Calcium levels correlate with cell cycle phase and affect the level of the cyclin B transcript in Dictyostelium discoideum

Cell cycle-related metabolism and mitochondrial dynamics in a replication-competent pancreatic beta-cell line

Cell replication is a fundamental attribute of growth and repair in multicellular organisms. Pancreatic beta-cells in adults rarely enter cell cycle, hindering the capacity for regeneration in diabetes. Efforts to drive beta-cells into cell cycle have so far largely focused on regulatory molecules such as cyclins and cyclin-dependent kinases (CDKs). Investigations in cancer biology have uncovered that adaptive changes in metabolism, the mitochondrial network, and cellular Ca²⁺ are critical for permitting cells to progress through the cell cycle. Here, we investigated these parameters in the replication-competent beta-cell line INS 832/13. Cell cycle synchronization of this line permitted evaluation of cell metabolism, mitochondrial network, and cellular Ca²⁺ compartmentalization at key cell cycle stages. The mitochondrial network is interconnected and filamentous at G1/S but fragments during the S and G2/M phases, presumably to permit sorting to daughter cells. Pyruvate anaplerosis peaks at G1/S, consistent with generation of biomass for daughter cells, whereas mitochondrial Ca²⁺ and respiration increase during S and G2/M, consistent with increased energy requirements for DNA and lipid synthesis. This synchronization approach may be of value to investigators performing live cell imaging of Ca²⁺ or mitochondrial dynamics commonly undertaken in INS cell lines because without synchrony widely disparate data from cell to cell would be expected depending on position within cell cycle. Our findings also offer insight into why replicating beta-cells are relatively nonfunctional secreting insulin in response to glucose. They also provide guidance on metabolic requirements of beta-cells for the transition through the cell cycle that may complement the efforts currently restricted to manipulating cell cycle to drive beta-cells through cell cycle.

Patterning of the angiosperm female gametophyte through the prism of theoretical paradigms

The FG (female gametophyte) of flowering plants (angiosperms) is a simple highly polar structure composed of only a few cell types. The FG develops from a single cell through mitotic divisions to generate, depending on the species, four to 16 nuclei in a syncytium. These nuclei are then partitioned into three or four distinct cell types. The mechanisms underlying the specification of the nuclei in the FG has been a focus of research over the last decade. Nevertheless, we are far from understanding the patterning mechanisms that govern cell specification. Although some results were previously interpreted in terms of static positional information, several lines of evidence now show that local interactions are important. In the present article, we revisit the available data on developmental mutants and cell fate markers in the light of theoretical frameworks for biological patterning. We argue that a further dissection of the mechanisms may be impeded by the combinatorial and dynamical nature of developmental cues. However, accounting for these properties of developing systems is necessary to disentangle the diversity of the phenotypic manifestations of the underlying molecular interactions.

Developmental decisions in Dictyostelium discoideum

Dictyostelium discoideum is an excellent system in which to study developmental decisions. Synchronous development is triggered by starvation and rapidly generates a limited number of cell types. Genetic and image analyses have revealed the elegant intricacies associated with this simple development system. Key signaling pathways identified as regulating cell fate decisions are likely to be conserved with metazoa and are providing insight into differentiation decisions under circumstances where considerable cell movement takes place during development.

Dictyostelium calcium-binding protein 4a interacts with nucleomorphin, a BRCT-domain protein that regulates nuclear number

Nucleomorphin from Dictyostelium discoideum is a nuclear calmodulin-binding protein that is a member of the BRCT-domain containing cell cycle checkpoint proteins. Two differentially expressed isoforms, NumA and NumB, share an extensive acidic domain (DEED) that when deleted produces highly multinucleated cells. We performed a yeast two-hybrid screen of a Dictyostelium cDNA library using NumA as bait. Here we show that nucleomorphin interacts with calcium-binding protein 4a (CBP4a) in a Ca(2+)-dependent manner. Further deletion analysis suggests this interaction requires residues found within the DEED domain. NumA and CBP4a mRNAs are expressed at the same stages of development. CBP4a belongs to a large family of Dictyostelium CBPs, for which no cellular or developmental functions had previously been determined. Since the interaction of CBP4a with nucleomorphin requires the DEED domain, this suggests that CBP4a may respond to Ca(2+)-signalling through modulating factors that might function in concert to regulate nuclear number.

Calcium regulates the expression of a Dictyostelium discoideum asparaginyl tRNA synthetase gene

In a screen for calcium-regulated gene expression during growth and development of Dictyostelium discoideum we have identified an asparaginyl tRNA synthetase (ddAsnRS) gene, the second tRNA synthetase gene identified in this organism. The ddAsnRS gene shows many unique features. One, it is repressed by lowering cellular calcium, making it the first known calcium-regulated tRNA synthetase. Two, despite the calcium-dependence, its expression is unaltered during the cell cycle, making this the first D. discoideum gene to show a calcium-dependent but cell cycle phase-independent expression. Finally, the N-terminal domain of the predicted ddAsnRS protein shows higher sequence similarity to Glutaminyl tRNA synthetases than to other Asn tRNA synthetases. These unique features of the AsnRS from this primitive eukaryote not only point to a novel mechanism regulating the components of translation machinery and gene expression by calcium, but also hint at a link between the evolution of GlnRS and AsnRS in eukaryotes.

ABC transporters required for endocytosis and endosomal pH regulation inDictyostelium

In Dictyostelium, the RtoA protein links both initial cell-type choice and physiological state to cell-cycle phase. rtoA– cells (containing a disruption of the rtoA gene) generally do not develop past the mound stage, and have an abnormal ratio of prestalk and prespore cells. RtoA is also involved in fusion of endocytic/exocytic vesicles. Cells lacking RtoA, although having a normal endocytosis rate, have a decreased exocytosis rate and endosomes with abnormally low pHs. RtoA levels vary during the cell cycle, causing a cell-cycle-dependent modulation of parameters such as cytosolic pH (Brazill et al., 2000). To uncover other genes involved in the RtoA-mediated differentiation, we identified genetic suppressors of rtoA. One of these suppressors disrupted two genes, mdrA1 and mdrA2, a tandem duplication encoding two members of the ATP binding cassette (ABC) transporter superfamily. Disruption of mdrA1/mdrA2 results in release from the developmental block and suppression of the defect in initial cell type choice caused by loss of the rtoA gene. However, this is not accomplished by re-establishing the link between cell type choice and cell cycle phase. MdrA1 protein is localized to the endosome. mdrA1–/mdrA2– cells (containing a disruption of these genes) have an endocytosis rate roughly 70% that of wild-type or rtoA– cells, whereas mdrA1–/mdrA2–/rtoA– cells have an endocytosis rate roughly 20% that of wild-type. The exocytosis rates of mdrA1–/mdrA2– and mdrA1–/mdrA2–/rtoA– are roughly that of wild-type. mdrA1–/mdrA2– endosomes have an unusually high pH, whereas mdrA1–/mdrA2–/rtoA– endosomes have an almost normal pH. The ability of mdrA1/mdrA2 disruption to rescue the cell-type proportion, developmental defects, and endosomal pH defects caused by rtoA disruption, and the ability of rtoA disruption to exacerbate the endocytosis defects caused by mdrA1/mdrA2 disruption, suggest a genetic interaction between rtoA, mdrA1 and mdrA2.

Biphasic expression of rnrB in Dictyostelium discoideum suggests a direct relationship between cell cycle control and cell differentiation

Cell differentiation in Dictyostelium is strongly affected by the cell cycle. Cell cycle control is well-understood in other systems, but this has had almost no impact on the study of Dictyostelium cell differentiation, in part because the cell cycle in Dictyostelium is unusual, lacking a G1 phase. Here we describe the cell-cycle regulated expression of rnrB, which codes for the small subunit of ribonucleotide reductase and is a marker of late G1 in many systems. There appear to be two expression peaks, one in mid-G2 and the other near the G2/M transition. Using Xgal/anti-BrdU double staining, we show that cells in asynchronously growing cultures express in both phases, with a gap between them during which the gene is transcriptionally silent. Cold-synchronized cells show exclusively G2/M expression, while mid-G2 expression is seen in high-density synchronized cells and can also be inferred in cells undergoing synchronization by either method. rnrB expression occurs in other systems shortly after cells pass a point (the "restriction point" or "start") at which they commit to complete their current cell cycle. We demonstrate a similar commitment point in Dictyostelium and show that this occurs shortly before the mid-G2 rnrB expression peak. The Dictyostelium cell cycle thus appears to include a well-defined though inconspicuous event, between early and mid-G2, with some features which are normally associated with the G1/S transition. Others have described a switch from stalk to spore differentiation preference at about this time. Since Dictyostelium cells switch back from spore to stalk preference approximately at the G2/M rnrB expression maximum, cell differentiation as well as rnrB expression may be regulated directly by fundamental cell cycle control processes.