A New Theory of Alzheimer's Disease: Innate Autoimmune Diseases

Human efforts to find a difficult cure for Alzheimer's disease (AD) highlight the continuing need to develop innovative mechanism-driven AD models
.
Recently, the journal Alzheimer's & Dementia published a review titled "Alzheimer's disease as an innate autoimmune disease (AD2): A new molecular paradigm," in which the authors propose a new model of Alzheimer's disease: an autoimmune disease (AD2).

In response to pathogen/injury-related molecular pattern stimulus events (e.
g.
, infection, trauma, ischemia, contamination), amyloid β protein (Aβ) is released as an early response cytokine, triggering an innate immune cascade in which Aβ exhibits dual immunomodulating/antimicrobial effects
.
However, the antimicrobial properties of Aβ lead to false attacks on "own" neurons due to electrophysiological similarities between neurons and bacteria in terms of transmembrane potential gradients and anionic charges on outer membrane macromolecules
.
The subsequent breakdown products of necrotic neurons cause further release of Aβ, leading to a chronic, self-perpetuating cycle
.

In AD2, amino acid metabolism (Trp, Arg) is the central controller in regulating AD autoimmunity
.
AD2 includes Aβ as an important molecular player, but rejects the "amyloid hypothesis," arguing that Aβ is a physiological oligomeric cytokine that is part of a larger conceptualization of the immunopathology of
AD.

Introduction narrative

The authors' team proposes a broad new molecular-level model of AD: "an autoimmune disease" ("AD-squared" or "AD2"
).
In the AD2 model, AD is interpreted as a brain-centric, innate immune disease involving concurrent autoimmune and autoinflammatory mechanisms
.
The AD2 model can be summarized as follows: Aβ is physiologically biosynthetic and released as an early response immunopeptide for different pathogen/injury-associated molecular pattern (PAMP-DAMP) immunostimulatory events (e.
g.
, infection, trauma, ischemia, air pollution, depression), and triggers an innate immune cascade in which oligomeric Aβ exhibits immunomodulatory and antimicrobial properties
.

The immunomodulatory properties of Aβ (mediated by oligomeric interactions of Aβ with trigger receptors expressed on myeloid cell 2 [TREM2], glycosaminoglycans [GAG], and Nod-like receptor protein 3 [NLRP3] receptors) enhance ongoing microglial activation and pro-inflammatory cytokine release, ultimately leading to apoptotic neuronal death
through a nonspecific autoinflammatory process in which bystander neurons are killed.

At the same time, Aβ exhibits antimicrobial properties (with or without bacteria), leading to false attacks on "self" neurons, which are caused
by electrophysiological similarities between neurons and bacteria in terms of transmembrane potential gradient (-80 mV) and anionic charge on outer membrane macromolecules (gangliosides in neurons).
cardiolipin or lipopolysaccharide [LPS]), which makes them equally susceptible to necrosis
due to penetration of cytotoxic membranes by antimicrobial peptides such as Aβ (AMPs).
Following this specific but misguided self-attack, the resulting necrotic neuronal breakdown products spread to adjacent neurons, triggering a further sustained release of Aβ, leading to a self-perpetuating autoimmune cycle
.

Some authors have hypothesized that AD is an autoimmune disease, but this is usually done within the established concept of autoimmunity
.
Traditionally, autoimmunity has been considered a chronic disease of adaptive immunity rather than innate immunity because adaptive immunity is highly specific, long-lasting, and "remembered" through antibody responses, which are immediate, non-specific, and not remembered; Therefore, autoantibodies are often seen as markers of autoimmunity
.
According to this statement, autoantibodies against Aβ, tau, microglia, glial fibrillary acidic proteins, voltage-gated potassium channels, and glutamate decarboxylase have been identified in patients with AD, but their role in the pathophysiology of the disease remains uncertain (are they pathogenic or are they merely secondary paraphenomena of unmasked proteins released from dying cells?).
）
。

The AD2 model differs and assumes that AD is an innate immune autoimmune disease rather than an adaptive immunity
.
Traditionally, any unintended self-inflicted host injury associated with innate immune dysfunction has been considered autoinflammatory rather than autoimmune, primarily a non-specific bystander effect
resulting from a pro-inflammatory state.
However, as AD2 speculates, AD is indeed an autoimmune disease innate immunity because Aβ, released as an antimicrobial immune peptide, inadvertently (but specifically) mistakes neurons for non-self, piercing the neuron's cell membrane
like a bacterium.

The impact of AD2 is manifold
.
The AD2 model strives to coordinate other diseases, including proteopathies, synaptic toxicity, and mitochondrial diseases, while recognizing Aβ as a physiological oligomeric immunopeptide
.
Since immune processes, including autoimmunity, are homeostasis-regulated (i.
e.
, pharmacologically modified glucocorticoid metabolism to manage routine adaptive autoimmune diseases such as systemic lupus erythematosus), it is equally reasonable to assume that existing neurochemical processes serve as modulators of brain innate immunity and a possible source
of "endogenous anti-AD molecules".
In the AD2 model, amino acid metabolism of L-tryptophan and L-arginine emerges
as innate immunomodulators.
Diagnostically AD2 suggests that levels of L-tryptophan or L-arginine metabolites in serum or cerebrospinal fluid (CSF) may be useful biomarkers
in addition to other neuroinflammatory markers.
Therapeutically , in order to achieve disease-remitting treatment strategies for AD, AD2 may downregulate congenital autoimmunity by: (1) molecular analogues of L-tryptophan or L-arginine metabolites, (2) enzyme inhibitors of L-tryptophan and/or L-arginine metabolic pathways, and (3) microbiome manipulation
of L-tryptophan metabolism.

First, authors need to re-evaluate Aβ with an unbiased and open mind, focusing on its role as an immunopeptide and AMP, considering the full implications of its AMP function, rather than obsessively searching for microbes
that may cause AD.
Second, they needed to better characterize the well-defined biomolecular basis
of the immunomodulatory effects of Aβ on the TREM2-GAG-NLRP3 network from the perspective of AD pathogenesis.
Third, authors need to explore the mechanistic implications of the Aβ antimicrobial-immunomodulatory duality, especially when
it interacts with (and interprets) other AD disease mechanism proposals, particularly synaptic toxicity and tauopathy.
Fourth, the authors' team needs to critically assess the capabilities
of widely used AD transgenic mouse models (and other evaluative bioassays).
Accurately generalize AD
in the context of the AD2 model.
Fifth, they need to rigorously evaluate L-tryptophan and L-arginine metabolism, not only based on the neurochemical basis of AD, but also to identify new diagnostic and therapeutic approaches
.

Study design and synthesis

The author team begins with an extensive review of journals and patent documents (including data from clinical neuroscience, systems biology, neurobiology, pathology, immunology, biochemistry, drug design, and molecular modeling) and uses various algorithms (e.
g.
, databases [PubChem, BioCyc/MetaCyc, Kyoto Encyclopedia of Genes and Genomes (KEGG), U.
S.
Patent and Trade Office (USPTO), World Intellectual Property Organization (WIPO)] searches), A comprehensive mechanism model
of AD was designed.
This model, called AD2, attempts to explain the complexity
of the etiology and pathogenesis of AD.

The AD2 model is as follows: AD is an underlying chronic, progressive neuroimmune system disease (Figure 1).

Whether it is a pathogen-associated molecular-pattern stimulus event (e.
g.
, infection, air pollution) or an injury-related molecular-pattern stimulus event (e.
g.
, trauma, ischemia, depression, obesity), Aβ is physiologically produced and released (in the form of monomeric and multiple oligomer aggregations) as an early response cytokine triggering an innate immune cascade
.
Once released, Aβ exhibits cytokine/chemokine-like properties with immunomodulatory and antibacterial effects, regardless of the stimuli that precipitate its release (i.
e.
, infection and depression); Thus, even in the absence of microorganisms, the released Aβ will exhibit immunomodulatory and "antibacterial" properties
.

The AD2 model provides a new paradigm for understanding AD at the molecular level
.
Like all models of complex phenomena, AD2 has limitations, and its ultimate success will depend on its ability to unambiguously understand AD at the molecular level and facilitate the development and implementation
of effective diagnostic and therapeutic approaches.

Figure 1 AD2 model

method

To create the AD2 model, the authors' team initially conducted a comprehensive literature search using PubMed (842 separate search terms resulted in a survey of 98,946 articles); Assisted search was conducted using Google, Bing, and Wolfram-Alpha search
engines.
Patent literature searches using the USPTO and WIPO search tools further expand searches
.
The scope of the identified papers includes biophysics, computational modeling, basic neuroscience, pathology, clinical neurology, etc
.

outcome

AD2: Aβ is an immunopeptide

AMP and cytokines (including chemokine isotypes) are essential innate immunopeptides
in the brain.
However, the term antimicrobial peptide is a misnomer because AMP has both immunomodulatory and antimicrobial effects
.
In fact, AMP, also known as host defense peptides, is a fundamental regulator of innate immunity through multiple pathways
, including GAG binding.

Conversely but similarly, chemokine-type cytokines may be antimicrobial and immunomodulatory and also bound in part by GAG
.
Therefore, AMPs and many cytokines have dual antibacterial and immunomodulatory functions
.
Structurally, this functional duality is achieved through their amphiphilic structure, with discrete adjacent motifs of cationic and hydrophobic amino acid residues
.
Many AMP and chemokines are physiologically oligomerized/aggregated as part of their normal function, thereby increasing bacterial killing and immunomodulatory activity
through GAG binding.
Because Aβ is a GAG-bound easily aggregable peptide with dual antibacterial and immunomodulatory activity, it can be said that it is located within the functional and structural range of immunopeptides defined by AMP and chemokines (in this respect, oligomerization is a physiological and not a pathological process of Aβ).

2.
The antimicrobial effect of AD2: Aβ is neurotoxic

Aβ is an AMP
.
This property has been proven
computationally and experimentally.
From a mechanistic point of view, AMP binds to invading microorganisms through coulombic interactions, whereby the positively charged fragment of AMP is electrostatically attracted to the negatively charged bacterial membrane surface, and then adjacent hydrophobic regions in AMP are inserted through the membrane, resulting in peptide-induced membrane rupture, resulting in cytoplasmic leakage and necrotic cell death—a process
driven by the transmembrane elevator degree electrostatic of bacteria 。 Bacterial transmembrane potentials are able to hyperpolarize/depolarize ion channels and mediate coordinated interbacterial signaling at the biofilm level; Due to their transmembrane electrical properties, bacteria communicate using ion channels and electrical impulses, similar to neurons
.

Unfortunately, AMP-like properties also allow for Aβ-mediated misguided attacks on "own" neurons due to specific electrophysiological similarities between neurons and bacteria in terms of transmembrane potential gradients (≈ –80 mV) and anionic charges on the outer membrane of macromolecules (gangliosides in neurons; LPS and cardiolipin in bacteria).

Similar to the binding of AMP to bacteria, Aβ binds electrostatically to neuronal membranes, or directly to phospholipids, or preferably as oligomers to
anionic GM1 or GAG membrane molecules.
Thus, Aβ exhibits "antibacterial" activity
against neurons even in the absence of bacteria.
This misleading but very specific molecular attack on autoneurons is an innate immune autoimmune phenomenon that produces persistent Aβ by releasing the GM1-Aβ
complex.

The role of Aβ as AMP can also consider two other Aβ neurochemical issues: non-amyloid production versus metabolic pathways of amyloid production; and the different contributions
of Aβ1-40 and Aβ1-42 to disease progression.
The non-amyloid production pathway involves α-secretase cleavage of amyloid precursor proteins (APP) to produce two fragments: the 83 amino acid C-terminal fragments retained in the membrane and the N-terminal extracellular domain (sAPPα)
released into the extracellular.
The amyloid production pathway leads to neurotoxic Aβ production: β-secretase (BACE1) mediates the first proteolysis step; Continuous cleavage of γ-secretase releases Aβ peptides
.
γ-secretase is an enzyme complex
composed of progerin 1 or 2 (PS1, PS2), nicastrin, prepharyngeal deficiency (APH-1), and progerin enhancer 2 (PEN2).
The resulting Aβ peptide is predominantly (90%) 40 residues in length, and a small portion (10%) contains 42 residues (Aβ1-42); Aβ1-42 is a more neurotoxic form, and elevated plasma levels of Aβ1-42 correlate better
with AD severity.

In the AD2 model, Aβ is an AMP immunopeptide that is physiologically released
in response to the presence of immune stressors, such as the presence of microorganisms.
Various studies have confirmed that immune threats such as bacterial and viral infections upregulate β and γ secretases in favor of the amyloid (over non-amyloid) pathway and Aβ1-42 (over Aβ1-40).

Aβ1-42 exhibits sequence homology to bacteriomycin and structural similarity to conventional antimicrobial peptides and viral fusion domains
.
Aβ1-40 has much
lower antimicrobial (and neurotoxic) activity.
Therefore, the amyloid Aβx-42 variant has stronger antimicrobial activity and is therefore the dominant neurotoxic isomer in the AD2 model
.

In addition, AD2 suggests the need to avoid indiscriminately labeling all Aβ as neurotoxic, identify Aβ1-42 as preferential pathogenicity, and treat other isomers, such as Aβ1-38, as potentially paradoxical protective effects
.

AD2: immunomodulatory role of Aβ in neurotoxicity

Aβ is an immunomodulatory peptide
.
Aβ affects the cellular and humoral components of the innate immunity of the brain, binds to microglia and affects the release and action
of cytokines.

Aβ is released as a molecular trigger, triggering a broad cascade of
immune responses.
Initially, oligosaccharide Aβ binds
to microglia via TREM2 receptors and/or membrane-associated glycosaminoglycans (e.
g.
, GAG such as heparan sulfate).
The binding of Aβ to TREM2 interrupts the role of TREM2 in maintaining neuronal integrity, leading to changes
in cytokine expression and apoptosis.
In addition, Aβ enhances the interaction of TREM2 with its signal transduction linker DAP12 protein, regulating downstream phosphorylation
of splenic tyrosine kinase (SYK) and GSK3β kinase.

Similarly, the multifaceted steady-state cell signaling process of GAG is disrupted
by Aβ binding.
The binding of Aβ to TREM2 and GAG also leads to the activation of the NLRP3 inflammasome, a polymeric protein complex that triggers further release of pro-inflammatory cytokines including IL-1β and IL-18, indicating additional neuroinflammatory changes; Oligomeric Aβ also has the ability to
interact directly with NLRP3.
Alterations in TREM2-GAG-NLRP3 system function promote persistent microglia-mediated innate immune dysregulation and TLR4 and CD14 co-receptor stimulation, thereby inducing the release of TNFα pro-inflammatory immune mediators
by NFκB 。 Aβ-triggered dysfunction of the TREM2-GAG-NLRP3 system distorts post-translational modifications of tau, resulting in an imbalance in the phosphorylation/hyperphosphorylation tau ratio; Overphosphorylated tau breaks down microtubules and isolates normal tau into tangles, disrupts cytoplasmic function, interferes with axon trafficking, and pathologically causes neuronal death, clinically leading to memory and cognitive impairment
.

Aβ-triggered dysfunction of the TREM2-GAG-NLRP3 system also skews the resting polarization of microglia, resulting in an imbalance in the ratio of pro-inflammatory/anti-inflammatory microglia; Excessive pro-inflammatory microglial activity (resulting in IL-1β, IL-6, TNFα, IFNγ, IL-12 release) is cytotoxic and intrinsic apoptotic pathway
can be activated by upregulating Bak, Bcl-x, and Bcl-2 cell death regulatory proteins 。 Overall, the pro-inflammatory cascade resulting from the immunomodulatory interaction of Aβ with the TREM2-GAG-NLRP3 system ultimately leads to nonspecific bystander cytotoxicity, leading to autoinflammation leading to neuronal death (enhancing the concomitant AMP-like interaction of Aβ with neurons, leading to specific cytotoxicity, via innate autoimmunity).

In the autoimmune component of AD2, neuronal membranes exhibit lipid heterogeneity, with synaptic regions most similar
to bacteria.
While neuronal axon membranes have a similar outer lobe lipid composition to other eukaryotic cells (i.
e.
, mostly zwitterionic [phosphatidylcholine], with anionic lipids shuttling to the inner lobe via ATP-driven flipase), neuronal synaptic membranes include highly abundant anionic lipids with sterol microdomains (phosphatidylserine, phosphatidylinositol) making the synaptic membrane more bacteria-like and vulnerable to direct Aβ/AMP attack
.

At the same time, Aβ triggers TREM2-GAG-NLRP3 system dysfunction, which directly leads to abnormal synaptic pruning and additional synapse loss.
TREM2, GAG, and NLRP3 receptors all contribute to normal synaptic function, plasticity, and elasticity—processes that regulate abnormal inhibition of
Aβ—respectively but complementarily.
In addition, Aβ oligomers induce abnormal stabilization of F-actin within dendritic spines, further impairing synaptic strength and
plasticity in related processes involving activation of the Rho-associated protein kinase (ROCK) pathway and phosphorylation of cofilin-1 actin-binding protein.

4.
AD2: AD is a chronic autoimmune disease

If Aβ is a typical innate immunopeptide, its effects will be acute, transient, and self-limited
.
However, the innate immune effects of Aβ immunopeptides are uniquely chronic and progressive
.
The long-term nature of the Aβ immune response stems from the antimicrobial effects of Aβ, whose autoimmunity crosses neurons, leading to rupture
of neuronal membranes.

The damaged membrane of necrotic Aβ-killed neurons releases the GM1-Aβ complex, which induces neighboring neurons to produce/release Aβ
.
The GM1-Aβ complex released from the membrane of broken necrotic neurons derives the binding
of the HHQK positive motif of Aβ to the negatively charged gangliosides in the outer membrane lobe of the neuron.
Thus, necrotic (but not apoptosis) neurons release GM1-Aβ complexes that spread to healthy neighboring neurons, triggering them to produce Aβ, a spontaneous process that transforms AD into a chronic, self-perpetuating process
.

AD2: the role of L-tryptophan and L-arginine

McGaha et al.
describe L-tryptophan as a "key regulator" and control system
of innate immunity in brain cancer.
L-tryptophan metabolism is a recognized innate immunomodulator, and enzymes such as indolemine-2,3-dioxygenase (IDO; L-tryptophan catabolism of initial and rate-determinants) is used as a drug target
.
In AD, L-tryptophan metabolism has the ability to prevent excessive inflammation through multiple biochemical mechanisms and is central to the inflammatory process in AD; Decreased plasma L-tryptophan levels in patients with AD; Acute L-tryptophan depletion in patients with AD leads to increased cognitive dysfunction; L-tryptophan malabsorption predisposes to AD; and L-tryptophan administration produced statistically significant improvements
in a rat dementia model.
The well-defined molecular mechanisms by which L-tryptophan and related metabolites regulate innate immunity are multiple; For example, 3-hydroxyanthranilic acid metabolites have the ability to
inhibit Aβ oligomerization and reduce IL-6 and TNFα inflammatory cytokine production.

Meanwhile, Morris designated L-arginine as the "master and commander" controller of innate immunity
.
In AD, there are region-specific changes in brain concentrations of L-arginine and its downstream metabolites (L-citrulline, L-ornithine, agmatine, putrescine, spermidine, spermine, glutamine); Correspondingly, the activity and protein expression of two key L-arginine metabolases, nitric oxide synthase and arginase, are also altered
in a region-specific manner.
Fleszar et al.
performed targeted metabolomic analysis
of nitric oxide/L-arginine pathway metabolites in dementia.
It is proven to be related
to pathology, severity, and structural changes in the brain.

In mouse models of AD, citrulline supplementation improved cognitive decline
by altering L-arginine and nitric oxide levels.
The study by Colton and colleagues strongly suggests that local immune-mediated L-arginine catabolism is a new and potentially key mechanism that mediates age-dependent and regional neuronal loss
in AD patients.
The well-defined molecular mechanisms by which L-arginine and related metabolites pharmacologically regulate innate immunity are multiple; For example, L-arginine alters the mTOR signaling pathway and reduces IL-1β, IL-6, TNFα, and inducible nitrate oxidase
.

Mondanelli et al.
have concluded that both the L-tryptophan and L-arginine metabolic pathways underlie innate immune control, and that the interaction between these two pathways may be at the center of reprogramming immune cell function at the center of immune-based diseases.
In addition, both L-tryptophan and L-arginine have been shown to be direct inhibitors of Aβ oligomerization (in addition to other innate immunomodulatory mechanisms), suggesting that amino acids have the ability to
enable multisite inhibitors of AD progression.
Background of
the AD2 model.
These findings suggest that altered L-tryptophan and L-arginine metabolism in different regions of the AD brain warrant further investigation to understand their role
in disease pathogenesis and/or progression.

AD2: "off-target" non-immune effects of Aβ

Due to their peptides, they have flexible conformational structures, cytokines and related immunopeptides that bind not only to immunity, but also to a range of non-immune receptors; Aβ is no exception
.
Aβ also binds to a variety of receptors, including α7nAChR, NMDA-R, CLAC-P/Collagen XXV, P75NTR, and insulin receptors
.
This receptor binding heterogeneity of Aβ as malleable immunopeptides (like most cytokines) adds additional complexity
to the diversity of AD pathologies and their diverse immunopathological pathogenesis.

discuss

1.
AD2: implications for the development of diagnostics

Several recent non-targeted metabolomics studies have confirmed changes in L-tryptophan metabolite levels in serum and spinal fluid in patients with AD, indicating the potential value
of the AD2 model in supporting future diagnostic assays.

AD2: Impact on treatment development

The AD2 model provides an opportunity for new drug designs and a new direction
for the development of neuroimmunotherapies that is different from protein aggregation targets.
AD2 has identified a variety of druggable biochemical pathways; L-tryptophan and L-arginine metabolism as innate immunomodulators is the logical first target
.
L-tryptophan metabolism provides a druggable biosynthase (IDO) that catalyzes the catabolism of L-tryptophan, as well as a multifunctional metabolite (3-hydroxyanthranilate) that targets multiple sites in the AD2 cascade, from cytokine release to microglial activation, to autophagy
.
L-arginine metabolism also provides a range of drug targets, including catabolase (arginase) and the bioequivalent of L-arginine metabolites (citrulline).

3.
Repurpose known drugs

Another alternative approach to drug design is to repurpose known drugs to address the various pathways identified in AD2, starting with
L-tryptophan and L-arginine metabolism.
Known drugs that have not been studied in AD, such as IDO inhibitors currently being evaluated in cancer trials, can be repurposed for non-oncological neurodegenerative indications
.

For example, indomethacin (2-[1-(4-chlorobenzoyl)-5-methoxy-2-methylindole-3-yl]acetic acid), flustatin ([E]-7-[3-(4-fluorobenzoyl)-1-propan-2-cycloindole-2-yl]-3,5-dihydroxyheptane-6-enoate), methamine (N-[2,3-methyl]-phthalenelic acid), and furosemide (4-chloro-N-[2-furanyl]-5-sulfonyl-phthalenebenzoic acid) has an indole or phthalate group associated with the structure of L-tryptophan metabolites (Figure 2); All of these previously documented anti-oligomer and/or anti-inflammatory activities In order to use furosemide as a potential platform for AD drug development, we demonstrate the utility of furosemide and its associated analogues as anti-immune agents for AD
.

4.
AD2: a new treatment

Figure 2 Reuse of known drugs

AD2 also makes the case for the search for new therapeutic avenues, such as microbiome approaches
.
The intestinal microbiota can change the host's utilization of L-tryptophan; L-tryptophan regulates major gut bacteria (such as Bifidobacterium infantis, Lactobacillus johnsonii, and Enterobacterium coli) through intestinal bacteria metabolization into indole, indole analogues, skatole, and tryptamine analogues, capable of "fine-tuning" L-tryptophan metabolism and selectively producing specific and unique therapeutic L-tryptophan metabolites as endogenously produced hypothetical therapeutic agents
.

conclusion

Given the complexity of AD, the central thesis of AD2 does have some potential limitations, namely that AD is primarily a congenital autoimmune disease
.
First, how AD2 explains the culture of AD pathology and the specificity of the neural networks affected by the disease
.
AD initially disrupts neuronal connections in memory and information-processing regions of the brain, including the hippocampus and the entorhinal cortex, which affects cortical regions
responsible for language, reasoning, and social behavior.
These brain centers enable default mode networks and multisensory integration mechanisms
necessary for brain-based communication.
At the cellular level, central brain regions are rich in synapses to facilitate information transmission and therefore require a lot of energy
from mitochondrial activity.

Therefore, these regions are preferentially susceptible to synaptic toxicity and mitochondrial toxic processes, both of which are at least partially explained
by the AD2 model.
Second, how AD2 explains the inconsistent association and nonlinear relationship
between Aβ burden and dementia progression.
As shown in Figure 1, AD2 conceptualizes the pathogenesis of AD as derived from two parallel processes: an autoimmune process directly involved in Aβ and an autoinflammatory process that, once initiated, can proceed independently of Aβ to produce neurotoxicity
through pro-inflammatory effects.
Although AD2 cannot predict the autoimmune/autoinflammatory ratio in any particular person, given the different contributions of autoimmunity and autoinflammatory pathways to AD progression, these parallel processes may explain the nonlinear association
between Aβ burden and cognitive impairment.
Similarly, AD2 can demonstrate, but does not fully explain, the precise duration of disease stages decades before symptoms, or the heterogeneity
of clinical manifestations of AD between people.

AD2 highlights the importance of future studies to better characterize the clear biomolecular basis of the effects of Aβ on the immunomodulation of the
TREM2-GAG-NLRP3 network from an autoinflammatory perspective.
AD2 also emphasizes the need to explore the mechanistic implications of the antimicrobial-immunomodulatory duality of Aβ, particularly when it interprets other disease mechanism recommendations, such as synaptic toxicity and tau lesions, to achieve a broadly covered AD model that unifies multiple different theories into a single comprehensive explanation
。 AD2 emphasizes that future research requires rigorous evaluation of L-tryptophan and L-arginine metabolism, not only from the neurochemical basis of AD, but also to identify new diagnostic and therapeutic approaches, utilizing traditional (small molecule drugs, whether repurposed or designed from scratch) and non-traditional (microbiome manipulation) strategies, while accepting the notion that "endogenous anti-AD molecules" (similar to the use of steroids to treat adaptive autoimmune diseases) may constitute future therapeutic directions; In doing so, AD2 emphasizes that an eventual cure may require reasonable multiple drug treatments, rather than a single "panacea.
"
In addition, AD2 emphasizes the need to continually critically evaluate the ability of widely used AD transgenic mouse models (as well as other evaluative in vitro and in vivo bioassays) to accurately generalize AD in the AD2 model environment—unreasonable belief that flawed analyses may lead to abandonment of potentially effective treatments
.
Finally, the mechanism details of AD2 should be used to improve risk reduction methods
.

More specifically, AD2 also points to critical next steps in
AD2 model validation.
The bottom line of AD2 is that Aβ is a cytokine-like immune peptide that cannot distinguish between bacteria and neurons, leading to a specific but erroneous attack on itself—defining AD as an innate autoimmune response
.
To verify this critical assertion, the key next steps are as follows
.
Step 1: Immunohistochemistry studies verify that Aβ has the properties of cytokines (or related immunopeptides), where cytokines are a broad structurally diverse class of secreted cell signaling peptides that facilitate interaction and communication between immunocompetent cells, synchronize immune system responses, affect the release and receptor binding of other cytokines, and stimulate cells to move towards sites of inflammation, infection, and trauma, thereby acting as messenger molecules for both innate and adaptive immune processes; and verify that immune-triggered events, such as infection or trauma, stimulate the biosynthesis and release of Aβ so it functions
as a cytokine.
Step 2: Verify by electrophysiological studies that Aβ aggressively attacks bacteria and neurons by similar mechanisms (correspondingly, other AMP/immunopeptides [e.
g.
, LL-37] attack neurons and bacteria in a similar manner) and verify that the attacked neurons release the Aβ-GM1 complex to spread to adjacent neurons, triggering the sustained release
of additional Aβ.

By treating Aβ as a cytokine and AD as an innate autoimmune/autoinflammatory disease (rather than just a disordered protein misfolding), AD2 provides a new molecular paradigm to explain the precise cascade of biochemical events that ultimately lead to AD; In turn, this detailed biomolecular understanding will not only help enhance the understanding of disease pathogenesis and risk factors, but will also improve diagnosis and treatment
by identifying unknown known targets, new targets, unevaluated known drugs, and new drugs as potential disease modifiers.

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