Use of polarized PC12 cells to monitor protein localization in the early biosynthetic pathway

Enhancing the Study of Quantal Exocytotic Events: Combining Diamond Multi-Electrode Arrays with Amperometric PEak Analysis (APE) an Automated Analysis Code

MicroGraphited-Diamond-Multi Electrode Arrays (μG-D-MEAs) can be successfully used to reveal, in real time, quantal exocytotic events occurring from many individual neurosecretory cells and/or from many neurons within a network. As μG-D-MEAs arrays are patterned with up to 16 sensing microelectrodes, each of them recording large amounts of data revealing the exocytotic activity, the aim of this work was to support an adequate analysis code to speed up the signal detection. The cutting-edge technology of microGraphited-Diamond-Multi Electrode Arrays (μG-D-MEAs) has been implemented with an automated analysis code (APE, Amperometric Peak Analysis) developed using Matlab R2022a software to provide easy and accurate detection of amperometric spike parameters, including the analysis of the pre-spike foot that sometimes precedes the complete fusion pore dilatation. Data have been acquired from cultured PC12 cells, either collecting events during spontaneous exocytosis or after L-DOPA incubation. Validation of the APE code was performed by comparing the acquired spike parameters with those obtained using Quanta Analysis (Igor macro) by Mosharov et al.

Functional characterization of all-trans retinoic acid-induced differentiation factor (ATRAID)

All-trans retinoic acid-induced differentiation (ATRAID) factor was first identified in HL60 cells. Several mRNA isoforms exist, but the respective proteins have not been fully characterized. In transfected cells expressing Myc-Flag-tagged ATRAID Isoform (Iso) A, B, and C, Iso C was found to be expressed at high levels, Iso A was found to be expressed at low levels due to rapid degradation, and the predicted protein expressed from Iso B was not detected. Iso C was present mainly in an N-glycosylated form. In subcellular fractionation experiments, Iso C localized to the membranous and nuclear fractions, while immunofluorescence analysis revealed that Iso C is located close to the plasma membrane, mainly in cytoplasmic vesicles and in the Golgi area. We confirm that Iso C colocalizes to some extent with endosomal/lysosomal markers LAMP1 and LAMP2. Furthermore, we show that ATRAID co-localizes with RAB11, a GTPase associated with recycling endosomes and implicated in regulating vesicular trafficking.

Evidence for the role of Rab11-positive recycling endosomes as intermediates in coronavirus egress from epithelial cells

After their assembly by budding into the lumen of the intermediate compartment (IC) at the endoplasmic reticulum (ER)–Golgi interface, coronaviruses (CoVs) are released from their host cells following a pathway that remains poorly understood. The traditional view that CoV exit occurs via the constitutive secretory route has recently been questioned by studies suggesting that this process involves unconventional secretion. Here, using the avian infectious bronchitis virus (IBV) as a well-established model virus, we have applied confocal microscopy to investigate the pathway of CoV egress from epithelial Vero cells. We report a novel effect of IBV infection on cellular endomembranes, namely, the compaction of the pericentrosomal endocytic recycling compartment (ERC) defined by the GTPase Rab11, which coincides with the previously described Golgi fragmentation, as well as virus release. Despite Golgi disassembly, the IC elements containing the major IBV membrane protein (M)—which mostly associates with newly formed virus particles—maintain their close spatial connection with the Rab11-positive endocytic recycling system. Moreover, partial colocalization of the M protein with Rab11 was observed, whereas M displayed negligible overlap with LAMP-1, indicating that IBV egress does not occur via late endosomes or lysosomes. Synchronization of virus release using temperature-shift protocols was accompanied by increased colocalization of M and Rab11 in vesicular and vacuolar structures in the pericentrosomal region and at the cell periphery, most likely representing IBV-containing transport carriers. In conclusion, these results add CoVs to the growing list of viruses exploiting the endocytic recycling apparatus defined by Rab11 for their assembly and/or release.

Pellino-2 in nonimmune cells: novel interaction partners and intracellular localization

Pellino-2 is an E3 ubiquitin ligase that mediates intracellular signaling in innate immune pathways. Most studies of endogenous Pellino-2 have been performed in macrophages, but none in nonimmune cells. Using yeast two-hybrid screening and co-immunoprecipitation, we identified six novel interaction partners of Pellino-2, with various localizations: insulin receptor substrate 1, NIMA-related kinase 9, tumor necrosis factor receptor-associated factor 7, cyclin-F, roundabout homolog 1, and disheveled homolog 2. Pellino-2 showed cytoplasmic localization in a wide range of nonimmune cells under physiological potassium concentrations. Treatment with the potassium ionophore nigericin resulted in nuclear localization of Pellino-2, which was reversed by the potassium channel blocker tetraethylammonium. Live-cell imaging revealed intracellular migration of GFP-tagged Pellino-2. In summary, Pellino-2 interacts with proteins at different cellular locations, taking part in dynamic processes that change its intracellular localization influenced by potassium efflux.

K+ regulates relocation of Pellino-2 to the site of NLRP3 inflammasome activation in macrophages

Pellino proteins are E3 ubiquitin ligases involved in the innate immune system. Recently, Pellino-2 was reported to modulate the activation of the mouse Nlrp3 inflammasome. We examined the intracellular localization of human Pellino-2 in THP1-derived macrophages during activation with LPS and ATP. We observed that Pellino-2 changed intracellular localization and colocalized with the inflammasome proteins NLRP3 and ASC late in the assembly of the inflammasome. Colocalization with NLRP3 and ASC was also seen in cells maintained in potassium-free medium. The colocalization and inflammasome activation were abrogated by several potassium channel inhibitors, supporting a role for potassium efflux in modulating intracellular localization of Pellino-2. The data suggest that Pellino-2 is essential for mediating the effect of potassium efflux on inflammasome activation.

Co-localization of Interleukin-1α and Annexin A2 at the plasma membrane in response to oxidative stress

Interleukin-1α (IL-1α) and Annexin A2 (AnxA2) are pleiotropic molecules with both intracellular and extracellular roles. They share several characteristics including unconventional secretion aided by S100 proteins, anchoring of the externalized proteins at the outer surface of the plasma membrane and response to oxidative stress. Although IL-1α and AnxA2 have been implicated in a variety of biological processes, including cancer, little is known about the mechanisms of their cellular release. In the present study, employing the non-cancerous breast epithelial MCF10A cells, we demonstrate that IL-1α and AnxA2 establish a close association in response to oxidative stress. Stress conditions lead to translocation of both proteins towards lamellipodia rich in vimentin and association of full-length IL-1α and Tyr23 phosphorylated AnxA2 with the plasma membrane at peripheral sites depleted of F-actin. Notably, membrane-associated IL-1α and AnxA2 preferentially localize to the outer edges of the MCF10A cell islands, suggesting that the two proteins participate in the communication of these epithelial cells with their neighboring cells. Similarly, in U2OS osteosarcoma cell line both endogenous IL-1α and transiently produced IL-1α/EGFP associate with the plasma membrane. While benign MFC10A cells present membrane-associated IL-1α and AnxA2 at the edges of their cell islands, the aggressive cancerous U2OS cells communicate in such manner also with distant cells.

Phosphorylation at serine 31 targets tyrosine hydroxylase to vesicles for transport along microtubules

Tyrosine hydroxylase (TH) catalyzes the conversion of l-tyrosine into l-DOPA, which is the rate-limiting step in the synthesis of catecholamines, such as dopamine, in dopaminergergic neurons. Low dopamine levels and death of the dopaminergic neurons are hallmarks of Parkinson's disease (PD), where α-synuclein is also a key player. TH is highly regulated, notably by phosphorylation of several Ser/Thr residues in the N-terminal tail. However, the functional role of TH phosphorylation at the Ser-31 site (THSer(P)-31) remains unclear. Here, we report that THSer(P)-31 co-distributes with the Golgi complex and synaptic-like vesicles in rat and human dopaminergic cells. We also found that the TH microsomal fraction content decreases after inhibition of cyclin-dependent kinase 5 (Cdk5) and ERK1/2. The cellular distribution of an overexpressed phospho-null mutant, TH1-S31A, was restricted to the soma of neuroblastoma cells, with decreased association with the microsomal fraction, whereas a phospho-mimic mutant, TH1-S31E, was distributed throughout the soma and neurites. TH1-S31E associated with vesicular monoamine transporter 2 (VMAT2) and α-synuclein in neuroblastoma cells, and endogenous THSer(P)-31 was detected in VMAT2- and α-synuclein-immunoprecipitated mouse brain samples. Microtubule disruption or co-transfection with α-synuclein A53T, a PD-associated mutation, caused TH1-S31E accumulation in the cell soma. Our results indicate that Ser-31 phosphorylation may regulate TH subcellular localization by enabling its transport along microtubules, notably toward the projection terminals. These findings disclose a new mechanism of TH regulation by phosphorylation and reveal its interaction with key players in PD, opening up new research avenues for better understanding dopamine synthesis in physiological and pathological states.

Defective PITRM1 mitochondrial peptidase is associated with Aβ amyloidotic neurodegeneration

Mitochondrial dysfunction and altered proteostasis are central features of neurodegenerative diseases. The pitrilysin metallopeptidase 1 (PITRM1) is a mitochondrial matrix enzyme, which digests oligopeptides, including the mitochondrial targeting sequences that are cleaved from proteins imported across the inner mitochondrial membrane and the mitochondrial fraction of amyloid beta (Aβ). We identified two siblings carrying a homozygous PITRM1 missense mutation (c.548G>A, p.Arg183Gln) associated with an autosomal recessive, slowly progressive syndrome characterised by mental retardation, spinocerebellar ataxia, cognitive decline and psychosis. The pathogenicity of the mutation was tested in vitro, in mutant fibroblasts and skeletal muscle, and in a yeast model. A Pitrm1^+/− heterozygous mouse showed progressive ataxia associated with brain degenerative lesions, including accumulation of Aβ‐positive amyloid deposits. Our results show that PITRM1 is responsible for significant Aβ degradation and that impairment of its activity results in Aβ accumulation, thus providing a mechanistic demonstration of the mitochondrial involvement in amyloidotic neurodegeneration.

A recombined allele of the lipase gene CEL and its pseudogene CELP confers susceptibility to chronic pancreatitis

Carboxyl-ester lipase is a digestive pancreatic enzyme encoded by the highly polymorphic CEL gene¹. Mutations in CEL cause maturity-onset diabetes of the young (MODY) with pancreatic exocrine dysfunction². Here we identified a hybrid allele (CEL-HYB), originating from a crossover between CEL and its neighboring pseudogene CELP. In a discovery cohort of familial chronic pancreatitis cases, the carrier frequency of CEL-HYB was 14.1% (10/71) compared with 1.0% (5/478) in controls (odds ratio [OR] = 15.5, 95% confidence interval [CI] = 5.1-46.9, P = 1.3 × 10⁻⁶). Three replication studies in non-alcoholic chronic pancreatitis cohorts identified CEL-HYB in a total of 3.7% (42/1,122) cases and 0.7% (30/4,152) controls (OR = 5.2, 95% CI = 3.2-8.5, P = 1.2 × 10⁻¹¹; formal meta-analysis). The allele was also enriched in alcoholic chronic pancreatitis. Expression of CEL-HYB in cellular models revealed reduced lipolytic activity, impaired secretion, prominent intracellular accumulation and induced autophagy. The hybrid variant of CEL is the first chronic pancreatitis gene identified outside the protease/antiprotease system of pancreatic acinar cells.

Endocytosis of secreted carboxyl ester lipase in a syndrome of diabetes and pancreatic exocrine dysfunction

Maturity-onset diabetes of the young, type 8 (MODY8) is characterized by a syndrome of autosomal dominantly inherited diabetes and exocrine pancreatic dysfunction. It is caused by deletion mutations in the last exon of the carboxyl ester lipase (CEL) gene, resulting in a CEL protein with increased tendency to aggregate. In this study we investigated the intracellular distribution of the wild type (WT) and mutant (MUT) CEL proteins in cellular models. We found that both CEL-WT and CEL-MUT were secreted via the endoplasmic reticulum and Golgi compartments. However, their subcellular distributions differed, as only CEL-MUT was observed as an aggregate at the cell surface and inside large cytoplasmic vacuoles. Many of the vacuoles were identified as components of the endosomal system, and after its secretion, the mutant CEL protein was re-internalized, transported to the lysosomes, and degraded. Internalization of CEL-MUT also led to reduced viability of pancreatic acinar and beta cells. These findings may have implications for the understanding of how the acinar-specific CEL-MUT protein causes both exocrine and endocrine pancreatic disease.

GCK-MODY diabetes as a protein misfolding disease: the mutation R275C promotes protein misfolding, self-association and cellular degradation

GCK-MODY, dominantly inherited mild hyperglycemia, is associated with more than 600 mutations in the glucokinase gene. Different molecular mechanisms have been shown to explain GCK-MODY. Here, we report a Pakistani family harboring the glucokinase mutation c.823C>T (p.R275C). The recombinant and in cellulo expressed mutant pancreatic enzyme revealed slightly increased enzyme activity (kcat) and normal affinity for α-D-glucose, and resistance to limited proteolysis by trypsin comparable with wild-type. When stably expressed in HEK293 cells and MIN6 β-cells (at different levels), the mutant protein appeared misfolded and unstable with a propensity to form dimers and aggregates. Its degradation rate was increased, involving the lysosomal and proteasomal quality control systems. On mutation, a hydrogen bond between the R275 side-chain and the carbonyl oxygen of D267 is broken, destabilizing the F260-L271 loop structure and the protein. This promotes the formation of dimers/aggregates and suggests that an increased cellular degradation is the molecular mechanism by which R275C causes GCK-MODY.

GCK-MODY diabetes associated with protein misfolding, cellular self-association and degradation

GCK-MODY, dominantly inherited mild fasting hyperglycemia, has been associated with >600 different mutations in the glucokinase (GK)-encoding gene (GCK). When expressed as recombinant pancreatic proteins, some mutations result in enzymes with normal/near-normal catalytic properties. The molecular mechanism(s) of GCK-MODY due to these mutations has remained elusive. Here, we aimed to explore the molecular mechanisms for two such catalytically 'normal' GCK mutations (S263P and G264S) in the F260-L270 loop of GK. When stably overexpressed in HEK293 cells and MIN6 β-cells, the S263P- and G264S-encoded mutations generated misfolded proteins with an increased rate of degradation (S263P>G264S) by the protein quality control machinery, and a propensity to self-associate (G264S>S263P) and form dimers (SDS resistant) and aggregates (partly Triton X-100 insoluble), as determined by pulse-chase experiments and subcellular fractionation. Thus, the GCK-MODY mutations S263P and G264S lead to protein misfolding causing destabilization, cellular dimerization/aggregation and enhanced rate of degradation. In silico predicted conformational changes of the F260-L270 loop structure are considered to mediate the dimerization of both mutant proteins by a domain swapping mechanism. Thus, similar properties may represent the molecular mechanisms for additional unexplained GCK-MODY mutations, and may also contribute to the disease mechanism in other previously characterized GCK-MODY inactivating mutations.

Diabetes and pancreatic exocrine dysfunction due to mutations in the carboxyl ester lipase gene-maturity onset diabetes of the young (CEL-MODY): a protein misfolding disease

CEL-maturity onset diabetes of the young (MODY), diabetes with pancreatic lipomatosis and exocrine dysfunction, is due to dominant frameshift mutations in the acinar cell carboxyl ester lipase gene (CEL). As Cel knock-out mice do not express the phenotype and the mutant protein has an altered and intrinsically disordered tandem repeat domain, we hypothesized that the disease mechanism might involve a negative effect of the mutant protein. In silico analysis showed that the pI of the tandem repeat was markedly increased from pH 3.3 in wild-type (WT) to 11.8 in mutant (MUT) human CEL. By stably overexpressing CEL-WT and CEL-MUT in HEK293 cells, we found similar glycosylation, ubiquitination, constitutive secretion, and quality control of the two proteins. The CEL-MUT protein demonstrated, however, a high propensity to form aggregates found intracellularly and extracellularly. Different physicochemical properties of the intrinsically disordered tandem repeat domains of WT and MUT proteins may contribute to different short and long range interactions with the globular core domain and other macromolecules, including cell membranes. Thus, we propose that CEL-MODY is a protein misfolding disease caused by a negative gain-of-function effect of the mutant proteins in pancreatic tissues.

The function of the intermediate compartment in pre-Golgi trafficking involves its stable connection with the centrosome

Because the functional borders of the intermediate compartment (IC) are not well defined, the spatial map of the transport machineries operating between the endoplasmic reticulum (ER) and the Golgi apparatus remains incomplete. Our previous studies showed that the IC consists of interconnected vacuolar and tubular parts with specific roles in pre-Golgi trafficking. Here, using live cell imaging, we demonstrate that the tubules containing the GTPase Rab1A create a long-lived membrane compartment around the centrosome. Separation of this pericentrosomal domain of the IC from the Golgi ribbon, due to centrosome motility, revealed that it contains a distinct pool of COPI coats and acts as a temperature-sensitive way station in post-ER trafficking. However, unlike the Golgi, the pericentrosomal IC resists the disassembly of COPI coats by brefeldin A, maintaining its juxtaposition with the endocytic recycling compartment, and operation as the focal point of a dynamic tubular network that extends to the cell periphery. These results provide novel insight into the compartmental organization of the secretory pathway and Golgi biogenesis. Moreover, they reveal a direct functional connection between the IC and the endosomal system, which evidently contributes to unconventional transport of the cystic fibrosis transmembrane conductance regulator to the cell surface.