Regulation Of Epilepsy And Multiple Myeloma By CRL4A(CRBN) E3Ubiquitin Ligase And The Underlying Mechanism

The ubiquitin pathway has various roles in regulating cell cycle, cell proliferation, apoptosis, signal transduction and immune system, especially in some diseases. We are focusing on the regulation of epilepsy and multiple myeloma by CRL4ACRBN E3ubiquitin ligase and the underlying mechanism.Ion channels regulate membrane excitation, and mutations of ion channels often cause serious neurological disorders including epilepsy. Compared with extensive analyses of channel protein structure and function, much less is known about the fine tuning of channel activity by post-translational modification. Here we report that the large conductance, Ca2+-and voltage-activated K+(BK) channels are targeted by the E3ubiquitin ligase CRL4ACRBN for poly-ubiquitination and retained in the endoplasmic reticulum (ER). Inactivation of CRL4ACRBN releases de-ubiquitinated BK channels from ER to plasma membrane, leading to markedly enhanced channel activity. Mice with CRL4ACRBN mutation in the brain or treated with a CRL4ACRBN inhibitor are very sensitive to seizure induction, which can be attenuated by blocking BK channels. Finally, the mutant mice develop spontaneous epilepsy when aged. Therefore, ubiquitination of BK channels prior to their cell surface expression is an important step to prevent systemic neuronal excitability and epileptogenesis. Multiple myeloma(MM) is a cancer of plasma cells, characterized by the infiltration of abnormal plasma cells in bone marrow. It’s one of the most prevalent blood cancers representing about1%of all cancers. The conventional and standard treatment of myeloma is chemotherapy or autologous stem-cell transplantation in the early years. With the discovery of new therapeutic targets and drug development, more novel drugs are introduced to treat multiple myeloma, such as bortezomib, thalidomide and related IMiD immunomodulatory agents. Multiple myeloma patients have witnessed a significantly improved survival after the approved treatment with the proteasome inhibitor bortezomib and IMiDs. The mechanism of treatment of MM by IMiDs was uncovered recently. IMiDs directly bind cereblon (CRBN), an adaptor protein of CRL4, and activate the CRL4CRBN ubiquitin ligase activity to target B cell specific transcription factors IKZF1and IKZF3, which are essential for MM cell survival, for ubiquitination and degradation. Paradoxically, IMiDs achieve better clinical response in MM patients when combined with bortezomib, which blocks IKZF1/3degradation and would in principle counteract IMiDs therapy. Here we report that CRBN is inherently unstable and, when unbound to CRL4, targeted by another E3ubiquitin ligase SCFFbxo7, for ubiquitination and degradation. Bortezomib stabilizes CRBN, and IMiDs further promote the loading of CRBN to CRL4, leading to enhanced degradation of IKZF1and IKZF3in MM cells and synergistic inhibition of MM cell proliferation. Genome-wide screening with a CRISPR-based library revealed some resistant genes to IMiDs in MM cells. Our findings provide a mechanistic frame work to guide sequential administration of bortezomib and IMiDs to achieve maximal therapeutic efficacy against MM.