Pam and Its Ortholog Highwire Interact with and May Negatively Regulate the TSC1·TSC2 Complex

Abstract

Tuberous sclerosis complex (TSC)1 is an autosomal dominant disorder characterized by benign hamartomas in brain, kidney, heart, lung, and skin. It is a developmental disorder with abnormalities in cell migration, differentiation, and proliferation. Neurological complications include seizures, mental retardation, and autism (1). The disease is caused by mutations in tumor suppressor genes TSC1 or TSC2 encoding hamartin and tuberin, respectively (2, 3). Hamartin and tuberin associate in vivo forming a complex with other proteins (4). Rodent models of Tsc1 and Tsc2 develop renal cyst adenomas with liver hemangiomas observed in mouse models (5-8). Mutations in either Drosophila Tsc1 or Tsc2 show increase in cell and organ size, whereas co-expression of dTsc1 and dTsc2 inhibits growth and reduces cell size (9 -12). Studies from fly also suggest that dS6 kinase may be an inhibitory target of dTsc1 and dTsc2 in growth control (10 -12). Recent studies have confirmed that mammalian TSC1 and TSC2 inhibit S6 kinase (S6K). Cells harboring mutations in either TSC1 or TSC2 have constitutively high phosphorylation of both S6K and its substrate S6 (8, 13-18). Furthermore, tuberin and hamartin function together to inhibit target of rapamycin-mediated signaling to S6K in mammals as well as in fly (15,16,18,19). Akt phosphorylates tuberin and inhibits tuberin-hamartin function (14,16,20,21). Furthermore, it is evident from the most recent reports that the small GTPase Rheb is a direct target of tuberin and hamartin in both the Drosophila and mammalian systems (22)(23)(24). These studies demonstrate Rheb GTPase-activating protein activity for tuberin, and the loss of tuberin and/or hamartin lead to an increase in GTP-bound Rheb, which in turn leads to activation of the mammalian target of rapamycin/S6K/4E-binding protein signaling pathway.Thus, recent studies have placed tuberin and hamartin in growth signaling pathways; however, critical CNS functions of these proteins remain unknown. Our recent work demonstrates that unlike kidney lesions, CNS lesions such as cortical tubers do not display a second somatic mutation and other mechanisms might play a role during tumorigenesis in the CNS (25). Furthermore, we have shown that the tuberinhamartin complex exists with neurofilament light chain and ezrin, radixin, and moesin (ERM) proteins in the growth cone and could play a role in neuronal migration (26). In our efforts to understand the tuberin-hamartin complex in the CNS, we have identified Pam (protein-associated with Myc) (27) as a physiologically relevant tuberin interactor and have shown that Pam exists in complex with tuberin-hamartin in the CNS. Our results also demonstrate that HIW, the Drosophila homolog of Pam, can genetically interact with dTsc1-dTsc2 in the fly. The evidence that the Pam homologs in worm (RPM-1) (28, 29) and in Drosophila (HIW) (30) are important regulators of synaptic growth suggests that the interaction of Pam with tuberin-hamartin will be highly relevant within the CNS. EXPERIMENTAL PROCEDUREScDNA Const...

Fbxo45 Forms a Novel Ubiquitin Ligase Complex and Is Required for Neuronal Development

“…Complete loss of function of fsn-1 in C. elegans results in defects that are characterized by the simultaneous presence of overdeveloped and underdeveloped neuromuscular junctions (NMJs) and which are similar to, but not as pronounced as, those observed in rpm-1 Ϫ/Ϫ mutants. These genetic findings support the notion that the functions of FSN-1 and RPM-1 are partially overlapping (21).Although PHR family members interact with many potential targets (11,24,26,31), genetic data have shown that one key substrate of RPM-1 and HIW is the mitogen-activated protein kinase kinase kinase known as DLK (dual leucine zipper kinase) in C. elegans and known as Wallenda in D. melanogaster, respectively. The abundance of this kinase is increased in rpm-1 or hiw mutants, and synaptic defects in the mutant worms and flies are suppressed by a loss of DLK or Wallenda.…”

Evidence for a Conserved Function in Synapse Formation Reveals Phr1 as a Candidate Gene for Respiratory Failure in Newborn Mice

“…The partial innervation of the diaphragm and the thinner phrenic nerve that we have observed in mice also raises the possibility that PHR1 regulates factors critical for proper axonal outgrowth (14). Thus, PHR1 activity would be important not only for the proper function of neuronal connections but also for their formation.…”

The Ubiquitin Ligase MYCBP2 Regulates Transient Receptor Potential Vanilloid Receptor 1 (TRPV1) Internalization through Inhibition of p38 MAPK Signaling

“…One group comprised proteins representing different signaling pathways. The second group included proteins involved in transcriptional regulation, which was not unexpected, because the involvement of MYCBP2 in the regulation of transcriptional and translational regulation through Myc and mTOR signaling pathways has been demonstrated previously by different groups (1,21,41). The largest group comprised cytoskeletal associated proteins involved in growth regulation and receptor or vesicle trafficking, such as ROCK2, myosin Va, and myosin VI (31,33,42).…”