The secreted tyrosine kinase VLK is essential for normal platelet activation and thrombus formation

Intravital Imaging of Thrombosis Models in Mice

Intravital microscopy is a powerful tool to study thrombosis in real time. The kinetics of thrombus formation and progression in vivo is studied after inflicting damage to the endothelium through mechanical, chemical, or laser injury. Mouse models of atherosclerosis are also used to induce thrombus formation. Vessels of different sizes and from different vascular beds such as carotid artery or vena cava, mesenteric or cremaster arterioles, can be targeted. Using fluorescent dyes, antibodies, or reporter mouse strains allows to visualize key cells and factors mediating the thrombotic processes. Here, we review the latest literature on using intravital microscopy to study thrombosis as well as thromboinflammation following transient middle cerebral artery occlusion, infection-induced immunothrombosis, and liver ischemia reperfusion.

Sulfenylation links oxidative stress to protein disulfide isomerase oxidase activity and thrombus formation

BACKGROUND

Oxidative stress contributes to thrombosis in atherosclerosis, inflammation, infection, aging, and malignancy. Oxidant-induced cysteine modifications, including sulfenylation, can act as a redox-sensitive switch that controls protein function. Protein disulfide isomerase (PDI) is a prothrombotic enzyme with exquisitely redox-sensitive active-site cysteines.

OBJECTIVES

We hypothesized that PDI is sulfenylated during oxidative stress, contributing to the prothrombotic potential of PDI.

METHODS

Biochemical and enzymatic assays using purified proteins, platelet and endothelial cell assays, and in vivo murine thrombosis studies were used to evaluate the role of oxidative stress in PDI sulfenylation and prothrombotic activity.

RESULTS

PDI exposure to oxidants resulted in the loss of PDI reductase activity and simultaneously promoted sulfenylated PDI generation. Following exposure to oxidants, sulfenylated PDI spontaneously converted to disulfided PDI. PDI oxidized in this manner was able to transfer disulfides to protein substrates. Inhibition of sulfenylation impaired disulfide formation by oxidants, indicating that sulfenylation is an intermediate during PDI oxidation. Agonist-induced activation of platelets and endothelium resulted in the release of sulfenylated PDI. PDI was also sulfenylated by oxidized low-density lipoprotein (oxLDL). In an in vivo model of thrombus formation, oxLDL markedly promoted platelet accumulation following an arteriolar injury. PDI oxidoreductase inhibition blocked oxLDL-mediated augmentation of thrombosis.

CONCLUSION

PDI sulfenylation is a critical posttranslational modification that is an intermediate during disulfide PDI formation in the setting of oxidative stress. Oxidants generated by vascular cells during activation promote PDI sulfenylation, and interference with PDI during oxidative stress impairs thrombus formation.

Is autologous platelet activation the key step in ovarian therapy for fertility recovery and menopause reversal?

Platelets are a uniquely mammalian physiologic feature. As the only non-marine vertebrates to experience menopause, humans have a substantial post-reproductive lifespan and are believed to have a limited, non-renewable oocyte supply. Ovarian reserve typically declines after about age 35yrs, marking losses which cannot be recovered by available fertility medications. When in vitro fertilization fails due to low or absent ovarian response, gonadotropin adjustments are often ineffectual and if additional oocytes are occasionally harvested, egg quality is usually poor. This problem was confronted by Greek researchers who developed a new surgical method to insert autologous platelet-rich plasma (PRP) into ovaries; the first ovarian PRP success to improve reproductive outcomes was published from Athens in 2016. This innovation influenced later research with condensed platelet-derived growth factors, leading to correction of oocyte ploidy error, normal blastocyst development, and additional term livebirths. Yet women's health was among the last clinical domains to explore PRP, and its role in 'ovarian rejuvenation' remains unsettled. One critical aspect in this procedure is platelet activation, a commonly overlooked step in the cytokine release cascade considered essential for successful transition of undifferentiated ovarian stem cells to an oocyte lineage. Poor activation of platelets thus becomes an unforced error, potentially diminishing or even negating post-treatment ovarian follicular response. To answer this query, relevant theory, current disagreements, and new data on platelet activation are presented, along with clinical challenges for regenerative fertility practice.

Vertebrate lonesome kinase modulates the hepatocyte secretome to prevent perivascular liver fibrosis and inflammation

Vertebrate lonesome kinase (VLK) is the only known extracellular tyrosine kinase, but its physiological functions are largely unknown. We show that VLK is highly expressed in hepatocytes of neonatal mice, but downregulated during adulthood. To determine the role of VLK in liver homeostasis and regeneration, we generated mice with a hepatocyte-specific knockout of the VLK gene (Pkdcc). Cultured progenitor cells established from primary hepatocytes of Pkdcc knockout mice produced a secretome, which promoted their own proliferation in 3D spheroids and proliferation of cultured fibroblasts. In vivo, Pkdcc knockout mice developed liver steatosis with signs of inflammation and perivascular fibrosis upon aging, combined with expansion of liver progenitor cells. In response to chronic CCl₄-induced liver injury, the pattern of deposited collagen was significantly altered in these mice. The liver injury marker alpha-fetoprotein (AFP) was increased in the secretome of VLK-deficient cultured progenitor cells and in liver tissues of aged or CCl₄-treated knockout mice. These results support a key role for VLK and extracellular protein phosphorylation in liver homeostasis and repair through paracrine control of liver cell function and regulation of appropriate collagen deposition.

This article has an associated First Person interview with the first author of the paper.

Summary: The secreted protein kinase VLK is released from hepatocytes and protects the liver from perivascular fibrosis and inflammation.