Notch Signaling Guides Fate of Intestinal Progenitor Cells

Activation of the Notch signaling pathway influences the fate of immature intestinal progenitor cells by regulating cell–cell communication, according to a study by HMS researchers. The findings, which appeared in the June 16 Nature, might provide a method for amplifying these multipotent stem cells and manipulating them into different types of cells for therapeutic use.

“The idea that you can experimentally manipulate a cell signaling pathway and consequently affect the differentiation and the proliferation of the stem cells is very interesting as it provides a tool to study this elusive, rare, and important cell population,” said senior author Spyros Artavanis-Tsakonas. Working in fruit flies, his lab was the first to uncover the existence of the Notch signaling pathway, a fundamental cell interaction mechanism that controls metazoan cell fate. His lab has extended its studies in mammalian systems, and its most recent efforts have provided a direct link between Notch signals and intestinal cell lineage specification.

Though the precise mechanisms that control the differentiation of intestinal epithelial cells remain largely unknown, previous research had indirectly implicated the Notch pathway in early cell development through downstream transcription factors regulated by Notch. The researchers hypothesized that activation of the Notch receptor would affect the expression of these transcription factors and tested their hypothesis with doubly transgenic mice. The scientists chose to use intestinal cells because the gut is an ideal system to look at the intricate balance between differentiation, proliferation, and apoptosis that governs both normal development and pathogenesis in diseases including cancer, according to Artavanis-Tsakonas, the Kurt J. Isselbacher/Peter D. Schwartz professor of cell biology at HMS.

Results showed that Notch activation amplifies the intestinal progenitor pool while simultaneously inhibiting differentiation of these early cells. “The study has shown that if you manipulate the Notch pathway in the intestine you can do two things,” Artavanis-Tsakonas said, “manipulate the differentiation pattern of the cells in the gut and actually stimulate proliferation of very early precursor cells, or stem cells.” The scientists, who were funded by grants from the National Institutes of Health, will continue their work on Notch signaling in hopes of identifying therapeutic applications based on regulating cell fate.

—Rachel Patzer

Brain Reorganization Accompanies Memory Consolidation in Sleep

The human brain reorganizes itself to aid in memory consolidation following a night of sleep, report HMS researchers. Their study demonstrates for the first time that learning enhancements resulting from sleep are accompanied by a large-scale neuroplastic change in the brain.

Scientists have long been puzzled over why humans sleep for one third of their life, and recent studies have suggested that sleeping is essential to solidify and improve memories. Though researchers used to believe the simple passage of time was the major factor in memory consolidation, recent evidence suggests it is more strictly determined by time spent sleeping or in a particular stage of sleep. Past studies have shown that memory performance can improve 20 to 30 percent with sleep, and according to Matthew Walker, lead researcher on the current paper and an HMS instructor in psychiatry at Beth Israel Deaconess, intervening sleep is required for such improvement. “It is not quite practice that makes perfect, but instead, practice with sleep that makes perfect,” he said.

Memory consolidation is believed to occur through changes in the connection strength between brain cells, and on a larger scale, between different brain regions. The findings by Walker and his colleagues, which appeared online June 17 in Neuroscience, demonstrate that this neuroplasticity in response to learning forms the basis of brain reorganization and memory development.

The research involved teaching 12 healthy college-aged subjects a skilled finger movement in the morning and evening. After a 12-hour period of either wakefulness or sleep, the subjects were retested while undergoing an MRI scan of their brain activity. Significant differences in patterns of activity were evident following sleep compared to wakefulness. After a night of sleep, the motor cortex and the cerebellum, parts of the brain that control speed and accuracy, were more active. And increased activity after sleep was also observed in the right frontal lobe and right temporal lobe, areas thought to help form memory sequences.

“The consequences of these findings are wide reaching,” Walker said, “considering the fact that Western civilization is consistently getting less and less sleep each year, particularly our schoolchildren.”

—Rachel Patzer

A Bug in Bronze

A bronze sculpture of the poliovirus (dark gray) with receptor (light gray) based on research by Jim Hogle was rendered by Ed Meyer for an exhibition opening in April at the Smithsonian Institution. Beforehand, the sculpture was on display locally at a reunion of polio survivors organized by Children’s Hospital Boston, Spaulding Rehabilitation Hospital, and HMS. Hogle, the Edward S. Harkness professor of biological chemistry and molecular pharmacology, and his colleagues used receptor-decorated liposomes to capture poliovirus, solving the structure of the virus–receptor–membrane complex with cryo-electron microscopy. Their findings appear in the July Nature Structural & Molecular Biology.

Cancer Regulators Revealed by RNAi

A recent paper by HMS researchers uncovers several new kinases and phosphatases associated with regulatory pathways in cancer. Led by postdoctoral fellows Jeffrey MacKeigan and Leon Murphy and by senior author John Blenis, the report also identifies a new group of phosphatases with tumor suppressor–like activity. Based on specific RNA interference (RNAi) libraries, the work is the first large-scale classification of kinase and phosphatase gene families to identify regulatory roles in cell survival and apoptosis. The findings, which appear in the June edition of Nature Cell Biology, may provide a blueprint for the development of novel anticancer strategies.

Kinases and phosphatases typically control the reversible process of phosphorylation of molecules in specific cell signaling cascades. In cancer, they are often dysregulated. “Many of these are potential drug targets for the treatment of a variety of human disorders, such as cancer, diabetes, immune system disorders, neurobiological disorders, and others, resulting from improper regulation of cell death,” said Blenis, an HMS professor of cell biology.

After evaluating the RNAi screen, the researchers determined that 11 percent of kinases control cell survival. The presence of known survival kinases such as SGK, Akt2, and PKC-delta were found along with several novel regulators of apoptosis and chemoresistance. Because resistance to chemotherapy is often a problem in cancer patients, the researchers sought to identify targets that when downregulated, would restore sensitivity to chemotherapeutics. “We have identified several kinases that when expression levels are reduced with RNAi, sensitize chemoresistant cancer cells to low concentrations of chemotherapeutic agents,” Blenis said, highlighting the possibility that inhibition of these novel targets may be a strategy in drug treatment.

In their experiments, the researchers also revealed a previously unrecognized role for phosphatases as negative regulators of apoptosis. A total of 32 percent of phosphatases and their regulatory subunits promote cell survival. One group of phosphatases was also identified as potential tumor suppressors because loss of function resulted in chemoresistance.

—Rachel Patzer

RNAi Screen Yields Tumor Suppressor Role

Using RNA interference libraries to screen for genes that suppress oncogenic transformation of human cells, a team of researchers from HMS and the Dana–Farber Cancer Institute uncovered a novel tumor suppressor role for a neuronal transcriptional repressor. The researchers found that the repressor protein may play a significant part in a pathway leading to some types of cancer. Their study appears in the June 17 Cell.

The repressor, RE1-silencing transcription factor (REST)/Neuron-restrictive silencer factor (NRSF), is typically known for its function as a transcription factor in cancer. Several kinds of tumor, such as those of the breast, lung, and ovary, inappropriately express neuron-specific genes. Occasionally this expression can result in an autoimmune response that leads to impairment of brain function in conditions called paraneoplastic neurological degenerations, or PND. The outcome suggests that the cancers contain defects in the regulation of these neuronal programs, and the idea that a repressor such as REST is involved is “very attractive,” according to lead researcher Stephen Elledge, the Gregor Mendel professor of genetics and of medicine. Transcription factors often control whole programs of gene expression during early development and therefore are logical candidates for alteration in cancer. “I view cancer as a reactivation of developmental pathways: normal cells rapidly proliferate in the embryo, mature, and then are supposed to turn off that developmental pathway,” Elledge explained. But in cancer, these pathways turn on once again.

In analyzing the relationship between REST function and tumor formation, the Elledge lab found that a significant number of tumors had deletions on the REST locus of chromosome 4, suggesting REST could be a target for frequent deletions in cancer. The scientists analyzed colon tumors and colon tumor cell lines for the presence of mutations in the regions of REST and found a frameshift mutation resulting in the early truncation of the protein. Cells forced to express the mutated form of REST gained cancerlike properties, further suggesting a tumor suppression role for REST.

The recent emergence of RNA interference (RNAi) has enabled a “very powerful” approach to finding cancer genes, according to Elledge. Its ability to knock down genes enabled the scientists to perform genetic screens, facilitating quick identification of specific targets. Other relatively established tumor suppressors, TGFBR2 and PTEN, were identified by the researchers in an RNAi-based screen of genes that suppress oncogenic transformation in human mammary epithelial cells. Finding genes they knew were involved in cancer was encouraging.

Elledge said the lab has since created a better library to screen genes, noting the process is basically “10 times better” than it was at the beginning of the study. The researchers received their funding from the National Cancer Institute, the National Institutes of Health, and the U.S. Department of Defense.

—Rachel Patzer

top