The endoplasmic reticulum kinase PERK interacts with the oxidoreductase ERO1 to metabolically adapt mitochondria

Endoplasmic reticulum (ER) homeostasis requires molecular regulators that tailor mitochondrial bioenergetics to the needs of protein folding. For instance, calnexin maintains mitochondria metabolism and mitochondria-ER contacts (MERCs) through reactive oxygen species (ROS) from NADPH oxidase 4 (NOX4). However, induction of ER stress requires a quick molecular rewiring of mitochondria to adapt to new energy needs. This machinery is not characterized. We now show that the oxidoreductase ERO1⍺ covalently interacts with protein kinase RNA-like ER kinase (PERK) upon treatment with tunicamycin. The PERK-ERO1⍺ interaction requires the C-terminal active site of ERO1⍺ and cysteine 216 of PERK. Moreover, we show that the PERK-ERO1⍺ complex promotes oxidization of MERC proteins and controls mitochondrial dynamics. Using proteinaceous probes, we determined that these functions improve ER-mitochondria Ca²⁺ flux to maintain bioenergetics in both organelles, while limiting oxidative stress. Therefore, the PERK-ERO1⍺ complex is a key molecular machinery that allows quick metabolic adaptation to ER stress.

Astrocytes show increased levels of Ero1α in multiple sclerosis and its experimental autoimmune encephalomyelitis animal model

Multiple sclerosis (MS) and its animal models are characterized by cellular inflammation within the central nervous system (CNS). The sources and consequences of this inflammation are currently not completely understood. Critical signs and mediators of CNS inflammation are reactive oxygen species (ROS) that promote inflammation. ROS originate from a variety of redox-reactive enzymes, one class of which catalyses oxidative protein folding within the endoplasmic reticulum (ER). Here, the unfolded protein response and other signalling mechanisms maintain a balance between ROS producers such as ER oxidoreductin 1α (Ero1α) and antioxidants such as glutathione peroxidase 8 (GPx8). The role of ROS production within the ER has so far not been examined in the context of MS. In this manuscript, we examined how components of the ER redox network change upon MS and experimental autoimmune encephalomyelitis (EAE). We found that unlike GPx8, Ero1α increases within both MS and EAE astrocytes, in parallel with an imbalance of other oxidases such of GPx7, and that no change was observed within neurons. This imbalance of ER redox enzymes can reduce the lifespan of astrocytes, while neurons are not affected. Therefore, Ero1α induction makes astrocytes vulnerable to oxidative stress in the MS and EAE pathologies.

Rab32 uses its effector reticulon 3L to trigger autophagic degradation of mitochondria-associated membrane (MAM) proteins

BACKGROUND

Rab32 is a small GTPase associated with multiple organelles but is particularly enriched at the endoplasmic reticulum (ER). Here, it controls targeting to mitochondria-ER contacts (MERCs), thus influencing composition of the mitochondria-associated membrane (MAM). Moreover, Rab32 regulates mitochondrial membrane dynamics via its effector dynamin-related protein 1 (Drp1). Rab32 has also been reported to induce autophagy, an essential pathway targeting intracellular components for their degradation. However, no autophagy-specific effectors have been identified for Rab32. Similarly, the identity of the intracellular membrane targeted by this small GTPase and the type of autophagy it induces are not known yet.

RESULTS

To investigate the target of autophagic degradation mediated by Rab32, we tested a large panel of organellar proteins. We found that a subset of MERC proteins, including the thioredoxin-related transmembrane protein TMX1, are specifically targeted for degradation in a Rab32-dependent manner. We also identified the long isoform of reticulon-3 (RTN3L), a known ER-phagy receptor, as a Rab32 effector.

CONCLUSIONS

Rab32 promotes degradation of mitochondrial-proximal ER membranes through autophagy with the help of RTN3L. We propose to call this type of selective autophagy "MAM-phagy".

Targeting N-myristoylation for therapy of B-cell lymphomas

Myristoylation, the N-terminal modification of proteins with the fatty acid myristate, is critical for membrane targeting and cell signaling. Because cancer cells often have increased N-myristoyltransferase (NMT) expression, NMTs were proposed as anti-cancer targets. To systematically investigate this, we performed robotic cancer cell line screens and discovered a marked sensitivity of hematological cancer cell lines, including B-cell lymphomas, to the potent pan-NMT inhibitor PCLX-001. PCLX-001 treatment impacts the global myristoylation of lymphoma cell proteins and inhibits early B-cell receptor (BCR) signaling events critical for survival. In addition to abrogating myristoylation of Src family kinases, PCLX-001 also promotes their degradation and, unexpectedly, that of numerous non-myristoylated BCR effectors including c-Myc, NFκB and P-ERK, leading to cancer cell death in vitro and in xenograft models. Because some treated lymphoma patients experience relapse and die, targeting B-cell lymphomas with a NMT inhibitor potentially provides an additional much needed treatment option for lymphoma.

Abstract 5156: Targeting N-myristoylation in B-cell lymphomas as a therapeutic strategy

Myristoylation is the N-terminal modification of proteins with the fatty acid myristate. This process is mediated by two ubiquitously expressed N-myristoyltransferases, NMT1 and NMT2, and is critical for membrane targeting and cell signaling. Because NMT expression is increased in some cancers, we used three robotic screens to evaluate the potential of the potent pan-NMT inhibitor PCLX-001 on 300 cancer cell lines spanning the spectrum of human cancers. We discovered a marked increase in the sensitivity of hematological cancer cell lines, including B-cell lymphomas, to myristoylation inhibition. PCLX-001 consistently reduced both lymphoma cell proliferation and viability at concentrations lower than those needed to inhibit the growth of or to kill benign immortalized B cells. In lymphoma cell lines, PCLX-001 treatment inhibited early B-cell receptor (BCR) signaling events by disrupting membrane targeting of several myristoylated Src family kinases and promoted their ubiquitin-mediated degradation. Unexpectedly, PCLX-001 also promoted the degradation of non-myristoylated transcriptional activators P-ERK, c-Myc, NFκB and CREB downstream in the BCR signaling cascade, leading to loss of survival signals and apoptosis. Furthermore, compared to clinically approved drugs dasatinib and ibrutinib, PCLX-001 was more potent in vitro at inhibiting B-cell signaling, had a wider breadth of efficacy, and had greater selectivity thus sparing normal B cells. PCLX-001 treatment reduced tumor size in a time and concentration dependent manner in three B-cell lymphoma xenograft models and resulted in complete disease regression in two of these models, including an R-CHOP refractory lymphoma patient-derived xenograft. To investigate the potential mechanisms responsible for the sensitivity of hematological cancers to PCLX-001, we examined the NMT expression levels in cancer cells using publically available databases. Contrary to the reported NMT overexpression in some cancers, we found that hematological cancer cell lines and tumors both display significant reduction in NMT2 expression. The decreased NMT2 expression is significantly correlated with lower EC50 and poorer patient prognosis. Using the CRISPR-based genetic alteration Cancer Dependency Map, we discovered that cancer cells are highly dependent on functional NMT1, and that NMT1 dependency increases with low NMT2 expression. PCLX-001 treatment may mimic the effect of genetic alteration of NMT1 in hematological cancer cells low in NMT2 by pharmacologically inhibiting the remaining NMT1 in these cells. This results in an effect reminiscent of synthetic lethality since the vast majority of normal cells express both NMTs and PCLX-001 selectively kills NMT2-deficient cancer cells while sparing normal cells. Our findings support the ongoing development and eventual clinical trials of PCLX-001 as a therapy for hematological cancers.

Citation Format: Erwan Beauchamp, Megan C. Yap, Maneka A. Perinpanayagam, Jay M. Gamma, Krista M. Vincent, Raymond Lai, Wei-Feng Dong, Manikandan Lakshmanan, Anandhkumar Raju, Vinay Tergaonkar, Soo Yong Tan, Soon Thye Lim, Lynne Postovit, Kevin D. Read, David W. Gray, Paul G. Wyatt, John R. Mackey, Luc G. Berthiaume. Targeting N-myristoylation in B-cell lymphomas as a therapeutic strategy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5156.

The ER chaperone calnexin controls mitochondrial positioning and respiration

Chaperones in the endoplasmic reticulum (ER) control the flux of Ca2+ ions into mitochondria, thereby increasing or decreasing the energetic output of the oxidative phosphorylation pathway. An example is the abundant ER lectin calnexin, which interacts with sarco/endoplasmic reticulum Ca2+ ATPase (SERCA). We found that calnexin stimulated the ATPase activity of SERCA by maintaining its redox state. This function enabled calnexin to control how much ER Ca2+ was available for mitochondria, a key determinant for mitochondrial bioenergetics. Calnexin-deficient cells compensated for the loss of this function by partially shifting energy generation to the glycolytic pathway. These cells also showed closer apposition between the ER and mitochondria. Calnexin therefore controls the cellular energy balance between oxidative phosphorylation and glycolysis.

Abstract 3046: Targeting N-myristoylation in B cell lymphomas as a therapeutic strategy

Treatment of aggressive lymphoma is toxic, expensive, and a substantial proportion of patients relapse and die. There is an urgent need for more effective treatments. Myristoylation is required for biological activity of >200 intracellular proteins. N-myristoyltransferases (NMTs) transfer the fatty acid myristate to N-terminal glycine residue; there are two isoforms, NMT1 and 2. Since they are critical to intracellular signaling, NMTs are potential anti-cancer targets. We tested a novel potent pan-NMT inhibitor, PCLX-001, in B cell lymphoma cell lines. In vitro assays included cell viability, immunoblotting, and metabolic labeling of lymphoma cell lines. Immunohistochemistry was performed on formalin fixed paraffin embedded lymphoma specimens from patients. In vivo experiments included cell line derived murine xenografts and a patient derived mouse xenograft treated with increasing concentrations of PCLX-001. PCLX-001 selectively killed lymphoma cells, while sparing normal cells in vitro and in 3 mouse xenograft models, eradicating tumors in two of these models including a patient-derived xenograft from a R-CHOP refractory lymphoma patient. While NMT2 is overexpressed in some cancers, loss of NMT2 expression is common in numerous cancers and occurs at the highest prevalence in lymphomas, where it is independently linked to a worse prognosis. This NMT2 suppression occurred through epigenetic mechanisms and may account for lymphoma sensitivity to NMT inhibition. The global myristoylation of lymphoma cell proteins, including that of the protein tyrosine kinase oncogene Src, is profoundly inhibited by PCLX-001. Loss of Src myristoylation is accompanied by loss of Src activity and may account for loss of prosurvival signals causing lymphoma cell death. Targeting NMT2 deficient B cell lymphoma with a pan-NMT inhibitor suppresses the residual NMT1 function provides a novel, selective, and effective therapeutic strategy.

Citation Format: John R. Mackey, Erwan Beauchamp, Megan C. Yap, Aishwarya Iyer, Maneka A. Perinpanayagam, Krista M. Vincent, Abass M. Al-Momany, Ryan J. Heit, Jacky Y. Sim, Raymond Lai, Wei-feng Dong, Manikandan Lakshmanan, Anandhkumar Raju, Vinay Tergaonkar, Soo Yong Tan, Soon Thye Lim, Lynne M. Postovit, Kevin D. Read, David W. Gray, Paul G. Wyatt, Luc G. Berthiaume. Targeting N-myristoylation in B cell lymphomas as a therapeutic strategy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3046.

Identification of a post-translationally myristoylated autophagy-inducing domain released by caspase cleavage of huntingtin

Huntington disease (HD) is a debilitating neurodegenerative disease characterized by the loss of motor control and cognitive ability that ultimately leads to death. It is caused by the expansion of a polyglutamine tract in the huntingtin (HTT) protein, which leads to aggregation of the protein and eventually cellular death. Both the wild-type and mutant form of the protein are highly regulated by post-translational modifications including proteolysis, palmitoylation and phosphorylation. We now demonstrate the existence of a new post-translational modification of HTT: the addition of the 14 carbon fatty acid myristate to a glycine residue exposed on a caspase-3-cleaved fragment (post-translational myristoylation) and that myristoylation of this fragment is altered in a physiologically relevant model of mutant HTT. Myristoylated HTT553-585-EGFP, but not its non-myristoylated variant, initially localized to the ER, induced the formation of autophagosomes and accumulated in abnormally large autophagolysosomal/lysosomal structures in a variety of cell types, including neuronal cell lines under nutrient-rich conditions. Our results suggest that accumulation of myristoylated HTT553-586 in cells may alter the rate of production of autophagosomes and/or their clearance through the heterotypic autophagosomal/lysosomal fusion process. Overall, our novel observations establish a role for the post-translational myristoylation of a caspase-3-cleaved fragment of HTT, highly similar to the Barkor/ATG14L autophagosome-targeting sequence domain thought to sense, maintain and/or promote membrane curvature in the regulation of autophagy. Abnormal processing or production of this myristoylated HTT fragment might be involved in the pathophysiology of HD.

Palmitoylation is the switch that assigns calnexin to quality control or ER Ca2+ signaling

The palmitoylation of calnexin serves to enrich calnexin on the mitochondria-associated membrane (MAM). Given a lack of information on the significance of this finding, we have investigated how this endoplasmic reticulum (ER)-internal sorting signal affects the functions of calnexin. Our results demonstrate that palmitoylated calnexin interacts with sarcoendoplasmic reticulum (SR) Ca(2+) transport ATPase (SERCA) 2b and that this interaction determines ER Ca(2+) content and the regulation of ER-mitochondria Ca(2+) crosstalk. In contrast, non-palmitoylated calnexin interacts with the oxidoreductase ERp57 and performs its well-known function in quality control. Interestingly, our results also show that calnexin palmitoylation is an ER-stress-dependent mechanism. Following a short-term ER stress, calnexin quickly becomes less palmitoylated, which shifts its function from the regulation of Ca(2+) signaling towards chaperoning and quality control of known substrates. These changes also correlate with a preferential distribution of calnexin to the MAM under resting conditions, or the rough ER and ER quality control compartment (ERQC) following ER stress. Our results have therefore identified the switch that assigns calnexin either to Ca(2+) signaling or to protein chaperoning.

Regulation of co- and post-translational myristoylation of proteins during apoptosis: interplay of N-myristoyltransferases and caspases

Myristoylation occurs cotranslationally on nascent proteins and post-translationally during apoptosis after caspase cleavages expose cryptic myristoylation sites. We demonstrate a drastic change in the myristoylated protein proteome in apoptotic cells, likely as more substrates are revealed by caspases. We show for the first time that both N-myristoyltransferases (NMTs) 1 and 2 are cleaved during apoptosis and that the caspase-3- or -8-mediated cleavage of NMT1 at Asp-72 precedes the cleavage of NMT2 by caspase-3 mainly at Asp-25. The cleavage of NMTs did not significantly affect their activity in apoptotic cells until the 8 h time point. However, the cleavage of the predominantly membrane bound NMT1 (64%) removed a polybasic domain stretch and led to a cytosolic relocalization (>55%), whereas predominantly cytosolic NMT2 (62%) relocalized to membranes when cleaved (>80%) after the removal of a negatively charged domain. The interplay between caspases and NMTs during apoptosis is of particular interest since caspases may not only control the rates of substrate production but also their myristoylation rate by regulating the location and perhaps the specificity of NMTs. Since apoptosis is often suppressed in cancer, the reduced caspase activity seen in cancer cells might also explain the higher NMT levels observed in many cancers.

Palmitoylated TMX and calnexin target to the mitochondria-associated membrane

The mitochondria-associated membrane (MAM) is a domain of the endoplasmic reticulum (ER) that mediates the exchange of ions, lipids and metabolites between the ER and mitochondria. ER chaperones and oxidoreductases are critical components of the MAM. However, the localization motifs and mechanisms for most MAM proteins have remained elusive. Using two highly related ER oxidoreductases as a model system, we now show that palmitoylation enriches ER-localized proteins on the MAM. We demonstrate that palmitoylation of cysteine residue(s) adjacent to the membrane-spanning domain promotes MAM enrichment of the transmembrane thioredoxin family protein TMX. In addition to TMX, our results also show that calnexin shuttles between the rough ER and the MAM depending on its palmitoylation status. Mutation of the TMX and calnexin palmitoylation sites and chemical interference with palmitoylation disrupt their MAM enrichment. Since ER-localized heme oxygenase-1, but not cytosolic GRP75 require palmitoylation to reside on the MAM, our findings identify palmitoylation as key for MAM enrichment of ER membrane proteins.

Rapid and selective detection of fatty acylated proteins using omega-alkynyl-fatty acids and click chemistry

Progress in understanding the biology of protein fatty acylation has been impeded by the lack of rapid direct detection and identification methods. We first report that a synthetic omega-alkynyl-palmitate analog can be readily and specifically incorporated into GAPDH or mitochondrial 3-hydroxyl-3-methylglutaryl-CoA synthase in vitro and reacted with an azido-biotin probe or the fluorogenic probe 3-azido-7-hydroxycoumarin using click chemistry for rapid detection by Western blotting or flat bed fluorescence scanning. The acylated cysteine residues were confirmed by MS. Second, omega-alkynyl-palmitate is preferentially incorporated into transiently expressed H- or N-Ras proteins (but not nonpalmitoylated K-Ras), compared with omega-alkynyl-myristate or omega-alkynyl-stearate, via an alkali sensitive thioester bond. Third, omega-alkynyl-myristate is specifically incorporated into endogenous co- and posttranslationally myristoylated proteins. The competitive inhibitors 2-bromopalmitate and 2-hydroxymyristate prevented incorporation of omega-alkynyl-palmitate and omega-alkynyl-myristate into palmitoylated and myristoylated proteins, respectively. Labeling cells with omega-alkynyl-palmitate does not affect membrane association of N-Ras. Furthermore, the palmitoylation of endogenous proteins including H- and N-Ras could be easily detected using omega-alkynyl-palmitate as label in cultured HeLa, Jurkat, and COS-7 cells, and, promisingly, in mice. The omega-alkynyl-myristate and -palmitate analogs used with click chemistry and azido-probes will be invaluable to study protein acylation in vitro, in cells, and in vivo.

Interrater Reliability of the TEMPA for the Measurement of Upper Limb Function in Adults With Traumatic Brain Injury

OBJECTIVE

To investigate the interrater reliability of the TEMPA in adults with traumatic brain injury (TBI).

PARTICIPANTS

Twenty adults with upper limb dysfunction after TBI who were participating in inpatient rehabilitation.

DESIGN

The TEMPA assessment was videotaped for each participant and 5 physical therapists independently rated these video recordings.

MAIN OUTCOME MEASURE

Functional rating and speed of execution for each item of the TEMPA.

RESULTS

Intraclass correlation coefficients (ICC) for the speed of execution and functional rating components of the TEMPA ranged from 0.898 to 1.000.

CONCLUSION

The excellent interrater reliability supports the use of the TEMPA in adults with TBI.