Storage pool diseases illuminate platelet dense granule biogenesis

Abstract

Platelet dense granules (DGs) are membrane bound compartments that store polyphosphate and small molecules such as ADP, ATP, Ca²⁺, and serotonin. The release of DG contents plays a central role in platelet aggregation to form a hemostatic plug. Accordingly, congenital deficiencies in the biogenesis of platelet DGs underlie human genetic disorders that cause storage pool disease and manifest with prolonged bleeding. DGs belong to a family of lysosome-related organelles, which also includes melanosomes, the compartments where the melanin pigments are synthesized. These organelles share several characteristics including an acidic lumen and, at least in part, the molecular machinery involved in their biogenesis. As a result, many genes affect both DG and melanosome biogenesis and the corresponding patients present not only with bleeding but also with oculocutaneous albinism. The identification and characterization of such genes has been instrumental in dissecting the pathways responsible for organelle biogenesis. Because the study of melanosome biogenesis has advanced more rapidly, this knowledge has been extrapolated to explain how DGs are produced. However, some progress has recently been made in studying platelet DG biogenesis directly in megakaryocytes and megakaryocytoid cells. DGs originate from an endosomal intermediate compartment, the multivesicular body. Maturation and differentiation into a DG begins when newly synthesized DG-specific proteins are delivered from early/recycling endosomal compartments. The machinery that orchestrates this vesicular trafficking is composed of a combination of both ubiquitous and cell type-specific proteins. Here, we review the current knowledge on DG biogenesis. In particular, we focus on the individual human and murine genes encoding the molecular machinery involved in this process and how their deficiencies result in disease.

Keywords: AP-3 complex; BLOC; HOPS; Hermansky–Pudlak syndrome; Rab38; protein traffic.

Figure 1. Model for the biogenesis of platelet dense granules

In megakaryocytes, multivesicular bodies (MVBs) mature into dense granules upon receiving newly synthesized specific transmembrane proteins through multiple vesicular trafficking pathways. Dense granule cargo is sorted by adaptor protein (AP) complexes at the level of the early/recycling endosome tubules that are likely stabilized by BLOC-1. The best understood pathway to dense granules is defined by AP-3, which binds sorting signals present in the cytosolic tails of cargo proteins and recruits clathrin, facilitating the formation of the coated vesicle (see inset). Rab38 is present on to the transport vesicle that mediates targeting of the vesicle to the maturing dense granule. Very little is known regarding BLOC-2 and BLOC-3 function in DG biogenesis. Based on melanosome research they are tentatively placed downstream BLOC-1 and AP-3, and may also work independently of AP-3. TPC2 regulates dense granule pH, the pool of releasable Ca²⁺, and a “kiss-and-run” mechanism of dense granule membrane dynamics and content exchange (see inset).

Figure 2. Schematic representation of protein complexes involved in dense granule biogenesis

The cartoon representation of BLOC-1, AP-3, HOPS/CORVET and Rab GGTase II complexes is based on structural information while BLOC-2 and BLOC-3 simply represent their subunit composition.