We identified the role of MMP7 in SCII and studied the mechanism on neuroapoptosis and inflammation. MMP-7 expression was noticeably increased at the 24 h time point after surgery. Silencing MMP-7 showed a protective effect after SCII by improving lower limb motor function and histologic evaluation, along with reducing the expression of MMP-7, apoptotic factor and inflammatory cytokine. It was concluded that silencing MMP-7 protected the rats against SCII by inhibiting the neuroapoptosis and inflammation.
During nervous system development and following injury, MMPs have been increasingly recognized as playing vital roles [24]. For example, MMP-9 was associated with the early process of neuropathic pain in CCI rats [25]; and in peripheral nerve regeneration, MMP-7 promoted myelin sheath formation and Schwann cell migration [15]. As the smallest member of MMPs, MMP-7 plays vital roles in many physiological processes [26]. In bactericidal and anti-inflammatory roles, MMP7-deficient mice are strongly protected against an otherwise lethal dose of LPS [27]. In our previous study [17], we suggested silencing lncRNA Gas5 protected against SCII in a rat-model by regulating MMP-7, but the specific mechanism of MMP-7 remained unclear. In this study, MMP-7 expression was greatly upregulated along with the reduction of Tarlov scores after SCII, suggesting a probable relation between MMP-7 and dyskinesia. Based on the 24 h time point after SCII, cellular distribution of MMP-7 with GFAP, Iba-1 and NeuN was evaluated by Double immunofluorescence labeling. The results revealed that MMP-7 was mostly expressed in NeuN-positive neurons and the fluorescent density of MMP-7 greatly increased after SCII, suggesting that MMP-7 could play vital roles in SCII at least partially through neurons.
RNA interference technology is widely utilized to regulate the expression of individual genes and study the cellular function. In addition, siRNA technology was used to perform targeted treatment of protein genes and study the roles of specific proteins [28]. In our research, the protein and RNA expression of MMP-7 was greatly reduced by intrathecal injection of si-MMP7. Meanwhile, the IL-1β and cleaved caspase-3 was greatly reduced by intrathecal injection of si-MMP7. IL-1β and cleaved caspase-3 are common inflammatory cytokine and apoptotic factors, respectively [29]. Therefore, silencing MMP-7 could inhibit the neuroapoptosis and inflammation. In addition, silencing MMP-7 could improve lower limb motor function and histologic evaluation and reduce the expression of MMP-7 in neurons. Overall, silencing MMP-7 exerted neuroprotection on rat spinal cords against SCII.
In a study of MS lesions, double staining showed that MMP-7 was evidently localized to Iba1-positive microglia at 3 and 5 weeks demyelination [16], indicating that MMP-7 was closely related to microglia function. In our study, MMP-7 was mostly expressed in NeuN-positive neurons, indicating that MMP-7 was closely related to neurons function. It may be due to different disease models, tissue sites and timing. Additionally, the function of neurons and microglia may be partially in series. Of the major cell types of spinal cords, neurons are closely associated with lower limb motor function after SCII [4]. Therefore, MMP-7, which was mainly expressed in NeuN-positive neurons, could affect lower limb motor function after SCII at least partly through neurons. As shown in Fig. 6F, MMP-7 fluorescence was co-localized with cleaved caspase-3 fluorescence, suggesting that MMP-7 might play partially roles by cleaved caspase-3. Neuroapoptosis was now known to be the vital factor related to neuronal loss during SCII [30]. Damage from ischemia reperfusion could inhibit neuronal repair systems and induce caspase-mediated neuroapoptosis [30, 31]. Therefore, MMP7, which was co-localized with NeuN and cleaved caspase-3 respectively, was closely related to the mechanism of neuroapoptosis induced by SCII. In addition, silencing MMP-7 could increase Tarlov scores and reduce the expression of MMP-7 in neurons and cleaved caspase-3 protein expression. Overall, silencing MMP-7 ameliorated SCII-induced neuroapoptosis through restraining the MMP-7/ cleaved caspase-3 axis.
This study will help researchers exploit potential therapeutic targets and diagnostic biomarkers for SCII. In the clinical treatment of MMP-7 after SCII, the detailed molecular mechanisms of MMP-7 need more additional research and cell experiments in vitro. The mechanism of neuroapoptosis has been elaborated, but more detailed mechanisms of inflammation or others require further study.