Breakthrough in optogenetics

Genuine Rhodopsin KR2

Dmitry Ludmirsky, "For Science"

A collaboration of scientists from the Moscow Institute of Physics and Technology (MIPT), the Institute of Structural Biology of Grenoble University and the European Accelerator Complex in Grenoble (France), the Julich Research Center, Aachen University and the Max Planck Institute (Germany) for the first time in the world revealed and studied the structure of the protein-rhodopsin KR2 in physiological conditions. This pioneering work promises a new breakthrough in one of the most relevant biomedical disciplines – optogenetics – and its practical applications such as the treatment of widespread neurological diseases. Clinical depression, increased anxiety, epilepsy, Parkinson's disease – all these pathologies will receive a new tool for effective therapy thanks to the fundamental discovery of an international group of researchers, in which a team of biophysicists from MIPT played a leading role. The work of scientists has been published in one of the most prestigious scientific journals – Science Advances, the publication of the American Association for the Advancement of Science (Kovalev et al., Structure and mechanisms of sodium-pumping KR2 rhodopsin).

A few years ago, a new, previously unknown type of ion transporter, the protein rhodopsin, called KR2, was discovered in the cell membrane of the marine bacterium Krokinobacter eikastus. It belongs to the group of photosensitive proteins that optogenetics uses. Under the influence of light, such proteins allow charged particles – ions – to enter or exit the cell. By introducing such ion transporters into the neural membrane, scientists are able to use directed light pulses to influence the potential of the cell membrane of neurons, controlling their activity. KR2 was able to purposefully remove a specific type of ions from the cell – sodium ions. He "pumps" them out of the cell, and does not pass them in both directions, so scientists use the English verb pump to denote such an active action. Accordingly, KR2 is referred to as a "pump". In addition, its mutant forms are able to pump not only sodium but also potassium ions through the cell membrane. Therefore, embedding KR2 into the cell membrane of neurons could theoretically provide the possibility of full control of the activity of nerve cells.

But the wave of research generated by the discovery of a new "pump" has also encountered some very mysterious properties of this rhodopsin. In particular, it turned out that several groups of researchers in the course of their work discovered and described a total of as many as five different structures of a promising protein. It is noteworthy that in part of these structures, five protein molecules were organized into a stable pentamer, while only protein monomers were present in the remaining ones.

"And a dramatic question arose: which of these structures should be considered correct? – says one of the main authors of the work, a graduate student of MIPT Kirill Kovalev. "Generally speaking, all the structures found turned out to be quite similar, but the devil is in the details: the possibilities of using the newly discovered object in scientific and clinical practice depend on them."

And as a result of the work of a group of scientists led by physicists, the origin of the frightening variety of structures of the new protein was revealed. It turned out to be generated by the fact that different groups of researchers studied KR2 under not completely identical conditions. Meanwhile, a protein with unique properties is synthesized by the organism of a bacterium living in the ocean under very specific environmental parameters: it is surrounded by a water column with a strictly defined salt concentration, acidity, and hydrogen pH. It is precisely and only under these conditions that the protein does what scientists expect from it – pumps sodium ions, while forming pentamers in the cell membrane.

Monomer (left) and pentamer of rhodopsin KR2 in the cell membrane (blue discs). In the monomeric state, sodium transport is blocked, the pore (orange) does not allow ions to penetrate into the protein. A drawing from an article in Science Advances.

A variety of "false" protein structures were, it turns out, either crystallization artifacts, or discovered and studied in such conditions when KR2 practically does not carry those properties for which the world community of optogenetics places great hopes on it.

"For the first time, we modeled the so-called physiological conditions for the existence and operation of KR2 and as a result described the "correct" structure of the new protein, which occurs with the proper properties of the environment. We have shown that the functional unit of the protein is precisely the pentamer," explains Valentin Gordeliy, head of the Center for Research on Molecular Mechanisms of Aging and Age–Related Diseases at MIPT and at the Institute of Structural Biology in Grenoble. – At the same time, it was possible to explain why serious errors arose in the previous numerous studies of the structure of the object."

Experts believe that knowledge of the true structure of the revolutionary for optogenetics rhodopsin KR2 in physiological conditions is not only fundamental for understanding the mechanism of the protein, but also opens up many new grandiose opportunities for studying the nervous system of living organisms, modeling new tools of optogenetics and their application in medical practice.

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