A role for Id2 in regulating photic entrainment of the mammalian circadian system.
Id2 was identified in DNA microarray screens for rhythmically expressed genes [3-5], and further analysis revealed a circadian pattern of expression of all four Id genes in multiple tissues including the suprachiasmatic nucleus.In vitro studies using an mPer1 and an AVP promoter reporter revealed the potential for ID1, ID2, and ID3 proteins to interact with the canonical basic HLH clock proteins BMAL1 and CLOCK.These data suggest that the Id genes may be important for entrainment and operation of the mammalian circadian system, potentially acting through BMAL1 and CLOCK targets.
Affiliation: Department of Genetics, Dartmouth Medical School, Hanover, NH 03755, USA. firstname.lastname@example.org
Inhibitor of DNA binding genes (Id1-Id4) encode helix-loop-helix (HLH) transcriptional repressors associated with development and tumorigenesis [1, 2], but little is known concerning the function(s) of these genes in normal adult animals. Id2 was identified in DNA microarray screens for rhythmically expressed genes [3-5], and further analysis revealed a circadian pattern of expression of all four Id genes in multiple tissues including the suprachiasmatic nucleus. To explore an in vivo function, we generated and characterized deletion mutations of Id2 and of Id4. Id2(-/-) mice exhibit abnormally rapid entrainment and an increase in the magnitude of the phase shift of the pacemaker. A significant proportion of mice also exhibit disrupted rhythms when maintained under constant darkness. Conversely, Id4(-/-) mice did not exhibit a noticeable circadian phenotype. In vitro studies using an mPer1 and an AVP promoter reporter revealed the potential for ID1, ID2, and ID3 proteins to interact with the canonical basic HLH clock proteins BMAL1 and CLOCK. These data suggest that the Id genes may be important for entrainment and operation of the mammalian circadian system, potentially acting through BMAL1 and CLOCK targets.
- Biological Clocks/physiology*
- Circadian Rhythm/physiology*
- Inhibitor of Differentiation Protein 2/genetics/metabolism*
- ARNTL Transcription Factors
- Basic Helix-Loop-Helix Transcription Factors/genetics/metabolism
- CLOCK Proteins
- Gene Expression Regulation
- Inhibitor of Differentiation Proteins/genetics/metabolism
- Mice, Knockout
- Motor Activity/physiology
- Promoter Regions, Genetic
- Protein Isoforms/genetics/metabolism
- Suprachiasmatic Nucleus/metabolism
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fig2: A Proportion of Id2 Mutant Mice Show Severe Disruption of Locomotor Activity Rhythms(A–D) Activity records of representative wild-type (+/+; [A] and [B]) and Id2 mutant (−/−; [C] and [D]) mice are shown in double-plotted format. Each horizontal line represents a 48 hr period, and the second 24 hr period is plotted to the right and below the first. Vertical bars represent periods of wheel-running activity (A and C) or general activity as measured by a passive infrared detector (B and D). Animals were individually housed in a 12:12 LD cycle for at least 14 days, transferred to DD for 30 days, and then transferred back to a LD cycle. The line above DD on the right indicates the transition from LD to DD. The timing of the respective light-dark cycles is indicated by the white-and-black bars above and below the records. Numbers on the left indicate the number of days in the study. Wild-type mice maintained robust rhythmicity in both LD and DD, as is reflected by the representative mice (A and B). Of the Id2 mutant mice, 75% showed robust to distinct rhythms in both LD and DD, as shown in the representative animal (C), whereas 25% became arrhythmic or showed very weak rhythmicity in DD, and in some cases in LD, as shown in the representative animal (D); see also Figure S4. Note that the animal in (B) was not provided with a running wheel, and that mouse (D) activity data were collected for a longer duration.(E–H) χ2 periodogram and Fourier analysis of the locomotor activity rhythms. Plots represent analysis of 10 days of data from a representative wild-type mouse in LD (E) and DD (F) and from an Id2 mutant mouse in LD (G) and DD (H). Analyses are for the mice whose actograms are as shown in (B) and (D). The analyses were undertaken on the last 10 day duration in LD and on the third 10 day duration in DD. For the periodogram analysis, the ascending straight line represents a statistical significance of p = 0.001. For Fourier analysis, a frequency of 0.042 cycles/hr corresponds to 1 cycle/24 hr, which is denoted by the arrowhead below the chart where statistically significant (statistical significance was determined by the Clocklab program). Note the changes in the y axis between charts, and a reduction in values, indicates a loss of rhythm amplitude in DD. In LD, all wild-type and Id2+/− mice had a significant rhythm with a period length of 23.9–24.0 hr and showed significant rhythms in DD with variable period lengths. In these example mice, the wild-type mouse had a significant rhythm in both LD and in DD, with period lengths of 24.0 hr and 23.5 hr, respectively. In the example Id2 mutant mouse, a significant rhythm was detected in LD but no significant rhythm was detected in DD. Also note that the rhythm in LD was robust in days 1–10 (G), but after the duration in DD, in the LD during days 44–57, only a weak rhythm was identified (analysis not shown: periodogram, n.s.; Fourier analysis, significant peak at ∼0.042 cycles/hr of 0.02).
Because Id2 had shown the most consistent rhythmic gene expression in all cell lines and tissues, we generated transgenic mice lacking the entire coding region and therefore for the Id2 gene (Figure S2 and Supplemental Discussion) and examined their circadian systems (Figure 2, Table S1). Homozygous mutants are morphologically dissimilar to their wild-type littermate controls, being on average 27% smaller (F2,56 = 9.1, p = 0.0004), and exhibit a high level of lethality during the postnatal period; our other broad observations of the Id2−/− mice, such as immunocompromised status, are consistent with the Id2 mice generated by Yokota and colleagues  (see Supplemental Data). Importantly, histological analysis of SCN coronal sections showed no gross anatomical difference between Id2+/+ and Id2−/− F2 littermates (Figure S3), indicating that any circadian phenotypes in the mutants are not due to a gross developmental defect in the basic organization of the SCN.