In the blood, lipoproteins ferry cholesterol and other lipids to cells throughout the body. In the cerebrospinal fluid, they may do the same, but because they are 200-fold more dilute there, scientists know little about them. Now, John Melchior at the Pacific Northwest National Laboratory in Richland, Washington, and colleagues have overcome this hurdle. They discovered a wide array of lipoproteins in the CSF, which even surpasses that of blood. In the August 31 Science Translational Medicine, the scientists reported 10 differently sized classes of CSF lipoproteins for a total of 303. These lipoproteins are involved in processes germane to neurodegeneration, including inflammation, immune responses, wound healing, and synapse function.
- Ten size-groups of lipoproteins cruise healthy cerebrospinal fluid.
- They comprise 303 proteins, of which half are not in plasma.
- Their pathways? Inflammation, immune response, and more.
Other scientists praised the work. “[It] represents a significant advancement in our understanding of which proteins are present in human CSF lipoprotein particles,” wrote Tobias Hartmann, Saarland University in Homburg, Germany. René Frank, University of Leeds, U.K., noted that some of the proteins in these particles have been associated with Alzheimer’s disease. “[This study is] a treasure trove of biochemical information on the protein composition of cognitively normal human donor CSF and the size profile of [its] lipoprotein particles and extracellular vesicles,” he wrote. “... It could be exciting to extend this elegant study of human CSF to that from living Alzheimer’s disease patient donors, comparing with CSF from healthy, and cognitively normal, aged-matched control donors.” (Comments below.)
Plasma lipoproteins come in four flavors: ultra-low-density, very-low-density (VLDL), low-density (LDL), and high-density (HDL). Lipoproteins include, you guessed it, fats—phospholipids surrounding a core of cholesterol and triglycerides—and proteins, commonly apolipoproteins. Their makeup varies widely, with HDLs housing up to 285 types of protein (reviewed by Davidson et al., 2022). Until now, the CSF lipoprotein list had a measly six, all apolipoproteins.
To plumb the CSF lipoproteome, first author Nathaniel Merrill analyzed commercially available pooled human CSF samples. He added fluorescent lipids to label the lipoproteins, which made them more detectable. Then, he used high-resolution size-exclusion chromatography (SEC) to separate them through three tandem columns, followed by mass spectrometry to identify the proteins (see image below).
Voila, Lipoproteins. After labeling them with a fluorescent lipid, lipoproteins were separated by size exclusion chromatography. A lipid removal agent (LRA) left just the proteins, which were then digested with trypsin and analyzed via mass spectrometry. [Courtesy of Merrill et al., Science Advances, 2023.]
Merrill detected 10 different-sized clusters of lipoproteins: one large VLDL and one large LDL, seven small-to-medium HDLs, and one very small HDL. Of the 303 proteins, half had not been detected in blood lipoprotein particles. The more abundant of these included the serine protease kallikrein-6, neurotransmitter-related secretogranin-1, and neuropeptide precursor VGF. CSF-specific proteins accounted for just 15 percent of the total lipoprotein content. The most abundant lipoproteins in CSF were also found in blood, e.g. APOA1, APOA2, APOE, and APOJ, aka clusterin.
To the authors, this means the CSF-specific proteins might dock onto lipoprotein particles to give them specific functions. These proteins eluted from the SEC column in single peaks, indicating they hitched a ride with only one class of lipoprotein. At the same time, apolipoproteins acted as the primary scaffolds for forming the particles, eluting with most lipoprotein species in the samples.
What might these proteins be doing in the brain? Gene ontology placed them into 15 functional clusters, including immune responses, inflammation, wound healing, proteolysis, and neurodevelopment. Many CSF-specific proteins were in the latter group. Machine-learning algorithms pieced together protein-protein relationships, finding about 150 groups of two to four that likely hitch rides on the same-sized lipoprotein (see image below).
One group of proteins caught the researchers’ attention. It involved neurexin 3 (NRXN3), amyloid precursor-like-protein 1 (APLP1), and plexin domain containing 2 (PLXDC2). All are substrates of β-secretase, one of two enzymes needed to cut Aβ from APP. NRXN3 and APLP1 also regulate synapse formation and function (Aoto et al., 2015; Schilling et al., 2017).
Commentators wondered how CSF lipoprotein composition changes with aging and during neurodegenerative diseases, such as AD (comments below). “It will be critical to profile changes of the CSF lipoproteome during the course of Alzheimer’s disease progression, particularly in individuals at high risk for AD, such as carriers of autosomal dominant AD mutations or of APOE4/4, and those carrying loss-of-function mutations in AD risk genes implicated in lipoprotein metabolism,” wrote Edoardo Marcora, Icahn School of Medicine, Mount Sinai, New York.—Chelsea Weidman Burke
- Davidson WS, Shah AS, Sexmith H, Gordon SM. The HDL Proteome Watch: Compilation of studies leads to new insights on HDL function. Biochim Biophys Acta Mol Cell Biol Lipids. 2022 Feb;1867(2):159072. Epub 2021 Nov 18 PubMed.
- Aoto J, Földy C, Ilcus SM, Tabuchi K, Südhof TC. Distinct circuit-dependent functions of presynaptic neurexin-3 at GABAergic and glutamatergic synapses. Nat Neurosci. 2015 Jul;18(7):997-1007. Epub 2015 Jun 1 PubMed.
- Schilling S, Mehr A, Ludewig S, Stephan J, Zimmermann M, August A, Strecker P, Korte M, Koo EH, Müller UC, Kins S, Eggert S. APLP1 Is a Synaptic Cell Adhesion Molecule, Supporting Maintenance of Dendritic Spines and Basal Synaptic Transmission. J Neurosci. 2017 May 24;37(21):5345-5365. Epub 2017 Apr 27 PubMed.
No Available Further Reading
- Merrill NJ, Davidson WS, He Y, Díaz Ludovico I, Sarkar S, Berger MR, McDermott JE, Van Eldik LJ, Wilcock DM, Monroe ME, Kyle JE, Bruce KD, Heinecke JW, Vaisar T, Raber J, Quinn JF, Melchior JT. Human cerebrospinal fluid contains diverse lipoprotein subspecies enriched in proteins implicated in central nervous system health. Sci Adv. 2023 Sep;9(35):eadi5571. Epub 2023 Aug 30 PubMed.