Regulation of Retinoid Receptors by Retinoic Acid and Axonal Contact in Schwann Cells Maria-Jesus Latasa, Jose Miguel Cosgaya* Department of Endocrine and Nervous System Physiopathology, Instituto de Investigaciones Biome´dicas, Consejo Superior de Investigaciones Cient´ıficas, Universidad Auto´ noma de Madrid, Madrid, Spain Abstract Background: Schwann cells (SCs) are the cell type responsible for the formation of the myelin sheath in the peripheral nervous system (PNS). As retinoic acid (RA) and other retinoids have a profound effect as regulators of the myelination program, we sought to investigate how their nuclear receptors levels were regulated in this cell type. Methodology/Principal Findings: In the present study, by using Schwann cells primary cultures from neonatal Wistar rat pups, as well as myelinating cocultures of Schwann cells with embryonic rat dorsal root ganglion sensory neurons, we have found that sustained expression of RXR-c depends on the continuous presence of a labile activator, while axonal contact mimickers produced an increase in RXR-c mRNA and protein levels, increment that could be prevented by RA. The upregulation by axonal contact mimickers and the transcriptional downregulation by RA were dependent on de novo protein synthesis and did not involve changes in mRNA stability. On the other hand, RAR-b mRNA levels were only slightly modulated by axonal contact mimickers, while RA produced a strong transcriptional upregulation that was independent of de novo protein synthesis without changes in mRNA stability. Conclusions/Significance: All together, our results show that retinoid receptors are regulated in a complex manner in Schwann cells, suggesting that they could have a prominent role as regulators of Schwann cell physiology. Citation: Latasa M-J, Cosgaya JM (2011) Regulation of Retinoid Receptors by Retinoic Acid and Axonal Contact in Schwann Cells. PLoS ONE 6(2): e17023. doi:10.1371/journal.pone.0017023 Editor: Christoph Winkler, National University of Singapore, Singapore Received October 15, 2010; Accepted January 18, 2011; Published February 28, 2011 Copyright: 2011 Latasa, Cosgaya. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by grants to JMC from the Spanish Ministerio de Educaci o´n y Ciencia (SAF2003-02528; SAF2004-05343; SAF2007-61693), Comunidad de Madrid-CSIC, and Fundacio´ n Mutua Madrilen˜ a. MJL was supported by the Ramon y Cajal program from the Spanish Ministerio de Eduaci o´n y Ciencia. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: jmcosgaya@iib.uam.es Introduction All-trans-retinoic acid (RA) and other retinoids are potent regulators of morphogenesis, cell growth and differentiation, especially in the nervous system, where their abundance is comparatively very high [1]. RA induces neurite outgrowth and neuronal differentiation of several neural cell types, including embryonic and adult stem cells, dorsal root ganglia neurons (DRGN), or neuroblastoma cells [2,3,4]. In PC12 cells, RA inhibits cell proliferation and mimics the effect of neurotrophins on the expression of several genes, although the retinoid does not cause neurite extension in parental PC12 cells [5,6,7]. In contrast, RA can stimulate neuritogenesis in cAMP-dependent protein kinase A deficient PC12 cell lines that express high levels of RA receptor [8]. Most of the studies on RA functions on the nervous system relate to its function as a morphogen during neural tube development or as a differentiation factor for central nervous system (CNS) neurons, but RA signaling also occurs in the peripheral nervous system (PNS). In the sciatic nerve of adult rats, all the required components of the RA signaling pathway (retinoid X receptors, RA receptors, RA synthesizing enzymes RALDH-1, - 2 and -3, as well as the cellular retinoid binding proteins CRBP-I, CRABP-I and -II) are expressed [9]. During regeneration after nerve crush or transection, local production and activation of RA in SC is greatly increased [9,10], suggesting a potential role for RA during PNS development and repair. Moreover, the fact that retinoid receptors seem to be present on SC [11] suggests that RA could also play a role in SC physiology during development or in pathological conditions during Wallerian degeneration and remyelination, and not only on axonal growth and regeneration. Furthermore, RA has been shown to exert a variety of functions on SC, including the induction of neuregulin receptor erbB3 [10], cooperation with BMP2 to induce GDNF [12], or downregulation of CNTF expression [13]. On the other hand, RA has been shown to induce differenti- ation of neuroblastoma cells towards a Schwann cell-resembling phenotype [14]. It is also a major part of a differentiation cocktail capable of inducing Schwann cell differentiation from neural stem cells [15], bone-marrow stromal cells [16], adipose tissue-derived stem cells [17] or human umbilical cord-derived mesenchymal stromal cells [18]. Additionally, it has been described that the loss of RA in zebrafish results in a lack of myelination, although RA signaling seems to be required at an early stage of nervous system development, long before the onset of myelin gene expression and even before the formation of overt glial-specific precursors [19]. More recently, we have been able to show that retinoids participate in the regulation of peripheral myelin formation [20]. PLoS ONE | www.plosone.org 1 February 2011 | Volume 6 | Issue 2 | e17023