Sarah R. Horn

Growth factor erv1-like modulates Drp1 to preserve mitochondrial dynamics and function in mouse embryonic stem cells.

Gfer links mitochondrial dynamics with pluripotency in mouse embryonic stem cells through its modulation of the mitochondrial fission GTPase Drp1.

Regulation of mitochondrial morphology by APC/CCdh1-mediated control of Drp1 stability

Mitochondria form an interconnected network that undergoes dynamin-related protein 1 (Drp1)-dependent fission during mitosis. We demonstrate that changes in mitochondrial dynamics as cells exit mitosis are driven through ubiquitylation of Drp1 by the (anaphase- promoting complex/cyclosome and its coactivator Cdh1) APC/CCdh1 complex. Inhibition Drp1 degradation prevents the normal regrowth of mitochondrial networks during G1 phase.

An Essential Role for the Glut1 PDZ-Binding Motif in Growth Factor Regulation of Glut1 Degradation and Trafficking

Cell surface localization of the Glut (glucose transporter), Glut1, is a cytokine-controlled process essential to support the metabolism and survival of haemopoietic cells. Molecular mechanisms that regulate Glut1 trafficking, however, are not certain. In the present study, we show that a C-terminal PDZ-binding motif in Glut1 is critical to promote maximal cytokine-stimulated Glut1 cell surface localization and prevent Glut1 lysosomal degradation in the absence of growth factor. Disruption of this PDZ-binding sequence through deletion or point mutation sharply decreased surface Glut1 levels and led to rapid targeting of internalized Glut1 to lysosomes for proteolysis, particularly in growth factor-deprived cells. The PDZ-domain protein, GIPC (G(alpha)-interacting protein-interacting protein, C-terminus), bound to Glut1 in part via the Glut1 C-terminal PDZ-binding motif, and we found that GIPC deficiency decreased Glut1 surface levels and glucose uptake. Unlike the Glut1 degradation observed on mutation of the Glut1 PDZ-binding domain, however, GIPC deficiency resulted in accumulation of intracellular Glut1 in a pool distinct from the recycling pathway of the TfR (transferrin receptor). Blockade of Glut1 lysosomal targeting after growth factor withdrawal also led to intracellular accumulation of Glut1, a portion of which could be rapidly restored to the cell surface after growth factor stimulation. These results indicate that the C-terminal PDZ-binding motif of Glut1 plays a key role in growth factor regulation of glucose uptake by both allowing GIPC to promote Glut1 trafficking to the cell surface and protecting intracellular Glut1 from lysosomal degradation after growth factor withdrawal, thus allowing the potential for a rapid return of intracellular Glut1 to the cell surface on restimulation.

Mitochondrial Localization of Reaper to Promote Inhibitors of Apoptosis Protein Degradation Conferred by GH3 Domain-Lipid Interactions

Morphological hallmarks of apoptosis result from activation of the caspase family of cysteine proteases, which are opposed by a pro-survival family of inhibitors of apoptosis proteins (IAPs). In Drosophila, disruption of IAP function by Reaper, HID, and Grim (RHG) proteins is sufficient to induce cell death. RHG proteins have been reported to localize to mitochondria, which, in the case of both Reaper and Grim proteins, is mediated by an amphipathic helical domain known as the GH3. Through direct binding, Reaper can bring the Drosophila IAP (DIAP1) to mitochondria, concomitantly promoting IAP auto-ubiquitination and destruction. Whether this localization is sufficient to induce DIAP1 auto-ubiquitination has not been reported. In this study we characterize the interaction between Reaper and the mitochondria using both Xenopus and Drosophila systems. We find that Reaper concentrates on the outer surface of mitochondria in a nonperipheral manner largely mediated by GH3-lipid interactions. Importantly, we show that mitochondrial targeting of DIAP1 alone is not sufficient for degradation and requires Reaper binding. Conversely, Reaper able to bind IAPs, but lacking a mitochondrial targeting GH3 domain (ΔGH3 Reaper), can induce DIAP1 turnover only if DIAP1 is otherwise targeted to membranes. Surprisingly, targeting DIAP1 to the endoplasmic reticulum instead of mitochondria is partially effective in allowing ΔGH3 Reaper to promote DIAP1 degradation, suggesting that co-localization of DIAP and Reaper at a membrane surface is critical for the induction of DIAP degradation. Collectively, these data provide a specific function for the GH3 domain in conferring protein-lipid interactions, demonstrate that both Reaper binding and mitochondrial localization are required for accelerated IAP degradation, and suggest that membrane localization per se contributes to DIAP1 auto-ubiquitination and degradation.

Mitochondrial fusion is regulated by Reaper to modulate Drosophila programmed cell death.

In most multicellular organisms, the decision to undergo programmed cell death in response to cellular damage or developmental cues is typically transmitted through mitochondria. It has been suggested that an exception is the apoptotic pathway of Drosophila melanogaster, in which the role of mitochondria remains unclear. Although IAP antagonists in Drosophila such as Reaper, Hid and Grim may induce cell death without mitochondrial membrane permeabilization, it is surprising that all three localize to mitochondria. Moreover, induction of Reaper and Hid appears to result in mitochondrial fragmentation during Drosophila cell death. Most importantly, disruption of mitochondrial fission can inhibit Reaper and Hid-induced cell death, suggesting that alterations in mitochondrial dynamics can modulate cell death in fly cells. We report here that Drosophila Reaper can induce mitochondrial fragmentation by binding to and inhibiting the pro-fusion protein MFN2 and its Drosophila counterpart dMFN/Marf. Our in vitro and in vivo analyses reveal that dMFN overexpression can inhibit cell death induced by Reaper or γ-irradiation. In addition, knockdown of dMFN causes a striking loss of adult wing tissue and significant apoptosis in the developing wing discs. Our findings are consistent with a growing body of work describing a role for mitochondrial fission and fusion machinery in the decision of cells to die.

The deubiquitinating enzyme DUBAI stabilizes DIAP1 to suppress Drosophila apoptosis

Deubiquitinating enzymes (DUBs) counteract ubiquitin ligases to modulate the ubiquitination and stability of target signaling molecules. In Drosophila, the ubiquitin–proteasome system has a key role in the regulation of apoptosis, most notably, by controlling the abundance of the central apoptotic regulator DIAP1. Although the mechanism underlying DIAP1 ubiquitination has been extensively studied, the precise role of DUB(s) in controlling DIAP1 activity has not been fully investigated. Here we report the identification of a DIAP1-directed DUB using two complementary approaches. First, a panel of putative Drosophila DUBs was expressed in S2 cells to determine whether DIAP1 could be stabilized, despite treatment with death-inducing stimuli that would induce DIAP1 degradation. In addition, RNAi fly lines were used to detect modifiers of DIAP1 antagonist-induced cell death in the developing eye. Together, these approaches identified a previously uncharacterized protein encoded by CG8830, which we named DeUBiquitinating-Apoptotic-Inhibitor (DUBAI), as a novel DUB capable of preserving DIAP1 to dampen Drosophila apoptosis. DUBAI interacts with DIAP1 in S2 cells, and the putative active site of its DUB domain (C367) is required to rescue DIAP1 levels following apoptotic stimuli. DUBAI, therefore, represents a novel locus of apoptotic regulation in Drosophila, antagonizing cell death signals that would otherwise result in DIAP1 degradation.

Composite lymphoma of follicular B-cell and peripheral T-cell types with distinct zone distribution in a 75-year-old male patient: a case study

Composite lymphoma of T-/B-cell type is rare, and follicular lymphoma composite with peripheral T-cell lymphoma (PTCL) has not previously been reported. We report such a case with both neoplastic components displaying a unique zone of distribution. A 75-year-old male patient presented with generalized lymphadenopathy. Sections of axillary lymph node demonstrated potentially 2 clonal processes, PTCL with aberrant CD20 expression and follicular lymphoma. Interestingly, the 2 neoplastic components were confined to their respective classic distribution zones, with PTCL occupying the interfollicular areas and follicular lymphoma residing in follicles. Both populations were detected by flow cytometry, but their immunophenotypes were insufficient to define clonality. Nonetheless, biclonality was demonstrated by lymphoid receptor gene rearrangement analyses. Molecular cytogenetics showed IGH/BCL2 fusion in the follicular lymphoma and amplification of IGH gene or trisomy/tetrasomy 14 in the PTCL. The current case underscores the complexity of composite lymphoma and advocates a multimodal approach to establishing the diagnosis.