Summary
Lysosomal Associated Membrane Protein-2 (LAMP-2) is an inherent component of lysosomal
granule membranes in diverse cell types, including platelets. We examined platelets
for evidence of LAMP-2 in dense granule membranes as CD63 has previously been shown
to be present in both lysosomal and dense granule membranes. Immunological techniques
were used to examine the localization of LAMP-2 in control platelets and those from
an individual with Hermansky-Pudlak syndrome (HPS), a condition characterised by platelet
dense granule deficiency. Immunoblotting studies demonstrated that LAMP-2 was enriched
in a dense granule preparation. Flow cytometry of thrombin-stimulated control platelets
was consistent with biphasic surface expression of LAMP-2. The early expression was
accompanied by dense granule, but minimal lysosomal granule, release. The late expression
was accompanied by additional lysosomal granule release only. Thrombin stimulation
of HPS platelets showed only late, lysosome-associated LAMP-2 expression. Immunoelectron
microscopy indicated the presence of LAMP-2 in the membranes of serotonin-containing
granules as identified by an anti-serotonin polyclonal antibody. These data indicate
that LAMP-2 is present in the membranes of platelet dense granules in addition to
lysosomal granules, and has a similar distribution to CD63.
