Because of the central importance of differential gene expression to every biological process, the study of gene regulation has been at the forefront of research in genetics and molecular biology. Of the major levels of gene regulation, the control of translation initiation and protein synthesis is the least understood, despite the fact that almost all developmental, neurological, and physiological processes are controlled at this level. Our lab has set out to attack this problem by identifying and studying genes that regulate translation initiation. We are investigating two key protein kinases, GCN2 and PERK, that phosphorylate the translation initiation factor eIF2 alpha and thereby regulate the expression of gene networks underlying critical developmental and physiological pathways. Recently, we have made several exciting discoveries regarding the potential functions of PERK and GCN2.
PERK is dominantly expressed in highly secretory cells, including hepatocytes, osteoblasts, exocrine-pancreas acinar cells, and the insulin-secreting pancreatic beta cells. We have shown that the activation of PERK is regulated by stimulus-coupled secretion, such as glucose stimulation of the insulin-secreting beta cells in the pancreas. The importance of PERK was underscored by our finding that those mice genetically deficient for PERK display neonatal onset of insulin dependent diabetes, exocrine pancreas failure, multiple skeletal dysplasias, severe metabolic dysfunctions, and growth retardation (Mol. Cell. Biol. 2002 22:6681; Endocrinology 2003 144:3505). The phenotype of the Perk knockout mouse parallels closely the human Wolcott-Rallison syndrome, which was recently shown to be associated with mutations in the Perk gene. By using our mouse model systems, we are exploring novel therapeutic approaches to treat Wolcott-Rallison patients in collaboration with a consortium of scientists and clinicians around the world.
GCN2 is expressed in a variety of tissues, including the liver and brain. We have generated mice genetically deficient for GCN2 and found that these mice are viable and fertile when reared under standard conditions, but have a decreased probability of completing development when they are deprived of amino acids. In addition our group has collaborated with other researchers to show that GCN2 is a key sensor of dietary amino acids in the brain that has direct effects on feeding behavior. (See Science 2005 307:1776). We have recently initiated new studies to investigate the potential role of PERK and GCN2 in the regulation of learning and memory functions associated with the hippocampus.
Our approach to determining the functions of PERK and GCN2 is to apply a battery of genetic, genomic/proteomic, molecular, biochemical, and physiological methods. To further explore the mechanisms underlying the multiple functions of PERK and GCN2 that are ablated in the knockout mice, we have generated conditional (tissue or stage specific) knockouts in mice using the Cre/loxP site-specific recombination system that we have transgenically introduced into the mouse genome. These conditional knockout mutations allow us to examine the function of these genes in isolation of other defects and to determine whether such defects are cell autonomous. Our genetic analysis of mice is complemented by cell culture experiments utilizing a variety of cell types to investigate the molecular basis of gene function and regulation.
Our research has direct biomedical implications for several human diseases, including diabetes, neurological disorders, cancer, osteoporosis, and growth defects. Many such diseases are caused by developmental and physiological defects that arise in late fetal and early neonatal development as organisms transition from in utero development, dominated by the developmental program and maternal environment, to neonatal-juvenile development which rapidly test the ability of the newly developed organ systems to grow and mature and carry out normal physiological functions. We are learning that PERK and GCN2, as key regulators of gene expression, are particularly important during this critical developmental-physiological transition.
