Discovery of redundant protein functions raises questions about the evolution of the nervous system

Five proteins share important roles in the formation and function of synapses and can substitute for each other. This discovery was made by a team of the Karl Landsteiner University of Health Sciences Krems (KL Krems) and the CavX Ph.D. program of the Medical University of Innsbruck, and their work is now published in the International Journal of Molecular Sciences.

Most of these proteins are components of so-called calcium channels, and only recently, the team discovered redundant functions for three of the proteins in synapse formation and neuronal signal transmission. The recent finding that two additional proteins (α₂δ-4 and Cachd1) can fulfill the same functions is surprising and raises questions about the evolution of the nervous system.

Ion channels serve to conduct signals in the nervous system, so it is essential that their function is tightly regulated. Proteins of the α₂δ-family play an important role in this process. They serve as regulatory subunits of calcium channels and hence for a long time they are known to regulate calcium currents.

Recently, however, Univ. Prof. Dr. Gerald Obermair, head of the Division of Physiology at the KL Krems, and his team were able to show that three of the four α₂δ-proteins are also chiefly involved in the formation of synapses and that they can substitute for each other in this fundamental function.

This caused a considerable stir, as α₂δ-proteins are associated with diseases such as epilepsy, autism, schizophrenia, and anxiety. Now Prof. Obermair and his research group have succeeded in showing that the last of the four proteins of this family and also another protein not only influence synapse formation, but also affect signal transmission.

Two is better, many are best

In contrast to the previously studied α₂δ-proteins (isoforms -1, -2, -3), the now investigated α₂δ-4 occurs predominantly in the retina of the eye but is hardly found in the brain. The current results, as Prof. Obermair explains, are even more surprising: "We were able to show in cell cultures that α₂δ-4 can exert very similar functions in the brain as the previously studied proteins α₂δ-1 to -3. Indeed, all these proteins can even replace each other in their most critical function. This seems wasteful and is remarkable in evolutionary terms."

On top of that, the team also studied a protein known as Cachd1. While this protein is structurally similar to the α₂δ-proteins, it is still unclear whether it also serves as a subunit of ion channels. Unlike α₂δ-4, however, it is abundantly found in the brain and has been linked to brain functions. This and its similarity to α₂δ-proteins were reasons enough to take a closer look at the functions of Cachd1.

"And indeed," elaborates Cornelia Ablinger, first author of the study and a student in the CavX Ph.D. program, "it turned out that Cachd1 can also take over the functions of α₂δ-proteins. Hence, it can modulate synapse formation and also affect channel function."

Further experiments with all α₂δ isoforms and Cachd1 showed that the ability to substitute for each other does not come without subtle differences. For example, analyses of synaptic calcium signals identified minute differences indicating specific modulatory roles of each protein. A finding that allows Prof. Obermair to speculate on the apparent redundancy of the proteins: "It may well be that in the course of evolution they diversified one after the other to adapt the critical control of nerve signal transmission to the requirements of increasingly complex organisms."

Experimental challenge

The surprising results were only made possible by ten years of preliminary work by Prof. Obermair's team. Actually, it was the ability of α₂δ proteins to substitute for each other that posed an experimental challenge. Particularly, because first a cellular nerve cell model had to be developed in which all three genes for α₂δ-1 to -3 were switched off. This endeavor turned out to be a big challenge, with a success rate of less than 5%.

However, once the team—which received a great deal of international attention—had overcome this hurdle, the questions could be experimentally tackled. The discovery that α₂δ-4 and Cachd1 can also modulate synapse formation and differentiation was made possible by the preliminary work. Precise measurements of calcium signals of individual synapses provided evidence that α₂δ-4 and Cachd1 can also modulate channel function.