Effect of primary congenital hypothyroidism upon expression of genes mediating murine brain glucose uptake

Brain development and bioenergetic changes

Bioenergetics describe the biochemical processes responsible for energy supply in organisms. When these changes become dysregulated in brain development, multiple neurodevelopmental diseases can occur, implicating bioenergetics as key regulators of neural development. Historically, the discovery of disease processes affecting individual stages of brain development has revealed critical roles that bioenergetics play in generating the nervous system. Bioenergetic-dependent neurodevelopmental disorders include neural tube closure defects, microcephaly, intellectual disability, autism spectrum disorders, epilepsy, mTORopathies, and oncogenic processes. Developmental timing and cell-type specificity of these changes determine the long-term effects of bioenergetic disease mechanisms on brain form and function. Here, we discuss key metabolic regulators of neural progenitor specification, neuronal differentiation (neurogenesis), and gliogenesis. In general, transitions between glycolysis and oxidative phosphorylation are regulated in early brain development and in oncogenesis, and reactive oxygen species (ROS) and mitochondrial maturity play key roles later in differentiation. We also discuss how bioenergetics interface with the developmental regulation of other key neural elements, including the cerebrospinal fluid brain environment. While questions remain about the interplay between bioenergetics and brain development, this review integrates the current state of known key intersections between these processes in health and disease.

Hypothyroidism and Cognitive Disorders during Development and Adulthood: Implications in the Central Nervous System

Thyroid hormones (THs) play a critical function in fundamental signaling of the body regulating process such as metabolism of glucose and lipids, cell maturation and proliferation, and neurogenesis, to name just a few. THs trigger biological effects both by directly affecting gene expression through the interaction with nuclear receptors (genomic effects) and by activating protein kinases and/or ion channels (short-term effects). For years, a close relationship between the THs hormones and the central nervous system (CNS) has been described, not only for neuronal cells but also for glial development and differentiation. A deficit in thyroid hormones triiodothyronine (T₃) and thyroxine (T₄) is observed in the hypothyroid condition, generated by a iodine deficiency or an autoimmune response of the body. In the hypothyroid condition, several cellular deregulation and alterations have been described in dendrite spine morphology, cell migration and proliferation, and impaired synaptic transmission in the hippocampus, among others. The aim of this review is to describe the role of the thyroid hormones with focus in brain function and neurodegenerative disorders.

T3 strongly regulates GLUT1 and GLUT3 mRNA in cerebral cortex of hypothyroid rat neonates

Experimental hypothyroidism alters the expression of the GLUT1 and GLUT4 glucose transporters in brown adipose tissue, skeletal muscle and heart. Congenital hypothyroidism disrupts the development and function of the CNS, and the importance of GLUT1 for proper brain function has been dramatically evidenced in the cases of GLUT1 deficiency syndrome. Because of this, we hypothesised that the expression of GLUT1 and GLUT3, glucose transporters expressed in brain cortex, may be altered in congenital hypothyroidism. GLUT3 mRNA was induced during postnatal development whereas GLUT1 mRNA was initially repressed and further induced; both processes were essentially similar in control and hypothyroid animals. Under these conditions GLUT1 protein expression was reduced in cerebral cortex from 15-day-old hypothyroid neonates, which suggests the existence of post-transcriptional alterations. The most striking differences were observed when hypothyroid animals at different developmental stages were treated acutely with T(3). GLUT1 and GLUT3 mRNA expression behaved in opposite ways in response to treatment with the hormone. Furthermore, the behaviour of each glucose transporter isoform against T(3) was not uniform but changed alongside development. In all, our data show that the regulation of GLUT1 and GLUT3 in cerebral cortex is regulated by T(3) in a complex way and suggest that alterations in the expression of glucose transporters induced by hypothyroidism might have a functional impact on brain glucose uptake.

Insulin-responsive glucose transporters-GLUT8 and GLUT4 are expressed in the developing mammalian brain

We investigated the spatial and temporal distribution of insulin-responsive facilitative glucose transporter isoforms GLUT4 and GLUT8 in the developing mouse brain. Employing Western blot analysis and specific antibodies, GLUT4 and GLUT8 peaked during the suckling phase. Immunohistochemical analysis revealed the presence of GLUT4 mainly in neurites in sensory and motor areas of cortical and subcortical structures of the brain from P7 until adulthood. In contrast, GLUT8 was found in the same anatomical structures within neurites and cell bodies. Most striking was the presence of GLUT8 in the cell bodies of the substantia nigra. We conclude that both GLUT4 and GLUT8 are present in murine brain, with highest concentrations noted during the suckling phase. These insulin-responsive isoforms may have a unique role in augmenting substrate delivery under conditions of increased demand.

Spontaneous circling behavior and dopamine neuron loss in a genetically hypothyroid mouse

The genetically hypothyroid mouse, Tshr(hyt), has a single point mutation resulting in a defective thyroid-stimulating hormone receptor, and therefore a non-functional thyroid gland. This is an autosomal recessive disorder and affected mice have been reported to have a number of somatic and behavioral deficits. This study reports a pronounced, spontaneous, asymmetrical circling behavior in the Tshr(hyt) mouse. The spontaneous circling behavior appeared in about 25% of the homozygous animals, in both males and females. The circling usually appeared by postnatal day 35 and continued throughout the lifespan of the animal. The circling was in one direction only, either clockwise or counterclockwise, with the directional preference being almost absolute. A stereological analysis of tyrosine hydroxylase immunoreactive neurons in the substantia nigra and adjacent ventral tegmental area of circling homozygous mice, non-circling homozygous mice and heterozygous mice revealed that the circlers had significantly fewer (40% reduction) midbrain dopamine neurons than those animals that did not circle. There was not an association between the direction of the circling and an asymmetry in the number of dopamine neurons in the midbrains of these mice. There was no difference in the number of dopamine neurons in the midbrain of the homozygous non-circlers and the heterozygous mice. These studies indicate that about 25% of genetically hypothyroid mice demonstrated a spontaneous, perseverative, unilateral circling behavior that was associated with a significant reduction in the number of their midbrain dopamine neurons. Thus congenitally hypothyroid mice are at risk for a reduction in the number of nigral dopamine neurons and an associated repetitive movement disorder.