Sequential acquisition of cacophony calcium currents, sodium channels and voltage‐dependent potassium currents affects spike shape and dendrite growth during postembryonic maturation of an identified Drosophila motoneuron

Abstract

During metamorphosis the CNS undergoes profound changes to accommodate the switch from larval to adult behaviors. In Drosophila and other holometabolous insects, adult neurons differentiate either from respecified larval neurons, newly born neurons, or are born embryonically but remain developmentally arrested until differentiation during pupal life. This study addresses the latter in the identified Drosophila flight motoneuron 5. In situ patch-clamp recordings, intracellular dye fills and immunocytochemistry address the interplay between dendritic shape, excitability and ionic current development. During pupal life, changes in excitability and spike shape correspond to a stereotyped, progressive appearance of voltage-gated ion channels. High-voltage-activated calcium current is the first current to appear at pupal stage P4, prior to the onset of dendrite growth. This is followed by voltage-gated sodium as well as transient potassium channel expression, when first dendrites grow, and sodium-dependent action potentials can be evoked by somatic current injection. Sustained potassium current appears later than transient potassium current. During the early stages of rapid dendritic growth, sodium-dependent action potentials are broadened by a calcium component. Narrowing of spike shape coincides with sequential increases in transient and sustained potassium currents during stages when dendritic growth ceases. Targeted RNAi knockdown of pupal calcium current significantly reduces dendritic growth. These data indicate that the stereotyped sequential acquisition of different voltage-gated ion channels affects spike shape and excitability such that activity-dependent calcium influx serves as a partner of genetic programs during critical stages of motoneuron dendrite growth.

Intra-neuronal Competition for Synaptic Partners Conserves the Amount of Dendritic Building Material

“…All MN5 dendrites (Ryglewski et al, 2014a) as well as cholinergic and GABAergic inputs form during pupal life (Kuehn and Duch, 2013). We employed targeted genetic manipulation to selectively alter cholinergic or GABAergic synaptic activity in different parts of the developing MN5 arbor.…”

“…MN5 firing causes global Ca 2+ influx into the soma and the dendrites (Figure 3A). Conversely, knockdown of the Drosophila Ca v 2 homolog cacophony reduces somatic and dendritic Ca 2+ influx upon firing and decreases dendrite arbor size (Ryglewski et al, 2014a). Therefore, overall MN5 firing activity and global Ca 2+ influx regulate total dendrite length, likely together with steroid hormones (Weeks and Levine, 1995).…”

Dendrites are dispensable for basic motoneuron function but essential for fine tuning of behavior

“…MN5 can be unambiguously identified from animal to animal and displays a stereotyped dendritic morphology comprising a total length of 6,500 μm and >4,000 branches (13). Dendritic architecture is tightly regulated during development by steroid hormones (24), calcium signaling (25), neural activity (19,20), and cell surface signals (26). We have previously shown that the Significance Approximately 100 billion neurons in our brain form 100 trillion synapses onto 100,000 miles of dendritic cable.…”

Voltage-gated calcium channels are abnormal in cultured spinal motoneurons in the G93A-SOD1 transgenic mouse model of ALS

“…However, we cannot exclude the possibility that the mSOD1 alters the timing and sequencing of spinal cord maturational processes involving Ca 2+ channels. Ca 2+ channels contribute to many events during neuronal development, including cell migration (Komuro and Rakic, 1992), axonal growth (Gomez and Spitzer, 1999), and dendrite growth (Ryglewski et al, 2014). The developmental onset theory for ALS pathogenesis is refutable by the observation that G93A-SOD1 mice are born with locomotion mechanisms overtly intact and spinal cord development appears histologically normal by Nissl staining.…”