The role of apoliproteins in neurological disorders - ISSN 2753-8176 (online)

Ana Pedro 1

1 Gwyntwr1386 Social Pharmacy, Regus Chester Business Park, Heronsway, Chester, CH49QR, UK , info@gwyntwr1386.com,

Apolipoproteins are “any of various proteins that combine with a lipid to form a lipoprotein, such as HDL and LDL”, or “The protein components of lipoproteins, which remain after the lipids to which the proteins are bound have been removed”(1). Apolipoproteins are reported to be linked to many neurodegenerative disorders. APOE plays a role in many brain disorders, including Alzheimer’s disease, mild cognitive impairment, multiple sclerosis, traumatic brain injury, Creutzfeldt-Jakob disease (CJD) and maybe in more some others like Parkinson’s disease (PD) (2).

We found out in a recent publication (3) that apoliproteins E (APOE) (total) and L1 (APOL1) are present in the plasma of autistic children and their parents but not in the plasma of their normal siblings. APOE present on high-density lipoproteins primarily function is lipid transport and cholesterol homeostasis in the central nervous system (CNS). APOE has an high expression in the prefrontal cortex and falls by approximately 50% from neonatal to adult ages (5). Moreover, APOE methylation in pediatric patients with autistic spectrum disorders (ASD) is much higher than that in the healthy controls and APOE hypermethylation is inversely correlated with lower expression (4). Also, a genetic study suggest that APOE2 and APOE4 variants are associated with ASD (6).

APOE4 is also associated with Alzheimer’s disease and other neurodegenerative disorders, and is expressed at low levels in brain and cerebrospinal fluid. Cultured astrocytes and neurons expressing APOE4 show reduced cholesterol and phospholipid secretion, decreased lipid-binding capacity, and increased intracellular degradation. Domain interaction, in which arginine-61 interacts ionically with glutamic acid-255, and a less stable conformation than APOE2 are supposed to be responsible for this dysfunction. Blocking this domain interaction by gene targeting (replacing arginine-61 with threonine) or by small-molecule structure correctors can increase CNS APOE4 levels and its lipid-binding capacity and decreases its intracellular degradation. Furthermore, plasma levels of APOE vary with APOE genotype (APOE2>APOE4) (7,8). In healthy controls, APOE2 is more abundant in plasma then APO4 (9). Total APOE is also less abundant in APOE4 carriers than in noncarriers and this gradient is also seen in cognitively impaired individuals. In cognitive normal individuals, mean plasma APOE is ≈5.7 mg/dL and total cerebrospinal fluid (CSF) APOE is 0.7 mg/dL (range,≈0.6–0.9 mg/dL; highest in APOE2 and lowest in APOE4 carriers). In plasma, this gradient reflects decreased LDL receptor binding of APOE2 (and thus higher plasma levels) and the preference of APOE4 for VLDL, due to its unique structural features (accelerates hepatic clearance and results in lower levels). In the brain, the lower APOE4 levels and thus lower levels of cholesterol transport could affect cholesterol homeostasis and neuronal plasticity (10). By other side, very recent proteomic analysis revealed that APOL1 is involved in lipid metabolism and transport, as a potential CSF biomarker for frontotemporal dementia (FTLD).Higher APOL1 immunoreactivity associated with neuronal and glia cells was observed in the frontal cortices of FTLD cases compared to controls (p<0.001) (11). The presence of total APOE and APOL1 in the plasma precipitates of all of the autistic children and their parents but not in the plasma of their normal sibling may underline a still yet unknown feature of CNS cholesterol transport and metabolism which might be important to understand the causes of ASD.

Moreover, apolipoproteins that are involved in cholesterol removal from the cells and transport excess cholesterol to bile seem to have a protective role in PD such as APOA1. Also, elevated levels of APOD in glial cells of substancia nigra can be associated with PD (2).

References

1. Elliot et al (2010).Apolipoproteins in the brain: implications for neurological and psychiatric disorders. Clin Lipidol. 2010 August 1; 51(4): 555–573. doi:10.2217/CLP.10.37.

2. Emamzadeh F (2017).Role of Apolipoproteins and α-Synuclein in Parkinson’s Disease.J Mol Neurosci (2017) 62:344–355

3. Pedro A (2022). The role of apoliproteins APOE and APOL1 in Autism DOI:10.13140/RG.2.2.23055.05286, ISSN 2753-8176 (online)

4.Hu et al. (2018).APOE hypermethylation is associated with autism spectrum disorder in a Chinese population. EXPERIMENTAL AND THERAPEUTIC MEDICINE 15: 4749-4754, 2018

5.Elliot et al (2010). Apolipoproteins in the brain: implications for neurological and psychiatric disorders. Clin Lipidol. 2010 August 1; 51(4): 555–573.

6. Giunco CT, de Oliveira AB, Carvalho-Salles AB, Souza DS, Silva AE, da Rocha SS and Fett-Conte AC: Association between APOE polymorphisms and predisposition for autism. Psychiatr Genet 19: 338, 2009.

7.Utermann G. Genetic polymorphism of apolipoprotein E – impact on plasma lipoprotein metabolism. In: Crepaldi G, Tiengo A, Baggio G, eds. Diabetes, Obesity and Hyperlipidemias – III. Amsterdam: Elsevier Science Publishers; 1985:1–28

8. Rasmussen KL, Tybjaerg-Hansen A, Nordestgaard BG, Frikke-Schmidt R. Plasma levels of apolipoprotein E and risk of dementia in the general population. Ann Neurol. 2015;77:301–311. doi: 10.1002/ana.24326.

9. Gupta VB, Wilson AC, Burnham S, et al; AIBL Research Group. Follow-up plasma apolipoprotein E levels in the Australian Imaging, Biomarkers and Lifestyle Flagship Study of Ageing (AIBL) cohort. Alzheimers Res Ther. 2015;7:16. doi: 10.1186/s13195-015-0105-6.

10. Mahley R. Central Nervous System Lipoproteins: ApoE and Regulation of Cholesterol Metabolism. Arterioscler Thromb Vasc Biol, 2016 Jul;36(7):1305-15

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11.Hok-A-Hin et al. Apolipoprotein L1 is increased in frontotemporal lobar degeneration post-mortem brain but not in ante-mortem cerebrospinal fluid. Neurobiol Dis 2022 Oct 1;172:105813.