Microglia and astrocytes are two key cellular regulators of inflammatory processes developing in the CNS. These two cell types can either exert pro-inflammatory or anti-inflammatory functions according to their polarization, classically categorized as M1 (pro-inflammatory) or M2 (anti- inflammatory) for microglia and A1 (pro-inflammatory) or A2 (anti-inflammatory) for astrocytes. Remarkably, many neuroprotective natural compounds function by rebalancing the pro-inflammatory phenotypes toward the anti-inflammatory ones [10,11]. It is important to underscore that categorizing these cells in this binary manner might not accurately represent the diverse phenotypes of microglia and astrocytes; thus, it is noteworthy to view them as existing along a spectrum rather than as entirely separate populations [2,12,13].

Microglia, the macrophage-lineage cells of the CNS, represent the initial cell type that reacts to danger. These immune cells can shift from one phenotype to another in response to distinct environmental conditions within the CNS. When exposed to PAMPs or DAMPs, such as lipopolysaccharide (LPS) or reactive species, respectively, as well as IFN-γ, microglial cells are activated in the M1 phenotype, expressing pro-inflammatory signatures such as interleukin (IL)-1β, IL-6, tumor necrosis factor (TNF)-α, chemokines, inducible nitric oxide synthase (iNOS), adenine dinucleotide phosphate (NADPH) oxidase and cyclooxygenase (COX)-2 [12,14]. This pro-inflammatory phenotype is associated with neural tissue damage [15]. NADPH oxidase produces ROS, such as superoxide anions, which in turn can combine with nitric oxide (NO) generated by iNOS to form peroxynitrite radicals [16]. When their concentration is greater than the cellular antioxidant capacity, reactive species are toxic, as they are capable of inducing DNA and protein damage as well as oxidizing the cellular membrane, resulting in lipid peroxidation and disruption of its properties, leading to necrotic cellular death [15,16,17]. TNF-α stimulates microglia in an autocrine manner, leading to an excessive release of glutamate, which consequently causes excitotoxic neuronal cell death [18]. Excessive glutamate dysregulates Ca²⁺ influx in neurons through hyper stimulation of the NMDA receptor, which in turn leads to RNS and ROS production, determining cellular death and exacerbating the neuroinflammatory response [19]. Several pathways are related to the pro-inflammatory switch of microglia.

LPS is recognized by, and thus activates, the Toll-like receptors (TLR)-4 signal pathway, which in turn culminates in a pro-inflammatory cascade and M1 microglia polarization. TLR4, activated by its ligand, leads to an enhanced activity of nuclear factor kappa-B (NF-κB) and mitogen-activated protein kinases (MAPKs) JNK, ERK and p38, which enhance pro-inflammatory mediator transcription [7,12,20]. ROS and ion fluxes can trigger NOD-like receptor pyrin-domain-containing 3 (NLRP3)-inflammasome signaling and, hence, the neurotoxic M1 activation of microglia [21,22]. Of note, the NLRP3 inflammasome pathway is associated with the induction of a pro-inflammatory form of programmed cellular death called pyroptosis [23]. The Janus kinase/signal transducer and activator of transcription 1 (JAK1-2/STAT1) pathway is likewise implicated in M1 polarization. IFN-γ acts through this pathway, and when it binds to its receptor, it triggers STAT1 phosphorylation, increasing its transcriptional activity, which in turn upregulates pro- inflammatory genes [12,24,25]. Ultimately, contact-dependent Notch signaling drives microglia to the pro-inflammatory phenotype [26,27].

Given the pro-inflammatory role of the aforementioned signaling pathways, their pharmacological targeting may be beneficial in regulating the shift of microglia from the M1 to M2 polarization state. This modulation holds promise for the treatment of a range of brain diseases [28,29].

As already mentioned, induced by cytokines such as IL-4, IL-13 and IL-10, microglia can exhibit an anti-inflammatory phenotype. M2-polarized microglial cells release anti-inflammatory cytokines such as IL-10, Transforming Growth Factor (TGF)-β and IL-1R antagonists, which collectively act in opposition to their pro-inflammatory counterparts [30,31]. Additional factors expressed by M2 microglia associated with the resolution of neuroinflammation and neuroprotection include Arginase1 (Arg1), which suppresses NO production by competing with iNOS for arginine as a substrate and determines the production of molecules (i.e., polyamines) involved in tissue repair, cell proliferation and survival [30,32,33]; CD206 (also known as macrophage mannose receptor 1), a phagocytic receptor that binds myeloperoxidase and lysosomal hydrolases and, because of that, plays a pivotal role in the resolution of neuroinflammation [30,34]; and neurotrophic factors such as BDNF [30]. Several pathways are positively associated with M2 polarization. Some examples comprise JAK1/STAT6 (triggered by IL-4), the cannabinoid receptor 2 (CB2)/peroxisome proliferator-activated receptor gamma (PPAR-γ) axis [12,35,36], triggering receptor expressed on myeloid cells 2 (TREM2) signaling [20,37,38] and the PI3K/Akt cascade [39]. Notably, the role of this latter pathway on M1 to M2 polarization seems to be a function of specific Akt isoforms [40]. Boosting these pathways or, conversely, inhibiting the pro-inflammatory ones with exogenous compounds could be beneficial in the treatment of neuroinflammation associated with various brain disorders.

Astrocytes play a fundamental role in maintaining brain homeostasis and are thus implicated in many CNS disorders. Notably, they contribute to blood–brain barrier (BBB) integrity, neuronal metabolism, synapse and neurotransmission regulation, potassium clearance, glymphatic flow control and host-defense mechanisms [41,42,43]. In the context of neuroinflammation, microglia and astrocytes interact to modulate the course of the response to an insult. Pro-inflammatory signals released by M1 microglia such as TNF-α, IL-1α and C1q stimulate reactive A1 astrocytes [44]. This particular astrocytal polarization exhibits a shared profile of secreted molecules to that of pro-inflammatory microglia, thus contributing to the augmentation of the neuroinflammatory burden [45,46].

Pointing out the remarkable role of microglia interaction with astrocytes in their A1 polarization, knock-out mice lacking microglia failed to induce A1 astrocytes after LPS treatment, while wild-type showed strong A1 induction [44]. Furthermore, it was demonstrated that the NLRP3 inflammasome pathway in microglia rather than in astrocytes is strongly associated with their pro-inflammatory shift and, accordingly, the knock-out of NLRP3 in microglia mitigates the neuronal dysfunction provoked by A1-like astrocytes, both in in vitro and in vivo settings [47,48]. A1 astrocytes release neurotoxic factors and promote glial scar formation, which overall can exert detrimental outcomes for brain repair and neuronal cell survival. For example, glial fibrillary acidic protein (GFAP) and chondroitin sulfate proteoglycans (CSPGs), the main components of glial scars, inhibit axonal regeneration [13,44,49,50]. Different signaling pathways and transcription factors are involved in glial scar formation driven by reactive astrocytes, including IL- 1/NF-κB, IL-6/STAT3, NOTCH/STAT3 and TGF-β/SMAD3; therefore, attenuating this neuroinflammatory-related process targeting these cell signaling cascades might be of benefit in CNS pathologies [51,52,53]. Analogously, M2 microglial cytokines trigger reactive A2 astrocytes, associated with anti-inflammatory cytokine release, neuroprotection and repair [44,45,46,54]. Pathways and transcription factors that are linked with enhanced A1 to A2 polarization include the PI3K/Akt axis [55] and STAT6, with this latter being associated with the upregulation of antioxidants genes in A2 astrocytes, such as nuclear factor erythroid 2-related factor 2 (Nrf2) and Arg1 [56].

It is worth emphasizing that comprehending the mechanisms implicated in the regulation of neuroinflammation holds significant relevance in order to modulate this immune response through external molecules.

Neurodegeneration is a characteristic of numerous brain pathologies in which CNS functions deteriorate over time [5].

Neurodegenerative diseases place a significant burden on developed societies with aging populations. While research in this field is highly active, there remains a deficiency in comprehending the etiopathogenesis of these disorders, which is crucial for the discovery of new molecular targets and the development of therapies. In recent years, the role of neuroinflammation has emerged as a distinguishing feature of neurogenerative diseases, making it a new promising molecular process to focus on [5,57,58].

Notably during aging, a major risk factor for the development of neurodegenerative diseases, inflammatory processes rise in the brain (inflammaging) and there is a tendency for an increase in microglia M1/M2 ratio [33,59]. Additionally, the upregulation of gene signatures of M1-polarized microglia has been found in post-mortem tissues of patients with neurodegenerative disorders [60].

Along the same lines, an increase in A1 pro-inflammatory astrocytes has been identified in post-mortem neural tissues of individuals affected by AD, HT, PD, MS and ALS [44]. These findings suggest that neurodegenerative diseases develop in the context of a neuroinflammatory microenvironment.

Protein aggregates within neurons and in the extracellular space are a common characteristic and a hallmark of neurodegenerative diseases such as PD, AD, FTLD, HD and ALS [5,61]. Various studies clearly showed a link between protein aggregates in neurodegenerative diseases and neuroinflammation. Here, some examples of recent research and findings about this intricate association are provided.

Alpha-Synuclein (αSyn) aggregates (a pathological hallmark of PD) have been shown to lead to neuroinflammation and neurodegeneration through double-stranded DNA breaks and the induction of the DNA sensor GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) immune pathway, in a microglial and astrocyte mixed culture and a mouse model of PD [62]. Furthermore, in this study, the authors found STING upregulation in autopsied PD patients relative to healthy controls. These findings (associated with the amelioration of motor dysfunction symptoms in mice with STING knock-out) imply that this immune pathway, and therefore neuroinflammation, is an important feature in a neurodegenerative disorder such as PD [62,63]. Similarly, in an αSyn-driven mouse model of PD, the NLRP3 inflammasome cascade has been found to be upregulated in microglia [64]. The NLRP3 inflammasome pathway is also upregulated in microglia of mice models of PD driven by mitochondrial dysfunction and oxidative stress (thus independently of αSyn aggregates) and it precedes dopaminergic neuron degeneration and motor deficits [64]. Accordingly, NLRP3 pharmacological inhibition was found to be neuroprotective in these PD mice models. Furthermore, in PD patients, the upregulation of inflammasome markers has been evidenced in the substantia nigra of post-mortem brains [64]. Thus, taken together with other similar studies, these observations indicate that the aforementioned inflammatory signaling pathway is a key feature of this neurodegenerative disease [64,65]. Beta-amyloid (Aβ) aggregates, an AD pathological hallmark, induce the NLRP3 pathway through TLR4 in the BV-2 microglia cell line, and its conditioned medium reduces HT-22 neuronal cell line viability [66]. In another research study investigating the activation mechanism of the NLRP3 pathway by Aβ in primary microglia, the results revealed that Aβ aggregates initiate a process that involves the spleen tyrosine kinase (Syk)-mediated inactivation of AMPK. This inactivation leads to mitochondrial stress and an increase in the production of ROS, which subsequently triggers the NLRP3 cascade [67]. Interestingly, and in alignment with these findings, markers of pyroptosis (i.e., cleaved gasdermin D), an NLRP3-related inflammatory type of programmed cellular death, has been found to be upregulated in the post-mortem AD brain [68]. Moreover, the upregulation of genes and proteins associated with both inflammasome activation and pyroptosis is also observed in CNS post-mortem tissues of individuals with ALS and MS [69,70], underscoring the pivotal role of these processes as hallmarks in the landscape of neurodegenerative diseases. Of note, acetylcholinesterase (AChE) favors Aβ aggregate formation, thereby decreasing its activity associated with the inhibition of Aβ-fibrillogenesis [71].

Taking into account the central role of neuroinflammation in the abovementioned neurodegenerative diseases (described as examples of the various other brain disorders with neuroinflammation as a characteristic component), targeting immune pathway effectors or their biochemical activators (i.e., ROS, pathological protein aggregates, etc.) could represent a promising therapeutic strategy in order to prevent and ameliorate symptoms and the progression of neurodegenerative disorders.