Projects at the Hidalgo lab   

 

Tolls as neurotrophic receptors and mechanisms of action in the CNS.

Mammalian neurotrophins exert their functions by binding to receptors of the Trk and p75 families, and activate signalling downstream to promote cell survival (via ERK, AKT and NFkB) or cell death (via JNK) and synaptic plasticity (PLCgamma, CREB).


Neurotrophin receptors of the canonical Trk or p75 protein families have not been found in Drosophila, provoking the question of how is neurotrophism implemented in insects. The neurotrophin family members in Drosophila are DNT1, DNT2 and Spz, which share sequence, structure and functional conservation with the mammalian neurotrophins. Spz is well-known ligand of Toll. Toll was first discovered in Drosophila for its function in establishing the embryonic dorso-ventral axis, and later found to have functions also in innate immunity. Mammalian Toll-Like Receptors (TLRs) were subsequently discovered by homology to Drosophila Toll, and found to also have functions in innate immunity, and alterations in their function results in multiple diseases - from inflammation and autoimmunity to kidney disease. The Toll receptor superfamily has universal functions in innate immunity throughout the animals.

We found that DNT1 and DNT2 bind Toll-6 and Toll-7 receptors in Drosophila and have functions in the central nervous system (CNS) at all stages, from the embryo to the adult brain. They are expressed in CNS interneurons and motoneurons. Loss of Toll-6 and Toll-7 function results in impaired larval locomotion and motoraxon targeting defects. Over-expression of Toll-6 or Toll-7 also impairs motoraxon targeting. The incidence of apoptosis in the CNS increases in Toll-6 or Toll-7 mutants, and apoptosis leads to cell loss, at least amongst interneurons. Over-expression of Toll-6 and Toll-7 can rescue naturally occurring cell death in the CNS.


DNT1 and DNT2 interact genetically with Toll-6 and Toll-7,  suggesting a preferential interaction of Toll-7 with DNT1, and Toll-6 with DNT2. These interactions are likely to be promiscuous. Over-expression of Toll-6 and Toll-7 in neurons rescues the apoptosis increase of DNT2 and DNT1 mutants, respectively.


In collaboration with Prof. Nick J. Gay and his team at the Department of Biochemistry, University of Cambridge, we have shown that DNT1 and DNT2 can bind Toll-6 and Toll-7 in vitro, in cell culture and in vivo and one of the downstream targets of Toll-6 and Toll-7 is NFkB. Interestingly, NFKB is also the downstream target of p75, the universal and most ancient of the neurotrophin receptors.


Thus, Toll-6 and Toll-7 function as receptors for DNT2 and DNT1.


Our findings suggest that either neurotrophism is implemented by a different mechanism in flies compared to humans, or that there might be a previously unforeseen relationship between the neurotrophin and TLR families also in mammals. In either case, these findings are important in order to continue using Drosophila to understand the CNS and brain, and to model brain diseases.


See: McIlroy G, Foldi I, Aurikko J, Wentzell JS, Lim MA, Fenton JC, Gay NJ and Hidalgo A (2013) Toll-6 and Toll-7 function as neurotrophin receptors in the Drosophila melanogaster CNS. Nature Neuroscience 16, 1248-1256. doi: 10.1038/nn.3474.