Redox regulation of cysteine-dependent enzymes in neurodegeneration

The Mia40/CHCHD4 Oxidative Folding System: Redox Regulation and Signaling in the Mitochondrial Intermembrane Space

Mitochondria are critical for several cellular functions as they control metabolism, cell physiology, and cell death. The mitochondrial proteome consists of around 1500 proteins, the vast majority of which (about 99% of them) are encoded by nuclear genes, with only 13 polypeptides in human cells encoded by mitochondrial DNA. Therefore, it is critical for all the mitochondrial proteins that are nuclear-encoded to be targeted precisely and sorted specifically to their site of action inside mitochondria. These processes of targeting and sorting are catalysed by protein translocases that operate in each one of the mitochondrial sub-compartments. The main protein import pathway for the intermembrane space (IMS) recognises proteins that are cysteine-rich, and it is the only import pathway that chemically modifies the imported precursors by introducing disulphide bonds to them. In this manner, the precursors are trapped in the IMS in a folded state. The key component of this pathway is Mia40 (called CHCHD4 in human cells), which itself contains cysteine motifs and is subject to redox regulation. In this review, we detail the basic components of the MIA pathway and the disulphide relay mechanism that underpins the electron transfer reaction along the oxidative folding mechanism. Then, we discuss the key protein modulators of this pathway and how they are interlinked to the small redox-active molecules that critically affect the redox state in the IMS. We present also evidence that the mitochondrial redox processes that are linked to iron–sulfur clusters biogenesis and calcium homeostasis coalesce in the IMS at the MIA machinery. The fact that the MIA machinery and several of its interactors and substrates are linked to a variety of common human diseases connected to mitochondrial dysfunction highlight the potential of redox processes in the IMS as a promising new target for developing new treatments for some of the most complex and devastating human diseases.

Proteolysis mediated by cysteine cathepsins and legumain-recent advances and cell biological challenges

Proteases play essential roles in protein degradation, protein processing, and extracellular matrix remodeling in all cell types and tissues. They are also involved in protein turnover for maintenance of homeostasis and protein activation or inactivation for cell signaling. Proteases range in function and specificity, with some performing distinct substrate cleavages, while others accomplish proteolysis of a wide range of substrates. As such, different cell types use specialized molecular mechanisms to regulate the localization of proteases and their function within the compartments to which they are destined. Here, we focus on the cysteine family of cathepsin proteases and legumain, which act predominately within the endo-lysosomal pathway. In particular, recent knowledge on cysteine cathepsins and their primary regulator legumain is scrutinized in terms of their trafficking to endo-lysosomal compartments and other less recognized cellular locations. We further explore the mechanisms that regulate these processes and point to pathological cases which arise from detours taken by these proteases. Moreover, the emerging biological roles of specific forms and variants of cysteine cathepsins and legumain are discussed. These may be decisive, pathogenic, or even deadly when localizing to unusual cellular compartments in their enzymatically active form, because they may exert unexpected effects by alternative substrate cleavage. Hence, we propose future perspectives for addressing the actions of cysteine cathepsins and legumain as well as their specific forms and variants. The increasing knowledge in non-canonical aspects of cysteine cathepsin- and legumain-mediated proteolysis may prove valuable for developing new strategies to utilize these versatile proteases in therapeutic approaches.

Redox regulation in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that results from the death of upper and lower motor neurons. Due to a lack of effective treatment, it is imperative to understand the underlying mechanisms and processes involved in disease progression. Regulations in cellular reduction/oxidation (redox) processes are being increasingly implicated in disease. Here we discuss the possible involvement of redox dysregulation in the pathophysiology of ALS, either as a cause of cellular abnormalities or a consequence. We focus on its possible role in oxidative stress, protein misfolding, glutamate excitotoxicity, lipid peroxidation and cholesterol esterification, mitochondrial dysfunction, impaired axonal transport and neurofilament aggregation, autophagic stress, and endoplasmic reticulum (ER) stress. We also speculate that an ER chaperone protein disulphide isomerase (PDI) could play a key role in this dysregulation. PDI is essential for normal protein folding by oxidation and reduction of disulphide bonds, and hence any disruption to this process may have consequences for motor neurons. Addressing the mechanism underlying redox regulation and dysregulation may therefore help to unravel the molecular mechanism involved in ALS.

Interplay between protein carbonylation and nitrosylation in plants

ROS and reactive nitrogen species (RNS) are key regulators of redox homeostasis in living organisms including plants. As control of redox homeostasis plays a central function in plant biology, redox proteomics could help in characterizing the potential roles played by ROS/RNS-induced posttranslational modification in plant cells. In this review, we focus on two posttranslational modifications: protein carbonylation (a marker of protein oxidation) and protein S-nitrosylation, both of which having recently emerged as important regulatory mechanisms during numerous fundamental biological processes. Here, we describe the recent progress in proteomic analysis of carbonylated and nitrosylated proteins and highlight the achievements made in understanding the physiological basis of these oxy/nitro modifications in plants. In addition, we document the existence of a relationship between ROS-based carbonylation and RNS-based nitrosylation thus supporting the finding that crosstalk between cellular signaling stress pathways induced by ROS and RNS could be mediated by specific protein modifications.