Role of Brain Derived Neurotrophic Factor (BDNF) in Anorexia Nervosa: Pathogenic Mechanism and Therapeutic Potential
Project location: ITALY, Milano
Project start date: October 2016 - Project end date: September 2018
Project number: 2016-046
Beneficiary: Università degli Studi di Milano
Anorexia nervosa (AN) is a severe psychiatric illness characterized by restricted eating, an intense fear of gaining weight, even when the patient is severely underweight, and by abnormally high levels of physical activity. AN has a complex aetiology, in which genetic, biological, psychological and sociocultural factors, and the interaction between them, seem to significantly contribute to disease susceptibility.
AN begins with a restrictive diet and weight loss and progresses to an out-of-control spiral, in which the positive experience of control on food intake results extremely rewarding for the patient thus reinforcing the dieting behavior. Among mental illnesses, AN has one of the highest mortality rates (10–20%) (Sullivan, 1995; Birmingham et al., 2005;Bulik et al., 2007). The onset of AN is primarily concentrated at puberty, with 90–95% of the cases occurring among females (DSM-5) (American Psychiatric Association, 2013) and an incidence rate of around 0.9%. It has been proposed that alterations in mood and greater anxiety, peculiar of young female and triggered by hormonal changes, may predispose and be responsible for an increase vulnerability to lose weight and refuse the idea of maintaining a normal body weight (Buchanan et al., 1992; Kaye et al., 2009).
Despite the greater incidence of AN in adolescent females and the high mortality rate of this neuropsychiatric disorder, little is known about its aetiology or the predictive factors. The physiological, biochemical and behavioral aspects of AN are poorly understood, making treatment elusive, often time-consuming and challenging. The search for useful pharmacological treatments coupled with psychosocial intervention and support work with patient’s family has been ongoing for many decades now, but with very limited success. In fact, patients with AN face a life-long risk of relapse, suggesting that the causal link between disturbed appetite and other phenomena, such as anxiety or obsessive-compulsive behaviors, remains unclear. Moreover, alterations in emotionality and feeding behavior might fuel the illness, accelerating the entrance in a vicious cycle of an apparent well-being resulting in exaggerated weight loss and in compulsive over-exercise.
It is also crucial to consider that the initial onset of AN occurs primarily during adolescence, a period of high vulnerability. The adolescent brain is undergoing dramatic changes and for this reason it might be sensitive to outside influences that could induce adaptive or maladaptive plasticity in the nervous system. The continuation of development during adolescence as well as the sensation seeking typical of this period of life, confer great vulnerability to many effects of the environment. In addition, adolescents show an immature inhibitory control system and a prevalence of impulsive and emotional responses to stimuli, suggesting that the late maturation of the prefrontal cortex (PFC) may contribute to confer liability to stressful situations as well as cultural and societal pressures. Due to the imposed socio-cultural model of the body and to vulnerable temperament and fragile personality trait, many teenager girls became anorexic because they see themselves as overweight, even when they are starved or clearly malnourished. Eating, food and weight control become obsessions. In fact, people with anorexia tend to show compulsive behaviors, becoming obsessed with food, and often show behaviors consistent with other addictions in their efforts to overly control their food intake and weight.
The imbalance in energy intake caused by restrained eating is known to alter incentive motivational system and cognitive ability to drive behaviors. Moreover, long-term alterations in energy intake are also associated with brain volume changes and, at the neurobiological level, they lead to dysfunction of various neuropeptides in the central nervous system (CNS) and in the peripheral nervous system. It has been shown that corticotrophin-releasing factor (CRF), opioid peptides, ghrelin and leptin are implicated in the pathogenesis of AN (Hasan and Hasan, 2011). The main targets of these factors in the CNS are the regions that regulate energy balance and food intake, especially the hypothalamus. It has been proposed, through biochemical and genetic studies, that the neurotrophin brain-derived neurotrophic factor (BDNF) is a key molecule that modulates orexigenic and anorexigenic pathways regulating energy homeostasis maintenance (Rosas-Varga et al., 2011). Despite these observations, the psychological and neurobiological symptoms are poorly connected with specific molecular mechanisms.
Based on the close link between anorexia and other psychiatric disorders, such as anxiety, depression, substance abuse and compulsive behaviors, pharmacological treatment with antidepressants, antipsychotics and mood stabilizers may help some anorexic patients when given as part of a complete treatment program. These drugs can help treating depression or anxiety, although none has been proven to decrease the desire to lose weight.
In this context, it is crucial to understand the contributions of neurobiological systems to the onset and the sustenance of AN and their interactions with the environment, in order to improve the treatment approach and increase treatment efficacy.
Thus, to investigate the core symptoms of AN and the cellular basis of its vulnerability, a rat model of AN, has been developed called activity-based anorexia (ABA). This model, which involves the simultaneous exposure of animals to a restricted feeding schedule while free access is allowed to an activity wheel, captures two hallmarks of AN. One is the voluntary excessive exercise, which is evoked by imposition of food restriction; the other is voluntary food restriction, as the food-restricted animals paradoxically begin to choose exercise over feeding, even during the period of food access (1-2h free access per day) (Carrera et al., 2014). When the ABA-inducing environment is imposed upon adolescent female rats, this combination of behaviors leads to severe body weight loss and mortality, unless removed from the ABA- inducing environment by around the fifth-sixth day (Routtenberg and Kuznesof, 1967; Barbarich-Marsteller et al., 2013; Chowdhury et al., 2013; Gutierrez, 2013). Conversely, rodents exposed only to restricted food access or running wheels maintain body weight, suggesting that the onset of the AN is due to the combination of food restriction and sustained physical activity.
Despite the high incidence of AN pathology in adolescent female population, the vast majority of ABA studies are descriptive investigations about the nutritional status and physical exercise on peripheral signals implicated in homeostasis regulation but little is know about the implications of these signals in brain homeostasis and responsiveness. To improve our knowledge about food related disorders, there is a need for identification of neuroplastic mechanisms involved in the AN-induced vulnerability and that might contribute to control abnormal behavior in AN patients.
Since anorexia nervosa may be considered as a psychiatric disorder and evidence exists that people suffering from anorexia nervosa show comorbidity with other psychiatric disorders such as depression, anxiety and addictive disorders (Cohen et al., 2010), there is considerable evidence that alterations in corticostriatal limbic and dorsal cognitive neural circuitry are related to the pathophysiology of AN. Moreover, it has been hypothesized that an altered balance of reward and inhibition, coupled with emotional dysregulation, contributes to altered response to salient stimuli, such as food, contributing to the development and maintenance of anorexic behavior (Wierenga et al., 2014). Clinically, the imbalance toward overactive cognitive control circuitry and underactive reward circuitry likely interferes with motivation for treatment and ability to learn from experience, unabling ill individuals to recover from the vicious cycle of the disease.
Recently, it is becoming more and more evident that appetite modulators, such as leptin and ghrelin, might influence not only the homeostatic control of hunger and satiety but also the emotional and rewarding component of food intake through the modulation of other molecules involved in neuroplasticity and cognition such as the neurotrophin BDNF. In fact, several lines of evidence suggest the involvement of BDNF in the pathophysiology of AN (Nakazato et al., 2012; Monteleone et al., 2013). According to genetic studies and a meta-analysis on the BDNF gene, a 30% higher incidence of eating disorders is seen among individuals with the Val/Met and the Met/Met polymorphism of the BDNF gene (Mercader et al., 2007; Gratacos et al., 2007), indicating that the BDNF Val66-Met (rs6265) polymorphism is linked to eating disorders.
In addition, several lines of evidence have shown that BDNF serum levels are significantly diminished in patients with anorexia nervosa and positively correlated with body mass index (Nakazato et al., 2003; Monteleone et al., 2004). One small study showed that decreased circulating BDNF levels in emaciated AN patients were not restored after partial weight recovery (Nakazato et al., 2006), whereas a larger investigation found a normalization of serum BDNF in weight-recovered patients with AN (Ehrlich et al., 2009). Despite these unclear results in recovered patients, increasing our knowledge on the putative involvement of BDNF in the pathophysiological mechanisms of AN may help to develop more specific and effective treatment approaches.
Furthermore, animal experiments provide evidence for a role of central BDNF in the regulation of food intake and energy homeostasis due to its abundance in the hypothalamus (one of the major regulatory centers for eating behavior in the central nervous system), hippocampus, prefrontal cortex, ventral tegmental area and amygdala. The action of BDNF in anorexia nervosa might be dual. On one hand, its decline may represent an homeostatic adaptive phenomenon aimed at promoting food intake in condition of chronic starvation by acting on the energy balance mechanisms in the hypothalamus. Among BDNF transgenic mice, BDNF+/- knockout mice exhibit hyperactivity, high anxiety and weight gain under stress (Kernie et al., 2000). Further, exogenous BDNF reverses the phenotype of these animals, suggesting that endogenous BDNF reduces feeding and body weight gain (Kernie et al. 2000). On the other hand, BDNF has been implicated in the positive modulation of hedonic eating by increasing mesolimbic dopaminergic activity (Cordeira et al., 2010), suggesting that an impaired BDNF transmission in the central reward pathway might sustain changes in reward processes, altered motivated behavior and cognitive performances in AN patients. In fact, in animal models, BDNF has been demonstrated to be involved in motivational and anticipatory aspects of eating behavior. Food anticipatory activity, which is the increase in locomotor activity prior to feeding in rodents undergoing restricted feeding paradigm, is considered an expression of the motivation to eat while the presence of wheel motor activity when food is available is believed to be an expression of the lack of motivation to eat. It has been recently shown that food anticipatory activity in mice was accompanied by a strong increase in BDNF expression levels in the hippocampus, whereas an increase of wheel activity in food-restricted mice when exposed to food was found in parallel with reduced hippocampal BDNF expression (Gelegen et al., 2008). Such findings indicate that the BDNF gene product is involved in the modulation of eating behavior and weight control.
Taking all these data together, it appears that dissecting the role of BDNF in anorexia nervosa may represent an adding value both for increasing the knowledge on the disease and opening new potential avenues for treatment.
Thus, the gap this project intends to fill is to investigate in detail the molecular mechanisms subserving the activity-based anorexia model dissecting the involvement of the BDNF system in the energy balance and homeostasis maintenance in different brain areas. Given that adolescence is an important developmental stage for neuro-maturational processes, this project will identify the contribution of neuroplastic molecules in AN female rats evaluating their short and long-term implications in brain function and behavior.
Moreover, the behavioral approach that we are planning to use will also permit to evaluate the response to challenging events that normally occur throughout life and to correlate molecular alterations and behavioral effect of the pathology, highlighting the possible comorbidity with other psychiatric diseases.
In addition, using a pharmacological and ‘psychological’ approach, either alone or in combination, to restore the dysregulation in neuronal pathways that may contribute to the development and maintenance of the disorder and to normalize the altered behavior induced by the pathology will diminish the risk of relapse.
In summary, the main goal of this project, which received a grant from the Nando and Elsa Peretti Foundation, with the identification of novel molecular mechanisms involved in the development and stabilization of AN during a highly vulnerability window such as adolescence, will provide novel information that should increase the understanding of the molecular and cellular basis of the pathology providing novel markers to device early and more effective therapeutic interventions to oppose to the deleterious effects of the psychopathology.
Moreover, taking into account that, at variance from other disorders, anorexia nervosa represents a neglected field for preclinical research, we believe that the results deriving from our proposal could fuel the research in this field and open new avenues for drug treatment.