A cell's 'fuel gauge' promotes healthy development
Salk Institute scientists have found how a cellular "fuel gauge", responsible for monitoring and managing cell energy, has another unexpected role. Without it, cells won't know when to "clean up" or how to recycle cell waste — a forerunner to diabetes and cancer.
A cellular fuel gauge made from a protein complex called AMPK, oversees energy input and output that keeps a cell running smoothly. If AMPK were a car sensor, it would tell you when to get gas or lower air conditioning to save energy.
Similarly, if a cell's fuel supply is low, AMPK slows down cell growth and metabolism. Previously, Reuben Shaw PhD, holder of the William R. Brody Chair at Salk Institute, had discovered AMPK stopped the revved-up metabolism of tumors and restored normal function to liver and other tissues of diabetics.
"Even though there's great interest in AMPK related to diabetes and cancer, frankly nothing was known about how this fuel gauge process changes in different cell populations during development."
Shaw PhD, Salk Institute.
So aside from giving new insight into stem cell therapies, the new work published March 2016 in Genes & Development, could help refine cancer treatment.
"To begin, we used CRISPR technology to edit out two important components of the AMPK pathway in embryonic stem cells," says Nathan Young, Salk research associate and first author of the paper. "At first we didn't see any difference, but things became interesting when we prompted the cells to differentiate."
Normally, embryonic stem cells have the capacity to generate specialized cells belonging to one of three broad cell groups called germ layers — (1) endoderm (2) ectoderm and (3) mesoderm. From these layers all cell types ultimately develop in an organism.
However, cells without a functioning AMPK signalling pathway fail to make endoderm (the innermost layer in an organism), and instead make too much ectoderm (the layer that should turn into skin).
"This was the first inclination that the AMPK metabolic pathway is telling cells what kind of specialized tissues to become."
This is a remarkable observation. When researchers looked closer at gene expression patterns in cells without AMPK, they found large numbers of genes relegated to one specific cell structure: the lysosome. The lysosome is a self-contained organelle full of corrosive enzymes ready to degrade cellular material for reuse — they are a cell's garbage disposal/recycling center.
Researchers observed the loss of lysosomes resulted from loss of Tfeb transcription factor as Tfeb turns on the expression of lysosome genes in times of starvation. By reintroducing Tfeb into dysfunctional cells, the team was able to restore normal development and differentiation.
Shaw: "It was thought that lysosomes and AMPK were connected somehow, but no one dreamed that you'd get no lysosomes if you don't have AMPK acting as a fuel gauge. Connecting the AMPK pathway to lysosomes begs the question of whether this pathway is part of anti-cancer pathways as well."
Currently, lysosome inhibitors are in dozens of clinical trials for breast, lung, pancreatic and brain cancers — even though the exact link between lysosomes and tumors is not understood.
Says Shaw: "We are decoding some of these underlying connections that might indicate when and how a cancer drug might be useful. This may help up make better, more specific ways of targeting lysosomes in cancer."
Faithful execution of developmental programs relies on the acquisition of unique cell identities from pluripotent progenitors, a process governed by combinatorial inputs from numerous signaling cascades that ultimately dictate lineage-specific transcriptional outputs. Despite growing evidence that metabolism is integrated with many molecular networks, how pathways that control energy homeostasis may affect cell fate decisions is largely unknown. Here, we show that AMP-activated protein kinase (AMPK), a central metabolic regulator, plays critical roles in lineage specification. Although AMPK-deficient embryonic stem cells (ESCs) were normal in the pluripotent state, these cells displayed profound defects upon differentiation, failing to generate chimeric embryos and preferentially adopting an ectodermal fate at the expense of the endoderm during embryoid body (EB) formation. AMPK−/− EBs exhibited reduced levels of Tfeb, a master transcriptional regulator of lysosomes, leading to diminished endolysosomal function. Remarkably, genetic loss of Tfeb also yielded endodermal defects, while AMPK-null ESCs overexpressing this transcription factor normalized their differential potential, revealing an intimate connection between Tfeb/lysosomes and germ layer specification. The compromised endolysosomal system resulting from AMPK or Tfeb inactivation blunted Wnt signaling, while up-regulating this pathway restored expression of endodermal markers. Collectively, these results uncover the AMPK pathway as a novel regulator of cell fate determination during differentiation.
Other authors were Anwesh Kamireddy, Jeanine Van Nostrand, Lillian Eichner, Maxim Nikolaievich Shokhirev and Yelena Dayn, all of the Salk Institute. The work was supported by the National Institutes of Health and the Leona M. and Harry B. Helmsley Charitable Trust.
About the Salk Institute for Biological Studies:
Every cure has a starting point. The Salk Institute embodies Jonas Salk's mission to dare to make dreams into reality. Its internationally renowned and award-winning scientists explore the very foundations of life, seeking new understandings in neuroscience, genetics, immunology and more. The Institute is an independent nonprofit organization and architectural landmark: small by choice, intimate by nature and fearless in the face of any challenge. Be it cancer or Alzheimer's, aging or diabetes, Salk is where cures begin.
The Transcription Factor TFEB Links mTORC1 Signaling to Transcriptional Control of Lysosome Homeostasis
Lysosomes are the major cellular site for clearance of defective organelles and digestion of internalized material. Demand on lysosomal capacity varies greatly, but the mechanisms that adjust lysosomal function to maintain cellular homeostasis are unknown. In this study, we identify an interaction between mTOR and the TFEB transcription factor on the surface of lysosomes that allows mTOR to transduce signals arising from changes in lysosomal status to TFEB and thus control the ability of TFEB to enter the nucleus. This occurs via regulation of the serine 211 phosphorylation-dependent binding of 14-3-3 proteins to TFEB. These results identify TFEB as a novel target of mTOR that couples the transcriptional regulation of genes encoding proteins of autophagosomes and lysosomes to cellular need. We further present evidence that the closely related MITF and TFE3 transcription factors are regulated in a similar manner, thus broadening the range of physiological contexts under which such regulation may prove important.
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May 4, 2016 Fetal Timeline Maternal Timeline News News Archive
Salk scientists reveal a close association between lysosomes, recycling centers of the cell,
and the development of the endoderm germ layer. This image shows a well-differentiated
structure made from normal embryonic stem cells, with all cell nuclei stained blue.
Only endoderm cells (GREEN) - destined to become the innermost cells of an organism -
contain high levels of lysosomes (RED).
Image Credit: Anwesh Kamireddy, The Salk Institute