Compartmentalization of secretory proteins in pancreatic zymogen granules as revealed by immunolabeling on cryo-fixed and molecular distillation processed tissue

Effects of mild ozonisation on gene expression and nuclear domains organization in vitro

In the last two decades, the use of ozone (O₃) as a complementary medical approach has progressively been increasing; however, its application is still limited due to the numerous doubts about its possible toxicity, despite the low concentrations used in therapy. For an appropriate and safe clinical application of a potentially toxic agent such as O₃, it is crucial to elucidate the cellular response to its administration. Molecular analyses and transmission electron microscopy were here combined to investigate in vitro the effects of O₃ administration on transcriptional activity and nuclear domains organization of cultured SH-SY5Y neuronal cells; low O₃ concentrations were used as those currently administered in clinical practice. Mild ozonisation did not affect cell proliferation or death, while molecular analyses showed an O₃-induced modulation of some genes involved in the cell response to stress (HMOX1, ERCC4, CDKN1A) and in the transcription machinery (CTDSP1). Ultrastructural cytochemistry after experiments of bromouridine incorporation consistently demonstrated an increased transcriptional rate at both the nucleoplasmic (mRNA) and the nucleolar (rRNA) level. No ultrastructural alteration of nuclear domains was observed. Our molecular, ultrastructural and cytochemical data demonstrate that a mild toxic stimulus such as mild ozonisation stimulate cell protective pathways and nuclear transcription, without altering cell viability. This could possibly account for the positive effects observed in ozone-treated patients.

Low Voltage Transmission Electron Microscopy in Cell Biology

Low voltage transmission electron microscopy (LVTEM) was employed to examine biological tissues with accelerating voltages as low as 5kV. Tissue preparation was modified to take advantage of the low-voltage techniques. Treatments with heavy metals, such as post-fixation with osmium tetroxide, on block and counterstaining were omitted. Sections (40nm) were thinner than usual and generated highly contrasted images. General appearance of the cells remains similar to that of conventional TEM. New features were however revealed. The matrix of the pancreatic granules displays heterogeneity with partitions that may correspond to the inner-segregation of their secretory proteins. Mitochondria revealed the presence of the ATP synthase granules along their cristea. The nuclear dense chromatin displayed a honeycomb organization while distinct beads, nucleosomes, aligned along thin threads were seen in the dispersed chromatin. Nuclear pore protein complexes revealed their globular nature. The intercalated disks in cardiac muscle displayed their fine structural organization. These features correlate well with data described or predicted by cell and molecular biology. These new aspects are not revealed when thicker and conventionally osmicated tissue sections were examined by LVTEM, indicating that major masking effects are associated with standard TEM techniques. Immunogold was adapted to LVTEM further enhancing its potential in cell biology.

Perspectives on low voltage transmission electron microscopy as applied to cell biology

Low voltage transmission electron microscopy (LVTEM) with accelerating voltages as low as 5 kV was applied to cell biology. To take advantage of the increased contrast given by LVTEM, tissue preparation was modified omitting all heavy metals such as osmium, uranium, and lead from the fixation, on block staining and counterstaining. Nonstained ultra-thin tissue sections (40 nm thick) generated highly contrasted images. While the aspect of the cells remains similar to that obtained by conventional TEM, some new substructures were revealed. The pancreatic acinar cells granules present a heterogeneous matrix with partitions corresponding to segregation of their different secretory proteins. Microvilli display their core of microfilaments anchored to the dense top membrane. Mitochondria revealed the presence of distinct particles along their cristea membranes that may correspond to the ATP synthase complexes or oxysomes. The dense nuclear chromatin displays a honey-comb appearance while distinct beads aligned along thin threads were seen in the dispersed chromatin. These new features revealed by LVTEM correlate with structures described or predicted through other approaches. Masking effects due to thickness of the tissue sections and to the presence of heavy metals must have prevented their observation by conventional TEM. Furthermore, the immunogold was adapted to LVTEM revealing nuclear lamin-A at the edge of the dense chromatin ribbons. Combining cytochemistry with LVTEM brings additional advantages to this new approach in cell biology.

Immunogold cytochemistry in neuroscience

The complexity of the central nervous system calls for immunocytochemical procedures that allow target proteins to be localized with high precision and with opportunities for quantitation. Immunogold procedures stand out as particularly powerful in this regard. Although these procedures have found wide application in the neuroscience community, they present limitations and pitfalls that must be taken into account. At the same time, these procedures offer potentials that remain to be fully realized.

Compartmentalization of pancreatic secretory zymogen granules as revealed by low-voltage transmission electron microscopy

Low-voltage (5-kV) transmission electron microscopy revealed a novel aspect of the pancreatic acinar cell secretory granules not previously detected by conventional (80-kV) transmission electron microscopy. Examination of ultra-thin (30-nm) sections of non-osmicated, stain-free pancreatic tissue sections by low-voltage electron microscopy revealed the existence of granules with non-homogeneous matrix and sub-compartments having circular or oval profiles of different electron densities and sizes. Such partition is completely masked when observing tissues after postfixation with osmium tetroxide by low-voltage transmission electron microscopy at 5 kV and/or when thicker sections (70 nm) are examined at 80 kV. This morphological partition reflects an internal compartmentalization of the granule content that was previously predicted by morphological, physiological, and biochemical means. It corresponds to the segregation of the different secretory proteins inside the granule as demonstrated by high-resolution immunocytochemistry and reflects a well-organized aggregation of the secretory proteins at the time of granule formation in the trans-Golgi. Such partition of the granule matrix undergoes changes under experimental conditions known to alter the secretory process such as stimulation of secretion or diabetes.

Roles of molecular chaperones in pancreatic secretion and their involvement in intestinal absorption

This review focuses on the contribution of molecular chaperones in the secretory process of digestive enzymes and their interaction with enterocytes. By using biochemistry and immunocytochemistry, we have shown that Grp94, Cpn10, Cpn60, and protein disulfide isomerase (PDI) are present all along the rough endoplasmic reticulum-Golgi-granule secretory pathway of the pancreatic acinar cells and are secreted into the acinar lumen. Two other molecular chaperones, Grp78 and the Hsp70, appear to be restricted to the rough endoplasmic reticulum and the trans-Golgi apparatus, respectively. We have found that chaperones can be associated with pancreatic enzymes along the secretory pathway. Indeed, double immunogold and immunocoprecipitation revealed an association between Cpn60 and the colipase-dependent lipase (CDL) and between Grp94 and the bile salt-dependent lipase (BSDL). These complexes are secreted into the acinar lumen and diverted to the duodenal lumen. These findings led us to investigate these enzyme-chaperone complexes in intestinal tissue. Grp94, Cpn60, and PDI are present on microvilli and on the endosomal compartment of enterocytes. Furthermore, we have shown that the Grp94-BSDL complexes are internalized by enterocytes through classical endocytosis. Upon dissociation of the BSDL-Grp94 complex in the late endosome, BSDL is transferred to the basolateral membrane. We propose that Grp94 interacts with specific receptors and/or could force the associated protein to adopt a specific conformation that allows its binding to corresponding membrane receptors and its internalization by enterocytes. These two hypotheses need not to be exclusive. The existence of such a pancreatic secretion-intestinal absorption link speaks in favor of a coordinated functional connection between these two entities, through molecular chaperones, in order to optimize intestinal activities.

High-resolution ultrastructural study of the rat glomerular basement membrane in long-term experimental diabetes

The ultrastructure of the glomerular basement membrane of the long-term diabetic and age-matched control rats was studied with the application of advanced high-resolution microscopy. By using the freeze substitution method for the preparation of the renal tissue, it was possible to observe that the glomerular basement membrane in control and diabetic animals is composed on only a single lamina densa without the presence of a lamina lucida interna or externa. High-resolution electron microscopy of the diabetic glomerular basement membrane showed significant alterations in its morphology and ultrastructure. First, the basement membrane in diabetic condition appeared to be split into two halves, endothelial and epithelial. In the epithelial half of the membrane, the network of distinct strands referred to as cords, which were clearly present in the glomerular basement membrane of age-matched control animals, became less distinct and showed a diffused appearance being evenly replaced by thin filaments. The openings of the network were filled with a granular material. In the endothelial half of the membrane, on the other hand, the cord network was variably lost in diabetic condition and, within the resulting vacant spaces, bundles of fibrils 12 nm in width, identified as basotubules, were deposited. Immunolabeling for type IV collagen was found to be enriched in the endothelial half of the basement membrane being associated with the bundles of basotubules. The ultrastructural changes reported by high-resolution microscopy could be related to the molecular alterations of the basement membrane components and to the loss in permselectivity occurring during diabetes.