Interaction of sortilin with apolipoprotein E3 enables neurons to use long-chain fatty acids as alternative metabolic fuel

Binding of apoE4 traps internalized sortilin in early endosomes, preventing discharge of cargo and recycling to the cell surface.

“Similarly to other receptors bound by apoE73,74, binding of apoE4 traps internalized sortilin in early endosome, preventing discharge of cargo and recycling to the cell surface15.”

• cites [73] Chen, Y., Durakoglugil, M. S., Xian, X. & Herz, J. ApoE4 reduces glutamate receptor function and synaptic plasticity by selectively impairing ApoE receptor recycling. Proc. Natl Acad. Sci. USA 107, 12011–12016 (2010). sourcePartly supported

  The Discussion (page unavailable) reports that ApoE4 sequesters Apoer2 in intracellular recycling compartments, reducing its expression at the neuronal surface. It does not identify sortilin as the trapped receptor or establish that cargo discharge is prevented in early endosomes. (LLM, conf=0.86)

  Experimental — accuracy not yet measured
• cites [74] Zhao, N. et al. Apolipoprotein E4 impairs neuronal insulin signaling by trapping insulin receptor in the endosomes. Neuron 96, 115–129 (2017). sourcePartly supported

  The Results section reports that apoE4 retains insulin receptors in endosomes and impairs their recycling to the cell surface. Our copy does not establish that the receptor is sortilin or that cargo discharge is prevented; searches for sortilin and its cargo trafficking found no supporting passage. No page numbers were provided. (LLM, conf=0.83)

  Experimental — accuracy not yet measured
• cites [15] Asaro, A. et al. ApoE4 disrupts interaction of sortilin with fatty acid-binding protein 7 essential to promote lipid signaling. J. Cell Sci. 134, jcs258894 (2021). sourceNot checked yet

The noxious propensity of apoE4 is due to an increase in isoelectric point compared to apoE3, resulting in unfolding of the protein in the acidic milieu of endosomes.

“This noxious propensity of apoE4 is due to an increase in the isoelectric point, as compared to apoE3, resulting in unfolding of the protein in the acidic milieu of endosomes75–79.”

• cites [75] Heeren, J., Beisiegel, U. & Grewal, T. Apolipoprotein E recycling: implications for dyslipidemia and atherosclerosis. Arterioscler Thromb. Vasc. Biol. 26, 442–448 (2006). sourceNot checked — no full text of the source
• cites [76] Heeren, J. et al. Impaired recycling of apolipoprotein E4 is associated with intracellular cholesterol accumulation. J. Biol. Chem. 279, 55483–55492 (2004). sourcePartly supported

  The Discussion (p. 9) says apoE4 is more prone than apoE3 to form a molten globule at low pH and proposes that acidification in early endosomes may change apoE4’s conformation. It does not attribute that difference to a higher isoelectric point, and it presents the endosomal mechanism as a possibility rather than an established cause. (LLM, conf=0.88)

  Experimental — accuracy not yet measured
• cites [77] Heeren, J. et al. Recycling of apoprotein E is associated with cholesterol efflux and high density lipoprotein internalization. J. Biol. Chem. 278, 14370–14378 (2003). sourceCould not verify

  The Experimental Procedures give an isoelectric point for apoE3 (pI 5.78), and the Discussion describes apoE in endosomes during recycling. Our copy does not compare apoE4 with apoE3 or say that acidic endosomes cause apoE4 to unfold (searched: isoelectric point, apoE4, unfolding). No page numbers were available for these passages. (LLM, conf=0.77)

  Experimental — accuracy not yet measured
• cites [78] Xian, X. et al. Reversal of ApoE4-induced recycling block as a novel prevention approach for Alzheimer’s disease. Elife 7, e40048 (2018). sourcePartly supported

  The Introduction says ApoE4 differs from ApoE3 in isoelectric point and is prone to unfolding in acidic endosomes. The Results and Discussion describe the proposed harmful mechanism more cautiously: ApoE4’s structural lability, together with reduced solubility near its isoelectric point, might cause the recycling block. They do not establish that the isoelectric-point change itself causes the unfolding, as the citing sentence implies. No page numbers are available in our copy. (LLM, conf=0.85)

  Experimental — accuracy not yet measured
• cites [79] Pohlkamp, T. et al. NHE6 depletion corrects ApoE4-mediated synaptic impairments and reduces amyloid plaque load. Elife 10, e72034(2021). sourcePartly supported

  The Introduction reports that ApoE4 has a higher isoelectric point than ApoE3 (about 6.4 versus 6.1). The Discussion proposes isoelectric precipitation and structural unfolding at early-endosomal pH as a mechanism for impaired recycling. The citing sentence presents that proposed mechanism as an established cause, which the source does not establish. (LLM, conf=0.84)

  Experimental — accuracy not yet measured

Loss of sortilin in E3KO iNs results in a drastic decrease in cellular content of PUFAs compared with the isogenic E3WT line.

“Loss of sortilin in E3KO iNs resulted in a drastic decrease in cellular content of PUFAs when compared with the isogenic E3WT line (Fig. 5a).”

Decreased transcript levels in E3KO iNs include those encoding ACADL, carnitine transporter SLC22A5, and fatty acid transport proteins SLC27A1, SLC27A5, and SLC27A6.

“With relevance to lipid homeostasis, decreased transcript levels included those encoding ACADL, carnitine transporter SLC22A5, as well as fatty acid transport proteins SLC27A1, SLC27A5 and SLC27A6.”

SORT1 deficiency decreases the levels of PUFAs and eCBs in human apoE3 neurons.

“SORT1 deficiency decreases the levels of PUFAs and eCBs in human apoE3 neurons”

Sortilin deficiency has no impact on transcription of genes encoding PPARδ and PPARγ in iNs.

“No impact on transcription was seen for genes encoding PPAR family members PPARδ and PPARγ (Fig. 6d).”

SORT1 deficiency does not alter concentrations of most PUFAs and eCBs in E4KO iNs compared with isogenic E4WT neurons.

“Importantly, SORT1 deficiency did not alter the concentrations of most PUFAs and eCBs in E4KO iNs when compared with isogenic E4WT neurons (Extended Data Fig. 9), a finding in agreement with apoE4 rendering WT neurons functionally deficient for sortilin.”

In mouse brain cortices, transcript levels of genes implicated in fatty acid metabolism and mitochondrial lipid consumption show no interaction between APOE and Sort1 genotypes whereby E3WT would be significantly different from the other three genotypes.

“No interactions between APOE and Sort1 genotypes were seen whereby transcript levels in E3WT would be significantly different from those in the other three genotypes (Extended Data Fig. 4).”

Most of these dysregulated genes show reduced transcript levels when neurons lack sortilin.

“Most of these genes showed reduced transcript levels when neurons lacked the receptor (Fig. 6c).”

In E3KO iNs, electrical activities significantly decrease in the absence of glucose, regardless of the absence or presence of palmitate.

“By contrast, electrical activities significantly decreased in E3KO iNs in the absence of glucose, regardless of the absence or presence of palmitate (Fig. 4i–k and Extended Data Fig. 8d).”

Some apoE genotype-dependent transcriptional changes are observed in astrocytes, including Slc16a1, Cpt2, and Acadl.

“Some apoE genotype-dependent transcriptional changes were observed in astrocytes (Slc16a1, Cpt2 and Acadl; Fig. 3c), in line with the established impact of apoE variants on astrocyte metabolism25.”

• cites [25] Lee, H. et al. ApoE4-dependent lysosomal cholesterol accumulation impairs mitochondrial homeostasis and oxidative phosphorylation in human astrocytes. Cell Rep. 42, 113183 (2023). sourceNot checked yet

In a second independent APOE ε3/ε3 iPS cell line, etomoxir robustly impacts maximal respiration in E3*WT but not in E3*KO neurons.

“When differentiated to iNs (Extended Data Fig. 6g,h), a robust impact of etomoxir on maximal respiration was seen in E3*WT (Extended Data Fig. 6i and Supplementary Fig. 9a) but not in E3*KO (Extended Data Fig. 6j and Supplementary Fig. 9b) neurons.”

When glucose is depleted and palmitate is added as an alternative fuel, mitochondrial respiration is sustained in E3WT synaptosomes but shuts down completely in E3KO, E4WT, and E4KO synaptosomes.

“In these experiments, mitochondrial respiration sustained in E3WT synaptosomes, but shut down completely in the three other genotypes, possibly impacting overall cell viability and (mitochondrial) functions even further (Fig. 2a).”

Insensitivity to etomoxir is also observed in synaptosomes isolated from female animals.

“Insensitivity to etomoxir, as seen in synaptosomes from male mice in Fig. 1h–j, was also observed in synaptosomes isolated from female animals (Extended Data Fig. 2 and Supplementary Fig. 3).”

Maximal respiration decreases in E3WT iNs when treated with UK5099 or etomoxir in the presence of normal glucose levels.

“Mirroring phenotypes seen in mouse synaptosomes, maximal respiration decreased in E3WT iNs when treated with UK5099 or etomoxir in the presence of normal glucose levels (Fig. 4a and Supplementary Fig. 8a).”

In E3WT iN cultures, burst duration, burst count, and network burst count significantly deteriorate when treated with media lacking glucose and palmitate.

“Burst duration, burst count and network burst count all significantly deteriorated in E3WT iN cultures when treated with media lacking glucose and palmitate (Fig. 4i–k).”

E4WT iNs are insensitive to etomoxir when exposed to conditioned medium from E4WT astrocytes.

“As shown in Fig. 4c, maximal respiration in E4WT iNs was insensitive to etomoxir when neurons were exposed to conditioned medium from E4WT astrocytes (Fig. 4e,g and Supplementary Fig. 10a).”

MCFAs heptanoic acid and octanoic acid increase maximal respiration in all four genotypes.

“MCFAs increased maximal respiration, an effect seen in all four genotypes (Fig. 2j–m and Supplementary Fig. 6).”

Genetic or apoE4-induced sortilin deficiency disrupts lipid fuel choice and synaptic activity in human neurons.

“Genetic or apoE4-induced sortilin deficiency disrupts lipid fuel choice and synaptic activity in human neurons”

When fully supplemented with glucose, electrical activities including burst duration, burst count, and network burst count are comparable in E3WT and E3KO iNs.

“When fully supplemented with glucose, electrical activities, including burst duration, burst count and network burst count, were comparable in both genotypes (Fig. 4h and Extended Data Fig. 8a,b).”

Sortilin deficiency decreases transcript levels of PPARA in iNs.

“Sortilin deficiency also decreased transcript levels of PPARA (encoding PPARα), a master transcriptional regulator of cellular lipid metabolism19 (Fig. 6d).”

• cites [19] Gervois, P., Torra, I. P., Fruchart, J. C. & Staels, B. Regulation of lipid and lipoprotein metabolism by PPAR activators. Clin. Chem. Lab. Med. 38, 3–11 (2000). sourceNot checked — no full text of the source

Microarray analysis identifies 30 transcripts that are dysregulated in E3KO compared to isogenic E3WT iNs.

“Using a microarray-based strategy to assess transcript levels of 84 PPAR targets, we identified 30 transcripts that were dysregulated in E3KO compared to isogenic E3WT iNs (Fig. 6a,b).”

Sensitivity to etomoxir is restored in E4WT iNs when exposed to medium conditioned by E3WT iAs.

“However, sensitivity to etomoxir was restored in E4WT iNs when exposed to medium conditioned by E3WT iAs (Fig. 4f,g and Supplementary Fig. 10b).”

Palmitate supplementation increases concentrations of citrate, malate, and fumarate in E3WT synaptosomes but not in E3KO synaptosomes.

“As documented by liquid chromatography–tandem mass spectrometry (LC–MS/MS)-based metabolomics, supplementation with palmitate increased the concentrations of citrate, malate and fumarate in E3WT synaptosomes (Extended Data Fig. 3a), an effect not seen in E3KO (Extended Data Fig. 3b).”

An interaction of APOE and Sort1 genotypes on transcript levels is seen only in neurons and includes the fatty acid oxidation enzyme Acadl and the carnitine acyltransferases Crat and Cpt1a.

“However, an interaction of APOE and Sort1 genotypes on transcript levels was only seen in neurons and included the fatty acid oxidation enzyme long-chain acyl-CoA dehydrogenase Acadl, and the carnitine acyltransferases Crat and Cpt1a (Fig. 3c).”

Depletion of PUFA and eCB contents in E3KO neurons is not due to alterations in levels of these lipids released by iAs, as concentrations of the tested lipids are comparable in conditioned media from E3WT and E3KO iAs.

“Depletion of PUFA and eCB contents in E3KO neurons was not due to alterations in levels of these lipids released by iAs as the concentrations of the tested lipids were comparable in conditioned media from E3WT and E3KO iAs (Fig. 5b).”

Basal respiration, proton leak, and ATP-linked respiration in synaptosomes do not show an interaction between Sort1 and APOE genotypes.

“Other mitochondrial parameters, including basal respiration (Fig. 1c), proton leak (Fig. 1e) or ATP-linked respiration (Fig. 1f) did not show such interaction of genotypes.”

Loss of sortilin in E3KO human neurons coincides with a compensatory increase in FABP6 protein levels.

“and with a concomitant compensatory increase in levels of FABP6, an alternative intracellular fatty acid carrier32 (Fig. 6i).”

LCFA, PUFA and eCB precursors show comparable levels in media conditioned by apoE3 or apoE4 expressing WT astrocytes.

“However, LCFA, PUFA and eCB precursors showed comparable levels in media conditioned by apoE3 (Fig. 5b) or apoE4 (Extended Data Fig. 9b) expressing WT astrocytes.”

Bezafibrate treatment confers sensitivity to etomoxir in E3KO iNs, as documented by a bezafibrate-induced decrease in maximal respiration in etomoxir-treated cells compared to control iNs treated with etomoxir without bezafibrate.

“Importantly, bezafibrate treatment also conferred sensitivity to etomoxir in E3KO iNs as documented by a bezafibrate-induced decrease in maximal respiration in cells treated with etomoxir as compared to control iNs treated with etomoxir in the absence of bezafibrate (Fig. 7b,c and Supplementary Fig. 11).”

ApoE genotype-dependent changes in transcriptional profiles occur in primary E3 versus E4 astrocytes.

“This conclusion is also supported by apoE genotype-dependent changes in transcriptional profiles in primary E3 versus E4 astrocytes seen in this study (Fig. 3).”

Electrical activity is comparable in E3WT and E3KO neurons fully supplemented with glucose.

“Along these lines, electrical activity is comparable in E3WT and E3KO neurons fully supplemented with glucose (Fig. 4h).”

Blockade of mitochondrial import of LCFAs by etomoxir decreases maximal respiration in E3WT mitochondria even in the presence of sufficient glucose supply.

“As shown in substrate oxidation stress tests, blockade of mitochondrial import of LCFAs by etomoxir decreases maximal respiration in E3WT mitochondria, even in the presence of sufficient glucose supply.”

Loss of sortilin in E3KO human neurons coincides with reduced FABP7 protein levels.

“Loss of sortilin coincided with reduced levels of FABP7, a known sortilin interactor15 and intracellular carrier for PUFAs and eCBs (Fig. 6h)”

Neurons sorted from E3WT mouse brains show induced expression of genes related to LCFA consumption compared to E3KO and E4 neurons.

“Induced expression of genes related to LCFA consumption in neurons sorted from E3WT mouse brains, as compared to E3KO and E4 neurons (Fig. 3), further substantiate the relevance of this pathway for in vivo brain lipid metabolism under normal (that is, full glucose) conditions.”

Bezafibrate treatment establishes sensitivity to etomoxir in E4WT and E4KO iNs.

“and established sensitivity to etomoxir (Fig. 7e,f,h,i and Supplementary Figs. 12 and 13).”

The etomoxir-induced decrease in maximal respiration is seen in mouse synaptosomes and human neurons.

“This effect is seen in mouse synaptosomes (Fig. 1g) and human neurons (Fig. 4a).”

E3KO iNs show decreased levels of synaptamide, 2-arachidonoylglycerol and linoleoyl ethanolamide.

“This conclusion is supported by decreased levels of synaptamide, 2-arachidonoylglycerol and linoleoyl ethanolamide (Fig. 5a), known ligands of this transcription factor57,58.”

Synaptosomes prepared from female mice also show an interaction of apoE and sortilin genotypes in defining sensitivity to etomoxir.

“Similarly to males, synaptosomes prepared from female mice also show an interaction of apoE and sortilin genotypes in defining sensitivity to etomoxir (Extended Data Fig. 2).”

Loss of sortilin activity in human E3KO neurons coincides with a drastic reduction in neuronal levels of PUFAs and their bioactive derivatives, most notably eCBs and eCB-like metabolites.

“Using targeted lipidomics, we show that loss of sortilin activity in human E3KO neurons coincides with a drastic reduction in neuronal levels of PUFAs and their bioactive derivatives, most notably eCBs and eCB-like metabolites (Fig. 5a).”

Bezafibrate treatment increases PPARα activity in E4WT and E4KO iNs.

“Similarly to the situation in E3KO, bezafibrate treatment also increased PPARα activity in E4WT and E4KO iNs (Fig. 7d,g)”

Induction of PPARα activity by bezafibrate increases transcript levels of PPARA and CPT1A in E3KO iNs.

“As expected, induction of PPARα activity by bezafibrate increased transcript levels of its targets PPARA and CPT1A in E3KO iNs (Fig. 7a).”

Sortilin deficiency per se does not impact synapse function in this model.

“These findings documented that sortilin deficiency per se did not impact synapse function in our model.”

The adapted neuronal differentiation protocol produces iNs from all four genotypes comparable in cellular appearance and neuronal marker expression to neurons differentiated using the standard astrocyte co-culture protocol.

“This protocol reliably produced iPS cell-derived neurons from all four genotypes (referred to as induced neurons, iNs), comparable in cellular appearance and neuronal marker expression to neurons differentiated using the standard astrocyte co-culture protocol (Extended Data Fig. 5e,f).”

APOE and SORT1 genotypes do not impact pluripotency of the iPS cell lines, as shown by testing expression of pluripotency markers.

“APOE and SORT1 genotypes did not impact pluripotency of the iPS cell lines as shown by testing expression of pluripotency markers using immunocytochemistry (Supplementary Fig. 7e,f) and quantitative PCR with reverse transcription (RT–qPCR; Supplementary Fig. 7d,g,h).”

All four iPS cell genotypes generate induced astrocytes comparable in appearance and expression of astrocyte markers GFAP and S100β.

“All four genotypes generated astrocytes (referred to herein as induced astrocytes, iAs) that were comparable in appearance as well as in expression of astrocyte markers glial fibrillary acid protein (GFAP) and S100β, as documented by immunocytochemistry (Extended Data Fig. 5b) and RT–qPCR (Extended Data Fig. 5c).”

The reduced ability of E3KO and E4 synaptosomes to use fatty acids as fuel is restricted to LCFAs requiring transport by the carnitine shuttle.

“This observation indicated that the reduced ability of E3KO and E4 synaptosomes to use fatty acids as fuel was restricted to LCFAs requiring transport by the carnitine shuttle.”

• AssumptionRests on an assumption: Only LCFAs require the carnitine shuttle system, whereas SCFAs and MCFAs can pass mitochondrial membranes.

Purity of sorted neuronal and astrocyte preparations is confirmed by the exclusive presence of neuronal marker transcripts NeuN/Rbfox3 and Baf53b/Actl6b in sorted neurons and astrocyte marker transcripts Aldh1l1 and Gfap in sorted astrocytes.

“Purity of our cell preparations was confirmed by the exclusive presence of neuronal marker transcripts NeuN/Rbfox3 (NeuN) and Baf53b/Actl6b (Baf53b) in sorted neurons (Fig. 3a) and astrocyte marker transcripts Aldh1l1 and Gfap in sorted astrocytes (Fig. 3b).”

Sortilin mediates neuronal uptake of polyunsaturated fatty acids carried by apolipoprotein E (apoE).

“Here we show that sortilin mediates neuronal uptake of polyunsaturated fatty acids carried by apolipoprotein E (apoE).”

Sortilin and apoE3 interactions distinctly affect fatty acid metabolism in neurons but not astrocytes.

“Apparently, sortilin and apoE3 interactions distinctly affected the fatty acid metabolism in neurons, but not astrocytes, a finding in line with the predominant neuronal expression of the receptor26.”

• cites [26] Sarret, P. et al. Distribution of NTS3 receptor/sortilin mRNA and protein in the rat central nervous system. J. Comp. Neurol. 461, 483–505 (2003). sourceNot checked — no full text of the source

In mouse models, sortilin-dependent uptake of lipids works well with apoE3, the most common isoform of the lipid carrier in humans.

“In mouse models, sortilin-dependent uptake of lipids works well with apoE3, the most common isoform of the lipid carrier in humans.”

• cites [14] Asaro, A. et al. Apolipoprotein E4 disrupts the neuroprotective action of sortilin in neuronal lipid metabolism and endocannabinoid signaling. Alzheimers Dement. 16, 1248–1258 (2020). sourceCheck pending
• cites [15] Asaro, A. et al. ApoE4 disrupts interaction of sortilin with fatty acid-binding protein 7 essential to promote lipid signaling. J. Cell Sci. 134, jcs258894 (2021). sourceCheck pending

Loss of sortilin expression in SORT1-defective iPS cell lines is confirmed by western blotting.

“Loss of sortilin expression was confirmed by western blotting (Supplementary Fig. 7c).”

Internalized lipids are converted into ligands for peroxisome proliferator-activated receptor alpha, inducing transcription profiles that enable neurons to use long-chain fatty acids as metabolic fuel when glucose is limited.

“Using humanized mouse strains and induced pluripotent stem cell-based cell models of brain lipid homeostasis, we demonstrate that internalized lipids are converted into ligands for peroxisome proliferator-activated receptor alpha inducing transcription profiles that enable neurons to use long-chain fatty acids as metabolic fuel when glucose is limited.”

E3KO and E4 genotypes are unable to utilize LCFAs as fuel for mitochondrial energy production.

“These findings documented an inability of E3KO and E4 genotypes to utilize LCFAs as fuel for mitochondrial energy production.”

Sortilin acts as a lipoprotein receptor in the brain, facilitating neuronal uptake of apolipoprotein E (apoE), the major carrier for lipids in brain interstitial fluids.

“Recently, we showed that sortilin also acts as a lipoprotein receptor in the brain, facilitating neuronal uptake of apolipoprotein E (apoE), the major carrier for lipids in brain interstitial fluids10.”

• cites [10] Carlo, A. S. et al. The pro-neurotrophin receptor sortilin is a major neuronal apolipoprotein E receptor for catabolism of amyloid-beta peptide in the brain. J. Neurosci. 33, 358–370 (2013). sourceNot checked yet

No discernible genotype-dependent differences in expression of neuronal markers MAP2, TUJ1 (TUBB3), NEFL, and RBFOX3 are seen by immunocytochemistry or RT-qPCR.

“No discernible genotype-dependent differences in expression of neuronal markers MAP2 as well TUJ1 (TUBB3), neurofilament light chain (NEFL) and RNA binding Fox-1 homologue 3 (RBFOX3) were seen by immunocytochemistry (Extended Data Fig. 5h) or RT–qPCR (Extended Data Fig. 5i).”

Disrupted interactions of sortilin with apoE do not impact global brain lipid handling.

“These findings argued that disrupted interactions of sortilin with apoE did not impact global brain lipid handling.”

Genetic or functional deficiency of sortilin disrupts the ability of neurons to support mitochondrial energy homeostasis through LCFA consumption.

“Jointly, our findings in mouse and human cell models corroborated our hypothesis that genetic (E3KO, E4KO) or functional (E4WT) deficiency of sortilin disrupts the ability of neurons to support mitochondrial energy homeostasis through LCFA consumption.”

iAs of all four genotypes produce and secrete similar amounts of apoE variants.

“Importantly, iAs of all four genotypes produced and secreted similar amounts of apoE variants as shown by comparative RT–qPCR (Extended Data Fig. 5c) and western blotting (Extended Data Fig. 5d).”

Levels of PUFAs and endocannabinoids are decreased in the brains of E3KO, E4WT, and E4KO mice compared to E3WT animals.

“We previously reported that levels of PUFAs and endocannabinoids (eCBs) were decreased in the brains of E3KO, E4WT and E4KO mice as compared to E3WT animals14.”

• cites [14] Asaro, A. et al. Apolipoprotein E4 disrupts the neuroprotective action of sortilin in neuronal lipid metabolism and endocannabinoid signaling. Alzheimers Dement. 16, 1248–1258 (2020). sourceCheck pending

Deteriorating lipoprotein receptor functions of sortilin in the ageing brain may aggravate deficits in neuronal energy production in the older population.

“As both dementias are characterized by a hypometabolism of glucose40–44,72, deteriorating lipoprotein receptor functions of sortilin in the ageing brain may well aggravate deficits in neuronal energy production in the older population.”

Endocytic uptake of lipidated apoE3 by sortilin delivers PUFAs to neurons to be converted into ligands for the transcriptional activator PPARα.

“We propose that endocytic uptake of lipidated apoE3 by this receptor delivers PUFAs to neurons to be converted into ligands for the transcriptional activator PPARα.”

LCFA consumption helps raise neuronal energy production to meet maximal demand even if sufficient glucose is available.

“These findings argue that LCFA consumption helps to raise neuronal energy production to meet maximal demand, even if sufficient glucose is available.”

Synaptamide, 2-arachidonoylglycerol and linoleoyl ethanolamide are known ligands of PPARα.

“This conclusion is supported by decreased levels of synaptamide, 2-arachidonoylglycerol and linoleoyl ethanolamide (Fig. 5a), known ligands of this transcription factor57,58.”

• cites [57] Artmann, A. et al. Influence of dietary fatty acids on endocannabinoid and N-acylethanolamine levels in rat brain, liver and small intestine. Biochim. Biophys. Acta 1781, 200–212 (2008). sourceNot checked — no full text of the source
• cites [58] Kozak, K. R. et al. 15-Lipoxygenase metabolism of 2-arachidonylglycerol. Generation of a peroxisome proliferator-activated receptor alpha agonist. J. Biol. Chem. 277, 23278–23286 (2002). sourceCheck pending

Loss of PUFAs is attributed to a prominent role of sortilin in neuronal uptake of lipidated apoE, providing essential lipids for cellular lipid metabolism and action.

“Loss of PUFAs is attributed to a prominent role of the receptor in neuronal uptake of lipidated apoE, providing essential lipids for cellular lipid metabolism and action10,14.”

• cites [10] Carlo, A. S. et al. The pro-neurotrophin receptor sortilin is a major neuronal apolipoprotein E receptor for catabolism of amyloid-beta peptide in the brain. J. Neurosci. 33, 358–370 (2013). sourceNot checked yet
• cites [14] Asaro, A. et al. Apolipoprotein E4 disrupts the neuroprotective action of sortilin in neuronal lipid metabolism and endocannabinoid signaling. Alzheimers Dement. 16, 1248–1258 (2020). sourceCheck pending

Availability of glucose becomes limited during various stress conditions, such as brain ageing.

“However, availability of glucose becomes limited during various stress conditions, such as brain ageing34–39.”

• cites [34] Ivancevic, V. et al. Regional cerebral glucose metabolism in healthy volunteers determined by fluordeoxyglucose positron emission tomography: appearance and variance in the transaxial, coronal, and sagittal planes. Clin. Nucl. Med. 25, 596–602 (2000). sourceNot checked — no full text of the source
• cites [35] Pardo, J. V. et al. Where the brain grows old: decline in anterior cingulate and medial prefrontal function with normal aging. Neuroimage 35, 1231–1237 (2007). sourceNot checked — no full text of the source
• cites [36] Ding, F., Yao, J., Rettberg, J. R., Chen, S. & Brinton, R. D. Early decline in glucose transport and metabolism precedes shift to ketogenic system in female aging and Alzheimer’s mouse brain: implication for bioenergetic intervention. PLoS ONE 8, e79977 (2013). sourceCheck pending
• cites [37] Wang, Y. et al. Midlife chronological and endocrinological transitions in brain metabolism: system biology basis for increased Alzheimer’s risk in female brain. Sci. Rep. 10, 8528 (2020). sourceCheck pending
• cites [38] Yin, F. et al. The perimenopausal aging transition in the female rat brain: decline in bioenergetic systems and synaptic plasticity. Neurobiol. Aging 36, 2282–2295 (2015). sourceCheck pending
• cites [39] Fattoretti, P. et al. Quantitative immunohistochemistry of glucose transport protein (Glut3) expression in the rat hippocampus during aging. J. Histochem. Cytochem. 49, 671–672 (2001). sourceCheck pending

PPARα-dependent induction of metabolic gene expression enables neurons to use LCFAs as metabolic fuel under basal as well as under glucose-restricted conditions.

“PPARα-dependent induction of metabolic gene expression enables neurons to use LCFAs as metabolic fuel under basal as well as under glucose-restricted conditions.”

Sortilin interacts with FABP7.

“As well as by quantity, sortilin may also distinguish itself from other apoE receptors by functionality as it interacts with FABP7 (ref. 15).”

• cites [15] Asaro, A. et al. ApoE4 disrupts interaction of sortilin with fatty acid-binding protein 7 essential to promote lipid signaling. J. Cell Sci. 134, jcs258894 (2021). sourceNot checked yet

Sortilin is in a unique position among the various apoE receptors to link extracellular apoE and intracellular FABP7 PUFA transport processes in neurons.

“Thus, sortilin is in a unique position among the various apoE receptors to link extracellular (that is, apoE) and intracellular (that is, FABP7) PUFA transport processes in neurons.”

Neurons in the E3WT brain express enzymes to metabolize LCFAs and MCFAs but not SCFAs, a less favoured lipid fuel.

“Accordingly, neurons in the E3WT brain express enzymes to metabolize LCFAs (and MCFAs) but not SCFAs, a less favoured lipid fuel.”

• AssumptionRests on an assumption: Purified synaptosomes lack the ability to acutely induce gene expression in response to changing experimental conditions.
• AssumptionRests on an assumption: The ability of synaptosomes to consume fatty acids of various chain lengths in mitochondrial stress tests is defined by the enzyme repertoire expressed in the mouse brain at the time of sample preparation.

Loss of sortilin activity in gene-targeted mice increases circulating levels of apoE in the brain due to impaired cellular clearance of the carrier.

“Loss of sortilin activity in gene-targeted mice increases circulating levels of apoE in the brain, due to impaired cellular clearance of the carrier10.”

• cites [10] Carlo, A. S. et al. The pro-neurotrophin receptor sortilin is a major neuronal apolipoprotein E receptor for catabolism of amyloid-beta peptide in the brain. J. Neurosci. 33, 358–370 (2013). sourceNot checked yet

The induced expression of LCFA consumption genes in E3WT mouse neurons substantiates the relevance of this pathway for in vivo brain lipid metabolism under normal full glucose conditions.

“Induced expression of genes related to LCFA consumption in neurons sorted from E3WT mouse brains, as compared to E3KO and E4 neurons (Fig. 3), further substantiate the relevance of this pathway for in vivo brain lipid metabolism under normal (that is, full glucose) conditions.”

Functional inactivation of sortilin adds another mechanistic explanation to the detrimental actions of apoE4.

“Functional inactivation of sortilin adds another mechanistic explanation to the detrimental actions of this most important genetic risk factor for sporadic Alzheimer disease.”

The need for LCFAs as metabolic fuel is a feature seen in mitochondria in soma and axonal endings.

“Although comparing mitochondria from human and mouse models, our findings suggest that the need for LCFAs as metabolic fuel is a feature seen in mitochondria in soma and axonal endings.”

Impaired neuronal uptake and conversion, rather than reduced exogenous availability of fatty acids, causes the major metabolic phenotypes seen in E3KO and E4 neurons.

“These findings support our concept that impaired neuronal uptake and conversion, rather than reduced exogenous availability of fatty acids, causes major metabolic phenotypes seen in E3KO and in E4 neurons herein.”

Sortilin deficiency reduces apoE uptake in primary mouse neurons by approximately 70%.

“Sortilin deficiency also reduces apoE uptake in primary mouse neurons by approximately 70%10.”

• cites [10] Carlo, A. S. et al. The pro-neurotrophin receptor sortilin is a major neuronal apolipoprotein E receptor for catabolism of amyloid-beta peptide in the brain. J. Neurosci. 33, 358–370 (2013). sourceNot checked yet

Impaired PPARα activity in human neurons lacking sortilin is substantiated by reduced expression of multiple transcriptional targets relevant to lipid handling, most notably CPT1A.

“Impaired activity of PPARα in human neurons lacking sortilin is substantiated by the reduced expression of multiple transcriptional targets with relevance to lipid handling, most notably CPT1A, the mitochondrial importer for LCFA (Fig. 6a–g).”

Neurons exposed to apoE4 revert to PPARα-independent mitochondrial import of medium-chain and short-chain fatty acids, which are energetically less favourable substrates to support energy production.

“Instead, they revert to PPARα-independent mitochondrial import of medium-chain and short-chain fatty acids, energetically less favourable substrates to support energy production.”

ApoE3 uptake requires the interaction of sortilin with the carrier FABP7, facilitating intraneuronal lipid transport.

“As shown in established cell lines before, apoE3 uptake also requires the interaction of sortilin with the carrier FABP7, facilitating intraneuronal lipid transport15.”

• cites [15] Asaro, A. et al. ApoE4 disrupts interaction of sortilin with fatty acid-binding protein 7 essential to promote lipid signaling. J. Cell Sci. 134, jcs258894 (2021). sourceCheck pending

Neurons exposed to apoE4-containing lipoproteins are unable to efficiently utilize LCFAs as metabolic fuel.

“Ultimately, neurons exposed to apoE4-containing lipoproteins are unable to efficiently utilize LCFAs as metabolic fuel.”

Loss of sortilin activity occurs in the brain of APOE ε4 carriers.

“These findings argue for loss of sortilin activity in the brain of APOE ε4 carriers, an assumption corroborated by rescue of sensitivity to etomoxir in E4WT iNs when exposed to apoE3-containing rather than apoE4-containing astrocyte media (Fig. 4e–g).”

Sortilin is essential for proper sorting and stable expression of FABP7 in established cell lines.

“Sortilin is essential for proper sorting and stable expression of FABP7 in established cell lines15, a mechanism now supported by data in human neurons (Fig. 6h).”

• cites [15] Asaro, A. et al. ApoE4 disrupts interaction of sortilin with fatty acid-binding protein 7 essential to promote lipid signaling. J. Cell Sci. 134, jcs258894 (2021). sourceNot checked yet

Impaired activation of PPARα is a likely reason for alterations in mitochondrial lipid handling in E3KO iNs.

“Given the drastic decrease in PUFA and eCB levels in E3KO iNs as compared to isogenic E3WT cells, impaired activation of PPARα is a likely reason for alterations in mitochondrial lipid handling in this genotype.”

Loss of PPARα-induced expression of CPT1A provides a possible molecular explanation for the insensitivity of E3KO or E4 neurons to etomoxir.

“Loss of PPARα-induced expression of CPT1A provided a possible molecular explanation for the insensitivity of E3KO or E4 neurons to etomoxir.”

Induction of LCFA import through CPT1A expression, among other metabolically relevant PPARα targets, explains the metabolic phenotypes distinguishing sortilin-expressing from sortilin-deficient neurons.

“Still, the ability of bezafibrate to rescue the sensitivity of E3KO and E4 iNs to etomoxir argues that induction of LCFA import through CPT1A expression, among other metabolically relevant targets of PPARα, explains the metabolic phenotypes distinguishing sortilin-expressing from sortilin-deficient neurons.”

The ability of bezafibrate to promote LCFA consumption in cells lacking sortilin indicates that these experimental conditions reflect consumption of intracellular lipid stores.

“Of note, the ability of bezafibrate to promote LCFA consumption in cells lacking the endocytic receptor sortilin argued that these experimental conditions reflect the consumption of intracellular lipid stores.”