Stem cell fate is influenced by a number of factors and interactions that require robust control for safe and effective regeneration of functional tissue. Coordinated interactions with soluble factors, other cells, and extracellular matrices define a local biochemical and mechanical niche with complex and dynamic regulation that stem cells sense. Decellularized tissue matrices and synthetic polymer niches are being used in the clinic, and they are also beginning to clarify fundamental aspects of how stem cells contribute to homeostasis and repair, for example, at sites of fibrosis. Multi-faceted technologies are increasingly required to produce and interrogate cells ex vivo, to build predictive models, and ultimately to enhance stem cell integration in vivo for therapeutic benefit.Control over stem cell trafficking, survival, proliferation, and differentiation within a complex in vivo milieu is extremely challenging. In studies of animal models and humans where stem cell engraftment has been quantified after injection, only a few percent of cells remain after several days or weeks [eg.(1)]. Many clinical trials are nonetheless underway, particularly with adult bone marrow derived mesenchymal stem cells (MSC) which are being investigated as treatments for diseases of non-hematopoietic tissues -primarily myocardial infarction and peripheral ischemia (2). Although FDA approval for human testing of cells differentiated from embryonic stem cells (ESC) is a recent landmark for the field (3), two widely reported clinical cases highlight some of the technical opportunities and challenges with stem cells in soft tissue repair. One patient in Spain was successfully transplanted with a re-engineered trachea in 2008: donor trachea was first decellularized using a detergent (without denaturing the collagenous matrix), and then this scaffold was re-cellularized in a rotating bioreactor using MSC-derived cartilage-like cells (4). Long term safety and efficacy will be important to monitor and understand. Indeed, in a second case, the cerebellum of a boy with ataxia telangiectasia (AT) was injected with human fetal neural stem cells (NSC), and four years later, a glio-neuronal brain tumor of stem cell origin was found (5). Upon implantation, stem cells and their derived lineages encounter a multitude of cues that can influence cell fate. Efforts to parse the molecular mechanisms for translation from bench to clinic will increasingly benefit from a wide range of new and established technologies. Here we briefly review salient features of microenvironments, mechanics, and material systems that are being pursued to control stem cells for both basic insight and application.Correspondence: discher@seas.upenn.edu, mooneyd@seas.harvard.edu, peter.zandstra@utoronto.ca. NIH Public Access Niche interactions and in vitro designsThe niche is the in vivo microenvironment that regulates stem cell survival, self-renewal, and differentiation. Key niche components and interactions include growth factors, cell-cell contacts, and cell-mat...

Refractoriness to induction chemotherapy and relapse after achievement of remission are the main obstacles to cure in acute myeloid leukaemia (AML). After standard induction chemotherapy, patients are assigned to different post-remission strategies on the basis of cytogenetic and molecular abnormalities that broadly define adverse, intermediate and favourable risk categories. However, some patients do not respond to induction therapy and another subset will eventually relapse despite the lack of adverse risk factors. There is an urgent need for better biomarkers to identify these high-risk patients before starting induction chemotherapy, to enable testing of alternative induction strategies in clinical trials. The high rate of relapse in AML has been attributed to the persistence of leukaemia stem cells (LSCs), which possess a number of stem cell properties, including quiescence, that are linked to therapy resistance. Here, to develop predictive and/or prognostic biomarkers related to stemness, we generated a list of genes that are differentially expressed between 138 LSC and 89 LSC cell fractions from 78 AML patients validated by xenotransplantation. To extract the core transcriptional components of stemness relevant to clinical outcomes, we performed sparse regression analysis of LSC gene expression against survival in a large training cohort, generating a 17-gene LSC score (LSC17). The LSC17 score was highly prognostic in five independent cohorts comprising patients of diverse AML subtypes (n = 908) and contributed greatly to accurate prediction of initial therapy resistance. Patients with high LSC17 scores had poor outcomes with current treatments including allogeneic stem cell transplantation. The LSC17 score provides clinicians with a rapid and powerful tool to identify AML patients who do not benefit from standard therapy and who should be enrolled in trials evaluating novel upfront or post-remission strategies.

Transforming growth factor-beta (TGFbeta) family members regulate many developmental and pathological events through Smad transcriptional modulators. How nuclear accumulation of Smad is coupled to the transcriptional machinery is poorly understood. Here we demonstrate that in response to TGFbeta stimulation the transcriptional regulator TAZ binds heteromeric Smad2/3-4 complexes and is recruited to TGFbeta response elements. In human embryonic stem cells TAZ is required to maintain self-renewal markers and loss of TAZ leads to inhibition of TGFbeta signalling and differentiation into a neuroectoderm lineage. In the absence of TAZ, Smad2/3-4 complexes fail to accumulate in the nucleus and activate transcription. Furthermore, TAZ, which itself engages in shuttling, dominantly controls Smad nucleocytoplasmic localization and can be retained in the nucleus by transcriptional co-factors such as ARC105, a component of the Mediator complex. TAZ thus defines a hierarchical system regulating Smad nuclear accumulation and coupling to the transcriptional machinery.

The small number of hematopoietic stem and progenitor cells in cord blood units limits their widespread use in human transplant protocols. We identified a family of chemically related small molecules that stimulates the expansion ex vivo of human cord blood cells capable of reconstituting human hematopoiesis for at least 6 months in immunocompromised mice. The potent activity of these newly identified compounds, UM171 being the prototype, is independent of suppression of the aryl hydrocarbon receptor, which targets cells with more-limited regenerative potential. The properties of UM171 make it a potential candidate for hematopoietic stem cell transplantation and gene therapy.