Chung Yu Chan

An acoustofluidic micromixer based on oscillating sidewall sharp-edges

Rapid and homogeneous mixing inside a microfluidic channel is demonstrated via the acoustic streaming phenomenon induced by the oscillation of sidewall sharp-edges. By optimizing the design of the sharp-edges, excellent mixing performance and fast mixing speed can be achieved in a simple device, making our sharp-edge-based acoustic micromixer a promising candidate for a wide variety of applications.

Spatial colocalization and functional link of purinosomes with mitochondria

Spatial control of cellular enzymes Purine is a building block of DNA and also a component of ATP that is used as an energy source in the cell. Enzymes involved in purine biosynthesis organize into dynamic bodies called purinosomes. French et al. found that purinosomes colocalize with mitochondria, organelles that generate ATP (see the Perspective by Ma and Jones). Dysregulation of mitochondria caused an increase in the number of purinosomes. This suggests a synergy, with the purinosomes supplying the purine required for ATP production and in turn using ATP in the biosynthetic pathway. A master regulator of cellular metabolism, mTOR, appears to mediate the association of purinosomes and mitochondria. This could make purine and ATP synthesis responsive to changes in the metabolic needs of the cell. Science, this issue p. 733; see also p. 670 Intracellular bodies composed of purine biosynthetic enzymes exhibit an mTOR-mediated association with mitochondria. [Also see Perspective by Ma and Jones] Purine biosynthetic enzymes organize into dynamic cellular bodies called purinosomes. Little is known about the spatiotemporal control of these structures. Using super-resolution microscopy, we demonstrated that purinosomes colocalized with mitochondria, and these results were supported by isolation of purinosome enzymes with mitochondria. Moreover, the number of purinosome-containing cells responded to dysregulation of mitochondrial function and metabolism. To explore the role of intracellular signaling, we performed a kinome screen using a label-free assay and found that mechanistic target of rapamycin (mTOR) influenced purinosome assembly. mTOR inhibition reduced purinosome-mitochondria colocalization and suppressed purinosome formation stimulated by mitochondria dysregulation. Collectively, our data suggest an mTOR-mediated link between purinosomes and mitochondria, and a general means by which mTOR regulates nucleotide metabolism by spatiotemporal control over protein association.

Probing cell–cell communication with microfluidic devices

Intercellular communication is a mechanism that regulates critical events during embryogenesis and coordinates signalling within differentiated tissues, such as the nervous and cardiovascular systems. To perform specialized activities, these tissues utilize the rapid exchange of signals among networks that, while are composed of different cell types, are nevertheless functionally coupled. Errors in cellular communication can lead to varied deleterious effects such as degenerative and autoimmune diseases. However, the intercellular communication network is extremely complex in multicellular organisms making isolation of the functional unit and study of basic mechanisms technically challenging. New experimental methods to examine mechanisms of intercellular communication among cultured cells could provide insight into physiological and pathological processes alike. Recent developments in microfluidic technology allow miniaturized and integrated devices to perform intercellular communication experiments on-chip. Microfluidics have many advantages, including the ability to replicate in vitro the chemical, mechanical, and physical cellular microenvironment of tissues with precise spatial and temporal control combined with dynamic characterization, high throughput, scalability and reproducibility. In this Focus article, we highlight some of the recent work and advances in the application of microfluidics to the study of mammalian intercellular communication with particular emphasis on cell contact and soluble factor mediated communication. In addition, we provide some insights into likely direction of the future developments in this field.

Quantitative analysis of purine nucleotides indicates that purinosomes increase de novo purine biosynthesis.

Background: Metabolic enzymes have been hypothesized to assemble into complex to respond to cellular metabolism changes. Results: De novo purine biosynthesis increases in purinosome-containing cells. Conclusion: Purine metabolism is adjusted by purinosome assembly. Significance: This study indicates that purinosome is a functional multienzyme complex. Enzymes in the de novo purine biosynthesis pathway are recruited to form a dynamic metabolic complex referred to as the purinosome. Previous studies have demonstrated that purinosome assembly responds to purine levels in culture medium. Purine-depleted medium or 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT) treatment stimulates the purinosome assembly in HeLa cells. Here, several metabolomic technologies were applied to quantify the static cellular levels of purine nucleotides and measure the de novo biosynthesis rate of IMP, AMP, and GMP. Direct comparison of purinosome-rich cells (cultured in purine-depleted medium) and normal cells showed a 3-fold increase in IMP concentration in purinosome-rich cells and similar levels of AMP, GMP, and ratios of AMP/GMP and ATP/ADP for both. In addition, a higher level of IMP was also observed in HeLa cells treated with DMAT. Furthermore, increases in the de novo IMP/AMP/GMP biosynthetic flux rate under purine-depleted condition were observed. The synthetic enzymes, adenylosuccinate synthase (ADSS) and inosine monophosphate dehydrogenase (IMPDH), downstream of IMP were also shown to be part of the purinosome. Collectively, these results provide further evidence that purinosome assembly is directly related to activated de novo purine biosynthesis, consistent with the functionality of the purinosome.

Purinosome formation as a function of the cell cycle

Significance We show that the assembly/disassembly of the purinosome is cell cycle-dependent and correlates with cellular demands for purine biosynthesis encountered during the cell cycle. The number of purinosome-containing cells fluctuates: it peaks in G1 phase in HeLa cells cultured under purine-depleted conditions, but remains high across G1, S, and G2/M phases in fibroblasts incapable of purine salvage owing to a deficiency in hypoxanthine-guanine phosphoribosyltransferase. Thus, purinosome function is a cellular biomarker for increased metabolic flux through the de novo purine biosynthetic pathway in response to cellular purine requirements. The de novo purine biosynthetic pathway relies on six enzymes to catalyze the conversion of phosphoribosylpyrophosphate to inosine 5′-monophosphate. Under purine-depleted conditions, these enzymes form a multienzyme complex known as the purinosome. Previous studies have revealed the spatial organization and importance of the purinosome within mammalian cancer cells. In this study, time-lapse fluorescence microscopy was used to investigate the cell cycle dependency on purinosome formation in two cell models. Results in HeLa cells under purine-depleted conditions demonstrated a significantly higher number of cells with purinosomes in the G1 phase, which was further confirmed by cell synchronization. HGPRT-deficient fibroblast cells also exhibited the greatest purinosome formation in the G1 phase; however, elevated levels of purinosomes were also observed in the S and G2/M phases. The observed variation in cell cycle-dependent purinosome formation between the two cell models tested can be attributed to differences in purine biosynthetic mechanisms. Our results demonstrate that purinosome formation is closely related to the cell cycle.

A polystyrene-based microfluidic device with three-dimensional interconnected microporous walls for perfusion cell culture

In this article, we present a simple, rapid prototyped polystyrene-based microfluidic device with three-dimensional (3D) interconnected microporous walls for long term perfusion cell culture. Patterned 3D interconnected microporous structures were created by a chemical treatment together with a protective mask and the native hydrophobic nature of the microporous structures were selectively made hydrophilic using oxygen plasma treatment together with a protective mask. Using this polystyrene-based cell culture microfluidic device, we successfully demonstrated the support of four days perfusion cell culture of hepatocytes (C3A cells).