A Mini Review on the Protective Effect of Lignans for the Treatment of Neurodegenerative Disorders

Nature is a rich source of numerous bioactive compounds that are categorized as secondary metabolites. Lignans are group of such compounds, generally called phytoestrogens widely present in many plants and vegetables, grains, seeds, nuts and tea. They have been used as folk medicine for the treatment of several clinical conditions like asthma, cardiovascular diseases, arthrosclerosis, colitis and many more. Structurally, lignans are characterized by two phenylpropane groups attached by a carbon bond. They have been divided in to several types on the basis of structure of their carbon skeleton, the way of cyclization and oxygen incorporation in the skeleton. Lignans from numerous plant species such as Kadsura polysperma, Kadsura ananosma, Schisandra wilsoniana, Schisandra chinensis, Schisandra arisanensis, Manglietiastrum sinicum, Pycnanthus angolensis, Cleistanthus indochinensis, Sargentodoxa cuneata, Tabebuia chrysotricha, Lindera glauca, Tilia amurensis and many more have been found to be beneficial for cancer, hepatitis, microbial and fungal infection. Neurodegenerative Diseases (NDD) represent a class of disorder each of which corresponds to a specific pathological condition while their molecular pathways have been found to be interlinked. Lignans isolated from particular species like Myristica fragrans, Adelostemma gracillimum, Schisandra chinensis, Torreya nucifera, Larrea tridentate, Eucommia ulmoides, Caulis clematidis, Saururus chinensis, Zea mays are helpful for NDDs including Alzheimer’s disease, Parkinson’s disease, glaucoma, amyotrophic lateral sclerosis, Huntington’s disease and Japanese encephalitis. In this review the individual lignans will be summarized in relation to their neuroprotective and cognitive enhancement activities.


Introduction
The word lignans depicting a group of dimerized phenylpropanoids in which the two carbons namely C6-C3 are attached by its central carbon ( Figure 1) was first introduced by Haworth in 1936 [1]. The natural derivatives of lignans are named as neolignans [2]. This polyphenolic phytoestrogen is found in more than 60 families of vascular plants such as rye, barley and fresh fruits and vegetables and predominantly in the hulls of flax and sesame seeds [3]. Neurodegenerative Diseases (NDD) refer to a progressive loss in particular regions of the brain from which there is no recovery. Cells, which are the fundamental units of life hold numerous quality control schemes to spot out and remove dysfunctional toxic cellular components like organelles and proteins. It is a fact that, at steady state, misfolded proteins are continuously formed inside the cell and are thus fixed by several in built mechanisms like chaperones, ubiquitin proteasome system and autophagy. These systems hold the key for a healthy cellular atmosphere, thus any altered cellular condition resulting in expression and aggregation of toxic proteins that overcome these quality control systems eventually result in death of the concerned and neighboring cells in a progressive manner. This phenomenon becomes the basis of NDD like Alzheimer's (AD), Parkinson's (PD), Huntington's Diseases (HD), Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). These NDD possess unique clinical manifestations for each, although the molecular pathways are quite similar. Huge research on these aspects revealed lot of critical information regarding the particular molecules associated with these diseases such as Amyloid-β (Aβ) in AD [4,5], α-synuclein in PD [6], huntingtin protein in HD [7], and transactive response DNAbinding protein 43 (TDP-43) in FTD and ALS [8]. Further it is evidenced that these NDDs arise due to involvement of multiple pathways such as abnormal protein metabolism, unregulated free radical production and metal and pesticide exposure. Several lignans are historically proven useful for many pathological conditions like cancer and NDDs and in this mini review, we have gathered the comprehensive information on those lignans and their sources. Pathology of the various NDDs and the feasibility of lignans for treating various NDDs are also discussed.

Lignans: Source, Chemistry and Pharmacological Properties Source
Lignans, a group of non-flavonoid polyphenols acting as antioxidants and defense molecules against bacterial and fungal pathogens are wide spread in plant kingdom particularly in plant-based foods, including seeds, fruits and vegetables and have recently attracted significant scientific attention due to their wide biological activities. These compounds are created naturally by peroxidases and laccases enzymes which induce structural diversity and bioactivities to them [9,10]. Generally, the lignans content of food does not exceed 0.2 mg/g while some foods like flaxseed [11] and sesame seeds [12,13]  Ocimum Scientific Publishers grains and seed coat of seeds. Among the dietary components, barley, oats, buckwheat, millet, wheat, sesame seeds, rye and flax contain quite high levels of lignans [14]. Legumes and nuts also have considerably good amount of lignans while fruits and vegetables such as grapes, kiwi, oranges, pineapple, asparagus, wine, coffee and even tea cannot be ignored [15][16][17][18]. Animal foods on the other hand are poor in lignans whereas exceptionally very less enterolignans like enterodiol and enterolactone are found in milk products which are produced due to the action of gut inhabiting bacteria [19].

Chemistry and Pharmacological properties of the different sub-groups of lignans
Basically lignans have two phenylpropane units joined by a C-C bond between the central atoms of the respective side chains. Lignans have been divided in to 8 subgroups such as aryltetralins, arylnaphthalene, furans, furofurans, dibenzylbutanes, dibenzylbutyrolactols, dibenzocyclooctadienes and dibenzylbutyrolactones based on the carbon skeleton, cyclization and the way oxygen is incorporated in the skeleton. Lignans occur in nature as glycosylated derivatives while their free forms are very rare. Out of the most common lignans secoisolariciresinol, lariciresinol, pinoresinol, matairesinol and 7-hydroxymatairesinol are found abundantly in nature. This section will elaborate on the different subgroups of lignans and their biological properties.
Dibenzocyclooctadienes: More than 130 dibenzocyclooctadiene lignans have been isolated from two plant genera Schisandra and Kadsura of family Schisandraceae. CD spectroscopy revealed the structures and configurations of 14 new dibenzocyclooctadiene lignans and ananolignans isolated from the seeds of K. ananosma, where two ananolignans showed the most promising in vitro neuroprotection against oxidative stress [20]. More recently, an investigation on S. wilsoniana fruits led to identification of dibenzocyclooctadiene lignans and marlignans whose cytotoxicity and anti-HIV-1 were evaluated and interestingly they displayed EC 50 of 3  [63]. Fruits of Broussonetia papyrifera (Moraceae) afforded [61] chushizisins A, B, C and D out of which compound A, B and D had DPPH radical-scavenging activity. Callislignan B from the leaves of Callistemon lanceolatus exhibited antibacterial activity against S. aureus [64]. Ligraminols from Acorus gramineus (Araceae) showed weak inhibitory activity against A549 proliferation [65]. Acorus gramineus Soland (Araceae) afforded surinamensinols A and B which had antiproliferative activities against several human cancer cell lines [66]. Moderate radical-scavenging activity was observed by neolignans from the roots of Nannoglottis carpesioides (Asteraceae) [67].

Neurodegenerative Disorders and their Pathology Alzheimer Disease (AD) and pathology
AD is a menacing progressive neurodegenerative disorder that is regarded as one of the most serious health problems worldwide. Affected individuals initially face difficulty in remembering newly learned information, suffer from disorientation, mood changes, confusion but later develop more serious memory loss . 05 .
Ocimum Scientific Publishers and behavior changes, suspicions about own relatives, associates and caregivers. AD brain shows clear deposition of Aβ and neurofibrillary tangles. Two major forms of AD include Early-Onset Familial AD (EOFAD) and Late-Onset AD (LOAD). EOFAD which represents less than 5% is rare but highly penetrant which arise due to mutations in different genes which transmit in an autosomal dominant fashion whereas the latter one arises without any familial link [71]. Till now, more than 160 mutations in genes such as Aβ Precursor Protein (APP) on chromosome 21 [72], Presenilin 1 (PSEN1) on chromosome 14 [73], and Presenilin 2 (PSEN2) on chromosome 1 [74] have been reported to cause EOFAD. On contrary LOAD which appears at age 65 or older represents the vast majority of all AD cases. It has been suggested that the ε4 allele of the apolipoprotein E gene on chromosome 19q13 exerts a risk for LOAD while the minor allele, ε2 has some protective effect [75]. Although there is no cure for AD there are drug and non-drug procedures that may manage the symptoms. Cholinesterase inhibitors such as donepezil, revastigmine and galantamine are being used for memory enhancement. A number of herbal remedies are promoted as memory enhancers to delay or prevent AD.
Several hypotheses have been proposed regarding AD pathology such as cholinergic, tau and amyloid hypothesis. The cholinergic hypothesis directs about a sharp decline in the level of Acetylcholine (ACh) which is a major neurotransmitter, due to its hydrolysis by Acetylcholinesterase (AChE) [76]. Amyloid hypothesis proposes the formation and deposition of amyloid beta (Aβ) protein in the brain of AD patients which is produced from Amyloid Precursor Protein (APP), a transmembrane glycoprotein involved in multiple biological processes like protein trafficking, neurogenesis and synaptogenesis and of course evolutionarily conserved [77]. The deposited Aβ also called senile or neuritic plaques are commonly surrounded by microglial cells and reactive astrocytes which are involved in neuroinflammatory cascade. Evidences are growing regarding cross linking of cholinergic and Aβ pathways [78]. Tau is an axonal phosphoprotein that stabilizes microtubule through binding with tubulin and promotes neurite outgrowth [79]. In the neurons, during the early developmental stages, it predominantly occurs in hyperphosphorylated condition but in adults, it is found in dephosphorylated form, which is essential for cytoskeletal integrity [80]. Glycogen synthase kinase-3β (GSK-3β) controls tau phosphorylation and its abnormal hyperphosphorylation makes it inaccessible for proteases and thus binding to tubulin, resulting in structural and functional disruption of synaptic metabolism ( Figure 2) [81]. The degree of cognitive impairment in AD is significantly correlated with the presence of neurofibrillary tangles [82]. A consecutive study . 06 .
Submit Manuscript [83] shows that a protein kinase p38γ phosphorylates tau at the threonine-205 to protect it from Aβ aggregation induced damage and the activity of this enzyme is mostly lost with AD progression. Vascular amyloid deposition or congophilic angiopathy is another pathological characteristic of AD where Aβ accumulates in the leptomeningial walls and within the gray matter of the cerebral cortex. Severe congophilic angiopathy leads to vascular rupture leading to accumulation of blood in the brain tissues causing hemorrhages [84]. Granulovacuolar degeneration and eosinophilic perineuronal lesions or Hirano Bodies are other pathological lesions found in AD which are relatively less understood [85].

Parkinson's Disease (PD) and pathology
PD the second of common neurodegenerative diseases is an extrapyramidal progressive motor dysfunction, affecting the midbrain region called the substantia nigra pars compacta. It affects 1 % of people over 60, 3.4 % over 70, and 4% over 80 years of age [86,87]. Tremor, rigidity and postural stability and bradykinesia are the preliminary symptoms of PD while depression, dementia, sleep abnormalities and autonomic failure become evident during its later stage [88].
The key symptoms of PD are correlated to the deficiency in the neurotransmitter dopamine. Therefore, the current treatment of PD involves drugs such as L-3,4-dihydroxyphenylalanine (L-DOPA) which is a precursor of dopamine, monoamine oxidase inhibitors, and dopamine receptor agonists which facilitate dopaminergic neurotransmission. Unfortunately, none of the currently available therapies can delay PD associated degeneration while constant use of L-DOPA frequently causes psychiatric reactions. Hence most of the ongoing PD related research is to modify the disease course by neuroprotection.
Two prominent pathological hallmarks displayed in postmortem brains of PD patients are the presence of Lewy bodies, aggregated ɑ-synuclein [89] and reactive microgliosis. Mostly 90-95% of PD is sporadic while its rare familial forms involving mutations in a number of genes have also been described. Missense mutations in the ɑ-synuclein gene have been linked to PD [90]. Mutations in the parkin genes lead to mitochondrial dysfunction, oxidative stress, and cell death [91][92][93]. Mutations in the DJ-1 gene makes the cells vulnerable to oxidative stress and leads to early onset autosomal recessive PD [94]. Majority of PD cases are sporadic and environmental factors play a critical role in PD etiology. Consistent uses of herbicides or pesticides [95] and exposure to organic solvents like carbon monoxide, and carbon disulfide [96] increase the risk of PD.

Amyotrophic Lateral Sclerosis (ALS) and pathology
ALS or Lou Gehrig's disease cause premature death of neurons that control voluntary muscles thereby causing paralysis.
Clinically it is characterized by progressive degeneration of motor neurons in the brain and spinal cord. Though the occurrence of ALS is low (0.005%) when compared to other dementia, its prevalence increases in people with age 55-75 years. Genetic factors such as MAPT (Microtubule-Associated Protein Tau) and mutations in genes SOD1 and ALS2 genes are majorly responsible for most Familial ALS (FALS) cases [97]. These mutations account for about 20% of FALS cases while up to 10% of the sporadic ALS there was no familial segregation [98].
Major neuropathological features of ALS include protein misfolding, oxidative stress, glutamatergic excitotoxicity and cytoskeletal dysfunctions leading to severe neurodegeneration. Further, intracellular inclusions such as Bunina bodies, perikaryal inclusions of neurofilament and Lewy body-like cytoplasmic inclusions are prominent microscopic features. Though in majority of ALS cases, brain does not exhibit prominent abnormalities, spinal cord frequently reveals atrophic anterior nerve roots and precentral gyrus [99]. Some ALS cases also display atrophy of the frontal or temporal cortex or both [100][101][102]. In some cases, the corticospinal tract displays grey matter abnormalities [103][104][105].

Huntington Disease (HD) and pathology
Huntington Disease (HD) is an autosomal dominant, progressive neurodegenerative disease characterized by abnormal movements (chorea), cognitive loss and psychiatric sickness. HD is caused by an extended repeat of CAG (usually more than 40) in the gene which encodes the huntingtin protein located in chromosome 4. Subsequently it results in atrophied striatum, cortical and extrastriatal regions. Mutant huntingtin which has been implicated in the toxicity of HD form aggregates by its N-terminal cleavage, further oligomerize and form inclusions. This in turn adversely affects a number of intracellular processes such as mitochondrial and transcriptional system of several genes, disrupted calcium signaling, abnormal protein interactions, alterations in proteosomal function and finally autophagy [106]. Overall, the pathological factors affect the brain causing cell loss and gliosis with the striatum being most affected.

Glaucoma and pathology
Glaucoma is considered as a set of optical nerve disorders that are associated with structural changes in the optic nerve head which lead to blindness. Primary Open-Angle Glaucoma (POAG) and Primary Angle-Closure Glaucoma (PACG) are the most common forms of glaucoma [107]. The junction between the iris and cornea (forming an angle) serve as drainer of aqueous humor from the anterior chamber of the eye [108] and in POAG, the trabecular meshwork becomes unblocked by iris tissue leading to opening of the angle and subsequent intraocular pressure gets transmitted to the axons of retinal . 07 .
Ocimum Scientific Publishers ganglia at the optic nerve which imparts a mechanical stress and finally cell death [109]. In PACG exactly the opposite happens when the peripheral iris blocks normal aqueous humor flow [110] which lead to increased intraocular pressure and optic nerve damage. Symptomatically glaucoma is characterized by a sudden and painful loss of vision [111], blurred vision and rainbows around lights accompanied with nausea and vomiting [112].

Effect of Lignans for the Treatment of Neurodegenerative Disorders Lignans with anti-AD effect
Alzheimer's Disease (AD) which cause progressive degeneration of neurons (involve multiple roadways such as Aβ toxicity microglia activation and release of inflammatory cytokines like IL-1β, TNF-α, NO and NF-kB [113]. Several lignans has been proved to target many of these pathways and thus seemed beneficial in both in vitro and in vivo conditions. Macelignan from Myristica fragrans, was found to mitigate glutamate induced neurotoxicity via anti-oxidant and anti-inflammatory properties along with reduced ROS production in murine hippocampal HT22 cells. This study also evidenced that macelignan mediates inhibition of Lipopolysaccharides (LPS)induced neuroinflammation in primary culture of rat microglial cells through reduced TNF-ɑ and IL-6 [114]. Subsequent in vivo report says that this lignans ameliorate LPS induced neuroinflammation in rats through its molecular mechanism extended to MAPK signalling and nuclear factor kB (NF-kB) [115]. Based on a review clarifying the accumulation of toxic ceramides in liver that cause neurodegenerative diseases such as AD due to improper lipid metabolism caused by diabetes type 2 [116], macelignan when administered to diabetic rats interestingly reduced phosphorylation of eukaryotic initiation factor, ER stress, glucose-regulated protein and CCAAT/ enhancer-binding protein homologous protein [117]. Another report suggests four new lignans from crude extract of Adelostemma gracillimum (Apocynaceae) root had protective effect against N-methyl-D-aspartate (NMDA)-induced cytotoxicity in rat primary cortical neurons [118]. Similarly, five novel lignans from Rubus idaeus rhizome methanol extract protected human neuroblastoma cells SHSY5Y cells from H 2 O 2 induced neurodegeneration through its in vitro antioxidative property [119]. Similarly studies on the effect of lignans from stem of Schisandra chinensis rattan on amyloid-β 1-42 induced microglia activation showed that they ameliorate microglia activation through activation of NF-kB/MAPK signaling pathway [120]. Schisandrin B obtained from the fruit of the same plant mitigated Aβ 1-42 induced DNA methylation in SHSY5Y cells evidenced by decreased mRNA and protein expression of DNA methyltransferases DNMT3A and DNMT1 [121]. In the same way honokiol attenuated Aβ 1-42 oligomer induced memory impairment in AD model mice via reduced mitochondrial apoptosis and inhibition of NFkβ signaling [122]. A study using hippocampal neurons from rats demonstrated that nordihydroguaiaretic acid protects hippocampal neurons against amyloid beta-peptide toxicity through ROS suppression and reduced calcium accumulation [123]. Similarly, several other lignans in methanolic extract like arctiin, (-)-traxillagenin, (-)-arctigenin, traxillaside, and 3(-)-4'-demethyltraxillagenin isolated from the bark of Torreya nucifera (Taxaceae) were found to be very potent neuroprotectors against glutamate-induced toxicity in rat cortical primary cells [124]. In a similar manner some lignans such as tricin, salcolin, and tetrahydro-4,6-bis(4-hydroxy-3methoxyphenyl)-1H,3H-furo [3,4-c] furan-1-one isolated from the ethanolic extract of Zea mays stems revealed both antiinflammatory as well as neuroprotective effect in LPS RAW 264.7 cells and glutamate-induced HT22 cells respectively [125]. Consecutively Flax Lignans (FLL) has also been in the queue of holding neuroprotective potential which has been supported by few studies. FLL reduced intracellular Ca 2+ and balanced expression of Bcel-2 and Bax, further reduced the upregulation of GluN2B subunit containing NMDA receptor in NMDA exposed cells [126]. Since NMDA receptor plays a role in the mechanism of synaptic transmission, the FLL can improve the learning abilities and memory of AD patients. Four polysperlignans another class of dibenzocyclooctadiene lignans isolated from K. polysperma stem when tested on Aβ or H 2 O 2 induced neurotoxicity on PC12 cells had statistically significant neuroprotective effects [127]. In the same way strong antioxidant activity was observed in H 2 O 2 treated PC12 cells by chushizisins obtained from fruits of Broussonetia papyrifera [61].
In another study 20 mg/Kg sesamin, the lignin present in more amount in sesame oil not only improved motor imbalance but also lowered malondialdehyde, caspase 3 activity, α-synuclein expression and ROS [130]. Another report by Kiyofuji et al., evidenced that supplementation of 10μM macelignan (a nutmeg lignan) to the mice midbrain slice cultures treated with IFN-γ and LPS showed dramatic effect on inflammation induced degeneration of dopaminergic neurons through expression of arginase-1 [131]. Induction of arginase-1 by the lignin is a good treatment approach for PD because studies have shown that the plaque removal will be better in cells which are positive for arginase-1. In another study, supplementation of 25 μM (-)-sesamin ameliorated 6-OHDA induced neurotoxicity in PC12 cells via reduced ERK1/2 and Bad (Bcl-2-associated death promoter) phosphorylation. Additionally, oral administration of 30 mg/kg body weight of sesamin showed increased dopamine, 3,4-dihydroxyphenylacetic acid, norepinephrine, as well as homovanillic acid in the substantia nigra-striatum of 6-OHDAinduced PD in rat model [132].

Lignans effective for other neurodegenerative disorders
Mutations in copper/zinc-superoxide dismutase (SOD1) has been found to be associated with familial ALS and a sharp increase in TNFα in the CNS of G93A-SOD1 mice (transgenic expressing mutant SOD1 enzymes) proposed a possible target for the therapeutic leads against ALS. Nordihydroguaiaretic Acid (NDGA), a lignan from Larrea tridentate showed 10% increase in lifespan of G93A-SOD1 mice model of ALS. It antagonized TNF-ɑ and 5-lipoxygenase and thus reduced neuronal damage [133]. A similar work done by Boston et al., revealed that subcutaneous injection of 1 mg of NDGA/day for 30 days in mice, caused persistent increase of glutamate uptake in spinal cord synaptosomes but when given to the SOD1-G93A transgenic mouse model of ALS at later stage it did not extend their life span [134].
Glaucoma a progressive neurodegenerative disease characterized by degeneration of retinal ganglion cells and axons requires development of potent neuroprotective leads for effective treatment. Glaucoma exhibit strong similarities with AD and PD including the selective loss of neuronal populations and trans synaptic degeneration [135]. Several lignans has been reported of having neuroprotection against glaucoma such as lignans from Eucommia ulmoides extract showed neuroprotection against glaucoma-associated optic neuropathy in glaucomatous rats by activating AMPK (AMP-activated protein kinase) signaling, which restores the energy balance of the cells [136].
Plant lignans has also been found to be effective for HD related oxidative damage. Recent studies show that NDGA improves ATP generation and mitochondria structure in oxidative stressinduced neuronal cells. In addition, it restores mitochondria structure mitochondrial membrane potential and synaptic structure in the striatum of R6/2 mice [137]. Some of the plant lignans are even proved to be beneficial for virus borne neurodegenerative diseases like Japanese Encephalitis (JE). For instance, Swarup et al., found that arctigenin provides complete protection against JE in BALB/c infected with Japanese Encephalitis Virus (JEV). Several factors contributed for the neuroprotective effect of the lignan including reduced caspase-3 activity, ROS, RNS, proinflammatory cytokines and viral load [138]. In the same way dihydrobenzofuran neolignans from Clematidis armandii showed potent antineuroinflammatory activity by suppressing TNF-α release in LPS-stimulated BV-2 cells [139]. Apoptotic neuronal cells contribute to neuronal loss in neurodegenerative diseases where some lignans like sauchinone from (Saururus chinensis) has been reported to protect C6 rat glioma cells from staurosporine induced apoptosis via decreased caspase-3 activity [140]. Some of the lignans like petaslignolide A from Petasites japonicus [141] and 9-hydroxypinoresinol were found to be protective against kainic acid induced seizure in mice [142]. Figure 3 shows the structure of lignans effective for these neurodegenerative diseases.

Conclusion
Plant secondary metabolites represent a huge group of bioactive molecules and lignans are one of such molecules which have been used as folk medicine from time immemorial.
Based on their structure lignans have been divided in to dibenzocyclooctadienes, arylnaphthalenes, aryltetralins, dibenzylbutanes, dibenzylbutyrolactols, dibenzylbutyrolactones, furofurans and furans. Similarly, neolignans which are the natural derivatives of lignan have been divided into Benzofurans, alkyl aryl ethers and benzodioxanes. In this review the individual lignans were summarized in relation to their neuroprotective and cognitive enhancement activities. Lignans from many plant species were found to be beneficial for cancer, hepatitis, microbial and fungal infections while specific lignans like macelignan, schisandrin B, arctiin, (-)-traxillagenin, traxillaside, 3(-)-4'-demethyltraxillagenin, flax lignans, schisantherin A, (-)-sesamin, nordihydroguaiaretic acid and sauchinone were helpful for NDDs including Alzheimer's disease, Parkinson's disease, glaucoma, amyotrophic lateral sclerosis, Huntington's disease and Japanese encephalitis. These molecules prevent neurodegeneration through their antioxidative and antiinflammatory effect; they also intervene the molecular pathways like DNA methylation, caspase 3, ERK etc. that lead to cellular apoptosis ( Figure 4). Literature survey proposed that lignans can serve as efficient neuroprotective molecules in cell line