Platelet life without TMEM163: no dense granules

In this issue of Blood, Yuan et al identified TMEM163 as a key regulator of platelet zinc homeostasis, playing a critical role in dense granule biogenesis in megakaryocytes.¹ 

Hermansky-Pudlak syndrome (HPS) is an inherited disease that is caused by mutations in genes that have an essential role in the assembly of cellular organelles, platelet dense (δ-delta) granules, melanosomes, lysosomes, lung lamellar bodies and cytotoxic T-cell lymphocyte granules.²  The abnormalities associated with HPS include hypopigmentation, visual deficiencies, prolonged bleeding times, and ceroid deposition in the lung and other organs. Adaptor protein-3 (AP-3) and biogenesis of lysosome-related organelles complex (BLOC) were identified as key players in the synthesis of lysosomal-related organelles. Platelet δ-granules are lysosomal-related organelles that are derived from endosomes and multivesicular bodies in megakaryocytes. Similar to their action in melanosomes, AP-3– and BLOC-dependent pathways play an important role in platelet δ-granule biogenesis.²  However, the exact molecular mechanisms involved in this process have not been determined in HPS and other platelet storage pool diseases.

Recently, transmembrane protein 163 (TMEM163) was identified as a new member of the zinc (Zn²⁺) transporter Slc30 family with a protein structure similar to that of ZnT3 and ZnT9.³  TMEM163 regulates Zn²⁺ transport in mammalian cells,³  particularly in cellular organelles, such as lysosomes, endosomes, and synaptic vesicles.^4,5  Transient receptor potential mucolipin 1 interacts with TMEM163, supporting translocation of TMEM163 from the plasma membrane to the lysosome, building a functional unit to regulate Zn²⁺ and proton (H⁺) transport. TMEM163 also regulates the protein expression and activity of the P2X purinergic receptors, indicating that TMEM163 has a diverse role in Zn²⁺-dependent signaling and gene regulation.^5,6 

Yuan et al showed that, in a megakaryocytic cell line, TMEM163 is selectively localized in precursors and maturated δ-granules, but not in α-granules. TMEM163 levels were significantly decreased in platelets isolated from mouse models and human patients lacking HPS-associated proteins (BLOC-1, BLOC-2, and AP-3). TMEM163-deficient platelets lack δ-granules, and Zn²⁺ abnormally accumulated in TMEM163-deficient megakaryocytic cells and platelets, indicating a crucial role for TMEM163-mediated Zn²⁺ transport in platelet δ-granule biogenesis. Furthermore, TMEM163 interacted with BLOC-1. Therefore, it was postulated that this interplay may regulate the trafficking of TMEM163 from early endosomes to maturated δ-granules (see figure). Consistent with the lack of δ-granules, TMEM163-deficient mice had reduced secretion of δ-granule–specific components, such as adenosine triphosphate and serotonin. The reduction in these second-wave mediators and consequent amplification of platelet reactivity through purinergic and serotonin receptors resulted in impaired α-granule secretion and platelet-aggregation responses. These platelet abnormalities influenced hemostasis, as shown by prolonged bleeding times in TMEM163-deficient mice. Previous studies have shown similar pathological symptoms using mouse or human platelets with storage pool deficiency; conversely, extracellular Zn²⁺ supplementation could rescue many hemostatic defects.⁷  Altogether, these results highlighted TMEM163 as an important Zn²⁺ transporter, contributing its function to platelet pathology of HPS and storage pool disease with δ-granule abnormalities.

Interestingly, TMEM163-deficient mice displayed normal platelet count and size with normal α-granule content, indicating that TMEM163 is not essential for platelet production or α-granule biogenesis. Although abnormal accumulation of Zn²⁺ was detected in TMEM163-deficient platelets, this did not influence the gene expression of other ZIP/ZnT transporters. Combined deficiency of Rab32/Rab38 small guanosine triphosphatases in mice results in HPS with profound defects in hemostasis.⁸  Rab32/Rab38 levels were normal in TMEM163-deficient mice, indicating a unique role for TMEM163-dependent pathways in δ-granule biogenesis.

In addition to TMEM163, other ZIP/ZnT isoforms may be involved in megakaryocyte and platelet Zn²⁺ homeostasis, which regulate distinct Zn²⁺-dependent signaling pathways.⁷  The expression profile and function of ZIP/ZnT isoforms could be dynamic during megakaryopoiesis, changing to optimize Zn²⁺ levels in the cytoplasm and intracellular organelles and regulate Zn²⁺-responsive genes and transcription factors to support platelet production (see figure). Granular-resident Zn²⁺ store in platelets is also involved in the regulation of the coagulation cascade.^7,9  These complex processes can be further analyzed in mouse models with ZIP/ZnT and HPS/TMEM163 deficiency to determine how defective Zn²⁺ transport and granular Zn²⁺ content are involved in platelet signaling and hemostatic complications in HPS and other storage pool diseases.

The study by Yuan et al has provided new insights into the regulatory mechanism of Zn²⁺ transport in δ-granules and suggests a link between abnormal δ-granule biogenesis and hemostasis. These findings may provide the basis for exploring Zn²⁺-dependent therapeutic strategies in HPS and other storage pool diseases.