Neurobiological Promises of the Bitter Diterpene Lactone Andrographolide

Andrographolide (ANDRO), a bitter diterpene lactone found in Andrographis paniculata (Burm.f.) Nees, possesses several biological effects such as antioxidant, anti-inflammatory, and organo-protective effects. Scientific reports suggest that it also has neuroprotective capacity in various test systems. The purpose of this review was to synthesize the neuropharmacological properties of ANDRO and highlight the molecular mechanisms of action that highlight these activities. A careful search was done in PubMed and Google Scholar databases using specific keywords. Findings suggest that ANDRO possess neuroprotective, analgesic, and antifatigue effects. Prominent effects were stated on neuro-inflammation, cerebral ischemia, Alzheimer's and Parkinson's diseases, multiple sclerosis, and brain cancer in mice and rats. Furthermore, ANDRO and its derivatives can enhance memory and learning capacity in experimental animals (rats) without causing any toxicity in the brain. Thus, ANDRO may be one of the most promising plant-based psychopharmacological lead compounds for new drug development.


Introduction
Neurological disorders are very common and important health issues in elderly people [1,2]. The inflammation plays a crucial role in several neurodegenerative diseases, including Alzheimer's (AD) disease and Parkinson's (PD), multiple sclerosis (MS), and amyotrophic lateral sclerosis (ALS). Oxidative stress is an important factor in the pathophysiology of many chronic diseases such as diseases of the central system or cancers [3,4]. Therefore, much attention has been given to the discovery and development of neurological drugs from various origins [5,6].
Current evidence suggests that plants are a promising source of phytochemicals for the treatment of various diseases [7][8][9], and thus, phytochemicals are considered one of the most popular tools for sound health maintenance throughout the world [10][11][12][13]. Diterpenes are evident in their many promising biological activities, including neuroprotective capacity [14,15].
In this sense, the present review is aimed at focusing on ANDRO neurobiological effects.
The research was done according to the consensus statement of researchers active in ethnopharmacology and with particular input by the ConSEFS Advisory group [32], and the name of the plant was verified according to the PlantList [33]. No language restrictions were imposed, and manuscripts were evaluated for dose/concentration, administration route, test systems, results, discussion, conclusion, and proposed mechanisms of action.
The following inclusion criteria were considered: (1) Studies developed in vitro, ex vivo, or in vivo, and humans and their derived tissues and cells (2) Studies with ANDRO and its derivatives (3) ANDRO or its derivative joint effects with other chemical compounds (4) Studies with or without proposing mechanisms of action After careful search and strict analysis, 38 reports (PubMed: 28; Google Scholar: 10) were included. The most important mechanism of ANDRO' effects on AD, PD, MS, cerebral ischemia, intracerebral haemorrhage, neuropathic pain, brain tumour, and depression is summarized in Table 1 and   [34,35]. CNS degeneration involves the progressive chronic loss of neural structure and function, resulting in functional and mental neurological deficiencies [7,13].
3.1.1. Alzheimer's Disease. Alzheimer's disease (AD) is a chronic neurodegenerative disease, which usually has a slow progression and gradually worsens over time. It is the cause of 60-70% of cases of dementia [36,37]. Current treatment protocols are not sufficient for effectively preventing AD signs and symptoms [38,39]. Many natural resources have been discovered in recent decades which can be used as adjuvant therapies in the treatment of AD. [8,40,41].
In some recent studies, ANDRO administration (2 and 4 mg/kg i.p.) suppressed the spatial learning and memory function impairment in Octodon degus [42], and the proposed mechanisms of ANDRO were as follows: (1) recovery of learning performance and spatial memory, (2) synaptic basal transmission recovery, (3) protection of synaptic proteins, and (4) lowering of amyloid-beta (Aβ) aggregate maturation and phosphorylated tau protein [42].
In another study, ANDRO (2 mg/kg) was found to improve learning/memory by activating Wnt signaling. Wnt is a signaling pathway that enhances glucose metabolism via gene expression and/or activity enhancement of hexokinase, phosphofructokinase, and AMP-activated protein kinase (AMPK) [20,43].
Other studies narrate that ANDRO treatment during chronic cerebral hypoperfusion suppressed astrocyte activation supported by decreased expression of the glial fibrillary acid protein (GFAP), enhanced brain-derived neurotrophic factor (BDNF) and tyrosine kinase receptor B (TrkB) expression, and reversed upregulated expression of TNF-α, IL-1β, and caspase-3. Thus, in the rat model of chronic cerebral hypoperfusion, ANDRO improved impaired spatial learning and memory [47].
Anxiety and depressive disorders are frequently comorbid with PD [52]. Thus, a forced swimming test was performed in mice to measure depressive symptoms. After ANDRO (5 mg/kg) treatment, the swimming time was noticeably enhanced showing an improvement of depressive symptoms [53]. Geng et al. [54] provided evidence that ANDRO attenuated DA neuron loss, oxidative stress, and preserved mitochondrial morphology and also reduced mitochondrial malfunctions, reduced cell death, and inhibited GTPase activity in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-(MPTP-) induced PD. Apoptosis is also an important factor that triggers PD.

Multiple Sclerosis.
Multiple sclerosis (MS) is a wellknown immune-mediated disorder, in which insulating covers of nerve cells in the spinal cord and brain are damaged in the CNS [58]. From the MS pathogenesis, it was found that CD4 + T-cell-mediated autoimmunity is crucial in MS pathogenesis, mainly for early disease initiation [59,60]. T-helper type 1 (Th1) cells, characterized by interferon-(IFN-) γ production, mediate the MS pathogenesis [61,62], but IL-17-expressing T-helper cells (Th17) are also involved. CD8 + , as well as CD4 + T cells, was equally immune-stained for IL-17 and IL-17 production inactive areas of MS lesions [63].
ANDRO inhibits the dendritic cells ability and generates peptide-major histocompatibility complexes required for T cell activation. In LPS-treated dendritic cells, ANDRO attenuated the upregulation of the maturation markers I-A b , CD40, and CD86 (B7.2) [16]. Besides, ANDRO also suppressed T cell function, IFN-γ, and IL-2 production [57]. These effects may contribute to ANDRO's therapeutic potential, ameliorating MS symptoms in autoimmune encephalomyelitis mice through inhibition of T-cell activation and antibody responses directed to the myelin sheath [16].

Andrographolide and Stroke
3.2.1. Ischemic Stroke. Cerebral ischemia occurs when the metabolic demand of brain is not satisfied due to an insufficient blood flow [48,64]. This involves cerebral hypoxia 3 Oxidative Medicine and Cellular Longevity leading to the death of brain tissues [65,66]. Cerebral ischemia is one of the serious causes of morbimortality worldwide. The treatment options against cerebral ischemia/ stroke are limited [67].
Some studies have shown that ANDRO (0.1 and 1 mg/kg i.p) lowered the infarct volume and neurological deficits and drastically reduced microglia cells in permanent middle cerebral artery occlusion-(pMCAO-) induced rat model [27]. On the other hand, ANDRO showed a neuroprotective effect by reducing inflammation in the MCAO-induced rat model. ANDRO abolished neuroinflammatory markers, such as IL-1β and TNF-α. ANDRO also suppressed NF-κB activation [27].

Hemorrhagic Stroke.
Intracerebral hemorrhage (ICH) occurs within brain tissue or ventricles [71] and is a prime CNS health problem all over the world with high morbimortality rates. So far, no effective strategies exist to treat this disorder, and nearly 20% of patients achieve therapeutic outcomes [72]. There are several causes of, including neuroinflammation and microglial activation. Therefore, prevention and secondary treatment of brain injury are important for patients with ICH [23,73].
In some studies, ANDRO (1 and 2 mg/kg) was able to reduce neurobehavioral damage, the water content in the brain, alleviate neuronal cell death, and degeneration in ICH-induced SBI rats [23]. Further experimental analysis shows that ANDRO has inhibitory effects on CD11b + and CD16+ microglia cells and attenuated TNF-α and IL-6 level by deactivating the NF-κB signaling pathway through reverse phosphorylation of IκBα and p65 in ICH rats [23]. Furthermore, in the ICH brain, ANDRO reduced caspase-1-caspase-recruitment domain (ASC) and NLRP3-ASC interaction, thereby inhibiting caspase-1/gasdermin D cleavage and IL-1β production [23]. In addition, ANDRO sup-presses NF-κB and NLPR3 inflammasome activation via p65 translocation assembly inhibition of NLRP3/ASC/caspase-1 complex [23] and can reduce SBI after ICH [15].
ANDRO is also evident to exert analgesic effects in Charles Foster male albino rats [76]. ANDRO (25 mg/ 20 mL) also attenuated mechanical and thermal hyperalgesia and downregulated the expression of the P2X7 receptor. Besides this, ANDRO decreased TNF-α and IL-1β expression, increased IL-10 expression, inhibited ERK signaling pathways activation, and also decreased the coexpression of GFAP and P2X7 receptors [77], thus reducing neuropathic pain in the HIV rat model.

Andrographolide and Brain Tumors.
Glioma is a tumor occurring in the glial cells of the brain or spine [78] and comprises about 30% of all brain tumors and 80% of all malignant brain tumors in CNS [79]. Targeted therapies for cancer are a rapidly advancing field for treating tumor, and natural products have become the best choice for researchers ( [80], Sharifi-Rad et al., 2021b).
3.5. Andrographolide and Depression. One of the most common neuropsychiatric disorders in the world is depression [85][86][87]. Characterized by a variety of signs and symptoms, antidepressants have been increasingly used for depression treatment in daily life, but their multiple side effects and high rates of failure have triggered the researchers' interest to find more effective and safer therapeutic strategies [88]. ANDRO (5 mg/kg) administration can improve depressivelike behaviour, as well as to attenuate the expression of proinflammatory mediators and cytokines, including NO, iNOS, COX-2, IL-1β, IL-6 and TNF-α, NF-κB signaling (p-p65, p-IκBα), and NLRP3 inflammasome assembly (NLRP3, ASC, and caspase-1) in the prefrontal cortex. Besides, ANDRO (5 mg/kg) increased Beclin1 expression and abrogated phosphorylated mTORC1 (p-mTOR), revealing autophagic activity in the prefrontal cortex of chronic unpredictable mild stress mice [54]. Beclin1 stimulates the initial stages of autophagy, and p-mTOR inhibits autophagy through ATG13 and ULK1/2 phosphorylation [89]. ANDRO-generated autophagy can attenuate depressive-like symptoms, inhibits inflammation, and shows antidepressive effects [54].

Conclusions
The antioxidant and anti-inflammatory effects of ANDRO and some of its derivatives are well-known. It is also widely recognized that substances with these kinds of properties are cytoprotective and can protect animal organs. This review gives insights on the neuro-pharmacological effects of ANDRO and its derivatives in several test systems. In light of these data, ANDRO can be considered one of the most important neuro-protective phytochemicals that can be considered as an adjuvant treatment in neurodegenerative diseases such as AD, PD, and MS. Further studies are needed to find solutions such as nanocarriers, to increase the bioavailability of ANDRO in order to cross the blood-brain barrier by incorporating in pharmaceutical formulations such as nanoparticles.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare no conflict of interest.

Authors' Contributions
All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis, interpretation, or in all these areas that is revising or critically reviewing the article, giving final approval of the version to be published, agreeing on the journal to which the article has been submitted, and confirming to be accountable for all aspects of the work.