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In vivo human apolipoprotein E isoform fractional turnover rates in the CNS.

Wildsmith KR, Basak JM, Patterson BW, Pyatkivskyy Y, Kim J, Yarasheski KE, Wang JX, Mawuenyega KG, Jiang H, Parsadanian M, Yoon H, Kasten T, Sigurdson WC, Xiong C, Goate A, Holtzman DM, Bateman RJ - PLoS ONE (2012)

Bottom Line: No isoform-specific differences in CNS ApoE3 and ApoE4 turnover rates were observed when measured in human CSF or mouse brain.However, CNS and peripheral ApoE isoform turnover rates differed substantially, which is consistent with previous reports and suggests that the pathways responsible for ApoE metabolism are different in the CNS and the periphery.We also demonstrate a slower turnover rate for CSF ApoE than that for amyloid beta, another molecule critically important in AD pathogenesis.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurology, Washington University School of Medicine, Saint Louis, Missouri, United States of America.

ABSTRACT
Apolipoprotein E (ApoE) is the strongest genetic risk factor for Alzheimer's disease and has been implicated in the risk for other neurological disorders. The three common ApoE isoforms (ApoE2, E3, and E4) each differ by a single amino acid, with ApoE4 increasing and ApoE2 decreasing the risk of Alzheimer's disease (AD). Both the isoform and amount of ApoE in the brain modulate AD pathology by altering the extent of amyloid beta (Aβ) peptide deposition. Therefore, quantifying ApoE isoform production and clearance rates may advance our understanding of the role of ApoE in health and disease. To measure the kinetics of ApoE in the central nervous system (CNS), we applied in vivo stable isotope labeling to quantify the fractional turnover rates of ApoE isoforms in 18 cognitively-normal adults and in ApoE3 and ApoE4 targeted-replacement mice. No isoform-specific differences in CNS ApoE3 and ApoE4 turnover rates were observed when measured in human CSF or mouse brain. However, CNS and peripheral ApoE isoform turnover rates differed substantially, which is consistent with previous reports and suggests that the pathways responsible for ApoE metabolism are different in the CNS and the periphery. We also demonstrate a slower turnover rate for CSF ApoE than that for amyloid beta, another molecule critically important in AD pathogenesis.

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Brain ApoE kinetics in ApoE3 and ApoE4 targeted replacement mice.ApoE was extracted from brains of ApoE3/E3 and ApoE4/E4 mice labeled with 13C6-leucine. Similar kinetics were observed for ApoE3 and ApoE4 mice with monoexponential slopes of 6.2±0.48%/h and 4.8±1.12%/h, respectively (blue: ApoE3, black: ApoE4, n = 3–6 mice per time point, P  = 0.2817, error bars represent SEM).
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pone-0038013-g005: Brain ApoE kinetics in ApoE3 and ApoE4 targeted replacement mice.ApoE was extracted from brains of ApoE3/E3 and ApoE4/E4 mice labeled with 13C6-leucine. Similar kinetics were observed for ApoE3 and ApoE4 mice with monoexponential slopes of 6.2±0.48%/h and 4.8±1.12%/h, respectively (blue: ApoE3, black: ApoE4, n = 3–6 mice per time point, P  = 0.2817, error bars represent SEM).

Mentions: Mice that have been genetically modified to express the human ApoE isoforms via the endogenous mouse ApoE promoter have become useful tools for studying the effect of human ApoE isoforms on various neurological conditions [5]. Several studies have analyzed ApoE levels in the brains of these mice, but the results have not been consistent [23], [24], [27], [28], [32]. In the current study, the brain kinetics of ApoE3 and ApoE4 were evaluated by pulse 13C6-leu labeling mice that were homozygous for either ApoE3 or ApoE4. The appearance of labeled ApoE in the mouse brain cortex was then quantified using LC/MS/MS. By plotting the ratio of labeled ApoE to unlabeled ApoE from the brains of different mice at various time points following the 13C6-leu pulse injection, a kinetic time course for ApoE3 and ApoE4 was obtained (Fig. 5A). ApoE turnover was characterized by a rapid rise in 13C6-leu labeling for the first hour following tracer injection, followed by a steady decrease in 13C6-leu labeling for the next 12 hrs. Similar to the analysis of the human data, the monoexponential slope FCR was calculated from the decreasing regions of the TTR time course for each ApoE isoform (Fig. 5B) (ApoE3 6.19±0.48%/h, ApoE4 4.80±1.12%/h, p = 0.2817). No statistically significant differences were observed between the FCR of ApoE3 and ApoE4. FSR values were not calculated because there were not a sufficient number of data points to accurately measure a linear front-end slope. We also measured brain ApoE levels in these mice by ELISA, and observed that ApoE4 mice had 12% less ApoE than ApoE3 mice (Table S1). Since the FCR values were not different between genotype, the small difference in protein levels is more likely due to changes in ApoE production. Therefore, we measured brain ApoE mRNA levels in the mice and found that ApoE4 mice have 20% lower mRNA levels than ApoE3 mice (Fig. S1).


In vivo human apolipoprotein E isoform fractional turnover rates in the CNS.

Wildsmith KR, Basak JM, Patterson BW, Pyatkivskyy Y, Kim J, Yarasheski KE, Wang JX, Mawuenyega KG, Jiang H, Parsadanian M, Yoon H, Kasten T, Sigurdson WC, Xiong C, Goate A, Holtzman DM, Bateman RJ - PLoS ONE (2012)

Brain ApoE kinetics in ApoE3 and ApoE4 targeted replacement mice.ApoE was extracted from brains of ApoE3/E3 and ApoE4/E4 mice labeled with 13C6-leucine. Similar kinetics were observed for ApoE3 and ApoE4 mice with monoexponential slopes of 6.2±0.48%/h and 4.8±1.12%/h, respectively (blue: ApoE3, black: ApoE4, n = 3–6 mice per time point, P  = 0.2817, error bars represent SEM).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3366983&req=5

pone-0038013-g005: Brain ApoE kinetics in ApoE3 and ApoE4 targeted replacement mice.ApoE was extracted from brains of ApoE3/E3 and ApoE4/E4 mice labeled with 13C6-leucine. Similar kinetics were observed for ApoE3 and ApoE4 mice with monoexponential slopes of 6.2±0.48%/h and 4.8±1.12%/h, respectively (blue: ApoE3, black: ApoE4, n = 3–6 mice per time point, P  = 0.2817, error bars represent SEM).
Mentions: Mice that have been genetically modified to express the human ApoE isoforms via the endogenous mouse ApoE promoter have become useful tools for studying the effect of human ApoE isoforms on various neurological conditions [5]. Several studies have analyzed ApoE levels in the brains of these mice, but the results have not been consistent [23], [24], [27], [28], [32]. In the current study, the brain kinetics of ApoE3 and ApoE4 were evaluated by pulse 13C6-leu labeling mice that were homozygous for either ApoE3 or ApoE4. The appearance of labeled ApoE in the mouse brain cortex was then quantified using LC/MS/MS. By plotting the ratio of labeled ApoE to unlabeled ApoE from the brains of different mice at various time points following the 13C6-leu pulse injection, a kinetic time course for ApoE3 and ApoE4 was obtained (Fig. 5A). ApoE turnover was characterized by a rapid rise in 13C6-leu labeling for the first hour following tracer injection, followed by a steady decrease in 13C6-leu labeling for the next 12 hrs. Similar to the analysis of the human data, the monoexponential slope FCR was calculated from the decreasing regions of the TTR time course for each ApoE isoform (Fig. 5B) (ApoE3 6.19±0.48%/h, ApoE4 4.80±1.12%/h, p = 0.2817). No statistically significant differences were observed between the FCR of ApoE3 and ApoE4. FSR values were not calculated because there were not a sufficient number of data points to accurately measure a linear front-end slope. We also measured brain ApoE levels in these mice by ELISA, and observed that ApoE4 mice had 12% less ApoE than ApoE3 mice (Table S1). Since the FCR values were not different between genotype, the small difference in protein levels is more likely due to changes in ApoE production. Therefore, we measured brain ApoE mRNA levels in the mice and found that ApoE4 mice have 20% lower mRNA levels than ApoE3 mice (Fig. S1).

Bottom Line: No isoform-specific differences in CNS ApoE3 and ApoE4 turnover rates were observed when measured in human CSF or mouse brain.However, CNS and peripheral ApoE isoform turnover rates differed substantially, which is consistent with previous reports and suggests that the pathways responsible for ApoE metabolism are different in the CNS and the periphery.We also demonstrate a slower turnover rate for CSF ApoE than that for amyloid beta, another molecule critically important in AD pathogenesis.

View Article: PubMed Central - PubMed

Affiliation: Department of Neurology, Washington University School of Medicine, Saint Louis, Missouri, United States of America.

ABSTRACT
Apolipoprotein E (ApoE) is the strongest genetic risk factor for Alzheimer's disease and has been implicated in the risk for other neurological disorders. The three common ApoE isoforms (ApoE2, E3, and E4) each differ by a single amino acid, with ApoE4 increasing and ApoE2 decreasing the risk of Alzheimer's disease (AD). Both the isoform and amount of ApoE in the brain modulate AD pathology by altering the extent of amyloid beta (Aβ) peptide deposition. Therefore, quantifying ApoE isoform production and clearance rates may advance our understanding of the role of ApoE in health and disease. To measure the kinetics of ApoE in the central nervous system (CNS), we applied in vivo stable isotope labeling to quantify the fractional turnover rates of ApoE isoforms in 18 cognitively-normal adults and in ApoE3 and ApoE4 targeted-replacement mice. No isoform-specific differences in CNS ApoE3 and ApoE4 turnover rates were observed when measured in human CSF or mouse brain. However, CNS and peripheral ApoE isoform turnover rates differed substantially, which is consistent with previous reports and suggests that the pathways responsible for ApoE metabolism are different in the CNS and the periphery. We also demonstrate a slower turnover rate for CSF ApoE than that for amyloid beta, another molecule critically important in AD pathogenesis.

Show MeSH
Related in: MedlinePlus