Molecular physiology of weight regulation in mice and humans

doi:10.1038/ijo.2008.245

Int J Obes (Lond). Author manuscript; available in PMC 2009 May 14.

Published in final edited form as:

Int J Obes (Lond). 2008 December; 32(Suppl 7): S98–108.

doi: 10.1038/ijo.2008.245

PMCID: PMC2682360

NIHMSID: NIHMS105335

Molecular physiology of weight regulation in mice and humans

RL Leibel

RL Leibel, Division of Molecular Genetics and Naomi Berrie Diabetes Center, Columbia University College of Physicians and Surgeons, New York, NY, USA;

Correspondence: Professor RL Leibel, Division of Molecular Genetics and Naomi Berrie Diabetes Center, Columbia University College of Physicians and Surgeons, Room 620, 1150 St., Nicholas Ave., New York, NY, 10032, USA. E-mail: rl232/at/columbia.edu

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Abstract

Evolutionary considerations relating to efficiency in reproduction, and survival in hostile environments, suggest that body energy stores are sensed and actively regulated, with stronger physiological and behavioral responses to loss than gain of stored energy. Many physiological studies support this inference, and suggest that a critical axis runs between body fat and the hypothalamus. The molecular cloning of leptin and its receptor—projects based explicitly on the search for elements in this axis—confirmed the existence of this axis and provided important tools with which to understand its molecular physiology. Demonstration of the importance of this soma-brain reciprocal connection in body weight regulation in humans has been pursued using both classical genetic approaches and studies of physiological responses to experimental weight perturbation. This paper reviews the history of the rationale and methodology of the cloning of leptin (Lep) and the leptin receptor (Lepr), and describes some of the clinical investigation characterizing this axis.

Keywords: leptin, energy homeostasis, genetics of body weight regulation

Introduction

The long-term relative stability of body weight in adult humans—despite annual energy intakes in the range of 700k cal to 1M cal per annum and large differences in levels of physical activity—has understandably led animal and clinical investigators to seek biological mechanisms by which such stability might be achieved. Thermodynamic considerations lead to the obvious arbiters of body mass: the net difference between energy intake and expenditure over time. A smaller role is played by so-called ‘partitioning’—the tendency for this difference to be reflected in relative amounts of fat and lean tissue. The large (almost 10-fold) difference in energy density of these tissues in vivo suggests that large changes in composition can occur with minimal effects on body mass. Commercial animal husbandry (lean pigs and cows, large-breasted chickens and so on),¹^,² and a substantial literature regarding the polygenics of rodent body composition³ support the existence of the phenomenon, though its molecular/biochemical bases and role in human biology are not clear.⁴ ‘Partitioning’ is also sometimes used to describe anatomic rather than chemical distribution of excess energy. For example, there are very different functional consequences to the storage (as fat) of such energy in subcutaneous versus intra-abdominal adipose tissue depots versus intra-myocellular fat.⁵^,⁶ Again, the mechanisms by which such differential deposition is achieved are not known.

Stability of body weight; facultative ‘wasting’ of energy in service of weight (fat) stability; likely predominance of energy intake

Over 100 years ago, Neumann⁷ noted the remarkable stability of his body weight over a period of a year despite no conscious effort to control either his energy intake or his expenditure. He used the term ‘luxuskonsumption’ to describe what he believed to be the body’s facultative ability to raise rates of energy expenditure to oppose relative excesses of energy intake. The so-called ‘Vermont Overfeeding Study’⁸ and others⁹^,¹⁰ involving voluntary over-feeding in human subjects have supported some aspects of Neumann’s inferences. The identification of the capacity of brown fat to uncouple substrate oxidation from ATP generation provides a plausible cellular/molecular mechanism in rodents.¹¹ The reciprocal process of reduction of energy expenditure beyond that explicable by loss of body tissue in circumstances of negative energy balance has been confirmed many times in animals and humans.¹²

The mechanisms for these increases and decreases in energy expenditure (not necessarily the reciprocals of the same process) are still not fully understood. Persistent net differences in energy balance of as little as 5%, in the context of annual energy throughputs such as those mentioned above, can easily account for substantial degrees of obesity in adults. However, the accuracy of measurements of energy expenditure in free-living subjects—for example, by the use of ‘doubly labeled water’¹³—is about 5%. The ability to measure energy intake in free-living subjects is much worse, probably not better than ±25%¹⁴ in most instances, though measurements to within 5% of estimated energy expenditure (by DLW) have been achieved (G Bray, personal communication, 2008). On the basis of studies in animals quantifying the relative contributions of energy intake and expenditure to body mass, and simple considerations regarding the ease with which extra energy can be ingested versus the effort needed to ‘burn off’ energy, it is likely that excessive intake is a more important contributor to risk of obesity than reduced energy expenditure,¹⁵ though individuals with lower energy expenditure may be more susceptible to weight gain.¹⁶ This differential may not be as extreme in the recidivism to obesity in formerly obese individuals (see below). That is, reduced energy expenditure may play an important role in regain of lost body weight.

‘Homeostatic’ responses to weight perturbation are comprised of coordinate changes in behaviors and ‘vegetative’ systems controlling energy expenditure. The former include qualitative and quantitative aspects of food intake and voluntary levels of physical activity and exercise; the latter constituted by autonomic, neuroendocrine and other mediators of resting and non-resting energy expenditure (NREE), muscle work efficiency and partitioning. While there may be heuristic merit to making such distinctions, they tend to obscure the critical fact that these responses are integrated and coordinated in a manner that opposes weight perturbation in either direction. There are apparent distinct genetic influences on virtually every one of these ‘endophenotypes’,¹⁷ and, in the aggregate, they constitute the biobehavioral substrates for the regulation of body weight. One of the remarkable insights deriving from the identification of rodent obesity monogenes (and their human orthologs) is the fact that single molecules (e.g. leptin, MC4R and NPY) occupying critical points in the regulatory physiology can affect both complex behaviors (food intake, timing, preference and spontaneous physical activity) and vegetative functions (resting energy expenditure and partitioning) in a coordinate fashion. The existence of such molecules was predictable from the integrated physiology. Likewise, the predictable redundancy in such a critical system accounts for the frequent finding that abrogation of a single component (e.g. NPY and ghrelin) does not necessarily have a major effect on body weight.¹⁸^–²⁰

‘Set point’

Regardless of the final effectors of body weight homeostasis, virtually all homeostatic (non-open loop) models call for a means by which the body can sense and respond to quantitative aspects of somatic energy stores. As the major store of disposable somatic energy is fat, theories relating to the aspect of body composition ‘sensed’ and signaled by this mechanism have focused on adipose tissue. In the 1940s, Hetherington and Ranson²¹ showed that mechanical injury in the region of the ventral medial hypothalamus led to extreme obesity in rats. Comparable lesions in the lateral hypothalamus resulted in anorexia and weight loss.²²^,²³ In both instances, the animals, once stable on their new somatic growth isobars, ‘defended’ their new body weights against both imposed weight gain and loss. These findings led to the suggestion that a ‘set point’ for body weight had been interrupted by these manipulations. Comparable types of hypothalamic injury in humans supported this notion,²⁴ as did precise regrowth of fat depots in response to lipectomy in rats.²⁵^,²⁶ Kennedy²⁷ and Hervey²⁸ suggested that body fat (rather than body weight per se) was being regulated, and Porte, Woods and others²⁹^–³² suggested that circulating insulin might provide the proposed fat mass-sensitive signal. A mathematical model for a fat-related signal to the brain—proportional to adipocyte volume—was proposed by me.³³ Parabiosis (continuous, low-volume blood exchange between living animals) conducted by Doug Coleman using mice segregating for two genetically distinct spontaneous mutations (ob and db) that lead to severe obesity, suggested that the two genes encoded proteins in the same pathway: one possibility was that they were enzymes in such a pathway; another was that ob might be a circulating factor (proportional to body fat) for which db was a receptor.³⁴ Similar studies by others in rodents³⁵^–³⁷ and apes³⁸ supported the notion of a circulating signal of fatness. We and others proposed that molecules released by fat, for example glycerol,³⁹ or the ratio of glycerol to free fatty acids,⁴⁰ or metabolites of free fatty acids (e.g. β-hydroxybutyrate), might provide such a signal. Direct effects on food intake of glycerol,³⁹ ketones⁴¹ and free fatty acids,⁴² have been shown, and these molecules may indeed play a role in the regulation of energy homeostasis. However, the small volumes of blood exchange sufficient to affect adiposity in parabiosis studies suggested the existence of more potent and specific molecules.

Molecular predicates of systemic responses to weight perturbations

A retrospective analysis of the weight-maintenance energy costs of weight-reduced obese human subjects⁴³ led us to propose that the weight-reduced state (in humans and animals) is actually a state of metabolic ‘deformation’, sensed by a homeostatic mechanism responding to changes in body fat as a state of energy deprivation (hence threat to survival and reproductive capacity). We found that such individuals showed 15–20% reductions in energy expenditure beyond what could be accounted for by reduced body mass and composition (Figure 1). These decreases in energy expenditure were not compensated for by proportional decreases in energy intake; in fact, hunger was increased, consistent with an integrated mechanism for defense of body fat stores. The combination of reduced energy expenditure and increased hunger could account for the <95% recidivism to obesity seen in otherwise successfully treated obese patients.⁴⁴^,⁴⁵

Figure 1

Weight-maintaining energy requirements in weight-reduced human subjects. Weight-maintaining energy intake requirements (kcalm⁻² d⁻¹) in 26 patients studied when obese and after substantial weight loss (reduced-obese state) in comparison (more ...)

In an effort to better define the molecular physiology of the weight-reduced state, and to identify specific molecules that might mediate responses to lost body fat, in 1985 my collaborators and I undertook two major projects: (1) to define prospectively the metabolic, behavioral and neuro-endocrine responses to experimental weight perturbations in obese and never-obese human subjects; and (2) to clone by positional genetic approaches the two genes that were mutant in the animals Coleman had used in his parabiosis experiments (ob and db) (Figure 2 and Figure 3). It is important to note that, at the time, the molecular genetic tools—and some of the computational approaches—that proved crucial to the success of the positional cloning project were not available; this point was made repeatedly by a skeptical NIH (National Institutes of Health) study section whose members also felt that, even if achieved, the identification of mouse-obesity genes was unlikely to shed much or any light on the situation in humans. Some of my comments in the section on ‘Motivation’ are relevant here. The project was initially supported by a grant from the President of Rockefeller University and by ‘Director’s Funds’ provided for a 3-year period by NIDDK Director, Phil Gorden. The human weight perturbation studies were based on the premise that the best way to study the relevant systemic biology was (1) to use each subject as his/her own control and (2) to eliminate as much environmental variability as possible by keeping subjects housed continuously in a clinical research center, eating nothing other than a defined liquid formula diet. These projects were conducted in parallel and took about 10 years to complete.

Figure 2

ob/ob mouse with sibling.

Figure 3

Schematic of effects on body weight and diabetes-related phenotypes of parabiosis of ob, db and +/+ mice. From these studies Coleman inferred that it ob might encode a secreted molecule, for which db was the receptor. Molecular cloning proved him right. (more ...)

Molecular cloning of ob and db

Jeff Friedman and I, and our associates, used a ‘reverse genetic/positional cloning’ strategy to clone ob and db. These approaches were used in the mid-1980s to map the genes for Huntington’s disease,⁴⁷ cystic fibrosis,⁴⁸ chronic granulomatous disease⁴⁹ and muscular dystrophy.⁵⁰ In the case of the mice, we had the great advantage of being able to use inbred strains in large, controlled crosses. At Doug Coleman’s suggestion, we relied primarily on crosses in which the respective mutations (segregating on C57BL/6J) were crossed (primarily) to DBA/2J, Cast/Ei or Spretus animals. Strain selection was based on the likely genetic distance from B6. As both ob and db mice are infertile, the ovaries of mice homozygous for the respective mutations (i.e. obese) were quartered and transplanted (by Dorcas Corow at the Jackson Lab) to the ovarian beds of carrier B6 mice whose ovaries had been excised. The F2 and N2 progeny of these crosses were used to produce increasingly detailed molecular genetic maps of the intervals around both mutations (proximal Chr6 for ob; mid Chr4 for db).⁵¹^–⁵³ Initially, we depended on restriction length polymorphisms⁵⁴ exhibited on Southern blots—a tedious technique made more difficult by the paucity of suitable polymorphic genetic ‘markers’. This problem was partially relieved by Nate Bahary’s painstaking physical dissection and phage cloning of specific portions of a Roberstonian chromosome comprised of Chrs 4 and 6, and ultimately by the identification of microsatellites in the mouse genome that were (more) highly polymorphic among strains.⁵⁵ Because of the large numbers of animals involved (ultimately about 4000 progeny), and because of the diabetes proneness conveyed by some of the genetic backgrounds (e.g. DBA and KsJ), it was advantageous to score these animals for relative adiposity (i.e. homozygosity for ob or db) by 8–12 weeks of age. We used a weight–length ratio (referred to locally as the ‘body mouse index’), and measures of plasma glucose and insulin to identify ob/ob and db/db animals. Molecular maps were generated using early versions of software developed by Eric Lander and his collaborators at the Whitehead Institute.⁵⁶ Large clones (YAC, BAC, P1) were also used to create a physical ‘contig’ for the regions around both mutations. Once the map around ob was in the vicinity of 500 kb, exonic trapping was used to obtain portions of genes in the interval that could be used for Northern analysis in mutant animals. A second mutation at ob had been identified at the Jackson Laboratory; we learned of this ob 2J mutation during a visit to Jackson for a Festschrift for Doug Coleman. We maintained a colony of these animals, which we used to test candidate gene expression using the trapped exons. In the spring of 1994, clone 2G7 was shown (versus wild-type animals) to be over-expressed in ob 1J and absent in ob 2J. Access to this allelic series was critical in confirming that 2G7 was a portion of the ob gene. The full-length gene was then cloned, and ob 1J shown to have a nonsense/mutation in codon 105;⁵⁷ the cause of the ob 2J mutation was not identified until 1997: a retroviral (ETn) insertion into the first intron that led to the production of chimeric transcripts and a functionally null allele.⁵⁸ The cognate protein (‘leptin’) was synthesized and shown to rescue the phenotype of the ob (not the db) mouse. Lou Tartaglia—then at Millennium Pharmaceuticals—used the protein to screen a phage expression library of choroid plexus—an experiment that I assured him would not work—and identified a candidate gene for db,⁵⁹^,⁶⁰ which we confirmed by mapping back to a physical map of the db region.⁶¹ Thus, within about 1 year, the sequences of both genes were reported. The locations of both genes in the human genome corresponded to predictions made on the basis of mouse-human synteny-homology maps.⁶²^,⁶³

The tools to complete these projects were not available when we started. This point was made to us repeatedly by study sections. Others picked up on the idea of cloning these genes, and we knew that we were not alone in these efforts. Fellow scientists who knew of our efforts would chide us occasionally regarding the prolonged gestation. These and other pressures created tensions and behaviors well-documented in several books reporting the history of this work;⁶⁴^,⁶⁵ likewise, the frenetic early efforts to exploit leptin financially and therapeutically.

The ob (‘leptin’) gene encoded a 16-kDa circulating molecule, for which db (‘leptin receptor’) was the receptor.⁵⁷ As noted, the db gene was identified in a choroid plexus expression library screened for clones that produced proteins that bound leptin.⁵⁹^,⁶⁰ The candidate gene for db was confirmed by mapping it back on a physical map of the region containing the db gene.⁶⁶ On the basis of the similar phenotypes of the db mouse and the Zucker rat (fatty, fa), and the fact that the mouse and rat mutations had both arisen, spontaneously, on several occasions, we hypothesized that the mutations were in the same gene (Figure 4). The molecular maps of the regions around db and fa showed essential conservation of genes and gene order,⁶⁷ supporting this inference, which was later proven by the molecular cloning of both the Zucker and corpulent mutations of fa.⁶⁶^,⁶⁸ Knowledge of the position of the fa mutation, and access to mapping crosses of the mutation, allowed us to rule out genes, such as Crh, Npy and Lpl,⁶⁹ that had been proposed as the genes accounting for the Zucker phenotype.⁷⁰ We subsequently identified a series of non-conservative amino acid substitutions in the human leptin receptor (LEPR), none of which seems to play a major roles in susceptibility to obesity.⁷¹

Figure 4

db/db mouse with fa/fa rat. These mutations were genetically mapped to homologous intervals of the mouse and rat genome and the correspondence subsequently proved by physical mapping.⁶¹^,⁶⁶^,⁶⁷

Human weight perturbation

Our weight-perturbation studies showed that obese and never-obese humans require identical amounts of energy intake when such intake (hence expenditure) is normalized to metabolic mass (lean body mass). Hence, a common misconception—that the obese can sustain body mass on highly restricted food intake—was shown to violate the widely accepted and biologically relevant First Law of Thermodynamics. These studies also showed that a 10% reduction in body weight reduces total energy expenditure by about 20% more than that predicted by the change in mass and body composition, on the basis of the relationship of energy expenditure to metabolic mass before weight reduction. These changes were qualitatively and quantitatively similar among lean, obese, male and female subjects, again consistent with our growing view that obese and lean individuals are metabolically equivalent at customary body weight, and ‘defend’ those weights by identical mechanisms, with equal force.⁷² Surprisingly, the major compartment of energy expenditure affected by maintenance of a reduced body weight was NREE, the cost of low levels of physical activity⁷² (Figure 5). In follow-up studies, we showed that this decline in NREE is, primarily, because of increased chemo-mechanical efficiency of skeletal muscle work at low levels of physical activity.⁷³ On the basis of our own studies and those of others, this metabolic phenotype does not seem to lessen with prolonged maintenance of reduced body weight.⁴⁵^,⁷⁴ In light of the consequences of small imbalances of energy intake and expenditure mentioned above, the 20% decline in energy expenditure provoked by as little as a 10% reduction in body weight is sufficient to account for the recidivism to obesity. The fact that responses are similar in lean and obese subjects suggests that each is defending their usual body weight/composition with equal physiological ‘force’.

Figure 5

Effects of experimental weight perturbation in human subjects studied at −10 and −20% below customary body weight. Each subject was studied at Wt initial (usual), at −10% and, in some instances, −20% as well. Summary bar (more ...)

Leptin

At the time of its identification, some believed that leptin’s primary role was to protect against increase of body fat.⁵⁷ In fact, the gene’s name (derived from the Greek word ‘leptos’ for light) was based on that premise. However, on the basis of the metabolic phenotypes of ob and db mice, the responses of the ob mouse to leptin replacement, and evolutionary considerations regarding the need for genes that would defend (rather than waste) body fat to enhance survival during periods of restricted access to food, we and others thought that the primary role of the leptin axis was to defend body fat, and that the ob and db animals, unable to make or respond to leptin, were in a physiological state equivalent to perpetual starvation. The several large—and unsuccessful—trials designed to use high doses of leptin (increasing blood levels as much as ten times the normal)⁷⁵ showed that the fat-suppression model of leptin action was incorrect. The behavioral and neuropeptide responses of fasted mice⁷⁶ and rats⁷⁷ to exogenous leptin were consistent with leptin’s primary biological role as a defender of body fat. To prove the validity of this concept in humans, we administered leptin to our weight-reduced subjects in doses just sufficient to restore circulating concentrations to those present before the loss of 10% of body weight by hypocaloric diet. Virtually all of the bioenergetic (including muscle work efficiency), sympathetic-autonomic and neuroendocrine changes (including reduced T3) characteristic of the weight-reduced state were reversed by this expedient, confirming the concept that the weight-reduced state is a state of relative leptin deficiency, and that the reduction in circulating leptin is responsible for the characteristic phenotypes ⁷⁸ (Figure 6). These experiments also call attention to the fact that the physiology of the weight-reduced state is not the same as that during maintenance of ‘usual’ body weight, and that alleviation of the physiology of the weight-reduced state may be, in some respects, a better (more physiologic) pharmacologic target than the induction of anorexia and weight loss per se.

Figure 6

Energy expenditure phenotypes—including skeletal muscle work efficiency—in weight-reduced human subjects. Percentage change (mean ± s.e.m.) from values of energy expenditure, skeletal muscle work efficiency and fuel utilization (more ...)

As anticipated, circulating concentrations of leptin are closely proportional to body fat mass.⁷⁹ On the basis of the general lack of response of humans to high doses of exogenous leptin and the striking reversal of metabolic phenotypes produced by replacement doses of leptin in fasting or underfed animals⁷⁶^,⁸⁰ and weight-reduced humans, ⁷⁸ we have proposed that the ‘endocrinology’ of the leptin axis is unlike more conventional systems in which response to hormone concentrations is approximately proportional to ambient hormone over a wide range of concentrations. Responses to leptin are more analogous to those related to circulating glucose concentrations: at high concentrations, there is little or no awareness of elevated blood glucose aside from, perhaps, polyuria. However, subphysiological concentrations of glucose are rightly ‘perceived’ as a threat to consciousness/survival, and are responded to by outpourings of catecholamines, corticosteroids and an acute sense of hunger. Similarly, high levels of leptin are not responded to, whereas reduced concentrations lead to increased drive to eat and reduced energy expenditure in an effort to conserve and restore body fat stores. These asymmetric physiological responses to leptin are schematized in Figure 7.⁸¹ Although there is evidence that prolonged elevation of ambient leptin can increase the production of suppressor of cytokine signaling 3 (SOCS3), a protein that interferes with Janus kinase 2/signal transducers and activators of transcription signaling by the LEPR,⁸² we have argued that the concept of ‘leptin resistance’, presumably mediated in part by such effects, need not be invoked if the analogy to glucose physiology is extended. For example, in research and clinical settings, we do not generally refer to the absence of conscious sensation of an ambient glucose concentration of 300 mg dl⁻¹ as ‘glucose resistance’. Rather, we infer that central nervous system biology has not evolved to sense this state, possibly because no effective compensating behaviors are possible. A similar argument can be made with regard to leptin: that is, that evolutionary ‘experience’ with a need to suppress food intake in response to adiposity has not been particularly important, and, in fact, might be ‘dangerous’ in animals seeking to survive in hostile environments. Hence, the strength of physiological ‘defense’ of body fat is weaker, though certainly not absent⁷²^,⁸³, against upward deflections than those that bring body fat to a point below a ‘threshold’ for minimum allowable body fat mass. This asymmetry provides the biological substrate for the secular trend in obesity, which is an apparent response of a compliant biology with strong environmental/hedonic pressures that favor positive energy balance. Susceptibility to excessive weight gain in such propitious circumstances is, however, not evenly distributed throughout the population. This point is made by the ‘tail’ on the upper end of the distribution of body mass index in the United States, for example.⁸⁴ This skewing is consistent with the premise that there are genetic factors mediating resistance to weight gain, just as there are genes responsible for the defense of body fat. These certainly may not be the same genes in most or any instances (Figure 7).

Figure 7

(a) Unlike dose–response characteristics of many hormones, leptin at high concentrations has little effect, whereas concentrations below a minimum ‘threshold’ invoke dramatic changes in hypothalamic neuropeptides and behaviors/metabolic (more ...)

Critical to the experimental utility of this model is the development of a cell/molecular basis for the hypothetical construct of a minimum threshold for body fat. We have described this concept in detail elsewhere,⁸⁵^,⁸⁶ but fundamentally we suggest that leptin provides a circulating signal that reflects adipose tissue mass, and that the responses of arcuate, brain stem and other brain regions to this signal depend on the expression and structural integrity of the many proteins that convey and integrate this and other afferent signals that mediate control of energy homeostasis. Subtle sequence variation in the relevant genes, and developmental influences that may affect both the imprinting of these genes and actual brain structure,⁸⁷^–⁸⁹ determine the efficiency with which the relevant peripheral signals are sensed and conveyed within the central nervous system (Figure 8). Hence, extreme situations such as absence of either signal (LEP null)⁹⁰ or receptors (LEPR null)⁹¹^,⁹² result in profound obesity; derangements in MC4R produce less severe phenotypes;⁹³ and genetic variation in FTO/FTM (by mechanisms unclear at this point) produce even more modest changes in body mass index.⁹⁴^,⁹⁵ An example of the additivity of such variants was provided by our observation that heterozygotes for ob or db were about 35% fatter than +/+ animals, and that the ob/+;db/+ double heterozygotes were about 50% fatter than +/+.⁹⁶ Congruent with these animal studies, Farooqi et al.⁹⁷ showed that humans heterozygous for null LEP mutations are also about 35% fatter than individuals with two normal alleles at this locus. In this model, body fat (hence circulating leptin) increases until the ‘signal’ at the level of the hypothalamus/brain stem is sufficiently intense to compensate for otherwise diminished signal transmission capacity resulting from variously hypomorphic alleles at steps in the signaling and response cascades. We have recently undertaken a simultaneous analysis of 25 genes in these pathways for allelic variation that could contribute to aggregate body fatness.⁹⁸ This sort of analyses, requiring large numbers of subjects, will be required ultimately to work out the gene × gene and gene × environment (and development) interactions that determine the function of this complex regulatory system.

Figure 8

Schematic of the linear relationship of plasma leptin concentration with body fat mass. A threshold (Figure 7) for leptin signaling is shown at the line labeled ‘initial’. This initial threshold is determined as per Figure 7. Here the (more ...)

Brain imaging

It is obvious that complex behaviors, responsive to both ‘vegetative’ signals emanating from the hypothalamic/brain stem pathways outlined above, as well as environmental cues relating to clock time, availability/palatability of food and ‘psychological factors’, determine the timing and duration of ingestive behaviors.⁹⁹ Although the utility of analogizing aspects of ingestive behavior to addictive behaviors is arguable,¹⁰⁰ there is unequivocal experimental evidence for the role of ‘higher’ brain regions in hedonic responses to food and ‘executive’ behaviors related to decisions to eat. The advent of functional and diffusion tensor magnetic resonance imaging of the brain now enables us to observe effects on blood flow (a reflection of neuronal activity) and pathway activity in specific brain regions¹⁰¹ in response to fasting/refeeding, weight perturbations and specific foods. We have examined region-specific functional magnetic resonance imaging (fMRI) patterns in human subjects undergoing the weight-perturbation studies described briefly above. Maintenance of a reduced body weight results in characteristic patterns of activation and suppression of functional magnetic resonance imaging activities in the hypothalamus, limbic system, amygdala, frontal and fusi-form gyri. These changes are largely reversed by the administration of doses of leptin sufficient to restore circulating levels of leptin to those present before the weight loss over 5 weeks.¹⁰² As ‘long’ isoforms of the LEPR are located primarily in the hypothalamus and brain stem, these effects on ‘higher’ brain regions are presumably transduced through cells in the hypothalamus and brain stem. Thus, some aspects of hedonic and other ‘conscious’ responses to food are mediated by vegetative centers in a manner comparable to conscious thirst responses to increased tonicity of serum.

It seems likely that we are in the very earliest stages of the use of neuroimaging to understand the complexities of human ingestive behaviors. It is not difficult to imagine that labeled metabolites, receptor ligands, diffusion tensor imaging and less restrained testing situations will open up important experimental approaches. Of great importance in this context, and many others, are more sophisticated quantitative and qualitative measurements of ingestive behaviors in free-living human subjects. These systems will not only enable deeper understanding of the biology of weight homeostasis, but could also provide mechanistic insights into modes of action of new agents being developed to treat obesity.

Motivation

It is stylish (and expedient) at present to emphasize the ‘translational’ aspects of biomedical research and to act as though such projections/predictions of what might go from ‘bench-to-bedside’ were readily made. As anyone who does this work for a living knows, they are not. Whatever successes in translation characterize the research described here are the result of asking one big question—‘how is body weight regulated in humans?’—and then using whatever tools (physiology, genetics, cell and molecular biology) and ‘models’ (animals, humans and cells) are available to answer the corollary questions that arise from the big one: what are the physiological responses to weight perturbation?; what genes mediate the defense of body weight?; how do these genes act in concert?; what developmental and post-developmental processes influence the underlying neural substrates for this homeostasis? It seems to me that students often do not get the message that a good way to do research is to pick an important question, and to pursue it relentlessly over time, trying to avoid experiments that are doable, but not so important, in favor of the more difficult, time-consuming and risky experiments that are likely to move one closer to the big answer. There are good, even urgent, reasons to ignore/invert this advice in service of one’s CV and career. But that strategy also deprives one of some of the thrills inherent in ‘big picture’ research. Einstein’s advice is relevant here: ‘I have little patience with scientists who take a board of wood, look for its thinnest part, and drill a great number of holes where drilling is easy.’

Note: The views expressed in this historical review are mine, but reflect the important intellectual interactions I have had over 30 years with many mentors, associates, students and collaborators. Among these are Jack Crawford, MD, who introduced me to ob mice as a pediatric endocrinology fellow at the Mass General in 1973–1975; Jules Hirsch, MD, who pioneered in the study of the functional anatomy of adipose tissue and who provided me with the opportunity and protected time to learn adipose tissue biology, energy metabolism and mouse genetics; Jules also secured funding from the Rockefeller University to start the mouse project that led to the cloning of ob and db; Jeff Friedman, MD, PhD, who had the courage as a starting assistant professor to engage in the collaborative effort that, over a period of 9 years, led to the identification of the ob and db genes; and to Doug Coleman, who provided expert advice, some critical animals, and steady encouragement. Extraordinary students and fellows who worked on the ob/db and fa projects included Nathan Bahary, MD, PhD, Wendy K Chung, MD, PhD, Yiying Zhang, PhD, Don Siegel, PhD, Streamson C, Chua, MD, PhD, Gary Truett, PhD, Jordan Smoller, MD, and others, and a large number of dedicated young technicians. My longstanding associate, Mike Rosenbaum, MD, for the past 20 years has led virtually all of the clinical studies related to bioenergetics of weight homeostasis, and the physiology of leptin in that context. More recently, Joy Hirsch, PhD at Columbia, has collaborated with us on functional magnetic resonance-imaging studies in weight-reduced subjects. Other collaborators on these studies include Rochelle Goldsmith, MD, Bo Bogardus, MD, Eric Ravussin, PhD, Krista Vandenborne, PhD, Louis Aronne, MD, and Laurel Mayer, MD. Members of the New York Obesity Research Center—Xavier Pi-Sunyer, MD, Steve Heymsfield, MD, Dympna Gallagher, PhD, Harry Kisileff, PhD, and Carol Boozer, PhD, have all helped me and my associates in various ways throughout the years. Extraordinary nursing staff and research dietitians have likewise been critical to our studies of energy homeostasis in human subjects. Finally, our research subjects themselves (human and otherwise) have contributed selflessly to our efforts to understand the molecular physiology and molecular genetics of obesity.

Acknowledgements

I am grateful to George Bray and Claude Bouchard for comments on an earlier version of this paper.

Footnotes

Conflict of interest

The author has declared no financial interests.

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