Role of Rab5 early endosomes in regulating Drosophila gut antibacterial response.

Interactions between prokaryotes and eukaryotes require a dialogue between MAMPs and PRRs. In Drosophila, bacterial peptidoglycan is detected by PGRP receptors. While the components of the signaling cascades activated upon PGN/PGRP interactions are well characterized, little is known about the subcellular events that translate these early signaling steps into target gene transcription. Using a Drosophila enteric infection model, we show that gut-associated bacteria can induce the formation of intracellular PGRP-LE aggregates which colocalized with the early endosome marker Rab5. Combining microscopic and RNA-seq analysis, we demonstrate that RNAi inactivation of the endocytosis pathway in the Drosophila gut affects the expression of essential regulators of the NF-κB response leading not only to a disruption of the immune response locally in the gut but also at the systemic level. This work sheds new light on the involvement of the endocytosis pathway in the control of the gut response to intestinal bacterial infection.

Peptidoglycan-dependent NF-κB activation in a small subset of brain octopaminergic neurons controls female oviposition.

When facing microbes, animals engage in behaviors that lower the impact of the infection. We previously demonstrated that internal sensing of bacterial peptidoglycan reduces Drosophila female oviposition via NF-κB pathway activation in some neurons (Kurz et al., 2017). Although we showed that the neuromodulator octopamine is implicated, the identity of the involved neurons, as well as the physiological mechanism blocking egg-laying, remained unknown. In this study, we identified few ventral nerve cord and brain octopaminergic neurons expressing an NF-κB pathway component. We functionally demonstrated that NF-κB pathway activation in the brain, but not in the ventral nerve cord octopaminergic neurons, triggers an egg-laying drop in response to infection. Furthermore, we demonstrated via calcium imaging that the activity of these neurons can be directly modulated by peptidoglycan and that these cells do not control other octopamine-dependent behaviors such as female receptivity. This study shows that by sensing peptidoglycan and hence activating NF-κB cascade, a couple of brain neurons modulate a specific octopamine-dependent behavior to adapt female physiology status to their infectious state.

Cytosolic and Secreted Peptidoglycan-Degrading Enzymes in Drosophila Respectively Control Local and Systemic Immune Responses to Microbiota.

Gut-associated bacteria produce metabolites that both have a local influence on the intestinal tract and act at a distance on remote organs. In Drosophila, bacteria-derived peptidoglycan (PGN) displays such a dual role. PGN triggers local antimicrobial peptide production by enterocytes; it also activates systemic immune responses in fat-body cells and modulates fly behavior by acting on neurons. How these responses to a single microbiota-derived compound are simultaneously coordinated is not understood. We show here that the PGRP-LB locus generates both cytosolic and secreted PGN-cleaving enzymes. Through genetic analysis, we demonstrate that the cytosolic PGRP-LB isoforms cell-autonomously control the intensity of NF-κB activation in enterocytes, whereas the secreted isoform prevents massive and detrimental gut-derived PGN dissemination throughout the organism. This study explains how Drosophila are able to uncouple the modulation of local versus systemic responses to a single gut-bacteria-derived product by using isoform-specific enzymes.

Activation of 5-HT2A Receptors Restores KCC2 Function and Reduces Neuropathic Pain after Spinal Cord Injury.

Downregulation of the potassium chloride cotransporter type 2 (KCC2) after a spinal cord injury (SCI) disinhibits motoneurons and dorsal horn interneurons causing spasticity and neuropathic pain, respectively. We showed recently (Bos et al., 2013) that specific activation of 5-HT2A receptors by TCB-2 [(4-bromo-3,6-dimethoxybenzocyclobuten-1-yl)methylamine hydrobromide] upregulates KCC2 function, restores motoneuronal inhibition and reduces SCI-induced spasticity. Here, we tested the potential analgesic effect of TCB-2 on central (thoracic hemisection) and peripheral [spared nerve injury (SNI)] neuropathic pain. We found mechanical and thermal hyperalgesia reduced by an acute administration of TCB-2 in rats with SCI. This analgesic effect was associated with an increase in dorsal horn membrane KCC2 expression and was prevented by pharmacological blockade of KCC2 with an intrathecal injection of DIOA [(dihydroindenyl)oxy]alkanoic acid]. In contrast, the SNI-induced neuropathic pain was not attenuated by TCB-2 although there was a slight increase of membrane KCC2 expression in the dorsal horn ipsilateral to the lesion. Up-regulation of KCC2 function by targeting 5-HT2A receptors, therefore, has therapeutic potential in the treatment of neuropathic pain induced by SCI but not by SNI.

Oligopeptide Transporters of the SLC15 Family Are Dispensable for Peptidoglycan Sensing and Transport in Drosophila.

Peptidoglycan (PGN) detection by PGN recognition proteins (PGRP) is the main trigger of the antibacterial immune response in Drosophila. Depending on the type of immune cell, PGN can be sensed either at the cell membrane by PGRP-LC or inside the cell by PGRP-LE, which plays a role similar to that of Nod2 in mammals. Previous work, mainly in cell cultures, has shown that oligopeptide transporters of the SLC15 family are essential for the delivery of PGN for Nod2 detection inside of the cells, and that this function might be conserved in flies. By generating and analyzing the immune phenotypes of loss-of-function mutations in 3 SLC15 Drosophila family members, we tested their role in mediating PGRP-LE-dependent PGN activation. Our results show that Yin, CG2930, and CG9444 are required neither for PGRP-LE activation by PGN nor for PGN transport from the gut lumen to the insect blood. These data show that, while intracellular PGN detection is an essential step of the antibacterial response in both insects and mammals, the types of PGN transporters and sensors are different in these animals.

Peptidoglycan sensing by octopaminergic neurons modulates Drosophila oviposition.

As infectious diseases pose a threat to host integrity, eukaryotes have evolved mechanisms to eliminate pathogens. In addition to develop strategies reducing infection, animals can engage in behaviors that lower the impact of the infection. The molecular mechanisms by which microbes impact host behavior are not well understood. We demonstrate that bacterial infection of Drosophila females reduces oviposition and that peptidoglycan, the component that activates Drosophila antibacterial response, is also the elicitor of this behavioral change. We show that peptidoglycan regulates egg-laying rate by activating NF-κB signaling pathway in octopaminergic neurons and that, a dedicated peptidoglycan degrading enzyme acts in these neurons to buffer this behavioral response. This study shows that a unique ligand and signaling cascade are used in immune cells to mount an immune response and in neurons to control fly behavior following infection. This may represent a case of behavioral immunity.

Hook2, a microtubule-binding protein, interacts with Par6α and controls centrosome orientation during polarized cell migration.

Polarity protein complexes function during polarized cell migration and a subset of these proteins localizes to the reoriented centrosome during this process. Despite these observations, the mechanisms behind the recruitment of these polarity complexes such as the aPKC/PAR6α complex to the centrosome are not well understood. Here we identify Hook2 as an interactor for the aPKC/PAR6α complex that functions to localize this complex at the centrosome. We first demonstrate that Hook2 is essential for the polarized Golgi re-orientation towards the migration front. Depletion of Hook2 results in a decrease of PAR6α at the centrosome during cell migration, while overexpression of Hook2 in cells induced the formation of aggresomes with the recruitment of PAR6α, aPKC and PAR3. In addition, we demonstrate that the interaction between the C-terminal domain of Hook2 and the aPKC-binding domain of PAR6α localizes PAR6α to the centrosome during cell migration. Our data suggests that Hook2, a microtubule binding protein, plays an important role in the regulation of PAR6α recruitment to the centrosome to bridge microtubules and the aPKC/PAR complex. This data reveals how some of the polarity protein complexes are recruited to the centrosome and might regulate pericentriolar and microtubule organization and potentially impact on polarized migration.

Cleavage of Na(+) channels by calpain increases persistent Na(+) current and promotes spasticity after spinal cord injury.

Upregulation of the persistent sodium current (I(NaP)) in motoneurons contributes to the development of spasticity after spinal cord injury (SCI). We investigated the mechanisms that regulate I(NaP) and observed elevated expression of voltage-gated sodium (Nav) 1.6 channels in spinal lumbar motoneurons of adult rats with SCI. Furthermore, immunoblots revealed a proteolysis of Nav channels, and biochemical assays identified calpain as the main proteolytic factor. Calpain-dependent cleavage of Nav channels after neonatal SCI was associated with an upregulation of I(NaP) in motoneurons. Similarly, the calpain-dependent cleavage of Nav1.6 channels expressed in human embryonic kidney (HEK) 293 cells caused the upregulation of I(NaP). The pharmacological inhibition of calpain activity by MDL28170 reduced the cleavage of Nav channels, I(NaP) in motoneurons and spasticity in rats with SCI. Similarly, the blockade of I(NaP) by riluzole alleviated spasticity. This study demonstrates that Nav channel expression in lumbar motoneurons is altered after SCI, and it shows a tight relationship between the calpain-dependent proteolysis of Nav1.6 channels, the upregulation of I(NaP) and spasticity.