Transcriptional Reorganization of Drosophila Motor Neurons and Their Muscular Junctions toward a Neuroendocrine Phenotype by the bHLH Protein Dimmed

Abstract

Neuroendocrine cells store and secrete bulk amounts of neuropeptides, and display morphological and molecular characteristics distinct from neurons signaling with classical neurotransmitters. In Drosophila the transcription factor Dimmed (Dimm), is a prime organizer of neuroendocrine capacity in a majority of the peptidergic neurons. These neurons display large cell bodies and extensive axon terminations that commonly do not form regular synapses. We ask which molecular compartments of a neuron are affected by Dimm to generate these morphological features. Thus, we ectopically expressed Dimm in glutamatergic, Dimm-negative, motor neurons and analyzed their characteristics in the central nervous system and the neuromuscular junction. Ectopic Dimm results in motor neurons with enlarged cell bodies, diminished dendrites, larger axon terminations and boutons, as well as reduced expression of synaptic proteins both pre and post-synaptically. Furthermore, the neurons display diminished vesicular glutamate transporter, and signaling components known to sustain interactions between the developing axon termination and muscle, such as wingless and frizzled are down regulated. Ectopic co-expression of Dimm and the insulin receptor augments most of the above effects on the motor neurons. In summary, ectopic Dimm expression alters the glutamatergic motor neuron phenotype toward a neuroendocrine one, both pre- and post-synaptically. Thus, Dimm is a key organizer of both secretory capacity and morphological features characteristic of neuroendocrine cells, and this transcription factor affects also post-synaptic proteins.

Substrates for Neuronal Cotransmission With Neuropeptides and Small Molecule Neurotransmitters in Drosophila

“…(D) In each neuromere of the larval ventral nerve cord (VNC) there is a pair of midline motor neurons, aCC and RP2 that signal with glutamate. The RP2 neurons also produce proctolin from Luo et al (2017) .…”

“…Also in Drosophila certain motor neurons express proctolin ( Anderson et al, 1988 ; Taylor et al, 2004 ). In Drosophila larvae especially the segmental mid-line motor neurons designated RP2 coexpress glutamate and proctolin ( Luo et al, 2017 ) ( Figure 3D ). Application of proctolin onto body wall muscle in Drosophila larvae indicated muscle fiber specific actions and induced dose dependent slow contractions ( Ormerod et al, 2016 ).…”

Function of Drosophila Synaptotagmins in membrane trafficking at synapses

“…Antisera raised against the two proteins demonstrate they are expressed in multiple neuropeptide releasing neurons in both the CNS and PNS [100]. Syt-α and Syt-β expression are also controlled by a master regulator of neuroendocrine neurons, the Dimmed transcription factor [108][109][110][334][335][336]. These data are consistent with Syt-α and Syt-β residing on DCVs to mediate Ca 2+ -dependent release of neuropeptides, though genetic analysis will be required to define their function.…”

Stac protein regulates release of neuropeptides

“…Interestingly, application of proctolin, the endogenous neuropeptide released by motor boutons (19), enhances body wall muscle contraction without affecting the membrane potential of muscles (47). The excitatory neurotransmitter released by motor neuron boutons is glutamate so the evoked synaptic potential at the NMJ is due to the action of glutamate (53). We examined whether Dstac might also regulate release of glutamate by recording the synaptic potential following stimulation of the motor nerve in mutant larvae.…”