Isoform Specificity of a Compound Targeting Actin Filaments Containing Tropomyosin Tpm1.8/1.9

The unbranched actin filaments in mammalian cells are usually composed of co-polymers of a specific tropomyosin isoform with actin. Genetic manipulation has revealed that the tropomyosins largely define the functional properties of actin filaments in an isoform-specific, non-redundant manner. Tropomyosin isoforms play a role in human diseases including cancers, thrombocytopenia and thrombocythaemia, endometrial decidualisation resistance, and ulcerative colitis. Hence, the development of compounds that target different tropomyosins are potentially valuable tools for cell biology as well as potential therapeutics. We have recently identified compounds that target Tpm1.8/1.9 and now address the isoform specificity of these compounds. Tpm1.8/1.9 is primarily enriched in the lamellipodium of migrating cells but not in stress fibre bundles unlike Tpm3.1/3.2 and Tpm4.2, which are enriched in stress fibres. Human fibroblasts also incorporate Tpm1.8/1.9 into fine filaments emanating from the perinuclear region. Exposure of human fibroblasts and SK-N-SH cells to the compounds 189-1 and 189-3 results in dispersal of Tpm1.8/1.9 from the lamellipodium and fine filaments to a diffuse organisation in the cytoplasm. In contrast, at doses that disperse Tpm1.8/1.9, 189-3 has no impact on the association of either Tpm3.1/3.2 or Tpm4.2 with actin filament bundles, whereas 189-1 also targets Tpm4.2. Tpm1.8/1.9 organisation becomes dispersed between 12- and 18-hour exposure to 189-3, and the organisation of Tpm1.8/1.9 returns within 4 h of drug washout. We conclude that the amino acid sequence differences located at 7 positions in the first 19 residues of these isoforms provide sufficient specificity to generate compounds that target Tpm1.8/1.9 alone.