Targeted Proteomics Analysis for Parvalbumin in Rat Brain Tissue

Thyroid hormones (THs) modulate the differentiation and fate-specificity of inhibitory neurons in the developing brain. A subset of GABAergic inhibitory neurons expresses the low molecular weight calcium-binding protein parvalbumin (Pvalb). The presence of Pvalb confers specific firing properties upon these neurons that are essential for neuronal wiring, solidification of synaptic contacts, and neuronal plasticity. TH regulates both the maturation and timing of expression of Pvalb, and these interneurons appear to be particularly sensitive to disruption of the thyroid system, including exposure to environmental contaminants. We sought to develop methods to improve detection and facilitate quantification of perturbations of this novel biomarker of TH disruption. We used liquid chromatography-tandem mass spectrometry (LC-MS/MS) in a targeted proteomics approach to examine Pvalb in the cortex of the rat brain. The anterior cortex was collected from adult Long Evans rats and homogenized in a buffer containing dithiothreitol and then denatured. An in-silico tryptic digest was used to cleave Pvalb, resulting in peptides ~ 8-25 amino acids in length and free of internal lysine and arginine. Using recombinant Pvalb, in-silico digestion and LC-MS/MS confirmed that peptide fragments SGFIEEDELGSILK and IGVEEFSTLVAES were abundant in the cortex of the rat brain and contained multiple transitions for quantitation and confirmation.  For quantitation, the digestant was spiked with isotope labeled Pvalb peptides, and the mixture was purified by mixed-mode solid-phase extraction prior to analysis by LC-MS/MS.   These preliminary findings demonstrate analyte specificity and sensitivity for the quantification of Pvalb protein and will be applied to ongoing efforts to identify the impact of TH-disrupting chemicals on the developing brain. This work does not reflect US EPA policy.

Impact/Purpose

Impact Statement: This work was conducted to improve our ability to provide quantitative information in the refinement of Adverse Outcome Pathways for thyroid disrupting chemicals and neurotoxicity. Inhibitory interneurons that express a specific calcium binding protein known as parvalbumin are critical for the formation of neural networks in the developing brain and control inhibitory function in the brain of the adult. We have recently reported that this population of neurons is particularly vulnerable to impairment in rat pups born to dams exposed to thyroid disrupting chemicals. This was discovered  using labor intensive immunohistochemistry, imaging, and cell counting procedures in histological material. It appears that alterations in parvalbumin expression in inhibitory neurons in the brain is emerging as a common outcome in chemically-induced thyroid disruption in rat, thyroid receptor mouse mutants, deiodinase and hormone transporter knock-out models, and in humans with mutations of the brain TH transporter, MCT8, adding a much needed tool to our arsenal of bioindicators of impaired neuronal structure and function in rodent models used in regulatory evaluations of thyroid disrupting chemicals. In the current work, we report on preliminary findings exploring the possibility of an analytical measure of the same protein marker in the brain for more refined quantification of parvalbumin with faster turnaround than traditional immunohistochemistry.

Citation

Ford, J. AND M. Gilbert. Targeted Proteomics Analysis for Parvalbumin in Rat Brain Tissue. Development and Neuro Tox Conference, Charleston, SC, June 24 - 28, 2023.