Regulatory elements that control the lineage-specific expression of myoD.

The molecular basis of skeletal muscle lineage determination was investigated by analyzing DNA control elements that regulate the myogenic determination gene myoD. A distal enhancer was identified that positively regulates expression of the human myoD gene. The myoD enhancer and promoter were active in myogenic and several nonmyogenic cell lines. In transgenic mouse embryos, however, the myoD enhancer and promoter together directed expression of a lacZ transgene specifically to the skeletal muscle lineage. These data suggest that during development myoD is regulated by mechanisms that restrict accessibility of myoD control elements to positive trans-acting factors.

Different impact of p53 and p21 on the radiation response of mouse tissues.

Mammalian tissues differ dramatically in their sensitivity to genotoxic stress, although the mechanisms determining these differences remain largely unknown. To analyse the role of p53 and p21 in determination of tissue specificity to DNA damage in vivo, we compared the effects of gamma radiation on DNA synthesis on whole-body sections of wild type, p53-deficient and p21-deficient mice. A dramatic reduction in 14C-thymidine incorporation after gamma irradiation was observed in the majority of rapidly proliferating tissues of wild type and p21-/- but not in p53-/- mice, confirming the key role of p53 in determination of tissue response to genotoxic stress in vivo and suggesting that p53-mediated inhibition of DNA synthesis does not depend on p21. Rapid radiation induced p53-dependent apoptosis was mapped to the areas of high levels of p53 mRNA in radiation sensitive tissues analysed (white pulp in the spleen and bases of crypts in small intestine), indicating that p53 regulation at the mRNA level is a determinant of cellular sensitivity to genotoxic stress. High p53 mRNA expression is inherited as a recessive trait in cell-cell hybrids suggesting the involvement of a negative control mechanism in the regulation of p53 gene expression.

A simple metabolic system for selection of hybrid hybridomas (tetradomas) producing bispecific monoclonal antibodies.

A metabolic selective system has been proposed for the selection of hybrid hybridomas (tetradomas) based on the introduction in one of the parental cell lines of two traits simultaneously--a recessive one (resistance to 8-azaguanine) and a dominant one (multidrug resistance). Tetradomas were selected in the presence of two selective agents: aminopterin and actinomycin D. Using this approach we produced tetradomas secreting bispecific MAbs to horseradish peroxidase and human alpha-fetoprotein.

Monoclonal antibody to fibronectin which inhibits extracellular matrix assembly.

A monoclonal antibody L8 specific to fibronectin was shown to inhibit fibronectin incorporation into the fibroblast extracellular matrix. Antibody L8 could not interact with fibronectin complexed with gelatin. The results suggest the existence of a specific site on the fibronectin molecule playing a critical role in the assembly of the fibronectin extracellular matrix. This site is located near the collagen-binding domain.

Dramatic heterogeneity of transgene expression in the mammary gland of lactating mice: a model system to study the synthetic activity of mammary epithelial cells.

We studied the expression of human serum albumin (HSA) driven by the ovine beta-lactoglobulin promoter in the mammary glands of lactating mice from five independent transgenic strains, by employing combined in situ hybridization and immunostaining techniques. Four strains displayed a heterogeneous pattern of expression: mice of strains 91 and 92 expressed the transgene in only a fraction of the lobules, whereas in strains 69 and 83 all lobules contained cells expressing HSA. In all four strains the patterns of expression within expressing lobules corresponded to the morphology of alveolar cells and to the extent of local milk secretion, suggesting that filling of alveolus with secreted material was accompanied by asynchronous downregulation of transgene expression. In situ hybridization to the endogenous milk protein genes alpha-lactalbumin, beta-casein, and whey acidic protein revealed a uniform pattern of expression in lactating mammary glands of transgeneic and in four out of five non-transgeneic mice. In the fifth control mouse, we detected downregulation of gene expression in lobules containing alveoli distended by secreted milk. The pattern of expression of the three endogenous genes was greatly disturbed after a short (3-hr) unilateral closure of mammary glands, and very much resembled the pattern of expression of the HSA transgenes. These results demonstrate that transgeneic mice provide a useful model to study the factors that regulate the synthetic activity of mammary epithelial cells.

[Physical and chemical properties of the condensate of breathed-out air in healthy people]. / Fiziko-khimicheskie svoistva kondensata vydykhaemogo vozdukha u zdorovykh liudei.

The physical-and-chemical condition of condensate of breathed-out moisture (CBM) was studied by the computer-aided analysis of axis-symmetric drops' form in healthy subjects; such condition was found to depend on sex, age and smoking-addiction of the examined persons. The surface tension of the mentioned moisture correlated with its viscoelastic index and with relaxation. The metabolites of nitric oxide, lipids, urea, lactic acid (not uric acid) and of hydrogen peroxide were found to influence the physical-and-chemical parameters of respiratory moisture. Hydrogen peroxide had a reverse correlation with the CBM viscoelastic module. A status of the pulmonary surfactant can be evaluated by using the studied physical-and-chemical CBM parameters.

The expression of the regulatory myosin light chain 2 gene during mouse embryogenesis.

The fast skeletal muscle myosin light chain 2 (MLC2) gene is expressed specifically in skeletal muscles of newborn and adult mice, and has no detectable sequence homology with any of the other MLC genes including the slow cardiac MLC2 gene. The expression of the fast skeletal muscle MLC2 gene during early mouse embryogenesis was studied by in situ hybridization. Serial sections of embryos from 8.5 to 12.5 days post coitum (d.p.c.) were hybridized to MLC2 cRNA and to probes for the myogenic regulatory genes MyoD1 and myogenin. The results revealed different temporal and spatial patterns of hybridization for different muscle groups. MLC2 transcripts were first detected 9.5 d.p.c. in the myotomal regions of rostral somites, already expressing myogenin. Surprisingly, at the same stage, a weak MLC2 signal was also detected in the cardiomyocytes. The cardiac expression was transient and could not be detected at later stages while the myotomal signal persisted and spread to the more caudal somites, very similar to the expression of myogenin. Beginning from 10.5 d.p.c., several extramyotomal premuscle cells masses have been demarcated by MyoD1 expression. MLC2 transcripts were detected in only one of these cell masses. Although, transcripts of myogenin were detected in all these cell masses, the number of expressing cells was significantly lower than that observed for MyoD1. By 11.5 d.p.c., all three hybridization signals colocalized in most extramyotomal muscle-forming regions, with the exception of the diaphragm and the hindlimb buds, where only few cells expressed MLC2 and more cells expressed MyoD1 than myogenin. At 12.5 d.p.c., all three studied genes displayed a similar spatial pattern of expression in most muscle-forming regions. However, in some muscles, the MyoD1 signal spread over more cells compared to myogenin or MLC2. Our results are consistent with the suggestion that multiple myogenic programs exist for myoblasts differentiating in the myotome and extramyotomal regions.

The expression of specific surface antigen of smooth muscle cells is related to proliferation.

Monoclonal antibody L1 has been obtained after immunization of BALB/c mice with long-term cultured smooth muscle cells (SMC) originally isolated from rat aortic media. Antibody L1 recognizes only the surface antigen of cultured SMC and does not react with other cultured rat cell types. It has been shown that in primary culture of SMC the L1-positive cells appear on the 2nd to 3rd day and their proportion increases up to the 7th day up to 40% in DMEM supplemented with 10% of fetal calf serum (FCS), up to 25% in DMEM with 5% of rat whole-blood serum, but up to only 5% in DMEM with 5% rat plasma-derived serum. These results are in agreement with data on [14C]thymidine incorporation and on flow cytometry. Using FACS II, the SMC were sorted into subpopulations on the 4th and 8th days of primary culture according to the intensity of their specific immunofluorescence. It has been found that the DNA profile in intensively labelled cells corresponds to that in an intensively proliferating population of cells. These findings suggest that antigen L1 appears to be the specific marker of modulated SMC entering the cell cycle.

Ectopic expression of MyoD1 in mice causes prenatal lethalities.

A variety of differentiated cell types can be converted to skeletal muscle following transfection with the myogenic regulatory gene MyoD1. To determine whether MyoD1 is a dominant muscle regulator in vivo, mouse fertilized eggs were microinjected with a beta-actin/MyoD1 gene. Ectopic expression of MyoD1 during mouse embryogenesis led to embryonic lethalities, the cause of which is not known. Transgenic embryos died before midgestation. The majority of tested embryos between 7.5 and 9.5 days, although retarded compared to control littermates, differentiated normally into tissues representative of all three germ layers. In most transgenic embryos there was no indication of myogenic conversion. The expression of the introduced gene was detected in all ectodermal and mesodermal tissues but was absent in all endodermal cells. Forced expression of MyoD1 was associated with the activation of myogenin and MLC2 (but not myf5 or MRF4) genes in non-muscle cell types, demonstrating the dominant regulatory function of MyoD1 during development. These results demonstrate that ectopic MyoD1 expression and activation of myogenin and MLC2 have no significant effects in the determination of cell lineages or the developmental fate of differentiated mesodermal and ectodermal cell lineages.

Dual regulation of beta-lactoglobulin/human serum albumin gene expression by the extracellular matrix in mammary cells from transgenic mice.

Mammary explants and epithelial cell cultures from transgenic mice carrying the human serum albumin (HSA) gene or minigenes behind the regulatory sequences of the ovine beta-lactoglobulin gene were analyzed. Previously we demonstrated that mammary explants from virgin female transgenic mice synthesize and secrete high levels of HSA during the first day in culture. Here we present a detailed analysis of endogenous and transgene expression during the first 20 h of mammary explant cultures. We show that HSA genes as well as endogenous milk protein genes are rapidly induced upon explantation. Unexpectedly, HSA was synthesized also in mammary explants from strains that do not secrete HSA into the milk, indicating the existence of a cryptic potential to express the transgene. Histological examination revealed that some luminal epithelial cells detached from the underlying extracellular matrix (ECM) soon after explantation. Epithelial cell cultures from nonsecreting strains grown on plastic rapidly induced transgene expression and secreted higher levels of HSA into the medium compared to cells grown on collagen. These results suggest that tissue organization and most likely the interaction of epithelial cells with the ECM are intimately involved in the control of HSA transgene expression.

Synthesis and secretion of caseins by the mouse mammary gland: production and characterization of new polyclonal antibodies.

Polyclonal antibodies to mouse alpha- and beta/gamma-caseins were raised in rabbits. These antibodies display tissue- and species specificity as shown by immunoblotting. Immunohistochemical analyses demonstrate that both alpha- and beta/gamma-caseins were synthesized and secreted from virtually all lactating mammary epithelial cells, in a pattern very similar to that of the mouse alpha-lactalbumin. Residual amounts of caseins were located also in the apical surface of epithelial cells surrounding the ducal lumen of virgin mammary gland sections. In contrast to the significant level of gamma-casein in the milk, the amount of this protein compared to alpha- or beta-caseins was extremely low in medium conditioned for 24 h by mammary explants of mid-pregnant mice immediately after explantation or after 4 days.

Immunohistochemical study of angiotensin-converting enzyme in human tissues using monoclonal antibodies.

The localization of angiotensin-converting enzyme (ACE) in human tissues has been studied by the PAP-method with the use of monoclonal antibody 9 B9 against human lung ACE. The enzyme was detected on the surface of endothelial cells in lung, myocardium, liver, intestine and testis as well as in the epithelial cells of the kidney proximal tubules and intestine. The monoclonal antibody 9 B9 did not react with ACE in the epithelial cells of the testis seminiferous tubules. These data suggest that the antibody 9 B9 recognizes epitope which is shared by the ACE molecule of endothelial cells and renal and intestinal epithelial cells but is not present in testicular ACE, or is not accessible there to the antibody.

The distal human myoD enhancer sequences direct unique muscle-specific patterns of lacZ expression during mouse development.

Transgenic mice carrying the bacterial lacZ reporter gene under the control of the regulatory elements of the human myoD gene have been produced. The developmental expression of the myoD reporter transgene in somites, limb buds, visceral arches, and cephalocervical regions was studied in transgenic embryos by beta-gal staining. In somites, the spatiotemporal pattern of transgene expression was different from other muscle-specific regulatory and structural genes and revealed that myoD-expressing cells arise in distinct patterns in somites that are dependent on position along the anterior-posterior (AP) body axis (occipital and cervical vs thoracic and more posterior myotomes). Transgene expression did not follow a strict anterior to posterior sequence of activation and therefore was not strictly correlated with somite developmental age. Moreover, the pattern of transgene expression along the dorsal-ventral myotomal axis was dependent on somite position along the anterior-posterior axis. While myoD expression is first detected after the myotome is well-formed, transgene expression in the dorsal and ventral medial lips of the dermatome suggests a function for myoD in the expansion of the myotome. Whole-mount in situ hybridization confirmed that these unique patterns of transgene expression in somites, as well as expression in limb buds, visceral arches, and other myogenic centers, are concordant with the distribution of endogenous myoD transcripts. These results shed new light on the developmental differences between myotomes at different positions along the AP and DV axis and demonstrate a unique axial pattern of somitic myoD expression, suggesting a specific role of myoD in myotome lineage determination and differentiation.

Beta-lactoglobulin/human serum albumin fusion genes do not respond accurately to signals from the extracellular matrix in mammary epithelial cells from transgenic mice.

Mammary epithelial cell cultures from transgenic mice carrying the human serum albumin (HSA) gene or minigenes behind the regulatory sequences of the ovine beta-lactoglobulin gene were analyzed. Previously, we demonstrated that non-HSA-secreting transgenic strains retain the potential to express the HSA transgene in vitro and that mammary epithelial cell cultures from non-HSA-secreting strains express higher levels of HSA when grown on tissue culture plastic than they do when grown on collagen. In this study we studied the expression of BLG/HSA fusion genes in epithelial cell cultures of additional transgenic strains and additional substrata. Our results show that: (1) The BLG/HSA fusion gene in only one of seven HSA-secreting or nonsecreting transgenic strains tested accurately responded to signals from the EHS matrix; (2) HSA DNA sequences dominantly affected the activity of BLG as well as the whey acidic protein promoters; and (3) HGF/SF induced both milk proteins and HSA gene expression. These results suggest that the response to the extra cellular matrix (ECM) plays a key role in the expression of BLG/HSA fusion genes and that the function of the regulatory elements within the promoter regions of milk protein genes involved in response to the ECM, in developmental and in tissue specificity, very much depend on the downstream gene sequences.

The consequences of a constitutive expression of MyoD1 in ES cells and mouse embryos.

A variety of differentiated cell types can be converted to skeletal muscle following transfection with the myogenic regulatory gene MyoD1. To determine whether multipotent embryonic stem (ES) cells respond similarly, cultures of two ES cell lines were electroporated with a MyoD1 cDNA driven by the beta-actin promoter. All transfected clones tested, carrying single copy of the exogenous gene, expressed high levels of MyoD1 mRNA. Surprisingly, although maintained in mitogen-rich medium, this ectopic expression was associated with a transactivation of the endogenous myogenin and myosin light chain 2 genes but not the endogenous MyoD1, MRF4, myf5, skeletal muscle actin or myosin heavy chain genes. Preferential myogenesis and the appearance of contracting skeletal muscle fibers was observed only when the transfected cells were allowed to differentiate, via embryoid bodies, in low mitogen-containing medium. Myogenesis was associated with the activation of MRF4 and myf5 genes and in a significant increase in the level of myogenin mRNA. Not all cells were converted to skeletal muscle, indicating that only a subset of stem cells can respond to MyoD1. Moreover, the continued expression of MyoD1 was not required for myogenesis. Interestingly, no preferential myogenesis was observed when the transfected ES cells were allowed to differentiate in vivo to teratocarcinomas. These results show that ES cells can respond to MyoD1, but environmental factors control the expression of its myogenic differentiation function. Second, MyoD1 function in ES cells, even under environmental conditions that favour differentiation, is not dominant (incomplete penetrance). Third, that the exogenous MyoD1 transactivates the endogenous myogenin and MLC2 genes in ES cells. No live transgenic mice could be produced following microinjection of the beta-actin/MyoD1 gene into the pronuclei of fertilized eggs. Transgenic embryos died before mid gestation. The majority of tested embryos between 7.5 and 9.5 days, although retarded compared to control litermates, differentiated into tissues representative of all three germ layers. The expression of the introduced gene was detected in all ectodermal and mesodermal tissues but was absent in all endodermal cells. These results demonstrate again that MyoD1 is not a dominant regulatory factor.