Gopal Niraula

All Publications

• Aligned Collagen Fibers Drive Distinct Traction Force Signatures to Regulate Contact Guidance. ACS nano Niraula, G., Foroozandehfar, A., Namanda, F. R., Schneider, I. C. 2025; 19 (33): 30165-30185

  Abstract

  Cellular forces on deposited nonfibrillar extracellular matrix (ECM) have been measured extensively. However, in vivo, cells exert traction forces on collagen fibers within the ECM. Oftentimes, collagen fibers are aligned, as seen in cancer, fibrosis, and during wound healing. How forces are transmitted on aligned collagen fibers and how the cytoskeleton regulates this is unknown. Here, we develop a fiber-traction force microscopy (f-TFM) approach that uses collagen fibers transferred to flexible substrates with fiducial markers on the collagen fibers and in the underlying flexible substrates. We find that the elastic modulus of the substrate determines the steady-state traction stress exerted by cells on aligned collagen fibers but does not affect traction force kinetics. Collagen fiber networks result in higher traction stresses than adsorbed collagen, particularly for randomly oriented fibers. In cells that weakly contact guide, formins and Arp2/3 modulate traction stress differently, with formins increasing traction stress magnitude, while Arp2/3 increases traction stress kinetics. However, both are important in driving traction force increases during cell turning on aligned collagen fibers. In cells that strongly contact guide, Arp2/3 and formins are less important than myosin II. In addition, there is a positive correlation between traction force and directionality on aligned collagen fibers for modest cell alignment. Further increases in traction stress are not required for high cell alignment. These findings underscore the complex interplay between the mechanics of collagen fiber networks, cytoskeletal regulators, and cellular traction forces, providing insights into how cells navigate complex fiber networks during migration.

  View details for DOI 10.1021/acsnano.5c06736

  View details for PubMedID 40811686

• Develop Tandem Tension Sensor to Gauge Integrin-Transmitted Molecular Forces. ACS sensors Niraula, G., Pyne, A., Wang, X. 2024; 9 (7): 3660-3670

  Abstract

  DNA-based tension sensors have innovated the imaging and calibration of mechanosensitive receptor-transmitted molecular forces, such as integrin tensions. However, these sensors mainly serve as binary reporters, only indicating if molecular forces exceed one predefined threshold. Here, we have developed tandem tension sensor (TTS), which comprises two consecutive force-sensing units, each with unique force detection thresholds and distinct fluorescence spectra, thereby enabling the quantification of molecular forces with dual reference levels. With TTS, we revealed that vinculin is not required for transmitting integrin tensions at approximately 10 pN (piconewtons) but is essential for elevating integrin tensions beyond 20 pN in focal adhesions (FAs). Such high tensions have emerged during the early stage of FA formation. TTS also successfully detected changes in integrin tensions in response to disrupted actin formation, inhibited myosin activity, and tuned substrate elasticity. We also applied TTS to examine integrin tensions in platelets and revealed two force regimes, with integrin tensions surpassing 20 pN at cell central regions and 13-20 pN integrin tensions at the cell edge. Overall, TTS, especially the construct consisting of a hairpin DNA (13 pN opening force) and a shearing DNA (20 pN opening force), stands as a valuable tool for the quantification of receptor-transmitted molecular forces within living cells.

  View details for DOI 10.1021/acssensors.4c00756

  View details for PubMedID 38968930

  View details for PubMedCentralID PMC11287754

• Development of a Ratiometric Tension Sensor Exclusively Responding to Integrin Tension Magnitude in Live Cells. ACS sensors Sarkar, A., Niraula, G., LeVine, D., Zhao, Y., Tu, Y., Mollaeian, K., Ren, J., Que, L., Wang, X. 2023; 8 (10): 3701-3712

  Abstract

  Integrin tensions are critical for cell mechanotransduction. By converting force to fluorescence, molecular tension sensors image integrin tensions in live cells with a high resolution. However, the fluorescence signal intensity results collectively from integrin tension magnitude, tension dwell time, integrin density, sensor accessibility, and so forth, making it highly challenging to specifically monitor the molecular force level of integrin tensions. Here, a ratiometric tension sensor (RTS) was developed to exclusively monitor the integrin tension magnitude. The RTS consists of two tension-sensing units that are coupled in series and always subject to the same integrin tension. These two units are activated by tension to fluoresce in separate spectra and with different activation rates. The ratio of their activation probabilities, reported by fluorescence ratiometric measurement, is solely determined by the local integrin tension magnitude. RTS responded sensitively to the variation of integrin tension magnitude in platelets and focal adhesions due to different cell plating times, actomyosin inhibition, or vinculin knockout. At last, RTS confirmed that integrin tension magnitude in platelets and focal adhesions decreases monotonically with the substrate rigidity, verifying the rigidity dependence of integrin tensions in live cells and suggesting that integrin tension magnitude could be a key biomechanical factor in cell rigidity sensing.

  View details for DOI 10.1021/acssensors.3c00606

  View details for PubMedID 37738233

  View details for PubMedCentralID PMC10788086