This is a negative feedback mechanism of Alzheimer's disease that contributes to at least the initial progression of the disease (the peroxynitrite nitration of NMDA receptors is likely the main negative feedback mechanism as the disease passes the early stages).

Overexpression and altered metabolism of amyloid precursor protein (APP) resulting in increased 4 kDa amyloid β peptide (Aβ) production are believed to play a major role in Alzheimer’s disease (AD). Therefore, reducing Aβ production in the brain is a possible therapy for AD. Because AD pathology is fairly restricted to the CNS of humans, we have established human cerebral primary neuron cultures to investigate the metabolism of APP. In many cell lines and rodent primary neuron cultures, phorbol ester activation of protein kinase C (PKC) increases the release of the secreted large N-terminal fragment of amyloid precursor protein (sAPP) and decreases Aβ release (Buxbaum et al., 1993; Gadzuba et al., 1993; Hung et al., 1993). In contrast, we find that PKC activation in human primary neurons increases the rate of sAPP release and the production of APP C-terminal fragments and 4 kDa Aβ. Our results indicate species- and cell type-specific regulation of APP metabolism. Therefore, our results curtail the use of PKC activators in controlling human brain Aβ levels.

http://www.jneurosci.org/content/18/8/2907.long

C-terminal fragment of amyloid precursor protein induces astrocytosis.