In this study, we provide evidence that accumulated ΔFosB increases locomotor activity, and that FosB antagonizes this effect similar to findings from in vitro
cellular assays (1
). On the other hand, stress tolerance appears to be the sum of ΔFosB and FosB. These effects may be partly mediated via E-cadherin, an indirect target of fosB
gene products. Together, these data suggest distinct patterns of behavioral abnormalities among the mutant mouse lines examined (). We propose that fosBG/G
mice exhibit behaviors that resemble depression, including decreased locomotion, increased immobility during forced swimming, and increased anxiety-like responses. In contrast, fosB+/d
mice exhibit behaviors that in some ways resemble manic-like symptoms, including hyperlocomotion, increased stress tolerance, and reduced anxiety-like responses. Interestingly, fosBd/d
mice exhibit significantly higher dopamine sensitivity, and most of the altered behaviors seen in fosB+/d
mice except for increased stress tolerance. These mice thus present a picture of blended responses perhaps reminiscent of certain aspects of bipolar disorder. Clearly, these interpretations are based on initial analyses in rodent models and require much further work for validation, including studying these molecular findings in the human disorders.
Figure 6 Summary of the effects of different levels of fosB gene products on locomotor activity and stress tolerance. FosB and accumulated ΔFosB amplify stress tolerance and E-cadherin expression cooperatively. Accumulated ΔFosB facilitates Akt (more ...)
The similar results obtained from fosBG/G
mice in the water maze () and repeated forced swim test () indicate that the water maze has the potential to reveal responses to stress. Whereas the repeated forced swim test revealed the stress vulnerability of fosBd/d
mice more clearly than the water maze, the difference between wild-type and fosB+/d
mice was more apparent in the water maze, suggesting that the repeated forced swim test may be better to detect initial stress vulnerability, while the water maze may be better to detect stress tolerance that develops over time. Data with paroxetine suggest that fosB
gene products are partly required for antidepressant responses to the drug, as reported recently (24
mice, the antagonistic relationship between FosB and accumulated ΔFosB, first seen in vitro
), was captured in measures of spontaneous locomotor activity (, Figure S2A in Supplement 1
), elevated plus maze (), and sensitivity to a D2 antagonist (, Figure S5G in Supplement 1
). The expression level of ΔFosB in fosB+/d
mice is much higher than that of FosB, but FosB has much higher trans-activity than ΔFosB (1
). These data suggest that lower expression levels of FosB may be sufficient to suppress the activity of accumulated ΔFosB, and thereby normalize some of the consequences of its accumulation (Figure S7 in Supplement 1
). Such antagonism is not seen in all cases, perhaps reflecting the different cell types or target genes involved. In ΔFosB bitransgenic mice, where ΔFosB expression is inducible and relatively restricted to D1 neurons in NAc and dorsal striatum, Kelz et al.
reported higher locomotor activity in a novel test chamber not on the first day but on the second day (14
). This finding corresponds to observations in fosB+/d
mice, which exhibited higher locomotor activity with a similar pattern (), thus suggesting that ΔFosB accumulation in striatum plays an important role in mediating locomotor activation via dopamine signaling.
We know that the fosB
gene is induced by dopaminergic signals (23
). Since Akt phosphorylation is also induced by dopamine (35
), it is possible that accumulated ΔFosB enhances Akt phosphorylation, and that the extent of spontaneous dopaminergic activation reflects levels of Akt phosphorylation. Perrotti et al
. reported that morphine induces ΔFosB accumulation in NAc (23
), and Russo et al
. reported that overexpression of a constitutively active form of Akt in NAc enhances locomotor sensitization induced by morphine (36
). Locomotor sensitization by morphine is regulated by indirect dopaminergic mechanisms such as increased dopamine release in NAc (37
). Cocaine directly increases dopamine release in NAc by inhibiting dopamine reuptake. Overexpression of ΔFosB enhanced cocaine locomotor sensitization (39
), suggesting that enhanced Akt phosphorylation by accumulated ΔFosB might facilitate locomotor sensitization by cocaine and by morphine. This is consistent with the significant up-regulation of Akt phosphorylation and enhanced locomotor sensitization by MPH observed in fosBd/d
mice (, S6C in Supplement 1
). The mechanism by which ΔFosB accumulation leads to increased Akt phosphorylation is not known, since ΔFosB could affect the expression of any of several regulators of Akt phosphorylation (40
). One possibility is that accumulated ΔFosB may repress the expression of a protein phosphatase, such as PP2A, since Akt phosphorylation is known to be suppressed by D2 signaling in striatum via the activation this phosphatase (41
). These and other possibilities now require direct investigation.
-knockout (KO) mice had been established by Brown et al
), and actually have a potential to express Δ3ΔFosB, an alternative-translation initiation product similar to Δ2ΔFosB (Figure S1 in Supplement 1
). Hiroi et al.
reported that fosB
-KO mice showed slightly higher locomotor activity when they were introduced into a novel test chamber (12
). This corresponds to our observation that fosBG/G
mice exhibit significantly higher locomotor activity in the open field on the first day (), suggesting that FosB may suppress exploratory behavior in the novel environment independent of the presence of ΔFosB. On the other hand, Zhu et al.
) reported that the fosB
-KO mice exhibit lower sensitivity to other types of stress paradigms (tail suspension test), and Brown et al.
) had reported that the fosB
-KO mice are defect in nurturing, and no difference in the Morris water maze test. These findings are not consistent wit h our results, since fosBG/G
mice exhibited higher stress vulnerability and no defect in nurturing (data not shown). These differences could reflect the potential influence of Δ3ΔFosB or the different genetic backgrounds (BALB-C in the prior studies by Brown et al.
and C57BL6/J in the present investigations).
Interestingly, the expression level of E-cadherin and the stress tolerance observed upon repeated forced swimming paralleled the combined expression of FosB and ΔFosB. Only with regard to the particular expression pattern and behavioral phenotype does FosB not antagonize but coordinates with ΔFosB. ΔFosB has been reported to accumulate after chronic stress in several brain regions (22
), and accumulated ΔFosB in the ventrolateral periaqueductal gray promotes active coping responses against stress, such as forced swimming (13
). More recently, overexpression of ΔFosB in the NAc suppresses the deleterious effects of social defeat (24
). Findings from the present study suggest that such antidepressant-like effects of ΔFosB require the presence of FosB.
E-cadherin is an important cell-cell adhesion molecule, and influences synapse formation in neuronal cells (44
). In rodent models of depression, the number of mature synapse buttons or dendritic spines is decreased for several neuronal cell types, and antidepressant treatments reverse these abnormalities (45
). Here, we found that the expression level of E-cadherin in fosB
mutant and wild-type mice parallels stress tolerance, suggesting that E-cadherin may be one mechanism underlying the stress tolerance induced by fosB
gene products. FosB might inhibit E-cadherin degradation via transactivation of an inhibitory binding protein against Hakai, which is an E3-ligase of E-cadherin, or directly inhibit the transcription of Hakai mRNA (48
). Accumulated ΔFosB might increase E-cadherin translation via TGF-β signaling, which is increased in fosB+/d
ES cells (3
). Future studies are needed to explore these possibilities directly.
Cdk5 and GluR2, putative ΔFosB targets identified in bitransgenic mouse models (14
), are expressed at similar levels among fosB
mutants and wild type. McClung and Nestler reported that the expression levels of various ΔFosB targets in these mice switched from up- to down-regulated, or vice versa, between two and eight weeks after turning on ΔFosB expression (9
). The constitutive changes in fosB
gene expression in our mutant mice might not result in altered expression of Cdk5 and GluR2.
In summary, the fosBG/G mutant is a definitive fosB-null mouse, and fosB+/d and fosBd/d mutants differentially express aberrant levels of ΔFosB and FosB from the endogenous fosB gene. These three mouse l ines exhibit behavioral abnormalities mimicking different types of mood disorders. Further studies of these mice will shed light on the mechanisms controlling mood disorder-related behaviors, and thus contributing to the development of improved treatments for these disorders.
fosB gene encodes FosB and ΔFosB proteins. ΔFosB is highly stable and accumulated in specific regions of brain after stress or antidepressant treatments. Using two types of new fosB mutant mice, we found that accumulated ΔFosB increases locomotor activity and elevates Akt phosphorylation independent of FosB, while it acts as anti-depressant cooperatively with FosB and the effect is paralleled with E-cadherin expression in striatum. These data suggests the importance of fosB gene in mood disorder.