Rab27a Is Required for Regulated Secretion in Cytotoxic T Lymphocytes

Abstract

Rab27a activity is affected in several mouse models of human disease including Griscelli (ashen mice) and Hermansky-Pudlak (gunmetal mice) syndromes. A loss of function mutation occurs in the Rab27a gene in ashen (ash), whereas in gunmetal (gm) Rab27a dysfunction is secondary to a mutation in the α subunit of Rab geranylgeranyl transferase, an enzyme required for prenylation and activation of Rabs. We show here that Rab27a is normally expressed in cytotoxic T lymphocytes (CTLs), but absent in ashen homozygotes (ash/ash). Cytotoxicity and secretion assays show that ash/ash CTLs are unable to kill target cells or to secrete granzyme A and hexosaminidase. By immunofluorescence and electron microscopy, we show polarization but no membrane docking of ash/ash lytic granules at the immunological synapse. In gunmetal CTLs, we show underprenylation and redistribution of Rab27a to the cytosol, implying reduced activity. Gunmetal CTLs show a reduced ability to kill target cells but retain the ability to secrete hexosaminidase and granzyme A. However, only some of the granules polarize to the immunological synapse, and many remain dispersed around the periphery of the CTLs. These results demonstrate that Rab27a is required in a final secretory step and that other Rab proteins also affected in gunmetal are likely to be involved in polarization of the granules to the immunological synapse.

The Dilute Locus and Griscelli Syndrome: Gateways Towards a Better Understanding of Melanosome Transport

“…Since the expression was found to be absent in brain, Ménasché et al (20) proposed that RAB27A-mutated Griscelli patients present that subset of the Griscelli phenotype in which next to the pigmentation disorder, immune disorder features, with an uncontrolled T-lymphocyte and macrophage activation syndrome, the hemophagocytic syndrome, predominate, with absence of early neurologic abnormalities. Indeed, RAB27A-deficient T cells exhibited reduced cytotoxicity and cytolytic granule exocytosis, whereas MYO5A-defective T cells did not (20,23). In the MYO5A-mutated Griscelli-phenotype the neurologic impairment is more prominent, consistent with the major role of myosin Va in brain (24).…”

Rab27a and Rab27b are involved in stimulation-dependent RANKL release from secretory lysosomes in osteoblastic cells

“…(28,44) The regulation of RANKL in osteoblastic cells is similar to that of FasL in cytotoxic Tlymphocytes; however, there are also some differences. (24,(46)(47)(48)(49) Both Rab27a and Rab27b are involved in stimulation-dependent RANKL release in osteoblastic cells (Figs. 1 and 2).…”

“…(19) Rab27a, which is expressed in many types of secretory organs, is reported to be involved in secretion from lysosomal organelles such as secretory granules in insulin-secreting cells, (20) dense granules in platelets, (21) dense-core vesicles in neuroendocrine cells, (22) melanosomes in melanocytes, (23) and lytic granules in cytotoxic T-lymphocytes (CTLs). (24) Several Rab27a effector molecules also have been identified. In melanocytes, Rab27a and its effector, melanophilin, are involved in the transport of melanosomes along actin filaments, (25) and the subsequent tethering, docking, and fusion steps at the plasma membrane are regulated by Rab27a and another effector, Slp2-a.…”

Macrophage activation syndrome as part of systemic juvenile idiopathic arthritis: diagnosis, genetics, pathophysiology and treatment

“…Rab27a is important in docking the perforin laden vesicle at the cell membrane for release. 48 Munc13-4 is likely important in activating Syntaxin 11 that in turn is important for vesicle fusion. 25,28 Absence of any these molecules prevents the release of perforin and therefore destruction of target antigenpresenting cells (Figure 2).…”