Mann, Karl

The aim of the study was to evaluate the effect of severe short‐term nutritional deprivation (ND) on the fractional synthesis rates (FSR) specific proteins and protein pools in the diaphragm muscle (DiaM). Male rats were divided into two groups: 20% of usual food intake for 4 days (ND) with water provided at libitum and free‐eating control (Ctl). [U‐13‐C] leucine was continuously infused through the jugular vein for 3–3.5 h after a priming dose of the isotope. Blood samples were collected from the carotid artery at baseline and every 30 min at precursor pool plateau. DiaM was excised at the end of the isotope infusion. Mixed muscle protein (MMP), myosin heavy chain (MHC), sarcoplasmic (Sarco) and mitochondrial (Mito) proteins were extracted from DiaM and the incorporation of the label into each protein(s) was determined using mass spectrometry. The FSR of MMP, MHC, Sarco and Mito in the rat DiaM were calculated using the precursor‐product relationship. ND resulted in the reduction of the FSRs of MMP (71%), MHC (69%), Sarco (44%) and Mito (55%) in DiaM. In summary, short‐term ND is associated with a global decrease in the synthetic rate of DiaM proteins, suggesting a compromise in the ability of the DiaM to synthesize functionally important proteins in the ND state. The underlying mechanisms for this depressed state of protein synthesis in DiaM related to ND remain to be elucidated.

GPI-PLD [glycosylphosphatidylinositol (GPI)-specific phospholipase D (PLD)] is a secreted mammalian enzyme that specifically cleaves GPI-anchored proteins. In addition, the enzyme has been shown to cleave GPI anchor intermediates in cell lysates. The biosynthesis of the GPI anchor is well characterized; however, the mechanisms by which the levels of GPI anchor intermediates are regulated are still unknown. To investigate whether GPI-PLD plays a role in this regulation, we isolated stable HeLa cells overexpressing the enzyme. GPI-PLD-HeLa (GPI-PLD-transfected HeLa) cells showed a 3-fold increase in intracellular GPI-PLD activity and drastically decreased the levels of GPI-anchored proteins when compared with untransfected HeLa controls. Intracellular cleavage of GPI-anchored proteins has been suggested to occur early in the secretory pathway and, in agreement with this proposal, GPI-PLD activity in GPI-PLD-HeLa cells was detected not only in the endoplasmic reticulum and Golgi apparatus, but also in the plasma membrane. The enzyme was also active in lipid rafts, membrane microdomains in which GPI-anchored proteins and GPI anchor intermediates are concentrated, indicating that intracellular GPI-PLD cleavage may also occur in this compartment. Pulse–chase paradigms revealed the turnover rate of the last intermediate of the GPI anchor pathway in GPI-PLD-HeLa cells to be accelerated compared with the controls. Furthermore, 1,10-phenanthroline, a GPI-PLD inhibitor, reversed this effect. Our studies demonstrated that GPI-PLD can cleave not only GPI-anchored proteins, but also GPI anchor intermediates intracellularly. This observation opens the possibility that GPI-PLD can influence the steady-state levels of GPI-anchored proteins by hydrolysing the anchor before and after its attachment to proteins.