The cytoskeleton, an intricate network of protein filaments, motor proteins, and cross-linkers, largely determines the mechanical properties of cells. Among the three filamentous components, F-actin, microtubules, and intermediate filaments (IFs), the IF network is by far the most extensible and resilient to stress. We present a multiscale approach to disentangle the three main contributions to vimentin IF network mechanics—single-filament mechanics, filament length, and interactions between filaments—including their temporal evolution. Combining particle tracking, quadruple optical trapping, and computational modeling, we derive quantitative information on the strength and kinetics of filament interactions. Specifically, we find that hydrophobic contributions to network mechanics enter mostly via filament-elongation kinetics, whereas electrostatics have a direct influence on filament–filament interactions.

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Near-infrared (NIR) fluorophores are emerging tools for biophotonics because of their reduced scattering, increased tissue penetration and low phototoxicity. However, the library of NIR fluorophores is still limited. Here, we...

The cytoskeleton, an intricate network of protein filaments, motor proteins, and crosslinkers, largely determines the mechanical properties of cells. Among the three filamentous components, F-actin, microtubules, and intermediate filaments (IFs), the IF network is by far the most extensible and resilient to stress. We present a multiscale approach to disentangle the three main contributions to vimentin IF network mechanics – single filament mechanics, filament length, and interactions between filaments – including their temporal evolution. Combining particle tracking, quadruple optical trapping and computational modeling, we derive quantitative information on the strength and kinetics of filament interactions. Specifically, we find that hydrophobic contributions to network mechanics enter mostly via filament elongation kinetics, whereas electrostatics have a direct influence on filament–filament interactions. These results indicate that cells might need to explicitly suppress attractive interactions to re-organize the extremely stable cellular vimentin network.

Near-infrared (NIR) fluorophores are emerging tools for biophotonics because of their reduced scattering, increased tissue penetration and low phototoxicity. However, the library of NIR fluorophores is still limited. Here, we report the NIR fluorescence of two benzene-fused oligo-BODIPYs in their hexameric (H) and octameric (O) forms. These dyes emit bright NIR fluorescence (H: maxima 943/1075 nm, O: maxima 976/1115 nm) that can be excited in the NIR (H = 921 nm, O = 956 nm) or non-resonantly over a broad range in the visible region. The emission bands of H show a bathochromic shift and peak sharpening with increasing dye concentration suggesting the presence of J-aggregates. Furthermore, the emission maxima of both H and O shift up to 20 nm in solvents of different polarity. These dyes can be used as NIR ink and imaged remotely on the macroscopic level with a stand-off distance of 20 cm. We furthermore demonstrate their versatility for biophotonics by coating microscale beads and performing microrheology via NIR video particle tracking (NIR-VPT) in biopolymer (F-actin) networks. No photodamaging of the actin filaments takes place, which is typically observed for visible fluorophores and highlights the advantages of these NIR dyes.