Aquaporin-4 Knockdown in the Substantia Nigra Exacerbates α-Synuclein Pathology, Neurodegeneration, and Motor Dysfunction in a Rat Model of Parkinson’s Disease

The water channel aquaporin-4 (AQP4) is a key participant in the molecular and cellular mechanisms providing the elimination of various metabolites and amyloid proteins from brain tissue. It is assumed that AQP4 dysfunction may provoke the development of Parkinson’s disease (PD) and lead to its aggravation. The aim of this study was to elucidate whether downregulated AQP4 expression in the substantia nigra pars compacta (SNpc) affects the development of α-synuclein pathology, neurodegeneration, and motor impairment in a rat model of the clinical stage of PD. Experiments were carried out on male Wistar rats aged 6–7 months. To downregulate AQP4 expression in the SNpc, a lentiviral construct containing a nucleotide sequence that encodes a short hairpin RNA (shRNA) interfering with natural AQP4 mRNA and thus silencing the Aqp4 gene (AQP4-LVC) was injected into the SNpc. To reproduce a model of the clinical stage of PD, the proteasome inhibitor lactacystin (LC) was bilaterally injected into the SNpc four weeks following AQP4-LVC administration. The study involved behavioral tests, immunohistochemical labeling, and immunoblotting. AQP4-LVC application caused a 42% decrease in AQP4 protein content in the SNpc after four weeks. The LC model of PD was characterized by the appearance of motor disorders, death of 57% of dopamine (DA)-ergic neurons in the SNpc and 56% of their axons in the striatum, attenuation of compensatory processes aimed at maintaining DA level in the nigrostriatal system, and an increase in the content of a total water-soluble form of α-synuclein, as well as its aggregated and Ser129-phosphorylated forms. Downregulation of AQP4 expression in the SNpc in an LC model of PD exacerbated motor dysfunction and caused the appearance of dystrophy signs, indicating the development of a terminal phase of the clinical stage of PD. The rapid progression of parkinsonian symptoms was associated with enhanced neurodegeneration and the depletion of compensatory processes in the nigrostriatal system, coupled with the accelerated formation of pathological α-synuclein aggregates. The obtained data indicate that AQP4 deficiency in the SNpc accelerates the development of Parkinson’s-like pathology.