Spinal cord injury (SCI) is a life-threatening neurological disorder in which structural destruction and functional failure of the central nervous system frequently result in severe loss of lower-limb motor function and even permanent paralysis [1]. Pathologically, SCI is a process that evolves through two consecutive phases, namely primary and secondary injury. The primary injury is caused by immediate mechanical force that physically disrupts neural tissue, whereas the secondary injury is a self-propagating cascade that enlarges the lesion, compromises quality of life, and intensifies long-term medical costs [2,3]. Importantly, the secondary cascade is considered partially reversible or at least delayable by pharmacologic intervention [4]. Microglia, the resident immune cells of the CNS, are the first responders after neural trauma and play a pivotal role in SCI pathophysiology [5]. Within minutes after injury they become activated and accumulate around the lesion, initially exerting protective functions such as phagocytosis and inflammatory modulation [6]. However, chronically activated microglia overproduce pro-inflammatory cytokines and neurotoxic mediators that amplify tissue damage and impede functional recovery [7]. Consequently, curbing excessive microglial activation has been shown to attenuate SCI. Tetramethylpyrazine, for example, limits SCI by suppressing microglial activation [8], and lupenone improves locomotor deficits in SCI mice by inhibiting microglial activation through modulation of the NF-κB pathway [9,10].

Peroxisome proliferator-activated receptor γ (PPARγ), a nuclear receptor super-family member that governs metabolic homeostasis, inflammatory responses, and cell differentiation, has emerged as a therapeutic target for metabolic, inflammatory, and central nervous system disorders [11]. In CNS trauma PPARγ confers neuroprotection by dampening inflammation, attenuating oxidative stress, fostering neuronal differentiation and axonal regeneration, and regulating metabolism and mitochondrial function [12,13]. After SCI, PPARγ expression rises in a time- and severity-dependent manner, predominantly in activated microglia and surviving neurons [14]. Pharmacologic activation of PPARγ shifts microglia from a pro-inflammatory toward an anti-inflammatory phenotype and thereby mitigates SCI [15]. TGN-020 likewise protects by activating the AQP4/PPARγ/mTOR axis, enhancing astrocytic autophagy and suppressing inflammation [16].

Aquilaria sinensis (Lour.) Gilg (Thymelaeaceae), the plant from which the Chinese medicinal “Chenxiang” (agarwood) is derived, yields resin-impregnated heartwood whose extracts exert protective effects on the central nervous system and are used clinically to relieve insomnia, anxiety, and depression [17,18]. Lee JS et al. reported that an ethanol extract of agarwood attenuates glutamate-induced neurotoxicity by inhibiting apoptosis- and inflammation-related signaling pathways [19], and follow-up work demonstrated that the same extract suppresses neuro-inflammation by blocking NF-κB nuclear translocation and decreasing pro-inflammatory cytokine release from microglia [20]. Our own studies showed that Zhenbao Pill, a multi-herb formula containing agarwood, improves SCI progression in rats by inhibiting neuronal apoptosis and modulating immunity [[21], [22]]. Agarwood-specific phenylethyl chromones have been identified as partial agonists of PPARγ [23]. Agarotetrol (AH1), a principal bioactive constituent of agarwood, is officially used as a quantitative marker for agarwood quality. Bioinformatic docking performed in the present study predicts that AH1 binds specifically to the ligand-binding domain (LBD) of PPARγ. Moreover, Ershi-wei Chenxiang pills, which contain AH1, have been reported to suppress inflammatory infiltration and neuronal apoptosis [24]. We therefore hypothesize that AH1 targets and activates PPARγ, thereby polarizing microglia toward an anti-inflammatory phenotype and ultimately ameliorating the progression of SCI.

In summary, this study was designed to investigate whether AH1 specifically binds to the LBD of PPARγ, activates downstream PPARγ signaling, thereby modulates microglial activation, and ultimately attenuates neuroinflammation following SCI.