A recent study has changed the traditional view of the evolutionary history of specific ion channels, which are essential proteins involved in electrical signaling within the nervous system. Researchers from Penn State have discovered that the Shaker family of ion channels were already present in tiny single-celled organisms long before the common ancestor of all animals, indicating that these channels existed before the nervous system even developed.
A recent study has changed the conventional view of the evolutionary timeline for certain proteins vital to electrical signaling in the nervous system. Researchers from Penn State have found that the well-known potassium ion channels from the Shaker family existed in minute single-celled organisms much earlier than the common ancestor of all animals. This implies that instead of evolving alongside the nervous system, these ion channels were already in place prior to its emergence.
The study was published in the Proceedings of the National Academy of Sciences.
“We often view evolution as a straightforward progression toward higher complexity, but that is not always how it works in nature,” explained Timothy Jegla, an associate professor of biology at Penn State and the study’s lead researcher. “Previously, it was believed that ion channels developed and diversified in tandem with the increasing complexity of the nervous systems in different animals. However, our research contradicts this notion. We’ve found that the oldest living animals, characterized by simple nerve nets, actually exhibit the greatest diversity in ion channels. This new evidence further supports the idea that many foundational elements for the nervous system were already present in our protozoan ancestors, before the nervous system itself emerged.”
Ion channels, which are found within cell membranes, manage the movement of charged particles called ions in and out of cells. This regulation is crucial for generating the electrical signals that enable communication in the nervous system. The Shaker family of ion channels is present across a wide variety of species, including humans, mice, and fruit flies, and it specifically controls the efflux of potassium ions to terminate electrical signals known as action potentials. Similar to transistors in computer chips, these channels respond to changes in the electric field by opening or closing.
“Much of our knowledge regarding the molecular workings of ion channels is derived from studies on the Shaker family,” Jegla stated. “We once believed that the Shaker family of voltage-gated potassium channels was exclusive to animals, but we’ve now identified genes responsible for this family of ion channels in several types of choanoflagellates, which are single-celled organisms that are the closest living relatives to animals.”
In earlier research, the team searched for these genes in two choanoflagellate species but could not find any. In the current investigation, they expanded their search to include 21 different choanoflagellate species and discovered evidence of Shaker family genes in three.
Various subfamilies of ion channels within the Shaker family exist throughout the animal kingdom. Previously, the team found that comb jellies, which possess relatively simple “nerve nets” believed to bear resemblance to the earliest animal nervous systems, contained only one type of these channels, known as Kv1. This led them to postulate that the common ancestor of animals likely had only Kv1, with other types developing later. However, Jegla and his colleagues discovered that the Shaker family genes found in choanoflagellates closely resembled the Kv2, Kv3, and Kv4 types.
“We had thought that Kv2 through Kv4 types evolved more recently, but our new findings indicate that the Kv2-4-like channels present in choanoflagellates are actually the oldest form,” Jegla mentioned.
This discovery also suggests that multiple subtypes existed at the base of the animal family tree, including Kv1, found in comb jellies, as well as the Kv2-4-like channels identified in choanoflagellates.
“The Kv2-4-like genes were lost in the living descendants of the earliest animal groups like comb jellies and sponges. We recognize their earlier existence solely due to the evidence from choanoflagellates,” Jegla added. “Gene loss is quite common in evolution, occurring as frequently as the emergence of new genes, although it can be challenging to pinpoint. Thanks to the affordability of genetic sequencing, we can now broadly sample species, allowing us to uncover more instances of gene loss, which will reshape our understanding of how our own gene families originated.”
This research further corroborates the notion that many components of the nervous system were already in existence prior to the full evolution of the nervous system, Jegla noted.
“The majority of the crucial proteins involved in electrical signaling, which facilitate neuronal communication and movement, were already present before animals appeared,” Jegla explained. “It appears that early animals were able to assemble a functional nervous system quite rapidly in their evolution, simply because the essential proteins were readily available.”
Jegla also highlighted that grasping how these ion channels evolved can enhance our understanding of their functionalities, thereby potentially aiding in developing treatments for disorders linked to ion channel malfunctions, such as heart arrhythmias and epilepsy.
Alongside Jegla, the research team consists of Benjamin Simonson, a graduate student in the molecular, cellular, and integrative biosciences program at the Huck Institutes of the Life Sciences and the Eberly College of Science at Penn State, who recently defended his dissertation, and David Spafford, an associate professor of biology at the University of Waterloo, specializing in choanoflagellate physiology. This work was funded by the Penn State Department of Biology and the Huck Institutes of the Life Sciences.
