Cancer Cells Invade Confined Microchannels via a Self-Directed Mesenchymal-to-Amoeboid Transition.

Cancer cell invasion through physical barriers in the extracellular matrix (ECM) requires a complex synergy of traction force against the ECM, mechanosensitive feedback, and subsequent cytoskeletal rearrangement. PDMS microchannels were used to investigate the transition from mesenchymal to amoeboid invasion in cancer cells. Migration was faster in narrow 3 µm-wide channels than in wider 10 µm channels, even in the absence of cell-binding ECM proteins. Cells permeating narrow channels exhibited blebbing and had smooth leading edge profiles, suggesting an ECM-induced transition from mesenchymal invasion to amoeboid invasion. Live cell labeling revealed a mechanosensing period in which the cell attempts mesenchymal-based migration, reorganizes its cytoskeleton, and proceeds using an amoeboid phenotype. Rho/ROCK (amoeboid) and Rac (mesenchymal) pathway inhibition revealed that amoeboid invasion through confined environments relies on both pathways in a time- and ECM-dependent manner. This demonstrates that cancer cells can dynamically modify their invasion programming to navigate physically confining matrix conditions.

Coupled circumferential and axial tension driven by actin and myosin influences in vivo axon diameter.

It has long been known that neuronal axons are contractile. They actively maintain rest tension along the longitudinal direction both in vitro and in vivo. Here we show evidence that embryonic drosophila axons also actively maintain contractility/tension along the circumferential direction. We used confocal microscopy and spatial light interference microscopy to monitor axonal diameter along their length. We observed a decrease in diameter when microtubules are disrupted and an increase in diameter when actin filaments or myosin II are disrupted. Interestingly, active diameter reduction occurred consistently when axons were subjected to manipulations known to increase axial tension, suggesting that tension can be coupled in the axial and circumferential direction. This is further supported by the remarkably similar time constants for diameter reduction and rest tension increase of slackened axons. We infer that the actomyosin-driven circumferential contraction/hoop tension applies a squeezing force on the microtubule bundle of the axons. This hoop tension is balanced by the restoring force of the microtubule bundle. Therefore, axonal diameter increased when actin/myosin disrupting drugs relaxed the hoop tension and decreased when microtubule disrupting drug relaxed the restoring force. Circumferential tension thus can regulate axonal diameter and volume, as well as potentially microtubules alignment, inter-tubular spacing, and, by extension, axonal transport.

Mechanism of Axonal Contractility in Embryonic Drosophila Motor Neurons In Vivo.

Several in vitro and limited in vivo experiments have shown that neurons maintain a rest tension along their axons intrinsically. They grow in response to stretch but contract in response to loss of tension. This contraction eventually leads to the restoration of the rest tension in axons. However, the mechanism by which axons maintain tension in vivo remains elusive. The objective of this work is to elucidate the key cytoskeletal components responsible for generating tension in axons. Toward this goal, in vivo experiments were conducted on single axons of embryonic Drosophila motor neurons in the presence of various drugs. Each axon was slackened mechanically by bringing the neuromuscular junction toward the central nervous system multiple times. In the absence of any drug, axons shortened and restored the straight configuration within 2-4 min of slackening. The total shortening was ∼40% of the original length. The recovery rate in each cycle, but not the recovery magnitude, was dependent on the axon's prior contraction history. For example, the contraction time of a previously slackened axon may be twice its first-time contraction. This recovery was significantly hampered with the depletion of ATP, inhibition of myosin motors, and disruption of actin filaments. The disruption of microtubules did not affect the recovery magnitude, but, on the contrary, led to an enhanced recovery rate compared to control cases. These results suggest that the actomyosin machinery is the major active element in axonal contraction, whereas microtubules contribute as resistive/dissipative elements.