Granulolysis in neurosecretory neurons of the rat supraoptico-posthypophyseal system

Lysosomal cathepsin creates chimeric epitopes for diabetogenic CD4 T cells via transpeptidation

The identification of the peptide epitopes presented by major histocompatibility complex class II (MHCII) molecules that drive the CD4 T cell component of autoimmune diseases has presented a formidable challenge over several decades. In type 1 diabetes (T1D), recent insight into this problem has come from the realization that several of the important epitopes are not directly processed from a protein source, but rather pieced together by fusion of different peptide fragments of secretory granule proteins to create new chimeric epitopes. We have proposed that this fusion is performed by a reverse proteolysis reaction called transpeptidation, occurring during the catabolic turnover of pancreatic proteins when secretory granules fuse with lysosomes (crinophagy). Here, we demonstrate several highly antigenic chimeric epitopes for diabetogenic CD4 T cells that are produced by digestion of the appropriate inactive fragments of the granule proteins with the lysosomal protease cathepsin L (Cat-L). This pathway has implications for how self-tolerance can be broken peripherally in T1D and other autoimmune diseases.

Site of synthesis, intracellular transport and secretion of glycoprotein in exocrine cells

The site of attachment of the first sugar, N-acetylgalactosamine, to the seryl and threonyl residues of the protein chain is unknown in exocrine cells. The subsequent sugars of the carbohydrate side chains, galactose and N-acetylglucosamine alternately, and the end-group sugars, galactose, N-acetylgalactosamine and fucose, are attached in the Golgi complex. Sulphate too is attached in that structure. In the stomach, sulphate is probably transferred in the most mature cisterna of the Golgi stacks, galactose and fucose in other cisternae, suggesting a gradient in transferase activities along the stack. The possibilities of regulating the amount and relative sugar composition of the glycoproteins are discussed. The secretory product is stored in granules. Their polygonal, large and swollen appearance and complex formation by loss of bordering membranes, as observed in many kinds of glycoprotein-secreting cells ('mucous cells') might be caused by ineffective fixation techniques. Direct vascular perfusion results in a picture no different from what is found in non-mucous cells. Whether secretion is merely exocytotic, as in non-mucous cells, or whether it is accompanied by a loss of membrane and even cytoplasm needs thorough investigation, with the effects of various fixation techniques being compared.

Electron microscopic and immunocytochemical study of rapidly frozen, freeze-substituted neural lobes of rats

Rapid freezing of freshly dissected or incubated neural lobes was explored as a means of obtaining ultrastructural preservation of the more natural state of this tissue. A quantitative assessment of the region of good fixation was made in order to determine the relative fractions occupied by axons, pituicytes and the extracellular space. The immunocytochemical distributions of neurophysins and the glycopeptide portion of the vasopressin precursor were evaluated using the immunogold technique in order to determine the relative numbers of oxytocin and vasopressin fibre types in the fixed region, and the subcellular localization of these antigens. The uncut surface of rat neural lobes was rapidly frozen against a highly polished copper plug and freeze-substituted in osmium-acetone either immediately after dissection (approximately 2 min), or after a 15 min incubation period in vitro in an oxygenated, balanced salt solution. Substituted neural lobes were prepared for either conventional electron microscopy, or for immunogold labelling of neurophysins and the glycopeptide precursor to vasopressin. Membranes, subcellular organelles and extracellular matrix were well preserved 10 microns deep to the contacted surface. The extracellular space accounted for approximately 30% of the cross-sectional area of the neuropil and could be divided into two domains: an extended perivascular space (28-29% of total area); and a narrow (approximately 24 nm; approximately 1% of total) space between closely apposed neurosecretory processes or between these processes and pituicytes. Pituicytes accounted for about 30% of the area and axons 20-25%. Pituicytes occupied close to 60% of the basal lamina at the neurohaemal contact zone, while axons occupied approximately 20%. There were no differences between neural lobes frozen immediately after dissection and those incubated for 15 min in any of these measures, suggesting minimal fluid redistribution. Gold particles were specifically localized over large (100-200 nm) dense core vesicles, and less frequently over multivesicular bodies and lysosomes. No etching of the plastic or reduction of osmium was necessary to achieve labelling. Specific labelling of one set of terminals and axons (about 80%) was observed with the monoclonal antibody previously shown to be specific for oxytocin-neurophysin, while in neighbouring sections the remaining 20% of the processes were labelled with the antiserum to the vasopressin precursor, or with non-specific antibodies to neurophysins. In conclusion, ultrarapid freezing preserves a large extracellular space in the neural lobe and provides for high resolution morphological and immunocytochemical studies of neurohypophysial structure.

Neuronal autophagy in experimental Creutzfeldt-Jakob's disease

We report an experimental model of Creutzfeldt-Jakob's disease (CJD) in mice leading to the formation of giant autophagic vacuoles (AV) in neurons of the cerebral cortex. These AV appear at the end of the incubation period (4-6 months postinoculation), together with spongy changes and clinical symptoms. Autophagy, a process of intracellular digestion of cell constituents by the lysosomal compartment, is known in many cell types, where it plays a role both in the physiological turnover and in pathological processes and is involved in protein metabolism. The same also occurs in neurons. Here autophagy is known to occur in the normal state and leads to residual bodies called lipofuscin granules. An increase in lipofuscin is known to occur in human and experimental CJD. Therefore, an increase in autophagy and in AV can be expected. In our experimental model, the activation of neuronal autophagy may be related to an alteration in neuronal protein metabolism.

Radioautographic study of glycoprotein synthesis and fate in the hypothalamo-neurohypophyseal system of vasopressin-deficient Brattleboro rats

L-3H-fucose was injected into the lateral cerebral ventricle of vasopressin-deficient Brattleboro and control Long-Evans rats which were subsequently killed at several time intervals after the injection. The hypothalamus and the neurohypophysis were processed for light- and electronmicroscopic radioautography. Other complementary experiments using immunocytochemical and enzyme-histochemical techniques were also undertaken. L-3H-fucose was incorporated into newly synthesized glycoproteins in the Golgi apparatus of supraoptic and paraventricular neurons, and later on labelled glycoproteins migrated to lysosomes and the plasma membrane surrounding the perikaryon. The Golgi apparatus of the vasopressin-deficient neurons remained heavily labelled as long as 3 days after injection, in sharp contrast with the normal neurons in which there was a remarkable decrease of label in the Golgi region between 4 and 24 h after the isotope administration. Labelled glycoproteins also migrated to the neurohypophysis and were mainly found in the axonal plasma membrane, vesicles and axoplasm. The renewal of glycoproteins in the neurohypophysis of Brattleboro rats was faster than in the normal rats and this was attributed to the lack of formation of products which are normally packaged in secretory granules in the perikaryon and released at the axon terminal in the neurohypophysis. Colchicine caused a disturbance in the topography of the organelles of the perikaryon and the most striking features were the displacement of Golgi stacks to the periphery of the perikaryon and an accumulation of mitochondria in this neuronal region. No secretory granules were observed in the vasopressin-deficient neurons of untreated or colchicine-treated Brattleboro rats. By contrast, secretory granules (most of them labelled with 3H-fucose) were concentrated in the perikaryon of colchicine-treated Long-Evans rats. In these rats, colchicine caused a severe block in the migration of 3H-fucose-labelled glycoproteins to the neurohypophysis, but this did not occur in the Brattleboro rats. The results of the experiments were interpreted in the light of the genetic defect known to occur in Brattleboro rats which causes the inability to produce vasopressin and also remarkable morphological and physiological changes in the affected neurons.

The neuronal endoplasmic reticulum: its cytochemistry and contribution to the endomembrane system. II. Axons and terminals

The morphology and cytochemistry of the endoplasmic reticulum (ER) in axons and terminals of a number of different types of neurons in brains from mice were investigated ultrastructurally. The neurohypophysis received particular attention because the morphology and enzyme cytochemical activities of many of the preterminal swellings of hypothalamo-neurohypophysial axons are altered by chronic salt-stress. Membrane contrast and enzyme cytochemical staining techniques were employed to characterize the axonal reticulum and to determine if organelles representing the lysosomal system in the axon and the tubular profiles participating in the anterograde axonal transport of native horseradish peroxidase (HRP) are associated with the ER. Potential enzyme cytochemical markers for the axonal ER included glucose-6-phosphatase (G6Pase), thiamine pyrophosphatase, nucleoside diphosphatase, and acid hydroxylase activities. The anterograde transport of HRP was analyzed in undamaged hypothalamo-neurohypophysial neurons and in facial and hypoglossal motoneurons of mice receiving the protein in the lateral cerebral ventricle. The ER pervaded the axon and appeared as parallel, 20-40-nm-wide tubules interconnected by oblique anastomoses. Membrane thickness of the axonal reticulum measured 60-100 A, which is similar to that of the perikaryal ER. Enzyme cytochemical activities associated with the ER or lysosomes were not conspicuous in axons and terminals under normal conditions but became prominent in some axons and preterminal swellings manifesting an autophagic appearance within neurohypophyses from salt-stressed mice. Only G6Pase activity was a marker for the ER in these axons and preterminals. Many ER profiles in non-incubated sections and in G6Pase cytochemical preparations of salt-stressed neurohypophyses were wrapped around or interspersed among secretory granules, multilamellar bodies, and vacuoles that may represent forms of lysosomes involved in autophagy and crinophagy. Acid hydrolase activities were localized within the vacuoles as well as within 80-130-nm-wide, blunt-ended tubules in pituitary stalk axons; similar reactive tubules were confluent with large secondary lysosomes in neurosecretory cell bodies and may be derived from these lysosomes. Morphologically identical tubules transporting HRP in the anterograde direction were observed only in the salt-stressed hypothalamo-neurohypophysial neuron. The HRP-positive tubules very likely are affiliated with the lysosomal system.

The influence of age on the hypothalamo-neurohypophyseal system of the mouse: a quantitative ultrastructural analysis of the posterior pituitary

A quantitative ultrastructural study of the posterior pituitary was undertaken. The posterior pituitary is the storage of the hormones vasopressin and oxytocin. Female C57BL/Icrfat mice which were under conditions of normal hydration were sampled at 8 and 32 months of age. There was a slight age-related decline in the volume fraction of cellular components with a corresponding increase in perivascular space. The proportion of neurosecretory endings and swellings showing autophagic activity (Dellman type II Herring bodies) increased significantly in the senescent mouse. The total pituicyte population showed a significant decrease in volume fraction, but with a significant increase in electron-dense types. Of the subcellular parameters examined, only the diameter of the neurosecretory granules showed a significant age-related decrease, the volume fraction remaining unaltered. The animals used in this study were from stocks found to be in a constant oestrus-metoestrus-like state unlikely to obscure changes in pituitary morphology. Over-all, the morphology of the posterior pituitary of the senescent female mouse under physiologically defined resting conditions indicates maintenance of the status quo.

Mechanisms of inactivation of hypothalamic regulatory hormones

The mechanisms of enzymic inactivation of thyrotropin-releasing hormone, luteinizing hormone-releasing hormone and somatostatin, the three fully-characterized hypothalamic regulatory hormones, and the possible physiological significance of the peptidases in neuroendocrine control has been reviewed. Application of the criteria of enzyme location (at the sites of biosynthesis, release, action, elimination and excretion), appropriate biochemical characteristics of the enzymes and changes in enzyme activity in physiological circumstances all suggest that the peptidases can contribute to the mechanisms controlling the hypothalamic hormones' release and actions. Besides their physiological function, the enzymes may also be directly involved in certain pathological conditions. There is evidence to indicate that the enzymes degrading the regulatory hormones may participate in the process of hormone activation as well as inactivation. A continuing investigation of the peptidases may lead to a better understanding of the established endocrine and other putative functions of these hypothalamic polypeptide hormones.

Ultrastructural observations on the hypothalamo-posthypophysical complex of the Brattleboro rat

The hypothalamo-posthypophysial complex of the homozygous Brattleboro rat is characterized by a hyperactivity of its neurons. The neurosecretory fibers, especially in the neurohypophysis, show numerous signs of autophagy and tubular proliferation of the axoplasmic reticulum. These structural alterations, as well as the nematosomes of nucleolus-like bodies encountered in the perikarya, may be related to the chronic hyperactivity of the neurons. They can be reduced by administration of exogenous vasopressin. The numerous liposomes in the pituicytes are paralleled in the neuronal perikarya by a great number of lysosomes. Small dense core vesicles observed in the neurosecretory endings and perikarya may indicate a secretory product distinct from oxytocin and vasopressin. In the homozygous Brattleboro rat, endings of the aminergic type are more numerous than in the normal rat. With respect to the role they may play in the secretory processes, their increase might be secondary to the vasopressin deficiency.

Sequence of ultrastructural changes induced by activation in the posterior neurosecretory cells in the brain of Rhodnius prolixus with special reference to the role of lysosomes

The posterior neurosecretory cell (PNC) group in the brain of Rhodnius prolixus is composed of five ultrastructurally identical cells. The PNC were examined in the unfed fifth instar and at seven stages (from 15 min to 14 days) after activation was initiated by feeding. Each stage examined revealed successive changes in morphology which can be related to the synthesis, maturation, storage and transport of neurosecretory material. It is suggested, in particular, that the lysosomal system (dense bodies and multivesicular bodies) may play a role in the maturation of the secretory granules.