Regulation of ΔFosB Induction and Activity
While ΔFosB, like all other Fos proteins, dimerizes with a Jun family member to form active AP1 complexes (Nestler 2008), there is evidence from cell-free preparations that ΔFosB can also homodimerize (Jorissen et al. 2007). In vitro evidence suggests that such ΔFosB homodimers might exert different functional effects compared with ΔFosB:Jun heterodimers, although further work is needed to study this hypothesis in vivo.
The FosB gene promoter has not been extensively studied. It contains a CRE (cAMP response element) and SRE (serum response element) site which, by analogy to the c-Fos gene, presumably mediate its rapid induction in response to an acute stimulus, although this has not been investigated directly. While several forms of chronic stimulation induce the accumulation of ΔFosB protein in brain, as will be described in the next sections, the induction by different stimuli differentially requires CREB (CRE binding protein, which activates CREs) vs. SRF (serum response factor, which activates SREs) (Vialou et al. 2012). The basis of such different requirements for ΔFosB induction remains unknown. Progress has also been made in defining the chromatin changes that occur at the FosB locus in concert with its induction or suppression (Kumar et al. 2005; Maze et al. 2010; Heller et al. 2014).
An additional protein product has been shown to be translated from ΔFosB mRNA in both cultured cells and brain based on the use of an alternative translation start site. This results in a protein termed Δ2ΔFosB which lacks the 78 N-terminal amino acids in ΔFosB. The function of Δ2ΔFosB is poorly understood; it lacks behavioral effects when overexpressed in brain (Ohnishi et al. 2015).
Role of ΔFosB in Drug Addiction
While ΔFosB induction occurs in many cell types, it has been best studied in the nervous system, where its unique temporal properties were first elaborated in a brain region called the nucleus accumbens, important for mediating reward, in response to chronic exposure to cocaine (Hope et al. 1994; Hiroi et al. 1997). At baseline, appreciable levels of ΔFosB are seen in this brain region, with virtually nondetectable levels seen throughout the rest of brain. The factors driving this uniquely high basal expression of the protein in nucleus accumbens remain unknown. A single dose of cocaine causes the rapid but transient induction of all Fos family proteins in this brain region, which dissipate within hours. By contrast, the repeated administration of cocaine causes the gradual accumulation of ΔFosB and the relative desensitization of other Fos family proteins, resulting in the molecular switch described above (see insert in Fig. 1) (Hope et al. 1994; Hiroi et al. 1997; Nestler 2008). Research has shown that chronic administration of every known drug of abuse shares this induction of ΔFosB in the nucleus accumbens, with equivalent induction seen in animals that volitionally self-administer the drugs to themselves. Importantly, such induction of ΔFosB has been documented in the nucleus accumbens of human cocaine addicts examined postmortem (Robison et al. 2013).
The predominant neuronal cell type in the nucleus accumbens is termed the medium spiny neuron (MSN), which account for ~95% of all neurons in this brain region. Two major MSN subtypes have been described based on the predominant type of dopamine receptor they express: D1-type MSNs and D2-type MSNs. ΔFosB induction in nucleus accumbens in response to chronic drug exposure occurs selectively in neurons and virtually exclusively in D1-type MSNs for all drugs of abuse with one exception. Opiate drugs of abuse induce ΔFosB roughly equally in D1- and D2-type MSNs (Lobo et al. 2013).
Several lines of evidence support the hypothesis that induction of ΔFosB in D1-type MSNs in nucleus accumbens increases an individual’s sensitivity to the rewarding effects of a drug of abuse and promote a state of addiction. The inducible overexpression of ΔFosB selectively in this neuronal cell type of adult animals, either by use of inducible bitransgenic mice or Cre-dependent viral gene transfer, increases the ability of cocaine to increase locomotor activity, place conditioning, intracranial self-stimulation, and drug self-administration (Nestler 2008; Muschamp et al. 2012; Grueter et al. 2013). In contrast, blockade of ΔFosB activity, via overexpression of a dominant negative antagonist, exerts the opposite effects. Overexpression of ΔFosB or a dominant negative antagonist in nucleus accumbens yields similar behavioral effects for other drugs of abuse including opiates. Together, these findings have led to the hypothesis that ΔFosB induction in D1-type MSNs in nucleus accumbens contributes to a shared mechanism of drug addiction (Nestler 2008). The consequences of ΔFosB induction in D2-type MSNs on drug-related behaviors are not known.
Drugs of abuse induce ΔFosB, with similar temporal patterns, in several other brain regions although in most cases its behavioral effects remain poorly defined. For example, chronic self-administration of cocaine induces ΔFosB in orbitofrontal cortex where it has been shown to contribute to the increase in compulsive- and impulsive-like behavior seen in the drug-addicted state (Nestler 2008).
Role of ΔFosB in Stress Responses
As reported originally for drugs of abuse, exposure to any of several types of stress produces a very similar pattern of Fos family expression in the nucleus accumbens, with acute stress causing the rapid but transient induction of all Fos proteins but chronic stress leading to the selective accumulation of ΔFosB and relative loss of induction of the other proteins (Nestler 2014). In contrast, one type of stress – prolonged social isolation during adulthood – results in the opposite change, a sustained suppression of the normally high levels of ΔFosB expressed under baseline conditions. Interestingly, depressed humans also show reduced ΔFosB levels in nucleus accumbens (Vialou et al. 2010).
Insight into the contribution of ΔFosB to stress responses came initially from the mapping of the cell-type specificity of its induction in the chronic social defeat stress paradigm. Here, roughly two-thirds of stressed mice – termed susceptible – show a range of depression-related behavioral abnormalities, whereas the remaining one-third avoid these deleterious effects and are termed resilient. While initial studies demonstrated roughly equal ΔFosB induction in both groups of mice (Vialou et al. 2010), a more recent study found clear differences in the cell type affected: ΔFosB induction occurs selectively in D1-type MSNs in resilient mice, whereas such induction occurs selectively in D2-type MSNs in susceptible mice (Lobo et al. 2013). Consistent with these observations, selective overexpression of ΔFosB in D1-type MSNs in adult nucleus accumbens promotes resilience in chronic stress paradigms (Vialou et al. 2010). Such induction also promotes the consumption of several natural rewards, such as drinking sucrose, eating a high fat diet, engaging in sexual behavior, and running on an exercise wheel (Nestler 2008, 2015). These actions align with ΔFosB’s ability to promote rewarding responses to drugs of abuse as well and suggest a general role for ΔFosB in enhancing reward and motivation under normal circumstances, effects which can contribute to an addiction syndrome in the face of particularly powerful drug rewards. Presumably, the suppression of basal levels of ΔFosB expression seen in the nucleus accumbens of depressed humans occurs in D1-type MSNs, consistent with a loss of reward, but this requires experimental confirmation.
Chronic stress induces ΔFosB in several other brain regions beyond the nucleus accumbens, although its roles in these regions in regulating responses to stress remain largely unknown (Nestler 2014).
Other ΔFosB Actions
ΔFosB has been shown to be induced by several other forms of chronic stimulation. It is induced in nucleus accumbens, dorsal striatum, and regions of frontal cortex by chronic exposure to antipsychotic drugs (Dietz et al. 2014). In the former two regions, this induction is selective for D2-type MSNs, while in the latter it occurs predominantly in pyramidal neurons (Lobo et al. 2013; Dietz et al. 2014). Such induction of ΔFosB might contribute to side effects of these mediations (Dietz et al. 2014). The protein is also very highly induced in dorsal striatum upon denervation of dopaminergic inputs to the dorsal striatum combined with repeated administration of L-Dopa. Evidence suggests that ΔFosB, under these conditions, promotes the development of dyskinesias (Feyder et al. 2016), which might contribute to dyskinetic side effects seen in patients with Parkinson’s disease who are treated with L-Dopa. Finally, ΔFosB is induced in hippocampal dentate granule cell neurons upon treatment with kainic acid, a neurotoxin, although the functional consequences of such induction are not known (Mandelzys et al. 1997).
Much less is known about the influence of ΔFosB in peripheral tissues. One notable exception is bone and fat, where ΔFosB overexpression has been shown to promote bone formation and inhibit adipogenesis, respectively (Sabatakos et al. 2000). Current research is aimed at understanding the role of endogenous ΔFosB in regulating these endpoints and explicating its mechanism of action.
ΔFosB Target Genes
As noted above, ΔFosB would be expected to regulate the expression of target genes through its binding (primarily as ΔFosB:Jun heterodimers but possibly as ΔFosB homodimers as well) to AP1 response elements located within the promoters of responsive genes. As with most transcription factors, ΔFosB binding can lead either to the induction or repression of its target genes. Numerous target genes have been identified for ΔFosB, mostly in addiction and depression models given the important role of the protein in these syndromes.
One important target for ΔFosB, in both addiction and depression models, is the gene encoding the GluA2 subunit of AMPA glutamate receptors, identified originally through a candidate gene approach (Nestler 2008). AMPA receptors containing GluA2 are Ca2+-impermeable and exhibit lower overall current when activated. ΔFosB induces GluA2 in nucleus accumbens MSNs, and this effect is associated with reduced AMPA responses and a higher number of so-called silent synapses (Vialou et al. 2010; Grueter et al. 2013; Robison et al. 2013). Consistent with these data, ΔFosB has also been shown to mediate the ability of chronic cocaine administration to induce the growth of new, immature thin dendritic spines on nucleus accumbens MSNs (Maze et al. 2010; Robison et al. 2013). GluA2 is likely just one of many synaptic proteins whose expression is controlled by ΔFosB. As just one example, ΔFosB mediates the induction of CDK5 (cyclin-dependent protein kinase 5) in nucleus accumbens in response to chronic cocaine administration (Kumar et al. 2005; Maze et al. 2010).
Another candidate target of ΔFosB is the gene encoding the opioid peptide, dynorphin. ΔFosB suppresses dynorphin expression in the nucleus accumbens (Nestler 2008). Since dynorphin acts to oppose reward mechanisms, this action of ΔFosB likely contributes to its pro-reward and pro-resilience behavioral effects in addiction and depression models, respectively. Another candidate gene suppressed by ΔFosB in nucleus accumbens is c-Fos, thus contributing to the selective induction of ΔFosB, as opposed to other Fos family proteins, as ΔFosB accumulates (see Fig. 1) (Nestler 2008).
Increasingly, unbiased genome-wide methods are being used to identify a broader range of ΔFosB target genes. McClung and Nestler (2003) used DNA microarrays to characterize genome-wide changes in gene expression in the nucleus accumbens upon overexpression of ΔFosB or a dominant negative antagonist in the absence or presence of cocaine administration. Renthal et al. (2009) used ChIP-chip (chromatin immunoprecipitation followed by analysis on promoter chips) to identify gene promoters where the binding of endogenous ΔFosB is altered in the nucleus accumbens in response to chronic cocaine administration. These studies identified numerous novel ΔFosB targets which are now being investigated in greater detail. An example is Sirt1, which encodes a protein deacetylase implicated in numerous cellular processes. While this work must be extended by use of more modern experimental approaches (in particular, RNA-seq and ChIP-seq), it illustrates how open-ended methods reveal fundamentally new insight into the biochemical pathways controlled by ΔFosB and, hence, involved in a range of neuropsychiatric syndromes.
Transcription factors control the transcription of target genes by recruiting to those genes a host of chromatin-regulatory proteins which influence nucleosome structure and ultimately the binding of the transcriptional machinery. Not surprisingly, therefore, ΔFosB binding to genes is associated with numerous chromatin modifications, such as histone acetylation and methylation as just two examples (Kumar et al. 2005; Renthal et al. 2009; Maze et al. 2010). An important area for future research is to determine what controls ΔFosB’s action in recruiting co-activators and thereby inducing certain genes, while recruiting co-repressors and suppressing other genes. This could be due to differences in the sequences that flank the AP1 sites in the two types of genes or, alternatively, to differences in the preexisting chromatin landscape of the affected genes.
ΔFosB is a unique transcription factor given its relatively slow accumulation in response to a repeated or chronic stimulus and its unusual stability which underlies its relatively prolonged expression even weeks after the stimulus ends. ΔFosB thus represents a novel mechanism governing relatively prolonged plasticity. The functional consequences of such ΔFosB accumulation are cell-type and stimulus specific. Its induction in D1-type MSNs in nucleus accumbens promotes reward and motivation which can increase an individual’s resilience to stress but, in excess, in the presence of a drug of abuse contributes to an addictive state. In contrast, its induction in D2-type MSNs seems to promote susceptibility to stress. ΔFosB has also been implicated in mediating some of the side effects of antipsychotic medications as well as of L-Dopa. ΔFosB has been used as a means of revealing novel features of the biology underlying these various conditions. In other words, given its several distinctive features, identification of its target genes provides new insight into the range of genes and biochemical pathways affected. In addition, because it is so well characterized and so many experimental tools are available for its manipulation, it has been used effectively to interrogate how signaling pathways in the brain control gene expression through the regulation of chromatin endpoints.
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