Maintenance of a reduced body weight is accompanied by a decrease in energy expenditure beyond that accounted for by reduced body mass and composition, as well as by an increased drive to eat. These effects appear to be due— in part—to reductions in circulating leptin concentrations due to loss of body fat. Gut microbiota have been implicated in the regulation of body weight. The effects of weight loss on qualitative aspects of gut microbiota have been studied in humans and mice, but these studies have been confounded by concurrent changes in diet composition, which influence microbial community composition. We studied the impact of 20% weight loss on the microbiota of diet-induced obese (DIO: 60% calories fat) mice on a high-fat diet (HFD). Weight-reduced DIO (DIO-WR) mice had the same body weight and composition as control (CON) adlibitum (AL) fed mice being fed a control diet (10% calories fat), allowing a direct comparison of diet and weight-perturbation effects. Microbial community composition was assessed by pyrosequencing 16S rRNA genes derived from the ceca of sacrificed animals. There was a strong effect of diet composition on the diversity and composition of the microbiota. The relative abundance of specific members of the microbiota was correlated with circulating leptin concentrations and gene expression levels of inflammation markers in subcutaneous white adipose tissue in all mice. Together, these results suggest that both host adiposity and diet composition impact microbiota composition, possibly through leptin-mediated regulation of mucus production and/or inflammatory processes that alter the gut habitat.
Previous genome-wide association studies have identified a strong
association between FTO and human obesity, although the
mechanism by which FTO affects obesity remains unknown. Anew
study suggests that the obesity risk alleles are gain-of-function.
To compare, in mice, the accuracy of estimates of energy expenditure using an energy balance technique (TEEbal : food energy intake and body composition change) versus indirect calorimetry (TEEIC).
In 32 male C57BL/6J mice energy expenditure was estimated using an energy balance (caloric intake minus change in body energy stores) method over a 37 day period. Energy expenditure was also measured in the same animals by indirect calorimetry. These measures were compared.
The two methods were highly correlated (r2 = 0.87: TEEbal = 1.07 * TEEIC − 0.22, p < 0.0001). By Bland-Altman analysis, TEEbal estimates were slightly higher (4.6±1.5%; p < 0.05) than TEEIC estimates (Bias = 0.55 kcal/24h).
TEEbal can be performed in “home cages” and provides an accurate integrated long-term measurement of energy expenditure while minimizing potentially confounding stress that may accompany the use of indirect calorimetry systems. The technique can also be used to assess long term energy intake.
Obesity; Indirect Calorimetry; Metabolism
“Diabetes and Oral Disease: Implications for Health Professionals” was a one-day conference convened by the Columbia University College of Dental Medicine, the Columbia University College of Physicians and Surgeons, and the New York Academy of Sciences on May 4, 2011in New York City. The program included an examination of the bidirectional relationship between oral disease and diabetes and the inter-professional working relationships for the care of people who have diabetes. The overall goal of the conference was to promote discussion among the healthcare professions who treat people with diabetes, encourage improved communication and collaboration among them and ultimately, improve patient management of the oral and overall effects of diabetes. Attracting over 150 members of the medical and dental professions from eight different countries, the conference included speakers from academia and government and was divided into four sessions. This report summarizes the scientific presentations of the event.
diabetes; oral disease; meeting report
Generation of surrogate sources of insulin-producing β-cells remains a goal of diabetes therapy. While most efforts have been directed at differentiating embryonic or induced pluripotent stem (iPS) cells into β-like-cells through endodermal progenitors, we have shown that gut endocrine progenitor cells of mice can be differentiated into glucose-responsive, insulin-producing cells by ablation of transcription factor Foxo1. Here we show that FOXO1 is present in human gut endocrine progenitor and serotonin-producing cells. Using gut organoids derived from human iPS cells, we show that FOXO1 inhibition using a dominant-negative mutant or lentivirus-encoded shRNA promotes generation of insulin-positive cells that express all markers of mature pancreatic β-cells, release C-peptide in response to secretagogues, and survive in vivo following transplantation into mice. The findings raise the possibility of using gut-targeted FOXO1 inhibition or gut organoids as a source of insulin-producing cells to treat human diabetes.
Type 2 diabetes (T2D) is a major risk factor for late-onset Alzheimer's disease (AD). A variety of metabolic changes related to T2D (e.g. hyperinsulinemia, hyperglycemia, and elevated branched-chain amino acids) have been proposed as mechanistic links, but the basis for this association remains unknown. Retromer-dependent trafficking is implicated in the pathogenesis of AD, and two key retromer proteins, VPS35 and VPS26, are deficient in the hippocampal formation of AD patients. We characterized VPS35 levels in five different mouse models of T2D/obesity to identify specific metabolic factors that could affect retromer in the brain. Mouse models in which hyperleucinemia was present displayed hippocampus-selective retromer deficiency. Wild-type lean mice fed a high leucine diet also developed hippocampal-selective retromer deficiency, and neuronal-like cells grown in high ambient leucine had reduced retromer complex proteins. Our results suggest that hyperleucinemia may account, in part, for the association of insulin resistance/T2D with AD.
Alzheimer's disease; Obesity; Diabetes; Retromer
The physiology of the weight reduced (WR) state suggests that pharmacologic agents affecting energy homeostasis may have greater efficacy in WR individuals. Our aim was to establish a protocol that allows for evaluation of efficacy of weight maintenance agents and to assess the effectiveness of AZD2820, a novel melanocortin 4 receptor agonist in such a paradigm.
Design and Methods
MC4R agonist was administered in stratified doses to mice who were either fed high fat diet ad libitum (AL) throughout the study; or stabilized at a 20% reduced body weight (BW), administered the drug for four weeks, and thereafter released from caloric restriction while continuing to receive the drug (WR).
After release of WR mice to AL feeding, the high-dose group (53.4nmol/day) regained 12.4% less BW than their vehicle-treated controls. In WR mice, 10.8nmol/day of the agonist was sufficient to maintain these animals at 95.1% of initial BW vs 53.4nmol/day required to maintain the BW of AL animals (94.5%).
In the WR state, the MC4R agonist was comparably efficacious to a 5-fold higher dose in the AL state. This protocol provides a model for evaluating the mechanisms and quantitative efficacy of weight-maintenance strategies and agents.
Obesity, which is frequently associated with diabetes, hypertension, and cardiovascular diseases, is primarily the result of a net excess of caloric intake over energy expenditure. Human obesity is highly heritable, but the specific genes mediating susceptibility in non-syndromic obesity remain unclear. We tested candidate genes in pathways related to food intake and energy expenditure for association with body mass index (BMI).
We re-analyzed 355 common genetic variants of 30 candidate genes in 7 molecular pathways related to obesity in 1,982 unrelated European Americans from the New York Health Project. Data were analyzed by using a Bayesian hierarchical generalized linear model. The BMIs were log-transformed and then adjusted for covariates including age, age2, gender, and diabetes status. The single nucleotide polymorphisms (SNPs) were modeled as additive effects.
With the stipulated adjustments, nine SNPs in eight genes were significantly associated with BMI: GHRL (rs35683), AGRP (rs5030980), CPE (rs1946816 and rs4481204), GLP1R (rs2268641), HTR2A (rs912127), NPY5R (Y5R1c52), SOCS3 (rs4969170), and STAT3 (rs4796793). We also found a gender-by-SNP interaction (rs1745837 in HTR2A), which indicated that variants in the gene HTR2A had a stronger association with BMI in males. In addition, NPY1R was detected as having a significant gene effect even though none of the SNPs in this gene was significant.
Variations in genes AGRP, CPE, GHRL, GLP1R, HTR2A, NPY1R, NPY5R, SOCS3, and STAT3 showed modest associations with BMI in European Americans. The pathways in which these genes participate regulate energy intake and thus these associations are mechanistically plausible in this context.
Obesity; genetic association; single nucleotide polymorphism (SNP); Bayesian hierarchical generalized linear model (BhGLM)
Maternal obesity can influence susceptibility to obesity and type 2 diabetes in progeny. We examined the relationship of maternal insulin resistance (IR), a metabolically important consequence of increased adiposity, to adverse consequences of obesity for fetal development. We used mice heterozygous for a null allele of the insulin receptor (Insr) to study the contributions of maternal IR to offspring phenotype without the potential confound of obesity per se, and how maternal consumption of high-fat diet (HFD) may, independently and interactively, affect progeny. In progeny fed a 60% HFD, body weight and adiposity were transiently (5–7 weeks) increased in wild-type (+/+) offspring of Insr+/− HFD-fed dams compared to offspring of wild-type HFD-fed dams. Offspring of HFD-fed wild-type dams had increased body weight, blood glucose, and plasma insulin concentrations compared to offspring of chow-fed wild-type dams. Quantification of proopiomelanocortin (POMC) and neuropeptide-Y (NPY) populations in the arcuate nucleus of the hypothalamus (ARH) of offspring of wild-type vs. Insr+/− dams was performed to determine whether maternal IR affects the formation of central feeding circuits. We found a 20% increase in the number of Pomc-expressing cells at postnatal day 9 in offspring of Insr+/− dams. In conclusion, maternal HFD consumption—distinct from overt obesity per se—was a major contributor to increased body weight, adiposity, IR, and liver triglyceride (TG) phenotypes in progeny. Maternal IR played a minor role in predisposing progeny to obesity and IR, though it acted synergistically with maternal HFD to exacerbate early obesity in progeny.
Hyperphagia is a central feature of inherited disorders (e.g., Prader–Willi Syndrome) in which obesity is a primary phenotypic component. Hyperphagia may also contribute to obesity as observed in the general population, thus raising the potential importance of common underlying mechanisms and treatments. Substantial gaps in understanding the molecular basis of inherited hyperphagia syndromes are present as are a lack of mechanistic of mechanistic targets that can serve as a basis for pharmacologic and behavioral treatments.
Design and Methods
International conference with 28 experts, including scientists and caregivers, providing presentations, panel discussions, and debates.
The reviewed collective research and clinical experience provides a critical body of new and novel information on hyperphagia at levels ranging from molecular to population. Gaps in understanding and tools needed for additional research were identified.
This report documents the full scope of important topics reviewed at a comprehensive international meeting devoted to the topic of hyperphagia and identifies key areas for future funding and research.
To understand the regulation of adipocyte size and adipokine expression in relation to gender, anatomic location, adiposity, and metabolic risk factors in adolescents with morbid obesity.
Design and Methods
Adipocyte size and adipokine expression in paired abdominal subcutaneous (SAT) and omental (VAT) surgical adipose tissues were related to gender, anatomic location, adiposity, and metabolic risk factors in a group of morbidly obese adolescents.
Significant depot- and/or gender-related differences in adipocyte size and adipokine expression were detected. Adjusted for BMI, adipocyte size in both depots was larger in males than in females and was a major predictor of mRNA levels of leptin, PAI-1, and adiponectin. Gender, but not adipocyte size, was significantly correlated with pro-inflammatory cytokine expression. BMI and waist circumference were correlated positively with VAT adipocyte size and negatively with SAT adipocyte size. VAT adiponectin and IL-6 expression levels were major predictors of HDL cholesterol concentrations, independent of gender, adiposity, and insulin sensitivity.
Adipose tissue morphology and function in obese adolescents are influenced by gender and anatomic location; the pattern of gender- and depot-related differences in adipocyte size and adipokine expression suggests that adolescent males, relative to the females, are at increased risk for obesity-related metabolic co-morbidities.
Perturbations in the functional integrity of the leptin axis are obvious candidates for mediation of altered adiposity. In a large number of genetic association studies in humans, the non-conservative LEPR Q223R allele has been inconsistently associated with adiposity. Subtle, long term effects of such genetic variants can be obscured by effects of the environment and other confounders that render definitive inferences difficult to reach. We directly assessed the biological effects of this variant in 129P3/J mice segregating for the humanized Lepr allele at codon 223. No effects of this allele were detected on body weight, composition, or energy expenditure in animals fed diets of varying fat content over periods as long as 235 days. In vitro, Q223R did not affect leptin signaling as reflected by activation of STAT3. We conclude that Q223R is unlikely to play a significant role in regulation of human adiposity. This approach to vetting of human allelic variation might be more widely employed.
genetics; obesity; leptin; mouse models
Two recent, large GWAS in European populations have associated a ∼47 Kb region that contains part of the FTO gene with high BMI. The functions of FTO and adjacent FTM in human biology are not clear. We examined expression of these genes in organs of mice segregating for monogenic obesity mutations, exposed to under/over feeding, and to 4 °C. Fto/Ftm expression was reduced in mesenteric adipose tissue of mice segregating for the Ay, Lepob, Leprdb, Cpefat or tub mutations and there was a similar trend in other tissues. These effects were not due to adiposity per se. Hypothalamic Fto and Ftm expression were decreased by fasting in lean and obese animals and by cold exposure in lean mice. The fact that responses of Fto and Ftm expression to these manipulations were almost indistinguishable suggested that the genes might be co-regulated. The putative overlapping regulatory region contains at least 2 canonical CUTL1 binding sites. One of these nominal CUTL1 sites includes rs8050136, a SNP associated with high body mass. The A allele of rs8050136 – associated with lower body mass than the C allele – preferentially bound CUTL1 in human fibroblast DNA. 70% knockdown of CUTL1 expression in human fibroblasts decreased FTO and FTM expression by 90 and 65 %, respectively. Animals and humans with various genetic interruptions of FTO or FTM have phenotypes reminiscent of aspects of the Bardet-Biedl obesity syndrome, a confirmed “ciliopathy”. FTM has recently been shown to be a ciliary basal body protein.
obesity; hypothalamus; adipose tissue; CUTL1
To examine genetic factors responsible for metabolic syndrome and atherosclerosis in a setting of LDL receptor deficiency in a cross between C57BL/6J (B6) and PERA/Ei (PERA) inbred mouse strains.
Methods and results
Comparison of metabolic phenotypes in B6 and PERA strains revealed the PERA genetic background to be dramatically more susceptible to hyperleptinemia, hyperglycemia, hypertriglyceridemia, elevated insulin levels and body fat increase than the B6 background. To facilitate genetic analysis, metabolic syndrome-related traits and atherosclerotic lesion area were measured in 167 [(PERA × B6.129S7-Ldlrtm1Her) × B6.129S7-Ldlrtm1Her]N2 male and female backcross mice, homozygous for the Ldlr null allele. Quantitative trait locus analysis was performed using 153 polymorphic microsatellite markers spanning the genome. On chromosome 4, we identified a locus influencing plasma triglyceride, insulin, and leptin concentrations, body weight, and atherosclerosis. Several other genetic loci were identified with separate effects on plasma insulin, body weight, HDL-C and atherosclerosis.
The PERA strain is highly susceptible to the development of metabolic syndrome after feeding a WTD. This susceptibility is due, in part, to a locus on murine chromosome 4 in which PERA alleles predispose to adiposity, increased insulin and accelerated atherogenesis in the absence of marked hyperlipidemia.
The mouse coat color mutant mahoganoid (md) darkens coat color and decreases the obesity of Ay mice that ectopically overexpress agouti-signaling protein. The phenotypic effects of md are similar to those of the recently identified coat color mutant mahogany (Atrnmg). We report the positional cloning of mahoganoid, encoding a novel 494–amino acid protein containing a C3HC4 RING (really interesting new gene) domain that may function as an E3 ubiquitin ligase. The mutations in the mahoganoid allelic series (md, md2J, md5J) are all due to large retroviral insertions. In md and md2J, the result is minimal expression of the normal size transcripts in all tissues examined. Unlike Atrnmg/Atrnmg animals, we observe no evidence of neurological deficit or neuropathology in md/md mice. Body weight and body mass index (a surrogate for adiposity) measurements of B6.C3H-md-A md/+ and md/md animals on 9% and 45% kcal fat diets indicate that mahoganoid does not suppress body weight in B6.C3H animals in a gene dose-dependent fashion.
Obesity is a major public health problem that increases risk for a broad spectrum of co-morbid conditions. Despite evidence for a strong genetic contribution to susceptibility to obesity, previous efforts to discover the relevant genes using positional cloning have failed to account for most of the apparent genetic risk variance.
Design and Methods
Deploying a strategy combining analysis of exome sequencing data in extremely obese members of four consanguineous families with segregation analysis, we screened for causal genetic variants. Filter-based analysis and homozygosity mapping were used to identify and prioritize putative functional variants.
We identified two novel frameshift mutations in the Leptin Receptor (LEPR) in two of the families.
These results provide proof-of-principle that whole-exome sequencing of families segregating for extreme obesity can identify causal pathogenic mutations. The methods described here can be extended to additional families segregating for extreme obesity and should enable the identification of mutations in novel genes that predispose to obesity.
genetics of obesity; leptin receptor
A low-error 16S rRNA amplicon sequencing method (LEA-Seq) plus whole genome sequencing of >500 cultured isolates were used to characterize bacterial strain composition in the fecal microbiota of 37 USA adults sampled for up to five years. Microbiota stability follows a power law function which, when extrapolated, suggests that most strains in an individual are residents for decades. Shared strains were recovered from family members, but not from unrelated individuals. Sampling individuals for up to 32 weeks while consuming a monotonous liquid diet indicated that changes in weight are more predictive of changes in strain composition than sampling interval. This combination of stability and responsiveness to physiologic change confirms the potential of the gut microbiota as a diagnostic tool and therapeutic target.
Here we outline how islet cells use autocrine and paracrine “circuits” of classical neurotransmitters and their corresponding receptors and transporters to communicate with vicinal β-cells to regulate glucose stimulated insulin secretion. Many of these same circuits operate in the CNS and can be visualized by molecular imaging. We discuss how these techniques might be applied to measuring the dynamics of β-cell function in real time.
β-cells; imaging; GSIS; PET
Ildr2, a modifier of diabetes susceptibility in obese mice, is expressed in most organs, including islets and hypothalamus, with reduced levels in livers of diabetes-susceptible B6.DBA mice congenic for a 1.8 Mb interval of Chromosome 1. In hepatoma and neuronal cells, ILDR2 is primarily located in the endoplasmic reticulum membrane. We used adenovirus vectors that express shRNA or are driven by the CMV promoter, respectively, to knockdown or overexpress Ildr2 in livers of wild type and ob/ob mice. Livers in knockdown mice were steatotic, with increased hepatic and circulating triglycerides and total cholesterol. Increased circulating VLDL, without reduction in triglyceride clearance suggests an effect of reduced hepatic ILDR2 on hepatic cholesterol clearance. In animals that overexpress Ildr2, hepatic triglyceride and total cholesterol levels were reduced, and strikingly so in ob/ob mice. There were no significant changes in body weight, energy expenditure or glucose/insulin homeostasis in knockdown or overexpressing mice. Knockdown mice showed reduced expression of genes mediating synthesis and oxidation of hepatic lipids, suggesting secondary suppression in response to increased hepatic lipid content. In Ildr2-overexpressing ob/ob mice, in association with reduced liver fat content, levels of transcripts related to neutral lipid synthesis and cholesterol were increased, suggesting “relief” of the secondary suppression imposed by lipid accumulation. Considering the fixed location of ILDR2 in the endoplasmic reticulum, we investigated the possible participation of ILDR2 in ER stress responses. In general, Ildr2 overexpression was associated with increases, and knockdown with decreases in levels of expression of molecular components of canonical ER stress pathways. We conclude that manipulation of Ildr2 expression in liver affects both lipid homeostasis and ER stress pathways. Given these reciprocal interactions, and the relatively extended time-course over which these studies were conducted, we cannot assign causal primacy to either the effects on hepatic lipid homeostasis or ER stress responses.
Diabetes is a disorder characterized by loss of β cell mass and/or β cell function, leading to deficiency of insulin relative to metabolic need. To determine whether stem cell–derived β cells recapitulate molecular-physiological phenotypes of a diabetic subject, we generated induced pluripotent stem cells (iPSCs) from subjects with maturity-onset diabetes of the young type 2 (MODY2), which is characterized by heterozygous loss of function of the gene encoding glucokinase (GCK). These stem cells differentiated into β cells with efficiency comparable to that of controls and expressed markers of mature β cells, including urocortin-3 and zinc transporter 8, upon transplantation into mice. While insulin secretion in response to arginine or other secretagogues was identical to that in cells from healthy controls, GCK mutant β cells required higher glucose levels to stimulate insulin secretion. Importantly, this glucose-specific phenotype was fully reverted upon gene sequence correction by homologous recombination. Our results demonstrate that iPSC-derived β cells reflect β cell–autonomous phenotypes of MODY2 subjects, providing a platform for mechanistic analysis of specific genotypes on β cell function.
The increasing prevalence of obesity and its co-morbidities reflects the interaction of genes that favor the storage of excess calories as fat with an environment that provides ad libitum availability of calorically dense foods and encourages an increasingly sedentary lifestyle. While weight reduction is difficult in and of itself, anyone who has every lost weight will confirm that it is much harder to keep the weight off once it has been lost. The over 80% recidivism rate to pre-weight loss levels of body fatness after otherwise successful weight loss is due to the coordinate actions of metabolic, behavioral, neuroendocrine, and autonomic responses designed to maintain body energy stores (fat) at a CNS- defined “ideal”. This “adaptive thermogenesis” creates the ideal situation for weight regain and is operant in both lean and obese individuals attempting to sustain reduced body weights. Much of this opposition to sustained weight loss is mediated by the adipocyte-derived hormone “leptin”. The multiplicity of systems regulating energy stores and opposing the maintenance of a reduced body weight illustrate that body energy stores in general and obesity in particular are actively “defended” by interlocking bioenergetic and neurobiological physiologies. Important inferences can be drawn for therapeutic strategies by recognizing obesity as a disease in which the human body actively opposes the “cure” over long periods of time beyond the initial resolution of symptomatology.
Obesity; Adaptive Thermogenesis; Thyroid; Autonomic; Weight Loss
Type 2 vesicular monoamine transporter (VMAT2), found in the brain, is also expressed by β-cells of the pancreas in association with insulin. Preclinical experiments suggested that 11C-dihydrotetrabenazine PET–measured VMAT2 binding might serve as a biomarker of β-cell mass. We evaluated the feasibility of 11C-dihydrotetrabenazine PET quantification of pancreatic VMAT2 binding in healthy subjects and patients with long-standing type 1 diabetes.
11C-Dihydrotetrabenazine PET was performed on 6 patients and 9 controls. VMAT2 binding potential (BPND) was estimated voxelwise by using the renal cortex as reference tissue. As an index of total pancreatic VMAT2, the functional binding capacity (the sum of voxel BPND × voxel volume) was calculated. Pancreatic BPND, functional binding capacity, and stimulated insulin secretion measurements were compared between groups.
The pancreatic mean BPND was decreased in patients (1.86 ± 0.05) to 86% of control values (2.14 ± 0.08) (P = 0.01). In controls, but not in patients, BPND correlated with stimulated insulin secretion (r2 = 0.50, P = 0.03). The average functional binding capacity was decreased by at least 40% in patients (P = 0.001). The changes in functional binding capacity and BPND were less than the near-complete loss of stimulated insulin secretion observed in patients (P = 0.001).
These results suggest that 11C-dihydrotetrabenazine PET allows quantification of VMAT2 binding in the human pancreas. However, BPND and functional binding capacity appear to overestimate β-cell mass given the near-complete depletion of β-cell mass in long-standing type 1 diabetes, which may be due to higher nonspecific binding in the pancreas than in the renal cortex.
β-cell mass; diabetes; VMAT2; 11C-dihydrotetrabenazine; PET
To examine the effects of weight loss and leptin administration following weight loss on calciotropic hormones and bone turnover.
This was a prospective, single blinded study of twelve subjects (8 women, 4 men; 2 non-obese, 10 obese; age range 19–46 years) who were studied on an in-patient basis while maintaining their usual weight [Wtinitial], and during maintenance of 10% weight loss while receiving twice daily injections of either a placebo [Wt−10%P] or replacement doses of leptin [Wt−10%L]. The main outcome measures were markers of bone formation (bone alkaline phosphatase and procollagen type 1 amino terminal propeptide), and resorption (N-telopeptide) as well as parathyroid hormone, calcium and 25-hydroxy vitamin D measured from fasting morning serum.
As expected, serum leptin declined with weight loss. Bone alkaline phosphatase decreased by 12.3 ± 3.9% between Wtinitial and Wt−10%P, and remained suppressed after leptin administration (both P<0.01 compared to baseline). N-telopeptides increased by 37.2 ± 11.3% from Wtinitial to Wt−10%L (P<0.01). Procollagen type 1 amino terminal propeptide, parathyroid hormone, calcium and 25-hydroxy vitamin D did not change.
These results suggest that both decreased bone formation and increased bone resorption underlie bone loss associated with weight loss. Leptin administration did not prevent the uncoupling of bone remodeling that accompanies weight loss.
leptin; bone; weight loss
Fertilized mouse zygotes can reprogram somatic cells to a pluripotent state. Human zygotes might therefore be useful for producing patient-derived pluripotent stem cells. However, logistical, legal and social considerations have limited the availability of human eggs for research. Here we show that a significant number of normal fertilized eggs (zygotes) can be obtained for reprogramming studies. Using these zygotes, we found that when the zygotic genome was replaced with that of a somatic cell, development progressed normally throughout the cleavage stages, but then arrested before the morula stage. This arrest was associated with a failure to activate transcription in the transferred somatic genome. In contrast to human zygotes, mouse zygotes reprogrammed the somatic cell genome to a pluripotent state within hours after transfer. Our results suggest that there is a previously unappreciated barrier to successful human nuclear transfer, and that future studies should focus on the requirements for genome activation.
zygote; reprogramming; mitosis; zygotic genome activation