Apolipoprotein E is released by rat sciatic nerve during segmental demyelination and remyelination

Spectral Characterization of Stem Cell-Derived Myelination within the Injured Adult PNS Using the Solvatochromic Dye Nile Red

Background: Myelin is an essential component of the peripheral and central nervous system, enabling fast axonal conduction and supporting axonal integrity; limited tools exist for analysis of myelin composition in-vivo. Objective: To demonstrate that the photophysical properties of myelin-incorporated solvatochromic dyes can be exploited to probe the biochemical composition of living peripheral nerve myelin at high spatial resolution. Methods: Using the myelin-incorporated fluorescent dye Nile Red we sequentially analyzed the spectral characteristics of remyelinating myelin membranes both in-vitro and in-vivo, including in living rats. Results: We demonstrated a consistent bi-phasic evolution of emission spectra during early remyelination, and visually report the reliable biochemical flux of myelin membrane composition in-vitro and in-vivo. Conclusions: Solvatochromic spectroscopy enables the analysis of myelin membrane maturity during remyelination, and can be performed in-vivo. As the formation of myelin during early-to-late remyelination likely incorporates fluctuating fractions of lipophilic components and changes in lateral membrane mobility, we propose that our spectrochemical data reflects the observation of these biochemical processes.

Neuronal Regulation of Neuroprotective Microglial Apolipoprotein E Secretion in Rat In Vitro Models of Brain Pathophysiology

Apolipoprotein E (ApoE) is mainly secreted by glial cells and is involved in many brain functions, including neuronal plasticity, β-amyloid clearance, and neuroprotection. Microglia--the main immune cells of the brain--are one source of ApoE, but little is known about the physiologic regulation of microglial ApoE secretion by neurons and whether this release changes under inflammatory or neurodegenerative conditions. Using rat primary neural cell cultures, we show that microglia release ApoE through a Golgi-mediated secretion pathway and that ApoE progressively accumulates in neuroprotective microglia-conditioned medium. This constitutive ApoE release is negatively affected by microglial activation both with lipopolysaccharide and with ATP. Microglial ApoE release is stimulated by neuron-conditioned media and under coculture conditions. Neuron-stimulated microglial ApoE release is mediated by serine and glutamate through N-methyl-D-aspartate receptors and is differently regulated by activation states (i.e. lipopolysaccharide vs ATP) and by 6-hydroxydopamine. Microglial ApoE silencing abrogated protection of cerebellar granule neurons against 6-hydroxydopamine toxicity in cocultures, indicating that microglial ApoE release is neuroprotective. Our findings shed light on the reciprocal cross-talk between neurons and microglia that is crucial for normal brain functions. They also open the way for the identification of possible pharmacologic targets that can modulate neuroprotective microglial ApoE release under pathologic conditions.

APOE gene polymorphisms and diabetic peripheral neuropathy

Genetic factors may influence the natural course of diabetic peripheral neuropathy and explain some of its variability. The aim of this review was to examine the association between apolipoprotein E (apoE) gene polymorphisms and diabetic peripheral neuropathy. Four relevant studies were identified. The two earlier works provided evidence that the ɛ4 allele is a risk factor for this complication, while the two more recent studies were negative. Important differences in the methodology used and in the populations included are obvious, rendering difficult the comparison between studies. In conclusion, the association between APOE gene polymorphisms and diabetic peripheral neuropathy is still unclear. Available evidence is rather limited and results have so far been contradictory. Future studies should employ more robust methodology, adjusting for potential confounders and for the prevalence of neuropathy in the general population with diabetes.

The ε4 allele of the APOE gene is associated with more severe peripheral neuropathy in type 2 diabetic patients

We examined the association between the ε4 allele of the apolipoprotein E gene and severity of peripheral neuropathy in 234 patients with type 2 diabetes mellitus (T2DM). Based on the Neuropathy Disability Score (NDS), patients were divided into group A (NDS ≤ 6: mild or no neuropathy) and group B (NDS > 6: severe neuropathy). In each group, patients were further divided into ε4 carriers and non-ε4 carriers. In multivariate analysis, a more than 5-fold increased risk of severe neuropathy was associated with ε4 carrier status (adjusted odds ratio [aOR]: 5.26, 95% confidence interval [CI]: 2.24-12.31, P = .0001). The other significant risk factors for severe neuropathy included male gender (aOR: 2.08, 95% CI: 1.05-4.14, P = .036), diabetes duration (aOR: 1.05, 95% CI: 1.00-1.09, P = .039), and hemoglobin A1c (aOR: 1.32, 95% CI: 1.05-1.66, P = .020). In conclusion, the ε4 carrier status appears to be associated with severe peripheral neuropathy in T2DM.

ApoE isoform-specific regulation of regeneration in the peripheral nervous system

Apolipoprotein E (apoE) is a 34 kDa glycoprotein with three distinct isoforms in the human population (apoE2, apoE3 and apoE4) known to play a major role in differentially influencing risk to, as well as outcome from, disease and injury in the central nervous system. In general, the apoE4 allele is associated with poorer outcomes after disease or injury, whereas apoE3 is associated with better responses. The extent to which different apoE isoforms influence degenerative and regenerative events in the peripheral nervous system (PNS) is still to be established, and the mechanisms through which apoE exerts its isoform-specific effects remain unclear. Here, we have investigated isoform-specific effects of human apoE on the mouse PNS. Experiments in mice ubiquitously expressing human apoE3 or human apoE4 on a null mouse apoE background revealed that apoE4 expression significantly disrupted peripheral nerve regeneration and subsequent neuromuscular junction re-innervation following nerve injury compared with apoE3, with no observable effects on normal development, maturation or Wallerian degeneration. Proteomic isobaric tag for relative and absolute quantitation (iTRAQ) screens comparing healthy and regenerating peripheral nerves from mice expressing apoE3 or apoE4 revealed significant differences in networks of proteins regulating cellular outgrowth and regeneration (myosin/actin proteins), as well as differences in expression levels of proteins involved in regulating the blood-nerve barrier (including orosomucoid 1). Taken together, these findings have identified isoform-specific roles for apoE in determining the protein composition of peripheral nerve as well as regulating nerve regeneration pathways in vivo.

Apolipoprotein E polymorphism and silent microangiopathy-related cerebral damage. Results of the Austrian Stroke Prevention Study

BACKGROUND AND PURPOSE

Microangiopathy-related cerebral damage (MARCD) includes white matter abnormalities and lacunar infarctions and represents a common MRI observation in subjects above 50 years of age. The risk factors of such brain abnormalities are not fully determined. The goal of this study was to determine whether the genetic heterogeneity of apolipoprotein E (apoE) contributes to the occurrence of MARCD.

METHODS

Brain MRI (1.5 T) was performed in 280 individuals (ages 50 to 75 years) without neuropsychiatric disease randomly selected from the official register of residents of the city of Graz, Austria. All study participants underwent apoE genotyping, carotid Doppler sonography, electrocardiography, echocardiography, and a complete blood chemistry panel. MARCD was defined as evidence of early confluent and confluent white matter hyperintensities or lacunes. Carotid atherosclerosis was graded on a five-point scale ranging from not present (0) to complete occlusion (5).

RESULTS

MARCD occurred in 61 individuals (21%). The distribution of apoE genotypes differed significantly between subjects with and without MARCD (P = .036). Subjects with such findings more commonly had the epsilon 2/epsilon 3 genotype (24.6% versus 10%) at similar frequencies of genotypes containing the epsilon 4 allele. The epsilon 2/epsilon 3 genotype was associated with lower levels of total cholesterol (P = .0009), LDL cholesterol (P = .00001), and apolipoprotein B (P = .00001). Also, there was a nonsignificant trend toward less cardiac disease. Other major vascular risk factors and carotid abnormalities were similar among the various genotypes. Multiple logistic regression analysis created a model of significant MARCD predictors, including age (odds ratio [OR], 1.1 per year), hypertension (OR, 3.4), and the apoE epsilon 2/epsilon 3 genotype (OR, 3.0).

CONCLUSIONS

These data suggest an association between the apoE epsilon 2/epsilon 3 genotype and MARCD despite favorable effects on the lipid profile and cardiac disease.

Macrophage recruitment in different models of nerve injury: lysozyme as a marker for active phagocytosis

Macrophages play critical roles in both degenerative and regenerative processes following peripheral nerve injury. These include phagocytosis of debris, stimulation of Schwann cell dedifferentiation and proliferation, and salvage of myelin lipids for reutilization during regeneration. To better define the role of macrophages, we studied models of primary demyelination (tellurium intoxication) and secondary demyelination (nerve crush and cut). Sections of paraformaldehyde-fixed rat sciatic nerves at various stages of demyelination were stained with monoclonal antibody ED1, a standard macrophage marker, and a polyclonal antiserum specific for lysozyme (LYS). Near the peak of demyelination in all three models, LYS immunoreactivity colocalized with ED1 staining. Macrophages present in nerve after the period of maximal phagocytosis of myelin were much less immunoreactive for LYS. These results suggest LYS is a good marker for macrophages which are active in phagocytosis. Tellurium intoxication, which causes synchronous demyelination and subsequent remyelination of only about 25% of myelin internodes, recruited more macrophages (and induced more lysozyme expression) than either nerve crush or cut, which cause demyelination of all internodes distal to the injury site. This suggests that Schwann cells may recruit macrophages soon after metabolic insult and prior to actual demyelination. The final signal for macrophage recruitment is not directly related to the amount of damaged myelin. In the models listed above, steady state mRNA levels for apolipoprotein E (ApoE; possible mediator of cholesterol salvage), LYS, and P0 (major structural protein of PNS myelin), were analyzed by Northern blot analysis. LYS mRNA levels peaked sharply in all models, with a temporal pattern consistent with the expected presence of activated, phagocytic macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)

Schwann cell nerve growth factor receptor expression during initiation of remyelination

Initiation of remyelination is a promising therapeutic strategy to treat patients with demyelinating diseases, but specific factors that control remyelination are not clear. We first reported that expression of nerve growth factor receptor (NGFR) was increased during initiation of remyelination (Fan and Gelman, Journal of Neuropathology and Experimental Neurology 49: 312, 1990). In this study, we characterized the timing and cellular localization of NGFR expression in a model of segmental demyelination and remyelination using immunohistochemistry and monoclonal antibody 192-IgG, and compared it to an axonal neuropathy. At the onset of demyelination induced by tellurium (Te) poisoning, NGFR antigenicity was selectively expressed within and around demyelinating internodes in rat sciatic nerve. Dual fluorescence staining with myelin-specific antigen showed that NGFR colocalized with demyelinated internodal units with relative specificity; Schwann cell S-100 protein showed a concomitant down-regulation in injured internodes. Peak expression of NGFR occurred during the transition between demyelination and remyelination (day 8 of Te), then declined exponentially. NGFR expression was most prominent in the cytoplasm of daughter Schwann cells as they established contact with denuded axons, and was sharply repressed as compact myelin began to accumulate. Rare colocalization with neurofilament antigens revealed intraxonal deposits of NGFR in segmental demyelination. In the nerve crush model, Schwann cell NGFR expression was not segmentally distributed and was upregulated for a longer period of time. Our data establish that NGFR expression in the peripheral nervous system is not strictly linked to axon elongation, and that it probably functions during the initiation of myelination.

Tellurium-induced alterations in 3-hydroxy-3-methylglutaryl-CoA reductase gene expression and enzyme activity: differential effects in sciatic nerve and liver suggest tissue-specific regulation of cholesterol synthesis

The demyelination of peripheral nerves that results from exposure of developing rats to tellurium is due to inhibition of squalene epoxidase, a step in cholesterol biosynthesis. In sciatic nerve, cholesterol synthesis is greatly depressed, whereas in liver, some compensatory mechanism maintains normal levels of cholesterol synthesis. This tissue specificity was further explored by examining, in various tissues, gene expression and enzyme activity of 3-hydroxy-3-methylglutaryl-CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis. Exposure to tellurium resulted in pronounced increases in both message levels and enzyme activity in liver, the expected result consequent to up-regulation of this enzyme in response to decreasing levels of intracellular sterols. In contrast to liver, levels of mRNA and enzyme activity in sciatic nerve were both decreased during the tellurium-induced demyelinating period. The temporal pattern of changes in 3-hydroxy-3-methylglutaryl-CoA reductase message levels in sciatic nerve seen following exposure to tellurium was similar to the down-regulation seen for mRNA specific for PNS myelin proteins. Possible mechanisms for differential control of cholesterol biosynthesis in sciatic nerve and liver are discussed.

Tellurium blocks cholesterol synthesis by inhibiting squalene metabolism: preferential vulnerability to this metabolic block leads to peripheral nervous system demyelination

Inclusion of 1.1% elemental tellurium in the diet of postweanling rats produces a peripheral neuropathy due to a highly synchronous primary demyelination of sciatic nerve; this demyelination is followed closely by remyelination. Sciatic nerves from animals fed tellurium for various times were removed and incubated ex vivo for 1 h with [14C]acetate, and radioactivity incorporated into individual lipid classes was determined. In nerves from rats exposed to tellurium, there was a profound and selective block in the conversion of radioactive acetate to cholesterol. Another radioactive precursor, [3H]water, gave similar results. We suggest that tellurium feeding inhibits squalene epoxidase activity and that the consequent lack of cholesterol destabilizes myelin, thereby causing destruction of the larger internodes. Ex vivo incubation experiments were also carried out with liver slices. As with nerve, tellurium feeding caused accumulation in squalene of label from radioactive acetate, whereas labeling of cholesterol was greatly inhibited. Unexpectedly, however, incorporation of label from [3H]water into both squalene and cholesterol was increased. Relevant is the demonstration that liver was the primary site of bulk accumulation of squalene, which accounted for 10% of liver dry weight at 5 days. Thus, accumulation of squalene (and other mechanisms, possibly including up-regulation of cholesterol biosynthetic pathways) drives squalene epoxidase activity at normal levels in liver even in the presence of inhibitors of this enzyme. This is reflected by continuing incorporation of [3H]water into cholesterol; incorporation of this precursor takes place at many of the postsqualene biosynthetic steps for sterol formation. [14C]Acetate entering the sterol pathway before squalene in liver is greatly diluted in specific activity when it reaches the large squalene pool, and thus increased squalene epoxidase activity does not transfer significant 14C label to sterols. In contrast to the situation with liver, synthesis of sterols is markedly depressed in sciatic nerve, and squalene does not accumulate to high levels.

Cholesterol from degenerating nerve myelin becomes associated with lipoproteins containing apolipoprotein E

Apolipoprotein E is synthesized and secreted by rat sciatic nerve consequent to several types of injury. It has been proposed that endoneurial apolipoprotein E, in analogy to its role in systemic cholesterol transport, is involved in the salvage and reutilization of myelin cholesterol during degeneration and regeneration. To test this hypothesis, nerve lipids were prelabeled via intraneural injection of [3H]acetate. Four weeks later the nerves were crushed. From 1 to 12 weeks later, crushed nerves were examined for extracellular lipoprotein-bound cholesterol label. By 2 weeks after injury, 10% of the endoneurial lipid label was in a soluble form that was releasable into incubation medium. This released fraction was enriched in labeled cholesterol, and its labeled lipid composition was constant, in contrast to the changing distribution of label in the nerve with time after injury. On a KBr gradient, the released lipid label cofractionated with the released apolipoprotein E at densities similar to that of lipoproteins. These data indicate that at least some myelin cholesterol in injured nerve becomes associated with apolipoprotein E-containing lipoproteins and thus is available for reutilization via the hypothesized model.

Clinical relevance of the quantification of apolipoprotein E in cerebrospinal fluid

Apolipoprotein E was measured in paired sera and cerebrospinal fluid samples from 483 neurological patients. The average apolipoprotein E concentration was 7.5 (+/- 3.1) mg/l in cerebrospinal fluid and 93.5 (+/- 29.7) mg/l in serum. Mean apolipoprotein B concentrations in 88 patients were 0.77 +/- 3.4 mg/l in cerebrospinal fluid and 1.06 +/- 0.31 g/l in serum. The apolipoprotein E concentration in cerebrospinal fluid was much greater than expected for passive diffusion from serum. By contrast to apolipoprotein B, there was no correlation between the apolipoprotein E cerebrospinal fluid/serum concentration quotient and the albumin concentration quotient. This suggests that apolipoprotein E in cerebrospinal fluid, unlike apolipoprotein B, is locally synthesized and that the use of a cerebrospinal fluid/serum concentration quotient is unnecessary. In a control group (n = 64) the mean cerebrospinal fluid apolipoprotein E value (+/- SD) was 5.9 (1.6) mg/l. Elevated cerebrospinal fluid apolipoprotein E concentrations were observed in acute (n = 22, 12.5 +/- 6.2 mg/l) and chronic inflammatory central nervous system diseases (n = 15, 10.4 +/- 2.4 mg/l).

Cholesterol synthesis is down-regulated during regeneration of peripheral nerve

The discovery of apolipoprotein E synthesis and secretion by injured peripheral nerve to the hypothesis that endoneurial apolipoprotein E serves to salvage degenerating myelin cholesterol. This salvaged cholesterol could then be reutilized by Schwann cells during remyelination via uptake through low-density lipoprotein receptors. As a test of this hypothesis, we measured the rate of cholesterol synthesis in rat sciatic nerve endoneurium during development and at various times following a crush injury at 50 days of age. In control nerves [14C]acetate incorporation into cholesterol and 3-hydroxy-3-methylglutaryl-CoA reductase activity were closely linked throughout development, indicating that reductase activity in nerve, as in other tissues, is a good indicator of cholesterol's synthetic rate. In the crushed nerves cholesterol synthesis fell to nearly zero during the first week after the crush. There was a partial recovery during the second to fourth weeks, but unlike that of other lipids, cholesterol synthesis remained well below control nerve values throughout most of the 15-week post-crush period examined. Thus, cholesterol synthesis is at very low levels during the myelination of regenerating axons. These results are consistent with a receptor-mediated down-regulation of cholesterol synthesis by lipoproteins, and would be expected if Schwann cells were utilizing an external source of cholesterol as postulated above.

Macrophage-like cells from explant cultures of rat sciatic nerve produce apolipoprotein E

Apolipoprotein E is synthesized and secreted by degenerating peripheral nerve, but the role of resident endoneurial cells in this process is not clear. To exclude the involvement of nonresident cells, we examined the cellular source of endoneurial apolipoprotein E in explant cultures of rat sciatic nerve. The cellular outgrowth from these explant cultures released apolipoprotein E into the culture medium. The cellular outgrowth contained fibroblasts, Schwann cells, and a population of cells with many phenotypic characteristics of macrophages, including the production of apolipoprotein E. No other cell type in the cultures appeared to contribute to this production. These data suggest that apolipoprotein E is produced by resident endoneurial cells in explant cultures and that these cells are macrophages.