Phototropism In plants, cryptochromes mediate
phototropism, or directional growth toward a light source, in response to blue light. This response is now known to have its own set of photoreceptors, the
phototropins. Unlike
phytochromes and phototropins, cryptochromes are not
kinases. Their
flavin chromophore is reduced by light and transported into the
cell nucleus, where it affects the
turgor pressure and causes subsequent stem elongation. To be specific,
Cry2 is responsible for blue-light-mediated
cotyledon and leaf expansion.
Cry2 overexpression in
transgenic plants increases blue-light-stimulated cotyledon expansion, which results in many broad leaves and no flowers rather than a few primary leaves with a flower. A double loss-of-function mutation in Arabidopsis thaliana Early Flowering 3 (elf3) and Cry2 genes delays flowering under continuous light and was shown to accelerate it during long and short days, which suggests that Arabidopsis CRY2 may play a role in accelerating flowering time during continuous light.
Photomorphogenesis Cryptochromes receptors cause plants to respond to blue light via
photomorphogenesis. They help control seed and seedling development, as well as the switch from the vegetative to the flowering stage of development. In
Arabidopsis, CRY1 is the primary inhibitor of hypocotyl elongation but CRY2 inhibits hypocotyl elongation under low blue light intensity. CRY2 promotes flowering under long-day conditions. CRY gene mediates photomorphogenesis in several ways. CRY C-terminal interacts with CONTITUTIVE PHOTOMORPHOGENIC 1 (COP1), a E3 ubiquitin ligase that represses photomorphogenesis and flowering time. The interaction inhibits COP1 activity and allows transcription factors such as ELONGATED HYPOCOTYL 5 (HY5) to accumulate. HY5 is a basic leucine zipper (bZIP) factor that promotes photomorphogenesis by binding to light-responsive genes. CRY interacts with G protein β-subunit AGB1, where HY5 dissociates from AGB1 and becomes activated. CRY interacts with PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) and PIF5, repressors of photomorphogenesis and promoter of hypocotyl elongation, to repress PIF4 and PIF5 transcription activity. Lastly, CRY can inhibit
auxin and
brassinosterioid (BR) signaling to promote photomorphogenesis.
Light capture Despite much research on the topic, cryptochrome
photoreception and
phototransduction in
Drosophila and
Arabidopsis thaliana is still poorly understood. Cryptochromes are known to possess two chromophores:
pterin (in the form of
5,10-methenyltetrahydrofolic acid (MTHF)) and flavin (in the form of FAD). Both may absorb a
photon, and in
Arabidopsis, pterin appears to absorb at a wavelength of 380 nm and flavin at 450 nm. Past studies have supported a model by which energy captured by pterin is transferred to flavin. Under this model of phototransduction, FAD would then be
reduced to FADH, which probably mediates the
phosphorylation of a certain domain in cryptochrome. This could then trigger a
signal transduction chain, possibly affecting
gene regulation in the
cell nucleus. A new hypothesis proposes that partner molecules sense the transduction of a light signal into a chemical signal in plant cryptochromes, which could be triggered by a photo-induced negative charge on the FAD cofactor or on the neighboring aspartic acid within the protein. This negative charge would electrostatically repel the protein-bound
ATP molecule and thereby also the protein C-terminal domain, which covers the
ATP binding pocket prior to photon absorption. The resulting change in protein conformation could lead to phosphorylation of previously inaccessible phosphorylation sites on the C-terminus and the given phosphorylated segment could then liberate the transcription factor HY5 by competing for the same binding site at the negative regulator of photomorphogenesis
COP1. A different mechanism may function in
Drosophila. The true ground state of the flavin cofactor in
Drosophila CRY is still debated, with some models indicating that the FAD is in an oxidized form, while others support a model in which the flavin cofactor exists in
anion radical form, •. Recently, researchers have observed that oxidized FAD is readily reduced to • by light. Furthermore, mutations that blocked photoreduction had no effect on light-induced degradation of CRY, while mutations that altered the stability of • destroyed CRY photoreceptor function. These observations provide support for a ground state of •. Researchers have also recently proposed a model in which is excited to its
doublet or quartet state by absorption of a photon, which then leads to a conformational change in the CRY protein. Also the ring eyes of the
demosponge larva of
Amphimedon queenslandica express a blue-light-sensitive cryptochrome (Aq-Cry2), which might mediate phototaxis. In contrast, the eyes of most animals use
photo-sensitive opsins expressed in photoreceptor cells, which communicate information about light from the environment to the nervous system. However,
A. queenslandica lacks a nervous system, like other
sponges. And it does not have an
opsin gene in its fully sequenced
genome either, despite having many other
G-protein-coupled receptors (GPCRs). Therefore, the sponge's unique eyes must have evolved a different mechanism to detect light and mediate phototaxis, possibly with cryptochromes or other proteins.
Iris function Isolated irises constrict in response to light via a photomechanical transduction response (PMTR) in a variety of species and require either
melanopsin or cryptochrome to do so. The iris of chicken embryos senses short-wavelength light via a cryptochrome, rather than opsins. Research by Margiotta and Howard (2020) shows that the PMTR of the chicken iris striated muscle occurs with
CRY gene activation by 430 nm blue light. The PMTR was inhibited in
CRY gene knockouts and decreased when flavin reductase was inhibited, but remained intact with the addition of melanopsin antagonists. Similarly, cytosolic
CRY1 and
CRY2 proteins were found in iris
myotubes, and decreasing transcription of these genes inhibited PMTRs. The greatest iris PMTRs therefore correspond with the development of striated, rather than smooth, muscle fibers through
CRY-mediated PMTRs.
Circadian rhythm Studies in animals and plants suggest that cryptochromes play a pivotal role in the generation and maintenance of circadian rhythms. Similarly, cryptochromes play an important role in the entrainment of circadian rhythms in plants. In
Drosophila, cryptochrome (dCRY) acts as a blue-light photoreceptor that directly modulates light input into the circadian clock, while in mammals, cryptochromes (CRY1 and CRY2) act as
transcription repressors within the circadian clockwork. Some insects, including the
monarch butterfly, have both a mammal-like and a
Drosophila-like version of cryptochrome, providing evidence for an ancestral clock mechanism involving both light-sensing and transcriptional-repression roles for cryptochrome.
Cry mutants have altered circadian rhythms, showing that
Cry affects the circadian pacemaker.
Drosophila with mutated
Cry exhibit little to no mRNA cycling. A point mutation in
cryb, which is required for flavin association in CRY protein, results in no PER or TIM protein cycling in either DD or LD. In addition, mice lacking
Cry1 or
Cry2 genes exhibit differentially altered free running periods, but are still capable of
photoentrainment. However, mice that lack both
Cry1 and
Cry2 are arrhythmic in both LD and DD and always have high
Per1 mRNA levels. These results suggest that cryptochromes play a photoreceptive role, as well as acting as negative regulators of Per gene expression in mice.
In Drosophila In
Drosophila, cryptochrome is only encoded by one
Cry gene (d
Cry) and functions as a blue light photoreceptor. Exposure to blue light induces a conformation similar to that of the always-active CRY mutant with a C-terminal deletion (CRYΔ). The half-life of this conformation is 15 minutes in the dark and facilitates the binding of CRY to other clock gene products, PER and
TIM, in a light-dependent manner. Once bound by dCRY, dTIM is committed to degradation by the ubiquitin-
proteasome system. Although light pulses do not entrain, full photoperiod LD cycles can still drive cycling in the
ventral-
lateral neurons in the
Drosophila brain. These data along with other results suggest that CRY is the cell-autonomous photoreceptor for body clocks in
Drosophila and may play a role in nonparametric entrainment (entrainment by short discrete light pulses). However, the lateral neurons receive light information through both the blue light CRY pathway and the
rhodopsin pathway. Therefore, CRY is involved in light perception and is an input to the circadian clock, however it is not the only input for light information, as a sustained rhythm has been shown in the absence of the CRY pathway, in which it is believed that the rhodopsin pathway is providing some light input. Recently, it has also been shown that there is a CRY-mediated light response that is independent of the classical circadian CRY-TIM interaction. This mechanism is believed to require a flavin
redox-based mechanism that is dependent on potassium channel conductance. This CRY-mediated light response has been shown to increase
action potential firing within seconds of a light response in
opsin-knockout
Drosophila. Cryptochrome, like many genes involved in circadian rhythm, shows circadian cycling in
mRNA and protein levels. In
Drosophila,
Cry mRNA concentrations cycle under a light-dark cycle (LD), with high levels in light and low levels in the dark. This cycling persists in constant darkness (DD), but with decreased amplitude. The transcription of the
Cry gene also cycles with a similar trend. CRY protein levels, however, cycle in a different manner than
Cry transcription and mRNA levels. In LD, CRY protein has low levels in light and high levels in dark, and, in DD, CRY levels increase continuously throughout the subjective day and night. Thus, CRY expression is regulated by the clock at the transcriptional level and by light at the
translational and posttranslational level. Overexpression of
Cry also affects circadian light responses. In
Drosophila,
Cry overexpression increases flies' sensitivity to low-intensity light. This light regulation of CRY protein levels suggests that CRY has a circadian role upstream of other clock genes and components.
In mammals In mammals, cryptochrome proteins are encoded by two genes,
Cry1 and
Cry2. Cry2 Cryptochrome is one of the four groups of mammalian clock genes/proteins that generate a transcription-translation negative-feedback loop (TTFL), along with
Period (PER),
CLOCK, and
BMAL1. In this loop, CLOCK and BMAL1 proteins are
transcriptional activators, which together bind to the
promoters of the
Cry2 and
Per genes and activate their transcription. The CRY2 and PER proteins then bind to each other, enter the nucleus, and inhibit CLOCK-BMAL1-activated transcription. The overall function of CRY2 is therefore to repress transcription of CLOCK and BMAL1.
Cry1 Cry1 encodes the CRY1 protein which is a mammalian circadian photoreceptor. In mice,
Cry1 expression displays circadian rhythms in the
suprachiasmatic nucleus, a brain region involved in the generation of circadian rhythms, with mRNA levels peaking during the light phase and reaching a minimum in the dark. These daily oscillations in expression are maintained in constant darkness. While CRY1 has been well established as a TIM homolog in mammals, the role of CRY1 as a photoreceptor in mammals has been controversial. Early papers indicated that CRY1 has both light-independent and -dependent functions. A study conducted by Selby CP et al. (2000) found that mice without rhodopsin but with cryptochrome still respond to light; however, in mice without either rhodopsin or cryptochrome,
c-Fos transcription, a mediator of light sensitivity, significantly drops. In recent years, data have supported
melanopsin as the main circadian photoreceptor, in particular melanopsin cells that mediate entrainment and communication between the
eye and the suprachiasmatic nucleus (SCN). One of the main difficulties in confirming or denying CRY as a mammalian photoreceptor is that when the gene is knocked out the animal goes arrhythmic, so it is hard to measure its capacity as purely a photoreceptor. However, some recent studies indicate that human CRY1 may mediate light response in peripheral tissues. Normal mammalian circadian rhythm relies critically on delayed expression of
Cry1 following activation of the
Cry1 promoter. Whereas rhythms in
Per2 promoter activation and
Per2 mRNA levels have almost the same phase,
Cry1 mRNA production is delayed by approximately four hours relative to
Cry1 promoter activation. This delay is independent of CRY1 or CRY2 levels and is mediated by a combination of
E/E'-box and D-box elements in the promoter and
RevErbA/
ROR binding elements (RREs) in the gene's first intron.
Transfection of arrhythmic
Cry1−/−
Cry2−/− double-knockout cells with only the
Cry1 promoter (causing constitutive
Cry1 expression) is not sufficient to rescue rhythmicity. Transfection of these cells with both the promoter and the first
intron is required for restoration of circadian rhythms in these cells. There is evidence that CRY1 can play a role in how sleep-wake patterns can be
inherited through families. There is a mutation,
CRY1Δ11, that causes a delay in one's circadian rhythm. CRY1Δ11 is a splicing variant that has deleted an
auto-inhibitory section of the gene. It causes a delay by increasing the affinity of CLOCK and
BMAL which in turn lengthens the period. This causes people with this mutation to have a later sleep midpoint than the rest of the population, causing a disorder known as
delayed sleep–wake phase disorder. CRY1 is also a key modulator in
DNA repair, specifically through temporal regulation. CRY1 has an impact in the cell cycle progression, particularly in the
G2/M checkpoint, and the
depletion of CRY1 leads to effects on DNA repair networks, including mismatch repair, UV, and
nucleotide excision. In
cancer, CRY1 is stabilized by DNA damage, which results in CRY1 expression being associated with worse outcomes in
prostate cancer. Because of its role in DNA repair and being
pro-tumorigenic, further research can use CRY1 as a
therapeutic target. Variants of CRY1 can have impacts on humans in regards to metabolic output. According to a 2021 study,
metabolic outputs, measured by
bowel movements, were severely different for participants who were
wild type in comparison to those with the CRY1Δ11 variant. The participants with the variant had a delayed sleep cycle and
delayed metabolic output when compared to the wild type.
Magnetoreception has been proposed for quantum magnetoreception in birds. Magnetoreception is a sense which allows an organism to detect a magnetic field to perceive direction, altitude or location. Experimental data suggests that cryptochromes in the
photoreceptor neurons of birds' eyes are involved in magnetic orientation during
migration. Cryptochromes are also thought to be essential for the light-dependent ability of
Drosophila to sense
magnetic fields. Magnetic fields were once reported to affect cryptochromes also in
Arabidopsis thaliana plants: growth behavior seemed to be affected by magnetic fields in the presence of blue (but not red) light. Nevertheless, these results have later turned out to be irreproducible under strictly controlled conditions in another laboratory, suggesting that plant cryptochromes do not respond to magnetic fields. Cryptochrome forms a pair of
radicals with correlated
spins when exposed to blue light. Radical pairs can also be generated by the light-independent dark reoxidation of the flavin cofactor by molecular oxygen through the formation of a spin-correlated FADH-superoxide radical pairs. Magnetoreception is hypothesized to function through the surrounding magnetic field's effect on the correlation (parallel or anti-parallel) of these radicals, which affects the lifetime of the activated form of cryptochrome. Activation of cryptochrome may affect the light-sensitivity of
retinal neurons, with the overall result that the animal can sense the magnetic field. Animal cryptochromes and closely related animal (6-4) photolyases contain a longer chain of electron-transferring tryptophans than other proteins of the cryptochrome-photolyase superfamily (a tryptophan tetrad instead of a triad). The longer chain leads to a better separation and over 1000× longer lifetimes of the photoinduced flavin-tryptophan radical pairs than in proteins with a triad of tryptophans. The absence of spin-selective recombination of these radical pairs on the nanosecond to microsecond timescales seems to be incompatible with the suggestion that magnetoreception by cryptochromes is based on the forward light reaction. == References ==