P0 phosphorylation in nerves from normal and diabetic rats: role of protein kinase C and turnover of phosphate groups

Diabetic neuropathy and neuropathic pain: a (con)fusion of pathogenic mechanisms?

Neuropathy is a common complication of long-term diabetes that impairs quality of life by producing pain, sensory loss and limb amputation. The presence of neuropathy in both insulin-deficient (type 1) and insulin resistant (type 2) diabetes along with the slowing of progression of neuropathy by improved glycemic control in type 1 diabetes has caused the majority of preclinical and clinical investigations to focus on hyperglycemia as the initiating pathogenic lesion. Studies in animal models of diabetes have identified multiple plausible mechanisms of glucotoxicity to the nervous system including post-translational modification of proteins by glucose and increased glucose metabolism by aldose reductase, glycolysis and other catabolic pathways. However, it is becoming increasingly apparent that factors not necessarily downstream of hyperglycemia can also contribute to the incidence, progression and severity of neuropathy and neuropathic pain. For example, peripheral nerve contains insulin receptors that transduce the neurotrophic and neurosupportive properties of insulin, independent of systemic glucose regulation, while the detection of neuropathy and neuropathic pain in patients with metabolic syndrome and failure of improved glycemic control to protect against neuropathy in cohorts of type 2 diabetic patients has placed a focus on the pathogenic role of dyslipidemia. This review provides an overview of current understanding of potential initiating lesions for diabetic neuropathy and the multiple downstream mechanisms identified in cell and animal models of diabetes that may contribute to the pathogenesis of diabetic neuropathy and neuropathic pain.

Polyphenols of marine red macroalga Symphyocladia latiuscula ameliorate diabetic peripheral neuropathy in experimental animals

Aims

Chronic hyperglycaemia activates the polyol pathway of glucose metabolism thereby stimulating the activation aldose reductase enzyme that in turn initiates a cascade of deleterious events, eventually, leading to nerve damage or neuropathy. Marine macroalgae and their isolated chemical constituents have been found to possess potential antidiabetic activity and have proved beneficial in the treatment of diabetes. In this study the neuroprotective effect of polyphenols isolated from the red macroalga Symphyocladia latiuscula was evaluated in experimental diabetic peripheral neuropathy.

Main methods

The polyphenolic fraction from Symphyocladia latiuscula was isolated. Diabetic peripheral neuropathy (DPN) was induced in animals by intraperitoneal injection of streptozotocin (45 mg/kg, b. w) and maintained for 6 weeks followed by treatment with SLPP or epalrestat. Nerve Conduction Velocity (NCV) and Compound Muscle Action Potential (CMAP) were measured using a non-invasive method followed by muscular grip strength test. Sciatic nerve aldose reductase activity, sorbitol accumulation, Na⁺K⁺-ATPase activity, production of pro-inflammatory cytokines and expression of AR and PKC were assessed.

Key findings

The Symphyocladia latiuscula polyphenols (SLPP) were found to inhibit aldose reductase activity as well as their expression in diabetic animals thereby improving the NCV, CMAP and muscle grip strength. Improvements in the sciatic nerve Na⁺K⁺-ATPase activity and intraneural accumulation of sorbitol, an index of aldose reductase overactivity, were evident with SLPP treatment. The production of pro-inflammatory cytokines (IL-6, IL-1β and TNF-α) and expression of protein kinase C (PKC) were also diminished.

Significance

The data suggest that the polyphenols of Symphyocladia latiuscula have neuroprotective potential against experimental DPN.

Diet-induced obesity and low testosterone increase neuroinflammation and impair neural function

BACKGROUND

Low testosterone and obesity are independent risk factors for dysfunction of the nervous system including neurodegenerative disorders such as Alzheimer's disease (AD). In this study, we investigate the independent and cooperative interactions of testosterone and diet-induced obesity on metabolic, inflammatory, and neural health indices in the central and peripheral nervous systems.

METHODS

Male C57B6/J mice were maintained on normal or high-fat diet under varying testosterone conditions for a four-month treatment period, after which metabolic indices were measured and RNA isolated from cerebral cortex and sciatic nerve. Cortices were used to generate mixed glial cultures, upon which embryonic cerebrocortical neurons were co-cultured for assessment of neuron survival and neurite outgrowth. Peripheral nerve damage was determined using paw-withdrawal assay, myelin sheath protein expression levels, and Na+,K+-ATPase activity levels.

RESULTS

Our results demonstrate that detrimental effects on both metabolic (blood glucose, insulin sensitivity) and proinflammatory (cytokine expression) responses caused by diet-induced obesity are exacerbated by testosterone depletion. Mixed glial cultures generated from obese mice retain elevated cytokine expression, although low testosterone effects do not persist ex vivo. Primary neurons co-cultured with glial cultures generated from high-fat fed animals exhibit reduced survival and poorer neurite outgrowth. In addition, low testosterone and diet-induced obesity combine to increase inflammation and evidence of nerve damage in the peripheral nervous system.

CONCLUSIONS

Testosterone and diet-induced obesity independently and cooperatively regulate neuroinflammation in central and peripheral nervous systems, which may contribute to observed impairments in neural health. Together, our findings suggest that low testosterone and obesity are interactive regulators of neuroinflammation that, in combination with adipose-derived inflammatory pathways and other factors, increase the risk of downstream disorders including type 2 diabetes and Alzheimer's disease.

Myelin protein zero: mutations in the cytoplasmic domain interfere with its cellular trafficking

The cytoplasmic domain of myelin protein zero (MPZ), the principal protein of peripheral myelin, undergoes phosphorylation on several serine residues and a tyrosine group that is maximal during peak nerve myelination. Mutations that could affect MPZ phosphorylation cause the inherited neuropathy, Charcot-Marie-Tooth disease Type 1B. To investigate a possible role for phosphorylation in regulation of MPZ trafficking within the cell, we expressed wild-type and mutated MPZ-enhanced green fluorescent protein (GFP) fusion proteins in cultured Schwann-like cells. Whereas wild-type protein is present almost entirely at the cell surface, mutation of serine 204 to alanine or at a nearby presumed PKC substrate motif (198RSTK201) causes 40-60% of protein to be retained in the cytoplasm. Mutation of S204 to aspartate, which introduces a permanent negative charge, also impairs MPZ movement to the plasma membrane. In contrast, tyrosine 191 mutation has no effect on MPZ cellular distribution. Simultaneous alteration of S204 and Y191 produces much less perturbation of MPZ trafficking than mutation of S204 alone. Colocalization studies showed that mutated MPZ-EGFP trapped in the cytoplasm associates with all organelles in the secretory pathway. Previous studies have shown that cytoplasmic mutations at serine, but not tyrosine phosphorylation sites, abolish MPZ adhesive properties. Our results suggest that this loss of adhesion may be due, at least in part, to a failure of sufficient MPZ to reach the cell surface and that this impaired trafficking is associated with deficient serine phosphorylation in the cytoplasmic domain.

C-peptide and diabetic neuropathy

Diabetic polyneuropathy (DPN) is the most common chronic complication of diabetes and affects Type 1 diabetic patients disproportionately. In the last two decades it has become increasingly evident that underlying metabolic, molecular and functional mechanisms and, ultimately, structural changes differ in DPN between the two major types of diabetes. In Type 1 diabetes, impaired insulin/C-peptide action has emerged as a prominent pathogenetic factor. C-peptide was long considered to be biologically inactive. During the last number of years it has been shown to have a number of insulin-like effects but without affecting blood glucose levels. Preclinical studies have demonstrated effects on Na(+)/K(+)-ATPase activity, endothelial nitric oxide synthase, expression of neurotrophic factors and regulation of molecular species underlying the degeneration of the nodal apparatus in Type 1 diabetic nerves, as well as DNA binding of transcription factors and modulation of apoptotic phenomena. In animal studies, these effects have translated into protection and improvement of functional abnormalities, promotion of nerve fibre regeneration, protection of structural changes and amelioration of apoptotic phenomena targeting central and peripheral nerve cell constituents. Several small-scale clinical trials confirm these beneficial effects on autonomic and somatic nerve function and blood flow in a variety of tissues. Therefore, evidence to date indicating that replacement of C-peptide in patients with Type 1 diabetes will retard and prevent chronic complication is real and encouraging. Large-scale clinical trials necessary to bring this natural substance into the clinical arena should, therefore, be encouraged and accelerated.

Protein kinase C changes in diabetes: is the concept relevant to neuropathy?

Protein kinase C (PKC) comprises a superfamily of isoenzymes, many of which are activated by 1,2-diacylglycerol (DAG) in the presence of phosphatidylserine. In order to be capable of DAG activation, PKC must first undergo a series of phosphorylation at three conserved sites. PKC isoforms phosphorylate a wide variety of intracellular target proteins and have multiple functions in signal transduction-mediated cellular regulation. An elevation in DAG levels and an increase in composite PKC activity and/or certain isoforms occurs in several nonneural tissues from diabetic animals, including the vasculature. The ability of isoform-specific PKC inhibitors to antagonize diabetes-induced abnormalities has implicated altered PKC beta activity in the onset of several diabetic complications, In contrast to many other tissues, DAG levels fall in diabetic nerve and a consistent pattern of change in PKC activity has not been observed. Treatments that alter PKC activity affect nerve Na+, K+-ATPase activity, but the mechanism involved is not well understood, Inhibition of PKC beta in diabetic rats appears to correct reduced nerve blood flow and decreased nerve conduction velocity. These and other findings indicate that changes in the neurovasculature exert adverse effects during the pathogenesis of diabetic neuropathy. Still unresolved is a clear-cut role for PKC in the development of abnormalities in neural cell metabolism. Further progress will depend on a more complete understanding of the functions of individual PKC isoforms in nerve. Future investigation could focus profitably on biochemical processes in nerve cells that modulate PKC activity and that are altered in diabetes, such as vascular endothelial growth factor levels and production of reactive oxygen species arising from oxidative stress.

Myelin P0: new knowledge and new roles

Protein zero (P0) is an integral transmembrane glycoprotein that serves as the major protein component of peripheral nerve myelin and is a member of the immunoglobulin (IgG) gene superfamily. As a cell adhesion molecule, P0 mediates homophilic adhesive interactions between Schwann cell plasma membranes and is a key structural constituent of both the major dense line and intraperiod line of compact myelin. Both the extracellular and cytoplasmic domains contribute to these interactions and evidence indicates that the post-translational modifications of the molecule, including glycosylation, acylation and phosphorylation, play an important modulatory role in adhesion and likely in the proper trafficking of P0 from the endoplasmic reticulum to the plasma membrane as well. Structural and genetic studies indicate that mutations in P0 producing human demyelinating diseases probably do so by perturbing or preventing homophilic interactions during myelination, or by producing cellular toxicity or an unstable myelin sheath. A variety of transcription factors, growth factors and neurosteroids both directly and indirectly influence P0 gene expression during maturation of the myelinating Schwann cell. Besides its structural function in myelin, P0 may have roles in the delivery of other Schwann cell proteins to their proper location, especially at or near nodes of Ranvier, and in neuronal-glial interactions.

Mutations in the cytoplasmic domain of P0 reveal a role for PKC-mediated phosphorylation in adhesion and myelination

Mutations in P0 (MPZ), the major myelin protein of the peripheral nervous system, cause the inherited demyelinating neuropathy Charcot-Marie-Tooth disease type 1B. P0 is a member of the immunoglobulin superfamily and functions as a homophilic adhesion molecule. We now show that point mutations in the cytoplasmic domain that modify a PKC target motif (RSTK) or an adjacent serine residue abolish P0 adhesion function and can cause peripheral neuropathy in humans. Consistent with these data, PKCalpha along with the PKC binding protein RACK1 are immunoprecipitated with wild-type P0, and inhibition of PKC activity abolishes P0-mediated adhesion. Point mutations in the RSTK target site that abolish adhesion do not alter the association of PKC with P0; however, deletion of a 14 amino acid region, which includes the RSTK motif, does abolish the association. Thus, the interaction of PKCalpha with the cytoplasmic domain of P0 is independent of specific target residues but is dependent on a nearby sequence. We conclude that PKC-mediated phosphorylation of specific residues within the cytoplasmic domain of P0 is necessary for P0-mediated adhesion, and alteration of this process can cause demyelinating neuropathy in humans.

Tyrosine phosphorylation of myelin protein PO

Po (M(r) 30 kDa), the major protein component of peripheral nervous system (PNS) myelin, is known to be phosphorylated by protein kinase C on serine residues at multiple sites. This study was conducted to assess whether other amino acids might be phosphorylated in the protein. Segments of rat sciatic nerve were incubated with 32P in either the presence or absence of phorbol ester. Labeled Po was isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subjected to partial acid hydrolysis. Upon separation of the hydrolysis products by either thin-layer electrophoresis or thin-layer chromatography, a radioactive spot was detected which comigrated with authentic phosphotyrosine. In other experiments, nerves were incubated with the tyrosine phosphatase inhibitors vanadate or vanadyl hydroperoxide (pervanadate). When the nerve homogenate proteins were separated on gels and probed with a monoclonal antibody to phosphotyrosine on Western blots, a positive immune reaction was obtained for a protein species which migrated with the same mobility as PO on Coomassie Blue-stained gels. In the absence of 2-mercaptoethanol, this immunoreactive band displayed increased mobility on gels which is characteristic of the migration pattern of Po. The same immunostaining results were obtained using a purified peripheral myelin fraction prepared from nerve homogenates. Furthermore, the positions of immunoreactive bands produced by anti-Po and antiphosphotyrosine antibodies coincided on the same immunoblot of myelin proteins and purified Po. These data indicate that one or more tyrosyl residues in Po can be phosphorylated in intact sciatic nerve.

Phosphorylation of myelin protein: recent advances

Since it was first described 25 years ago, phosphorylation has come to be recognized as a widespread and dynamic post-translation modification of myelin proteins. In this review, the phosphorylation characteristics of myelin basic protein, protein zero (P0), myelin-associated glycoprotein and 2'3' cyclic nucleotide 3'-phosphodiesterase are summarized. Emphasis is placed on recent advances in our knowledge concerning the protein kinases involved and the sites of phosphorylation in the amino acid sequences, where known. The possible roles of myelin protein phosphorylation in modulating myelin structure, the process of myelin assembly and mediation of signal transduction events are discussed.