An abnormal calcium uptake pump in Chediak-Higashi neutrophil lysosomes

Endolysosomal Ca2+ signaling in cardiovascular health and disease

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) regulates a plethora of functions in the cardiovascular (CV) system, including contraction in cardiomyocytes and vascular smooth muscle cells (VSMCs), and angiogenesis in vascular endothelial cells and endothelial colony forming cells. The sarco/endoplasmic reticulum (SR/ER) represents the largest endogenous Ca²⁺ store, which releases Ca²⁺ through ryanodine receptors (RyRs) and/or inositol-1,4,5-trisphosphate receptors (InsP₃Rs) upon extracellular stimulation. The acidic vesicles of the endolysosomal (EL) compartment represent an additional endogenous Ca²⁺ store, which is targeted by several second messengers, including nicotinic acid adenine dinucleotide phosphate (NAADP) and phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂], and may release intraluminal Ca²⁺ through multiple Ca²⁺ permeable channels, including two-pore channels 1 and 2 (TPC1-2) and Transient Receptor Potential Mucolipin 1 (TRPML1). Herein, we discuss the emerging, pathophysiological role of EL Ca²⁺ signaling in the CV system. We describe the role of cardiac TPCs in β-adrenoceptor stimulation, arrhythmia, hypertrophy, and ischemia-reperfusion injury. We then illustrate the role of EL Ca²⁺ signaling in VSMCs, where TPCs promote vasoconstriction and contribute to pulmonary artery hypertension and atherosclerosis, whereas TRPML1 sustains vasodilation and is also involved in atherosclerosis. Subsequently, we describe the mechanisms whereby endothelial TPCs promote vasodilation, contribute to neurovascular coupling in the brain and stimulate angiogenesis and vasculogenesis. Finally, we discuss about the possibility to target TPCs, which are likely to mediate CV cell infection by the Severe Acute Respiratory Disease-Coronavirus-2, with Food and Drug Administration-approved drugs to alleviate the detrimental effects of Coronavirus Disease-19 on the CV system.

Annexins: Ca2+ Effectors Determining Membrane Trafficking in the Late Endocytic Compartment

Despite the discovery of annexins 40 years ago, we are just beginning to understand some of the functions of these still enigmatic proteins. Defined and characterized by their ability to bind anionic membrane lipids in a Ca²⁺-dependent manner, each annexin has to be considered a multifunctional protein, with a multitude of cellular locations and diverse activities. Underlying causes for this considerable functional diversity include their capability to associate with multiple cytosolic and membrane proteins. In recent years, the increasingly recognized establishment of membrane contact sites between subcellular compartments opens a new scenario for annexins as instrumental players to link Ca²⁺ signalling with the integration of membrane trafficking in many facets of cell physiology. In this chapter, we review and discuss current knowledge on the contribution of annexins in the biogenesis and functioning of the late endocytic compartment, affecting endo- and exocytic pathways in a variety of physiological consequences ranging from membrane repair, lysosomal exocytosis, to cell migration.

On the move, lysosomal CAX drives Ca2+ transport and motility

Acidic Ca²⁺ stores are important sources of Ca²⁺ during cell signaling but little is known about how Ca²⁺ enters these stores. In this issue, Melchionda et al. (2016. J. Cell Biol.http://dx.doi.org/10.1083/jcb.201510019) identify a Ca²⁺/H⁺ exchanger (CAX) that is required for Ca²⁺ uptake and cell migration in vertebrates.

Intracellular two-phase Ca2+ release and apoptosis controlled by TRP-ML1 channel activity in coronary arterial myocytes

Activation of the death receptor Fas has been reported to produce a two-phase intracellular Ca(2+) release response in coronary arterial myocytes (CAMs), which consists of local Ca(2+) bursts via lysosomal transient potential receptor-mucolipin 1 (TRP-ML1) channels and consequent Ca(2+) release from the sarcoplasmic reticulum (SR). The present study was designed to explore the molecular mechanism by which lysosomal Ca(2+) bursts are coupled with SR Ca(2+) release in mouse CAMs and to determine the functional relevance of this lysosome-associated two-phase Ca(2+) release to apoptosis, a common action of Fas activation with Fas ligand (FasL). By confocal microscopy, we found that transfection of CAMs with TRP-ML1 small interfering (si)RNA substantially inhibited FasL (10 ng/ml)-induced lysosome Ca(2+) bursts and consequent SR Ca(2+) release. In contrast, transfection of CAMs with plasmids containing a full-length TRP-ML1 gene enhanced FasL-induced two-phase Ca(2+) release. We further demonstrated that FasL significantly increased the colocalization of the lysosomal marker Lamp1 with ryanodine receptor 3 and enhanced a dynamic trafficking of lysosomes to the SR. When CAMs were treated with TRP-ML1 siRNA, FasL-induced interactions between the lysosomes and SR were substantially blocked. Functionally, FasL-induced apoptosis and activation of calpain and calcineurin, the Ca(2+) sensitive proteins that mediate apoptosis, were significantly attenuated by silencing TRP-ML1 gene but enhanced by overexpression of TRP-ML1 gene. These results suggest that TRP-ML1 channel-mediated lysosomal Ca(2+) bursts upon FasL stimulation promote lysosome trafficking and interactions with the SR, leading to apoptosis of CAMs via a Ca(2+)-dependent mechanism.

Lysosomes shape Ins(1,4,5)P3-evoked Ca2+ signals by selectively sequestering Ca2+ released from the endoplasmic reticulum

Most intracellular Ca(2+) signals result from opening of Ca(2+) channels in the plasma membrane or endoplasmic reticulum (ER), and they are reversed by active transport across these membranes or by shuttling Ca(2+) into mitochondria. Ca(2+) channels in lysosomes contribute to endo-lysosomal trafficking and Ca(2+) signalling, but the role of lysosomal Ca(2+) uptake in Ca(2+) signalling is unexplored. Inhibition of lysosomal Ca(2+) uptake by dissipating the H(+) gradient (using bafilomycin A1), perforating lysosomal membranes (using glycyl-L-phenylalanine 2-naphthylamide) or lysosome fusion (using vacuolin) increased the Ca(2+) signals evoked by receptors that stimulate inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] formation. Bafilomycin A1 amplified the Ca(2+) signals evoked by photolysis of caged Ins(1,4,5)P(3) or by inhibition of ER Ca(2+) pumps, and it slowed recovery from them. Ca(2+) signals evoked by store-operated Ca(2+) entry were unaffected by bafilomycin A1. Video-imaging with total internal reflection fluorescence microscopy revealed that lysosomes were motile and remained intimately associated with the ER. Close association of lysosomes with the ER allows them selectively to accumulate Ca(2+) released by Ins(1,4,5)P(3) receptors.

Lysosomal Ca(2+) homeostasis: role in pathogenesis of lysosomal storage diseases

Disrupted cellular Ca(2+) signaling is believed to play a role in a number of human diseases including lysosomal storage diseases (LSD). LSDs are a group of ∼50 diseases caused predominantly by mutations in lysosomal proteins that result in accumulation of macromolecules within the lysosome. We recently reported that Niemann-Pick type C (NPC) is the first human disease to be associated with defective lysosomal Ca(2+) uptake and defective NAADP-mediated lysosomal Ca(2+) release. These defects in NPC cells leads to the disruption in endocytosis and subsequent lipid storage that is a feature of this disease. In contrast, Chediak-Higashi Syndrome cells have been reported to have enhanced lysosomal Ca(2+) uptake whilst the TRPML1 protein defective in mucolipidosis type IV is believed to function as a Ca(2+) channel. In this review we provide a summary of the current knowledge on the role of lysosomal Ca(2+) signaling in the pathogenesis of this group of diseases.

Acidic Ca(2+) stores come to the fore

Changes in the concentration of cytosolic Ca(2+) form the basis of a ubiquitous signal transduction pathway. Accumulating evidence implicates acidic organelles in the control of Ca(2+) dynamics in organisms across phyla. In this special issue, we discuss Ca(2+) signalling by these "acidic Ca(2+) stores" which include acidocalcisomes, vacuoles, the endo-lysosomal system, lysosome-related organelles, secretory vesicles and the Golgi complex. Ca(2+) release from these morphologically very different organelles is mediated by members of the TRP channel superfamily and two-pore channels. Inositol trisphosphate and ryanodine receptors which are traditionally viewed as endoplasmic reticulum Ca(2+) release channels can also mobilize acidic Ca(2+) stores. Ca(2+) uptake into acidic Ca(2+) stores is driven by Ca(2+) ATPases and Ca(2+)/H(+) exchangers. In animal cells, the Ca(2+)-mobilizing messenger NAADP plays a central role in mediating Ca(2+) signals from acidic Ca(2+) stores through activation of two-pore channels. These signals are important for several physiological processes including muscle contraction and differentiation. Dysfunctional acidic Ca(2+) stores have been implicated in diseases such as acute pancreatitis and lysosomal storage disorders. Acidic Ca(2+) stores are therefore emerging as essential components of the Ca(2+) signalling network and merit extensive further study.

Endolysosomal calcium regulation and disease

Until recently, the mechanisms that regulate endolysosomal calcium homoeostasis were poorly understood. The discovery of the molecular target of NAADP (nicotinic acid-adenine dinucleotide phosphate) as the two-pore channels resident in the endolysosomal system has highlighted this compartment as an important calcium store. The recent findings that dysfunctional NAADP release leads to defective endocytic function which in turn results in secondary lipid accumulation in the lysosomal storage disease Niemann-Pick type C, is the first evidence of a direct connection between a human disease and defective lysosomal calcium release. In the present review, we provide a summary of the current knowledge on mechanisms of calcium homoeostasis within the endolysosomal system and how these mechanisms may be affected in human metabolic disorders.

Alterations in erythrocyte membrane lipid and fatty acid composition in Chediak-Higashi syndrome

Chediak-Higashi syndrome (CHS) is an autosomal recessive disease characterized by the presence of abnormally large cytoplasmic organelles in all body granule producing cells. The molecular mechanism for this disease is still unknown. Functional disorders in membrane-related processes have been reported. Erythrocyte membranes from four CHS patients and 15 relatives including obligatory heterozygous were studied to examine potential alterations in the lipid and fatty acid profile of erythrocyte membranes associated with this syndrome. Plasma concentrations of cholesterol, triglycerides, phospholipids, and apolipoproteins AI and B100, and the lipid components of very low-, intermediate-, low- and high-density lipoproteins were also determined. CHS erythrocyte membranes were found to be enriched with lipids in relation to protein and to show: (1) an increase in cholesterol and choline-containing phospholipids (sphingomyelin and phosphatidylcholine) that predominate in the outer monolayer, which is higher than the increase in phosphatidylserine and phosphatidylethanolamine, that are chiefly limited to the inner monolayer in normal red blood cells; (2) a relative palmitic acid and saturated fatty acid increase and arachidonic acid and unsaturated fatty acid decrease, this resulting in a lower unsaturation index than controls. Changes in CHS erythrocyte membrane lipids seem to be unrelated to serum lipid disorders as plasma lipid and apolipoprotein concentrations were apparently in the normal range, with the exception of a modest hypertriglyceridemia in patients and relatives and a decreased concentration of HDL cholesterol in patients. These findings indicate that CHS erythrocyte membranes contain an abnormal lipid matrix with which membrane proteins are defectively associated. The anomalous CHS membrane composition can be explained on the postulated effects of the CHS1/Lyst gene.

History and implications of the neutrophil glycoprotein deficiencies

This review focuses on the initial clinical descriptions and subsequent investigation of the syndrome of recurrent infections associated with neutrophil membrane glycoprotein deficiencies. Characterization of the missing group of three glycoprotein heterodimers and their role in adhesion-related neutrophil function is summarized. Study of the clinical consequences of these genetically determined membrane glycoprotein defects has also contributed to the understanding of the role of normal neutrophils in both host defense and host tissue damage.