Cellular circadian oscillators in the suprachiasmatic nucleus remain coupled in the absence of connexin-36

Abstract

In mammals, the master circadian clock resides in the suprachiasmatic nucleus (SCN). The SCN is characterized by robust circadian oscillations of clock gene expression and neuronal firing. The synchronization of circadian oscillations among individual cells in the SCN is attributed to intercellular coupling. Previous studies have shown that gap junctions, specifically those composed of connexin-36 (Cx36) subunits, are required for coupling of electrical firing among SCN neurons at a time scale of milliseconds. However, it remains unknown whether Cx36 gap junctions also contribute to coupling of circadian (~24 h) rhythms of clock gene expression. Here, we investigated circadian expression patterns of the clock gene Period 2 (Per2) in the SCN of Cx36-deficient mice using luminometry and single-cell bioluminescence imaging. Surprisingly, we found that synchronization of circadian PER2 expression rhythms is maintained in SCN explants from Cx36-deficient mice. Since Cx36 expression levels change with age, we also tested circadian running-wheel behavior of juvenile (3–4 weeks old) and adult (9–30 weeks old) Cx36-deficient mice. We found that impact of connexin-36 expression on circadian behavior changes greatly during postnatal development. However, consistent with the intact synchrony among SCN cells in cultured explants, Cx36-deficient mice had intact locomotor circadian rhythms, although adults displayed a lengthened period in constant darkness. Our data indicate that even though Cx36 may be required for electrical coupling of SCN cells, it does not affect coupling of molecular clock gene rhythms. Thus, electrical coupling of neurons and coupling of circadian clock gene oscillations can be regulated independently in the SCN.

Connexin30 and Connexin43 show a time-of-day dependent expression in the mouse suprachiasmatic nucleus and modulate rhythmic locomotor activity in the context of chronodisruption

“…This shows that glial Cx43 and Cx30 are dispensable for circadian rhythm generation. This is in contrast to changes in period length of circadian rhythms in locomotor activity observed in Cx36-deficient mice [28, 29]. Thus, in the SNC Cx36-mediated neuronal coupling is functionally more important than glial coupling through Cx30 or Cx43.…”

“…Within the SCN, neuronal gap junctions are important for electric coupling, which is crucial to generate a coherent circadian rhythm of neuronal firing [27]. Importantly, deletion of neural connexin Cx36 significantly impairs the SCN electric coupling resulting in changes of circadian period length [28, 29]. Moreover, astrocytic-neuronal signalling regulates rhythmic SCN neuronal activity and thus circadian rhythm generation [30].…”

Astrocyte Clocks and Glucose Homeostasis

“…Secondly, astrocytes form a syncytium, via gap junctions, that allow the propagation of small signaling molecules through the glial network ( 69 ). Pharmacological inhibition of gap junctions in SCN slides ( 70 , 71 ) and mouse models with deletion of the neuronal connexin-36, impairs the circadian pattern of neuronal activity without affecting the long-term synchronization of clock gene expression ( 72 ) and with mild effect on behavioral rhythms ( 73 ). Similarly, studies in mouse models with deletion of astrocytic specific connexins indicate that the astrocytic coupling in the SCN is dispensable for circadian rhythm generation and light-entrainment ( 74 ).…”

Circuit development in the master clock network of mammals

“…Connexin36 is a subunit of gap junctions specifically expressed in neurons, and it has been shown that inhibiting the function of gap junctions eliminates electrical coupling among SCN neurons and deleting connexin36 decreases the amplitude of daily rhythms in behaviour (Long et al., ; Wang, Chen & Wang, ). However, recent work reveals that loss of connexin36 in adult mice does not impair circadian function at the behavioural or molecular levels (Diemer et al, ). Collectively, these results suggest that signalling through connexin36–expression gap junctions modulates short‐term electrical coupling among SCN neurons, but this mechanism is not necessary for long‐term synchronization among SCN neurons at the level of the molecular clock.…”