Long Non-Coding RNA Transcripts in Crucian Carp Brain - Annotation and Expression Patterns in Anoxia and Reoxygenation

Long non-coding RNA transcripts (lncRNAs) are important regulators of various cellular processes, including gene expression. However, whether they are involved in the regulation of anoxia-induced transcriptomic changes in crucian carp (Carassius carassius), a species that can survive without oxygen for months at low temperatures, has remained unexplored. The existing genome annotation includes mainly protein-coding genes, which are more straightforward to annotate than the lower-expressed lncRNAs. Therefore, this study aimed to annotate lncRNAs in the crucian carp genome, investigate their potential involvement in regulating mRNA abundance in response to anoxia and reoxygenation, and further characterize any differentially regulated lncRNAs and their potential interaction partners (nearby genes). Using next-generation RNA sequencing of brain samples from crucian carp across normoxia, anoxia, and reoxygenation conditions (n=10 per group), 145,264 transcripts were assembled with a reference-guided approach. Using the bioinformatic tools CPC2, CNCI, CPAT, and FEELnc, we identified 6,072 lncRNAs among the assembled transcripts. We observed a substantial transcriptomic response to anoxia, with a total of 56,440 differentially expressed (DE) transcripts (adjusted p-value < 0.05; | log2(fold change)| > 0.38), including 1,321 lncRNAs. The FEELnc classification module identified multiple proximal RNA transcripts that may help to predict lncRNA functions, and due to their proximity to potential interaction partners (IPs), were likely cis-acting. Most (60%) of the interaction sites between DElncRNAs and their targets were located between genes (intergenic), predominantly on the same strand in intergenic subtypes, and primarily nested in the genic context. The log2-fold changes in RNA abundance of DE predicted IPs exhibited a positive correlation with the log2-fold changes of their corresponding DElncRNAs. These findings indicate that lncRNAs are at least co-expressed with and may regulate transcription of specific genes in crucian carp during anoxia and reoxygenation.