Stress proteins accumulate in cultured retinal glial cells during herpes simplex viral infection

Retinal microglia - A key player in healthy and diseased retina

Microglia, the resident immune cells of the brain and retina, are constantly engaged in the surveillance of their surrounding neural tissue. During embryonic development they infiltrate the retinal tissues and participate in the phagocytosis of redundant neurons. The contribution of microglia in maintaining the purposeful and functional histo-architecture of the adult retina is indispensable. Within the retinal microenvironment, robust microglial activation is elicited by subtle changes caused by extrinsic and intrinsic factors. When there is a disturbance in the cell-cell communication between microglia and other retinal cells, for example in retinal injury, the activated microglia can manifest actions that can be detrimental. This is evidenced by activated microglia secreting inflammatory mediators that can further aggravate the retinal injury. Microglial activation as a harbinger of a variety of retinal diseases is well documented by many studies. In addition, a change in the microglial phenotype which may be associated with aging, may predispose the retina to age-related diseases. In light of the above, the focus of this review is to highlight the role played by microglia in the healthy and diseased retina, based on findings of our own work and from that of others.

Generation and Characterization of UL21-Null Herpes Simplex Virus Type 1

UL21 of herpes simplex virus type 1 (HSV-1) is an accessory gene that encodes a component of the tegument. Homologs of this protein have been identified in the alpha, beta, and gamma herpesvirus subfamilies, although their functions are unclear. To clarify the functions of UL21, we generated a UL21-null HSV-1 mutant. Growth analysis showed that the synthesis of infectious UL21-null HSV-1 in glial cells was delayed and that the overall yield was low. The plaque sizes of the UL21-null mutant were smaller than those of wild-type HSV-1. We identified several candidate UL21-interacting proteins, including intermediate filaments, by yeast two-hybrid screening. The distribution of glial fibrillary acidic protein (GFAP), which is the main component of intermediate filaments, was altered in UL21-null mutant-infected glial cells compared to wild-type virus-infected cells. These results will help clarify the function of UL21 and broaden our understanding of the life cycle of HSV.

Marek disease virus encodes a basic-leucine zipper gene resembling the fos/jun oncogenes that is highly expressed in lymphoblastoid tumors

Marek disease virus (MDV) is a herpesvirus of chickens that induces T lymphomas within 3 weeks of infection. The short latency and polyclonal nature of MDV-induced tumors have suggested that the virus may encode one or more direct-acting oncogenes. To date, however, no MDV-specific tumor antigens or candidate transforming genes have been demonstrated. In this paper, we report the identification of a MDV gene encoding a protein with homology to the leucine-zipper class of nuclear oncogenes. It also contains a proline-rich domain characteristic of another class of transcription factors. This gene, designated meq, maps to the long repeat of MDV and is one of the few genes that are highly expressed in MDV-induced T-cell tumors. To our knowledge, a herpesvirus gene closely related to the fos/jun family of oncogenes has not been reported previously.

Induction of stress proteins in SV-40 transformed human RPE-derived cells by organic oxidants

The expression and induction of heat shock proteins (HSPs) were examined in cultured SV-40-transformed human retinal pigment epithelial (RPE)-derived cells following exposure to chemical oxidants. Concentrations of hydrogen peroxide and the organic oxidants tert-butyl hydroperoxide, cumene hydroperoxide and linoleic acid hydroperoxide were used under conditions where cell viability was between 75% and 90% as determined by the trypan blue exclusion tests. The types of HSPs that are either induced and/or elevated from constitutive levels in cultured transformed cells were separated both by SDS-PAGE and by two-dimensional gel electrophoresis. Subsequent immunoblotting was performed with both a monoclonal antibody (MAb C92) specific for only the stress-inducible form of HSP 70, namely HSP 72, and with a monoclonal antibody (MAb N27) specific for both the stress-inducible HSP 72 and its constitutive form, HSP 73. As positive controls for comparison, other types of stressing agents were used that included heat shock at 41 degrees C for various times and exposure to the proline analogue, azetidine-2-carboxylic acid (AzC). Protein immunoblotting analysis demonstrate that: (a) Stress-inducible HSP 72 is present at low levels in non-stressed cultured transformed RPE-derived cells and in fresh bovine retina and RPE, but is not detectable in non-stressed cultured lung fibroblasts until induction with heat shock; and (b) Stress-inducible HSP 72 is elevated from constitutive levels in RPE-derived cultured cells after exposure to various oxidants. After cellular exposure to both organic oxidants and to AzC in the presence of L-[35-S]-methionine, two-dimensional gel electrophoresis confirmed the elevation of newly synthesized HSP 72. Thus, these results indicate that cultured human SV-40-transformed RPE-derived cells are capable of elevated HSP 72 biosynthesis under conditions of oxidative stress produced by organic oxidants.

Herpes simplex virus 1 infection upregulates stress protein expression in cultured retinal neurons

The production of a 57K stress protein (StrP) after herpes simplex virus type 1 (HSV-1) infection was examined in cultured neonatal rat retinal cells. StrP expression in individual cells was identified using a monoclonal antibody, TI56. Indirect immunofluorescence of uninfected retinal cultures showed that approximately 40% of cells expressed neurofilament (NF+) and 5% expressed a low level of StrP. Following HSV infection the proportion of NF+ cells decreased while the proportion of StrP positive cells became greater and the intensity of staining increased. The number of cells labelled with a polyclonal anti-HSV antibody increased with time after infection. Retinal neurons in culture can be infected with HSV, after which StrP expression is significantly upregulated.

Response of cultured Müller cells to heat shock--an immunocytochemical study of heat shock and intermediate filament proteins in response to temperature elevation

Müller cells from adult rabbit retina were cultured at normal (37 degrees C) temperature and under heat shock conditions (42-43 degrees C) for either 1.5 hr or 4 hr. The presence of heat shock proteins of three different molecular weights, and of vimentin, was identified by the use of monoclonal antibodies and of GFAP by a suitable polyclonal antibody coupled in each instance with immunofluorescence microscopy or immuno-electron microscopy. Heat shock conditions resulted in the expression of HSP of molecular weights in the ranges 57-61 kD, 68 kD and 90 kD. The high level of expression of GFAP in unshocked Müller cell cultures (80-90% of cells) was reduced after 1.5 hr of heat shock to 20-30% of cells, and after 4 hr to only 5%. Although the expression of vimentin (90% of untreated cells) was only slightly reduced by heat shock (60-80% of cells) the intracellular staining pattern of vimentin was greatly altered by heat shock. Changes in the expression of HSP, GFAP and vimentin can all be regarded as part of the Müller cell response to heat shock conditions.