Inhibition of Cpeb3 ribozyme elevates CPEB3 protein expression and polyadenylation of its target mRNAs and enhances object location memory.

ABSTRACT

A self-cleaving ribozyme that maps to an intron of the cytoplasmic polyadenylation element-binding protein 3 (Cpeb3) gene is thought to play a role in human episodic memory, but the underlying mechanisms mediating this effect are not known. We tested the activity of the murine sequence and found that the ribozyme's self-scission half-life matches the time it takes an RNA polymerase to reach the immediate downstream exon, suggesting that the ribozyme-dependent intron cleavage is tuned to co-transcriptional splicing of the Cpeb3 mRNA. Our studies also reveal that the murine ribozyme modulates maturation of its harboring mRNA in both cultured cortical neurons and the hippocampus inhibition of the ribozyme using an antisense oligonucleotide leads to increased CPEB3 protein expression, which enhances polyadenylation and translation of localized plasticity-related target mRNAs, and subsequently strengthens hippocampal-dependent long-term memory. These findings reveal a previously unknown role for self-cleaving ribozyme activity in regulating experience-induced co-transcriptional and local translational processes required for learning and memory.

Stored within DNA are the instructions cells need to make proteins. In order for proteins to get made, the region of DNA that codes for the desired protein (known as the gene) must first be copied into a molecule called messenger RNA (or mRNA for short). Once transcribed, the mRNA undergoes further modifications, including removing redundant segments known as introns. It then travels to molecular machines that translate its genetic sequence into the building blocks of the protein. Following transcription, some RNAs can fold into catalytic segments known as self-cleaving ribozymes which promote the scission of their own genetic sequence. One such ribozyme resides in the intron of a gene for CPEB3, a protein which adds a poly(A) tail to various mRNAs, including some involved in learning and memory. Although this ribozyme is found in most mammals, its biological role is poorly understood. Previous studies suggested that the ribozyme cleaves itself at the same time as the mRNA for CPEB3 is transcribed. This led Chen et al. to hypothesize that the rate at which these two events occur impacts the amount of CPEB3 produced, resulting in changes in memory and learning. If the ribozyme cleaves quickly, the intron is disrupted and may not be properly removed, leading to less CPEB3 being made. However, if the ribozyme is inhibited, the intron remains intact and is efficiently excised, resulting in higher levels of CPEB3 protein. To test how the ribozyme impacts CPEB3 production, Chen et al. inhibited the enzyme from cutting itself with antisense oligonucleotides (ASOs). The ASOs were applied to in vitro transcription systems, neurons cultured in the laboratory and the brains of living mice in an area called the hippocampus. The in vitro and cell culture experiments led to higher levels of CPEB3 protein and the addition of more poly(A) tails to mRNAs involved in neuron communication. Injection of the ASOs into the brains of mice had the same effect, and also improved their memory and learning. The findings of Chen et al. show a new mechanism for controlling protein production, and suggest that ASOs could be used to increase the levels of CPEB3 and modulate neuronal activity. This is the first time a biological role for a self-cleaving ribozyme in mammals has been identified, and the approach used could be applied to investigate the function of two other self-cleaving ribozymes located in introns in humans.