The Portfolio Diet and Incident Type 2 Diabetes: Findings From the Women's Health Initiative Prospective Cohort Study

OBJECTIVE

A plant-based dietary pattern, the Portfolio Diet, has been shown to lower LDL cholesterol and other cardiovascular disease risk factors. However, no study has evaluated the association of this diet with incident type 2 diabetes.

RESEARCH DESIGN AND METHODS

This analysis included 145,299 postmenopausal women free of diabetes at baseline in the Women's Health Initiative (WHI) Clinical Trials and Observational Study from 1993 to 2021. Adherence to the diet was assessed with a score based on six components (high in plant protein [soy and pulses], nuts, viscous fiber, plant sterols, and monounsaturated fat and low in saturated fat and cholesterol) determined from a validated food-frequency questionnaire. We used Cox proportional hazards models to estimate hazard ratios (HRs) and 95% CIs of the association of the Portfolio Diet, alongside the Dietary Approaches to Stop Hypertension (DASH) and Mediterranean diets, with incident type 2 diabetes, with adjustment for potential confounders.

RESULTS

Over a mean follow-up of 16.0 years, 13,943 cases of incident type 2 diabetes were identified. In comparisons of the highest with the lowest quintiles of adherence, the HRs for risk of incident type 2 diabetes were 0.77 (95% CI 0.72, 0.82) for the Portfolio Diet, 0.69 (0.64, 0.73) for the DASH diet, and 0.78 (0.74, 0.83) for the Mediterranean diet. These findings were attenuated by 10% after additional adjustment for BMI.

CONCLUSIONS

Greater adherence to the plant-predominant Portfolio, DASH, and Mediterranean diets was prospectively associated with lower risk of type 2 diabetes in postmenopausal women.

RF26 | PMON332 Safety and Efficacy of Treatment with Lonapegsomatropin in Children with Growth Hormone Deficiency at Week 130 in the enliGHten Trial

Background

Once-weekly lonapegsomatropin (TransCon hGH) is a long-acting prodrug of somatropin, recently approved for the treatment of pediatric growth hormone deficiency (GHD) by the US FDA. In the pivotal 52-week phase 3 heiGHt trial and the 26-week fliGHt trial, lonapegsomatropin demonstrated safety and efficacy in both treatment-naïve and treatment-experienced children with GHD.

Methods

Results are reported from Week 130 (data snapshot date: September 01, 2021) from the open-label extension trial enliGHten. Upon entry into enliGHten, all participants received lonapegsomatropin via vial/syringe, with subsequent switch to TransCon hGH Auto-Injector when available.

Results

A total of 298 participants (98% of parent trials) continued from the heiGHt or fliGHt trials into enliGHten. At Week 130, 36 participants had completed the trial and 248 participants remain enrolled. The height standard deviation score (SDS) mean (standard deviation [SD]) at Week 130 was -0.64 (0.85) compared with baseline value of -1.56 (0.88) at the start of enliGHten. Participants continued to approach the average parental height SDS mean (SD) of -0.39 (0.77). The annualized height velocity mean (SD) at Week 130 was 9.32 cm/year (0.45). Average IGF-1 SDS mean (SD) at Week 130 was 1.46 (1.18) compared with 0.52 (1.58) at enliGHten baseline. For the 36 participants who had completed the trial, 14 had reached bone age of >14 years (girls) or >16 years (boys), and 24 had completed the study based on investigator judgement that treatment for growth hormone deficiency was no longer necessary. The difference between mean (SD) height SDS at last visit and average parental height SDS was -0.05 (0.75) for the 36 completers, and 61.1% of these participants had met or exceeded the average parental height SDS. The mean (SD) total duration of hGH treatment for participants who completed the trial (including daily treatment and lonapegsomatropin) was 3.53 (0.88) years, and the mean (SD) duration of treatment with lonapegsomatropin was 2.20 (0.66). With continued lonapegsomatropin treatment, the AE profile remained consistent with what was observed in the parent trials, with no new safety signals.

Conclusions

Children and adolescents treated with lonapegsomatropin showed continued improvement of height SDS through their 3rd year of therapy. Lonapegsomatropin continued to demonstrate a safety profile comparable to that of daily somatropin therapy.

Presentation: Monday, June 13, 2022 12:30 p.m. - 12:35 p.m., Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.

Switching to Weekly Lonapegsomatropin from Daily Somatropin in Children with Growth Hormone Deficiency: The fliGHt Trial

INTRODUCTION

The phase 3 fliGHt Trial evaluated the safety and tolerability of once-weekly lonapegsomatropin, a long-acting prodrug, in children with growth hormone deficiency (GHD) who switched from daily somatropin therapy to lonapegsomatropin.

METHODS

This multicenter, open-label, 26-week phase 3 trial took place at 28 sites across 4 countries (Australia, Canada, New Zealand, and the USA). The trial enrolled 146 children with GHD, 143 of which were previously treated with daily somatropin. All subjects received once-weekly lonapegsomatropin 0.24 mg human growth hormone/kg/week. The primary outcome measure was safety and tolerability of lonapegsomatropin over 26 weeks. Secondary outcome measures assessed annualized height velocity (AHV), height standard deviation score (SDS), and IGF-1 SDS at 26 weeks.

RESULTS

Subjects had a mean prior daily somatropin dose of 0.29 mg/kg/week. Treatment-emergent adverse events (AEs) reported were similar to the published AE profile of daily somatropin therapies. After switching to lonapegsomatropin, the least-squares mean (LSM) AHV was 8.7 cm/year (95% CI: 8.2, 9.2) at Week 26 and LSM height SDS changed from baseline to Week 26 of +0.25 (95% CI: 0.21, 0.29). Among switch subjects, the LSM for average IGF-1 SDS was sustained at Weeks 13 and 26, representing an approximate 0.7 increase from baseline (prior to switching from daily somatropin therapy). Patient-reported outcomes indicated a preference for weekly lonapegsomatropin among both children and their parents.

CONCLUSIONS

Lonapegsomatropin treatment outcomes were as expected across a range of ages and treatment experiences. Switching to lonapegsomatropin resulted in a similar AE profile to daily somatropin therapy.

Safety and Efficacy of Lonapegsomatropin in Children With Growth Hormone Deficiency: enliGHten Trial 2-Year Results

PURPOSE

The objectives of the ongoing, Phase 3, open-label extension trial enliGHten are to assess the long-term safety and efficacy of weekly administered long-acting growth hormone lonapegsomatropin in children with growth hormone deficiency.

METHODS

Eligible subjects completing a prior Phase 3 lonapegsomatropin parent trial (heiGHt or fliGHt) were invited to participate. All subjects were treated with lonapegsomatropin. Subjects in the United States switched to the TransCon hGH Auto-Injector when available. Endpoints were long-term safety, annualized height velocity, pharmacodynamics [insulin-like growth factor-1 SD score (SDS) values], and patient- and caregiver-reported assessments of convenience and tolerability.

RESULTS

Lonapegsomatropin treatment during enliGHten was associated with continued improvements in height SDS through week 104 in treatment-naïve subjects from the heiGHt trial (-2.89 to -1.37 for the lonapegsomatropin group; -3.0 to -1.52 for the daily somatropin group). Height SDS also continued to improve among switch subjects from the fliGHt trial (-1.42 at fliGHt baseline to -0.69 at week 78). After 104 weeks, the average bone age/chronological age ratio for each treatment group was 0.8 (0.1), showing only minimal advancement of bone age relative to chronological age with continued lonapegsomatropin treatment among heiGHt subjects. Fewer local tolerability reactions were reported with the TransCon hGH Auto-Injector compared with syringe/needle.

CONCLUSIONS

Treatment with lonapegsomatropin continued to be safe and well-tolerated, with no new safety signals identified. Children treated with once-weekly lonapegsomatropin showed continued improvement of height SDS through the second year of therapy without excess advancement of bone age.

Efficacy and Safety of up to 2 Years of Treatment With TransCon hGH (Lonapegsomatropin) in Treatment-Naïve and Treatment-Experienced Children With Growth Hormone Deficiency

Background: Once-weekly TransCon hGH (lonapegsomatropin) is an investigational long-acting prodrug of somatropin in development for GHD. In the pivotal 52-week phase 3 heiGHt trial, lonapegsomatropin demonstrated superior annualized height velocity (AHV) compared to the same weekly dose of daily somatropin in treatment-naïve children with GHD. In the 26-week fliGHt trial, switch from daily somatropin to lonapegsomatropin provided continued growth and maintained a good safety profile. Methods: Results are reported from heiGHt and fliGHt subjects who continued into the open-label long-term extension enliGHten trial (data cut: June 1^st 2020). Subjects received either lonapegsomatropin (Group A; vial/syringe) or daily somatropin (Group B; pen device) in heiGHt, or lonapegsomatropin in fliGHt (Group C; vial/syringe). Upon entry into enliGHten, all subjects received lonapegsomatropin via vial/syringe, with subsequent switch to TransCon hGH Auto-Injector when available. Average IGF-1 was obtained on post-dose Day 5 (±1) in enliGHten. A by-visit ANCOVA model was used for numeric efficacy endpoints. Results: A total of 298 (98%) subjects continued into enliGHten. (A: n=103; B: n=55; C: n=140). The treatment difference in LS mean ∆ height SDS (A vs B) at the end of heiGHt (Week 52, 1.10 vs 0.96, P=0.015) was sustained through Week 104 (1.61 vs 1.49, P=0.158). For Group C, height SDS improved from −1.42 at fliGHt baseline to −0.69 at Week 78. AHV was within the expected range for 2^nd year therapy. Among children who switched (B), an attenuation in the expected 2^nd year decline of AHV suggested that lonapegsomatropin had an improved treatment effect relative to the previous daily somatropin. Mean (SD) average IGF-1 SDS remained stable and generally within the expected range for all groups (Week 104, A: 0.95 [1.22], B: 1.04 [1.25]; Week 78, C:1.81 [1.08]). An improvement in injection site tolerability was observed after switching to the TransCon hGH Auto-Injector; subjects and parents also indicated overall ease-of-use of the device (assessed by the Device Usability Questionnaire). With continued lonapegsomatropin treatment, the AE profile remained consistent with what was observed in the parent trials, with no new safety signals. Throughout enliGHten and the parent trials, non-neutralizing low-titer anti-hGH binding antibodies were detected post-dose in a total of 15 subjects (5.0%). Lab parameters were stable and generally remained within the normal range throughout the trials. As of the data cut, 2 subjects have achieved near adult height (AHV <2 cm/year over the last 9 months or bone age >14 [females] or >16 [males]) and thus have completed the trial. Conclusions: Children treated with lonapegsomatropin showed continued improvement of height SDS through their 2nd year of therapy. Lonapegsomatropin continued to demonstrate a safety profile comparable to that of daily somatropin therapy.

Neuroligin-2-derived peptide-covered polyamidoamine-based (PAMAM) dendrimers enhance pancreatic β-cells' proliferation and functions

Pancreatic β-cell membranes and presynaptic areas of neurons contain analogous protein complexes that control the secretion of bioactive molecules. These complexes include the neuroligins (NLs) and their binding partners, the neurexins (NXs). It has been recently reported that both insulin secretion and the proliferation rates of β-cells increase when cells are co-cultured with full-length NL-2 clusters. The pharmacological use of full-length protein is always problematic due to its unfavorable pharmacokinetic properties. Thus, NL-2-derived short peptide was conjugated to the surface of polyamidoamine-based (PAMAM) dendrimers. This nanoscale composite improved β-cell functions in terms of the rate of proliferation, glucose-stimulated insulin secretion (GSIS), and functional maturation. This functionalized dendrimer also protected β-cells under cellular stress conditions. In addition, various novel peptidomimetic scaffolds of NL-2-derived peptide were designed, synthesized, and conjugated to the surface of PAMAM in order to increase the biostability of the conjugates. However, after being covered by peptidomimetics, PAMAM dendrimers were inactive. Thus, the original peptide-based PAMAM dendrimer is a leading compound for continued research that might provide a unique starting point for designing an innovative class of antidiabetic therapeutics that possess a unique mode of action.

Alarming increase in HbA1c and near misdiagnosis of diabetes mellitus resulting from a clinical laboratory instrument upgrade and haemoglobin variant

A 29-year-old woman was referred for new-onset diabetes mellitus after her glycosylated haemoglobin (HbA1c) was found to be 10.2%. Three years earlier, the patient's HbA1c-measured by the same clinical laboratory-had been 5.5%. The newer HbA1c was discordant with fasting glucose levels and a lack of diabetes-associated symptoms. The laboratory reported that their assay methodology remained unchanged and also that no haemoglobin variants were detected. Further investigation, however, revealed, first, that the patient carried a haemoglobin alpha chain mutation (Hb Wayne) that can sometimes cause assay interference and, second, that although the laboratory's assay methodology had not changed, their assay instrument had. Depending on assay methodology, haemoglobin variants can cause HbA1c assay interference and the presence of these variants may not be detected by the performing laboratory. Interference may not only be dependent on assay methodology but also on the assay instrument used.

Cell-sized lipid vesicles for cell-cell synaptic therapies

Cell-sized lipid vesicles (CLVs) have shown great promise for therapeutic and artificial cell applications, but their fragility and short shelf life has hindered widespread adoption and commercial viability. We present a method to circumvent the storage limitations of CLVs such as giant unilamellar vesicles (GUVs) and single-compartment multisomes (SCMs) by storing them in a double emulsion precursor form. The double emulsions can be stored for at least 8 months and readily converted into either GUVs or SCMs at any time. In this study, we investigate the interfacial parameters responsible for this morphological change, and we also demonstrate the therapeutic potential of CLVs by utilizing them to present a transmembrane protein, neuroligin-2, to pancreatic β-cells, forming cell-cell synapses that stimulate insulin secretion and cellular growth.

Extracellular CADM1 interactions influence insulin secretion by rat and human islet β-cells and promote clustering of syntaxin-1

Contact between β-cells is necessary for their normal function. Identification of the proteins mediating the effects of β-cell-to-β-cell contact is a necessary step toward gaining a full understanding of the determinants of β-cell function and insulin secretion. The secretory machinery of the β-cells is nearly identical to that of central nervous system (CNS) synapses, and we hypothesize that the transcellular protein interactions that drive maturation of the two secretory machineries upon contact of one cell (or neural process) with another are also highly similar. Two such transcellular interactions, important for both synaptic and β-cell function, have been identified: EphA/ephrin-A and neuroligin/neurexin. Here, we tested the role of another synaptic cleft protein, CADM1, in insulinoma cells and in rat and human islet β-cells. We found that CADM1 is a predominant CADM isoform in β-cells. In INS-1 cells and primary β-cells, CADM1 constrains insulin secretion, and its expression decreases after prolonged glucose stimulation. Using a coculture model, we found that CADM1 also influences insulin secretion in a transcellular manner. We asked whether extracellular CADM1 interactions exert their influence via the same mechanisms by which they influence neurotransmitter exocytosis. Our results suggest that, as in the CNS, CADM1 interactions drive exocytic site assembly and promote actin network formation. These results support the broader hypothesis that the effects of cell-cell contact on β-cell maturation and function are mediated by the same extracellular protein interactions that drive the formation of the presynaptic exocytic machinery. These interactions may be therapeutic targets for reversing β-cell dysfunction in diabetes.

Nutritional energy stimulates NAD+ production to promote tankyrase-mediated PARsylation in insulinoma cells

The poly-ADP-ribosylation (PARsylation) activity of tankyrase (TNKS) regulates diverse physiological processes including energy metabolism and wnt/β-catenin signaling. This TNKS activity uses NAD⁺ as a co-substrate to post-translationally modify various acceptor proteins including TNKS itself. PARsylation by TNKS often tags the acceptors for ubiquitination and proteasomal degradation. Whether this TNKS activity is regulated by physiological changes in NAD⁺ levels or, more broadly, in cellular energy charge has not been investigated. Because the NAD⁺ biosynthetic enzyme nicotinamide phosphoribosyltransferase (NAMPT) in vitro is robustly potentiated by ATP, we hypothesized that nutritional energy might stimulate cellular NAMPT to produce NAD⁺ and thereby augment TNKS catalysis. Using insulin-secreting cells as a model, we showed that glucose indeed stimulates the autoPARsylation of TNKS and consequently its turnover by the ubiquitin-proteasomal system. This glucose effect on TNKS is mediated primarily by NAD⁺ since it is mirrored by the NAD⁺ precursor nicotinamide mononucleotide (NMN), and is blunted by the NAMPT inhibitor FK866. The TNKS-destabilizing effect of glucose is shared by other metabolic fuels including pyruvate and amino acids. NAD⁺ flux analysis showed that glucose and nutrients, by increasing ATP, stimulate NAMPT-mediated NAD⁺ production to expand NAD⁺ stores. Collectively our data uncover a metabolic pathway whereby nutritional energy augments NAD⁺ production to drive the PARsylating activity of TNKS, leading to autoPARsylation-dependent degradation of the TNKS protein. The modulation of TNKS catalytic activity and protein abundance by cellular energy charge could potentially impose a nutritional control on the many processes that TNKS regulates through PARsylation. More broadly, the stimulation of NAD⁺ production by ATP suggests that nutritional energy may enhance the functions of other NAD⁺-driven enzymes including sirtuins.

Coculture analysis of extracellular protein interactions affecting insulin secretion by pancreatic beta cells

Interactions between cell-surface proteins help coordinate the function of neighboring cells. Pancreatic beta cells are clustered together within pancreatic islets and act in a coordinated fashion to maintain glucose homeostasis. It is becoming increasingly clear that interactions between transmembrane proteins on the surfaces of adjacent beta cells are important determinants of beta-cell function. Elucidation of the roles of particular transcellular interactions by knockdown, knockout or overexpression studies in cultured beta cells or in vivo necessitates direct perturbation of mRNA and protein expression, potentially affecting beta-cell health and/or function in ways that could confound analyses of the effects of specific interactions. These approaches also alter levels of the intracellular domains of the targeted proteins and may prevent effects due to interactions between proteins within the same cell membrane to be distinguished from the effects of transcellular interactions. Here a method for determining the effect of specific transcellular interactions on the insulin secreting capacity and responsiveness of beta cells is presented. This method is applicable to beta-cell lines, such as INS-1 cells, and to dissociated primary beta cells. It is based on coculture models developed by neurobiologists, who found that exposure of cultured neurons to specific neuronal proteins expressed on HEK293 (or COS) cell layers identified proteins important for driving synapse formation. Given the parallels between the secretory machinery of neuronal synapses and of beta cells, we reasoned that beta-cell functional maturation might be driven by similar transcellular interactions. We developed a system where beta cells are cultured on a layer of HEK293 cells expressing a protein of interest. In this model, the beta-cell cytoplasm is untouched while extracellular protein-protein interactions are manipulated. Although we focus here primarily on studies of glucose-stimulated insulin secretion, other processes can be analyzed; for example, changes in gene expression as determined by immunoblotting or qPCR.

Altered pancreatic islet function and morphology in mice lacking the Beta-cell surface protein neuroligin-2

Neuroligin-2 is a transmembrane, cell-surface protein originally identified as an inhibitory synapse-associated protein in the central nervous system. Neuroligin-2 is also present on the pancreatic beta-cell surface, and there it engages in transcellular interactions that drive functional maturation of the insulin secretory machinery; these are necessary for normal insulin secretion. The effects of neuroligin-2 deficiency on brain and neuronal function and morphology and on behavior and coordination have been extensively characterized using neuroligin-2 knockout mice. The effects of absent neuroligin-2 expression on islet development and function, however, are unknown. Here, to help test whether neuroligin-2 is necessary for normal islet development, we characterized islet morphology in mice lacking neuroligin-2. To test whether–as predicted by our earlier co-culture studies–absence of neuroligin-2 impairs beta cell function, we compared glucose-stimulated insulin secretion by islets from mutant and wild-type mice. Our results show that while islets from neuroligin-2-deficient mice do not to appear to differ architecturally from wild-type islets, they are smaller, fewer in number, and contain beta cells with lower insulin content. Evaluation of transcript levels suggests that upregulation of neuroligin-1 helps compensate for loss of neuroligin-2. Surprisingly, under both basal and stimulating glucose levels, isolated islets from the knockout mice secreted more of their intracellular insulin content. Rat islets with shRNA-mediated neuroligin-2 knockdown also exhibited increased insulin secretion. Neurexin transcript levels were lower in the knockout mice and, consistent with our prior finding that neurexin is a key constituent of the insulin granule docking machinery, insulin granule docking was reduced. These results indicate that neuroligin-2 is not necessary for the formation of pancreatic islets but that neuroligin-2 influences islet size and number. Neuroligin-2–perhaps through its effects on the expression and/or activity of its binding partner neurexin–promotes insulin granule docking, a known constraint on insulin secretion.

Transcellular neuroligin-2 interactions enhance insulin secretion and are integral to pancreatic β cell function

Normal glucose-stimulated insulin secretion is dependent on interactions between neighboring β cells. Elucidation of the reasons why this cell-to-cell contact is essential will probably yield critical insights into β cell maturation and function. In the central nervous system, transcellular protein interactions (i.e. interactions between proteins on the surfaces of different cells) involving neuroligins are key mediators of synaptic functional development. We previously demonstrated that β cells express neuroligin-2 and that insulin secretion is affected by changes in neuroligin-2 expression. Here we show that the effect of neuroligin-2 on insulin secretion is mediated by transcellular interactions. Neuroligin-2 binds with nanomolar affinity to a partner on the β cell surface and contributes to the increased insulin secretion brought about by β cell-to-β cell contact. It does so in a manner seemingly independent of interactions with neurexin, a known binding partner. As in the synapse, transcellular neuroligin-2 interactions enhance the functioning of the submembrane exocytic machinery. Also, as in the synapse, neuroligin-2 clustering is important. Neuroligin-2 in soluble form, rather than presented on a cell surface, decreases insulin secretion by rat islets and MIN-6 cells, most likely by interfering with endogenous neuroligin interactions. Prolonged contact with neuroligin-2-expressing cells increases INS-1 β cell proliferation and insulin content. These results extend the known parallels between the synaptic and β cell secretory machineries to extracellular interactions. Neuroligin-2 interactions are one of the few transcellular protein interactions thus far identified that directly enhance insulin secretion. Together, these results indicate a significant role for transcellular neuroligin-2 interactions in the establishment of β cell function.

Neurexin-1α contributes to insulin-containing secretory granule docking

Neurexins are a family of transmembrane, synaptic adhesion molecules. In neurons, neurexins bind to both sub-plasma membrane and synaptic vesicle-associated constituents of the secretory machinery, play a key role in the organization and stabilization of the presynaptic active zone, and help mediate docking of synaptic vesicles. We have previously shown that neurexins, like many other protein constituents of the neurotransmitter exocytotic machinery, are expressed in pancreatic β cells. We hypothesized that the role of neurexins in β cells parallels their role in neurons, with β-cell neurexins helping to mediate insulin granule docking and secretion. Here we demonstrate that β cells express a more restricted pattern of neurexin transcripts than neurons, with a clear predominance of neurexin-1α expressed in isolated islets. Using INS-1E β cells, we found that neurexin-1α interacts with membrane-bound components of the secretory granule-docking machinery and with the granule-associated protein granuphilin. Decreased expression of neurexin-1α, like decreased expression of granuphilin, reduces granule docking at the β-cell membrane and improves insulin secretion. Perifusion of neurexin-1α KO mouse islets revealed a significant increase in second-phase insulin secretion with a trend toward increased first-phase secretion. Upon glucose stimulation, neurexin-1α protein levels decrease. This glucose-induced down-regulation may enhance glucose-stimulated insulin secretion. We conclude that neurexin-1α is a component of the β-cell secretory machinery and contributes to secretory granule docking, most likely through interactions with granuphilin. Neurexin-1α is the only transmembrane component of the docking machinery identified thus far. Our findings provide new insights into the mechanisms of insulin granule docking and exocytosis.

An AP-3-dependent mechanism drives synaptic-like microvesicle biogenesis in pancreatic islet beta-cells

Pancreatic islet beta-cells contain synaptic-like microvesicles (SLMVs). The origin, trafficking, and role of these SLMVs are poorly understood. In neurons, synaptic vesicle (SV) biogenesis is mediated by two different cytosolic adaptor protein complexes, a ubiquitous AP-2 complex and the neuron-specific AP-3B complex. Mice lacking AP-3B subunits exhibit impaired GABAergic (inhibitory) neurotransmission and reduced neuronal vesicular GABA transporter (VGAT) content. Since beta-cell maturation and exocytotic function seem to parallel that of the inhibitory synapse, we predicted that AP-3B-associated vesicles would be present in beta-cells. Here, we test the hypothesis that AP-3B is expressed in islets and mediates beta-cell SLMV biogenesis. A secondary aim was to test whether the sedimentation properties of INS-1 beta-cell microvesicles are identical to those of bona fide SLMVs isolated from PC12 cells. Our results show that the two neuron-specific AP-3 subunits beta3B and mu3B are expressed in beta-cells, the first time these proteins have been found to be expressed outside the nervous system. We found that beta-cell SLMVs share the same sedimentation properties as PC12 SLMVs and contain SV proteins that sort specifically to AP-3B-associated vesicles in the brain. Brefeldin A, a drug that interferes with AP-3-mediated SV biogenesis, inhibits the delivery of AP-3 cargoes to beta-cell SLMVs. Consistent with a role for AP-3 in the biogenesis of GABAergic SLMV in beta-cells, INS-1 cell VGAT content decreases upon inhibition of AP-3 delta-subunit expression. Our findings suggest that beta-cells and neurons share molecules and mechanisms important for mediating the neuron-specific membrane trafficking pathways that underlie synaptic vesicle formation.

Expression of neurexin, neuroligin, and their cytoplasmic binding partners in the pancreatic beta-cells and the involvement of neuroligin in insulin secretion

The composition of the beta-cell exocytic machinery is very similar to that of neuronal synapses, and the developmental pathway of beta-cells and neurons substantially overlap. beta-Cells secrete gamma-aminobutyric acid and express proteins that, in the brain, are specific markers of inhibitory synapses. Recently, neuronal coculture experiments have identified three families of synaptic cell-surface molecules (neurexins, neuroligins, and SynCAM) that drive synapse formation in vitro and that control the differentiation of nascent synapses into either excitatory or inhibitory fully mature nerve terminals. The inhibitory synapse-like character of the beta-cells led us to hypothesize that members of these families of synapse-inducing adhesion molecules would be expressed in beta-cells and that the pattern of expression would resemble that associated with neuronal inhibitory synaptogenesis. Here, we describe beta-cell expression of the neuroligins, neurexins, and SynCAM, and show that neuroligin expression affects insulin secretion in INS-1 beta-cells and rat islet cells. Our findings demonstrate that neuroligins and neurexins are expressed outside the central nervous system and help confer an inhibitory synaptic-like phenotype onto the beta-cell surface. Analogous to their role in synaptic neurotransmission, neurexin-neuroligin interactions may play a role in the formation of the submembrane insulin secretory apparatus.

Release of glutamate decarboxylase-65 into the circulation by injured pancreatic islet beta-cells

The enzyme glutamate decarboxylase-65 (GAD65) is a major autoantigen in autoimmune diabetes. The mechanism whereby autoreactivity to GAD65, an intracellular protein, is triggered is unknown, and it is possible that immunoreactive GAD65 is released by injured pancreatic islet beta-cells. There is a great need for methods by which to detect and monitor ongoing islet injury. If GAD65 were released and, furthermore, were able to reach the circulation, it could function as a marker of beta-cell injury. Here, a novel GAD65 plasma immunoassay is used to test the hypotheses that beta-cell injury induces GAD65 discharge in vivo and that discharged GAD65 reaches the bloodstream. Plasma GAD65 levels were determined in rats treated with alloxan, and with diabetogenic and low, subdiabetogenic doses of streptozotocin. beta-Cell injury resulted in GAD65 release into the circulation in a dose-dependent manner, and low-dose streptozotocin resulted in a more gradual increase in plasma GAD65 levels than did diabetogenic doses. Plasma GAD65 levels were reduced in rats that had undergone partial pancreatectomy and remained undetectable in mice. Together, these data demonstrate that GAD65 can be released into the circulation by injured beta-cells. Autoantigen shedding may contribute to the pathogenesis of islet autoimmunity in the multiple low-dose streptozocin model and perhaps, more generally, in other forms of autoimmune diabetes. These results demonstrate that, as is true with other tissues, islet injury, at least in some circumstances, can be monitored by use of discharged, circulating proteins. GAD65 is the first such confirmed protein marker of islet injury.

A highly sensitive immunoassay resistant to autoantibody interference for detection of the diabetes-associated autoantigen glutamic acid decarboxylase 65 in blood and other biological samples

BACKGROUND

Glutamic acid decarboxylase-65 (GAD65) is a major autoantigen in autoimmune diabetes and is discharged from injured islet beta cells. GAD65 may also be released by transplanted islets undergoing immunological rejection. To test hypotheses regarding the utility of GAD65 as a biomarker for transplant rejection or diabetes-associated islet damage and also regarding the timing and instigators of GAD65 release in humans or animal models, a sensitive assay capable of measuring GAD65 in serum or plasma will be necessary. Ideally, this assay would also be resistant to interference by anti-GAD65 autoantibodies.

METHODS

A novel, magnetic bead-based assay was developed based on GAD65 capture by a monoclonal antibody directed to the only region of the protein known not to be significantly targeted by autoantibodies. A subsequent denaturation step allows sensitive immunodetection to proceed using anti-GAD65 polyclonal antibodies that would otherwise potentially be blocked by bound autoantibodies.

RESULTS

The GAD65 assay worked equally well with serum and plasma as with a solution of bovine serum albumin (BSA). The limit of blank was 31 pg/mL and did not differ significantly in the BSA solution (27 pg/mL). Mean recovery of GAD65 from the plasma of control subjects and GAD65 autoantibody-positive and -negative subjects with type 1 diabetes was 101 +/- 4.6%, 88 +/- 7.8%, and 99 +/- 7.0% (+/- SEM), respectively. The assay was used to quantify both recombinant GAD65 and the GAD65 content of human and rodent islets and other tissue extracts that were added to human plasma samples.

CONCLUSIONS

A sensitive, autoantibody-resistant GAD65 assay has been developed that is compatible with detection in serum and plasma and therefore will likely also work with a variety of other biologic fluids. This assay may enable the use of circulating GAD65 as a biomarker of islet damage or transplant rejection and will facilitate in vivo studies of the pathogenesis of anti-GAD65 autoreactivity.

Identification and characterization of a novel isoform of the vesicular gamma-aminobutyric acid transporter with glucose-regulated expression in rat islets

Pancreatic islets are unique outside the nervous system in that they contain high levels of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), synthesized by the enzyme glutamic acid decarboxylase (GAD). Since the role that GABA plays in the islet and the mechanisms whereby the two major GAD isoforms (GAD65 and GAD67) function as diabetes-associated autoantigens are unknown, continued characterization of the islet GAD-GABA system is important. We previously demonstrated that the GABA and glycine transporter vesicular inhibitory amino acid transporter (VIAAT also known as VGAT) is present in rat islets. Here we identify a novel 52 kDa variant of VIAAT in rat islets: VIAAT-52 (V52). V52 is an amino-terminally truncated form of VIAAT (V57) that likely results from utilization of a downstream start site of translation. V57 and V52 display different patterns of post-translational modification and cellular expression. Our results have indicated that islet content of V52, but not V57, is responsive to changes in glucose concentration and other extracellular conditions. VIAAT is expressed in the islet alpha cells, but there have been conflicting findings regarding the presence of VIAAT in the beta cells. Here we have also provided additional evidence for the presence of VIAAT in islet beta cells and show that the beta cell line INS-1 expresses V57. V52 may be better adapted than V57 to the unique rat alpha cell GAD-GABA system, which lacks GAD65 and in which VIAAT traffics to secretory granules rather than just to synaptic microvesicles.

Immune reactivity to GAD25 in type 1 diabetes mellitus

While both isoforms of glutamic acid decarboxylase (GAD) function as important autoantigens in autoimmune diabetes mellitus-GAD65 in humans and GAD67 in the NOD mouse-GAD67 is not synthesized in human pancreatic islets and is thought not to be an autoantigen in human diabetes. We have recently shown, however, that human islets contain a GAD67 splice variant: GAD25. Given the evidence that GAD67 could be a key diabetogenic autoantigen in the NOD mouse and the high prevalence of GAD65 autoantibodies in human type 1 diabetes, it became important to ask whether there is also immune reactivity to GAD25 in type 1 diabetes-possibly implicating it in the pathogenesis of the disease-and whether GAD25 reactivity could, like GAD65 reactivity, function as a clinically useful marker for the disease. We also hypothesized that the presence of autoantibodies to the smaller splice variant could be a cause of the up to 30% prevalence of GAD67 autoreactivity associated with type 1 diabetes. We therefore analyzed GAD25 reactivity in 105 newly-diagnosed children with type 1 diabetes and 74 control subjects. While 14 (13%) of the diabetic subjects were positive for GAD67 autoantibodies, only 3 (3%) were positive for GAD25 reactivity, none of which were GAD67 antibody-positive. Analysis of reactivity to a GAD67 chimera was consistent with GAD67 binding activity being due to cross-reactive GAD65 antibodies. Immunostaining confirmed the presence of GAD25 in human islets, revealing GAD25-positive cells to be sparse. Our results indicate that autoreactivity to GAD25 is rare in newly diagnosed type 1 diabetes and does not underlie GAD67 reactivity.

Expression of the vesicular inhibitory amino acid transporter in pancreatic islet cells: distribution of the transporter within rat islets

gamma-Aminobutyric acid (GABA) is stored in microvesicles in pancreatic islet cells. Because GAD65 and GAD67, which catalyze the formation of GABA, are cytoplasmic, the existence of an islet vesicular GABA transporter has been postulated. Here, we test the hypothesis that the putative transporter is the vesicular inhibitory amino acid transporter (VIAAT), a neuronal transmembrane transporter of GABA and glycine. We sequenced the human VIAAT gene and determined that the human and rat proteins share over 98% sequence identity. In vitro expression of VIAAT and immunoblotting of brain and islet lysates revealed two forms of the protein: an approximately 52-kDa and an approximately 57-kDa form. By immunoblotting and immunohistochemistry, we detected VIAAT in rat but not human islets. Immunohistochemical staining showed that in rat islets, the distribution of VIAAT expression parallels that of GAD67, with increased expression in the mantle. GABA, too, was found to be present in islet non-beta-cells. We conclude that VIAAT is expressed in rat islets and is more abundant in the mantle and that expression in human islets is very low or nil. The rat islet mantle differs from rat and human beta-cells in that it contains only GAD67 and relatively increased levels of VIAAT. Cells that express only GAD67 may require higher levels of VIAAT expression.

Alternative splicing of GAD67 results in the synthesis of a third form of glutamic-acid decarboxylase in human islets and other non-neural tissues

Two forms of glutamic-acid decarboxylase (GAD) have been identified in mammalian tissues: a 65-kDa form (GAD65) and a 67-kDa form (GAD67). Alternate splicing produces one or two smaller variants of GAD67 in the brain of embryonic mice and rats. Additionally, a short, heretofore unidentified transcript homologous to GAD67 has been detected in human testis RNA. Because GAD, the enzyme responsible for gamma-aminobutyric acid production and a key autoantigen in type I diabetes, has unclear function in non-neural tissue, it is important to understand its pattern of expression. Unlike GAD65, GAD67 is not produced in human pancreatic islets. Here, we describe a novel splice variant of GAD67 that is produced in human islets, testis, adrenal cortex, and perhaps other endocrine tissues, but not in brain. This transcript directs the synthesis of a protein without GAD enzymatic activity: GAD25. A unique peptide sequence at the carboxyl terminus of GAD25 is highly conserved between mice, rats, and humans. We conclude that humans produce a third form of GAD in non-neural tissues and that human islets, although they do not synthesize full-length GAD67, do express this shortened variant.

Association between baseline plasma leptin levels and subsequent development of diabetes in Japanese Americans

OBJECTIVE

Plasma leptin levels correlate strongly with increased total adipose tissue, a known risk factor for type 2 diabetes, yet the role of leptin in the etiology of diabetes remains unclear. We sought to determine whether leptin is a risk factor for development of diabetes in Japanese Americans.

RESEARCH DESIGN AND METHODS

We compared baseline leptin levels in 370 nondiabetic Japanese Americans who remained nondiabetic for 5-6 years of follow-up with those of 40 nondiabetic Japanese Americans who developed diabetes during follow-up. All participants had computed tomography measurements of baseline subcutaneous chest, abdomen, thigh, and intra-abdominal fat, with total fat defined as the sum of all these measurements.

RESULTS

The mean age was 51.7 +/- 11.7 years for men and 51.9 +/- 12.0 years for women. The 23 men who developed diabetes had significantly higher leptin levels than the 212 men who remained nondiabetic (P < 0.01). Among men, baseline leptin levels predicted diabetes risk independent of baseline total fat, insulin, insulin resistance, glucose, or age in separate multiple logistic regression models (relative risk adjusted for baseline total fat = 1.80 per SD increase [2.7 ng/ml], 95% CI 1.02-3.17). This association was particularly strong among men in the top decile for intra-abdominal fat. In contrast, the 17 women who developed diabetes had leptin levels similar to those of the 158 women who remained nondiabetic (P = 0.31).

CONCLUSIONS

Among Japanese Americans, increased baseline leptin levels are associated with increased risk of developing diabetes in men but not in women.

Increased plasma leptin levels are associated with fat accumulation in Japanese Americans

Although the hormone leptin seems to play a role in ensuring the maintenance of adequate energy stores and thereby protects against starvation, its role in the regulation of body weight and adiposity under normal circumstances is unclear. Overweight individuals have markedly elevated circulating leptin levels, suggesting that leptin's effect on food intake and thermogenesis is diminished or absent in obesity. Recent evidence, though, indicates that weight gain in Pima Indians is associated with relatively decreased levels of the hormone. Because it is important to understand whether a deficiency in circulating leptin contributes to the development of obesity, we sought to determine whether there is a relationship between leptin levels and subsequent changes in adiposity in a more typical population. We compared baseline plasma leptin concentrations to changes over 5 years in body weight, BMI, and computed tomography-determined total fat in 492 second- and third-generation Japanese Americans. Subjects were of 100% Japanese ancestry; male subjects had a mean BMI at baseline of 25.4 kg/m2 and a mean age of 54 years; female subjects had a mean BMI of 23.1 kg/m2 and a mean age of 53 years. Changes in weight (men: r = 0.17, P < 0.05; women: r = 0.20, P < 0.05), BMI (men: r = 0.17, P < 0.05; women: r = 0.18, P < 0.05), and total fat (men: r = 0.19, P < 0.05; women: r = 0.20, P < 0.01) were positively correlated with baseline leptin levels adjusted for baseline adiposity, fasting insulin, and age. In Japanese Americans, then, relatively increased leptin levels are associated with greater subsequent gains in weight and adiposity. We concluded that in this population, fat accumulation is associated not with leptin deficiency but possibly with leptin resistance and is preceded by increased leptin levels.

Endoplasmic reticulum-mediated quality control of type I collagen production by cells from osteogenesis imperfecta patients with mutations in the pro alpha 1 (I) chain carboxyl-terminal propeptide which impair subunit assembly

A heterozygous single base change in exon 49 of COL1A1, which converted the codon for pro alpha 1(I) carboxyl-terminal propeptide residue 94 from tryptophan (TGG) to cysteine (TGT) was identified in a baby with lethal osteogenesis imperfecta (OI64). The C-propeptide mutations in OI64 and in another lethal osteogenesis imperfecta cell strain (OI26), which has a frameshift mutation altering the sequence of the carboxyl-terminal half of the propeptide (Bateman, J. F., Lamande, S. R., Dahl, H.-H. M., Chan, D., Mascara, T. and Cole, W. G. (1989) J. Biol. Chem. 264, 10960-10964), disturbed procollagen folding and retarded the formation of disulfide-linked trimers. Although assembly was delayed, the presence of slowly migrating, overmodified alpha 1(I) and alpha 2(I) chains indicated that mutant pro alpha 1(I) could associate with normal pro alpha 1(I) and pro alpha 2(I) to form pepsin-resistant triple-helical molecules, a proportion of which were secreted. Further evidence of the aberrant folding of mutant procollagen in OI64 and OI26 was provided by experiments demonstrating that the endoplasmic reticulum resident molecular chaperone BiP, which binds to malfolded proteins, was specifically bound to type I procollagen and was coimmunoprecipitated in the osteogenesis imperfecta cells but not control cells. Experiments with brefeldin A, which inhibits protein export from the endoplasmic reticulum, demonstrated that unassembled mutant pro alpha 1(I) chains were selectively degraded within the endoplasmic reticulum resulting in reduced collagen production by the osteogenesis imperfecta cells. This biosynthetic deficiency was reflected in the inability of OI64 and OI26 cells to produce a substantial in vitro collagenous matrix when grown in the continuous presence of ascorbic acid to allow collagen matrix formation. Both these carboxyl-terminal propeptide mutants showed a marked reduction in collagen accumulation to 20% (or less) of control cultures, comparable to the reduced collagen content of tissues from OI26.

Mutations in the carboxyl-terminal propeptide of the pro alpha 1(I) chain of type I collagen result in defective chain association and produce lethal osteogenesis imperfecta

Fibroblast cell strains from three infants with perinatal lethal osteogenesis imperfecta (OI) carry unique mutations in COL1A1 (the gene encoding the pro alpha 1(I) chain of type I procollagen) that impair chain association. The three mutations are: substitution of arginine for leucine at position 170 in the carboxyl-terminal propeptide, substitution of histidine for aspartic acid at position 59 of the propeptide, and a 6-base pair deletion that results in loss of 2 amino acid residues, glutamic acid and tyrosine, at positions 119 and 120 of the propeptide. In each cell strain the rate of association of the abnormal chain into molecules is slowed and formation of interchain disulfide bonds is impaired. The substitution at residue 59 alters the most highly conserved region of the propeptide and has a much greater effect on the incorporation of pro alpha 2(I) chains into disulfide-bonded trimers than the other two mutations. The molecules that contain the abnormal chains are extensively overmodified by post-translational modifying enzymes, yet have a normal thermal stability. These findings extend the range of lethal mutations in the type I collagen genes and help to identify regions of the carboxyl-terminal propeptide that may be important for chain-chain recognition and molecular assembly.

BiP binds type I procollagen pro alpha chains with mutations in the carboxyl-terminal propeptide synthesized by cells from patients with osteogenesis imperfecta

Of 20 fibroblast cell strains from patients with osteogenesis imperfecta (OI), a disease caused by mutations in the genes encoding type I procollagen, three had increased synthesis of BiP (GRP78), an hsp70-related, endoplasmic reticulum-resident protein. All three strains carry unique mutations in pro alpha 1(I) chains which impair type I procollagen chain association. Immunoprecipitation and pulse-chase experiments show that BiP (immunoglobulin heavy chain-binding protein) stably binds pro alpha 1(I) chains in these three cell strains after a brief lag. Ascorbate, which increases procollagen synthesis, increases BiP synthesis and content in these three strains and not in the others. In one of these three strains, BiP content is constitutively elevated prior to ascorbate treatment, and BiP is less inducible. This strain also has relatively high levels of synthesis and content of GRP94, another endoplasmic reticulum-resident stress protein. Pretreating each of the three cell strains to increase their BiP content reduces subsequent ascorbate-mediated BiP induction. BiP synthesis in the 17 other OI strains examined, which had a variety of type I procollagen mutations, was normal. These results suggest that BiP is induced by and binds procollagen with specific types of mutations: ones in the carboxyl-terminal propeptide that interfere with chain association. The recognition by BiP of such procollagen in OI cell strains shows that BiP plays a role in the physiological response to the production of some disease-producing abnormal proteins.

Defective folding and stable association with protein disulfide isomerase/prolyl hydroxylase of type I procollagen with a deletion in the pro alpha 2(I) chain that preserves the Gly-X-Y repeat pattern

We have studied the folding, processing, and association with two endoplasmic reticulum (ER) resident proteins of the abnormal type I procollagen molecules produced by a strain of fibroblasts harboring a 4.5 kilobase deletion in an allele of COL1A2 (Willing, M. C., Cohn, D.H., Starman, B. Holbrook, K.A., Greenberg, C.R., and Byers, P.H. (1988) J. Biol. Chem. 263, 8398-8404). By sequencing cDNA, we found that the mutant allele encodes pro alpha 2(I) chains that are shortened by 180 amino acids but retain the Gly-X-Y repeat pattern crucial for collagen triple helix formation. The type I procollagen molecules that incorporated the shortened chain were retained intracellularly and were stable. The triple helical domain in these molecules did not attain a normal conformation and remained accessible to posttranslational modifying enzymes amino-terminal to the deletion site for a prolonged period. The abnormal molecules folded into a triple helical conformation more slowly than the normal molecules, and the amino-terminal ends of the pro alpha 1(I) chains failed to become protease-resistant. While the abnormal procollagen molecules were not bound by the ER-resident protein BiP, they stably associated with protein disulfide isomerase, the beta-subunit of prolyl-4-hydroxylase. These results indicate that some mutations in type I collagen genes both transiently delay folding and permanently disrupt the structure of the triple helix and suggest that binding to prolyl-4-hydroxylase helps to retain certain abnormal procollagen molecules within the ER.