MOF maintains transcriptional programs regulating cellular stress response.

MOF (MYST1, KAT8) is the major H4K16 lysine acetyltransferase (KAT) in Drosophila and mammals and is essential for embryonic development. However, little is known regarding the role of MOF in specific cell lineages. Here we analyze the differential role of MOF in proliferating and terminally differentiated tissues at steady state and under stress conditions. In proliferating cells, MOF directly binds and maintains the expression of genes required for cell cycle progression. In contrast, MOF is dispensable for terminally differentiated, postmitotic glomerular podocytes under physiological conditions. However, in response to injury, MOF is absolutely critical for podocyte maintenance in vivo. Consistently, we detect defective nuclear, endoplasmic reticulum and Golgi structures, as well as presence of multivesicular bodies in vivo in podocytes lacking Mof following injury. Undertaking genome-wide expression analysis of podocytes, we uncover several MOF-regulated pathways required for stress response. We find that MOF, along with the members of the non-specific lethal but not the male-specific lethal complex, directly binds to genes encoding the lysosome, endocytosis and vacuole pathways, which are known regulators of podocyte maintenance. Thus, our work identifies MOF as a key regulator of cellular stress response in glomerular podocytes.

MOZ (MYST3, KAT6A) inhibits senescence via the INK4A-ARF pathway.

Cellular senescence is an important mechanism that restricts tumour growth. The Ink4a-Arf locus (also known as Cdkn2a), which encodes p16(INK4A) and p19(ARF), has a central role in inducing and maintaining senescence. Given the importance of cellular senescence in restraining tumour growth, great emphasis is being placed on the identification of novel factors that can modulate senescence. The MYST-family histone acetyltransferase MOZ (MYST3, KAT6A), first identified in recurrent translocations in acute myeloid leukaemia, has been implicated in both the promotion and inhibition of senescence. In this study, we investigate the role of MOZ in cellular senescence and show that MOZ is a potent inhibitor of senescence via the INK4A-ARF pathway. Primary mouse embryonic fibroblasts (MEFs) isolated from Moz-deficient embryos exhibit premature senescence, which was rescued on the Ink4a-Arf(-/-) background. Importantly, senescence resulting from the absence of MOZ was not accompanied by DNA damage, suggesting that MOZ acts independently of the DNA damage response. Consistent with the importance of senescence in cancer, expression profiling revealed that genes overexpressed in aggressive and highly proliferative cancers are expressed at low levels in Moz-deficient MEFs. We show that MOZ is required to maintain normal levels of histone 3 lysine 9 (H3K9) and H3K27 acetylation at the transcriptional start sites of at least four genes, Cdc6, Ezh2, E2f2 and Melk, and normal mRNA levels of these genes. CDC6, EZH2 and E2F2 are known inhibitors of the INK4A-ARF pathway. Using chromatin immunoprecipitation, we show that MOZ occupies the Cdc6, Ezh2 and Melk loci, thereby providing a direct link between MOZ, H3K9 and H3K27 acetylation, and normal transcriptional levels at these loci. This work establishes that MOZ is an upstream inhibitor of the INK4A-ARF pathway, and suggests that inhibiting MOZ may be one way to induce senescence in proliferative tumour cells.

Pro-apoptotic BIM is an essential initiator of physiological endothelial cell death independent of regulation by FOXO3.

The growth of new blood vessels by angiogenesis is essential for normal development, but can also cause or contribute to the pathology of numerous diseases. Recent studies have shown that BIM, a pro-apoptotic BCL2-family protein, is required for endothelial cell apoptosis in vivo, and can contribute to the anti-angiogenic effect of VEGF-A inhibitors in certain tumor models. Despite its importance, the extent to which BIM is autonomously required for physiological endothelial apoptosis remains unknown and its regulation under such conditions is poorly defined. While the transcription factor FOXO3 has been proposed to induce Bim in response to growth factor withdrawal, evidence for this function is circumstantial. We report that apoptosis was reduced in Bim(-/-) primary endothelial cells, demonstrating a cell-autonomous role for BIM in endothelial death following serum and growth factor withdrawal. In conflict with in vitro studies, BIM-dependent endothelial death in vivo did not require FOXO3. Moreover, endothelial apoptosis proceeded normally in mice lacking FOXO-binding sites in the Bim promoter. Bim mRNA was upregulated in endothelial cells starved of serum and growth factors and this was accompanied by the downregulation of miRNAs of the miR-17∼92 cluster. Bim mRNA levels were also elevated in miR-17∼92(+/-) endothelial cells cultured under steady-state conditions, suggesting that miR-17∼92 cluster miRNAs may contribute to regulating overall Bim mRNA levels in endothelial cells.

Consequences of the combined loss of BOK and BAK or BOK and BAX.

The multi-BCL-2 homology domain pro-apoptotic BCL-2 family members BAK and BAX have critical roles in apoptosis. They are essential for mitochondrial outer-membrane permeabilization, leading to the release of apoptogenic factors such as cytochrome-c, which promote activation of the caspase cascade and cellular demolition. The BOK protein has extensive amino-acid sequence similarity to BAK and BAX and is expressed in diverse cell types, particularly those of the female reproductive tissues. The BOK-deficient mice have no readily discernible abnormalities, and its function therefore remains unresolved. We hypothesized that BOK may exert functions that overlap with those of BAK and/or BAX and examined this by generating Bok(-/-)Bak(-/-) and Bok(-/-)Bax(-/-) mice. Combined loss of BOK and BAK did not elicit any noticeable defects, although it remains possible that BOK and BAK have critical roles in developmental cell death that overlap with those of BAX. In most tissues examined, loss of BOK did not exacerbate the abnormalities caused by loss of BAX, such as defects in spermatogenesis or the increase in neuronal populations in the brain and retina. Notably, however, old Bok(-/-)Bax(-/-) females had abnormally increased numbers of oocytes from different stages of development, indicating that BOK may have a pro-apoptotic function overlapping with that of BAX in age-related follicular atresia.

Breaking an absolute species barrier: transgenic mice expressing the mink PrP gene are susceptible to transmissible mink encephalopathy.

Transmissible mink encephalopathy (TME) is a rare disease of the North American mink, which has never been successfully transmitted to laboratory mice. We generated transgenic mice expressing the mink prion protein (PrP) and inoculated them with TME or the mouse-adapted scrapie strain 79A. TME infected mink PrP-transgenic mice on a murine PrP knockout background. The absolute species barrier between the infectious agent of TME and mice was therefore broken. Following TME and 79A infection of mice carrying both mink and murine PrP(C), only proteinase-resistant PrP homologous to the incoming agent was detectable. The presence of the murine PrP(C) prolonged the incubation time of TME substantially.

Purification of a pluripotent neural stem cell from the adult mouse brain.

The adult mammalian central nervous system (CNS) contains a population of neural stem cells (NSCs) with properties said to include the generation of non-neural progeny. However, the precise identity, location and potential of the NSC in situ remain unclear. We purified NSCs from the adult mouse brain by flow cytometry, and directly examined the cells' properties. Here we show that one type of NSC, which expresses the protein nestin but only low levels of PNA-binding and HSA proteins, is found in both ependymal and subventricular zones and accounts for about 63% of the total NSC activity. Furthermore, the selective depletion of the population of this stem cell in querkopf mutant mice (which are deficient in production of olfactory neurons) suggests that it acts as a major functional stem cell in vivo. Most freshly isolated NSCs, when co-cultured with a muscle cell line, rapidly differentiated in vitro into myocytes that contain myosin heavy chain (MyHC). This demonstrates that a predominant, functional type of stem cell exists in the periventricular region of the adult brain with the intrinsic ability to generate neural and non-neural cells.

The murine gene, Traube, is essential for the growth of preimplantation embryos.

Little is known about the genetic control of preimplantation development. We have isolated, characterized, and mutated a previously undescribed mouse gene, Traube (Trb), essential for preimplantation development. Similar protein coding sequences are found in rats, humans, and yeast. The TRB protein contained two amino-terminal acidic domains, a leucine zipper, and three putative nuclear localization signals. The Trb gene was expressed at low levels ubiquitously early in development and became restricted to the liver and the central nervous system from E11.5 onward. Myc-tagged TRB protein was localized to the nucleus, and in a large proportion of the cells to the nucleoli. The Trb mutant embryos halted in development at the compacted morula stage at E2.5. At E3.5 they started to decompact and a day later they disintegrated and died. The observed defect was cell autonomous, as mutant cells failed to participate in the formation of chimeric embryos. The Trb mutant embryos showed a 50% reduction of the total cell number. The mutant embryos exhibited a paucity of ribosomes, polyribosomes, and rough endoplasmic reticulum. This paucity of ribosomes together with the localization of TRB to the nucleoli, the site of ribosome synthesis, suggests that TRB is involved in the synthesis of ribosomes.

Taube nuss is a novel gene essential for the survival of pluripotent cells of early mouse embryos.

The cells of the inner cell mass constitute the pluripotent cell population of the early embryo. They have the potential to form all of the tissues of the embryo proper and some extra-embryonic tissues. They can be considered a transient stem cell population for the whole of the embryo, and stem cells maintaining the same capacity can be isolated from these cells. We have isolated, characterised and mutated a novel gene, taube nuss (Tbn), that is essential for the survival of this important cell population. The taube nuss protein sequence (TBN) was highly conserved between human, mouse, Xenopus laevis, Drosophila melanogaster, Caenorhabditis elegans and Arabidopsis thaliana, particularly in a domain that is not present in any published proteins, showing that TBN is the founding member of a completely new class of proteins with an important function in development. The Tbn gene was expressed ubiquitously as early as E2. 5 and throughout embryonic development. It was also expressed in adult brain with slightly higher levels in the hippocampus. The Tbn mutant embryos developed normally to the blastocyst stage and contained inner cell masses. They hatched from the zonae pellucidae, implanted and induced decidual reactions, but failed to develop beyond E4.0. At this time the trophoblast cells were viable, but inner cell masses were not detectable. At E3.75, massive TUNEL-positive DNA degradation and chromatin condensation were visible within the inner cell masses, whereas the cell membranes where intact. Caspase 3 was expressed in these cells. In vitro, the inner cell mass of mutant embryos failed to proliferate and died after a short period in culture. These results indicate that the novel protein, taube nuss, is necessary for the survival of the inner cell mass cells and that inner cell mass cells died of apoptosis in the absence of the taube nuss protein. As cell pruning by apoptosis is a recognised developmental process at this stage of development, the taube nuss protein may be one of the factors regulating the extent of programmed cell death at this time point.

Uncx4.1 is required for the formation of the pedicles and proximal ribs and acts upstream of Pax9.

The expression of the homeobox gene Uncx4.1 in the somite is restricted to the caudal half of the newly formed somite and sclerotome. Here we show that mice with a targeted mutation of the Uncx4.1 gene exhibit defects in the axial skeleton and ribs. In the absence of Uncx4.1, pedicles of the neural arches and proximal ribs are not formed. In addition, dorsal root ganglia are disorganized. Histological and marker analysis revealed that Uncx4.1 is not necessary for somite segmentation. It is required to maintain the condensation of the caudal half-sclerotome, from which the missing skeletal elements are derived. The loss of proximal ribs in Pax1/Pax9 double mutants and the data presented here argue for a role of Uncx4.1 upstream of Pax9 in the caudolateral sclerotome. Our results further indicate that Uncx4.1 may be involved in the differential cell adhesion properties of the somite.

Querkopf, a MYST family histone acetyltransferase, is required for normal cerebral cortex development.

In order to find, and mutate, novel genes required for regulation of neurogenesis in the cerebral cortex, we performed a genetic screen in mice. As the result of this screen, we created a new mouse mutant, querkopf. The querkopf mutation is due to an insertion into a MYST family histone acetyltransferase gene. Mice homozygous for the querkopf mutation have craniofacial abnormalities, fail to thrive in the postnatal period and have defects in central nervous system development. The defects in central nervous system development are particularly prominent in the cerebral cortex, which is disproportionally smaller than in wild-type mice. A large reduction in the size of the cortical plate was already apparent during embryogenesis. Homozygous mice show a lack of large pyramidal cells in layer V of the cortex, which is reflected in a reduction in the number of Otx1-positive neurons in this layer during postnatal development. Homozygous mice also show a reduction in the number of GAD67-positive interneurons throughout the cortex. Our results suggest that Querkopf is an essential component of a genetic cascade regulating cell differentiation in the cortex, probably acting in a multiprotein complex regulating chromatin structure during transcription.

Mice lacking HSP90beta fail to develop a placental labyrinth.

The 90 kDa heat-shock proteins (HSP90s) play important roles during stress situations as general chaperones and under physiological conditions in the conformational activation of specific protein substrates. Vertebrates express two cytosolic HSP90s (HSP90alpha and HSP90beta) ubiquitously. We have mutated the Hsp90beta gene in murine embryonic stem cells and generated Hsp90beta mutant mice. Heterozygous animals were phenotypically normal. Interestingly, homozygous embryos developed normally until embryonic day 9.0/9.5. Then, although Hsp90beta is expressed ubiquitously, they exhibited phenotypic abnormalities restricted to the placenta. The mutant concepti failed to form a fetal placental labyrinth and died a day later. Fusion between the allantois and the chorionic plate occurred, allantoic blood vessels invaded the chorion, but then did not expand. Mutant trophoblast cells failed to differentiate into trilaminar labyrinthine trophoblast. Despite conspicuous similarities between HSP90alpha and HSP90beta at the molecular level, our data suggest that HSP90beta has a key role in placenta development that cannot be performed by the endogenous HSP90alpha alone. Analysis of chimeric concepti consisting of mutant embryos and tetraploid embryos or ES cells revealed that wild-type allantois was able to induce mutant trophoblast to differentiate. In contrast, trophoblast wild type at the Hsp90beta locus was unable to differentiate when in contact with mutant allantois. Therefore, the primary defect caused by the Hsp90beta mutation resided in the allantois. The allantois mesoderm is thought to induce trophoblast differentiation. Our results show that Hsp90beta is a necessary component of this induction process.

Winged helix transcription factor Foxb1 is essential for access of mammillothalamic axons to the thalamus.

Our aim was to study the mechanisms of brain histogenesis. As a model, we have used the role of winged helix transcription factor gene Foxb1 in the emergence of a very specific morphological trait of the diencephalon, the mammillary axonal complex. Foxb1 is expressed in a large hypothalamic neuronal group (the mammillary body), which gives origin to a major axonal bundle with branches to thalamus, tectum and tegmentum. We have generated mice carrying a targeted mutation of Foxb1 plus the tau-lacZ reporter. In these mutants, a subpopulation of dorsal thalamic ventricular cells "thalamic palisade" show abnormal persistence of Foxb1 transcriptional activity; the thalamic branch of the mammillary axonal complex is not able to grow past these cells and enter the thalamus. The other two branches of the mammillary axonal complex (to tectum and tegmentum) are unaffected by the mutation. Most of the neurons that originate the mammillothalamic axons suffer apoptosis after navigational failure. Analysis of chimeric brains with wild-type and Foxb1 mutant cells suggests that correct expression of Foxb1 in the thalamic palisade is sufficient to rescue the normal phenotype. Our results indicate that Foxb1 is essential for diencephalic histogenesis and that it exerts its effects by controlling access to the target by one particular axonal branch.

A new gene trap construct enriching for insertion events near the 5' end of genes.

The gene trap approach is based on the integration of a gene trap vector into the genome. This can be done either by electroporation of a plasmid construct or by infection with a viral vector. Commonly used viral gene trap vectors have been shown to select for integrations near the 5' end of genes. To date, no plasmid vector with a similar tendency has been reported. In this paper we describe a new plasmid vector, pKC199beta geo. This vector contained a short splice acceptor fragment from the Hoxc9 gene, a full length lacZ gene, including an ATG, and a reduced activity, mutant neomycin phosphotransferase gene as a selectable marker. This vector enriched the population of trapped genes in our gene trap screen for insertion events in the 5' end of genes. In the two cases examined the beta-galactosidase activity pattern accurately reflected the endogenous promotor activity.

Efficiency assessment of the gene trap approach.

The trapping of genes in murine embryonic stem (ES) cells offers three features in one experimental approach: 1) analysis of the expression patterns of unknown genes by using a simple staining method, 2) rapid cloning of unknown genes, and 3) generation of mutant mouse lines. We performed a gene trap screen aimed at the discovery of new genes regulating embryonic development. We have processed 209 gene trap events for expression patterns in chimeric murine embryos. Randomly tested, beta-galactosidase-positive ES cell clones resulted in vivo in 35% gene trap events showing no beta-galactosidase activity, 39% gene trap events with ubiquitous beta-galactosidase activity, and 26% gene trap events showing beta-galactosidase activity restricted to specific cell types or organs. In vitro preselection reduced gene trap events with ubiquitous beta-galactosidase activity to 10% and increased the gene trap events with restricted beta-galactosidase activity to 64%, making the screening procedure for genes expressed in a restricted manner 2.5-fold more efficient. In five of the seven gene trap insertions into genes in which the expression pattern during embryogenesis was known, the beta-galactosidase marker gene reproduced faithfully the expression pattern of the trapped gene. 5'-Rapid amplification of cDNA ends (5'-RACE) of 28 gene trap events revealed 19 novel mouse genes, 8 known mouse genes, and 1 random transsplicing event. Twelve of the 25 mouse lines that crossed to homozygosity showed overt abnormalities. The genomic structure was investigated in four of these gene trap events, which caused obvious abnormalities. In all four cases, the splice-acceptor gene trap construct was inserted into an exon. One of the 13 gene trap events that did not result in overt abnormalities was examined for the presence of wild-type mRNA. Homozygous animals were found to produce normal levels of wild-type mRNA. Evidently, gene trapping does not always provide all three of the features mentioned above. In this paper, we discuss the efficiency of gene trapping and ways in which some problems may be overcome.

Distribution of a murine protein tyrosine phosphatase BL-beta-galactosidase fusion protein suggests a role in neurite outgrowth.

We have generated a gene trap insertion into the protein tyrosine phosphatase-BL (PTP-BL) locus, which produces a fusion of the N-terminal half of PTP-BL with beta-galactosidase. During development, beta-galactosidase activity was seen in all epithelial cells: strong staining was observed in the stomach and kidney epithelium, the ependymal layer of the central nervous system, and the surface ectoderm. Particularly prominent beta-galactosidase activity was seen in the peripheral nervous system, which correlated with neurite outgrowth. In epithelial cells, staining was seen in the apical portion of the cells. In nerves, beta-galactosidase activity was associated with growth cones as well as with Schwann cells. This suggests that the amino-terminal portion of PTP-BL contains sequences sufficient to target the fusion protein to specific subcellular compartments. In situ hybridization with a PTP-BL probe demonstrated that all tissues in which beta-galactosidase activity was seen were genuine sites of expression of the PTP-BL gene, although differences in the stability of the PTP-BL protein and the PTP-BL-beta-galactosidase fusion protein may exist. The distribution of beta-galactosidase activity in the peripheral nervous system, together with the structure of the wild-type protein, suggests that this phosphatase may have a role in regulation of the cytoskeleton during the development of the peripheral nervous system.

Compensation for a gene trap mutation in the murine microtubule-associated protein 4 locus by alternative polyadenylation and alternative splicing.

One of the features expected of the gene trap approach is the functional mutation of a gene, allowing its loss-of-function phenotype analysis. We have mutated the murine microtubule-associated protein 4 (MAP-4) locus by inserting a splice-acceptor gene trap construct. Because the MAP-4 gene has been cloned, sufficient information is available to investigate the efficiency of the gene trap insertion in disrupting the protein-coding region. The fusion mRNA contains the first 905 bases of the MAP-4 mRNA and is expected to code for a truncated, nonfunctional MAP-4 protein missing, among others, the microtubule-binding domain. Activity of the lacZ marker gene of the gene trap construct was observed in all tissues throughout development and in all cells examined in adult animals. However, some cells and tissues showed higher levels of activity than others: for example, blood vessel endothelium, heart, aspects of the developing nervous system, surface ectoderm of embryonic day 11.5 embryos, and the ependymal layer and blood vessel endothelium in adult brain. MAP-4 binds to microtubules and is thought to modulate their stability. It is expressed differentially in different tissues as 5.5-kb, 6.5-kb, 8-kb, 9-kb, and 10-kb mRNA species from a single copy gene in mice. Northern hybridization with a 5', MAP-4-specific probe revealed a 3.3-kb splice variant, which has not been described previously, that was expressed as the most abundant MAP-4 mRNA species in the brain but not in other tissues. Mice homozygous for the reported gene trap insertion in the MAP-4 locus (MAP-4gt/gt) are viable and appear to be phenotypically normal. They exhibited normal levels of all MAP-4 mRNA species in brain and kidney, showing that the simian virus 40-polyadenylation signal of the gene trap construct was ignored and also showing compensation for the gene trap insertion by splicing around the gene trap construct.

Germ line chimeras from female ES cells.

Murine embryonic stem cells (ES cells) are pluripotent cells that can contribute to all tissues of developing mice including the germ line when aggregated with 8-cell-stage embryos or injected into the blastocoel cavity of murine blastocysts. As well as for in vitro studies, they are used to introduce mutations into the murine genome, thereby producing new mutant mouse lines. All ES cell lines so far described that contribute to the germ line have been male. However, for many studies female ES cell lines may be essential. Female ES cells may be particularly useful for mutating genes located on the X chromosome which would otherwise be hemizygously lethal in males. We show here that female ES cells are able to form germ line chimeras and to sex convert a male host embryo.

Initiation in vitro of growth of bovine primordial follicles.

Factors that control the onset of primordial follicle growth are unknown. We have tested the hypothesis that primordial follicles from fetal calves can survive and initiate growth in vitro in serum-free conditions. Superficial pieces of ovarian cortex, containing mostly primordial follicles, were isolated from bovine fetuses 6-8 mo old and cultured for 0, 2, 4, or 7 days in Waymouth MB 752/1 medium supplemented with insulin, transferrin, selenium, linoleic acid, and BSA (ITS+). Histological examination of cortical pieces after 2, 4, and 7 days in culture showed that the number of healthy primordial follicles had decreased by 88%, 90%, and 94%, respectively (p < 0.01), whereas the number of healthy primary follicles had increased to 260%, 209%, and 197%, respectively, of the number present on Day 0 (p < 0.05). The percentage of follicles that showed signs of atresia did not change with time in culture and was about 28% and 50% for primordial and primary follicles, respectively. After 7 days in culture, the mean diameter of the few remaining healthy primordial follicles was 1.2 times the average diameter of primordial follicles present on Day 0 (p < 0.01). In contrast, after 2, 4, and 7 days in culture, primary follicles were 1.2, 1.3, and 1.4 times larger in diameter, respectively, relative to Day 0 (p < 0.01). There was little change in the diameter of oocytes in primordial follicles during culture, whereas in primary follicles an increase in oocyte diameter became apparent after 4 and 7 days (1.1 and 1.2 times, respectively, p < 0.01). That follicle growth was initiated in vitro was further confirmed by immunolocalization of proliferating cell nuclear antigen (PCNA), a marker for cell growth and proliferation, in cultured and freshly isolated pieces of ovarian cortex. In freshly isolated tissue, PCNA staining was absent from pre-granulosa cells and oocytes of the quiescent primordial follicles but was intense in granulosa cells and oocytes of the few growing primary follicles. After 2, 4, and 7 days in culture, PCNA was expressed intensely in the oocyte and many granulosa cells of newly activated primary follicles. These results demonstrate that bovine primordial follicles can enter the growth phase in vitro and that PCNA expression by granulosa cells and oocytes is closely associated with the onset of primordial follicle growth. The fact that a high percentage of primordial follicles initiated growth in vitro suggests that the ovarian stroma exerts inhibitory control over the initiation of primordial follicle growth in vivo. The culture system we describe may provide the means to test this hypothesis and others.

Senescence of aortic endothelial cells in culture: effects of basic fibroblast growth factor expression on cell phenotype, migration, and proliferation.

Bovine aortic endothelial cells (BAEC) can be isolated in large numbers without major contamination by other cells and maintained in culture with a limited life span for about 100 population doublings. In order to study phenotypic changes of BAEC during long-term culture, stocks of different passages of BAEC were established and their morphological, migratory, and proliferative properties analyzed. Early-passage BAEC (passages 5-15) rapidly produce dense, cobblestone-like monolayers. Their growth beyond the monolayer configuration is characterized by the formation of an irregular network of spindle-shaped, crisscrossing BAEC growing either on top or beneath the monolayer, and by the assembly of elongated BAEC into well-differentiated capillary-like tubes. In contrast, senescent BAEC (passages 35-45) form perfect cobblestone monolayers that contain several, often multinucleated giant cells and a few capillary-like tubes but not the crisscrossing networks of their early-passage counterparts. The rates of BAEC migration and proliferation gradually decline during in vitro senescence. This decline is neutralized by exogenous basic fibroblast growth factor (bFGF) which elevates the migratory and proliferative capacities of early-passage and senescent BAEC to uniformly high levels. Northern blot analysis shows a gradual decline in bFGF message and an increase in laminin message during in vitro BAEC senescence. The present study supports the concept of autocrine growth regulation of BAEC and associates a decreased bFGF message with decreased rates of migration and proliferation as well as loss of the crisscrossing BAEC morphotype in senescent cultures.