Rnd3 is necessary for the correct oligodendrocyte differentiation and myelination in the central nervous system.

Rho small GTPases are proteins with key roles in the development of the central nervous system. Rnd proteins are a subfamily of Rho GTPases, characterized by their constitutive activity. Rnd3/RhoE is a member of this subfamily ubiquitously expressed in the CNS, whose specific functions during brain development are still not well defined. Since other Rho proteins have been linked to the myelination process, we study here the expression and function of Rnd3 in oligodendrocyte development. We have found that Rnd3 is expressed in a subset of oligodendrocyte precursor cells and of mature oligodendrocytes both in vivo and in vitro. We have analyzed the role of Rnd3 in myelination using mice lacking Rnd3 expression (Rnd3gt/gt mice), showing that these mice exhibit hypomyelination in the brain and a reduction in the number of mature and total oligodendrocytes in the corpus callosum and striatum. The mutants display a decreased expression of several myelin proteins and a reduction in the number of myelinated axons. In addition, myelinated axons exhibit thinner myelin sheaths. In vitro experiments using Rnd3gt/gt mutant mice showed that the differentiation of the precursor cells is altered in the absence of Rnd3 expression, suggesting that Rnd3 is directly required for the differentiation of oligodendrocytes and, in consequence, for the correct myelination of the CNS. This work shows Rnd3 as a new protein involved in oligodendrocyte maturation, opening new avenues to further study the function of Rnd3 in the development of the central nervous system and its possible involvement in demyelinating diseases.

RhoE is spatiotemporally regulated in the postnatal mouse CNS.

Rnd proteins are a family of small GTPases that have been involved in axon path finding and CNS development by their control of actin cytoskeleton dynamics. Rnd proteins are constitutively activated and, subsequently, their functions determined by their localization and expression levels. In this work we have analyzed by Western blot and immunohistochemistry the levels and localization of Rnd3/RhoE during mouse postnatal development. CNS was found to be the main tissue for RhoE protein expression, which was detected in all regions of the adult brain and spinal cord, with the highest levels in the olfactory bulb and cortex. RhoE protein levels were considerably higher in all the regions of the CNS the first 2-3 weeks of postnatal development, undergoing later a decrease that led to low levels in the adult. Immunohistochemical detection of RhoE at postnatal day 21 showed an intense and widespread labelling throughout the CNS. RhoE immunoreactivity was detected in the granular and mitral cells and anterior olfactory nuclei of the olfactory bulb and in all cerebral layers. In the striatum, diencephalon, mesencephalon, pons, medulla oblongata and spinal cord, RhoE was widely distributed with higher intensity in the motoneurones and in some brainstem nuclei such as the red nucleus or the reticulotegmental nucleus. The pyramidal cells of CA1-3 and the polymorph layer, but not the granular cells of the dentate gyrus in the hippocampus were strongly labelled. At earlier stages the labelling was nearly similar; however, a prominent labelling was detected in the cells of the rostral migratory stream and in the external granule cells of the cerebellum. Our results suggest that RhoE can play important roles in the postnatal development and maturation of the CNS, especially in the migratory processes affecting the neurones.