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. 2012 Oct 18;76(2):383-95.
doi: 10.1016/j.neuron.2012.08.028. Epub 2012 Oct 17.

Drosophila CPEB Orb2A Mediates Memory Independent of Its RNA-binding Domain

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Free PMC article

Drosophila CPEB Orb2A Mediates Memory Independent of Its RNA-binding Domain

Sebastian Krüttner et al. Neuron. .
Free PMC article

Abstract

Long-term memory and synaptic plasticity are thought to require the synthesis of new proteins at activated synapses. The CPEB family of RNA binding proteins, including Drosophila Orb2, has been implicated in this process. The precise mechanism by which these molecules regulate memory formation is however poorly understood. We used gene targeting and site-specific transgenesis to specifically modify the endogenous orb2 gene in order to investigate its role in long-term memory formation. We show that the Orb2A and Orb2B isoforms, while both essential, have distinct functions in memory formation. These two isoforms have common glutamine-rich and RNA-binding domains, yet Orb2A uniquely requires the former and Orb2B the latter. We further show that Orb2A induces Orb2 complexes in a manner dependent upon both its glutamine-rich region and neuronal activity. We propose that Orb2B acts as a conventional CPEB to regulate transport and/or translation of specific mRNAs, whereas Orb2A acts in an unconventional manner to form stable Orb2 complexes that are essential for memory to persist.

Figures

Figure 1
Figure 1
Generation of the orb2attP Allele (A) Most of the orb2 open reading frame was replaced with the acceptor site, attP, for the phi-C31 recombinase by ends-out homologous recombination. Dashed lines indicate deleted region. orb2 modified alleles were generated by insertion into the orb2attP allele of the relevant donor construct bearing the donor site, attB, and the modified genomic orb2 fragment, by phi-C31 mediated transgenesis. (B) orb2 modified alleles were verified molecularly by Southern blots (SB), PCR, and RT-PCR. SB (left panel) was performed using probe a indicated in (A) and EcoRI, SpeI restriction digest. PCR (middle panel) amplification was performed using primers b and c indicated in (A). Obtained products were of the predicted size. RT-PCR (right panel) fragments were amplified using primers e and f indicated in (A). Product sizes were as predicted and were verified by DNA sequencing. RpS8 was used as an internal control. orb2attP/orb2+GFP heterozygotes were used for SB and PCR. Adult rare homozygous escapers orb2attP were used for RT-PCR. (C) LIs (green bars) of males carrying the indicated orb2 alleles (on the left) with the corresponding Orb2A and Orb2B protein organization (in the middle), tested in single-pair assays with mated females as trainers and testers for long-term memory (LTM) and short-term memory (STM). Control Canton-S flies, wild-type rescue allele (orb2+GFP), Q domain-deleted allele (Orb2ΔQGFP), and transheterozygote (Orb2ΔQGFP/orb2attP) were tested. p values are for H0: LI = 0, ∗∗p < 0.01, ∗∗∗p < 0.001 (permutation test). See also Table S1 for LTM and Table S2 for STM.
Figure 2
Figure 2
Orb2 Is Enriched in the Nervous System and Localizes to Pre- and Postsynaptic Compartments (A) Confocal projections of the Drosophila embryo, larvae, and adult brain of orb2+GFP animals (upper panels) stained with antibody to GFP (green) and counterstained with the general neuropil marker, antibody to either FasII or nc82 (red). Scale bar is 50 μm. Confocal projections of the adult Drosophila ventral nerve cord (VNC), mushroom body calyx (ca), lobes (α, γ), and cell bodies of the Kenyon cells (KC) (lower panels) of orb2+GFP animals stained with antibody to GFP (green) and counterstained with either general neuropil marker, antibody nc82 (red), or Kenyon cell body marker, antibody to Dachshund (DAC) (red). Scale bar is 50 μm except close up of the calyx and MB lobes, 10 μm. (B) Immuno-EM of the Drosophila orb2+GFP and canton-S brains in the region of calyx. See also Figure S1. (A) A GFP positive presynaptic cell (two asterisks) labeled by dark DAB precipitates is shown with an electron-dense active zone (arrow). Three GFP negative neurons indicated by a single asterisk display clear electron-dense active zones (arrowheads) with associated synaptic vesicles. Occasionally T bars (T), the presumptive docking site for vesicles, are also visible. (B) A GFP positive neuron (two asterisks) that is postsynaptic to a GFP negative neuron (asterisk) with an active zone (arrowhead) and synaptic vesicles is shown. The closely aligned pre- and postsynaptic membranes of the synaptic cleft are visible. (C) A similar region of the MB calyx to those above, but from a control Canton-S animal, is shown. Arrowheads indicate the presynaptic specializations: electron-dense active zones, associated synaptic vesicles, and T bars. In all panels, mitochondria are labeled with M and scale bar is 500 nm. (C) Schematic of the strategy to generate isoform specific orb2 alleles. (D) Confocal projections of the Drosophila brain and the VNC of orb2ΔAGFP (upper panels) and orb2ΔBGFP (lower panels) mutant flies stained with the antibody to GFP (green) and counterstained with the general neuropil marker, antibody nc82 (red). Scale bar is 50 μm. See also Figures S2 and S3.
Figure 3
Figure 3
The Q Domain in Orb2A Is Both Required and Sufficient for Long-Term Memory LIs (green bars) of males carrying the indicated orb2 alleles (left) with the corresponding Orb2A and Orb2B protein organization (middle) tested in single-pair assays with mated females as trainers and testers for long-term memory. p values are for H0: LI = 0, ∗∗p < 0.01, ∗∗∗p < 0.001 and H0 LI = LI1 (bars) (permutation test). See also Figure S4 and Table S4.
Figure 4
Figure 4
The RBD Is Essential for Function of Orb2B but Not of Orb2A in Long-Term Memory (A and B) LIs (green bars) of males carrying the indicated orb2 alleles (left) with the corresponding Orb2A and Orb2B protein organization (middle), tested in single-pair assays with mated females as trainers and testers for long-term memory (LTM) (A and B) and short-term memory (STM) (A). p values are for H0: LI = 0, ∗∗p < 0.01, ∗∗∗p < 0.001 and H0: LI = LI1 (bars) (permutation test). See also Table S5B. (C) LIs (green bars) of 247-Gal4, orb2mCPEB2RBD/tub-Gal80ts, UAS-orb2B or UAS-orb2BRBD males, cultured according to two temperature regimes (left) in tests for long-term memory. p values are for H0: LI = 0, ∗∗p < 0.01, ∗∗∗p < 0.001 (permutation test). See also Table S5C.
Figure 5
Figure 5
Orb2A and Orb2B Form Multimeric Complexes Mediated by the Q Domain (A) Representative confocal images of the Drosophila S2 cells expressing either Orb2AGFP or Orb2BGFP wild-type (upper panels) or with the Q domain deleted (lower panels) stained with the antibody to GFP (green) and counterstained with DAPI (blue). (B) Immunoprecipitation (IP), SDD-AGE, and western blot (WB) analysis of cell extracts from the Drosophila S2 cells coexpressing Orb2AGFP and Orb2BGFP isoforms either wild-type or with the Q domain deleted. (C) Immunoprecipitation (IP), SDD-AGE, and western blot (WB) analysis of the orb2+GFP, orb2ΔAGFP, and orb2ΔBGFP/+ head extracts. Note that Orb2A is detectable only with the 5× protein input (12.5 mg). (D) Mass spectroscopy (MS) analysis of the immunoprecipitates from head extracts of the animals carrying the indicated orb2 alleles (left). In the middle, schematics of the Orb2 protein organization are shown. On the right, % coverage of the total and the unique Orb2B peptides identified by MS after immunoprecipitation of either wild-type or Q domain-deleted Orb2AGFP.
Figure 6
Figure 6
Neuronal Stimulation Induces Orb2 Multimers through the Q Domain of Orb2A (A and B) Adult flies of indicated genotypes, after being starved for 16 hr, were fed with either tyramine or dopamine and sucrose for 6 hr and then continued on sucrose only. At the indicated time points, head extracts were analyzed by IP and WB for Orb2 multimers. Tubulin was used as a loading control. (C) Adult flies of indicated genotypes, after being starved for 16 hr, were fed with tyramine and sucrose for 6 hr. Head extracts were analyzed by IP and WB for Orb2 oligomers after 24 hr (upper panel). LIs (green bars) of males carrying the indicated orb2 alleles (left) with the corresponding Orb2A and Orb2B protein organization (middle), tested in single-pair assays with mated females as trainers and testers for long-term memory (lower panel). p values are for H0, LI = 0, ∗∗∗p < 0.001 and H0, LI = LI1 (bar) (permutation test). See also Figure S4.

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