The activity of MAPK9 occurs in an organised manner within a signalling cascade that involves phosphorylation steps [20]. MAPK pathway activation occurs primarily through upstream kinases called MAP kinase kinase kinases (MAP3Ks), such as apoptosis signal-regulating kinase 1 (ASK1), MLKs, TAK1, and MEKK1-4. MAP3K activation occurs in response to cellular stimuli, including oxidative stress, endoplasmic reticulum stress, inflammatory cytokines, and excitotoxicity (Figure 2). Afterwards, MAP3K phosphorylates the downstream kinases known as MAP kinase kinases (MAP2Ks), such as MKK4 and MKK7, and these proteins phosphorylate the TPY motif of JNK family proteins, causing JNK2 activation [29].

The involvement of the stress-activated pathway via MAPK9 is critical in influencing cell fate in disease states. Oxidative stress constitutes one of the most powerful activators of JNK2 signalling. The accumulation of excessive amounts of reactive oxygen species (ROS) causes the disruption of the cell redox balance and initiates the phosphorylation cascades mediated by ASK1, which in turn results in the constant activation of JNK [30]. Furthermore, inflammatory cytokines like tumour necrosis factor-alpha (TNF-α) and interleukin (IL)-1β activate JNK signalling in response to receptor-mediated signals [31]. In neurodegenerative disorders, the activation of the MAPK9 pathway leads to continuous neuroinflammation and neurological dysfunction. Overexpression of c-Jun activation correlates strongly with the development of neuronal cell death and neurodegeneration. Increased phosphorylation of c-Jun in degenerating neurons under conditions of oxidative stress and excitotoxicity has been reported in experimental models [32].

From a physiological perspective, MAPK9 has several vital functions in maintaining cellular homeostasis and neural health. When activated under normal conditions, JNK2 triggers various adaptive stress responses to protect cells from damage and support their recovery [33]. The temporary activation of JNK signalling might increase antioxidant levels and facilitate the disposal of damaged cellular structures via autophagy. On the contrary, prolonged activation usually triggers JNK-mediated apoptotic cascades. In general, JNK-induced apoptosis occurs via two major processes: nuclear activation of AP-1, which induces pro-apoptotic genes, and the mitochondrial pathway, involving caspase activation and cytochrome c release [34].

In addition to the apoptotic pathway, MAPK9 has been found to play a role in neuronal development and differentiation. Signalling through JNK affects cytoskeletal remodelling, axon growth, and neuronal migration during early brain development [35]. JNK signalling has been shown to affect neuritogenesis and synapse formation, processes crucial to the formation of functional neural circuits. In one instance, JNK-dependent phosphorylation of microtubule-binding proteins plays a critical role in the formation of neuronal architecture and intracellular transport. Also, MAPK9 is involved in synaptic plasticity, a vital process essential for learning and memory [36]. Regulated JNK signalling influences the dynamics of synaptic protein turnover, receptor trafficking, and long-term potentiation (LTP). On the other hand, dysregulated JNK signalling leads to deterioration of synaptic function and impaired cognition, especially in AD patients, where increased JNK signalling activity is associated with synaptic loss and memory impairment. There have been reports that aberrant JNK signalling activity is implicated in the phosphorylation of tau protein. The combination of MAPK9/JNK2 forms a vital stress-response kinase that mediates signals from the external environment and within the cell to control neuronal survival, inflammatory response, and cell death [37].

Neuroinflammation is considered an important feature of several neurodegenerative disorders, in which microglial cells and astrocytes become chronically activated, resulting in the continuous production of pro-inflammatory factors that lead to neuronal death [40, 41]. However, the importance of MAPK9 (JNK2) lies in its ability to regulate neuroinflammatory pathways in response to cellular and oxidative stress. After activation, the MAPK9 pathway increases the expression of pro-inflammatory factors such as TNF-α, IL-1β, and IL-6 [31, 42].

Numerous studies have shown the importance of JNK signalling in neuroinflammation. For instance, genetic depletion or inhibition of JNK2 significantly reduces α-synuclein production and inflammatory responses in neuronal models of PD [43], indicating that JNK2 is a key factor in the neuroinflammation associated with the pathology. Additionally, studies of neuroinflammation in AD models have found elevated activation of MAPK pathway components in microglia, and this increase, in turn, corresponded to the induction of inflammation. Therefore, MAPK9 serves as a mediator between cellular stress and inflammation [44]. Hence, blocking MAPK9 activity may be a useful approach for treating neuroinflammation.

Programmed cell death, also known as neuronal apoptosis, is a significant pathophysiological hallmark of neurodegeneration, as it underlies the progressive loss of neurons seen in conditions such as AD, PD, and HD [45, 46]. MAPK9 (JNK2) is involved in inducing apoptosis by transducing intracellular signals in response to cellular stress from oxidative stress, inflammation, mitochondrial dysfunction, and accumulation of abnormal proteins [47]. Following its activation, MAPK9 phosphorylates transcription factors such as c-Jun, thus activating pro-apoptotic genes. Several studies have also shown that modulation of JNK signalling results in better autophagy-mediated degradation of aggregated Aβ and α-synuclein [48, 49].

For example, in PD models, treatment with the neurotoxin 6-hydroxydopamine (6-OHDA) led to an increase in JNK pathway activity and c-Jun phosphorylation, followed by apoptosis [50]. JNK inhibition showed a decrease in neuronal death and enhanced survival. Likewise, in AD models, Aβ peptides were found to activate the JNK signalling pathway, which led to mitochondrial dysfunction as well as neuronal death [51]. Inhibition of JNK caused a reduction in the activation of caspases, thus protecting neurons from amyloid neurotoxicity.

MAPK9 also plays a vital role in the pathogenesis of tau in AD. Tau proteins normally play a significant role in stabilising microtubules within the neurons [52]. However, in cases of hyperphosphorylation, tau leads to its aggregation, resulting in the formation of neurofibrillary tangles [53]. JNK2 phosphorylates tau, resulting in microtubule destabilisation. Increased levels of phospho-JNK and tau proteins in postmortem AD brain samples indicate a link between MAPK9 activity and AD pathogenesis.

Synaptic dysfunction in AD may be related to JNK signalling. Continuous activation of MAPK9 disrupts synaptic plasticity and neurotransmission, subsequently resulting in memory problems. Research shows that continuous activation of JNK prevents LTP, a physiological phenomenon necessary for learning and memory [54]. Therefore, MAPK9-induced synaptic damage in AD is a contributing factor towards impaired cognition.

Another factor linked to the aggregation of alpha-synuclein, a hallmark feature of PD, is MAPK9. The misfolding of α-synuclein leads to the stimulation of inflammation and oxidative stress, which then activates the JNK pathway [55]. This, in turn, causes increased protein aggregation and cell death, forming a vicious cycle. Furthermore, the experimental model that uses 6-OHDA, a neurotoxin, to produce a Parkinson-like degeneration of dopamine neurons has been extensively used to investigate this mechanism. In these studies, 6-OHDA induces the activation of JNK signalling, including MAPK9, that results in c-Jun phosphorylation and apoptosis gene expression [56]. The pharmacological inhibition of JNK signalling increases cell survival by preventing neuronal cell death.

Furthermore, a widely used model of induced Parkinsonism consists of treating mice with the toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). It was found that exposure to MPTP increases JNK activation in dopaminergic neurons. Moreover, pharmacological inhibition of JNK or the knockout of JNK2 decreases neurodegeneration and maintains mouse motor function [57]. Also, according to Ruan et al. [58], the protective effect of nicotine against PD was studied using an MPTP-induced mouse model, which induces loss of dopaminergic neurons. The results demonstrated that nicotine administration prior to MPTP exposure improved motor activity, reduced anxiety-related behaviours, and protected dopaminergic neurons in the substantia nigra and striatum. In addition, it inhibited neuronal cell death, reduced phosphorylated α-synuclein levels, and also suppressed neuroinflammation-related microglial activation. Mechanistically, nicotine was observed to regulate MAPK signalling pathways by inhibiting the JNK pathway and increasing ERK signalling, without altering the p38 pathway. This resulted in a decrease in degeneration and neuroinflammation processes in the brain.

The relevance of MAPK9 (JNK2) as a kinase associated with various stresses that contribute to neuronal death has recently been elucidated and is considered alongside classical experimental models of neurodegenerative diseases such as PD and AD. It has been found that MAPK9 is a major mediator of oxidative stress, excitotoxic injury, and inflammatory damage, each of which is prevalent in many neurological disorders [20, 21]. Activation of JNK signalling has been implicated in delayed neuronal cell death following cerebral ischemia, with increased activation of JNK2 and JNK3 observed after ischemic injury [59].

The activation of MAPK9 cascades is initiated in traumatic brain injury (TBI) when mechanical injury affects brain tissue. Such injury induces neuronal cell death via c-Jun phosphorylation, followed by upregulation of pro-apoptotic gene expression. Animal experiments indicate that blocking JNK signalling decreases lesion size and enhances recovery after injury. The involvement of MAPK9 has also been found in multiple sclerosis (MS), in which demyelination induced by inflammation is linked to increased JNK activity in glia and neurons. In experimental demyelination, JNK suppression was shown to reduce inflammatory damage and maintain neuronal structural integrity. Collectively, these studies reveal the involvement of MAPK9 in several types of neurodegenerative disorders in which neuronal damage results from cellular stress, thus making it a valuable candidate for developing neuroprotective therapies [60].

Furthermore, recent evidence has also shown that MAPK9 is involved in vascular changes that contribute to neurodegeneration [61, 62]. Chronic cerebral hypoperfusion, dysfunctional endothelium, and neurovascular unit damage lead to cognitive impairment because of increased oxidative stress, inflammatory response, and blood-brain barrier (BBB) damage [62, 63]. The activation of the JNK pathway in cerebrovascular endothelial cells leads to increased synthesis of pro-inflammatory factors, increased vascular permeability, and more severe neurotoxic effects after ischemia. Therefore, the development of vascular inflammation can potentiate the effect of MAPK9 on neuronal apoptosis and promote the development of vascular dementia and mixed neurodegenerative diseases.

Moreover, the common pathway in both traumatic and vascular neurodegeneration includes the MAPK9 signalling pathway, which could also play an important role in chronic traumatic encephalopathy (CTE), which is characterised by progressive neuroinflammation and brain injuries resulting from repetitive mild brain injuries. Contemporary studies on CTE emphasise the possible value of various diagnostic modalities, including diffusion tensor imaging (DTI), functional magnetic resonance imaging (fMRI), magnetic resonance spectroscopy (MRS), susceptibility weighted imaging (SWI) and positron emission tomography (PET), together with cerebrospinal fluid biomarkers, including phosphorylated tau, TREM2, CCL11, NfL and GFAP [64]. Importantly, both stroke and neurodegenerative disease have similar secondary pathological mechanisms such as cerebral hypoperfusion, dysfunction of the endothelium, disruption of the BBB, oxidative stress, neurovascular inflammation, mitochondrial dysfunction, and secondary neuron death. These overlapping secondary pathological mechanisms can lead to persistent activation of MAPK9/JNK signalling and exacerbate neuronal damage after the primary insult. Thus, the targeting of MAPK9-induced secondary injury mechanisms could be relevant for treatment of both conditions [65]. This implies that neuroprotection targeting MAPK9 might be applicable to other diseases as well, given similarities in secondary injury mechanisms. MAPK9/JNK is a stress-signalling pathway, not a disease-specific one. The role of MAPK9 in neurodegeneration may vary in patients with AD, PD, stroke-induced neurodegeneration, and CTE since there will be different stresses acting upstream [64, 65]. Thus, treatment of MAPK9-mediated neurodegeneration needs to be aetiology-based. Table 1 summarises some neurodegenerative diseases associated with MAPK9 dysregulation.

The protective effect of phytochemicals from medicinal plants against neurological damage occurs through several mechanisms. The first mechanism involved in the neuroprotective properties of phytochemicals is their antioxidant activity. Oxidative stress is one of the main causes of neurological disorders, as it leads to the oxidation of macromolecules and impairment of neurons’ function [40]. Phytochemicals can protect neurons from oxidative damage by donating electrons to free radicals and strengthening the antioxidant system in cells (Figure 3). Some phytochemicals, such as curcumin, quercetin, and resveratrol, stimulate activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) and increase production of such enzymes as superoxide dismutase, catalase, and glutathione peroxidase [81].

Another essential mechanism underlying the neuroprotective action of phytochemicals is their anti-inflammatory effects. Neuroinflammation plays an important role in neuronal death through microglial activation and the subsequent release of inflammatory cytokines. A number of phytochemicals have been found to block inflammation via NF-κB, MAPKs, and JNK pathways. EGCG and curcumin, for example, have been found to block microglial activation and down-regulate TNF-α, IL-1β, and IL-6 [82]. In addition, resveratrol has been found to suppress inflammation induced by the release of inflammatory cytokines in models of PD and cerebral ischemia.

Phytochemicals also possess anti-apoptotic properties by regulating both the intrinsic and extrinsic apoptotic pathways. Diseases associated with neurodegeneration are characterised by progressive neuronal cell death caused by the activation of apoptotic cascades. Various phytoconstituents can prevent apoptosis by modulating Bcl-2 proteins, caspases, and mitochondrial-related mechanisms. It has been found that curcumin and berberine can downregulate Bax gene expression and upregulate the expression of anti-apoptotic Bcl-2 proteins, thereby preventing mitochondrial membrane depolarisation. Quercetin also prevents the activation of caspase-3 [83].

A number of medicinal herbs have been explored to discover their neuroprotective actions and usefulness in neurodegenerative disorders. The plant Curcuma longa, popularly known as turmeric, is one of the most commonly studied medicinal herbs because of the presence of curcumin, which has excellent antioxidant and anti-inflammatory activities [84]. Curcumin has proven capabilities for inhibiting the formation of Aβ and reducing inflammation and oxidative stress in AD.

The other medicinal plant under discussion is Camellia sinensis (Green tea), which contains catechins such as EGCG. The major effects of EGCG include reducing oxidative stress, inhibiting protein aggregation, and attenuating the inflammatory response in neurodegeneration models. There are reports indicating that green tea catechins enhance cognitive function and protect dopaminergic neurons in PD models [85].

Ginkgo biloba is yet another well-known medicinal plant used for cognitive enhancement and neuroprotection. The active components of Ginkgo biloba are flavonoids and terpenoids, which enhance cerebral blood flow and provide protection against apoptosis and oxidative stress. Studies show promising results in improving memory and cognitive functions in mild cognitive impairment (MCI) and AD [86].

Withania somnifera (Ashwagandha) is one of the valuable medicinal herbs used in Ayurveda. Active phytochemicals present in the plant, called withanolides, possess antioxidant, anti-inflammatory, and anti-apoptotic activities. In experiments, it was found that Ashwagandha exerts its protective effect on neurons in AD, PD, and cerebral ischemia through anti-oxidation [87]. Bacopa monnieri is an Ayurvedic herb famous for its ability to enhance cognition and protect nerve cells. Bacosides are the main active components of Bacopa monnieri; they stimulate synaptic activity, decrease oxidative stress, and improve memory. Laboratory studies suggest that Bacopa monnieri prevents neurons from damage due to Aβ exposure and positively affects cognitive functions in neurodegenerative diseases [88].

In totality, the medicinal properties of plants and their bioactive phytochemicals are strong therapeutic candidates for neurodegenerative diseases because of their multitarget mechanisms. The modulation of oxidative stress, neuroinflammation, apoptosis, and mitochondrial dysfunction by these plants makes them strong candidates in the development of neuroprotective drugs focusing on the MAPK9 pathway.

Curcumin, the main polyphenol extracted from Curcuma longa, also known as turmeric, is one of the best-studied natural agents used in neurodegenerative disorders. The ability of curcumin to act as an antioxidant, anti-inflammatory, and antiapoptotic compound has been demonstrated experimentally, and many of its actions can be attributed to the inhibition of the MAPK9/JNK pathway.

For instance, a study carried out by Wang et al. [89] demonstrated that curcumin reduced JNK-3 expression and phosphorylation and attenuated neuronal apoptosis in the hippocampus of rats with Aβ-induced AD. Curcumin has also been found to inhibit activation of NF-κB and pro-inflammatory cytokines, including TNF-α and IL-1β, which could explain its anti-inflammatory effects. Moreover, curcumin prevents mitochondrial depolarisation and cytochrome c release, protecting the mitochondria [90]. As for PD, curcumin showed protective effects when administered to patients suffering from rotenone-induced and MPTP-induced neurodegeneration. In particular, curcumin inhibited JNK-mediated apoptosis, resulting in decreased loss of dopaminergic neurons and improved motor function [91].

Several in vitro and in vivo studies have reported that curcumin attenuates JNK phosphorylation and reduces neuronal apoptosis induced by oxidative stress or Aβ exposure. However, these effects vary across experimental models, doses, and formulations, and may also involve indirect regulation through upstream antioxidant and anti-inflammatory pathways, including Nrf2 and NF-κB, rather than direct inhibition of MAPK9.

Resveratrol is a polyphenolic compound naturally occurring in grapes, berries, and red wine. This compound possesses strong antioxidant and anti-inflammatory activities, along with exhibiting neuroprotective effects in multiple models of neurodegeneration [92]. The most prevalent pathway through which resveratrol exerts neuroprotective activity is its antioxidant and anti-apoptotic actions by inhibiting oxidative stress and JNK signalling pathways. Inhibitory effects of resveratrol on JNK signalling pathways involve decreased production of ROS and improved antioxidant defense by activation of sirtuin 1 (SIRT1) and Nrf2 signalling pathways. Although resveratrol can decrease the activation of JNK in various models of neurodegenerative disorders [93], the effectiveness of the treatment is determined by many factors, including the disease model used, dosage, and the duration of the treatment, as well as the decrease in oxidative stress and SIRT1/Nrf2 activation rather than MAPK9 inhibition.

Furthermore, resveratrol modulates neuroprotective pathways involving neuron survival and synaptic transmission. In models of ischemic stroke, resveratrol alleviated neuronal injury by decreasing the inflammatory response and inhibiting phosphorylation of JNK [94]. Moreover, it increased the synthesis of ATP through promoting mitochondrial biogenesis in neurons. Based on multiple actions described above, resveratrol is considered an efficient phytochemical capable of modulating MAPK9-related neurodegeneration.

Quercetin is a type of flavonoid found abundantly in fruits, vegetables, onions, apples, and various medicinal herbs. Quercetin is notable due to its strong antioxidant properties and ability to regulate cellular signalling pathways involved in oxidative stress and inflammation. Quercetin’s protective actions on the nervous system involve ROS scavenging and regulation of MAPK signalling pathways, including JNK activation.

In vitro experiments have established that quercetin can reduce neuronal injury induced by oxidative stress due to its ability to remove free radicals and increase the activity of antioxidant enzymes [95]. In neurons exposed to hydrogen peroxide, quercetin has been found to decrease JNK phosphorylation and apoptosis of neurons under oxidative stress conditions. Yet, clinical studies have not been conducted, and its neuroprotective action most probably involves the regulation of more than one signalling pathway, both antioxidative and anti-inflammatory [96]. Quercetin also acts as an anti-inflammatory agent by suppressing NF-κB activation and the secretion of pro-inflammatory cytokines. In AD animal models, quercetin has been demonstrated to reverse Aβ-mediated neuronal damage and enhance cognitive function. Moreover, quercetin provides protection against mitochondrial dysfunction by stabilising mitochondrial membranes and reducing ROS generation. In PD animal models, quercetin caused a decrease in dopaminergic neuron degeneration and improvement of motor dysfunction caused by oxidative stress-induced MAPK signalling pathways. The aforementioned evidence highlights the potential of quercetin in treating MAPK9-mediated neurological disorders.

EGCG, the main catechin in Camellia sinensis (green tea), is a widely studied phytochemical known for its neuroprotective properties through interactions with the MAPK pathway. The first mechanism underlying the neuroprotective action of EGCG is its ability to inhibit protein aggregation. It was found that EGCG prevents the accumulation of Aβ protein and reduces protein aggregation. Moreover, EGCG can reduce JNK activation in response to exposure to Aβ and induce a decrease in cell apoptosis [97]. Another effect of EGCG is the inhibition of hyperphosphorylation of the tau protein, thereby inhibiting the development of neurofibrillary tangles.

Another neuroprotective property of EGCG is mitochondrial protection. It reduces ROS generation, stimulates ATP production, and maintains mitochondrial membrane stability. EGCG protects dopaminergic neurons from oxidative stress and mitochondria-mediated apoptosis in animal models of PD. Moreover, EGCG can inhibit microglial activation and inflammatory cytokine production. Thus, these studies have shown a decreased activation of JNK due to treatment with EGCG in animal models of AD and PD. However, such observations are not always consistent, and the effect might be a consequence of decreased oxidative stress, mitochondrial preservation, and neuroinflammation suppression.

The isoquinoline alkaloid berberine, isolated from herbs such as Berberis vulgaris, has great potential as a neuroprotective agent due to its anti-inflammatory and antioxidant properties. Specifically, berberine regulates several pathways related to neuronal survival, including the MAPK and NF-κB pathways. Several studies have found that berberine can inhibit JNK phosphorylation in neuroinflammatory and ischemic models. Nevertheless, it appears from existing studies that such an effect is part of a more complex regulation of MAPKs and NF-κB signalling pathways, which complicates ascribing neuroprotective effects only to MAPK9 inhibition [98].

Moreover, berberine alleviates oxidative stress through increased antioxidant enzyme activity and inhibition of ROS production. For instance, berberine enhances cognitive outcomes and reduces Aβ levels in Alzheimer’s models. Additionally, the phytochemical acts on mitochondria by boosting ATP synthesis and protecting mitochondrial membranes.

Other new phytochemicals that can regulate MAPK9-mediated pathways include luteolin, kaempferol, and ginsenosides. Particularly, luteolin is a flavonoid found in celery and green peppers that can inhibit the activation of microglia and block JNK-induced inflammatory signalling. On the other hand, kaempferol reduces oxidative stress and neuronal death in ischemia models. Ginsenosides are compounds derived from Panax ginseng that induce neuroprotection by modulating MAPK and mitochondrial pathways. Medicinal plant bioactives constitute novel candidates in MAPK9-related neurodegenerative diseases. This is attributed to their ability to regulate numerous pathological processes, including neuroinflammation, oxidative stress, apoptosis, and mitochondrial dysfunction. Table 2 highlights some reported medicinal plants and active phytochemicals targeting MAPK9 (JNK2) in neurodegenerative diseases.

While there are many reports on the ability of several phytochemicals to inhibit JNK phosphorylation, care should be taken when drawing conclusions from these results. The majority of these data are based on in vitro and animal experiments, in which various disease models and doses were used. Furthermore, most of the time, the decrease in JNK phosphorylation was due to a reduction in oxidative stress, improvement in mitochondrial function or alteration of upstream signalling pathways instead of inhibition of MAPK9 itself.

While JNK3 has been found to be the most studied isoform in relation to neurodegenerative disorders due to its primary neuronal expression, inhibition of JNK3 alone may not be enough to inhibit the complicated inflammatory and oxidative responses involved in the pathogenesis of such disorders. In contrast to JNK3, MAPK9 (JNK2) has been found to be expressed in neurons, astrocytes, microglial cells, and immune cells [14]. This enables MAPK9 to regulate not only neuronal apoptosis but also neuroinflammation. From experimental evidence, it has been seen that JNK2 participates in cytokine secretion, oxidative stress enhancement, mitochondrial damage, and protein aggregation [106]. However, the widespread occurrence of JNK2 has its own dangers, as the side effects in the system after long-term inhibition need to be considered. Thus, future research efforts should be concentrated on the creation of brain-specific inhibitors for MAPK9 that could provide maximum protection to the neurons without causing any toxic effects outside the nervous system. JNK2 and JNK3 proteins can thus be considered complementary members of the JNK pathway rather than competing therapeutic targets.

Bioactive molecules extracted from medicinal plants that could act on MAPK9/JNK2, a MAPK pathway, have shown considerable neuroprotective effects against neurodegeneration in animal studies. Nevertheless, the clinical efficacy of these drugs is often determined by the permeability of the molecules through the BBB. The BBB acts as a natural barrier that regulates the transport of foreign particles into brain tissue while protecting it from harm caused by external sources [107]. As such, several neuroprotective phytochemicals fail to enter the brain due to the BBB.

Most natural bioactives have physicochemical constraints, including low solubility, poor membrane permeability, and rapid metabolic degradation, resulting in minimal BBB permeation [108]. In addition to acting as a barrier to drug delivery, BBB dysfunction is a key pathology associated with neurodegenerative diseases. It has been observed that MAPK9-mediated inflammatory signalling leads to endothelial cell damage, loss of tight junction protein function, and dysfunction of neurovascular units, and thus facilitates immune cell entry and chronic neuroinflammation. This means that therapy targeting MAPK9 can serve a double purpose.

There is substantial evidence for curcumin’s anti-inflammatory and MAPK9-inhibitory actions, although its clinical application is hindered by limited brain uptake and poor pharmacokinetics [41, 90, 109]. Furthermore, the antioxidant and anti-apoptotic actions of quercetin and resveratrol are significant in mitigating cellular damage, although they exhibit poor CNS bioavailability when administered orally [110]. Despite these challenges, phytochemicals such as EGCG and berberine have exhibited BBB penetration and protective effects in experimental models of neurodegenerative conditions. Knowledge of the physiological factors that determine BBB permeation of compounds is vital to the design of therapies based on plant derivatives.

Another key drawback is that poor bioavailability poses a significant constraint in the use of medicinal plants’ phytochemicals for therapeutic purposes. Many phytochemicals get degraded and metabolised quickly, thereby posing a challenge to their therapeutic efficiency due to reduced bioavailability. Curcumin is one such compound, which, despite having excellent neuroprotective effects, shows poor bioavailability due to its poor absorption rate, metabolic rate, and short plasma half-life [111]. Another such compound is resveratrol, which shows a high first-pass effect [112]. For example, Almeida et al. [113] conducted a randomised, double-blind, placebo-controlled clinical study assessing the pharmacokinetics and safety profile of trans-resveratrol in a randomised, double-blind and placebo-controlled study among healthy human volunteers. They were administered doses ranging from 25 to 150 mg, six times per day. The study indicated that trans-resveratrol is highly absorbed and achieves maximum plasma levels within 0.8 to 1.5 hours following dosing. While the drug showed high tolerance and produced only minor side effects, its plasma levels were still low because of low bioavailability and quick clearance from the body [113].

Some approaches used to address the above-mentioned barriers to the effective delivery of phytochemicals include structural modification, formulation optimisation, and various encapsulation techniques. Nano-curcumin (NCMN) has demonstrated improved bioavailability, enhanced ability to cross the BBB, and more pronounced neuroprotective effects in AD [107, 108].

Development of novel drug delivery systems has greatly increased the potential of neuroprotective phytochemicals. Nanotechnological formulations, including nanoparticles, liposomes, polymeric micelles, dendrimers, and solid lipid nanoparticles, serve as efficient tools to increase BBB penetration and targeted delivery to neuronal cells. Such systems protect phytochemicals against degradation, improve their solubility, and facilitate controlled drug delivery [108].

The use of nanoparticle delivery of curcumin led to higher brain uptake and decreased neurotoxicity caused by Aβ deposition in AD models [116, 117]. Also, delivery of EGCG via nanoparticles was effective in protecting mitochondria and inhibiting dopaminergic neuronal loss in PD models [118]. The development of intranasal drug delivery systems is gaining popularity as it does not require passage across the BBB and uses the olfactory route to deliver drugs directly into the brain. Such an approach can increase the efficiency of therapy for MAPK9-dependent neurodegenerative diseases.

Another potentially effective approach to enhancing neuroprotective efficacy is combination therapy. Given that neurological diseases such as AD and PD have several interrelated pathological pathways, monotherapies are unlikely to deliver significant benefits. A combination of phytochemicals from medicinal plants with pharmaceutical drugs or other biologically active compounds could offer improved efficacy through synergistic effects. A number of studies have shown the benefits of using combination therapies of phytochemicals. Combination therapy of curcumin and donepezil was more effective in improving cognitive function and neuroinflammation than each agent alone in AD [119]. Beltagy et al. [119] examined the therapeutic value of NCMN, both alone and in combination with donepezil, in a rat model of AD. The findings revealed that NCMN significantly enhanced cognitive function, locomotor ability, and neuronal protection through antioxidant, anti-inflammatory, anti-tau hyperphosphorylation, and anti-cholinesterase properties. It should be noted that NCMN also acted as an active agent in modulating the PI3K/AKT/GSK-3β signalling pathway, which is very important in neuronal protection and survival. The use of NCMN along with donepezil showed better results than either alone [119].

Similarly, Resveratrol combined with levodopa (LD) was more effective at increasing dopaminergic neuronal survival and reducing oxidative stress in PD. For example, Liu et al. [120] studied the synergistic effect of LD and resveratrol on rotenone-induced neurodegeneration in PD using cellular and animal models. Their study results revealed that the combination of LD and resveratrol inhibited ROS generation, improved mitochondrial function, and prevented cell death. Additionally, they inhibited inflammation by downregulating the toll-like receptor (TLR)-2 and TLR-4 signalling pathways, thereby reducing the secretion of inflammatory cytokines. In rotenone-exposed rats, there were improvements in motor function, enhanced superoxide dismutase (SOD) activity, and diminished malondialdehyde (MDA) levels [120]. Thus, combination therapies can reduce doses of pharmaceutical drugs and lower their side effects. In addition, combination therapies targeting MAPK9 and other related signalling pathways could provide more effective neuroprotection.

Precision medicine holds promise for advancing personalised neuroprotective strategies against MAPK9. Neurodegenerative disorders are characterised by significant heterogeneity in terms of their pathogenesis due to genetic predispositions, differences in inflammation profiles, disease progression, and responsiveness to therapy among different patients [121]. Modern advances in genomics, proteomics, transcriptomics, and computational biology have enabled the identification of biomarkers that correlate with MAPK9 dysfunction and neuronal stress responses. These biomarkers can help classify patients and apply an individualized phytochemical approach. For instance, individuals exhibiting signs of increased oxidative stress or activation of the MAPK9 pathway are likely to respond positively to antioxidant compounds such as curcumin, quercetin, or EGCG [122]. Moreover, innovative computational techniques, such as molecular docking, artificial intelligence (AI), and network pharmacology, can help identify novel plant bioactives with selective activity against MAPK9.

On balance, bioactive compounds from medicinal plants targeting MAPK9 hold great promise for the treatment of neurodegenerative conditions (Table 3). Despite the considerable difficulties posed by issues such as the penetration of these drugs across the BBB, insufficient bioavailability, and limited clinical translatability, several potential solutions are emerging.

Phytochemical therapeutics offer a unique advantage in neuroprotection by simultaneously targeting multiple disease mechanisms, including MAPK9-mediated oxidative stress, neuroinflammation, and neuronal apoptosis. However, challenges related to bioavailability, blood-brain barrier penetration, standardisation, and limited clinical evidence must be addressed before these compounds can be widely adopted as effective therapies for neurodegenerative diseases.

While having shown promising preclinical efficacy, the clinical development of phytochemicals that target MAPK9 faces challenges due to low aqueous solubility, fast metabolism, inability to penetrate the BBB, and inconsistency in pharmacokinetic profiles. Thus, nanoformulation technology has been recognised as an appropriate way to overcome these obstacles rather than merely being used as carriers for the drugs. For instance, polymeric nanoparticles, solid lipid nanoparticles, nanostructured lipid carriers, liposomes, dendrimers, and nanoemulsions enhance the stability of phytochemicals, enable prolonged circulation in the bloodstream, control release, and increase BBB passage via receptor-mediated or adsorptive endocytosis [123]. Similarly, NCMN has significantly higher oral bioavailability and accumulation in the brain compared with curcumin, leading to better antioxidant, anti-neuroinflammatory, and anti-apoptotic effects in AD mouse models [124]. Furthermore, nanoencapsulated EGCG and resveratrol have an increased ability to enter neurons and protect mitochondria [125].

Yet, there are some limitations associated with the use of nanoformulations as well. The effectiveness of these formulations will be determined by factors such as the particle size, surface charge, entrapment efficiency, stability of the formulation, repeatability, and safety of the manufacturing process, which need to be optimised to facilitate successful transition to the clinical setting [126]. In addition, there are no examples of the nanoformulations targeting MAPK9 that have passed preclinical testing; thus, comparative analysis of various nano-carriers under the same conditions is still very rare.

Effective translation of the MAPK9 phytochemicals in the clinic will necessitate the development of innovative imaging and biomarker approaches in order to optimise patient stratification, therapeutic response monitoring and pharmacological drug development. Advanced neuroimaging modalities such as structural MRI, DTI, fMRI, and PET can be used to assess the presence of brain atrophy, white matter pathology, neuroinflammation, and brain metabolism non-invasively [127]. Imaging techniques involving the use of radiolabeled ligands against Aβ, tau, and activated microglia might be considered as an approach to elucidate whether targeting MAPK9 signalling leads to a decrease in the disease-related burden. Fluid biomarkers, including p-tau, NfL, GFAP, inflammatory cytokines, and markers of oxidative stress, can serve as a quantitative measure of neuronal injury and disease progression [128]. As MAPK9 modulates oxidative stress, apoptosis, and neuroinflammation, integration of imaging with biomarkers will allow early detection of treatment responses and personalised medicine strategies.

An equally important element to consider here is the utilisation of an extended set of biomarkers, since there are multiple pathological mechanisms which interact in order to bring about neurodegenerative disorders. As opposed to a single biomarker, a multimodal panel should be used in this case, consisting of neurofilament light chain, total and phosphorylated tau, glial fibrillary acidic protein, TNF-α, IL-1β, IL-6, MDA, glutathione, superoxide dismutase, as well as BBB integrity markers. Furthermore, incorporation of transcriptomic, proteomic and phosphoproteomic signatures that indicate the activation of MAPK9 would make the procedure even more efficient. Altogether, with the help of neuroimaging in combination with an extended biomarker panel, it will be possible to implement precision medicine that will enable early detection of disease progression, as well as a more accurate objective measurement of MAPK9-targeted therapies.

Molecular docking remains one of the most utilised methods in structure-based drug design. The technique predicts the best way that a compound docks in the receptor site of a protein and also measures the strength of interaction between a protein and a ligand [129]. In neurodegenerative diseases, molecular docking has emerged as an important technique for identifying bioactive compounds from medicinal plants that can inhibit MAPK9 activity. Furthermore, with the availability of high-resolution crystal structures of MAPK9, molecular docking becomes possible and enables the identification of the binding energy of phytochemicals to crucial catalytic and allosteric sites of the protein [130]. Molecular docking studies often assess hydrogen-bond formation, hydrophobicity, electrostatic attraction, and binding energy to establish the feasibility of the interaction [131].

Among several phytochemicals, curcumin, quercetin, resveratrol, luteolin, kaempferol, and berberine exhibit good docking potential towards MAPK9/JNK proteins. For instance, molecular docking studies have revealed that curcumin can form stable hydrogen bonds with residues in the ATP-binding site of JNK enzymes, suggesting potential inhibition of their activity [132]. Likewise, quercetin can strongly interact with catalytic residues taking part in the phosphorylation process, thereby possibly preventing the subsequent activation of c-Jun and inflammatory pathways [133].

Virtual screening can be used in addition to docking to quickly test a large number of chemicals against the target protein. Phytochemical libraries, such as ZINC, IMPPAT, NPASS, and Traditional Chinese Medicine Libraries, may be screened for MAPK9 inhibitors [134–136]. This technique saves time by eliminating the need to test a large pool of compounds in the laboratory. Some recent studies have successfully identified new types of flavonoids and polyphenols with high affinity for JNK proteins using virtual screening approaches [137, 138]. For example, dos Santos Maia et al. [139] assessed the possibility of utilising lignans as multitarget drugs for AD by applying the QSAR, docking, and Absorption, Distribution, Metabolism, Excretion, and Toxicity (ADMET) approaches. As a result, 139 lignans were found to exhibit activity against several targets implicated in neurodegenerative processes, including MAPK9-linked JNK-3, PTP1B, NOX1, NQO1, PDE5, Nrf2, COX-2, and iNOS. Six lignans, including austrobailignan 6, anolignan C, 7-epi-virolin, compound 64, ococymosin, and mappiodoinin B, exhibited high neuroprotective and antioxidant properties. It was concluded that lignans are suitable candidates for multitarget drug development against AD.

Although docking gives a static view of interactions between the protein and ligand, MD studies give a dynamic view by observing the interactions that occur during a period of time under physiological conditions [140]. It is now mandatory to validate docking results using MD simulation and also to determine the stability of plant secondary metabolite-MAPK9 complexes. Thus, using Newtonian mechanics, the MD simulation technique helps researchers analyse the atomic movement of protein-ligand interactions. Various properties like root mean square deviation (RMSD), root mean square fluctuation (RMSF), radius of gyration (Rg), hydrogen bonding occupancy, solvent accessible surface area (SASA), and binding energy calculations help evaluate the stability and functional aspects of protein-ligand interactions [141].

For example, studies investigating the complex between curcumin and JNKs have exhibited protein-ligand interactions that were stable over 100 ns of MD simulation trajectories, indicating that curcumin can stabilise itself at the kinase’s active site under physiological conditions [142]. Likewise, EGCG and resveratrol have been found to adopt stable conformations upon binding to JNKs in MD simulations, making them likely candidates for neuroprotective inhibitors [143, 144]. Therefore, MD simulation is extremely useful for studying the induced-fit effect on MAPK9 induced by ligand binding, as it may help identify new allosteric binding sites and thus develop more selective inhibitors. Additionally, free energy calculations using MM/PBSA or MM/GBSA provide quantitative data on binding energy, making computational predictions more reliable before proceeding to experimentation.

AI and ML techniques are transforming modern pharmaceutical discovery through fast screening and analysis of large biological and chemical datasets. They show promise in significantly accelerating the identification of medicinal plant compounds that inhibit MAPK9 and other protein kinases linked to neurodegenerative diseases.

ML methods could be used with available data on kinase inhibitors to predict biological activities, toxicity, pharmacological characteristics, and BBB penetration capacity of phytochemicals [145]. Random forests, support vector machines, gradient boosting, and deep learning methods achieved strong performance in predicting compound-target interactions [146].

In the case of natural product discovery, AI techniques could be applied to select phytochemicals with the highest likelihood of inhibiting MAPK9 before any docking or laboratory experiments. Moreover, ML techniques can reveal hidden structural features associated with kinase inhibition, helping to discover novel chemical scaffolds in medicinal plants [147, 148].

These advances in generative AI technology have further increased opportunities to discover neuroprotective drugs. New phytochemical compounds designed using GANs, VAEs, and transformer architectures could be developed with enhanced pharmacokinetic profiles to offset problems arising from their low bioavailability and poor target selectivity. Moreover, AI-driven predictions of compound permeability across the BBB and of their ADME/Tox properties could reduce the likelihood of failure and increase the likelihood of success in the discovery of treatments for neurological disorders.

Whereas traditional medicines act on a single target site at a time, bioactive compounds from medicinal plants often exert their therapeutic effects through multiple molecular targets. Network pharmacology has become an increasingly popular field of systems biology that studies the complex mechanism of action of plants [149]. Network pharmacology combines bioinformatics, pharmacology, genomics, proteomics, and systems biology to study network interactions among natural compounds, proteins, signalling pathways, and diseases. It is especially useful for neurodegenerative conditions because of the presence of several interrelated pathological factors, including oxidative stress, neuroinflammation, mitochondrial dysfunction, protein aggregation, and apoptosis.

Network pharmacology techniques help determine key molecular hubs and signalling pathways modulated by phytochemicals from medicinal plants. Studies done on the compounds curcumin and resveratrol have shown involvement in the MAPK9, NF-κB, PI3K/Akt, Nrf2, and apoptotic pathways, among others [150]. This shows the importance and the wide spectrum of activity of these compounds. Similarly, studies on the plants Withania somnifera and Bacopa monnieri have shown regulation of multiple neuronal targets, including those governing neuronal survival and synaptic transmission [151, 152].

By integrating information from transcriptomics and proteomics datasets, researchers will be able to establish how phytochemicals modulate cellular signalling pathways in their entirety, rather than individual proteins. The application of molecular docking, MD simulations, AI, ML, and network pharmacology is revolutionising the screening of bioactive compounds from medicinal plants for MAPK9 targeting.