Loss of PIKfyve in platelets causes a lysosomal disease leading to inflammation and thrombosis in mice

Abstract

PIKfyve is a lipid kinase that is essential for the synthesis of phosphatidylinositol-3,5-bisphosphate [PtdIns(3,5)P2], and for the regulation of membrane dynamics within the endolysosomal system in mammals. Depletion of intracellular pools of PtdIns(3,5)P2 in humans and in mice is associated with neurodegeneration and early lethality. However, the biological role of PtdIns(3,5)P2 in non-neural tissues is not well understood. Platelets are hematopoietic cells that function in a variety of physiological responses. Essential to many of these functions is the activation-dependent release of effectors from distinct storage granules - alpha granules, dense granules, and lysosomes - that derive from the endolysosomal system. Here we show that platelet-specific ablation of the PIKfyve gene in mice results in accelerated arterial thrombosis, but also unexpectedly to multiorgan defects that impair development, body mass, fertility, and survival by inducing inappropriate inflammatory responses characterized by macrophage accumulation in multiple tissues. Platelet depletion in vivo significantly impairs the progression of multiorgan defects in these mice, confirming that these defects reflect a platelet-specific process. Although PIKfyve-null platelets generate and release normal amounts of alpha granule and dense granule contents, they develop defective maturation and excessive storage of lysosomal enzymes, which are released upon platelet activation. Remarkably, impairing the secretion of lysosomes from PIKfyve-deficient platelets in vivo significantly attenuates the multiorgan defects in mice, suggesting that platelet lysosome secretion contributes to pathogenesis. Together, these results demonstrate that PIKfyve is an essential regulator for the biogenesis of platelet lysosomes, and highlight the previously unrecognized and important pathological contributions of platelet lysosomes in inflammation, arterial thrombosis, and macrophage biology.

Lysosome enlargement during inhibition of the lipid kinase PIKfyve proceeds through lysosome coalescence

“…Disruption of PIKfyve activity causes several major physiological defects including neurological decay, lethal embryonic development, and inflammatory disease, among others (Ikonomov et al, 2011;Min et al, 2014;Chow et al, 2007;McCartney et al, 2014). It is unclear how these defects arise, but they likely relate to disruption of endo/lysosome homeostasis, including impaired hydrolytic function, reduced rates of autophagic flux, and endo/lysosome swelling (Martin et al, 2013;de Lartigue et al, 2009;Min et al, 2014;Kim et al, 2016).…”

“…It is unclear how these defects arise, but they likely relate to disruption of endo/lysosome homeostasis, including impaired hydrolytic function, reduced rates of autophagic flux, and endo/lysosome swelling (Martin et al, 2013;de Lartigue et al, 2009;Min et al, 2014;Kim et al, 2016). Endo/lysosome vacuolation remains the most dramatic and visible defect in PIKfyve-abrogated cells.…”

“…http://dx.doi.org/10.1101/295246 doi: bioRxiv preprint first posted online Apr. 5, 2018; targeting lysosomal proteins to lysosomes (Ikonomov et al, 2009a;Rutherford et al, 2006;Min et al, 2014). Given this, we suspected that TFEB activation may not contribute to lysosome swelling in cells acutely suppressed for PIKfyve.…”

“…Inactivation of PIKfyve or of its regulators, Vac14/ArPIKfyve and Fig4/Sac3, causes a variety of physiological problems including embryonic lethality, neurodegeneration and immune malfunction (Cai et al, 2013;Chow et al, 2007;Jin et al, 2008;Ho et al, 2012;Mccartney et al, 2014;Sbrissa et al, 2007;Ferguson et al, 2010;Min et al, 2014;Lenk et al, 2016;Ikonomov et al, 2011). At the cellular level, these defects relate to impaired autophagic flux, altered delivery to lysosomes, dysregulation of lysosomal Ca 2+ transport, and massive enlargement of lysosomes (Ferguson et al, 2009;Dong et al, 2010;de Lartigue et al, 2009;Ferguson et al, 2010;Kim et al, 2014;Ho et al, 2012;Mccartney et al, 2014;Kim et al, 2016).…”

Sympathetic stimulation facilitates thrombopoiesis by promoting megakaryocyte adhesion, migration, and proplatelet formation

“…1,2 Platelets are generated by megakaryocytes (MKs), which are derived from hematopoietic stem cells (HSCs) through several consecutive stages accompanied by the cells' migration from the osteoblastic niche to the vascular niche in the bone marrow (BM). 3,4 This entire process is called thrombopoiesis.…”