Researchers at the Crick are tackling the big questions about human health and disease, and new findings are published every week. Our faculty have picked some of the most significant papers published by Crick scientists, all of which are freely available thanks to our open science policy.
Functional cross-talk between allosteric effects of activating and inhibiting ligands underlies PKM2 regulation
This work reveals that amino acids, rather than fructose 1,6-bisphosphate, are the relevant cellular regulators of pyruvate kinase M2 (PKM2), a critical node in cancer metabolism. It further elucidates the molecular mechanism of PKM2 regulation by amino acids with a new algorithm that predicts allosteric pathways in proteins, a major and difficult problem in structural biology.
CD1d-mediated lipid presentation by CD11c+ cells regulates intestinal homeostasis
Intestinal homeostasis requires a continuous dialogue between commensal bacteria and intestinal immune cells. Natural Killer T (NKT) cells are a population of CD1d-restricted lipid-reactive lymphocytes contributing to the regulation of mucosal immunity, but the mechanisms underlying this are poorly understood. Here we show that lipid presentation by CD1d+ intestinal dendritic cells and macrophages controls NKT cell function and activation which in turn regulates commensal bacteria and immune cell populations in the gut. These results reveal an NKT cell-dendritic cell crosstalk as a key mechanism for the regulation of intestinal homeostasis.
CD9 identifies pancreatic cancer stem cells and modulates glutamine metabolism to fuel tumour growth
This work identifies a cancer stem cell (CSC) population in pancreatic ductal adenocarcinoma (PDAC) marked by the tetraspanin CD9. We showed that CD9Hi CSCs are required for the epithelial and mesenchymal cellular heterogeneity seen in PDAC. We found that CD9 assembles a protein complex involved in regulating PDAC metabolism on the cell surface. CD9 depletion dramatically inhibited PDAC growth, identifying CD9 as a therapeutic PDAC target. These findings suggest that the cellular composition of pancreatic cancer is controlled by a CSC population.
Tissue curvature and apicobasal mechanical tension imbalance instruct cancer morphogenesis
This study introduces a new technique, FLASH, which enables immunostaining of whole organs for imaging and opens up the possibility of analysing a plethora of antigens and tissues that were previously impossible to study in 3D. By achieving this feat, we were able to study epithelial deformation from the moment of transformation within the intact pancreas, to show that early tumours adopt different shapes depending on tissue curvature, due to the distribution of intracellular forces. The work connects cell mechanics with the biology of tumour development in an unprecedented manner.
Mouse retinal cell behaviour in space and time using light sheet fluorescence microscopy
We successfully performed the first lightsheet 3D/4D imaging of mouse retinas (focussing on vessels and neurons) to demonstrate that current confocal methods distort vessel tissue. This brings a much improved way to observe and quantify the devastating changes to vessels and neurons in retinopathy mouse models.
Phospho-dependent regulation of SAMHD1 oligomerisation couples catalysis and restriction
This study explained the mechanism of SAMHD1 regulation by phosphorylation/tetramerisation and correlated restriction activity with the capacity of SAMHD1 to form long lived, stable tetramers. These data form the basis of the prevailing model for SAMHD1 restriction of HIV-1 where dNTP-stabilised SAMHD1 tetramers deplete and maintain low levels of dNTPs in the non-permissive cells resistant to HIV-1 infection.
A protease cascade regulates release of the human malaria parasite Plasmodium falciparum from host red blood cells
This study showed that egress involves an enzyme cascade in which the serine protease SUB1 activates a second, cysteine protease called SERA6, enabling SERA6 to rapidly and precisely cleave the major red cell cytoskeletal protein β-spectrin and dismantle the cytoskeleton. It provides the first plausible model to explain how the parasite accomplishes timely rupture of its host cell membrane.
Autocatalytic activation of a malarial egress protease is druggable and requires a protein cofactor
A study led by the Blackman lab has shed new light on a key pathway that allows the malaria parasite to escape from the host’s red blood cells. Their findings identify a target that could be used to develop a new class of antimalarial drug designed to prevent disease progression.
Reconstitution of a functional human thymus by postnatal stromal progenitor cells and natural whole-organ scaffolds
In this paper we define the heterogeneity and the clonogenic potential of human thymus stroma; characterise progenitor cells capable of extensive expansion in vitro, thereby achieving clinically relevant numbers with resilience to long-term storage; and report an epithelial-mesenchymal hybrid phenotype of thymus epithelial cells in vivo and in vitro that affects cell behaviour, a unique feature among any epithelia so far reported. We describe a protocol for organs that lack a main vascular access that allowed us to specify the role of natural ECM in supporting organ morphogenesis ex vivo and in vivo; and reconstitute a functional human thymus long-term in vivo.
Versatile humanized niche model enables study of normal and malignant human hematopoiesis
Immunodeficient mouse models have been instrumental in improving our understanding of human healthy haemopoietic stem cells and their hierarchical organisation as well as of the functional and phenotypic heterogeneity of leukaemic stem cells in acute myeloid leukaemia. However, xenotransplantation models failed at reconstituting the human bone marrow niche which remains of mouse origin. Using a bioengineered scaffold, we developed a new versatile humanised bone marrow niche which supports the engraftment of both normal and leukaemia stem cells in vivo. This 3D scaffold represents a suitable model to study and dissect the human bone marrow composition and test the effect of specific stroma cell types and niche factor functions during both normal human haemopoiesis and leukaemia.
Stabilization of reversed replication forks by telomerase drives telomere catastrophe
This study defined the mechanism leading to critically short telomeres in the absence of RTEL1 and showed that telomerase, which extends telomeres in normal cells, is pathological when forks encounter an obstacle within the telomere. We showed that replication forks stall and reverse at persistent t-loops, which creates a pseudo-telomere substrate that is inappropriately stabilised by telomerase. Removing telomerase or blocking replication fork reversal rescued telomere dysfunction in Rtel1 deficient cells. We proposed that when persistent t-loops stall the replisome, telomerase inhibits fork restart, triggering the excision of the t-loop by SLX1/4 and loss of a substantial part of the telomere.
Rad51 paralogs remodel pre-synaptic Rad51 filaments to stimulate homologous recombination
This study was the first to demonstrate that RAD51 paralogues bind to and structurally remodel the pre-synaptic RAD-51-ssDNA filament to a stabilised, “open”, and flexible conformation, which facilitates strand exchange with the template duplex. We showed that RAD51 paralogues act by binding the end of the presynaptic filament, which induces a conformational change that stabilises RAD-51 bound to ssDNA and primes the filament for strand exchange. These observations established for the first time the underlying mechanism of HR stimulation by Rad51 paralogues and revealed a new paradigm for the action of HR mediator proteins.
CDK phosphorylation of TRF2 controls t-loop dynamics during the cell cycle
Evidence suggested that the telomere adopts a lasso-like t-loop configuration, which safeguards chromosome ends from being recognised as DNA double strand breaks. However, the regulation and physiological importance of t-loops in end-protection was uncertain. This study uncovered a phospho-switch in TRF2 that coordinates the timely assembly and disassembly of t-loops during the cell cycle, which protects telomeres from replication stress and an unscheduled DNA damage response. These results were the first to definitively establish the t-loop as a physiologically important structure required to suppress checkpoint activation at telomere ends.
TRF2-independent chromosome end protection during pluripotency
This work revealed that telomere protection is solved by distinct mechanisms in pluripotent and somatic tissues. In somatic cells, TRF2 sequesters the telomere within a t-loop, preventing telomere end-to-end fusions and inviability. In contrast, TRF2 is dispensable for telomere protection in pluripotent cells; ESCs lacking TRF2 grow normally, do not fuse their telomeres and form functional t-loops. Upon differentiation this unique attribute of stem cells is lost and TRF2 assumes its full role in end protection. The retention of end protection in the presence of t-loops, but absence of TRF2, confirmed that t-loops are a key mediator of telomere end protection irrespectively of how they form.
Decoding of position in the developing neural tube from antiparallel morphogen gradients
Like many developing tissues, the vertebrate neural tube is patterned by antiparallel morphogen gradients. Using quantitative gene expression and signalling measurements we derived and validated a characteristic decoding map that relates morphogen input to the positional identity of neural progenitors. This revealed a strategy that minimises patterning errors in response to the joint input of noisy opposing gradients. The study illustrates how we integrate quantitative data, developmental and microfluidic experiments with phenological and mechanistic models.
Species-specific pace of development is associated with differences in protein stability
Despite evolutionarily conservation of molecular mechanisms, the speed of development varies substantially between species. Using in vitro directed differentiation of embryonic stem cells to motor neurons, we show that the programme of motor neuron differentiation runs twice as fast in mouse as in human. We provide evidence that a two-fold increase in protein stability and cell cycle duration in human cells compared to mouse can account for the slower pace of human development, indicating that global differences in kinetic parameters play a major role in interspecies differences in developmental tempo. This study establishes a new experimental system in which to address fundamental questions.
Nervous system regionalization entails axial allocation before neural differentiation
The prevailing view of neural induction in vertebrate embryos had been that cells are initially induced with anterior (forebrain) identity and then caudalising signals convert a proportion to posterior fates (spinal cord). Using chromatin accessibility, to define how cells adopt region-specific neural fates, combined with genetic and biochemical perturbations, we found that contrary to the established model, cells commit to a regional identity before acquiring neural identity. These findings prompt a revision to textbook models of neural induction. The study illustrates our adoption of new genomic methods (ATACseq) to address long-standing questions, and our capacity to productively collaborate with computational biologists.
Restriction of memory B cell differentiation at the germinal center B cell positive selection stage
Memory B cells (MBCs) are key for protection from reinfection. However, it is mechanistically unclear how germinal center (GC) B cells differentiate into MBCs. MYC is transiently induced in cells fated for GC expansion and plasma cell (PC) formation, so-called positively selected GC B cells. We found that these cells coexpressed MYC and MIZ1 (MYC-interacting zinc-finger protein 1 [ZBTB17]). MYC and MIZ1 are transcriptional activators; however, they form a transcriptional repressor complex that represses MIZ1 target genes. Mice lacking MYC-MIZ1 complexes displayed impaired cell cycle entry of positively selected GC B cells and reduced GC B cell expansion and PC formation. Notably, absence of MYC-MIZ1 complexes in positively selected GC B cells led to a gene expression profile alike that of MBCs and increased MBC differentiation. Thus, at the GC positive selection stage, MYC-MIZ1 complexes are required for effective GC expansion and PC formation and to restrict MBC differentiation. We propose that MYC and MIZ1 form a module that regulates GC B cell fate.
Permissive selection followed by affinity-based proliferation of GC light zone B cells dictates cell fate and ensures clonal breadth
Memory B cells (MBCs) and plasma cells (PCs) are formed during the so-called germinal center (GC) B cell reaction. In the GC reaction B cells mutate their B cell receptor (BCR) genes and those that acquire a higher-affinity BCR for a pathogen antigen are presumably selected to survive and differentiate, whereas B cells carrying a lower-affinity BCR die. However, this cannot explain retention of GC B cells with varied BCR affinities and the formation of MBCs that normally carry lower-affinity BCRs. This work re-defines selection of GC B cells as permissive to ensure clonal diversity and broad protection.
Published in Proceedings of the National Academy of Sciences of USA
D-Cycloserine destruction by alanine racemase and the limit of irreversible inhibition
D-cycloserine is an antibiotic used for decades to treat drug resistant tuberculosis. Its inhibition mechanism came into question when in a previous paper we determined alanine racemase activity in “fully inhibited” cells. This study demonstrated a previously unknown path during the assumed irreversible inhibition of alanine racemase that leads to the destruction of the antibiotic, meaning that alanine racemase is not irreversibly inhibited by the drug. The paper highlights the complexity of studying the chemical mechanisms of inhibition of enzymes and points to a novel strategy to design D-cycloserine analogues with improved properties.