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The primary goal of the Marks Lab is to understand the molecular basis of intracellular membrane trafficking underlying the formation of cell type-specific lysosome-related organelles (LROs). The lab’s best-developed model system is the melanosome, an LRO in skin melanocytes and eye pigment cells within which melanin pigments are synthesized. The biogenesis of melanosomes and other LROs is disrupted in a group of genetic diseases called the Hermansky-Pudlak syndromes (HPS); the team and its collaborators have shown that the products of HPS-associated genes in skin melanocytes regulate two pathways of membrane protein transport from endosomes to newly forming melanosomes and a retrograde pathway from melanosomes.
The Marks Lab also seeks to understand how melanosome function is impacted by specific proteins including: melanosomal transmembrane transporters such as OCA2 and SLC45A2 that are defective in patients with distinct forms of oculocutaneous albinism; PMEL, a functional amyloid that forms the matrix upon which melanins polymerize; and lysosomal proteins, such as the transporter MFSD12, that impact melanogenesis indirectly.
With other collaborators, the lab has extended its analyses of LRO biogenesis to dense and alpha granules in platelets and megakaryocytes, lamellar bodies in lung epithelial type II cells, and phagosomes in dendritic cells. These organelles are also disrupted in different forms of HPS, and the Marks Lab is testing whether the models it has developed in melanocytes apply to organelle biogenesis in these other systems. For example, whereas melanosome biogenesis in skin melanocytes is constitutive, dense granule biogenesis is limited to a late stage of megakaryocyte differentiation into proplatelet strings.
Additionally, the lab has identified defects in innate immunity to bacterial infection from a mouse HPS2 model, partly explaining the recurrent bacterial infections observed in HPS2 patients, and is uncovering mechanisms by which the endosomal adaptor AP-3 influences toll-like receptor recruitment to phagosomes, inflammasome signaling from phagosomes, and autophagy activation in dendritic cells.
- HPS types 7, 8 and 9 are due to mutations in genes encoding subunits of BLOC-1, a complex required for the generation of tubular cargo carriers destined for melanosomes. The lab employs a variety of approaches to probe how BLOC-1 interacts with membranes and with other HPS-encoded protein complexes to better understand its mechanism of action.
- Fusion of transport carriers with target organelles is mediated by SNARE proteins. The lab is using a number of approaches to define the SNAREs involved in fusion of BLOC-1-dependent carriers with maturing melanosomes and to understand how their localization and activity are regulated.
- HPS types 3, 5 and 6 are due to mutations in genes encoding subunits of BLOC-2, a complex that directs BLOC-1-dependent cargo carriers with melanosomes. The lab is using biochemical and cell biological approaches to dissect the mechanism by which it functions.
- The lab is testing the role of lipid modifying enzymes in regulating the recruitment of BLOC-1 and BLOC-2 to membranes and in regulating the activity of other components of the membrane transport process.
- A mouse model of HPS bears a point mutation altering a single amino acid within the VPS33A subunit of the HOPS/CORVET endolysosomal tethering complex. The lab is employing a number of approaches to determine how HOPS or CORVET specifically impacts melanosome biogenesis and the mechanism by which it does so.
- Pigment cell-specific transmembrane proteins SLC45A2 and OCA2 are a transporter and channel, respectively, that modulate melanosome pH at different stages of melanosome maturation and that are targets of mutation in different forms of oculocutaneous albinism. The lab is using a variety of approaches to understand how they differentially contribute to this function and why they are both required to modulate melanosome pH.
- A GWAS study from Sarah Tishkoff's lab at the University of Pennsylvania identified several genes that show altered expression in light vs. dark skinned Africans. One newly identified gene, MFSD12, encodes a transmembrane transporter on lysosomes and not on melanosomes; decreased MFSD12 expression darkens pigmentation. Using melanocytes that express or are depleted of MFSD12, the lab is analyzing lysosome: melanosome contacts, melanosome and lysosome pH, and lysosomal signaling to uncover the mechanism of pigment regulation by MFSD12.
- HPS patients and mouse models all suffer from excessive bleeding and bruising due to defects in the formation of platelet dense granules, but little is known about the dense granule membrane or how and when it forms. The Marks Lab is using inducible megakaryocytic cell lines, primary megakaryocytes, and platelets from mice and humans to identify dense granule proteins, determine when during differentiation to platelets dense granules form, and understand how HPS complexes function in their generation.
- Some patients with HPS types 1 and 4 (lacking components of BLOC-3) develop a severe form of inflammatory bowel disease, but it is not known why. Together with Edward M. Behrens, MD, chief of the Division of Rheumatology, the lab is assessing the role of BLOC-3 in regulating cytokine production by cells of the innate immune system as a potential underlying susceptibility factor for the development of inflammatory disease.