A proline- and glutamine-rich protein promotes apoptosis in neuronal cells

During development, excess neurons are eliminated by programmed cell death. Similarly, conditionally immortalized (SV40-T-ts) rat hippocampal and sept al cells undergo cell death following differentiation with several factors such as fibroblast growth factor, constitutively activated Raf-1, or phorbo l esters. The mechanism by which cell death occurs has not been identified. Using RNA differential display, we have identified and characterized a nov el immediate early gene (denoted PQR for proline- and glutamine-rich) induc ed during differentiation of both rat hippocampal and septal cell lines. Th e 44-kDa PQR protein, rich in PQ, PH, and QQ repeats, is homologous to a mu rine protein (TDAG51) required for Fas-mediated apoptosis in T cells. To de termine whether PQR acts as a mediator of apoptosis in neuronal cells, the hippocampal H19-7 cells were microinjected with either a plasmid expressing PQR cDNA or an antibody against PQR. Microinjection of differentiating H19 -7 cells with a neutralizing antibody against PQR increased the number of s urviving cells by 50%. Transient expression of PQR in both differentiating and nondifferentiating H19-7 cells decreased the number of surviving cells by 35-50%; this reduction was reversed by microinjection of PQR antibody. F inally, levels of Fas transcripts are not increased in the neuronal cells, indicating that the mechanism of action differs from that in T cells. These results demonstrate that PQR can be induced by growth factors and differen tiating agents and can itself induce apoptosis in hippocampal H19-7 cells. Furthermore, these data suggest that PQR can function more generally as a m ediator of apoptosis and provide a possible mechanism for induction of prog rammed cell death during neuronal development.