Humans carrying the CORD7 (cone-rod dystrophy 7) mutation possess increased verbal IQ and working memory. This autosomal dominant syndrome is caused by the single-amino acid R844H exchange (human numbering) located in the 310 helix of the C2A domain of RIMS1/RIM1 (Rab3-interacting molecule 1). RIM is an evolutionarily conserved multi-domain protein and essential component of presynaptic active zones, which is centrally involved in fast, Ca2+-triggered neurotransmitter release. How the CORD7 mutation affects synaptic function has remained unclear thus far. Here, we established Drosophila melanogaster as a disease model for clarifying the effects of the CORD7 mutation on RIM function and synaptic vesicle release. To this end, using protein expression and X-ray crystallography, we solved the molecular structure of the Drosophila C2A domain at 1.92 Å resolution and by comparison to its mammalian homolog ascertained that the location of the CORD7 mutation is structurally conserved in fly RIM. Further, CRISPR/Cas9-assisted genomic engineering was employed for the generation of rim alleles encoding the R915H CORD7 exchange or R915E,R916E substitutions (fly numbering) to effect local charge reversal at the 310 helix. Through electrophysiological characterization by two-electrode voltage clamp and focal recordings we determined that the CORD7 mutation exerts a semi-dominant rather than a dominant effect on synaptic transmission resulting in faster, more efficient synaptic release and increased size of the readily releasable pool but decreased sensitivity for the fast calcium chelator BAPTA. In addition, the rim CORD7 allele increased the number of presynaptic active zones but left their nanoscopic organization unperturbed as revealed by super-resolution microscopy of the presynaptic scaffold protein Bruchpilot/ELKS/CAST. We conclude that the CORD7 mutation leads to tighter release coupling, an increased readily releasable pool size and more release sites thereby promoting more efficient synaptic transmitter release. These results strongly suggest that similar mechanisms may underlie the CORD7 disease phenotype in patients and that enhanced synaptic transmission may contribute to their increased cognitive abilities.

Adhesion-type GPCRs (aGPCRs) participate in a vast range of physiological processes. Their frequent association with mechanosensitive functions suggests that processing of mechanical stimuli may be a common feature of this receptor family. Previously, we reported that the Drosophila aGPCR CIRL sensitizes sensory responses to gentle touch and sound by amplifying signal transduction in low-threshold mechanoreceptors (Scholz et al., 2017). Here, we show that Cirl is also expressed in high-threshold mechanical nociceptors where it adjusts nocifensive behaviour under physiological and pathological conditions. Optogenetic in vivo experiments indicate that CIRL lowers cAMP levels in both mechanosensory submodalities. However, contrasting its role in touch-sensitive neurons, CIRL dampens the response of nociceptors to mechanical stimulation. Consistent with this finding, rat nociceptors display decreased Cirl1 expression during allodynia. Thus, cAMP-downregulation by CIRL exerts opposing effects on low-threshold mechanosensors and high-threshold nociceptors. This intriguing bipolar action facilitates the separation of mechanosensory signals carrying different physiological information.

IntroductionNeurotransmitter release at presynaptic active zones (AZs) requires concerted protein interactions within a dense 3D nano-hemisphere. Among the complex protein meshwork the (M)unc-13 family member Unc-13 of Drosophila melanogaster is essential for docking of synaptic vesicles and transmitter release.MethodsWe employ minos-mediated integration cassette (MiMIC)-based gene editing using GFSTF (EGFP-FlAsH-StrepII-TEV-3xFlag) to endogenously tag all annotated Drosophila Unc-13 isoforms enabling visualization of endogenous Unc-13 expression within the central and peripheral nervous system.Results and discussionElectrophysiological characterization using two-electrode voltage clamp (TEVC) reveals that evoked and spontaneous synaptic transmission remain unaffected in unc-13GFSTF 3rd instar larvae and acute presynaptic homeostatic potentiation (PHP) can be induced at control levels. Furthermore, multi-color structured-illumination shows precise co-localization of Unc-13GFSTF, Bruchpilot, and GluRIIA-receptor subunits within the synaptic mesoscale. Localization microscopy in combination with HDBSCAN algorithms detect Unc-13GFSTF subclusters that move toward the AZ center during PHP with unaltered Unc-13GFSTF protein levels.

1 Adhesion-type G protein-coupled receptors (aGPCRs) participate in a vast range of 2 physiological processes. Correspondingly, these receptors are associated with diverse human 3 diseases, such as developmental disorders, defects of the nervous system, allergies and 4 cancer. Several aGPCRs have recently been linked to mechanosensitive functions suggesting 5 that processing of mechanical stimuli may be a common feature of this receptor family. CIRL 6 (ADGRL/Latrophilin, LPHN), one of the oldest members of the aGPCR family, sensitizes 7 sensory responses of larval Drosophila to gentle touch and sound by amplifying 8 mechanosensory signal transduction in low-threshold mechanoreceptors (Scholz et al., 2015; 9 2017). In the present study, we show that Cirl is also expressed in high-threshold mechanical 10 nociceptors where it adjusts nocifensive behaviour under physiological and pathophysiological 11 conditions. Optogenetic in vivo experiments indicate that CIRL quenches cAMP levels in both 12 mechanosensory submodalities. However, contrasting its effect in touch sensitive neurons, 13 CIRL dampens the response of nociceptors to mechanical stimulation. Consistent with this 14 finding, rat nociceptors display a drop in Cirl1 expression during allodynia. Taken together, 15 these results demonstrate that CIRL exerts opposing modulatory functions in low-threshold 16 mechanosensors and high-threshold nociceptors. This intriguing bipolar action likely facilitates 17 the separation of mechanosensory signals carrying different physiological information. 18 19 20 2 Mechanosensation encompasses the distinct submodalities of touch, proprioception, and 3 mechanical nociception. Touch plays an important discriminative role and contributes to social 4 interactions (Abraira and Ginty, 2013; McGlone et al., 2014). Nociception reports incipient or 5 potential tissue damage. It triggers protective behaviours and can give rise to pain sensations 6 (Basbaum et al., 2009). Thus, physically similar signals can carry fundamentally different 7 physiological information, depending on stimulus intensity. Whereas innocuous touch 8 sensations rely on low-threshold mechanosensory neurons, noxious mechanical stimuli 9 activate high-threshold mechanosensory neurons, i.e. nociceptors. While mechanisms to 10 differentiate these mechanosensory submodalities are essential for survival, little is known how 11 this is achieved at cellular and molecular levels.12 13 The activity of nociceptors can be increased through sensitization, e.g. upon inflammation, and 14 decreased through antinociceptive processes, leading to pain relief. In both cases, G protein-15 coupled receptors (GPCRs) play an important modulatory role. Receptors that couple to 16 heterotrimeric Gq/11 or Gs proteins, like the prostaglandin EP2 receptor, increase the excitability 17 of nociceptors by activating phospholipase C and adenylate cyclase pathways, respectively. 18 In contrast, Gi/o-coupled receptors, which are gated by soluble ligands like morphine and 19 endogenous opioid neu...

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Neurotransmitter release at presynaptic active zones (AZs) requires concerted protein interactions within a dense 3D nano-hemisphere. Among the complex protein mesh- work the (M)unc-13 family member Unc-13 of Drosophila melanogaster is essential for docking of synaptic vesicles and transmitter release. We employ MiMIC-based gene editing using GFSTF (EGFP-FlAsH-StrepII-TEV- 3xFlag) to endogenously tag all annotated Drosophila Unc-13 isoforms enabling visualization of endogenous Unc-13 expression within the central and peripheral nervous system. Electrophysiological characterization using two-electrode voltage clamp (TEVC) reveals that evoked and spontaneous synaptic transmission remain unaffected in unc-13 GFSTF 3 rd instar larvae and acute presynaptic homeostatic potentiation (PHP) can be induced at control levels. Furthermore, multi-color structured-illumination shows precise co-localization of Unc-13 GFSTF , Bruchpilot and GluRIIA-receptor subunits within the synaptic mesoscale. Localization microscopy in combination with HDBSCAN algorithms detect Unc-13 GFSTF nanoclusters that move towards the AZ center during PHP with unaltered Unc-13 GFSTF protein levels.