Neurogenesis

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.

We use the Drosophila eye to study the signals which control neurogenesis. Prior to photoreceptor differentiation an extensive proliferative phase generates a large pool of undifferentiated cells, which are then specified sequentially through reiterative use of the Notch and EGF receptor pathways. Neurogenesis in the Drosophila eye occurs in a spatio-temporal manner making it particularly well suited for studying temporal controls during differentiation.

Insulin receptor signalling and neurogenesis
We have shown that the conserved insulin receptor (InR)/Tor pathway plays a key role in controlling the timing of neuronal differentiation in Drosophila (Bateman and McNeill 2004, Cell 119, p.87-96). By using mutants in various components of the InR/Tor pathway, we showed that activation of this pathway causes precocious differentiation of neurons. Conversely, inhibition of InR/Tor signalling significantly delays neurogenesis. Correct timing of neuronal differentiation is essential for tissue pattern formation and consequently mutations in components of InR/Tor signalling cause pattern defects in the adult. One of the aims of our research is to determine the molecular mechanism by which InR/Tor signalling regulates the timing of neuronal differentiation in Drosophila.

Neurogenesis and disease
We are also interested in diseases in which Inr/Tor signalling plays a role. One such disease is Tuberous Sclerosis Complex (TSC). TSC affects 1 in 6000 live births and is caused by mutations in one of two genes (TSC1 or TSC2). The pathology of TSC is typified by the formation of benign tumours in the brain, kidneys and other organs, beginning in early childhood. One of the most debilitating manifestations among patients with TSC is the high prevalence and severity of epilepsy, with patients suffering from up to hundreds of seizures per day. TSC1/2 are core components of the InR/Tor pathway and we have shown that loss of TSC1 causes precocious neuronal differentiation in Drosophila. The demonstration that TSC1 controls the timing of neuronal differentiation and hence neuronal patterning in Drosophila is intriguing, since abnormal neuronal development and migration are a major part of the neuropathology of TSC.