Involvement of Calpain/p35-p25/Cdk5/NMDAR Signaling Pathway in Glutamate-Induced Neurotoxicity in Cultured Rat Retinal Neurons

We investigated possible involvement of a calpain/p35-p25/cyclin-dependent kinase 5 (Cdk5) signaling pathway in modifying NMDA receptors (NMDARs) in glutamate-induced injury of cultured rat retinal neurons. Glutamate treatment decreased cell viability and induced cell apoptosis, which was accompanied by an increase in Cdk5 and p-Cdk5(T15) protein levels. The Cdk5 inhibitor roscovitine rescued the cell viability and inhibited the cell apoptosis. In addition, the protein levels of both calpain 2 and calpain-specific alpha-spectrin breakdown products (SBDPs), which are both Ca2+-dependent, were elevated in glutamate-induced cell injury. The protein levels of Cdk5, p-Cdk5(T15), calpain 2 and SBDPs tended to decline with glutamate treatments of more than 9 h. Furthermore, the elevation of SBDPs was attenuated by either D-APV, a NMDAR antagonist, or CNQX, a non-NMDAR antagonist, but was hardly changed by the inhibitors of intracellular calcium stores dantrolene and xestospongin. Moreover, the Cdk5 co-activator p35 was significantly up-regulated, whereas its cleaved product p25 expression showed a transient increase. Glutamate treatment for less than 9 h also considerably enhanced the ratio of the Cdk5-phosphorylated NMDAR subunit NR2A at Ser1232 site (p-NR2A(S1232)) and NR2A (p-NR2A(S1232)/NR2A), and caused a translocation of p-NR2A(S1232) from the cytosol to the plasma membrane. The enhanced p-NR2A(S1232) was inhibited by roscovitine, but augmented by over-expression of Cdk5. Calcium imaging experiments further showed that intracellular Ca2+ concentrations ([Ca2+](i)) of retinal cells were steadily increased following glutamate treatments of 2 h, 6 h and 9 h. All these results suggest that the activation of the calpain/p35-p25/Cdk5 signaling pathway may contribute to glutamate neurotoxicity in the retina by up-regulating p-NR2A(S1232) expression.