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Role of Rnd proteins in the regulation of dendrite, spine and synapse formation in the developing hippocampus and their implication in hippocampal-dependent memory

Final Report Summary - RHOHIPPOMEMO (Role of Rnd proteins in the regulation of dendrite, spine and synapse formation in the developing hippocampus and their implication in hippocampal-dependent memory)

The hippocampus constitutes one of the main brain structures that are crucial for memory and emotion. Proper development of hippocampal neuronal networks is therefore vital to the mental health of an individual and perturbation of this process leads to memory disorders. These disorders seriously affect the quality of everyday life and the ability of individuals to contribute to the community resulting in the dependence of the memory impaired patients, posing a serious burden to the health systems of industrialized countries. Given the very high human and financial cost of these disorders, their prevention and treatment represent a major challenge in modern societies. In this context, a better understanding of the mechanisms regulating hippocampal development and function appears fundamental. I thus proposed a project that tackles this major research theme.
In particular, during neuronal development, the precise control of dendrite and spine formation is critical for normal neuronal activity and brain function. These dendrites are branched projections of a neuron and are concerned with receiving and processing information from other neurons. More precisely, the fine dendritic protrusions called spines serve as the main sites of contacts between neurons in the brain. Over the past few years, it has become clear that the family of small RhoGTPases, which are well-known regulators of the cell “skeleton”, play a major role in the regulation of spine formation and maintenance (Govek et al., 2005; Jan and Jan, 2010; Newey et al., 2005; Saneyoshi et al., 2010). Given the importance of these molecules in this process, it is therefore not surprising that genetic alterations in the signaling of RhoGTPases have been associated with mental retardation (Govek et al., 2005; Newey et al., 2005). As a result, therapeutic intervention that targets RhoGTPase activity or that of their associated molecules represents a possibility for preventing or curing memory disorders.
Among the RhoGTPases, I have focused my attention on relatively newly characterized members of this family: the Rnd proteins. Discovered within the past twenty years, the Rnd family represents a sub-group of the RhoGTPases and consists of three proteins: Rnd1, Rnd2 and Rnd3. Interestingly, my preliminary results showed that all Rnds are highly and differently expressed in the hippocampus during the first postnatal weeks, which corresponds with the most active period of dendrite/spine formation, and to a lesser extent at adult stages which altogether suggested a sustained role of Rnd proteins in hippocampal neuron development and function. As a better understanding of this role could ultimately provide further insights into the pathogenesis of many cognitive disorders, we proposed to study the role of Rnd proteins in the regulation of dendrite and spine formation in the developing hippocampus and their implication in the function of hippocampal neurons at an integrated level focusing on memory.
To reach these objectives, we combined multiple approaches from molecular to behavioural analyses. To study the role of Rnds in dendrite and spine formation in vivo, we silenced Rnd expression in the hippocampus specifically at the time of dendrite formation and evaluate the impact on dendrite morphology and spine number. To assess the implication of Rnds in hippocampal-dependent functions, we used mice carrying conditional null mutations in the hippocampus. Since the hippocampus has been implicated in a broad range of memory processes involving learning and memory of space, such as detection of novel environments, contextual conditioning and spatial navigation, we assessed these behaviors with multiple tasks. We also investigated the impact of Rnd deletion in mood behavior given the role the hippocampus in anxiety and depression.
In accordance with our initial hypothesis, our data indicate that the three members of the Rnd family have a critical role in the establishment of dendrite and spine structures during hippocampal development and their activity is distinct according to the different subregions of the hippocampus. In addition, our behavioral analyses show that the deletion of Rnd1 in hippocampal neurons induces an anxiety-like behavior whereas the loss of Rnd2 impairs working memory.
Further studies are now needed to clarify the downstream signaling mediating Rnd effect on dendrite and/or spines and to deepen their role in hippocampal-dependent functions. This is of fundamental importance from a basic scientific perspective, with the goal of providing mechanistic insights into the critical aspect of memory regulation, as well as from a clinical perspective, with the goal of providing effective therapies for a range of disorders involving cognitive impairments. Alterations in the RhoGTPase signaling pathway have been shown to contribute to mental retardation (MR). MR is a common cause of intellectual disability and affects about 2–3% of children and young adults. The discovery of new molecules that modulate the RhoGTPase signaling pathways, as well as research focusing on the molecular mechanisms of the signaling cascades involved, may allow the development of therapeutic strategies to counteract some aspects of MR, e.g. by the activation of compensatory molecular pathways.
A better understanding of the mechanisms regulating memory processes is relevant not only for the treatment of neurodevelopmental cognitive disorders but also for all the other memory disorders that can be related for example to aging, stress, post-traumatic stress disorders and other pathologies (depression…). These disorders concern a large part of the population, and have a very high human and financial cost. The rapidly growing elderly population and the increasing occurrence of memory disorders make the promotion of “healthy ageing” and the treatment of memory diseases a major challenge in European countries. However, the development of treatments is hindered mainly because the biological basis of memory is still unclear. For this reason a better understanding of the mechanisms regulating memory processes appears fundamental. Novel discoveries in this field of research may have phenomenal applications and a major impact on society and economy.
Besides its potential impact on society, this project has involved the generation and characterization of mice carrying conditional null mutations in Rnd genes (Rnd1flox/flox, Rnd2flox/flox and Rnd3flox/flox mice), which now constitute a useful tool for a large scientific community. Indeed, because of the functions of Rnds in many cellular processes and their implication for example in cancer, these mice may be really useful in other fields of research and may allow to reveal new information about their roles in vivo. In addition, Rnd mutant mice may provide animal models for human diseases. Indeed, recent data suggest that Rnd3 might be involved in motoneuron diseases. Finally, recent findings have showed the involvement of Rho GTPase signaling in the development and progression of neurodegenerative diseases, which further highlight that studies on Rnd functions in vivo represents uncharted territory that holds therapeutic potential.

Contact details of the researcher funded by the project RhoHippoMemo:
Dr Emilie PACARY
Neurocentre Magendie INSERM U862
Neurogenesis and Physiopathology Team
146, Rue Léo Saignat
33000 Bordeaux, France
emilie.pacary@inserm.fr

Contact details of the scientist in charge of the project RhoHippoMemo:
Dr Djoher Nora ABROUS
Neurocentre Magendie INSERM U862
Group Leader of the “Neurogenesis and Physiopathology” Team
146, Rue Léo Saignat
33000 Bordeaux, France
nora.abrous@inserm.fr