The tau mutation shortens the period of rhythmic photoreceptor outer segment disk shedding in the hamster

The outer segments of vertebrate retinal photoreceptors undergo periodic shedding of membrane from their distal tips. This circadian rhythm of disk shedding persists with a period of about 24 h in the absence of external time cues. A circadian oscillator controlling photoreceptor disk shedding may exist in the eye, but in addition, the circadian clock in the hypothalamic suprachiasmatic nucleus (SCN) may also influence ocular rhythms including that of disk shedding. The tau mutation directly affects the SCN, and shortens the period of locomotor activity from 24 h in wild-type hamsters to 20 h in homozygous mutants. Here we show that homozygous tau-mutant hamsters in a 20-h light/dark cycle exhibit a 20-h oscillation in the rate of disk shedding, with peak phagosome numbers in the retinal pigmented epithelium occurring just after light onset. The numbers of phagosomes are significantly elevated from mid-dark levels prior to light onset, indicating that the disk shedding cycle anticipates dawn. Under conditions of constant darkness, the disk shedding rhythm in tau-mutant hamsters persists with a period of approximately 20 h. These results indicate that a rhythm of retinal photoreceptor outer segment disk shedding exists in the hamster eye, and that the period of this rhythm is shortened by the tau mutation.