Advancing Renal Amyloidosis Care: The Role f Modern Diagnostic Techniques with the Potential of Enhancing Patient Outcomes

Published: May 28, 2024

1. Characterizing Proteomic Variations in Amyloidosis

Amyloidosis is the accumulation of misfolded proteins outside cells in an insoluble β-pleated sheet form. When seen under polarized light and stained with Congo red, these amyloid deposits can be identified by their distinctive apple-green–orange birefringence. Electron microscopy has shown that these deposits comprise rigid, non-branching fibrils measuring 7.5 to 10 nm in diameter [1,2]. Localized amyloidosis arises when amyloids are produced and deposited in the same place. In systemic amyloidosis, amyloidogenic proteins are synthesized in one region, such as the bone marrow or liver, and then deposited in another, such as the heart or kidneys [3]. The mechanisms that cause amyloidosis, whether systemic or localized, are generally classified into three broad categories: (1) the increased production of proteins that result in a predisposition toward amyloid formation, (2) mutations in proteins that increase the likelihood of misfolding compared to their normal counterparts, and (3) a natural proclivity of certain normal (wild-type) proteins to form amyloid [4]. Systemic amyloidosis can be hereditary or acquired. Notably, the absence of a family history does not preclude hereditary amyloidosis. Approximately half of the people with hereditary amyloidosis have no known family history of the disease [4,5,6].

Amyloidosis is classified based on the originating protein and is typically categorized as either a wild-type (acquired) or germline pathogenic variant (mutant). Proper amyloid typing is vital to avoid diagnostic mistakes [4,5,6]. Forty-two proteins are amyloidogenic, with at least 17 being capable of causing systemic disease [7]. Presently, six amyloidogenic proteins have the ability to form amyloids in both the wild-type and mutant forms. These proteins include transthyretin (ATTR), beta-2 microglobulin (Aβ2M), serum amyloid A (AA), apolipoprotein A-IV (ApoAIV), Aβ protein (Aβ), and prion protein (APrP) [8,9]. Other examples of proteins include leukocyte chemotactic factor 2 (ALECT2), fibrinogen α-chain (AFib), gelsolin (AGel), apolipoproteins A-I and A-II (AApoAI and AApoAII), and apolipoprotein C-III (AApoCIII) [10]. The subtypes of amyloidosis and their respective pathogenic mechanisms are illustrated

Besides amyloidogenic proteins, multiple other proteins have been identified that contribute to our understanding of the physiology of the disease. Proteins such as clusterin, vitronectin, apolipoprotein E (ApoE), ApoA4, and serum amyloid P-component (SAP) constitute amyloid signature proteins found in all types of amyloidosis and across all affected tissues. While some proteins, such as SAP, may promote amyloid development, others, such as clusterin, may function as amyloid chaperones and reduce amyloid production. Amyloid plaques contain proteins from the affected organs, which can provide information about the degree of amyloid accumulation, the processes that lead to cell damage, and the physiological reactions of various tissues to amyloid accumulation [11]. Proteins deposited in amyloid plaques (expanded proteome) and proteins overexpressed in plaques compared to controls (plaque-specific proteome) have been investigated in 2650 cases from eight different types of kidney amyloidosis [immunoglobulin light chain (AL), heavy chain (AH), a mix of heavy and light chain amyloid (IGH), ALECT2, AA, AFib, AApoAIV, and AApoCII] using laser microdissection–mass spectrometry. The proteomic content was higher in the more indolent types of amyloid (non-AL) than in AL [12]. The proteomic “density” of amyloid plaques can serve as an indicator of more gradual disease progression [13]. AA, AFib, and AApoCII amyloidosis were the most proteomically distinct, driven by increased complement pathway proteins, whereas AL and ALECT2 showed significant proteomic heterogeneity [14].

Complement pathway proteins are important sources of diversity across various amyloidosis types and proteomic subgroups. They may be synthesized in situ and may signify an unusual response to pathogens. A noticeable reduction in ubiquitins and heat shock proteins in patients with AL, AH, AApoCII, and AFib was noted, which may reflect the overutilization of these proteins by the ubiquitin–proteasome system [14]. This may also apply to heat shock proteins, a widely recognized cluster of chaperones that counteract amyloid formation [15]. Ubiquitins and heat shock proteins can be directly captured within aggregates of misfolded proteins, further exacerbating the collapse of cellular proteostasis [16,17,18].

The proteome specific to kidney AL plaques comprised 24 proteins, including those linked to kidney damage (α1 antitrypsin and heat shock protein β1). Hierarchical clustering of AL cases using their plaque-specific proteomes revealed four distinct clusters. One cluster, associated with better kidney survival, was distinguished by its higher proteomic content and the presence of 14-3-3 proteins; yet, it had reduced levels of amyloidogenic light chains (LCs) and the majority of signature proteins. The 14-3-3 proteins are a family of structurally similar phospho-binding proteins that regulate major cellular functions and act as molecular chaperones for several other proteins involved in a variety of cellular processes such as signal transduction, proliferation, differentiation, apoptosis, and autophagy [19]. They interact directly with different forms of amyloid and help to eliminate them [20,21]. Proteins linked to kidney diseases, including heat shock protein family B (small) member 1 (HSPB1), S100A6, transgelin (TAGLN), and TIMP metallopeptidase inhibitor 3 (TIMP3), were shown to be enriched in the proteome unique to AL. HSPB1 enhances autophagy during acute kidney injury (AKI) and is similarly activated by misfolded proteins [22]. S100A6, a protein that binds calcium, is elevated in AKI and potentially reduces β-amyloid accumulation in animal studies of Alzheimer’s disease [23,24]. TAGLN associates with cytoskeletal proteins and is upregulated by the amyloid precursor protein in Alzheimer’s disease [25]. Lastly, TIMP3, a component also found in cardiac amyloid proteome, may play a role in matrix remodeling and fibrosis in AL [13]. Glutathione-S transferase (GST) and phosphatidylethanolamine-binding protein 1 (PEB1) showed the highest upregulation in the proteome of early-stage amyloidosis. Both function as antioxidants, and PEB1 plays a key role in regulating the ferroptosis pathway [26,27]. Finally, the accumulation behavior of amyloidogenic proteins is independent of other proteins, suggesting that kidney toxicity in this condition cannot be solely attributed to amyloid levels. Investigating the functions of the proteins regarding disease development could provide a foundation for creating new treatments [12].

Authors

Charlotte Delrue, Amélie Dendooven, Annelore Vandendriessche, Reinhart Speeckaert, Sander De Bruyne and Marijn M. Speeckaert