The Role of FBXO41 in Neuronal Function and Hippocampal Development

In this thesis we characterized the function of Fbxo41 in neuronal signaling and brain development. We show that FBXO41 interacts with SKP1 and cullin 1 in an E3 ubiquitin ligase SKP1-cullin-F-Box protein (SCF) complex and localizes at centrosomes, from which primary cilia emanate (Chapter 2). Accumulation of FBXO41 at centrosomes disassembles primary cilia and affects Sonic Hedgehog (SHH) signaling. FBXO41 mediates cilia disassembly through aurora a kinase in proliferative cells, but not in neurons, while actin polymerization impacts FBXO41 dependent disassembly in both cell types. In addition, we identify the phorbol ester PDBU as a trigger of neuronal cilia disassembly, also in the absence of FBXO41. Hence, FBXO41 is sufficient but not required for cilia disassembly in neurons. Cilia are important for brain function and maturation (reviewed in 1), we therefore tested the effects of FBXO41 depletion on neurotransmission (Chapter 3) and dentate gyrus development (Chapter 4). In FBXO41 knockout (KO) neurons, neurite length, dendritic branching and synaptic density is normal compared to WT controls. However, synaptic transmission is less efficient. Hippocampal glutamatergic neurons lacking FBXO41 show decreased release probability which, with the relatively small sample size in our study, does not accumulate in significant effects on evoked postsynaptic current (EPSC) amplitudes. However, the effect on striatal GABAergic neurons appears more robust with a reduced readily releasable vesicle pool and decreased charge transfer during repetitive stimulation. These neurons also have less Synaptophysin at synapses, which may point to reduced vesicle numbers at GABAergic terminals. Dentate gyrus (DG) development is affected in Fbxo41 KO with a smaller overall size and less radial glial-like (RGL) cells and immature neurons, and delayed migration of neurons from the hilus to the granule cell layer (GCL). During brain development, neurons not only have to organize but also establish efficient connections to produce functional networks. In Fbox41 KO, network activity in the DG is perturbed with delayed increase in activity and synchronicity during early postnatal development. In order to better characterize the role of FBXO41 during development, we generated a conditional Fbxo41 KO (cKO) mouse model (Chapter 5). By deleting exon 4 of the Fbxo41 gene we were able to delete full length Fbxo41, but a truncated variant arose probably due to exon skipping, which was expressed at lower levels compared to full length FBXO41. Neurons lacking full length FBXO41 have increased axonal length and an increased paired pulse ratio. Overall, our studies uncovered several traits of FBXO41 (Fig. 6.1): it 1. is an E3 ligase; 2. accumulates at centrosomes, and regulates cilia length; 3. contributes to efficient neurotransmission; 4. regulates development of the dentate gyrus.