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disease (AD) is one of the most well-known diseases
that follow this process.
AD was first reported by a German physician, Alois
Alzheimer in 1907, and is the most common subtype
of dementia and a neurodegenerative disease charac-
terized by the death of nerve cells in the cerebralcortex. The disease affects 25 million people worldwide
in 2000 and it is expected to increase to 114 million by
2050 (Wimo et al., 2003). AD affects one in three in-
dividual over the age of 85 years (Hebert et al., 2003).
AD in the early stages presents with a decline in
cognitive function, particularly short term memory.
As the disease progresses, patients complain of pro-
blems in reading, speaking and logical thinking, and
long-term memory is also impaired. In the later stages
of the disease, language deficits, depression, aggres-
sion, agitation and psychosis are commonly observed
in AD patients. As the disease progresses further, pa-tients eventually depends for total care from caregivers.
ALZHEIMERS DISEASE (AD)
AD has two characteristic pathologic hallmarks,
senile plaques (SPs) and neurofibrillary tangles (NFTs).
SPs are an extracellular accumulation of beta-amyloid
(A) surrounded by dystrophic neurites and microglia.
A originates from the proteolysis of the amyloid
precursor protein (APP) through sequential cleavage
by beta-site amyloid precursor protein-cleaving enzyme
1 (BACE-1, -secretase) and -secretase (Selko, 1997).The excessive production of Aor decreased Aclear-
ance leads to the accumulation and aggregation of A,
which is toxic to cells (Selkoe, 2001; Tanzi and Bertram,
2005). High amounts of this toxic Ais believed to be
a major cause of AD pathology (amyloid hypothesis)
(Kawahara and Kuroda, 2000). Genetic mutations at
the cleavage sites inAPP was the etiology behind the
familial Alzheimers disease found in a Swedish family.
This family showed early disease progression to as
early as 30 years. The cause of AD in this Swedish
family is implicated for the increased production of A
and the aggregationof Ato oligomers. Soluble, mono-meric Aself-aggregates into multiple forms including
oligomers (2 to 6 peptides), fibrils and -pleated sheets
(insoluble fibers). Among them, oligomers of A are
the most neurotoxic (Walsh and Selkoe, 2007). The
fact that the severity of cognitive impairment correlates
well with the levels of Aoligomers, while the total A
burden in AD patients does not, support the impor-
tance of Aoligomers in the AD pathology (Lue et al.,
1999). Aggregated Aoligomers stimulate an array of
biological signaling pathways by direct or indirect
interactions with neuronal membranes and cause
oxidative stress and inflammatory responses, which
leads to an impairment of the neuronal synapses and
dendrites (Heneka and O'Banion, 2007; Roberson and
Mucke, 2006). The accumulation of A eventually
causes the pathological characteristics of AD, includ-
ing neuronal loss and disintegration of the neuralcircuits.
On the other hand, the presence of neurofibrillar
tangles (NFTs) is another pathological hallmark of
AD. NFTs are the intraneuronal accumulation of pair-
ed helical filaments (PHFs) with hyperphosphorylated
tau being the major protein subunits of PHFs (Kosik
et al., 1986; Wood et al., 1986; Kondo et al., 1988).
Normally, the tau protein is abundant in neurons of
the CNS. The tau protein promotes the assembly of
tubulin to microtubules, and stabilizes microtubules
and vesicle transport. However, hyperphosphorylation
of tau makes it insoluble, and drastically decreasesthe affinity in microtubules. Therefore, instead of pro-
moting microtubule assembly, it aggregates itself to
PHFs (Iqbal et al., 2009). In contrast to A, there is no
genetic mutation of tau in AD, but the degree of cogni-
tive decline in AD patients correlates significantly
with the levels of phosphorylated tau and total tau in
the cerebrospinal fluid (Wallin et al., 2006). In addi-
tion, experimental evidence indicates that Aalso acts
by inducing an increased phosphorylation of tau at the
disease-relevant sites (Busciglio et al., 1995; Greenberg
and Kosik, 1995; Takashima et al., 1998; Zheng et al.,
2002), and can cause tau aggregation into PHF-likefilaments (Rank et al., 2002). A-Induced neurodegen-
eration in cultured neurons or A-induced cognitive
deterioration in mice mandates phosphorylation of the
tau protein (Rapoport et al., 2002; Roberson et al.,
2007). The hyperphosphorylated tau-induced neuro-
toxicity occurs downstream of Aand tau phosphory-
lation is considered as the limiting factor in A-
induced cell death (Leschik et al., 2007). This suggests
that tau phosphorylation plays a key role in the
disease progression of AD induced by A.
THERAPEUTIC AGENTS AVAILABLE FORAD
The agents used in pharmacotherapy of AD are the
acetylcholinesterase (AChE) inhibitors, which were
the first approved medication for the treatment of AD.
Postmortem studies of the brains in AD patients re-
vealed lower levels of the neurotransmitter, acetylcho-
line (ACh) and the enzyme choline acetyl transferase
(ChAT). A shortage of ACh in the brain shows a strong
correlation with the deteriorated cognitive function in
AD patients (Francis et al., 1999). To increase the
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Potential Therapeutic Agents against Alzheimer's Disease from Natural Sources 1591
neurotransmitter ACh in the synapses, synthetic
compounds that inhibit AChE, an enzyme responsible
for the breakdown of ACh in the nerve terminals, was
introduced in clinics (Table I). Tacrine was the first
U.S. Food and Drug Administration (FDA)-approved
AChE inhibitor for the treatment of cognitive loss in
AD patients since 1993. Since then, several otherAChE inhibitors have been approved by the US FDA
including donepezil in 1996, rivastigmine in 2000, and
galantamine in 2001 (van Marum, 2008). AChE in-
hibitors can help alleviate the cognitive symptoms of
AD patients, but they do not halt or delay the disease
progression. In addition, they are accompanied with
unwanted side effects such as nausea, diarrhea and
hepatotoxicity, and have a limited utility in AD
patients due to their poor oral bioavailability (Lleo et
al., 2006).
In addition, cholinergic and nicotinic agonists have
been proposed to increase ACh release in nerve ter-minals but these are not as effective as AChE inhibitors
as far as therapeutic effectiveness is concerned.
More recently glutamate-mediated neurotoxicity
was proposed as a possible etiology and therapeutic
target have been directed towards this pathway in the
treatment of AD. Memantine, a N-methyl-D-aspartate
(NMDA) receptor antagonist, has been approved for
the treatment in AD from FDA in 2004 (Lipton, 2007).
Memantine exerts its neuroprotective effect at least in
part though the inhibition of excitotoxicity, which
leads to neuronal injury or death though over-activa-
tion of the NMDA receptors by excessive exposure toneurotransmitter glutamate (Sonkusare et al., 2005).
It was reported that memantine improved the lan-
guage function and overall cognitive abilities signifi-
cantly in moderate to severe AD patients (Ferris et al.,
2009; Mecocci et al., 2009).
NATURAL PRODUCTS AS POTENTIALTHERAPEUTIC AGENTS TO TREAT AD
Recent advancement in pharmacology is based on
the development of concepts such as genomics, pro-
teomics and chemical genomics, which allowed the
discovery of new molecular drug-targets. In addition,
achievement of chemical diversity by combinatorial
chemistry also leads to the discovery of a range of
molecular targets to screen for potential therapeutic
agents through high-throughput screens (Newman
and Cragg, 2007). Despite the advances in extractionand isolation techniques practiced in natural product
chemistry, there are few systemic approaches to be
followed during the discovery of biologically active
natural products. Furthermore, 63% of the low molec-
ular drugs developed from 1981 to 2006 are natural pro-
ducts or natural product-derived compounds (Newman
and Cragg, 2007). These reports suggest that natural
products have strong potential to develop biological
active compounds with anti-AD activity, but these
natural products have attracted relatively little at-
tention as a potentially valuable resource for drug
discovery against AD. Only some natural sources suchas Ginkgo bilobaL. (Ginkgoaceae) and Huperzia serrata
Trevis (Pteridophyta) have been studied extensively
as natural therapeutic agents to treat AD patients.
Therefore, it was envisioned that natural products
may produce biologically active compounds against
AD.
Modulators of -, - and -secretasesA is considered as a major cause of AD. A is a
proteolytic product of APP by - and -secretases. APP
is an evolutionary conserved type 1 transmembrane
glycoprotein (Rosen et al., 1989). Although the precisefunction of APP is not known, it is believed that APP
is involved in the development of CNS and stress or
injury response (Panegyres, 2001). APP undergoes
proteolytic processing through either non-amyloido-
genic or amyloidogenic pathways (Nunan and Small,
2000). During the non-amyloidogenic pathway, APP is
cleaved by membrane-bound enzyme -secretase
within its A domain. This results in the release of
extracellular secretion of soluble -secretase-cleaved
sAPP fragments and the production of a short
membrane bound COOH-terminal fragment, -CTF
Table I.AChE inhibitors approved by US FDA for the treatment of AD
General name Brand name Side effects
Tacrine CognexPrimarily hepatotoxicity.In addition, nausea, indigestion, vomiting, diarrhea, abdominal pain, skin rash
Donepezil Aricept Headache, generalized pain, fatigue, dizziness, nausea, vomiting, diarrhea, anorexia,weight loss, muscle cramping, insomnia
Rivastigmine Exelon Weight loss, anorexia, dizziness
Galantamine Razadyne Nausea, vomiting, diarrhea, anorexia, weight loss, vascular disease (heart attack, stroke)
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or C83, which precludes the formation of A(Esch et
al., 1990) (Fig. 1). Subsequent cleavage of C83 by -
secretase induces the extracellular release of a 3-kDa
peptide p3, whereas APP intracellular domain (AICD)
is secreted into the cytoplasm (Cao and Sudhof, 2004).
Therefore, any substances that can enhance the acti-vity of -secretase can reduce the production of neuro-
toxic Aand should be beneficial for the treatment of
AD.
In the amyloidogenic pathway, APP undergoes se-
quential proteolytic processing by - and -secretases.
-Secretase (BACE1) cleaves APP at the N-terminal
part of the Adomain (Yan et al., 1999) (Fig. 1). This
process releases extracellular sAPP fragments and
retains the membrane bound -C-Terminal Fragment
(-CTF) or C99 fragment. Subsequent cleavage of -
CTF (C99) by -secretase at the C-terminal part of the
A domain releases APP in the intracellular domain
(AICD) and secretes A to the extracellular spaces
(Seubert et al., 1993). -Secretase is a membrane pro-
tein complex composed of presenilin (PS), nicastrin,
Aph-1 and Pen-2. PS1 and PS2 are essential for -secretase-induced proteolytic cleavage of APP, and
mutations in PSs enhance the proteolytic processing
of APP by -secretase thereby increasing the release
of A in mice (Selkoe, 1998). Furthermore, families
with mutations in PSs are associated with Familial
Alzheimers Disease, which has an onset as early as
their 30s (St George-Hyslop and Petit, 2005). There-
fore, any agents that could modulate the - and -
secretases activities can suppress the cleavage of APP
and subsequently the secretion of Awould be desirable.
Fig. 1.Proteolytic processing of Amyloid Precursor Protein (APP) to beta-amyloid (A). The cleavage of APP by -secretaseinitiates the proteolytic process of APP in the center of the A domain and precludes the generation of A. This non-amyloidogenic process releases extracellular p3 proteins and the amyloid intracellular domain (AICD). Meanwhile, thecleavage of APP by -secretase leads to the release of extracellular secretion of soluble APP-(sAPP) and membrane-boundC99 fragment. The sequential cleavage of C99 fragment by -secretase generates AICD and A. The generated soluble andmonomeric Aundergoes self-aggregation and oligomers of Aare the most toxic to neurons. Thus, the modulation of -, -and -secretases activities, and Aaggregation by any substances including natural products would be desirable for AD. Forexample, Ginkgo bilobaextracts (EGb761) enhance the activity of -secretase (A: -secretase enhancer). Catechins in greentea and resveratrol derivatives inhibit the activity of -secretase (B: -secretase inhibitors), whereas DAPT and LY411375inhibit that of -secretase (C: -secretase inhibitor). In addition, curcuminoids such as curcumin and demethoxycurcumininhibit the aggregation of Ainto oligomers (D: Aaggregation inhibitor).
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Potential Therapeutic Agents against Alzheimer's Disease from Natural Sources 1593
-Secretase enhancerSome natural products have been reported to en-
hance the non-amyloidogenic pathway. Garlic (Allium
sativumL., Alliaceae) has been used for both culinary
and medicinal purposes since ancient times. Garlic
was reported to be beneficial to prevent heart diseaseincluding atherosclerosis, high cholesterol and high
blood pressure and cancer. Garlic also reduces platelet
aggregation, hyperlipidemia and blood sugar levels
(Borek, 2006). In addition, Chauhan reported the
beneficial effects of a garlic extract on AD (Chauhan,
2006). According to this report, garlic extracts, aged at
room temperature for 20 months after extracting slic-
ed fresh garlic with 20% aqueous ethanol, increased
the -secretase-cleaved sAPP fragments by 58.9%.
Aged garlic extracts also decreased the levels of soluble
and fibrillar A and A-immunoreactive plaques by
2.3-3 and 1.67 fold, respectively, in an AlzheimersSwedish double mutant mouse model (Tg2576 mice).
Among the major water-soluble and lipid-soluble con-
stituents of the aged garlic extract, diallyl-disulfide
(DADS, 1) and S-allyl-cysteine (SAC, 2) (Fig. 2) showed
positive effects in reducing the levels of Aproduction,
but the degree of inhibition was slightly lower than
aged garlic extracts, probably due to the synergistic
effect of other multi-potent phytochemicals present in
aged garlic extracts. In addition, aged garlic extracts
ameliorated early cognitive impairment and prevent-
ed the aggravation of memory tasks in Tg2576 mice,
which might be correlated with the Mild Cognitive Im-
pairment (MCI) stage of AD (Chauhan and Sandoval,
2007).
Epigallocatechin-3-gallate (EGCG, 3) (Fig. 2), the
main phenolic constituent of green tea, reduces the
cleavage of APP and the production of A in murine
neuroblastoma cells (N2a) transfected with thehuman Swedish mutant APP. EGCG also inhibits the
generation of A in primary neurons cultured from
Tg2576 mice by promoting -secretase proteolytic pro-
cess, thereby increasing the production of -secretase-
cleaved sAPP fragments (Rezai-Zadeh et al., 2005).
Treatment of PS2 mutant AD mice with EGCG en-
hanced the activity of -secretase and reduced the
activities of - and -secretases, subsequently decreas-
ing the levels of A, which was accompanied by an
improved memory function (Lee et al., 2009). These
effects have been suggested to be mediated via the
ERK and NF-B pathways in mice. The oral or in-traperitoneal administration of EGCG to Tg2576 mice
improved the working memory. The beneficial effect of
EGCG in Tg2576 mice was accompanied by a signifi-
cant decrease in the soluble and insoluble forms of the
Aand the Aburden in the brain (Rezai-Zadeh et al.,
2008). Long-term oral administration of A improved
the spatial learning and memory and prevented the
decrease in the proteins involved in the synaptic func-
tion and synaptic structure including brain-derived
neurotrophic factor (BDNF) and post-synaptic density
protein-95 (PSD95) (Li et al., 2009). Furthermore,
EGCG and fish oil, which consists of omega-3 fattyacids such as eicosapentaenoic acid (EPA) and docosa-
hexaenoic acid (DHA), significantly enhance the acti-
vity of -secretase and the production of -secretase-
cleaved sAPPfragments in N2a cells. The co-admin-
istration of EGCG and fish oil has synergistic effects
on the decrease in AD-like pathology in Tg2576 mice,
suggesting the beneficial effects of EGCG and fish oil
in the treatment of AD (Giunta et al., 2010).
Omega-3 fatty acids, such as DHA (4) (Fig. 2) and
EPA are also known to reduce the risk of AD in epi-
demiological studies (Kalmijn et al., 2004; Morris et
al., 2003). In addition, DHA enhanced APP processingby increasing the -secretase activity to -secretase-
cleaved sAPPfragments and reduced Aproduction.
Hence, DHA reduces the overall plaque burden in
Tg2576 mice (Lim et al., 2005). DHA is a polyunsat-
urated omega-3 fatty acid found in fish. The low levels
of DHA in the blood have been linked to cognitive
impairment, and changes in learning behavior in rats
(Catalan et al., 2002; Ikemoto et al., 2001). A defici-
ency of DHA in plasma has been linked to cognitive
decline in AD patients (Yurko-Mauro, 2010). In parti-
cular, a deficiency of DHA in the brain membranes ofFig. 2.Structures of -secretase modulators
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1594 S.-Y. Park
AD patients suggests the loss of protection from the
free-radical mediated lipid peroxidation of the mem-
branes (Montine et al., 2002).
Cryptotanshinone (5) (Fig. 2) is a natural product-
derived -secretase enhancer, a diterpene isolated
from the roots of Salvia miltiorrhizaBunge (Lamina-ceae). S. miltiorrhiza has been used as traditional
Chinese medicine to prevent cardiac diseases, arthritis
and other inflammatory diseases (Gao et al., 1979).
Recently, cryptotanshinone was reported to improve
the learning and memory, and cognitive deficits in
APP/PS1 mice (Kim et al., 2007), and the main mo-
lecular mechanisms responsible for the beneficial
effects of cryptotanshinone on the AD-like pathology
was the reduced production of A and increased -
secretase-cleaved sAPP fragments through the en-
hanced -secretase activity (Mei et al., 2009), sug-
gesting that cryptotanshinone has good therapeuticpotential for AD.
Ginkgo biloba extracts (EGb761) (Ginkgoaceae) are
another class of well-known natural product in clinical
studies for AD. EGb761 was reported to enhance the
cognitive function in AD, but the precise mechanism
of its pharmacological effects is unclear. However, it
was reported that EGb761 increased the release of -
secretase-cleaved sAPP fragments, but the expres-
sion of -secretase was unaffected by EGb761, sug-
gesting that EGb761 increases the APP metabolism to
produce sAPP fragments by activating the non-
amyloidogenic pathway (Colciaghi et al., 2004).L-3-n-Butylphthalide isolated fromApium graveolens
L. (Apiaceae) was reported to be neuroprotective in
cultured neuronal cells, in ischemic and A-infused
animal models (Peng et al., 2008; Peng et al., 2009).
Recently, L-3-n-butylphthalide was reported to display
beneficial effects on learning and memory in triple-
transgenic AD mice. L-3-n-Butylphthalide significantly
improved the cognitive deficits, which was accompani-
ed by a reduced Aburden (Peng et al., 2009) in the
brain. The decrease in the Aburden might be related
to the enhancement of -secretase-cleaved-APPs
fragment formation and the activity of -secretaseactivity, which activates a non-amyloidogenic pathway
in APP processing and excludes the formation of A
(Peng et al., 2010).
-Secretase inhibitorsBased on efforts to discover compounds that modu-
late the production of A, some natural products have
been reported to exert its neuroprotection by inhibit-
ing the amyloidogenic pathway. For example, five -
secretase inhibitors were isolated from the root ex-
tract of Angelica dahurica (Fisch.) Benth. Et Hooker
(Umbelliferae), which is a traditional folk remedy used
in Korea to treat headache, bleeding and menstrual
discomfort. -Secretase inhibitors isolated from A.
dahuricaroots were types of furanocoumarins (Fig. 3)
including isoimperatorin (6), imperatorin (7), (+)-oxy-
peucedanin (8), (+)-bakangelicol (9) and (+)-byakan-gelicin (10) (Marumoto and Miyazawa, 2010). Owing
to their low molecular weight, these compounds can
easily cross the blood-brain-barrier (BBB) and reach
the brain.
Protoberberine alkaloids from Coptis chinensis Franch.
(Ranunculaceae), epiberberine (11) and groenlandi-
cine (12) (Fig. 3), exert significant inhibitory activity
against -secretase with IC50values of 8.55 and 19.68
M (Jung et al., 2009). In particular, groenlandicine
not only decreases the production of Aby inhibiting
-secretase, but can also have potent inhibitory effects
on AChE and reactive oxygen species (ROS).In addition, an extract of Polygala tenuifoliaWild.
(Polygalaceae) is commonly used as a traditional
Chinese medicine to treat memory loss (Lv et al.,
2009). An extract of P. tenuifolia was reported to
alleviate scopolamine-induced cognitive impairment
in rats (Park et al., 2002). Efforts to determine the
major constituents responsible for the beneficial
effects on memory loss allowed the isolation of the
final product, tenuifolin (Lv et al., 2009). Tenuigenin
is another active constituent in the root extract of P.
tenuifolia (Jia et al., 2004). Both compounds have
beneficial effects on AD by reducing the A burdenthat acts by inhibiting the -secretase activity thereby
decreasing the Aproduction.
Recently resveratrol derivatives were reported to
reduce the activity of -secretase, in spite of resvera-
trol itself does not demonstrate any related activity
(Fig. 3). A new resveratrol dimer, (+)-vitisinol E and
four known resveratrol oligomers, (+)-epsilon-viniferin
(13), (+)-ampelopsin A (14), (+)-vitisin A (15) and (-)-
vitisin B (16), isolated from Vitis viniferaL. (Vitaceae),
the common grape vine, exhibited strong inhibition on
the -secretase activity in a dose-dependent manner
(Choi et al., 2009). In addition, trans/cis-resveratrolmixture, oxyresveratrol (17), cis-scirpusin A (18) and
veraphenol (19), which were isolated from the rhizomes
of Smilax chinaL. (Smilaceae), showed potent inhibitory
activity against -secretase (Jeon et al., 2007).
Isoflavones including neocorylin, bakuchiol, bava-
chromene, isobavachromene, bavachalcone, 7,8-dihydro-
8-(4-hydrophenyl)-2,2-dimethyl-2-H,6-H-[1,2-B:5,4-B']
dipyran-6-one, isolated from Psoralea corylifolia L.
(Fabaceae) were also strong inhibitors of -secretase
activity (Choi et al., 2008b). In addition, luteolin and
rosmarinic acid was identified as the active principles
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Potential Therapeutic Agents against Alzheimer's Disease from Natural Sources 1595
of -secretase inhibition from Perilla frutescens var.
acuta (Laminaceae) (Choi et al., 2008a).
Catechins primarily included in green tea are fla-
vonoids that exert a range of pharmacological activi-
Fig. 3.Structures of -secretase inhibitors
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1596 S.-Y. Park
ties. In particular, the neuroprotective effects with
strong antioxidant and anti-inflammatory properties
make catechins attractive compounds (Zaveri, 2006).
Furthermore, an improvement in learning and memory
by the catechins in green tea suggest promising roles
in treating neurodegenerative disorders (Lin et al.,2007). (-)-Epicatechin gallate (20), (-)-epigallocatechin
gallate (21), (-)-gallocatechin gallate (22), (-)-catechin
gallate (23), (-)-gallocatechin (24), and (-)-epigallo-
catechin (25) (Fig. 3) inhibited -secretase with IC50