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Mesial temporal lobe epilepsy (MTLE) is the most frequent form of partial epilepsy. Granule cell dispersion, resulting from aberrant neuronal migration in the hippocampus, is pathognomonic of MTLE. Reelin, a secreted neurodevelopmental glycoprotein has a crucial role in controlling the radial migration of neurons. Several animal studies have implicated Reelin in the MTLE pathogenesis. The aim of this study was to investigate the Reelin signaling pathway in the MTLE patients. Therefore, we studied each step in the Reelin signalling pathway for the gene and protein expressions, in the hippocampal tissue obtained from patients undergoing surgery for MTLE and compared it with age matched normal autopsy cases. We found statistically significant decrease (P<0.001) in the Reelin mRNA expression in MTLE patients. Among the two reelin receptors, apolipoprotein E receptor 2 (ApoER2) was significantly increased whereas very low density lipoprotein receptor (VLDLR) was decreased among the patients. Disabled 1 (Dab1), the downstream target of reelin, was found to be decreased. Dab1 in turn inhibits Cofilin, which is responsible for cytoskeletal reorganization, thus limiting aberrant neuronal migration. Statistically significant over expression of Cofilin protein was found in the patient group. Matrix metalloproteinase 9 (MMP-9) and tissue inhibitor of metalloproteases-1 (TIMP-1), both of which are involved in processing of Reelin, were down regulated in 70-85% of cases. In summary, the whole pathway was found to be deranged in MTLE. These results indicate that Reelin signaling pathway is disturbed at various points in the MTLE patients and might be involved in the pathogenesis & progression of MTLE. Our results extend the existing information regarding the components of the Reelin pathway and further, establish a link between pathway disturbance and MTLE.

INTRODUCTION Granule cell dispersion (GCD) is pathognomonic of hippocampal sclerosis seen in mesial temporal lobe epilepsy (MTLE). Current animal studies associate GCD to deficiency of Reelin, an extracellular matrix protein. The present study evaluated complete Reelin signalling pathway in MTLE in human tissue hippocampus and anterior temporal lobe (ATL). ATL was included, as ATL resection along with amygdalo-hippocampectomy results in better disease control. METHODS Hematoxylin and Eosin (H&E) staining was done to identify the MTLE. Gene expression profiles of Reelin and its receptors (very low density lipoprotein-VLDLR and Apolipoprotein E 2-ApoER2), its downstream molecule Disabled 1 (Dab1) and its upstream inhibitor tissue inhibitor of metalloproteinases-1 (TIMP-1) were compared in normal and epileptic human tissue. Quantitative real-time PCR was done to determine the gene expression in the hippocampus and ATL. Western blotting was done to quantify the protein expression. Ethical clearance was taken from the Institute Ethical committee. RESULTS Typical features of MTLE were seen histologically. Width of granule cell layer was found to be broadened than normal controls. The expression of Reelin was found to be in same order to that of control. But its expression has been reduced as compared to the other genes. Reactive increase in the expression of both receptors was found. In hippocampus increase was more for ApoER2 while in ATL VLDLR showed increased expression. Similarly increased expression of Dab1 was seen, more in the hippocampus. Significantly increased expression was found in TIMP-1. CONCLUSION Data suggests a causative role of Reelin deficiency in MTLE. The increase in TIMP-1 would inhibit the Reelin cleavage, leading to deficiency of active Reelin. Downstream components of the Reelin show reactive increase. Reelin deficiency in ATL validates the surgical significance of ATL resection. Establishing a causative role of Reelin in MTLE in human tissue might have significant therapeutic potential.

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