Figure 1.

The innate immune receptor engagement and possible signaling pathways in actin cytoskeleton remodeling. Within the common paradigm of innate immune recognition, there is no doubt the first signal for recognition of Francisella spp. must originate from the cell surface receptors. Francisella is recognized via TLR (Toll like receptor) heterodimers, especially TLR1 or 6, TLR2, and TLR4 that lead to myddosome formation followed by activation of NFκB (nuclear factor kappa-light-chain-enhancer of activated B cells), p38 protein and the activation of inflammatory cytokine genes. The signaling through TLR2 and TLR4 is possible only from the phagosome. Thus, TLR-MyD88 (myeloid differentiation primary response 88) complex is moved to the lipid rafts followed by the endocytosis. Nevertheless, there should be other signal from unknown receptor that ensures the moving of TLR-MyD88 complex into the phagosome by activation of actin cytoskeleton remodeling processes. From published data, the signaling pathways via S6K1 (ribosomal protein S6 kinase beta-1), ERK1/2 (extracellular signal-regulated kinase 1), 14-3-3 protein, or PAKs (p21-activated kinases) seem to be important for the actin remodeling, the actin cytoskeleton activation, and the regulation of transcription/translation needed for signal transduction and functional restructuralization.