Investigation of the effects of sars-CoV-2 spike/s1 protein, neuropilin-1 and toll-like receptor 4 inhibitors on different physiological processes in the nervous system

As of 2019, Severe Acute Respiratory Syndrome-2 (SARS-CoV-2), which caused the pandemic, has been reported to cause disorders in the functioning of the nervous system. In addition, a small number of studies have found that the S1 protein, one of the subunits of the SARS-CoV-2 Spike protein, directly or indirectly causes physiological disorders in the nervous system. Within the scope of the thesis study, the physiological effects of Neuropilin-1 (Nrp-1) and Toll-like Receptor 4 (TLR4) inhibitors, which have been reported to be one of the units of SARS-CoV-2 Spike/S1 protein and its target receptors, on neuronal cells were discussed. For this purpose, the appropriate concentrations of the substances were first determined by cytotoxic assays on L929 and T98G cell lines. Following this step, to investigate the effects of the substances on neuronal processes, appropriate experimental groups were established with the physiological identities of SH-SY5Y cells, both at the neuroblastoma stage, neuronal differentiation stage and after differentiation into neuronal cells. The effects of both direct and combined administration of the substances on neuronal differentiation potential (neurogenesis) were examined. We also evaluated the effects of the substances on the neuroprotection process in SH-SY5Y cells differentiated into neurons. In iv addition, the effects of the substances on the recovery after a nerve injury and on the neuronal migration process in a transwell system were analyzed. The data obtained showed that the SARS-CoV-2 Spike/S1 protein exerts similar effects on the functions of Nrp-1 as the Nrp-1 inhibitor. Similar to Nrp-1 inhibitor, S1 protein was found to negatively affect neuronal differentiation, slow down axonal growth in a timedependent manner, impair neuroprotection in a time-dependent manner, induce apoptosis, have a negative effect on nerve injury recovery, and negatively affect cell adhesion and neuronal migration. In contrast, blocking TLR4, another target receptor of S1 protein, with TAK242 did not have a similar effect on these processes as S1 and EG00229. In addition, Nrp-1 and TLR4 receptors have been found to be directly or indirectly related to each other in some physiological processes in the nervous system. Nevertheless, EG00229 prevented the negative effects of S1 depending on time and quantity parameters. Taken together, the results suggest that SARS-CoV-2 or S1 protein infiltrating the nervous system may directly cause pathophysiological effects in the nervous system by disrupting Nrp-1 function. The findings will shed light on the studies of drugs or therapeutic agents that can be designed to prevent the damages of SARS-CoV-2 in the nervous system. Keywords: SARS-CoV-2, Spike protein, Neuropilin-1, Toll-like receptor 4, Neurogenesis, Axonal growth, Neuroprotection, Neuronal migration