Nuclear receptor signalling
Cellular effects of retinoic acid (RA) are mediated by binding to nuclear receptors - the retinoic acid receptors (RARs) and retinoid X receptors (RXRs). There are three subtypes of each receptor, alpha, beta and gamma, and multiple isoforms of each subtype due to alternative splicing and differential promoter usage. RARs mediate gene expression by forming heterodimers with RXRs, whereas RXRs can mediate gene expression either as homodimers or by forming heterodimers with orphan receptors, which are also members of the nuclear receptor. The RAR/RXR heterodimers regulate transcription by binding to retinoic acid response elements (RAREs) in the upstream regions of target genes. Because the RAR genes contain RAREs, one notable effect of RA is its ability to induce the expression of the RARs themselves, thus stimulating various RA signalling pathways.
RARbeta2 signalling and neurite regeneration
We have shown that the RARbeta2 receptor induces neurite regeneration in vitro in both embryonic and adult neurons. In the adult spinal cord little or no expression of RARbeta2 can be detected, however when this tissue is transduced with RARbeta2 using lentivirus vectors neurite outgrowth occurs. By microarray analysis we have identified a number of genes involved in neurite regeneration, which are regulated by RARbeta2.
RARalpha signalling and neuronal survival
By generating retinoid deficient rats we have shown that RARalpha as opposed to other RAR receptors is required for the survival of motoneurons, Purkinje neurons and cerebral cortex neurons the same receptor deficit is found in human pathology samples of spontaneous cases of motoneuron disease and Alzheimer’s disease (AD). By using both in vitro and in vivo assays a number of target genes known to be involved in AD have been identified which are regulated by RARalpha signalling. Current work involves manipulating the retinoid pathway in mouse models of neurodegeneration and assaying for target genes and behavioural analysis including open field, novel object recognition and T maze.
RAR signalling and stem/progenitor cell differentiation
We have identified specific roles of RARbeta and alpha signalling in neural progenitor cell (NPC) differentiation. This will allow the transplantation of stem cells with a defined lineage into the injured nervous system or the stimulation of endogenous progenitor cells in the injured CNS both of which may lead to functional repair.
Screening for novel retinoids
Retinoids are small molecules which have been shown to cross the blood brain barrier, and therefore have therapeutic potential for the treatment of CNS disorders. However, to date very few retinoids with drug like properties have been developed. We have set up screening assays for both binding (IC50) and potency (EC50) of retinoids at the RARs from which we can identify specific receptor agonists. These will be used in models of CNS injury described above.