Post-translational modifications of apolipoprotein A-I and Po proteins in the avian peripheral nerve

Effects of olanzapine on serum protein phosphorylation patterns in patients with schizophrenia

PURPOSE

Previous studies have shown that blood serum phosphoproteins are altered in schizophrenia patients in comparison to controls. However, it is not known whether phosphoproteins are also changed in response to treatment with antipsychotics.

EXPERIMENTAL DESIGN

Blood samples were taken from patients (n = 23) at baseline and after 6 weeks of olanzapine treatment. Immobilized metal ion affinity chromatography (IMAC) was used for enrichment of serum phosphoproteins and these were analyzed by label-free LC-MS in expression mode (LC-MS(E) ).

RESULTS

We identified 11 proteins that were changed significantly in overall abundance and 45 proteins that showed changes in phosphorylation after the antipsychotic treatment. The altered phosphoproteins were mainly involved in the acute phase response, lipid and glucose homeostasis (LXR), retinoic acid signaling (RXR), and complement pathways. Some of the proteins showed a marked increase in phosphorylation, including apolipoprotein A-I (3.4-fold), alpha-1-anti-chymotrypsin (3.1-fold), and apolipoprotein B-100 (2.2-fold). In addition, several proteins showed either decreased phosphorylation (e.g. complement C4A, collagen alpha-1 chain, complement factor H) or a mixture of increased and decreased phoshphorylation (e.g. afamin, complement C5, complement factor B). Finally, 24 of the altered phosphoproteins showed opposite directional changes in a comparison of baseline schizophrenia patients before and after treatment with olanzapine. These included alpha-1B-glycoprotein, apolipoprotein A-IV, vitamin D-binding protein, and prothrombin.

CONCLUSIONS AND CLINICAL RELEVANCE

These data demonstrate the potential for future studies of serum phosphoproteins as a readout of physiological function and might have utility in studies aimed at identification of biomarkers for drug response prediction or monitoring.

Mass spectrometry-based analytical tools for the molecular protein characterization of human plasma lipoproteins

Lipoproteins are a heterogeneous population of blood plasma particles composed of apolipoproteins and lipids. Lipoproteins transport exogenous and endogenous triglycerides and cholesterol from sites of absorption and formation to sites of storage and usage. Three major classes of lipoproteins are distinguished according to their density: high-density (HDL), low-density (LDL) and very low-density lipoproteins (VLDL). While HDLs contain mainly apolipoproteins of lower molecular weight, the two other classes contain apolipoprotein B and apolipoprotein (a) together with triglycerides and cholesterol. HDL concentrations were found to be inversely related to coronary heart disease and LDL/VLDL concentrations directly related. Although many studies have been published in this area, few have concentrated on the exact protein composition of lipoprotein particles. Lipoproteins were separated by density gradient ultracentrifugation into different subclasses. Native gel electrophoresis revealed different gel migration behaviour of the particles, with less dense particles having higher apparent hydrodynamic radii than denser particles. Apolipoprotein composition profiles were measured by matrix-assisted laser desorption/ionization-mass spectrometry on a macromizer instrument, equipped with the recently introduced cryodetector technology, and revealed differences in apolipoprotein composition between HDL subclasses. By combining these profiles with protein identifications from native and denaturing polyacrylamide gels by liquid chromatography-tandem mass spectrometry, we characterized comprehensively the exact protein composition of different lipoprotein particles. We concluded that the differential display of protein weight information acquired by macromizer mass spectrometry is an excellent tool for revealing structural variations of different lipoprotein particles, and hence the foundation is laid for the screening of cardiovascular disease risk factors associated with lipoproteins.

Characterization and partial purification of a novel 36 kDa peripheral myelin protein recognized by the sera of patients with neurological disorders

Sera of some patients with acquired sensory neuropathy, chronic inflammatory demyelinating polyradiculoneuropathy and motor neuron disease have high titres of IgG autoantibodies to a minor human peripheral nerve glycoprotein of approximately 36 kDa. This protein cofractionated with PNS myelin and was also found in bovine and rat nerve but not in CNS myelin or other nonneural human tissues. The N-terminal sequence revealed that this protein is related to the major myelin protein P0. Monoclonal antibodies to P0 and to the carbohydrate epitope HNK-1 did not recognize the 36-kDa protein, and the human anti-36-kDa antibodies did not bind to P0. IgG binding to this protein was not abolished after periodate oxidation or deglycosylation, suggesting that the epitope recognized by the human antibodies is peptidic. Differential glycosylation did not account for the differences in the apparent molecular weight between these two proteins. Overall our results indicate that the 36-kDa protein is a variant of P0.

Secretion and processing of apolipoprotein A-I in the avian sciatic nerve during development

Apolipoprotein A-I (apo A-I), a major apolipoprotein synthesized by liver and intestine to facilitate transport of plasma lipids as lipoproteins, has been detected also in the avian sciatic nerve. The mRNA and protein levels of apo A-I have been shown to increase during the period of rapid myelination (LeBlanc et al.: J Cell Biol 109:1245-1256, 1989). In order to assess the synthesis of apo A-I protein and the processing of apo A-I isoforms during development, endoneurial slices of avian sciatic nerves from chicks during active myelination at 15 and 17 days embryonic and 1 day posthatch age were incubated with [35]S-methionine. The incubations were fractionated into secreted and intracellular fractions, and incorporation of the label was assessed for apo A-I protein. The pattern of labeling of Po protein, as a marker of myelination, was also determined in the intracellular and compact myelin fractions. Methionine incorporation into Po protein was highest in the intracellular compartment at the 15-day embryonic stage and decreased thereafter, with a corresponding increase in the myelin fraction. During these developmental periods, the levels of nascent apo A-I increased in both the secreted and intracellular fractions. The synthesis of apo A-I specifically increases in the secreted fraction compared with total protein synthesis. The processing of the pro-apo A-I is also developmentally regulated. In the intracellular compartment, there are approximately equal proportions of the acidic and basic isoforms. However, with increasing age, a higher proportion of the apo A-I is secreted as acidic isoforms. It is concluded that the secretion and processing of apo A-I is developmentally regulated in the chick sciatic nerve, in parallel with the process of active myelination.

Biosynthesis and compartmentalization of Po, apolipoprotein A-I, and lipids in the myelinating chick sciatic nerve

Myelin deposition in developing chick sciatic nerve is associated with rapid synthesis of lipids, the major myelin protein Po and apo A-I, a major constituent of plasma lipoproteins. In order to understand possible roles of apo A-I in myelin assembly the synthesis and appearance of Po, apo A-I and lipids was studied in an intracellular fraction, an intralamellar fraction thought to be related to, or derived from, myelin and compact myelin from rapidly myelinating sciatic nerve of 1 day chicks. Incorporation with methionine or pulse-chase experiments indicated that initial synthesis of Po occurs in the intracellular fraction followed by movement to the intralamellar fraction and myelin. Incorporation of labelled oleate into phospholipids suggested that initial synthesis occurs in the intracellular and intralamellar fractions with slow movement to myelin. Incorporation of labelled galactose into cerebrosides suggested that initial synthesis occurs partially in myelin with slow loss from this fraction to the intralamellar fraction. However, incorporation of methionine into apo A-I indicated that initial synthesis occurred in the intracellular fraction with some transfer to the intralamellar fraction and secretion of a major portion into the incubation medium. It is concluded that the subcellular distribution of nascent apo A-I is not well coordinated with the distribution of other nascent constituents of the myelin membrane. The accumulation of nascent Po, phospholipids and cerebrosides in the intralamellar fraction compared to compact myelin suggests that this fraction may play a role as a precursor membrane or as a storage site for assembly of myelin constituents into compact myelin.