Simone Kühnle

Role of the ubiquitin ligase E6AP/UBE3A in controlling levels of the synaptic protein Arc

Inactivation of the ubiquitin ligase E6 associated protein (E6AP) encoded by the UBE3A gene has been associated with development of the Angelman syndrome. Recently, it was reported that in mice, loss of E6AP expression results in increased levels of the synaptic protein Arc and a concomitant impaired synaptic function, providing an explanation for some phenotypic features of Angelman syndrome patients. Accordingly, E6AP has been shown to negatively regulate activity-regulated cytoskeleton-associated protein (Arc) and it has been suggested that E6AP targets Arc for ubiquitination and degradation. In our study, we provide evidence that Arc is not a direct substrate for E6AP and binds only weakly to E6AP, if at all. Furthermore, we show that down-regulation of E6AP expression stimulates estradiol-induced transcription of the Arc gene. Thus, we propose that Arc protein levels are controlled by E6AP at the transcriptional rather than at the posttranslational level.

Physical and Functional Interaction of the HECT Ubiquitin-protein Ligases E6AP and HERC2

Deregulation of the ubiquitin-protein ligase E6AP contributes to the development of the Angelman syndrome and to cervical carcinogenesis suggesting that the activity of E6AP needs to be under tight control. However, how E6AP activity is regulated at the post-translational level under non-pathologic conditions is poorly understood. In this study, we report that the giant protein HERC2, which is like E6AP a member of the HECT family of ubiquitin-protein ligases, binds to E6AP. The interaction is mediated by the RCC1-like domain 2 of HERC2 and a region spanning amino acid residues 150–200 of E6AP. Furthermore, we provide evidence that HERC2 stimulates the ubiquitin-protein ligase activity of E6AP in vitro and within cells and that this stimulatory effect does not depend on the ubiquitin-protein ligase activity of HERC2. Thus, the data obtained indicate that HERC2 acts as a regulator of E6AP.

Mutation of HERC2 causes developmental delay with Angelman-like features

Background Deregulation of the activity of the ubiquitin ligase E6AP (UBE3A) is well recognised to contribute to the development of Angelman syndrome (AS). The ubiquitin ligase HERC2, encoded by the HERC2 gene is thought to be a key regulator of E6AP. Methods and results Using a combination of autozygosity mapping and linkage analysis, we studied an autosomal-recessive neurodevelopmental disorder with some phenotypic similarities to AS, found among the Old Order Amish. Our molecular investigation identified a mutation in HERC2 associated with the disease phenotype. We establish that the encoded mutant HERC2 protein has a reduced half-life compared with its wild-type counterpart, which is associated with a significant reduction in HERC2 levels in affected individuals. Conclusions Our data implicate a model in which disruption of HERC2 function relates to a reduction in E6AP activity resulting in neurodevelopmental delay, suggesting a previously unrecognised role of HERC2 in the pathogenesis of AS.

GM-CSF Restores Innate, But Not Adaptive, Immune Responses in Glucocorticoid-Immunosuppressed Human Blood In Vitro

Infection remains the major complication of immunosuppressive therapy in organ transplantation. Therefore, reconstitution of the innate immunity against infections, without activation of the adaptive immune responses, to prevent graft rejection is a clinically desirable status in transplant recipients. We found that GM-CSF restored TNF mRNA and protein expression without inducing IL-2 production and T cell proliferation in glucocorticoid-immunosuppressed blood from either healthy donors or liver transplant patients. Gene array experiments indicated that GM-CSF selectively restored a variety of dexamethasone-suppressed, LPS-inducible genes relevant for innate immunity. A possible explanation for the lack of GM-CSF to restore T cell proliferation is its enhancement of the release of IL-1βR antagonist, rather than of IL-1β itself, since exogenously added IL-1β induced an IL-2-independent Con A-stimulated proliferation of glucocorticoid-immunosuppressed lymphocytes. Finally, to test the in vivo relevance of our findings, we showed that GM-CSF restored the survival of dexamethasone- or cyclosporine A-immunosuppressed mice from an otherwise lethal infection with Salmonella typhimurium. In addition to this increased resistance to infection, GM-CSF did not induce graft rejection of a skin allotransplant in cyclosporine A-immunosuppressed mice. The selective restoration potential of GM-CSF suggests its therapeutic use in improving the resistance against infections upon organ transplantation.

Role of ubiquitin and the HPV E6 oncoprotein in E6AP-mediated ubiquitination

Significance Deregulation of components of the ubiquitin–proteasome system contributes to the development of various diseases. A prominent example is the ubiquitin ligase E6AP/UBE3A, which is associated with three disorders: in complex with the E6 oncoprotein of human papillomaviruses, it contributes to cervical carcinogenesis; loss of E6AP expression results in the development of Angelman syndrome; and increased E6AP expression has been associated with autism spectrum disorders. This indicates that E6AP has to be tightly controlled; however, only little is known about how this is achieved. By analyzing the role of ubiquitin and the E6 oncoprotein in E6AP-mediated ubiquitination, we provide evidence that E6AP exists in an active state and a latent state and that its activity is controlled by allosteric effectors. Deregulation of the ubiquitin ligase E6 associated protein (E6AP) encoded by the UBE3A gene has been associated with three different clinical pictures. Hijacking of E6AP by the E6 oncoprotein of distinct human papillomaviruses (HPV) contributes to the development of cervical cancer, whereas loss of E6AP expression or function is the cause of Angelman syndrome, a neurodevelopmental disorder, and increased expression of E6AP has been involved in autism spectrum disorders. Although these observations indicate that the activity of E6AP has to be tightly controlled, only little is known about how E6AP is regulated at the posttranslational level. Here, we provide evidence that the hydrophobic patch of ubiquitin comprising Leu-8 and Ile-44 is important for E6AP-mediated ubiquitination, whereas it does not affect the catalytic properties of the isolated catalytic HECT domain of E6AP. Furthermore, we show that the HPV E6 oncoprotein rescues the disability of full-length E6AP to use a respective hydrophobic patch mutant of ubiquitin for ubiquitination and that it stimulates E6AP-mediated ubiquitination of Ring1B, a known substrate of E6AP, in vitro and in cells. Based on these data, we propose that E6AP exists in at least two different states, an active and a less active or latent one, and that the activity of E6AP is controlled by noncovalent interactions with ubiquitin and allosteric activators such as the HPV E6 oncoprotein.