Mechanism and implications of traditional Chinese medicine in amyotrophic lateral sclerosis therapy

Amyotrophic lateral sclerosis (ALS) is a life-threatening neurodegenerative disease causing progressive degeneration of motor neurons, ultimately resulting in death. Till now, no medicinal strategy has been proved to be completely successful in ameliorating the disease’s symptoms, except riluzole that only has a moderate effect. A limited therapeutic intervention of ALS encourages patients to opt for alternative medicine and therapies. Traditional Chinese herbal medicines (TCM) have been shown to overcome neuroinflammation, excitatory amino acid toxicity, oxidative stress, apoptosis and autophagy. In this regard, Chinese herbal medicines (CHMs) have been explored as a therapeutic option to manage clinical manifestations of ALS and other neurological diseases. In this review, we summarize the therapeutic benefits of CHMs on various neurodegenerative disorders, particularly ALS. The mechanistic details of various Chinese herbs along with their active molecules that delay the disease onset have also been discussed.


Introduction
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease associated with the death of motor neurons resulting in muscle atrophy and dysfunction. Patients usually die within 3-5 years of diagnosis of ALS, owing to its extremely fatal consequences (Wijesekera and Leigh 2009). The prevalence of ALS is one to two individuals out of a million per year, with approximately 90% events being sporadic (sALS) and 10% cases are marked as familial (fALS) (Robberecht and Philips 2013). Genetic mutations like those in Cu/Zn superoxide dismutase 1 (SOD1, 20%), TAR DNA-binding protein 43 (TDP-43, 1-5%) and fused in sarcoma (FUS, 1-5%) are common causes of fALS as reviewed by Rental et al. (Renton et al. 2014). Recently, a hexanucleotide repeat enlargement (G4C2) n within the cryptography region of chromosome 9 open reading frame 72 (C9ORF72) factor of the western population has been reported as a common cause of fALS (~ 40%) (DeJesus-Hernandez et al. 2011;Renton et al. 2011).
ALS is a heterogeneous disease not only genetically, but also clinically. The age and site of emergence, progression rate and extent of cognitive dysfunction are highly variable in this pathology. Neurons of the anterior and temporal cortex of the brain are found to be affected in some patients. It was observed that in 15% of ALS cases, frontotemporal dementia is present with frontotemporal lobe degeneration (FTLD) as the main symptom (Ringholz et al. 2005). Similarly, 15% of FTLD patients show symptoms like motor neuron degeneration similar to that of ALS (Ferrari et al. 2011). Thus, both FTLD and ALS are commonly referred to as ALS/FTLD (Robberecht and Philips 2013). Interestingly, ALS/FTLD shares several common gene mutations (Kumar et al. 2016a).
The etiology of ALS is multi-factorial (Ferraiuolo et al. 2011;Shaw 2005). The pathogenesis of ALS is mediated by a variety of cellular pathways associated with glutamate-induced oxidative stress, inflammation, excitotoxicity, mitochondrial dysfunction, protein misfolding and aggregation, and inefficient protein quality control (Dunkel et al. 2012;Kumar et al. 2016b). Even today, 1 3 targeted and successful therapeutic strategy to manage neurological symptoms remains one of the essential issues of ALS. At present, there is no effective treatment available except riluzole, the only drug approved by the US Food and Drug Administration (FDA) for the treatment of ALS (Bensimon et al. 1994;Wokke 1996).
Even after many years, riluzole remains the simplest accredited treatment for ALS. Riluzole increases the survival by about 2-3 months in ALS patients (Hinchcliffe and Smith 2017;Miller et al. 2003Miller et al. , 2007Miller et al. , 2012, an impact that was reproduced well in clinical trials (Bensimon et al. 1994(Bensimon et al. , 2002Lacomblez et al. 1996). However, the efficacy of riluzole is questionable, as there are no clinical studies demonstrating the usefulness of riluzole on muscular atrophy, muscle spasticity, dysarthria, dysphagia and lifestyle improvements in ALS patients (Miller et al. 2012). A restriction of riluzole and lack of alternative treatment of ALS demands the development of complementary and/or alternative medicine (CAM) for ALS therapy (Bedlack et al. 2015). Traditional Chinese medicine (TCM), the main form of CAM, is one of the better available choices for ALS therapy. This review highlights the advantages of Chinese herbal medicine (CHM) as an alternative ALS therapy. We further emphasized on the mechanism of action of each CHMs that are used in treatment of ALS (Fig. 1).

Traditional Chinese Medicine
Traditional Chinese medicine (TCM) is one of the world's oldest clinical structures based on natural herbs. TCM components are considered as a source of 'vital energy' which provides mental and physical strength, and is required for growth, reproduction, cognitive features and daily activities. TCM typically utilizes Chinese herbs that help in body repair and maintain internal balance (Iriti et al. 2010;Ou et al. 2003). Herbal formulations are essential for Chinese, Ayurvedic, and Unani medicine, and are globally accepted because these contain hundreds of bioactive components which may be exploited in effective therapies. Indeed, over 50% of currently used pharmaceutical drugs are being synthesized from plants and herbs (Pan et al. 2013a).
TCM is one of the better choices for the treatment of ALS because of its large number of benefits Sucher 2013;Tu et al. 2015;Zhang et al. 2014b). The clinical manifestations of ALS involve muscular atrophy, respiratory failure and fatique. An inclusive TCM approach is beneficial for ALS treatment as it involves Chinese herbs that improves the Qi of the spleen and lung. Most of the TCM decoctions and herbs used "nourish the spleen and enrich vitality" and meet the TCM guidelines for treating weakness and atrophy. Accordingly, TCM decoctions and formulations are often used in the treatment of ALS. For example, Fuyuan Shengji granule, Qilong Yiqi powder, jiweiling injection, Huqian Wan (Hidden Tiger Pill) and Fig. 1 CHMs act as potential therapeutic agents in ALS by targeting oxidative stress, neuroinflammation, excitotoxicity and protein degradation pathways Yisui Tang formula are effective for ALS patients Zhang et al. 2014b).
Recently, Jiawei Sijunzi decoction (JWSJZ) derived from Sijuni decoction plus herbs Radax astragali and Desertliving cistanche has been used for the treatment of ALS (Pan et al. 2013b). Pan et al. investigated the clinical efficacy and safety of JWSJZ decoction for ALS treatment. They observed that JWSJZ decoction was safer and slightly superior in delaying the development of ALS. Moreover, Dihuang Yinzi (DHYZ) is a long-established TCM prescribed for neurological disorder Yinfei syndrome that focuses on speech and language disorder symptoms (yin syndrome), and motility disorders such as motor paralysis (fei syndrome). Recently, DHYZ (decoction of 12 herbs) has been shown to improve bulbar paralysis, minimal use of ventilator support and prolonged survival time in ALS patients (Qiu et al. 2016). It has also been widely used for the treatment of several neurological disorders such as stroke (Yu et al. 2015a), Parkinson's disease (Gu et al. 2015) and spinal cord injury (Li et al. 2012b). Thus, several studies have established the tremendous ability of CHMs to fight against ALS because of their neuroprotective function (Zhang et al. 2014b).

CHMs as antioxidative drugs
Oxidative stress within the cell plays a key role in determining the onset and progression of neurodegenerative diseases (Barber and Shaw 2010; Barnham et al. 2004;Lin and Beal 2006;Niedzielska et al. 2016;Radi et al. 2014). Superoxide dismutase 1 (SOD1) acts as an important antioxidative enzyme that catalyzes the conversion of harmful superoxide species into hydrogen peroxide and oxygen, thereby protecting the body from the deleterious effects of free radicals. More than 150 mutations in SOD1 have been found in fALS patients, clearly indicating the involvement of oxidative stress in ALS (Radunovic and Leigh 1999). SOD1 mutations are proposed to reduce the superoxide dismutase activity of SOD1, thereby raising the levels of free radicals inside neurons. Excessive free radicals directly damage the cellular proteins, enzymes, DNA, membrane phospholipids and other biomolecules through peroxidation that ultimately leads to cell death. Moreover, the mutant protein reacts with peroxynitrite to form highly reactive intermediate that nitrates tyrosine residues of proteins that gradually injure neurons and other cell types. Oxidative damage associated with SOD1 mutation has been studied in cellular and rodent models of ALS Parakh et al. 2013), in spinal cord and cortex motor neurons (Ferrante et al. 1997;Shaw et al. 1995b), post mortem tissue of ALS patients (Abe et al. 1995(Abe et al. , 1997 and in CSF of ALS patients (Ihara et al. 2005). Elevated expression levels of oxidative stress-sensitive proteins such as HO-1, NQO1 and GCLC have also been detected in the brainstem of SOD1-G93A mice at the late stage. A number of reports suggest that CHMs increases the antioxidant activity of the enzymatic/non-enzymatic machinery, scavenges the intracellular reactive oxygen species (ROS) and modulates gene expression and regulation. Table 1 shows the summary of some of the CHMs (Fig. 2) with high antioxidant abilities in connection with ALS.
Ginkgo biloba L. or maidenhair tree is a conventional medicinal plant which belongs to the Ginkgoaceae family. Ginkgo biloba leaves extract is a rich source of flavonol glycosides (such as myricetin, kaempferol and quercetin (1)) ( Fig. 2), terpene trilactones, called ginkgolides (2) (A, B, C and J), proanthocyanidins, ginkgolic acids, ginkgols bilobols and some non-flavonoid glycosides (Singh et al. 2008;van Beek 2002). According to a survey at http://www.Clini calTr ials.gov and PubMed, approximately 500 clinical studies have evaluated the valuable role of Ginkgo biloba on a large number of human disorders (Birks and Grimley Evans 2009;Iriti et al. 2010). Ferrante et al. (Ferrante et al. 2001) reported that Gingko biloba extract has a gender-specific neuroprotective role in transgenic SOD1 G93A mouse model system of ALS. The authors showed that oral application of Gingko biloba extract decreased the motor neuron loss and significantly ameliorated the survival time in male SOD1 G93A mice. However, no significant improvement in neuronal function was observed in female SOD1 G93A mice.

Panax ginseng
Panax ginseng C.A. Meyer (or Chinese ginseng) belongs to the Araliaceae family and is a common medicinal herb from the Panax genus which has a 2000 years history in TCM (Vogler et al. 1999;Yun 2001). Primary active ingredients of ginseng are ginsenosides, protopanaxadiol (3) and protopanaxatriol (Qi et al. 2011). Ginseng shows a range of pharmacological activities such as antioxidant (Zhang et al. 1996), anticancerous (Shin et al. 2000), anti-inflammatory (Lee et al. 2008), neuroprotective Radad et al. 2006;Rausch et al. 2006;Van Kampen et al. 2003) and antiapoptotic (Nakaya et al. 2004). Jiang et al.  reported that 40 and 80 mg/kg of ginseng roots show beneficial effect in diminishing the motor dysfunction as well as prolong the survival time in SOD1 G93A mice. It was suggested that the protective effects of ginseng could be due to increased activity of nerve growth factor (NGF), enhanced antioxidant effect and altered nitric oxide level.

Radix Astragali
Radix Astragali is commonly used in Chinese medicine as a key herb for ALS treatment. Astragaloside IV (4) is the chief bioactive component and is considered as the main marker or quality index in Pharmacopoeia of the People's Republic  Decreases apoptosis and LDH activity Cheng et al. (2008) and Luo and Sun (2011) Anti-neuroinflammation Scutellaria baicalensis Georgi (Huáng Qín)

Cannabis
Cannabis (marijuana), one of the fundamental herbs of conventional Chinese medicine, has long been used to treat several diseases such as gastrointestinal illness, brain tumor, breast cancer, AIDS, Alzheimer's disease and ALS/MND. Cannabis appears to target a number of known pathophysiology of ALS (Bilsland and Greensmith 2008; Pryce and Baker 2015). Preclinical data have shown that cannabis has strong antioxidative, anti-inflammatory and neuroprotective effects. In SOD1 G93A ALS mouse model, cannabis treatment has resulted in delayed progression of the disease and increased survival rate in mice even when dispensed after onset of symptoms (Raman et al. 2004).
Carter and Rosen (2001) and Amtmann et al. (2004) have proved that cannabis may extend neuronal cell survival and is effective in relieving symptoms of appetite loss, depression, muscular spasticity, excess saliva and weakness.  showed that cannabinol, a non-psychotropic cannabinoid, substantially delays disease onset through more than 2 weeks without affecting survival in SOD1 G93A mice. Weber et al. (2010) reported that treatment with delta-9 tetrahydrocannabinol (THC) (5), the main bioactive component of cannabis, at onset of the disease delayed motor dysfunction and increased survival in SOD1 G93A mice. Shoemaker et al. (2007) reveal that daily administration of selective cannabinoid receptor 2 (CB2) agonist AM-1241 prolonged the survival in SOD1 G93A mice. Moreover, treatment with WINSS, 212-2, a synthetic cannabinoid, improved motor neuron survival in SOD1 G93A mice (Bilsland et al. 2006).

Epigallocatechin gallate
Epigallocatechin gallate (EGCG) (6), the water-soluble flavan-3-ol compound of green tea, is commonly employed as a free-radical scavenger and antioxidant in neurological diseases such as ALS (Mandel et al. 2004). Xu et al. (2006) reported that oral application of 10 mg/kg EGCG delays the progression of disease, increases the number of motor neurons and improves the survival time in SOD1 G93A mice. Koh et al. (2004) demonstrated the neuroprotective action of EGCG against oxidative stress-induced apoptosis in both wild-type and SOD1 G93A motor neuron cells. They showed that the neuroprotective effects of EGCG could be explained by up-regulation of PI3K/Akt and GSK-3 pathways and down-regulation of caspase-3, PARP and mitochondrial damage. Koh et al. (2006) showed neuroprotection in SOD1 G93A mice treated with EGCG. EGCG administration  Rhizoma curcumae longae (Jiang huang)

Curcumin
Hinders α-synuclein accumulation through induction of autophagy Hasima and Ozpolat (2014) and Jiang et al. (2013) delayed the disease progression and motor dysfunction and increased the survival time in SOD1 G93A mice.

Diallyl trisulfide
Diallyl trisulfide (DATS) (7) is the most potent organosulfur compound formed by the breakdown of allicin present in the garlic bulbs of Liliaceae allium. It can cross the blood-brain barrier (BBB) and is known to possess antiatherogenic, anticancerous, antioxidant, antiapoptotic and neuroprotective properties. Sun et al. have demonstrated that DATS prevents motor neurons against excitotoxicity and plays a neuroprotective role in ALS (Sun et al. 2009). Guo et al. (Guo et al. 2011) have reported that oral administration of DATS (80 mg/kg/day) prolongs disease time span and extends the life span for about 1 week in SOD1 G93A mice.

Madecassoside
Madecassoside (8) is a bioactive component of triterpenoid saponins derived from Centella asiatica (L) Urban. Several studies have demonstrated the protective effect of madecassoside by preventing neuronal apoptosis and enhancing the antioxidant activity (Du et al. 2014;Lin et al. 2014;Luo et al. 2014;Xu et al. 2013).

CHMs as anti-inflammatory drugs
Neuroinflammation is a primary pathological landmark of neurodegenerative disorders (Khandelwal et al. 2011) and thus represents a crucial therapeutic target (Yong and Rivest 2009). In ALS, activation of microglia, astrocytes and complement system occurs due to impairment of motor neurons in CNS that leads to the secretion of various inflammatory modulators such as tumor necrosis factor-α (TNF-α), cyclooxygenase (COX-2), leukotriene B4, inducible nitric oxide synthase (iNOS) and prostaglandin E2. These active molecules initiate the inflammatory response that ultimately culminates in neurodegeneration (Troost et al. 1990;Zhao et al. 2013). It is known that inhibiting neuroinflammation and microglial activation can attenuate the risk of ALS . Table 1 summarizes the various Chinese herbs along with their cellular targets in ALS.

Celastrol
Celastrol (10) is a triterpenoid obtained from traditional Chinese herb, Tripterygium wilfordii Hook F. It was initially used as an anti-inflammatory compound, but was later shown to have robust antioxidative outcomes and also elevates expression level of HSP-70 (Trott et al. 2008). Kiaei et al. (2005) reported that celastrol, at 2 mg/kg/day delays disease onset, improves motor function and prolongs survival in SOD1 G93A mice. They showed down-regulation of TNF-α and iNOS expression, and decrease in immunoreactivity of glial fibrillary acidic protein and CD-40 in the spinal cord of celastrol-treated SOD1 G93A mice in contrast to control SOD1 G93A mice.

Resveratol
Resveratrol (3,5,4′-trihydroxystilbene) (11), a polyhydroxy diphenyl ethylene, is appreciably found in plants such as Veratrum nigrum, mulberry, grape, peanut and Rhizoma polygoni cuspidate. It acts as an antioxidant, antiaging, antidiabetic, anti-ischemic and anti-inflammatory molecule (Athar et al. 2007;Baur and Sinclair 2006;Patel et al. 2011;Richard et al. 2011). Resveratrol has demonstrated therapeutic benefits on Parkinson's disease (PD) (Khan et al. 2010), Alzheimer's disease (AD) (Li et al. 2012a) and Huntington disease (HD) (Ho et al. 2010). Yanez et al. (2011) reported that resveratrol protects neurons from the motor cortex of rat brain against  Wang et al. (2011) showed that resveratrol increases the Sirtutin 1 (SIRT1) expression in the human SOD1 G93A motor neuron-like cell culture. It improves the cell viability, increases the level of ATP and prevents apoptosis. Markert et al. (2010) reported that resveratrol decreased the severity of ALS by mitigating the abnormality in p53 acetylation in SOD1 G93A mice. Barber et al. (2009) also demonstrated the neuroprotective role of resveratrol in a cellular model of ALS via its antioxidant activity. Han et al. (2012) reported that intraperitoneal application of resveratrol at 20 mg/kg delays the ALS onset and prolongs survival in SOD1 G93A mice. The authors reported the up-regulation of HSP25 and HSP70 as well as SIRT1 activation as a possible mechanism of neuroprotection.
Similarly, Mancuso et al. (2014) also showed that resveratrol improves the survival time in SOD1 G93A mice and delays the onset of disease symptoms. They reported that resveratrol activates cellular pathways that regulate autophagy such as SIRT1 and AMP-activated protein kinase (AMPK) pathways, reduces microglial reactivity and improves mitochondrial biogenesis.

Curcumin
Curcumin (12) is a known polyphenolic compound obtained from the Chinese herb Rhizome Curcuma longa (Jiang huang). Curcumin has been shown to possess effective therapeutic potential against neurodegenerative diseases (Bandyopadhyay et al. 2008;Sharma et al. 2009) because of its ability to cross the blood-brain barrier, act as an antioxidant, anti-inflammatory and amyloid fibrillation inhibitor (Garcia-Alloza et al. 2007;Yang et al. 2005). The anti-inflammatory property of curcumin has been associated with the inhibition of the NF-κB signaling pathway and reduction of intracellular inflammatory mediators such as TNF-α, IL-6 and IL-6 (Sikora et al. 2010). In the PD model, curcumin activates autophagy in neuronal cells by inhibiting the mTOR/p70S6K signaling thereby reducing α-synuclein accumulation (Jiang et al. 2013). Dong et al. (2014) showed that dimethoxy curcumin improves the abnormalities of voltage-gated sodium channels and action potentials, thus reducing the excitotoxicity in a motor neuron-like cellular model of mutated TDP-43 Q331K in ALS. Similarly, Lu et al. (2012) reported that dimethoxy curcumin improves mitochondrial dysfunction in mutated TDP-43-transfected NSC-34 cells. Cashman et al. (2012) examines the effect of curcumin on inflammation in monocytes from ALS patients and showed that curcumin and its analogs promote clearance of defective SOD1 in ALS.

Caffeic acid phenethyl ester
Caffeic acid phenethyl ester (CAPE) (13), a naturally derived bioactive phenol, serves as antioxidant and antiinflammatory drug showing neuroprotection in various neurological diseases (dos Santos et al. 2014;Fontanilla et al. 2011). CAPE was shown to inhibit caspase-3 activation and p38 phosphorylation, thereby protecting neurons against glutamate excitotoxicity (Wei et al. 2008). It has also been reported that oral application of CAPE at 10 mg/kg/day increased disease duration and extended the survival of SOD1 G93A mice (Fontanilla et al. 2011). This neuroprotection was achieved by the reduction in activated p38 MAP kinase as well as diminished microglia astrocytes activation, indicating that both apoptosis and inflammation were inhibited.

Withaferin A
Withaferin A (14) is a steroidal lactone isolated from the medicinal plant of Ayurvedic importance, Withania somnifera. It has been shown that withaferin A reduces inflammation and improves motor defects in TDP-43 transgenic mice , and also attenuates glutamate-induced and microglia-mediated cell death in motor neuron culture (Swarup et al. 2011). Patel et al. (Patel et al. 2015) reported that intraperitoneal injection of 4 mg/ kg of withaferin A prevents weight loss, attenuates neuronal loss and improves the survival time in SOD1 G93A and SOD1 G37R mouse models of ALS. Withaferin A shows its neuroprotective effects by up-regulation of HSP70 and HSP25 as well as down-regulation of inflammatory cytokines IL-10.

dl-3-n-Butylphthalide
dl-3-n-Butylphthalide (15), found in Chinese celery seeds, has antioxidative, antiapoptotic and anti-inflammatory activities (Huang et al. 2010;Xu and Feng 1999;Zhang et al. 2010). In 2002, it was recommended in China for the treatment of ischemic stroke. It also improved cognitive impairment and β-amyloid-mediated neurodegeneration in rat model of AD (Peng et al. 2009). Recently, oral treatment of 60 mg/kg/day of dl-3-n-butylphthalide in female SOD1 G93A mice showed a decrease in weight loss, reduced motor neuron death and increase in life span. This neuroprotective effect is because of up-regulation of Nrf2 and HO-1, down-regulation of NF-κB and TNF-α and reduction in GFAP and CD11b immunoreactivity, and ultimately inhibition of astrogliosis (Feng et al. 2012). Recently, a 6-month clinical trial in China established the safe and efficient use of dl-3-n-butylphthalide on patients with sub-cortical vascular cognitive dysfunction without dementia (Jia et al. 2016).

CHMs as antiexcitotoxic drugs
Glutamate is considered as an important excitatory neurotransmitter in the central nervous system (CNS). Rise in glutamate levels in extracellular space due to delayed neurotransmitter clearance and activation of glutamate receptors cause neuronal injury. Such neurotoxicity that arises due to excitatory neurotransmitters is termed as excitotoxicity (Kawahara and Kwak 2005;Matyja et al. 2006). This excitotoxicity causes cell death due to huge calcium influx, ROS, mitochondrial dysfunction, energy imbalance and proteolysis (Arundine and Tymianski 2003; Guo et al. 2003;Zhao et al. 2008). Activation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and N-methyld-aspartate (NMDA) receptors leads to excitotoxicity (Van Den Bosch et al. 2006). Increase in glutamate levels in ALS patients (Perry et al. 1990;Shaw et al. 1995a) indicates the involvement of excitotoxicity in ALS. Moreover, the benefit of riluzole, which acts as an antiexcitotoxic drug (Ludolph and Jesse 2009), also suggests the neuronal damage due to excitotoxicity in ALS patients. Some of the CHMs targeting excitatory pathways in ALS are discussed below (Table 1).

Huperzine-A
Huperzine-A (Hup-A) (16) is a lycopodium alkaloid extracted from Huperzia serrata (Thunb) Trev. (Qian Ceng Ta). It has been approved as an AD drug by the FDA of the People's Republic of China. The neuroprotective role of Hup-A was mainly explained by its ability to inhibit glutamate neurotransmission (Ved et al. 1997). Gordon et al. (2001) showed that Hup-A binds to NMDA receptor and blocks its interaction with glutamate, thus preventing glutamate toxicity. Later, Hemendinger et al. (2008) illustrated the neuroprotective effect of Hup-A against various cell death inducers using a model system of motor neuron-like cell line. These studies indicate that Hup-A can be used as a complementary treatment for ALS.

Ferulic acid
Ferulic acid (18), extracted from a Chinese herb Angelica Sinensis Radix, is a water-soluble phenolic acid and can easily pass through the blood-brain barrier. Jin et al. (2007) reported that sodium ferulate provides neuroprotection by preventing glutamate-induced apoptosis in cultured cortical neuronal cells. Moreover, Luo and Sun (2011) demonstrated the neuroprotective effect to hypoxia and excitotoxicity of ferulic acid in treated PC12 cells. Recently, Ren et al. (2017) illustrated the neuroprotective role of ferulic acid against ischemia-induced cerebral injury both in vivo and in vitro.
The study suggests that FA exerts its effects by protecting against apoptosis and oxidative stress induced by cell injury in the brain.

CHMs as autophagy modulator
Misfolded and aggregated proteins are primarily removed from the system by ubiquitin-proteasome and autophagy-lysosome pathways (Ciechanover and Kwon 2015;Gadhave et al. 2016;Nedelsky et al. 2008;Takalo et al. 2013). These pathways clear aggregation-prone misfolded proteins from the cytoplasm and provide neuroprotection in HD (Ravikumar et al. 2004), AD (Caccamo et al. 2010), PD (Crews et al. 2010) and ALS (Ciura et al. 2016;Majcher et al. 2015;Nassif and Hetz 2011;Navone et al. 2015). Autophagy is negatively regulated by the activation of mammalian target of rapamycin (mTOR) kinase pathway (Ravikumar et al. 2010). It has been observed that inducing autophagy by rapamycin (mTOR inhibitor) has beneficial effects in neurodegeneration observed in TDP-43-related pathogenesis (Ballou and Lin 2008;Wang et al. 2012). However, rapamycin was observed to exacerbate the motor neuron degeneration in ALS mice model (Zhang et al. 2011b). Other autophagy activators such as progesterone or trehalose showed promising results in SOD1 G93A mice in delaying disease onset and prolonging survival (Castillo et al. 2013;Kim et al. 2013;Zhang et al. 2014a). These results showed that autophagy is the major metabolic route of the pathological inclusions, and hence modulation of autophagy can be a possible therapeutic approach to combat ALS.

Berberine
Berberine (20) is an isoquinoline alkaloid derived from a wide variety of herbs such as Rhizome coptidis (Huang lian) and Cortex phellodendri (Huang be). It induces autophagy by inhibiting the mTOR signaling pathway (Ahmed et al. 2015). Berberine is well established as an antibacterial, antidiabetic, antihypertensive, antiproliferative and anti-inflammatory, and can be safely administered to humans (Fan et al. 2015;Jeong et al. 2009;Yin et al. 2008). Moreover, researchers have demonstrated the neuroprotective role of berberine in PD, HD and AD by reducing the deposition of β-amyloids, or increasing the degradation of mutant protein by the autophagic pathway (Durairajan et al. 2012;Jiang et al. 2015;Jung et al. 2009;Kim et al. 2014). Recently, Chang et al. (Chang et al. 2016) showed that berberine increases the degradation rate and decreases the aggregate formation of truncated TDP-43 fragments in Neuro 2a cells through activation of autophagic degradation pathway, suggesting its potential neuroprotective role in TDP-43 proteinopathies.

Neferine
Neferine (21) is the bioactive component of Nelumbo nucifera (Lian hua) and has been employed to maintain glucose and lipid homeostasis (Pan et al. 2009). It has been shown that neferine reduces toxicity of mutant protein in HD by activating the AMPK-mTOR-dependent autophagic pathway in HD . Neferine also exhibited antioxidative and anti-inflammatory activity in neurodegenerative diseases by inhibiting lipid peroxidation and NF-κB activation (Jung et al. 2010).

Rottlerin
Rottlerin (22) is extracted from Mallotus philippensis (Cu kang chai) and exhibits anti-inflammatory properties through regulation of inflammatory mediators for instance COX, heme oxygenase, NF-κB, lipoxygenase and protein kinase C (Maioli et al. 2012). Rottlerin inhibits the progression of amyloid Aβ, prion protein and α-synuclein aggregates (Maioli et al. 2012), clearly suggesting its role in neurodegenerative disorders. Rottlerin also induced cell death via autophagy by inhibiting the PI3K/Akt/ mTOR signaling pathway. However, it is not yet known that the anti-inflammatory effects of rottlerin are linked to autophagy.

Timosaponin AIII
Timosaponin AIII (23) is an active component of Rhizoma anemarrhenae (Zhi mu). Timosaponin AIII regulates the cytosolic Ca 2+ concentration (Wang et al. 2002) and superoxide generation by arachidonic acid (Zhang et al. 1999), thus showing antioxidative and anti-inflammation properties. It also shows neuroprotection by activating autophagy and facilitating the clearance of ubiquitinated proteins that are prone to aggregation (Lok et al. 2011).

Celastrol
Celastrol (10) is the bioactive component of Radix tripterygii wilfordii (Lei gong teng) and exhibits antioxidative and anti-inflammatory properties. It also prevents neurodegeneration by inducing autophagy via targeting JNK and the PTEN-Akt/mTOR pathway ).

Onjisaponin B
Onjisaponin B (24), a bioactive ingredient present in Radix polygalae (RP) (Yuan zhi), is commonly used in several TCM extracts such as "Kai Xin San"  and "Ding Zhi Wan" (Dong et al. 2013) for the treatment of poor memory , anxiety (Yao et al. 2010), insomnia or depression ). Onjisaponin B was reported to reduce cellular toxicity in PD and HD by accelerating the clearance of mutated α-synuclein and huntingtin, respectively (Wu et al. 2013. In spite of having extensive literature on molecular mechanisms and features of many autophagy modulators derived from CHMs, we are nevertheless a long way away from translating those traditional medicines into clinical application in neurodegenerative diseases including ALS

Conclusions
Even after 20 years since FDA approval of riluzole as the only drug for ALS, several attempts were taken to formulate a better therapeutic candidate to curb ALS. Some potential drugs, that has been found to be effective against ALS in preclinical animal studies, failed to replicate well in human clinical trials. An increasing number of CHMs have been described in context of ALS therapeutics. Such compounds have also been used therapeutically to manage an array of diseases since many decades. CHMs explained in this review reveal therapeutic applications in numerous pathophysiologies of ALS and act as an antioxidant, antiexcitotoxicity, anti-inflammatory and antiapoptotic agent. Such a multi-targeted approach of CHMs involving various cellular pathways makes them suitable therapeutics for ALS. Moreover, most of these CHMs have long been described as formulations in the TCM with common pharmacological action, toxicity or side-effects. Hence, clinical trials employing these bioactive compounds may serve as a safe and reliable strategy. Positive findings will possibly broaden the scope of CHM applications in ALS and related neurodegenerative diseases.