**10. Erythrocytes, cholesterol, and other lipids**

The erythrocyte membrane contains lipids like phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, sphingomyelin, and membrane proteins. The membrane also contains cholesterol, which is important for membrane stability and deformability of erythrocytes [139]. The parasite Plasmodium does not synthesize cholesterol, and vesicles containing cholesterol have been observed to transport from erythrocytes to the Plasmodium parasite [140]. Cholesterol can be chelated by beta-methyl-cyclodextrin, which has been used against Plasmodium infections [141]. Many studies point to an association between lipid constituents of erythrocytes and divers diseases [142]. Many studies have found that the omega-3, also called n-3 fatty acids, are promoting health in various ways, whereas omega-6 (n-6) may be less health promoting and the saturated fatty acids are further less health promoting. Trans-fatty acids are usually not considered health promoting, although this is somewhat controversial considering their occurrence in dairy products. Similar results are found across many studies, irrespective of whether plasma or erythrocyte levels are measured. One reason for less clear health promotion of omega-6 fatty acids is that the omega-6 arachidonic acid is a precursor for pro-inflammatory prostaglandins, thromboxanes, and leukotrienes [143], collectively called eicosanoids. Omega-3 fatty acids on the other hand have been reported to reduce signaling through the pro-inflammatory toll-like receptor 4 (TLR4) [144]. Dietary levels of omega-6 fatty acids are usually well in excess of those of omega-3, which may lead to undesirable enzymatic competition for the fatty acid substrate. Erythrocyte membrane constituents are of interest as diagnostics, since erythrocytes are more long-term indicators than plasma levels, which are influenced by the most recent meal. Erythrocyte levels of omega-3 fatty acids were associated with less cardiovascular events in the Framingham heart study [145]. Erythrocyte omega-6 fatty acid levels showed no association with cardiovascular events in the same study [146]. Erythrocyte omega-3 polyunsaturated fatty acids were negatively correlated with cancer [147]. Erythrocyte omega-3 fatty acids were associated with less risk of islet autoimmunity in children with diabetes [148]. A study of erythrocytes in preclinical Alzheimer's disease found increased levels of the omega-6 fatty acid arachidonic acid and decreased levels of the omega-3 fatty acid docosapentaenoic acid in participants with high neocortical betaamyloid load prior to cognitive impairment [149]. Decreased omega-3 fatty acid levels in erythrocyte membranes of schizophrenia patients has given rise to a modified version of the dopamine hypothesis of schizophrenia [150]. A study of erythrocyte fatty acids in schizophrenia patients found that omega-3 polyunsaturated fatty acids were significantly lower and the omega-6:omega-3 fatty acid ratio was significantly higher in the group with dominantly negative symptoms as compared to the group with dominantly positive symptoms [151]. A significant deficit of erythrocyte omega-3 docosahexaenoic acid has been noted in bipolar disorder type I [152], whereas erythrocyte fatty acid profiles had no predictive value in autism spectrum disorder [153].

Trans-fatty acids in erythrocytes were found to be associated with increased risk of coronary heart disease in prospective [154] or cross-sectional studies [155]. However, trans-fatty acids in erythrocytes appear to be decreasing in Europe as seen in samples from 2008 to 2015 [156].

### **11. Erythrocytes and ribonucleic acid (RNA)**

Although ribonucleic acid (RNA) has been reported in erythrocytes since long (**Figure 1**), the purity of blood cell preparations was often questioned [157]. The use of single-cell transcriptomics has provided further confirmation of the presence of RNA in erythrocytes [158]. Around 8000 messenger-RNA transcripts have now been found in erythrocytes. Analysis of messenger-RNA has shown that erythrocytes can be divided into at least seven different categories [158]. These seem to represent different stages of development from reticulocytes to cells in transition to mature and finally senescing erythrocytes. The most abundant messenger-RNA transcripts are related to erythropoiesis and may be seen as a residue from that process. Alternatively, messenger-RNA may serve as template for protein synthesis, which would require the presence of ribosomes and other parts of translation, which may possibly exist at a low level in erythrocytes [159]. Hundreds of different microRNAs (miRNAs) have also been confirmed in erythrocytes [159]. MiRNAs are usually about 22 nucleotides in length and function to reduce, or silence, the expression of genes. Important parts of this mechanism are the RNAse III enzymes Drosha and Dicer, the endonuclease Argonaute, and the RNA-binding protein DGCR8. Drosha, Dicer, and Argonaute are all magnesium-dependent enzymes [160–162]. MiRNA genes are transcribed by RNA polymerase II to a pri-miRNA. Drosha and DGCR8 then process the pri-miRNA to a pre-miRNA which is exported from the nucleus through exportin-5. Once in the cytoplasm, the miRNA is further processed by Dicer and bound to Argonaute2 forming the RNA-induced silencing (RISC) complex. Some of the miRNAs of the erythrocyte like miR-451, miR-144, and miR-486 are involved in erythropoiesis [163]. MiR-451 is also associated with malaria, and miR-144 is correlated with hypoxia at high altitudes [159]. An attractive hypothesis is that erythrocytic RNA enclosed in microvesicles is used as a means of communication between erythrocytes and other cells.

Microvesicles are extracellular vesicles 0.1–1 micrometer in size, formed by budding from the plasma membrane. Microvesicles are distinguished from exosomes, which are smaller and released as preformed vesicles, 10–100 nanometer in size. Microvesicles and exosomes may contain proteins, lipids, and RNA. Microvesicles are particularly known as carriers of miRNA. Erythrocytes are known to form microvesicles under blood storage, but microvesicles are also formed as a normal physiological process [164]. As mentioned, microvesicles may fulfill a function as information carriers between different cells in the body. Microvesicles also function as an efficient protection against proteases or RNases that would otherwise degrade

#### *Erythrocytes as Messengers for Information and Energy Exchange between Cells DOI: http://dx.doi.org/10.5772/intechopen.108321*

the content [165]. A connection between erythrocyte microvesicle formation and diseases or pathological conditions have been proposed in several cases. However, a more comprehensive analysis of miRNA content of microvesicles derived from healthy erythrocytes compared to those from patients with disease conditions seems to be needed.

Transport of miRNA or messenger-RNA from erythrocytes to other cells may be a way of regulating gene expression in target cells [159]. The known functions of these miRNAs in other cells of the body could accordingly be relevant also in the context of erythrocytes. MiRNAs are generally edited by the gene silencing mechanism in cells, giving rise to several variants of each miRNA family, often indicated in the nomenclature by extra suffixes, like the expressed strand being indicated by a 3-p or 5-p suffix. Some abundant and notable erythrocyte miRNAs are miR451, miR144, miR16, and let-7. The miR451 family is abundant in erythrocytes and functions in erythropoiesis by downregulating the Ywhaz gene, whose protein product 14–3-3-zeta keeps the transcription factor FoxO3 in the cytosol [166]. FoxO3 positively regulates antioxidant enzymes like catalase and glutathione peroxidase. CRISPR-Cas9-mediated mutagenesis of miR451 confirmed that miR451 is necessary for erythroid differentiation and expression of transferrin receptor 1, also known as CD71 [163]. The miR144 family is expressed together with miR451 and was similarly found to be necessary for erythroid differentiation and CD71 expression [163]. Plasmodium-infected erythrocytes produce microvesicles containing miR451, miR16, and let-7, among others [167–169].
