Neural circuits must rapidly detect and interpret sensory stimuli to accurately guide behavior. In the visual system, retinal circuits begin to distinguish light and dark stimuli at synapses between photoreceptors and ON- and OFF-bipolar cells, respectively. Bipolar cells develop without a need for other retinal cell types or activity, and thus, are thought to rely on autonomous gene expression. Through a genetic screen, we identified an essential role for pregnancy-associated plasma protein-aa (pappaa) in visually guided behaviors mediated by OFF-bipolar cells. By characterizing the identified pappaa mutant, our work describes the first role for pappaa in the development of neural circuits and identifies a critical factor in the formation and function of the retinal circuits that specifically guide behavioral responses to dark stimuli.
Despite a grossly normal retina and retinotectal projection, larvae harboring nonsense mutations in pappaa show an almost complete failure to initiate an O-bend response to sudden darkness or to turn away from a gradient of darkness. In contrast, pappaa mutant larvae facing a light target accurately swim towards the light. Together, the mutants’ behavior suggest that the ON-bipolar retinal pathway is intact, but the OFF-bipolar pathway mediating dark induced behavior is disrupted. Indeed, selective pharmacological stimulation of the OFF-bipolar pathway in pappaa mutants restores their dark induced behavioral deficiencies. pappaa acts as a metalloprotease to stimulate local IGF1 signaling. To gain insight into when pappaa-IGF1 signaling is required for dark induced visually guided behavior, we used genetic and pharmacological approaches to manipulate IGF1 signaling in pappaa wild type and mutant larvae. Induction of a transgenic, dominant negative IGF1R during temporally restricted periods indicated that IGF1 signaling is critical during the period of retinal synaptogenesis for larvae to respond to dark stimuli. Moreover, stimulation of IGF1 signaling in pappaa mutants during this stage improved dark induced behavior in the mutants. Throughout this period of retinal synaptogenesis, pappaa is exclusively expressed by bipolar cells in the retina and not in the tectum, consistent with a developmental role for pappaa in OFF-bipolar cell synaptic connections. Currently, we are performing ERG recordings and high-resolution anatomical analyses of OFF-bipolar cells to identify the specific requirement of PAPPAA-IGF1 signaling for OFF-bipolar cell synaptic function.