Signaling functions of the NFkappaB1 gene products in macrophages and T cells

The NF-kappaB family of transcription factors plays a pivotal role in the regulation of genes involved in diverse biological processes, including immune responses, inflammation, apoptosis, and oncogenesis. In mammalian systems, the NF-kappaB family includes five members, NF-kappaB1, NF-kappaB2, RelA, RelB and c-Rel, characterized by their structural homology in an evolutionary conserved domain, the Rel homology domain. The different NF-kappaB members form homo- or heterodimers that mediate transactivation of specific target genes. The NF-kappaB complexes are normally sequestered in the cytoplasm by a family of inhibitory proteins, IkappaBs, with IkappaBalpha being the prototypical IkappaB member that controls a so-called canononical pathway of NF-kappaB activation. Various stimuli such as proinflammatory cytokines, antigens, and TLR agonists, activate an IkappaB kinase (IKK) complex, which phosphorylates IkappaBalpha at specific serines. Phosphorylated IkappaBalpha is rapidly ubiquitinated and targeted to the 26S proteasome for degradation. These sequential signaling events lead to the liberation of NF-kappaB complexes, which concurrently move to the nucleus to regulate target gene expression by binding specific kappaB enhancers. Despite the extensive studies of the canonical NF-kappaB signaling pathway, many missing links exist for a comprehensive understanding of NF-kappaB activation. This thesis research focuses on the function of the nfkappaB1 gene that encodes two major proteins, the precursor protein p105 and its processing product, the mature NF-kappaB1 subunit p50. In addition to generating p50, p105 functions as an IkappaB-like molecule, although how p105 regulates NF-kappaB function is not well understood. At least in some cells, nfkappaB1 also produces a splicing variant mRNA that encodes the C-terminal portion of p105, known as IkappaBgamma based on its sequence homology to IkappaBalpha. The in vivo function of IkappaBgamma is also unclear, although it is generally thought to act similarly to p105. Studies in this dissertation were directed towards to elucidating the physiological role of the nfkappaB1 gene products, including p105, p50 and IkappaBgamma, in immune and inflammatory responses. To obtain genetic evidence, I utilized three mouse models, the nfkappaB1 knockout (-/-) mice that lack all of the nfkappaB1 gene products, the p50 knockin (KI) mice that express only p50, and the IkappaBgamma transgenic (Tg) mice that specifically express IkappaBgamma. The major findings are summarized below: 1. Aberrant activation of p50 causes T-cell dependent inflammation. Prior studies suggest that p50 is constitutively expressed in the nucleus of immune cells of the p50KI mice. This mouse model is thus useful for the study of the pathological consequences of deregulated p50 activation. Interestingly, the p50KI mice were found to spontaneously develop intestinal inflammation with typical features of human inflammatory bowel disease (IBD). This inflammatory disorder is mediated by T cells, since it…