TBC1D1 interacting proteins, VPS13A and VPS13C, regulate GLUT4 homeostasis in C2C12 myotubes

Proteins involved in the spaciotemporal regulation of GLUT4 trafficking represent potential therapeutic targets for the treatment of insulin resistance and type 2 diabetes. A key regulator of insulin- and exercise-stimulated glucose uptake and GLUT4 trafficking is TBC1D1. This study aimed to identify proteins that regulate GLUT4 trafficking and homeostasis via TBC1D1. Using an unbiased quantitative proteomics approach, we identified proteins that interact with TBC1D1 in C2C12 myotubes including VPS13A and VPS13C, the Rab binding proteins EHBP1L1 and MICAL1, and the calcium pump SERCA1. These proteins associate with TBC1D1 via its phosphotyrosine binding (PTB) domains and their interactions with TBC1D1 were unaffected by AMPK activation, distinguishing them from the AMPK regulated interaction between TBC1D1 and AMPKα1 complexes. Depletion of VPS13A or VPS13C caused a post-transcriptional increase in cellular GLUT4 protein and enhanced cell surface GLUT4 levels in response to AMPK activation. The phenomenon was specific to GLUT4 because other recycling proteins were unaffected. Our results provide further support for a role of the TBC1D1 PTB domains as a scaffold for a range of Rab regulators, and also the VPS13 family of proteins which have been previously linked to fasting glycaemic traits and insulin resistance in genome wide association studies.

Solvent exposure of Tyr10 as a probe of structural differences between monomeric and aggregated forms of the amyloid-β peptide

Aggregation of amyloid-β (Aβ) peptides is a characteristic pathological feature of Alzheimer's disease. We have exploited the relationship between solvent exposure and intrinsic fluorescence of a single tyrosine residue, Tyr10, in the Aβ sequence to probe structural features of the monomeric, oligomeric and fibrillar forms of the 42-residue Aβ1-42. By monitoring the quenching of Tyr10 fluorescence upon addition of water-soluble acrylamide, we show that in Aβ1-42 oligomers this residue is solvent-exposed to a similar extent to that found in the unfolded monomer. By contrast, Tyr10 is significantly shielded from acrylamide quenching in Aβ1-42 fibrils, consistent with its proximity to the fibrillar cross-β core. Furthermore, circular dichroism measurements reveal that Aβ1-42 oligomers have a considerably lower β-sheet content than the Aβ1-42 fibrils, indicative of a less ordered molecular arrangement in the former. Taken together these findings suggest significant differences in the structural assembly of oligomers and fibrils that are consistent with differences in their biological effects.

The impact of diabetes on arterial reconstructions for multilevel arterial occlusive disease11Presented at the 28th Annual Symposium on Vascular Surgery of the Society for Clinical Vascular Surgery, Rancho Mirage, California, March 15–19, 2000

PURPOSE

Critical limb ischemia due to multilevel arterial occlusive disease often may be treated with an inflow procedure alone; however, a subset patients require a subsequent infrainguinal revascularization for persistence of their symptoms. As diabetic patients typically exhibit a pattern of extensive distal arterial disease, we sought to determine if the presence of diabetes mellitus altered the need for an outflow procedure after inflow bypass.

METHODS

A total of 504 patients undergoing inflow bypass for occlusive disease and lower extremity ischemia between 1990 and 1998 were entered prospectively into a computerized vascular registry. Inflow bypass procedures performed were as follows: aortofemoral (370; 73%), axillofemoral (56; 11%), femorofemoral (81; 16%). Of these patients, 79 required subsequent outflow bypass for unresolved ischemic symptoms. Multiple logistic regression analysis was used to analyze the effects of diabetes and multiple other risk factors on the need for an additional outflow procedure.

RESULTS

The indications for surgery were limb salvage (78%) and disabling claudication (22%). Overall morbidity was 17.7% (hematoma, 3.8%; wound infection, 2.5%; graft occlusion, 1.3%; myocardial infarction, 2.5%; acute renal failure,1.3%; pulmonary failure, 2.5%; pneumonia, 3.8%). Overall mortality was 0%. Diabetic patients comprised a greater proportion of the total number of patients requiring inflow bypass (301 of 504) as well as a greater proportion of patients requiring inflow and outflow procedures (47 of 79). Diabetes was determined not to be an independent risk factor for the need for multiple revascularization procedures by multiple logistic regression analysis (P >0.10).

CONCLUSION

Although patients with diabetes are predisposed to the development of distal arterial occlusive disease, in this study the subgroup of diabetic patients who present with aortoiliac occlusive disease were no more likely than patients without diabetes to require multiple levels of revascularization. These findings provide little rationale for simultaneous inflow and outflow procedures based on the presence of diabetes alone.

Combined iliac angioplasty and infrainguinal revascularization surgery are effective in diabetic patients with multilevel arterial disease

The success of percutaneous transluminal angioplasty (PTA) in the treatment of common and external iliac atherosclerotic lesions has been established for the general population. However, several studies have suggested that the presence of diabetes may reduce the effectiveness of iliac angioplasty, particularly in the setting of limb-threatening ischemia requiring concomitant lower extremity revascularization. This study compared the results of iliac artery PTA performed in conjunction with infrainguinal bypass for limb-threatening ischemia for diabetic (DM) and nondiabetic (non-DM) patients. Between 1991 and 2000, 159 PTA were performed in 126 patients (DM = 99/79%, non-DM = 27/21%) in conjunction with subsequent infrainguinal bypass for limb-threatening ischemia (gangrene = 42%, ulcer = 36%, rest pain = 22%). These patients were followed prospectively using a computerized vascular registry. Stents were placed in 34 (21.4%) cases for suboptimal angioplasty results. In this study the combined use of standard surgical and endoluminal modalities for the treatment of multilevel arterial occlusive disease resulted in excellent cumulative patency and limb salvage rates. The presence of diabetes did not alter these favorable results. Multimodal vascular therapy may be used effectively in diabetic patients with limb-threatening ischemia due to multiple levels of arterial occlusion.