Targeting of the 5-HT1A serotonin receptor to neuronal dendrites is mediated by Yif1B

Expression and lysosomal targeting of CLN7, a major facilitator superfamily transporter associated with variant late-infantile neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses (NCLs) constitute a group of progressive neurodegenerative disorders resulting from mutations in at least eight different genes. Mutations in the most recently identified NCL gene, MFSD8/CLN7, underlie a variant of late-infantile NCL (vLINCL). The MFSD8/CLN7 gene encodes a polytopic protein with unknown function, which shares homology with ion-coupled membrane transporters. In this study, we confirmed the lysosomal localization of the native CLN7 protein. This localization of CLN7 is not impaired by the presence of pathogenic missense mutations or after genetic ablation of the N-glycans. Expression of chimeric and full-length constructs showed that lysosomal targeting of CLN7 is mainly determined by an N-terminal dileucine motif, which specifically binds to the heterotetrameric adaptor AP-1 in vitro. We also show that CLN7 mRNA is more abundant in neurons than astrocytes and microglia, and that it is expressed throughout rat brain, with increased levels in the granular layer of cerebellum and hippocampal pyramidal cells. Interestingly, this cellular and regional distribution is in good agreement with the autofluorescent lysosomal storage and cell loss patterns found in brains from CLN7-defective patients. Overall, these data highlight lysosomes as the primary site of action for CLN7, and suggest that the pathophysiology underpinning CLN7-associated vLINCL is a cell-autonomous process.

Substitution of 5-HT1A receptor signaling by a light-activated G protein-coupled receptor

Understanding serotonergic (5-HT) signaling is critical for understanding human physiology, behavior, and neuropsychiatric disease. 5-HT mediates its actions via ionotropic and metabotropic 5-HT receptors. The 5-HT(1A) receptor is a metabotropic G protein-coupled receptor linked to the G(i/o) signaling pathway and has been specifically implicated in the pathogenesis of depression and anxiety. To understand and precisely control 5-HT(1A) signaling, we created a light-activated G protein-coupled receptor that targets into 5-HT(1A) receptor domains and substitutes for endogenous 5-HT(1A) receptors. To induce 5-HT(1A)-like targeting, vertebrate rhodopsin was tagged with the C-terminal domain (CT) of 5-HT(1A) (Rh-CT(5-HT1A)). Rh-CT(5-HT1A) activates G protein-coupled inward rectifying K(+) channels in response to light and causes membrane hyperpolarization in hippocampal neurons, similar to the agonist-induced responses of the 5-HT(1A) receptor. The intracellular distribution of Rh-CT(5-HT1A) resembles that of the 5-HT(1A) receptor; Rh-CT(5-HT1A) localizes to somatodendritic sites and is efficiently trafficked to distal dendritic processes. Additionally, neuronal expression of Rh-CT(5-HT1A), but not Rh, decreases 5-HT(1A) agonist sensitivity, suggesting that Rh-CT(5-HT1A) and 5-HT(1A) receptors compete to interact with the same trafficking machinery. Finally, Rh-CT(5-HT1A) is able to rescue 5-HT(1A) signaling of 5-HT(1A) KO mice in cultured neurons and in slices of the dorsal raphe showing that Rh-CT(5-HT1A) is able to functionally compensate for native 5-HT(1A). Thus, as an optogenetic tool, Rh-CT(5-HT1A) has the potential to directly correlate in vivo 5-HT(1A) signaling with 5-HT neuron activity and behavior in both normal animals and animal models of neuropsychiatric disease.