(19b) Cellular Adaptations Against Hydraulic Resistance Towards Higher Motility

During development, immune surveillance and cancer metastasis cell migration plays crucial roles. Metastatic cancer cells migrate in-vivo through preexisting tissue tracks and they confront environments of high hydraulic resistance at multiple sites of the metastatic cascade – at the interstitial spaces due to crowding of extracellular and degraded proteins, mucus lining of organs and inside blood vessels. Additionally, there is elevated hydraulic resistance in the local microenvironment of a growing tumor due to tumor-growth induced solid stress and elevated interstitial fluid pressure. However, the molecular mechanisms responsible for resistance sensation by cells and their downstream effectors in confined cell migration are still elusive. Intriguingly, we observed enhanced confined motility of various cell types in response to elevated hydraulic resistance. This prompted us to hypothesize that cells display distinct migration characteristics in response to hydraulic resistance in confinement, and this plasticity enable them to adapt and overcome the pathologically-relevant elevated resistances. Combining state-of-the-art bioengineering tools with sophisticated molecular biology and live cell imaging techniques, we have deciphered the mechanisms by which cells overcome the elevated hydraulic resistance.

To recreate in the in vitro setting confining tissue tracks encountered in vivo, we fabricated stiff (3,000 MPa) polydimethyl-siloxane (PDMS)- and compliant (10 kPa) polyacrylamide (PA)- based microchannels and studied the migratory phenomena of both cancerous and non-cancerous cell lines under elevated hydraulic resistances. Specifically, we examined MDA-MB-231 breast cancer cells, human osteosarcoma (HOS) cells as well as primary dermal fibroblasts and human aortic smooth muscle cells (hAOSMCs). Of note, all these cell types displayed increased motility at high hydraulic resistance even in two-dimensional (2D) wound healing assays, as evidenced by the markedly reduced times for wound closure (e.g., from 20 hours down to 11 hours for MDA-MB-231 cells and from 23 hours to 15 hours for fibroblasts). Also, migration speeds through confining channels dramatically increased in response to elevated hydraulic resistance (e.g., from 50 µm/h to 80 µm/h for MDA-MB-231 cells and from 60 µm/h to 72 µm/h for fibroblasts). Importantly, these increased speeds were accompanied by marked phenotypic changes. Specifically, MDA-MB-231 cells, which exhibit a primarily bleb-based phenotype under physiologically-relevant resistances, transition to a mesenchymal phenotype at elevated resistances. Although high concentrations of latrunculin A (2 µM) failed to abrogate MDA-MB-231 cell locomotion in confining channels at physiologically-relevant resistances, which is in line with the osmotic engine model (OEM), they were sufficient to completely halt cell migration inside microchannels at elevated resistances. Moreover, shRNA-based gene silencing of sodium/hydrogen exchanger 1 (NHE1) in MDA-MB-231 cells had a markedly larger impact on confined migration under increased resistance. The increased dependency of cells upon actin polymerization and NHE1-dependent water permeation under higher resistance also hints at an intricate crosstalk between different migratory pathways. Finally, optogenetic tools are being used in confining cells experiencing different hydraulic resistances to elucidate the involvement of additional players in the crosstalk of actin cytoskeleton and OEM.

Collectively, our data suggest that hydraulic resistance regulates the efficiency, modes and mechanisms of cell migration. Furthermore, we showcase that increasing resistance to pathologically relevant levels induces plasticity, primarily in cancerous cells, and these studies will enhance our understanding of the mechanisms underlying migration and phenotypic deregulation, enabling better design of drugs specifically targeting the adaptability of these cells.