The convergence of imaging techniques with molecular biology, biochemistry and computing is a rap... more The convergence of imaging techniques with molecular biology, biochemistry and computing is a rapidly growing area. A TRENDS Guide to Imaging Technologies outlines the latest developments and applications within the life sciences. This collection features nine reviews that cover specific areas, such as molecular imaging and atomic scale imaging, and the latest developments in the application of imaging to neurosciences and in small animal models. These integrated approaches provide great promise for insights into biological processes and in the diagnosis of disease.
This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and e... more This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identi®ed presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modi®able synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The¯awed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed``sprouting program,'' which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without speci®c involvement of the neuronal nucleus. 7
We have reported that the local administration of serine protease inhibitors (amyloid precursor p... more We have reported that the local administration of serine protease inhibitors (amyloid precursor protein with the Kunitz insert (APP K + ), aprotinin, and leupeptin) to the rat sciatic nerve determines a sprouting response of myelinated axons, proliferation of Schwann cells, and demyelination, 5 to 7 days later. Further study of these nerves with the electron microscope revealed (i) a sprouting response of nonmedullated axons, (ii) the appearance of fine axons with a few turns of compact myelin, (iii) abnormal contacts of axons with basal laminae, with fibroblast-like cells, and between them, (iv) the occurrence of hemidesmosome-and desmosome-like junctions between Schwann cell processes, and between Schwann cells and axons, and (v) the appearance of amorphous and fibrillary extracellular deposits alongside the axolemma. The adjacent proximal and distal segments were normal, i.e., axons remained continuous, and the alterations were confined to the segment exposed to the protease inhibitors. Heated APP K +, APP without the Kunitz insert (APP K -), bovine serum albumin, and saline, did not elicit cytological alterations. Our results suggest that these inhibitors of serine proteases (i) set free a sprouting drive of axons by disrupting an ongoing repressive mechanism; (ii) modify the adhesive properties of axons and Schwann cells, and (iii) alter the natural history of an extracellular material. The imbalance of an extracellular protease system may participate in the pathogenesis of Alzheimer's disease.
We studied the cytological alterations produced in the rat sciatic nerve by the amyloid precursor... more We studied the cytological alterations produced in the rat sciatic nerve by the amyloid precursor protein (APP) containing the Kunitz insert (APP K+) and other protease inhibitors. Conditioning of nerve segments with APP K+, aprotinin or leupeptin for 5 days or more resulted in mitosis of Schwann cells, demyelination of fibres, and a <10-fold increase in Schwann cells, associated with demyelinated fibres. Altered fibres nevertheless involved a small part of the population. Nerve segments proximal and distal to the conditioned region showed almost no alteration. Conditioning with saline, heated APP K+, or APP without the Kunitz insert was not effective. We conclude that APP K+ and other protease inhibitors induce Schwann cells to enter the cell cycle, and once committed to proliferate they resorb their myelin. These functional properties of APP may be relevant to the pathogenesis of Alzheimer's disease.
Abstract: The hallmark event of Alzheimer's disease (AD) is the deposition of amyloid as insolubl... more Abstract: The hallmark event of Alzheimer's disease (AD) is the deposition of amyloid as insoluble fiber masses in extracellular neuritic plaques and around the walls of cerebral blood vessels. The main component of amyloid is a hydrophobic peptide, named amyloid β-peptide (βA4), which results from the processing of a much longer membrane amyloid precursor protein (APP). This review focuses on the structural features of βA4 and the factors that determine βA4 insolubilization. Theoretical and experimental studies of the primary structure of βA4 have shown that it is composed of a completely hydrophobic C-terminal domain, which adopts β-strand structure, and an N-terminal region, whose sequence permits different secondary structures. In fact, this region can exist as an α-helical or β-strand conformation depending on the environmental condition (pH and hydrophobicity surrounding the molecule). The effects of pH and hydrophobicity on βA4 structure may elucidate the mechanisms determining its aggregation and amyloid deposition in AD.
The calibers and microtubular content of axons were studied in normal and regenerating fibers of ... more The calibers and microtubular content of axons were studied in normal and regenerating fibers of the sural nerve from 17 to 122 days after a lesion of the sciatic nerve of young adult rats. During this period (70–175 days of age), the cross-sectional area of control myelinated axons almost doubled but that of nonmedullated axons did not change. In regenerating nerves, after 122 days of recovery, the cross-sectional area of myelinated fibers was still 38% below that of the normal side. In contrast, the regenerating nonmedullated population was richer in fine (< 0.2 μm2) and in coarse (> 0.9 μm2) fibers than on the control side; the cross-sectional area averages were 0.50 and 0.54–0.70 μm2 for the normal and regenerating populations, respectively.The microtubular density of normal 3-μm myelinated fibers averaged 24.0 microtubules/μm2. In regenerating fibers of the same size the density varied between 19.2 and 23.2 microtubules/μm2. Microtubular density values of normal and regenerating fibers were not statistically different. In nonmedullated fibers, the microtubular content (expressed as microtubular density or number of microtubules per axon) correlated with the caliber of the fiber. In these correlations, only minor differences were observed between regenerating and uninjured fibers.Our results indicate that nonmedullated fibers terminate their radial growth well before myelinated fibers do, and that axonal microtubular content correlates with the local size of the fiber and is largely insensitive to regeneration.
Regenerating axons need proteins to grow and we explored whether a local supply is necessary. Cru... more Regenerating axons need proteins to grow and we explored whether a local supply is necessary. Crushed peroneal nerves were entubulated with silicone sleeves, plain or loaded with cycloheximide (CHX); some nerves were frozen to kill resident cells. When a plain sleeve was placed distal to the crush, axons regrew 5.0 mm in 3 days (pinch test), and 4.6 mm when the sleeve was placed around a frozen nerve (n.s). CHX administered distal to the crush reduced the elongation by~58% (P Ͻ 0.01) in unfrozen or frozen nerves whilst its administration central to the crush was ineffectual. Immunostaining of nerves with GAP-43 gave similar values. Under the electron microscope, axonal sprouts were less frequent when CHX was used irrespective of the cellular or acellular condition of the nerve. Therefore, an inhibitor of protein synthesis reduces axonal regrowth, an effect mediated neither by parent neurones nor by resident cells. We propose that axons synthesize proteins.
The convergence of imaging techniques with molecular biology, biochemistry and computing is a rap... more The convergence of imaging techniques with molecular biology, biochemistry and computing is a rapidly growing area. A TRENDS Guide to Imaging Technologies outlines the latest developments and applications within the life sciences. This collection features nine reviews that cover specific areas, such as molecular imaging and atomic scale imaging, and the latest developments in the application of imaging to neurosciences and in small animal models. These integrated approaches provide great promise for insights into biological processes and in the diagnosis of disease.
This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and e... more This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identi®ed presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modi®able synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The¯awed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed``sprouting program,'' which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without speci®c involvement of the neuronal nucleus. 7
We have reported that the local administration of serine protease inhibitors (amyloid precursor p... more We have reported that the local administration of serine protease inhibitors (amyloid precursor protein with the Kunitz insert (APP K + ), aprotinin, and leupeptin) to the rat sciatic nerve determines a sprouting response of myelinated axons, proliferation of Schwann cells, and demyelination, 5 to 7 days later. Further study of these nerves with the electron microscope revealed (i) a sprouting response of nonmedullated axons, (ii) the appearance of fine axons with a few turns of compact myelin, (iii) abnormal contacts of axons with basal laminae, with fibroblast-like cells, and between them, (iv) the occurrence of hemidesmosome-and desmosome-like junctions between Schwann cell processes, and between Schwann cells and axons, and (v) the appearance of amorphous and fibrillary extracellular deposits alongside the axolemma. The adjacent proximal and distal segments were normal, i.e., axons remained continuous, and the alterations were confined to the segment exposed to the protease inhibitors. Heated APP K +, APP without the Kunitz insert (APP K -), bovine serum albumin, and saline, did not elicit cytological alterations. Our results suggest that these inhibitors of serine proteases (i) set free a sprouting drive of axons by disrupting an ongoing repressive mechanism; (ii) modify the adhesive properties of axons and Schwann cells, and (iii) alter the natural history of an extracellular material. The imbalance of an extracellular protease system may participate in the pathogenesis of Alzheimer's disease.
We studied the cytological alterations produced in the rat sciatic nerve by the amyloid precursor... more We studied the cytological alterations produced in the rat sciatic nerve by the amyloid precursor protein (APP) containing the Kunitz insert (APP K+) and other protease inhibitors. Conditioning of nerve segments with APP K+, aprotinin or leupeptin for 5 days or more resulted in mitosis of Schwann cells, demyelination of fibres, and a <10-fold increase in Schwann cells, associated with demyelinated fibres. Altered fibres nevertheless involved a small part of the population. Nerve segments proximal and distal to the conditioned region showed almost no alteration. Conditioning with saline, heated APP K+, or APP without the Kunitz insert was not effective. We conclude that APP K+ and other protease inhibitors induce Schwann cells to enter the cell cycle, and once committed to proliferate they resorb their myelin. These functional properties of APP may be relevant to the pathogenesis of Alzheimer's disease.
Abstract: The hallmark event of Alzheimer's disease (AD) is the deposition of amyloid as insolubl... more Abstract: The hallmark event of Alzheimer's disease (AD) is the deposition of amyloid as insoluble fiber masses in extracellular neuritic plaques and around the walls of cerebral blood vessels. The main component of amyloid is a hydrophobic peptide, named amyloid β-peptide (βA4), which results from the processing of a much longer membrane amyloid precursor protein (APP). This review focuses on the structural features of βA4 and the factors that determine βA4 insolubilization. Theoretical and experimental studies of the primary structure of βA4 have shown that it is composed of a completely hydrophobic C-terminal domain, which adopts β-strand structure, and an N-terminal region, whose sequence permits different secondary structures. In fact, this region can exist as an α-helical or β-strand conformation depending on the environmental condition (pH and hydrophobicity surrounding the molecule). The effects of pH and hydrophobicity on βA4 structure may elucidate the mechanisms determining its aggregation and amyloid deposition in AD.
The calibers and microtubular content of axons were studied in normal and regenerating fibers of ... more The calibers and microtubular content of axons were studied in normal and regenerating fibers of the sural nerve from 17 to 122 days after a lesion of the sciatic nerve of young adult rats. During this period (70–175 days of age), the cross-sectional area of control myelinated axons almost doubled but that of nonmedullated axons did not change. In regenerating nerves, after 122 days of recovery, the cross-sectional area of myelinated fibers was still 38% below that of the normal side. In contrast, the regenerating nonmedullated population was richer in fine (< 0.2 μm2) and in coarse (> 0.9 μm2) fibers than on the control side; the cross-sectional area averages were 0.50 and 0.54–0.70 μm2 for the normal and regenerating populations, respectively.The microtubular density of normal 3-μm myelinated fibers averaged 24.0 microtubules/μm2. In regenerating fibers of the same size the density varied between 19.2 and 23.2 microtubules/μm2. Microtubular density values of normal and regenerating fibers were not statistically different. In nonmedullated fibers, the microtubular content (expressed as microtubular density or number of microtubules per axon) correlated with the caliber of the fiber. In these correlations, only minor differences were observed between regenerating and uninjured fibers.Our results indicate that nonmedullated fibers terminate their radial growth well before myelinated fibers do, and that axonal microtubular content correlates with the local size of the fiber and is largely insensitive to regeneration.
Regenerating axons need proteins to grow and we explored whether a local supply is necessary. Cru... more Regenerating axons need proteins to grow and we explored whether a local supply is necessary. Crushed peroneal nerves were entubulated with silicone sleeves, plain or loaded with cycloheximide (CHX); some nerves were frozen to kill resident cells. When a plain sleeve was placed distal to the crush, axons regrew 5.0 mm in 3 days (pinch test), and 4.6 mm when the sleeve was placed around a frozen nerve (n.s). CHX administered distal to the crush reduced the elongation by~58% (P Ͻ 0.01) in unfrozen or frozen nerves whilst its administration central to the crush was ineffectual. Immunostaining of nerves with GAP-43 gave similar values. Under the electron microscope, axonal sprouts were less frequent when CHX was used irrespective of the cellular or acellular condition of the nerve. Therefore, an inhibitor of protein synthesis reduces axonal regrowth, an effect mediated neither by parent neurones nor by resident cells. We propose that axons synthesize proteins.
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Papers by Jaime Alvarez