Abstract
Thyroid hormone (TH) plays important roles in the developing brain. TH deficiency in early life leads to severe developmental impairment in the hippocampus. However, the mechanisms of TH action in the developing hippocampus are still largely unknown. In this study, we generated 3,5,3’-tri-iodo-l-thyronine (T3)-free neuronal supplement, based on the composition of neuronal supplement 21 (NS21), to examine the effect of TH in the developing hippocampus using primary cultured neurons. Effects of TH on neurons were compared between cultures in this T3-free culture medium (-T3 group) and a medium in which T3 was added (+T3 group). Morphometric analysis and RT-qPCR were performed on 7, 10, and 14 days in vitro (DIV). On 10 DIV, a decreased dendrite arborization in -T3 group was observed. Such difference was not observed on 7 and 14 DIV. Brain-derived neurotrophic factor (Bdnf) mRNA levels also decreased significantly in -T3 group on 10 DIV. We then confirmed protein levels of phosphorylated neurotrophic tyrosine kinase type 2 (NTRK2, TRKB), which is a receptor for BDNF, on 10 DIV by immunocytochemistry and Western blot analysis. Phosphorylated NTRK2 levels significantly decreased in -T3 group compared to +T3 group on 10 DIV. Considering the role of BDNF on neurodevelopment, we examined its involvement by adding BDNF on 8 and 9 DIV. Addition of 10 ng/ml BDNF recovered the suppressed dendrite arborization induced by T3 deficiency on 10 DIV. We show that the lack of TH induces a developmental delay in primary hippocampal neurons, likely caused through a decreased Bdnf expression. Thus, BDNF may play a role in TH-regulated dendritogenesis.
Highlights
Thyroid hormone (TH) plays important roles in general body growth, cardiovascular system coordination, skeletal muscle regulation, and central nervous system (CNS) development [1, 2]
We verified only the astrocyte number in the glial cells, because Xie et al reported that oligodendrocyte and microglia could not survive in the neurobasal medium plus B27 supplement which is the origin of the neuronal supplement 21 (NS21) [42]
Based on the simultaneous decrease of Brain-derived neurotrophic factor (Bdnf) mRNA levels and abnormal dendrite arborization on 10 days in vitro (DIV), we examined whether brain-derived neurotrophic factor (BDNF) is involved in this process
Summary
Thyroid hormone (TH) plays important roles in general body growth, cardiovascular system coordination, skeletal muscle regulation, and central nervous system (CNS) development [1, 2]. Perinatal TH deficiency induces brain dysfunction such as disrupted motor coordination, memory impairment, and decline in intelligence quotient (IQ) score [5, 6]. Impairment of spatial memory caused by perinatal TH deficiency has been reported in the Morris water maze test and in the object location test [10,11,12]. The mechanisms of these behavioral changes are considered to be induced by the reduction of synaptic transmission efficiency and neurotransmitter release in the hippocampus in TH-deficient mice [10] and rats [13,14,15,16]. A decrease in the volume of the hippocampal CA1 and CA3 region, as well as a decrease in granule and pyramidal cell dendrite arborization, has been reported in the rat hippocampus [17,18,19,20]
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