Bilaterian Giant Ankyrins Have a Common Evolutionary Origin and Play a Conserved Role in Patterning the Axon Initial Segment

Abstract

In vertebrate neurons, the axon initial segment (AIS) is specialized for action potential initiation. It is organized by a giant 480 Kd variant of ankyrin G (AnkG) that serves as an anchor for ion channels and is required for a plasma membrane diffusion barrier that excludes somatodendritic proteins from the axon. An unusually long exon required to encode this 480Kd variant is thought to have been inserted only recently during vertebrate evolution, so the giant ankyrin-based AIS scaffold has been viewed as a vertebrate adaptation for fast, precise signaling. We re-examined AIS evolution through phylogenomic analysis of ankyrins and by testing the role of ankyrins in proximal axon organization in a model multipolar Drosophila neuron (ddaE). We find giant isoforms of ankyrin in all major bilaterian phyla, and present evidence in favor of a single common origin for giant ankyrins and the corresponding long exon in a bilaterian ancestor. This finding raises the question of whether giant ankyrin isoforms play a conserved role in AIS organization throughout the Bilateria. We examined this possibility by looking for conserved ankyrin-dependent AIS features in Drosophila ddaE neurons via live imaging. We found that ddaE neurons have an axonal diffusion barrier proximal to the cell body that requires a giant isoform of the neuronal ankyrin Ank2. Furthermore, the potassium channel shal concentrates in the proximal axon in an Ank2-dependent manner. Our results indicate that the giant ankyrin-based cytoskeleton of the AIS may have evolved prior to the radiation of extant bilaterian lineages, much earlier than previously thought.

The Drosophila small conductance potassium channel (SK) negatively regulates nociception

“…First, it suggests that the enhanced Ca ++ signals that we observed in nociceptor soma are potentially caused by back propagation of action potentials rather than a hyperexcitable soma or dendrites. Second, the proximal axonal localization is consistent with recently described evidence that other Drosophila K + channels may be localized to the axon initial segment in these neurons (Jegla et al, 2016). It is possible that SK regulates the after hyperpolarization of action potentials as in other systems, and this in turn, regulates firing frequency.…”

Cortical anchoring of the microtubule cytoskeleton is essential for neuron polarity and functioning

“…Mutating the sequence for the presumed extra microtubule binding site in the C. elegans UNC-116 motor tail reduced the capacity of the motor to organize microtubules minus-end out (Yan et al, 2013), arguing in favor of kinesin-1 driven microtubule-microtubule sliding. This kinesin-1 dependent microtubule sliding in Drosophila supposedly is independent of the kinesin light chain adaptors (Jolly et al, 2010;Winding et al, 2016). However, we found that, similar to depletion of UNC-116 (kinesin-1), depletion of KLC-2 also was able to suppress the microtubule sliding defect in the unc-33 (CRMP) mutant.…”

“…In Drosophila, microtubules are slid over other microtubules by kinesin-1 and potentially also by cortically anchored dynein, during the earliest phases of neurite outgrowth (Del Castillo et al, 2015). To accomplish this microtubule-microtubule sliding, kinesin-1 contains an additional microtubule binding site in its tail, which when mutated leads to developmental defects in both the axon and the dendrite, without affecting kinesin-1 cargo transport (Jolly et al, 2010;Winding et al, 2016). Mutating the sequence for the presumed extra microtubule binding site in the C. elegans UNC-116 motor tail reduced the capacity of the motor to organize microtubules minus-end out (Yan et al, 2013), arguing in favor of kinesin-1 driven microtubule-microtubule sliding.…”

“…Neurons typically express giant Ankyrin isoforms that may have a common evolutionary origin and that have essential functions in neuron development (Jegla et al, 2016). Interestingly, these giant Ankyrin proteins evolved several ways to connect the submembrane cytoskeleton to the microtubule network.…”

Diversity of Axonal and Dendritic Contributions to Neuronal Output

“…Moreover, recent studies suggest that their dendrites and axons behave very much like their vertebrate counterparts (Sánchez-Soriano et al, 2005;Rolls et al, 2007). Consistent with this, although the presence of a distinctive AIS in invertebrates has long been questioned, recent evidence demonstrated that cultured drosophila neurons express a specific isoform of ankyrin in the proximal axonal region (Rolls et al, 2007;Jegla et al, 2016), much like ankyrin-G in vertebrate neurons. In summary, many neurons in the animal kingdom are unipolar, departing from the classical neuronal morphology depicted in most textbooks.…”