Mitosis of Schwann cells and demyelination are induced by the amyloid precursor protein and other protease inhibitors in the rat sciatic nerve

Inhibition of calpains fails to improve regeneration through a peripheral nerve conduit

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Calpain inhibitor leupeptin locally applied to transected sciatic nerve of rats.

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Axon number and myelination not significantly increased 3 months after lesion.

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No difference in behavioral tests after nerve regeneration.

Intramuscular injection of the calpain inhibitor leupeptin promotes peripheral nerve regeneration in primates (Badalamente et al., 1989 [13]), and direct positive effects of leupeptin on axon outgrowth were observed in vitro (Hausott et al., 2012 [12]). In this study, we applied leupeptin (2 mg/ml) directly to collagen-filled nerve conduits in the rat sciatic nerve transection model. Analysis of myelinated axons and retrogradely labeled motoneurons as well as functional ‘CatWalk’ video analysis did not reveal significant differences between vehicle controls and leupeptin treated animals. Therefore, leupeptin does not improve nerve regeneration via protease inhibition in regrowing axons or in surrounding Schwann cells following a single application to a peripheral nerve conduit suggesting indirect effects on motor endplate integrity if applied systemically.

The MMP-9/TIMP-1 axis controls the status of differentiation and function of myelin-forming Schwann cells in nerve regeneration

Background

Myelinating Schwann cells (mSCs) form myelin in the peripheral nervous system. Because of the works by us and others, matrix metalloproteinase-9 (MMP-9) has recently emerged as an essential component of the Schwann cell signaling network during sciatic nerve regeneration.

Methodology/Principal Findings

In the present study, using the genome-wide transcriptional profiling of normal and injured sciatic nerves in mice followed by extensive bioinformatics analyses of the data, we determined that an endogenous, specific MMP-9 inhibitor [tissue inhibitor of metalloproteinases (TIMP)-1] was a top up-regulated gene in the injured nerve. MMP-9 capture followed by gelatin zymography and Western blotting of the isolated samples revealed the presence of the MMP-9/TIMP-1 heterodimers and the activated MMP-9 enzyme in the injured nerve within the first 24 h post-injury. MMP-9 and TIMP-1 co-localized in mSCs. Knockout of the MMP-9 gene in mice resulted in elevated numbers of de-differentiated/immature mSCs in the damaged nerve. Our comparative studies using MMP-9 knockout and wild-type mice documented an aberrantly enhanced proliferative activity and, accordingly, an increased number of post-mitotic Schwann cells, short internodes and additional nodal abnormalities in remyelinated nerves of MMP-9 knockout mice. These data imply that during the first days post-injury MMP-9 exhibits a functionally important anti-mitogenic activity in the wild-type mice. Pharmacological inhibition of MMP activity suppressed the expression of Na_v1.7/1.8 channels in the crushed nerves.

Conclusion/Significance

Collectively, our data established an essential role of the MMP-9/TIMP-1 axis in guiding the mSC differentiation and the molecular assembly of myelin domains in the course of the nerve repair process. Our findings of the MMP-dependent regulation of Na_v channels, which we document here for the first time, provide a basis for therapeutic intervention in sensorimotor pathologies and pain.

Leupeptin enhances cell surface localization of fibroblast growth factor receptor 1 in adult sensory neurons by increased recycling

Fibroblast growth factors (FGFs) act as trophic factors during development and regeneration of the nervous system. FGFs mediate their responses by activation of four types of FGF receptors (FGFR1-4). FGFR1 is expressed in adult sensory neurons of dorsal root ganglia (DRG), and overexpression of FGFR1 enhances FGF-2-induced elongative axon growth in vitro. Ligand-induced activation of FGFR1 is followed by endocytosis and rapid lysosomal degradation. We previously reported that the lysosomal inhibitor leupeptin prevents degradation of FGFR1 and promotes FGF-2-induced elongative axon growth of DRG neurons overexpressing FGFR1. Therefore, we analyzed the effects of leupeptin on intracellular sorting of FGFR1 in PC12 pheochromocytoma cells and DRG neurons. Leupeptin increased colocalization of FGFR1 with lysosomes. Furthermore, leupeptin enhanced the cell surface localization of FGFR1 by increased receptor recycling and this effect was abolished by the recycling inhibitor monensin. In addition, a lysine mutant of FGFR1, which is preferentially recycled back to the cell surface, promoted elongative axon growth of DRG neurons similar to leupeptin. In contrast, the lysosomal inhibitor bafilomycin had no effect on surface localization of FGFR1, inhibited axon growth of DRG neurons and abolished the effects of leupeptin on receptor recycling. Together, our results strongly imply that increased recycling of FGFR1 promotes axon elongation, but not axonal branching, of adult DRG neurons in vitro.

Remodeling of motor nerve terminals in demyelinating axons of periaxin-null mice

Myelin formation around axons increases nerve conduction velocity and influences both the structure and function of the myelinated axon. In the peripheral nervous system, demyelinating forms of hereditary Charcot-Marie-Tooth (CMT) diseases cause reduced nerve conduction velocity initially and ultimately axonal degeneration. Several mouse models of CMT diseases have been generated, allowing the study of the consequences of disrupting Schwann cell function on peripheral nerve fibers. Nevertheless, the effect of demyelination at the level of the neuromuscular synapse has been largely overlooked. Here we show that in mice lacking functional Periaxin (Prx) genes, a model of a recessive type of CMT disease known as CMT4F, neuromuscular junctions (NMJs) develop profound morphological changes in the preterminal region of motor axons. These changes include extensive preterminal branches that originate in demyelinated regions of the nerve fiber and axonal swellings associated with residually-myelinated regions of the fiber. Using intracellular recording from muscle fibers we detected asynchronous failure of action potential transmission at high but not low stimulation frequencies, a phenomenon consistent with branch point failure. Taken together, our morphological and electrophysiological findings suggest that preterminal branching due to segmental demyelination near the neuromuscular synapse in Periaxin KO mice may underlie some characteristics of disabilities, including coordination deficits, present in this mouse model of CMT disease. These results reveal the importance of studying how demyelinating diseases might influence NMJ function and contribute to clinical disability.

Lengthening of G2/mitosis in cortical precursors from mice lacking β-amyloid precursor protein

The beta-amyloid precursor protein (APP) is expressed within the nervous system, even at the earliest stages of embryonic development when cell growth and proliferation is particularly important. In order to study the function of APP at these early developmental stages, we have studied the development of the cerebral cortex in both wild type and App-/- mutant mice. Here, we demonstrate that APP mRNA is expressed in cortical precursor cells and that APP protein is concentrated within their apical domains during interphase. However, during mitosis, APP re-localizes to the peripheral space surrounding the metaphase plate. In APP-deficient cortical precursors, the duration of mitosis is increased and a higher proportion of cortical precursor cells contained nuclei in late G2. We conclude that during cortical development APP plays a role in controlling cell cycle progression, particularly affecting G2 and mitosis. These observations may have important implications for our understanding of how APP influences the progression of Alzheimer's disease, since degenerating cortical neurons have been shown to up-regulate cell cycle markers and re-enter the mitotic cycle before dying.

Structure and function of amyloid in Alzheimer's disease

This review is focused on the structure and function of Alzheimer's amyloid deposits. Amyloid formation is a process in which normal well-folded cellular proteins undergo a self-assembly process that leads to the formation of large and ordered protein structures. Amyloid deposition, oligomerization, and higher order polymerization, and the structure adopted by these assemblies, as well as their functional relationship with cell biology are underscored. Numerous efforts have been directed to elucidate these issues and their relation with senile dementia. Significant advances made in the last decade in amyloid structure, dynamics and cell biology are summarized and discussed. The mechanism of amyloid neurotoxicity is discussed with emphasis on the Wnt signaling pathway. This review is focused on Alzheimer's amyloid fibrils in general and has been divided into two parts dealing with the structure and function of amyloid.

The N‐terminal copper‐binding domain of the amyloid precursor protein protects against Cu2+neurotoxicity in vivo

The amyloid precursor protein (APP) contains a Cu binding domain (CuBD) localized between amino acids 135 and 156 (APP135-156), which can reduce Cu2+ to Cu1+ in vitro. The physiological function of this APP domain has not yet being established; nevertheless several studies support the notion that the CuBD of APP is involved in Cu homeostasis. We used APP synthetic peptides to evaluate their protective properties against Cu2+ neurotoxicity in a bilateral intra-hippocampal injection model. We found that human APP135-156 protects against Cu2+-induced neurotoxic effects, such as, impairment of spatial memory, neuronal cell loss, and astrogliosis. APP135-156 lacking two histidine residues showed protection against Cu2+; however, APP135-156 mutated in cysteine 144, a key residue in the reduction of Cu2+ to Cu1+, did not protect against Cu2+ neurotoxicity. In accordance with recent reports, the CuBD of the Caenorhabditis elegans, APL-1, protected against Cu2+ neurotoxicity in vivo. We also found that Cu2+ neurotoxicity is associated with an increase in nitrotyrosine immunofluorescence as well as with a decrease in Cu2+ uptake. The CuBD of APP therefore may play a role in the detoxification of brain Cu.

Characterisation of matrix metalloproteinases and the effects of a broad-spectrum inhibitor (BB-1101) in peripheral nerve regeneration

The effect of treatment with a broad-spectrum inhibitor (BB1101) of the matrix metalloproteinases (MMPs) on nerve regeneration and functional recovery after nerve crush was examined. Drug treatment had no effect on latency but from 63 days the compound muscle action potential was significantly increased and was no different to that in the sham-operated controls at 72 days. Levels of MMP mRNA expression, and the localisation and activity of MMP proteins, were examined in rats for a 2 month period following a nerve crush injury, and compared with sham-operated controls. The mRNA of all the MMPs studied was up-regulated by 5-10 days after nerve crush, and they remained up-regulated for 40-63 days, except for MMP-9 which was down-regulated by 10 days. MMP immunoreactivity was localised to Schwann cells, macrophages and endothelial cells, and with the exception of membrane type 1-MMP (MT1-MMP), it was more intense after nerve crush compared with sham-operated controls. Regenerating axons showed immunoreactivity for MMP-2 and MMP-3. In situ zymography confirmed that the activity of MMPs in the nerve was increased following crush but that the activity was greatly reduced in rats treated with BB-1101. Thus despite the inhibition of MMPs by BB-1101, the drug did not appear to essentially affect nerve degeneration or regeneration following nerve crush but that it could be beneficial in promoting the more effective reinnervation of muscles possibly by actions at the level of the muscles.

Comparison of matrix metalloproteinase expression during Wallerian degeneration in the central and peripheral nervous systems

The matrix metalloproteinases (MMPs) are a large family of zinc-dependent enzymes which are able to degrade the protein components of the extracellular matrix. They can be placed into subgroups based on structural similarities and substrate specificity. Aberrant expression of these destructive enzymes has been implicated in the pathogenesis of immune-mediated neuroinflammatory disorders. In this study we investigate the involvement of MMPs, from each subgroup, in Wallerian degeneration in both the central and peripheral nervous systems. Wallerian degeneration describes the process initiated by transection of a nerve fibre and entails the degradation and removal of the axon and myelin from the distal stump. A similar degenerative process occurs as the final shared pathway contributing to most common neuropathies. MMP expression and localisation in the peripheral nervous system are compared with events in the CNS during Wallerian degeneration. Within 3 days after axotomy in the peripheral nervous system, MMP-9, MMP-7 and MMP-12 are elevated. These MMPs are produced by Schwann cells, endothelial cells and macrophages. The temporospatial expression of activated MMP-9 correlates with breakdown of the blood-nerve barrier. In the CNS, 1 week after optic nerve crush, four MMPs are induced and primarily localised to astrocytes, not microglia or oligodendrocytes. In the degenerating optic nerve, examined at later time points (4, 8, 12 and 18 weeks), MMP expression was down-regulated. The absence of MMPs in oligodendrocytes and mononuclear phagocytes during Wallerian degeneration may contribute to the slower removal of myelin debris observed in the CNS. The low level of the inactive pro-form of MMP-9 in the degenerating optic nerve may explain why the blood-brain barrier remains intact, while the blood-nerve barrier is rapidly broken down. We conclude that the difference in the level of expression, activation state and cellular distribution of MMPs may contribute to the different sequence of events observed during Wallerian degeneration in the peripheral compared to the CNS.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Protein synthesis in axons and terminals: significance for maintenance, plasticity and regulation of phenotype. With a critique of slow transport theory

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 identified presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modifiable 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 flawed 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 specific involvement of the neuronal nucleus.

Acetylcholinesterase and inhibitors: effects upon normal and regenerating nerves of the rat

In peripheral nerves, the function of acetylcholinesterase (AChE) is not related to hydrolysis of acetylcholine. To test for a trophic role, AChE or its inhibitors were administered locally to normal and regenerating nerves of rats. In the normal nerve, neither AChE nor serum albumin affected the cytological pattern of the nerve. BW284c51, a specific inhibitor of AChE, resulted in demyelination, proliferation of Schwann cells and sprouting of axons after 5-7 days. Edrophonium or propidium, other specific inhibitors of AChE, did so to a much lesser extent. Vehicle, and iso-OMPA (inhibitor of pseudocholinesterases) did not affect the cytology of the nerve. Elongation of regenerating axons was evaluated at day 3 post-crush. Native AChE applied distal to the crush reduced the elongation of regenerating axons (- 36%), while serum albumin, heated AChE and filtered AChE did not. BW284c51, edrophonium or propidium enhanced the axonal elongation (33%) when they were administered for 2 days before, but not after, the crush. Iso-OMPA or vehicle administered before or after the crush were not effective. Thus, AChE reduces elongation of regenerating axons, while inhibition of AChE enhances elongation and affects the cytology of the normal nerve as well. We propose that AChE has a trophic role in mammalian peripheral nerves.

Mechanisms of neuronal death in Down's syndrome

There is growing evidence that neuronal death in Down's syndrome is due to apoptotic mechanisms. The phenomena, however, that trigger and regulate programmed cell death in the Down's syndrome-related neurodegeneration are still much debated. In vitro evidence has suggested that the main factor responsible for neuronal death in this condition is the accumulation of beta-amyloid, due to the overexpression of its precursor protein. Another hypothesis argues for the importance of reactive oxygen species in neuronal death. However, the in vivo findings do not entirely support either theories. We propose that neuronal apoptosis, as well as the formation of Alzheimer-type pathology, in Down's syndrome is due to an aberrant re-entry of neurones into the cell division cycle. Due to the simultaneous overexpression of conflicting cell cycle regulatory signals the mitogenic amyloid precursor and the differentiation factor S100, the cell cycle is abandoned. Subsequently the cell cycle arrest may lead to either the formation of Alzheimer-related pathology or to apoptotic cell death.

Peripheral axons of Wlds mice, which regenerate after a delay of several weeks, do so readily when transcription is inhibited in the distal stump

We have raised the hypothesis that differentiated Schwann cells repress regrowth of axons but become permissive upon dedifferentiation. WId(S) mouse is a strain in which severed peripheral nerves do not degenerate for several weeks, and axonal regeneration does not occur either [5,11]. In this strain, we studied the role of resident cells upon axonal regeneration by inhibiting transcription. Regeneration was assessed with the pinch test, electron microscopy and Dil (a fluorescent lipid soluble dye). After a crush, WId(S) axons did not regenerate but they did so when the crush was made through a nerve segment treated with actinomycin D (ActD), an inhibitor of transcription. In contrast, when the crush was made distal to the treated segment no regeneration ensued. Our results support the notion that normal resident cells of peripheral nerves repress axonal growth.

Cerebrospinal fluid of HTLV-1 associated myelopathy patients induces axonal sproutings and Schwann cell proliferation in the rat sciatic nerve

HTLV-1 (human T-cell leukemia virus type I) associated myelopathy (HAM) is a demyelinating disease. We showed that the CSF of patients and heated CSF of normal subjects induce a segmentary demyelination in rat nerves, and potentiate trypsin in vitro. Here we further characterize the neuropathy induced by the CSF of patients. Peroneal nerves injected 5-8 days before with native or heated CSF of patients, besides extensive demyelination, presented proliferation of myelinating and nonmyelinating Schwann cells, axonal sprouting, fine fibres with a few turns of myelin, disarray of nonmedullated bundles, desmosome-like junctions, and coated pits and vesicles in Schwann cells and axons. The normal CSF was innocuous to the nerve in its native form, but after heating, it induced a neuropathy in all, similar to that elicited by the CSF of patients. Our findings indicate that the CSF of HAM patients contains a thermostable pathogen for nerves of the rat; a thermostable pathogen also occurs in the normal CSF although its activity is checked by endogenous thermolabile factors. We suggest that the pathogen present in the CSF of HAM patients participates in the disease.

Cellular and molecular basis of estrogen's neuroprotection. Potential relevance for Alzheimer's disease

Alzheimer's disease (AD) is one of the most common types of dementia among the aged population, with a higher prevalence in women. The reason for this latter observation remained unsolved for years, but recent studies have provided evidence that a lack of circulating estrogen in postmenopausal women could be a relevant factor. Moreover, follow-up studies among postmenopausal women who had received estrogen-replacement therapy (ERT), suggested that they had a markedly reduced risk of developing AD. In addition, studies among older women who already had AD indeed confirmed that a decrease in estrogen levels was likely to be an important factor in triggering the pathogenesis of the disease. In this review article, we will discuss the evidence suggesting that estrogen may have a protective role against AD, mainly through its action as: a trophic factor for cholinergic neurons, a modulator for the expression of apolipoprotein E (ApoE) in the brain, an antioxidant compound decreasing the neuronal damage caused by oxidative stress, and a promoter of the physiological nonamyloidogenic processing of the amyloid precursor protein (APP), decreasing the production of the amyloid-beta-peptide (A beta), a key factor in the pathogenesis of AD.

The amyloid precursor protein gene: a neuropeptide gene with diverse functions in the central nervous system

The amyloid precursor protein (APP) is a member of a family of proteins found in the central nervous system with a fundamental role in the pathogenesis of Alzheimer's disease. This review describes the experimental evidence that has provided functional insights into this protein and emphasizes the importance of APP in many neurobiological processes.

Segmental demyelination induced by cerebrospinal fluid of progressive spastic paraparesis: correlation with altered proteolytic parameters

Progressive spastic paraparesis (PSP) is a demyelinating disease of the central nervous system. We studied the ability of the cerebrospinal fluid (CSF) of patients to induce alterations in rat peroneal nerves, and to modify the proteolytic activity of trypsin in vitro. Subperineurial injection of native or heated CSF of patients induced segmental demyelination and other cytological alterations 5-7 days later, in the infiltrated zone, while proximal and distal regions were normal. The CSF of normal subjects did not induce demyelination, but upon heating, it did so. Trypsin was strongly inhibited by the normal CSF but upon heating, its inhibitory activity was replaced by a strong potentiation. In contrast, native and heated CSF of patients potentiated trypsin. Our findings indicate that (1) the normal CSF contains a thermostable factor that potentiates trypsin whose function is overruled by thermolabile protease inhibitors; (2) the CSF of PSP patients has a reduced inhibitory activity and a conserved ability to potentiate trypsin; and (3) the CSF is endowed with a pathogenic power that correlates with an unchecked potentiating activity. We propose that the imbalance of a protease system may play a role in the pathogenesis of PSP lesions.

Sprouting and abnormal contacts of nonmedullated axons, and deposition of extracellular material induced by the amyloid precursor protein (APP) and other protease inhibitors

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 myclin, (iii) abnormal contracts 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 Kappa +, 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.

Extracellular matrix regulates the amount of the beta-amyloid precursor protein and its amyloidogenic fragments

We have studied the influence of the extracellular matrix (ECM) on the amount of beta-amyloid precursor protein (APP) and C-terminal amyloid-bearing fragments in 313 fibroblasts. After incubation with ECM components, the cellular APP content of 3T3 cells changed. Besides, different substrata including collagen, fibronectin, laminin, vitronectin, and heparin, determined changes in the amount of a C-terminal 22 kDa-fragment. The regulation of amyloidogenic fragments by the ECM was transient; in fact, when 3T3 cells were plated on tissue culture dishes coated with collagen or vitronectin, maximal levels of the 22 kDa fragment were observed 12 h after plating; in the presence of fibronectin, the maximum level of the amyloidogenic fragment was obtained 36 h after plating. These results indicate that the ECM modulates in a transient way the generation of APP-derived polypeptides containing the amyloid-beta-peptide (A beta). The ECM does not have a generalized effect on 3T3 fibroblasts, because no significant differences in cell attachment, growth rate, whole-cell polypeptide pattern beta 1 integrin and alpha-tubulin levels were observed on cells grown on various matrix proteins. Laminin, collagen, and heparin also influence the level of an amyloidogenic fragment of 35 kDa in Neuro 2A neuronal cells, without a significant change in the neuronal marker acetylcholinesterase. In this case, however, a long-lasting response to ECM molecules was observed. These observations provide evidence that ECM molecules influence APP biogenesis, including the generation of amyloidogenic fragments containing the A beta peptide. Our studies might prove significant to understand the localized increment of beta-amyloid deposition in selected areas of the brain of Alzheimer's patients.