Unraveling stem cells
Neuroscientists document the first steps in the process of a stem cell transforming into a different cell type.
How do neurons become neurons? They begin as stem cells with the potential to become any cell in the body — they are undifferentiated.
Until now, however, exactly how differentiation happens has been something of a mystery. Now, research by University of California Santa Barbara (UCSB) neuroscientists has deciphered some of the earliest changes occurring as stems cells transform into neurons or other cell types.
Working with human embryonic stems cells in petri dishes, postdoctoral fellow Jiwon Jang discovered a new pathway key to cell differentiation. The findings appear in the journal Cell.
"Jiwon's discovery is very important because it details the way stem cells work and begin to undergo differentiation. It's a very fundamental piece of knowledge that had been missing in the field."
Kenneth S. Kosik PhD, Harriman Professor of Neuroscience Research, Department of Molecular, Cellular, and Developmental Biology, UCSB, and senior author.
When stem cells begin to differentiate, they form precursor cell types: neuroectoderms (NE) with the potential to become brain cells, such as neurons; or mesendoderms (ME), which ultimately become cells that make up organs, muscles, blood and bone.
Jang discovered a number of steps along the Primary cilium, Autophagy Nrf2 (PAN) axis, so named by Kosik and Jang. This newly identified pathway appears to pre-determine a stem cell's fate.
"The PAN axis is very important in cell fate decisions. The length of the G1 phase induces cilia to protrude. The longer those cellular antennae are exposed, the more signals they pick up."
Jiwon Jang, postdoctoral fellow Department of Molecular, Cellular and Developmental Biology, University of California Santa Barbara.
For some time, scientists have known Gap 1 (G1) is the first of four phases in the cell cycle. But, they weren't clear about its role in stem cell differentiation. Jang's research demonstrates that in stem cells destined to become neurons, the longer phase of G1 triggers them to morph into neuroectoderms.
During the elongated G1 interval, cells develop primary cilia, external antenna-like protrusions capable of sensing their environment. These cilia activate a cells' trash disposal — or autophagy.
Cilia on cell surface Image credit: studyblue.com
Another important component of the cell cycle is Nrf2 a leucine Zipper (bZIP). Nrf2 monitors cells looking for dangerous free radicals — atoms, molecules or an ion, anything that might be without an electron, causing strand breaks in DNA. bZIP binds together two regions of DNA before transcribing it into RNA, critical for making a gene readable.
"Nrf2 levels are very high in stem cells — as stem cells are the future. Without Nrf2 watching out for the integrity of the genome, future progeny are in trouble. Nrf2 is like a guardian to the cell and makes sure it functions properly," says Kosik, co-director of the campus's Neuroscience Research Institute.
Levels of Nrf2 start to decline during the long G1 interval; levels that don't usually diminish until a cell begins to differentiate.
According to Jang: "We thought that if cells are identical, they would differentiate in the same way. But, that is not what we found. Cell fate is controlled by the long G1 cycle extending cilia's exposure to signals in their environment. That is one cool concept."
•NE-specific G1 lengthening initiates NE derivation through primary cilia
•Increased ciliogenesis activates autophagy and presages NE differentiation
•NE precursor-specific autophagy inactivates Nrf2
•Nrf2 suppresses NE fate by directly controlling OCT4 and NANOG expression
Under defined differentiation conditions, human embryonic stem cells (hESCs) can be directed toward a mesendoderm (ME) or neuroectoderm (NE) fate, the first decision during hESC differentiation. Coupled with lineage-specific G1 lengthening, a divergent ciliation pattern emerged within the first 24 hr of induced lineage specification, and these changes heralded a neuroectoderm decision before any neural precursor markers were expressed. By day 2, increased ciliation in NE precursors induced autophagy that resulted in the inactivation of Nrf2 and thereby relieved transcriptional activation of OCT4 and NANOG. Nrf2 binds directly to upstream regions of these pluripotency genes to promote their expression and repress NE derivation. Nrf2 suppression was sufficient to rescue poorly neurogenic iPSC lines. Only after these events had been initiated did neural precursor markers get expressed at day 4. Thus, we have identified a primary cilium-autophagy-Nrf2 (PAN) control axis coupled to cell-cycle progression that directs hESCs toward NE.
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