We have described a small gene family, the tubby like proteins (TULP). Mutations in its founding member, tubby (TUB), cause a tripartite phenotype of obesity, retinal and cochlear degeneration in mice, which is similar to the phenotypes observed in individuals with Alstrom or Bardet-Biedl syndromes which are caused by mutations in ciliary associated proteins. The late onset, slowly progressing obesity in tubby mice is reflective of common forms of maturity onset human obesity. Polymorphisms in the TUB gene have been shown to be associated with obesity, and mutations in TULP1 cause retinitis pigmentosa and Leber's Congenital Amaurosis in human populations. Furthermore, a null mutation for Tulp3 leads to embryonic lethality in mice. A strong conservation of the tubby gene family through evolution in higher eukaryotes and the associated diseases observed when they are mutated, indicate that TULPs play an important role in the normal function of the cells in which they are expressed. Currently, the molecular function of TULPs is not known. However, accumulation of rhodopsin containing vesicles in the retina of tubby and Tulp1 mutant mice, and interaction with genes and proteins involved in intracellular trafficking, suggest a role for TULPs in this process. To begin to understand the cellular function of TUB, we will carry out experiments in genetically modified mice that will test whether TUB plays a role in cellular transport using high resolution imaging of labeled TUB protein in co-localization studies with markers of intracellular trafficking. We will also use yeast two hybrid analysis and immunoprecipitation to identify proteins that directly interact with TUB. These studies will define pathways in which TUB participates. Similar to the lack of knowledge about the cellular function of TULPs, there is also limited knowledge about how disruptions in TULPs lead to the observed disease phenotypes. To test whether the obesity in tubby mice is a result of its disruption in the CNS or in adipocytes or a combination of both, we will analyze tissue specific targeted mutations that inactivate TUB at both of these sites individually. We will test the hypothesis that tubby mice have chronic sympathetic activation to brown fat, thus limiting their obesity. We will test the hypothesis that mutations of TULP3 will lead to obesity and retinal disease in the adult animal by generating a tamoxifen inducible allele of TULP3. And finally, by positional cloning we will identify a genetic modifier of the tubby mutation that can prevent obesity in tubby mice. This suite of experiments will provide entry points into the pathways in which TULPs act and insight into their cellular function and, thereby, reveal their mechanisms of action.
Our long term goal is to understand the role that tubby like proteins (TULPs) play in the cell and how defects in these proteins and in the primary cilium, a cellular structure that TULPs are associated with, lead to dysfunction within several organ systems and the organism. This project will further our understanding of how intracellular trafficking associated with the primary cilium contributes to obesity, a connection that has only recently been recognized with the analyses of Bardet-Biedl and Alstrom syndromes. Since these syndromes comprise diseases that are common in the general population such as obesity, type 2 diabetes, and blindness, understanding TULP function may also contribute to a better understanding of the common forms of these diseases.
|Won, Jungyeon; Shi, Lan Ying; Hicks, Wanda et al. (2012) Translational vision research models program. Adv Exp Med Biol 723:391-7|
|Maddox, Dennis M; Ikeda, Sakae; Ikeda, Akihiro et al. (2012) An allele of microtubule-associated protein 1A (Mtap1a) reduces photoreceptor degeneration in Tulp1 and Tub Mutant Mice. Invest Ophthalmol Vis Sci 53:1663-9|