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1590 S.-Y. Park

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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-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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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

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