Whole-transcriptome changes in gene expression accompany aging of sensory neurons in Aplysia californica

Abstract

BackgroundLarge-scale molecular changes occur during aging and have many downstream consequences on whole-organism function, such as motor function, learning, and memory. The marine mollusk Aplysia californica can be used to study transcriptional changes that occur with age in identified neurons of the brain, because its simplified nervous system allows for more direct correlations between molecular changes, physiological changes, and their phenotypic outcomes. Behavioral deficits in the tail-withdrawal reflex of aged animals have been correlated with reduced excitation in sensory neurons that control the reflex. RNASeq was used to investigate whole-transcriptome changes in tail-withdrawal sensory neurons of sexually mature and aged Aplysia to correlate transcriptional changes with reduced behavioral and physiological responses.ResultsPaired-end sequencing resulted in 210 million reads used for differential expression analysis. Aging significantly altered expression of 1202 transcripts in sensory neurons underlying the tail-withdrawal reflex, with an approximately equal number of these genes up- and down regulated with age. Despite overall bidirectionality of expression changes, > 80% of ion channel genes that were differentially expressed had decreased expression with age. In particular, several voltage-gated K+ and Ca2+ channels were down regulated. This marked decrease in ion channel expression may play an important role in previously observed declines in aged sensory neuron excitability. We also observed decreased expression of genes and pathways involved in learning and memory. Genes involved in the stress response showed increased expression in aged Aplysia neurons.ConclusionsSignificantly altered expression of many genes between sexually mature and aged Aplysia suggests large molecular changes that may impact neuronal function. Decreased ion channel mRNA observed could mean fewer receptors present in aged neurons, resulting in reduced excitability of PVC sensory neurons, ultimately leading to reduced tail-withdrawal reflex observed in aged Aplysia. Significant changes in other genes and pathways, such as stress response and learning and memory, have previously been shown to occur with age in many vertebrate organisms. This suggests that some effects of aging are common across many animal phyla.Electronic supplementary materialThe online version of this article (10.1186/s12864-018-4909-1) contains supplementary material, which is available to authorized users.

Changes in Metabolism and Proteostasis Drive Aging Phenotype in Aplysia californica Sensory Neurons

“…Pairwise Wilcoxon test (fdr-corrected p ≤ 0.05) revealed a significant decrease in TWR through the Mature life stage followed by a significant increase in TWR in the Aged life stages. This U-shaped pattern with age is different from the pattern described in Kempsell and Fieber (2014 ; Figure 2B ); however, the steady increase in time to execute TWR after age 10 months is consistent with earlier reports ( Kempsell and Fieber, 2014 ; Greer et al, 2018 ).…”

“…Mortality was similarly within previously reported ranges. Reflex behavior data fell within expected norms and verified that normal aging occurred during late mature and aged stages ( Greer et al, 2018 ). TTR conformed to the Aplysia Stages of Aging model ( Kempsell and Fieber, 2014 ), with 10-month animals representing aged stage AI and 11- to 12-month animals representing stage AII.…”

“…Animals were weighed and then placed in solo cages to acclimate overnight. The next morning, reflex performance was assessed in each individual based on time to right (TTR) behavior and time to relax the tail following tail pinch, called the tail withdrawal reflex (TWR), as described previously ( Greer et al, 2018 ). Each reflex behavior was assessed in triplicate, with a minimum of 5 min rest between replicates and 15 min rest between TTR and TWR.…”

“…Reflex data were analyzed with a Kruskal–Wallis and pairwise Wilcoxon post hoc test with Benjamini–Hochberg false discovery rate correction. Animals were then housed in solo cages and fasted for 2 days after behavioral assessment before sacrifice and sampling to prevent behavioral assessment from biasing expression in target neurons as described previously ( Greer et al, 2018 ).…”

“…Furthermore, phylogenetic analysis has demonstrated that Aplysia represents an evolutionarily closer model to vertebrates than currently used ecdysozoan models, suggesting that Aplysia may offer a more effective model for human neuronal aging than do ecdysozoans ( Moroz et al, 2006 ). As with Drosophila and C. elegans , a reference genome and transcriptome are available for Aplysia (GCF_000002075.1), which has allowed for investigation of transcriptional changes in aging Aplysia neurons; however, there is a distinct absence of transcriptional profiling across the aging process as has been done in other models ( Moroz and Kohn, 2010 ; Kadakkuzha et al, 2013 ; Moroz and Kohn, 2013 ; Greer et al, 2018 ). Sampling of multiple time points is imperative for a full understanding of the transcriptional dynamics of aging and addressing which phenomena may be drivers of age-related dysfunctions.…”