Switch of rhodopsin expression in terminally differentiated Drosophila sensory neurons

In this article, a switch of rhodopsin expression in terminally differentiated Drosophila sensory neurons is studied. According to the article, during metamorphosis, all Rh6 photoreceptors die, whereas the Rh5 photoreceptors “switch fate” by turning off Rh5 and turning on Rh6 expression. To understand the relationship between the two photoreceptors, the authors tracked them through metamorphosis. They labeled the photoreceptors using a histone protein to incorporate with chromatin in order to detect neurons during pupation. From this, the authors determined that all Rh6 photoreceptors degenerate during early metamorphosis and fail to appear later, as stated previously. However, this metamorphic analysis alone does not verify the death of Rh6 photoreceptors and transformation of Rh5 photoreceptors.

Three independent experiments were conducted to further verify these results. The first experiment ablated Rh5 photoreceptors expressing pro-apoptotic genes. By doing this, the experiment showed that an absence of Rh5 photoreceptors resulted. Also, if the cell death of Rh6 is prevented, all 12 photoreceptors will exist and all will express Rh6. A second experiment inhibited larval Rh6 photoreceptors by expressing a dominant negative form of EGFR (a cell growth receptor). This resulted in the normal expression of Rh6 photoreceptors.

Something must trigger the switch from Rh5 to Rh6. It was also shown that EcR (Ecdysome receptor) functions autonomously both for death of larval Rh6 and the sensory switch of Rh5 to express Rh6 by expressing the dominant negative form of EcR in Rh6 photoreceptors. Therefore, the ability to switch function of neurons provides “hard-wired sensory plasticity.” This goes along with the third experiment performed. The experimenters examined the expression of Sal (a Rh5 subtype) and Svp (a Rh6 subtype) in adult subjects. However, it was determined by the experimenters that Sal nor Svp is sufficient to alter larval photoreceptor response because late expression of either does not affect rhodopsin expression.

It was found that, instead of Sal or Svp, the transcription factor Senseless (Sens) mediates the cellular behaviors of Rh5 and Rh6 photoreceptors. The expression of Sens in Rh6 can indeed rescue them from death, without interfering with Sal or Svp. This was tested by misexpressing Sens in Rh6 photoreceptors.

This article greatly relates to what we have learned in class about light reception. In lecture we learned the structure and function of rhodopsin and photoreceptors. Essentially, this article furthered our understanding of rhodopsin photoreceptors by investigating and analyzing, in great detail, photoreceptors of light sensory neurons in Drosophila. By looking at the structure and function of different photoreceptors in Drosophila sensory neurons, the experimenters were able to test the source of rescue of Rh6 from death and therefore determine how Rh5 is turned off while Rh6 on the other hand is turned on. By studying photoreceptors in sensory neurons of an organism we can determine which photoreceptor is the source of a particular inexplicable event which occurs.

Sprecher, S.G. & Desplan, C. (2008) Switch of rhodopsin expression in terminally differentiated Drosophila sensory neurons. Nature, 454, 533-537