The complexity of the nervous system presents a challenge to elucidation of molecular and cellular mechanisms underlying psychiatric conditions such as autism spectrum disorders. Innovative methods that can reduce this complexity by focusing on relevant systems may help elevate signal above noise. Dysregulation of the serotonergic system has long been implicated in autism. By specific profiling of serotonergic neurons using the Translating Ribosome Affinity Purification (TRAP) methodology, we uncovered a number of enriched transcripts in these cells, including the RNA-binding protein Celf6, of unknown function in the CNS. A rare variant introducing a premature stop codon in human CELF6 was found to be significantly associated with autism in human datasets. We hypothesized that disruption of Celf6 in mice would result in perturbation of some autism-related behaviors.
To assess the contribution of Celf6 to the etiology of autism by analysis of relevant behaviors in a murine model.
Celf6-/- (n=23) and WT (n=23) C57Bl6/j mouse pups were tested for ultrasonic vocalization induced by maternal separation. Emitted vocalization was measured from the audio recordings, as well as temporal and spectral characteristics of the emitted calls, using automated computational analyses. A separate cohort of Celf6-/- (n=11) and WT (n= 9) adult mice was tested for exploratory behavior in the holeboard maze as well as sociability in the three-chambered social approach assay. Mice of both genotypes were also tested for levels of neurotransmitters by mass spectrometry of brain tissue.
Celf6-/- mouse pups showed a 60% reduction in the amount of elicited ultrasonic vocalization (p=0.001), measured by the number of emitted calls. Celf6-/- mouse pups did not differ significantly in body weight or temperature, suggesting differences were not due to gross developmental delays nor environmental conditions at the time of testing. We found no significant change to Celf6-/- temporal characteristics (call duration and inter-call intervals) nor spectral complexity (as measured by percentage of frequency jumps), suggesting that the deficit was not one of physical production. Furthermore, although there was no significant difference in sociability between genotypes, WT adults significantly increased exploratory activity following familiarization with a food reward (p=0.002) compared to Celf6-/- mice. Finally, we found that Celf6-/- mice had a significant ~20% reduction to brain serotonin levels (p<0.002) but not other neurotransmitters such as GABA or glycine.
Our findings suggest Celf6-/- pups do not respond as readily as WT animals to a social cue and this is not due to physical inabilities. Furthermore, Celf6-/- mice do not appear to modify behavior as adults in response to reward. These data point to both communication deficits and resistance to change in the Celf6-/- model. Future investigations will probe the molecular role of Celf6 in the the development and maintenance of the serotonergic system, adding to the body of knowledge on the cellular mechanisms underlying these complex behaviors. Our results show that expression profiling of specific cell populations facilitates screening for genes contributing to relevant phenotypes in psychiatric disorder, and they identify Celf6 as a gene contributing to the expression of some autistic-like behaviors.