Yeast Interfering RNA Larvicides

Mysore et al. recently reported in the Malaria Journal their successful development of a number of Anopheles larvicides consisting of yeast expressing specific siRNAs.

RNAi has attracted considerable attention as a pest control tool, particularly in the agricultural space.  In the field of vector biology, no so much.  There have been some interesting reports of using bacteria- and Pichia-expressed dsRNA as oral delivery systems for down-regulating genes in Aedes larvae.  Directly feeding dsRNA to larvae can also result in target gene ‘silencing’.

Mysore et al. have extended some of this earlier work and have taken it in a slightly different direction with interesting results.

The current work was preceded by a screen using siRNAs designed against a panel of Anopheles genes that are orthologous to genes in Drosophila that are known to result in larval lethality when their expression is down-regulated or eliminated.

Three genes were used in the current study:

Sac 1 – in D. melanogaster it is a “Sac1 phosphatase (Sac1) is a lipid phosphatase that dephosphorylates phosphatidylinositol 4-phosphate to generate phosphatidylinositol. Sac1 roles include dorsal closure, Hedgehog signaling and axon guidance.” (from Flybase).

otk – in D. melanogasterOff-track (Otk) associates with PlexA to receive a repulsive signal from Sema-1a contributing to axon guidance in the central nervous system and motor neurons.  Otk is also associated with non-canonical Wnt signaling, opposing canonical Wnt signaling activation.” (from Flybase).

lrc – in D. melanogaster this is CG6700 and was referred to by the authors as leukocyte receptor complex member (This is not an official Flybase name but an accurate descriptor nonetheless)

Drawing of lateral view of Anopheles larvae showing palmate hairs at water surface; larva larger than in slide 4

Lateral view of Anopheles larvae.  image from https://www.utep.edu/leb/mosquito/larvslide19.htm

photo of Anopheles larva

Lateral view of Anopheles larvae. photo from https://ipmworld.umn.edu/curtis-malaria-vectors

Mysore et al. . designed two siRNAs for each gene; confirm them and then, using yeast expression vectors, transformed them into S. cerevisiae.  siRNAs were used to increase specificity and reduce the probability for off target effects.

The authors performed a number of experiments including testing larvicidal activity of live yeast and dead yeast.  Larvicidal activities were from about 80% to 100% depending on how the siRNAs were delivered.  They provided good evidence for significant gene silencing using PCR and indirect immunoflourescenc.

One of the more interesting finding is that dead yeast that had expressed the Anopheles specific siRNAs were highly effective larvicides.  The authors report their ability to create dried inactivated yeast interfering RNA tablets that were quite effective.

The details of these studies are worth examining in detail.

This image of a larval habitat of Anopheles gambiae in Afirca is from Huang, J., et al., Daily temperature profiles in and around Western Kenyan larval habitats of. Vol. 5. 2006. 87.

In the field of vector biology and malaria control specifically, the effective implementation of all of the current tools for controlling vector populations and treating malaria still result in what is referred to as residual transmission.  This nagging and significant transmission represents a major challenge and one way it is being combated is through ‘larval source reduction’.   Larvicides could play a big role.  The yeast system described by Mysore et al. . has a number of interesting features that make it imaginable that some form of this technology might work in Africa.

Mysore, K., et al., Yeast interfering RNA larvicides targeting neural genes induce high rates of Anopheles larval mortality. Malaria Journal, 2017. 16(1): p. 461.

 

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