Highlights d Light-sensitive, multilayered human retinal organoids with functional synapses d 285,441 transcriptomes from light-responsive human retinas and retinal organoids d Organoid cell types converge to adult peripheral retinal cell types d Linking retinal diseases to human retinal and retinal organoid cell types

Evolution has tuned the nervous system of most animals to produce stereotyped behavioural responses to ethologically relevant stimuli. For example, female Drosophila avoid laying eggs in the presence of geosmin, an odorant produced by toxic moulds.Using this system, we now identify third order olfactory neurons that are essential for an innate aversive behaviour. Connectomics data place these neurons in the context of a complete synaptic circuit from sensory input to descending output. We find multiple levels of valence-specific convergence, including a novel form of axo-axonic input onto second order neurons conveying another danger signal, the pheromone of parasitoid wasps. However we also observe a massive divergence as geosmin-responsive second order olfactory neurons connect with a diverse array of ~75 cell types. Our data suggest a transition from a labelled line organisation in the periphery to one in which olfactory information is mapped onto many different higher order populations with distinct behavioural significance.

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Brain-derived neurotrophic factor (BDNF) plays important functions in cell survival and differentiation, neuronal outgrowth and plasticity. In Alzheimer’s disease (AD), BDNF signaling is known to be impaired, partially because amyloid β (Aβ) induces truncation of BDNF main receptor, TrkB-full length (TrkB-FL). We have previously shown that such truncation is mediated by calpains, results in the formation of an intracellular domain (ICD) fragment and causes BDNF loss of function. Since calpains are Ca2+-dependent proteases, we hypothesized that excessive intracellular Ca2+ build-up could be due to dysfunctional N-methyl-d-aspartate receptors (NMDARs) activation. To experimentally address this hypothesis, we investigated whether TrkB-FL truncation by calpains and consequent BDNF loss of function could be prevented by NMDAR blockade. We herein demonstrate that a NMDAR antagonist, memantine, prevented excessive calpain activation and TrkB-FL truncation induced by Aβ25–35. When calpains were inhibited by calpastatin, BDNF was able to increase the dendritic spine density of neurons exposed to Aβ25135. Moreover, NMDAR inhibition by memantine also prevented Aβ-driven deleterious impact of BDNF loss of function on structural (spine density) and functional outcomes (synaptic potentiation). Collectively, these findings support NMDAR/Ca2+/calpains mechanistic involvement in Aβ-triggered BDNF signaling disruption.