Role of epithelial-mesenchymal transition in repair of the lacrimal gland after experimentally induced injury.

PURPOSE: Ongoing studies demonstrate that the murine lacrimal gland is capable of repair after experimentally induced injury. It was recently reported that repair of the lacrimal gland involved the mobilization of mesenchymal stem cells (MSCs). These cells expressed the type VI intermediate filament protein nestin whose expression was upregulated during the repair phase. The aim of the present study was to investigate the roles of vimentin, a type III intermediate filament protein and a marker of epithelial-mesenchymal transition (EMT) in repair of the lacrimal gland. METHODS: Injury was induced by direct injection of interleukin (IL)-1 into the exorbital lacrimal gland. MSCs were prepared from injured glands using tissue explants. Expression of vimentin and the transcription factor Snai1, a master regulator of EMT, was determined by RT-PCR, Western blotting analysis, and immunofluorescence. RESULTS: These data show that vimentin expression, at both the mRNA and the protein levels, was upregulated during the repair phase (2-3 days postinjury) and returned to the control level when repair ended. Temporal expression of Snai1 mirrored that of vimentin and was localized in cell nuclei. Cultured MSCs isolated from injured lacrimal glands expressed Snai1 and vimentin alongside nestin and alpha smooth muscle actin (another biomarker of EMT). There was a strong positive correlation between Snai1 expression and vimentin expression. CONCLUSIONS: It was found that EMT is induced during repair of the lacrimal gland to generate MSCs to initiate repair, and that mesenchymal-epithelial transition is then activated to form acinar and ductal epithelial cells.

Isolation and propagation of mesenchymal stem cells from the lacrimal gland.

PURPOSE: Previously, it was reported that the murine lacrimal gland is capable of repair after experimentally induced injury and that the number of stem/progenitor cells was increased during the repair phase (2-3 days after injury). The aim of the present study was to determine whether these cells can be isolated from the lacrimal gland and propagated in vitro. METHODS: Lacrimal gland injury was induced by injection of interleukin (IL)-1, and injection of saline vehicle served as control. Two and half days after injection, the lacrimal glands were removed and used to prepare explants or acinar cells for tissue culture. Cells derived from the explants and the acinar cells were grown in DMEM supplemented with 10% fetal bovine serum. Cells were stained for the stem cells markers, nestin, vimentin, ABCG2, and Sca-1. Cell proliferation was measured using an antibody against Ki67 or a cell-counting kit. The adipogenic capability of these cells was also tested in vitro. RESULTS: Results show that nestin-positive cells can be isolated from IL-1-injected, but not saline-injected, lacrimal glands. A population of nestin-positive cells was also positive for vimentin, an intermediate filament protein expressed by mesenchymal cells. In addition, cultured cells expressed two other markers of stem cells, ABCG2 and Sca-1. These cells proliferated in vitro and can be induced to form adipocytes, attesting to their mesenchymal stem cell property. CONCLUSIONS: Murine lacrimal glands contain mesenchymal stem cells that seem to play a pivotal role in tissue repair.

Perturbing the ubiquitin pathway reveals how mitosis is hijacked to denucleate and regulate cell proliferation and differentiation in vivo.

BACKGROUND: The eye lens presents a unique opportunity to explore roles for specific molecules in cell proliferation, differentiation and development because cells remain in place throughout life and, like red blood cells and keratinocytes, they go through the most extreme differentiation, including removal of nuclei and cessation of protein synthesis. Ubiquitination controls many critical cellular processes, most of which require specific lysines on ubiquitin (Ub). Of the 7 lysines (K) least is known about effects of modification of K6. METHODOLOGY AND PRINCIPAL FINDINGS: We replaced K6 with tryptophan (W) because K6 is the most readily modified K and W is the most structurally similar residue to biotin. The backbone of K6W-Ub is indistinguishable from that of Wt-Ub. K6W-Ub is effectively conjugated and deconjugated but the conjugates are not degraded via the ubiquitin proteasome pathways (UPP). Expression of K6W-ubiquitin in the lens and lens cells results in accumulation of intracellular aggregates and also slows cell proliferation and the differentiation program, including expression of lens specific proteins, differentiation of epithelial cells into fibers, achieving proper fiber cell morphology, and removal of nuclei. The latter is critical for transparency, but the mechanism by which cell nuclei are removed has remained an age old enigma. This was also solved by expressing K6W-Ub. p27(kip), a UPP substrate accumulates in lenses which express K6W-Ub. This precludes phosphorylation of nuclear lamin by the mitotic kinase, a prerequisite for disassembly of the nuclear membrane. Thus the nucleus remains intact and DNAseIIß neither gains entry to the nucleus nor degrades the DNA. These results could not be obtained using chemical proteasome inhibitors that cannot be directed to specific tissues. CONCLUSIONS AND SIGNIFICANCE: K6W-Ub provides a novel, genetic means to study functions of the UPP because it can be targeted to specific cells and tissues. A fully functional UPP is required to execute most stages of lens differentiation, specifically removal of cell nuclei. In the absence of a functional UPP, small aggregate prone, cataractous lenses are formed.

Expression and characterization of the integrase of bovine immunodeficiency virus.

In the pol gene of bovine immunodeficiency virus (BIV) there is a sequence, located between the reverse-transcriptase and integrase (IN)-encoding sequences, that is not found in HIV-1 pol gene and encodes a 74-residue polypeptide with homology to dUTPases. We have expressed two BIV IN versions that differ in their amino termini. The longer version, containing the 74-residue sequence, did not show any detectable 3'-end processing and strand transfer IN activities and performed only the IN-associated disintegration. Consequently, the shorter version, lacking the dUTPase-related residues, performed all three activities and is most likely similar to the viral enzyme. A comparison between BIV IN and the well-studied HIV-1 IN, with substrates that mimic the U5 LTR sequences of BIV, HIV-1 and another bovine lentivirus, Jembrana disease virus, revealed that the extra 3'-end sequence beyond the conserved "CA" is probably less important for IN activities than the sequence upstream to the "CA".

The catalytic properties of the recombinant reverse transcriptase of bovine immunodeficiency virus.

Bovine immunodeficiency virus (BIV) is a lentivirus with no proven pathogenesis in infected cattle. Yet, in experimentally infected rabbits, it causes an AIDS-like disease. Consequently, we expressed two recombinant isoforms of BIV reverse transcriptase (RT), which differ in their C-termini, and studied their catalytic properties. Both isoforms prefer Mg(+2) over Mn(+2) with most DNA polymerase and ribonuclease-H substrates. The processivity of DNA synthesis by the BIV RTs is higher than that of HIV-1 RT, whereas the fidelity of synthesis is even lower than that of the HIV-1 enzyme. The ribonuclease-H cleavage pattern suggests that the spatial distance between the polymerase and ribonuclease-H active sites of the two BIV RT isoforms equals 20 nt, unlike the 17 nt distance observed in almost all other RTs. The longer BIV RT version is somewhat less active than the shorter version, suggesting that the extra 74 residues (with homology to dUTPases) might obstruct efficient catalysis.

Peptides derived from the reverse transcriptase of human immunodeficiency virus type 1 as novel inhibitors of the viral integrase.

Recent studies have shown that the integrase (IN) of HIV-1 is inhibited in vitro by HIV-1 reverse transcriptase (RT). We further investigated the specific protein sequences of RT that were involved in this inhibition by screening a complete library of RT-derived peptides for their inhibition of IN activities. Two 20-residue peptides, peptide 4286, derived from the RT DNA polymerase domain, and the one designated 4321, from the RT ribonuclease H domain, inhibit the enzymatic activities of IN in vitro. The former peptide inhibits all three IN-associated activities (3'-end processing, strand transfer, and disintegration), whereas the latter one inhibits primarily the first two functions. We showed the importance of the sequences and peptide length for the effective inhibition of IN activities. Binding assays of the peptides to IN (with no DNA substrate present) indicated that the two inhibitory peptides (as well as several non-inhibitory peptides) interact directly with IN. Moreover, the isolated catalytic core domain of IN also interacted directly with the two inhibitory peptides. Nevertheless, only peptide 4286 can inhibit the disintegration activity associated with the IN core domain, because this activity is the only one exhibited by this domain. This result was expected from the lack of inhibition of disintegration of full-length IN by peptide 4321. The data and the three-dimensional models presented suggested that the inhibition resulted from steric hindrance of the catalytic domain of IN. This information can substantially facilitate the development of novel drugs against HIV INs and thus contribute to the fight against AIDS.

The mature reverse transcriptase molecules in virions of mouse mammary tumor virus possess protease-derived sequences.

Our efforts to express in bacteria the enzymatically active reverse transcriptase (RT) of mouse mammary tumor virus (MMTV) have shown that the RT is active only after adding 27 amino acid residues, which are derived from the end of the pro gene, to the amino-terminus of the RT (Biochem, J. (1998) 329, 579-587). In the present study we have tested whether the mature RT found in virions is also fused to protease-derived sequences. To this end, we have analyzed the RT molecules in virions of MMTV by using two antisera directed against peptides, derived from either the carboxyl-terminus of MMTV protease or the middle of MMTV RT. The data suggest that the mature RT, located in virions, contains at its amino-terminus sequences from the carboxyl-terminus of the protease protein. This finding supports previous suggestions that MMTV RT is a transframe protein (derived from both pro and pol reading frames of MMTV) and that amino acid residues located at the carboxyl-terminus of the protease have a dual usage as integral parts of both the protease and the RT enzymes.

Expression and characterization of a recombinant novel reverse transcriptase of a porcine endogenous retrovirus.

The study of porcine endogenous retroviruses (PERVs) becomes increasingly important due to the potential use of pig cells, tissues, and organs as a source for xenogenic cell therapy and xenotransplantation into humans. Consequently, we have constructed a plasmid that induces in bacteria the synthesis of a soluble and highly active reverse transcriptase (RT) of PERV-B. The purified PERV RT was studied biochemically in comparison with the RT of murine leukemia virus (MLV), because of the high-sequence homology between these two RTs. The data show that in several properties the two enzymes are similar, particularly regarding the monomeric subunit composition of the proteins in solution, the high resistance to deoxynucleoside analogues, and the pattern of RNA cleavage by the ribonuclease H activity (RNase H) of the RTs. However, in several cases there are apparent differences between the two RTs, most notable the divalent cation preference (Mn(+2) versus Mg(+2)) in the DNA polymerase reactions. As already shown for viral PERV RT, the novel recombinant PERV RT exhibits a relatively high resistance to several deoxynucleoside analogue inhibitors, suggesting that they might not be very efficient in inhibiting the replication of PERV virions. Therefore, the availability of large amounts of the recombinant RT can be useful for a wide screening of novel drugs against infectious PERV.

The processivity and fidelity of DNA synthesis exhibited by the reverse transcriptase of bovine leukemia virus.

We have recently expressed in bacteria the enzymatically active reverse transcriptase (RT) of bovine leukemia virus (BLV) [Perach, M. & Hizi, A. (1999) Virology 259, 176-189]. In the present study, we have studied in vitro two features of the DNA polymerase activity of BLV RT, the processivity of DNA synthesis and the fidelity of DNA synthesis. These properties were compared with those of the well-studied RTs of human immunodeficiency virus type 1 (HIV-1) and murine leukaemia virus (MLV). Both the elongation of the DNA template and the processivity of DNA synthesis exhibited by BLV RT are impaired relative to the other two RTs studied. Two parameters of fidelity were studied, the capacity to incorporate incorrect nucleotides at the 3' end of the nascent DNA strand and the ability to extend these 3' end mispairs. BLV RT shows a fidelity of misinsertion higher than that of HIV-1 RT and lower than that of MLV RT. The pattern of mispair elongation by BLV RT suggests that the in vitro error proneness of BLV RT is closer to that of HIV-1 RT. These fidelity properties are discussed in the context of the various retroviral RTs studied so far.

Inhibition of the integrases of human immunodeficiency viruses type 1 and type 2 by reverse transcriptases.

We present evidence that the integrases (INs) of HIV types 1 and 2 are inhibited in vitro by the reverse transcriptases (RTs) of HIV-1, HIV-2 and murine leukaemia virus. Both 3'-end processing and 3'-end joining (strand transfer) activities of IN were affected by the RTs. Full inhibitions were accomplished with most RT and IN combinations tested at around equimolar RT/IN ratios. The disintegration activity of IN was also inhibited by RTs. Neither DNA synthesis nor the ribonuclease H (RNase H) domain of RT were involved in IN inhibition, since specific DNA polymerase inhibitors did not affect the level of IN inhibition, and the p51 isoform of HIV-1 RT (which lacks the RNase H domain) is as effective in inhibiting IN as the heterodimeric p66/p51 isoform. On the other hand, the catalytic activities of HIV RTs were not affected by the INs, showing that RTs can inhibit IN activities, whereas INs do not inhibit RTs. We postulate that sequences and/or three-dimensional protein structures common to RTs interact with INs and inhibit their activities. We show evidence for this hypothesis and discuss the possible sites of IN involved in this interaction.