Four human spectrin genes are now recognized. Two encode alpha spectrins (αI and αII), the other two encode beta spectrins (βI and βII). Multiple alternatively spliced transcripts have also been identified for all but al spectrin, yielding a subtle but rich diversity of possible ap spectrin heterodimer species in most cells. The role of these isoforms and the factors that control their assembly into the triton-insoluble cortical membrane skeleton are poorly understood. RT-PCR analysis using primers flanking regions of alternative mRNA spicing for αII, βI, and βII spectrin have been used to explore the diversity of isoform expression in cultured fibroblasts, MDCK cells, and PC12 cells. Factors that stimulate assembly or redistribution of the spectrin skeleton in these cells were also sought. Several isoforms of spectrin are expressed in each of these cell lines, and PC12 cells altered the balance of one isoform moderately in response to NGF stimulation. These three cell lines also illustrate different ways that the assembly of the cortical skeleton may be regulated. In SV40tsA58 temperature sensitive large T transformed fibroblasts, spectrin redistributes from a largely cytoplasmic distribution to focal membrane patches upon transformation; in MDCK cells, cell-cell contact initiates spectrin assembly; in PC12 cells, stimulation with growth factor (NGF) induces a redistribution of spectrin from membrane to cytoplasmic pools. Collectively, these results suggest that both cell-cell contact and growth factor mediated signaling mechanisms control the assembly and isoform composition of the spectrin cytoskeleton, and highlight the complexity of this process.
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