MicroRNA function in Drosophila memory formation
Introduction
MicroRNAs (miRs) are small (∼22 nt) non-coding RNAs that provide post-transcriptional regulation of gene expression [1, 2]. They have been implicated in many different aspects of biology, from development to tumorigenesis [3, 4, 5]. Recent studies extend their influence into the biology of memory formation and memory disorders [6, 7, 8•], with miRs being offered as early biomarkers of Alzheimer disease (AD) [9] and as potential therapeutic targets [10, 11].
MiRs are usually transcribed from the genome as long primary miR hairpins (pri-miRs) by RNA polymerase II (Figure 1) [12]. In some cases, miRs are spliced-out from introns by the spliceosome and termed ‘miRtrons’ [13]. Pri-miRs are then processed in the nucleus by the Drosha/Pasha microprocessor complex into ∼70 nt long precursor-miRs (pre-miRs). Exportin5 actively (with Ran-GTP) translocates pre-miRs to the cytoplasm where they undergo the next processing step by the Dicer/Loquatious complex to produce a mature duplex composed of a guide strand and its passenger. The guide strand is preferentially inserted in a protein complex [14] called the RNA-induced silencing complex (RISC), made up of a member of the Argonaute (Ago) family of proteins and multiple other ribonucleoproteins (RNP). RISC guides the miR to the mRNA target based on sequence complementarity between the miR recognition element (MRE) in the 3′UTR of the mRNA, and a ‘seed region’ (nt 2–8) at the 5′ end of the miR [15]. RISC inhibits mRNA translation or triggers degradation depending on the degree of complementarity between the miR and the mRNA [16], thereby producing post-transcriptional control over the gene expression.
From a pure conceptual viewpoint, miRs are attractive molecular candidates for influencing memory formation [6, 7, 17]. On the one hand, they might quickly release sequestered ‘memory mRNAs’ for translation, either centrally or locally, in response to relevant neural activity [6, 7, 8•, 17, 18]. The release of mRNAs for translation could be particularly important for modifying the function and structure of synapses tagged for memory formation [19, 20, 21]. This would enable miRs to physiologically influence the dynamics of synaptic mRNA expression for intermediate-term or long-term memory formation. Given that long-term memory sparks changes in nuclear gene expression [22, 23, 24, 25], miRs might also influence the quality or quantity of mRNAs translated at the soma to marshal the required cellular differentiation for this form of memory. On the other hand, miRs regulate nervous system development [6, 26] and this provides an instructive role in building the neuronal circuits involved in memory formation [27]. Roles for miRs in neurodevelopmental [10, 28, 29], neurodegenerative [30], and neuropsychiatric disorders have been established [17, 28]. These disorders usually present with associated symptoms of learning disability and/or memory loss [8•, 10, 28, 29, 31, 32].
The fruit fly, Drosophila melanogaster, offers a facile organism to dissect the roles of miRs in memory formation. This model system provides simple and quantifiable behaviors to study memory, including olfactory classical conditioning [33] and long-term odor habituation [34, 35]. The fly's olfactory nervous system (Figure 2a), the brain region principally involved in olfactory memory, has been extensively characterized with a relatively detailed description of its neuronal circuits and constituent cell types [36, 37, 38, 39, 40]. Moreover, there exists very significant homology between the insect and the mammalian olfactory nervous system [41], such that conceptual insights made from the fly are easily extended to mammalian olfactory memory formation. In addition, the fly offers an extensive genetic toolset that includes genomic mutants, RNAi libraries, overexpression constructs and methodology for temporal and cell type-specific control of transgene expression [42, 43, 44]. As one example, ‘sponge technology’, when combined with specific Gal4 drivers, allows cell type-specific and temporal inhibition of individual miRs [45, 46] to test their importance in memory formation [47••, 48••, 49••].
There are many important questions to answer concerning the roles for miRs in Drosophila olfactory memory: (i) Which individual miRs are involved in memory formation? (ii) Where in the memory neural circuit does each miR function? (iii) When during the life cycle of the fly is each miR required? (iv) What specific phases of olfactory memory — short-term, intermediate-term, or long-term memory — are under the influence of individual miRs? (v) What specific aspects of neuronal physiology are affected by miR regulation? (vi) What are the target mRNAs for the miRs that are involved in olfactory memory formation? (vii) Does the dysregulation of miR expression always cause poor learning and memory, or can certain miRs be classed as memory suppressor whose normal function is to constrain memory formation? These and many related questions are tractable using the fly as the model system.
Section snippets
The RISC pathway is involved in olfactory memory formation
The antennal lobe (AL, Figure 2a) is the first relay center for olfactory information in the fly brain [41]. At the synaptic regions of the AL — the glomeruli — the axons of olfactory receptor neurons (ORn) transmit sensory information to the dendrites of projection neurons (Pn) and local interneurons (Ln). Pn then convey olfactory information to the mushroom body neurons (MBn) and neurons of the lateral horn (LH). The AL has been well studied because of its role in processing and coding olfactory
MiRs involved in memory formation
Beyond the speculated involvement of miR-280 and its regulated mRNAs including CaMKII in the Pn, Li et al. were the first to show that an individual miRNA, miR-276a, modulates memory formation through its regulation of dopamine receptor (DopR) expression [47••]. DopR is a central actor in olfactory memory formation, currently thought to convey the signal for the unconditioned stimulus (electric shocks) to the MBn [57]. Li and co-workers showed that partial miR-276a inhibition in the MBn using
From disease models to microRNAs
An alternative approach for identifying miRNAs that may function in memory formation is to identify miRs that are dysregulated in fly models of human diseases linked with memory deficits. Several fly models for human disease have been developed and characterized [31, 42, 43, 71].
Kong et al. [72•] overexpressed Aβ throughout the nervous system as a fly model of Alzheimer's disease [73] and identified 17 miRs that were dysregulated in fly heads (8 increased/9 decreased). Performance after
Conclusion
MicroRNAs provide a rapid cellular mechanism by modulating the expression of clusters of genes at post-transcriptional level. These features, along with the regulation they offer in synaptic compartments make them a particularly attractive class of molecules for modulating memory formation. Past research has clearly shown that the molecular machinery required for the biosynthesis of miRs is critical for normal LTM formation. In addition, several individual Drosophila miRs, including miR-276a,
Conflict of interest
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgments
Research in the authors’ laboratory was supported by grants R35NS097224 and P01NS090994 from the National Institute for Neurological Disorders and Stroke to R.L.D.
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Cited by (0)
- 1
Current address: Neurogenetics and Memory, Genetics & Development Department, Institute of Human Genetics/CNRS UPR1142, 141, rue de la Cardonille, 34396 Montpellier Cedex 5, France.
- 2
These authors made equal contributions to this review.