Age related and hypothyroidism related changes on the stoichiometry of neurofilament subunits in the developing rat brain.

Hypothyroidism in the developing brain results in progressive intraneuronal accumulation of neurofilament (NF) proteins in the proximal hillock regions of axons, analogous to the pathological intraneuronal accumulation of NF in common neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and Amyotrophic lateral sclerosis. A preferential decline in the expression of the light chain of NF occurs in all the three diseases leading to an absolute change in stoichiometry of the NF subunits. Using the developing hypothyroid rat cerebra as a model, we have tried to elucidate if age or hypothyroidism causes a change in the stoichiometry or molar ratio of the NF subunits which could be responsible for their aberrant intraneuronal accumulation. Western blotting and chemiluminescence assay of cytoskeletal preparations from normal and hypothyroid developing rats at postnatal days 5 (PND5), PND15 and PND25 shows that in the normal cerebra, the expression of NFL and NFM were abundant during the first 2 weeks, corresponding to the onset of axonal outgrowth and synaptogenesis, whereas that of NFH was predominant during the second and third weeks corresponding to the period of maturation of synapses, axonal caliber and transport processes. These results show that consistent with the requirement for neuronal differentiation during synaptogenesis, the molar ratios NFH:NFM:NFL changed significantly from 1:3:9 at PND5 to 1:2:6 at PND25. Hypothyroidism caused a 40-60% decline in the expression of all three subunits. However, at all three ages examined, differences in the molar ratios of the NF subunits between normal and hypothyroid cerebra were insignificant suggesting that factors other than alteration in the stoichiometry of NF subunits are associated with their aberrant intraneuronal aggregation.