Can We Treat Neuroinflammation in Alzheimer's Disease?

TL;DR: This review highlights the importance of a broad multimodal approach to treat successfully the neuroinflammation underlying AD and introduces new players underlying neuro inflammation in AD: the activity of the endocannabinoid system and the intestinal microbiota as neuroprotectors.

Abstract: Alzheimer's disease (AD), considered the most common type of dementia, is characterized by a progressive loss of memory, visuospatial, language and complex cognitive abilities. In addition, patients often show comorbid depression and aggressiveness. Aging is the major factor contributing to AD; however, the initial cause that triggers the disease is yet unknown. Scientific evidence demonstrates that AD, especially the late onset of AD, is not the result of a single event, but rather it appears because of a combination of risk elements with the lack of protective ones. A major risk factor underlying the disease is neuroinflammation, which can be activated by different situations, including chronic pathogenic infections, prolonged stress and metabolic syndrome. Consequently, many therapeutic strategies against AD have been designed to reduce neuro-inflammation, with very promising results improving cognitive function in preclinical models of the disease. The literature is massive; thus, in this review we will revise the translational evidence of these early strategies focusing in anti-diabetic and anti-inflammatory molecules and discuss their therapeutic application in humans. Furthermore, we review the preclinical and clinical data of nutraceutical application against AD symptoms. Finally, we introduce new players underlying neuroinflammation in AD: the activity of the endocannabinoid system and the intestinal microbiota as neuroprotectors. This review highlights the importance of a broad multimodal approach to treat successfully the neuroinflammation underlying AD.

Summary

1. Introduction

• Alzheimer's disease (AD) is the most common type of dementia, and is characterized by a progressive loss of memory, visuospatial and complex cognitive abilities, such as language and reasoning, which ultimately lead to a total inability to perform any type of daily activity [1, 2] .
• Oftentimes, the patient presents comorbid psychopathologies, including depression, psychosis, anxiety, aggressive and antisocial behavior.
• Histologically, AD has been traditionally characterized by the appearance of neurofibrillary tangles (NFTs) and amyloid plaques [3] .
• An imbalance between the formation and clearance of Aβ peptides results in their aggregation and accumulation in amyloid plaques [8] .

2. Neuroinflammation in AD

• Neuroinflammation is a process regulated by brain resident macrophages, the microglia cells, which are required to recognize and eliminate any toxic component in the central nervous system (CNS) (for a review, see [16] ).
• The activation of this system, the so-called inflammasome, initiates the inflammatory cascade, which results in the secretion of several pro-inflammatory cytokines, such as tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ) and interleukins 1β, 6 and 18 (IL-1β, IL-6 and IL-18, respectively).
• Under pathological conditions, microglia cells do not go back to their resting state, thus causing a chronic inflammation process, with the overproduction of pro-inflammatory cytokines and reduction of neuroprotective factors that in sustained situations become highly toxic, leading to neurodegeneration [20] .
• Naproxen, a non-selective COX inhibitor was administered (220 mg/twice day for two years) to 195 pre-symptomatic AD subjects (aged 55+) with a familial history of AD.
• Furthermore, the specific TNF-α inhibitor, Etanercept, was evaluated in a small group of 41 AD patients (55+) with mild to severe AD (SMMSE score between 10 and 27), to test its anti-inflammatory effect and subsequent improvement of cognitive function.

3. Targeting Insulin Resistance to Treat AD

• Several situations can bring about insulin resistance: metabolic syndrome caused by high fat diet, sedentarism, obesity, genetic predisposition and neuroinflammation [39] .
• Most importantly, these data support that proper stratification by disease stage, Apoe4 carrier status and different types of insulin must be considered for a better therapeutic effect [42] .
• These strategies include the activation of adenosine monophosphate (AMP)-activated protein kinase (AMPK).
• Indeed, Lactoferrin antioxidant function is highly dependent on its iron binding capacity [49] .
• In an 18 months clinical trial in dyslipidemic patients, although artovastatin effectively corrected dyslipidemia and inflammatory markers, cognitive function was not evaluated in the study [65] ; furthermore, a randomized clinical trial demonstrated no beneficial effects of artovastatin treatment on AD patients' symptoms [66] .

4. Nutraceuticals as a Treatment of AD

• In the last years, several molecules isolated from plants, also known as nutraceuticals, have been proposed as useful tools for ameliorating cognitive functions and reducing neuroinflammation in animal models of AD.
• There is still no clear evidence of its application to AD patients.
• One study showed improvement in cognitive function when administered to older population compared to control group [72] .
• Another nutraceutical commonly used as a herbal medicine and food supplement given its anti-inflammatory properties is ginsenoside (ginseng saponin) [92] .
• In addition, the authors have demonstrated that ABA can improve memory in an animal model of AD, reducing neuroinflammatory markers and restoring insulin-mediating molecule expression [101] [102] [103] .

5. Targeting the Endocannabinoid System in Preclinical Models of AD

• The endocannabinoid system (ECS) is a lipid-based signaling mechanism involved in the control of neuronal and brain immune function, acting as a natural defense mechanism against pathological conditions [104] .
• Data regarding the participation of the CB1 in AD are somewhat conflicting [108] .
• In contrast, cerebral CB2 expression is sparse under normal conditions, but after specific insults (i.e., neuroinflammation), its expression augments in neurons, M1 pro-inflammatory microglia and astrocytes.
• It had no effect on Neuropsychiatric Inventory (NPI).
• To their knowledge, the potential therapeutic effect of CB2 have not yet been tested in AD patients, this is now considered a promising therapeutic target in AD, given their participation in inflammatory regulation and also given the crosstalk between acetylcholine transmission and endocannabinoid function that has been revealed recently [122] .

6. Gut Microbiota, Neuroinflammation and AD

• The microbiota of the gastrointestinal tract is becoming increasingly relevant in the study of neuroinflammatory diseases, such as AD.
• A growing body of evidence supports that the GM maintains a close relationship with the activity of the CNS, though the microbiota-gut-brain axis.
• In addition, LPS and amyloid peptides stimulate the epithelium immune cells triggering the innate immune response [140] .
• It is well known that the food habits are a determining factor in the composition of the microbiota.
• It is safe to assume that the consumption of a balanced diet high in fiber and low in fat and the use of pro/prebiotics will contribute to reduce neuroinflammation in AD, and therefore, aid in ameliorating its neurological and neuropsychiatric symptoms.

7. Conclusions

• Neuroinflammation and insulin resistance are considered major neuropathological events underlying the onset and progression of AD; therefore, multiple strategies that target these processes have been developed to effectively treat this disease.
• A recent metanalysis has reviewed the results of dietary supplementation in clinical trials against AD.
• Finally, a multidomain intervention with exercise and diet are the main strategies that showed very promising results in preventing cognitive decline in 2654 people at risk [158] (see Table 5 ).
• Taking together all clinical studies revised, the authors conclude that strategies targeting neuroinflammation together with insulin resistance have, finally, demonstrated to be a promising therapeutic potential in AD, especially at early stages.
• Many molecules have produced inconclusive results, and other methods, such as promoting neuroprotection via CB2 boosting or restoring GM, are still at the preclinical stage.

Figures

Figure 3. Working hypothesis, namely, the role of the ECS, on neuroinflammation in AD. Inflammatory cytokines activate CB2 signaling in microglia, which, in turn, initiates a feedback mechanism to finish this process. Due to the chronic inflammatory process in the AD brain, the CB2 pathway remain active; however, it fails to reduce inflammation.

Figure 1. Amyloid precursor protein (APP) processing. The α and γ secretases are involved in the non-amyloidogenic pathway, whereas the β and the γ secretases are involved in the amyloidogenic pathway, generating the Aβ toxic oligomer.

Table 2. Clinical studies testing anti-inflammatory molecules with potential therapeutic value for Alzheimer disease treatment. Other: other symptoms or biomarkers evaluated. NT (not tested); U (unspecified). Acetate, propionate and butyrate are short-chain fatty acids. ADAS-Cog: Alzheimer disease assessment scale–cognitive; ADFACS: Alzheimer’s Disease Functional Assessment and Change Scale; ADCS-ADL: ADCS Activity of Daily Living; CDR: Clinical Dementia Rating; MMSE: Mini Mental Status Evaluation Test; NPI: Neuropsychiatric Inventory; PAL-WMS-R: Paired-Associate Learning Wechsler Memory Scale.

Table 3. Clinical studies testing nutraceuticals with a potential therapeutic value for Alzheimer’s disease treatment. Other: other symptoms or biomarkers evaluated; NT: not tested; U: unspecified; ADAS-Cog: Alzheimer disease assessment scale–cognitive; ADCS-ADL: ADCS Activity of Daily Living; AVL: Auditory Verbal Learning Test; CCR: Cambridge Contextual Reading Test; CN: Category naming test; COWA: Controlled Oral Word Association Test; DSS: Digit Symbol Substitution Test; MMSE: Mini Mental Status Evaluation Test; MoCA: Montreal Cognitive Assessment; WAIS-R: Wechsler Adults Intelligence Scale.

Table 5. Clinical studies testing a multidomain intervention as a treatment against Alzheimer’s disease. Other: other symptoms or biomarkers evaluated; NT: not tested; NTB; Neuropsychological Test Battery.

Table 4. Clinical studies focused on the regulation of the activity of the gut microbiota as a potential treatment against Alzheimer’s disease. Other: other symptoms or biomarkers evaluated; NT: not tested; CDT: clock Drawing test; MMSE: Mini-Mental Status; RBANS: Repeatable Battery for the Assessment of Neuropsychological Status.

Table 2. Clinical studies testing anti-inflammatory molecules with potential therapeutic value for Alzheimer disease treatment. Other: other symptoms or biomarkers evaluated. NT (not tested); U (unspecified). Acetate, propionate and butyrate are short-chain fatty acids. ADAS-Cog: Alzheimer disease assessment scale–cognitive; ADFACS: Alzheimer’s Disease Functional Assessment and Change Scale; ADCS-ADL: ADCS Activity of Daily Living; CDR: Clinical Dementia Rating; MMSE: Mini Mental Status Evaluation Test; NPI: Neuropsychiatric Inventory; PAL-WMS-R: Paired-Associate Learning Wechsler Memory Scale.

Table 1. Clinical studies testing antioxidant molecules with potential therapeutic value for Alzheimer’s disease treatment. Other: other symptoms or biomarkers evaluated; NT: not tested; U: unspecified; ADAS-Cog: Alzheimer disease assessment scale–cognitive; APS: Alzheimer Progression Score; BADLS: Bristol Activities of Daily Living Scale; CGI-I: Clinical Global Impression-improvement; MMSE: Mini Mental Status Evaluation Test; NPI: Neuropsychiatry Inventory.

Figure 2. Insulin signaling cascade. The scheme shows the negative feedback mechanism that mTORC1 exerts over IRS1/2. Activation of AMPK inhibits mTORC1, thus improving insulin signaling. In pathological situations, insulin resistance reduces Akt activity, leading to higher GSK-3β activity and subsequent Tau hyperphosphorylation, an important hallmark of AD.

Full text

International Journal of
Molecular Sciences
Review
Can We Treat Neuroinflammation in
Alzheimer’s Disease?
Sandra Sánchez-Sarasúa
†
, Iván Fernández-Pérez
†
, Verónica Espinosa-Fernández ,
Ana María Sánchez-Pérez * and Juan Carlos Ledesma *
Neurobiotechnology Group, Department of Medicine, Health Science Faculty, Universitat Jaume I,
12071 Castellón, Spain; sarasuad@uji.es (S.S.-S.); ivfernan@uji.es (I.F.-P.); veronica.espinosa@uji.es (V.E.-F.)
* Correspondence: sanchean@med.uji.es (A.M.S.-P.); ledesma@uji.es (J.C.L.)
† These authors contributed equally to this work.
Received: 3 November 2020; Accepted: 16 November 2020; Published: 19 November 2020

 
Abstract:
Alzheimer’s disease (AD), considered the most common type of dementia, is characterized
by a progressive loss of memory, visuospatial, language and complex cognitive abilities. In addition,
patients often show comorbid depression and aggressiveness. Aging is the major factor contributing
to AD; however, the initial cause that triggers the disease is yet unknown. Scientific evidence
demonstrates that AD, especially the late onset of AD, is not the result of a single event, but rather
it appears because of a combination of risk elements with the lack of protective ones. A major risk
factor underlying the disease is neuroinflammation, which can be activated by different situations,
including chronic pathogenic infections, prolonged stress and metabolic syndrome. Consequently,
many therapeutic strategies against AD have been designed to reduce neuro-inflammation, with very
promising results improving cognitive function in preclinical models of the disease. The literature
is massive; thus, in this review we will revise the translational evidence of these early strategies
focusing in anti-diabetic and anti-inflammatory molecules and discuss their therapeutic application in
humans. Furthermore, we review the preclinical and clinical data of nutraceutical application against
AD symptoms. Finally, we introduce new players underlying neuroinflammation in AD: the activity
of the endocannabinoid system and the intestinal microbiota as neuroprotectors. This review
highlights the importance of a broad multimodal approach to treat successfully the neuroinflammation
underlying AD.
Keywords:
Alzheimer’s disease; neuroinflammation; insulin resistance; nutraceuticals;
endocannabinoid system; gut microbiota
1. Introduction
Alzheimer’s disease (AD) is the most common type of dementia, and is characterized by a
progressive loss of memory, visuospatial and complex cognitive abilities, such as language and
reasoning, which ultimately lead to a total inability to perform any type of daily activity [
1
,
2
].
Oftentimes, the patient presents comorbid psychopathologies, including depression, psychosis, anxiety,
aggressive and antisocial behavior. Histologically, AD has been traditionally characterized by the
appearance of neurofibrillary tangles (NFTs) and amyloid plaques [
3
]. NFTs are the intracellular
aggregation of hyperphosphorylated Tau, a microtubule-associated protein that provides axonal
cytoskeleton stability. Under pathological conditions (e.g., neuroinflammation and insulin resistance),
Tau undergoes hyperphosphorylation, and consequently, conformational changes that reduce its
affinity for microtubules [
3
], leading to neurodegeneration [
4
]. Misfolded Tau can spread via migration
to neighbor healthy neurons, worsening the condition [
4
,
5
]. Senile amyloid plaques are formed by
amyloid
β
(A
β
) peptide accumulation [
6
]. The old amyloid hypothesis to explain AD postulates that
Int. J. Mol. Sci. 2020, 21, 8751; doi:10.3390/ijms21228751 www.mdpi.com/journal/ijms

Int. J. Mol. Sci. 2020, 21, 8751 2 of 23
soluble A
β
oligomers, and A
β
deposits in plaques, together with NFTs are ultimately responsible for
neuronal death.
A
β
-peptide is generated from the amyloid protein precursor (APP) proteolysis. APP is a
transmembrane glycoprotein expressed in a wide variety of cells and located on chromosome 21
(21q21.3, in mammals) and it undergoes proteolysis by secretases in two possible pathways (see Figure 1).
The amyloidogenic pathway starts by the action of the
β
-site APP-cleaving enzyme 1 secretase (BACE 1),
followed by the action of the
γ
-secretase (presenilin). This sequential cleavage releases different lengths
of A
β
peptides (39–42aa), depending on
γ
-secretase action. Long peptides (A
β
42
) are more prone to
aggregation. The non-amyloidogenic pathway starts by the
α
-secretase action followed, as above,
by
γ
-secretase, where no pathological peptides are generated. In healthy conditions, both pathways
would compete in APP proteolysis [
7
], and the clearance of amyloid products is carried out by microglia,
the resident brain macrophages. An imbalance between the formation and clearance of A
β
peptides
results in their aggregation and accumulation in amyloid plaques [8].
Figure 1.
Amyloid precursor protein (APP) processing. The
α
and
γ
secretases are involved in the
non-amyloidogenic pathway, whereas the
β
and the
γ
secretases are involved in the amyloidogenic
pathway, generating the Aβ toxic oligomer.
Soluble A
β
oligomers can also be neurotoxic, since they induce intracellular oxidative stress
and synaptic dysfunction [
9
,
10
] through the aberrant interaction with numerous receptors (NMDA,
AMPA, acetylcholine, insulin, BDNF and receptors for advanced glycosylation end products; for a
review, see [
6
]). Since deposition of A
β
s in amyloid plaques have been observed in other dementias
and in non-demented aged people, it is nowadays only considered a specific AD hallmark if it
is observed in addition to other signs, such as NFTs, neurodegeneration, insulin resistance and
neuroinflammation [
11
,
12
]. Importantly, inflammation and insulin resistance start the pathological
process years before the appearance of AD’s first clinical symptoms [13–15].
2. Neuroinflammation in AD
Neuroinflammation is a process regulated by brain resident macrophages, the microglia cells,
which are required to recognize and eliminate any toxic component in the central nervous system
(CNS) (for a review, see [
16
]). Microglia has a high capacity for mobility, and they can switch between
two different phenotypes, M1 and M2, characterized by a different morphology and cytokine profile.
The M2 phenotype is the resting type that actively monitors the brain in healthy conditions [
17
].
The switch to M1 begins with the recognition of the pathogen-associated molecular patterns (PAMPs)

Int. J. Mol. Sci. 2020, 21, 8751 3 of 23
or the damage-associated molecular patterns (DAMPS) by the pattern recognition receptors (PRRs).
This includes the ‘toll-like receptors’ (TLRs) in microglia membrane (both plasma and endosomal
membrane), cytoplasmic NOD-like receptors (NLR), intracellular retinoic acid-inducible gene-I-like
receptors and transmembrane C-type lectin receptors (for a review, see [
18
]). PAMPS and DAMPS range
from bacterial wall components, the lipopolysaccharides (LPS) and virus capsid proteins, to debris
released by dying cells and A
β
oligomers [
19
]. The activation of this system, the so-called inflammasome,
initiates the inflammatory cascade, which results in the secretion of several pro-inflammatory cytokines,
such as tumor necrosis factor-
α
(TNF-
α
), interferon-
γ
(IFN-
γ
) and interleukins 1
β
, 6 and 18 (IL-1
β
,
IL-6 and IL-18, respectively). Pro-inflammatory cytokines purpose is to orchestrate the neutralization
and elimination of toxic molecules and/or cellular debris. In normal conditions, once the toxic
stimuli have been cleared, microglia swifts to the anti-inflammatory (M1) phenotype and secretes
anti-inflammatory cytokines such as interleukins 4, 10 and 18 (IL-4, IL-10 and IL-18, respectively),
brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF), whose role is to terminate the
innate immune response and contribute to restore the synaptic function. However, under pathological
conditions, microglia cells do not go back to their resting state, thus causing a chronic inflammation
process, with the overproduction of pro-inflammatory cytokines and reduction of neuroprotective
factors that in sustained situations become highly toxic, leading to neurodegeneration [20].
Therefore, the chronic neuroimmune system activation underlies the initiation and progression
in many dementias, and surely, is involved in the late onset of AD [
21
–
23
]. Not only A
β
activates
the microglia [
24
], but also misfolded Tau interaction with microglia triggers inflammation [
25
].
The elimination of the microglial receptor, NLR family pyrin domain containing 3 (NLRP3) has shown
to reduce brain A
β
levels in rodent models of AD [
26
,
27
]; since then, NLRP3 inflammasome has
been deeply studied and characterized in AD [
28
,
29
]. In addition to the neurological symptoms,
neuroinflammation also underlies the psychiatric signs associated with AD, and for that reason,
targeting neuroinflammation has also been proposed to treat those comorbid disturbances [30].
According to the neuroinflammation hypothesis underlying AD, there is a lower incidence of AD
among users of chronic non-steroidal anti-inflammatory molecules (NSAIDs) [
31
,
32
]. NSAIDs inhibit
mostly the cyclooxygenase (COX) activity, which synthesizes prostaglandin (PG) from arachidonic acid.
At least two isoforms have been described, COX-1 and 2. COX-1 is expressed constitutively; in contrast,
COX-2 is induced by inflammation and cellular stress, increasing PG production [
33
]. Anti-inflammatory
compounds, inhibiting COX activity, Naproxen and Celecoxib have been tested in clinical trials against
AD. Naproxen, a non-selective COX inhibitor was administered (220 mg/twice day for two years) to
195 pre-symptomatic AD subjects (aged 55+) with a familial history of AD. The progression of the disease
was evaluated with the Alzheimer’s Progression Score (APS). Naproxen reduced the rate of the APS,
though not significantly [
34
]. Celecoxib, a selective COX-2 inhibitor, was administrated (200 mg/twice
day for 2 years) in 677 pre-symptomatic subjects (70+) with at least one first-degree relative with AD.
No improvement in the cognitive symptoms in the Alzheimer’s Disease Anti-inflammatory Prevention
Trial (ADAPT) in the AD patients compared to the placebo group was found [
35
]. None of these clinical
trials analyzed inflammation biomarkers; therefore, these studies cannot test the neuro-inflammation
hypothesis underlying AD progression. In addition, these clinical data would shift the focus to different
inflammation pathways, other than the COX-PG pathway.
Furthermore, the specific TNF-
α
inhibitor, Etanercept, was evaluated in a small group of
41 AD patients (55+) with mild to severe AD (SMMSE score between 10 and 27), to test its
anti-inflammatory effect and subsequent improvement of cognitive function. The weekly 50 mg
subcutaneous administration was well tolerated; however, after 24 weeks of treatment, Etanercept did
not show significant beneficial effects in cognition, behavior, systemic cytokine levels or global function
compared to the placebo-treated group [
36
]. The failure of this clinical trial involves many factors,
including insulin resistance [
37
]; thus, inhibiting specifically the TNF-
α
action may not be sufficient to
counteract the inflammasome activity, and hence, to effectively prevent disease, perhaps due to the

Int. J. Mol. Sci. 2020, 21, 8751 4 of 23
short period of time of assays,. In Table 1, the clinical studies testing anti-inflammatory molecules with
potential therapeutic value for Alzheimer disease treatment are presented.
Table 1.
Clinical studies testing antioxidant molecules with potential therapeutic value for Alzheimer’s
disease treatment. Other: other symptoms or biomarkers evaluated; NT: not tested; U: unspecified;
ADAS-Cog: Alzheimer disease assessment scale–cognitive; APS: Alzheimer Progression Score; BADLS:
Bristol Activities of Daily Living Scale; CGI-I: Clinical Global Impression-improvement; MMSE: Mini
Mental Status Evaluation Test; NPI: Neuropsychiatry Inventory.
Compound (Dose). Source
Patients
(Years Old)
Study
Design
Inflammatory/AD
Biomarkers
Cognitive Effect Other
Citation
Year
Naproxen
(220 mg/twice daily)
Derived from
propionic acid
195
(>55)
2 years NT = APS progression -
[34]
2020
Celecoxib
(200 mg/twice daily)
Derived from
propionic acid
2356
(70–85)
3 years NT = ADAPT score -
[35]
2015
Etanercept
(50 mg/once weekly
subcutaneous)
U
41
(70–74)
24 weeks
= TNF-α levels
= IL-6 levels
= IL-10 levels
= IL-12p70 levels
= CRP levels
= ADAS-cog score
= BADLS score
= CGI-I
= Cornell Scale score
= MMSE score
= NPI score
-
[36]
2015
3. Targeting Insulin Resistance to Treat AD
The late onset of AD is strongly associated with insulin resistance; in fact, AD has been
often recognized as Type 3 diabetes [
38
]. Several situations can bring about insulin resistance:
metabolic syndrome caused by high fat diet, sedentarism, obesity, genetic predisposition and
neuroinflammation [
39
]. Insulin resistance increases Tau aberrant phosphorylation, the expression of
APP and the formation of A
β
oligomers and its deposition. In addition, insulin resistance augments
oxidative and endoplasmic reticulum stress, mitochondrial dysfunction and pro-inflammatory
cascades [
40
]. Not surprisingly, Type 2 Diabetes mellitus (T2DM) has been associated with cognitive
impairment [41].
For this reason, administration of intranasal (IN) insulin has been considered as a potential
therapeutic strategy against AD. A systematic review on this strategy concluded that whereas IN
insulin administration showed improvement in verbal memory and story recall, it was not effective
on other aspects of cognition. Interestingly, the authors conclude that the treatment is affected
by the Apoe4 isoform, where Apoe4 (–) patients displayed more benefits compared to Apoe4 (+)
patients. This systematic review concluded that current data do not demonstrate that IN insulin can
be used as a treatment for dementia of AD or mild cognitive impairment (MCI), although it is very
safe, not interfering with systemic glucose levels. Most importantly, these data support that proper
stratification by disease stage, Apoe4 carrier status and different types of insulin must be considered
for a better therapeutic effect [42].
In Insulin resistance situations, the administration of insulin is not effective in the long term;
thus, other treatments have been developed instead to enhance the insulin sensitivity, rather than
overload the system with insulin. These strategies include the activation of adenosine monophosphate
(AMP)-activated protein kinase (AMPK). AMPK activation inhibits the mammalian target of rapamycin
(mTOR)/p70 ribosomal S6 kinase (p70S6K) activity [
43
]. The mTOR/p70S6K pathway is activated by
insulin and phosphorylates the insulin receptor substate 1 (IRS1) on serine residues as a negative
feedback loop to reduce insulin signaling [
44
,
45
] (see Figure 2). Interestingly, AMPK activity displays
an anti-inflammatory effect, decreasing inflammatory cells proliferation and their adhesion to the
blood vessel endothelium. AMPK activity also reduces amyloidogenesis, Tau hyperphosphorylation
and the activation of autophagic degradation [
43
]. In agreement with this, a pilot study in non-diabetic
subjects (aged 55–80 years) diagnosed with MCI, Metformin (an AMPK activator) administration,
ameliorated learning, memory and attentional abilities, evaluated by the Paired Associates Learning
(PAL) scale and DMS Percent Correct Simultaneous. Despite the improvement in behavior, no changes

Int. J. Mol. Sci. 2020, 21, 8751 5 of 23
in the cerebrospinal fluid (CSF) of A
β
42
, and total or phosphorylated Tau levels were found [
46
],
further suggesting the idea that the amyloid hypothesis does not accurately explain AD.
Figure 2.
Insulin signaling cascade. The scheme shows the negative feedback mechanism that
mTORC1 exerts over IRS1/2. Activation of AMPK inhibits mTORC1, thus improving insulin signaling.
In pathological situations, insulin resistance reduces Akt activity, leading to higher GSK-3
β
activity
and subsequent Tau hyperphosphorylation, an important hallmark of AD.
Milk-derived proteins have also been proposed as possible antioxidant and anti-inflammatory
compounds, given their capability to reduce insulin resistance. For example, lactoferrin (a multifunctional
iron-binding glycoprotein) administration increases insulin sensitivity in adipose tissue explants from
obese subjects [
47
,
48
]. Indeed, Lactoferrin antioxidant function is highly dependent on its iron binding
capacity [
49
]. In metabolic syndromes, iron accumulation is considered an important factor underlying
insulin resistance and oxidative stress; accordingly, iron-chelators have a positive effect ameliorating the
physiopathology of obesity. Lactoferrin therapeutic potential against AD was demonstrated in a pilot
study with AD patients. Short-term administration of lactoferrin (250 mg/day for three months) reduced
serum oxidative levels and neuroinflammatory markers, and regulated neurotransmitters serum levels
concomitant with improved cognitive performance, compared to control [50].
Moreover, deficiency in micronutrients such as vitamin B12 (critical for mental health [
51
])
has been associated with insulin resistance [
51
–
53
]. Interestingly, combined treatment of folic acid
and vitamin B12 has been shown to improve AD cognitive performance in a randomized trial of
240 patients diagnosed with MCI for 6 months, concomitant with a reduction in serum inflammatory
markers [
54
]. Additionally, Vitamin B12 in combination with anti-psychotic drugs (Risperidone and
Quetiapine) reduced blood levels of the pro-inflammatory cytokines IL-8 and TNF-
α
and augmented
the expression of the anti-inflammatory cytokine TGF-
β
, compared to non-treated AD patients [
55
].
The same medication formulation was tested in psychotic patients for the expression of the Cluster
of Differentiation 68 (CD68), a protein expressed by monocytes and macrophages that has been
shown to correlate positively with psychotic symptoms in AD patients. This treatment reduced
CD68 expression [
56
], and therefore, has been proposed as a good strategy against AD. In addition,
CD68 has been shown to bind and internalize oxidized Low-Density Lipoprotein (oxLDL), a cholesterol
carrier [
57
], suggesting a relationship of CS68 with intracellular lipid accumulation and atherogenesis.
In this line of research, pharmacological treatments used to treat other diseases, such as
hypertension (i.e., calcium channel blockers) [
58
,
59
] or hypercholesterolemia (i.e., statins) [
60
,
61
],
were postulated as therapeutic agents against AD, given their alleged anti-inflammatory and insulin
sensitizing properties. The results from a randomized clinical trial demonstrated that, for instance,
the calcium channel blocker nilvadipine has no beneficial effects in a clinical trial against treating
AD [
62
]. On the other hand, statins’ potential therapeutic effect against Alzheimer seems controversial.
Simvastatin has been shown to improve memory deficits only at higher doses (80 mg/daily for

Frequently asked questions

Q1. What is the role of iron in metabolic syndromes?

In metabolic syndromes, iron accumulation is considered an important factor underlying insulin resistance and oxidative stress; accordingly, iron-chelators have a positive effect ameliorating the physiopathology of obesity. 

Q2. What are the contributions in "Can we treat neuroinflammation in alzheimer’s disease?" ?

The literature is massive ; thus, in this review the authors will revise the translational evidence of these early strategies focusing in anti-diabetic and anti-inflammatory molecules and discuss their therapeutic application in humans. Furthermore, the authors review the preclinical and clinical data of nutraceutical application against AD symptoms. Finally, the authors introduce new players underlying neuroinflammation in AD: the activity of the endocannabinoid system and the intestinal microbiota as neuroprotectors. This review highlights the importance of a broad multimodal approach to treat successfully the neuroinflammation underlying AD. 

Q3. What is the name of the protein that provides axonal stability?

NFTs are the intracellular aggregation of hyperphosphorylated Tau, a microtubule-associated protein that provides axonal cytoskeleton stability. 

Q4. What is the role of resveratrol in the treatment of AD?

Resveratrol (RV), a polyphenol no flavonoid found in fruits, including nuts, berries and grape skin, is a Sirtuin activator, stimulates cell survival and prevents apoptosis, neuroinflammation and oxidative stress [74,75]. 

Q5. What are the main phyla of bacteria found in healthy human GM?

The main phyla of bacteria encountered in healthy human GM are Firmicutes and Bacteroidetes (90%), and the remaining 10% contains Actinobacteria, Proteobacteria, Fusobacteria and Verrucomicrobia [123,124]. 

Q6. What is the purpose of pro-inflammatory cytokines?

Pro-inflammatory cytokines purpose is to orchestrate the neutralization and elimination of toxic molecules and/or cellular debris. 

Q7. What is the investigated nutraceutical to treat AD?

Because of its anti-oxidant and anti-inflammatory properties, polyphenol compounds belong to the most investigated nutraceuticals to treat human pathologies. 

Q8. What is the role of the endocannabinoid system in AD?

The endocannabinoid system (ECS) is a lipid-based signaling mechanism involved in the control of neuronal and brain immune function, acting as a natural defense mechanism against pathological conditions [104]. 

Q9. What is the role of CS68 in the oxidative stress of adi?

In addition, CD68 has been shown to bind and internalize oxidized Low-Density Lipoprotein (oxLDL), a cholesterol carrier [57], suggesting a relationship of CS68 with intracellular lipid accumulation and atherogenesis. 

Q10. What is the reason for the failure of this clinical trial?

The failure of this clinical trial involves many factors, including insulin resistance [37]; thus, inhibiting specifically the TNF-α action may not be sufficient to counteract the inflammasome activity, and hence, to effectively prevent disease, perhaps due to theshort period of time of assays,. 

Q11. What are the gram-negative bacteria that produce lipopolysaccharides?

Bacteroidetes are gram-negative bacteria that generate lipopolysaccharides (LPS), and several species of bacteria such as Escherichia Coli, Salmonella enterica, Salmonella typhimurium, Bacillus subtilis, Mycobacterium tuberculosis and Staphylococcus aureus, which produce amyloid peptides in the gut [138]. 

Q12. How many AD patients were treated with a selective COX-2 inhibitor?

a selective COX-2 inhibitor, was administrated (200 mg/twice day for 2 years) in 677 pre-symptomatic subjects (70+) with at least one first-degree relative with AD. 

Q13. What are the common phytohormones used in the treatment of AD?

Other phytohormones, such as Abscisic acid (ABA), has been demonstrated to have antiglucemic effects in patients with type 2 diabetes [97]. 

Q14. What is the role of vitamin B12 in the treatment of AD?

In this line of research, pharmacological treatments used to treat other diseases, such as hypertension (i.e., calcium channel blockers) [58,59] or hypercholesterolemia (i.e., statins) [60,61], were postulated as therapeutic agents against AD, given their alleged anti-inflammatory and insulin sensitizing properties. 

Q15. What is the main reason why IN insulin is not effective?

A systematic review on this strategy concluded that whereas IN insulin administration showed improvement in verbal memory and story recall, it was not effective on other aspects of cognition. 

Q16. What is the main reason for the lack of clinical studies?

These data conclude that although there is a promising therapeutic evidence in correcting dyslipidemia with statins treatment in AD, more studies are needed to establish their therapeutic applications in AD patients. 

Q17. What is the way to treat AD?

Taken together, all the clinical data presented in this section (Table 2) suggest that targeting insulin resistance is a promising strategy to fight AD; however, longer longitudinal studies and larger cohort studies with stratified patients will provide a better profile of successful treatment. 

Q18. What are the main strategies that have shown promising results in preventing cognitive decline in AD?

a multidomain intervention with exercise and diet are the main strategies that showed very promising results in preventing cognitive decline in 2654 people at risk [158] (see Table 5). 

Q19. How many patients have been tested in a clinical trial?

According to these premises, MD as an intervention strategy against AD has been tested in a six-year clinical trial with over 500 patients, aged 55–80.