The membrane receptor Toll and the related Toll-like receptors (TLRs) are best known for their universal function in innate immunity. However, Toll/TLRs were initially discovered in a developmental context, and recent studies have revealed that Toll/TLRs carry out previously unanticipated functions in development, regulating cell fate, cell number, neural circuit connectivity and synaptogenesis. Furthermore, knowledge of their molecular mechanisms of action is expanding and has highlighted that Toll/TLRs function beyond the canonical NF-κB pathway to regulate cell-to-cell communication and signalling at the synapse. Here, we provide an overview of Toll/TLR signalling and discuss how this signalling pathway regulates various aspects of development across species.

Cell number plasticity is coupled to circuitry in the nervous system, adjusting cell mass to functional requirements. In mammals, this is achieved by neurotrophin (NT) ligands, which promote cell survival via their Trk and p75NTR receptors and cell death via p75NTR and Sortilin. Drosophila NTs (DNTs) bind Toll receptors instead to promote neuronal survival, but whether they can also regulate cell death is unknown. In this study, we show that DNTs and Tolls can switch from promoting cell survival to death in the central nervous system (CNS) via a three-tier mechanism. First, DNT cleavage patterns result in alternative signaling outcomes. Second, different Tolls can preferentially promote cell survival or death. Third, distinct adaptors downstream of Tolls can drive either apoptosis or cell survival. Toll-6 promotes cell survival via MyD88–NF-κB and cell death via Wek-Sarm-JNK. The distribution of adaptors changes in space and time and may segregate to distinct neural circuits. This novel mechanism for CNS cell plasticity may operate in wider contexts.

Sarcomeres are extremely highly ordered macromolecular assemblies where structural organization is intimately linked to their functionality as contractile units. Although the structural basis of actin and Myosin interaction is revealed at a quasiatomic resolution, much less is known about the molecular organization of the I-band and H-zone. We report the development of a powerful nanoscopic approach, combined with a structure-averaging algorithm, that allowed us to determine the position of 27 sarcomeric proteins in Drosophila melanogaster flight muscles with a quasimolecular, ∼5- to 10-nm localization precision. With this protein localization atlas and template-based protein structure modeling, we have assembled refined I-band and H-zone models with unparalleled scope and resolution. In addition, we found that actin regulatory proteins of the H-zone are organized into two distinct layers, suggesting that the major place of thin filament assembly is an M-line–centered narrow domain where short actin oligomers can form and subsequently anneal to the pointed end.

Neurotrophism, structural plasticity, learning and long-term memory in mammals critically depend on neurotrophins binding Trk receptors to activate tyrosine kinase (TyrK) signalling, but Drosophila lacks full-length Trks, raising the question of how these processes occur in the fly. Paradoxically, truncated Trk isoforms lacking the TyrK predominate in the adult human brain, but whether they have neuronal functions independently of full-length Trks is unknown. Drosophila has TyrK-less Trk-family receptors, encoded by the kekkon (kek) genes, suggesting that evolutionarily conserved functions for this receptor class may exist. Here, we asked whether Keks function together with Drosophila neurotrophins (DNTs) at the larval glutamatergic neuromuscular junction (NMJ). Starting with an unbiased approach, we tested the evelen LRR and Ig-containing (LIG) proteins encoded in the Drosophila genome for expression in the central nervous system (CNS) and potential interaction with DNTs. Kek-6 was expressed in the CNS, could interact genetically with DNTs and could bind DNT2 both in signaling essays and in co-immunoprecipitations. There is promiscuity in ligand binding, as Kek-6 could also bind DNT1, and Kek-5 could also bind DNT2. In vivo, Kek-6 is found presynaptically in motoneurons, and binds DNT2 produced by the muscle, which functions as a retrograde factor at the NMJ. Kek-6 and DNT2 regulate NMJ growth, bouton formation and active zone homeostasis. Kek-6 does not antagonise the alternative DNT2 receptor Toll-6, but rather the two receptors contribute in distinct manners to NMJ structural plasticity. Using pull-down assays, we identified and validated CaMKII and VAP33A as intracellular partners of Kek-6, and show that together they regulate NMJ growth and active zone formation. These functions of Kek-6 could be evolutionarily conserved, raising the intriguing possibility that a novel mechanism of structural synaptic plasticity involving truncated Trk-family receptors independently of TyrK signaling may also operate in the human brain. AUTHOR SUMMARY: A long-standing paradox had been to explain how brain structural plasticity, learning and long-term memory might occur in Drosophila in the absence of canonical Trk receptors for neurotrophin (NT) ligands. NTs link structure and function in the brain enabling adjustments in cell number, dendritic, axonal and synaptic patterns, in response to neuronal activity. These events are essential for brain development, learning and long-term memory, and are thought to depend on the tyrosine-kinase function of the NT Trk receptors. However, paradoxically, the most abundant Trk isoforms in the adult human brain lack the tyrosine kinase, and their neuronal function is unknown. Remarkably, Drosophila has kinase-less receptors of the Trk family encoded by the kekkon (kek) genes, suggesting that deep evolutionary functional conservation for this receptor class could be unveiled. Here, we show that Kek-6 is a receptor for Drosophila neurotrophin 2 (DNT2) that regulates structural synaptic plasticity via CaMKII and VAP33A, well-known factors regulating synaptic structure and plasticity, and vesicle release. Our findings suggest that in mammals truncated Trk-family receptors could also have synaptic functions in neurons independently of Tyrosine kinase signalling. This might reveal a novel mechanism of brain plasticity, with important implications for understanding also the human brain, in health and disease.