Abstract
<title>Abstract</title> <p> Background Parkinson’s disease (PD) is characterised by progressive dopaminergic neurodegeneration associated with mitochondrial dysfunction, oxidative stress, and α-synuclein accumulation. Although insulin-like growth factor II (IGF-II) has shown neuroprotective effects in experimental models, its underlying mechanisms remain unclear. This study investigated whether IGF-II protection depends on activation of the sphingosine kinase 1 (SphK1)/sphingosine-1-phosphate (S1P)/S1P <sub>1</sub> receptor signalling pathway. Methods Neuroprotective effects were evaluated in SN4741 dopaminergic neurons exposed to 1-methyl-4-phenylpyridinium (MPP <sup>+</sup> ) and in a chronic MPTP/probenecid mouse model of PD. Mitochondrial function, cell viability, sphingolipid metabolism, antioxidant responses, protein expression, and behavioural outcomes were assessed. Results MPP <sup>+</sup> induced mitochondrial dysfunction, oxidative stress, prohibitin-2 depletion, and neuronal death. IGF-II restored mitochondrial integrity and cell survival, increased SphK activity, normalised intracellular and extracellular S1P levels, preserved S1P <sub>1</sub> receptor expression, and selectively upregulated SphK1. Pharmacological inhibition of SphK1 or S1P <sub>1</sub> abolished IGF-II-mediated protection, demonstrating the essential role of this pathway. IGF-II also promoted nuclear translocation of nuclear factor erythroid 2-related factor 2 (Nrf2) and restored antioxidant enzyme activities. In vivo, IGF-II prevented cognitive deficits and anxiety-like behaviour, reduced α-synuclein accumulation, and preserved SphK1 expression in the substantia nigra of MPTP-treated mice. Conclusions IGF-II exerts neuroprotective effects through activation of the SphK1/S1P/S1P <sub>1</sub> signalling axis, linking mitochondrial preservation, antioxidant defence, and regulation of α-synuclein pathology. Targeting this sphingolipid-dependent pathway may represent a promising disease-modifying therapeutic strategy for PD. </p>