Platelet secretory granules or secretory lysosomes?

In human blood platelets one can identify three types of secretory organelles: dense granules or bodies, alpha granules and lysosomes. Using routine transmission electron microscopy (TEM) dense granules appear as vacuoles with an electron dense core. This perception is obviously based on a fixational artefact due to the use of osmium tetroxide and organic solvents during TEM. For example during cryosectioning when these substances are not used, dense granules are difficult to identify [1] or are believed to be in the form of vacuoles which have leaked their contents [2]. In the so called whole mounts preparation, where platelets are not exposed to any fixatives or staining, dense granules appear as dense bodies [3]. Alpha granules in TEM show moderate electron density with a denser centre. Platelet lysosomes in TEM appear morphologically similar to alpha granules. Dense bodies are said to contain predominantly adenine nucleotides and serotonin, whereas alpha granules contain a large variety of secretory proteins. Lysosomes are characterised by an acidic interior, the presence of acid hydrolases and lysosomal membrane markers. This classification is fully accepted even though increasing evidence accumulates, pointing to certain interrelatedness between these three compartments.

Thus mepacrine, a typical lysosome labelling drug was found to accumulate in platelet dense bodies [4] as well in alpha granules [5]. This is apparently due to the acidic interior of both [[4], [5]]. An acidic interior is however one of the characteristic features of lysosomes. Dense bodies of platelets were recently called lysosome-related organelles because similarly to lysosomes they show an acidic interior, accumulate mepacrine and exhibit the lysosomal membrane glycoprotein marker CD63 or another lysosomal marker LAMP2 [6]. Interestingly dense granules also share the presence of P-selectin and glycoprotein 1b and IIbIIIa with alpha granules [4]. A deficiency in dense granules is characteristic for the Hermansky-Pudlak platelet syndrome with hemorrhagic diathesis, which is a generalised lysosomal disorder where other tissues are also involved [[6], [7]].

It is of interest that dense bodies [2] and alpha granules [1] share multivesicular bodies (MVB) as their precursors. MVBs are well known lysosomal components characterised by the presence of lysosomal enzymes and products of endocytosis as reviewed in [7]. Indeed MVBs as precursors of alpha granules were found to contain acid hydrolases and the lysosomal marker CD63. Importantly the lysosomal membrane marker CD63 was detected also in some newly formed platelet alpha granules [1], obviously owing to the protective technique of cryosectioning. The presence of some lysosomal enzymes in an isolated dense bodies fraction was found biochemically [8].

The lysosomal nature of platelet alpha granules is further suggested by their endocytic activities. It was shown for example that alpha granules are able to incorporate plasma fibrinogen, albumin or factor V – for references see [9]. It should be mentioned that isolated platelet alpha granules show the presence of lysosomal acid phosphatases biochemically [8] and have an acidic interior as mentioned above [5]. The fact that acid phosphatases are difficult to detect in alpha granules histochemically [1], can be explained by the lower sensitivity of histochemical methods compared to biochemical methods, obviously due to tightly packed glycoproteins in alpha granules. Subcellular fractionation of platelet secretory organelles showed that the typical lysosomal enzyme acid phosphatase can be detected practically in every fraction, which means also in the alpha granules fraction, as reviewed in [[7], [9]]. One should bring up here the not well known fact that platelet lysosomal acid phosphatase cannot be solubilised even by detergents and exists always in a form firmly bound to lipoprotein membranes – as discussed in [[7], [9]].

From the above one can see that platelet dense bodies as well as alpha granules share many features common for lysosomes. It is therefore not surprising that granules with secretory products in hematopoietic cells were recently labelled as secretory lysosomes. There is increasing evidence suggesting that even lysosomes can be involved in regulated exocytosis [10]. Finally one is tempted to say that dense bodies of platelets with their unstable ultrastructure and much lower frequency then alpha granules may represent a transient form of secretory lysosomes. Lysosomes in general as well as platelet alpha granules are known for their heterogeneity [7].

This lysosomal perception of platelet secretory organelles would help for example to explain the presence of P-selectin in both dense granules and alpha granules. In an analogy P-selectin was found to accumulate in secretory lysosomes of Rbl-2H3 cells – a basophilic leukemia cell line known to store and secrete serotonin and lysosomal enzymes [11]. It remains to be seen whether this lysosomal perception of platelet secretory granules will prove to be viable.