All of the steps of gene expression, from RNA transcription to protein translation to posttranslational modifications of proteins, offer critical control points for dynamic regulation. However, the best understood is regulation of gene transcription, which is the focus of the remainder of this chapter. The precise control of transcription by extracellular signals such as neurotransmitters, growth factors, and cytokines permits the regulation of processes such as cell proliferation and differentiation and assists cells in adapting to their environments. Each cell in an organism contains a complete copy of that organism’s genome. However, selective expression of this common genome is required for the formation of distinct cell types during development, including the formation of thousands of types of neurons in the brain. In the fully differentiated adult brain, a subset of an organism’s genes remains accessible for expression and the rate of their expression is under constant control in response to the external environment. Indeed, such regulation of gene transcription is now thought to be a key mechanism for neural and behavioral plasticity.
Transcription Factors: Key Regulators of Gene Expression
The rate of transcription of a given gene is determined first by the state of its nearby chromatin. Many genes are inaccessible for transcription in neurons due to their presence in silenced chromatin. Conversely, many neural genes are not expressed in nonneural cells due to the action of the transcriptional repressor, RE1-silencing transcription factor (REST; also called neuron-restrictive silencer factor [NRSF]). Other genes available for transcription exist in varying states of activation (see 4–18), some being actively transcribed, others not transcribed but in a permissive or poised state, that is, able to be turned on in response to the right stimulus. The activation of such permissive genes involves several steps that are only briefly summarized here. The key step is the binding of a class of protein, named transcription factors, to short DNA sequences, called response elements, present along the regulatory regions of genes, that is, promoters or enhancers. Transcription factor binding provides part of the thermodynamic energy to spread nucleosomes apart. The factors also recruit to the gene enzymes that modify histones, such as HATs, which further loosens the nucleosome complex. More than 100 proteins may bind to the vicinity of an activated gene. This basal transcription apparatus contains RNA polymerase (more specifically, RNA polymerase II for virtually all genes that encode mRNAs) and a host of proteins that move nucleosomes along a strand of DNA, allowing for its transcription into RNA.
Transcription factors typically contain physically distinct functional domains 4–21. These domains contribute to the categorization of transcription factors into various families. Many transcription factors are active only when they form dimers or higher-order complexes. Multimerization domains are diverse and include so-called leucine zippers (see 4–2). Whether transcription factor dimers are homodimers or heterodimers, both partners commonly contribute to the DNA-binding domain and to the activation domain.
A generalized polymerase II promoter. Two regulatory elements, including a hypothetical activator, or response element, and the TATA box are located on a stretch of DNA. The TATA element binds the TATA-binding protein (TBP), which associates with multiple general transcription factors (TFIIA, B, E, F, and H). This basal transcription apparatus recruits RNA polymerase (pol) II and also forms the substrate for interactions with various activator proteins, which typically contain DNA-binding domains, dimerization domains, and transcription activation domains. Several of these proteins may be modified by phosphorylation.
Interestingly, dimerization sometimes can be a mechanism for the negative control of transcription, as illustrated by the cAMP response element binding protein (CREB) family of transcription factors. CREB normally binds to its cAMP response elements (CREs) as homodimers. However, another gene, cAMP response element modulator (CREM), encodes a truncated protein inducible cAMP repressor (ICER), which can dimerize with CREB. However, because ICER lacks an activation domain, the resulting CREB–ICER dimers cannot activate transcription. ICER is thus an endogenous inhibitor (or dominant negative antagonist) of CREB-mediated transcription.
Although transcription factors may directly contact several proteins in the basal transcription complex, they sometimes interact with this apparatus through the mediation of coactivator or adapter proteins (see 4–21). In either scenario, transcription factors that bind at a distance from the core promoter can interact with the basal transcription apparatus because the DNA forms loops that bring distant regulatory regions (eg, enhancers) in contact with each other.
Many activator proteins become involved in the assembly of the mature transcription apparatus only after modification, such as phosphorylation, has occurred in response to extracellular signals. This, too, is illustrated by CREB. CREB can activate transcription only when it is phosphorylated on a particular serine residue (ser133) because phosphorylation of this residue permits CREB to interact with an adapter protein known as CREB-binding protein (CBP), which in turn contacts and activates the basal transcription apparatus in part via its HAT activity. Interestingly, mutations in CBP cause Rubinstein–Taybi syndrome, an autosomal dominant disorder characterized by mental retardation, bone and palatal abnormalities, and cardiac dysfunction 4–9.
Transcription Factors: Targets of Signaling Pathways
Two major mechanisms of transcriptional regulation by extracellular signals are illustrated in 4–22. One of these mechanisms involves transcription factors that are present at significant levels under basal conditions and are rapidly stimulated by signaling cascades to activate or repress the transcription of responsive target genes. By means of the other major mechanism, transcription factors that are expressed at very low levels under basal conditions are induced by a physiologic signal that enables them to regulate the expression of a series of genes.
Intracellular pathways underlying the regulation of gene expression. The stimulation of neurotransmitter, hormone, or neurotrophic factor receptors activates specific second messenger and protein phosphorylation pathways, which produce effects on neuronal function through the phosphorylation of numerous proteins. Changes in gene expression occur by means of two basic mechanisms. When constitutively expressed transcription factors, such as CREB, are phosphorylated by protein kinases, their transcriptional activity is altered, and this causes changes in the expression of specific target genes. Among the target genes are those that encode other transcription factors, such as c-Fos. Once induced, these transcription factors alter the expression of still other target genes.
A critical step in the extracellular regulation of gene expression is the transduction of signals from the cell membrane to the nucleus, which can be accomplished by several mechanisms. Some transcription factors translocate to the nucleus in response to their activation. These include steroid hormone receptors, whose translocation is triggered by the binding of their ligand, and nuclear factor-kappaB (NF-κB), a transcription factor retained in the cytoplasm by a binding protein (IκB) that masks its nuclear localization signal. Signal-regulated phosphorylation of IκB by I kappa kinase (IκK) leads to the dissociation of NF-κB, which in turn is permitted to enter the nucleus to bind DNA; IκB subsequently undergoes proteolysis in the cytoplasm 4–23. Other transcription factors must be phosphorylated or dephosphorylated directly before they can bind to DNA; for example, the phosphorylation of STATs by the protein tyrosine kinase JAK in the cytoplasm permits STAT multimerization, which in turn permits nuclear translocation and the construction of an effective DNA-binding site in the multimer.
Regulation of NF-κB. An active NF-κB transcription factor complex (composed of p50 and Rel-A) is retained in the cytoplasm, where it is inactive, by IκB. Cellular signals activate NF-κB via phosphorylation of IκB, which leads to its rapid degradation. p50-Rel-A is then free to enter the nucleus where it binds to its specific response elements and regulates the transcription of specific target genes. Phosphorylation of IκB is catalyzed in most cases by I kappa kinase (IκK), which in turn is activated via receptors for inflammatory cytokines, such as tumor necrosis factor-α. Other cellular kinases can also lead to NF-κB activation via indirect actions that lead to IκK activation.
Some transcription factors are already bound to their cognate regulatory elements in the nucleus under basal conditions and are converted into transcriptional activators by phosphorylation. CREB, for example, is bound to CREs before cell stimulation 4–24. The critical nuclear translocation step for CREB involves the activation of protein kinases such as PKA, which, after entering the nucleus, phosphorylates CREB. Alternatively, CREB activation can involve the nuclear translocation of second messengers, such as Ca2+ bound to calmodulin. After entering the nucleus, these second messengers activate CaM-kinase IV that in turn phosphorylates CREB 4–24. As previously mentioned, phosphorylation converts CREB into a transcriptional activator by permitting it to recruit CBP into the transcription complex.
Regulation of CREB phosphorylation. Several signaling pathways converge on the phosphorylation of CREB at a single serine residue (ser133). Neurotransmitters that stimulate adenylyl cyclase (AC) increase CREB phosphorylation by activating protein kinase A (PKA). Activated PKA catalytic subunits translocate to the nucleus, where they phosphorylate (P) ser133. Neurotransmitters that inhibit adenylyl cyclase cause the opposite cascade and inhibit CREB phosphorylation. Increased Ca2+ permeates the nucleus, where it activates Ca2+/calmodulin-dependent protein kinase type IV (CaMK IV), which phosphorylates ser133. Growth factor–regulated pathways also lead to CREB phosphorylation by means of the Ras-Raf-MEK pathway that leads to the activation of extracellular signal–regulated kinases (ERKs). ERKs translocate to the nucleus and phosphorylate and activate ribosomal S6 kinase (RSK), which in turn phosphorylates CREB at ser133.
The remainder of this chapter focuses on several transcription factor families that have received a great deal of attention as mediators of neural and behavioral plasticity in the adult.
CREB Family of Transcription Factors
CREB regulates transcription by binding to CREs present in a subset of genes. As their name suggests, CREs enable cAMP to activate (and in some cases repress) genes and have more recently been shown to confer responsiveness of genes to Ca2+ and to the MAP-kinase pathway as well. CREs have been identified in many genes expressed in the nervous system, including those that encode neuropeptides, neurotransmitter synthetic enzymes, signaling proteins, and other transcription factors.
The consensus CRE sequence, TGACGTCA, illustrates the palindromic nature of many transcription factor binding sites: the DNA sequence of the site’s two complementary DNA strands, which run in opposite directions, are identical 4–25. Many regulatory elements are perfect or approximate palindromes because many transcription factors bind to DNA as dimers, whereby each member is responsible for recognizing half of the binding site. While the idealized CRE site is a palindrome, several (nonpalindromic) variations in the sequence exist in CREB-regulated genes.
The palindromic structure of consensus CREs and AP-1 elements. Palindromes or near palindromes are common features of regulatory elements that bind transcription factors as dimers. In general, perfect palindromes are the strongest binding sites for these factors. An intact CGTCA sequence (arrows) may be an absolute requirement for CREs.
Regulation of CREB by cAMP, Ca2+, and growth factors
cAMP, Ca2+, and growth factors activate CREB by causing its phosphorylation at ser133. cAMP activates PKA, whereas Ca2+ activates CaM-kinase IV, both of which phosphorylate ser133 4–24. CREB is also phosphorylated on ser133 by a growth factor–activated kinase, known as ribosomal S6 kinase (specifically RSK-90), which is phosphorylated and activated by MAP-kinases. The activation of CREB thereby illustrates a striking convergence of several signaling pathways on the phosphorylation of a single amino acid residue in the protein. As mentioned earlier, such phosphorylation of CREB activates it by enabling its interaction with CBP. CREB contains sites other than ser133 that can be phosphorylated, which may assist in fine-tuning the regulation of CREB-mediated transcription.
CREB mediation of neural plasticity
The activation of a single transcription factor by convergent signaling pathways is particularly important in the nervous system because it may represent a mechanism for long-term neural adaptations. A case in point is learning and memory, which depend on the temporally coordinated arrival of two different signals that subsequently must be integrated in target neurons and their circuits. CREB has been shown to be important for learning and memory in several species. Genetic knockout of CREB in Aplysia, Drosophila, and rodents impairs the formation of new memory, while overexpression of wild-type or constitutively active forms of CREB causes the opposite effects. Likewise, CREB is required for cellular models of learning and memory, such as long-term potentiation. The role of CREB in these processes is discussed in greater detail in Chapter 14.
CREB, like many transcription factors, is a member of a large family of related proteins. Other members of this family may compensate for CREB when it is inactivated, and also provide independent forms of positive and negative gene regulation. Among the proteins that are closely related to CREB are activating transcription factors (ATFs) and CREMs, which are generated by distinct genes. Several alternative splice forms of CREB, ATFs, and CREMs have been identified. All of these proteins bind CREs as dimers, and many can heterodimerize with CREB itself. The similarities between ATF1 and CREB are especially striking because both are activated by cAMP and Ca2+ signaling pathways. Many ATF proteins and CREM isoforms also appear to activate transcription; however, some CREMs, such as ICER, act to repress it, as stated earlier.
The dimerization domain used by CREB proteins, and several other families of transcription factors, is called a leucine zipper, mentioned above. This domain is composed of an α helix in which every seventh residue is a leucine; the leucines line up along one face of the α helix two turns apart. The aligned leucines of the two dimerization partners interact hydrophobically to stabilize the dimer. The many proteins, in addition to CREB, that utilize these mechanisms of dimerization belong to a superfamily known as basic-leucine zipper (bZIP) proteins.
AP-1 Family of Transcription Factors
Activator protein-1 (AP-1) transcription factors are indispensable in the regulation of neural gene expression by extracellular signals. AP-1 proteins bind DNA, as heterodimers or homodimers, to the DNA sequence TGACTCA, which is called the AP-1 sequence. This consensus sequence is a heptamer that forms a palindrome flanking a central C or G base (see 4–25). Although it differs from the CRE sequence by only a single base, AP-1 sites strongly prefer AP-1 proteins as opposed to CREB, which requires an intact CGTCA sequence. As a result, this single base difference between CRE and AP-1 sites significantly influences which transcription factors, and hence which intracellular signaling pathways, can regulate a particular gene. Many genes expressed in the nervous system contain AP-1 sites in their regulatory regions. Among these are genes that encode neuropeptides, neurotransmitter receptors, neurotransmitter synthetic enzymes, and cytoskeletal proteins.
AP-1 transcription factors bind to DNA as dimers that comprise two families of proteins, Fos and Jun, both of which are bZIP proteins. The known members of the Fos family are c-Fos, Fos-related antigen-1 (FRA1), FRA2, and FosB and its alternative spliced variant ΔFosB. The known members of the Jun family are c-Jun, JunB, and JunD. Most AP-1 complexes are formed by one Fos family member and one Jun family member. Unlike Fos proteins, certain Jun proteins also can form homodimers that bind to AP-1 sites, albeit with lower affinity than Fos–Jun heterodimers. Transcriptional regulation is further complicated by the fact that some AP-1 proteins can heterodimerize, by means of the leucine zipper, with members of the CREB–ATF family. AP-1 proteins also can form higher-order complexes with apparently unrelated families of transcription factors. They can, for example, complex with and inhibit the transcriptional activity of steroid hormone receptors.
Among Fos and Jun proteins, only JunD is expressed constitutively at high levels in many cell types. The other AP-1 proteins tend to be expressed at low or even undetectable levels under basal conditions, but with stimulation may be induced to high levels of expression. Thus, unlike regulation by constitutively expressed transcription factors such as CREB, regulation by most Fos/Jun heterodimers requires new transcription and translation of the transcription factors themselves. Most AP-1 proteins have short half-lives, which means that they exert highly transient effects of some stimulus on gene expression. An exception is ΔFosB (a truncated product of FosB) that, unlike all other Fos family proteins, is highly stable and thereby can mediate relatively persistent changes in gene expression 4–26. Accordingly, ΔFosB has been implicated in long-lasting neural and behavioral plasticity, in particular, drug addiction (Chapter 16).
Changes in the composition of AP-1 complexes over time. A. Several waves of Fos family proteins are induced by acute stimuli in neurons. c-Fos, which is induced most rapidly, degrades within several hours; several others, such as FosB, ΔFosB, Fos-related antigen-1 (FRA1), and FRA2, are induced somewhat later and persist somewhat longer than c-Fos. A portion of the acutely induced ΔFosB is stabilized by phosphorylation, which allows the protein to persist in the brain. B. With repeated stimulation, each stimulus induces a low level of stabilized ΔFosB isoforms, as indicated by the horizontal lines. The result is a gradual increase in total ΔFosB levels, as indicated by the stepped line; such an increase gradually induces significant levels of a long-lasting AP-1 complex, which contributes to several forms of long-lasting neural plasticity.
Activation of cellular genes by AP-1
The genes that encode most AP-1 transcription factors are termed immediate early genes (IEGs). IEGs, a prototype of which is the c-Fos gene, are activated rapidly (within minutes) and transiently and do not require new protein synthesis. Late response genes, in contrast, are induced or repressed more slowly (over hours) and depend on the synthesis of new proteins. The term IEG initially applied to viral genes in eukaryotic cells that are activated immediately after infection by commandeering host cell transcription factors. Viral IEGs generally encode transcription factors that activate the late expression of viral genes.
The application of IEG terminology to nonviral genes has created some confusion. Many cellular genes are induced independently of protein synthesis but with a time course whose duration is between that of classic IEGs and late response genes. Moreover, many cellular genes that are regulated as IEGs encode proteins that are not transcription factors; for example, any gene induced by CREB could potentially show temporal features of induction of an IEG. Despite these complications, the concept of IEG-encoded transcription factors has assisted our understanding of gene regulation in the nervous system. In addition, several IEGs (in particular, c-Fos) have been used as cellular markers of neural activation because of their rapid induction from low basal levels in response to neuronal depolarization and various second messenger and growth factor pathways 4–27.
Example of using c-Fos induction to map patterns of neuronal activation in the brain. Acute administration of cocaine (a drug of abuse; see Chapter 16) or haloperidol (an antipsychotic drug; see Chapter 17) induces c-Fos in striatum, but with different patterns. Cocaine induces c-Fos more medially, while haloperidol induces c-Fos more laterally. These findings have provided insight into the subtypes of striatal neurons, and the circuits in which they operate, that are affected by these two drug treatments.
Activation by multiple signaling pathways
The best characterized cellular IEG is c-Fos. Because this gene contains binding sites for CREB 4–28, it is not surprising that it can be activated rapidly by neurotransmitters or drugs that stimulate the cAMP or Ca2+ pathways. The c-Fos gene also can be induced by the Ras/MEK/MAP-kinase pathway, discussed earlier in this chapter. This activation may occur in part via phosphorylation and activation of CREB by RSK. However, growth factor induction of c-Fos also occurs via a CREB-independent mechanism: subtypes of ERK translocate into the nucleus where they phosphorylate the transcription factor Elk-1 (also called the ternary complex factor [TCF]). Elk-1 subsequently complexes with the serum response factor (SRF) to bind to and activate the serum response element (SRE) in the c-Fos gene (see 4–28). SREs are also present in many other growth factor–inducible genes.
Regulatory region of the c-Fos gene. A CRE site binds CREB, a serum response element (SRE) binds serum response factor (SRF) and ternary complex factor (TCF or Elk-1), and a SIF-inducible element (SIE) binds STAT proteins; these three elements represent a small number of all known transcription factor–binding sites on the gene. Proteins that bind at these sites are constitutively present in cells and are activated by phosphorylation. CREB can be activated by protein kinase A, Ca2+/calmodulin-dependent protein kinases (CaMKs), or ribosomal S6 kinases (RSKs) (see 4–24); Elk-1 can be activated by the MAP-kinases ERK1 and ERK2; and the STAT proteins can be activated by the JAK protein tyrosine kinases. Because the activation of c-Fos by any of multiple signaling pathways requires only signal-induced phosphorylation rather than new protein synthesis, it can be triggered very rapidly by a wide array of stimuli. RTKs, receptor tyrosine kinases; STATs, signal transducers and activators of transcription; JAK, Janus kinase.
Still another mechanism of c-Fos induction involves cytokine-activated signaling pathways that trigger the activation of STATs. As previously mentioned, STATs are activated in response to their phosphorylation by JAKs, which in turn are activated by a variety of cytokines and other signals, as stated earlier. After STATs are activated, they form multimeric complexes, translocate to the nucleus, and bind to their specific DNA response elements, called STAT sites. The c-Fos gene (see 4–28) contains a STAT site (also called SIF-inducible element [SIE]), which mediates the induction of the gene by cytokines. STAT sites are found in many genes expressed in the nervous system.
Regulation by phosphorylation
Several AP-1 proteins are regulated at the posttranslational level by phosphorylation. The best studied example is c-Jun, which is phosphorylated in response to the activation of a MAP-kinase signaling pathway, called SAP-kinases or JNKs, by some form of cellular stress (see 4–15). JNKs phosphorylate c-Jun in its transcriptional activation domain, and thereby increase its ability to activate transcription. This process has been implicated in the modulation of synaptic transmission and may, under unique circumstances, trigger the activation of apoptosis (programmed cell death) pathways (Chapter 18).
Steroid Hormone Receptor Superfamily
Steroid hormones (eg, glucocorticoids, estrogen, testosterone, and mineralocorticoids) and related signals (retinoids, thyroid hormones, and vitamin D3) are small, lipid-soluble ligands that diffuse readily across cell membranes. Unlike the receptors for neurotransmitters and peptide hormones, which are located in the cell membrane, the receptors for these ligands are localized in the cytoplasm. In response to ligand binding, steroid hormone receptors translocate to the nucleus, where they regulate the expression of certain genes by binding to specific hormone response elements (HREs) in their regulatory regions. Thus, these receptors are sometimes referred to as the nuclear receptor superfamily. Steroid, thyroid, and retinoid hormones and their nuclear receptors are critical for nervous system function, and are extremely important in CNS pharmacology (Chapters 10 and 15).
The mechanisms by which the glucocorticoid receptor (GR) operates are representative of those utilized by most receptors in this superfamily. Under basal conditions, the GR is retained in the cytoplasm by a large multiprotein complex of chaperone proteins, including the heat shock protein Hsp90 and the immunophilin Hsp56. After it is bound by glucocorticoid, the GR dissociates from its chaperones, translocates to the nucleus, and subsequently binds to glucocorticoid response elements (GREs). GREs typically are 15 bases in length and consist of 2 palindromic half-sites. GRs and related nuclear receptors have a modular structure similar to that of other transcription factors 4–29. The DNA-binding domain of the GR is characterized by multiple cysteines organized around a central zinc ion, an arrangement often referred to as a zinc finger. Many other transcription factors possess this DNA-binding domain.
The glucocorticoid receptor (GR). A. GR domains. B. Transcriptional regulation by glucocorticoids and the GR. The idealized promoters represented here contain two glucocorticoid response elements (GREs) or one AP-1 site. GRs act on GREs to activate (top) or inhibit (center) transcription, and also inhibit AP-1-mediated transcription by binding directly to a Fos–Jun dimer (bottom). The TATA box binds the general transcription factors required for activation.
GREs can confer either positive or negative regulation on the genes to which they are linked (see 4–29). A well-characterized negative GRE is located in the proopiomelanocortin gene mentioned above (Chapters 7 and 10); the GRE permits glucocorticoids to repress the gene, which encodes ACTH, and thus acts as an important feedback mechanism by inhibiting further glucocorticoid synthesis.
GRs are responsible for many important physiologic actions that do not appear to be mediated by DNA binding. Rather, they can interfere with transcriptional activity mediated by other transcription factors—particularly those mediated by AP-1 and NF-κB proteins (see 4–29). Glucocorticoids can also interfere with NF-κB activity by inducing expression of IκB, the protein that holds NF-κB in the cytoplasm.
Additional members of the nuclear hormone receptor superfamily have been identified in recent years. Some respond to cholesterol or lipids and are important in the regulation of intermediary metabolism 4–5. The endogenous ligands for many others are not yet known. Increasing evidence supports the involvement of many such nuclear receptors in neural functioning. Nerve growth factor inducible factor B (NGFI-B) and Nurr1, for example, are regulated in the brain by exposure to certain psychotropic drugs, including cocaine and antipsychotic agents.
4–5 Transcription Factors as Drug Targets
Transcription factors and other proteins involved in the regulation of gene expression may represent viable targets for drugs, particularly if they are expressed in a limited number of cell types or if they exhibit unique temporal patterns (see ΔFosB in 4–26). Conversely, transcription factors such as CREB or c-Fos would not be useful for this purpose because they are ubiquitous. However, efforts to exploit any of these proteins should not be hampered by their location in the nucleus; most drugs that penetrate the blood–brain barrier also cross cell and nuclear membranes.
We know that protein–protein interaction domains can be targeted by small molecules. Because most transcription factors bind DNA as dimers or multimeric complexes, these interaction domains may possess the diversity and specificity characteristic of successful pharmaceutical targets. An example is provided by the thiazolidinedione drugs (eg, rosiglitazone and pioglitazone), which are used to treat type II diabetes. These drugs bind to and activate peroxisome proliferator–activated receptor-γ (PPARγ), a member of the steroid hormone superfamily of nuclear receptors. Endogenous ligands for PPARγ may include prostaglandins. The PPARγ–thiazolidinedione complex dimerizes with retinoid X receptor (RXR)—another member of the nuclear receptor family—and the dimer binds to target response elements in DNA. Among the genes that contain such response elements are those that encode for proteins that enhance sensitivity to insulin. PPARγ agonists are also being evaluated as neuroprotective agents in a range of neurodegenerative disorders and stroke (Chapters 18 and 20).
The development of small molecule drugs that target protein–DNA-binding sites represents another approach in exploiting the transcriptional machinery in drug discovery efforts.
Other Transcription Factors
The CREB, AP-1, NF-κB, STAT, and steroid hormone receptor families represent just a small portion of the transcription factors that are expressed in neurons and glia. However, many other transcription factors are crucial in neural signaling. CAATT enhancing binding protein (C/EBP) and its family members mediate some of the effects of the cAMP pathway on gene expression. Egr family members (eg, Egr-1/Zif268/NGFI-A) are zinc finger transcription factors that, like c-Fos, are induced rapidly and transiently in the brain by stimuli whose temporal features resemble those of IEGs. The induction of EGR family proteins has been correlated with induction of hippocampal long-term potentiation (Chapter 14); however, the target genes of these proteins remain poorly characterized. Circadian genes, the prototype of which is clock, are transcription factors that control diurnal variations in cell function through the regulation of gene expression (Chapter 13).
The functional mechanisms addressed in this chapter are far more intricate than their descriptions suggest. Regulatory regions of genes are usually far longer than the coding regions of genes, and regulatory information is contained not only in the 5´ promoter regions of genes but also in transcribed sequences and in 3´ untranscribed regions. Moreover, within the 5´ regions, this chapter focuses on a relatively small number of response elements. Most genes contain many regulatory sites, and these sites do not function in isolation but influence one another. Consequently, the temporal and spatial synthesis of multiple signaling pathways affects the expression of most genes. Unraveling this complexity is a daunting task, particularly in vivo, but is likely to yield important clues for understanding neural and behavioral plasticity. One goal of future research, outlined in 4–5, is to take advantage of the enormous complexity and specificity of these mechanisms to generate agents that act more rapidly and more effectively in the treatment of neuropsychiatric disorders.