Interleukin-6 (IL-6) receptor/IL-6 fusion protein (Hyper IL-6) effects on the neonatal mouse brain: possible role for IL-6 trans-signaling in brain development and functional neurobehavioral outcomes

Adverse neurodevelopmental outcomes are linked to perinatal production of inflammatory mediators, including interleukin 6 (IL-6). While a pivotal role for maternal elevation in IL-6 has been established in determining neurobehavioral outcomes in the offspring and considered the primary target mediating the fetal inflammatory response, questions remain as to the specific actions of IL-6 on the developing brain. CD-1 male mice received a subdural injection of the bioactive fusion protein, hyper IL-6 (HIL-6) on postnatal-day (PND)4 and assessed from preweaning until adulthood. Immunohistochemical evaluation of astrocytes and microglia and mRNA levels for pro-inflammatory cytokines and host response genes indicated no evidence of an acute neuroinflammatory injury response. HIL-6 accelerated motor development and increased reactivity to stimulation and number of entries in a light/dark chamber, decreased ability to learn to withhold a response in passive avoidance, and effected deficits in social novelty behavior. No changes were observed in motor activity, pre-pulse startle inhibition, or learning and memory in the Morris water maze or radial arm maze, as have been reported for models of more severe developmental neuroinflammation. In young animals, mRNA levels for MBP and PLP/DM20 decreased and less complexity of MBP processes in the cortex was evident by immunohistochemistry. The non-hydroxy cerebroside fraction of cerebral lipids was increased. These results provide evidence for selective effects of IL-6 signaling, particularly trans-signaling, in the developing brain in the absence of a general neuroinflammatory response. These data contribute to our further understanding of the multiple aspects of IL-6 signaling in the developing brain.

Normal metabolism but different physical properties of myelin from mice deficient in proteolipid protein

Proteolipid protein (PLP) is the primary protein component of CNS myelin, yet myelin from the PLP(null) mouse has only minor ultrastructural abnormalities. Might compensation for a potentially unstable structure involve increased myelin synthesis and turnover? This was not the case; neither accumulation nor in vivo synthesis rates for the myelin-specific lipid cerebroside was altered in PLP(null) mice relative to wild-type (wt) animals. However, the yield of myelin from PLP(null) mice, assayed as levels of cerebroside, was only about 55% of wt control levels. Loss of myelin occurred during initial centrifugation of brain homogenate at 20,000g for 20 min, which is sufficient to sediment almost all myelin from wt mice. Cerebroside-containing fragments from PLP(null) mice remaining in the supernatant could be sedimented by more stringent centrifugation, 100,000g for 60 min. Both the rapidly and the more slowly sedimenting cerebroside-containing membranes banded at the 0.85/0.32 M sucrose interface of a density gradient, as did myelin from wt mice. These results suggest at least some myelin from PLP(null) mice differs from wt myelin with respect to physical stability (fragmented into smaller particles during dispersion) and/or density. Alternatively, slowly sedimenting cerebroside-containing particles could be myelin precursor membranes that, lacking PLP, were retarded in their processing toward mature myelin and thus differ from mature myelin in physical properties. If this is so, recently synthesized cerebroside should be preferentially found in these "slower-sedimenting" myelin precursor fragments. Metabolic tracer experiments showed this was not the case. We conclude that PLP(null) myelin is physically less stable and/or less dense than wt myelin.

Alterations in metabolism and gene expression in brain regions during cuprizone-induced demyelination and remyelination

Exposure of mice to the copper chelator, cuprizone, results in CNS demyelination. There is remyelination after removal of the metabolic insult. We present brain regional studies identifying corpus callosum as particularly severely affected; 65% of cerebroside is lost after 6 weeks of exposure. We examined recovery of cerebroside and ability to synthesize cerebroside and cholesterol following removal of the toxicant. The temporal pattern for concentration of myelin basic protein resembled that of cerebroside. We applied Affymetrix GeneChip technology to corpus callosum to identify temporal changes in levels of mRNAs during demyelination and remyelination. Genes coding for myelin structural components were greatly down-regulated during demyelination and up-regulated during remyelination. Genes related to microglia/macrophages appeared in a time-course (peaking at 6 weeks) correlating with phagocytosis of myelin and repair of lesions. mRNAs coding for many cytokines had peak expression at 4 weeks, compatible with intercellular signaling roles. Of interest were other genes with temporal patterns correlating with one of the three above patterns, but of function not obviously related to demyelination/remyelination. The ability to correlate gene expression with known pathophysiological events should help in elucidating further function of such genes as related to demyelination/remyelination.

Cerebroside synthesis as a measure of the rate of remyelination following cuprizone-induced demyelination in brain

We studied markers of myelin content and of the rate of myelination in brains of mice between 8 and 20 weeks of age. During the 12-week time-course, control animals showed slight increases in the content of oligodendroglial-specific cerebroside, as well as cholesterol (enriched in, but not specific to, myelin). In contrast, synthesis of these lipids, as assayed by in vivo incorporation of (3)H(2)O, was substantial, indicating turnover of 0.4% and 0.7% of total brain cerebroside and cholesterol, respectively, each day. We also studied mice exposed to a diet containing 0.2% of the copper chelator, cuprizone. After 6 weeks 20%, and by 12 weeks, over 30% of brain cerebroside was gone. Demyelination was accompanied by down-regulation of mRNA expression for enzymes controlling myelin lipid synthesis (ceramide galactosyl transferase for cerebroside; hydroxymethylglutaryl-CoA reductase for cholesterol), and for myelin basic protein. Synthesis of myelin lipids was also greatly depressed. The 20% cerebroside deficit consequent to 6 weeks of cuprizone exposure was restored 6 weeks after return to a control diet. During remyelination, expression of myelin-related mRNA species, as well as cerebroside and cholesterol synthesis were restored to normal. However, in contrast to the steady state metabolic turnover in the control situation, all the cerebroside and cholesterol made were accumulated. To the extent that accumulating cerebroside is targeted for eventual inclusion in myelin (discussed) the rate of its synthesis is proportional to remyelination. With our assay, in vivo rates of cerebroside synthesis can be determined for a time window of the order of hours. This offers greater temporal resolution and accuracy relative to classical methods assaying accumulation of myelin components at time intervals of several days. We propose this experimental design, and the reproducible cuprizone model, as appropriate for studies of how to promote remyelination.

Parameters related to lipid metabolism as markers of myelination in mouse brain

Myelination, during both normal development and with respect to disorders of myelination, is commonly studied by morphological and/or biochemical techniques that assay as their end-points the extent of myelination. The rate of myelination is potentially a more useful parameter, but it is difficult and time-consuming to establish, requiring a complete developmental study with labor-intensive methodology. We report herein development of methodology to assay the absolute rate of myelination at any desired time during development. This involves intraperitoneal injection of (3)H(2)O to label body water pools, followed by determination of label in the myelin-specific lipid, cerebroside. The absolute amount of cerebroside synthesized can then be calculated from the specific radioactivity of body water and knowledge of the number of hydrogens from water incorporated into cerebroside. During development, the rate of cerebroside synthesis correlated well with the rate of accumulation of the myelin-specific components, myelin basic protein and cerebroside. For purposes of control, we also tested other putative, albeit less quantitative, indices of the rate of myelination. Levels of mRNA for ceramide galactosyltransferase (rate-limiting enzyme in cerebroside synthesis) and for myelin basic protein did not closely correlate with myelination at all times. Cholesterol synthesis closely matched the rate of cholesterol accumulation but did not track well with myelination. Synthesis of fatty acids did not correlate well with accumulation of either fatty acids (phospholipids) or myelin markers. We conclude that measurement of cerebroside synthesis rates provides a good measure of the rate of myelination. This approach may be useful as an additional parameter for examining the effects of environmental or genetic alterations on the rate of myelination.

Diurnal and dietary-induced changes in cholesterol synthesis correlate with levels of mRNA for HMG-CoA reductase

We determined the extent to which diurnal variation in cholesterol synthesis in liver is controlled by steady-state mRNA levels for the rate-limiting enzyme in the pathway, hydroxymethylglutaryl (HMG)-CoA reductase. Rats 30 days of age and maintained on a low-cholesterol diet since weaning were injected intraperitoneally with (3)H(2)O. The specific radioactivity of the whole-body water pool soon became constant, allowing for expression of values for incorporation of label into cholesterol as absolute rates of cholesterol synthesis. In liver, there was a peak of cholesterol synthesis from 8 pm to midnight, a 4-fold increase over synthesis rates from 8 am to noon. Increases in synthesis were quantitatively in lock step with increases in mRNA levels for HMG-CoA reductase occurring 4 h earlier. In a parallel experiment, rats received 1% cholesterol in the diet from weaning to 30 days of age. Basal levels of hepatic cholesterol synthesis were greatly diminished and there was little diurnal variation of cholesterol synthesis or of levels of mRNA for HMG-CoA reductase. Levels of mRNA for the low density lipoprotein receptor and scavenger receptor-B1 (putative high density lipoprotein receptor) showed little diurnal variation, regardless of diet. This suggests that diurnal variation of hepatic cholesterol synthesis is driven primarily by varying the steady-state mRNA levels for HMG-CoA reductase. Other tissues were also examined. Adrenal gland also showed a 4-fold diurnal increase in accumulation of recently synthesized cholesterol. In contrast to liver, however, there was little corresponding change in mRNA expression for HMG-CoA reductase. Much of this newly synthesized cholesterol may be of hepatic origin, imported into adrenal by SR-B1, whose mRNA was up-regulated 2-fold. In brain, there was no diurnal variation in either cholesterol synthesis or mRNA expression, and no influence of high- or low-cholesterol diets on synthesis rates or HMG-CoA reductase mRNA levels.

Peripheral nerve regeneration and cholesterol reutilization are normal in the low-density lipoprotein receptor knockout mouse

Following peripheral nerve injury, cholesterol from degenerating myelin is retained locally within macrophages and subsequently reutilized by Schwann cells for synthesis of new myelin during nerve regeneration. Substantial evidence indicates this conservation and reutilization of cholesterol is accomplished via lipoprotein-mediated intercellular transport, although the identities of the lipoproteins and their receptors are unresolved. Because Schwann cells in regenerating nerve are reported to express the low-density lipoprotein (LDL) receptor (LDLR), we used the LDLR knockout mouse to examine the potential role of this receptor in cholesterol reutilization. Sciatic nerves were crushed in knockout and wild-type mice and examined 3 days to 10 weeks later. Morphometric analyses and measures of mRNA levels for myelin protein P(0), indicate that axon regeneration and myelination proceed normally in the LDLR knockout mouse. We therefore measured hydroxy-methylglutaryl-coenzyme A (HMG-CoA) reductase activity and mRNA levels to determine whether Schwann cells compensated for the absence of the LDLR by upregulating cholesterol synthesis. Unexpectedly, these measures remained at the same downregulated levels found in regenerating nerves of wild-type animals. The apparently normal nerve regeneration, coupled with the lack of any compensatory upregulation of cholesterol synthesis in the LDLR knockout mice, indicates that other lipoprotein receptors must be primarily involved in cholesterol uptake by Schwann cells.

Gene expression in brain during cuprizone-induced demyelination and remyelination

When C57BL/6J mice, 8 weeks of age, received 0.2% Cuprizone in their diet, extensive demyelination in corpus callosum was detectable after 3 weeks, and there was massive demyelination by 4 weeks. As expected, the accumulation of phagocytically active microglia/macrophages correlated closely with demyelination. When Cuprizone was removed from the diet, remyelination was soon initiated; after 6 weeks of recovery, myelin levels were near-normal and phagocytic cells were no longer prominent. Steady-state levels of mRNA for myelin-associated glycoprotein, myelin basic protein, and ceramide galactosyltransferase were already profoundly depressed after 1 week of Cuprizone exposure and were only 10-20% of control values after 2 weeks. Unexpectedly, upregulation of mRNA for these myelin genes did not correlate with initiation of remyelination but rather with accumulation of microglia/macrophages. After 6 weeks of exposure to Cuprizone, mRNA levels were at control levels or higher-in the face of massive demyelination. This suggests that in addition to effecting myelin removal, microglia/macrophages may simultaneously push surviving oligodendroglia or their progenitors toward myelination.

Control of cholesterol biosynthesis in Schwann cells

Cholesterol accounts for over one-fourth of total myelin lipids. We found that, during development of the rat sciatic nerve, expression of mRNA for hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme in cholesterol biosynthesis, was up-regulated in parallel with mRNA for P0, the major structural protein of PNS myelin, and with ceramide galactosyltransferase (CGT), the rate-limiting enzyme in cerebroside biosynthesis. To help establish the nature of this coordinate regulation of myelin-related genes, we examined their steady-state mRNA levels in cultured primary Schwann cells. We also assayed synthesis of cholesterol and cerebroside to distinguish how much control of synthetic activity for these two myelin lipids involved mRNA levels for HMG-CoA reductase and CGT, and how much involved post-mRNA control mechanisms. Addition of forskolin to cells cultured in media supplemented with normal calf serum resulted in up-regulation of P0 and CGT mRNA expression and cerebroside synthesis, without corresponding increases in HMG-CoA reductase mRNA or cholesterol synthesis. Cholesterol synthesis increased approximately threefold in Schwann cells cultured with lipoprotein-deficient serum, without any increase in HMG-CoA reductase mRNA. Furthermore, addition of either serum lipoproteins or 25-hydroxycholesterol decreased cholesterol synthesis without altering HMG-CoA reductase mRNA levels. We conclude that, as in other tissues, cholesterol synthesis in Schwann cells is regulated primarily by intracellular sterol levels. Much of this regulation occurs at posttranscriptional levels. Thus, the in vivo coordinate up-regulation of HMG-CoA reductase gene expression in myelinating Schwann cells is secondary to intracellular depletion of cholesterol, as it is compartmentalized within the myelin. It is probably not due to coordinate control at the level of mRNA expression.

Monocyte chemoattractant protein 1 is responsible for macrophage recruitment following injury to sciatic nerve

Following injury to the peripheral nervous system, circulating monocytes/macrophages are recruited to the damaged tissue, where they play vital roles during both nerve degeneration and subsequent regeneration. Monocyte chemoattractant protein-1 (MCP-1), a member of the C-C or beta-chemokine family, is a powerful leukocyte recruitment/activation factor that is relatively specific for monocytes/macrophages. Because these are the predominant leukocyte type recruited by injured nerve, we hypothesized that upregulation of MCP-1 expression is involved in recruitment of these cells. Indeed, assay of steady-state levels of MCP-1 mRNA in rat sciatic nerve during tellurium-induced primary demyelination indicated up-regulation of this chemokine with a peak after 3 days of tellurium exposure, preceding the peak of accumulation of phagocytic macrophages (assayed as lysozyme mRNA levels) by 6 days. Increasing levels of MCP-1 mRNA expression, induced by increasing levels of tellurium exposure, resulted in corresponding increases in subsequent recruitment of macrophages. In situ hybridization suggested that MCP-1 mRNA was localized in Schwann cells. No expression of MIP-2, which is a C-X-C or alpha-chemokine that is specific for recruitment of neutrophils, was detected, consistent with the lack of recruitment of significant numbers of these cells. In addition, we also investigated the response seen following nerve transection (axonal degeneration and secondary demyelination with no subsequent regeneration) and nerve crush (degeneration followed by regeneration). In these latter two nerve injury models, there was also a marked, early up-regulation of MCP-1 mRNA, with a time course that is compatible with a role for this chemokine in macrophage recruitment. We conclude that MCP-1 is involved in recruiting monocytes/macrophages to injured peripheral nerve and that the specificity of leukocyte types recruited results from specificity of chemokine production.

Regenerating sciatic nerve does not utilize circulating cholesterol

The rapid accumulation of myelin in the peripheral nervous system during the early postnatal period requires large amounts of cholesterol, a major myelin lipid. All of the cholesterol accumulating in the developing rat sciatic nerve is synthesized locally within the nerve, rather than being derived from the supply in lipoproteins in the systemic circulation (Jurevics and Morell, J. Lipid Res. 5:112-120; 1994). Since this lack of utilization of circulating cholesterol may relate to exclusion by the blood-nerve barrier, we examined the sources of cholesterol needed for regeneration following nerve injury, when the blood-nerve barrier is breached. One sciatic nerve was crushed or transected, and at various times later, the rate of cholesterol accumulation was compared with the rate of local in vivo synthesis of cholesterol within the nerve, utilizing intraperitoneally injected 3H2O as precursor. The accumulation of additional cholesterol in nerve during regeneration and remyelination could all be accounted for by that locally synthesized within the nerve. There was also an increase in cholesterol esters in injured nerve segments; in crushed nerves, these levels decreased during regeneration and remyelination, consistent with reutilization of cholesterol originally salvaged by phagocytic macrophages and Schwann cells. Thus, regeneration and remyelination following injury in sciatic nerve utilizes both salvaged cholesterol and cholesterol synthesized locally within the nerve, but not cholesterol from the circulation.

Carbon disulfide neurotoxicity in rats: IV. Increased mRNA expression of low-affinity nerve growth factor receptor--a sensitive and early indicator of PNS damage

Expression of the low-affinity nerve growth factor receptor (NGF-R) in the peripheral nervous system is regulated by Schwann cell-axonal contact. Steady-state mRNA levels for NGF-R are very low in the mature peripheral nervous system, but are markedly upregulated in sciatic nerve during both primary demyelination (tellurium exposure) and secondary demyelination (Wallerian degeneration). Upregulation also occurs in various subdegenerative axonopathy models where there is axonal atrophy, suggesting its usefulness as a marker for subtle perturbations in normal axon-Schwann cell interactions (Roberson et al., Mol Brain Res 1995; 28:231-238). To further test this hypothesis, we examined NGF-R mRNA expression in sciatic nerves of rats exposed to carbon disulfide (CS2), a toxicant known to cause a distal axonopathy. Adult rats were exposed to CS2 gas (50, 500, or 800 ppm, 6 hr/day, 5 days/wk) for 2-13 weeks. RNA was isolated from sciatic nerves and levels of mRNA for NGF-R determined by Northern blot analysis. NGF-R mRNA expression increased in a dose- and time-dependent manner. Message levels were already increased after 2 wks of exposure to 800 ppm CS2, and increased further with continued exposure. Morphological alterations were not apparent in the sciatic nerve, even at the highest dosage levels with the longest exposure times. Upregulation of NGF-R mRNA is thus an indicator of subtle alterations in the normal axon-Schwann cell relationship and provides a sensitive measure of CS2 neurotoxicity. Assay of this marker may also be useful as a rapid and very sensitive general screen for other compounds which are potentially toxic to the peripheral nervous system.

Tellurium causes dose-dependent coordinate down-regulation of myelin gene expression

Exposure of developing rats to a diet containing elemental tellurium systemically inhibits cholesterol synthesis at the level of squalene epoxidase. At high tellurium exposure levels (> 0.1% in the diet), there is an associated segmental demyelination of the PNS. Low levels of dietary tellurium (0.0001%) led to in vivo inhibition of squalene epoxidase activity in sciatic nerve, and inhibition increased with increasing exposure levels. With increasing dose and increasing exposure times, there was an increasing degree of demyelination and increasing down-regulation of mRNA levels for myelin P0 protein, ceramide galactosyltransferase (rate-limiting enzyme in cerebroside synthesis), and HMG-CoA reductase (rate-limiting enzyme in cholesterol synthesis). Because these were all down-regulated in parallel, we conclude there is coordinate regulation of the entire program for myelin synthesis in Schwann cells. An anomaly was that at early time points and low tellurium levels, mRNA levels for HMG-CoA reductase were slightly elevated, presumably in response to tellurium-induced sterol deficits. We suggest the eventual down-regulation relates to a separate mechanism by which Schwann cells regulate cholesterol synthesis, related to the need for coordinate synthesis of myelin components. Levels of mRNA for the low-affinity nerve growth factor receptor (indicator of alterations in axon-Schwann cell interactions) and for lysozyme (marker for phagocytic macrophages) were both up-regulated in a dose- and time-dependent manner which correlated with the presence of segmental demyelination. Levels of mRNA coding for myelin-related proteins were down-regulated at low tellurium exposure levels, without demyelination or up-regulation of nerve growth factor receptor. This suggests the down-regulation is related to the tellurium-induced cholesterol deficit, and not to the loss of axonal contact associated with early stages of demyelination or to the entry of activated macrophages.

Alterations in gene expression associated with primary demyelination and remyelination in the peripheral nervous system

Primary demyelination is an important component of a number of human diseases and toxic neuropathies. Animal models of primary demyelination are useful for isolating processes involved in myelin breakdown and remyelination because the complicating events associated with axonal degeneration and regeneration are not present. The tellurium neuropathy model has proven especially useful in this respect. Tellurium specifically blocks synthesis of cholesterol, a major component of PNS myelin. The resulting cholesterol deficit in myelin-producing Schwann cells rapidly leads to sychronous primary demyelination of the sciatic nerve, which is followed by rapid synchronous remyelination when tellurium exposure is discontinued. Known alterations in gene expression for myelin proteins and for other proteins involved in the sequence of events associated with demyelination and subsequent remyelination in the PNS are reviewed, and new data regarding gene expression changes during tellurium neuropathy are presented and discussed.

Tissue-specific coordinate regulation of enzymes of cholesterol biosynthesis: sciatic nerve versus liver

Exposure of weanling rats to a diet containing the element tellurium results in specific inhibition of squalene epoxidase, an obligate enzyme in cholesterol biosynthesis. Liver responds to the resulting intracellular sterol deficit by up-regulating, in parallel and to the same extent, expression of mRNA for squalene epoxidase and for HMG-CoA reductase, the major rate-limiting enzyme in the pathway. This increased mRNA expression, coupled with additional translational and posttranslational activation of the pathway allows normal levels of cholesterol synthesis in liver despite tellurium-induced inhibition of squalene epoxidase. The response to tellurium challenge in sciatic nerve is very different. In this tissue, cholesterol synthesis is prominent because of the large amount of cholesterol required for synthesis and maintenance of myelin. Although nerve shows an initial (at 1 day) up-regulation of mRNA expression for both enzymes in response to tellurium exposure, this is followed quickly by parallel down-regulation of both enzymes, in concert with down-regulation of mRNA expression for myelin proteins. We suggest that the tellurium-induced deficit in sterols leads to a coordinate down-regulation of synthesis of myelin components. The initial early up-regulation of cholesterol biosynthesis in sciatic nerve due to the cholesterol deficit is countered by down-regulation which is coordinated with overall control of the program of myelin assembly. This tissue-specific control of cholesterol synthesis in sciatic nerve is a point of vulnerability to toxicants, and may be related to the need for coordinate synthesis of all components of myelin.

Schwann cells as targets for neurotoxicants

Schwann cells subserve a variety of roles in the peripheral nervous system (PNS), including ionic homeostasis, and protection and possible metabolic support of axons. It is, however, the myelinating subtype of these glia which appear most sensitive to toxic insults. Myelinating Schwann cells must synthesize large amounts of myelin proteins (P0 is the major myelin protein) and lipids (cholesterol is most prominent) within a short, tightly-programmed developmental window. Schwann cells are preferentially vulnerable to neurotoxic insults during this period of maximal metabolic stress. The hydrophobicity of myelin (reservoir for lipid-soluble toxicants) and possible specialized energy-requiring mechanisms for maintenance of myelin structure are points of vulnerability for the mature myelin sheath. Fortunately, Schwann cells are highly plastic; they dedifferentiate to more primitive precursor cells following a demyelinating insult, but are able to redifferentiate and remyelinate axons during subsequent nerve regeneration. For study of such processes, a useful model system is exposure of developing rats to the element tellurium; this produces a highly synchronous primary demyelination of PNS which is followed closely by rapid remyelination. Interpretation of the metabolic events involved in simplified by the nearly complete lack of axonal degeneration. We have uncovered the primary lesion (block in cholesterol biosynthesis) and elucidated some of the steps involved in the demyelination-remyelination response. Particularly useful have been studies of gene expression of certain proteins (nerve growth factor receptor, myelin proteins, macrophage-specific lysozyme) which have enabled us to define some of the cellular responses to this toxicant-induced injury. A generally applicable result that has emerged from these and other similar studies is that upregulation of NGF-R mRNA is a sensitive marker of nerve damage; it may be useful as a screen for potentially neurotoxic compounds.

Tellurite specifically affects squalene epoxidase: investigations examining the mechanism of tellurium-induced neuropathy

A peripheral neuropathy characterized by a transient demyelinating/remyelinating sequence results when young rats are fed a tellurium-containing diet. The neuropathy occurs secondary to a systemic block in cholesterol synthesis. Squalene accumulation suggested the lesion was at the level of squalene expoxidase, a microsomal monooxygenase that uses NADPH cytochrome P450 reductase to receive its necessary reducing equivalents from NADPH. We have now demonstrated directly specificity for squalene epoxidase; our in vitro studies show that squalene epoxidase is inhibited 50% in the presence of 5 microM tellurite, the presumptive in vivo active metabolite. Under these conditions, the activities of other monooxygenases, aniline hydroxylase and benzo(a)pyrene hydroxylase, were inhibited less than 5%. We also present data suggesting that tellurite inhibits squalene epoxidation by interacting with highly susceptible -SH groups present on this monooxygenase. In vivo studies of specificity were based on the compensatory response to feeding of tellurium. Following tellurium intoxication, there was up-regulation of squalene epoxidase activity both in liver (11-fold) and sciatic nerve (fivefold). This induction was a specific response, as demonstrated in liver by the lack of up-regulation following exposure to the nonspecific microsomal enzyme inducer, phenobarbital. As a control, we also measured the microsomal monooxygenase activities of aniline hydroxylase and benzo(a)pyrene hydroxylase. Although they were induced following phenobarbital exposure, activities of these monooxygenases were not affected following tellurium intoxication, providing further evidence of specificity of tellurium intoxication for squalene epoxidase.

NGFR-mRNA expression in sciatic nerve: a sensitive indicator of early stages of axonopathy

Expression of the low-affinity nerve growth factor receptor (NGFR) in the sciatic nerve (particularly Schwann cells) is high during development but is downregulated upon establishment of the mature axon-Schwann cell relationship. NGFR is re-expressed by Schwann cells if this relationship is altered by degeneration of axons (axotomy) or myelin (tellurium intoxication). To determine the sensitivity of NGFR expression to axonal injury, we have assayed NGFR-mRNA levels in proximal and distal regions of nerves exposed to the axonopathic agents acrylamide and isoniazid, as well as in proximal and distal stumps of axotomized nerves. NGFR-mRNA was elevated in all three models and correlated regionally with sites of axonal perturbation. In distal regions of acrylamide- and isoniazid-intoxicated nerves, NGFR-mRNA was elevated at least 2 days prior to visible signs of axonal degeneration as assayed by morphological techniques utilizing light microscopy. NGFR-mRNA was also elevated in proximal regions of axotomized and acrylamide-intoxicated nerves prior to signs of axonal degeneration. In these models, increased mRNA expression correlated with alterations in the size distribution of axonal cross sections. The common response in all of these situations indicates that NGFR expression, in addition to being a marker for axonal degeneration, is also a sensitive indicator of less profound perturbations in normal axon-Schwann cell interactions, including early stages of axonopathy. We suggest that assay for NGFR-mRNA may be utilized as a rapid and simple method (relative to more labor-intensive morphological methods) to screen for peripheral neurotoxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Primary demyelination induced by exposure to tellurium alters Schwann cell gene expression: a model for intracellular targeting of NGF receptor

Exposure of developing rats to tellurium results in a highly synchronous segmental demyelination of peripheral nerves with sparing of axons; this demyelination is followed closely by a period of rapid remyelination. Demyelination occurs subsequent to a tellurium-induced block in the synthesis of cholesterol, the major myelin lipid. We utilized the techniques of Northern blotting, in situ hybridization, and immunocytochemistry to examine temporal alterations in Schwann cell gene expression related to demyelination and remyelination. Tellurium-induced demyelination is associated with downregulation of myelin protein expression and a corresponding upregulation of NGF receptor (NGF-R) and glial fibrillary acidic protein (GFAP) expression. Steady-state mRNA levels (expressed on a "per nerve" basis) for P0, the major myelin protein, were decreased by about 50% after 5 d of tellurium exposure, while levels of mRNA for NGF-R and GFAP were markedly increased (about 15-fold). In situ hybridization of teased fibers suggested that the increase in steady-state mRNA levels for NGF-R was primarily associated with demyelinated internodes and not with adjacent unaffected internodes. Although P0 message was almost totally absent from demyelinating internodes, it was also reduced in normal-appearing internodes as well. This suggests that limiting the supply of a required membrane component (cholesterol) may lead to partial downregulation of myelin gene expression in all myelinating Schwann cells. In partially demyelinated internodes, NGF-R and GFAP immunofluorescence appeared largely confined to the demyelinated regions. This suggests specific targeting of these proteins to local areas of the Schwann cell where there is myelin loss. These results demonstrate that demyelination is associated with reversion of the affected Schwann cells to a precursor cell phenotype. Because axons remain intact, our results suggest that these changes in Schwann cell gene expression do not require input from a degenerating axon, but instead may depend on whether concerted synthesis of myelin is occurring.

Neurofilament and tubulin mRNA expression in Schwann cells

Feeding of elemental tellurium to weanling rats blocks synthesis of cholesterol (a major component of myelin), and causes demyelination of the sciatic nerve. Expression of mRNA for myelin-specific genes in Schwann cells is downregulated. We now demonstrate specificity for Schwann cell injury in that expression of mRNAs for neurofilament subunits and for class II beta-tubulin (parameters sensitive to axonal injury) is unaltered in neurons of the dorsal root ganglia. An unexpected result was that in tellurium-treated rats there was marked upregulation of expression of mRNAs coding for the light and medium neurofilament subunits ("neuron-specific" proteins) as well as that for class II beta-tubulin (the major neuronal beta-tubulin isotype) in Schwann cells. Expression of these "neuronal" mRNA species was also detected in distal stumps of transected nerves at times when Schwann cells were undergoing dedifferentiation.

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.

Primary demyelination induced by exposure to tellurium alters mRNA levels for nerve growth factor receptor, SCIP, 2',3'-cyclic nucleotide 3'-phosphodiesterase, and myelin proteolipid protein in rat sciatic nerve

Weanling rats fed a diet containing tellurium develop a peripheral neuropathy characterized by a highly synchronous primary demyelination; this demyelination is followed closely by a period of rapid remyelination. The demyelination is related to the inhibition of squalene epoxidase activity, which results in a block in cholesterol synthesis. Expression of mRNA for the major structural proteins of PNS myelin, myelin basic protein and P0, is coordinately down-regulated during the demyelinating phase and then up-regulated during the remyelinating phase (Toews et al., J. Neurosci. Res., 26 (1990) 501-507). We now report tellurium-induced alterations in gene expression for several proteins which are not major structural components of myelin in the peripheral nervous system. Expression of mRNA for nerve growth factor receptor in sciatic nerve was very low in control animals, but was markedly up-regulated after 3-5 days of exposure to tellurium, a time corresponding to the beginning of demyelination. Levels remained elevated during the subsequent period of remyelination. Expression of mRNA for SCIP (a presumptive transcription factor) was also up-regulated in sciatic nerve following tellurium exposure, with a time course similar to that for nerve growth factor receptor. When examined as a fraction of total RNA, steady-state mRNA levels for 2',3'-cyclic nucleotide 3'-phosphodiesterase and the myelin proteolipid protein were decreased during the demyelinating phase; however, this decrease could be largely accounted for by increased levels of total RNA. When analyzed on a 'per nerve' basis, steady-state mRNA levels for these two proteins were actually increased about 2-fold by 9 days after beginning tellurium exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Tellurium-induced neuropathy: a model for reversible reductions in myelin protein gene expression

Inclusion of 1.1% tellurium in the diet of developing rats causes a highly synchronous primary demyelination of peripheral nerves, which is followed closely by a period of rapid remyelination. The demyelination is related to the inhibition of squalene epoxidase activity, which results in a block in cholesterol synthesis and accumulation of squalene. We now report that the demyelination resulting from this limiting of the supply of an intrinsic component of myelin (cholesterol) leads to repression of the expression of mRNA for myelin-specific proteins. Tellurium exposure resulted in an increase in total RNA (largely rRNA) in sciatic nerve, which could not be accounted for by cellular proliferation; these increased levels of rRNA may be a reactive response of Schwann cells to toxic insult and may relate to the higher levels of protein synthesis required during remyelination. In contrast, steady-state levels of mRNA, determined by Northern blot analysis, for P0 and myelin basic protein were markedly decreased (levels after 5 days of tellurium exposure were only 10-15% of control levels as a fraction of total RNA and 25-35% of control levels when the increased levels of total RNA were taken into account). Message levels increased during the subsequent period of remyelination and reached near-normal levels 30 days after beginning tellurium exposure. Although message levels for the myelin-associated glycoprotein showed a similar temporal pattern, levels did not decrease as greatly and subsequently increased sooner than did levels for P0 and myelin basic protein. The coordinate alterations in message levels for myelin proteins indicate that Schwann cells can down-regulate and then up-regulate the synthesis of myelin in response to alterations in the supply of membrane components.

Lipid metabolism during early stages of Wallerian degeneration in the rat sciatic nerve

We examined changes in biosynthetic capacity of sciatic nerve during the early stages of Wallerian degeneration, utilizing a model that permits exclusion of nonresident cells from degenerating nerve. Sciatic nerve segments were placed in either 5-microns pore (allowing infiltration of nonresident cells) or 0.22-microns pore (excluding nonresident cells) Millipore diffusion chambers and then implanted in the peritoneal cavity of the same 32-34-day-old rat. At times up to 7 days postsurgery, nerve segments from the chambers, as well as control segments from the contralateral sciatic nerve, were removed and their capacity to incorporate radioactive precursors into lipids and proteins assayed in vitro. In nerve segments from both the 0.22- and 5-microns pore chambers, incorporation of [14C]acetate into total lipids was decreased relative to control by 50% at 12 h postsurgery and by 85% at day 3. This decreased incorporation of [14C]acetate reflects primarily decreased de novo synthesis of cholesterol and of fatty acyl residues incorporated into glycerolipids and sphingolipids. There was a preferentially decreased synthesis of cerebrosides and cholesterol (components enriched in myelin) relative to other lipids, while cholesterol esters and free fatty acids (products of membrane degradation) accounted for a greater proportion of the greatly reduced levels of total lipid label. In contrast to [14C]acetate, incorporation of [3H]glycerol into lipids was increased up to fourfold, relative to control, 1 day after nerve transection.(ABSTRACT TRUNCATED AT 250 WORDS)

Acrylamide-induced increases in deposition of axonally transported glycoproteins in rat sciatic nerve

The axonal transport of proteins, glycoproteins, and gangliosides in sensory neurons of the sciatic nerve was examined in adult rats exposed to acrylamide via intraperitoneal injection (40 mg/kg of body weight/day for nine consecutive days). The L5 dorsal root ganglion was injected with either [35S]methionine to label proteins or [3H]glucosamine to label, more specifically, glycoproteins and gangliosides. At times ranging from 2 to 6 h later, the sciatic nerve and injected ganglion were excised and radioactivity in consecutive 5-mm segments determined. In both control and acrylamide-treated animals, outflow profiles of [35S]methionine-labeled proteins showed a well defined crest which moved down the nerve at a rate of approximately 340 mm/day. Similar outflow profiles and transport rates were seen for [3H]glucosamine-labeled glycoproteins in control animals. However, in animals treated with acrylamide, the crest of transported labeled glycoprotein was severely attenuated as it moved down the nerve. This finding suggests that in acrylamide-treated animals, axonally transported glycoproteins were preferentially transferred (unloaded or exchanged against unlabeled molecules) from the transport vector to stationary axonal structures. We also examined the clearance of axonally transported glycoproteins distal to a ligature on the nerve. The observed impairment of clearance in acrylamide-treated animals relative to controls is supportive of the above hypothesis. Acrylamide may directly affect the mechanism by which axonally transported material is unloaded from the transport vector. Alternatively, the increased rate of unloading might reflect an acrylamide-induced increase in the demand for axonally transported material.

Tellurium-induced neuropathy: metabolic alterations associated with demyelination and remyelination in rat sciatic nerve

Rats fed a diet containing 1.25% elemental tellurium initiated on postnatal day 20 undergo a transient neuropathy characterized by synchronous demyelination of peripheral nerves. In sciatic nerve, the extent of demyelination was maximal after 5 days of tellurium exposure; there was a loss of 25% of the myelin, as assayed by concentration of myelin-specific P0 protein. Tellurium-induced alterations in the metabolic capacity of Schwann cells were examined by measuring the synthesis of myelin lipids in vitro in isolated sciatic nerve segments. Exposure to tellurium resulted in an early marked decrease of approximately 50% in overall incorporation of [14C]acetate into lipids, with a preferential depression in synthesis of cerebrosides, cholesterol, and ethanolamine plasmalogens (components enriched in myelin). Most dramatically, within 1 day of initiation of tellurium exposure, there was a profound increase in [14C]acetate-derived radioactivity in squalene; 23% of incorporated label was in this intermediate of cholesterol biosynthesis, compared to less than 0.5% in controls. In association with the remyelinating phase seen after 5 days of tellurium exposure, synthesis of myelin components gradually returned to normal levels. After 30 days, metabolic and morphologic alterations were no longer apparent. We suggest that the sequence of metabolic events in sciatic nerve following tellurium treatment initially involves inhibition of the conversion of squalene to 2,3-epoxysqualene, and that this block in the cholesterol biosynthesis pathway results, either directly or indirectly, in the inhibition of the synthesis of myelin components and breakdown of myelin.

Metabolism of functional groups modifying the CNS myelin-associated glycoprotein

We have examined the metabolic turnover of the peptide backbone of the CNS myelin-associated glycoprotein (MAG) and of the fucose and sulfate groups modifying this protein. Rats (20 or 90 days old) were injected intracranially with mixtures of [3H]fucose and [14C]glycine, [3H]glycine and [35S]sulfuric acid, or [3H]fucose and [35S]sulfuric acid. At times ranging from 30 min to 4 weeks later, myelin was isolated, and radioactivity in MAG was determined following electrophoretic separation. Following the peak of incorporation, glycine-derived radioactivity in the MAG peptide backbone declined several-fold during the first week and was then metabolically stable (half-life much greater than 1 month). Declines with time in [3H]fucose- and [35S]sulfate-derived radioactivity in MAG were similar to that of [3H]glycine, an observation indicating that the fucose and sulfate groups modifying MAG are metabolized together with the peptide backbone as a single metabolic entity. These results were confirmed by experiments involving selective immunoprecipitation of MAG. The rates of incorporation of labeled glycine, fucose, and sulfate into MAG all decreased approximately 12-fold between 20 days of age and adulthood, a finding providing further evidence for concerted turnover of the entire molecule. Because of this concerted turnover, we suggest that functional groups modifying MAG serve some permanent structural role in protein configuration.

Early metabolic responses of retinal neurons to trimethyltin intoxication

Chronic systemic exposure of rats to the neuronotoxic compound trimethyltin (TMT) results in increased incorporation of radioactive precursors into retinal proteins and glycoproteins. Because this increased metabolic activity is accompanied by minimal subcellular pathological alterations and almost no neuronal necrosis, we suggested that it may represent an early, reactive (compensatory) response (Brain Res. 398, 298-304; 1986). We have now investigated the development of this metabolic response to TMT in more detail. Beginning at 30 d of age, rats received weekly doses of TMT (4 mg/kg body wt) by gavage for up to 7 wk; rates of incorporation of [35S]methionine and [3H]fucose into retinal proteins and glycoproteins, respectively, were then determined using in vitro retinal incubations. The apparent rates of protein synthesis and glycoprotein glycosylation in retinas from TMT-treated animals were normal or slightly decreased after 1-3 wkly doses, but were increased after 4 doses and more markedly increased after 7 doses. Glycoprotein glycosylation was increased to a greater degree (192% of control after 7 wk of dosing) than was protein synthesis (134% of control). The increased incorporation in retinas from TMT-treated animals persisted when retinas were incubated with "flooding" concentrations of precursor (1 mM), suggesting that these increases were not owing to alterations in the size of retinal precursor pools. The preferential increase in glycoprotein glycosylation was partially owing to a selective increase in glycosylation of two molecular species with apparent mol wt of 32 and 45 KDa. Quantitative autoradiographic analysis of newly synthesized proteins and glycoproteins indicated that the TMT-induced increase in metabolic activity was not specific or selective for any retinal layer or cell type. We suggest that the preferential activation of glycoprotein glycosylation, and in particular the increased glycosylation of the 32 and 45 KDa glycoprotein species, may represent part of a compensatory metabolic response of retinal neurons to TMT-induced neuronal injury.

Deposition and transfer of axonally transported phospholipids in rat sciatic nerve

Radioactive glycerol, ethanolamine, or choline injected into the vicinity of the cell bodies of rat sciatic nerve sensory fibers is incorporated into phospholipid. Some newly synthesized ethanolamine and choline phosphoglycerides are subsequently committed to transport down the sciatic nerve axons at a rate of several hundred millimeters per day. Most labeled choline phosphoglycerides move uniformly down the axons; in contrast, the crest of moving ethanolamine phosphoglycerides is continually attenuated. These data, as well as differences in the clearance of these phospholipids distal to a nerve ligature, suggest that various classes of labeled phospholipids are differentially unloaded from the transport vector (possibly by exchange with unlabeled lipid in stationary axonal structures) during movement down the axons. The extent of unloading appears to be defined by the base moiety; both diacyl and plasmalogen species of ethanolamine phosphoglycerides exchange extensively with stationary axonal lipids, while most choline phosphoglycerides continue down the axons. Autoradiographic studies with 3H-choline and 3H-ethanolamine demonstrated that most unloaded phospholipid is initially deposited in axonal structures; some of this unloaded lipid is subsequently transferred to the axon/myelin interface (axolemma?) and then to myelin. Although transported ethanolamine phosphoglycerides exchange more extensively with lipids in stationary axonal structures than do choline phosphoglycerides, at early times more label from 3H-choline is found in myelin. A model to resolve this seeming discrepancy is proposed, wherein a differential topographic localization of phospholipid classes in the membrane of the transport vector allows for a preferential extensive exchange of transported ethanolamine phosphoglycerides with lipids in stationary axonal structures, while choline phosphoglycerides become available for rapid transfer to myelin by a process involving vesicle fusion with axolemma.

Rapid axonal transport in focally demyelinated sciatic nerve

Focal demyelination was produced in rat sciatic nerve by unilateral intraneural injection of anti-galactocerebroside serum. A functional lesion was confirmed by the presence of nerve conduction block. Histologically, this corresponded to demyelination of 50-70% of the fibers in nerve cross sections; axonal structures appeared intact. At the time of maximal demyelination (7 d), 35S-methionine or 3H-fucose was injected bilaterally into the spinal cord ventral horn. At later times (5 hr-7 d), the sciatic nerve was removed and radioactivity in successive nerve segments was quantitated. The transport rates (approximately 260 mm/d) and the composition of transported proteins and glycoproteins (separated on 7-15% polyacrylamide gradient gels) were not altered in lesioned nerves relative to contralateral control nerves. Light microscopic autoradiographic analysis revealed a similar localization of axonally transported and deposited glycoproteins in demyelinated and control fibers. Initially (8 hr), the majority of label was over axons. Labeled glycoproteins remaining in the nerve after 1 week were retained mainly in axolemmal regions. We conclude that acute focal primary demyelination does not lead to major alterations in the transport or deposition of newly synthesized macromolecules.

Axonal transport through nodes of Ranvier

Axonally transported glycoproteins are shown to accumulate at nodes of Ranvier. We hypothesize that the increased labeling in nodal regions results from the rheological effects of axonal constriction as well as from selective deposition of some transported labeled molecules.

Axonal transport characteristics of gangliosides in sensory axons of rat sciatic nerve

The distribution of axonally transported gangliosides and glycoproteins along the sciatic nerve was examined from 3 h to 4 weeks following injection of[3H]glucosamine into the fifth lumbar dorsal root ganglion of adult rats. Incorporation of labeled precursor into these glycoconjugates reached a maximal level in the ganglion within 6 h. Outflow patterns of radioactivity for glycoproteins showed a well-defined crest with a transport rate of approximately 330 mm/day. In contrast, the crest of transported gangliosides was continuously attenuated, implying a significant deposition along the axon, and an alternative method of calculating velocity was required. Analysis of accumulation of labeled material at double ligatures demonstrated both anterograde and retrograde transport of glycoproteins and gangliosides and allowed for the calculation of an anterograde transport rate of about 270 mm/day for each. Additional evidence of ganglioside transport is provided in that the TLC pattern of transported radioactive gangliosides accumulating at a ligature is significantly different from the pattern seen in the dorsal root ganglion or following intraneural administration of the labeled precursor. These data indicate that gangliosides are transported at the same rapid rate as glycoproteins but are subject to a more extensive exchange with stationary material than are glycoproteins.

Metabolism of phosphate and sulfate groups modifying the P0 protein of peripheral nervous system myelin

We have examined the metabolism of phosphate and sulfate groups modifying the P0 protein, the major protein of peripheral nervous system myelin, using an in vitro incubation system. Incorporation of [3H]leucine into the P0 peptide backbone decreased approximately 25-fold between 10 and 90 days of age, a finding reflecting a decreased rate of myelin synthesis in the older animals. In contrast, incorporation of [32P]phosphate into P0 decreased only four- to fivefold, a result indicating that phosphate groups are metabolized independently of the peptide backbone. Developmental decreases in the incorporation of sulfate groups into P0 were similar to those seen for leucine, an observation suggesting that this modifying group is metabolized together with the peptide backbone as a single metabolic entity. The time course of labeling of P0 isolated from the starting homogenate and from myelin was also compared. Results are consistent with sulfation of P0 protein taking place before insertion of newly synthesized P0 into myelin. In contrast, incorporation of phosphate into P0 appears to involve both the newly synthesized pool and the preexisting pool of P0 in myelin. Presumably, entry of phosphate into P0 in myelin involves turnover of preexisting phosphate groups and rephosphorylation by myelin protein kinases. Developmental decreases in the specific activity of P0 phosphate groups in myelin are consistent with the presence of a small, rapidly turning-over pool of phosphorylated P0 (perhaps associated with the axon-myelin interface), which does not increase to the same extent as the marked increase in bulk myelin that occurs during development.

Increased synthesis of membrane macromolecules is an early response of retinal neurons to trimethyltin intoxication

We studied the synthesis and axonal transport of proteins and glycoproteins in the visual system of adult Long-Evans rats that had received 4 weekly doses of trimethyltin hydroxide (TMT, 4 mg/kg b. wt.) by gastric intubation. One week following the last dose, an in vitro assay was used to study the rate of incorporation of radioactive precursors into various macromolecules of isolated retinas. Retinas from TMT-treated rats showed increased apparent rates of synthesis, relative to retinas from control rats, for proteins [( 35S]methionine precursor) and glycoproteins [( 3H]fucose precursor). Gel electrophoretic analysis of newly synthesized proteins indicated that the increased synthesis was a generalized effect, i.e. it was not restricted to a select subset of proteins. The axonal transport of these macromolecules by retinal ganglion cells to axons (optic tract) and nerve endings (superior colliculus) was examined in vivo following intraocular precursor injection. The amount of material transported, relative to that synthesized in the retina, was not appreciably altered in TMT-treated rats, indicating that TMT did not selectively impair axonal transport. The biochemical changes were accompanied by minimal ultrastructural alterations and little neuronal necrosis in the retina. We suggest that TMT induces increased synthesis of membrane macromolecules in retinal neurons; this may reflect an early reactive (compensatory) response rather than a regressive (degenerative) response of retinal neurons to TMT. Our data do not support the hypothesis that TMT induces a functional impairment of neuronal endoplasmic reticulum or Golgi apparatus.

Developmental pattern for phosphatidylserine decarboxylase in rat brain

In adult rats, a significant portion of brain ethanolamine glycerophospholipids are synthesized by a pathway involving phosphatidylserine decarboxylase, a mitochondrial enzyme. We have now examined whether this enzyme plays a particularly prominent role during development. Activities for both phosphatidylserine decarboxylase and succinate dehydrogenase (another mitochondrial enzyme) were determined in brain homogenates from rats 5 days of age to adulthood. Succinate dehydrogenase activity, expressed on a per unit brain protein basis, increased markedly during development. This pattern has been reported previously and is as expected from the postnatal increase in oxidative metabolism. In contrast, phosphatidylserine decarboxylase activity decreased 40% from 5 to 30 days of age. The apparent Km for brain phosphatidylserine decarboxylase was 85 microM in both young (8- and 20-day-old) and adult animals. Parallel studies in vivo were carried out to determine the contribution of the phosphatidylserine decarboxylase pathway, relative to pathways utilizing ethanolamine directly, to the synthesis of brain ethanolamine glycerophospholipids. Animals were injected intracranially with a mixture of L-[G-3H]serine and [2-14C]ethanolamine and incorporation into the base moieties of the phospholipids determined. The 3H/14C ratio of ethanolamine glycerophospholipids decreased about 50% during development. Our studies in vitro and in vivo both suggest that phosphatidylserine decarboxylase plays a significant role in the synthesis of brain ethanolamine glycerophospholipids at all ages, although it is relatively more prominent early in development.

The effect of lead toxicity and milk deprivation of myelination in the rat

During a defined postnatal developmental period, the 2nd through the 28th postnatal day, rats were exposed daily to either an oral administration of 200 mg lead (as lead acetate) per kilogram of body weight, an 8-hr maternal milk deprivation schedule, or a combination of the two insults. On the 29th day the rats were killed. Either lead exposure or milk deprivation alone decreased brain (10%) and body (15%) weights, and an additive effect was observed in rats exposed to both lead and milk deprivation (brain: 20%; body: 35%). Neither the lead nor the deprivation insult alone produced a perturbation in the process of myelination. However, when the two conditions were combined an interaction was evident as a 25% decrease in myelin accumulation in females. No effect was seen in males. The myelination deficit in females was specific in that neither accumulation of glial fibrillary acidic protein (a marker for astroglial cells) nor neurofilament protein (a marker for neurons, especially axons) was perturbed. Tissue lead concentrations did not suggest that this increased sensitivity in females was due to a selective increase in their body burden of lead.

Axonal transport of glycoconjugates in the rat visual system

Long-Evans rats at 45 days of age were injected intraocularly with 25 mu Ci of [3H]glucosamine. Incorporation of radioactivity into retinal gangliosides, glycoproteins, and glycosaminoglycans (GAGs) was determined at various times after injection. Portions of all three classes of radioactive macromolecules were committed to rapid axonal transport in the retinal ganglion cells. With respect to gangliosides about 60% of those synthesized in the retina were retained in that structure, 30% were committed to transport to regions containing the nerve terminal structures (lateral geniculate body and superior colliculus), and about 10% were deposited in stationary structures of the axons (optic nerve and tract). With the exception of ganglioside GD3 the molecular species distribution of gangliosides synthesized in the retina matched that committed to transport. In contrast to gangliosides a smaller fraction of newly synthesized retinal glycoprotein (less than 12% of that synthesized in the retina) was committed to rapid transport to nerve ending regions and only about 0.5% was retained in the nerve and tract. The molecular-weight distribution of glycoproteins committed to transport differed quantitatively from that of the retina. With respect to GAGs an even smaller portion (1-2%) of that synthesized in the retina was committed to rapid transport; of this portion almost all was recovered in nerve terminal-containing structures. A constant proportion of each retinal GAG species was transported to the superior colliculus. We suggest that most of the retinal gangliosides are synthesized in neurons and preferentially in ganglion cells (possibly a function of the large surface membrane area supported by these cells). Subcellular fractionation experiments indicated that transported gangliosides, glycoproteins, and GAGs may be preferentially distributed into different subcellular compartments.

Resistance to disruption of multilamellar fragments of central nervous system myelin

Single-bilayer vesicles of myelin are desirable for studying myelin development and metabolism. Accordingly, our interest was drawn to a procedure for vesiculating myelin (Steck et al., Biochim, Biophys. Acta 509, 397-408, 1978). We used X-ray diffraction analysis to examine these putative vesicle preparations because much larger amounts of material can be surveyed by this method than by electron microscopy. The sharpness (width) of the rings in the X-ray diffraction pattern varies inversely with the number of bilayers per multilayer structure. We therefore expected to see the diffuse diffraction pattern characteristic of single bilayers. Diffraction patterns were recorded from isolated rat brain myelin before and after the vesiculation procedure. Both patterns showed sharp rings, indicating numerous multilayered structures. Average values ranging from 7 to 10 bilayers per multilayer were calculated in both cases. This procedure did produce a small fraction of single-bilayer structures, which were isolated by differential centrifugation; however, these accounted for only about 1% of the total myelin present. The diffraction pattern of this material showed the diffuse band typical of single-bilayer structures, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated it had the same protein composition as in normal myelin. Similar results were also obtained using either fresh or frozen bovine brain myelin. Variations of the published vesiculation procedure (incubation in 0.1 M NaCl or in buffers containing glycerol; disruption by sonication or use of a Tissumizer) also were not effective in breaking down the multilamellar fragments into thinner structures. The conclude that the multilamellar fragments of isolated CNS myelin resist disruption into single-bilayer structures.

Differences in the kinetics of axonal transport for individual lipid classes in rat sciatic nerve

Lipid precursors ([2-3H]glycerol for phospholipids and [3H]acetate for cholesterol) were injected into the L-5 dorsal root ganglion of adult rats. At various times, animals were killed, the ganglion and consecutive 5-mm segments of sciatic nerve were dissected, and lipids were extracted and analyzed by TLC. Individual lipid classes exhibited markedly different transport patterns. The crest of radioactive phosphatidylcholine moved as a sharply defined front at about 300 mm/day, with a relatively flat plateau behind the moving crest. Although some radioactive phosphatidylethanolamine also moved at the same rate, the crest was continually attenuated as it moved so that a gradient of radioactive phosphatidylethanolamine along the axon was maintained for several days. Transported diphosphatidylglycerol exhibited a defined crest, as did phosphatidylcholine, but moved at about half the rate. Labeled cholesterol was transported at a rapid rate similar to that for phosphatidylcholine and phosphatidylethanolamine, but like phosphatidylethanolamine, the initial moving crest of radioactivity was continually attenuated. Relative to the phospholipids, cholesterol showed a more prolonged period of accumulation in the axons and was more metabolically stable. We propose that most labeled phosphatidylcholine, phosphatidylethanolamine, and cholesterol is transported in similar (or the same) rapidly moving membranous particles. Once incorporated into these particles, molecules of phosphatidylcholine tend to maintain associated with them during transport. In contrast, molecules of phosphatidylethanolamine and cholesterol in these transported particles exchange extensively with unlabeled molecules in stationary axonal structures. Diphosphatidylglycerol, localized in a specialized organelle, the mitochondrion, is transported at a slower rate than other phospholipids, and does not exchange with other structures.

Turnover of axonally transported phospholipids in nerve endings of retinal ganglion cells

We have investigated the metabolic turnover of axonally transported phospholipids in myelinated axons (optic tract) and nerve endings (superior colliculus) of retinal ganglion cells. One week following intraocular injection of [2-3H]glycerol, turnover rates for individual phospholipid classes in the retina (which contains a number of other cell types in addition to the ganglion cells) were all very similar to each other, with apparent half-lives of approximately 7 days. Apparent half-lives of labeled phospholipids in superior colliculus (presumably primarily in retinal ganglion cell nerve endings) were 10 days for both choline and inositol phosphoglycerides and 13 days for both serine and diacylethanolamine phosphoglycerides. Subcellular fractionation data obtained from superior colliculus at various times after injection suggested that apparent turnover rates determined for nerve ending phospholipids probably were not significantly affected by transfer of axonally transported 3H lipids into myelin. Apparent half-lives for phospholipids in optic tract were somewhat longer than in superior colliculus, ranging from 11 to 18 days. The slower turnover rates in optic tract may, in part, reflect the transfer of some axonal lipids to the more metabolically stable pool of lipids in the myelin ensheathing the retinal ganglion cell axons. In both optic tract and superior colliculus, apparent half-lives for axonally transported phospholipids labeled with [32P]phosphate were only slightly longer than for [2-3H]glycerol, while those for [14C]choline and [3H]acetate were markedly longer, indicating differing degrees of metabolic conservation or reutilization of these precursors relative to glycerol.

Effect of inorganic lead exposure on myelination in the rat

The effect of defined lead burdens on myelination of the central and peripheral nervous systems was studied in neonatal Long-Evans rats. Pups were exposed to inorganic lead (100 or 400 mg Pb as lead acetate/kg body wt/day by gastric intubation) from day 2 following birth to 30 days of age. Accumulation of myelin in forebrain was not affected by the 100-mg dosage, but at the 400mg/kg dosage level, myelin accumulation was reduced by approximately 42% on a per gram forebrain basis relative to vehicle-intubated animals. The deficit was over 50% on a per forebrain basis, since there was also a slight reduction in brain weight. This lead effect was observed at both 15 and 30 days of age. Accumulation of myelin in optic nerve (determined on the basis of proteolipid protein concentration) was also reduced by 30% relative to controls by this dosage level. However, myelination in sciatic nerve (determined on the basis of Po protein concentration) was not affected by this exposure regimen. Myelin deficits were greater than could be accounted for by undernutrition secondary to lead exposure and were not due to a developmental delay in the onset of myelination.

Cholesterol is a component of the rapid phase of axonal transport

Twenty-two-day-old rats were injected intraocularly with [3H]acetate and killed between 1 hr and 35 days later. Cholesterol was isolated from the retinas, optic tracts, lateral geniculate bodies, and superior colliculi. Within the retina, radioactivity was rapidly incorporated into cholesterol with maximal labeling present one hour after injection. Transported labeled cholesterol (contralaterally corrected for systemic background labeling) was present in the superior colliculus by three hours. Radioactive cholesterol accumulated in all visual structures throughout the 35-day period, but the rate of accumulation was maximal at about the time of arrival of the initial pulse of radioactivity. Colchicine treatment of the retina blocked transport of cholesterol but not its synthesis by the retina. The results indicate that cholesterol is rapidly transported in the visual system and also released from the retina for a prolonged period after its synthesis.

Myelin and microsomes from rhesus monkey spinal cord: isolation and effects of trauma

Dispersions of rhesus monkey spinal cord and brain were separated into large particle (crude mitochondrial plus nuclear) and small particle (crude microsomal) fractions; myelin was isolated from each of these preparative fractions. In brain preparations, almost all myelin was found in the large particle fraction; in contrast, almost half the myelin from spinal cord preparations was found in the small particle fraction. In addition, much larger amounts of partially degraded myelin were found in the fraction floating on 0.32 M sucrose and in the cytosol fraction of the spinal cord preparation in comparison to those of the brain preparations. These results suggest that rhesus monkey spinal cord myelin is more fragile than brain myelin; upon dispersion of spinal cord, more small myelin vesicles (isolated from the crude microsomal fraction) and more 0.32 M sucrose floating fraction (partially degraded myelin) are formed. After trauma of the spinal cord, the proportion of small vesicle myelin was increased at the expense of large vesicle myelin, lending further support to the hypothesis that spinal cord myelin is more fragile than brain myelin. Although the lipid compositions were similar, spinal cord myelin had a lower protein content and a lower 2',3'-cyclic nucleotide phosphodiesterase specific activity than did brain myelin. The lipid composition of microsomes from brain differed somewhat from that of spinal cord microsomes.

Axonal transport and metabolism of [3H]fucose- and [35S]-sulfate-labeled macromolecules in the rat visual system

The axonal transport of labeled macromolecules in retinal ganglion cells of rats was investigated from 1 to 20 days following intraocular injection of [3H]fucose and [35S]sulfate. Maximal incorporation of [3H]fucose into acid insoluble material in the retina was at 8 h, followed by a biphasic decline. Transported [3H]fucose (98% as glycoprotein) was in the optic nerve at 1 h, the optic tract and lateral geniculate body by 2 h, and the superior colliculus by 3 h after injection, indicating a rate of transport of approximately 200 mm/day. Radioactivity continued to accumulate in the superior colliculus for at least 8 h and began to decline rapidly by 24 h. Between 3 and 6 days levels rose again in both optic tract and superior colliculus before starting a gradual decline, indicating that a wave of rapidly transported material was delayed in leaving the retina. When proteins in the superior colliculus were fractionated by gel electrophoresis, the composition of the two fucosylated protein transport phases could be partially resolved. Radioactivity in individual gel peaks represented primarily in the first phase decayed with an average half-life of one day, althouth that in one prominent protein of molecular weight 280,000 turned over with a half-life of the order of 12 h. Radioactive peaks primarily in the second phase decayed with an average half-life of more than a week. Incorporation of [35S]sulfate into acid insoluble material in the retina was maximal at 1-2 h, after which there was a rapid loss of label. The appearance of [35S]sulfate in the optic tract, lateral geniculate body and superior colliculus preceded by a short time that of the [3H]fucose; indicating a shorter retinal processing time for this label. The total transported [35S]sulfate in the superior colliculus peaked by 4-8 h and had fallen by 65% at one day; no prominent second wave of transport was observed as was the case for [3H]fucose. Acid insoluble [35S]sulfate in the superior colliculus was equally divided between glycopeptides and glycosaminoglycans at all times examined, indicating that these macromolecules are transported at the same rate. [35S]Sulfate incorporated into various proteins fractionated by gel electrophoresis had heterogeneous turnover rates, the average being around 12 h. Radioactivity in one group of proteins, of molecular weight around 90,000, decayed with a half-life of only a few hours.