New insights into the pathogenesis of Hermansky-Pudlak syndrome

Melanin Biopolymers in Pharmacology and Medicine-Skin Pigmentation Disorders, Implications for Drug Action, Adverse Effects and Therapy

Melanins are biopolymeric pigments formed by a multi-step oxidation process of tyrosine in highly specialized cells called melanocytes. Melanin pigments are mainly found in the skin, iris, hair follicles, and inner ear. The photoprotective properties of melanin biopolymers have been linked to their perinuclear localization to protect DNA, but their ability to scavenge metal ions and antioxidant properties has also been noted. Interactions between drugs and melanins are of clinical relevance. The formation of drug-melanin complexes can affect both the efficacy of pharmacotherapy and the occurrence of adverse effects such as phototoxic reactions and discoloration. Because the amount and type of melanin synthesized in the body is subject to multifactorial regulation-determined by both internal factors such as genetic predisposition, inflammation, and hormonal balance and external factors such as contact with allergens or exposure to UV radiation-different effects on the melanogenesis process can be observed. These factors can directly influence skin pigmentation disorders, resulting in hypopigmentation or hyperpigmentation of a genetic or acquired nature. In this review, we will present information on melanocyte biology, melanogenesis, and the multifactorial influence of melanin on pharmacological parameters during pharmacotherapy. In addition, the types of skin color disorders, with special emphasis on the process of their development, symptoms, and methods of treatment, are presented in this article.

Physical Contacts Between Mitochondria and WPBs Participate in WPB Maturation

BACKGROUND

Weibel-Palade bodies (WPBs) are endothelial cell-specific cigar-shaped secretory organelles containing various biologically active molecules. WPBs play crucial roles in thrombosis, hemostasis, angiogenesis, and inflammation. The main content of WPBs is the procoagulant protein vWF (von Willebrand factor). Physical contacts and functional cross talk between mitochondria and other organelles have been demonstrated. Whether an interorganellar connection exists between mitochondria and WPBs is unknown.

METHODS

We observed physical contacts between mitochondria and WPBs in human umbilical vein endothelial cells by electron microscopy and living cell confocal microscopy. We developed an artificial intelligence-assisted method to quantify the duration and length of organelle contact sites in live cells.

RESULTS

We found there existed physical contacts between mitochondria and WPBs. Disruption of mitochondrial function affected the morphology of WPBs. Furthermore, we found that Rab3b, a small GTPase on the WPBs, was enriched at the mitochondrion-WPB contact sites. Rab3b deficiency reduced interaction between the two organelles and impaired the maturation of WPBs and vWF multimer secretion.

CONCLUSIONS

Our results reveal that Rab3b plays a crucial role in mediating the mitochondrion-WPB contacts, and that mitochondrion-WPB coupling is critical for the maturation of WPBs in vascular endothelial cells.

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Pallidin function in Drosophila surface glia regulates sleep and is dependent on amino acid availability

The Pallidin protein is a central subunit of a multimeric complex called biogenesis of lysosome-related organelles complex 1 (BLOC1) that regulates specific endosomal functions and has been linked to schizophrenia. We show here that downregulation of Pallidin and other members of BLOC1 in the surface glia, the Drosophila equivalent of the blood-brain barrier, reduces and delays nighttime sleep in a circadian-clock-dependent manner. In agreement with BLOC1 involvement in amino acid transport, downregulation of the large neutral amino acid transporter 1 (LAT1)-like transporters JhI-21 and mnd, as well as of TOR (target of rapamycin) amino acid signaling, phenocopy Pallidin knockdown. Furthermore, supplementing food with leucine normalizes the sleep/wake phenotypes of Pallidin downregulation, and we identify a role for Pallidin in the subcellular trafficking of JhI-21. Finally, we provide evidence that Pallidin in surface glia is required for GABAergic neuronal activity. These data identify a BLOC1 function linking essential amino acid availability and GABAergic sleep/wake regulation.

A gain-of-function TPC2 variant R210C increases affinity to PI(3,5)P2 and causes lysosome acidification and hypopigmentation

Albinism is a group of inherited disorders mainly affecting skin, hair and eyes. Here we identify a de novo point mutation, p.R210C, in the TPCN2 gene which encodes Two Pore Channel 2 (TPC2) from a patient with albinism. TPC2 is an endolysosome and melanosome localized non-selective cation channel involved in regulating pigment production. Through inside-out recording of plasma membrane targeted TPC2 and direct recording of enlarged endolysosomal vacuoles, we reveal that the R210C mutant displays constitutive channel activation and markedly increased affinity to PI(3,5)P₂. Mice harboring the homologous mutation, R194C, also exhibit hypopigmentation in the fur and skin, as well as less pigment and melanosomes in the retina in a dominant inheritance manner. Moreover, mouse embryonic fibroblasts carrying the R194C mutation show enlarged endolysosomes, enhanced lysosomal Ca²⁺ release and hyper-acidification. Our data suggest that R210C is a pathogenic gain-of-function TPC2 variant that underlies an unusual dominant type of albinism.

VDAC1 negatively regulates melanogenesis through the Ca2+-calcineurin-CRTC1-MITF pathway

This study revealed an important and novel role of mitochondrial VDAC1 in regulating melanogenesis in resting melanocytes through a Ca²⁺-regulated pathway that is independent of the alpha-MSH/UVB pathway.

Melanocytes produce melanin for protecting DNA from ultraviolet exposure to maintain genomic stability. However, the precise regulation of melanogenesis is not fully understood. VDAC1, which is mainly localized in the outer mitochondrial membrane, functions as a gatekeeper for the entry or exit of Ca²⁺ between mitochondria and the cytosol and participates in multiple physiological processes. Here, we showed a novel role of VDAC1 in melanogenesis. Depletion of VDAC1 increased pigment content and up-regulated melanogenic genes, TYR, TYRP1, and TYRP2. Knockdown of VDAC1 increased free cytosolic Ca²⁺ in cultured melanocytes at the resting state, which activated calcineurin through the Ca²⁺-calmodulin-CaN pathway. The activated CaN dephosphorylated CRTC1 to facilitate its nuclear translocation and ultimately up-regulated the transcription of the master regulator of melanogenesis MITF. Consistently, depletion of Vdac1 in mice led to up-regulation of the transcription of MITF and thereafter its targeted melanogenic genes. These findings suggest that VDAC1 is an important negative regulator of melanogenesis, which expands our knowledge about pigment production and implies its potential role in melanoma.

Identification of a Novel MLPH Missense Mutation in a Chinese Griscelli Syndrome 3 Patient

Melanophilin (MLPH) functions as a linker between RAB27A and myosin Va (MYO5A) in regulating skin pigmentation during the melanosome transport process. The MYO5A-MLPH-RAB27A ternary protein complex is required for anchoring mature melanosomes in the peripheral actin filaments of melanocytes for subsequent transfer to adjacent keratinocytes. Griscelli syndrome type 3 (GS3) is caused by mutations in the MLPH gene. So far, only five variants of MLPH associated with GS3 have been reported. Here, we reported the first patient with GS3 in a Chinese population. The proband carried a novel homozygous missense mutation (c.73G>C; p.D25H), residing in the conserved Slp homology domain of MLPH, and presented with hypopigmentation of the hair, eyebrows, and eyelashes. Light microscopy revealed the presence of abnormal pigment clumping in his hair shaft. In silico tools predicted this MLPH variant to be likely pathogenic. Using immunoblotting and immunofluorescence analysis, we demonstrated that the MLPH (D25H) variant had an inhibitory effect on melanosome transport by exhibiting perinuclear melanosome aggregation in melanocytes, and greatly reduced its binding to RAB27A, although the protein level of MLPH in the patient was not changed. Our findings suggest that MLPH (D25H) is a pathogenic variant that expands the genetic spectrum of the MLPH gene.