Randolph S. Ashton

The influence of hydrogel modulus on the proliferation and differentiation of encapsulated neural stem cells

There has been an increasing interest in understanding how the mechanical properties of the microenvironment influence stem cell fate. We describe studies of the proliferation and differentiation of neural stem cells (NSCs) encapsulated within three-dimensional scaffolds--alginate hydrogels--whose elastic moduli were varied over two orders of magnitude. The rate of proliferation of neural stem cells decreased with increase in the modulus of the hydrogels. Moreover, we observed the greatest enhancement in expression of the neuronal marker beta-tubulin III within the softest hydrogels, which had an elastic modulus comparable to that of brain tissues. To our knowledge, this work represents the first demonstration of the influence of modulus on NSC differentiation in three-dimensional scaffolds. Three-dimensional scaffolds that control stem cell fate would be broadly useful for applications in regenerative medicine and tissue engineering.

Astrocytes regulate adult hippocampal neurogenesis through ephrin-B signaling

Neurogenesis in the adult hippocampus involves activation of quiescent neural stem cells (NSCs) to yield transiently amplifying NSCs, progenitors, and, ultimately, neurons that affect learning and memory. This process is tightly controlled by microenvironmental cues, although a few endogenous factors are known to regulate neuronal differentiation. Astrocytes have been implicated, but their role in juxtacrine (that is, cell-cell contact dependent) signaling in NSC niches has not been investigated. We found that ephrin-B2 presented from rodent hippocampal astrocytes regulated neurogenesis in vivo. Furthermore, clonal analysis in NSC fate-mapping studies revealed a previously unknown role for ephrin-B2 in instructing neuronal differentiation. In addition, ephrin-B2 signaling, transduced by EphB4 receptors on NSCs, activated β-catenin in vitro and in vivo independently of Wnt signaling and upregulated proneural transcription factors. Ephrin-B2+ astrocytes therefore promote neuronal differentiation of adult NSCs through juxtacrine signaling, findings that advance our understanding of adult neurogenesis and may have future regenerative medicine implications.

Multivalency of Sonic Hedgehog Conjugated to Linear Polymer Chains Modulates Protein Potency

A potently active multivalent form of the protein Sonic hedgehog (Shh) was produced by bioconjugation of a modified recombinant form of Shh to the linear polymers poly(acrylic acid) (pAAc) and hyaluronic acid (HyA) via a two-step reaction exploiting carboimiide and maleimide chemistry. Efficiency of the conjugation was approximately 75% even at stoichiometric ratios of 30 Shh molecules per linear HyA chain (i.e., 30:1 Shh/HyA). Bioactivity of the conjugates was tested via a cellular assay across a range of stoichiometric ratios of Shh molecules to HyA linear chains, which was varied from 0.6:1 Shh/HyA to 22:1 Shh/HyA. Results indicate that low conjugation ratios decrease Shh bioactivity and high ratios increase this activity beyond the potency of monomeric Shh, with approximately equal activity between monomeric soluble Shh and conjugated Shh at 7:1 Shh/HyA. In addition, high-ratio constructs increased angiogenesis determined by the in vivo chick chorioallantoic membrane (CAM) assay. These results are captured by a kinetic model of multiple interactions between the Shh/HyA conjugates and cell surface receptors resulting in higher cell signaling at lower bulk Shh concentrations.

Defined Human Pluripotent Stem Cell Culture Enables Highly Efficient Neuroepithelium Derivation Without Small Molecule Inhibitors

The embryonic neuroepithelium gives rise to the entire central nervous system in vivo, making it an important tissue for developmental studies and a prospective cell source for regenerative applications. Current protocols for deriving homogenous neuroepithelial cultures from human pluripotent stem cells (hPSCs) consist of either embryoid body‐mediated neuralization followed by a manual isolation step or adherent differentiation using small molecule inhibitors. Here, we report that hPSCs maintained under chemically defined, feeder‐independent, and xeno‐free conditions can be directly differentiated into pure neuroepithelial cultures ([mt]90% Pax6+/N‐cadherin+ with widespread rosette formation) within 6 days under adherent conditions, without small molecule inhibitors, and using only minimalistic medium consisting of Dulbeccos modified Eagles medium/F‐12, sodium bicarbonate, selenium, ascorbic acid, transferrin, and insulin (i.e., E6 medium). Furthermore, we provide evidence that the defined culture conditions enable this high level of neural conversion in contrast to hPSCs maintained on mouse embryonic fibroblasts (MEFs). In addition, hPSCs previously maintained on MEFs could be rapidly converted to a neural compliant state upon transfer to these defined conditions while still maintaining their ability to generate all three germ layers. Overall, this fully defined and scalable protocol should be broadly useful for generating therapeutic neural cells for regenerative applications. Stem Cells 2014;32:1032–1042

Deterministic HOX Patterning in Human Pluripotent Stem Cell-Derived Neuroectoderm

Summary Colinear HOX expression during hindbrain and spinal cord development diversifies and assigns regional neural phenotypes to discrete rhombomeric and vertebral domains. Despite the precision of HOX patterning in vivo, in vitro approaches for differentiating human pluripotent stem cells (hPSCs) to posterior neural fates coarsely pattern HOX expression thereby generating cultures broadly specified to hindbrain or spinal cord regions. Here, we demonstrate that successive activation of fibroblast growth factor, Wnt/β-catenin, and growth differentiation factor signaling during hPSC differentiation generates stable, homogenous SOX2+/Brachyury+ neuromesoderm that exhibits progressive, full colinear HOX activation over 7 days. Switching to retinoic acid treatment at any point during this process halts colinear HOX activation and transitions the neuromesoderm into SOX2+/PAX6+ neuroectoderm with predictable, discrete HOX gene/protein profiles that can be further differentiated into region-specific cells, e.g., motor neurons. This fully defined approach significantly expands capabilities to derive regional neural phenotypes from diverse hindbrain and spinal cord domains.

Microfluidic separation of DNA.

DNA separation is important for numerous applications in biotechnology and medicine. Efforts to improve DNA separation in microdevices have led to advances in capillary electrophoresis and the development of novel separation strategies. Current research on microcapillary electrophoresis materials is focused on the development of separation matrices with low injection viscosities and wall-coating capabilities. Microcapillary injector geometries are being designed to allow increased control of sample plug volumes. Novel separation strategies using entropic traps, arrays of pillars and Brownian ratchets are also being developed.

The effect of multivalent Sonic hedgehog on differentiation of human embryonic stem cells into dopaminergic and GABAergic neurons

Stem cell differentiation is regulated by complex repertoires of signaling ligands which often use multivalent interactions, where multiple ligands tethered to one entity interact with multiple cellular receptors to yield oligomeric complexes. One such ligand is Sonic hedgehog (Shh), whose posttranslational lipid modifications and assembly into multimers enhance its biological potency, potentially through receptor clustering. Investigations of Shh typically utilize recombinant, monomeric protein, and thus the impact of multivalency on ligand potency is unexplored. Among its many activities, Shh is required for ventralization of the midbrain and forebrain and is therefore critical for the development of midbrain dopaminergic (mDA) and forebrain gamma-aminobutyric acid (GABA) inhibitory neurons. We have designed multivalent biomaterials presenting Shh in defined spatial arrangements and investigated the role of Shh valency in ventral specification of human embryonic stem cells (hESCs) into these therapeutically relevant cell types. Multivalent Shh conjugates with optimal valencies, compared to the monomeric Shh, increased the percentages of neurons belonging to mDA or forebrain GABAergic fates from 33% to 60% or 52% to 86%, respectively. Thus, multivalent Shh bioconjugates can enhance neuronal lineage commitment of pluripotent stem cells and thereby facilitate efficient derivation of neurons that could be used to treat Parkinsons and epilepsy patients.

High‐Throughput Screening of Gene Function in Stem Cells Using Clonal Microarrays

We describe a microarray‐based approach for the high‐throughput screening of gene function in stem cells and demonstrate the potential of this method by growing and isolating clonal populations of both adult and embryonic neural stem cells. Clonal microarrays are constructed by seeding a population of cells at clonal density on micropatterned surfaces generated using soft lithographic microfabrication techniques. Clones of interest can be isolated after assaying in parallel for various cellular processes and functions, including proliferation, signal transduction, and differentiation. We demonstrate the compatibility of the technique with both gain‐ and loss‐of‐function studies using cell populations infected with cDNA libraries or DNA constructs that induce RNA interference. The infection of cells with a library prior to seeding and the compact but isolated growth of clonal cell populations will facilitate the screening of large libraries in a wide variety of mammalian cells, including those that are difficult to transfect by conventional methods.

High-content imaging with micropatterned multiwell plates reveals influence of cell geometry and cytoskeleton on chromatin dynamics

Understanding the mechanisms underpinning cellular responses to microenvironmental cues requires tight control not only of the complex milieu of soluble signaling factors, extracellular matrix (ECM) connections and cell-cell contacts within cell culture, but also of the biophysics of human cells. Advances in biomaterial fabrication technologies have recently facilitated detailed examination of cellular biophysics and revealed that constraints on cell geometry arising from the cellular microenvironment influence a wide variety of human cell behaviors. Here, we create an in vitro platform capable of precise and independent control of biochemical and biophysical microenvironmental cues by adapting microcontact printing technology into the format of standard six- to 96-well plates to create MicroContact Printed Well Plates (μCP Well Plates). Automated high-content imaging of human cells seeded on μCP Well Plates revealed tight, highly consistent control of single-cell geometry, cytoskeletal organization, and nuclear elongation. Detailed subcellular imaging of the actin cytoskeleton and chromatin within live human fibroblasts on μCP Well Plates was then used to describe a new relationship between cellular geometry and chromatin dynamics. In summary, the μCP Well Plate platform is an enabling high-content screening technology for human cell biology and cellular engineering efforts that seek to identify key biochemical and biophysical cues in the cellular microenvironment.

TFG facilitates outer coat disassembly on COPII transport carriers to promote tethering and fusion with ER–Golgi intermediate compartments

Significance The endoplasmic reticulum (ER) serves as a platform for the packaging of most secretory proteins into conserved coat protein complex II (COPII)-coated transport carriers destined for ER–Golgi intermediate compartments (ERGIC) in animal cells. In this work, we demonstrate that Trk-fused gene (TFG), a protein implicated in multiple neurodegenerative diseases and oncogenesis, functions in this pathway by interacting directly with the COPII protein Sec23. Specifically, we show that TFG outcompetes interactions between the inner and outer layers of the COPII coat, indicating that TFG promotes the uncoating process after transport carriers undergo scission from the ER. Moreover, we demonstrate that TFG simultaneously captures and concentrates COPII transport carriers at the ER/ERGIC interface to enable the rapid movement of secretory cargoes to the ERGIC. The conserved coat protein complex II (COPII) mediates the initial steps of secretory protein trafficking by assembling onto subdomains of the endoplasmic reticulum (ER) in two layers to generate cargo-laden transport carriers that ultimately fuse with an adjacent ER–Golgi intermediate compartment (ERGIC). Here, we demonstrate that Trk-fused gene (TFG) binds directly to the inner layer of the COPII coat. Specifically, the TFG C terminus interacts with Sec23 through a shared interface with the outer COPII coat and the cargo receptor Tango1/cTAGE5. Our findings indicate that TFG binding to Sec23 outcompetes these other associations in a concentration-dependent manner and ultimately promotes outer coat dissociation. Additionally, we demonstrate that TFG tethers vesicles harboring the inner COPII coat, which contributes to their clustering between the ER and ERGIC in cells. Together, our studies define a mechanism by which COPII transport carriers are retained locally at the ER/ERGIC interface after outer coat disassembly, which is a prerequisite for fusion with ERGIC membranes.