Tumor-induced lymph node alterations detected by MRI lymphography using gadolinium nanoparticles.

Contrast-enhanced MRI lymphography shows potential to identify alterations in lymph drainage through lymph nodes (LNs) in cancer and other diseases. MRI studies have typically used low molecular weight gadolinium contrast agents, however larger gadolinium-loaded nanoparticles possess characteristics that could improve the specificity and sensitivity of lymphography. The performance of three gadolinium contrast agents with different sizes and properties was compared by 3T MRI after subcutaneous injection. Mice bearing B16-F10 melanoma footpad tumors were imaged to assess tumor-induced alterations in lymph drainage through tumor-draining popliteal and inguinal LNs versus contralateral uninvolved drainage. Gadolinium lipid nanoparticles were able to identify tumor-induced alterations in contrast agent drainage into the popliteal LN, while lower molecular weight or albumin-binding gadolinium agents were less effective. All of the contrast agents distributed in foci around the cortex and medulla of tumor-draining popliteal LNs, while they were restricted to the cortex of non-draining LNs. Surprisingly, second-tier tumor-draining inguinal LNs exhibited reduced uptake, indicating that tumors can also divert LN drainage. These characteristics of tumor-induced lymph drainage could be useful for diagnosis of LN pathology in cancer and other diseases. The preferential uptake of nanoparticle contrasts into tumor-draining LNs could also allow selective targeting of therapies to tumor-draining LNs.

Differential cytokine responses in human and mouse lymphatic endothelial cells to cytokines in vitro.

BACKGROUND: Inflammatory cytokines dysregulate microvascular function, yet how cytokines affect lymphatic endothelial cells (LEC) are unclear. METHODS AND RESULTS: We examined effects of TNF-α, IL-1 beta, and IFN-gamma on LEC proliferation, endothelial cell adhesion molecule (ECAM) expression, capillary formation, and barrier changes in murine (SV-LEC) and human LECs (HMEC-1a). RESULTS: All cytokines induced ICAM-1, VCAM-1, MAdCAM-1, and E-selectin in SV-LECs; TNF-α, IL-1 beta; and IFN-gamma induced ECAMs (but not MAdCAM-1) in HMEC-1a. IL-1 beta increased, while IFN-gamma and TNF-α reduced SV-LEC proliferation. While TNF-α induced, IFN-gamma decreased, and IL-1 beta did not show any effect on HMEC-1a proliferation. TNF-α, IL-1 beta, and IFN-gamma each reduced capillary formation in SV-LEC and in HMEC-1a. TNF-α and IL-1 beta reduced barrier in SV-LEC and HMEC-1a; IFN-gamma did not affect SV-LEC barrier, but enhanced HMEC-1a barrier. Inflammatory cytokines alter LEC growth, activation and barrier function in vitro and may disturb lymphatic clearance increasing tissue edema in vivo. CONCLUSION: Therapies that maintain or restore lymphatic function (including cytokines blockade), may represent important strategies for limiting inflammation.

p19/Arf and p53 suppress sentinel lymph node lymphangiogenesis and carcinoma metastasis.

The ability of tumor cells to metastasize is increasingly viewed as an interaction between the primary tumor and host tissues. Deletion of the p19/Arf or p53 tumor suppressor genes accelerates malignant progression and metastatic spread of 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced squamous cell carcinomas, providing a model system to address mechanisms of metastasis. Here, we show that benign pre-metastatic papillomas from wild-type mice trigger lymphangiogenesis within draining lymph nodes, whereas there is no growth of primary tumor lymphatic vessels. Lymph node lymphangiogenesis is greatly accelerated in papilloma-bearing p19/Arf- or p53-deficient mice, which coincides with the greater propensity of these tumors to progress to carcinomas and to metastasize. The extent of accumulation of B cells within the tumor-draining lymph nodes of wild-type mice predicted the level of lymph node lymphangiogenesis and metastatic potential. Arf or p53 deficiency strongly accelerated lymph node immune cell accumulation, in a manner that was associated with the extent of lymph node lymphatic sinus growth. This immune cell accumulation and lymph node lymphangiogenesis phenotype identifies host anti-tumor responses that could drive metastatic spread of cancers via the lymphatics.

Blocked B cell differentiation and emigration support the early growth of Myc-induced lymphomas.

Avian leukosis virus induces lymphoma in chickens after proviral integration within the c-Myc gene, and subsequent expansion of Myc-overexpressing lymphocytes within transformed bursal follicles. The clonal expansion of these follicles allowed us to examine how Myc influences cell differentiation, growth, and apoptosis in lymphoid progenitors soon after the onset of Myc overexpression. Immunohistochemical analysis of developmental markers established that Myc overexpression consistently blocks lymphocyte differentiation at a late embryonic stage. Myc-transformed follicles also grow much more rapidly than normal follicles. This rapid growth is not mediated by suppression of apoptosis, as normal and Myc-transformed follicles showed similar rates of cell death by TUNEL immunohistochemical analysis of cells undergoing DNA degradation. Measurements of DNA synthesis and mitotic index showed modest effects of Myc to increase lymphocyte proliferation, as normal lymphocytes already divide rapidly. The major mechanism mediating rapid growth of transformed follicles instead involved failure of myc-overexpressing lymphocytes to emigrate from transformed follicles, while normal lymphocytes actively emigrate after hatching, as measured by BrdU pulse-chase labeling and immunohistochemical measurements. This failure to undergo the normal program of differentiation and subsequent bursal retention of lymphocytes accounts for most of the growth of transformed follicles, while Myc-induced proliferation makes a smaller contribution.

Analysis of gene expression during myc oncogene-induced lymphomagenesis in the bursa of Fabricius.

The transcriptional effects of deregulated myc gene overexpression are implicated in tumorigenesis in a spectrum of experimental and naturally occurring neoplasms. In follicles of the chicken bursa of Fabricius, myc induction of B-cell neoplasia requires a target cell population present during early bursal development and progresses through preneoplastic transformed follicles to metastatic lymphomas. We developed a chicken immune system cDNA microarray to analyze broad changes in gene expression that occur during normal embryonic B-cell development and during myc-induced neoplastic transformation in the bursa. The number of mRNAs showing at least 3-fold change was greater during myc-induced lymphomagenesis than during normal development, and hierarchical cluster analysis of expression patterns revealed that levels of several hundred mRNAs varied in concert with levels of myc overexpression. A set of 41 mRNAs were most consistently elevated in myc-overexpressing preneoplastic and neoplastic cells, most involved in processes thought to be subject to regulation by Myc. The mRNAs for another cluster of genes were overexpressed in neoplasia independent of myc expression level, including a small subset with the expression signature of embryonic bursal lymphocytes. Overexpression of myc, and some of the genes overexpressed with myc, may be important for generation of preneoplastic transformed follicles. However, expression profiles of late metastatic tumors showed a large variation in concert with myc expression levels, and some showed minimal myc overexpression. Therefore, high-level myc overexpression may be more important in the early induction of these lymphomas than in maintenance of late-stage metastases.

Angiogenesis is an early event in the generation of myc-induced lymphomas.

Angiogenesis was identified as an early consequence of myc gene overexpression in two models of retroviral lymphomagenesis. Avian leukosis virus (ALV) induces bursal lymphoma in chickens after proviral c-myc gene integration, while the HB-1 retrovirus carries a v-myc oncogene and also induces metastatic lymphoma. Immunohistochemical studies of the effects of increased c-myc or v-myc overexpression revealed early angiogenesis within myc-transformed bursal follicles, which persisted in lymphomas and metastases. Abnormal vessel growth was consistently detected within 13 days after transplantation of a few myc-overexpressing progenitors into ablated bursal follicles, suggesting that these angiogenic changes may support the initial expansion of tumor precursors, as well as later stage lymphomagenesis. Conditioned media from myc-overexpressing B cell lines promoted proliferation of vascular endothelium in vitro, while media from B cells expressing low myc levels showed little effect. Moreover, ectopic myc overexpression in the low myc B cell lines increased production of the endothelial growth activity, indicating that myc induces secretion of angiogenic factors from B cells. These findings demonstrate that myc overexpression in lymphocytes generates an angiogenic phenotype in vitro as well as in vivo. Oncogene (2000).

Resistance to avian leukosis virus lymphomagenesis occurs subsequent to proviral c-myc integration.

Most chicken strains are highly susceptible to avian leukosis virus (ALV) induction of bursal lymphoma, involving proviral integration within the c-myc proto-oncogene, while certain strains are genetically resistant to lymphomagenesis. A nested PCR assay was developed to analyse the appearance of proviral c-myc integrations after ALV infection of lymphoma-susceptible birds, and to determine whether these integrations arise in lymphoma-resistant birds. Proviral c-myc integrations are detected in bursa and other tissues from 6 day-old lymphoma-susceptible birds infected as embryos. The abundance of bursal cells carrying these integrations increases roughly 40-fold by 35 days of age, indicating that these cells hyperproliferate within the bursal environment. Bursal cells with proviral c-myc integrations also arise soon after infection of lymphoma-resistant embryos. However, these cells expand much more slowly than cells from lymphoma-susceptible birds. Both strains show the same rate of viral infection, so that resistance to lymphomagenesis occurs at a step subsequent to proviral c-myc integration. Proviral c-erbB gene integrations arise at the same frequency in bursa and other tissues of both strains, and they do not increase in abundance during development. These findings indicate that the mechanism of resistance to lymphomagenesis involves specific inhibition of cells with proviral c-myc integrations within the bursa.

Differential selection of cells with proviral c-myc and c-erbB integrations after avian leukosis virus infection.

Avian leukosis virus (ALV) infection induces bursal lymphomas in chickens after proviral integration within the c-myc proto-oncogene and induces erythroblastosis after integration within the c-erbB proto-oncogene. A nested PCR assay was used to analyze the appearance of these integrations at an early stage of tumor induction after infection of embryos. Five to eight distinct proviral c-myc integration events were amplified from bursas of infected 35-day-old birds, in good agreement with the number of transformed bursal follicles arising with these integrations. Cells containing these integrations are remarkably common, with an estimated 1 in 350 bursal cells having proviral c-myc integrations. These integrations were clustered within the 3' half of c-myc intron 1, in a pattern similar to that observed in bursal lymphomas. Bone marrow and spleen showed a similar number and pattern of integrations clustered within 3' c-myc intron 1, indicating that this region is a common integration target whether or not that tissue undergoes tumor induction. While all tissues showed equivalent levels of viral infection, cells with c-myc integrations were much more abundant in the bursa than in other tissues, indicating that cells with proviral c-myc integrations are preferentially expanded within the bursal environment. Proviral integration within the c-erbB gene was also analyzed, to detect clustered c-erbB intron 14 integrations associated with erythroblastosis. Proviral c-erbB integrations were equally abundant in the bone marrow, spleen, and bursa. These integrations were randomly situated upstream of c-erbB exon 15, indicating that cells carrying 3' intron 14 integrations must be selected during induction of erythroblastosis.

VBP and RelA regulate avian leukosis virus long terminal repeat-enhanced transcription in B cells.

The avian leukosis virus (ALV) long terminal repeat (LTR) contains a compact transcription enhancer that is active in many cell types. A major feature of the enhancer is multiple CCAAT/enhancer element motifs that could be important for the strong transcriptional activity of this unit. The contributions of the three CCAAT/enhancer elements to LTR function were examined in B cells, as this cell type is targeted for ALV tumor induction following integration of LTR sequences next to the c-myc proto-oncogene. One CCAAT/enhancer element, termed a3, was found to be the most critical for LTR enhancement in transiently transfected B lymphoma cells, while in chicken embryo fibroblasts all three elements contributed equally to enhancement. Gel shift assays demonstrated that vitellogenin gene-binding protein (VBP), a member of the PAR subfamily of C/EBP factors, is a major component of the nuclear proteins binding to the a3 CCAAT/enhancer element. VBP activated transcription through the a3 CCAAT/enhancer element, supporting the idea that VBP is important for LTR enhancement in B cells. A member of the Rel family of proteins was also identified as a component of the a3 protein binding complex in B cells. Gel shift and immunoprecipitation assays indicated that this factor is RelA. Gel shift assays demonstrated that while RelA does not bind directly to the LTR CCAAT/enhancer elements, it does interact with VBP to potentiate VBP DNA binding activity. The synergistic interaction of VBP and RelA increased CCAAT/enhancer element-mediated transcription, indicating that both factors may be important for viral LTR regulation and also for expression of many cellular genes.

The avian C/EBPgamma gene encodes a highly conserved leucine zipper transcription factor.

The C/EBP family of transcription factors regulates viral and cellular CCAAT/enhancer element-mediated transcription. We report the isolation and characterization of genomic and cDNA clones encoding avian CCAAT/enhancer-binding protein-gamma (C/EBP gamma). A partial cDNA clone for a C/EBP-related gene was previously identified by expression library screening for proteins binding the A1 CCAAT/enhancer motif from the avian leukosis virus long terminal repeat [W. Bowers and A. Ruddell (1992) J. Virol. 66, 6578-6586]. Additional cDNA and genomic clones were generated and sequenced to identify the complete protein coding sequence of this gene. Sequence analysis indicates that this gene encodes the avian homolog of C/EBP gamma. As with the murine C/EBP gamma homolog, the avian C/EBP gamma gene is comprised of two exons, with the open reading frame encoded in exon 2. The 150-aa C/EBP gamma protein is highly conserved, as the avian protein shows more than 80% identity with the murine and human homologs. The sequence of the initiation methionine (-3 caaAUGa +4) from the 150-aa open reading frame has a non-optimal Kozak initiation sequence. In vitro transcription and translation assay of this avian cDNA followed by radioimmunoprecipitation assay using a murine C/EBP gamma antiserum indicates that this non-optimal initiation codon is used to express a 22-kDa DNA-binding protein.

The VBP and a1/EBP leucine zipper factors bind overlapping subsets of avian retroviral long terminal repeat CCAAT/enhancer elements.

Two long terminal repeat (LTR) enhancer-binding proteins which may regulate high rates of avian leukosis virus (ALV) LTR-enhanced c-myc transcription during bursal lymphomagenesis have been identified (A. Ruddell, M. Linial, and M. Groudine, Mol. Cell. Biol. 9:5660-5668, 1989). The genes encoding the a1/EBP and a3/EBP binding factors were cloned by expression screening of a lambda gt11 cDNA library from chicken bursal lymphoma cells. The a1/EBP cDNA encodes a novel leucine zipper transcription factor (W. Bowers and A. Ruddell, J. Virol. 66:6578-6586, 1992). The partial a3/EBP cDNA clone encodes amino acids 84 to 313 of vitellogenin gene-binding protein (VBP), a leucine zipper factor that binds the avian vitellogenin II gene promoter (S. Iyer, D. Davis, and J. Burch, Mol. Cell. Biol. 11:4863-4875, 1991). Multiple VBP mRNAs are expressed in B cells in a pattern identical to that previously observed for VBP in other cell types. The LTR-binding activities of VBP, a1/EBP, and B-cell nuclear extract protein were compared and mapped by gel shift, DNase I footprinting, and methylation interference assays. The purified VBP and a1/EBP bacterial fusion proteins bind overlapping but distinct subsets of CCAAT/enhancer elements in the closely related ALV and Rous sarcoma virus (RSV) LTR enhancers. Protein binding to these CCAAT/enhancer elements accounts for most of the labile LTR enhancer-binding activity observed in B-cell nuclear extracts. VBP and a1/EBP could mediate the high rates of ALV and RSV LTR-enhanced transcription in bursal lymphoma cells and many other cell types.

Transcriptional regulation of ALV bursal lymphomagenesis.

Avian leukosis virus (ALV) induces bursal lymphoma in chickens after rare integration of proviral long terminal repeat (LTR) sequences next to the c-myc proto-oncogene. Labile LTR-binding proteins are essential for c-myc hyperexpression, since LTR-enhanced transcription, as well as the activity of the LTR-binding proteins, is specifically decreased following protein synthesis inhibition. This lability is restricted to hematopoetic cells of ALV-susceptible chicken strains, suggesting that the labile proteins play an important role in susceptibility to lymphomagenesis. Five proteins that interact with the ALV LTR enhancer were identified from bursal lymphoma cells: two proteins are stable (b and c), while three proteins (a1, a3, and b*) are labile in pre-B cell types. Two cDNAs (a1/EBP and a3/EBP) encoding leucine zipper proteins that bind to the ALV LTR enhancer were cloned using a lambda gt11 cDNA expression library made from bursal lymphoma cells. We are presently studying the roles of a1/EBP and a3/EBP in labile regulation of LTR-enhanced c-myc transcription and susceptibility to bursal lymphomagenesis.

a1/EBP: a leucine zipper protein that binds CCAAT/enhancer elements in the avian leukosis virus long terminal repeat enhancer.

Avian leukosis virus (ALV) induces bursal lymphoma in chickens after integration of proviral long terminal repeat (LTR) enhancer sequences next to the c-myc proto-oncogene. Labile LTR-binding proteins appear to be essential for c-myc hyperexpression, since both LTR-enhanced transcription and the activities of LTR-binding proteins are specifically decreased after inhibition of protein synthesis (A. Ruddell, M. Linial, W. Schubach, and M. Groudine, J. Virol. 62:2728-2735, 1988). This lability is restricted to hematopoietic cells from ALV-susceptible chicken strains, suggesting that the labile proteins play an important role in lymphomagenesis. The major labile activity binding to the a1 LTR region (A. Ruddell, M. Linial, and M. Groudine, Mol. Cell. Biol. 12:5660-5668, 1989) was purified from bursal lymphoma cells by conventional and oligonucleotide affinity chromatography, yielding three proteins of 35, 40, and 42 kDa. More than one of these species binds the a1 LTR region, as judged by gel shift analysis. A gene encoding an a1-binding protein (designated a1/EBP) was cloned by screening a bursal lymphoma cDNA library for fusion proteins binding the a1 LTR site. DNase I footprinting and gel shift assays indicate that the a1/EBP fusion protein binds multiple LTR CCAAT/enhancer elements in a pattern similar to that of the purified B-cell protein. DNA sequence analysis shows that this 2.2-kb cDNA encodes a 209-amino-acid open reading frame containing carboxy-terminal basic and leucine zipper motifs, indicating that a1/EBP encodes a novel member of the leucine zipper family of transcription factors.

A cytogenetic and genetic characterization of a group of closely linked second chromosome mutations that suppress position-effect variegation in Drosophila melanogaster.

Characterization of a group of dominant second chromosome suppressor of position-effect variegation (PEV) (Su(var)) mutants has revealed a variety of interesting properties, including: maternal-effect suppression of PEV, homozygous lethality or semilethality and male-specific hemizygous lethality, female infecundity, acute sensitivity to the amount of heterochromatin in the cell and sensitivity to sodium butyrate. Deficiency/duplication mapping and complementation tests have revealed that eight of the mutants define at least two genes in section 31 of the left arm of chromosome 2 and they suggest that a ninth corresponds to an additional nonessential Su(var) gene within or near this region. The effects of specific deficiencies and a duplication on PEV indicate that the expression of one or more of the Su(var) genes in this region of the chromosome is dose-dependent, i.e., capable of haplo-abnormal suppression and triplo-abnormal enhancement. Interestingly, the appearance of certain visible phenotypes among a subset of the mutants suggests that they may possess antimorphic properties. Our results are consistent with the hypothesis that two of these Su(var) genes encode structural components of heterochromatin. We also report that two previously isolated mutants located in 31E and 31F-32A act as recessive suppressors of PEV.

Chromosomal structure is altered by mutations that suppress or enhance position effect variegation.

We examined the genetic, morphological, and molecular effects of position effect variegation in Drosophila, and the effects of mutations that either suppress [Su(var)] or enhance [E(var)] this phenomenon. All eight Su(var) mutations examined strongly suppress the inactivation of variegating alleles of the genes white [In(l) wm4], brown [In(2R) bwVDe2] and Stubble [T(2; 3) SbV]. The E(var) mutation enhances variegation of these loci. The chromosomal region 3C-E (26 bands) which includes the white locus is usually packaged as heterochromatin in salivary glands of the variegating strain wm4. Addition of any of the Su(var) mutations restores a more euchromatic morphology to this region. In situ hybridization to polytene chromosomes and DNA blot analyses of gene copy number demonstrate that the DNA of the w+ gene is less accessible to its probe in the variegating wm4 strain than it is in the wild-type or variegation-suppressed strains. Blot analysis of larval salivary gland DNA indicates that the white gene copy number does not vary among the strains. Hence, the differences in binding of the w+ gene probe in the variegating and variegation-suppressed strains reflect differences in chromosomal packaging rather than alterations in gene number. The effects of variegation and the Su(var) mutations on chromatin structure were analyzed further by DNAse I digestion and DNA blot hybridization. In contrast to their dramatic effects on chromosomal morphology and gene expression, the Su(var) mutations had negligible effects on nuclease sensitivity of the white gene chromatin. We suggest that the changes in gene expression resulting from position effect variegation and the action of the Su(var) mutations involve alterations in chromosomal packaging.

Tissue-specific lability and expression of avian leukosis virus long terminal repeat enhancer-binding proteins.

Avian leukosis virus (ALV) induces bursal lymphomas in chickens, after proviral integration next to the cellular myc proto-oncogene, and subsequent c-myc hyperexpression. Our previous work suggested that labile or short-lived cellular proteins interact with the viral long terminal repeat (LTR) enhancer, and binding of these proteins appeared to be essential for high rates of LTR-enhanced transcription (A. Ruddell, M. Linial, W. Schubach, and M. Groudine, J. Virol. 62:2728-2735, 1988). This lability is specific for B-lymphoid cell types, since T cells and fibroblasts show stable high rates of LTR-enhanced transcription and stable LTR-binding activity. Moreover, the lability of these proteins may be important in determining susceptibility to bursal lymphoma. In this study, we separated and characterized the labile and stable LTR-binding proteins and examined their lability and expression in different cell types. Gel shift and DNase I footprinting analyses indicated that at least five proteins interact with the 140-base-pair LTR enhancer region. These proteins were distinct by several criteria, including lability or stability after inhibition of protein synthesis, resistance to heat denaturation, chromatographic behavior, and expression in different cell types. Two binding proteins were present in many cell types and were specifically labile in B cells. A third binding protein showed hematopoietic-cell-type-specific expression and was also labile in B cells. These findings indicate that there is tissue-specific modulation of the lability and expression of ALV LTR-binding proteins, which may be important for regulation of LTR transcription enhancement and ALV bursal lymphomagenesis.

Lability of leukosis virus enhancer-binding proteins in avian hematopoeitic cells.

Bursal lymphomas induced by avian leukosis virus (ALV) are characterized by integration of long terminal repeat (LTR) enhancer sequences next to the myc proto-oncogene and by subsequent myc hyperexpression. Nuclear runoff transcription analyses have shown that protein synthesis inhibition specifically decreases transcription of LTR-enhanced genes in bursal lymphoma cell lines (M. Linial, N. Gunderson, and M. Groudine, Science 230:1126-1132, 1985). Here, we show that LTR-enhanced transcription is also labile in nontransformed bursa, bone marrow, and spleen but not in other ALV-infected tissues from lymphoma-susceptible chickens. The bursal cells demonstrated this lability of LTR-enhanced transcription only at an early stage of development, when chickens are susceptible to ALV-induced lymphomagenesis. Mature bursal cells show stable LTR transcription enhancement (unaffected by inhibition of protein synthesis) and are not susceptible to lymphomagenesis. In lymphoma-resistant chicken strains, LTR-enhanced transcription was stable in all tissues during development. These data suggest that lability of LTR transcription enhancement in hematopoietic cells is involved in susceptibility to lymphomagenesis, and we propose a model for the action of these labile enhancing factors. Gel shift analysis of nuclear proteins from lymphoma cells indicated that four or more binding proteins specifically interact with the three LTR enhancer regions. These proteins can be separated by their differential sensitivity to heat treatment or protein synthesis inhibition. The lability of a subset of these binding proteins correlates with lability of LTR-enhanced transcription in certain lymphoid cell types, suggesting that these proteins are essential for LTR transcription enhancement.

Biphasic pattern of histone gene expression during Drosophila oogenesis.

The expression of histone genes during Drosophila oogenesis was compared to periods of DNA synthesis as well as to the pattern of actin gene expression. Accumulation of histone mRNAs was measured by RNA blot hybridization. Relatively low levels of histone mRNAs are present in egg chambers prior to stage 10, during the period of nurse and follicle cell polyploidization. Surprisingly, histone mRNAs accumulate rapidly and selectively after stage 10, coinciding with the onset of nurse cell degeneration and well after DNA synthesis and actin mRNA accumulation have ceased. A large proportion of the histone mRNAs is associated with polysomes at all times, indicating that expression of histone genes is not strictly coupled to DNA synthesis. The burst of histone mRNA accumulation near the end of oogenesis may provide a store of maternal histone mRNA to support the rapid cleavages that occur during early embryogenesis. These and previous results suggest that genes are independently regulated during differentiation of the Drosophila egg chamber.

Preferential expression of actin genes during oogenesis of Drosophila.

The expression of actin genes was examined during oogenesis of Drosophila. Accumulation of actin proteins was quantitated by a two-stage electrophoresis procedure. Egg chambers accumulate actins preferentially, resulting in a twofold enrichment over other nonyolk proteins. RNA gel blot hybridization experiments demonstrated a concomitant twofold selective increase of actin mRNA levels over that of other mRNAs, suggesting regulation of actin genes at the pretranslational level. Despite an abrupt arrest of actin protein accumulation near the end of oogenesis, the bulk of the actin mRNAs remains associated with polysomes of constant size. It appears that this shut-off of actin protein accumulation is due to an overall decrease in translational efficiency, rather than actin mRNA degradation or its dissociation from polysomes.