Substitution of Leu for Pro-193 in the insulin receptor in a patient with a genetic form of severe insulin resistance

Unraveling stress resilience: Insights from adaptations to extreme environments by Astyanax mexicanus cavefish

Extreme environmental conditions have profound impacts on shaping the evolutionary trajectory of organisms. Exposure to these conditions elicits stress responses, that can trigger phenotypic changes in novel directions. The Mexican Tetra, Astyanax mexicanus, is an excellent model for understanding evolutionary mechanisms in response to extreme or new environments. This fish species consists of two morphs; the classical surface-dwelling fish and the blind cave-dwellers that inhabit dark and biodiversity-reduced ecosystems. In this review, we explore the specific stressors present in cave environments and examine the diverse adaptive strategies employed by cave populations to not only survive but thrive as successful colonizers. By analyzing the evolutionary responses of A. mexicanus, we gain valuable insights into the genetic, physiological, and behavioral adaptations that enable organisms to flourish under challenging environmental conditions.

The Mexican Tetra, Astyanax mexicanus, as a Model System in Cell and Developmental Biology

Our understanding of cell and developmental biology has been greatly aided by a focus on a small number of model organisms. However, we are now in an era where techniques to investigate gene function can be applied across phyla, allowing scientists to explore the diversity and flexibility of developmental mechanisms and gain a deeper understanding of life. Researchers comparing the eyeless cave-adapted Mexican tetra, Astyanax mexicanus, with its river-dwelling counterpart are revealing how the development of the eyes, pigment, brain, cranium, blood, and digestive system evolves as animals adapt to new environments. Breakthroughs in our understanding of the genetic and developmental basis of regressive and constructive trait evolution have come from A. mexicanus research. They include understanding the types of mutations that alter traits, which cellular and developmental processes they affect, and how they lead to pleiotropy. We review recent progress in the field and highlight areas for future investigations that include evolution of sex differentiation, neural crest development, and metabolic regulation of embryogenesis.

Subterranean life: Behavior, metabolic, and some other adaptations of Astyanax cavefish

The ability of fishes to adapt to any aquatic environment seems limitless. It is enthralling how new species keep appearing at the deep sea or in subterranean environments. There are close to 230 known species of cavefishes, still today the best-known cavefish is Astyanax mexicanus, a Characid that has become a model organism, and has been studied and scrutinized since 1936. There are two morphotypes for A. mexicanus, a surface fish and a cavefish. The surface fish lives in central and northeastern Mexico and south of the United States, while the cavefish is endemic to the "Sierra del Abra-Tanchipa region" in northeast Mexico. The extensive genetic and genomic analysis depicts a complex origin for Astyanax cavefish, with multiple cave invasions and persistent gene flow among cave populations. The surface founder population prevails in the same region where the caves are. In this review, we focus on both morphotype's main morphological and physiological differences, but mainly in recent discoveries about behavioral and metabolic adaptations for subterranean life. These traits may not be as obvious as the troglomorphic characteristics, but are key to understand how Astyanax cavefish thrives in this environment of perpetual darkness.

The integrative biology of type 2 diabetes

Obesity and type 2 diabetes are the most frequent metabolic disorders, but their causes remain largely unclear. Insulin resistance, the common underlying abnormality, results from imbalance between energy intake and expenditure favouring nutrient-storage pathways, which evolved to maximize energy utilization and preserve adequate substrate supply to the brain. Initially, dysfunction of white adipose tissue and circulating metabolites modulate tissue communication and insulin signalling. However, when the energy imbalance is chronic, mechanisms such as inflammatory pathways accelerate these abnormalities. Here we summarize recent studies providing insights into insulin resistance and increased hepatic gluconeogenesis associated with obesity and type 2 diabetes, focusing on data from humans and relevant animal models.

Leucine Rich Repeat Proteins: Sequences, Mutations, Structures and Diseases

Mutations in the genes encoding Leucine Rich Repeat (LRR) containing proteins are associated with over sixty human diseases; these include high myopia, mitochondrial encephalomyopathy, and Crohn's disease. These mutations occur frequently within the LRR domains and within the regions that shield the hydrophobic core of the LRR domain. The amino acid sequences of fifty-five LRR proteins have been published. They include Nod-Like Receptors (NLRs) such as NLRP1, NLRP3, NLRP14, and Nod-2, Small Leucine Rich Repeat Proteoglycans (SLRPs) such as keratocan, lumican, fibromodulin, PRELP, biglycan, and nyctalopin, and F-box/LRR-repeat proteins such as FBXL2, FBXL4, and FBXL12. For example, 363 missense mutations have been identified. Replacement of arginine, proline, or cysteine by another amino acid, or the reverse, is frequently observed. The diverse effects of the mutations are discussed based on the known structures of LRR proteins. These mutations influence protein folding, aggregation, oligomerization, stability, protein-ligand interactions, disulfide bond formation, and glycosylation. Most of the mutations cause loss of function and a few, gain of function.

Insulin resistance in cavefish as an adaptation to a nutrient-limited environment

Periodic food shortages are a major challenge faced by organisms in natural habitats. Cave-dwelling animals must withstand long periods of nutrient deprivation, as-in the absence of photosynthesis-caves depend on external energy sources such as seasonal floods. Here we show that cave-adapted populations of the Mexican tetra, Astyanax mexicanus, have dysregulated blood glucose homeostasis and are insulin-resistant compared to river-adapted populations. We found that multiple cave populations carry a mutation in the insulin receptor that leads to decreased insulin binding in vitro and contributes to hyperglycaemia. Hybrid fish from surface-cave crosses carrying this mutation weigh more than non-carriers, and zebrafish genetically engineered to carry the mutation have increased body weight and insulin resistance. Higher body weight may be advantageous in caves as a strategy to cope with an infrequent food supply. In humans, the identical mutation in the insulin receptor leads to a severe form of insulin resistance and reduced lifespan. However, cavefish have a similar lifespan to surface fish and do not accumulate the advanced glycation end-products in the blood that are typically associated with the progression of diabetes-associated pathologies. Our findings suggest that diminished insulin signalling is beneficial in a nutrient-limited environment and that cavefish may have acquired compensatory mechanisms that enable them to circumvent the typical negative effects associated with failure to regulate blood glucose levels.

Sequencing analysis of insulin receptor defects and detection of two novel mutations in INSR gene

Mutations in the insulin receptor gene cause the inherited insulin resistant syndromes Leprechaunism and Rabson–Mendenhall syndrome. These recessive conditions are characterized by intrauterine and post-natal growth restrictions, dysmorphic features, altered glucose homeostasis, and early demise. The insulin receptor gene (INSR) maps to the short arm of chromosome 19 and is composed of 22 exons. Here we optimize the conditions for sequencing this gene and report novel mutations in patients with severe insulin resistance.

Methods

PCR amplification of the 22 coding exons of the INSR gene was performed using M13-tailed primers. Bidirectional DNA sequencing was performed with BigDye Terminator chemistry and M13 primers and the product was analyzed on the ABI 3100 genetic analyzer. Data analysis was performed using Mutation Surveyor software comparing the sequence to a reference INSR sequence (Genbank NC_000019).

Results

We sequenced four patients with Leprechaunism or Rabson–Mendenhall syndromes as well as seven samples from normal individuals and confirmed previously identified mutations in the affected patients. Three of the four mutations identified in this group caused premature insertion of a stop codon. In addition, the INSR gene was sequenced in 14 clinical samples from patients with suspected insulin resistance and one novel mutation was found in an infant with a suspected diagnosis of Leprechaunism.

Discussion

Leprechaunism and Rabson–Mendenhall syndrome are very rare and difficult to diagnose. Diagnosis is currently based mostly on clinical criteria. Clinical availability of DNA sequencing can provide an objective way of confirming or excluding the diagnosis.

Two novel mutations identified in familial cases with Donohue syndrome

Donohue syndrome (DS) is a rare and lethal autosomal recessive disease caused by mutations in the insulin receptor (INSR) gene, manifesting marked insulin resistance, severe growth retardation, hypertrichosis, and characteristic dysmorphic features. We report the clinical, molecular, and biochemical characterization of three new patients with DS, and address genotype–phenotype issues playing a role in the pathophysiology of DS. A female infant born to first-degree cousins Muslim Arab parents and two brothers born to first-degree cousins Druze parents presented classical features of DS with hypertrophic cardiomyopathy and died in infancy. Each patient was found homozygous for one missense mutation within the extracellular domain of the INSR gene. Western blot analysis identified the proreceptor of INSR, but not its mature subunits alpha and beta. Of 95 healthy Muslims, no heterozygous was found and of 52 healthy Druze from the same village, one was heterozygous. This study presents two novel familial mutations in the alpha subunit of the INSR which appear to impair post-translational processing of the INSR, resulting loss of its function. Both mutations cause DS with hypertrophic cardiomyopathy and early death. Identification of the causative mutation enables prevention of this devastating disease.

Rabson-Mendenhall syndrome with recurrent cerebral infarcts caused by a novel INSR mutation

Rabson-Mendenhall syndrome (RMS) is an autosomal recessive disorder characterized by extreme insulin resistance and certain characteristic phenotypic features. The primary defect lies in the insulin receptor and involves biallelic mutations that lead to a loss of function through various postulated defects. We present a 9-year-old female patient with RMS complicated by multiple cerebral infarcts, in whom genetic analysis revealed a novel insulin receptor mutation.

Two novel mutations in the insulin binding subunit of the insulin receptor gene without insulin binding impairment in a patient with Rabson-Mendenhall syndrome

Homozygous or compound heterozygous mutations within the insulin binding domain of the human insulin receptor (INSR) are usually associated with severe impairment of insulin binding leading to Donohue syndrome ("Leprechaunism"), which is characterized by excessive hyperglycemia with hyperinsulinism, pre- and postnatal growth retardation, distinct dysmorphism and early death. Missense mutations in the beta subunits are commonly associated with a milder impairment of insulin binding and milder phenotype with prolonged survival and less dysmorphism, the so called Rabson-Mendenhall syndrome. We report on a 13-year-old girl with Donohue syndrome like dysmorphism, hyperinsulinism and prolonged survival due to two novel INSR missense mutations within the insulin binding domain. Unexpectedly, insulin binding assays and investigations of activation of central insulin signaling pathways in fibroblasts revealed no significant alterations. Instead, immunofluorescence studies showed abnormal perinuclear distribution of the INSR alpha and beta subunits. Our data indicate that the quality of insulin binding activity is correlated with survival, not with the dysmorphic phenotype, and it is not always a valid parameter for predicting INSR mutations as proposed.

Identification of novel C253Y missense and Y864X nonsense mutations in the insulin receptor gene in type A insulin-resistant patients

Mutations in the insulin receptor gene have been reported in cases of type A insulin resistance. We report herein two cases of type A insulin resistance, which involve some novel mutations. Case 1 is a heterozygote of the C253Y missense mutation and case 2 is a heterozygote of the Y864X nonsense mutation. In the C253Y missense mutation in exon 3, a cysteine residue is replaced with tyrosine in the cysteine-rich domain of the alpha subunit. The Y864X in exon 13 results in a truncated receptor, which is devoid of most of the beta subunit. This mutant receptor could not be expressed on a cell membrane since the transmembrane domain is missing. Other significant mutations were not found for the entire coding regions and splice/donor sites.

Insulin receptor antibodies inhibit insulin uptake by the liver: in vivo 123I-insulin scintigraphic scanning and in vitro characterization in autoimmune hypoglycemia

BACKGROUND

Insulin receptor antibodies can induce severe hypoglycemia or insulin resistance in rare autoimmune syndromes. In vitro properties of these antibodies occasionally explain the clinical features of the syndrome, but direct evidence of their in vivo activity is poor. We studied a 58-year-old male with rheumatoid arthritis who presented with hypoglycemic coma.

METHODS AND RESULTS

Antibodies were detected by inhibition of 125I-insulin binding to human insulin receptor-3T3 cells by the patient's serum. By immunofluorescence, they were immunoglobulin G of all four subclasses, immunoprecipitated insulin receptors from biotin-labeled cells, and triggered phosphorylation of the beta subunit of the insulin receptor. Insulin binding on the patient's red blood cells was markedly reduced. A biodistribution study after intravenous 123I-Tyr A14 insulin showed a marked inhibition of tracer uptake by the liver, reaching 10% of the injected dose (controls, mean +/- SD, 21.1 +/- 1.7%; n = 10). Time activity curves generated on the liver and on the heart were parallel, with a T1/2 of 11.5 minutes for both, suggesting that no specific uptake occurred in the liver, where tracer activity represented only the blood pool. Clearance of insulin from the blood was indeed slower than in controls and mainly occurred through the kidneys. Analysis of plasma 123I-insulin immunoreactivity and trichloroacetic acid precipitate showed that insulin degradation did not occur as in normal controls.

CONCLUSIONS

In this patient with hypoglycemic syndrome, insulin receptor antibodies with in vitro insulin-like activity are capable of blocking in vivo the access of insulin to the liver receptor compartment, as directly demonstrated by the markedly altered biodistribution of intravenously injected 123I-insulin.

Analysis of a naturally occurring mutation in sucrase-isomaltase: glutamine 1098 is not essential for transport to the surface of COS-1 cells

A glutamine for proline substitution at position 1098 was previously shown to result in accumulation of brush-border sucrase-isomaltase in the Golgi apparatus. The substitution is present in a highly homologous region of the protein, and results in a comparable accumulation when introduced into the same region in lysosomal alpha-glucosidase. To study the importance of the glutamine-1098, we analyzed the transport compatibility of two mutants in which glutamine-1098 is substituted by lysine or alanine. Both mutants were transported to the cell surface and processed comparable to wild type. We concluded that glutamine-1098 is not essential for transport to the cell surface.

Major circadian variations of glucose homeostasis in a patient with Rabson-Mendenhall syndrome and primary insulin resistance due to a mutation (Cys284-->Tyr) in the insulin receptor alpha-subunit

We have performed clinical, in vitro biochemical, and genetic studies of a patient with severe insulin resistance, considerable growth restriction, and Rabson-Mendenhall syndrome (patient RM-3). The blood IGF-I level was undetectable in this patient, although the GH level was moderately decreased. During the postprandial period, glycemia, ketonuria, and plasma glucagon were very elevated despite high doses of exogenous insulin (glucose levels up to 30 mmol/L). In the postabsorptive state, blood glucose was normalized with small amounts of insulin; ketonuria, and glucagon levels were reduced but remained supranormal. Erythrocytes and cultured skin fibroblasts from the patient displayed a decrease in cell surface insulin receptors (IRs). The ability of physiologic concentrations of insulin to stimulate metabolic processes was altered in patient fibroblasts. Analysis of the IR gene by denaturing gradient gel electrophoresis and direct sequencing showed a homozygous missense mutation in exon 3, replacing Cys284 by Tyr in the alpha-subunit. In conclusion, marked primary insulin resistance was evidenced in patient cells as a result of a structural alteration in the IR alpha-subunit. The in vitro studies could not account alone for the in vivo metabolic alterations because glucose homeostasis varied considerably during the day in the patient.

Mutant insulin receptors in syndromes of insulin resistance

To date, mutations of the insulin receptor remain the only well-established causes of severe insulin resistance. There is a broad correlation between the extent of impairment of signal transduction seen when the mutant receptors are expressed in vitro with the severity of the clinical phenotype. Thus leprechaunism, Rabson-Mendenhall syndrome and Type A insulin resistance appear to represent points on a continuum of severity of receptor dysfunction, rather than completely distinct syndromes. In other syndromes of insulin resistance, insulin receptor abnormalities remain the exception. However, functional studies of expressed naturally occurring insulin receptor mutations have acted as experiments of nature and greatly aided attempts to dissect the structure-function relationships of the receptor. The next few years will no doubt begin to reveal the contributions made by defects in the post-receptor signalling cascade to the syndromes of insulin resistance in man.