1. Introduction
Obesity-induced neuroinflammation is a type of brain aseptic chronic inflammation characterized by high pro-inflammatory cytokines, reactive microglia, and astrogliosis
[1][2][3][4]. The intensity of the inflammation induced by obesogenic diets is lower than that caused by infections
[5]. However, this neuroinflammation can be evidenced even in normal-weight individuals without insulin resistance or other metabolic disorders
[6]. Neuroinflammation induced by a hyperlipidemic diet (lipid, 45 kcal%) was first described in the hypothalamus, evidenced by an increase in c-Jun N-terminal kinase (JNK) and nuclear factor kappa-B (NF-κB) signaling and a reduction in insulin and leptin signaling
[1]. Hypothalamic inflammation promotes leptin resistance
[7], gliosis, and neuronal death
[8]. High-fat diet (HFD)-induced gliosis implicates the activation of astrocytes and microglia
[9][10], promoting the release of pro-inflammatory cytokines such as interleukin 1β (IL-1β) and tumor necrosis factor-alpha (TNF-α), resulting in the overexpression of cyclooxygenases and the production of reactive oxygen species
[11].
2. Inflammation
2.1. Peripheral Inflammation
Obesity produces a chronic low-grade inflammation within the peripheral tissues; adipose tissue is one of the most sensitive to obesity-induced inflammation
[5][12]. In lean individuals, adipose tissue contains multiple immune cells that operate in the T helper 2 (Th2) state, including homeostatic anti-inflammatory macrophages, regulatory T (Treg) cells, type 2 innate lymphoid cells (ILC2), invariant natural killer T (iNKT) cells, natural killer (NK) cells, and eosinophils
[13]. In obesity, this immune profile shifts towards a pro-inflammatory state, hallmarked by the proliferation and recruitment of neutrophils, inflammatory macrophages, B cells, cytotoxic T lymphocytes (CD8
+ T) cells, and T helper (Th) 1 and Th17 cells, along with a reduced abundance of eosinophils, Treg cells, iNKT cells, and ILC2
[13]. Saturated fatty acids directly promote inflammation, facilitating the absorption of lipopolysaccharides (LPS)
[14] and activating macrophages, microglia, and astrocytes, similar to LPS by binding to toll-like receptor-4 (TLR4), which triggers NF-κB signaling and promotes cytokine release
[15]. In the same context, TNF-α decreases the sensitivity of insulin receptor 1 in adipocytes
[16], which can be reversed by the inactivation of TNF-α receptors
[17]. Similarly, a TLR4 knockout (KO) demonstrated a reduced preference for fat and sugar intake
[18]. TLR4 has a critical role in propagating the activation of the NOD-like receptor protein 3 (NLRP3) inflammasome activated by saturated fatty acids
[19].
Another aspect that has recently gained interest is the role of microbiota. In this regard, many publications support the involvement of gut microbiota in the pathophysiology of obesity. In rodent models of diet-induced obesity, gut microbiota modifications were associated with increased intestinal permeability, allowing the passage of food or bacterial antigens that contribute to low-grade inflammation and insulin resistance
[20]. The perturbation of the intestinal microbiota and changes in intestinal permeability are considered to be a trigger of inflammation in obesity
[21]. In the same sense, metabolic endotoxemia originating from dysbiotic gut microbiota has been identified as a primary mediator for triggering the chronic low-grade inflammation responsible for the development of obesity
[22]. Animal studies have demonstrated that gut microbiota could promote adiposity and weight gain by altering the host gene expression, the metabolic and inflammatory pathways, and the gut–brain axis
[23].
2.2. Central Inflammation or Neuroinflammation
Obesity-induced neuroinflammation was first described in the hypothalamus, evidenced by the upregulation of JNK and NF-κB signaling and a reduced insulin and leptin profile caused by exposure to an HFD
[1]. Hypothalamic inflammation leads to leptin resistance
[7] and changes in neural projections as well as gliosis and neuronal death
[8]. In neuroinflammation, gliosis is characterized by reactive astrocytes and microglia
[10][24]. Activated microglia can release various pro-inflammatory cytokines such as IL-1β and TNF-α as well as cyclooxygenases and reactive oxygen species
[11]. The deletion of adapter proteins for toll-like receptors protected mice from weight gain and the development of leptin resistance when fed an HFD
[25].
2.2.1. Neuroinflammatory Mechanisms
Blood-Brain Barrier
The functionality of the BBB depends on a strict architecture. In the ventromedial hypothalamus, the barrier specializes in admitting the dynamic passage of hormones and nutrients from the blood to the energy-sensing arcuate nucleus of the hypothalamus (ARC) and the export of newly synthesized hormones to the pituitary. At the median eminence (ME) level, the barrier has fenestrated capillaries that allow the faster transport of substances into the nutrient-sensing hypothalamic nuclei adjacent to it. However, ME tanycytes, specialized radial glia cells lining the walls of the third ventricle, form a physical barrier to control the correct transport of nutrients and metabolic hormones into the brain parenchyma
[26][27][28].
Several mechanisms have been proposed to explain how diet-induced inflammation occurs. One possible mechanism is the alteration of the blood–brain barrier (BBB). A Western Diet (WD) (high fat/high sucrose) consumption increases the BBB permeability, thus allowing immune cell infiltration and leading to hypothalamic inflammation
[29]. In this sense, the interactions between the C-reactive protein and the BBB increase the paracellular permeability and induce reactive gliosis
[30]. In leptin receptor-deficient (db/db) mice, inflammatory changes in the BBB participated in obesity-related cognitive alterations; rescued cognitive deficits were achieved by reducing the BBB permeability
[31]. In this sense, an Evans blue stain entered the central nervous system (CNS) of mice fed with an HFD
[32][33][34]; a possible explanation of the mechanism arose from the reduction of tight junction transcripts such as occludin, claudin-5, and claudin-12 in the thalamus and midbrain, increasing the permeability of the BBB in the hippocampus
[33][35].
Fatty Acids
High-fat and high-sugar diets upregulate the inflammatory NF-κB pathway in the hypothalamus, which is a binding site for regulating energy homeostasis
[36]. An inflammatory phenotype was visualized when microglia were treated with saturated fatty acids in vitro
[37]. Hypothalamic inflammation contributes to developing and maintaining the obese phenotype; exposure to an HFD for three days produced neuroinflammation, gliosis, and the markers of neuronal injuries in rodents
[2]. Furthermore, just one day of an HFD could increase the expression of IL-6 and TNF-α as well as microglial activation
[4]. Lipids in the hypothalamus play a potential role in the development of obesity and related metabolic diseases, suggesting that the WD affects lipid accumulation and synthesis in the brain
[38], leading to an onset of an increase in inflammatory cytokines, oxidative stress, transcription factor changes, neuron malfunctions, or cell death
[39]. A WD drives the inflammatory responses in the hypothalamus, eventually leading to metabolic disorders
[40].
2.2.2. Role of Glial Cells in Obesity
Microglia sense their surrounding environment and express pro-inflammatory (M1) or anti-inflammatory (M2) phenotypes
[41] in response to the presence of pathogen-associated molecular patterns (PAMPs)
[42]. Microglia cells have been proposed to be a critical target of obesity-related inflammation
[43]. In this context, systemic injection of lipopolysaccharide (LPS) promoted microglial activation in the hypothalamus
[44]. LPS induces a greater expression of the primary histocompatibility complex class 1 (MHC-I), pro-inflammatory cytokines (TNF-α, IL-1, and IL-6), and the activation of cyclooxygenase 1 (COX-1) and NF-κB
[45][46]. In a mouse model of diet-induced obesity, a partial substitution of the fatty acid composition of a diet of flax seed oil (rich in C18:3) or olive oil (rich in C18:1) corrected hypothalamic inflammation, which was evaluated by a JNK and NF-κB activity reduction
[47]. At the hypothalamic level, metabolic inflammation increased the activation of IKKb/NF-κB up to two times compared to a chronic HFD, and up to five or six times in the case of hyperphagic obese animals
[25]. Similarly, microglial cell cultures treated with palmitic acid (a long-chain saturated fatty acid) have a rapid TLR4-dependent microglial activation
[48]. Investigations into the ARC observed that microglia activation was critical for altering the energy balance and inducing weight gain during long-term HFD exposure in mice
[49], leading to enhanced susceptibility to obesity and a possible suitable pharmacological target. On the other hand, diet-induced microgliosis in the hippocampus has been identified in patients and experimental models of Alzheimer’s disease, providing a potential mechanistic link between obesity/type 2 diabetes and cognitive impairments
[50]. A high-caloric diet also increases NLRP3 expression, indicating inflammasome activation and IL-1β production in the hippocampus and amygdala-derived microglia
[51] (
Figure 1). Interestingly, minocycline (a second-generation tetracycline) is considered to be an inhibitor of microglia-induced neuroinflammation
[52], inhibiting intracellular signaling pathways such as p38, ERK1/2, and NF-κB and the release of pro-inflammatory factors, including IL-1β, IL-18, IL-6, and NOS2
[53]. Several studies have shown that treatment with minocycline improves depressive symptoms, decreases the expression of pro-inflammatory cytokines associated with the hyperactivity of the hypothalamic–pituitary–adrenal axis (HPA)
[54], and the administration of drugs of abuse in the reward system
[55]. Minocycline, an FDA-approved tetracyclic antibiotic with anti-inflammatory properties
[56], has been associated with a reduction in HFD-induced weight gain as well as an improvement in insulin sensitivity, a decline in active microglia, and the restoration of alterations in autophagy-related gene networks in the PVN
[57]. Microglial activation could influence the energy balance, but the promotion of leptin resistance and impairments in adipose thermogenesis are not yet clear
[58].
Figure 1. Activating the NLR family pyrin domain-containing 3 (NLRP3) inflammasome signaling pathway requires two signals. Signal 1 or priming is provided by pathogen-associated molecular patterns (PAMPs), danger-associated molecular patterns (DAMPs), cholesterol, or the activation of toll-like receptors (TLRs) or cytokine receptors, leading to the nuclear factor kappa-B NF-κB activation that upregulates the levels of several inflammasome components such as the protein NLRP3, pro-IL-1β, and pro-IL-18. Signal 2 or activation is provided by numerous PAMPs or DAMPs, including viruses, cholesterol, potassium efflux, reactive oxygen species (ROS), extracellular ATP, and lysosomal dysfunctions, among others. ASC, an adaptor protein, recruits NLRP3 and pro-caspase-1 to form the NLRP3 inflammasome complex. Caspase-1 promotes the processing of interleukins for their subsequent release. This figure was created with BioRender.com under a subscription and has a license from BioRender to use the figure in journal publications.
2.2.3. Inflammasomes
The NLRP3 inflammasome is part of the innate immune system activating caspase-1, promoting the release of pro-inflammatory cytokines IL-1β/IL-18 in response to microbial infections and cell damage
[59]. At the molecular level, inflammasomes comprise three components: (1) a sensor such as a NOD-like receptor (NLR) or an AIM-2-like receptor (ALR); (2) an apoptosis-associated adapter protein (ASC) containing a caspase recruitment domain; and (3) cysteine inflammatory caspase-1 aspartate
[59]. Inflammasomes are also involved in the cleavage of gasdermin-D (GSDM-D), the induction of pyroptosis
[60], and obesity-induced inflammation
[61]. The NLRP3 inflammasome is one of the most extensively studied inflammasomes
[62]. This complex belongs to the nucleotide-binding oligomerization domain-like receptor family (NOD-like) pyrin domain-containing 3 (NLRP3)
[61][63]. The NLRP3 inflammasome is activated by molecular patterns associated with cell damage (DAMPS) and favors the proteolytic cleavage of pro-interleukin 1β and 18 through caspase-1, generating the respective active proteins (IL-1β and IL-18), which tend to the inflammatory response
[64]. In this context, the NLRP3 inflammasome can be activated by cholesterol crystals and ceramides
[65] due to the exacerbation of lipolysis in obesity
[66], but can also be activated by bacteria, fungi, and viruses; however, NLRP3 is associated with metabolic and inflammatory conditions such as obesity
[67][68]. Unlike other inflammasome complexes, NLRP3 is unique. It mediates the recognition of DAMPs directly involved in cellular metabolism
[69]; NLRP3 expression is dependent on NF-κB and, therefore, can be regulated by the components of a diet.
Astrogliosis is characterized by an increase in glial fibrillar acid protein (GFAP), promoting a pro-inflammatory phenotype and hypertrophic morphology. An HFD increases the expression of astrocytic and microglial markers such as GFAP and ionized calcium-binding adapter 1 (Iba-1), respectively, in the hypothalamus
[42].