Golgi proteins in circulating human platelets are distributed across non-stacked, scattered structures

Platelets are small, anucleate cell fragments that are central to hemostasis, thrombosis, and inflammation. They are derived from megakaryocytes from which they inherit their organelles. As platelets can synthesize proteins and contain many of the enzymes of the secretory pathway, one might expect all mature human platelets to contain a stacked Golgi apparatus, the central organelle of the secretory pathway. By thin section electron microscopy, stacked membranes resembling the stacked Golgi compartment in megakaryocytes and other nucleated cells can be detected in both proplatelets and platelets. However, the incidence of such structures is low and whether each and every platelet contains such a structure remains an open question. By single-label, immunofluorescence staining, Golgi glycosyltransferases are found within each platelet and map to scattered structures. Whether these structures are positive for marker proteins from multiple Golgi subcompartments remains unknown. Here, we have applied state-of-the-art techniques to probe the organization state of the Golgi apparatus in resting human platelets. By the whole cell volume technique of serial-block-face scanning electron microscopy (SBF-SEM), we failed to observe stacked, Golgi-like structures in any of the 65 platelets scored. When antibodies directed against Golgi proteins were tested against HeLa cells, labeling was restricted to an elongated juxtanuclear ribbon characteristic of a stacked Golgi apparatus. By multi-label immunofluorescence microscopy, we found that each and every resting human platelet was positive for cis, trans, and trans Golgi network (TGN) proteins. However, in each case, the proteins were found in small puncta scattered about the platelet. At the resolution of deconvolved, widefield fluorescence microscopy, these proteins had limited tendency to map adjacent to one another. When the results of 3D structured illumination microscopy (3D SIM), a super resolution technique, were scored quantitatively, the Golgi marker proteins failed to map together indicating at the protein level considerable degeneration of the platelet Golgi apparatus relative to the layered stack as seen in the megakaryocyte. In conclusion, we suggest that these results have important implications for organelle structure/function relationships in the mature platelet and the extent to which Golgi apparatus organization is maintained in platelets. Our results suggest that Golgi proteins in circulating platelets are present within a series of scattered, separated structures. As separate elements, selective sets of Golgi enzymes or sugar nucleotides could be secreted during platelet activation. The establishment of the functional importance, if any, of these scattered structures in sequential protein modification in circulating platelets will require further research.

STEM tomography reveals that the canalicular system and α-granules remain separate compartments during early secretion stages in blood platelets

ESSENTIALS: How platelets organize their α-granule cargo and use their canalicular system remains controversial. Past structural studies were limited due to small sampling volumes or decreased resolution. Our analyses revealed homogeneous granules and a closed canalicular system that opened on activation. Understanding how platelets alter their membranes during activation and secretion elucidates hemostasis.

SUMMARY

BACKGROUND

Platelets survey the vasculature for damage and, in response, activate and release a wide range of proteins from their α-granules. Alpha-granules may be biochemically and structurally heterogeneous; however, other studies suggest that they may be more homogeneous with the observed variation reflecting granule dynamics rather than fundamental differences.

OBJECTIVES

Our aim was to address how the structural organization of α-granules supports their dynamics.

METHODS

To preserve the native state, we prepared platelets by high-pressure freezing and freeze-substitution; and to image nearly entire cells, we recorded tomographic data in the scanning transmission electron microscope (STEM).

RESULTS AND CONCLUSIONS

In resting platelets, we observed a morphologically homogeneous α-granule population that displayed little variation in overall matrix electron density in freeze-substituted preparations (i.e., macro-homogeneity). In resting platelets, the incidence of tubular granule extensions was low, ~4%, but this increased by > 10-fold during early steps in platelet secretion. Using STEM, we observed that the initially decondensing α-granules and the canalicular system remained as separate membrane domains. Decondensing α-granules were found to fuse heterotypically with the plasma membrane via long, tubular connections or homotypically with each other. The frequency of canalicular system fusion with the plasma membrane also increased by about three-fold. Our results validate the utility of freeze-substitution and STEM tomography for characterizing platelet granule secretion and suggest a model in which fusion of platelet α-granules with the plasma membrane occurs via long tubular connections that may provide a spatially limited access route for the timed release of α-granule proteins.