Human Gas7 isoforms homologous to mouse transcripts differentially induce neurite outgrowth

Gas7, a growth-arrest-specific protein, is expressed preferentially in the brain and is required for neurite outgrowth in cultured cerebellar and peripheral murine neurons. Gas7 interacts with F-actin and colocalizes with the terminal part of actin microfilament in cells in which membrane outgrowth is present. Gas7 isoforms were discovered in murine brain by alternative splicing. This work reports the identification of two human Gas7 cDNA: hGas7-a with 2,427 nucleotides, which encodes 330 amino acids, and hGas7-b with 2,610 nucleotides, which encodes 412 amino acids according to predicted open-reading-frames. The predicted hGas7-b protein is 97% homologous to murine homologues, whereas the hGas7-a is homologous to the mouse Gas7-cb form that is expressed preferentially in cerebellum. Alignment analysis of the Gas7 protein sequences revealed a high homology to that in humans: 99% for the monkey, 97% in murine, and around 75% for the puffer fish and chicken. The hGas7-b protein comprises a WW domain, which often associates with other domains that are typically present in proteins in signal transduction processes, and an FCH domain, which participates in rearranging the cytoskeleton. The hGas7-a comprises only the FCH domain. Analysis of the human Gas7 sequences using the DNA database revealed that the two forms resulted from the canonical alternative splicing of a Gas7 genomic sequence. The abundance of both hGas7 mRNA levels, determined by quantitative PCR in tissues including brain, breast cancer, placenta, and head-neck cancer, revealed that the level of hGas7-a was 14 times that of hGas7-b in these tissues. Transfection of cells with hGas7-a or hGas7-b cDNA yielded the predicted 38-kDa or 50-kDa protein, respectively. The ectopic expression of hGas7 caused neurite-like cell processes in both mouse Neuro-2a and human SH-SY5Y neuroblastoma cells. Interestingly, the hGas7-a preferentially elicited the small lamellipodia, whereas the hGas7-b elicited the small filopodia phenotype. These findings reveal the evolutionary conservation of the structure and function of Gas7. They also suggest that the FCH domain in Gas7 may participate in the development of lamellipodia, and the WW domain may participate in the fine-tuning of the filopodia.