How does biological glass grow? Previously unknown role of the protein actin proven

Microscopic image of spicules of the glass sponge
Using modern bioanalytical methods, an international team of researchers, led by biomineralogists from TU Bergakademie Freiberg, was able to detect the protein actin in glass structures of sponges.

As a freely movable structure, the protein is responsible for the formation of fibres up to three metres long, the team reports in the current issue of the scientific journal Advanced Science.

For over 500 million years, the more than 15,000 species of horn and glass sponges have formed fascinating skeletal structures from natural silica-containing glass fibres. This is how the sponges form their three-dimensional tissue and hold on to the substrate. Individual glass fibre skeletal structures can grow up to three metres long and up to six cubic metres in size. Various organic biopolymers are responsible for the formation of this basic skeleton. However, it was previously unknown which proteins and mechanisms control the diversity of glass fibre forms in the investigated water temperature range from -1.9 to 24 degrees Celsius (about 28 to 75 degrees Fahrenheit).

Analytics clarify structure of bio-glass fibres at nano and molecular level

In the first step, the researchers at TU Bergakademie Freiberg demineralised the different glass fibres of more than ten sponge species with a special hydrofluoric acid solution. Modern microscopy methods then enabled the team to gain a precise insight into the nano- and molecular structures of the fibres: At their centre, they clearly identified a structural protein called actin. When the team inhibited this protein with an inhibitor in laboratory experiments with young sponges, growth of the glass fibres stopped immediately. Prof. Hermann Ehrlich, professor of biomineralogy, therefore concludes: "Actin is responsible for the branching and the very complicated architecture of the glass fibre skeleton in all marine and freshwater sponges studied. Along the actin structure, which acts as a scaffold, so-called silicatein molecules attach themselves, react there to form silicon oxide and thus shape the most diverse tissue skeletons in nature. The actin structure can move freely in any direction and leave so-called sclerocyte cells." Sclerocytes are specialised cells that form the biological glass fibres.

A previously unknown role for the protein emerges: "Since actin was understood to be a purely intracellular protein, no one had previously looked for it in extracellular structures such as biological glass fibres," says Prof. Hermann Ehrlich. The team of researchers, with scientists from Germany as well as from the USA, Poland, Russia and the Czech Republic, now proposes that F-actin acquired the ability to form new glass fibres outside the sclerocyte cell in the evolutionary development of glass sponges.

Background: Origin of the cultivated sponges

The breeding and purchase of the sponges used from a breeding facility in Tunisia complied with all requirements of the Nagoya Protocol for access to genetic resources and equitable benefit-sharing.

Original publication: Herrmann Ehrlich et al.: Arrested in Glass: Actin within Sophisticated Architectures of Biosilica in Sponges, Advanced Science
https://doi.org/10.1002/advs.202105059

Fragen beantwortet / Contact: 
Prof. Dr. rer. nat. Herrmann Ehrlich, herrmann.ehrlich@esm.tu-freiberg.de