Expression of an expanded CGG-repeat RNA in a single pair of primary sensory neurons impairs olfactory adaptation in Caenorhabditis elegans

Abstract

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a severe neurodegenerative disorder that affects carriers of premutation CGG-repeat expansion alleles of the fragile X mental retardation 1 (FMR1) gene; current evidence supports a causal role of the expanded CGG repeat within the FMR1 mRNA in the pathogenesis of FXTAS. Though the mRNA has been observed to induce cellular toxicity in FXTAS, the mechanisms are unclear. One common neurophysiological characteristic of FXTAS patients is their inability to properly attenuate their response to an auditory stimulus upon receipt of a small pre-stimulus. Therefore, to gain genetic and cell biological insight into FXTAS, we examined the effect of expanded CGG repeats on the plasticity of the olfactory response of the genetically tractable nematode, Caenorhabditis elegans (C. elegans). While C. elegans is innately attracted to odors, this response can be downregulated if the odor is paired with starvation. We found that expressing expanded CGG repeats in olfactory neurons interfered with this plasticity without affecting either the innate odor-seeking response or the olfactory neuronal morphology. Interrogation of three RNA regulatory pathways indicated that the expanded CGG repeats act via the C. elegans microRNA (miRNA)-specific Argonaute ALG-2 to diminish olfactory plasticity. This observation suggests that the miRNA-Argonaute pathway may play a pathogenic role in subverting neuronal function in FXTAS.

Fragile X–associated tremor/ataxia syndrome

“…Remarkably, Juang et al . observed an analogous loss of habituation—olfactory habituation—in a nematode ( Caenorhabditis elegans ) model in which an expanded CGG‐repeat expression vector (99 CGG repeats) is transcriptionally active solely in a single pair of olfactory neurons.…”

“…The primary odor‐sensory AWC neurons allow worms to chemotax toward attractive volatile chemicals and govern both the primary olfactory response and a secondary adaptive response, which requires neuronal plasticity , . A single AWC neuron senses the chemoattractant butanone and directs nematodes to move toward this odor source; prolonged odor exposure in the absence of food reduces the animal's attraction to butanone 100,101 because of processes that occur within the AWC neuron , . Remarkably, expression of the expanded CGG‐repeat element (but not the normal CGG repeat) in only a single pair of AWC neurons interfered with nematode's ability to attenuate the prolonged exposure to the odorant, while preserving normal odor detection .…”

“…A single AWC neuron senses the chemoattractant butanone and directs nematodes to move toward this odor source; prolonged odor exposure in the absence of food reduces the animal's attraction to butanone 100,101 because of processes that occur within the AWC neuron , . Remarkably, expression of the expanded CGG‐repeat element (but not the normal CGG repeat) in only a single pair of AWC neurons interfered with nematode's ability to attenuate the prolonged exposure to the odorant, while preserving normal odor detection . The authors found that the abnormal response to the expanded CGG repeat is mediated by the miRNA‐specific Argonaute (ALG‐2), suggesting that this pathway may play a role in the pathogenesis of neuronal function in FXTAS.…”

Non-coding RNA in Fragile X Syndrome and Converging Mechanisms Shared by Related Disorders

“…For instance, in neuronal cells and brain tissues from FXTAS patients, RNA aggregates containing pre-mutation CGG repeats decrease mature miRNA levels by sequestering RNA-binding proteins which are crucial to miRNA biogenesis, such as DGCR8 and DROSHA (Sellier et al, 2013). In olfactory neurons of C. elegans , expression of the expanded CGG repeats weakens the olfactory plasticity formation by interacting with the C. elegans specific argonaute ALG-2 (Juang et al, 2014). Additionally, miRNA-277, 424, 101, 129-5p, and 221 have also been indicated in the pathogenesis of FXTAS (Tan et al, 2012; Alvarez-Mora et al, 2013; Zongaro et al, 2013).…”

Models and mechanisms of repeat expansion disorders: a worm’s eye view

“…Therefore, worms have received limited attention for modeling Fragile X syndromes. Recently, a transgenic worm model of Fragile X was generated by expressing the human 5’UTR with 0 or 99 CGG upstream of the GFP coding sequence (Juang, Ludwig et al 2014) (Table 1). Expression was driven in the sensory AWC neuron, which mediates both primary and adaptive olfactory responses to attractive volatile chemical stimulants (Colbert and Bargmann 1995, L’Etoile, Coburn et al 2002).…”