The turnover of myelin in the adult rat

Untargeted Lipidomics after D2O Administration Reveals the Turnover Rate of Individual Lipids in Various Organs of Living Organisms

The administration of low doses of D₂O to living organisms was used for decades for the investigation of metabolic pathways and for the measurement of the turnover rate for specific compounds. Usually, the investigation of the deuterium uptake in lipids is performed by measuring the deuteration level of the palmitic acid residue using GC-MS instruments, and to our knowledge, the application of the modern untargeted LC-MS/MS lipidomics approaches was only reported a few times. Here, we investigated the deuterium uptake for >500 lipids for 13 organs and body liquids of mice (brain, lung, heart, liver, kidney, spleen, plasma, urine, etc.) after 4 days of 100% D₂O administration. The maximum deuteration level was observed in the liver, plasma, and lung, while in the brain and heart, the deuteration level was lower. Using MS/MS, we demonstrated the incorporation of deuterium in palmitic and stearic fragments in lipids (PC, PE, TAG, PG, etc.) but not in the corresponding free forms. Our results were analyzed based on the metabolic pathways of lipids.

Mechanisms of demyelination and neurodegeneration in globoid cell leukodystrophy

Globoid cell leukodystrophy (GLD), also known as Krabbe disease, is a lysosomal storage disorder causing extensive demyelination in the central and peripheral nervous systems. GLD is caused by loss-of-function mutations in the lysosomal hydrolase, galactosylceramidase (GALC), which catabolizes the myelin sphingolipid galactosylceramide. The pathophysiology of GLD is complex and reflects the expression of GALC in a number of glial and neural cell types in both the central and peripheral nervous systems (CNS and PNS), as well as leukocytes and kidney in the periphery. Over the years, GLD has garnered a wide range of scientific and medical interests, especially as a model system to study gene therapy and novel preclinical therapeutic approaches to treat the spontaneous murine model for GLD. Here, we review recent findings in the field of Krabbe disease, with particular emphasis on novel aspects of GALC physiology, GLD pathophysiology, and therapeutic strategies.

Qki regulates myelinogenesis through Srebp2-dependent cholesterol biosynthesis

Myelination depends on timely, precise control of oligodendrocyte differentiation and myelinogenesis. Cholesterol is the most abundant component of myelin and essential for myelin membrane assembly in the central nervous system. However, the underlying mechanisms of precise control of cholesterol biosynthesis in oligodendrocytes remain elusive. In the present study, we found that Qki depletion in neural stem cells or oligodendrocyte precursor cells in neonatal mice resulted in impaired cholesterol biosynthesis and defective myelinogenesis without compromising their differentiation into Aspa⁺Gstpi⁺ myelinating oligodendrocytes. Mechanistically, Qki-5 functions as a co-activator of Srebp2 to control transcription of the genes involved in cholesterol biosynthesis in oligodendrocytes. Consequently, Qki depletion led to substantially reduced concentration of cholesterol in mouse brain, impairing proper myelin assembly. Our study demonstrated that Qki-Srebp2-controlled cholesterol biosynthesis is indispensable for myelinogenesis and highlights a novel function of Qki as a transcriptional co-activator beyond its canonical function as an RNA-binding protein.

The Role of Lipids, Lipid Metabolism and Ectopic Lipid Accumulation in Axon Growth, Regeneration and Repair after CNS Injury and Disease

Axons in the adult mammalian nervous system can extend over formidable distances, up to one meter or more in humans. During development, axonal and dendritic growth requires continuous addition of new membrane. Of the three major kinds of membrane lipids, phospholipids are the most abundant in all cell membranes, including neurons. Not only immature axons, but also severed axons in the adult require large amounts of lipids for axon regeneration to occur. Lipids also serve as energy storage, signaling molecules and they contribute to tissue physiology, as demonstrated by a variety of metabolic disorders in which harmful amounts of lipids accumulate in various tissues through the body. Detrimental changes in lipid metabolism and excess accumulation of lipids contribute to a lack of axon regeneration, poor neurological outcome and complications after a variety of central nervous system (CNS) trauma including brain and spinal cord injury. Recent evidence indicates that rewiring lipid metabolism can be manipulated for therapeutic gain, as it favors conditions for axon regeneration and CNS repair. Here, we review the role of lipids, lipid metabolism and ectopic lipid accumulation in axon growth, regeneration and CNS repair. In addition, we outline molecular and pharmacological strategies to fine-tune lipid composition and energy metabolism in neurons and non-neuronal cells that can be exploited to improve neurological recovery after CNS trauma and disease.

Mature myelin maintenance requires Qki to coactivate PPARβ-RXRα-mediated lipid metabolism

Lipid-rich myelin forms electrically insulating, axon-wrapping multilayers that are essential for neural function, and mature myelin is traditionally considered metabolically inert. Surprisingly, we discovered that mature myelin lipids undergo rapid turnover, and quaking (Qki) is a major regulator of myelin lipid homeostasis. Oligodendrocyte-specific Qki depletion, without affecting oligodendrocyte survival, resulted in rapid demyelination, within 1 week, and gradually neurological deficits in adult mice. Myelin lipids, especially the monounsaturated fatty acids and very-long-chain fatty acids, were dramatically reduced by Qki depletion, whereas the major myelin proteins remained intact, and the demyelinating phenotypes of Qki-depleted mice were alleviated by a high-fat diet. Mechanistically, Qki serves as a coactivator of the PPARβ-RXRα complex, which controls the transcription of lipid-metabolism genes, particularly those involved in fatty acid desaturation and elongation. Treatment of Qki-depleted mice with PPARβ/RXR agonists significantly alleviated neurological disability and extended survival durations. Furthermore, a subset of lesions from patients with primary progressive multiple sclerosis were characterized by preferential reductions in myelin lipid contents, activities of various lipid metabolism pathways, and expression level of QKI-5 in human oligodendrocytes. Together, our results demonstrate that continuous lipid synthesis is indispensable for mature myelin maintenance and highlight an underappreciated role of lipid metabolism in demyelinating diseases.

Maturation of Brain Microstructure and Metabolism Associates with Increased Capacity for Self-Regulation during the Transition from Childhood to Adolescence

Children ages 9-12 years face increasing social and academic expectations that require mastery of their thoughts, emotions, and behavior. Little is known about the development of neural pathways integral to these improving capacities during the transition from childhood to adolescence. Among 234 healthy, inner-city male and female youth (species Homo sapiens) 9-12 years of age followed by the Columbia Center for Children's Environmental Health, we acquired diffusion tensor imaging, multiplanar chemical shift imaging, and cognitive measures requiring self-regulation. We found that increasing age was associated with increased fractional anisotropy and decreased apparent diffusion coefficient, most prominently in the frontal and cingulate cortices, striatum, thalamus, deep white matter, and cerebellum. Additionally, we found increasing age was associated with increased N-acetyl-l-aspartate (NAA) in the anterior cingulate and insular cortices, and decreased NAA in posterior cingulate and parietal cortices. Age-associated changes in microstructure and neurometabolite concentrations partially mediated age-related improvements in performance on executive function tests. Together, these findings suggest that maturation of key regions within cortico-striatal-thalamo-cortical circuits subserve the emergence of improved self-regulatory capacities during the transition from childhood to adolescence.SIGNIFICANCE STATEMENT Few imaging studies of normal brain development have focused on a population of inner-city, racial/ethnic minority youth during the transition from childhood to adolescence, a period when self-regulatory capacities rapidly improve. We used DTI and MPCSI to provide unique windows into brain maturation during this developmental epoch, assessing its mediating influences on age-related improvement in performance on self-regulatory tasks. Our findings suggest that rapid maturation of cortico-striato-thalamo-cortical circuits, represented as progressive white-matter maturation (increasing FA and increasing NAA, Ch, Cr concentrations accompanying advancing age) in frontal regions and related subcortical projections and synaptic pruning (decreasing NAA, Ch, Cr, Glx) in posterior regions, support age-related improvements in executive functioning and self-regulatory capacities in youth 9-12 years of age.

Axoglial interactions in myelin plasticity: Evaluating the relationship between neuronal activity and oligodendrocyte dynamics

Myelin is a critical component of the vertebrate nervous system, both increasing the conduction velocity of myelinated axons and allowing for metabolic coupling between the myelinating cells and axons. An increasing number of studies demonstrate that myelination is not simply a developmentally hardwired program, but rather that new myelinating oligodendrocytes can be generated throughout life. The generation of these oligodendrocytes and the formation of myelin are influenced both during development and adulthood by experience and levels of neuronal activity. This led to the concept of adaptive myelination, where ongoing activity-dependent changes to myelin represent a form of neural plasticity, refining neuronal functioning, and circuitry. Although human neuroimaging experiments support the concept of dynamic changes within specific white matter tracts relevant to individual tasks, animal studies have only just begun to probe the extent to which neuronal activity may alter myelination at the level of individual circuits and axons. Uncovering the role of adaptive myelination requires a detailed understanding of the localized interactions that occur between active axons and myelinating cells. In this review, we focus on recent animal studies that have begun to investigate the interactions between active axons and myelinating cells and review the evidence for-and against-the ability of neuronal activity to alter myelination at an axon-specific level.

Turning to myelin turnover

Neural plasticity in the adult central nervous system involves the adaptation of myelination, including the formation of novel myelin sheaths by adult-born oligodendrocytes. Yet, mature oligodendrocytes slowly but constantly turn over their pre-existing myelin sheaths, thereby establishing an equilibrium of replenishment and degradation that may also be subject to adaptation with consequences for nerve conduction velocity. In this short review we highlight selected approaches to the normal turnover of adult myelin in vivo, from injecting radioactive precursors of myelin constituents in the 1960s to current strategies involving isotope labeling and tamoxifen-induced gene targeting.

Limited daily feeding and intermittent feeding have different effects on regional brain energy homeostasis during aging

Albeit aging is an inevitable process, the rate of aging is susceptible to modifications. Dietary restriction (DR) is a vigorous nongenetic and nonpharmacological intervention that is known to delay aging and increase healthspan in diverse species. This study aimed to compare the impact of different restricting feeding regimes such as limited daily feeding (LDF, 60% AL) and intermittent feeding (IF) on brain energy homeostasis during aging. The analysis was focused on the key molecules in glucose and cholesterol metabolism in the cortex and hippocampus of middle-aged (12-month-old) and aged (24-month-old) male Wistar rats. We measured the impact of different DRs on the expression levels of AMPK, glucose transporters (GLUT1, GLUT3, GLUT4), and the rate-limiting enzyme in the cholesterol synthesis pathway (HMGCR). Additionally, we assessed the changes in the amounts of cholesterol, its metabolite, and precursors following LDF and IF. IF decreased the levels of AMPK and pAMPK in the cortex while the increased levels were detected in the hippocampus. Glucose metabolism was more affected in the cortex, while cholesterol metabolism was more influenced in the hippocampus. Overall, the hippocampus was more resilient to the DRs, with fewer changes compared to the cortex. We showed that LDF and IF differently affected the brain energy homeostasis during aging and that specific brain regions exhibited distinct vulnerabilities towards DRs. Consequently, special attention should be paid to the DR application among elderly as different phases of aging do not respond equally to altered nutritional regimes.

Cholesterol in myelin biogenesis and hypomyelinating disorders

The largest pool of free cholesterol in mammals resides in myelin membranes. Myelin facilitates rapid saltatory impulse propagation by electrical insulation of axons. This function is achieved by ensheathing axons with a tightly compacted stack of membranes. Cholesterol influences myelination at many steps, from the differentiation of myelinating glial cells, over the process of myelin membrane biogenesis, to the functionality of mature myelin. Cholesterol emerged as the only integral myelin component that is essential and rate-limiting for the development of myelin in the central and peripheral nervous system. Moreover, disorders that interfere with sterol synthesis or intracellular trafficking of cholesterol and other lipids cause hypomyelination and neurodegeneration. This review summarizes recent results on the roles of cholesterol in CNS myelin biogenesis in normal development and under different pathological conditions. This article is part of a Special Issue entitled Brain Lipids.

Aging induces tissue-specific changes in cholesterol metabolism in rat brain and liver

Disturbance of cholesterol homeostasis in the brain is coupled to age-related brain dysfunction. In the present work, we studied the relationship between aging and cholesterol metabolism in two brain regions, the cortex and hippocampus, as well as in the sera and liver of 6-, 12-, 18- and 24-month-old male Wistar rats. Using gas chromatography-mass spectrometry, we undertook a comparative analysis of the concentrations of cholesterol, its precursors and metabolites, as well as dietary-derived phytosterols. During aging, the concentrations of the three cholesterol precursors examined (lanosterol, lathosterol and desmosterol) were unchanged in the cortex, except for desmosterol which decreased (44 %) in 18-month-old rats. In the hippocampus, aging was associated with a significant reduction in lanosterol and lathosterol concentrations at 24 months (28 and 25 %, respectively), as well as by a significant decrease of desmosterol concentration at 18 and 24 months (36 and 51 %, respectively). In contrast, in the liver we detected age-induced increases in lanosterol and lathosterol concentrations, and no change in desmosterol concentration. The amounts of these sterols were lower than in the brain regions. In the cortex and hippocampus, desmosterol was the predominant cholesterol precursor. In the liver, lathosterol was the most abundant precursor. This ratio remained stable during aging. The most striking effect of aging observed in our study was a significant decrease in desmosterol concentration in the hippocampus which could reflect age-related reduced synaptic plasticity, thus representing one of the detrimental effects of advanced age.

Cholesterol and myelin biogenesis

Myelin consists of several layers of tightly compacted membranes wrapped around axons in the nervous system. The main function of myelin is to provide electrical insulation around the axon to ensure the rapid propagation of nerve conduction. As the myelinating glia terminally differentiates, they begin to produce myelin membranes on a remarkable scale. This membrane is unique in its composition being highly enriched in lipids, in particular galactosylceramide and cholesterol. In this review we will summarize the role of cholesterol in myelin biogenesis in the central and peripheral nervous system.

Cyclical and dose-dependent responses of adult human mature oligodendrocytes to fingolimod

Fingolimod is a sphingosine-1-phosphate (S1P) analogue that has been used in clinical trials as a systemic immunomodulatory therapy for multiple sclerosis. Fingolimod readily accesses the central nervous system, raising the issue of its direct effects on neural cells. We assessed the effects of active fingolimod on dissociated cultures of mature, myelin-producing oligodendrocytes (OLGs) derived from adult human brain. Human OLGs express S1P receptor transcripts in relative abundance of S1P5>S1P3>S1P1, with undetectable levels of S1P4. Low doses of fingolimod (100 pmol/L to 1 nmol/L) induced initial membrane elaboration (2 days), subsequent retraction (4 days), and recurrence of extension with prolonged treatment (8 days). Higher doses (10 nmol/L to 1 mumol/L) caused the opposite modulation of membrane dynamics. Retraction was rescued by co-treatment with the S1P3/S1P5 pathway antagonist, suramin, and was associated with RhoA-mediated cytoskeletal signaling. Membrane elaboration was mimicked using the S1P1 agonist SEW2871. Fingolimod rescued human OLGs from serum and glucose deprivation-induced apoptosis, which was reversed with suramin co-treatment and mimicked using an S1P5 agonist. High doses of fingolimod induced an initial down-regulation of S1P5 mRNA levels relative to control (4 hours), subsequent up-regulation (2 days), and recurrent down-regulation (8 days). S1P1 mRNA levels were inversely regulated compared with S1P5. These results indicate that fingolimod modulates maturity- and species-specific OLG membrane dynamics and survival responses that are directly relevant for myelin integrity.

Turnover of myelin lipids in aging brain

Turnover rates of myelin membrane components in mouse brains were determined by a method using stable isotope-labeling and mass spectrometry. The half-replacement times based on incorporation rates of newly synthesized molecules for young adult mice were 359 days for cholesterol, 20 days for phosphatidylcholine, 25 days for phosphatidylethanolamine, 94 days for cerebroside and 102 days for ganglioside GM1. The turnover rates of half-lives of myelin components were calculated from the decay curves of initially labeled molecules, and they were about the same as the half-replacement times. Individual components were thus revealed to be metabolized at different rates, and their turnover rates were differently affected by aging. As was observed with phospholipids, myelin pools appeared to be compartmentalized into rapidly and slowly exchanging pools. The turnover rates of cerebroside and GM1 decreased between the young and adult periods and slightly increased in senescence. The latter phenomenon may indicate an enhanced myelin turnover in senescence. The present study reveals the dynamic aspects of myelin membrane turnover during the life span of mouse.

Substrate-reduction therapy enhances the benefits of bone marrow transplantation in young mice with globoid cell leukodystrophy

Globoid cell leukodystrophy is an autosomal recessive disease with progressive demyelination caused by a deficiency of the lysosomal enzyme galactosylceramidase. Bone marrow transplantation (BMT) is a therapeutic option for patients with late-onset disease and for patients with early onset disease that had an early diagnosis owing to an affected sibling. This therapy, however, typically is not effective for early onset disease when the diagnosis occurs after several months of life. In an effort to enable a broader range of patients to benefit from BMT, we tested whether combining substrate-reduction therapy with BMT would result in a greater benefit than either treatment alone in the twitcher mouse model of globoid cell leukodystrophy. Twitcher mice treated with L-cycloserine, an inhibitor of 3-ketodyhydrosphingosine synthase, and transplanted with 50 +/- 5 x 10(6) bone marrow cells on d 10 had a mean life-span of 112 d compared with 51 d for BMT alone (p < 0.001) or L-cycloserine alone, which was previously reported to be 56 d. L-Cycloserine treatment also was initiated neonatally to determine whether it would allow for a delayed BMT to have therapeutic value. Twitcher mice given only BMT at 18 d or only a short course of L-cycloserine died at 36 and 37 d, respectively. Twitcher mice given a short course of L-cycloserine + BMT at 18 d lived to 58 d (p = 0.0006). In conclusion, substrate-reduction therapy enhanced the value of BMT in twitcher mice, suggesting that this combination strategy might benefit patients with globoid cell leukodystrophy.

L-cycloserine slows the clinical and pathological course in mice with globoid cell leukodystrophy (twitcher mice)

Globoid cell leukodystrophy (Krabbe's disease) is an autosomal recessive disease that affects the lysosomal enzyme galactosylceramidase. Galactosylceramidase removes galactose from galactosylceramide and psychosine, which are derived from sphingosine. In the present study, L-cycloserine (an inhibitor of 3-ketodyhydrosphingosine synthase) was administered to the twitcher mouse, an authentic model of globoid cell leukodystrophy. Twitcher mice treated with L-cycloserine had a significantly longer life span and a delayed onset of weight loss than vehicle-injected twitcher mice. Pathological features such as macrophage infiltration and astrocyte gliosis also were less in treated twitcher mice. These results indicate that substrate reduction therapy may have therapeutic value for individuals with residual enzymatic activity, e.g., individuals with late onset disease or individuals with partial enzyme replacement via bone marrow transplantation. In these cases, a reduction in galactosylceramide and psychosine synthesis would enable residual enzymatic activity to keep up with the accumulation of these substrates that would otherwise lead to pathology.

Release of intracellular calcium stores leads to retraction of membrane sheets and cell death in mature mouse oligodendrocytes

The ability of mature oligodendrocytes (OLs) to recover from insult is important in repair of damage following demyelination. Since regulation of Ca2+ levels within cells plays a critical role in function and survival, this study investigates the effects of changes in cytoplasmic Ca2+ on the viability of cultured mouse OLs and their ability to maintain membrane sheets. Mature OLs in culture respond rapidly to the calcium ionophore A23187 and promptly return to resting Ca2+ levels when the ionophore is removed. Longer exposure to 0.1-1.0 microM A23187 leads to microtubule disruption, membrane sheet retraction and eventual cell death; nuclear lysis occurs in many of the OLs, as reported by Scolding, et al. (1) for rat OLs. In our cultures, mature OLs were more susceptible to nuclear lysis than were immature OLs or astroglia. Release of intracellular Ca2+ stores with thapsigargin at 5-10 microM also leads to retraction of membrane sheets. Following 6 hours of continuous exposure to thapsigargin, the effects on membrane sheets are reversed over the next 12 hours. After 18 hours of continuous exposure to thapsigargin, only occasional nuclear lysis is observed, but a number of the mature OLs show signs of DNA fragmentation, indicating that apoptotic death is occurring. Our results suggest that mature OLs cannot survive a prolonged influx of extracellular calcium as readily as immature OLs and astroglia, but have mechanism to withstand similar increases in cytoplasmic Ca2+ following sustained release of intracellular stores.

Characterization of guanylyl cyclase in purified myelin

This study was undertaken to characterize the enzymatic properties of the particulate guanylyl cyclase previously shown to be present at a high level of activity in purified rat brain myelin. Significant activation was achieved by both Lubrol-PX and Triton X-100, the latter being somewhat more effective. A pH optimum of 7.8 was observed, compared to 7.4 for microsomes. Employing 1.2 mM GTP with 1% Triton X-100, linearity of response was observed up to 60 min and approximately 1.2 mg of myelin protein. Kinetic analysis revealed Km values of 0.258mM and 0.486mM for myelin and microsomes, respectively, similar values being obtained by Lineweaver-Burke analysis or Direct Linear Plot. Vmax values were 20 and 266 pmol/mg protein/min for myelin and microsomes, respectively. Washing of the myelin with 0.5 M NaCl or 0.1% Na taurocholate did not remove a significant amount of guanylyl cyclase activity, indicating the enzyme to be intrinsic to the myelin sheath.

Metabolic relationships between proteins of myelin and paranodally shedded, partially degraded myelin fragments in the rabbit CNS

The "close-to-node" regions of myelinated nerve fibres, i.e., the paranodal end segments, are generally thought to be sites of high metabolic activity and myelin sheath turnover. Data on turnover rates of individual myelin constituents are conflicting but there exists a common belief that myelin is metabolized as independent molecules rather than as a unit. The occurrence of paranodal Marchi-positive bodies, with morphological and biochemical properties consistent with partially degraded myelin, prompted us to examine the temporal dynamics of the incorporation of radioactive precursor label in the major proteins of myelin and the Marchi-positive bodies. 3H-leucine was administered intrathecally in adult rabbits. After various survival times, the spinal cord was subfractionated by ultracentrifugation in a discontinuous two-step 0.32 M/0.85 M sucrose gradient. Myelin was collected from the interface and a floating fraction, heavily enriched in Marchi-positive bodies, was recovered on top of the 0.32 M sucrose. By scintillation counting and by gel fluorography combined with immunoblotting, a gradual appearance with time of partially degraded peptides of myelin-associated protein and 2',3'-cyclic nucleotide 3'-phosphodiesterase was seen in the floating fraction but not in myelin. The temporal dynamics of the specific activities of these two proteins and myelin-basic protein and proteolipid protein were consistent with a typical source-product relationship between myelin and the material in the floating fraction. In conjunction with earlier morphological and biochemical findings, these data may suggest that Marchi-positive bodies appear as a consequence of myelin catabolism.

Polyneuropathy, ophthalmoplegia, leukoencephalopathy, and intestinal pseudo-obstruction: POLIP syndrome

We describe 5 individuals (from three separate families) with a progressive neurological disorder characterized by sensorimotor peripheral polyneuropathy, cranial neuropathies (external ophthalmoplegia, deafness), and the syndrome of chronic intestinal pseudo-obstruction. Magnetic resonance imaging showed widespread abnormality of the cerebral and cerebellar white matter in the 2 patients studied. Autopsy examination in 3 revealed widespread endoneurial fibrosis and demyelination in the peripheral nervous system, possibly secondary to axonal atrophy, and poorly defined changes in cerebral white matter (leukoencephalopathy). The cranial nerves and spinal roots were less severely involved and the neurons in the brainstem and spinal cord were intact. The fatal gastrointestinal dysmotility was due to a severe visceral neuropathy. We suggest that these patients manifested a hereditary disorder with distinctive clinical, radiological, and neuropathological features, and propose the acronym POLIP to emphasize the distinctive tetrad of polyneuropathy, ophthalmoplegia, leukoencephalopathy, and intestinal pseudo-obstruction.

Regulation of sulfotransferase activity by vitamin K in mouse brain

We have shown previously that the administration of warfarin to 16-day-old mice results in a significant reduction in levels of sulfatides, and to a lesser degree a reduction of other sphingolipids in brain. Vitamin K stimulates biosynthesis of sulfatides in warfarin-treated mice. We now report that warfarin inhibits brain sulfotransferase activity. This inhibition is reversed by vitamin K. The treatment of normal mice with vitamin K stimulates the activities of sulfotransferase and arylsulfatase and the turnover rate of brain sulfatides. The ability of vitamin K to influence the activity of biosynthetic and catabolic enzymes and the turnover of sulfatides suggests a possible regulatory role for vitamin K in the maturing brain.

Metabolism of exogenous galactosylceramide in the twitcher mouse brain

The in vivo metabolism of galactosylceramide (gal-cer) in normal mice and in twitcher mice, a model of human GLD, was examined following intracerebral administration of gal-cer containing [1-14C] stearic acid. In normal mice, gal-cer was hydrolyzed to ceramide within 6 hours and ceramide was hydrolyzed to sphingosine and fatty acid. Most of the released fatty acid was immediately incorporated into other lipids. About 75% of injected gal-cer was hydrolyzed 80 hours after the injection, while in the twitcher mouse, only 17% of gal-cer was hydrolyzed. These results show that degradation of gal-cer is impaired in the twitcher mouse brain, but contradict to the fact that there was no evidence of any accumulation of gal-cer in the brain. This discrepancy may be due to the different sorting routes of biosynthesized and exogenously-administered gal-cer in the mouse brain. Most of the biosynthesized gal-cer is incorporated into myelin, while the injected gal-cer is incorporated into lysosomes.

Cellular and molecular aspects of myelin protein gene expression

The cellular and molecular aspects of myelin protein metabolism have recently been among the most intensively studied in neurobiology. Myelination is a developmentally regulated process involving the coordination of expression of genes encoding both myelin proteins and the enzymes involved in myelin lipid metabolism. In the central nervous system, the oligodendrocyte plasma membrane elaborates prodigious amounts of myelin over a relatively short developmental period. During development, myelin undergoes characteristic biochemical changes, presumably correlated with the morphological changes during its maturation from loosely-whorled bilayers to the thick multilamellar structure typical of the adult membrane. Genes encoding four myelin proteins have been isolated, and each of these specifies families of polypeptide isoforms synthesized from mRNAs derived through alternative splicing of the primary gene transcripts. In most cases, the production of the alternatively spliced transcripts is developmentally regulated, leading to the observed protein compositional changes in myelin. The chromosomal localizations of several of the myelin protein genes have been mapped in mice and humans, and abnormalities in two separate genes appear to be the genetic defects in the murine dysmyelinating mutants, shiverer and jimpy. Insertion of a normal myelin basic protein gene into the shiverer genome appears to correct many of the clinical and cell biological abnormalities associated with the defect. Most of the dysmyelinating mutants, including those in which the genetic defect is established, appear to exhibit pleiotropy with respect to the expression of other myelin genes. Post-translational events also appear to be important in myelin assembly and metabolism. The major myelin proteins are synthesized at different subcellular locations and follow different routes of assembly into the membrane. Prevention of certain post-translational modifications of some myelin proteins can result in the disruption of myelin structure, reminiscent of naturally occurring myelin disorders. Studies on the expression of myelin genes in tissue culture have shown the importance of epigenetic factors (e.g., hormones, growth factors, and cell-cell interactions) in modulating myelin protein gene expression. Thus, myelinogenesis has proven to be very useful system in which to examine cellular and molecular mechanisms regulating the activity of a nervous system-specific process.

Membrane interactions in nerve myelin: II. Determination of surface charge from biochemical data

In our accompanying paper (Inouye and Kirschner, 1988) we calculated the surface charge density at the extracellular surfaces in peripheral and central nervous system (PNS; CNS) myelins from observations on the dependency of the width of the extracellular space on pH and ionic strength. Here, we have determined the surface charge density of the membrane surfaces in myelin from its chemical composition and the localization of some of its molecular components. We then analyzed the attractive and repulsive forces between the apposed surfaces and calculated equilibrium periods for comparison with the measured values. The biochemical model accounts for the observed isoelectric range of the myelin period and, with the surface charge reduced (possibly by divalent cation binding or a space charge approximation), the model also accounts for the dependency of period on pH above the isoelectric range. At the extracellular (and cytoplasmic) surfaces the contribution of lipid (with pI approximately 2) to the net surface charge is about the same in both PNS and CNS myelin, whereas the contribution of protein depends on which ones are exposed at the two surfaces. The protein conformation and localization modulate the surface charge of the lipid, resulting in positively-charged cytoplasmic surfaces (pI approximately 9) and negatively-charged extracellular surfaces (pI approximately 2-4). The net negative charge at the extracellular surface is due in CNS myelin to lipid, and in PNS myelin to both lipid and (PO) glycoprotein. The net positive charge at the cytoplasmic surface is due in CNS myelin mostly to basic protein, and in PNS myelin to PO glycoprotein and basic protein. The invariance of the cytoplasmic packing may be due to specific short-range interactions. Our models demonstrate how the particular myelin proteins and their localization and conformation can account for the differences in inter-membrane interactions in CNS and PNS myelins.

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.

Regulation of arylsulphatase A and sulphogalactolipid turnover by cortisol in myelinogenic cultures of cells dissociated from embryonic mouse brain

Myelinogenic cultures of cells dissociated from embryonic mouse brain were used to study the regulation of myelination-associated molecules by cortisol. Cortisol in physiological concentrations (0.03 microM) caused an increased accumulation of myelination-associated sulphogalactolipids. It also stimulated the myelin- and oligodendroglia-specific cyclic nucleotide phosphohydrolase. The increase in sulphogalactolipid content was caused by a cortisol-concentration-dependent inhibition in arylsulphatase A activity and not by an increase in either cerebroside sulphotransferase activity or an increase in availability of adenosine 3'-phosphate 5'-phosphosulphate. Of several steroid hormones tested only the glucocorticoid types brought about these changes. The relationship between net sulphogalactolipid accumulation and arylsulphatase A inhibition induced by cortisol was confirmed by sulphogalactolipid turnover studies. Depending on whether a single-phase or a two-phase decay calculation is used, the turnover of sulphogalactolipid with cortisol present was decreased at 22 days in culture by either 62% or 65% respectively of that without cortisol. This decrease in turnover can be attributed completely to the decrease of arylsulphatase activity by cortisol to 63% of the value for normal cells grown under the same conditions.

Partial purification and characterization of neutral proteases in lymph nodes of rats with experimental allergic encephalomyelitis

Two kinds of neutral protease activities in lymph nodes from Lewis rats with acute experimental allergic encephalomyelits (EAE) have been separated and partially purified and characterized. A soluble enzyme preparation enriched by gel filtration and ion-exchange chromatography hydrolyzes myelin basic protein, polylysine, and other basic proteins with an optimum pH at 6.0-6.5. It is inhibited by p-chloromercuribenzoate, and thus appears to be a mixture of thiol proteases. Another fraction containing proteolytic enzyme activity is strongly bound to the insoluble lymph node residue, and it also hydrolyzes myelin basic protein and histone, but not polylysine. It has a pH optimum above 7.5, is inhibited by phenylmethylsulfonyl fluoride, thus resembling elastase, but does not hydrolyze elastin-Congo red. The insoluble enzyme preparation hydrolyzes basic protein to 4-5 peptides in a pattern on polyacrylamide gels resembling that of the hydrolysis of basis protein by whole lymphocytes; the soluble enzyme mixture produces small fragments not retained on gels. Lymphocytes are a major component of the cells infiltrating the nervous system in experimental allergic encephalomyelitis, and neutral proteases contained in these cells may contribute to the degradation of myelin, especially of the basic protein.

Turnover of myelin proteins of rat brain, determined in fractions separated by sedimentation in a continuous sucrose gradient

Rats that received intracranial injections of [3H]leucine at 14 days of age were killed on days 17, 24, 38, 55, and 89 post-injection. Brains were homogenized and the myelin membranes separated in a sucrose density gradient. At day 17 sodium dodecylsulfate polyacrylamide gels of water-shocked, delipidated membrane fractions showed a difference in the specific activity of myelin proteins across the gradient. A decrease in specific activity was found in all of the proteins in the denser fractions, compared with the lighter fractions. As time after injection progressed, the difference became more pronounced; a two- to threefold decrease in specific activity was seen across the gradient in the various myelin proteins. The proteins of the lightest membrane fractions retained their high specific activity throughout the experiment in spite of extensive new myelin synthesis. Taking this new myelin into account, the decrease in specific activity in the denser myelin fractions could be explained by isotope dilution. Therefore, proteins present in at least some of the myelin are essentially stable.

Is Na + ATPase a myelin-associated enzyme?

The Na + K ATPase activity associated with purified myelin has been investigated. On the basis of marker enzyme studies, the Na + K ATPase activity of myelin was higher than could be accounted for by microsomal contamination. Fractions prepared from white matter-enriched areas of rat brain showed a threefold enrichment in Na + K ATPase activity in myelin as compared with the white matter homogenate. The ATPase activity in myelin was stimulated fourfold by treatment with sodium deoxycholate, but the activity in the whole brain homogenate and the microsomal fraction was only doubled. This discontinuity temperature for Na + K ATPase activity was significantly higher for the myelin fraction (29 degrees C) than for the microsomal fraction (21 degrees C), but the energies of activation, both above and below the discontinuity temperature, were the same for both fractions, Myelin Na + K ATPase had a lower affinity for strophanthidin than the microsomal enzyme, but both fractions were inhibited to the same extent by 10-3 M-strophanthidin. The evidence thus indicated that much of the ATPase activity of myelin is not the result of microsomal contamination. Although the possibility of axolemmal contamination cannot be ruled out conclusively, indirect evidence suggest that this is not a significant factor and that Na + K ATPase may be a myelin-associated enzyme.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.

A quantitative lifespan study of changes in cell number, cell division and cell death in various regions of the mouse forebrain

A quantitative study of changes in total cell number was carried out in the indusium griseum and anterior commissure from fetal life to old age in the mouse brain. The changes in the number of mitotic and pyknotic cells were recorded in the indusium griseum, anterior commissure, subependymal and ependymal layers over the same period. The number of neurons which are produced and which migrate to the indusium griseum are in excess of the number eventually required and the surplus neurons are lost by cell death in late gestation and early postnatal life while synaptogenesis and neuronal differentiation is taking place. This neuronal loss is associated with a rapid turnover of glia. Most first generation glia, or their immediate precursors, are produced prenatally, in parallel but one day behind neurons. There is no large burst of mitotic activity in the postnatal brain which gives rise to the myelination gliosis which is probably largely a migratory phenomenon. Cell division continues throughout life in all parts of the brain studied. The greatest mitotic activity is centred in the subependymal layer where mitotic cells substantially outnumber pyknotic ones. There is a gradual decrease in mitotic activity in the subependymal layer up to 9 months of age with fairly constant mitotic activity thereafter. Mitotic activity in the indusium griseum levels out at 3 months postnatum with mitotic and pyknotic cells present in roughly equal numbers thereafter. Mitotic activity in all parts of the anterior commissure levels out at 6 months postnatum and remains constant thereafter. Mitotic and pyknotic cells are present in similar numbers except for a peak in pyknotic cells at 9 months. Cell number in the indusium griseum and anterior commissure is fairly constant between 3 and 9 months, but glial number begins to decrease in all parts of the anterior commissure from 12 to 22 months. In the indusium griseum the number of glia increased slightly between 6 and 22 months. The number of neurons fluctuated during the first week after birth then remained constant until 18 months. There was a significant decrease in the number of neurons between 18 and 22 months.

Baisc protein hydrolysis in lymphocytes of Lewis rats with experimental allergic encephalomyelitis

Lymphocytes from lymph nodes of Lewis rats with acute experimental allergic encephalomyelitis (EAE) contain high amounts of acid and neutral proteinases which hydrolyze myelin basic protein. The activity at neutral pH is also expressed by whole lymphocytes in isotonic medium, with about 50% more activity released by homogenization. Neutral proteinase activity in lymphocytes increases with the onset of acute EAE while the activity of those from Freund's adjuvant-injected controls increases somewhat later. The total neutral proteinase activity appears to be membrane-bound, most likely in the lysosomes, but half the total was associated with the nuclear fraction. The basic protein proteinase was compared with an enzyme described earlier, especially active toward polylysine, and some differences were noted. It appears that two enzymes may be present in lymphocytes which hydrolyze basic protein at a neutral pH. An increase in neutral proteinase activity was observed in some, but not all, lymphocyte preparations from patients in various stages of multiple sclerosis. The finding that whole activated lymphocytes are capable of hydrolyzing basic protein suggests that these cells which are believed to be precursors of mononuclear cells migrating into the central nervous system may be active agents in the early stages of myelin dissolution in experimental allergic encephalomyelitis. At present, such a mechanism is only theoretical, and the possibility that activated lymphocytes may be a factor in demyelination in multiple sclerosis is even more speculative.

Turnover of myelin proteins in mouse brain in vivo

The incorporation of tyrosine into proteins was measured after the subcutaneous implantation of a pellet of [14C]tyrosine in mice. This method keeps the specific radioactivity of free tyrosine fairly constant and makes it possible to follow incorporation up to a 10-day period. At the end of 10 days most of the protein-bound tyrosine was replaced (i.e. most protein turned over) in lung, liver, heart, kidney and spleen; about half was replaced in brain, one-quarter in muscle. The rate of protein turnover in myelin was approx. 40% of that of whole brain proteins; at 10 days one-fifth of the myelin proteins were replaced. All protein components of myelin measured were in a dynamic state; incorporation decreased in the following order, Wolfgram greater than DM-20 greater than basic greater than proteolipid proteins. The incorporation of tyrosine into each protein fraction was greater in the 0-5-day than in the 5-10-day period, indicating heterogeneity of metabolic rates. The results show that after myelination at least a portion of each protein component of myelin is undergoing significant metabolic turnover. In the adult, myelin components are not stable, but turnover is heterogeneous, and each protein may be compartmentalized. Turnover can be influenced by a variety of factors.

Evidence for defective incorporation of proteins in myelin of the quaking mutant mouse

The defect in myelinogenesis present in the Quaking mutant mouse was investigated using a double radioisotope technique for comparing the incorporation of amino acid into myelin proteins of normal and mutant mice. Quaking mice and littermate controls received intracranial injections of 150 muCi [3H]glycine and 25 muCi of [14C]glycine respectively. After 2 h their brains were combined and jointly processed to obtain subcellular fractions. The 3H/14C ratio for the myelin subfraction was 1.88 as compared to a 3H/14C ratio of 3.0 for the other subfractions, indicating a 40% decrease in glycine incorporation into myelin of Quaking mice. Myelin proteins were separated by discontinuous gel electrophoresis in the presence of sodium dodecyl sulfate (SDS) and the 3H/14C ratios determined in each gel slice. In contrast to the microsomal subfractions which gave a 3H/14C ratio of 2.6 across the gel, the 3H/14C ratio of myelin showed large variations with values ranging from 0.54 for proteolipid protein to 2.0 for some of the high molecular weight proteins. During development, the Quaking mutant exhibited a preferential depression in glycine incorporation into proteolipid protein in 18-day-old mice, while in older animals (32-54 days) the fast migrating basic protein, as well as the proteolipid protein, was labeled to a significantly lesser extent.

Synthesis and turnover of cerebrosides and phosphatidylserine of myelin and microsomal fractions of adult and developing rat brain

The synthesis and turnover of cerebrosides and phospholipids was followed in microsomal and myelin fractions of developing and adult rat brains after an intracerebral injection of [U-14C]serine. The kinetics of incorporation of radioactivity into microsomal and myelin cerebrosides indicate the possibility of a precursor-product relationship between cerebrosides of these membranes. The specific radioactivity of myelin cerebrosides was corrected for the deposition of newly formed cerebrosides in myelin. Multiphasic curves were obtained for the decline in specific radioactivity of myelin and microsomal cerebrosides, suggesting different cerebroside pools in these membranes. The half-life of the fast turning-over pool of cerebrosides of myelin was 7 and 22 days for the developing and adult rat brain respectively. The half-life of the slowly turning-over pool of myelin cerebrosides was about 145 days for both groups of animals. The half-life of the rapidly turning-over microsomal cerebrosides was calculated to be 20 and 40 h for the developing and adult animals respectively. The half-life of the intermediate and slowly turning-over microsomal cerebrosides was 11 and 60 days respectively, for both groups of animals. The amount of incorporation of radioactivity into microsomal cerebrosides from L-serine was greatly decreased in the adult animals, and greater amounts of the precursor were directed towards the synthesis of phosphatidylserine. In the developing animals, considerable amounts of cerebrosides were synthesized from L-serine, besides phosphatidylserine. The time-course of incorporation indicated that a precursor-product relationship exists between microsomal and myelin phosphatidylserine. The half-life of microsomal phosphatidylserine was calculated to be about 8 h for the fast turning-over pool in both groups of animals.