Abstracts - Society for Developmental Biology

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implantation subpopulations of embryonic and placental stem cells (ESC and TSC, respectively) normally lose potency and
differentiate to produce the first essential functions. This is defined by studies of gene expression and null mutant lethals.
We study stress responsesof embryos/stem cells. At lower stresses anti-apoptotic and anabolic-to-catabolic shift responses
conserve energy, but there is no differentiation. At higher exposures cell growth is diminished and differentiation induced.
Since fewer cells produce more essential differentiated product/cell we call this compensatory differentiation. ESC and
TSC increase early and suppress later essential lineages, a stress response called “prioritized” differentiation. Stressactivated
protein kinase (SAPK) does not mediate stress-induced loss of nuclear potency factors, but mediates increases in
nuclear differentiation factors. SAPK is also the mediator of prioritized differentiation, increasing early, and suppressing
later lineages in a stress dose-dependent manner proportional to the amount of SAPK activated. AMP-activated protein
kinase (AMPK) mediates stress-induced loss of potency factor proteins. AMPK rescues potency mRNA, thus enabling
reversibility. Prioritized differentiation gives an understanding of how embryos/stem cells adapt to stress and will produce
biomarkers of stressed reproduction, drug and chemical toxicity tests, and insights into changes that affect pre- and postnatal
dysfunctions.
Program/Abstract # 276
The f-box protein atrogin enhances foxo in Drosophila melanogaster
Connors, Colleen; Staveley, Brian, Memorial University of Newfoundland, St. John's, Canada
Muscle atrophy can occur as the result of a wide range of conditions including diabetes, AIDS, sepsis and food
deprivation. Atrogin encodes an F-box protein that is the substrate recognition component of an SCF ubiquitin ligase
complex. It has been shown that atrogin acts in muscle degradation and is highly expressed during skeletal muscle atrophy
in Homo sapiens, Mus musculus,and Salmo salar among others.Specifically, atrogin can target proteins for degradation
that are essential for muscle synthesis such as myoD and eIF3f. However, not all target proteins of atrogin are targeted for
proteolysis. Foxo, a transcription factor and member of the insulin receptor signalling pathway, is ubiquitinated by atrogin
where the ubiquitin chain prevents the negative regulation of foxo by akt. Therefore, foxo localizes in the nucleus, and a
positive feedback loop between atrogin and foxo is activated, as atrogin is a target gene of foxo. An atrogin candidate had
been identified in Drosophila melanogaster and is well conserved between arthropods and mammals. Analysis shows that
overexpression of the gene can enhance phenotypes when co-expressed with foxo in the eye. Also, atrogin overexpression
can increase survivorship during nutritional stress, whereas a reduction of atrogin impairs the ability to endure starvation.
Taken together these findings may suggest a conserved role for atrogin in the regulation of foxo. In addition,
overexpression has been shown to cause a rapid degeneration of climbingability, and reduced longevity under standard
conditions. The potential for atrogin in degenerative phenotypes in Drosophila is being investigated. Funding by an
NSERC CGS to C.B. Connors and an NSERC Discovery Grant to B.E. Staveley.
Program/Abstract # 277
JNK phosphorylation of hnRNP K is required for axon outgrowth during nervous system development in Xenopus
laevis
Hutchins, Erica J.; Szaro, Ben G., State University of New York, Albany, United States
The RNA-binding protein hnRNP K is required for axon outgrowth. Its suppression in Xenopus embryos causes defects in
the translation of mRNAs of multiple cytoskeletal genes. Studies in cell lines have established that hnRNP K shuttles
between the nucleus and the cytoplasm to bind and regulate the fates of its target RNAs, from splicing to export and
translation. At each step, hnRNP K is regulated through post-translational modifications that alter its nucleic acid and
protein interactions, and its subcellular localization. How this happens in developing neurons to coordinate cytoskeletal
gene expression with the extracellular signals directing axon outgrowth is unknown. We have identified a JNK
phosphorylation site within hnRNP K that is essential for its function during neuronal development. Treatment with
SP600125, a pharmacological inhibitor of JNK, prevented formation of axons in primary neuronal cultures; a phosphomimetic
mutation of the JNK site on hnRNP K successfully rescued axon outgrowth in the presence of SP600125,
implicating hnRNP K as a major substrate on which JNK acts to affect axonogenesis. We propose a mechanism whereby
JNK controls translation of hnRNP K’s target mRNAs, and by extension axon outgrowth, at the point of translation
initiation through prevention of 80S ribosome assembly. JNK has long been implicated in the intracellular signaling
pathways that mediate effects of several receptors on axon outgrowth, although a mechanism of its action had not
previously been described. These data suggest a role for hnRNP K as a central regulatory component linking extracellular
signals that regulate axon outgrowth directly with the expression of key axonal structural components. Funded by NSF IOS
951043.