|FBJ murine osteosarcoma viral oncogene homolog B|
|Symbols||; AP-1; G0S3; GOS3; GOSB|
|RNA expression pattern|
The FOS gene family consists of 4 members: FOS, FOSB, FOSL1, and FOSL2. These genes encode leucine zipper proteins that can dimerize with proteins of the JUN family (e.g., c-Jun, JunD), thereby forming the transcription factor complex AP-1. As such, the FOS proteins have been implicated as regulators of cell proliferation, differentiation, and transformation. FosB and its truncated splice variants ΔFosB and (further truncated) Δ2ΔFosB are all involved in osteosclerosis, even though Δ2ΔFosB lacks a known transactivation domain, preventing it from affecting gene transcription through the AP-1 complex.
The ΔFosB splice variant has been identified as playing a central, crucial (necessary and sufficient) role in the development of many forms of behavioral plasticity and neuroplasticity involved in both behavioral addictions (associated with natural rewards) and drug addictions. ΔFosB differs from the full length FosB and further truncated Δ2ΔFosB in its capacity to produce these effects, as only accumbal ΔFosB overexpression is associated with pathological responses to drugs.
ΔFosB or Delta FosB is a truncated splice variant of FosB. ΔFosB has been implicated as a critical factor in the development of virtually all forms of behavioral and drug addictions. In the brain's reward system, it is linked to changes in a number of other gene products, such as CREB and sirtuins. In the body, ΔFosB regulates the commitment of mesenchymal precursor cells to the adipocyte or osteoblast lineage.
In the nucleus accumbens, ΔFosB functions as a "sustained molecular switch" and "master control protein" in the development of an addiction; in other words, once "turned on" (sufficiently overexpressed) ΔFosB triggers a series of transcription events that ultimately produce an addictive state (i.e., compulsive reward-seeking involving a particular stimulus); this state is sustained for months after cessation of drug use due to the abnormal and exceptionally long half-life of ΔFosB isoforms. ΔFosB expression in D1-type nucleus accumbens medium spiny neurons directly and positively regulates drug self-administration and reward sensitization. Based upon the accumulated evidence, a medical review from late 2014 argued that accumbal ΔFosB expression can be used as an addiction biomarker and that the degree of accumbal ΔFosB induction by a drug is a metric for how addictive it is relative to others.
Role in addiction
|• addiction – a state characterized by compulsive engagement in rewarding stimuli, despite adverse consequences|
|• reinforcing stimuli – stimuli that increase the probability of repeating behaviors paired with them|
|• rewarding stimuli – stimuli that the brain interprets as intrinsically positive or as something to be approached|
|• addictive drug – a drug that is both rewarding and reinforcing|
|• addictive behavior – a behavior that is both rewarding and reinforcing|
|• sensitization – an amplified response to a stimulus resulting from repeated exposure to it|
|• drug tolerance – the diminishing effect of a drug resulting from repeated administration at a given dose|
|• drug sensitization or reverse tolerance – the escalating effect of a drug resulting from repeated administration at a given dose|
|• drug dependence – an adaptive state associated with a withdrawal syndrome upon cessation of repeated drug intake|
|• physical dependence – dependence that involves persistent physical–somatic withdrawal symptoms (e.g., fatigue)|
|• psychological dependence – dependence that involves emotional–motivational withdrawal symptoms (e.g., dysphoria and anhedonia)|
|(edit | history)|
Current models of addiction from chronic addictive drug use involve alterations in gene expression in the mesocorticolimbic projection. The most important transcription factors that produce these alterations are ΔFosB, cyclic adenosine monophosphate (cAMP) response element binding protein (CREB), and nuclear factor kappa B (NFκB). ΔFosB is the most significant gene transcription factor in addiction since its viral or genetic overexpression in D1-type medium spiny neurons in the nucleus accumbens is necessary and sufficient for many of the neural adaptations seen in drug addiction; it has been implicated in addictions to alcohol, cannabinoids, cocaine, nicotine, phenylcyclidine, opiates, and substituted amphetamines. ΔJunD, a transcription factor, and G9a, a histone methyltransferase, both directly oppose the induction of ΔFosB (i.e., increases in its expression). Increases in nucleus accumbens ΔJunD expression using viral vectors (a genetically engineered virus) can reduce or, with a large increase, even block many of the neural and behavioral alterations seen in chronic drug abuse (i.e., the alterations mediated by ΔFosB).
ΔFosB also plays an important role in regulating behavioral responses to natural (non-drug) rewards, such as palatable food, sex, and exercise. Natural rewards, like drugs of abuse, induce gene expression of ΔFosB in the nucleus accumbens, and chronic acquisition of these rewards can result in a similar pathological addictive state through ΔFosB overexpression. Consequently, ΔFosB is the key transcription factor involved in addictions to natural rewards (i.e., behavioral addictions) as well; in particular, ΔFosB in the nucleus accumbens is critical for the reinforcing effects of sexual reward. Research on the interaction between natural and drug rewards suggests that dopaminergic psychostimulants (e.g., amphetamine) and sexual behavior act on similar biomolecular mechanisms to induce ΔFosB in the nucleus accumbens and possess bidirectional cross-sensitization effects[note 1] that are mediated through ΔFosB. This phenomenon is notable since, in humans, a dopamine dysregulation syndrome, characterized by drug-induced compulsive engagement in natural rewards (specifically, sexual activity, shopping, and gambling), has also been observed in some individuals taking dopaminergic medications.
ΔFosB inhibitors (drugs or treatments that oppose its action or reduce its expression) may be an effective treatment for addiction and addictive disorders.
Plasticity in cocaine addiction
ΔFosB levels have been found to increase upon the use of cocaine. Each subsequent dose of cocaine continues to increase ΔFosB levels with no ceiling of tolerance. Elevated levels of ΔFosB leads to increases in brain-derived neurotrophic factor (BDNF) levels, which in turn increases the number of dendritic branches and spines present on neurons involved with the nucleus accumbens and prefrontal cortex areas of the brain. This change can be identified rather quickly, and may be sustained weeks after the last dose of the drug.
Transgenic mice exhibiting inducible expression of ΔFosB primarily in the nucleus accumbens and dorsal striatum exhibit sensitized behavioural responses to cocaine. They self-administer cocaine at lower doses than control, but have a greater likelihood of relapse when the drug is withheld. ΔFosB increases the expression of AMPA receptor subunit GluR2 and also decreases expression of dynorphin, thereby enhancing sensitivity to reward.
Other functions in the brain
|c-Fos||↓||molecular switch enabling the
chronic induction of ΔFosB[note 2]
|dynorphin||↓||downregulation of κ-opioid feedback loop|
|NFκB||↑||expansion of dendritic processes and
regulation of cell survival pathways
|Cdk5||↑||expansion of dendritic processes|
|GluR2||↑||decreased sensitivity to glutamate|
|Form of neural or behavioral plasticity||Type of reinforcer||Sources|
|Opiates||Psychostimulants||High fat or sugar food||Sexual reward||Exercise||Environmental enrichment|
in the nucleus accumbens
|Escalation of intake||Yes||Yes||Yes|||
conditioned place preference
|Reinstatement of drug-seeking behavior||↑||↑||↓||↓|||
in the nucleus accumbens
|Sensitized dopamine response
in the nucleus accumbens
|Altered striatal dopamine signaling||↓DRD2, ↑DRD3||↑DRD1, ↓DRD2, ↑DRD3||↑DRD1, ↓DRD2, ↑DRD3||↑DRD2||↑DRD2|||
|Altered striatal opioid signaling||↑μ-opioid receptors||↑μ-opioid receptors
|↑μ-opioid receptors||↑μ-opioid receptors||No change||No change|||
|Changes in striatal opioid peptides||↑dynorphin||↑dynorphin||↓enkephalin||↑dynorphin||↑dynorphin|||
|Mesocorticolimbic Synaptic Plasticity|
|Number of dendrites in the nucleus accumbens||↓||↑||↑|||
|Dendritic spine density in
the nucleus accumbens
- In simplest terms, this means that when either amphetamine or sex is perceived as "more alluring or desirable" through reward sensitization, this effect occurs with the other as well.
- In other words, c-Fos repression allows ΔFosB to accumulate within nucleus accumbens dopamine neurons more rapidly because it is selectively induced in this state.
- "Entrez Gene: FOSB FBJ murine osteosarcoma viral oncogene homolog B".
- Siderovski DP, Blum S, Forsdyke RE, Forsdyke DR (October 1990). "A set of human putative lymphocyte G0/G1 switch genes includes genes homologous to rodent cytokine and zinc finger protein-encoding genes". DNA Cell Biol. 9 (8): 579–587. doi:10.1089/dna.1990.9.579. PMID 1702972.
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- Sabatakos G, Rowe GC, Kveiborg M, Wu M, Neff L, Chiusaroli R, Philbrick WM, Baron R (May 2008). "Doubly truncated FosB isoform (Delta2DeltaFosB) induces osteosclerosis in transgenic mice and modulates expression and phosphorylation of Smads in osteoblasts independent of intrinsic AP-1 activity". J. Bone Miner. Res. 23 (5): 584–595. doi:10.1359/jbmr.080110. PMC 2674536. PMID 18433296.
- Ruffle JK (November 2014). "Molecular neurobiology of addiction: what's all the (Δ)FosB about?". Am J Drug Alcohol Abuse 40 (6): 428–437. doi:10.3109/00952990.2014.933840. PMID 25083822.
ΔFosB as a therapeutic biomarker
The strong correlation between chronic drug exposure and ΔFosB provides novel opportunities for targeted therapies in addiction (118), and suggests methods to analyze their efficacy (119). Over the past two decades, research has progressed from identifying ΔFosB induction to investigating its subsequent action (38). It is likely that ΔFosB research will now progress into a new era – the use of ΔFosB as a biomarker. If ΔFosB detection is indicative of chronic drug exposure (and is at least partly responsible for dependence of the substance), then its monitoring for therapeutic efficacy in interventional studies is a suitable biomarker (Figure 2). Examples of therapeutic avenues are discussed herein. ...
ΔFosB is an essential transcription factor implicated in the molecular and behavioral pathways of addiction following repeated drug exposure. The formation of ΔFosB in multiple brain regions, and the molecular pathway leading to the formation of AP-1 complexes is well understood. The establishment of a functional purpose for ΔFosB has allowed further determination as to some of the key aspects of its molecular cascades, involving effectors such as GluR2 (87,88), Cdk5 (93) and NFkB (100). Moreover, many of these molecular changes identified are now directly linked to the structural, physiological and behavioral changes observed following chronic drug exposure (60,95,97,102). New frontiers of research investigating the molecular roles of ΔFosB have been opened by epigenetic studies, and recent advances have illustrated the role of ΔFosB acting on DNA and histones, truly as a ‘‘molecular switch’’ (34). As a consequence of our improved understanding of ΔFosB in addiction, it is possible to evaluate the addictive potential of current medications (119), as well as use it as a biomarker for assessing the efficacy of therapeutic interventions (121,122,124). Some of these proposed interventions have limitations (125) or are in their infancy (75). However, it is hoped that some of these preliminary findings may lead to innovative treatments, which are much needed in addiction.
- Robison AJ, Nestler EJ (November 2011). "Transcriptional and epigenetic mechanisms of addiction". Nat. Rev. Neurosci. 12 (11): 623–637. doi:10.1038/nrn3111. PMC 3272277. PMID 21989194.
ΔFosB has been linked directly to several addiction-related behaviors ... Importantly, genetic or viral overexpression of ΔJunD, a dominant negative mutant of JunD which antagonizes ΔFosB- and other AP-1-mediated transcriptional activity, in the NAc or OFC blocks these key effects of drug exposure14,22–24. This indicates that ΔFosB is both necessary and sufficient for many of the changes wrought in the brain by chronic drug exposure. ΔFosB is also induced in D1-type NAc MSNs by chronic consumption of several natural rewards, including sucrose, high fat food, sex, wheel running, where it promotes that consumption14,26–30. This implicates ΔFosB in the regulation of natural rewards under normal conditions and perhaps during pathological addictive-like states.
- Olsen CM (December 2011). "Natural rewards, neuroplasticity, and non-drug addictions". Neuropharmacology 61 (7): 1109–1122. doi:10.1016/j.neuropharm.2011.03.010. PMC 3139704. PMID 21459101.
Cross-sensitization is also bidirectional, as a history of amphetamine administration facilitates sexual behavior and enhances the associated increase in NAc DA ... As described for food reward, sexual experience can also lead to activation of plasticity-related signaling cascades. The transcription factor delta FosB is increased in the NAc, PFC, dorsal striatum, and VTA following repeated sexual behavior (Wallace et al., 2008; Pitchers et al., 2010b). This natural increase in delta FosB or viral overexpression of delta FosB within the NAc modulates sexual performance, and NAc blockade of delta FosB attenuates this behavior (Hedges et al, 2009; Pitchers et al., 2010b). Further, viral overexpression of delta FosB enhances the conditioned place preference for an environment paired with sexual experience (Hedges et al., 2009). ... In some people, there is a transition from “normal” to compulsive engagement in natural rewards (such as food or sex), a condition that some have termed behavioral or non-drug addictions (Holden, 2001; Grant et al., 2006a). ... In humans, the role of dopamine signaling in incentive-sensitization processes has recently been highlighted by the observation of a dopamine dysregulation syndrome in some patients taking dopaminergic drugs. This syndrome is characterized by a medication-induced increase in (or compulsive) engagement in non-drug rewards such as gambling, shopping, or sex (Evans et al, 2006; Aiken, 2007; Lader, 2008)."Table 1"
- Nestler EJ (December 2013). "Cellular basis of memory for addiction". Dialogues Clin. Neurosci. 15 (4): 431–443. PMC 3898681. PMID 24459410.
DESPITE THE IMPORTANCE OF NUMEROUS PSYCHOSOCIAL FACTORS, AT ITS CORE, DRUG ADDICTION INVOLVES A BIOLOGICAL PROCESS: the ability of repeated exposure to a drug of abuse to induce changes in a vulnerable brain that drive the compulsive seeking and taking of drugs, and loss of control over drug use, that define a state of addiction. ... A large body of literature has demonstrated that such ΔFosB induction in D1-type NAc neurons increases an animal's sensitivity to drug as well as natural rewards and promotes drug self-administration, presumably through a process of positive reinforcement ... A large body of literature has demonstrated that such ΔFosB induction in D1-type NAc neurons increases an animal's sensitivity to drug as well as natural rewards and promotes drug self-administration, presumably through a process of positive reinforcement ... Another ΔFosB target is cFos: as ΔFosB accumulates with repeated drug exposure it represses c-Fos and contributes to the molecular switch whereby ΔFosB is selectively induced in the chronic drug-treated state.41 Many other ΔFosB targets have been shown to mediate the ability of certain drugs of abuse to induce synaptic plasticity in the NAc and associated changes in the dendritic arborization of NAc medium spiny neurons, as will be discussed below.
- Ohnishi YN, Ohnishi YH, Vialou V, Mouzon E, LaPlant Q, Nishi A, Nestler EJ (October 2014). "Functional role of the N-terminal domain of ΔFosB in response to stress and drugs of abuse". Neuroscience 284C: 165–170. doi:10.1016/j.neuroscience.2014.10.002. PMID 25313003.
- Nakabeppu Y, Nathans D (February 1991). "A naturally occurring truncated form of FosB that inhibits Fos/Jun transcriptional activity". Cell 64 (4): 751–759. doi:10.1016/0092-8674(91)90504-R. PMID 1900040.
- Blum K, Werner T, Carnes S, Carnes P, Bowirrat A, Giordano J, Oscar-Berman M, Gold M (2012). "Sex, drugs, and rock 'n' roll: hypothesizing common mesolimbic activation as a function of reward gene polymorphisms". J. Psychoactive Drugs 44 (1): 38–55. doi:10.1080/02791072.2012.662112. PMC 4040958. PMID 22641964.
It has been found that deltaFosB gene in the NAc is critical for reinforcing effects of sexual reward. Pitchers and colleagues (2010) reported that sexual experience was shown to cause DeltaFosB accumulation in several limbic brain regions including the NAc, medial pre-frontal cortex, VTA, caudate, and putamen, but not the medial preoptic nucleus. Next, the induction of c-Fos, a downstream (repressed) target of DeltaFosB, was measured in sexually experienced and naive animals. The number of mating-induced c-Fos-IR cells was significantly decreased in sexually experienced animals compared to sexually naive controls. Finally, DeltaFosB levels and its activity in the NAc were manipulated using viral-mediated gene transfer to study its potential role in mediating sexual experience and experience-induced facilitation of sexual performance. Animals with DeltaFosB overexpression displayed enhanced facilitation of sexual performance with sexual experience relative to controls. In contrast, the expression of DeltaJunD, a dominant-negative binding partner of DeltaFosB, attenuated sexual experience-induced facilitation of sexual performance, and stunted long-term maintenance of facilitation compared to DeltaFosB overexpressing group. Together, these findings support a critical role for DeltaFosB expression in the NAc in the reinforcing effects of sexual behavior and sexual experience-induced facilitation of sexual performance. ... both drug addiction and sexual addiction represent pathological forms of neuroplasticity along with the emergence of aberrant behaviors involving a cascade of neurochemical changes mainly in the brain's rewarding circuitry.
- Nestler EJ (October 2008). "Review. Transcriptional mechanisms of addiction: role of DeltaFosB". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 363 (1507): 3245–3255. doi:10.1098/rstb.2008.0067. PMC 2607320. PMID 18640924.
Recent evidence has shown that ΔFosB also represses the c-fos gene that helps create the molecular switch—from the induction of several short-lived Fos family proteins after acute drug exposure to the predominant accumulation of ΔFosB after chronic drug exposure—cited earlier (Renthal et al. in press). The mechanism responsible for ΔFosB repression of c-fos expression is complex and is covered below. ...
Examples of validated targets for ΔFosB in nucleus accumbens ... GluR2 ... dynorphin ... Cdk5 ... NFκB ... c-Fos
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- Robison AJ, Nestler EJ (November 2011). "Transcriptional and epigenetic mechanisms of addiction". Nat. Rev. Neurosci. 12 (11): 623–637. doi:10.1038/nrn3111. PMC 3272277. PMID 21989194.
ΔFosB serves as one of the master control proteins governing this structural plasticity.
- Nestler EJ, Barrot M, Self DW (September 2001). "DeltaFosB: a sustained molecular switch for addiction". Proc. Natl. Acad. Sci. U.S.A. 98 (20): 11042–11046. doi:10.1073/pnas.191352698. PMC 58680. PMID 11572966.
Although the ΔFosB signal is relatively long-lived, it is not permanent. ΔFosB degrades gradually and can no longer be detected in brain after 1–2 months of drug withdrawal ... Indeed, ΔFosB is the longest-lived adaptation known to occur in adult brain, not only in response to drugs of abuse, but to any other perturbation (that doesn't involve lesions) as well.
- Malenka RC, Nestler EJ, Hyman SE (2009). "Chapter 15: Reinforcement and Addictive Disorders". In Sydor A, Brown RY. Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd ed.). New York: McGraw-Hill Medical. pp. 364–375. ISBN 9780071481274.
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Recent evidence has shown that ΔFosB also represses the c-fos gene that helps create the molecular switch—from the induction of several short-lived Fos family proteins after acute drug exposure to the predominant accumulation of ΔFosB after chronic drug exposure—cited earlier (Renthal et al. in press).
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The 35-37 kD ΔFosB isoforms accumulate with chronic drug exposure due to their extraordinarily long half-lives. ... As a result of its stability, the ΔFosB protein persists in neurons for at least several weeks after cessation of drug exposure. ... ΔFosB overexpression in nucleus accumbens induces NFκB
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G9a appears to be a critical control point for epigenetic regulation in NAc, as we know it functions in two negative feedback loops. It opposes the induction of ΔFosB, a long-lasting transcription factor important for drug addiction (Robison and Nestler, 2011), while ΔFosB in turn suppresses G9a expression (Maze et al., 2010; Sun et al., 2012a). Interestingly, a prior history of cocaine exposure, followed by one month of withdrawal, leads to enhanced inducibility of the FosB gene in response to a subsequent cocaine challenge (Damez-Werno et al., 2012a). This priming is not associated with changes in the upstream signaling pathways that control FosB expression, thus pointing to a chromatin mechanism. Indeed, the priming is associated with reduced H3K9me2 binding at the FosB gene as well as with enrichment of a particular phosphorylated form of RNA polymerase II which has been associated with gene priming in cell culture systems (Damez-Werno et al., 2012a). FosB gene priming thus represents an important example of latent regulation that is mediated via chromatin mechanisms, although much further work is needed to understand the underlying mechanisms involved.
- Pitchers KK, Vialou V, Nestler EJ, Laviolette SR, Lehman MN, Coolen LM (February 2013). "Natural and drug rewards act on common neural plasticity mechanisms with ΔFosB as a key mediator". J. Neurosci. 33 (8): 3434–3442. doi:10.1523/JNEUROSCI.4881-12.2013. PMC 3865508. PMID 23426671.
Drugs of abuse induce neuroplasticity in the natural reward pathway, specifically the nucleus accumbens (NAc), thereby causing development and expression of addictive behavior. ... Together, these findings demonstrate that drugs of abuse and natural reward behaviors act on common molecular and cellular mechanisms of plasticity that control vulnerability to drug addiction, and that this increased vulnerability is mediated by ΔFosB and its downstream transcriptional targets. ... Sexual behavior is highly rewarding (Tenk et al., 2009), and sexual experience causes sensitized drug-related behaviors, including cross-sensitization to amphetamine (Amph)-induced locomotor activity (Bradley and Meisel, 2001; Pitchers et al., 2010a) and enhanced Amph reward (Pitchers et al., 2010a). Moreover, sexual experience induces neural plasticity in the NAc similar to that induced by psychostimulant exposure, including increased dendritic spine density (Meisel and Mullins, 2006; Pitchers et al., 2010a), altered glutamate receptor trafficking, and decreased synaptic strength in prefrontal cortex-responding NAc shell neurons (Pitchers et al., 2012). Finally, periods of abstinence from sexual experience were found to be critical for enhanced Amph reward, NAc spinogenesis (Pitchers et al., 2010a), and glutamate receptor trafficking (Pitchers et al., 2012). These findings suggest that natural and drug reward experiences share common mechanisms of neural plasticity
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Role of ΔFosB in Depression
More recently, we have shown that induction of ΔFosB in nucleus accumbens in response to chronic stress represents a positive, adaptive mechanism to help the animal cope with the stress. In the social defeat paradigm, for example, animalsthat are resilient to the deleterious effects of defeat stress show greater induction of ΔFosB than vulnerable animals. Moreover, chronic administration of antidepressant medications induces ΔFosB in nucleus accumbens and the behavioral effects of these treatments can be blocked by blockade of ΔFosB activity in this brain region. Together, these data demonstrate that ΔFosB is a novel mechanism of resilience and a potentially important mediator of antidepressant action. ...
Interesting comparisons and contrasts with CREB are evident. Both ΔFosB and CREB are induced by stress and by drugs of abuse, yet they exert opposite effects on behavior. CREB reduces behavioral responses to emotional stimuli and induces a depression-like state in the extreme, whereas ΔFosB sensitizes reward and induces antidepressant-like responses. Also, the CREB signal is relatively short-lived, while the ΔFosB signal is long-lived.
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- Image legend
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