UV induces common cutaneous amyloid-like melanosomal protein aggregates

Abstract Misfolding of aggregation-prone proteins underpins diseases known as proteinopathies. One of these proteins, alpha-synuclein, is a component of aggregates in neurodegenerative conditions such as Parkinson’s disease. The melanosomal protein PMEL, which forms physiologic amyloid scaffold structures on which melanin is organized in melanosomes, similarly ectopically accumulates in the dermis in many forms of cutaneous hyperpigmentation. Here, we demonstrate in a wide range of common clinical pigmentary disorders, as well as in primary melanocyte and mouse models examined by molecular, proteomic, and electron microscopic tools, that melanocytic alpha-synuclein is a prominent component of intracellular protein aggregates bound to similar proteins as in Parkinson’s disease, as well as melanized extracellular protein deposits. Using the Real Time Quaking-Induced Conversion Assay (RT-QuIC), we demonstrate that UV induces misfolded melanosomal proteins to self-propagate, augmenting this pathology in prion-like fashion. CUT&RUN chromatin profiling and single-cell RNA-seq demonstrate that melanocytes utilize microphthalmia-associated transcription factor (MITF)-regulated autophagy to counteract protein aggregation, identifying aggregate removal as a core function of tanning. In contrast to extracellular aggregation, impaired intracellular aggregate removal contributes to melanocyte senescence, which conversely exacerbates chronic hypopigmentation and photoaging-related discoloration. These findings identify melanosomal proteinopathy as a common contributor to melanocyte dysfunction and suggest aggregate-focused management approaches. Competing Interest Statement D.E. Fisher discloses ownership and consulting relationships with Soltego, Tasca, Swiss Rockets, Coherent Medicines, Biocoz, AME Therapeutics, and a consulting relationship with Pierre Fabre. These interests were reviewed and are managed by Massachusetts General Hospital and Partners HealthCare in accordance with their conflict-of-interest policies. B.T.H. owns stock in Novartis; he serves on the SAB of Dewpoint and has an option for stock. He serves on a scientific advisory board or is a consultant for AbbVie, Alexion, Ambagon, Aprinoia Therapeutics, Arbor Bio, Arvinas, Avrobio, AstraZenica, Biogen, Bioinsights, BMS, Cell Signaling, Cure Alz Fund, CurieBio, Dewpoint, Eisai, Etiome, Latus, Merck, Novartis, Paragon, Pfizer, Sanofi, Sofinnova, SV Health, Takeda, TD Cowen, Vigil, Violet, Voyager, WaveBreak. His laboratory is supported by research grants from the National Institutes of Health, Cure Alzheimers Fund, Tau Consortium, and the JPB Foundation and sponsored research agreement from Abbvie and Sanofi. He has a collaborative project with Biogen and Neurimmune. These interests were reviewed and are managed by Massachusetts General Hospital in accordance with their conflict-of-interest policies. A.B. is cofounder and shareholder of Casma Therapeutics and former advisory board member of Avilar Therapeutics.