Palmitoylation signaling: a novel mechanism of mitochondria dynamics and diverse pathologies

Control of mitochondria-associated endoplasmic reticulum membranes by protein S-palmitoylation: Novel therapeutic targets for neurodegenerative diseases

Mitochondria-associated endoplasmic reticulum membranes (MAMs) are dynamic coupling structures between mitochondria and the endoplasmic reticulum (ER). As a new subcellular structure, MAMs combine the two critical organelle functions. Mitochondria and the ER could regulate each other via MAMs. MAMs are involved in calcium (Ca²⁺) homeostasis, autophagy, ER stress, lipid metabolism, etc. Researchers have found that MAMs are closely related to metabolic syndrome and neurodegenerative diseases (NDs). The formation of MAMs and their functions depend on specific proteins. Numerous protein enrichments, such as the IP3R-Grp75-VDAC complex, constitute MAMs. The changes in these proteins govern the interaction between mitochondria and the ER; they also affect the biological functions of MAMs. S-palmitoylation is a reversible protein post-translational modification (PTM) that mainly occurs on protein cysteine residues. More and more studies have shown that the S-palmitoylation of proteins is closely related to their membrane localization. Here, we first briefly describe the composition and function of MAMs, reviewing the component and biological roles of MAMs mediated by S-palmitoylation, elaborating on S-palmitoylated proteins in Ca²⁺ flux, lipid rafts, and so on. We try to provide new insight into the molecular basis of MAMs-related diseases, mainly NDs. Finally, we propose potential drug compounds targeting S-palmitoylation.

Inhibition of acetyl-CoA carboxylase impaired tubulin palmitoylation and induced spindle abnormalities

Tubulin s-palmitoylation involves the thioesterification of a cysteine residue in tubulin with palmitate. The palmitate moiety is produced by the fatty acid synthesis pathway, which is rate-limited by acetyl-CoA carboxylase (ACC). While it is known that ACC is phosphorylated at serine 79 (pSer⁷⁹) by AMPK and accumulates at the spindle pole (SP) during mitosis, a functional role for tubulin palmitoylation during mitosis has not been identified. In this study, we found that modulating pSer⁷⁹-ACC level at the SP using AMPK agonist and inhibitor induced spindle defects. Loss of ACC function induced spindle abnormalities in cell lines and in germ cells of the Drosophila germarium, and palmitic acid (PA) rescued the spindle defects in the cell line treated transiently with the ACC inhibitor, TOFA. Furthermore, inhibition of protein palmitoylating or depalmitoylating enzymes also induced spindle defects. Together, these data suggested that precisely regulated cellular palmitate level and protein palmitoylation may be required for accurate spindle assembly. We then showed that tubulin was largely palmitoylated in interphase cells but less palmitoylated in mitotic cells. TOFA treatment diminished tubulin palmitoylation at doses that disrupt microtubule (MT) instability and cause spindle defects. Moreover, spindle MTs comprised of α-tubulins mutated at the reported palmitoylation site exhibited disrupted dynamic instability. We also found that TOFA enhanced the MT-targeting drug-induced spindle abnormalities and cytotoxicity. Thus, our study reveals that precise regulation of ACC during mitosis impacts tubulin palmitoylation to delicately control MT dynamic instability and spindle assembly, thereby safeguarding nuclear and cell division.

Role of ABCA1 in Cardiovascular Disease

Cholesterol homeostasis plays a significant role in cardiovascular disease. Previous studies have indicated that ATP-binding cassette transporter A1 (ABCA1) is one of the most important proteins that maintains cholesterol homeostasis. ABCA1 mediates nascent high-density lipoprotein biogenesis. Upon binding with apolipoprotein A-I, ABCA1 facilitates the efflux of excess intracellular cholesterol and phospholipids and controls the rate-limiting step of reverse cholesterol transport. In addition, ABCA1 interacts with the apolipoprotein receptor and suppresses inflammation through a series of signaling pathways. Thus, ABCA1 may prevent cardiovascular disease by inhibiting inflammation and maintaining lipid homeostasis. Several studies have indicated that post-transcriptional modifications play a critical role in the regulation of ABCA1 transportation and plasma membrane localization, which affects its biological function. Meanwhile, carriers of the loss-of-function ABCA1 gene are often accompanied by decreased expression of ABCA1 and an increased risk of cardiovascular diseases. We summarized the ABCA1 transcription regulation mechanism, mutations, post-translational modifications, and their roles in the development of dyslipidemia, atherosclerosis, ischemia/reperfusion, myocardial infarction, and coronary heart disease.

Synaptic Plasticity Dysfunctions in the Pathophysiology of 22q11 Deletion Syndrome: Is There a Role for Astrocytes?

The 22q11 deletion syndrome (DS) is the most common microdeletion syndrome in humans and gives a high probability of developing psychiatric disorders. Synaptic and neuronal malfunctions appear to be at the core of the symptoms presented by patients. In fact, it has long been suggested that the behavioural and cognitive impairments observed in 22q11DS are probably due to alterations in the mechanisms regulating synaptic function and plasticity. Often, synaptic changes are related to structural and functional changes observed in patients with cognitive dysfunctions, therefore suggesting that synaptic plasticity has a crucial role in the pathophysiology of the syndrome. Most interestingly, among the genes deleted in 22q11DS, six encode for mitochondrial proteins that, in mouse models, are highly expressed just after birth, when active synaptogenesis occurs, therefore indicating that mitochondrial processes are strictly related to synapse formation and maintenance of a correct synaptic signalling. Because correct synaptic functioning, not only requires correct neuronal function and metabolism, but also needs the active contribution of astrocytes, we summarize in this review recent studies showing the involvement of synaptic plasticity in the pathophysiology of 22q11DS and we discuss the relevance of mitochondria in these processes and the possible involvement of astrocytes.

Drp1-dependent mitochondrial fission in cardiovascular disease

Mitochondria are highly dynamic organelles undergoing cycles of fusion and fission to modulate their morphology, distribution, and function, which are referred as 'mitochondrial dynamics'. Dynamin-related protein 1 (Drp1) is known as the major pro-fission protein whose activity is tightly regulated to clear the damaged mitochondria via mitophagy, ensuring a strict control over the intricate process of cellular and organ dynamics in heart. Various posttranslational modifications (PTMs) of Drp1 have been identified including phosphorylation, SUMOylation, palmitoylation, ubiquitination, S-nitrosylation, and O-GlcNAcylation, which implicate a role in the regulation of mitochondrial dynamics. An intact mitochondrial homeostasis is critical for heart to fuel contractile function and cardiomyocyte metabolism, while defects in mitochondrial dynamics constitute an essential part of the pathophysiology underlying various cardiovascular diseases (CVDs). In this review, we summarize current knowledge on the critical role of Drp1 in the pathogenesis of CVDs including endothelial dysfunction, smooth muscle remodeling, cardiac hypertrophy, pulmonary arterial hypertension, myocardial ischemia-reperfusion, and myocardial infarction. We also highlight how the targeting of Drp1 could potentially contribute to CVDs treatments.

Methods for Identification of Substrates/Inhibitors of FCP/SCP Type Protein Ser/Thr Phosphatases

Protein phosphorylation is the most widespread type of post-translational modification and is properly controlled by protein kinases and phosphatases. Regarding the phosphorylation of serine (Ser) and threonine (Thr) residues, relatively few protein Ser/Thr phosphatases control the specific dephosphorylation of numerous substrates, in contrast with Ser/Thr kinases. Recently, protein Ser/Thr phosphatases were reported to have rigid substrate recognition and exert various biological functions. Therefore, identification of targeted proteins by individual protein Ser/Thr phosphatases is crucial to clarify their own biological functions. However, to date, information on the development of methods for identification of the substrates of protein Ser/Thr phosphatases remains scarce. In turn, substrate-trapping mutants are powerful tools to search the individual substrates of protein tyrosine (Tyr) phosphatases. This review focuses on the development of novel methods for the identification of Ser/Thr phosphatases, especially small C-terminal domain phosphatase 1 (Scp1), using peptide-displayed phage library with AlF4−/BeF3−, and discusses the identification of putative inhibitors.

ABHD10 is an S-depalmitoylase affecting redox homeostasis through peroxiredoxin-5

S-palmitoylation is a reversible lipid post-translational modification that has been observed on mitochondrial proteins, but both the regulation and functional consequences of mitochondrial S-palmitoylation are poorly understood. Here, we show that perturbing the “erasers” of S-palmitoylation, acyl protein thioesterases (APTs), with either pan-active inhibitors or a new mitochondrial-targeted APT inhibitor, diminishes the antioxidant buffering capacity of mitochondria. Surprisingly, this effect was not mediated by the only known mitochondrial APT, but rather by a resident mitochondrial protein with no known endogenous function, ABHD10. We show that ABHD10 is a new member of the APT family of regulatory proteins and identify peroxiredoxin 5 (PRDX5), a key antioxidant protein, as the first target of ABHD10 S-depalmitoylase activity. We then discover that ABHD10 regulates the S-palmitoylation status of the nucleophilic active site residue of PRDX5, providing a direct mechanistic connection between ABHD10-mediated S-depalmitoylation of PRDX5 and its antioxidant capacity.