Genetic rat manipulation as an efficient model to unravel the role of Nogo-A and Staufen 2 proteins in cognitive processes

Abstract

Gene targeting technologies enabling a germline complete gene ablation mice had an enormous impact on the analysis of gene functions in vivo during the past two decades. However, the complete loss of a gene function often leads either to embryonic or early postnatal lethality or to molecular compensation that compensate for the function of the missing gene. Tissue or cell specific knockouts mostly avoid those drawbacks, but are currently confined mainly to mice as a model system. Indeed, development of gene inactivation technologies in rats is still far behind those available for mice, even if the elucidation of gene functions in transgenic rats would have several important advantages over using mice. The larger body size of a rat simplifies interventions such as microsurgery or multiple-electrode electrophysiological recording in vivo. Furthermore, higher order cognitive functions are more developed in this social rodent species than in the more solitarily living mice. Indeed, many behavioral tests are more advanced or validated for the rat, especially regarding the behavioral assessment of complex neuropsychiatric disease phenotypes, such as negative symptoms in schizophrenia or complex cognitive phenotypes. In the present study we used two different miRNA-based knockdown rat models to study the impact of those genes on emotional behavior and learning and memory processes. The first transgenic rat model is deficient for the Nogo-A protein within the entire animal. Nogo-A is expressed in CNS oligodendrocytes as well as in subpopulations of neurons and is known to suppress neurite growth and regeneration. In vivo studies in rats have shown successful regeneration of corticospinal tract axons over long distances and a significant enhancement of functional recovery using either neutralizing antibodies against Nogo-A or peptides blocking the Nogo receptor NgR. However, only few studies analyzed the role of Nogo-A on behavioral processes. Here, we show that Nogo-A deficient rats display behavioral phenotypes related to schizophrenia, such as difficulty in reversal learning, lower exploration and most importantly a reduced social contact behavior. Our behavior observations extend those described for Nogo-A knockout mice. In the second transgenic rat model were realized the first inducible tissue-specific gene inactivation rats described so far by knocking down Staufen2 (Stau2) protein production within excitatory neurons of the forebrain using an artificial miRNA targeting the respective protein. Staufen2 is a double-stranded RNA-binding protein essential for the localization of mRNAs in diverse cell types. In neurons, Stau2 regulates the dendritic localization and local translation of a subset of mRNAs that play a pivotal role in synaptic plasticity. In vitro experiments have shown that Stau2 is involved in the formation of dendritic spines, thereby modifying synaptic plasticity. However, no studies of Staufen function have been performed in vivo. Using our Stau2 deficient rats, we could show that the animals have an unaltered spatial reference memory and fear conditioning. However, Stau2 deficient rats have a highly significant impairment of spatial working memory. In addition, the transgenic animals have significant difficulties to detect spatial novelty. These behavioral finding fit very well to the in vivo electrophysiological data recorded by our collaborations, who could demonstrate that Stau2 deficient rats have an enhancement for LTP and an impairment of LTD. Together these findings suggest that Stau2 transported mRNAs are responsible for modulating synaptic plasticity at dendritic spines.