Peroxisome Proliferator-Activator Receptor γ: A Link between Macrophage CD36 and Inflammation in Malaria Infection

Association of a 7.9 kb Endogenous Retrovirus Insertion in Intron 1 of CD36 with Obesity and Fat Measurements in Sheep

BACKGROUND

Endogenous retroviruses (ERVs) enhance genetic diversity in vertebrates, including sheep. This study investigates the role of Ov-ERV-R13-CD36 within CD36 gene and its association with phenotypic traits in sheep. Analyzing 58 sheep genomes revealed that ERVs constitute approximately 6.02% to 10.05% of the genomic content. We identified 31 retroviral insertion polymorphisms (RIPs) from 28 ERV groups. Among these, Ov-ERV-R13-CD36, which is specifically classified as a beta retrovirus, was selected for further analysis due to its location in CD36 gene, known for its role in fat metabolism, obesity (OB), body weight (BW), and body condition score (BCS). We assessed the association of Ov-ERV-R13-CD36 with OB and BCS across six sheep breeds, utilizing data from 1,355 individuals.

RESULTS

Genomic analyses confirmed that Ov-ERV-R13-CD36 is located within CD36 gene on Chromosome 4, with polymorphisms across various sheep genomes. In a subset of 43 genomes, 22 contained the Ov-ERV-R13-CD36 insertion, while 21 exhibited wild-type variants. The studied animals showed variability in BCS and fat content associated with the Ov-ERV-R13-CD36 variant. Notably, Rahmani sheep exhibited a significantly higher BCS (4.62), categorized as obese, while Barki sheep displayed the lowest BCS (2.73), classified as thin to average. The association analysis indicated that sheep with the RIP-/- genotype correlated with higher OB and BCS, particularly in Rahmani and Romanov x Rahmani breeds.

CONCLUSIONS

Findings suggest that Ov-ERV-R13-CD36 within CD36 gene correlates with beneficial economic traits associated with OB and BCS, particularly in Rahmani and Romanov x Rahmani breeds. This indicates that Ov-ERV-R13-CD36 could be a valuable genetic marker for breeding programs aimed at enhancing traits like fat deposition and body condition in sheep.

Kinetics of monocyte subpopulations during experimental cerebral malaria and its resolution in a model of late chloroquine treatment

Cerebral malaria (CM) is one of the most severe forms of malaria and is a neuropathology that can lead to death. Monocytes have been shown to accumulate in the brain microvasculature at the onset of neurological symptoms during CM. Monocytes have a remarkable ability to adapt their function to their microenvironment from pro-inflammatory to resolving activities. This study aimed to describe the behavior of monocyte subpopulations during infection and its resolution. C57BL/6 mice were infected with the Plasmodium berghei ANKA strain and treated or not with chloroquine (CQ) on the first day of the onset of neurological symptoms (day 6) for 4 days and followed until day 12 to mimic neuroinflammation and its resolution during experimental CM. Ly6C monocyte subpopulations were identified by flow cytometry of cells from the spleen, peripheral blood, and brain and then quantified and characterized at different time points. In the brain, the Ly6C^int and Ly6C^low monocytes were associated with neuroinflammation, while Ly6C^hi and Ly6C^int were mobilized from the peripheral blood to the brain for resolution. During neuroinflammation, CD36 and CD163 were both involved via splenic monocytes, whereas our results suggest that the low CD36 expression in the brain during the neuroinflammation phase was due to degradation. The resolution phase was characterized by increased expressions of CD36 and CD163 in blood Ly6C^low monocytes, a higher expression of CD36 in the microglia, and restored high expression levels of CD163 in Ly6C^hi monocytes localized in the brain. Thus, our results suggest that increasing the expressions of CD36 and CD163 specifically in the brain during the neuroinflammatory phase contributes to its resolution.

Assessment of direct binding interaction between CD36 and its potential lipid ligands using a peptide mimic of the receptor labeled with a fluorophore

Cluster of differentiation 36 (CD36) is a cell-surface receptor that recognizes diverse substances. We have presented indirect evidence that a short segment of the receptor comprising amino acids 149-168 contains a site for binding of its lipid ligands (e.g., distinct fatty acids and aldehydes). However, experimental support for their direct interactions is yet to be achieved. For this, we devised a fluorescence intensity assay, where a synthetic peptide consisting of CD36 amino acids 149-168 labeled with fluorescein isothiocyanate (FITC-CD36_149-168) and its variant peptides were used as positive and negative probes, respectively. First, we obtained results indicating that 1-palmitoyl-2-(5-keto-6-octenedioyl)phosphatidylcholine (an established CD36 ligand) but not 1-palmitoyl-2-arachidonyl-phosphatidylcholine (a non-ligand of the receptor) bound in a saturable and specific manner to FITC-CD36_149-168. Strikingly, the assay allowed us to provide the first evidence supporting direct and specific binding between the CD36 segment and fatty aldehydes (e.g., Z-11-hexadecenal). However, this method failed to illustrate specific interactions of the segment with fatty acids, such as oleic acid. Nonetheless, our findings offer further insight into the biologically relevant ligands and the role of CD36. In addition, we suggest that this fluorescence-based technique provides a convenient means to evaluate protein (peptide)-lipid interactions.

Erythropoietin Promotes Infection Resolution and Lowers Antibiotic Requirements in E. coli- and S. aureus-Initiated Infections

Endogenous mechanisms underlying bacterial infection resolution are essential for the development of novel therapies for the treatment of inflammation caused by infection without unwanted side effects. Herein, we found that erythropoietin (EPO) promoted the resolution and enhanced antibiotic actions in Escherichia coli (E. coli)- and Staphylococcus aureus (S. aureus)-initiated infections. Levels of peritoneal EPO and macrophage erythropoietin receptor (EPOR) were elevated in self-limited E. coli-initiated peritonitis. Myeloid-specific EPOR-deficient mice exhibited an impaired inflammatory resolution and exogenous EPO enhanced this resolution in self-limited infections. Mechanistically, EPO increased macrophage clearance of bacteria via peroxisome proliferator-activated receptor γ (PPARγ)-induced CD36. Moreover, EPO ameliorated inflammation and increased the actions of ciprofloxacin and vancomycin in resolution-delayed E. coli- and S. aureus-initiated infections. Collectively, macrophage EPO signaling is temporally induced during infections. EPO is anti-phlogistic, increases engulfment, promotes infection resolution, and lowers antibiotic requirements.

Diversity of Human and Macaque Airway Immune Cells at Baseline and during Tuberculosis Infection

Immune cells of the distal airways serve as "first responders" of host immunity to the airborne pathogen Mycobacterium tuberculosis (Mtb). Mtb infection of cynomolgus macaques recapitulates the range of human outcomes from clinically silent latent tuberculosis infection (LTBI) to active tuberculosis of various degrees of severity. To further advance the application of this model to human studies, we compared profiles of bronchoalveolar lavage (BAL) cells of humans and cynomolgus macaques before and after Mtb infection. A simple gating strategy effectively defined BAL T-cell and phagocyte populations in both species. BAL from Mtb-naive humans and macaques showed similar differential cell counts. BAL T cells of macaques were composed of fewer CD4⁺cells but more CD8⁺ and CD4⁺CD8⁺ double-positive cells than were BAL T cells of humans. The most common mononuclear phagocyte population in BAL of both species displayed coexpression of HLA-DR, CD206, CD11b, and CD11c; however, multiple phagocyte subsets displaying only some of these markers were observed as well. Macaques with LTBI displayed a marked BAL lymphocytosis that was not observed in humans with LTBI. In macaques, the prevalence of specific mononuclear phagocyte subsets in baseline BAL correlated with ultimate outcomes of Mtb infection (i.e., LTBI versus active disease). Overall, these findings demonstrate the comparability of studies of pulmonary immunity to Mtb in humans and macaques. They also indicate a previously undescribed complexity of airway mononuclear phagocyte populations that suggests further lines of investigation relevant to understanding the mechanisms of both protection from and susceptibility to the development of active tuberculosis within the lung.

Monocytes and macrophages in malaria: protection or pathology?

Recruitment and activation of monocytes and macrophages are essential for clearance of malaria infection, but these have also been associated with adverse clinical outcomes. In this review we discuss recent discoveries on how distinct molecular interactions between monocytes, macrophages, and malaria parasites may alter the balance between protection and pathology in malaria-infected individuals. The immunopathology of severe malaria often originates from excessive immune activation by parasites. The involvement of monocytes and macrophages in these events is highlighted, and priorities for future research to clarify the roles of these cells in malaria are proposed. Knowledge of the factors influencing the balance between protection and pathology can assist in the design of therapeutics aimed at modulating monocyte and macrophage functions to improve outcomes.

Comparative Studies of Vertebrate Platelet Glycoprotein 4 (CD36)

Platelet glycoprotein 4 (CD36) (or fatty acyl translocase [FAT], or scavenger receptor class B, member 3 [SCARB3]) is an essential cell surface and skeletal muscle outer mitochondrial membrane glycoprotein involved in multiple functions in the body. CD36 serves as a ligand receptor of thrombospondin, long chain fatty acids, oxidized low density lipoproteins (LDLs) and malaria-infected erythrocytes. CD36 also influences various diseases, including angiogenesis, thrombosis, atherosclerosis, malaria, diabetes, steatosis, dementia and obesity. Genetic deficiency of this protein results in significant changes in fatty acid and oxidized lipid uptake. Comparative CD36 amino acid sequences and structures and CD36 gene locations were examined using data from several vertebrate genome projects. Vertebrate CD36 sequences shared 53-100% identity as compared with 29-32% sequence identities with other CD36-like superfamily members, SCARB1 and SCARB2. At least eight vertebrate CD36 N-glycosylation sites were conserved which are required for membrane integration. Sequence alignments, key amino acid residues and predicted secondary structures were also studied. Three CD36 domains were identified including cytoplasmic, transmembrane and exoplasmic sequences. Conserved sequences included N- and C-terminal transmembrane glycines; and exoplasmic cysteine disulphide residues; TSP-1 and PE binding sites, Thr92 and His242, respectively; 17 conserved proline and 14 glycine residues, which may participate in forming CD36 'short loops'; and basic amino acid residues, and may contribute to fatty acid and thrombospondin binding. Vertebrate CD36 genes usually contained 12 coding exons. The human CD36 gene contained transcription factor binding sites (including PPARG and PPARA) contributing to a high gene expression level (6.6 times average). Phylogenetic analyses examined the relationships and potential evolutionary origins of the vertebrate CD36 gene with vertebrate SCARB1 and SCARB2 genes. These suggested that CD36 originated in an ancestral genome and was subsequently duplicated to form three vertebrate CD36 gene family members, SCARB1, SCARB2 and CD36.