A Role of Phosphatidylserine in the Function of Recycling Endosomes

Cells internalize proteins and lipids in the plasma membrane (PM) and solutes in the extracellular space by endocytosis. The removal of PM by endocytosis is constantly balanced by the replenishment of proteins and lipids to PM through recycling pathway. Recycling endosomes (REs) are specific subsets of endosomes. Besides the established role of REs in recycling pathway, recent studies have revealed unanticipated roles of REs in membrane traffic and cell signalling. In this review, we highlight these emerging issues, with a particular focus on phosphatidylserine (PS), a phospholipid that is highly enriched in the cytosolic leaflet of RE membranes. We also discuss the pathogenesis of Hermansky Pudlak syndrome type 2 (HPS2) that arises from mutations in the AP3B1 gene, from the point of view of dysregulated RE functions.

Hermansky–Pudlak Syndrome Type 6 Accompanied with Bowel Vascular Malformation: Clinical Case

Background. Hermansky–Pudlak syndrome type 6 is rare hereditary disease caused by pathogenic variants in base sequence, deletions, and insertions in the HPS6 gene encoding the transmembrane protein of the same name. This disease occurs with hemorrhagic syndrome, oculocutaneous albinism, and inflammatory bowel diseases (in some cases). The clinical picture of the disease, including the gastrointestinal tract pathology, has not been studied completely due to the syndrome rarity.Clinical case description. We would like to present the description of clinical case of the patient with Hermansky–Pudlak syndrome type 6 accompanied with bowel vascular malformation. The patient diagnosed with “oculocutaneous albinism” at the age of 4.5 has shown recurrent intestinal bleedings, blood hemoglobin concentration decrease to 45 g/l; platelet count, mean platelet volume and platelet distribution width remained within the reference values. Slight decrease of Quick’s value to 68% (normal range 70–120%) was revealed. The study of platelet morphology has revealed a decrease in the number of dense granules: < 3 in 25% platelets, < 6 — in 64%. Ultrasound investigation has revealed signs of vascular malformation in ascending colon: significant changes of diameter (widening) and shape of intestinal wall vessels. Molecular genetic analysis (NGS) has revealed the nucleotide variant c.1133T>G (p.Leu378Arg) in homozygous state in the HPS6 gene. The same variant in homozygous state was revealed in the younger proband sister who also had vascular changes in the ascending colon wall.Conclusion. Differential diagnosis of Germanic–Pudlak syndrome type 6 should be performed with other types of this syndrome as well as with syndrome and non-syndrome forms of oculocutaneous albinism. Molecular genetic confirmation of the diagnosis is suggested via massive parallel sequencing (NGS) methods (exome sequencing included) due to the rarity of Hermansky–Pudlak syndrome.

Modeling of lung phenotype of Hermansky–Pudlak syndrome type I using patient-specific iPSCs

Background Somatic cells differentiated from patient-specific human induced pluripotent stem cells (iPSCs) could be a useful tool in human cell-based disease research. Hermansky–Pudlak syndrome (HPS) is an autosomal recessive genetic disorder characterized by oculocutaneous albinism and a platelet dysfunction. HPS patients often suffer from lethal HPS associated interstitial pneumonia (HPSIP). Lung transplantation has been the only treatment for HPSIP. Lysosome-related organelles are impaired in HPS, thereby disrupting alveolar type 2 (AT2) cells with lamellar bodies. HPSIP lungs are characterized by enlarged lamellar bodies. Despite species differences between human and mouse in HPSIP, most studies have been conducted in mice since culturing human AT2 cells is difficult. Methods We generated patient-specific iPSCs from patient-derived fibroblasts with the most common bi-allelic variant, c.1472_1487dup16, in HPS1 for modeling severe phenotypes of HPSIP. We then corrected the variant of patient-specific iPSCs using CRISPR-based microhomology-mediated end joining to obtain isogenic controls. The iPSCs were then differentiated into lung epithelial cells using two different lung organoid models, lung bud organoids (LBOs) and alveolar organoids (AOs), and explored the phenotypes contributing to the pathogenesis of HPSIP using transcriptomic and proteomic analyses. Results The LBOs derived from patient-specific iPSCs successfully recapitulated the abnormalities in morphology and size. Proteomic analysis of AOs involving iPSC-derived AT2 cells and primary lung fibroblasts revealed mitochondrial dysfunction in HPS1 patient-specific alveolar epithelial cells. Further, giant lamellar bodies were recapitulated in patient-specific AT2 cells. Conclusions The HPS1 patient-specific iPSCs and their gene-corrected counterparts generated in this study could be a new research tool for understanding the pathogenesis of HPSIP caused by HPS1 deficiency in humans.

A Novel Likely Pathogenic Variant in the BLOC1S5 Gene Associated with Hermansky-Pudlak Syndrome Type 11 and an Overview of Human BLOC-1 Deficiencies

Hermansky-Pudlak syndrome (HPS) is a heterogeneous disorder combining oculocutaneous albinism (OCA) and a platelet function disorder of varying severity as its most prominent features. The genes associated with HPS encode for different BLOC- (biogenesis of lysosome-related organelles complex) complexes and for the AP-3 (adaptor protein-3) complex, respectively. These proteins are involved in maturation, trafficking, and the function of lysosome-related organelles (LROs) such as melanosomes and platelet δ-granules. Some patients with different types of HPS can develop additional complications and symptoms like pulmonary fibrosis, granulomatous colitis, and immunodeficiency. A new type of HPS has recently been identified associated with genetic alterations in the BLOC1S5 gene, which encodes the subunit Muted of the BLOC-1 complex. Our aim was to unravel the genetic defect in two siblings with a suspected HPS diagnosis (because of OCA and bleeding symptoms) using next generation sequencing (NGS). Platelet functional analysis revealed reduced platelet aggregation after stimulation with ADP and a severe secretion defect in platelet δ-granules. NGS identified a novel homozygous essential splice site variant in the BLOC1S5 gene present in both affected siblings who are descendants of a consanguine marriage. The patients exhibited no additional symptoms. Our study confirms that pathogenic variants of BLOC1S5 cause the recently described HPS type 11.