Peroxisome proliferator-activated receptor (PPAR) belong to subgroup 1 of the nuclear receptor superfamily. They are known to form heterodimers with the retinoid X receptors (RXRs) when activated by endogenous or exogenous ligands and to bind to a co-activator such as PGC-1α.
1. Peroxisome Proliferator-Activated Receptor (PPARs): Types, Distribution and Functions
PPARs belong to subgroup 1 of the nuclear receptor superfamily
[1]. They are known to form heterodimers with the RXR when activated by endogenous or exogenous ligands and to bind to a co-activator such as PGC-1α. The activated PPAR complex binds to peroxisomal proliferative-response elements (PPREs), promoting gene transcription. Three isoforms of these receptors are known as the α, γ, and β/δ isoforms. However, the
PPARγ gene generates three transcripts by alternative splicing encoding for further γ isoforms
[2], which are involved in lipid metabolism, mitochondrial biogenesis, cellular energy production, glucose and amino acid regulation, and thermogenesis
[3]. Also, PPARs are activated by lipids consumed in the diet (fatty acids) or their metabolites (such as eicosanoids), and they are considered to be lipid sensors
[4].
PPARs are ubiquitously expressed in the organism, as shown in
Table 1, while the β/δ isoform is mainly expressed in the CNS, however, the γ isoform is the most studied therapeutic target in several neurodegenerative diseases
[5]. According to data retrieved from the Genotype-Tissue Expression (GTEx) project, the region with the most abundant expression is the caudate nucleus for α isoform, the cerebellar hemisphere for γ, and the cerebellum for β/δ. In addition, several nuclei from the basal ganglia are included, including the same ones that play an essential role in the motor deterioration of diseases such as PD and Huntington’s disease (HD). On the other hand,
Table 1 highlights the agonists corresponding to each isoform. It is worth mentioning that fatty acids and their derivatives could activate all isoforms and are considered pan-agonists or/and endogenous agonists. According to some research groups, there is a connection between the three isoforms called “the PPARs triad”. Activation of the triad regulates neuroprotection by promoting PPAR-dependent genes, including positive feedback on PPARs themselves
[6]. PPARγ increases the levels of the β/δ isoform, and vice versa, PPARβ/δ increases PPARγ levels. In addition, the β/δ isoform regulates α and γ activation, inducing the production of their endogenous agonists
[7]. According to the PPAR triad theory, PPARγ is essential for triad maintenance even in regions of the CNS where the abundance of the β/δ isoform predominates.
Table 1 shows the ligands for PPAR divided into endogenous and exogenous, but these ligands can also act as selective agonists for one isoform, agonists with dual effect, or pan-agonists. The neuroprotection that agonists can provide depending on their mode of action is discussed in what follows.
Several animal models of PD, HD, and Alzheimer’s disease (AD) have shown a neuroprotective effect of PPARγ activation by agonists
[5][8][9]. Glitazones (rosiglitazone, pioglitazone, and lobeglitazone) are the most widely studied PPARγ ligands, indicating the importance of this isoform
[10]. The main effects of specific PPARγ activation include prevention of mitochondrial dysfunction, reduction of ROS, LP production, increased PGC-1α production, suppression of autophagy, maintenance of mitochondrial membrane potential (ΔΨm), inhibition of the proinflammatory cytokines, preservation of dopaminergic neurons, and reduction of macrophage infiltration
[11][12][13][14][15].
On the other hand, recent studies point to the PPARα isoform being the target for preventing damage in AD, PD, depression, and schizophrenia
[16][17][18]. Fibrates (fenofibrate, clofibrate) are the main agonists of the α isoform, and recent studies have shown a neuroprotective effect, especially in the case of gemfibrozil
[19][20]. The different neuroprotective mechanisms related to PPARα activation are: (a) maintenance of glutamate homeostasis; (b) regulation in the metabolism of amyloid beta (Aβ) peptide; (c) cholinergic/dopaminergic signaling in the CNS; (d) attenuation of behavioral changes and dopaminergic dysfunction; (e) antidepressant activity; and (f) decreased proinflammatory signals and astrogliosis
[16][17][21][22].
Regarding the β/δ isoform, some specific agonists known are L-165041, GW0742, and KD3010; the last one reported as safe in the Phase 1b Clinical Trial for metabolic disorders treatment, including obesity
[23][24][25]. Activation of the β/δ PPAR isoform resulted in neuronal protection in various brain pathologies, such as cerebral ischemia, multiple sclerosis, amyotrophic lateral sclerosis, HD, PD, and AD
[24][26][27][28][29][30][31]. In addition, neuroprotective effects conferred by PPAR β/δ activation include the regulation of ceramide metabolism, the reduction of (Aβ) aggregates, anti-inflammatory and antiapoptotic activity, prevention in mitochondrial dysfunction, decreased neutrophil infiltration, diminished oxidative stress and synthesis of antioxidant enzymes, ultimately leading to the restoration of cognitive functions
[24][25][27][28][32].
Some ligands also exhibit dual effects, for example, 4-hydroxynonenal (4-HNE)-mediated PPAR β/δ antagonist/PPAR γ agonist has been verified to counteract the primary and secondary signs of PD neurodegeneration
[33]. In addition, MHY908, a PPAR α/γ dual agonist, prevents the loss of dopaminergic neurons and motor deficits in a PD model
[34].
Finally, the known endogenous agonists are considered to be pan-agonists. Belonging to these ligands are some fatty acids. However, agonists of similar lipidic nature that are exogenous is also found (consumed by the diet; see
Table 1), as in the case of exogenous fatty acids (oleic acid, eicosapentaenoic acid, and docosahexaenoic acid) that promote PPARγ receptor expression
[35]. Also, synthetic pan agonists (GFT1803 and bezafibrate) have been reported to prevent brain glucose hypometabolism and neuronal loss, attenuate microgliosis and the development of behavioral features in models of AD and Tau pathology
[36][37].
Table 1. Expression of PPARs in the brain and their neuroprotective effects.
Receptor Isoform/Agonist |
Brain Expression (TPM *) |
Neuroprotection Effects |
Ref. |
PPAR-α Endogenous: Fatty acids such as palmitic, stearic, palmitoleic, oleic, linoleic, AA and EPA. Exogenous: WY-14643, clofibrate, gemfibrozil, nafenopin, bezafibrate, and fenofibrate. |
Cd (4.799), Sc (4.660), SN (4.562), Acc (4.402), Acb (4.398), Cx (4.241) Amg (4.204), Pu (3.801), FroCx (3.677), Hy (3.597), Cb (3.561), HiF (3.057), CbH (2.399). |
Participates in neurotransmission processes, decreases neuroinflammation, oxidative stress, and Aβ aggregation. |
[9][38] |
PPAR-β/δ Endogenous: EPA, linoleic acid, 13-S-HODE, and 4-HNE. Exogenous: WY-14643, GW0742, GW501516, KD3010, and L-165041. |
Cb (46.72), CbH (42.24), Cx (36.76), FroCx (34.37), HiF(27.25), Sc (26.71), Acc (26.45), SN (22.51), Hy (21.20), Acb (20.36), Cd (20.27), Pu (18.38), Amg (2.77). |
Prevents damage in neurodegeneration (AD, PD, HD, MS, and ALS) ischemia, CNS traumatic injury, and neuroinflammation. |
[24][29][30][31][33][39] |
PPAR-γ Endogenous: AA, EPA, and 15 deoxy PGJ12. Exogenous: Pioglitazone, Rosiglitazone Ibuprofen, piroxicam, ciglitazone, and GW1929. |
CbH (2.744), Cb (2.425), FroCx (2.175), Acc (1.835), Cx (1.834), Acb (1.546), Sc (1.473), HiF (1.344), Amg (1.268), Hy (1.058), Cd (0.9625), SN (0.8271), Pu (0.7124). |
This entry is adapted from the peer-reviewed paper 10.3390/ijms24043264