Janell Hostettler

Oligodendrocytes and progenitors become progressively depleted within chronically demyelinated lesions.

To understand mechanisms that may underlie the progression of a demyelinated lesion to a chronic state, we have used the cuprizone model of chronic demyelination. In this study, we investigated the fate of oligodendrocytes during the progression of a demyelinating lesion to a chronic state and determined whether transplanted adult oligodendrocyte progenitors could remyelinate the chronically demyelinated axons. Although there is rapid regeneration of the oligodendrocyte population following an acute lesion, most of these newly regenerated cells undergo apoptosis if mice remain on a cuprizone diet. Furthermore, the oligodendrocyte progenitors also become progressively depleted within the lesion, which appears to contribute to the chronic demyelination. Interestingly, even if the mice are returned to a normal diet following 12 weeks of exposure to cuprizone, remyelination and oligodendrocyte regeneration does not occur. However, if adult O4+ progenitors are transplanted into the chronically demyelinated lesion of mice treated with cuprizone for 12 weeks, mature oligodendrocyte regeneration and remyelination occurs after the mice are returned to a normal diet. Thus, the formation of chronically demyelinated lesions induced by cuprizone appears to be the result of oligodendrocyte depletion within the lesion and not due to the inability of the chronically demyelinated axons to be remyelinated.

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.

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 3H2O, 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.

Control of cholesterol biosynthesis in Schwann cells

Abstract: 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.

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 PLPnull 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 PLPnull mice relative to wild‐type (wt) animals. However, the yield of myelin from PLPnull 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 PLPnull 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 PLPnull 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 PLPnull myelin is physically less stable and/or less dense than wt myelin.

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 P0, 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. J. Neurosci. Res. 59:581–586, 2000

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 synchronous 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.