Multi-Target Effects of ß-Caryophyllene and Carnosic Acid at the Crossroads of Mitochondrial Dysfunction and Neurodegeneration: From Oxidative Stress to Microglia-Mediated Neuroinflammation

Abstract

Inflammation and oxidative stress are interlinked and interdependent processes involved in many chronic diseases, including neurodegeneration, diabetes, cardiovascular diseases, and cancer. Therefore, targeting inflammatory pathways may represent a potential therapeutic strategy. Emerging evidence indicates that many phytochemicals extracted from edible plants have the potential to ameliorate the disease phenotypes. In this scenario, ß-caryophyllene (BCP), a bicyclic sesquiterpene, and carnosic acid (CA), an ortho-diphenolic diterpene, were demonstrated to exhibit anti-inflammatory, and antioxidant activities, as well as neuroprotective and mitoprotective effects in different in vitro and in vivo models. BCP essentially promotes its effects by acting as a selective agonist and allosteric modulator of cannabinoid type-2 receptor (CB2R). CA is a pro-electrophilic compound that, in response to oxidation, is converted to its electrophilic form. This can interact and activate the Keap1/Nrf2/ARE transcription pathway, triggering the synthesis of endogenous antioxidant "phase 2" enzymes. However, given the nature of its chemical structure, CA also exhibits direct antioxidant effects. BCP and CA can readily cross the BBB and accumulate in brain regions, giving rise to neuroprotective effects by preventing mitochondrial dysfunction and inhibiting activated microglia, substantially through the activation of pro-survival signalling pathways, including regulation of apoptosis and autophagy, and molecular mechanisms related to mitochondrial quality control. Findings from different in vitro/in vivo experimental models of Parkinson's disease and Alzheimer's disease reported the beneficial effects of both compounds, suggesting that their use in treatments may be a promising strategy in the management of neurodegenerative diseases aimed at maintaining mitochondrial homeostasis and ameliorating glia-mediated neuroinflammation.

Keywords: CB2R/PPARγ pathway; Keap1/Nrf2/ARE transcription pathway; NLRP3 inflammasome; PINK/parkin and mitophagy; carnosic acid; mitochondrial dynamics and biogenesis; mitochondrial protection; neurodegeneration; neuroglia; neuroinflammation and oxidative stress; phytochemicals; ß-caryophyllene.

Figure 1

Chemical structures and main natural sources of β-caryophillene and carnosic acid.

Figure 2

Schematic representation of microglial polarization, metabolic reprogramming, and immune responses under physiological and neuropathological conditions. In response to different environmental and cellular stresses, microglia promptly activate pro- or anti-inflammatory states to preserve tissue homeostasis. In addition to changes in morphology, phagocytosis capacity, and secretion of cytokines, activation of microglia results in changes in cellular energy demand. Under neuropathological conditions (e.g., Parkinson’s disease (PD) and Alzheimer’s disease (AD)), pro-inflammatory microglial activation (M1-like) requires metabolic shifts towards glucose uptake and glycolysis, with the consequent expression of glucose transporter 1 (GLUT1), as well as the production of lactate and reactive species. Conversely, under physiological and anti-inflammatory conditions, microglia (M2-like) mostly rely on oxidative phosphorylation.

Figure 3

Current model of multi-target protective effects of carnosic acid (CA) at the interface of mitochondrial dysfunction, neuroinflammation, and neurodegeneration. Upon stressful conditions, including oxidative injury, accumulation of altered/misfolded proteins, and inflammatory stimuli, mitochondria are impaired with ensuing release of mitochondrial-derived DAMPs (mtDAMPs) such as ROS, Ca²⁺, oxidized mtDNA, cardiolipin, ATP, and transcription factor A mitochondria (TFAM). In surrounding neurons and glial cells, mtDAMPs bind to TLR9, purinergic P2X7 receptor, RAGE, and TLR, promoting further mitochondrial damage and triggering pro-inflammatory, pro-apoptotic cascades that contribute to exacerbation of proinflammatory microglial activation, with the consequent degeneration of dopaminergic neurons. CA provides neuroprotective effects, decreasing oxidative stress by upregulating antioxidant defence, reducing the expression of proinflammatory cytokines via inhibiting the NFκB pathway and/or NLRP3 inflammasome activation, and preserving mitochondrial function. Therefore, Keap1 degradation and Nrf2/HO-1/NQO1 signalling pathway activation, upregulation of the PINK1/parkin pathway, and modulation of the PARIS/PGC-1α axis contribute to counteracting oxidative stress and energy imbalance, as well as mitochondrial disruption by promoting mitophagy, mitobiogenesis, mitochondrial fusion, and UPS machinery. Additionally, CA decreases the CEBPβ–NFκB interaction and reduces Aβ aggregation/deposition by inactivating the amyloidogenic proteolytic pathway of amyloid precursor protein (APP).

Figure 4

Current model of multi-target protective effects of β-caryophillene (BCP) at the intersection of mitochondrial dysfunction, neuroinflammation, and neurodegeneration. Upon stressful conditions, including oxidative injury, accumulation of altered/misfolded proteins, and inflammatory stimuli, mitochondria are impaired with the ensuing release of mtDAMPs such as ROS, Ca²⁺, oxidized mtDNA, cardiolipin, ATP, and TFAM. In surrounding neurons and glial cells, mtDAMPs bind to TLR9, the purinergic P2X7 receptor, RAGE, and TLR, promoting further mitochondrial damage and triggering pro-inflammatory, pro-apoptotic cascades that contribute to exacerbation of proinflammatory microglial activation, with the consequent degeneration of dopaminergic neurons. BCP provides neuroprotective effects, decreasing oxidative stress by upregulating antioxidant defence, reducing the expression of proinflammatory cytokines by inhibiting the NFκB pathway, and preserving mitochondrial function upregulating respiratory enzyme complex (I–IV) activities. Therefore, Keap1 degradation and Nrf2/HO-1/NQO1 signalling pathway activation, upregulation of the CB2R/PPARγ axis, and inhibition of the “JAK2-STAT3-BACE1” signalling pathway suppress TLR4, NO, PGE2, iNOS, and COX-2 expression, the secretion of pro-inflammatory cytokines, as well as Aβ oligomerization and deposition.

Figure 5

Potential hypotensive and neuroprotective actions of carnosic acid and β-caryophillene in ocular tissues. Considering the localisation and the functional involvement of CB₂R in the trabecular meshwork, ciliary body, and retina, BCP-mediated beneficial effects could be hypothesized. Potential neuroprotective efficacy of CA has also been demonstrated in different ocular tissues, including retinal ganglion cells and photoreceptors.