Synaptic Plasticity Induced by Differential Manipulation of Tonic and Phasic Motoneurons in Drosophila

Abstract

Structural and functional plasticity induced by neuronal competition is a common feature of developing nervous systems. However, the rules governing how postsynaptic cells differentiate between presynaptic inputs are unclear. In this study, we characterized synaptic interactions following manipulations of tonic Ib or phasic Is glutamatergic motoneurons that coinnervate postsynaptic muscles of male or female Drosophila melanogaster larvae. After identifying drivers for each neuronal subtype, we performed ablation or genetic manipulations to alter neuronal activity and examined the effects on synaptic innervation and function at neuromuscular junctions. Ablation of either Ib or Is resulted in decreased muscle response, with some functional compensation occurring in the Ib input when Is was missing. In contrast, the Is terminal failed to show functional or structural changes following loss of the coinnervating Ib input. Decreasing the activity of the Ib or Is neuron with tetanus toxin light chain resulted in structural changes in muscle innervation. Decreased Ib activity resulted in reduced active zone (AZ) number and decreased postsynaptic subsynaptic reticulum volume, with the emergence of filopodial-like protrusions from synaptic boutons of the Ib input. Decreased Is activity did not induce structural changes at its own synapses, but the coinnervating Ib motoneuron increased the number of synaptic boutons and AZs it formed. These findings indicate that tonic Ib and phasic Is motoneurons respond independently to changes in activity, with either functional or structural alterations in the Ib neuron occurring following ablation or reduced activity of the coinnervating Is input, respectively.

Distinct Target-Specific Mechanisms Homeostatically Stabilize Transmission at Pre- and Post-synaptic Compartments

“…Two different types of motor neurons innervate most muscles in Drosophila , called type Is (phasic) and type Ib (tonic; Atwood et al, 1993 ; Kurdyak et al, 1994 ; Lnenicka and Keshishian, 2000 ). Differences in GluR composition have been noted at terminals of Is and Ib inputs (Schmid et al, 2008 ), and there is evidence that these motor neuron subtypes may possess different plasticity rules (Newman et al, 2017 ; Li et al, 2018b ; Aponte-Santiago et al, 2020 ). Although no major differences in the adaptations related to hypo- and hyper-innervation were observed between Is and Ib inputs (Davis and Goodman, 1998 ), future work may uncover input-specific distinctions.…”

Synaptic Properties and Plasticity Mechanisms of Invertebrate Tonic and Phasic Neurons

“…The differential organization of the SSR around Ib and Is boutons indicates the muscle is capable of distinguishing between the two inputs and forming distinct postsynaptic specializations. This hypothesis is supported by the observation that glutamate receptor subtypes are differentially expressed at PSDs opposing the two inputs, with Ib synapses enriched for the GluRIIA subtype and Is terminals containing more of the GluRIIB subtype (Petersen et al, 1997;Marrus et al, 2004a;Rasse et al, 2005;Schmid et al, 2008;Aponte-Santiago et al, 2020). Drosophila glutamate receptors form tetramers with three shared subunits and a 4th subunit of either GluRIIA or GluRIIB (Schuster et al, 1991;Petersen et al, 1997;Marrus et al, 2004b;Featherstone et al, 2005;Qin et al, 2005).…”

“…Type I motoneurons are glutamatergic and subdivided into the Ib class with “big” boutons (3–6 μm in diameter) and the Is class that has “small” boutons (2–4 μm in diameter) as shown in Figure 1A . The Ib and Is neuronal subtypes have distinct morphological and electrophysiological properties that led to their classification as tonic (Ib) or phasic (Is) based on some similarities to crustaceans ( Johansen et al, 1989 ; Atwood et al, 1993 ; Jia et al, 1993 ; Kurdyak et al, 1994 ; Msghina et al, 1998 ; Lnenicka and Keshishian, 2000 ; Hoang and Chiba, 2001 ; Lu et al, 2016 ; Newman et al, 2017 ; Aponte-Santiago et al, 2020 ). Around 30 Ib motoneurons individually innervate each of the 30 muscles in a hemi-segment during late embryogenesis, while three Is neurons per hemi-segment innervate subsets of muscles to coordinate contraction of specific subgroups.…”

“…Around 30 Ib motoneurons individually innervate each of the 30 muscles in a hemi-segment during late embryogenesis, while three Is neurons per hemi-segment innervate subsets of muscles to coordinate contraction of specific subgroups. Innervation of abdominal muscles by Is motoneurons typically follows innervation by their Ib counterparts, with some muscles occasionally lacking Is input altogether ( Ashley et al, 2019 ; Aponte-Santiago et al, 2020 ). The terminal axons of Ib and Is motoneurons continue to grow over the muscle surface during the subsequent 6 days of larval development to eventually form ∼10 to 100 individual synaptic boutons depending on size of each specific muscle ( Zito et al, 1999 ).…”

“…The location of muscles M1 and M2 are indicated. Panels (C–F) are modified from Aponte-Santiago et al (2020) .…”