Mechanisms for Targeting of Proteins to Secretory Lysosomes of Haematopoietic Cells

Abstract: The cells of the innate immune system are essential for the host's first line defence against infection. Among these cells the neutrophil granulocyte has a central role by the production of antibiotic proteins and peptides. These are targeted for storage in secretory lysosomes and other neutrophil granules for subsequent release by regulated secretion in an inflammatory process. However, the granule content is very potent and may cause tissue damage when released. Basic knowledge about biosynthesis, targeting and secretion of these proteins is therefore important for the understanding of the role of neutrophils in the emergence of inflammatory diseases.

The aim of this thesis was to define mechanisms underlying targeting of proteins to secretory lysosomes of haematopoietic cells.

For this purpose, we investigated the role of protein aggregation in the targeting of luminal proteins to secretory lysosomes. The results suggested that aggregation does not facilitate sorting-for-entry into forming granules destined for the secretory lysosomes. Rather, a native conformation is required in order to allow retention and sorting and avoid elimination from the cell by constitutive secretion.

We also compared the ability of a native versus a non-native protein for endosomal uptake. The results indicated a native conformation to be required for internalization of cell surface located proteins.

Taken together, these results led to suggestions about protein quality controls existing at post-ER levels in the secretory pathway, possibly operating in the Golgi complex and at the plasma membrane. In addition, the results indicated the suggested post-ER protein quality controls to eliminate proteins from the cell by constitutive secretion, instead of targeting failing proteins for intracellular degradation as is observed in ER protein quality control.

Moreover, targeting for storage of a protein such as the soluble TNF receptor to secretory lysosomes was achieved by addition of a transmembrane sorting signal. This might be useful in the use of secretory lysosomes of haematopoietic cells as vehicles for targeting of therapeutic molecules for local release at sites of persistent inflammation.