Gene Regulatory Pathways in Vertebrate Pituitary Organogenesis

Simon Rhodes, Andrew Cagle, Marin Garcia, Chad Hunter, Parinda Kiratipranon, Jesse Savage, Aaron Showalter, Brooke West, and Benjamin Yaden.

Department of Biology and the Center for Regenerative Biology and Medicine
Indiana University-Purdue University Indianapolis

Protein and peptide hormones secreted from the vertebrate pituitary gland are critical for physiological homeostasis. These hormones regulate growth, reproduction, the stress response, and metabolism. Development of the hormone-secreting cells of the anterior pituitary gland requires the collective actions of numerous gene regulatory proteins, including the LHX3, LHX4, PROP-1, and PIT-1 homeodomain transcription factors. Mutations in the genes encoding these factors are associated with severe hormone deficiency diseases. In addition, mutational analyses of the orthologous genes in mice have revealed that these genes are components of a complex regulatory cascade that culminates in the differentiation of specialized pituitary cell types. We have analyzed these genes and the encoded proteins in several species. For example, studies of LHX3, a gene that is important in both pituitary and nervous system development, demonstrate that human LHX3 produces two mRNAs that encode at least three proteins. LHX3 mRNAs display different expression patterns in differentiated pituitary cell types and the LHX3 protein isoforms have different gene activation properties. Molecular analyses reveal that specific activities are conferred by modular protein domains that differentially regulate DNA binding, interaction with protein partners, alteration of local DNA topology, nuclear localization, and trans-activation and trans-repression functions. Comparative studies demonstrate that some molecular properties also are found in homologous proteins that control nervous system development in invertebrates. By contrast, other activities appear to be more recent and are restricted to mammalian systems. The PROP-1 gene exhibits another type of complexity. We have identified two alleles of this gene in cattle that encode different protein products with distinct gene regulatory abilities, indicating that polymorphic changes can impact the actions of regulatory proteins in normal populations. Molecular studies have revealed how subtle molecular changes can affect the gene regulatory properties of the PROP-1 transcription factor. In summary, multiple RNAs can be produced from pituitary transcription factor genes to form a complex "regulatory transcriptome". These RNAs encode multiple protein isoforms with distinct properties comprising an expanded "regulatory proteome". Supported by the NIH, NSF, and USDA.