Abstract
The actin cytoskeleton is important in many cellular processes such as motility, and establishing and maintaining cell morphology. Members of the tropomyosin protein family associate with the actin cytoskeleton along the major groove of actin filaments (F-actin), stabilising them and regulating actin-filament dynamics. To date over 40 non-muscle tropomyosin isoforms have been identified, which are encoded by 4 different genes (α, β, γ, δ). Individual tropomyosin isoforms define functionally distinct F-actin populations. Previous studies have shown that tropomyosins sort to distinct subcellular compartments at different stages of development in polarised cells. Neuronal growth cones are highly dynamic polarised structures, dependent on a constant reorganisation of the actin cytoskeleton. By eliminating tropomyosins in a knockout (KO) mouse model, we investigated the role of two tropomyosin isoforms, Tm5NM1 and Tm5NM2 (γTm gene products) in growth cone dynamics and neurite outgrowth. Growth cone protrusion rates were significantly increased in one day old Tm5NM1/2 KO hippocampal neurons compared to WT controls. Neuritogenesis was significantly affected by the elimination of Tm5NM1/2, with a slight decrease in neurite length and an increase in neuronal branching in neurons cultured for four days. At the molecular level, the depletion of Tm5NM1/2 had no impact on the protein levels and activity of ADF/cofilin in hippocampal neurons while in cortical neurons a subtle but significant increase in ADF/cofilin activity was observed. The subtle phenotype in the early stages of neuritogenesis observed from eliminating Tm5NM1/2 may be explained with functional compensation by other tropomyosin isoforms. Functional compensation for the loss of Tm5NM1/2 may be provided by isoforms Tm5a/5b, TmBr2 and Tm4 as they localise to the growth cones, structures where Tm5NM1/2 are normally found. These results suggest that Tm5NM1/2 may not be required for early stages of neuritogenesis but may still play a fine-tuning role for this process.
