Phototransduction Motifs and Variations

Overview

• Main Concept: Understanding how light absorbed by photoreceptors is converted into electrical signals, a process known as phototransduction.
• Key Component: Opsins, a class of photoproteins, play a crucial role in this mechanism.

Photoreceptors and Phototransduction

• Photoreceptors initiate visual processing by absorbing light and converting it into signals for the nervous system.
• The process of phototransduction is a conserved mechanism across different species.

Opsins

• Opsins are responsible for the light-induced signaling cascades in photoreceptors.
• Two main groups:
  • c-opsins
  • r-opsins
• Additional types include Go-opsin, more closely related to c-opsins.

Evolutionary Perspective

• Opsins have evolved with various photoreceptor types for different species.
• The divergence between protostome invertebrates and deuterostomes occurred approximately 550 million years ago.

Phototransduction Mechanisms in Vertebrates and Invertebrates

Vertebrate Rods (Figure 2)

• Active Form: Rhodopsin converted to an active form (Rh*), initiating the phototransduction cascade.
• Pathway: Involves Gt activation, cyclic GMP (cGMP) hydrolysis, and closing of CNG channels.
• Feedback: Includes negative feedback via Ca2+ pathways.

Pigment Cycles (Figure 3)

• Rods and Cones:
  • Utilize cycles with retinal pigment epithelial cells and Mller cells.
• Drosophila Photoreceptors: Employs blue light to photoisomerize retinal, with photoregeneration via long-wavelength light.

Drosophila Rhabdomeric Photoreceptors (Figure 4)

• Activation: Light absorption converts rhodopsin to an active state.
• Pathway: Involves Gq activation, PLC activity, and channel activation.
• Feedback: Ca2+ influx affects multiple sites, including PKC, arrestin, and TRP/TRPL channels.

Phototransduction Motifs (Figure 5)

• Two primary motifs:
  • Cyclic Nucleotide-Mediated Pathway: Includes Gt and Go pathways with distinct end results.
  • Phospholipase C (PLC)-Mediated Pathway: Involves hydrolysis of PIP2 and subsequent channel gating.

Key Differences

• Feedback Mechanisms: Varied feedback processes exist within the different pathways.
• Rare Pathways: Identified noncanonical pathways within certain species or cell types.

Additional Resources

• Includes similar articles, cited by other works, and reference materials for further study.

Grants and Funding

• Supported by various NIH grants and the Biotechnology and Biological Sciences Research Council in the UK.

Conclusion

• The study of phototransduction pathways reveals both highly conserved mechanisms and unique variations across species.