INTRODUCTION
The transcription factor, DeltaFosB, is known to be involved in some of the physiological mechanisms linked to addiction and compulsive behaviors. Levels of DeltaFosB have been shown to increase in multiple regions of the brain in response to repeated drug exposure as well as stress, certain antipsychotic or antidepressant medications, electroconvulsive seizures, and certain lesions. DeltaFosB is a truncated, highly stable splice variant of the FosB transcription factor whose expression is regulated by alternative splicing of the FosB gene. Due to this splicing, DeltaFosB is known to have a half life five times that of FosB, which is thought to be a basis for addiction [1].
STRUCTURE
The transcriptional factor DeltaFosB is unique in that it is a more stable form of FosB. There are two proposed regions towards the C-terminal that cause instability in FosB. DeltaFosB lacks a sequence of 101 amino acids (238-338) that are present in the FosB protein. This missing sequence contains destabilizing regions that contribute to proteasome-dependent and proteasome-independent FosB degradation. Proteins that undergo degradation are typically targeted by polyubiquitylation. HA-ubiquitin has been detected in FosB immunoprecipitations, but not in DeltaFosB, which is consistent with findings that DeltaFosB is not degraded by polyubiquitylation.
In general, Transcription Factors contain three domains, a Trans-Activating Domain, a DNA Binding Domain, and a Signal Sensing Domain. Three domains have been consistently noted on FosB. (See Figure 1) The first is a bZIP domain, located on amino acids 155-218. The second is a basic motif, located on amino acids 157-182, and the third is a leucine-zipper, located on amino acids 183-211.
Alternative splicing removes two destabilizing elements from FosB generating the more stable DeltaFosB transcription factor. A 140-nucleotide sequence is removed from exon 4 of the primary FosB transcript, resulting in a one-nucleotide frameshift and the formation of an early stop codon (TGA). This results in premature termination of normal FosB translation; therefore, proteins translated from DeltaFosB mRNA are missing an important amino acid sequence which is normally present towards the C-terminal of full-length FosB proteins. FosB contains a sequence of amino acids (278-337) that is normally targeted for degradation by proteasomes. The DeltaFosB variant lacks this sequence resulting in increased stability as it is not recognized and therefore not degraded by proteasomes. The other region identified for destabilization of FosB, independent of preteasome degradation, is a sequence of amino acids (238-277). The mechanism for destabilization in this region is not as well understood as the proteasome-dependent destabilizing region[2].
FIGURE:
This is a crystal structure of FosB from Staphylococcus aureus. This is a similar structure to DeltaFosB, although it retains 2 destabilizing elements which are spliced in the DeltaFosB variant. The DeltaFosB transcription factor should include 3 domains: a transactivating domain, a DNA binding domain, and a signal sensing domain. DeltaFosB includes is a basic motif, a Leucine zipper, and a bZIP domain scene
PDB ID: 4NB0
Structure and Function of the Genomically Encoded Fosfomycin Resistance Enzyme, FosB, from Staphylococcus aureus.
(2014) Biochemistry 53: 755-765
FUNCTION
FosB is encoded by the FOSB gene located on human chromosome 19. The Fos gene family consists of four main members (Fos, FosB, FosL1, and FosL2) that heterodimerize with Jun family proteins. The Leucine Zipper motif found in FosB is conserved in DeltaFosB, therefore the heterodimerization with Jun family proteins will form the Activator Protein-1 (AP-1) transcription factor complex. AP-1 complexes function to regulate gene expression by binding to AP-1 sites on promoter sequences. DeltaFosB makes more than 50 AP-1 dimeric complexes that bind to promoter regions of a wide variety of mammalian genes. For example, DeltaFosB has been shown to upregulate gene expression of NFκB, CDK5, and GluR2 involved with addiction [3].
DeltaFosB isoforms accumulate in the dopaminergic pathways of the brain with chronic drug use because of their long half-lives. The transcription factor will remain expressed in neurons for weeks following the discontinuation of drug use. This is made possible by the increased stability resulting from the lack of degron domains present in the C-terminus of FosB and the phosphorylation of DeltaFosB at its N terminus. These alterations that result in the accumulation of DeltaFosB could potentially explain the long-term effects of drug withdrawal resulting from changes in gene expression [4].
It is likely that DeltaFosB plays a role in mediating some of the neural and behavioral sensitivity resulting from chronic drug exposure. Kelz and colleagues (1999) found that mice induced to express high levels of DeltaFosB in the area of the nucleus accumbens that is associated with production of the transcription factor had heightened responses to cocaine exposure. DeltaFosB-expressing mice were seen to have higher levels of activity compared to their negative counterparts upon initial cocaine injection as well as sustained increase in activity upon subsequent cocaine injections. Expression of DeltaFosB has also been shown to increase responses to the rewarding effects of cocaine [5]. DeltaFosB-positive mice spent approximately 3 times longer in a compartment associated with drug exposure than did mice not expressing DeltaFosB.
PSYCHOLOGICAL INFLUENCES
The regulation of gene expression is believed to be related to some of the behavioral abnormalities associated with drug dependency. Due to genetic changes that are related to chronic drug use, transcription factors have been implicated in the mechanisms of addiction [1]. Addiction is thought to develop through certain pathways in the brain following chronic drug use. These areas include the nucleus accumbens, the ventral tegmental area, and the mesolimbic/dopaminergic pathway. Studies have shown that with long term use of amphetamine, cocaine, ethanol, methamphetamine, morphine, neuroleptics, nicotine, and THC, DeltaFosB accumulates primarily in the NAc and dorsal striatum.
Studies have shown that when DeltaFosB has been overexpressed in mice, drug-seeking behavior is increased. Additionally, it increases sensitivity to opioids, and causes greater dependence and tolerance to morphine.
It has also been found that levels of DeltaFosB are higher when drugs are self-administered rather than experimentally injected. This implies that DeltaFosB levels may be affected by behavioral rather than purely physiological effects of the drug.
This effect is underscored by a study which measured levels of DeltaFosB following a period of increased sexual activity [6]. After a period of abstinence of 1, 7, or 28 days, levels of DeltaFosB in the Nucleus Accumbens were measured. As found in drug studies, DeltaFosB persisted in the NAc neurons of sexually active rats for at least 28 days after abstaining from the reward behavior. It can be concluded that there are similarities in the effects of both natural and drug rewards on the mesolimbic system.
