Thyroid hormone is required for pruning, functioning and long-term maintenance of afferent inner hair cell synapses.

Functional maturation of afferent synaptic connections to inner hair cells (IHCs) involves pruning of excess synapses formed during development, as well as the strengthening and survival of the retained synapses. These events take place during the thyroid hormone (TH)-critical period of cochlear development, which is in the perinatal period for mice and in the third trimester for humans. Here, we used the hypothyroid Snell dwarf mouse (Pit1(dw)) as a model to study the role of TH in afferent type I synaptic refinement and functional maturation. We observed defects in afferent synaptic pruning and delays in calcium channel clustering in the IHCs of Pit1(dw) mice. Nevertheless, calcium currents and capacitance reached near normal levels in Pit1(dw) IHCs by the age of onset of hearing, despite the excess number of retained synapses. We restored normal synaptic pruning in Pit1(dw) IHCs by supplementing with TH from postnatal day (P)3 to P8, establishing this window as being critical for TH action on this process. Afferent terminals of older Pit1(dw) IHCs showed evidence of excitotoxic damage accompanied by a concomitant reduction in the levels of the glial glutamate transporter, GLAST. Our results indicate that a lack of TH during a critical period of inner ear development causes defects in pruning and long-term homeostatic maintenance of afferent synapses.

Effect of PIP3 on adhesion molecules and adhesion of THP-1 monocytes to HUVEC treated with high glucose.

BACKGROUND: Phosphatidylinositol-3,4,5-triphosphate (PIP3), a well-known lipid second messenger, plays a key role in insulin signaling and glucose homeostasis. Using human umbilical vein endothelial cells (HUVEC) and THP-1 monocytes, we tested the hypothesis that PIP3 can downregulate adhesion molecules and monocyte adhesion to endothelial cells. METHODS: HUVEC and monocytes were exposed to high glucose (HG, 25 mM, 20 h) with or without PIP3 (0-20 nM), or PIT-1 (25 µM), an inhibitor of PIP3. RESULTS: Both HG and PIT-1 caused a decrease in cellular PIP3 in monocytes and HUVEC compared to controls. Treatment with PIT-1 and HG also increased the ICAM-1 (intercellular adhesion molecule 1) total protein expression as well as its surface expression in HUVEC, CD11a (a subunit of lymphocyte function-associated antigen 1, LFA-1) total protein expression as well as its surface expression in monocytes, and adhesion of monocytes to HUVEC. Exogenous PIP3 supplementation restored the intracellular PIP3 concentrations, downregulated the expression of adhesion molecules, and reduced the adhesion of monocytes to HUVEC treated with HG. CONCLUSION: This study reports that a decrease in cellular PIP3 is associated with increased expression of adhesion molecules and monocyte-endothelial cell adhesion, and may play a role in the endothelial dysfunction associated with diabetes.

Effects of manganese exposure on dopamine and prolactin production in rat.

Although manganese (Mn) has been shown to increase prolactin (PRL) by decreasing dopamine (DA) in the hypothalamus, the mechanism of Mn-induced regulation of the hypothalamic-hypophyseal-pituitary axis is unclear. We assessed the effects of inhaled Mn on hypothalamic DA and pituitary PRL production and evaluated the role of pituitary-specific transacting factor 1 (Pit-1), a transacting factor of PRL gene, in Mn-induced changes in PRL secretion in the rat brain. Male rats exposed to Mn for 4 or 13 weeks (1.5 mg/m3, 6 h/day, 5 days/week) showed a progressive and significant decrease in hypothalamic DA, whereas PRL and Pit-1 mRNA levels increased in response to Mn exposure. These results suggest that exposure to Mn decreases hypothalamic DA and promotes the production of PRL in the pituitary and that Pit-1 might be a regulator of DA and PRL.

Osteoblast autonomous Pi regulation via Pit1 plays a role in bone mineralization.

The complex pathogenesis of mineralization defects seen in inherited and/or acquired hypophosphatemic disorders suggests that local inorganic phosphate (P(i)) regulation by osteoblasts may be a rate-limiting step in physiological bone mineralization. To test whether an osteoblast autonomous phosphate regulatory system regulates mineralization, we manipulated well-established in vivo and in vitro models to study mineralization stages separately from cellular proliferation/differentiation stages of osteogenesis. Foscarnet, an inhibitor of NaP(i) transport, blocked mineralization of osteoid formation in osteoblast cultures and local mineralization after injection over the calvariae of newborn rats. Mineralization was also down- and upregulated, respectively, with under- and overexpression of the type III NaP(i) transporter Pit1 in osteoblast cultures. Among molecules expressed in osteoblasts and known to be related to P(i) handling, stanniocalcin 1 was identified as an early response gene after foscarnet treatment; it was also regulated by extracellular P(i), and itself increased Pit1 accumulation in both osteoblast cultures and in vivo. These results provide new insights into the functional role of osteoblast autonomous P(i) handling in normal bone mineralization and the abnormalities seen in skeletal tissue in hypophosphatemic disorders.

The role of transcription factors implicated in anterior pituitary development in the aetiology of congenital hypopituitarism.

The anterior pituitary gland is a central regulator of growth, reproduction and homeostasis, and is the end-product of a carefully orchestrated pattern of expression of signalling molecules and transcription factors leading to the development of this complex organ secreting six hormones from five different cell types. Naturally occurring and transgenic murine models have demonstrated a role for many of these molecules in the aetiology of combined pituitary hormone deficiency (CPHD). These include the transcription factors HESX1, PROP1, POU1FI, LHX3, LHX4, TBX19 (TPIT), SOX3 and SOX2. The expression pattern of these transcription factors, their interaction with co-factors and their impact on target genes dictate the phenotype that results when the gene encoding the relevant transcription factor is mutated. The highly variable phenotype may consist of isolated hypopituitarism, or more complex disorders such as septo-optic dysplasia (SOD) and holoprosencephaly. Since mutations in any one transcription factor are uncommon, and since the overall incidence of mutations in known transcription factors is low in patients with CPHD, it is clear that many genes remain to be identified, and characterization of these will further elucidate the pathogenesis of these complex conditions, and also shed light on normal pituitary development.