Myelin basic protein stimulates insulin and glucagon secretion from rat pancreatic islets in vitro and in vivo

Glucose-Induced Changes in Gene Expression in Human Pancreatic Islets: Causes or Consequences of Chronic Hyperglycemia

Dysregulation of gene expression in islets from patients with type 2 diabetes (T2D) might be causally involved in the development of hyperglycemia, or it could develop as a consequence of hyperglycemia (i.e., glucotoxicity). To separate the genes that could be causally involved in pathogenesis from those likely to be secondary to hyperglycemia, we exposed islets from human donors to normal or high glucose concentrations for 24 h and analyzed gene expression. We compared these findings with gene expression in islets from donors with normal glucose tolerance and hyperglycemia (including T2D). The genes whose expression changed in the same direction after short-term glucose exposure, as in T2D, were considered most likely to be a consequence of hyperglycemia. Genes whose expression changed in hyperglycemia but not after short-term glucose exposure, particularly those that also correlated with insulin secretion, were considered the strongest candidates for causal involvement in T2D. For example, ERO1LB, DOCK10, IGSF11, and PRR14L were downregulated in donors with hyperglycemia and correlated positively with insulin secretion, suggesting a protective role, whereas TMEM132C was upregulated in hyperglycemia and correlated negatively with insulin secretion, suggesting a potential pathogenic role. This study provides a catalog of gene expression changes in human pancreatic islets after exposure to glucose.

Interaction of myelin basic protein and 2',3'-cyclic nucleotide phosphodiesterase with mitochondria

The content and distribution of myelin basic protein (MBP) isoforms (17 and 21.5 kDa) as well as 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase) were determined in mitochondrial fractions (myelin fraction, synaptic and nonsynaptic mitochondria) obtained after separation of brain mitochondria by Percoll density gradient. All the fractions could accumulate calcium, maintain membrane potential, and initiate the opening of the permeability transition pore (mPTP) in response to calcium overloading. Native mitochondria and structural contacts between membranes of myelin and mitochondria were found in the myelin fraction associated with brain mitochondria. Using Western blot, it was shown that addition of myelin fraction associated with brain mitochondria to the suspension of liver mitochondria can lead to binding of CNPase and MBP, present in the fraction with liver mitochondria under the conditions of both closed and opened mPTP. However, induction of mPTP opening in liver mitochondria was prevented in the presence of myelin fraction associated with brain mitochondria (Ca2+ release rate was decreased 1.5-fold, calcium retention time was doubled, and swelling amplitude was 2.8-fold reduced). These results indicate possible protective properties of MBP and CNPase.

Identification and characterization of a MBP isoform specific to hypothalamus in orange-spotted grouper (Epinephelus coioides)

Myelin basic protein (MBP), as a major component of the myelin sheath, has been revealed to play an important role in forming and maintaining myelin structure in vertebrate nervous system. In teleost, hypothalamus is an instinctive brain center and plays significant roles in many physiological functions, such as energy metabolism, growth, reproduction, and stress response. In comparison with other MBP identified in vertebrates, a smallest MBP is cloned and identified from the orange-spotted grouper hypothalamic cDNA plasmid library in this study. RT-PCR analysis and Western blot detection indicate that the EcMBP is specific to hypothalamus, and expresses mainly in the tuberal hypothalamus in adult grouper. Immunofluorescence localization suggests that EcMBP should be expressed by oligodendrocytes, and the expressing cells should be concentrated in hypothalamus and the area surrounding hypothalamus, such as NPOpc, VC, DP, NLTm, and NDLI. The studies on EcMBP expression pattern and developmental behaviour in the brains of grouper embryos and larvae reveal that the EcMBP-expressing cells are only limited in a defined set of cells on the border of hypothalamus, and suggest that the EcMBP-expressing cells might be a subpopulation of oligodendrocyte progenitor cells. This study not only identifies a smallest MBP isoform specific to hypothalamus that can be used as a molecular marker of oligodendrocytes in fish, but also provides new insights for MBP evolution and cellular distribution.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.

Reversed-phase capillary liquid chromatography coupled on-line to capillary electrophoresis immunoassays

Capillary reversed-phase liquid chromatography (RPLC) was coupled on-line to competitive capillary electrophoresis immunoassay (CEIA) to improve concentration sensitivity of the competitive CEIA and to provide a means for detecting multiple species that cross-react with antibody. A competitive CEIA for glucagon was used for demonstration of this technique. Five-microliter samples were injected onto a 4-cm-long by 50-micron-i.d. RPLC column. Sample was desorbed by gradient elution, mixed on-line with fluorescently labeled glucagon and anti-glucagon, incubated in a continuous-flow reaction capillary, and analyzed by capillary electrophoresis with flow-gated injection and laser-induced fluorescence detection. Electrophoretic analysis of the reactor stream was performed every 1.5 s, allowing nearly continuous monitoring of the RPLC separation. Preconcentration achieved by RPLC allowed improvement in the detection limit from 760 to 20 pM. Addition of the RPLC column also allowed multiple cross-reactive species to be differentiated by first separating them chromatographically and then detecting them with the immunoassay. The technique was used to measure glucagon secretion from single islets of Langerhans and to differentiate cross-reactive forms of glucagon with one assay.

Binding of soluble myelin basic protein to various conformational forms of alpha2-macroglobulin

Myelin basic protein is known to be released into the circulation following traumatic injuries or demyelination within the central nervous system, resulting in the generation of potentially immunogenic myelin basic protein material. In this investigation we have studied the binding of bovine and human myelin basic protein to human alpha2-macroglobulin, which was found to be the only major myelin basic protein-binding protein in human plasma. Myelin basic protein bound to all three conformational forms of alpha2-macroglobulin studied, i.e., native alpha2-macroglobulin, methylamine-treated alpha2-macroglobulin, and chymotrypsin-treated alpha2-macroglobulin. Zinc chloride (1 mM) or 1 mM iodoacetamide partly blocked the complex formation between myelin basic protein and alpha2-macroglobulin, while 1 mM magnesium chloride, 1 mM calcium chloride, or 1 mM EDTA had no effect on binding. Chymotrypsin and trypsin can degrade myelin basic protein to fragments which do not bind to alpha2-macroglobulin. However, when myelin basic protein was complexed with any of the conformational forms of alpha2-macroglobulin, no significant release of Na[125I]-labeled myelin basic protein occurred after proteinase treatment. The results suggest that binding of myelin basic protein to alpha2-macroglobulin may protect extracellular compartments in vivo from immunogenic myelin basic protein fragments and alpha2-macroglobulin may participate in the specific clearance of myelin basic protein from the circulation.

Disappearance of 125I-labelled myelin basic protein from blood circulation and its degradation and accumulation in various tissues in rats

Following acute central nervous system myelin injury, immunoreactive myelin basic protein (MBP) has been detected in the cerebrospinal fluid, blood and urine. In order to clarify the fate of MBP in the circulation, distribution and degradation of intravenously injected bovine MBP was followed in anaestethized rats for 5 to 240 min by using 125I-labelled MBP. Five minutes after injection of a dose of 60-400 ng of MBP, 44% of the label was recovered in the liver, 6.3% in the kidneys, 4.7% in the lungs and 15% in the blood circulation, the corresponding figures at a dose of 0.8 mg being 51, 7.4, 0.8 and 22%. The liver discarded the label fastest, 3% of the dose remaining 4 h after injection. The amount of label in urine increased simultaneously, the recovery at 4 h being 5.5% of the lower and 4.2% of the higher MBP dose. The percentage of total dose of the label per gram of tissue at 5 min (= distribution percentage, DP-5) was 3-4% in the liver and kidney and 1.6% in the spleen. The label content in the pancreas was increased at 15-60 min, compared to the DP-5 of 0.3% with a two-fold maximum at 30 min. The duodenum concentrated MBP in a similar manner as the pancreas but not as extensively. The DP-5 of 0.1% in the thymus was concentrated two-fold with a maximum at 60 min. A slight concentration occurred in the heart. The DP-5 of 0.03% in muscle, testis and brain was concentrated 3-fold at 60 min, 3.6-fold at 60-240 min and 2-fold at 30-60 min in the aforementioned tissues, respectively. In spite of degradation of the label in tissues, the distribution of high molecular weight (HMW = TCA-precipitable) MBP was similar. Other experiments showed that the kidney, lung and duodenum contained most of the HMW MBP at 20 h. Upon continuous release of MBP, the pancrease, thymus, duodenum, muscle and testis would thus cumulatively concentrate MBP, and the kidney, lung and duodenum would be quantitatively most affected. MBP was previously shown to enter into cells of pancreative islets and to stimulate insulin and glucagon release. It could have biological effects in other tissues as well. These effects could explain some peripheral symptoms present in neurological disorders.

Evidence supporting membrane fusion as the mechanism of myelin basic protein-induced insulin release from rat pancreatic islets

In order to clarify insulinotropic effects of the myelin basic protein (MBP) we studied mode of association and distribution of MBP in the pancreatic islets and tested the insulin-releasing activity of various MBP peptides. Rat pancreatic islets were first stimulated in a static incubation with 10 microM bovine MBP (bMBP) at a substimulatory (3.5 mM) glucose concentration. The islets exposed to MBP released significantly more insulin and glucagon in a second incubation in the absence of added stimulant and in the presence of 11.5 mM arginine than the incubated, non-stimulated islets and islets initially stimulated with 15 mM glucose. Response to stimulation with 15 mM glucose in the second incubation by islets exposed first to MBP was impaired compared to incubated, non-stimulated islets. Immunoelectron microscopy showed that MBP had entered into the islet cells and associated with membranes of intracellular vacuoles, most of which represented enlarged, often fused insulin granules. MBP was also present at the islet edge and in the intercellular spaces. Of the purified MBP peptides of sizes of 4.8-13.6 kDa, produced from the digestion with brain acid proteinase and with pepsin and covering the entire bMBP sequence, only the large peptides (1-88, 9.8 kDa and 43-169, 13.6 kDa) stimulated insulin secretion significantly. Heterogeneous peptide mixtures, obtained from a time-course digestion of bMBP by myelin calcium-activated neutral protease, consisting of peptides of approximate molecular weights of 8-11 kDa and larger, also stimulated insulin release. The glucagon-releasing activity of MBP peptides was low and followed the same pattern as the insulin-releasing activity. The present results suggest that MBP-induced fusion of the membranes of hormone granules is involved in MBP-induced insulin release. The hormone-releasing activity of the large peptides in addition to that of the intact molecule is explained as being due to the ability of these peptides to associate with membranes. MBP-induced hormone release and related effects could be associated with neuropathological conditions such as stroke and multiple sclerosis.