Opn5, also known as neuropsin, is an atypical opsin that functions as a photoreceptive protein. It is involved in various physiological functions related to light-sensing, such as photoperiodism, circadian rhythm regulation, and myopia prevention. It binds retinal as a chromophore and has distinct molecular characteristics among vertebrate species [2,3].

In mouse models, Opn5-null mice show several phenotypes. For example, they have overactive brown adipose tissue, increased body temperature, and exaggerated thermogenesis when cold-challenged, indicating that Opn5-expressing neurons in the hypothalamic preoptic area regulate thermogenesis in brown adipose tissue [4]. In addition, Opn5-null mice exhibit impaired photoentrainment and phase-shifting to UVA light, although Opn5 alone is not sufficient for photoentrainment of the suprachiasmatic nuclei clock [6]. In the retina, Opn5-null retinas fail to photoentrain to light cycles ex vivo, suggesting its role in local light-dependent regulation of the retinal circadian clock [5]. Also, violet light-induced suppression of lens-induced myopia in mice is dependent on retinal Opn5 expression, highlighting its role in myopia prevention [1].

In conclusion, Opn5 plays essential roles in multiple physiological processes including thermogenesis, circadian photoentrainment, and myopia prevention as revealed through mouse knockout models. These studies contribute to understanding the mechanisms of related diseases and potentially developing new strategies for myopia prevention and thermoregulation-related disorders [1,4,5,6].