Despite research efforts, treatments currently used for neurodegenerative diseases as already seen provide marginal symptomatic benefit. In recent years, research has focused on products of natural origin and thus phytocompounds that could be a valuable aid in combating various diseases [
49,
50,
51,
52]. Among the most studied medicinal plants
, Andrographis paniculata (
Ap) and especially its main constituent, Andrographolide (Andro), a diterpene lactone, have attracted particular interest.
Ap is a plant of Asian origin already known since ancient times under the name “King of Bitters”.
Ap is known for its anti-inflammatory, antibacterial, antiviral, antineoplastic, hepatoprotective, hypolipidemic, and hypoglycemic properties [
53,
54]. More specifically, in preclinical studies in rats, the aqueous extract of
Ap was able to result in a reduction in the expression of inflammatory markers such as TNF-α, IL-1β, IL-6 and oxidative stress makers such as ROS and Thiobarbituric Acid Reactive Substances (TBARS) with an enhancement of SOD (Superoxide Dismutase), CAT (Catalase) and GSH (Glutathione) activity [
55]. In addition, through pharmacokinetic analyses conducted on Wistar rats treated with
Ap and Andro extract, it was seen that the compound is able to easily cross the blood-brain barrier and generate effects at the neuronal level due to its high distribution capacity in the brain [
56]. Especially in the hippocampus
Ap is able to reduce the activity of LPS-induced cholinesterases, thus showing anti-inflammatory activity and improving mnemonic processes, as confirmed by spatial learning tests [
55]. The role of Andro has also been studied in the context of neuroinflammation. A study performed in vivo, by LPS-induced neuroinflammation, showed that treatment with Andro reduced cortical levels of certain chemokines such as CCL2, CCL5. Furthermore, the same authors observed the anti-inflammatory effect of Andro in vitro by inducing neuroinflammation of astrocytes with both LPS and IL-1β. In the LPS-induced neuroinflammation model, Andro treatment was able to inhibit LPS-induced TNF- α release through inhibition of NFkB and JNK, additionally increasing Nrf2 levels and HO-1 expression through the p38/MAPK and ERK-dependent signaling pathway [
57]. While in the IL-1β-induced neuroinflammation model, the compound had been able to reduce the release of CCL5, phosphorylation of NFkB p65 and IkBα, and glial fibrillary acidic protein (GFAP) [
58]. To investigate the role of Andro in LPS-induced neuroinflammation and memory, Das et al. evaluated the expression of TLR4 and its endogenous ligand High Mobility Group Box 1 (HMGB1) in primary mixed glial cells (PMLCs) in adult prefrontal cortices. Andro treatment was successful in reducing TLR4 expression and LPS-induced p-NFκB-p65 activation. In microglia, on the other hand, a reduction in CD-68 expression levels and an increase in arginase-1 levels were observed. While at the cortical level Andro was able to reduce the expression of GFAP, exerting a neuroprotective action in astrocytes through inhibition of the release of pro-inflammatory molecules such as iNOS, COX-2, nitrates, NLRP3, and caspase-1. Finally, in the context of cognitive memory processes Andro had been able to antagonize overexpression of PKC and phosphorylation of cAMP responsive element binding protein (CREB), which normally result up-regulated in cognitive deficits. The same authors also observed that at the level of the prefrontal cortex Andro had reduced levels of β-amyloid, APP, ptau, BACE-1 (β-secretase-1), activation of the pro-apoptotic caspase-3 and bax genes and increased levels of the anti-apoptotic gene Bcl-2, improving neuronal viability. At the hippocampal level, on the other hand, an increase in Post Synaptic Density Protein 95 (PSD-95) and synapsin was observed, resulting in increased neuronal plasticity, reducing the expression levels of inflammatory cytokines such as TNF-α and Macrophage Inflammatory Protein-1 (MIP-1), and an increasing those of TGF-β and IL-10 [
59]. In another study conducted by Adedayo et al, the effect of Andro in counteracting the amnesic effect of scopolamine, similar to that found in AD, was evaluated. Rats treated with the aqueous extract of
Ap, showed an improvement in cognitive function, as assessed by the Y-Labyrinth Test and the Morris Water Maze (MWM) Test, and through other analyses performed, significant reductions in AChE and Buttiril-Che, of some Monoamine Oxidases (MAO), purinergic enzymes such as ATPdase, ADPdase, while the levels of 5-Nucletidase and Adenosine Deaminase at the hippocampal level were found to be increased [
60]. Because neuroinflammation also plays a key role in diseases such as depression and schizophrenia [
61,
62], some studies have wanted to investigate the possible role of Andro in these diseases. In an in vivo model of schizophrenia induced by the administration of phencyclidine (PCP), Andro reduced the levels of IL-1β and TNF-α, p-p65, p-IκBα, p-p38 and p-ERK1/2 in the prefrontal cortex, improved oxidative stress instead through the activation of antioxidant enzymes such as SOD, CAT and GSH-Px and increased the levels of NRF-2, HO-1 and NQO-1. Cognitive improvement was also confirmed by behavioral tests that showed increased locomotor activity [
63]. Andro was also found to be effective in the treatment of stress-induced depression. In fact, mice subjected to chronic unpredictable mild stress (CUMS) and treated with Andro showed reduced levels of NO, iNOS, IL-1β, IL-6, TNF-α, COX-2, p-p65, p-IκBα, and NLR family pyrin domain containing 3 (NLRP3) in the prefrontal cortex, compared with untreated mice. In addition, Andro exerted a pro-autophagic action through increased Beclin-1 expression and decreased p-mTOR [
64]. In a recent study, the effects of Andro in the treatment of neurological disorders induced by heavy metal exposure were evaluated. Specifically, the effects of Andro on aluminum intoxication in Drosophila were evaluated. Andro treatment on flies resulted in improved survival, improved locomotor performance, and improved learning and memory through reduced AChE and MAO activity and increased catalase activity [
65]. In 2005, Tobar et al., also observed the protective action of Andro in the treatment of Multiple Sclerosis, an autoimmune neurodegenerative disease in which immune defenses and particularly lymphocytes, attack components of the nervous system. Indeed, in dendritic cells (DCs) pulsed with hen’s egg ovalbumin (OVA) Andro counteracted the generation of peptide-MHC complexes required for T-cell activation by inhibiting the up-regulation of maturation markers I-Ab, CD40 and CD86 in LPS-treated dendritic cells. Andro treatment was also tested in vivo on C57Bl/6 mice with experimental autoimmune encephalomyelitis (induced with the peptide MOG35-55) by significantly reducing the incidence of the disease as also demonstrated by reduced production of IFN and IL-2 [
66]. Therefore, based on the data reported in the literature, we can state that both
Ap and Andro are effective in treating the inflammatory and degenerative effects that occur during various disease states in the nervous system [
54,
55,
67,
68]. Here we will examine these effects on diseases involving the nervous system focusing on those most prevalent in the world population.
4.1. Alzheimer Disease
In vitro studies showed that andrographolide and some of its derivatives were able to activate α-secretase, which is involved in the inhibition of Aβ formation, and to inhibit β-secretase, which is involved in Aβ formation, without the protective treatment causing toxicity [
69]. On hippocampal neuronal cells of HT-22 mice, treatment with Andro increased the levels of nuclear factor erythroid-derived 2-like 2 / Kelch-like ECH-associated protein 1 (Nrf2/Keap), Antioxidant Response Element (ARE) gene, and HO-1 enzyme. Since these factors are major players in anti-inflammatory and antioxidant responses, again Andro was able to induce a cytoprotective response in the brain [
70]. On mouse microglia BV-2 (microglial cells derived from C57/BL6 murine), on the other hand, Andro reduced the expression of Aβ improving neuronal viability and, through the NF-kB-mediated signaling pathway, reduced the levels of TNF-α, COX-2, PGE2, i-NOS, NO, and cytokines such as IL-1β, IL-6, protecting neurons from damage produced by inflammation [
71].In addition to Andro, its analogs have also been used on these same cells, achieving complementary results. Indeed, the analogs have shown the ability to inhibit LPS-induced NO production but also iNOS expression, TNF-α and IL-6 production [
72]. One enzyme known to be involved in learning and memory processes and especially in tau protein phosphorylation and increased β-amyloid production is Glycogen Synthase Kinase-3β (GSK3β) [
73]. Andro treatment on primary hippocampal neuronal cultures induced inhibition of GSK3β enzyme and reduction of its active form [
74]. The same treatment conducted on HEK293 cells (Human embryonic kidney cells) showed comparable results. Furthermore, because GSK3β is involved in the Wnt/β-catenin signaling pathway known for its role in neurogenesis, the possible involvement of Andro on this pathway was also investigated [
75]. Andro treatment on hippocampal cells showed an induction of Wnt gene transcription by restoring its proper activity [
74]. Several studies on the effects of
Ap have also been conducted in mouse models. In fact, in the literature, we find several transgenic mouse models used to reproduce Alzheimer’s disease [
76,
77]. In one study in which Andro was tested on 2-month-old mice that had mutations on genes encoding for APP and presenilin (PS1), known proteins involved in AD, increased expression of the Wnt/β-catenin signaling pathway was observed [
78]. In another experiment in which mice were used instead with ages 7-12 months, Andro was able to alter the maturation of amyloid plaques in the cortex and hippocampus; in fact, in the early stages of the disease, the number of plaques was found to be reduced. Furthermore, in the same study, Andro was also observed to exert its effects on tau protein phosphorylation, leading to a significant increase in post-synaptic proteins including Shank, GluN2B, GluA2, PSD-95, and an increase in the inactive form of GSK-3β. While behavioral tests found that Andro-treated mice showed improved learning and lower latency values and improved spatial memory performance [
79]. In addition to transgenic mouse models, some work has also been conducted on Octodon degus mice, which are capable of naturally reproducing the neurodegeneration and neurological signs of Alzheimer’s with advancing age [
77]. Rivera et al., performed several behavioral tests on these 12- and 56-month-old rodents to assess their degree of spatial learning and memory after Andro administration. The results obtained showed neuroprotective effects that resulted in recovery of memory and learning, enhancement of excitatory postsynaptic field potential (fEPSP), protection of certain proteins such as synaptophysin (SYP), increased vesicular glutamate transporter 1 (vGluT1) and NMDA receptor subunit GluN2A. Their results also showed a decrease in phosphorylated tau protein and Aβ aggregate maturation in aged mice [
80]. In contrast, on these same animals, Lindsay et al. reported the neuroprotective effects of Andro through a reduction in Aβ, GFAP, IL-6, COX-2, and oxidative stress markers such as 4-HNE (4-Hydroxynonenal) and N-Tyr (N-Tyrosin) in the brain [
81]. It is well known that impaired glucose metabolism at the neuronal level may be related to several neurodegenerative diseases, including AD [
82,
83,
84,
85,
86,
87]. Cognitive deficits and symptoms comparable to AD are also reproducible following intracerebroventricular administration of streptozotocin (SZT). Rats that underwent this treatment and treated with Andro show improved spatial memory in the Morris Water Maze (Morris Water) and the Elevated Plus Maze test compared with the untreated group of animals. Andro also reduced levels of neuroinflammatory markers such as TNF-α, IL-1B and IL-16, reduced levels of the neurotransmitter glutamate (GLU), and increased levels of GABA. Because STZ administration induces increased levels of AChE and p-tau, the group of animals that had received Andro treatment showed a significant reduction in the expression levels of AChE and p-tau, as well as a reduction in oxidative stress through attenuation of MDA (Malondialdehyde) and nitrite, and increased levels of GSH, SOD and catalase compared with the group treated with STZ alone [
88]. Comparable results were also obtained on diabetic rats given oral administrations of STZ and subsequently treated with Andro [
89]. In another study on rat primary hippocampal neurons, treatment with Andro resulted in increased glucose uptake through increased GLUT translocation and increased ATP production resulting in promotion of AMPK-dependent glycolysis [
90].
Figure 1.
Graphic representation of molecular mechanisms underlying Alzheimer’s Disease on which Andrographis paniculata acts.
Figure 1.
Graphic representation of molecular mechanisms underlying Alzheimer’s Disease on which Andrographis paniculata acts.
4.2. Parkinson’s disease
As already widely described, inflammation and apoptosis are important factors in various neurodegenerative diseases and Parkinson’s is one of them [
91,
92]. To study the effects of Andro in this disease, rat midbrain glia cultures were pre- and post-treated concomitantly with LPS-induced dopaminergic neurodegeneration. The results obtained showed that Andro was able to attenuate LPS-induced dopaminergic neurodegeneration by reducing the activation of microglia and inflammatory factors such as ROS, TNF-α, NO and PGE2. In addition, pretreatment with Andro on BV2 microglia cells reduced the expression of COX-2 and iNOS. However, in this work, the neuroprotection of Andro on neurodegeneration induced by 1-methyl-4-phenyl-pyridine (MPP), a metabolite of MPTP used to induce Parkinson’s, was also tested, although pretreatment with Andro failed to reduce neuronal damage [
93]. In another study conducted instead on mice subjected to intraperitoneal administration of MPTP, treatment with Andro produced positive results in all behavioral tests performed including the catalepsy test, grip strength (grip strenght) and rota rod, leading to improved motor conditions [
64]. In vitro, it was seen that LPS- and MPP-induced microglia activation was reduced following Andro administration resulting in decreased NLRP3 inflammasome activation, and these data were confirmed by inhibition of the microglial expression marker Iba-1 both in vitro and in vivo on mouse brain [
94,
95]. Andro has also been shown to ameliorate mitochondrial dysfunction through inhibition of ROS formation and maintenance of mitochondrial membrane potential (in vitro) by also promoting autophagosome formation and elimination of damaged mitochondria [
95]. The neuroprotective effects of Andro on MPP-induced damage were also studied on neuroblastoma cells (SH-SY5Y). In these cells, andrographolide-lipoic acid (AL-1) conjugate protected against MPP-induced damage leading to increased cell viability and inhibited phosphorylation of NF-κB p65 and IκBα. While in MPTP-treated mice, AL-1 protected against the loss of TH-positive dopaminergic neurons in the substantia nigra pars compacta, attenuated dopamine loss in the striatum, and improved motor functions as demonstrated by behavioral tests [
96]. However, in a later study performed by Ketterman et al, in SH-SY5H cells, administration of Andro did not demonstrate protective effects against oxidative stress [
94]. Dysregulation of GLU levels is involved in several processes that may result in neuronal damage [
97,
98,
99]. In this regard Yang et al., in 2014, studied the neuroprotective effect of
Ap extract on HT22 neuronal cells in the mouse hippocampus following glutamate-induced damage. The results showed that
Ap can significantly reduce GLU-induced neuronal mortality as well as reduce cytosolic lactate dehydrogenase (LDH) levels, reduce Ca2+ influx, and reduce intracellular ROS production induced by GLU in a dose-dependent manner. In addition, the same authors also observed a significant decrease in the phosphorylation of MAPK, p38, ERK and JNK, while Western blotting analysis revealed a restoration of the levels of anti-apoptotic proteins such as Bcl-2, Bid, Bax and a reduction in apoptosis-inducing factor (AIF) after Andro treatment [
70].
Figure 2.
Graphic representation of molecular mechanisms underlying Parkinson’s disease on which Andrographis paniculata acts.
Figure 2.
Graphic representation of molecular mechanisms underlying Parkinson’s disease on which Andrographis paniculata acts.
4.3. Brain Ischemia-reperfusion injury
Cerebral ischemia is a pathological condition characterized by reduced blood supply that can affect different areas of the brain and induce major neuronal damages [
100]. In a study conducted on mouse brain endothelial cells, no positive effects were found following the administration of Andro, as the compound not only did not improve the viability of these cells but induced a marked reduction in the number of neuronal cells as a result of increased LDH release and increased apoptosis, blocking cell growth in the G0/G1 phase [
101]. However, from many other studies instead, especially in recent years, the protective action of Andro and its derivatives on ischemic damage seems to emerge and be confirmed. More specifically, in a rat mouse model with permanent middle cerebral artery occlusion (pMCAO), Andro reduced infarct volume and microglia activation in peri-infarct areas by going against increased levels of inflammatory markers such as IL-1β, TNF-α, PGE2 and the transcription factor NF-kB [
102]. In another study, however, rats undergoing ischemic brain injury/reperfusion (CI/R) treated with Andro showed decreased cerebral infarction, reduced superoxide anion and nitrotyrosine, reduced expression of gp91phox/NOX2, IL-1β and Hypoxia Inducible Factor (HIF), as well as reduced p65NFkB, thus confirming the protective effects of Andro in counteracting an ischemic attack [
103]. Mice with CI/R-induced oxidative brain damage and treated with Andro also reported reduced magnitude of infarction and subsequent neurological deficits, accompanied by reduced free radical production, nitrotyrosine, CD11b, NOX2 and iNOS formation [
104].
Another andrographolide derivative, triacetylandrographolide (CX-10), has been used to test possible neuroprotective effects against cerebral ischemia. On the macrophage cell line RAW264.7, both Andro and CX-10 counteracted NO and TNF-α production. While in BALB rats, CX-10 reduced LPS-induced TNF-α production. In Sprague-Dawley rats with middle cerebral artery occlusion (MCAO), reduced infarct size as well as improved motor performance were observed. At the level of brain tissues, CX-10 reduced the levels of TNF-α and IL-1β and increased the activity of antioxidant enzymes such as SOD, CAT, and GSH-P. Western blot analysis also showed the positive effects of Andro in reducing the expression levels of TLR4, NF-κB, TNF-α, iNOS proteins by increasing the expression of Nuclear factor erythroid 2-related factor 2 (Nrf2) and HO-1 [
103]. In mouse Cerebral Endothelial Cells (CECs), treated with 10μM Andro, an increase in HO-1 expression mediated by phosphorylation of ERK1/2, p38 MAPK and JNK1/2, and an increase in Heme oxygenase 2 (HO-2) protein, via the Nrf2 signalling pathway, was observed. In this way, Andro was able to protect neuronal cells from cell death induced by oxygen-glucose deprivation (OGD), thus improving cell viability. While in rats subjected to MCAO, Andro’s antioxidant effect in counteracting free radical production was observed, and a reduction in cerebral oedema and infarct volume was observed in brain tissue analyses [
105].
Figure 3.
Graphic representation of molecular mechanisms underlying Ischemia on which Andrographis paniculata acts.
Figure 3.
Graphic representation of molecular mechanisms underlying Ischemia on which Andrographis paniculata acts.