Serotonin uptake and storage in human platelet density subpopulations

Impact of Plasma 5 Hydroxyindoleacetic Acid, a Serotonin Metabolite, on Clinical Severity in Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is characterized by dysregulated vascular permeability. The clinical outcomes remain poor, and the disease burden is widespread. We demonstrated that plasma 5-hydroxyindoleacetic acid (5-HIAA), a serotonin metabolite, is a pivotal severity indicator of ARDS. Serotonin is an effector of cellular contraction and a modulator of vascular permeability. Plasma 5-HIAA levels were significantly elevated in severe ARDS cases with shock status (p = 0.047) and positively correlated with SOFA (p < 0.0001) and APACHE-II score (p < 0.0001). In the longitudinal analysis, plasma 5-HIAA levels were also a strong independent predictor of mortality rate (p = 0.005). This study indicates that plasma 5-HIAA is a biomarker of ARDS severity and highlights the importance of evaluating vascular leakage levels for ARDS treatment.

Tryptophan metabolism is dysregulated in individuals with Fanconi anemia

Fanconi anemia (FA) is a complex genetic disorder associated with progressive marrow failure and a strong predisposition to malignancy. FA is associated with metabolic disturbances such as short stature, insulin resistance, thyroid dysfunction, abnormal body mass index (BMI), and dyslipidemia. We studied tryptophan metabolism in FA by examining tryptophan and its metabolites before and during the stress of hematopoietic stem cell transplant (HSCT). Tryptophan is an essential amino acid that can be converted to serotonin and kynurenine. We report here that serotonin levels are markedly elevated 14 days after HSCT in individuals with FA, in contrast to individuals without FA. Kynurenine levels are significantly reduced in individuals with FA compared with individuals without FA, before and after HSCT. Most peripheral serotonin is made in the bowel. However, serotonin levels in stool decreased in individuals with FA after transplant, similar to individuals without FA. Instead, we detected serotonin production in the skin in individuals with FA, whereas none was seen in individuals without FA. As expected, serotonin and transforming growth factor β (TGF-β) levels were closely correlated with platelet count before and after HSCT in persons without FA. In FA, neither baseline serotonin nor TGF-B correlated with baseline platelet count (host-derived platelets), only TGF-B correlated 14 days after transplant (blood bank-derived platelets). BMI was negatively correlated with serotonin in individuals with FA, suggesting that hyperserotonemia may contribute to growth failure in FA. Serotonin is a potential therapeutic target, and currently available drugs might be beneficial in restoring metabolic balance in individuals with FA.

A zinc transporter, transmembrane protein 163, is critical for the biogenesis of platelet dense granules

Lysosome-related organelles (LROs) are a category of secretory organelles enriched with ions such as calcium, which are maintained by ion transporters or channels. Homeostasis of these ions is important for LRO biogenesis and secretion. Hermansky-Pudlak syndrome (HPS) is a recessive disorder with defects in multiple LROs, typically platelet dense granules (DGs) and melanosomes. However, the underlying mechanism of DG deficiency is largely unknown. Using quantitative proteomics, we identified a previously unreported platelet zinc transporter, transmembrane protein 163 (TMEM163), which was significantly reduced in BLOC-1 (Dtnbp1sdy and Pldnpa)-, BLOC-2 (Hps6ru)-, or AP-3 (Ap3b1pe)-deficient mice and HPS patients (HPS2, HPS3, HPS5, HPS6, or HPS9). We observed similar platelet DG defects and higher intracellular zinc accumulation in platelets of mice deficient in either TMEM163 or dysbindin (a BLOC-1 subunit). In addition, we discovered that BLOC-1 was required for the trafficking of TMEM163 to perinuclear DG and late endosome marker-positive compartments (likely DG precursors) in MEG-01 cells. Our results suggest that TMEM163 is critical for DG biogenesis and that BLOC-1 is required for the trafficking of TMEM163 to putative DG precursors. These new findings suggest that loss of TMEM163 function results in disruption of intracellular zinc homeostasis and provide insights into the pathogenesis of HPS or platelet storage pool deficiency.

Advances in Platelet Subpopulation Research

Although lacking a nucleus, platelets are increasingly recognized not only for their complexity, but also for their diversity. Some 50 years ago platelet subpopulations were characterized by size and density, and these characteristics were thought to reflect platelet aging. Since, our knowledge of platelet heterogeneity has grown to recognize that differences in platelet biochemistry and function exist. This includes the identification of vanguard and follower platelets, platelets with differing procoagulant ability including "COAT-platelets" which enhance procoagulant protein retention on their surface, and most recently, the identification of platelet subpopulations with a differential ability to generate and respond to nitric oxide. Hence, in this mini-review, we summarize the current knowledge of platelet subpopulation diversity focusing on their physical, biochemical, and functional heterogeneity. In addition, we review how platelet subpopulations may change between health and disease and how differences among platelets may influence response to anti-platelet therapy. Finally, we look forward and discuss some of the future directions and challenges for this growing field of platelet research.

Correlation of membrane anisotropy with function in subpopulations of human blood platelets

Human blood platelets were fractionated on a discontinuous Percoll gradient into high density (HD), intermediate (ID), and low density (LD) platelets. The subpopulations were characterized with regard to [14C]serotonin uptake and release, cAMP content, aggregation, and membrane anisotropy, which is inversely related to membrane fluidity. Membrane anisotropy, which was high in LD platelets, was found to decrease with increasing density (LD greater than ID greater than HD). LD platelets showed significant lower cAMP levels and [14C]serotonin uptake than the total platelet population (TPP) and ID and HD platelets. Upon ADP and serotonin stimulation the cAMP content was reduced in all platelet populations with the exception of HD platelets in which cAMP was unchanged. Upon thrombin stimulation the cAMP content was reduced only in TPP and LD platelet population and it was increased in HD platelet population. Thrombin activation changed the anisotropy only in LD platelets. Thrombin at a concentration of 0.001 U/ml reduced whereas 0.01 and 0.05 U thrombin/ml increased the membrane anisotropy significantly. As compared with TPP and the other subpopulations, LD platelets were most sensitive upon ADP and thrombin stimulated [14C]serotonin release as well as upon ADP, serotonin and thrombin induced aggregation. The findings suggest that the differing functional abilities of the platelet subpopulations are correlated to the various membrane anisotropies observed in these fractions.

Subcellular composition of platelet density subpopulations prepared using continuous Percoll gradients

Normal human platelets have been separated by density on continuous Percoll gradients and the subcellular composition of platelets of different density has been analysed. The number and concentration of dense granules increased significantly with platelet density, as did the concentrations of the dense granule constituents calcium and serotonin. The amount of serotonin per granule in the low density (LD) platelets was only two-thirds of the corresponding amount in the high density (HD) platelets. Platelets of all densities were able to sequester exogenous serotonin and release it in response to thrombin stimulation with similar efficiencies. The concentrations of the alpha-granule constituents von Willebrand factor and beta-thromboglobulin increased significantly with platelet density but the concentrations of the lysosomal enzyme beta-N-acetylglucosaminidase and total sialic acid did not differ significantly in the density subpopulations. The concentrations of the cytosolic enzymes lactate dehydrogenase and glucose-6-phosphate dehydrogenase were slightly higher in the LD population than in the other density subpopulations. The concentration of glycogen showed a marked positive relationship with platelet density and calculations suggested that glycogen was an important determinant of platelet density heterogeneity. The findings of the present study are compatible with recent suggestions that LD platelets may be slightly younger than HD platelets in normal human subjects.

Vervet monkey (Cercopithecus aethiops sabaeus) whole blood serotonin level is determined by platelet uptake sites

Whole blood serotonin levels in adult male vervet monkeys living in social groups are sensitive to the animals' social environment. The mechanisms that translate different behavioral and environmental cues into altered whole blood serotonin levels are unknown. In this study, we have measured platelet number, size, serotonin content, and serotonin uptake, as well as the serum concentrations of tryptophan, Mg+2 and Ca+2. Results showed that whole blood serotonin levels, platelet serotonin content, and the serotonin uptake parameter Vmax were stable within animals on repeated sampling. The whole blood serotonin level was highly positively associated with platelet serotonin content, and the platelet serotonin content was highly positively associated with Vmax. These findings suggested that whole blood serotonin levels were a function of the number of platelet uptake sites.