Background: Autophagy is a catabolic process for the autophagosome-lysosomal degradation of bulk cytoplasmic contents (1, 2). Selective autophagy targets the degradation of distinct sets of substrates and organelles (3-5). One of the best studied examples of selective autophagy involves the clearance of damaged mitochondria through a process called mitophagy. Several pathways have been described for various contexts of mitophagy, including the FUNDC1 pathway, the BNIP3 and BNIP3L/Nix pathway, and the PINK1/Parkin pathway. FUNDC1 is a mitochondrial protein that is phosphorylated by the autophagy kinase ULK1 and regulates hypoxia induced mitophagy (6, 7). BNIP3L/Nix and BNIP3 are members of the Bcl-2 family of apoptosis regulators that are expressed on mitochondria, induced by hypoxia, and have have been shown to play a role in mitophagy (8). BNIP3L/Nix is also important in the autophagic maturation of erythroid cells (9). FUNDC1, BNIP3 and BNIP3L/Nix bind to LC3 family members, targeting the mitochondria to the autophagosome.Non-hypoxic induction of mitophagy can be regulated by the PINK1/Parkin pathway, which plays causative roles in neurodegenerative disease, most notably Parkinson’s disease (10, 11). PINK1 is a mitochondrial serine/threonine kinase that is stabilized on the outer mitochondrial membrane of damaged mitochondria. Substrates of PINK1 include the E3 ubiquitin ligase Parkin and ubiquitin itself (12-14). Phosphorylation of Parkin as well as binding to phosphorylated ubiquitin leads to accumulation of ubiquitinated chains on multiple mitochondrial proteins. Ubiquitinated proteins are recognized by selective cargo receptors including SQSTM1/p62, Optineurin, and NDP52 (15-16). Autophagy cargo receptors contain an LC3-interacting region (LIR) required for binding to Atg8/LC3 family members and targeting to the autophagosome (3).
Background: Autophagy is a catabolic process for the autophagosome-lysosomal degradation of bulk cytoplasmic contents (1,2). Selective autophagy targets the degradation of distinct sets of substrates and organelles and can occur through the utilization of a number of autophagy cargo receptors (3-5). Autophagy cargo receptors contain an LC3-interacting region (LIR) required for interaction with Atg8/LC3 family members targeted to the autophagosome. SQSTM1/p62-like receptors (SLRs) are a family of autophagy cargo receptors that contain domains for binding to ubiquitin. This family includes prototypical member SQSTM1/p62, NBR1, NDP52, Optineurin, and TAX1BP1. Targets of SLRs include ubiquitylated protein aggregates (aggrephagy), organelles such as mitochondria (mitoophagy) and peroxisomes (pexophagy), and intracellular bacteria (xenophagy).Upon binding of cargo to these receptors, the complex is delivered to the autophagosome where both the cargo and receptor are degraded through the autophagic process. While some redundancy may exist among SLR family members, they can have unique activities. Many SLRs can have additional roles as scaffolding proteins for various signaling pathways. For example, SQSTM1/p62 interacts with KEAP1, a cytoplasmic inhibitor of NRF2, a key transcription factor involved in cellular responses to oxidative stress (6). Thus, accumulation of SQSTM1/p62 can lead to an increase in NRF2 activity.
Background: Mitogen-activated protein kinases (MAPKs) are a widely conserved family of serine/threonine protein kinases involved in many cellular programs, such as cell proliferation, differentiation, motility, and death. The p44/42 MAPK (Erk1/2) signaling pathway can be activated in response to a diverse range of extracellular stimuli including mitogens, growth factors, and cytokines (1-3), and research investigators consider it an important target in the diagnosis and treatment of cancer (4). Upon stimulation, a sequential three-part protein kinase cascade is initiated, consisting of a MAP kinase kinase kinase (MAPKKK or MAP3K), a MAP kinase kinase (MAPKK or MAP2K), and a MAP kinase (MAPK). Multiple p44/42 MAP3Ks have been identified, including members of the Raf family, as well as Mos and Tpl2/COT. MEK1 and MEK2 are the primary MAPKKs in this pathway (5,6). MEK1 and MEK2 activate p44 and p42 through phosphorylation of activation loop residues Thr202/Tyr204 and Thr185/Tyr187, respectively. Several downstream targets of p44/42 have been identified, including p90RSK (7) and the transcription factor Elk-1 (8,9). p44/42 are negatively regulated by a family of dual-specificity (Thr/Tyr) MAPK phosphatases, known as DUSPs or MKPs (10), along with MEK inhibitors, such as U0126 and PD98059.
Background: Autophagy is a catabolic process for the autophagosomic-lysosomal degradation of bulk cytoplasmic contents (1,2). Autophagy is generally activated by conditions of nutrient deprivation, but it has also been associated with a number of physiological processes, including development, differentiation, neurodegenerative diseases, infection, and cancer (3).Atg8 is a ubiquitin-like protein that is critical for autophagosome formation. Atg8 is synthesized as a precursor protein that is processed by the cysteine protease Atg4, followed by lipidation with phosphatidylethanolamine (PE) in a ubiqutin-like conjugation pathway involving Atg7 and Atg3 (4). This processing of Atg8, which is described as a conversion from type-I to type-II forms, is frequently described as a marker for autophagy. The type-II form of Atg8 is incorporated into maturing autophagosomes and leads to the recruitment of additional autophagy components, including cargo receptors like SQSTM1/p62. While yeast has a single Atg8 gene, many eukaryotes have at least six orthologs, including three microtubule-associated protein 1 light chain 3 (MAP1LC3/LC3) family members (LC3A, LC3B, and LC3C) and three GABAA receptor associated protein (GABARAP) family members (GABARAP, GABARAPL1/GEC1, and GABARAPL2/GATE-16). While highly conserved, these various family members can have important differences in their post-translational processing, expression profile, and protein interactions including distinct cargo receptor. This complexity within the Atg8 family is critical for selective mechanisms of autophagy that have been reported (5, 6).
Background: The 14-3-3 family of proteins plays a key regulatory role in signal transduction, checkpoint control, apoptotic and nutrient-sensing pathways (1,2). 14-3-3 proteins are highly conserved and ubiquitously expressed. There are at least seven isoforms, β, γ, ε, σ, ζ, τ, and η that have been identified in mammals. The initially described α and δ isoforms are confirmed to be phosphorylated forms of β and ζ, respectively (3). Through their amino-terminal α helical region, 14-3-3 proteins form homo- or heterodimers that interact with a wide variety of proteins: transcription factors, metabolic enzymes, cytoskeletal proteins, kinases, phosphatases, and other signaling molecules (3,4). The interaction of 14-3-3 proteins with their targets is primarily through a phospho-Ser/Thr motif. However, binding to divergent phospho-Ser/Thr motifs, as well as phosphorylation independent interactions has been observed (4). 14-3-3 binding masks specific sequences of the target protein, and therefore, modulates target protein localization, phosphorylation state, stability, and molecular interactions (1-4). 14-3-3 proteins may also induce target protein conformational changes that modify target protein function (4,5). Distinct temporal and spatial expression patterns of 14-3-3 isoforms have been observed in development and in acute response to extracellular signals and drugs, suggesting that 14-3-3 isoforms may perform different functions despite their sequence similarities (4). Several studies suggest that 14-3-3 isoforms are differentially regulated in cancer and neurological syndromes (2,3).
Background: Amyloid β (Aβ) precursor protein (APP) is a 100-140 kDa transmembrane glycoprotein that exists as several isoforms (1). The amino acid sequence of APP contains an amyloid domain, which can be processed and released by two-step proteolytic cleavage (1). The extracellular deposition and accumulation of the released Aβ fragments form the main components of amyloid plaques in Alzheimer's disease (1). Several fragments corresponding to progressive APP processing at alternative cleavage sites have been identified (2). These include Aβ (1-37), Aβ (1-39), Aβ (1-40), and Aβ (1-42) (2). These fragments can also be N-terminally modified to generate pyroglutamate-3 Aβ (pE3-peptide) (3). Fragment-specific and pan-Aβ antibodies are used to detect and examine relative levels of individual Aβ fragments.