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Christopher K. Hwang, Shyam S. Chaurasia, P M. Iuvone; NPAS2 in the Retina: Circadian Expression, Localization, and Role in Visual Function. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2722.
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© ARVO (1962-2015); The Authors (2016-present)
Neuronal PAS-domain protein 2 (NPAS2) is a transcription factor involved in the circadian clock in the mammalian brain. However, in the mammalian retina, the localization and function of NPAS2 have not been described. This study was conducted to examine the expression pattern of NPAS2 in the retina and its effects on retinal gene expression and visual function.
Studies were performed using C57BL/6 wildtype (WT) and Npas2-/- mice, which express mutated, non-functional NPAS2 protein fused to β-galactosidase. Visualization of β-galactosidase in the retina was achieved through enzyme activity assay and immunohistochemistry. Laser capture microdissection and quantitative real-time PCR were performed to isolate the photoreceptor, inner nuclear, and ganglion cell layers at five time points for analysis of gene expression pattern. Contrast sensitivity was measured using optokinetic tracking at mid-day and mid-night time points on regular light-dark cycle (LD) and after two days of constant darkness (DD2). To further evaluate the link between adenylate cyclase 1 (Adcy1) and contrast sensitivity, Adcy1-/- mice and dopamine D4 receptor null (Drd4-/-) mice, whose Adcy1 transcript rhythm is abolished in the retina, were also studied. Scotopic and photopic electroretinogram (ERG) studies were conducted to measure retinal responses to light.
NPAS2-β-galactosidase is localized to a subset of cells in the ganglion cell layer (GCL). Npas2 transcript is rhythmically expressed in the GCL, peaking in the early morning. In Npas2-/- mice, rhythmic expression of Adcy1 transcript is abolished in the GCL, but retained in the photoreceptor layer. A circadian rhythm of contrast sensitivity is observed in both WT and Npas2-/- mice, with higher contrast sensitivity in the daytime. However, Npas2-/- mice exhibit significant day-time contrast sensitivity deficits in LD as well as in DD2. A similar reduction in daytime contrast sensitivity is observed in Adcy1-/- mice and Drd4-/- mice. Npas2-/-, Adcy1-/-, and Drd4-/- mice exhibit normal visual acuity. No diurnal variation in visual acuity is observed. There is no difference in scotopic and photopic ERG amplitudes and implicit times between WT and Npas2-/- mice.
These findings establish that NPAS2 is rhythmically expressed, regulates Adcy1 expression in the GCL, and modulates contrast sensitivity. Abolishment of the Adcy1 transcript rhythm in the GCL appears to be sufficient to reduce contrast sensitivity to the level seen in Adcy1-/- and Drd4-/- mice. NPAS2 does not seem to affect gene expression or retinal function outside the GCL. The results suggest that dopamine may modulate daytime contrast sensitivity through an action on NPAS2-expressing cells in the GCL.
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