Kristin K. Baldwin

How common polymorphisms in noncoding genome regions can regulate cellular function remains largely unknown. Here we show that cardiac fibrosis, mimicked using a hydrogel with controllable stiffness, affects the regulation of the phenotypes of human cardiomyocytes by a portion of the long noncoding RNA ANRIL, the gene of which is located in the disease-associated 9p21 locus. In a physiological environment, cultured cardiomyocytes derived from induced pluripotent stem cells obtained from patients who are homozygous for cardiovascular-risk alleles (R/R cardiomyocytes) or from healthy individuals who are homozygous for nonrisk alleles contracted synchronously, independently of genotype. After hydrogel stiffening to mimic fibrosis, only the R/R cardiomyocytes exhibited asynchronous contractions. These effects were associated with increased expression of the short ANRIL isoform in R/R cardiomyocytes, which induced a c-Jun N-terminal kinase (JNK) phosphorylation-based mechanism that impaired gap junctions (particularly, loss of connexin-43 expression) following stiffening. Deletion of the risk locus or treatment with a JNK antagonist was sufficient to maintain gap junctions and prevent asynchronous contraction of cardiomyocytes. Our findings suggest that mechanical changes in the microenvironment of cardiomyocytes can activate the regulation of their function by noncoding loci.

Friedreich ataxia (FRDA) is a rare childhood neurodegenerative disorder with no effective treatment. FRDA is caused by transcriptional silencing of the FXN gene and consequent loss of the essential mitochondrial protein frataxin. Based on the knowledge that a GAA•TTC repeat expansion in the first intron of FXN leads to heterochromatin formation and gene silencing, we have shown that members of the 2-aminobenzamide family of histone deacetylase inhibitors (HDACi) reproducibly increase FXN mRNA levels in induced pluripotent stem cell (iPSC)-derived FRDA neuronal cells and in peripheral blood mononuclear cells from patients treated with the drug in a phase I clinical trial. How the reduced expression of frataxin leads to neurological and other systemic symptoms in FRDA patients remains unclear. Similarly to other triplet repeat disorders, it is not known why only specific cells types are affected in the disease, primarily the large sensory neurons of the dorsal root ganglia and cardiomyocytes. The combination of iPSC technology and genome editing techniques offers the unique possibility of addressing these questions in a relevant cell model of the disease, without the confounding effect of different genetic backgrounds. We derived a set of isogenic iPSC lines that differ only in the length of the GAA•TTC repeats, using “scarless” gene-editing methods (helper-dependent adenovirus-mediated homologous recombination). To uncover the gene expression signature due to GAA•TTC repeat expansion in FRDA neuronal cells and the effect of HDACi on these changes, we performed transcriptomic analysis of iPSC-derived central nervous system (CNS) and isogenic sensory neurons by RNA sequencing. We find that multiple cellular pathways are commonly affected by the loss of frataxin in CNS and peripheral nervous system neurons and these changes are partially restored by HDACi treatment.

The transcriptional programs that establish neuronal identity evolved to produce the rich diversity of neuronal cell types that arise sequentially during development. Remarkably, transient expression of certain transcription factors can also endow non-neural cells with neuronal properties. The relationship between reprogramming factors and the transcriptional networks that produce neuronal identity and diversity remains largely unknown. Here, from a screen of 598 pairs of transcription factors, we identify 76 pairs of transcription factors that induce mouse fibroblasts to differentiate into cells with neuronal features. By comparing the transcriptomes of these induced neuronal cells (iN cells) with those of endogenous neurons, we define a ‘core’ cell-autonomous neuronal signature. The iN cells also exhibit diversity; each transcription factor pair produces iN cells with unique transcriptional patterns that can predict their pharmacological responses. By linking distinct transcription factor input ‘codes’ to defined transcriptional outputs, this study delineates cell-autonomous features of neuronal identity and diversity and expands the reprogramming toolbox to facilitate engineering of induced neurons with desired patterns of gene expression and related functional properties.

The reprogramming of differentiated cells into induced pluripotent stem cells (iPSCs) is usually achieved by exogenous induction of transcription by factors acting in the nucleus. In contrast, during development, signaling pathways initiated at the membrane induce differentiation. The central idea of this study is to identify antibodies that can catalyze cellular de-differentiation and nuclear reprogramming by acting at the cell surface. We screen a lentiviral library encoding ∼ 100 million secreted and membrane-bound single-chain antibodies and identify antibodies that can replace either Sox2 and Myc (c-Myc) or Oct4 during reprogramming of mouse embryonic fibroblasts into iPSCs. We show that one Sox2-replacing antibody antagonizes the membrane-associated protein Basp1, thereby de-repressing nuclear factors WT1, Esrrb and Lin28a (Lin28) independent of Sox2. By manipulating this pathway, we identify three methods to generate iPSCs. Our results establish unbiased selection from autocrine combinatorial antibody libraries as a robust method to discover new biologics and uncover membrane-to-nucleus signaling pathways that regulate pluripotency and cell fate.

Olfactory circuits function at birth, yet are continuously remodeled through the integration of adult-born interneurons into the olfactory bulb in a manner that preserves olfactory perceptual stability throughout adult life. The mechanisms that ensure appropriate circuit stability in this dynamic context remain poorly understood. Since interneurons sculpt the excitatory output of mitral and tufted (MT) neurons to the olfactory cortex, we predicted that MT neurons might instruct interneuron wiring in the adult brain. By blocking synaptic transmission from MT neurons we show that MT neuronal activity is critical to maintain olfactory bulb integrity and interneuron survival. Blocking interneuron death uncovered a second activity-dependent checkpoint regulating dendrite branching. In contrast, cortical circuits and MT neurons remain stable in the face of these silent and degenerating olfactory circuits. These studies identify a circuit-specific role for non-sensory activity in regulating integration of neurons into the adult brain, as predicted by previous computational models.