Multipole magnetic tweezers measure organelle mechanical properties

14 Mar 2019 Belle Dumé
The magnetic bead exerting 50-piconewtons of force on the nuclear envelope caused deformation of the nucleus. Credit: Wang et al., Sci. Robot. 4, eaav6180 (2019)

Being able to directly probe organelles (specialized structures inside biological cells) and measure their properties is important for understanding subcellular activity, diagnosing diseases associated with this activity and developing new therapies. Doing this is no easy task, however. A team of researchers in Canada, China, and the US has now developed a new magnetic tweezer system that can measure the mechanical properties of organelles for the first time. The device makes use of a magnetic bead placed inside the cell that can be used to precisely manipulate intracellular structures at any location in 3D and apply a controllable force of up to 60 piconewtons to them for long periods. By relating the applied force and the amount by which the organelle deforms, as a result, the nanobot can measure properties such as viscoelasticity and plasticity of the cell nucleus, mitochondria, and endoplasmic reticulum.

“The mechanical properties of the biggest organelle inside a cell – the nucleus – are altered in cancer cells, cells with progeria, and cells infected with malaria,” explains lead author of this research study Xian Wang of the University of Toronto. “Directly probing the mechanical properties of the cell nucleus inside single cells would thus help us better understand the structural differences between diseased and healthy cells.”

Current techniques to manipulate subcellular structures often use invasive tools like micropipettes that can damage organelles. Optical tweezers that use laser beams are also popular, but the force that they generate is not high enough to mechanically move organelles.

Sub-micron bead

The new tweezer consists of six magnetic coils with sharp poles placed in different planes around a microscope coverslip seeded with live cells. The researchers apply electric current to each of the coils to generate a high magnetic gradient in the workspace. They then place a magnetic bead, with a diameter smaller than 1 micron, onto the coverslip, where the cells easily take it up through endocytosis. The bead can be pushed in different directions inside the cell by applying varying magnetic fields and so be made to “prove” the insides of the cell.

The device is integrated in a confocal microscope so that it can be imaged with high resolution. By observing the confocal microscope images in real time, the researchers were able to measure the elasticity and viscosity of a cell’s nucleus and observe variations in stiffness along different parts of the organelle. For example, they found that the nuclei of late-stage bladder cancer cells that they studied were less stiff when poked compared to early-stage ones.

The device provides a way to directly quantify the mechanical properties of intracellular organelles without having to freeze-dry and cut them up into slices first, explains Wang. What is more, we can exert forces of around 50 pN, which is an order of magnitude higher than that possible with lasers. And thanks to a generalized predictive control (GPC) algorithm, we can precisely control its position inside the cell to within a couple of hundred nanometres down the Brownian limit.A generalized predictive control algorithm

“When the magnetic bead is small, we require a high-resolution imaging tool to observe it and so control its position based on image feedback,” says Wang. “However, the problem is that high resolution means low imaging speed, which results in a large positioning error using a traditional control algorithm.”

To overcome this problem, the researchers adapted this algorithm for a sub-micron bead and developed their GPC, which predicts the bead position when no image feedback is available based on the previous camera frame. “This is how we are able to control it with an average of 0.4 microns (about half the bead’s body length),” says Wang.

“The idea of multipole magnetic tweezers is not new,” he tells Physics World. “We have simply further developed this concept and the method for sub-micron position control and pico-Newton force control on the smallest bead ever in this kind of study.”

Mechanical load during metastasis

In their experiments, the researchers found that the cell nucleus stiffens after repeated applied mechanical load. This type of load is what the cell experiences during metastasis when it has to squeeze through the tiny spaces between the endothelium cells of blood vessels.

“Previous research has revealed that nuclear deformation determines whether the cell can pass through a confined space,” explains Wang. “Nuclei of cancer cells are usually softer than those of healthy ones, which means they can do this and metastasize to other organs in the body. In our work, we discovered that late-stage cancer cells do not stiffen much after a force is applied to them so they can continue to move invasively.”

This study focused on characterizing the biophysical properties of intracellular organelles and the researchers say they are now looking to apply their technology to diagnosis and treatment. “Based on our current results, early-stage and late-stage cancer cell nuclei have distinct mechanical properties and these might potentially be used as a marker for diagnosis when no apparent physiological or morphological difference is present.

“For treatment, we are working on understanding and developing large scale tools – for example, applying a large force intracellularly to trigger cell apoptosis, for targeted cancer therapies.”

The team, reporting its work in Science Robotics 10.1126/scirobotics.aav6180, includes Helen McNeil and Yonit Tsatskis at Mount Sinai Hospital and Sevan Hopyan at The Hospital for Sick Children (SickKids).

15/3/2019 PHYSICSWORLD.COM