Hypothyroidism selectively reduces the rate and amount of transport for specific SCb proteins in the hyt/hyt mouse optic nerve

Abstract

Thyroid hormone significantly affects molecular and neuroanatomical properties of the developing nervous system. Altered connectivity in hypothyroidism may reflect reductions in process growth, alterations in process maintenance, or changes in synaptogenesis or synaptic maintenance. These events are dependent on microtubules, neurofilaments, microfilaments, and associated molecular components. Reductions in delivery of microtubules and neurofilaments to the distal axon by slow component a (SCa) of axonal transport may contribute to the neuroanatomical abnormalities of hypothyroidism (Stein et al., J Neurosci Res 28:121-133, 1991). However, hypothyroidism might also affect the axon and synaptic connections by altering slow component b (SCb), which includes actin microfilaments and proteins that contribute to synaptic function, i.e., clathrin, HSC70 (clathrin uncoating ATPase), spectrin, and calmodulin. To determine the effect of hypothyroidism on SCb proteins, slow axonal transport was analyzed in optic nerves of hyt/hyt hypothyroid mice, which have severe primary hypothyroidism, and euthyroid control mice. Clathrin, spectrin, HSC70, and actin showed significant reductions in transport velocity in hyt/hyt optic nerves relative to euthyroid nerves, but the transport rate for calmodulin was less affected. However, the amount of calmodulin was significantly elevated in hyt/hyt nerve over euthyroid nerves. Hypothyroidism selectively reduces transport of SCb proteins, which are thought to play significant roles in synaptic function and in the growth cone. The effects of hypothyroidism on microtubules and neurofilaments combined with actions on SCb suggest that changes in neuronal function associated with reduced thyroid hormone during development and maturity (i.e., alterations in neuronal connectivity, nerve conduction, and synaptic function) may be mediated in part by effects on slow axonal transport.

Thyroid Hormones and Motoneurons Development

“…HRP retrograde reaction p r o d u c t i s o b s e r v e d a s d a r k b l u e i n t r a c e llular granules varying in quantity from motoneuron to motoneuron even in the same trigeminal motor nucleus (Kawagishi et al, 1992). As hypothyroidism reduces neuronal process growth, synaptogenesis, axonal transport velocity (Biesiada et al, 1996;Stein et al, 1991) and neurotransmitter synthesis (Barakat-Walter et al, 2000;Behzadi & Ganji, 2005), it was of especial interest to investigate the alteration in the morphological characteristics of masseter motoneurons as well as HRP uptake and transport from the neuromuscular junction. In this regard the labeling quality of HRP backfilled masseteric motoneurons along with their size distribution profile under www.intechopen.com developmental hypothyroidism were evaluated.…”