Quantitative trait loci mapping and gene network analysis implicate protocadherin‐15 as a determinant of brain serotonin transporter expression

Abstract

Presynaptic serotonin (5-hydroxytryptamine, 5-HT) transporters (SERT) regulate 5-HT signaling via antidepressant-sensitive clearance of released neurotransmitter. Polymorphisms in the human SERT gene (SLC6A4) have been linked to risk for multiple neuropsychiatric disorders, including depression, obsessive-compulsive disorder and autism. Using BXD recombinant inbred mice, a genetic reference population that can support the discovery of novel determinants of complex traits, merging collective trait assessments with bioinformatics approaches, we examine phenotypic and molecular networks associated with SERT gene and protein expression. Correlational analyses revealed a network of genes that significantly associated with SERT mRNA levels. We quantified SERT protein expression levels and identified region- and gender-specific quantitative trait loci (QTLs), one of which associated with male midbrain SERT protein expression, centered on the protocadherin-15 gene (Pcdh15), overlapped with a QTL for midbrain 5-HT levels. Pcdh15 was also the only QTL-associated gene whose midbrain mRNA expression significantly associated with both SERT protein and 5-HT traits, suggesting an unrecognized role of the cell adhesion protein in the development or function of 5-HT neurons. To test this hypothesis, we assessed SERT protein and 5-HT traits in the Pcdh15 functional null line (Pcdh15av-3J), studies that revealed a strong, negative influence of Pcdh15 on these phenotypes. Together, our findings illustrate the power of multidimensional profiling of recombinant inbred lines in the analysis of molecular networks that support synaptic signaling, and that, as in the case of Pcdh15, can reveal novel relationships that may underlie risk for mental illness.

Physical Interactions and Functional Relationships of Neuroligin 2 and Midbrain Serotonin Transporters

“…Although DAT and NET can be readily detected in midbrain extracts, no associations with NLGN2 were obtained, revealing a unique relationship with SERT-dependent signaling. Consistent with this finding, we also observed no significant correlation between Nlgn2 and DAT mRNA ( Ye et al, 2014a , b ). SERT is synthesized in the somatic comparted and exported for expression on both somatodendritic and axonal membranes ( Tao-Cheng and Zhou, 1999 ; Descarries and Riad, 2012 ).…”

“…To determine whether loss of SERT protein in Nlgn2 null mice impacts nerve terminal 5-HT uptake capacity, we measured uptake of radiolabeled 5-HT uptake in midbrain and hippocampal synaptosomes, detecting significant reductions at a saturating substrate concentration (1 μM, Figure 6 ). Altogether, these findings reveal a dependence of SERT expression and activity on NLGN2 levels, likely arising from the changes in SERT mRNA expression predicted by our BXD studies ( Ye et al, 2014a , b ).…”

“…Also within this group, we identified NLGN2 based on two different peptides (AA 41–50 and 231–243). This finding drew our attention as we had previously identified Nlgn2 in our study of genes whose expression significantly correlated with midbrain SERT mRNA levels across BXD recombinant inbred lines ( Ye et al, 2014a , b ). Adding significance to this finding was our recovery of GABA A receptor α2 subunit (two peptides WT, one peptide null).…”

“…Recent studies from our group and others have revealed that SERT trafficking, activity and sensitivity to signaling pathways are influenced by an array of interacting proteins that include signaling enzymes (e.g., NOS1, PKG, PP2A; Bauman et al, 2000 ; Chanrion et al, 2007 ; Steiner et al, 2009 ), cell-surface receptors (e.g., A3AR and IL-1R; Zhu et al, 2010 , 2011 ), and scaffolding and cell adhesion proteins (e.g., Hic-5, ITGB3; Carneiro and Blakely, 2006 ; Carneiro et al, 2008 ). Recently, in order to nominate novel regulators of SERT expression and 5-HT homeostasis, we implemented a gene-driven, bioinformatics approach based on mRNA correlations captured from midbrain transcriptomes of recombinant-inbred mice (BXD; Ye et al, 2014a , b ). Attesting to the potential utility of this approach, we identified multiple genes whose expression is well-known to influence 5-HT neuron identity, synthesis and release, including the transcription factor gene Fev (also known as Pet1 ), the 5-HT biosynthetic enzyme tryptophan hydroxylase type 2 gene ( Tph2) , and the vesicular monoamine transporter type 2 (VMAT2) gene ( Slc18a2) .…”

Rodent models of treatment-resistant depression

“…The BXD mice have been used to identify a cluster of genes associated with the serotonin transporter mRNA levels (Ye et al, 2014), a finding which could utilized to further examine antidepressant resistance in behavioral models of antidepressant sensitivity. A quantitative trait loci analysis on an F2 intercross of BALB/cJ and A/J inbred mice mapped three loci on chromosomes 7, 12, and 19 related to citalopram response in the TST (Crowley et al, 2006).…”