Endocrine Abstracts

Allan-Herndon-Dudley syndrome (AHDS) is a genetic disorder characterized by intellectual disability and movement disorders. AHDS patients have mutations in the monocarboxylate transporter 8 (MCT8) that impedes thyroid hormone (TH) transport to the brain during crucial phases of brain development. In mice, the resulting structural and functional pathologies of human AHDS patients can be mimicked by knocking out murine MCT8 and the Solute Carrier Organic Anion Transporter Family Member 1C1 (SLCO1C1). Murine models such as these will thus guide us in understanding the disease and in finding AHDS treatments. To investigate the expression pattern of MCT8 we employed antibody stainings and a novel MCT8-Cre line, backed by analysis of single cell RNA sequencing data. MCT8 immunofluorescence showed strong expression in the choroid plexus and in tanycytes, both in the adult brain as well as in the embryonic and early postnatal brain. At postnatal day P8, neuronal MCT8 expression was strong enough to be detected in the cortex and hippocampus. Next, to visualize MCT8-expressing cells using an antibody independent method, we generated a new MCT8-Cre knock-in mouse and crossed it with CAG-Sun1/sfGFP mice that express a GFP-marked nuclear lamina protein only after Cre-mediated stop-cassette-excision. Fluorescence microscopy of adult brains showed reporter expression in virtually all nuclei. This near ubiquitous fluorescence suggested very early embryonal activation of MCT8 which happens after the blastocyst stage, since they show no fluorescence. Indeed, using publicly available single cell RNA sequencing data we saw that MCT8 expression starts at but not before E5.25, which coincides with implantation. To trace MCT8 expressing cells in later stages of the developing embryo, we currently investigate an alternative strategy based on a tamoxifen-inducible MCT8-CreERT2 line. Last, we wanted to explore potential treatment options against AHDS by attaching TH to peptides targeting the brain. As delivery peptide, we used a 13 amino acid derivative of leptin, a hormone secreted by fat tissue that acts as satiety signal in the hypothalamus. Leptin enters the brain via the leptin receptor (LepR), which like MCT8 is highly expressed in the choroid plexus and tanycytes. First, we compared the ability of leptin and the leptin derivative to bind and activate the LepR. However, while leptin showed strong activity, the leptin derivative was inactive, highlighting the need for peptides with better binding and uptake characteristics.