Reddien Lab Research Summary
Regeneration is one of the great mysteries
of biology. Planarians are bilaterally symmetric metazoans
that possess almost unlimited regenerative capacities
and that have been a classic regeneration model for
over a century. Since planarian regeneration involves
a population of adult pluripotent stem cells (the neoblasts),
planarians are an excellent organism for studies of
in vivo stem cell regulation. We use RNA interference
(RNAi) for high-throughput studies of gene function
in the planarian S. mediterranea. Our aim is to understand
how planarian neoblasts control regeneration.
RNAi screening. Planarians have not
historically been accessible to extensive genetic manipulation.
We developed a highly effective RNAi strategy that introduces
large-scale gene inhibition studies to planaria. 1065
genes have been inhibited during regeneration.
Phenotypes associated with the RNAi of 240 genes identify
many specific defects in the process of regeneration
and define the major categories of defects planarians
display following gene perturbations. 85% of these 240
genes have homology to genes in other organisms. We
assessed the effects of inhibiting genes on tissue homeostasis
in intact animals and on neoblast proliferation in amputated
animals, identifying candidate stem cell, regeneration,
and homeostasis regulators. Our RNAi screen establishes
planarians as a powerful model for the molecular genetic
study of stem cells, regeneration, and tissue homeostasis.
Neoblast regulation. We identified
a planarian gene, smedwi-2, that is needed for regeneration
and homeostasis. We found that smedwi-2 and the related
smedwi-1 gene are expressed in dividing neoblasts. The
SMEDWI-2 protein is homologous to Drosophila PIWI, a
germline stem cell regulator. Irradiation of planarians
specifically eliminates neoblasts and causes defects
that closely resemble those seen in smedwi-2(RNAi) animals.
Therefore, smedwi-2(RNAi) neoblasts may be dysfunctional.
We used FACS, antibody labeling, and BrdU to study the
neoblasts of smedwi-2(RNAi) animals. Our data indicate
a primary defect not in neoblast maintenance, but in
the function of neoblast progeny to replace aged cells
in homeostasis and to replace missing cells in regeneration.
Other genes with similar RNAi-induced defects from the
RNAi screen are being examined.
How do planarians specify what to regenerate?
Planarians can regenerate entire heads, sides, and tails.
Regeneration from irregularly shaped fragments involves
a robust ability to generate bilateral symmetry: new
tissue is produced in a regeneration blastema at the
wound site and remaining body regions are rearranged
to produce symmetrical and properly proportioned animals.
We identified three genes that are needed for asymmetric
regeneration. These genes are candidate components
of a BMP signaling pathway that controls the dorsal-ventral
patterning of many animal embryos: a BMP1/Tolloid-like
gene (smedolloid-1), a SMAD4-like gene (smedsmad4-1),
and a DPP-like gene (smeddpp-1). Intact adult animals
rely upon the action of this pathway to maintain their
body plans: RNAi of smedsmad4-1 in normal adults causes
a slow transformation in which animals develop two ventral
sides. Animal fragments with perturbed DPP signaling
and left-right symmetry regenerate blastemas with midline
defects. smeddpp-1 is expressed on the dorsal midline
suggesting this pathway normally regulates midline regeneration.
Animal fragments with perturbed DPP signaling but that
are asymmetric about the midline cannot produce blastemas.
We suggest the patterning activity of the midline is
necessary for the initiation of lateral blastema formation
in asymmetric fragments. This work provides a
starting point for investigations into how planarians
“know” what part of their body is missing.
The invertebrate animals C. elegans and Drosophila have
served as potent research subjects for the discovery
of basic principles in developmental biology.
We believe that the processes prominent in planarians
— regeneration, extensive tissue turnover as part
of homeostasis, and adult pluripotent stem cells —
combined with our ability to systematically study planarian
gene function, provide a new and powerful venue for
exploration of biology.
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