Signaling to the Nucleus Regulation of Gene Expression
Regulation of gene expression is one molecular mechanism that would be expected to lead to relatively stable changes within neurons. According to this scheme, repeated exposure to a drug of abuse or stress, by causing repeated perturbation of intracellular signal transduction pathways, would lead eventually to changes in nuclear function and to altered rates of transcription of particular target genes. Altered expression of these genes would lead to altered activity of the neurons in which those changes occur and, ultimately, to changes in the neural circuits in which those neurons operate. The result would be stable changes in behavior.
The rate of expression of a particular gene is controlled by its location within nucleosomes and by the activity of the cell’s transcriptional machinery. A nucleosome is a tightly wound span of DNA that is bound to histones and other nuclear proteins. Transcription of a gene requires the unwinding of a nucleosome, which makes the gene accessible to a basal transcription complex. This complex is comprised of RNA polymerase (which transcribes the new RNA strand) and numerous regulatory proteins (some of which unwind nucleosomes via histone acetyl transferase activity). Transcription factors are proteins that bind to specific sites (response elements; also called promoter or enhancer elements) present within the regulatory regions of certain genes and thereby increase or decrease the rate at which those genes are transcribed. Transcription factors act by enhancing (or inhibiting) the activity of the basal transcription complex, in some cases by altering nucleosomal structure through changes in histone acetyl transferase or histone deacetylase activity of the complex.
Regulation of transcription factors is the best-understood mechanism by which changes in gene expression occur in the adult brain. Most transcription factors are regulated by phosphorylation. Accordingly, repeated exposure to a drug of abuse, by causing repeated perturbation of synaptic transmission and hence of protein kinases or protein phosphatases, would lead eventually to changes in the phosphorylation state of particular transcription factors. This would lead to altered expression of target genes for these transcription factors. Among such target genes are those for additional transcriptional factors, which – via alterations in their levels – would alter the expression of additional target genes and so on. Drugs of abuse and stress could conceivably produce stable changes in gene expression via regulation of many other types of nuclear proteins, but such actions are just now being explored for the first time.
The Transcription Factor CREB
CREB was the first discovered and best characterized member of a family of related proteins that bind to a particular DNA sequence termed the CRE (consensus nucleotide sequence, TGACGTCA). CRE sites are found within the regulatory region of certain genes. CREB is activated upon its phosphorylation by any of several protein kinases and thereby mediates the effects of diverse types of extracellular stimuli on the expression of CRE-containing genes. CREB has been implicated in learning and memory. Likewise, we have obtained evidence that CREB is a critical mediator of aspects of drug addiction and depression.
The FOS-JUN (AP-1) Family of Transcription Factors
AP-1 transcription factors play a central role in the regulation of neural gene expression by extracellular signals. The name AP-1 was originally applied to a transcriptional activity, then called Activator Protein-1, that was subsequently found to be composed of multiple proteins which bind as heterodimers (and a few as homodimers) to the DNA sequence TGACTCA, the AP-1 sequence. AP-1 proteins are divided into two groups, the Fos and Jun families.
Fos family proteins are known to be induced rapidly, but transiently, in response to many types of stimuli, including drugs of abuse. However, the induction of these proteins desensitizes with repeated drug exposure. In contrast, one Fos family protein, termed ΔFosB, shows avery different pattern of induction. ΔFosB, which is derived from the fosB gene via alternative splicing, is unique in that it is a highly stable protein and consequently accumulates to high levels after repeated drug administration.