Multiparameter analysis of progeny of individual cells by laser scanning cytometry

BACKGROUND: Effectiveness of antitumor drugs to suppress unrestricted proliferation of cancer cells is commonly measured by cell clonogenicity assays. Assays of clonogenicity are also used in studies of stem/progenitor cells and in analysis of carcinogenic transformation. The conventional assays are limited to providing information about frequency of colonies (cloning efficiency) and do not reveal the qualitative (phenotype) attributes of individual colonies that may yield clues on mechanisms by which cell proliferation was affected by the studied agent. METHODS: Laser scanning cytometry (LSC) was adapted to identify and characterize size and phenotype of colonies of MCF-7 cells growing in microscope slide chambers, untreated and treated with the cytotoxic ribonuclease, onconase (Onc). Individual colonies were located and data representing each colony were segmented based on >650-nm fluorescence excited by a He-Ne laser of the cells whose protein was stained with BODIPY 630/650-X. The DNA of the cells was stained with propidium iodide (red fluorescence) whereas specific proteins (estrogen receptor [ER] or tumor suppressor p53) were detected immunocytochemically (green fluorescence), each excited by an Ar ion laser. RESULTS: A plethora of attributes of individual colonies were measured, such as (a) morphometric features (area, circumference, area/circumference ratio, DNA or protein content per area ratio), (b) number of cells (nuclei), (c) DNA content, (d) protein content and protein/DNA ratio, and (e) expression of ER or p53 per colony, per total protein, per nucleus or per DNA, within a colony. Also cell cycle distribution within individual colonies and heterogeneity of colonies with respect to all the measured features could be assessed. The colonies growing in the presence of Onc had many of the above attributes different than the colonies from the untreated cultures. CONCLUSIONS: Analysis of the features of cell colonies by LSC provides a wealth of information about the progeny of individual cells. Changes in colony size and phenotype, reflecting altered cell shape, cell size, colony protein/DNA ratio, and expression of individual proteins, may reveal mechanisms by which drugs suppress the proliferative capacity of the cells. This may include inducing growth imbalance and differentiation and modulating expression of the genes that may be associated with cell cycle, apoptosis, or differentiation in a progeny of individual cells. Extensions of LSC may make it applicable for automatic analysis of cloning efficiency and multiparameter analysis of cell colonies in soft agar. Such analyses may be useful in studies of the mechanisms and effectiveness of antitumor drugs, in the field of carcinogenesis, and for analyzing primary cultures and assessing tumor prognosis and drug sensitivity. The assay can also be adapted to analysis of microbial colonies.     

The novel Drosophila lysosomal enzyme receptor protein mediates lysosomal sorting in mammalian cells and binds mammalian and Drosophila GGA adaptors

Biogenesis of lysosomes depends in mammalian cells on the specific recognition and targeting of mannose 6-phosphate-containing lysosomal enzymes by two mannose 6-phosphate receptors (MPR46, MPR300), key components of the extensively studied receptor-mediated lysosomal sorting system in complex metazoans. In contrast, the biogenesis of lysosomes is poorly investigated in the less complex metazoan Drosophila melanogaster. We identified the novel type I transmembrane protein lysosomal enzyme receptor protein (LERP) with partial homology to the mammalian MPR300 encoded by Drosophila gene CG31072. LERP contains 5 lumenal repeats that share homology to the 15 lumenal repeats found in all identified MPR300. Four of the repeats display the P-lectin type pattern of conserved cysteine residues. However, the arginine residues identified to be essential for mannose 6-phosphate binding are not conserved. The recombinant LERP protein was expressed in mammalian cells and displayed an intracellular localization pattern similar to the mammalian MPR300. The LERP cytoplasmic domain shows highly conserved interactions with Drosophila and mammalian GGA adaptors known to mediate Golgi-endosome traffic of MPRs and other transmembrane cargo. Moreover, LERP rescues missorting of soluble lysosomal enzymes in MPR-deficient cells, giving strong evidence for a function that is equivalent to the mammalian counterpart. However, unlike the mammalian MPRs, LERP did not bind to the multimeric mannose 6-phosphate ligand phosphomannan. Thus ligand recognition by LERP does not depend on mannose 6-phosphate but may depend on a common feature present in mammalian lysosomal enzymes. Our data establish a potential important role for LERP in biogenesis of Drosophila lysosomes and suggest a GGA function also in the receptor-mediated lysosomal transport system in the fruit fly.     

Molecular analysis of mutagenesis in mammalian cells

Mammalian cells are constantly facing various types of mutagens. However, due to the high complexity of the cell genome, the molecular analysis of mutagenesis has not yet been possible. Therefore, we have used simian virus 40 (SV40) as a biological and molecular probe to characterize mutagenesis at the nucleotide level. By using a reversion assay from a temperature-sensitive phenotype towards a wild-type phenotype, we have analysed mutagenesis induced by u.v.-light and by apurinic sites (Ap sites). We report here experiments allowing us to quantify and to compare the mutagenic efficiency of various DNA lesions measured on the SV40 genome. The Ap sites are very mutagenic in this type of assay. The molecular analysis of u.v.-induced mutagenesis reveals that mutations correspond to single base-pair substitutions always located opposite Py-Py lesions. The mutations are almost equally distributed between transition and transversion types, and between the 5' and the 3' side of the Py-Py targets. These results demonstrate for the first time in animal cells the existence of targeted mutations induced by u.v.-light. We propose therefore, the use of SV40 as an efficient biological and molecular probe for assaying mutagenic pathways in mammalian cells.     

Flow sorting of tumor cells for morphometric analysis,particularly of rare cells

Using flow cytometric DNA measurement and sorting combined with morphometric light microscopy, different groups of cells were studied in a human melanoma pleural effusion, a human melanoma lymph node metastasis and a mouse tumor, as well as in normal reference tissues. Beside cells of the predominant tumor cell population, three types of rare tumor cells were studied after enrichment by sorting: a) giant cells from the >8c region, comprising about 5% of the tumor cells, b) binucleated and multinucleated cells with unequal nuclear sizes within the same cell, found at frequencies of about 1.5%, and c) <2c cells which were derived from the so-called “debris”-region of the DNA histogram, found at frequencies of about 1 to 6%. All these rare cells were found only in the malignant tumors and not in the benign reference tissues. Morphometry showed that the increase in the cellular DNA content in the different fractions of tumor cells was combined with an increase in the cellular and nuclear sizes. However, the n/c-ratio was constant in the whole range of tumor cell fractions, including the fractions from the the <2c and the >8c regions. The n/c-ratio of the <2c cells and giant cells differed from that of corresponding normal cells underlining their origin from the predominant tumor cell population. The possible linkage between the occurrence of the three rare cell types and genetic instability of tumors related to faulty nucleus and cell division is discussed.     

Light scatter analysis and sorting of cells activated in mixed leukocyte culture

Cells activated in unidirectional mixed leukocyte cultures (MLC) have been analyzed on the basis of their light scatter characteristics. C57BL/6 spleen cells were cultured with irradiated (2000 rads) DBA/2 spleen cells for 5 days and the resulting suspension of activated cells was passed on a FACS II flow cytometer. Correlated parameter analysis of forward light scatter (FLS) and perpendicular light scatter (PLS) indicated that the MLC consisted of a heterogenous mixture of viable cells, dead cells, and subcellular debris. However, by appropriate gating of the FLS/PLS distribution, viable cells could be identified as a biphasic FLS histogram. Sorting and morphological analyses of these two FLS peaks demonstrated that they corresponded to almost pure populations of small lymphocytes (lower peak) and lymphoblasts (upper peak), respectively. Furthermore, when sorted cells were tested for their ability to lyse antigenically relevant (DBA/2) tumor target cells in a 51Cr release assay, lymphoblasts were found to exhibit 40-fold greater cytolytic activity (on a per-cell basis) than small lymphocytes.     

DNA analysis and sorting of rat testis cells using two-parameter flow cytometry

By use of two-parameter flow cytometry of rat testis cell suspensions stained with mithramycin for DNA (the peak amplitude of the fluorescence signal versus total fluorescence intensity integrated over time), eight cell compartments could be distinguished without pre-enrichment of the samples. Cells in these compartments were identified by sorting and subsequent microscopic examination.     

Regulation of stress-induced intracellular sorting and chaperone function of Hsp27 (HspB1) in mammalian cells

In vitro, small Hsps (heat-shock proteins) have been shown to have chaperone function capable of keeping unfolded proteins in a form competent for Hsp70-dependent refolding. However, this has never been confirmed in living mammalian cells. In the present study, we show that Hsp27 (HspB1) translocates into the nucleus upon heat shock, where it forms granules that co-localize with IGCs (interchromatin granule clusters). Although heat-induced changes in the oligomerization status of Hsp27 correlate with its phosphorylation and nuclear translocation, Hsp27 phosphorylation alone is not sufficient for effective nuclear translocation of HspB1. Using firefly luciferase as a heat-sensitive reporter protein, we demonstrate that HspB1 expression in HspB1-deficient fibroblasts enhances protein refolding after heat shock. The positive effect of HspB1 on refolding is completely diminished by overexpression of Bag-1 (Bcl-2-associated athanogene), the negative regulator of Hsp70, consistent with the idea of HspB1 being the substrate holder for Hsp70. Although HspB1 and luciferase both accumulate in nuclear granules after heat shock, our results suggest that this is not related to the refolding activity of HspB1. Rather, granular accumulation may reflect a situation of failed refolding where the substrate is stored for subsequent degradation. Consistently, we found 20S proteasomes concentrated in nuclear granules of HspB1 after heat shock. We conclude that HspB1 contributes to an increased chaperone capacity of cells by binding unfolded proteins that are hereby kept competent for refolding by Hsp70 or that are sorted to nuclear granules if such refolding fails.     

Role for phosphatidylinositol 3-kinase in the sorting and transport of newly synthesized lysosomal enzymes in mammalian cells

Previous work with the yeast Saccharomyces cerevisiae has demonstrated a role for a phosphatidylinositol-specific PI 3-kinase, the product of the VPS34 gene, in the targeting of newly synthesized proteins to the vacuole, an organelle functionally equivalent to mammalian lysosomes (Schu, P. V., K. Takegawa, M. J. Fry, J. H. Stack, M. D. Waterfield, and S. D. Emr. 1993. Science [Wash. DC]. 260:88-91). The activity of Vps34p kinase is significantly reduced by the PI 3-kinase inhibitors wortmannin, a fungal metabolite, and LY294002, a quercetin analog (Stack, J. H., and S. D. Emr. 1994. J. Biol. Chem. 269:31552-31562). We show here that at concentrations which inhibit VPS34-encoded PI 3- kinase activity, wortmannin also inhibits the processing and delivery of newly synthesized cathepsin D to lysosomes in mammalian cells with half-maximal inhibition of delivery occurring at 100 nM wortmannin. As a result of wortmannin action, newly synthesized, unprocessed cathepsin D is secreted into the media. Moreover, after accumulation in the trans- Golgi network (TGN) at 20 degrees C, cathepsin D was rapidly missorted to the secretory pathway after addition of wortmannin and shifting to 37 degrees C. At concentrations that inhibited lysosomal enzyme delivery, both wortmannin and LY294002 caused a highly specific dilation of mannose 6-phosphate receptor (M6PR)-enriched vesicles of the prelysosome compartment (PLC), which swelled to approximately 1 micron within 15 min after treatment. With increasing time, the inhibitors caused a significant yet reversible change in M6PR distribution. By 3 h of treatment, the swollen PLC vacuoles were essentially depleted of receptors and, in addition, there was a fourfold loss of receptors from the cell surface. However, M6PRs were still abundant in the TGN. These results are most consistent with the interpretation that PI 3-kinase regulates the trafficking of lysosomal enzymes by interfering with a M6PR-dependent sorting event in the TGN. Moreover, they provide evidence that trafficking of soluble hydrolases to mammalian lysosomes and yeast vacuoles rely on similar regulatory mechanisms.     

Flow sorting of tumor cells for morphometric analysis, particularly of rare cells.

Using flow cytometric DNA measurement and sorting combined with morphometric light microscopy, different groups of cells were studied in a human melanoma pleural effusion, a human melanoma lymph node metastasis and a mouse tumor, as well as in normal reference tissues. Beside cells of the predominant tumor cell population, three types of rare tumor cells were studied after enrichment by sorting: a) giant cells from the greater than 8c region, comprising about 5% of the tumor cells, b) binucleated and multinucleated cells with unequal nuclear sizes within the same cell, found at frequencies of about 1.5%, and c) less than 2c cells which were derived from the so-called "debris"-region of the DNA histogram, found at frequencies of about 1 to 6%. All these rare cells were found only in the malignant tumors and not in the benign reference tissues. Morphometry showed that the increase in the cellular DNA content in the different fractions of tumor cells was combined with an increase in the cellular and nuclear sizes. However, the n/c-ratio was constant in the whole range of tumor cell fractions, including the fractions from the the less than 2c and the greater than 8c regions. The n/c-ratio of the less than 2c cells and giant cells differed from that of corresponding normal cells underlining their origin from the predominant tumor cell population. The possible linkage between the occurrence of the three rare cell types and genetic instability of tumors related to faulty nucleus and cell division is discussed.     

Systems analysis of N-glycan processing in mammalian cells

N-glycosylation plays a key role in the quality of many therapeutic glycoprotein biologics. The biosynthesis reactions of these oligosaccharides are a type of network in which a relatively small number of enzymes give rise to a large number of N-glycans as the reaction intermediates and terminal products. Multiple glycans appear on the glycoprotein molecules and give rise to a heterogeneous product. Controlling the glycan distribution is critical to the quality control of the product. Understanding N-glycan biosynthesis and the etiology of microheterogeneity would provide physiological insights, and facilitate cellular engineering to enhance glycoprotein quality. We developed a mathematical model of glycan biosynthesis in the Golgi and analyzed the various reaction variables on the resulting glycan distribution. The Golgi model was modeled as four compartments in series. The mechanism of protein transport across the Golgi is still controversial. From the viewpoint of their holding time distribution characteristics, the two main hypothesized mechanisms, vesicular transport and Golgi maturation models, resemble four continuous mixing-tanks (4CSTR) and four plug-flow reactors (4PFR) in series, respectively. The two hypotheses were modeled accordingly and compared. The intrinsic reaction kinetics were first evaluated using a batch (or single PFR) reactor. A sufficient holding time is needed to produce terminally-processed glycans. Altering enzyme concentrations has a complex effect on the final glycan distribution, as the changes often affect many reaction steps in the network. Comparison of the glycan profiles predicted by the 4CSTR and 4PFR models points to the 4PFR system as more likely to be the true mechanism. To assess whether glycan heterogeneity can be eliminated in the biosynthesis of biotherapeutics the 4PFR model was further used to assess whether a homogeneous glycan profile can be created through metabolic engineering. We demonstrate by the spatial localization of enzymes to specific compartments all terminally processed N-glycans can be synthesized as homogeneous products with a sufficient holding time in the Golgi compartments. The model developed may serve as a guide to future engineering of glycoproteins.     

Topological analysis of plasmid chromatin from yeast and mammalian cells

Yeast has proven to be a powerful system for investigation of chromatin structure. However, the extent to which yeast chromatin can serve as a model for mammalian chromatin is limited by the significant number of differences that have been reported. To further investigate the structural relationship between the two chromatins, we have performed a DNA topological analysis of pRSSVO, a 5889 base-pair plasmid that can replicate in either yeast or mammalian cells. When grown in mammalian cells, pRSSVO contains an average of 33 negative supercoils, consistent with one nucleosome per 181 bp. This is close to the measured nucleosome repeat length of 190 bp. However, when grown in yeast cells, pRSSVO contains an average of only 23 negative supercoils, which is indicative of only one nucleosome per 256 bp. This is dramatically different from the measured nucleosome repeat length of 165 bp. To account for these observations, we suggest that yeast chromatin is composed of relatively short ordered arrays of nucleosomes with a repeat of 165 bp, separated by substantial gaps, possibly corresponding to regulatory regions.