Analysis of mRNA poly(A) tail length in relation to long-term potentiation of synaptic transmission in the rat dentate gyrus in vivo

Abstract

Thoughts and experiences are stored in a capacious and dynamic central nervous system consisting of a vast number of communicating cells interconnected via synapses. New experiences generate neuronal activity which in turn modifies the strength of synaptic transmission at existing synapses. The mammalian nervous system is composed of billions of neurons which are a highly compartmentalized cell type with a great capacity to control their local protein composition. For long-term modifications of synapses to occur, neurons must be able to regulate gene expression and translation in a spatially and temporally specific manner depending on where and what type of synaptic activity it receives, so that proteins can be synthesized and modifications cemented into structure. The messengers of our genetic code, mRNA, can be compartmentalized into synapses where they can be translated on demand and are thus crucial to dendritic protein synthesis. At the end of each mRNA is a poly(A) tail – a stretch of adenosines attached to the 3′ end. For certain transcripts, changes in the length of this poly(A) tail are known to occur in the cytoplasm during neuronal activation. This may provide a mechanism for synaptic activity evoked changes in translational efficiency. Until recently, the level of accuracy in measuring the length of the poly(A) tail has been limited, but a new technology of direct RNA sequencing on nanopores has emerged which make it possible and efficient to sequence entire mRNA molecules, including the poly(A) tails. In this study, we use a well-established model system of long-term synaptic plasticity in a part of the hippocampal formation called the dentate gyrus, in adult anesthetized rats. After inducing activity-dependent plasticity via high frequency stimulation, the mRNAs from the dentate gyrus were extracted and nanopore sequenced to perform a genome-wide study of poly(A) tail length. Mean poly(A) tail lengths of approximately 15,000 genes were analyzed along with the difference in abundance of gene transcripts between stimulated and control groups to reveal a correlation between tail length and gene expression. A trend emerged showing a longer tail was correlated with genes that greatly increased their abundance after stimulation, while a shorter tail was correlated with genes that decreased their abundance after stimulation. Further study was then conducted on the expression of certain proteins in the whole dentate gyrus as well as in isolated synaptic compartments (synaptoneurosomes) of the dentate gyrus to investigate relationships between mRNA poly(A) tail length and dendritic protein synthesis. From this analysis, a tendency for decreased expression of polyadenylate-binding protein 1 (PABPC1) was observed in synaptoneurosomes following stimulation, which could present a possible model for poly(A) tail length-dependent gene regulation in synaptic compartments during synaptic plasticity.