Rab5 regulates the antimicrobial phagocytosis of hemocytes through class B scavenger receptor in Procambarus clarkii

Abstract

The crucial involvement of Rab5 in regulating phagocytosis through mediating receptors, such as the Class B scavenger receptor (SRB), has been documented in higher animal species. However, little is known about the molecular mechanisms of Rab5-mediated phagocytosis in invertebrates. The present study aims to investigate whether the phagocytic function of hemocytes in Procambarus clarkii can be regulated by Rab5 through SRB. We first cloned the open reading frame of SRB from P. clarkii (defined as PcSRB), encoding a putative protein containing a typical CD36 domain. PcSRB was widely expressed in all tested tissues of P. clarkii and induced by viral and bacterial stimulation. The extracellular segment of PcSRB (PcSRB-EX) can agglutinate as well as bind with both Gram-positive (G+) and Gram-negative (G−) bacteria. Knockdown of PcSRB resulted in down-regulation of the phagocytic capacity and bactericidal activity against Aeromonas hydrophila, along with impaired expression of several phagocytosis-related genes and antimicrobial peptide (AMP) genes. PcSRB interference also affected the mRNA expression level of Tolls, indicating that PcSRB might function as a recognition receptor or co-receptor in the Toll signaling pathway. Furthermore, Rab5 interference significantly impaired the expression of PcSRB at both gene and protein levels, which confirmed its significant regulatory effect on PcSRB. Subsequently, we observed a significant decrease in the phagocytic activity and bacteria clearance of hemocytes when Rab5 and PcSRB were simultaneously knocked down. Additionally, there was a notable down-regulation in the expression levels of genes related to phagocytosis and AMPs. Overall, these findings provide preliminary evidence that Rab5 mediates PcSRB to influence the phagocytosis of P. clarkii hemocytes against A. hydrophila.