Dilation of the endoplasmic reticulum in beta cells due to molecular overcrowding? Kinetic simulations of extension limits and consequences on proinsulin synthesis

Mitochondrial ROS in Slc4a11 KO Corneal Endothelial Cells Lead to ER Stress

Recent studies from Slc4a11 -/- mice have identified glutamine-induced mitochondrial dysfunction as a significant contributor toward oxidative stress, impaired lysosomal function, aberrant autophagy, and cell death in this Congenital Hereditary Endothelial Dystrophy (CHED) model. Because lysosomes are derived from endoplasmic reticulum (ER)-Golgi, we asked whether ER function is affected by mitochondrial ROS in Slc4a11 KO corneal endothelial cells. In mouse Slc4a11 -/- corneal endothelial tissue, we observed the presence of dilated ER and elevated expression of ER stress markers BIP and CHOP. Slc4a11 KO mouse corneal endothelial cells incubated with glutamine showed increased aggresome formation, BIP and GADD153, as well as reduced ER Ca2+ release as compared to WT. Induction of mitoROS by ETC inhibition also led to ER stress in WT cells. Treatment with the mitochondrial ROS quencher MitoQ, restored ER Ca2+ release and relieved ER stress markers in Slc4a11 KO cells in vitro. Systemic MitoQ also reduced BIP expression in Slc4a11 KO endothelium. We conclude that mitochondrial ROS can induce ER stress in corneal endothelial cells.

Foot-and-mouth disease virus induces PERK-mediated autophagy to suppress the antiviral interferon response

Foot-and-mouth disease virus (FMDV) is a picornavirus that causes contagious acute infection in cloven-hoofed animals. FMDV replication-associated viral protein expression induces endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), in turn inducing autophagy to restore cellular homeostasis. We observed that inhibition of BiP (also known as HSPA5 and GRP78), a master regulator of ER stress and UPR, decreased FMDV infection confirming their involvement. Further, we show that the FMDV infection induces UPR mainly through the PKR-like ER kinase (PERK; also known as EIF2AK3)-mediated pathway. Knockdown of PERK and chemical inhibition of PERK activation resulted in decreased expression of FMDV proteins along with the reduction of autophagy marker protein LC3B-II [the lipidated form of LC3B (also known as MAP1LC3B)]. There are conflicting reports on the role of autophagy in FMDV multiplication. Our study systematically demonstrates that during FMDV infection, PERK-mediated UPR stimulated an increased level of endogenous LC3B-II and turnover of SQSTM1, thus confirming the activation of functional autophagy. Modulation of the UPR and autophagy by pharmacological and genetic approaches resulted in reduced numbers of viral progeny, by enhancing the antiviral interferon response. Taken together, this study underscores the prospect of exploring PERK-mediated autophagy as an antiviral target.

Nonfunctional mutant Wrn protein leads to neurological deficits, neuronal stress, microglial alteration, and immune imbalance in a mouse model of Werner syndrome

Werner syndrome (WS) is a premature aging disorder caused by mutations in a RecQ-family DNA helicase, WRN. Mice lacking part of the helicase domain of the WRN orthologue exhibit many phenotypic features of WS, including metabolic abnormalities and a shorter lifespan. Yet, little is known about the impact of WRN mutations on the central nervous system in both humans and mouse models of WS. In the current study, we have performed a longitudinal behavioral assessment on mice bearing a Wrn helicase deletion. Behavioral tests demonstrated a loss of motor activity and coordination, reduction in perception, increase in repetitive behavior, and deficits in both spatial and social novelty memories in Wrn mutant mice compared to age-matched wild type mice. These neurological deficits were associated with biochemical and histological changes in the brain of aged Wrn mutant mice. Microglia, resident immune cells that regulate neuronal plasticity and function in the brain, were hyper-ramified in multiple regions involved with the behavioral deficits of Wrn mutant mice. Furthermore, western analyses indicated that Wrn mutant mice exhibited an increase of oxidative stress markers in the prefrontal cortex. Supporting these findings, electron microscopy studies revealed increased cellular aging and oxidative stress features, among microglia and neurons respectively, in the prefrontal cortex of aged Wrn mutant mice. In addition, multiplex immunoassay of serum identified significant changes in the expression levels of several pro- and anti-inflammatory cytokines. Taken together, these findings indicate that microglial dysfunction and neuronal oxidative stress, associated with peripheral immune system alterations, might be important driving forces leading to abnormal neurological symptoms in WS thus suggesting potential therapeutic targets for interventions.

Misfolded proinsulin in the endoplasmic reticulum during development of beta cell failure in diabetes

The endoplasmic reticulum (ER) is broadly distributed throughout the cytoplasm of pancreatic beta cells, and this is where all proinsulin is initially made. Healthy beta cells can synthesize 6000 proinsulin molecules per second. Ordinarily, nascent proinsulin entering the ER rapidly folds via the formation of three evolutionarily conserved disulfide bonds (B7-A7, B19-A20, and A6-A11). A modest amount of proinsulin misfolding, including both intramolecular disulfide mispairing and intermolecular disulfide-linked protein complexes, is a natural by-product of proinsulin biosynthesis, as is the case for many proteins. The steady-state level of misfolded proinsulin-a potential ER stressor-is linked to (1) production rate, (2) ER environment, (3) presence or absence of naturally occurring (mutational) defects in proinsulin, and (4) clearance of misfolded proinsulin molecules. Accumulation of misfolded proinsulin beyond a certain threshold begins to interfere with the normal intracellular transport of bystander proinsulin, leading to diminished insulin production and hyperglycemia, as well as exacerbating ER stress. This is most obvious in mutant INS gene-induced Diabetes of Youth (MIDY; an autosomal dominant disease) but also likely to occur in type 2 diabetes owing to dysregulation in proinsulin synthesis, ER folding environment, or clearance.

Steady-state structural fluctuation is a predictor of the necessity of pausing-mediated co-translational folding for small proteins

Translational pausing coordinates protein synthesis and co-translational folding. It is a common factor that facilitates the correct folding of large, multi-domain proteins. For small proteins, pausing sites rarely occurs in the gene body, and the 3'-end pausing sites are only essential for the folding of a fraction of proteins. The determinant of the necessity of the pausings remains obscure. In this study, we demonstrated that the steady-state structural fluctuation is a predictor of the necessity of pausing-mediated co-translational folding for small proteins. Validated by experiments with 5 model proteins, we found that the rigid protein structures do not, while the flexible structures do need 3'-end pausings to fold correctly. Therefore, rational optimization of translational pausing can improve soluble expression of small proteins with flexible structures, but not the rigid ones. The rigidity of the structure can be quantitatively estimated in silico using molecular dynamic simulation. Nevertheless, we also found that the translational pausing optimization increases the fitness of the expression host, and thus benefits the recombinant protein production, independent from the soluble expression. These results shed light on the structural basis of the translational pausing and provided a practical tool for industrial protein fermentation.

Role of the unfolded protein response in topography-induced osteogenic differentiation in rat bone marrow mesenchymal stem cells

The topography of biomaterials can significantly influence the osteogenic differentiation of cells. Understanding topographical signal transduction is critical for developing biofunctional surfaces, but the current knowledge is insufficient. Recently, numerous reports have suggested that the unfolded protein response (UPR) and osteogenic differentiation are inter-linked. Therefore, we hypothesize that the UPR pathway may be involved in the topography-induced osteogenesis. In the present study, different surface topographies were fabricated on pure titanium foils and the endoplasmic reticulum (ER) stress and UPR pathway were systematically investigated. We found that ER stress and the PERK-eIF2α-ATF4 pathway were activated in a time- and topography-dependent manner. Additionally, the activation of the PERK-eIF2α-ATF4 pathway by different topographies was in line with their osteogenic induction capability. More specifically, the osteogenic differentiation could be enhanced or weakened when the PERK-eIF2α-ATF4 pathway was promoted or inhibited, respectively. Furthermore, tuning of the degree of ER stress with different concentrations of thapsigargin revealed that mild ER stress promotes osteogenic differentiation, whereas excessive ER stress inhibits osteogenic differentiation and causes apoptosis. Taken together, our findings suggest that the UPR may play a critical role in topography-induced osteogenic differentiation, which may help to provide new insights into topographical signal transduction.

STATEMENT OF SIGNIFICANCE

Suitable implant surface topography can effectively improve bioactivity and eventual bone affinity. However, the mechanism of topographical signaling transduction is unclear and criteria for designation of an appropriate implant surface topography is lacking. This study shows that the ER stress and PERK-eIF2α-ATF4 pathway were activated by micro- and micro/nano-topographies, which is corresponding to the osteogenic induction abilities of these topographies. Furthermore, we have found that mild ER stress improves osteogenic differentiation, whereas excessive ER stress inhibits osteogenic differentiation and causes apoptosis. Our findings demonstrate that the UPR plays a critical role in the topography induced osteogenic differentiation, which may help to provide new insights into the topographical signaling transduction.

Insulin therapy for pre-hyperglycemic beta-cell endoplasmic reticulum crowding

Insulin therapy improves β-cell function in early stages of diabetes by mechanisms that may exceed alleviation of glucotoxicity. In advance type 2 diabetes, hyperglycemia causes β-cell damage and ultimately β-cell loss. At such an advanced stage, therapeutic modalities are often inadequate. Growing evidence indicates that in early stages of type-2 diabetes and some types of monogenic diabetes linked with malfunctioning endoplasmic-reticulum (ER), the β-cell ER fails to process sufficient proinsulin once it becomes overloaded. These changes manifest with ER distention (ER-crowding) and deficiency of secretory granules. We hypothesize that insulin therapy may improves β-cell function by alleviating ER-crowding. To support this hypothesis, we investigated pre-diabetic β-cell changes in hProC(A7)Y-CpepGFP transgenic mice that develop prolonged pre-diabetes due to proinsulin dysmaturation and ER-crowding. We attenuated the β-cell ER proinsulin synthesis with a treat-to-target insulin therapy while avoiding hypoglycemia and weight gain. Alleviation of ER-crowding resulted in temporary improvement in proinsulin maturation, insulin secretion and glucose tolerance. Our observations suggest that alleviation of pre-diabetic ER-crowding using a treat-to-target insulin therapy may improve β-cell function and may prevent further metabolic deterioration.

Endoplasmic reticulum stress: its role in disease and novel prospects for therapy

The endoplasmic reticulum (ER) is a multifunctional organelle required for lipid biosynthesis, calcium storage, and protein folding and processing. A number of physiological and pathological conditions, as well as a variety of pharmacological agents, are able to disturb proper ER function and thereby cause ER stress, which severely impairs protein folding and therefore poses the risk of proteotoxicity. Specific triggers for ER stress include, for example, particular intracellular alterations (e.g., calcium or redox imbalances), certain microenvironmental conditions (e.g., hypoglycemia, hypoxia, and acidosis), high-fat and high-sugar diet, a variety of natural compounds (e.g., thapsigargin, tunicamycin, and geldanamycin), and several prescription drugs (e.g., bortezomib/Velcade, celecoxib/Celebrex, and nelfinavir/Viracept). The cell reacts to ER stress by initiating a defensive process, called the unfolded protein response (UPR), which is comprised of cellular mechanisms aimed at adaptation and safeguarding cellular survival or, in cases of excessively severe stress, at initiation of apoptosis and elimination of the faulty cell. In recent years, this dichotomic stress response system has been linked to several human diseases, and efforts are underway to develop approaches to exploit ER stress mechanisms for therapy. For example, obesity and type 2 diabetes have been linked to ER stress-induced failure of insulin-producing pancreatic beta cells, and current research efforts are aimed at developing drugs that ameliorate cellular stress and thereby protect beta cell function. Other studies seek to pharmacologically aggravate chronic ER stress in cancer cells in order to enhance apoptosis and achieve tumor cell death. In the following, these principles will be presented and discussed.

Spatial localisation of chaperone distribution in the endoplasmic reticulum of yeast

In eukaryotes, the endoplasmic reticulum (ER) serves as the first membrane-enclosed organelle in the secretory pathway, with functions including protein folding, maturation and transport. Molecular chaperones, of the Hsp70 family of proteins, participate in assisting these processes and are essential to cellular function and survival. BiP is a resident Hsp70 chaperone in the ER of Saccharomyces cerevisiae. In this study the authors have created a partial differential equation model to examine how BiP interacts with the membrane-bound co-chaperone Sec63 in translocation, a process in which BiP assists in guiding a nascent protein into the ER lumen. It has been found that when Sec63 participates in translocation through localisation at the membrane, the spatial distribution of BiP is inhomogeneous, with more BiP at the surface. When translocation is inhibited through a disabling of Sec63's membrane tether, the concentration of BiP throughout the ER becomes homogeneous. The computational simulations suggest that Sec63's localisation and the resulting binding to BiP near the membrane surface of the ER enable a heterogeneous distribution of BiP within the ER, and may facilitate BiP's role in translocation. [Includes supplementary material].

Programmed cell death occurs asymmetrically during abscission in tomato

Abscission occurs specifically in the abscission zone (AZ) tissue as a natural stage of plant development. Previously, we observed delay of tomato (Solanum lycopersicum) leaf abscission when the LX ribonuclease (LX) was inhibited. The known association between LX expression and programmed cell death (PCD) suggested involvement of PCD in abscission. In this study, hallmarks of PCD were identified in the tomato leaf and flower AZs during the late stage of abscission. These included loss of cell viability, altered nuclear morphology, DNA fragmentation, elevated levels of reactive oxygen species and enzymatic activities, and expression of PCD-associated genes. Overexpression of antiapoptotic proteins resulted in retarded abscission, indicating PCD requirement. PCD, LX, and nuclease gene expression were visualized primarily in the AZ distal tissue, demonstrating an asymmetry between the two AZ sides. Asymmetric expression was observed for genes associated with cell wall hydrolysis, leading to AZ, or associated with ethylene biosynthesis, which induces abscission. These results suggest that different abscission-related processes occur asymmetrically between the AZ proximal and distal sides. Taken together, our findings identify PCD as a key mechanism that occurs asymmetrically during normal progression of abscission and suggest an important role for LX in this PCD process.

Biophysical alteration of the secretory track in β-cells due to molecular overcrowding: the relevance for diabetes

Recent data demonstrate that accumulation of misfolded proteins within the early part of the secretory track of β-cells causes impaired insulin synthesis and development of diabetes. The molecular mechanism of this cellular dysfunction remains largely unknown. Using basic molecular principles and computer simulations, we suggested recently that hyperglycemic conditions can generate substantial molecular crowding effects in the secretory track of β-cells leading to significant alterations of the insulin biosynthesis capabilities. Here, we review the major molecular mechanisms that may be implicated in the alteration of insulin synthesis in susceptible β-cells. Steric repulsions and volume exclusion in the endoplasmic reticulum (ER) increase the propensity of misfolding of proinsulin (the precursor molecule of insulin). In addition, similar forces might act in the next secretory compartments (Golgi and vesicles) leading to (i) altered packaging of proinsulin in vesicles (ii) entrapment of proinsulin convertases and/or restricted accessibility for these convertases to the cleavage sites on the surface of the proinsulin and (iii) depressed kinetic rate of the transformation of the native proinsulin in active insulin and C-peptide. These concepts are expressed in simple mathematical terms relating the kinetic coefficient of proinsulin to insulin conversion to the levels of proinsulin misfolding and hyperglycemic stress. The present approach is useful for understanding molecular phenomena associated with the pathogenesis of diabetes. It also offers practical means for predicting the state of pancreatic β-cells from measurements of the insulin to proinsulin ratio in the blood. This is of immediate clinical relevance and may improve the diagnosis of diabetes.

β-Cell dysfunction under hyperglycemic stress: a molecular model

BACKGROUND

Pancreatic β cells respond to chronic hyperglycemia by increasing the synthesis of proinsulin (the precursor molecule of insulin). Prolonged stimulations lead to accumulation of misfolded proinsulin in the secretory track, delayed insulin secretion, and release of unprocessed proinsulin in the blood. The molecular mechanisms connecting the state of endoplasmic reticulum overloading with the efficiency of proinsulin to insulin conversion remain largely unknown.

METHODS

Computer simulations can help us to understand mechanistic features of the β-cell secretory defect and to design experiments that may reveal the molecular basis of this dysfunction. We used molecular crowding concepts and statistical thermodynamics to dissect possible biophysical mechanisms underlying the alteration of the secretory track of β cells and to elucidate the chemistry aspects of the secretory dysfunction. We then used numerical algorithms to relate the degree of biophysical alteration of these secretory compartments with the change of proinsulin to insulin conversion rate.

RESULTS

Our computer simulations suggest that overloading the endoplasmic reticulum initiates downstream molecular crowding effects that affect protein translational mechanisms, including proinsulin misfolding, delayed packing of proinsulin in secretory vesicles, and low kinetic coefficient of proinsulin to insulin conversion.

CONCLUSIONS

Together with previous experimental data, the present study can help us to better understand chemistry aspects related to the secondary translational mechanisms in β cells and how hyperglycemic stress can alter secretory function. This can give a further impetus to the development of novel software to be used in a clinical setup for prediction and assessment of diabetic states in susceptible patients.

Endoplasmic reticulum overcrowding as a mechanism of beta-cell dysfunction in diabetes

This study suggests a molecular mechanism that explains the accumulation of denaturated proinsulin in the endoplasmic reticulum (ER) of beta-cells. Such states were frequently observed in beta-cells experiencing increased demand for insulin production and were shown to lead to secretory dysfunction and diabetes. Here, a self-consistent kinetic model is used to investigate changes in protein translation due to ER overloading. The model is based on a molecular theory that relates the molecular composition and level of molecular crowding in the ER to the kinetic rates of protein folding/misfolding and transit to the Golgi apparatus (GA). This study suggests that molecular crowding forces can increase protein misfolding and impair the transport to the GA, thus overwhelming the quality control mechanism in the ER. A continual accumulation of toxic residues in the ER enhances even further the molecular crowding, accelerating protein denaturation. This article shows that molecular crowding affects differently the transit of various proteins through the ER. Apparently, the molecular crowding level that can inhibit the transport of native proinsulin to the GA influences to a lesser extent the transit of proamylin, a much smaller peptide cosynthesized with proinsulin in the ER. Smaller-volume misfolded proinsulin species may also win the passage competition through the ER and move on the secretory track. However, misfolded proinsulin fails the conversion to active insulin. This study can help us to decipher circumstances leading to the alteration of the secretory function in susceptible beta-cells and the onset of diabetes.

Cholic acid micelles--controlling the size of the aqueous cavity by PEGylation

Data show that cholic acid (CA) micelles are less densely packed and much smaller than micelles formed by typical surfactants, suggesting that CA derivatives can be used to synthesize drug nanocarriers. Presumably, the formation of internal cavities is favored by the facial characteristics of the CA molecule, i.e. the convex molecular structure that is hydrophobic on one side and hydrophilic on the other. Here, we present a thermodynamical approach to quantify the effect of facial characteristics on forces governing the self-assembling process of CA molecules. We show that facial characteristics favor the entrapment of water molecules at interfaces upon CA aggregation, which weakens the attraction between CA hydrophobic moieties. Our computer simulations suggest that these effects contribute significantly to the tendency of CA molecules to form small "hollow-core" micelles. The attachment of polyethylene glycol (PEG) molecular chains to CA increases the repulsive forces in the system, reducing even further the micelle size. We use the present molecular model and experimental critical micelle concentration (cmc) data for CA-PEG systems to predict the change of the micelle size and cavity volume with the increase of the PEG chain length (x). Our computations indicate that the CA-PEG micelles are good candidates for drug delivery. The structural stability of CA-PEG micelles was further assessed by molecular dynamics simulations. We also tested the drug loading efficiency of this system and found an average of 0.5 mg paclitaxel load per 20 mg of CA-PEG polymer. The present study helps to identify critical parameters that control structural properties of the CA based nanocarriers and suggests practical means to optimize the ratio between micelle size and volume of the internal cavity.

The honeymoon phase: intersection of metabolism and immunology

PURPOSE OF REVIEW

Even though the honeymoon phase in type 1 diabetes mellitus has been well known, its underlying pathogenic mechanisms remain poorly described. The common explanation that it occurs due to 'beta-cell rest' on initiation of insulin therapy seems inadequate based on recent observations and studies and its underlying immunological aspects overlooked.

RECENT FINDINGS

In this article, we will review the metabolic and immunological aspects of the honeymoon phase and we will present our current model of the pathophysiology of this phase. Our view is that it is one of many phases of remission occurring basically due to development of adaptive immune tolerance along the course of type 1 diabetes. We will also review new findings of the interplay between metabolic factors (ambient glucose level) and immune function.

SUMMARY

The honeymoon phase provides a unique model to understand the pathogenesis of type 1 diabetes. Research to unravel its immune pathogenesis is needed. It may turn out that the optimum form of intervention in type 1 diabetes is one that combines enhancement of antigen-specific adaptive immune tolerance with optimized metabolic control in order to keep cytotoxic T cells anergic.