Neurons are among the most polarized cells in nature, having emerged more than a half-billion years ago in metazoans to receive, process, and transmit information. The basic instructions to polarize a neuron appear to be intrinsically encoded, but what drives neurons to their extreme morphology is largely unknown. In a recent article in Genes and Development, the Shi Lab describes a molecular program that induces the early morphology of neurons through a deeply conserved, metazoan-specific zinc finger protein Unkempt. They find that ectopic expression of Unkempt confers neuronal-like morphology to cells of different nonneuronal lineages, while its depletion in mouse embryonic brain disrupts the shape of migrating neurons. The authors show that Unkempt functions as a sequence-specific RNA-binding protein that targets coding regions of a defined set of ubiquitously expressed messages linked to protein metabolism and regulation of the cytoskeleton. They further demonstrate that RNA binding is required for Unkempt-induced remodeling of cellular shape and is directly coupled to a reduced production of the encoded proteins. Thus, during embryonic development, Unkempt controls a translationally regulated cell morphology program to ensure proper structuring of the nervous system.
Small RNA molecules are familiar as negative regulators of endogenous protein-coding genes, but their more deeply conserved function is to ensure genomic stability by keeping repetitive and parasitic elements in check. In the fission yeast Schizosaccharomyces pombe, small RNAs accomplish this task by guiding heterochromatin formation at DNA regions flanking the centromere of each chromosome. The small RNA effector complex that targets the heterochromatin machinery to pericentromeric domains, called RITS, includes an Argonaute protein and a subunit bearing glycine-tryptophan (GW) repeats. The GW motif is a feature of Argonaute-interacting proteins that is conserved across several kingdoms of life and in silencing pathways both nuclear and cytoplasmic. A recent publication from the Moazed Lab has now uncovered a mechanism of ordered assembly for the GW protein-containing RITS complex that prevents Argonautes not yet programmed with a guide RNA from associating with GW repeat-containing proteins. In addition, they show that the poorly characterized fungal protein Arb1 is essential for loading small RNAs onto the S. pombe Argonaute in vitro and for RITS assembly in vivo. Altogether, the results demonstrate that a GW protein can act as a sensor of an Argonaute’s small RNA loading state. Finally, the work suggests that GW proteins play an evolutionarily conserved role in restricting the recruitment of downstream silencing machineries as varied as RNA deadenylases and chromatin-modifying enzymes exclusively to mature, competent Argonaute complexes.
(Holoch and Moazed, Nat. Struct. Mol. Biol., PMID 25730778)
The Dimensions of Harvard Medical School, in the Transit Gallery of Gordon Hall, is a collection of photographs and profiles that captures a wide representation of the HMS community. Featured are 5 members of the Cell Biology community: faculty members Joan Brugge, Tomas Kirchhausen, and Davie Van Vactor; Nikon Imaging Center Director Jennifer Waters; and Research Operations Manager Karen Easley. Their profiles are on display until April 7th.
Mass spectrometry-based proteomics enables the global identification and quantification of proteins and their posttranslational modifications in complex biological samples. However, proteomic analysis requires a complete and accurate reference set of proteins and is therefore largely restricted to model organisms with sequenced genomes. In collaboration, the Gygi and Kirschner labs demonstrated the feasibility of deep genome-free proteomics by using a reference proteome derived from heterogeneous mRNA data. They identify more than 11,000 proteins from the egg of the African clawed frog and estimate each protein's abundance with approximately 2-fold precision. To facilitate proteomics in nonmodel organisms, the researchers made their platform available as an online resource that converts heterogeneous mRNA data into a protein reference set. This work was published in Current Biology.
Figure: Xenopus laevis ovary
Autoimmune destruction of insulin-producing β-cells leaves individuals with type 1 diabetes (T1D) insulin dependent for life. Strategies that replace or regenerate β-cells, including islet transplantation and β-cell generation from non-β-cell precursors are potential therapies in T1D. However, insufficient β-cell proliferation, survival, and insulin secretory response to glucose can limit the benefits of these strategies. As such, molecular pathways or strategies that simultaneously enhance β-cell mass and glucose signaling are of therapeutic interest. The Danial Lab has identified such strategy. Previous work showed that the BCL-2 family protein BAD stimulates the β-cell glucose response and insulin secretion through phosphorylation of a defined residue within an amphipathic α-helix known as the BCL-2 Homology (BH)-3 domain. This modification neutralizes BAD’s apoptotic function by preventing its capacity to bind and inactivate pro-survival BCL-2, BCL-XL and BCL-w proteins, and simultaneously triggers its ability to directly activate the glucose-metabolizing enzyme glucokinase (GK). However, whether beyond neutralizing BAD’s apoptotic activity, BAD phosphorylation has active, cell autonomous effects on b-cell survival was not known. In the February 2015 issue of Cell Reports, new findings from the Danial Lab show that genetic and pharmacologic approaches to mimic BAD phosphorylation within its BH3 helix not only stimulate insulin secretion but protect β-cells from death induced by T1D-related stress stimuli, including inflammatory, ER and oxidative stress. β-cell survival in this setting is not merely due to the inability of phospho-BAD to suppress pro-survival BCL-2 proteins but requires its activation of GK. Strikingly, the β-cell-autonomous benefits of phospho-BAD mimicry lead to improved donor islet engraftment in transplanted diabetic mice, increased β-cell viability in islet grafts, restoration of insulin release, and diabetes reversal. These findings highlight the utility of BAD phospho-BH3 mimetic approaches in augmenting functional β-cell mass in diabetes.
Proteins that are translocated into the endoplasmic reticulum (ER) undergo quality control so that only correctly folded proteins are moved on in the secretory pathway. If a protein cannot reach its native folded state, it is ultimately transported back into the cytosol, poly-ubiquitinated, and degraded by the proteasome, a process called ER-associated protein degradation (ERAD). How proteins are retro-translocated across the ER membrane and moved into the cytosol is only poorly understood. Previous work demonstrated that the ubiquitin ligase Hrd1p is the only membrane component needed for a basic ERAD process. Now, the Rapoport group shows that key steps of basic ERAD can be recapitulated with purified components (Stein et al., Cell 158, 1375-1388). Hrd1p binds misfolded protein substrates, discriminating them from folded proteins. Subsequently, both Hrd1p and substrate are poly-ubiquitinated. Next, the Cdc48p ATPase complex binds and uses the energy of ATP hydrolysis to release substrate from Hrd1p. Finally, ubiquitin chains are trimmed by the de-ubiquitinating enzyme Otu1p, which is recruited and activated by the Cdc48p complex. The Cdc48-dependent extraction of poly-ubiquitinated proteins from the membrane can be reproduced with reconstituted proteoliposomes. These results provide a major step forward towards the goal to recapitulate ERAD with purified components and to elucidate the molecular mechanism of the process.
The endoplasmic reticulum (ER) is an important membrane-bound organelle in all eukaryotic cells. Depending on cell type and functional state, the ER membrane can adopt different morphologies, including a network of interconnected tubules, and sheets that can contain fenestrations or be stacked on top of each other. How these different morphologies are generated is unclear. A collaboration of different groups, including the Rapoport group, resulted in a recent paper that presents a comprehensive theoretical model, which explains the formation and interconversion of virtually all known ER morphologies (Shemesh et al., 2014, Proc. Natl. Acad. Sci. USA PMID:25404289). The model is based on two types of membrane-shaping proteins (R- and S-type proteins), exemplified by the reticulons and lunapark, which both stabilize the high membrane curvature in cross-sections of tubules and sheet edges, but favor straight or concave sheet edges, respectively. Dependent on the concentrations of R- and S-type proteins, membrane morphologies can be generated that consist of tubules, sheets, sheet fenestrations, and sheet stacks with helicoidal connections. In a tubular ER network, lunapark stabilizes three-way junctions, i.e. small triangular sheets with concave edges. The model agrees with experimental observations and explains how curvature-stabilizing proteins determine ER morphology.
Cell celebrated its 40th anniversary by revisiting "landmark" Cell publications, an esteemed list that includes six papers from five faculty members: Dan Finley, Wade Harper, Yang Shi, Bruce Spiegelman, and Junying Yuan. One of these papers, Yang Shi's research identifying the first histone demethylase, LSD1, was also honored and showcased as an "Annotated Classic."
Randy King, the Harry C. McKenzie Professor of Cell Biology at Harvard Medical School, has been awarded the American Association of Medical Colleges Excellence in Teaching award for his involvement and innovation in the laboratory, in the classroom and across the curriculum at HMS.
The Elizabeth D. Hay Professorship in Cell Biology honors the legacy of Betty Hay, a pioneering figure at Harvard Medical School and in the field of cell biology. We are pleased to announce that Dean Jeffrey Flier has named Dr. Junying Yuan, Professor of Cell Biology, as the inaugural recipient of this endowed chair.
Dr. Hay was the first woman to be appointed as a full professor in a preclinical department at the Medical School in 1969, and served as the Chair of the Department of Anatomy (later the Department of Cell Biology) for sixteen years. Through her expertise in electron microscopy, Dr. Hay was the first to show that the extracellular matrix plays a vital role in determining cell behaviors, including cell shape, cell-to-cell signaling, wound repair, cell adhesion, and tissue function. Her work provided some of the first evidence for the existence of ECM receptors (now known as integrins) on the epithelial surface, and her discoveries formed the foundations for an entirely new field of cellular biology. She was also responsible for showing that cell differentiation is not an irreversible process, and the first to describe the transition of epithelial cells into mesenchymal cells, a process central to development and cancer metastasis.
Junying Yuan joined the Department of Cell Biology as an Assistant Professor in 1996 and was appointed full professor in 2000. As a graduate student, Dr. Yuan studied the relatively unknown field of cell death, and her work would eventually help lead Bob Horvitz to a Nobel Prize in 2002. Her research provided the first evidence of the existence of a targeted cellular death mechanism--apoptosis--that counterbalances cell proliferation, which transformed the cell death field from a descriptive science into a molecular discipline. More recently, Dr. Yuan discovered a regulated form of necrosis, called necroptosis, and has spent the last few years studying its molecular mechanism and identifying small molecule inhibitors, some of which have generated significant interest as possible therapeutics.
Congratulations again to Dr. Yuan!