Pneumococcal attachment to epithelial cells is enhanced by the secreted peptide VP1 via its control of hyaluronic acid processing
Authors: Cuevas Rolando A. , Ebrahimi Elnaz , Gazioglu Ozcan , Yesilkaya Hasan , Hiller N. Luisa 
Published: October 2019

The Gram-positive bacterium Streptococcus pneumoniae (pneumococcus) is an important human pathogen. It can either asymptomatically colonize the nasopharynx or spread to other tissues to cause mild to severe diseases. Nasopharyngeal colonization is a prerequisite for all pneumococcal diseases. We describe a molecular pathway utilized by pneumococcus to adhere to host cells and promote colonization. We demonstrate that the secreted peptide VP1 enhances pneumococcal attachment to epithelial cells. Transcriptional studies reveal that VP1 triggers the expression of operons involved in the transport and metabolism of hyaluronic acid (HA), a glycosaminoglycan present in the host extracellular matrix. Genetic experiments in the pneumococcus reveal that HA processing locus (HAL) promotes attachment. Further, overexpression of HAL genes in the Δvp1 background, reveal that the influence of VP1 on attachment is mediated via its effect on HA. In addition, VP1 also enhances degradation of the HA polymer, in a process that depends on the HAL genes. siRNA experiments to knockdown host HA synthesis support this conclusion. In these knockdown cells, attachment of wild-type pneumococci is decreased, and VP1 and HAL genes no longer contribute to the attachment. Finally, experiments in a murine model of colonization reveal that VP1 and HAL genes are significant contributors to colonization. Our working model, which combines our previous and current work, is that changes in nutrient availability that influence CodY and Rgg144 lead to changes in the levels of VP1. In turn, VP1 controls the expression of a genomic region involved in the transport and metabolism of HA, and these HAL genes promote adherence in an HA-dependent manner. VP1 is encoded by a core gene, which is highly induced in vivo and is a major contributor to host adhesion, biofilm development, colonization, and virulence. In conclusion, the VP1 peptide plays a central role in a pathway that connects nutrient availability, population-level signaling, adhesion, biofilm formation, colonization, and virulence.

AUTHOR SUMMARY

Streptococcus pneumoniae (the pneumococcus) is a major human pathogen. This bacterium asymptomatically colonizes the human upper respiratory tract from where it can disseminate to other tissues causing mild to severe disease. Colonization is a prerequisite for dissemination and disease, such that the molecules that control colonization are high-value candidates for therapeutic interventions. Pneumococcal colonization is a population-level response, which requires attachment to host cells and biofilm development. VP1 is a signaling peptide, highly induced in the presence of host cells and in vivo, promotes biofilm development, and serves as a potent virulence determinant. In this study, we build on the molecular mechanism of VP1 function to reveal novel bacterial and host molecules that enhance adherence and colonization. Our findings suggest that host hyaluronic acid serves as an anchor for pneumococcal cells, and that genes involved in the transport and metabolism of HA promote adherence. These genes are triggered by VP1, which in turn, is controlled by regulators that respond to nutrient status of the host. Finally, our results are strongly supported by studies in a murine model of colonization. We propose that VP1 serves as a marker for colonization and a target for drug design.