Expression of the growth arrest specific genes in rat embryonic fibroblasts undergoing senescence

Senescent vs. non-senescent cells in the human annulus in vivo: cell harvest with laser capture microdissection and gene expression studies with microarray analysis

BACKGROUND

Senescent cells are well-recognized in the aging/degenerating human disc. Senescent cells are viable, cannot divide, remain metabolically active and accumulate within the disc over time. Molecular analysis of senescent cells in tissue offers a special challenge since there are no cell surface markers for senescence which would let one use fluorescence-activated cell sorting as a method for separating out senescent cells.

METHODS

We employed a novel laser capture microdissection (LCM) design to selectively harvest senescent and non-senescent annulus cells in paraffin-embedded tissue, and compared their gene expression with microarray analysis. LCM was used to separately harvest senescent and non-senescent cells from 11 human annulus specimens.

RESULTS

Microarray analysis revealed significant differences in expression levels in senescent cells vs non-senescent cells: 292 genes were upregulated, and 321 downregulated. Genes with established relationships to senescence were found to be significantly upregulated in senescent cells vs. non-senescent cells: p38 (MPAK14), RB-Associated KRAB zinc finger, Discoidin, CUB and LCCL domain, growth arrest and DNA-damage inducible beta, p28ING5, sphingosine-1-phosphate receptor 2 and somatostatin receptor 3; cyclin-dependent kinase 8 showed significant downregulation in senescent cells. Nitric oxidase synthase 1, and heat shock 70 kDa protein 6, both of which were significantly down-regulated in senescent cells, also showed significant changes. Additional genes related to cytokines, cell proliferation, and other processes were also identified.

CONCLUSIONS

Our LCM-microarray analyses identified a set of genes associated with senescence which were significantly upregulated in senescent vs non-senescent cells in the human annulus. These genes include p38 MAP kinase, discoidin, inhibitor of growth family member 5, and growth arrest and DNA-damage-inducible beta. Other genes, including genes associated with cell proliferation, extracellular matrix formation, cell signaling and other cell functions also showed significant modulation in senescent vs non-senescent cells. The aging/degenerating disc undergoes a well-recognized loss of cells; understanding senescent cells is important since their presence further reduces the disc's ability to generate new cells to replace those lost to necrosis or apoptosis.

Increased cell senescence is associated with decreased cell proliferation in vivo in the degenerating human annulus

BACKGROUND CONTEXT

During disc degeneration, there is a well-recognized loss of cells. This puts the remaining cell population at high risk for any further decrease in cell function or cell numbers. Cell senescence has recently been shown to be present in the aging/degenerating human disc. Senescent cell are viable, metabolically active, persist, and accumulate over time, but cannot divide. Little is known about the relationship between renewal of the disc cell population via cell proliferation and disc cell senescence.

PURPOSE

To determine the percentage of senescent cells and proliferating cells in the human annulus in vivo.

STUDY DESIGN/SETTING

Human annulus specimens were obtained from surgical subjects and control donors in a study approved by the authors' Human Subjects Institutional Review Board.

PATIENT SAMPLE

One Thompson Grade I disc, 4 Grade II discs, 9 Grade III discs, and 12 Grade IV discs were studied.

OUTCOME MEASURES

The percentages of senescent cells and the percentage of proliferating cells.

METHODS

Immunohistochemistry was used to detect senescent cells using an antisenescence-associated beta-galactosidase antibody, and an antiproliferation antibody (Ki67). An average of 410 cells/specimens was counted to determine the percent senescence, and an average of 229 cells was counted to determine the percent proliferation.

RESULTS

Cell proliferation was low in both surgical and control normal donor annulus tissue (4.09%+1.77 (26), mean+SD (n)). There was no significant difference in the percentage of proliferating cells for more degenerate discs versus healthier discs (4.7%+1.6 (21) for Grades III and IV vs. 5.3%+1.9 (5) for Grades I and II). More degenerated Grades III and IV discs contained significantly greater percentages of senescent annulus cells than did the healthier Grades I and II discs (44.4%+20.0 (21) vs. 18.8%+11.0 (5), respectively; p=.011). A significant negative correlation was present between the percentage of senescent cells versus the percentage of proliferating cells, r=-0.013, p=.013. No correlation was present between age and the percentage of senescent cells or age and the percentage of proliferating cells.

CONCLUSIONS

Because senescent cells cannot divide, senescence may reduce the disc's ability to generate new cells to replace cells lost to necrosis or apoptosis. Senescent cells also accumulate in the disc over time, such that their metabolic patterns may contribute to the pathologic changes seen in degenerating discs. Novel data presented here show a significant negative correlation between the percentage of senescent cells and the percentage of proliferating cells during disc degeneration. Molecular work is underway in our lab to help us determine whether senescent cells in the disc secrete factors that can result in decreased proliferation in neighboring cells.

IGF-1 rescues human intervertebral annulus cells from in vitro stress-induced premature senescence

The aging human intervertebral disc contains a sizeable population of senescent cells. Since senescent cells cannot divide, senescence reduces the disc's ability to generate new cells to replace existing ones lost to necrosis or apoptosis. The objectives of the present work were: (1) to develop a reliable in vitro model for stress-induced premature senescence in human annulus cells, and (2) to investigate the potential for insulin-like growth factor-1 (IGF-1) to prevent or ameliorate senescence in vitro. The developed experimental model employs a 2 h exposure to 50 microM hydrogen peroxide; immunocytochemical localization of senescence associated-beta-galactosidase at pH 6.0 was used as the marker for senescent cells, and the percentage of senescent cells quantified after 3 days of culture. Nine sets of annulus cells were obtained from eight human surgical disc specimens; cells were tested with 0, 50, 100 or 500 ng/ml IGF-1. Although 50 or 100 ng/ml IGF-1 did not significantly alter the percentage of senescent cells, a significant reduction was present following exposure to 500 ng/ml IGF-1 (control, 56.3% +/- 8.5 (9); mean +/- SEM, (n) vs. treated, 39.6% +/- 6.6 (9), p = 0.0009). These novel findings point to the value of continued research towards development of future biologic therapies designed to reduce cell senescence in degenerating human discs.

Microarray analysis of laser capture microdissected-anulus cells from the human intervertebral disc

STUDY DESIGN

Five Thompson Grade I/II discs (Group 1), 7 Grade III discs (Group 2), and 3 Grade IV discs (Group IV) were studied here in a project approved by the authors' Human Subjects Institutional Review Board.

OBJECTIVES

Our objective was to use laser capture microdissection (LCM) to harvest cells from the human anulus and to derive gene expression profiles using microarray analysis.

SUMMARY OF BACKGROUND DATA

Appropriate gene expression is essential in the intervertebral disc for maintenance of extracellular matrix (ECM), ECM remodeling, and maintenance of a viable disc cell population. During disc degeneration, cell numbers drop, making gene expression studies challenging.

METHODS

LCM was used to harvest cells from paraffin-embedded sections of human anulus tissue. Gene profiling used Affymetrix GeneChip Human X3P arrays. ANOVA and SAM permutation analysis were applied to dCHIP normalized, filtered, and log-transformed gene expression data ( approximately 33,500 probes), and data analyzed to identify genes that were significantly differentially expressed between the 3 groups.

RESULTS

We identified 47 genes that were significantly differentially expressed between the 3 groups (P < 0.001 and lowest q values). Compared with the healthiest discs (Grade I/II), 13 genes were up-regulated and 19 down-regulated in both the Grade III and the Grade IV discs. Genes with biologic significance regulated during degeneration involved cell senescence, low cell division rates, hypoxia-related genes, heat-shock protein 70 interacting protein, neuropilin 2, and interleukin-23p19 (interleukin-12 family).

CONCLUSIONS

Results expand our understanding of disc aging and degeneration and show that LCM is a valuable technique that can be used to collect mRNA amounts adequate for microarray analysis from the sparse cell population of the human anulus.

Newt orthologue ofGrowth arrest-specific 6 (NvGas6) is implicated in stress response during newt forelimb regeneration

Red-spotted newts are capable of regenerating various structures and organs through the process of epimorphic regeneration. Receptor tyrosine kinases (RTKs) and their ligands are important for normal cellular development and physiology but most have not yet been characterised during regeneration. We have isolated a newt orthologue of Growth arrest-specific 6 (NvGas6), and examined its expression during forelimb regeneration and within a blastema cell line (B1H1). During limb regeneration, NvGas6 expression increases upon amputation, peaks during maximal blastema cell proliferation, and is subsequently downregulated during redifferentiation. Transcripts are localised to the wound epithelium and distal mesenchymal cells during dedifferentiation and proliferative phases, and scattered within redifferentiating tissues during later stages. In B1H1 cultures, NvGas6 is upregulated under reduced serum conditions and myogenesis. Treatment with mimosine and colchicine or exposure to heat shock or anoxia results in upregulation of NvGas6 expression. Taken together, our findings suggest that during regeneration, NvGas6 expression may be upregulated in response to cellular stress.

Posttranscriptional and posttranslational regulation of C/EBP delta in G0 growth-arrested mammary epithelial cells

Previous work from our laboratory demonstrated that CCAAT/enhancer-binding protein delta (C/EBP delta) functions in the initiation and maintenance of G(0) growth arrest in mouse mammary epithelial cells (MECs). In this report, we investigated the posttranscriptional and posttranslational regulation of C/EBP delta in G(0) growth-arrested mouse MECs. The results of transcriptional inhibitor studies demonstrated that the C/EBP delta mRNA exhibits a relatively short half-life in G(0) growth-arrested mouse MECs (t(1/2) approximately 35 min). In contrast, C/EBP delta mRNA has a longer half-life in G(0) growth-arrested mouse fibroblast cells (t(1/2) >100 min). Oligo/RNase H cleavage analysis and rapid amplification of cDNA ends-poly(A) test both confirmed the short C/EBP delta mRNA half-life observed in MECs and demonstrated that the C/EBP delta mRNA poly(A) tail is relatively short (approximately 100 nucleotides). In addition, the poly(A) tail length was not shortened during C/EBP delta mRNA degradation, which suggested a deadenylation-independent pathway. The C/EBP delta protein also exhibited a relatively short half-life in G(0) growth-arrested mouse MECs (t(1/2) approximately 120 min). The C/EBP delta protein was degraded in a ubiquitin-dependent manner, primarily in the nucleus, during G(0) growth arrest. In conclusion, these studies indicated that the C/EBP delta mRNA and protein content are under tight regulation in G(0) growth-arrested mouse MECs, despite the general concept that G(0) growth arrest is associated with a decrease in cellular activity.

Customized rapid subtraction hybridization (RaSH) gene microarrays identify overlapping expression changes in human fetal astrocytes resulting from human immunodeficiency virus-1 infection or tumor necrosis factor-alpha treatment

Genes displaying altered expression as a function of human immunodeficiency virus (HIV)-1 infection of cultured primary human fetal astrocytes (PHFA) were previously identified using a rapid subtraction hybridization (RaSH) method. This scheme identified both known and novel genes displaying elevated expression, astrocyte elevated genes (AEG), and decreased expression, astrocyte suppressed genes (ASG), in PHFA as a consequence of infection with HIV-1 or treatment with HIV-1 envelope glycoprotein (gp120). RaSH also identified both known and novel genes displaying enhanced (HR) or reduced (HS) expression in HIV-1 resistant versus HIV-1 susceptible human T-cell clones. In the present study, a customized microarray approach employing these RaSH-derived genes was used to distinguish overlapping gene expression changes occurring in PHFA as a function of treatment with HIV-1 and the neurotoxic agent tumor necrosis factor (TNF)-alpha. RaSH cDNAs were spotted (microarrayed) on nylon membranes and probed with temporally isolated reverse transcribed cDNAs from HIV-1-infected and TNF-alpha-treated PHFA. This strategy identified genes displaying parallel changes after TNF-alpha treatment as observed following HIV-1 infection. Confirmation of genuine differential expression was achieved by Northern blotting. These studies document that TNF-alpha can induce a set of corresponding changes in specific AEGs and ASGs as does HIV-1 infection in PHFA. Furthermore, this customized microarray approach with RaSH-derived clones represents an efficient and sensitive methodology for elucidating molecular changes in PHFA occurring as a consequence of treatment with pharmacological agents affecting astrocyte physiology.

Identification and cloning of human astrocyte genes displaying elevated expression after infection with HIV-1 or exposure to HIV-1 envelope glycoprotein by rapid subtraction hybridization, RaSH

Neurodegeneration and dementia are common complications of AIDS caused by human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system. HIV-1 target cells in the brain include microglia, infiltrating macrophages and astrocytes, but rarely neurons. Astrocytes play an important role in the maintenance of the synaptic micro-environment and in neuronal signal transmission. To investigate potential changes in cellular gene expression associated with HIV-1 infection of astrocytes, we employed an efficient and sensitive rapid subtraction hybridization approach, RaSH. Primary human astrocytes were isolated from abortus brain tissue and low-passage cells were infected with HIV-1. To identify genes that display both early and late expression modifications after HIV-1 infection and to avoid cloning genes displaying normal cell cycle fluctuations in astrocytes, RNAs were isolated and pooled from 6, 12, 24 h and 3 and 7 day uninfected and infected cells and used for RaSH. Temporal cDNA libraries were prepared from double-stranded cDNAs that were enzymatically digested into small fragments, ligated to adapters, PCR amplified, and hybridized by incubation of tester and driver PCR fragments. By subtracting temporal cDNAs derived from uninfected astrocytes from temporal cDNAs made from HIV-1 infected cells, genes displaying elevated expression in virus infected cells, termed astrocyte elevated genes (AEGs), were identified. Both known and novel AEGs, not reported in current DNA databases, are described that display early or late expression kinetics following HIV-1 infection or treatment with recombinant HIV-1 envelope glycoprotein (gp120). For selected AEGs, expression of their protein products was also tested by Western blotting and found to display elevated expression following HIV-1 infection. The comparable pattern of regulation of the AEGs following HIV-1 infection or gp120 treatment suggest that HIV-1 exposure of astrocytes, even in the absence of productive infection, can induce changes in cellular gene expression.

COP-1, a member of the CCN family, is a heparin-induced growth arrest specific gene in vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) hyperplasia is responsible for the failure of 15-30% of vascular surgical procedures such as coronary artery bypass grafts and angioplasties. We and others have shown that heparin suppresses VSMC proliferation in vivo and in cell culture. We hypothesize that heparin inhibits VSMC proliferation by binding to cell surface receptors, resulting in selective modulation of mitogenic signal transduction pathways and altered transcription of a specific subset of growth regulatory genes. To test this idea, we used subtractive hybridization to identify differentially expressed mRNAs in heparin-treated and untreated VSMC. We identified a heparin induced mRNA identical to Cop-1, a member of the CCN family of proteins which are secreted, cysteine-rich modular proteins involved in growth regulation and migration. Cop-1 from smooth muscle cells appears to have a different expression pattern and possibly different functions than Cop-1 from other cells. Cop-1 mRNA is expressed at high levels in quiescent VSMC and at low levels in proliferating VSMC, an expression pattern highly characteristic of growth arrest specific genes. Cop-1 mRNA is expressed at high levels in heparin treated VSMC and COP-1 protein is secreted into culture medium. In tissues, Cop-1 expression is observed in the uninjured rat aorta suggesting a possible role for Cop-1 in vivo. We found PDGF, but not EGF, inhibits the expression of Cop-1 in VSMC. Neither TGF-beta nor interferon-beta, two inhibitors of VSMC proliferation, were able to induce Cop-1 expression. In addition, heparin does not induce Cop-1 mRNA in endothelial cells and VSMC resistant to the antiproliferative effect of heparin. Conditioned medium from cells over-expressing COP-1 protein inhibits VSMC proliferation in culture. Together, our data indicate that COP-1 may play a role in the antiproliferative mechanism of action of heparin.

Temporal expression of neuronal connexins during hippocampal ontogeny

Communication through gap junction channels provides a major signaling mechanism during early brain histogenesis, a developmental time during which neural progenitor cells are inexcitable and do not express ligand-gated channel responses to the major CNS neurotransmitters. Expression of different gap junction types during neurogenesis may therefore define intercellular pathways for transmission of developmentally relevant molecules. To better understand the molecular mechanism(s) by which growth and differentiation of neurons are modulated by gap junction channels, we have been examining the developmental effects of a specific set of cytokines on differentiation and gap junction expression in a conditionally immortalized mouse embryonic hippocampal neuronal progenitor cell line (MK31). When multipotent MK31 cells are in an uncommitted state, they uniformly express the neuroepithelial intermediate filament class VI marker, nestin, are strongly coupled by gap junctions composed of connexin43 (Cx43) and express connexin45 (Cx45) at the mRNA level. As these cells undergo neuronal lineage commitment and exit from cell cycle, they begin to express the early neurofilament marker, NF66, and coupling strength and expression of Cx43 begin to decline with concurrent expression of other connexin proteins, including Cx26, Cx33, Cx36, Cx40 and Cx45. Terminal neuronal differentiation is heralded by the expression of more advanced neurofilament proteins, increased morphologic maturation, the elaboration of inward currents and action potentials that possess mature physiological properties, and changing profiles of expression of connexin subtypes, including upregulation of Cx36 expression. These important developmental transitions are regulated by a complex network of cell cycle checkpoints. To begin to examine the precise roles of gap junction proteins in traversing these developmental checkpoints and in thus regulating neurogenesis, we have focused on individual members of two classes of genes involved in these seminal events: ID (inhibitor of differentiation)-1 and GAS (growth arrest-specific gene)5. When MK31 cells were maintained in an uncommitted state, levels of ID-1 mRNA were high and GAS5 transcripts were essentially undetectable. Application of cytokines that promote neuronal lineage commitment and cell cycle exit resulted in down-regulation of ID-1 and upregulation of GAS5 transcripts, whereas additional cytokine paradigms that promoted terminal neuronal differentiation resulted in the delayed down-regulation of GAS5 mRNA. Stable MK31 transfectants were generated for ID-1 and GAS5. In basal conditions, cellular proliferation was enhanced in the ID-1 transfectants and inhibited in the GAS5 transfectants when compared with control MK31 cells. When cytokine-mediated neurogenesis was examined in these transfected cell lines, constitutive expression of ID-1 inhibited and constitutive expression of GAS5 enhanced initial and terminal stages of neuronal differentiation, with evidence that terminal neuronal maturation in both transfectant lines was associated with decreased cellular viability, possibly due to the presence of conflicting cell cycle-associated developmental signals. These experimental reagents will prove to be valuable experimental tools to help define the functional interrelationships between changing profiles of connexin protein expression and cell cycle regulation during neuronal ontogeny in the mammalian brain. The present review summarizes the current state of research involving the temporal expression of such connexin types in differentiating hippocampal neurons and speculates on the possible role of these intercellular channels in the development and plasticity of the nervous system. In addition, we describe the functional properties and expression pattern of the newly discovered neuronal-specific gap junctional protein, Cx36, in the developing mouse fetal hippocampus and in the rat retina and brain.