Improved immunoelectron microscopic method for localizing cytoskeletal proteins in Lowicryl K4M embedded tissues

Hotspot DNMT3A mutations in clonal hematopoiesis and acute myeloid leukemia sensitize cells to azacytidine via viral mimicry response

Somatic mutations in DNA methyltransferase 3A (DNMT3A) are among the most frequent alterations in clonal hematopoiesis (CH) and acute myeloid leukemia (AML), with a hotspot in exon 23 at arginine 882 (DNMT3A^R882). Here, we demonstrate that DNMT3A^R882H-dependent CH and AML cells are specifically susceptible to the hypomethylating agent azacytidine (AZA). Addition of AZA to chemotherapy prolonged AML survival solely in individuals with DNMT3A^R882 mutations, suggesting its potential as a predictive marker for AZA response. AML and CH mouse models confirmed AZA susceptibility specifically in DNMT3A^R882H-expressing cells. Hematopoietic stem cells (HSCs) and progenitor cells expressing DNMT3A^R882H exhibited cell autonomous viral mimicry response as a result of focal DNA hypomethylation at retrotransposon sequences. Administration of AZA boosted hypomethylation of retrotransposons specifically in DNMT3A^R882H-expressing cells and maintained elevated levels of canonical interferon-stimulated genes (ISGs), thus leading to suppressed protein translation and increased apoptosis.

Well-Differentiated Neuroendocrine Carcinoma of the Lung: A Clinicopathologic and Ultrastructural Study of 10 Cases

The clinico-pathologic characteristics of 10 resected pulmonary tumors, which proved to be well-differentiated neuroendocrine carcinomas (WDNC) on the basis of light microscopic, immunocytochemical, ultrastructural and immunoelectron microscopic investigations, were evaluated. The tumors showed a wide spectrum of histologic features that could be referred to three basic patterns: 1) a carcinoid-like pattern; 2) an organoid pattern characterized by palisading cells at the edge of cellular areas, and 3) a prevalent adenocarcinoma-like pattern. The second pattern was the most distinct even though it often mimicked the small cell/large cell subtype of small cell carcinoma (SCC) owing to its association with marked atypia and poor differentiation. All but one of the patients were males and smokers. The mean age was 58 years. Half of the tumors were centrally located including those showing the adenocarcinoma-like pattern. Disease-free and overall survival and type of tumor dissemination in four patients were similar to those of SCC. Five evaluable patients were alive and disease-free after a mean follow-up of 74 months. Two of these were initially diagnosed as SCC. We conclude that, because of its impact on prognosis, the diagnosis of WDNC appears to be relevant although other factors able to adversely affect the clinical course remain undefined.

Clarification of the peripherally located F-actin network around the primary afferent neurons

The primary afferent neurons are enriched for F-actin in the peripheries, giving rise to ring-like appearances when stained with fluorescence probes for actin. A variety of data, mainly obtained from in vitro experiments, suggested that the satellite cell (SC) envelopes around sensory neurons may be a candidate for the F-actin positive structures. However, immunohistochemical studies on dorsal root ganglia (DRGs) suggested that F-actin was located exclusively within the cortical regions of the sensory neurons. Given this discrepancy, the present study has reexamined the distribution of F-actin in the DRGs at both the fluorescence light and electron microscopic levels. Double staining of F-actin with S-100 beta and neural cell adhesion molecule showed that the fluorescence signals for F-actin around the DRG neurons were restricted to the inner margins of the SC envelope. Subsequent ultrastructural observations, by use of a modified anti-FITC system, demonstrated that the F-actin-positive sites corresponded mostly to the adneuronal SC lamellae, while the cortical cytoskeletons in the neurons seemed to contribute little to the typical F-actin staining. The high concentration of F-actin in the SC envelope has not yet described before, and is thought to assist in understanding the functions of the SCs in the ganglia.

Histochemical and immunohistochemical protocols for routine biopsies embedded in Lowicryl resin

Tissue processing and analysis require good preservation of both the shape and content of cells. Lowicryl resin is one of the few embedding media that allow good preservation of both tissue architecture and cellular contents. Therefore, different histochemical and immunohistochemical reactions can be applied to semithin sister sections from one biopsy. Further examination of a zone of interest can be carried out under the electron microscope. The hydrophilic property of Lowicryl resins makes possible different histochemical reactions; however, the technique used for paraffin sections must be adapted for each reaction. Antigenic preservation of cells by low temperature embedding allows immunolabeling on either semithin sections or in the zone of interest on ultrathin sections. We have shown the application and adaptation of different histochemical and immunohistochemical reactions on semithin and ultrathin sections from hepatic biopsies that were large, but thin. The variety of techniques that can be used on sister Lowicryl sections of a single biopsy makes this medium useful for extensive pathological studies of precious needle biopsies.

The peripheral cytoplasm of adrenocortical cells: zone-specific responses to ACTH

BACKGROUND

Differences in the cytoskeletal protein actin in cells from the zona glomerulosa and zona fasciculata would be of considerable interest because there is persuasive evidence that rat corticosteroids are secreted by mechanisms that are somewhat zone-specific. We have previously shown evidence that actin may be involved in steroid secretion, possibly in connection with changes in adrenocortical microvilli. However, the cells upon which the data were based were not separated according to zone of origin.

METHODS

Immunogold electron microscopy and morphometric procedures were used to determine whether ACTH-induced changes in the peripheral cytoplasm of isolated adrenocortical cells occur in both zona fasciculata and zona glomerulosa cells.

RESULTS

Actin immunoreactivity was more concentrated in the cytoplasm adjacent to the plasma membrane (including the cytoplasm within the microvilli) than it was in the internal cytoplasm in cells from both zones (4-6 times more concentrated in zona glomerulosa cells and 3-6 times more concentrated in zona fasciculata cells). However, the mean aggregate microvillar surface length (microvillar index) of untreated zona fasciculata cells (previously reported (Loesser and Malamed, 1987)) was 23% greater than that of untreated zona glomerulosa cells. Although ACTH (at a maximal steroidogenic concentration) had no effect on the peripheral cytoplasmic actin concentration of zona glomerulosa cells, there was a 24% increase in the aggregate microvillar length. In contrast, in zona fasciculata cells, ACTH treatment was accompanied by an increase in peripheral cytoplasmic actin concentration of 58-64% and an increase in aggregate microvillar surface length of 40% (previously reported (Loesser and Malamed, 1987)), almost twice that for zona glomerulosa cells.

CONCLUSION

The results suggest that ACTH-induced hormone release from zona fasciculata cells is mediated by increases in peripheral cytoplasmic actin and aggregate microvillar length; in zona glomerulosa cells such changes are small or absent.

Ultrastructural characterization of poorly differentiated rhabdomyosarcomas

Twenty-five rhabdomyosarcomas, histopathologically and immunocytochemically classified as embryonal (12 cases), alveolar (11 cases), and pleomorphic (two cases), were studied by routine and immunogold electron microscopy, to identify, in poorly differentiated cells, reproducible markers useful for diagnostic purposes. Rhabdomyoblasts with rudimentary sarcomeres were observed in 12 cases (48%). Large cells (named oval cells) with an oval shape and an eccentric nucleus, like rhabdomyoblasts but lacking organized myofilaments, were observed in 20 cases (80%). By immunogold electron microscopy, performed on four of 25 cases, both oval cells and rhabdomyoblasts showed abundant intermediate filaments positive for desmin. Oval cells may be used as markers of poorly differentiated rhabdomyosarcomas.

Actin distribution along the lateral wall of gerbil outer hair cells

Outer hair cells can contract parallel to their long axis, and it has been hypothesized that actin may play a role in this contraction. In this study, actin distribution was examined in the gerbil organ of Corti using postembedment immunoelectron microscopy. In addition to regions typically labelled by actin antibodies and observed by epifluorescence--the cuticular plate, stereocilia, and supporting cell processes--these procedures preserved the ultrastructure of the cell and allowed us to demonstrate actin reactivity along the lateral wall of the outer hair cells between the subsurface cisterns and the plasma membrane. This region is the location of structures (pillars and cortical cytoskeleton) though to be associated with contraction of the outer hair cells.

The use of fixed-frozen tissue in routine ultrastructural evaluation and immunogold labeling

A method of freezing fixed surgical tissue specimens for long-term storage and subsequent resin embedding was developed. The method entails fixation of the specimens in Karnowsky's liquid, cryoprotecting with 10% sucrose, freezing in melting Freon gas, and storage at -80 degrees C. When needed, even after years, the frozen tissue pieces are thawed in lukewarm (37 degrees C) buffer and embedded in resin (Epon or Lowicryl). Ultrastructural detail as well as immunocytochemical reactivity of a wide variety of cellular constituents are well preserved. The method is practical and allows retrospective embedding in Lowicryl resin (a technique not easily applicable in everyday routine) for subsequent ultrastructural immunolocalization studies.

Localization of the membrane binding sites of Era in Escherichia coli

Era is a GTP-binding protein that is essential for normal cell growth and division in Escherichia coli. In view of the fact that eukaryotic proteins similar to Era are membrane-associated and important in membrane signalling pathways, experiments were carried out to establish the intracellular location of Era. Immunoelectron microscopy was employed to demonstrate that Era resides at or very near the internal surface of the cytoplasmic membrane, which is a location expected for a membrane signalling protein. In addition, Era occurs in patches that may correspond to areas that are potential sites of septation.

Localization of the alpha 1 and alpha 2 subunits of the dihydropyridine receptor and ankyrin in skeletal muscle triads

We have studied the subcellular distribution of the alpha 1 and alpha 2 subunits of the dihydropyridine (DHP) receptor and ankyrin in rat skeletal muscle with immunofluorescence and immunogold labeling techniques. All three proteins were concentrated in the triad junction formed between the T-tubules and sarcoplasmic reticulum. The alpha 1 and alpha 2 subunits of the DHP receptor were colocalized in the junctional T-tubule membrane, supporting their proposed association in a functional complex and the possible participation of the alpha 2 subunit in excitation-contraction coupling. Ankyrin label in the triad showed a distribution different from that of the DHP receptor subunits. In addition, ankyrin was found in longitudinally oriented structures outside the triad. Thus, ankyrin might be involved in organizing the triad and in immobilizing integral membrane proteins in T-tubules and the sarcoplasmic reticulum.

Post embedding immunoelectron microscopy of human breast cancer: a comparison of three acrylic resins

The suitability of three acrylic resins for the immunoelectron microscopical localization of cell surface and cytoskeletal antigens in surgically excised, immersion fixed human breast cancer, using an immunogold system, has been assessed. Good localization of milk fat globule membrane was achieved with LR White, LR Gold and Lowicryl K11M, although the embedding schedule for LR White had to be modified. The best results were achieved with Lowicryl K11M. Only scanty labelling of actin and cytokeratin was seen in LR White embedded tissue, whereas there was clear localization in LR Gold and Lowicryl K11M embedded samples. Tubulin and alpha-actinin was detected at low level in tissues in the low temperature embedding resins, but not in LR White embedded samples. The morphology of the latter was poorer, and there was greater variability in ultrastructure and labelling. Of the two low temperature embedding resins, Lowicryl K11M gave slightly better results. However, the advantages could be outweighed by the problem incurred in achieving the low temperatures, and by poorer handling properties than LR Gold.

Ultrastructural cytoskeleton alterations and modification of actin expression in the NIH/3T3 cell line after transformation with Ha-ras-activated oncogene

Cytoskeleton alterations of NIH/3T3 fibroblast monolayers transfected with Ha-ras-activated oncogene were studied by immunofluorescence, immunoelectron microscopy, and immunoelectrophoretic analysis of actin isoforms. Transformation foci were found to consist of cells with a round shape and rare stress fibers that spread sparsely, forming rare focal contacts and fibronexuses. The loss of stress fibers in transformed cells was confirmed by staining with rhodamine-phalloidin and with a fluorescinated anti-non-muscle cell actin antibody. The transformed cells were anchored to the substrate prominently by filaments that contained fibronectin, as showed by immunoelectron microscopy. A down-regulation of alpha-actin isoform was observed by immunofluorescence and immunoblotting analysis using a specific monoclonal antibody. The diffuse distribution of alpha-actin, lacking a specific association with stress fibers, challenges the hypothesis of a connection between alpha-actin down-regulation and stress fiber loss.

An improved method for Lowicryl K4M electron microscopic embedding of brain tissue

The Lowicryl K4M low-temperature embedding technique was modified for brain tissue to facilitate antigen localization in neuronal structures by using postembedding immunoelectron microscopy. Rat brains fixed by perfusion with a mixture of 1% glutaraldehyde and 1% paraformaldehyde, were sectioned into 200-microns-thick coronal sections, and the appropriate area of the sections further microdissected into small samples (200 x 700 x 700 microns). These samples were processed for K4M embedding using prolonged incubation times at each step. This resulted in a homogeneously infiltrated tissue with good preservation of ultrastructure and antigen reactivity. Despite the lack of osmium fixation, the morphology of the synapses was clearly identifiable.

Distribution of Na+ channels and ankyrin in neuromuscular junctions is complementary to that of acetylcholine receptors and the 43 kd protein

We have used immunogold electron microscopy to study the organization of the acetylcholine receptor, 43 kd protein, voltage-sensitive Na+ channel, and ankyrin in the postsynaptic membrane of the rat neuromuscular junction. The acetylcholine receptor and the 43 kd protein are concentrated at the crests of the postsynaptic folds, coextensive with the subsynaptic density. In contrast, Na+ channels and ankyrin are concentrated in the membranes of the troughs and in perijunctional membranes, both characterized by discontinuous submembrane electron-dense plaques. This configuration of interspersed postsynaptic membrane domains enriched in either Na+ channels or acetylcholine receptors may facilitate the initiation of the muscle action potential. Furthermore, the results support the involvement of ankyrin in immobilizing Na+ channels in specific membrane domains, analogous to the proposed involvement of the 43 kd protein in acetylcholine receptor immobilization.

Comparison of LR white resin, Lowicryl K4M and Epon postembedding procedures for immunogold staining of actin in the testis

The efficiency of various postembedding procedures for actin immunogold detection was compared using testicular tissue as a model. Whatever the fixative, testes embedded in LR White resin or in Lowicryl K4M showed few differences as regard ultrastructural preservation and gave similar actin antigenicity preservation. A purified polyclonal antibody (IgG) and a monoclonal antibody (IgM) visualized with gold secondary antibody yielded high labeling intensity whereas the IgG-protein-A gold association was less efficient. Crude antisera gave a low specific staining/background ratio. Samples of testes, fixed in different conditions, were also embedded in Epon, omitting propylene oxide and lowering polymerization temperature to 40 degrees-50 degrees C. This slight modification improved ultrastructural preservation which was better than with hydrophilic resins, as well as made possible immunogold detection of actin though antigenicity preservation was lesser than with these resins. Thus, in Epon embedded samples actin labeling, using IgG antiactin-gold secondary antibody, was similar to that observed after hydrophilic resin-protein-A gold procedures. In addition to actin labeling of various somatic cells it was confirmed that actin is a consistent component of the subacrosomal space of spermatids during the greater part of spermiogenesis in rat.