Stem cell therapy of the liver--fusion or fiction?

Mesenchymal stromal cells: promising treatment for liver cirrhosis

Liver fibrosis is a wound-healing process that occurs in response to severe injuries and is hallmarked by the excessive accumulation of extracellular matrix or scar tissues within the liver. Liver fibrosis can be either acute or chronic and is induced by a variety of hepatotoxic causes, including lipid deposition, drugs, viruses, and autoimmune reactions. In advanced fibrosis, liver cirrhosis develops, a condition for which there is no successful therapy other than liver transplantation. Although liver transplantation is still a viable option, numerous limitations limit its application, including a lack of donor organs, immune rejection, and postoperative complications. As a result, there is an immediate need for a different kind of therapeutic approach. Recent research has shown that the administration of mesenchymal stromal cells (MSCs) is an attractive treatment modality for repairing liver injury and enhancing liver regeneration. This is accomplished through the cell migration into liver sites, immunoregulation, hepatogenic differentiation, as well as paracrine mechanisms. MSCs can also release a huge variety of molecules into the extracellular environment. These molecules, which include extracellular vesicles, lipids, free nucleic acids, and soluble proteins, exert crucial roles in repairing damaged tissue. In this review, we summarize the characteristics of MSCs, representative clinical study data, and the potential mechanisms of MSCs-based strategies for attenuating liver cirrhosis. Additionally, we examine the processes that are involved in the MSCs-dependent modulation of the immune milieu in liver cirrhosis. As a result, our findings lend credence to the concept of developing a cell therapy treatment for liver cirrhosis that is premised on MSCs. MSCs can be used as a candidate therapeutic agent to lengthen the survival duration of patients with liver cirrhosis or possibly reverse the condition in the near future.

In vitro Differentiation of Human TERT-Transfected Multi-Lineage Progenitor Cells (MLPC) into Immortalized Hepatocyte-Like Cells

Background

Research directed towards drug development, metabolism, and liver functions often utilize primary hepatocytes (PH) for preliminary in vitro studies. Variability in the in vitro functionality of PH and the unsuitability of hepatocarcinoma cells for these studies have driven researchers to look to ESC, iPS, and other stem cell types using differentiation protocols to provide more reliable and available cells. This study describes the development of hepatocyte-like cells through the in vitro differentiation of human TERT-immortalized cord blood-derived multi-lineage progenitor cells (MLPC). The E12 clonal cell line derived from polyclonal TERT-transfected cells was used throughout the study.

Methods

E12 MLPC were subjected to a three-step differentiation protocol using alternating combinations of growth factors, cytokines, and maturational factors. Cells at various stages of differentiation were analyzed for consistency with PH by morphology, immunohistochemistry, urea production, and gene expression.

Results

E12 MLPC were shown to significantly change morphology with each stage of differentiation. Coincidental with the morphological changes in the cells, immunohistochemistry data documented the differentiation to committed endoderm by the expression of SOX-17 and GATA-4; the progression to committed hepatocyte-like cells by the expression of a large number of markers including α-fetoprotein and albumin; and the final differentiation by the expression of nuclear and cytoplasmic HNF4. Fully differentiated cells demonstrated gene expression, urea production, and immunohistochemistry consistent with PH. A methodology and medium formulation to continuously expand the E12-derived hepatocyte-like cells is described.

Conclusion

The availability of immortalized hepatocyte-like cell lines could provide a consistent tool for the study of hepatic diseases, drug discovery, and the development of cellular therapies for liver disorders. Utilization of these techniques could provide a basis for the development of bridge therapies for liver failure patients awaiting transplant.

Development of immortalized human hepatocyte-like hybrid cells by fusion of multi-lineage progenitor cells with primary hepatocytes

Human primary hepatocytes (PHs) are critical to studying liver functions, drug metabolism and toxicity. PHs isolated from livers that are unacceptable for transplantation have limited expansion and culture viability in vitro, in addition to rapidly deteriorating enzymatic functions. The unsuitability of immortalized hepato-carcinoma cell lines for this function has prompted studies to develop hepatocyte-like cells from alternative sources like ESC, iPS, and other stem cell types using differentiation protocols. This study describes a novel technique to produce expandable and functional hepatocyte-like cells from the fusion of an immortalized human umbilical cord blood derived cell line (E12 MLPC) to normal human primary hepatocytes. Multi-lineage progenitor cells (MLPC) comprise a small subset of mesenchymal-like cells isolated from human umbilical cord blood. MLPC are distinguishable from other mesenchymal-like cells by their extended expansion capacity (up to 80 cell doublings before senescence) and the ability to be differentiated into cells representative of endo-, meso- and ectodermal origins. Transfection of MLPC with the gene for telomerase reverse transcriptase (TERT) resulted in clonal cell lines that were capable of differentiation to different cellular outcomes while maintaining their functional immortality. A methodology for the development of immortalized hepatocyte-like hybrid cells by the in vitro fusion of human MLPC with normal human primary hepatocytes is reported. The resultant hybrid cells exhibited homology with hepatocytes by morphology, immunohistochemistry, urea and albumin production and gene expression. A medium that allows stable long-term expansion of hepatocyte-like fusion cells is described.

Regenerative Cells for Transplantation in Hepatic Failure

Human embryonic stem (ES) cells and induced pluripotent stem (iPS) cells have an enormous potential; however, their potential clinical application is being arrested due to various limitations such as teratoma formation followed by tumorigenesis, emergent usage, and the quality control of cells, as well as safety issues regarding long-term culture are also delaying their clinical application. In addition, human ES cells have two crucial issues: immunogenicity and ethical issues associated with their clinical application. The efficient generation of human iPS cells requires gene transfer, yet the mechanism underlying pluripotent stem cell induction has not yet been fully elucidated. Otherwise, although human adult regenerative cells including mesenchymal stem cells have a limited capacity for differentiation, they are nevertheless promising candidates for tissue regeneration in a clinical setting. This review highlights the use of regenerative cells for transplantation in hepatic failure.

Hybrid cells differentiate to hepatic lineage cells and repair oxidative damage

Hybrid cells derived from stem cells play an important role in organogenesis, tissue regeneration and cancer formation. However, the fate of hybrid cells and their range of function are poorly understood. Fusing stem cells and somatic cells induces somatic cell reprogramming, and the resulting hybrid cells are embryonic stem cell-like cells. Therefore, we hypothesize that fusion-induced hybrid cells may behave like ES cells in certain microenvironments. In this study, human hepatic cells were induced to apoptosis with H(2)O(2), and then co-cultured with hybrid cells that had been derived from mouse ES cells and human hepatic cells using a transwell. After co-culturing, the degree of apoptosis was evaluated using Annexin-V/PI double-staining analysis, flow cytometry and Western-blot. We observed that H(2)O(2)-induced cell apoptosis was inhibited by co-culture. In addition, the activity of injury-related enzymes (GSH-Px, LDH and SOD) and the level of albumin release in the co-culture system trended toward the level of normal undamaged hepatic cells. The stably increased levels of secretion of ALB in the co-culture system also confirmed that co-culture with hybrid cells helped in recovery from injury. The fate of the hybrid cells was studied by analyzing their gene expression and protein expression profiles. The results of RT-PCR indicated that during co-culturing, like ES cells, hybrid cells differentiated into hepatic lineage cells. Hybrid cells transcripted genes from both parental cell genomes. Via immunocytochemical analysis, hepatic directional differentiation of the hybrid cells was also confirmed. After injecting the hybrid cells into the mouse liver, the GFP-labeled transplanted cells were distributed in the hepatic lobules and engrafted into the liver structure. This research expands the knowledge of fusion-related events and the possible function of hybrid cells. Moreover, it could indicate a new route of differentiation from pluripotent cells to tissue-specific cells via conditional co-culture.

Architectural and immunohistochemical characterization of biliary ductules in normal human liver

The canals of Hering or biliary ductules have been described to connect the bile canaliculi with the interlobular bile ducts, and thus forming the distal part of the biliary tree. Studies in the last two decades suggested that the cells constructing these ductules could behave as hepatic progenitor cells. The canals of Hering are confined to the periportal space in the rat, while they have been reported to spread beyond the limiting plate in human liver. The distribution of the distal biliary ductules in normal human hepatic tissue has been investigated in our recent experiments. We could demonstrate the presence of interlobular connective tissue septa in a rudimentary form in healthy livers. The canals of Hering run in these septa in line with the terminal branches of the portal vein and hepatic arteries. This arrangement develops in the postnatal period but regresses after early childhood. The canals of Hering can be identified by the unique epithelial membrane antigen (EMA)-/CD56+/CD133+ immunophenotype. The canals of Hering leave the periportal space and spread into the liver parenchyma along rudimentary interlobular septa outlining the hepatic lobules. Our observations refine the original architectural description of the intraparenchymal portion of the canals of Hering in the human liver. The distinct immunophenotype supports their unique biological function.

Clonal mesenchymal stem cells derived from human bone marrow can differentiate into hepatocyte-like cells in injured livers of SCID mice

There is increasing evidence that human mesenchymal stem cells (hMSCs) can be a valuable, transplantable source of hepatocytes. Most of the hMSCs preparations used in these studies were likely heterogeneous cell populations, isolated by adherence to plastic surfaces or by density gradient centrifugation. Therefore, the participation of other unknown trace cell populations cannot be rigorously discounted. Here we report the isolation and establishment of a cloned human MSC line (chMSC) from human bone marrow primary culture, through which we confirmed the hepatic differentiation capability of authentic hMSCs. chMSCs expressed markers of mesenchymal cells, but not markers of hematopoietic stem cells. In vitro, chMSCs can differentiate into either mesenchymal cells or cells exhibiting hepatocyte-like phenotypes. When transplanted intrasplentically into carbon tetrachloride-injured livers of SCID mice, EGFP-tagged chMSCs engrafted into the host liver parenchyma, exhibited typical hepatocyte morphology, form a three-dimensional architecture, and differentiate into hepatocyte-like cells expressing human albumin and alpha-1-anti-trypsin. By confocal microscopy, ultrafine intercellular nanotubular structures were visible between adjacent transplanted and host hepatocytes. We postulate that these structures may assist in the phenotype conversion of chMSCs, possibly by exchange of cytoplasmic components between native hepatocytes and transplanted cells. Thus, a clonal pure population of hMSCs, which can be expanded in culture, may have potential as a cellular source for substitution damaged cells in hepatic injury.

Transdifferentiation of stem cells: a critical view

Recently a large amount of new data on the plasticity of stem cells of various lineages have emerged, providing new perspectives especially for the therapeutic application of adult stem cells. Previously unknown possibilities of cell differentiation beyond the known commitment of a given stem cell have been described using keywords such as "blood to liver," or "bone to brain." Controversies on the likelihood, as well as the biological significance, of these conversions almost immediately arose within this young field of stem cell biology. This chapter will concentrate on these controversies and focus on selected examples demonstrating the technical aspects of stem cell transdifferentiation and the evaluation of the tools used to analyze these events.

Characterization of adipose tissue-derived cells isolated with the Celution system

BACKGROUND

The therapeutic potential of using stem cells is tremendous. Mesenchymal stromal cells (MSC) have now been isolated in various tissues including bone marrow (BM), muscle, skin and adipose tissue. Among them, adipose tissue could be one of the most suitable cell sources for cell therapy, because of its easy accessibility, minimal morbidity and abundance of stem cells. The large numbers of stem cells in adipose tissue means that clinically relevant stem cell numbers could be extracted from the tissue, potentially eliminating the need for in vitro expansion. To utilize these characteristics of adipose tissue fully, Cytori Therapeutics Inc. has developed a closed system called Celution to isolate and concentrate stem cells and regenerative cells automatically from adipose tissue.

METHODS

Adipose tissue-derived cells were isolated using the Celution system. The output from the Celution was characterized using multicolor FACS analysis with CD31, CD34, CD45, CD90, CD105 and CD146. The multidifferentiation potential of the cells was analyzed using adipogenic and osteogenic media.

RESULTS

Our results showed that cells from the Celution are composed of heterogeneous cell populations including adipose-derived stem cells (ASC) (CD31- CD34+ CD45- CD90+ CD105- CD146-), endothelial (progenitor) cells (CD31+ CD34+ CD45- CD90+ CD105- CD146+) and vascular smooth muscle cells (CD31- CD34+ CD45- CD90+ CD105- CD146+). We also confirmed the output contains cells able to differentiate into adipogenic and osteogenic phenotypes. Our results show that cells isolated with the Celution and manually are equivalent.

DISCUSSION

Cells from adipose tissue can be processed by Celution within the time frame of a single surgical procedure. This system could provide a 'real-time' treatment setting that is cost-effective and safe.

Functional modulation of ES-derived hepatocyte lineage cells via substrate compliance alteration

Pluripotent embryonic stem cells represent a promising renewable cell source to generate a variety of differentiated cell types including hepatocyte lineage cells, and may ultimately be incorporated into extracorporeal bioartificial liver devices and cell replacement therapies. Recently, we and others have utilized sodium butyrate to directly differentiate hepatocyte-like cells from murine embryonic stem cells cultured in a monolayer configuration. However, to incorporate stem cell technology into clinical and pharmaceutical applications, and hopefully increase the therapeutic potential of these differentiated cells for liver disease treatment, a major challenge remains in sustaining differentiated functions for an extended period of time in their secondary culture environment. In the present work, we have investigated the use of polyacrylamide hydrogels with defined mechanical compliances as a cell culture platform for improving and/or stabilizing functions of these hepatocyte-like cells. Several functional assays, e.g., urea secretion, intracellular albumin content, and albumin secretion, were performed to characterize hepatic functions of cells on polyacrylamide gels with stiffnesses of 5, 46.6, and 230 kPa. In conjunction with the mechanical and cell morphological characterization, we showed that hepatic functions of sodium butyrate differentiated cells were sustained and further enhanced on compliant substrates. This study promises to offer insights into regulating stem cell differentiation via mechanical stimuli, and assist us with designing a variety of dynamic culture systems for applications in tissue and cellular engineering.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

MAGfect: a novel liposome formulation for MRI labelling and visualization of cells

Cellular entry of imaging probes, such as contrast agents for magnetic resonance imaging (MRI), is a key requirement for many molecular imaging studies, particularly imaging intracellular events and cell tracking. Here, we describe the successful development and in vitro analysis of MAGfect, a novel liposome formulation containing a lipidic gadolinium contrast agent for MRI, Gd-DOTA-Chol , designed to enter and label cells. Liposome formulation and cell incubation time were optimised for maximum cellular uptake of the imaging probe in a variety of cell lines. MRI analysis of cells incubated with MAGfect showed them to be highly MRI active. This formulation was examined further for cytotoxicity, cell viability and mechanism of cell labelling. One of the key advantages of using MAGfect as a labelling vehicle arises from its potential for additional functions, such as concomitant drug or gene delivery and fluorescent labelling. The gadolinium liposome was found to be an effective vehicle for transport of plasmid DNA (pDNA) into cells and expression levels were comparable to the commercial transfection agent Trojene.

Hepatic progenitor cells in human fetal liver express the oval cell marker Thy-1

Hepatic progenitor cells play a major role in regenerating diseased liver. In rodents, progenitors forming hepatocytes or cholangiocytes are identified by the stem cell marker Thy-1. The aim of this study was to ascertain whether progenitor cells expressing Thy-1 could be identified in human fetal liver. Midtrimester human fetal liver was immunostained for Thy-1, cytokeratins 18 and 19, vimentin, CD34, CD45, and fibrinogen. Thy-1+ and Thy-1+CD34+ populations were purified using fluorescence-activated cell sorting (FACS). Immunofluorescence and mRNA expression were used to examine the bipotential nature of purified stem cells. We found that Thy-1+ cells were concentrated in portal tracts but were also scattered in parenchyma. In FACS-prepared cells, 0.18-3.08% (median 0.65%, n = 14) of cells were Thy-1+. Immunophenotyping revealed that some Thy-1+ cells coexpressed cytokeratins 18 and 19, others, fibrinogen and cytokeratin 19. RT-PCR demonstrated that Thy-1+ cells expressed mRNA for Thy-1, cytokeratin 18, and cytokeratin 19, and Thy-1+CD34+ cells expressed mRNA for alpha-fetoprotein, transferrin, and hepatocyte nuclear factor-4alpha. Thy-1+ cells were identified in fetal liver. These cells expressed several lineage markers, including coexpression of biliary and hepatocellular proteins and mRNA. These data suggest that Thy-1 is a marker of liver stem cells in human fetal liver.

Functional characterization of cell hybrids generated by induced fusion of primary porcine mesenchymal stem cells with an immortal murine cell line

Bone marrow mesenchymal stem cells (MSC) integrate into various organs and contribute to the regeneration of diverse tissues. However, the mechanistic basis of the plasticity of MSC is not fully understood. The change of cell fate has been suggested to occur through cell fusion. We have generated hybrid cell lines by polyethylene-glycol-mediated cell fusion of primary porcine MSC with the immortal murine fibroblast cell line F7, a derivative of the GM05267 cell line. The hybrid cell lines display fibroblastic morphology and proliferate like immortal cells. They contain tetraploid to hexaploid porcine chromosomes accompanied by hypo-diploid murine chromosomes. Interestingly, many hybrid cell lines also express high levels of tissue-nonspecific alkaline phosphatase, which is considered to be a marker of undifferentiated embryonic stem cells. All tested hybrid cell lines retain osteogenic differentiation, a few of them also retain adipogenic potential, but none retain chondrogenic differentiation. Conditioned media from hybrid cells enhance the proliferation of both early-passage and late-passage porcine MSC, indicating that the hybrid cells secrete diffusible growth stimulatory factors. Murine F7 cells thus have the unique property of generating immortal cell hybrids containing unusually high numbers of chromosomes derived from normal cells. These hybrid cells can be employed in various studies to improve our understanding of regenerative biology. This is the first report, to our knowledge, describing the generation of experimentally induced cell hybrids by using normal primary MSC.

Alginate-PLL microencapsulation: effect on the differentiation of embryonic stem cells into hepatocytes

The emergence of hepatocyte based clinical and pharmaceutical technologies, has been limited by the absence of a stable hepatocyte cell source. Embryonic stem cells may represent a potential solution to this cell source limitation problem since they are highly proliferative, renewable, and pluripotent. Although many investigators have described techniques to effectively differentiate stem cells into a variety of mature cell lineages, their practicality is limited by: (1) low yields of fully differentiated cells, (2) absence of large scale processing considerations, and (3) ineffective downstream enrichment protocols. Thus, a differentiation platform that may be modified to induce and sustain differentiated cell function and scaled to increase differentiated cell yield would improve current stem cell differentiation strategies. Microencapsulation provides a vehicle for the discrete control of key cell culture parameters such as the diffusion of growth factors, metabolites, and wastes. In addition, both cell seeding density and bead composition may be manipulated. In order to assess the feasibility of directing stem cell differentiation via microenvironment regulation, we have developed a murine embryonic stem cell (ES) alginate poly-l-lysine microencapsulation hepatocyte differentiation system. Our results indicate that the alginate microenvironment maintains cell viability, is conducive to ES cell differentiation, and maintains differentiated cellular function. This system may ultimately assist in developing scalable stem cell differentiation strategies.

Cardiac stem cells and mechanisms of myocardial regeneration

This review discusses current understanding of the role that endogenous and exogenous progenitor cells may have in the treatment of the diseased heart. In the last several years, a major effort has been made in an attempt to identify immature cells capable of differentiating into cell lineages different from the organ of origin to be employed for the regeneration of the damaged heart. Embryonic stem cells (ESCs) and bone marrow-derived cells (BMCs) have been extensively studied and characterized, and dramatic advances have been made in the clinical application of BMCs in heart failure of ischemic and nonischemic origin. However, a controversy exists concerning the ability of BMCs to acquire cardiac cell lineages and reconstitute the myocardium lost after infarction. The recognition that the adult heart possesses a stem cell compartment that can regenerate myocytes and coronary vessels has raised the unique possibility to rebuild dead myocardium after infarction, to repopulate the hypertrophic decompensated heart with new better functioning myocytes and vascular structures, and, perhaps, to reverse ventricular dilation and wall thinning. Cardiac stem cells may become the most important cell for cardiac repair.

Whole fetal liver transplantation--a new approach to cell therapy

We recently developed a novel rat model for liver repopulation, heterografting of microliver slices, aimed at overcoming the limitations inherent in both whole liver and hepatocyte transplantations. The aim of the present study was to evaluate the potential of whole fetal liver transplantations to survive and differentiate within the adult liver, using the adult liver slice transplantation model. Embryonic day 14 whole fetal livers from dipeptidyl peptidase IV+/+ wild-type Fischer 344 rats were transplanted into the livers of dipeptidyl peptidase IV-/- mutant rats. Adult hepatic markers, dipeptidyl peptidase IV, albumin, glycogen, and proliferation cell nuclear antigen- proliferation cell nuclear antigen (PCNA) were assessed in the transplanted liver tissue by immunohistochemistry. Two groups of 9 rats each were transplanted with 3 fetal livers per recipient. Two months later the rats were sacrificed and the markers were detected in the transplanted tissues. In conclusion, the results of this study raise the possibility that fetal liver transplantation could serve as a model for genetic metabolic liver diseases.

Factors influencing stem cell differentiation into the hepatic lineage in vitro

A major area of research in transplantation medicine is the potential application of stem cells in liver regeneration. This would require well-defined and efficient protocols for directing the differentiation of stem cells into the hepatic lineage, followed by their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying hepatogenesis and liver metabolism. The development of pharmokinetic and cytotoxicity/genotoxicity screening tests for newly developed biomaterials and drugs, could also utilize protocols developed for the hepatic differentiation of stem cells. Hence, this review critically examines the various strategies that could be employed to direct the differentiation of stem cells into the hepatic lineage in vitro.

Identity Deception: Not a Crime for a Stem Cell

Stem cell transdifferentiation in the adult organism is the most common and questioned mechanism of growth and repair. Recent data suggest that adult stem cells are capable of generating mature cells beyond their own tissue boundaries, a process called developmental plasticity. To date, the most versatile cell discovered is the bone marrow progenitor cell.

Cancer stem cell biology: from leukemia to solid tumors

The biology of stem cells and their intrinsic properties are now recognized as integral to tumor pathogenesis in several types of cancer. This observation has broad ramifications in the cancer research field and is likely to impact our understanding of the basic mechanisms of tumor formation and the strategies we use to treat cancers. A role for stem cells has been demonstrated for cancers of the hematopoietic system, breast and brain. Going forward it is likely that stem cells will also be implicated in other malignancies. Hence, a detailed understanding of stem cells and how they mediate tumor pathogenesis will be critical in developing more effective cancer therapies.

The fatty acid constitution and ordering state of membranes in dominant temperature-sensitive lethal mutation and wild-type Drosophila melanogaster larvae

The ordering state and changes in fatty acid composition of microsomal (MS) and mitochondrial (MC) membranes of two dominant temperature-sensitive (DTS) lethal mutations and the wild-type Oregon-R strain larvae of Drosophila melanogaster have been studied at 18 and 29 degrees C and after temperature-shift experiments. The membranes of wild-type larvae have a stable ordering state, with "S" values between 0.6 (18 degrees C) and 0.5 (29 degrees C) in both membranes which remained unchanged in shift experiments, although the ratios of saturated/unsaturated fatty acids were changed as expected. The strongly DTS mutation 1(2) 10DTS forms very rigid membranes at the restrictive temperature (29 degrees C) which cannot be normalized after shift down, while shift up or development at the permissive temperature results in normal ordering state. This mutant is less able to adjust MS and MC fatty acid composition in response to the growth temperature than the wild type. The less temperature-sensitive 1(2)2DTS allele occupies an intermediate state between Oregon-R and 1(2)10DTS in both respects. We assume and the genetical data suggest that the DTS mutant gene product is in competition with the wild-type product, resulting in a membrane structure which is not able to accommodate to the restrictive temperature.