Neuronal 3',3,5-triiodothyronine (T3) uptake and behavioral phenotype of mice deficient in Mct8, the neuronal T3 transporter mutated in Allan-Herndon-Dudley syndrome.
Thyroid hormone transport into cells requires plasma membrane transport proteins. Mutations in one of these, monocarboxylate transporter 8 (MCT8), have been identified as underlying cause for the Allan-Herndon-Dudley syndrome, an X-linked mental retardation in which the patients also present with abnormally high 3',3,5-triiodothyronine (T(3)) plasma levels. Mice deficient in Mct8 replicate the thyroid hormone abnormalities observed in the human condition. However, no neurological deficits have been described in mice lacking Mct8. Therefore, we subjected Mct8-deficient mice to a comprehensive immunohistochemical, neurological, and behavioral screen. Several behavioral abnormalities were found in the mutants. Interestingly, some of these behavioral changes are compatible with hypothyroidism, whereas others rather indicate hyperthyroidism. We thus hypothesized that neurons exclusively dependent on Mct8 are in a hypothyroid state, whereas neurons expressing other T(3) transporters become hyperthyroid, if they are exposed directly to the high plasma T(3). The majority of T(3) uptake in primary cortical neurons is mediated by Mct8, but pharmacological inhibition suggested functional expression of additional T(3) transporter classes. mRNAs encoding six T(3) transporters, including L-type amino acid transporters (LATs), were coexpressed with Mct8 in isolated neurons. We then demonstrated Lat2 expression in cultured neurons and throughout murine brain development. In contrast, LAT2 is expressed in microglia in the developing human brain during gestation, but not in neurons. We suggest that lack of functional complementation by alternative thyroid hormone transporters in developing human neurons precipitates the devastating neurodevelopmental phenotype in MCT8-deficient patients, whereas Mct8-deficient mouse neurons are functionally complemented by other transporters, for possibly Lat2.
Wirth
Eva K
EK
Neuroscience Research Center, Charité Universitätsmedizin Berlin, Berlin, Germany.
Roth
Stephan
S
Blechschmidt
Cristiane
C
Hölter
Sabine M
SM
Becker
Lore
L
Racz
Ildiko
I
Zimmer
Andreas
A
Klopstock
Thomas
T
Gailus-Durner
Valerie
V
Fuchs
Helmut
H
Wurst
Wolfgang
W
Naumann
Thomas
T
Bräuer
Anja
A
de Angelis
Martin Hrabé
MH
Köhrle
Josef
J
Grüters
Annette
A
Schweizer
Ulrich
U
D027182:Amino Acid Transport System y+ / Q000378:metabolism; D000818:Animals; D001522:Behavior, Animal; D001921:Brain / Q000196:embryology / Q000254:growth & development / Q000378:metabolism; D002478:Cells, Cultured; D027301:Fusion Regulatory Protein 1, Light Chains / Q000378:metabolism; D006801:Humans; D006980:Hyperthyroidism / Q000503:physiopathology; D007037:Hypothyroidism / Q000503:physiopathology; D026901:Membrane Transport Proteins / Q000172:deficiency* / Q000235:genetics / Q000378:metabolism; D038901:X-Linked Intellectual Disability / Q000235:genetics*; D051379:Mice; D018345:Mice, Knockout; D017628:Microglia / Q000378:metabolism; D027501:Monocarboxylic Acid Transporters; D009474:Neurons / Q000502:physiology*; D009483:Neuropsychological Tests; D010641:Phenotype; D012333:RNA, Messenger / Q000378:metabolism; D027981:Symporters; D013577:Syndrome; D014284:Triiodothyronine / Q000378:metabolism*
D027182:Amino Acid Transport System y+; D027301:Fusion Regulatory Protein 1, Light Chains; D026901:Membrane Transport Proteins; D027501:Monocarboxylic Acid Transporters; D012333:RNA, Messenger; C518286:SLC7A8 protein, mouse; C484257:Slc16a2 protein, mouse; D027981:Symporters; D014284:Triiodothyronine