A Potential Role for Shed Soluble Major Histocompatibility Class I Molecules as Modulators of Neurite Outgrowth

The neurobiological activities of classical major histocompatibility class I (MHCI) molecules are just beginning to be explored. To further examine MHCI's actions during the formation of neuronal connections, we cultured embryonic mouse retina explants a short distance from wildtype thalamic explants, or thalami from transgenic mice (termed “NSE-Db”) whose neurons express higher levels of MHCI. While retina neurites extended to form connections with wildtype thalami, we were surprised to find that retina neurite outgrowth was very stunted in regions proximal to NSE-Db thalamic explants, suggesting that a diffusible factor from these thalami inhibited retina neurite outgrowth. It has been long known that MHCI-expressing cells release soluble forms of MHCI (sMHCI) due to the shedding of intact MHCI molecules, as well as the alternative exon splicing of its heavy chain or the action proteases which cleave off it's transmembrane anchor. We show that the diffusible inhibitory factor from the NSE-Db thalami is sMHCI. We also show that COS cells programmed to express murine MHCI release sMHCI that inhibits neurite outgrowth from nearby neurons in vitro. The neuroinhibitory effect of sMHCI could be blocked by lowering cAMP levels, suggesting that the neuronal MHCI receptor's signaling mechanism involves a cyclic nucleotide-dependent pathway. Our results suggest that MHCI may not only have neurobiological activity in its membrane-bound form, it may also influence local neurons as a soluble molecule. We discuss the involvement of complement proteins in generating sMHCI and new theoretical models of MHCI's biological activities in the nervous system

Discrepant NOXA (PMAIP1) transcript and NOXA protein levels: a potential Achilles' heel in mantle cell lymphoma.

Mantle cell lymphoma (MCL) is an aggressive lymphoid neoplasm with transient response to conventional chemotherapy. We here investigated the role of the Bcl-2 homology domain 3-only protein NOXA for life-death decision in MCL. Surprisingly, NOXA (PMAIP1) mRNA and NOXA protein levels were extremely discrepant in MCL cells: NOXA mRNA was found to be highly expressed whereas NOXA protein levels were low. Chronic active B-cell receptor signaling and to a minor degree cyclin D1 overexpression contributed to high NOXA mRNA expression levels in MCL cells. The phoshatidyl-inositol-3 kinase/AKT/mammalian target of rapamycin pathway was identified as the major downstream signaling pathway involved in the maintenance of NOXA gene expression. Interestingly, MCL cells adapt to this constitutive pro-apoptotic signal by extensive ubiquitination and rapid proteasomal degradation of NOXA protein (T½∼15-30 min). In addition to the proteasome inhibitor Bortezomib, we identified the neddylation inhibitor MLN4924 and the fatty acid synthase inhibitor Orlistat as potent inducers of NOXA protein expression leading to apoptosis in MCL. All inhibitors targeted NOXA protein turnover. In contrast to Bortezomib, MLN4924 and Orlistat interfered with the ubiquitination process of NOXA protein thereby offering new strategies to kill Bortezomib-resistant MCL cells. Our data, therefore, highlight a critical role of NOXA in the balance between life and death in MCL. The discrepancy between NOXA transcript and protein levels is essential for sensitivity of MCL to ubiquitin-proteasome system inhibitors and could therefore provide a druggable Achilles' heel of MCL cells