Muscular blob shows new direction for tissue engineering

A microscope view of the new, controllable blob of muscle proteins (Image: Harvard University)

A quivering blob of muscle proteins in a Harvard lab could lead to controllable biomaterials to replace damaged body tissue.

Under a microscope, the "active gel" looks like a throbbing tangle of fibres immersed in jelly. Created by David Weitz and his colleagues at Harvard University, it is made from a molecular net of the muscle protein actin held into shape by another protein, filamin. Each actin strand has around 300 molecules of another muscle protein, myosin, attached.

The gel stiffens when exposed to ATP, the chemical that cells use to store and release energy. It becomes 1000 times firmer, a change in elasticity of the same order as Jell-O setting, says Weitz.

The myosin molecules flex like miniature biceps, bunching up the actin strands and causing the network to "tense up".

Natural mover

"What we're trying to do is unravel the design principles that nature uses to make mechanical structures," says Weitz.

Unlike the materials typically used by engineers, which have fixed properties, many natural materials and structures can adapt theirs as circumstance require. Muscle is a good example, says Weitz, and the network he has created is a step toward replicating such properties.

"This bridges ideas that have been out there," says Margaret Gardel, a researcher at the University of Chicago not involved with the work. The blob is similar to the adaptable but tough protein skeleton that as well as holding cells in shape also allows them to shape-shift as required, she says.

Weitz thinks his active gel design could be used to give a new twist to tissue engineering, which usually involves using a static scaffold to guide the growth of replacement tissues from stem cells.

Scaffolds with tunable elasticity could allow more complex structures to be grown, says Weitz. For example, a floppy, untensed blob could be moved into position and then set in place with a pulse of ATP.

Because the physical properties of nearby surfaces are known to affect what kind of tissue stem cells grow into, a scaffold with controllable stiffness could direct a collection of stems cells to grow into different cell types to sculpt more intricate tissues that contain different kinds of cell.