Increased intracellular pH is necessary for adult epithelial and embryonic stem cell differentiation

Drosophila Glypicans Regulate Follicle Stem Cell Maintenance and Niche Competition

Adult stem cells reside in specialized microenvironments called niches, which provide signals for stem cells to maintain their undifferentiated and self-renewing state. To maintain stem cell quality, several types of stem cells are known to be regularly replaced by progenitor cells through niche competition. However, the cellular and molecular bases for stem cell competition for niche occupancy are largely unknown. Here, we show that two Drosophila members of the glypican family of heparan sulfate proteoglycans (HSPGs), Dally and Dally-like (Dlp), differentially regulate follicle stem cell (FSC) maintenance and competitiveness for niche occupancy. Lineage analyses of glypican mutant FSC clones showed that dally is essential for normal FSC maintenance. In contrast, dlp is a hypercompetitive mutation: dlp mutant FSC progenitors often eventually occupy the entire epithelial sheet. RNA interference knockdown experiments showed that Dally and Dlp play both partially redundant and distinct roles in regulating Jak/Stat, Wg, and Hh signaling in FSCs. The Drosophila FSC system offers a powerful genetic model to study the mechanisms by which HSPGs exert specific functions in stem cell replacement and competition.

Functional and molecular characterization of transmembrane intracellular pH regulators in human dental pulp stem cells

OBJECTIVE

Homeostasis of intracellular pH (pH_i) plays vital roles in many cell functions, such as proliferation, apoptosis, differentiation and metastasis. Thus far, Na⁺-H⁺ exchanger (NHE), Na⁺-HCO₃^- co-transporter (NBC), Cl^-/HCO₃^- exchanger (AE) and Cl^-/OH^- exchanger (CHE) have been identified to co-regulate pH_i homeostasis. However, functional and biological pH_i-regulators in human dental pulp stem cells (hDPSCs) have yet to be identified.

DESIGN

Microspectrofluorimetry technique with pH-sensitive fluorescent dye, BCECF, was used to detect pH_i changes. NH₄Cl and Na⁺-acetate pre-pulse were used to induce intracellular acidosis and alkalosis, respectively. Isoforms of pH_i-regulators were detected by Western blot technique.

RESULTS

The resting pH_i was no significant difference between that in HEPES-buffered (nominal HCO₃^--free) solution or CO₂/HCO₃-buffered system (7.42 and 7.46, respectively). The pH_i recovery following the induced-intracellular acidosis was blocked completely by removing [Na⁺]_o, while only slowed (-63%) by adding HOE694 (a NHE1 specific inhibitor) in HEPES-buffered solution. The pH_i recovery was inhibited entirely by removing [Na⁺]_o, while adding HOE 694 pulse DIDS (an anion-transporter inhibitor) only slowed (-55%) the acid extrusion. Both in HEPES-buffered and CO₂/HCO₃-buffered system solution, the pH_i recovery after induced-intracellular alkalosis was entirely blocked by removing [Cl^-]_o. Western blot analysis showed the isoforms of pH_i regulators, including NHE1/2, NBCe1/n1, AE1/2/3/4 and CHE in the hDPSCs.

CONCLUSIONS

We demonstrate for the first time that resting pH_i is significantly higher than 7.2 and meditates functionally by two Na⁺-dependent acid extruders (NHE and NBC), two Cl^--dependent acid loaders (CHE and AE) and one Na⁺-independent acid extruder(s) in hDPSCs. These findings provide novel insight for basic and clinical treatment of dentistry.

Cell fate decisions: emerging roles for metabolic signals and cell morphology

Understanding how cell fate decisions are regulated is a fundamental goal of developmental and stem cell biology. Most studies on the control of cell fate decisions address the contributions of changes in transcriptional programming, epigenetic modifications, and biochemical differentiation cues. However, recent studies have found that other aspects of cell biology also make important contributions to regulating cell fate decisions. These cues can have a permissive or instructive role and are integrated into the larger network of signaling, functioning both upstream and downstream of developmental signaling pathways. Here, we summarize recent insights into how cell fate decisions are influenced by four aspects of cell biology: metabolism, reactive oxygen species (ROS), intracellular pH (pHi), and cell morphology. For each topic, we discuss how these cell biological cues interact with each other and with protein-based mechanisms for changing gene transcription. In addition, we highlight several questions that remain unanswered in these exciting and relatively new areas of the field.

Methods for Imaging Intracellular pH of the Follicle Stem Cell Lineage in Live Drosophila Ovarian Tissue

Changes in intracellular pH (pHi) play important roles in the regulation of many cellular functions, including metabolism, proliferation, and differentiation. Typically, pHi dynamics are determined in cultured cells, which are amenable to measuring and experimentally manipulating pHi. However, the recent development of new tools and methodologies has made it possible to study pHi dynamics within intact, live tissue. For Drosophila research, one important development was the generation of a transgenic line carrying a pHi biosensor, mCherry::pHluorin. Here, we describe a protocol that we routinely use for imaging live Drosophila ovarioles to measure pHi in the epithelial follicle stem cell (FSC) lineage in mCherry::pHluorin transgenic wild type lines; however, the methods described here can be easily adapted for other tissues, including the wing discs and eye epithelium. We describe techniques for expressing mCherry::pHluorin in the FSC lineage, maintaining ovarian tissue during live imaging, and acquiring and analyzing images to obtain pHi values.

Smad5 acts as an intracellular pH messenger and maintains bioenergetic homeostasis

Both environmental cues and intracellular bioenergetic states profoundly affect intracellular pH (pHi). How a cell responds to pHi changes to maintain bioenergetic homeostasis remains elusive. Here we show that Smad5, a well-characterized downstream component of bone morphogenetic protein (BMP) signaling responds to pHi changes. Cold, basic or hypertonic conditions increase pHi, which in turn dissociates protons from the charged amino acid clusters within the MH1 domain of Smad5, prompting its relocation from the nucleus to the cytoplasm. On the other hand, heat, acidic or hypotonic conditions decrease pHi, blocking the nuclear export of Smad5, and thus causing its nuclear accumulation. Active nucleocytoplasmic shuttling of Smad5 induced by environmental changes and pHi fluctuation is independent of BMP signaling, carboxyl terminus phosphorylation and Smad4. In addition, ablation of Smad5 causes chronic and irreversible dysregulation of cellular bioenergetic homeostasis and disrupted normal neural developmental processes as identified in a differentiation model of human pluripotent stem cells. Importantly, these metabolic and developmental deficits in Smad5-deficient cells could be rescued only by cytoplasmic Smad5. Cytoplasmic Smad5 physically interacts with hexokinase 1 and accelerates glycolysis. Together, our findings indicate that Smad5 acts as a pHi messenger and maintains the bioenergetic homeostasis of cells by regulating cytoplasmic metabolic machinery.

Structural Stability of the Coiled-Coil Domain of Tumor Susceptibility Gene (TSG)-101

The tumor susceptibility gene-101 coiled coil domain (TSG101cc) is an integral component of the endosomal maturation machinery and cytokinesis, and also interacts with several transcription factors. The TSG101cc has been crystallized as a homotetramer but is known to interact with two of its binding partners as a heterotrimer. To investigate this apparent discrepancy, we examined the solution thermodynamics of the TSG101cc. Here, we use circular dichroism, differential scanning calorimetry, analytical ultracentrifugation, fluorescence, and structural thermodynamic analysis to investigate the structural stability and the unfolding of the TSG101cc. We demonstrate that TSG101cc exists in solution primarily as a tetramer, which unfolds in a two-state manner. Surprisingly, no homodimeric or homotrimeric species were detected. Structural thermodynamic analysis of the homotetrameric structure and comparison with known oligomeric coiled-coils suggests that the TSG101cc homotetramer is comparatively unstable on a per residue basis. Furthermore, the homotrimeric coiled-coil is predicted to be much less stable than the functional heterotrimeric coiled-coil in the endosomal sorting complex required for transport 1 (ESCRT1). These results support a model whereby the tetramer–monomer equilibrium of TSG101 serves as the cellular reservoir of TSG101, which is effectively outcompeted when its binding partners are present and the heteroternary complex can form.

SMAD5 signaling: more than meets the nuclei

SMADs are essential transcriptional effectors of transforming growth factor-β (TGFβ)/TGFβ-related signaling that underlies embryonic development and adult homeostasis. A recent study by Fang et al. in Cell Research adds to this biological complexity by demonstrating an atypical cytoplasmic role for SMAD5 in modulating the bioenergetic homeostasis (i.e., glycolysis and mitochondrial respiration) of cells in response to fluctuations in intracellular pH that is independent of receptor signaling.

Cancer cell behaviors mediated by dysregulated pH dynamics at a glance

Dysregulated pH is a common characteristic of cancer cells, as they have an increased intracellular pH (pHi) and a decreased extracellular pH (pHe) compared with normal cells. Recent work has expanded our knowledge of how dysregulated pH dynamics influences cancer cell behaviors, including proliferation, metastasis, metabolic adaptation and tumorigenesis. Emerging data suggest that the dysregulated pH of cancers enables these specific cell behaviors by altering the structure and function of selective pH-sensitive proteins, termed pH sensors. Recent findings also show that, by blocking pHi increases, cancer cell behaviors can be attenuated. This suggests ion transporter inhibition as an effective therapeutic approach, either singly or in combination with targeted therapies. In this Cell Science at a Glance article and accompanying poster, we highlight the interconnected roles of dysregulated pH dynamics in cancer initiation, progression and adaptation.

A basic guide to stem cell differentiation

Increasing intracellular pH promotes the differentiation of adult and embryonic stem cells.