The physiological reactions after spinal cord injury are accompanied by local synthesis of the transcriptional activator retinoic acid (RA). RA exerts its effects by binding to retinoic acid receptors (RAR) which heterodimerize with retinoid X receptors (RXR) and then act as ligand-activated transcription factors. To identify possible cellular targets of RA we investigated protein levels and cellular distribution of retinoid receptors in the rat spinal cord at 4, 7, 14 and 21 days after a contusion injury. In the nonlesioned spinal cord, immunoreactivity for RARalpha, RXRalpha, RXRbeta and RXRgamma was localized in the cytosol of neurons, that of RXRalpha and RXRbeta in astrocytes and that of RARalpha, RXRalpha and RXRgamma in some oligodendrocytes. After contusion injury RARalpha and all RXRs appeared in the cell nuclei of reactive microglia and macrophages. This nuclear staining began at 4 days, was most prominent at 7 and 14 days and had decreased at 21 days after injury. A similar nuclear translocation was also observed for the RARalpha, RXRalpha and RXRbeta staining in neurons situated around the border of the contusion. These observations suggest that RA participates as a signal for the physiological responses of microglia and neurons after CNS injury.

In mammalian peripheral nerves a crush lesion causes interactions between injured neurons, Schwann cells and haematogenous macrophages that can lead to successful axonal regeneration. We suggest that the transcriptional activator retinoic acid (RA), takes part in gene regulation after peripheral nerve injury and that RA signalling is activated via the cellular retinoic acid binding protein (CRABP)-II and cellular retinol binding protein (CRBP)-I. With RT-PCR and immunoblotting all necessary components of the RA signalling pathway were detected in the sciatic nerve of adult rats. These are retinoic acid receptors, retinoid X receptors, the retinoic acid synthesizing enzymes RALDH-1, RALDH-2, and RALDH-3, in addition, the cellular retinoid binding proteins CRBP-I, CRABP-I and CRABP-II. Enzyme activity of RALDH-2 was detectable in the nerve, and using a transgenic reporter mouse we found local activation of RA responsive elements in the regenerating nerve. Sciatic nerve crush as well as transection resulted in a more than 10-fold up-regulation of CRBP-I, which is thought to facilitate the synthesis of RA. Both kinds of injury also caused a 15-fold increase in transcript and protein concentration of CRABP-II, a possible mediator of RA transfer to its nuclear receptors.

It was investigated whether retinoic acid (RA) and the proinflammatory cytokines IL-1beta, IL-6, and TNFalpha influence the intracellular distribution of retinoic acid receptors (RAR) and retinoid X receptors (RXR) in Schwann cells. This question arose because nuclear translocation of RARalpha, RXRalpha, and RXRbeta was observed after nerve injury, and because mutual interactions exist between the signal transduction pathways of RA and proinflammatory cytokines. Schwann cell primary cultures from the rat sciatic nerve were incubated with IL-1beta, IL-6, and TNFalpha, with all-trans RA and with a combination of IL-1beta and RA. After incubation periods ranging from 5 min to 5 h, the intracellular distributions of RARalpha, RARbeta, RXRalpha, and RXRbeta were analyzed. All three cytokines caused a shift of RARalpha from the cytosolic compartments into the cell nuclei. This was also observed with RA, and combining RA with IL-1beta produced an additive effect. IL-1beta and IL-6 also affected the distribution of RARbeta, although immunoreactivity of this receptor always remained stronger in the cytosol. No effect of the cytokines on RXRalpha or RXRbeta was observed, whereas RA treatment caused a stronger nuclear signal of both receptors. Effects on the subcellular localization of retinoid receptors may provide a link in a feedback loop between RA/RAR and cytokines.

The transcriptional activator retinoic acid (RA) supports axonal regeneration of several neuronal cell populations in vitro, and it has been suggested that its receptor RARbeta2 may be used to support axonal regeneration in the adult mammalian spinal cord. We have previously shown that spinal cord injury induces activity of the RA synthesizing enzyme retinaldehyde dehydrogenase (RALDH)2 in NG2-positive cells. This report quantifies the increase of RALDH2 protein in the injured spinal cord and characterizes the RALDH2/NG2 expressing cells probably as a unique RA synthesizing subpopulation of activated oligodendrocyte precursors or "polydendrocytes". In the uninjured spinal cord low levels of RALDH2 are present in oligodendrocytes as well as in the meninges and in blood vessels. Following injury there is a significant increase in RALDH2 in these latter two tissues and, given that the RALDH2/NG2 positive cells are clustered in the same area, this implies that these are specific foci of RA synthesis. It is presumed that these cells release RA in a paracrine fashion in the region of the wound; however, the RALDH2/NG2-immunoreactive cells expressed the retinoid receptors RARalpha, RARbeta, RXRalpha and RXRbeta, suggesting that RA also serves an autocrine function.