Characterization of the circadian system of NGFI-A and NGFI-A/NGFI-B deficient mice

Phenotyping of light-activated neurons in the mouse SCN based on the expression of FOS and EGR1

Light-sensitive neurons are located in the ventral and central core of the suprachiasmatic nucleus (SCN), whereas stably oscillating clock neurons are found mainly in the dorsal shell. Signals between the SCN core and shell are believed to play an important role in light entrainment. Core neurons express vasoactive intestinal polypeptide (VIP), gastrin-releasing peptide (GRP), and Neuroglobin (Ngb), whereas the shell neurons express vasopressin (AVP), prokineticin 2, and the VIP type 2 (VPAC2) receptor. In rodents, light has a phase-shifting capacity at night, which induces rapid and transient expression of the EGR1 and FOS in the SCN. Methods: The present study used immunohistochemical staining of FOS, EGR1, and phenotypical markers of SCN neurons (VIP, AVP, Ngb) to identify subtypes/populations of light-responsive neurons at early night. Results: Double immunohistochemistry and cell counting were used to evaluate the number of SCN neurons expressing FOS and EGR1 in the SCN. The number of neurons expressing either EGR1 or FOS was higher than the total number of neurons co-storing EGR1 and FOS. Of the total number of light-responsive cells, 42% expressed only EGR1, 43% expressed only FOS, and 15% expressed both EGR1 and FOS. Light-responsive VIP neurons represented only 31% of all VIP neurons, and EGR1 represents the largest group of light-responsive VIP neurons (18%). VIP neurons expressing only FOS represented 1% of the total light-responsive VIP neurons. 81% of the Ngb neurons in the mouse SCN were light-responsive, and of these neurons expressing only EGR1 after light stimulation represented 44%, whereas 24% expressed FOS. Although most light-responsive neurons are found in the core of the SCN, 29% of the AVP neurons in the shell were light-responsive, of which 8% expressed EGR1, 10% expressed FOS, and 11% co-expressed both EGR1 and FOS after light stimulation. Discussion: Our analysis revealed cell-specific differences in light responsiveness between different peptidergic and Ngb-expressing neurons in different compartments of the mouse SCN, indicating that light activates diverse neuronal networks in the SCN, some of which participate in photoentrainment.

NPAS4 regulates the transcriptional response of the suprachiasmatic nucleus to light and circadian behavior

The suprachiasmatic nucleus (SCN) is the master circadian pacemaker in mammals and is entrained by environmental light. However, the molecular basis of the response of the SCN to light is not fully understood. We used RNA/chromatin immunoprecipitation/single-nucleus sequencing with circadian behavioral assays to identify mouse SCN cell types and explore their responses to light. We identified three peptidergic cell types that responded to light in the SCN: arginine vasopressin (AVP), vasoactive intestinal peptide (VIP), and cholecystokinin (CCK). In each cell type, light-responsive subgroups were enriched for expression of neuronal Per-Arnt-Sim (PAS) domain protein 4 (NPAS4) target genes. Further, mice lacking Npas4 had a longer circadian period under constant conditions, a damped phase response curve to light, and reduced light-induced gene expression in the SCN. Our data indicate that NPAS4 is necessary for normal transcriptional responses to light in the SCN and critical for photic phase-shifting of circadian behavior.

Altered light induced EGR1 expression in the SCN of PACAP deficient mice

The brain’s biological clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus and generates circadian rhythms in physiology and behavior. The circadian clock needs daily adjustment by light to stay synchronized (entrained) with the astronomical 24 h light/dark cycle. Light entrainment occurs via melanopsin expressing retinal ganglion cells (mRGCs) and two neurotransmitters of the retinohypothalamic tract (RHT), PACAP and glutamate, which transmit light information to the SCN neurons. In SCN neurons, light signaling involves the immediate-early genes Fos, Egr1 and the clock genes Per1 and Per2. In this study, we used PACAP deficient mice to evaluate PACAP’s role in light induced gene expression of EGR1 in SCN neurons during early (ZT17) and late (ZT23) subjective night at high (300 lux) and low (10 lux) white light exposure. We found significantly lower levels of both EGR1 mRNA and protein in the SCN in PACAP deficient mice compared to wild type mice at early subjective night (ZT17) exposed to low but not high light intensity. No difference was found between the two genotypes at late night (ZT23) at neither light intensities. In conclusion, light mediated EGR1 induction in SCN neurons at early night at low light intensities is dependent of PACAP signaling. A role of PACAP in shaping synaptic plasticity during light stimulation at night is discussed.

Mice Lacking EGR1 Have Impaired Clock Gene (BMAL1) Oscillation, Locomotor Activity, and Body Temperature

Early growth response transcription factor 1 (EGR1) is expressed in the suprachiasmatic nucleus (SCN) after light stimulation. We used EGR1-deficient mice to address the role of EGR1 in the clock function and light-induced resetting of the clock. The diurnal rhythms of expression of the clock genes BMAL1 and PER1 in the SCN were evaluated by semi-quantitative in situ hybridization. We found no difference in the expression of PER1 mRNA between wildtype and EGR1-deficient mice; however, the daily rhythm of BMAL1 mRNA was completely abolished in the EGR1-deficient mice. In addition, we evaluated the circadian running wheel activity, telemetric locomotor activity, and core body temperature of the mice. Loss of EGR1 neither altered light-induced phase shifts at subjective night nor affected negative masking. Overall, circadian light entrainment was found in EGR1-deficient mice but they displayed a reduced locomotor activity and an altered temperature regulation compared to wild type mice. When placed in running wheels, a subpopulation of EGR1-deficient mice displayed a more disrupted activity rhythm with no measurable endogenous period length (tau). In conclusion, the present study provides the first evidence that the circadian clock in the SCN is disturbed in mice deficient of EGR1.

Molecular modulators of the circadian clock: lessons from flies and mice

Circadian timekeeping is a ubiquitous mechanism that enables organisms to maintain temporal coordination between internal biological processes and time of the local environment. The molecular basis of circadian rhythms lies in a set of transcription-translation feedback loops (TTFLs) that drives the rhythmic transcription of core clock genes, whose level and phase of expression serve as the marker of circadian time. However, it has become increasingly evident that additional regulatory mechanisms impinge upon the TTFLs to govern the properties and behavior of the circadian clock. Such mechanisms include changes in chromatin architecture, interactions with other transcription factor networks, post-transcriptional control by RNA modifications, alternative splicing and microRNAs, and post-translational regulation of subcellular trafficking and protein degradation. In this review, we will summarize the current knowledge of circadian clock regulation-from transcriptional to post-translational-drawing from literature pertaining to the Drosophila and murine circadian systems.

EGR1 regulates hepatic clock gene amplitude by activating Per1 transcription

The mammalian clock system is composed of a master clock and peripheral clocks. At the molecular level, the rhythm-generating mechanism is controlled by a molecular clock composed of positive and negative feedback loops. However, the underlying mechanisms for molecular clock regulation that affect circadian clock function remain unclear. Here, we show that Egr1 (early growth response 1), an early growth response gene, is expressed in mouse liver in a circadian manner. Consistently, Egr1 is transactivated by the CLOCK/BMAL1 heterodimer through a conserved E-box response element. In hepatocytes, EGR1 regulates the transcription of several core clock genes, including Bmal1, Per1, Per2, Rev-erbα and Rev-erbβ, and the rhythm amplitude of their expression is dependent on EGR1's transcriptional function. Further mechanistic studies indicated that EGR1 binds to the proximal region of the Per1 promoter to activate its transcription directly. When the peripheral clock is altered by light or feeding behavior transposition in Egr1-deficient mice, the expression phase of hepatic clock genes shifts normally, but the amplitude is also altered. Our data reveal a critical role for EGR1 in the regulation of hepatic clock circuitry, which may contribute to the rhythm stability of peripheral clock oscillators.

The transcription factor Zif268/Egr1, brain plasticity, and memory

The capacity to remember our past experiences and organize our future draws on a number of cognitive processes that allow our brain to form and store neural representations that can be recalled and updated at will. In the brain, these processes require mechanisms of neural plasticity in the activated circuits, brought about by cellular and molecular changes within the neurons activated during learning. At the cellular level, a wealth of experimental data accumulated in recent years provides evidence that signaling from synapses to nucleus and the rapid regulation of the expression of immediate early genes encoding inducible, regulatory transcription factors is a key step in the mechanisms underlying synaptic plasticity and the modification of neural networks required for the laying down of memories. In the activated neurons, these transcriptional events are thought to mediate the activation of selective gene programs and subsequent synthesis of proteins, leading to stable functional and structural remodeling of the activated networks, so that the memory can later be reactivated upon recall. Over the past few decades, novel insights have been gained in identifying key transcriptional regulators that can control the genomic response of synaptically activated neurons. Here, as an example of this approach, we focus on one such activity-dependent transcription factor, Zif268, known to be implicated in neuronal plasticity and memory formation. We summarize current knowledge about the regulation and function of Zif268 in different types of brain plasticity and memory processes.

Entrainment of circadian rhythm by ambient temperature cycles in mice

Much is known about how environmental light-dark cycles synchronize circadian rhythms in animals. The ability of environmental cycles of ambient temperature to synchronize circadian rhythms has also been investigated extensively but mostly in ectotherms. In the present study, the synchronization of the circadian rhythm of running-wheel activity by environmental cycles of ambient temperature was studied in laboratory mice. Although all mice were successfully entrained by a light-dark cycle, only 60% to 80% of the mice were entrained by temperature cycles (24-32 degrees C or 24-12 degrees C), and attainment of stable entrainment seemed to take longer under temperature cycles than under a light-dark cycle. This suggests that ambient temperature cycles are weaker zeitgebers than light-dark cycles, which is consistent with the results of the few previous studies using mammalian species. Whereas 80% of the mice were entrained by 24-h temperature cycles, only 60% were entrained by 23-h cycles, and none was entrained by 25-h cycles. The results did not clarify whether entrainment by temperature cycles is caused directly by temperature or indirectly through a temperature effect on locomotor activity, but it is clear that the rhythm of running-wheel activity in mice can be entrained by ambient temperature cycles in the nonnoxious range.

Proinflammatory role of leukocyte-derived Egr-1 in the development of murine postoperative ileus

BACKGROUND & AIMS

Early growth response gene-1 (Egr-1) is an important inflammatory transcription factor. We hypothesize that leukocyte-derived Egr-1 plays a key inflammatory role in causing postoperative ileus.

METHODS

Wild-type, Egr-1 knockout, and chimera mice (constructed by irradiation followed by injection with Egr-1(+/+) or Egr-1(-/-) bone marrow) were subjected to surgical manipulation of the gastrointestinal tract to induce ileus. Reverse-transcription polymerase chain reaction, Western blot, and immunohistochemistry quantified and localized Egr-1. Lumenal transit of nonabsorbable fluorescein isothiocyanate-labeled dextran and in vitro organ bath techniques measured functional gastrointestinal motility. Inflammatory mediator expressions were measured by Griess reaction, enzyme-linked immunosorbent assay, and multiplex Luminex assay.

RESULTS

Intestinal manipulation rapidly and significantly induced Egr-1 messenger RNA and protein within the inflamed muscularis externa. Egr-1 was colocalized early to smooth muscle and enteric neurons and later in extravasated monocytes after surgery when postoperative ileus was functionally prominent. The functional severity of postoperative ileus was significantly ameliorated in mice deficient in Egr-1(-/-) and chimera wild-type mice transplanted with Egr-1(-/-) bone marrow, whereas knockout mice with Egr-1(+/+) bone marrow again displayed significant ileus. Motility was mechanistically associated in Egr-1(-/-) gene deficiency with a down-regulation in the release of nitric oxide, prostanoids, monocyte chemoattractant protein-1, macrophage inflammatory protein-1alpha, interleukin-6, interleukin-1, and granulocyte colony-stimulating factor, as well as a decrease in the recruitment of leukocytes into the manipulated muscle wall of the intestine compared with wild-type mice.

CONCLUSIONS

Leukocyte-derived Egr-1 plays an early critical inflammatory role in the initiation of the postoperative inflammatory response, which leads to a prolonged decreased in gastrointestinal motility after intestinal surgery.

Identification of novel light-induced genes in the suprachiasmatic nucleus

Background

The transmission of information about the photic environment to the circadian clock involves a complex array of neurotransmitters, receptors, and second messenger systems. Exposure of an animal to light during the subjective night initiates rapid transcription of a number of immediate-early genes in the suprachiasmatic nucleus of the hypothalamus. Some of these genes have known roles in entraining the circadian clock, while others have unknown functions. Using laser capture microscopy, microarray analysis, and quantitative real-time PCR, we performed a comprehensive screen for changes in gene expression immediately following a 30 minute light pulse in suprachiasmatic nucleus of mice.

Results

The results of the microarray screen successfully identified previously known light-induced genes as well as several novel genes that may be important in the circadian clock. Newly identified light-induced genes include early growth response 2, proviral integration site 3, growth-arrest and DNA-damage-inducible 45 beta, and TCDD-inducible poly(ADP-ribose) polymerase. Comparative analysis of promoter sequences revealed the presence of evolutionarily conserved CRE and associated TATA box elements in most of the light-induced genes, while other core clock genes generally lack this combination of promoter elements.

Conclusion

The photic signalling cascade in the suprachiasmatic nucleus activates an array of immediate-early genes, most of which have unknown functions in the circadian clock. Detected evolutionary conservation of CRE and TATA box elements in promoters of light-induced genes suggest that the functional role of these elements has likely remained the same over evolutionary time across mammalian orders.

Nuclear receptors, metabolism, and the circadian clock

As ligand-dependent transcription factors, the nuclear receptor superfamily governs a remarkable array of rhythmic physiologic processes such as metabolism and reproduction. To provide a "molecular blueprint" for nuclear receptor function in circadian biology, we established a diurnal expression profile of all mouse nuclear receptors in critical metabolic tissues. Our finding of broad expression and tissue-specific oscillation of nuclear receptors along with their key target genes suggests that diurnal nuclear receptor expression may contribute to established rhythms in metabolic physiology and that nuclear receptors may be involved in coupling peripheral circadian clocks to divergent metabolic outputs. Conversely, nuclear receptors may serve peripheral clock input pathways, integrating signals from the light-sensing central clock in the suprachiasmatic nucleus and other environmental cues, such as nutrients and xenobiotics. Interplay between the core circadian clock and nuclear receptors may define a large-scale signaling network that links biological timing to metabolic physiology.

Enhanced circadian photoresponsiveness after prolonged dark adaptation in seven species of diurnal and nocturnal rodents

Previous studies in mice and Syrian hamsters have described an enhancement of circadian photoresponsiveness after exposure to darkness for several weeks. The present study investigated the generality of the phenomenon in 3 diurnal and 4 nocturnal rodent species. In four of the species tested, phase delays of the running-wheel activity rhythm evoked by 1-h light pulses were several-fold larger after 3 to 4 weeks of exposure to darkness than after a single day. This drastic change in photoresponsiveness has important implications for the understanding of the process of photic entrainment. Differences between species that showed a significant effect of dark adaptation and species that showed no effect were not accounted for by temporal niche (diurnal versus nocturnal) or photic sensitivity (albino versus pigmented). Further research is needed to elucidate the mechanisms responsible for inter-species differences in the occurrence of enhanced photoresponsiveness after dark adaptation and to identify the neural substrates of this phenomenon in species that exhibit it.

Evidence for Nr4a1 as a cold-induced effector of brown fat thermogenesis

Acute cold exposure leads to norepinephrine release in brown adipose tissue (BAT) and activates uncoupling protein (UCP)1-mediated nonshivering thermogenesis. Chronic sympathetic stimulation is known to initiate mitochondrial biogenesis, UCP1 expression, hyperplasia of BAT, and recruitment of brown adipocytes in white adipose tissue (WAT) depots. Despite distinct functions of BAT and WAT in energy balance, only a few genes are exclusively expressed in either tissue. We identified NUR77 (Nr4a1), an orphan receptor, to be induced transiently in brown adipocytes in response to beta-adrenergic stimulation and in BAT of cold-exposed mice. Subsequent reporter gene assays demonstrated an inhibitory action of NUR77 on basal and peroxisome proliferator-activated receptor (PPAR)gamma/retinoid X receptor (RXR)alpha-mediated transactivation of the Ucp1 enhancer in heterologous cotransfection experiments. Despite this function of NUR77 in the control of Ucp1 gene expression, nonshivering thermogenesis was not affected in Nur77 knockout mice. However, we observed a superinduction of Nor1 in BAT of cold-exposed knockout mice. We conclude that NUR77 is a cold-induced negative regulator of Ucp1, but phenotypic consequences in knockout mice are compensated by functional redundancy of Nor1.

Nerve growth factor-induced circadian phase shifts and MAP kinase activation in the hamster suprachiasmatic nuclei

Circadian rhythms are entrained by light and by several neurochemical stimuli. In hamsters housed in constant darkness, i.c.v. administration of nerve growth factor (NGF) at various times in their circadian cycle produced phase shifts of locomotor activity rhythms that were similar in direction and circadian timing to those produced by brief pulses of light. Moreover, the effect of NGF and light were not additive, indicating signalling points in common. These points include the immediate-early gene c-fos and ERK1/2, a component of the mitogen-activated protein kinases (MAPK) family. NGF activates c-FOS and ERK1/2-MAPK in the suprachiasmatic nuclei, the site of a circadian clock in mammals, when administered during the subjective night but not during the day. The effect of NGF on ERK1/2 activation was not inhibited by the administration of MK-801, a glutamate/NMDA receptor antagonist. These results suggest that NGF, acting through MAPK activation, plays a role in photic entrainment of the mammalian circadian clock.

Restricted expression and photic induction of a novel mouse regulatory factor X4 transcript in the suprachiasmatic nucleus

The regulatory factor X (RFX) family of transcription factors is characterized by a unique and highly conserved 76-amino acid residue DNA-binding domain. Mammals have five RFX genes, but the physiological functions of their products are unknown, with the exception of RFX5. Here a mouse RFX4 transcript was identified that encodes a peptide of 735 amino acids, including the DNA-binding domain. Its expression was localized in the suprachiasmatic nucleus, the central pacemaker site of the circadian clock. Also, light exposure was found to induce its gene expression in a subjective night-specific manner. Polyclonal antibodies were prepared, and an 80-kDa band was detected in the suprachiasmatic nucleus by Western hybridization. A histochemical study showed a localization of the products in the nucleus. This is the first report on mouse RFX4, which contains the RFX DNA-binding motif. Our investigation may provide clues to the physiological function of RFX4.

The ERK/MAP kinase pathway couples light to immediate-early gene expression in the suprachiasmatic nucleus

Signalling via the p42/44 mitogen-activated protein kinase (MAPK) pathway has been identified as an intermediate event coupling light to entrainment of the mammalian circadian clock located in the suprachiasmatic nucleus (SCN). Given this observation, it was of interest to determine where within the entrainment process the MAPK pathway was functioning. In this study, we examined the role of the MAPK pathway as a regulator of light-induced gene expression in the SCN. Towards this end, we characterized the effect pharmacological disruption of the MAPK cascade has on the expression of the immediate-early genes c-Fos, JunB and EGR-1. We report that uncoupling light from MAPK pathway activation attenuated the expression of all three gene products. In the absence of photic stimulation, inhibition of the MAPK pathway did not alter basal gene product expression levels. Light-induced activation of cAMP response element (CRE)-dependent transcription, as assessed using a CRE-LacZ transgenic mouse strain, was also disrupted by blocking MAPK pathway activation. These results reveal that the MAPK cascade functions as one of the first transduction steps leading from light to rapid transcriptional activation, an essential event in the entrainment process. MAPK pathway-dependent gene expression in the SCN may result, in part, from stimulation of CRE-dependent transcription.

Anatomical and temporal differences in the regulation of ZIF268 (NGFI-A) protein in the hamster and mouse suprachiasmatic nucleus

Several immediate-early genes have been shown to be induced in the rodent circadian pacemaker, the suprachiasmatic nucleus, by retinal illumination at night. We compared spontaneous and light-evoked levels of the immediate-early gene protein ZIF268 (NGFI-A) in the Syrian hamster and C57BL/6J mouse suprachiasmatic nucleus. Exposure of both hamsters and mice to light pulses early and late in the subjective night caused increased ZIF268 immunoreactivity in the region of the suprachiasmatic nucleus that receives retinal innervation. In contrast to hamsters, mice also showed spontaneous increases in ZIF268 at both subjective night phases at the lateral edges of the suprachiasmatic nucleus. Light also evoked a significant increase in ZIF268 levels during the subjective day in the lateral suprachiasmatic nucleus, with few labeled cells in the ventral and dorsal suprachiasmatic nucleus. These results demonstrate a novel circadian pattern and regional differentiation of ZIF268 immunoreactivity in the suprachiasmatic nucleus of mice that differ from those in other rodents. There are pronounced species differences in both spontaneous and light-evoked expression of ZIF268 immunoreactivity.

Experience-dependent plasticity of mouse visual cortex in the absence of the neuronal activity-dependent marker egr1/zif268

Neuronal activity elicits a rapid increase in the expression of several immediate early genes (IEGs). To clarify a role for IEG response in activity-dependent development, we examined the contribution of the egr1/zif268 gene during visual cortical processing and plasticity in mice. We first analyzed the expression of egr1 mRNA in wild-type (WT) mice using Northern blot hybridization. In the visual cortex, expression of egr1 mRNA increased dramatically after eye opening, systemic injection of kainate, or 30 min of photostimulation after a brief (5 d) period of dark adaptation. Thus, the expression of egr1 is regulated by synaptic activity in the mouse visual cortex, as it is in other species (e.g., monkeys, cats, and rats). To evaluate whether this transcription factor is directly involved in activity-dependent plasticity, mice lacking Egr1 were deprived of the use of one eye during the developmental critical period [postnatal day 24 (P24)-P34]. Extracellular in vivo single-unit recordings from the binocular zone of the visual cortex revealed that visual responses developed normally in egr1 knock-out (KO) mice. Moreover, a similarly significant shift of responsiveness in favor of the open eye was produced in both KO and WT mice by either brief (4 d) or long-term (>2 weeks) occlusion of one eye. There was no apparent compensation among egr2, egr3, or c-fos mRNA and protein expression in the visual cortex of egr1 KO mice. Taken together, these results indicate that egr1 is a useful marker of sensory input in mice but is not intrinsically necessary for the experience-dependent plasticity of the visual cortex. Our findings underscore a mechanistic distinction between sensory plasticity and long-lasting forms of synaptic potentiation in the hippocampus, for which egr1/zif268 was recently found to be essential.

Effects of cannabinoids in Krox-24 targeted mice

Krox-24 is an immediate early gene encoding a zinc-finger transcription factor implicated in several adaptive responses, and its induction by cannabinoids has been reported. We used mice targeted in the Krox-24 gene to specifically dissect the role of this protein in the acute and chronic central actions of cannabinoids. We report here on the ability of cannabinoids to activate G-proteins and to inhibit adenylyl cyclase, and to elicit behavioral responses in wild-type and mutant mice. The behavioral parameters and the biochemical correlates of abstinence after delta9-THC withdrawal were evaluated. We show that Krox-24 is not involved in the acute analgesic effects of delta9-THC and in the SR precipitated delta9-THC withdrawal syndrome.

The genomic action potential

Neurons compute in part by integrating, on a time scale of milliseconds, many synaptic inputs and generating a digital output-the "action potential" of classic electrophysiology. Recent discoveries indicate that neurons also perform a second, much slower, integration operating on a time scale of minutes or even hours. The output of this slower integration involves a pulse of gene expression which may be likened to the electrophysiological action potential. Its function, however, is not directed toward immediate transmission of a synaptic signal but rather toward the experience-dependent modification of the underlying synaptic circuitry. Commonly termed the "immediate early gene" (IEG) response, this phenomenon is often assumed to be a necessary component of a linear, deterministic cascade of memory consolidation. Critical review of the large literature describing the phenomenon, however, leads to an alternative model of IEG function in the brain. In this alternative, IEG activation is not directed at the consolidation of memories of a specific inducing event; instead, it sets the overall gain or efficiency of memory formation and directs it to circuits engaged by behaviorally significant contexts. The net result is a sharpening of the selectivity of memory formation, a recruitment of temporally correlated associations, and an ultimate enhancement of long-term memory retrieval.

The EGR family of transcription-regulatory factors: progress at the interface of molecular and systems neuroscience

The EGR family of transcription regulatory factors, which is implicated in orchestrating the changes in gene expression that underlie neuronal plasticity, has attracted the attention of both molecular and systems neuroscientists. In this article, the advances made in both these fields of research are reviewed. Recent systems-based studies underscore the remarkable sensitivity and specificity of the induction of the expression of genes encoding EGR-family members in naturally occurring plasticity paradigms. However, they also challenge conventional views of the role of this family in plasticity. Recent molecular studies have identified the gonadotropin subunit, luteinizing hormone beta, as an EGR1-regulated gene in vivo and uncovered an essential role for EGR3 in muscle-spindle development. In addition, the discovery of novel proteins that are capable of suppressing EGR-mediated transcription cast doubt over the prevalent assumption that changes in EGR mRNA or protein levels provide an accurate measure of EGR-driven transcriptional activity.