BACE2 processes PMEL to form the melanosome amyloid matrix in pigment cells

Abstract

Amyloids are often associated with pathologic processes such as in Alzheimer's disease (AD), but can also underlie physiological processes such as pigmentation. Formation of pathological and functional amyloidogenic substrates can require precursor processing by proteases, as exemplified by the generation of Aβ peptide from amyloid precursor protein (APP) by beta-site APP cleaving enzyme (BACE)1 and γ-secretase. Proteolytic processing of the pigment cell-specific Melanocyte Protein (PMEL) is also required to form functional amyloid fibrils during melanogenesis, but the enzymes involved are incompletely characterized. Here we show that the BACE1 homologue BACE2 processes PMEL to generate functional amyloids. BACE2 is highly expressed in pigment cells and Bace2 −/− but not Bace1 −/− mice display coat color defects, implying a specific role for BACE2 during melanogenesis. By using biochemical and morphological analyses, combined with RNA silencing, pharmacologic inhibition, and BACE2 overexpression in a human melanocytic cell line, we show that BACE2 cleaves the integral membrane form of PMEL within the juxtamembrane domain, releasing the PMEL luminal domain into endosomal precursors for the formation of amyloid fibrils and downstream melanosome morphogenesis. These studies identify an amyloidogenic substrate of BACE2, reveal an important physiological role for BACE2 in pigmentation, and highlight analogies in the generation of PMEL-derived functional amyloids and APPderived pathological amyloids.

Amyloid-based nanosensors and nanodevices

“…90 Other examples for functional amyloids in humans are amyloid fibers that serve as templates for the biosynthesis of melanin. [91][92][93] Furthermore, it has been shown that human serum albumin, lysozyme and other native proteins have a predisposition under certain situations to give way to amyloid fibril structures. 94,95 It is conceivable that the transition of a non-fibrillar protein to a self-aggregated amyloid fibrillar state does not necessarily only play a role in disease pathogenesis but also has a key function in signalling, communication and memorystorage.…”

“…Briefly, the melanin precursor (MP) is activated by a tyrosinase (T) resulting in the activated melanin precursor (AMP), which interacts with the amyloidogenic sequence PmeI17 (PmeI) leading to mature melanin. [91][92][93] assembled to microtubes, has been explored for effectively deliver drugs. Using rhodamine as a model drug, it was shown that the FF microtubes released the drug in a steady-state profile, following first-order kinetics.…”

“…The amyloid sequence PmeI drives a fast amyloid formation process: mature melanin is formed with PmeI serving as a template that organizes the melanin precursors and accelerates the covalent polymerization of these molecules into the mature melanin molecule. [91][92][93] Amyloidogenic protein sequences have been suggested to play other physiological roles, namely involving fibrin and tissue-type plasminogen activator. Fibrin results from thrombin-mediated proteolysis of fibrinogen.…”

The Kringle-like Domain Facilitates Post-endoplasmic Reticulum Changes to Premelanosome Protein (PMEL) Oligomerization and Disulfide Bond Configuration and Promotes Amyloid Formation

“…1A). A subsequent proteolytic cleavage by BACE2 within endosomes (33) or by an ADAM (a disintegrin and a metalloproteinase) protease at the plasma membrane (34) cleaves M␤ into two fragments; the C-terminal fragment (CTF) contains the transmembrane domain and is eventually degraded in a ␥-secretase-dependent manner in lysosomes (35), whereas the other fragment (M␤Ј) is liberated from the membrane covalently bound to M␣ (33,36). No longer anchored to the membrane, most of the M␣ polymerizes into amyloid fibrils within endosomes (30,37) and concomitantly or subsequently undergoes additional proteolytic processing events to form smaller fragments; some of these fragments, called M␣Ј, are derived from the central RPT domain and are detected within preparations of the parallel arrays of functional amyloid fibrils characteristic of early stage melanosomes (27,29,38).…”

“…No longer anchored to the membrane, most of the M␣ polymerizes into amyloid fibrils within endosomes (30,37) and concomitantly or subsequently undergoes additional proteolytic processing events to form smaller fragments; some of these fragments, called M␣Ј, are derived from the central RPT domain and are detected within preparations of the parallel arrays of functional amyloid fibrils characteristic of early stage melanosomes (27,29,38). Proprotein convertase and BACE2 cleavage are necessary for functional amyloid fibril formation, as interference with either of these two proteolytic processing events results in the formation of disordered PMEL aggregates instead of ordered fibrils (30,33). However, these cleavage events are not sufficient to induce fibril formation because a small fraction of cleaved PMEL is secreted from cells in culture (26,36,39) in a non-amyloid form.…”