The currently accepted model of biological membranes involves a heterogeneous, highly dynamic organization, where certain lipids and proteins associate to form cooperative platforms ("rafts") for cellular signaling or transport processes. Ceramides, a lipid species occurring under conditions of cellular stress and apoptosis, are considered to stabilize these platforms, thus modulating cellular function. The present study focuses on a previously unrecognized effect of ceramide generation. In agreement with previous studies, we find that ceramide leads to a depletion of sphingomyelin from mixtures with palmitoyl oleoyl phosphatidylcholine bilayers, forming a ceramide-sphingomyelin-rich gel phase that coexists with a fluid phase rich in palmitoyl oleoyl phosphatidylcholine. Interestingly, however, this latter phase has an almost fourfold smaller bending rigidity compared to a sphingomyelin-palmitoyl oleoyl phosphatidylcholine mixture lacking ceramide. The significant change of membrane bulk properties can have severe consequences for conformational equilibria of membrane proteins. We discuss these effects in terms of the lateral pressure profile concept for a simple geometric model of an ion channel and find a significant inhibition of its activity.

Patients with previous diabetic foot ulcer are prone to re-ulceration and (re)amputation, to various comorbidities, have significantly impaired quality of life and increased mortality. We aimed to evaluate the risk of foot related complications and mortality in a high-risk population of patients with healed diabetic foot syndrome over a decade. 91 patients with recently healed diabetic foot ulcer were invited for follow-up at 1, 6 and 11 years after inclusion. Patient characteristics at inclusion were: 40 women, 65 ± 11 years, diabetes type 1 (n = 6) or 2 (n = 85), BMI 28.5 ± 4.4 kg/m 2 , and HbA1c 68 ± 17 mmol/mol. Comorbidities included neuropathy (n = 91), peripheral artery disease (PAD), history of minor (n = 25) or major (n = 5, 5.5%) amputation, nephropathy (n = 40) and retinopathy (n = 53). Ulceration recurred in 71 (65%) patients, time to first recurrence was 1.8 ± 2.4 years (mean ± SD). 21 patients had to undergo (re)amputation (minor n = 19, major n = 2), time to amputation was 3.6 ± 1.9 years. Over time, 3 further major amputations were required in patients with an initial minor amputation. Thirty-three (36%) of the initially included patients completed the follow-up period of 11.0 ± 0.6 years. 58 patients (64%) died during the observational period, time to death was 5 ± 3 years in this group. We found overall high mortality of 64% throughout the follow-up period of 11 years in high-risk patients with healed diabetic foot syndrome. Presence of PAD, prior amputation and nephropathy as well as poor glycemic control were significantly predictive for death.

We mimicked the effect of sphingomyelinase activity on lipid mixtures of palmitoyl-oleoyl-phosphatidylcholine, sphingomyelin, ceramide, and 10 mol % cholesterol. Using x-ray diffraction experiments in combination with osmotic stress we found, in agreement with previous studies, that ceramide induces a coexistence of L(α) and L(β) domains. A detailed structural analysis of the coexisting domains demonstrated an increase of lipid packing density and membrane thickness in the L(α) domains upon increasing overall ceramide levels. This provides evidence for a ceramide-driven accumulation of cholesterol in the L(α) domains, in support of previous reports. We further determined the bending rigidities of the coexisting domains and found that the accumulation of cholesterol in the L(α) domains stabilizes their bending rigidity, which experiences a dramatic drop in the absence of cholesterol. Deriving experimental estimates for the spontaneous curvature and Gaussian modulus of curvature, we show, using a simple geometric model for ion channels, that in this way changes in the conformational equilibrium of membrane proteins can be kept small.

We applied x-ray diffraction, calorimetry, and infrared spectroscopy to lipid mixtures of palmitoyl-oleoyl phosphatidylcholine, sphingomyelin, and ceramide. This combination of experimental techniques allowed us to probe the stability and structural properties of coexisting lipid domains without resorting to any molecular probes. In particular, we found unstable microscopic domains (compositional/phase fluctuations) in the absence of ceramide, and macroscopically separated fluid and gel phases upon addition of ceramide. We also observed phase fluctuations in the presence of ceramide within the broad phase transition regions. We compare our results with fluorescence spectroscopy data and complement the previously reported phase diagram. We also obtained electron paramagnetic resonance data to assess the possible limitations of techniques employing a single label. Our study demonstrates the necessity of applying a combination of experimental techniques to probe local/global structural and fast/slow motional properties in complex lipid mixtures.