Importin-α2 mediates brain development, learning and memory consolidation in Drosophila

Abstract

Neuronal development and memory consolidation are conserved processes that rely on nuclear-cytoplasmic transport of signaling molecules to regulate gene activity and initiate cascades of downstream cellular events. Surprisingly, few reports address and validate this widely accepted perspective. Here we show that Importin-α2 (Imp-α2), a soluble nuclear transporter that shuttles cargoes between the cytoplasm and nucleus, is vital for brain development, learning and persistent memory in Drosophila melanogaster. Mutations in importin-α2 (imp-α2, known as Pendulin or Pen and homologous with human KPNA2) are alleles of mushroom body miniature B (mbmB), a gene known to regulate aspects of brain development and influence adult behavior in flies. Mushroom bodies (MBs), paired associative centers in the brain, are smaller than normal due to defective proliferation of specific intrinsic Kenyon cell (KC) neurons in mbmB mutants. Extant KCs projecting to the MB β-lobe terminate abnormally on the contralateral side of the brain. mbmB adults have impaired olfactory learning but normal memory decay in most respects, except that protein synthesis-dependent long-term memory (LTM) is abolished. This observation supports an alternative mechanism of persistent memory in which mutually exclusive protein-synthesis-dependent and -independent forms rely on opposing cellular mechanisms or circuits. We propose a testable model of Imp-α2 and nuclear transport roles in brain development and conditioned behavior. Based on our molecular characterization, we suggest that mbmB is hereafter referred to as imp-α2^mbmB.

Keywords:

• Conditioning
• consolidation
• Drosophila
• Importin
• Kenyon cells
• learning
• long-term memory
• mushroom body
• nuclear pore complex
• nucleocytoplasmic transport
• proliferation

Acknowledgements

For Troy, I have warm memories of thoughtful contemplation and lively conversation with you, often on warm evenings at the Würzburger Hofbräu Biergarten. We thank Bernard Mechler for flies, reagents and advice; Jerry Yin for suggesting and hosting a preliminary experiment to evaluate of CREB-Imp-α2 interaction; Michael Ginsburg for analysis of embryonic development in mbmB; UNLV colleagues, staff and lab members (past and present) for material and collegial support; Brian Hobbs for constant support in every possible way; and Josh Dubnau, Kim Hutson de Belle and two anonymous reviewers for helpful comments on an earlier version of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by grants from the National Science Foundation (NSF), National Center for Research Resources [grant number IBN-9982969], [grant number IBN-0237395] and National Institutes of Health (NIH) [grant number 2 P20RR016464 to JSdB].