Calcium-dependent transglutaminase of rat sympathetic ganglion in development and after nerve injury

A molecular mechanism of optic nerve regeneration in fish: the retinoid signaling pathway

The fish optic nerve regeneration process takes more than 100 days after axotomy and comprises four stages: neurite sprouting (1-4 days), axonal elongation (5-30 days), synaptic refinement (35-80 days) and functional recovery (100-120 days). We screened genes specifically upregulated in each stage from axotomized fish retina. The mRNAs for heat shock protein 70 and insulin-like growth factor-1 rapidly increased in the retinal ganglion cells soon after axotomy and function as cell-survival factors. Purpurin mRNA rapidly and transiently increased in the photoreceptors and purpurin protein diffusely increased in all nuclear layers at 1-4 days after injury. The purpurin gene has an active retinol-binding site and a signal peptide. Purpurin with retinol functions as a sprouting factor for thin neurites. This neurite-sprouting effect was closely mimicked by retinoic acid and blocked by its inhibitor. We propose that purpurin works as a retinol transporter to supply retinoic acid to damaged RGCs which in turn activates target genes. We also searched for genes involved in the second stage of regeneration. The mRNA of retinoid-signaling molecules increased in retinal ganglion cells at 7-14 days after injury and tissue transglutaminase and neuronal nitric oxide synthase mRNAs, RA-target genes, increased in retinal ganglion cells at 10-30 days after injury. They function as factors for the outgrowth of thick, long neurites. Here we present a retinoid-signaling hypothesis to explain molecular events during the early stages of optic nerve regeneration in fish.

Transglutaminase is linked to neurodegenerative diseases

Transglutaminase catalyzes a covalent bond between peptide-bound glutamine residues and either lysine-bound peptide residues or mono- or polyamines. Multiple lines of evidence suggest that transglutaminase is involved in neurodegenerative diseases including Alzheimer disease, progressive supranuclear palsy, Huntington disease (HD), and Parkinson disease. In all of the neurodegenerative diseases examined to date, transglutaminase enzyme activity is upregulated in selectively vulnerable brain regions, transglutaminase proteins are associated with inclusion bodies characteristic of the diseases, and prominent proteins in the inclusion bodies are modified by transglutaminase enzymes. These prominent proteins in the inclusion bodies, including tau, alpha-synuclein, and huntingtin protein, are modified by transglutaminase in vitro and alpha-synuclein and huntingtin protein are modified in cells in culture. Similar changes in transglutaminase and transglutaminase-modified proteins are replicated in transgenic mouse models of the neurodegenerative diseases, including Huntington disease and progressive supranuclear palsy. Lastly, inhibition of transglutaminase either via drug treatments or molecular approaches is beneficial for the treatment of HD transgenic mice but has yet to be explored for the other neurodegenerative diseases. Further research is needed to determine the specific role(s) that transglutaminase plays in the pathophysiology of neurodegenerative diseases with possible implications for transglutaminase as a therapeutic target.

Upregulation of retinal transglutaminase during the axonal elongation stage of goldfish optic nerve regeneration

Fish CNS neurons can repair their axons following nerve injury, whereas mammalian CNS neurons cannot regenerate, and become apoptotic within 1-2 weeks after the nerve lesion. One explanation for these differences is that one, or several molecules are upregulated in fish CNS neurons during nerve regeneration, and this same molecule is downregulated in mammalian CNS neurons before the development of apoptosis caused by nerve injury. A molecule satisfying these criteria might successfully rescue and repair the mammalian CNS neurons. In this study, we looked for such a candidate molecule from goldfish retinas. Transglutaminase derived from goldfish retina (TG(R)) was characterized as a regenerating molecule after optic nerve injury. A full-length cDNA for TG(R) was isolated from the goldfish retinal cDNA library prepared from axotomized retinas. Levels of TG(R) mRNA and protein increased only in the retinal ganglion cells (RGCs) between 10 and 40 days after optic nerve transection. Recombinant TG(R) protein enhanced neurite outgrowth from adult fish RGCs in culture. Specific interference RNA and antibodies for TG(R) inhibited neurite outgrowth both in vitro and in vivo. In contrast, the level of TG(R) protein decreased in rat RGCs within 1-3 days after nerve injury. Furthermore, the addition of recombinant TG(R) to retinal cultures induced striking neurite outgrowth from adult rat RGCs. These molecular and cellular data strongly suggest that TG(R) promotes axonal elongation at the surface of injured RGCs after optic nerve injury.

Reversible changes in goldfish brain polyamine concentrations and synthetic enzymes after cold exposure

Exposure of goldfish to the cold (5 degrees C) caused a sharp increase in brain putrescine level during the first week. Such increase continued at a minor rate for the whole period of exposure (2 months). In contrast, the content of spermidine and spermine remained unchanged. Putrescine increase was concomitant with a remarkable rise in ornithine decarboxylase activity (ODC), which reached a maximum stimulation after 1 week of cold exposure, and declined thereafter, remaining significantly higher than the control for the entire period of study. Cold exposure caused also a reduction of S-adenosylmethionine decarboxylase (AdoMetDC) activity and an increase of ornithine level, whereas methionine content was unchanged. When fish exposed to cold temperature were returned to 20 degrees C, the modifications observed on brain polyamine metabolism were completely reversed. Supported by previous observations, our results suggest that the changes in the polyamine metabolism induced in goldfish brain by cold exposure could represent an homeostatic mechanism carried out by the goldfish to minimize the possible effects of thermal changes.

Axonal Regeneration of Fish Optic Nerve after Injury

Since Sperry's work in the 1950s, it has been known that the central nervous system (CNS) neurons of lower vertebrates such as fish and amphibians can regenerate after axotomy, whereas the CNS neurons of mammals become apoptotic after axotomy. The goldfish optic nerve (ON) is one of the most studied animal models of CNS regeneration. Morphological changes in the goldfish retina and tectum after ON transection were first researched in the 1970s-1980s. Many biochemical studies of neurite outgrowth-promoting substances were then carried out in the 1980s-1990s. Many factors have been reported to be active substances that show increased levels during fish ON regeneration, as shown by using various protein chemistry techniques. However, there are very few molecular cloning techniques for studying ON regeneration after injury. In this review article, we summarize the neurite outgrowth-promoting factors reported by other researchers and describe our strategies for searching for ON regenerating molecules using a differential hybridization technique in the goldfish visual system. The process of goldfish ON regeneration after injury is very long. It takes about half a year from the start of axonal regrowth to complete restoration of vision. The process has been classified into three stages: early, middle and late. We screened for genes with increased expression during regeneration using axotomized goldfish retinal and tectal cDNA libraries and obtained stage-specific cDNA clones that were upregulated in the retina and tectum. We further discuss functional roles of these molecules in the regeneration processes of goldfish ON.

NMDA-evoked excitotoxicity increases tissue transglutaminase in cerebellar granule cells

In neuronal cells, excessive activation of glutamate receptors causes excitotoxic damage culminating in apoptotic and necrotic cell death. The molecular mechanism of excitotoxicity has been associated with excessive Ca(2+) influx and overload, triggering biochemical events that lead to cell death and tissue degeneration. Following mild insults via NMDA-receptor activation, central neurons undergo several biochemical modifications recognizable as early events in apoptotic machinery.Tissue transglutaminase, the most ubiquitous among cell transglutaminases, catalyzes the Ca(2+)-dependent protein cross-linking probably associated with morphological changes in several neurodegenerative disorders. The possible involvement of this enzyme in excitotoxicity-mediated events was investigated in primary cultures of cerebellar granule cells exposed for 30 min to NMDA (100 microM) in Locke's buffer. Under these conditions time-dependent increases in transglutaminase activity were observed. Tissue transglutaminase expression reached the highest levels within 3-4 h of NMDA exposure. Similarly, high levels of incorporation of fluorescent substrates were observed in living cells. Confocal laser microscopy analysis showed that fluorescein-labelled structures were distributed within the cytoplasm and close to the membranes of NMDA-exposed cells. These effects were dependent on the Ca(2+) influx triggered by the excitotoxic stimulus. Morphological changes in NMDA-treated cells gave evidence of significant cell damage which appeared within 5-6 h of NMDA exposure. These results suggest that increases in tissue transglutaminase may be associated to the effects of NMDA-induced excitotoxicity. Therefore, it is reasonable to hypothesize that if tissue transglutaminase levels and activity are up-regulated under such conditions, the protein cross-linking could be likely involved in excitotoxic response.

Therapeutic effects of cystamine in a murine model of Huntington's disease

The precise cause of neuronal death in Huntington's disease (HD) is unknown. Proteolytic products of the huntingtin protein can contribute to toxic cellular aggregates that may be formed in part by tissue transglutaminase (Tgase). Tgase activity is increased in HD brain. Treatment in R6/2 transgenic HD mice, using the transglutaminase inhibitor cystamine, significantly extended survival, improved body weight and motor performance, and delayed the neuropathological sequela. Tgase activity and N(Sigma)-(gamma-L-glutamyl)-L-lysine (GGEL) levels were significantly altered in HD mice. Free GGEL, a specific biochemical marker of Tgase activity, was markedly elevated in the neocortex and caudate nucleus in HD patients. Both Tgase and GGEL immunoreactivities colocalized to huntingtin aggregates. Cystamine treatment normalized transglutaminase and GGEL levels in R6/2 mice. These findings are consistent with the hypothesis that transglutaminase activity may play a role in the pathogenesis of HD, and they identify cystamine as a potential therapeutic strategy for treating HD patients.

Injury-induced "switch" from GTP-regulated to novel GTP-independent isoform of tissue transglutaminase in the rat spinal cord

We recently found that alternative transcripts of tissue transglutaminase (tTG or TG2) were present in hippocampal brain regions of Alzheimer's disease (AD), but not in control, non-demented, age-matched brains. Since antecedent non-severe trauma has been implicated in AD and other neurodegenerative diseases, such as Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), we were interested in whether alternative transcripts might be detected in a model of neurotrauma, controlled-contusion spinal cord injury (SCI) in the rat. Implicated in diverse roles from growth and differentiation to apoptotic cell death, only bifunctional tTG, of the nine member TG family, has dual catalytic activities: guanine trinucleotide (GTP) hydrolyzing activity (GTPase), as well as protein cross-linking. These functions imply two physiological functions: programmed cell life and death. These may have profound roles in the nervous system since studies in cultured astrocytes found tTG short (S) mRNA transcripts induced by treatment with injury-related cytokines. In the developing rat spinal cord, tTG activity is concentrated in ventral horn alpha motoneurons, but neither studies of spinal cord tTG gene expression, nor evaluation of the GTP-regulated isoforms in tissues, have been reported. We now report increased tTG protein and gene expression occurring rapidly after SCI. In parallel, novel appearance of a second, short form transcript, in addition to the normal long (L) isoform, occurs by 8 h of injury. Up-regulation of tTG message and activity following neural injury. with appearance of a truncated GTP-unregulated S form, may represent new approaches to drug targets in neurotrauma.

Tissue transglutaminase is essential for neurite outgrowth in human neuroblastoma SH-SY5Y cells

Tissue transglutaminase is a normal constituent of the central and peripheral nervous systems and in rats transglutaminase activity in brain and spinal cord is highest during fetal stages when axonal outgrowth is occurring. Further, treatment of human neuroblastoma SH-SY5Y cells with retinoic acid results in the cells withdrawing from the cell cycle and extending neurites, in the same time frame that tissue transglutaminase expression significantly increases. Considering these and other previous findings, this study was carried out to determine whether tissue transglutaminase is involved in neuronal differentiation of SH-SY5Y cells. For these studies SH-SY5Y cells stably overexpressing wild-type tissue transglutaminase, an inactive tissue transglutaminase mutant (C277S) or an antisense tissue transglutaminase construct (which decreased endogenous tissue transglutaminase below detectable levels) were used. SH-SY5Y cells overexpressing wild-type tissue transglutaminase spontaneously differentiated into a neuronal phenotype when grown in low-serum media. In contrast, cells overexpressing inactive tissue transglutaminase or the antisense tissue transglutaminase continued to proliferate and exhibit a flat polygenic morphology even when maintained in low-serum conditions. In addition, increased tissue transglutaminase expression in response to retinoic acid was abolished in the antisense tissue transglutaminase cells, and antisense and mutant tissue transglutaminase expressing cells did not extend neurites in response to retinoic acid. Moreover, wild-type and inactive tissue transglutaminase exhibited differential intracellular localization. These data indicate that tissue transglutaminase is necessary and sufficient for neuronal differentiation of human neuroblastoma SH-SY5Y cells.

Transglutaminase-induced cross-linking of tau proteins in progressive supranuclear palsy

The mechanisms leading to the abnormal self-polymerization of tau into straight and paired helical filaments (PHFs) and neurofibrillary tangles (NFT) in Alzheimer disease (AD) and progressive supranuclear palsy (PSP) are not known. However, transglutaminase-induced cross-linking of PHF-tau was observed in AD and thus may also contribute to the formation of NFT in other neurodegenerative disorders including PSP. Tissue homogenates from PSP and normal age-matched controls were used to immunoaffinity-purify proteins containing transglutaminase-induced epsilon-(gamma-glutamyl) lysine cross-links. The immunoaffinity-purified proteins were then examined on immunoblots with a PHF-tau antibody, PHF-1. There were significantly higher levels of epsilon-(gamma-glutamyl) lysine cross-linking of PHF-tau in globus pallidus and pons regions of PSP cases compared to barely detectable cross-links in controls. The occipital cortex, an area spared from neurofibrillary pathology in PSP, showed no detectable cross-linking of PHF-tau protein in either PSP cases or control cases. Double-label immunofluorescence demonstrated the colocalization of the cross-link and PHF-tau in NFT in pons of PSP Previous studies and present data are consistent with the hypothesis that transglutaminase-induced cross-linking may be a factor contributing to the abnormal polymerization and stabilization of tau in straight and PHFs leading to neurofibrillary tangle formation in neurodegenerative diseases, including PSP and AD.

Tissue transglutaminase: a possible role in neurodegenerative diseases

Tissue transglutaminase is a multifunctional protein that is likely to play a role in numerous processes in the nervous system. Tissue transglutaminase posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines. This action results in the formation of protein crosslinks or the incorporation of polyamines into substrate proteins, modifications that likely have significant effects on neural function. Tissue transglutaminase is a unique member of the transglutaminase family as in addition to catalyzing the calcium-dependent transamidation reaction, it also binds and hydrolyzes ATP and Guanosine 5'-triphosphate and may play a role in signal transduction. Tissue transglutaminase is a highly regulated and inducible enzyme that is developmentally regulated in the nervous system. In vitro, numerous substrates of tissue transglutaminase have been identified, and several of these proteins have been shown to be in situ substrates as well. Several specific roles for tissue transglutaminase have been described and there is evidence that tissue transglutaminase may also play a role in apoptosis. Recent findings have provided evidence that dysregulation of tissue transglutaminase may contribute to the pathology of several neurodegenerative conditions including Alzheimer's disease and Huntington's disease. In both of these diseases tissue transglutaminase and transglutaminase activity are elevated compared to age-matched controls. Further, immunohistochemical studies have demonstrated that there is an increase in tissue transglutaminase reactivity in affected neurons in both Alzheimer's and Huntington's disease. Although intriguing, many issues remain to be addressed to definitively establish a role for tissue transglutaminase in these neurodegenerative diseases.

Differential expression of multiple transglutaminases in human brain. Increased expression and cross-linking by transglutaminases 1 and 2 in Alzheimer's disease

The transglutaminase (TGase) family of enzymes, of which seven different members are known in the human genome, participate in many biological processes involving cross-linking proteins into large macromolecular assemblies. The TGase 2 enzyme is known to be present in neuronal tissues and may play a role in neuronal degenerative diseases such as Alzheimer's disease (AD) by aberrantly cross-linking proteins. In this paper, we demonstrate by reverse transcriptase-polymerase chain reaction and immunological methods with specific antibodies that in fact three members, the TGase 1, TGase 2, and TGase 3 enzymes, and are differentially expressed in various regions of normal human brain tissues. Interestingly, the TGase 1 and 3 enzymes and their proteolytically processed forms are involved in terminal differentiation programs of epithelial cell development and barrier function. In addition, we found that the levels of expression and activity of the TGase 1 and 2 enzymes were both increased in the cortex and cerebellum of AD patients. Furthermore, whereas normal brain tissues contain approximately 1 residue of cross-link/10,000 residues, AD patient cortex and cerebellum tissues contain 30-50 residues of cross-link/10,000 residues. Together, these findings suggest that multiple TGase enzymes are involved in normal neuronal structure and function, but their elevated expression and cross-linking activity may also contribute to neuronal degenerative disease.

Transglutaminase activity in serum and cerebrospinal fluid in sporadic amyotrophic lateral sclerosis: a possible use as an indicator of extent of the motor neuron loss

The activity of transglutaminase (TGase), a marker enzyme for tissue degeneration, was examined in serum and cerebrospinal fluid (CSF) obtained from patients with sporadic amyotrophic lateral sclerosis (SALS), and compared to those from patients without SALS. When the serum TGase activity values from SALS patients were compared against the 'ALS-scale', which is used for clinical evaluation of the progression of ALS, the TGase activity values were higher at the initial stage of the disease than in non-ALS subjects, whereas they became extremely low at the late stage of ALS. The TGase activity in the CSF taken at later than middle stage from ALS patients with definite clinical motor dysfunctions was significantly lower than in that from non-ALS subjects. We have previously demonstrated marked reduction of tissue TGase activity in all regions of spinal cord tissue transections from ALS patients, not only in ventral but also lateral and dorsal regions, relative to that in non-ALS patients. These results suggest that some TGase may be leaked out of the spinal cord tissue into the CSF and then into the blood-stream during the progression of ALS, and the enzymic activity finally becomes depleted at the terminal stages of the disease when most of the spinal motor neuronal perikarya have been destroyed.

Transglutaminase activity is increased in Alzheimer's disease brain

Transglutaminase is a calcium-activated enzyme that crosslinks substrate proteins into insoluble, often filamentous aggregates resistant to proteases. Because the neurofibrillary tangles in Alzheimer's disease have similar characteristics, and because tau protein, the major component of these tangles is an excellent substrate of transglutaminase in vitro, transglutaminase activity and levels were measured in control and Alzheimer's disease brain. Frozen prefrontal cortex and cerebellum samples from Alzheimer's disease and control cases matched for age and postmortem interval were used in the analyses. Total transglutaminase activity was significantly higher in the Alzheimer's disease prefrontal cortex compared to control. In addition the levels of tissue transglutaminase, as determined by quantitative immunoblotting, were elevated approximately 3-fold in Alzheimer's disease prefrontal cortex compared to control. To our knowledge, this is the first demonstration that transglutaminase is increased in Alzheimer's disease brain. There were no significant differences in transglutaminase activity or levels in the cerebellum between control and Alzheimer's disease cases. Because the elevation of transglutaminase in the Alzheimer's disease samples occurred in the prefrontal cortex, where neurofibrillary pathology is usually abundant, and not in the cerebellum, which is usually spared in Alzheimer's disease, it can be suggested that transglutaminase could be a contributing factor in neurofibrillary tangle formation.

Voltage-sensitive calcium channels mediate calcium entry into cultured mammalian sympathetic neurons following neurite transection

Calcium ion entry following mechanical neurite transection was examined in cultured sympathetic neurons loaded with the Ca2+ indicator fluo-3. Neurite transection produced a rapid [Ca2+]i rise in the cell soma which preceded any [Ca2+]i rise in the neurite (n = 30). Blocking sodium channels with tetrodotoxin had no effect on the Ca2+ rise, but inactivating voltage-sensitive Ca2+ channels by bath-applying 140 mM potassium prior to the transection, and the simultaneous application of nimodipine and omega-conotoxin GVIA, blockers of L-type and N-type Ca2+ channels, respectively, considerably attenuated the Ca2+ rise in the soma and neurites. These data contradict the intuitive hypothesis that Ca2+ entry following mechanical neurite transection occurs via non-specific influx pathways produced by cell-membrane disruption and provide direct evidence in mammalian neurons that immediate, traumatically-induced, increases in neuronal [Ca2+]i are amenable to pharmacological manipulation.

Alteration of transglutaminase activity in rat and human spinal cord after neuronal degeneration

We measured the activity of transglutaminase (TG), a Ca(2+)-dependent enzyme and a biochemical marker of cell degeneration, in the adult rat spinal cord after unilateral occlusion of a branch of the dorsal spinal artery, and compared it to the enzyme activity in the tissue on the contralateral side without ischemic damage. The affected half of the spinal cord showed a significant rise in intrinsic (endogenous) TG activity one day after ischemic insult while no apparent morphological changes were observed in the tissue. However, the enzymic activity on the affected side gradually decreased to reach the level in the non-affected tissue, accompanying severe degeneration of neuronal cells at 7 days after the surgery, then it declined to nearly half the level in the intact tissue 30 days after the operation. We also determined the TG activity in transverse sections of the human spinal cord obtained at autopsy from 5 amyotrophic lateral sclerosis (ALS) and 9 non-ALS patients. TG activity in thoracic and lumbar cords was markedly low in ALS patients not only in ventral and lateral regions but also in the dorsal portion. These findings imply that the reduced TG activity in the ALS spinal cord is one of the characteristic features of the disease reflecting exhaustion of the enzyme in the tissue resulting from degeneration of the spinal neurons through cross-linkage of soluble intraneuronal cytoplasmic proteins.

Transglutaminase C in cerebellar granule neurons: regulation and localization of substrate cross-linking

Covalent cross-linking of cell surface proteins by the calcium-dependent enzyme transglutaminase C may be implicated in cell-cell interactions and growth regulation. We demonstrate the presence of the enzyme in rat cerebellar cortex during postnatal development. Transglutaminase C was induced in cerebellar granule neurons in culture by retinoic acid, dibutyryl- and 8-bromo-cyclic AMP analogues and by cultivation on a biomatrix substratum. Cyclic AMP analogues stimulated transglutaminase activity in protein synthesis-dependent and -independent phases. The enzyme was distributed at focal adhesion sites on the axon. By calcium-dependent covalent incorporation of the primary amine acceptor substrate, 5-(biotinamido)pentylamine, an increase in the Ca(2+)-dependent cross-linking of at least 11 substrate proteins in the presence of retinoic acid and dibutyryl-cyclic AMP was detected. Of these substrates, a subset was labelled on the surface of living granule neurons. A low-molecular-weight substrate, p18, was tentatively identified as the retinoic acid-inducible neurite-promoting factor, midkine. Transglutaminase-mediated amine incorporation, midkine and isopeptide cross-links were co-localized to axonal adhesion sites. The results provide evidence of transglutaminase C-catalysed protein cross-linking activity in cerebellar granule neurons and its possible implication in cell-substratum interactions.

Cross-linking of beta-amyloid protein precursor catalyzed by tissue transglutaminase

Alzheimer's disease is characterized by progressive dementia, cortical atrophy with synaptic loss, and the accumulation of neurofibrillary tangles and senile plaques containing beta-amyloid. The beta-amyloid protein precursor (beta-APP), may normally be involved in cell adhesion related to synaptic maintenance. Loss of synapses correlates with dementia, suggesting that synaptic deficits may underlie the disease. Synapse stability may depend on the action of tissue transglutaminase (tTG), an enzyme capable of crosslinking large, multi-domain extracellular glycoproteins, that is active and present at synapses. We now show that beta-APP is a substrate for tTG in vitro that results in dimers and multimers by silver staining and immunoblotting. This novel post-translational modification suggests further roles for beta-APP in synaptic function as well as in Alzheimer's disease.

Blockade effect of nerve growth factor on GM1 ganglioside-induced activation of transglutaminase in superior cervical sympathetic ganglia excised from adult rat

The activity of transglutaminase (TG), a Ca(2+)-dependent enzyme indicating tissue degradation or differentiation, showed in isolated adult rat superior cervical ganglia (SCG) a rapid (within 15 to 30 min) and marked (approx. 5- to 8-fold) increase with the addition of either GM1 ganglioside (GM1, 5 nM), which is rich in synapses, or sialyl cholesterol (SC, 20 microM), a synthetic sialic acid-containing compound, to the incubation medium at 37 degrees C. Under the same incubation conditions, addition of GM1 or SC decreased protein kinase C (PKC) activity (-26% to -39%) in the cytosolic fraction of the SCG, but increased the enzymic activity (+39% to +61%) in the particulate (cell membrane) fraction, suggesting that a sialic acid-containing compound (GM1 or SC) promotes PKC translocation from the cytosol to the membrane in ganglionic neurons. By contrast, addition of a promoting factor for survival of sympathetic neurons even in adulthood, nerve growth factor, (NGF, 0.25 micrograms/ml) to the medium significantly decreased ganglionic TG activity (-43%). This inhibition was completely antagonized by the co-addition of NGF-monoclonal antibody (0.75 microgram/ml). An effective blockade of GM1- or SC-induced stimulation of ganglionic TG activity was seen by further addition of NGF to the medium. Also, NGF almost abolished the translocation of ganglionic PKC activity induced by the sialic acid-containing compounds, although either NGF or 12-O-tetradecanoylphorbol ester (TPA) alone stimulated the cytosolic PKC activity (approx. +30%) in the tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of arginine, S-adenosylmethionine and polyamines on nerve regeneration

INTRODUCTION

Axon growth and axon regeneration are complex processes requiring an adequate supply of certain metabolic precursors and nutrients.

MATERIAL AND METHODS

This article reviews the studies examining some of the processes of protein modification fundamental to both nerve regeneration and to the continuous and adequate supply of specific factors such as arginine, S-adenosylmethionine and polyamines.

RESULTS

The process of arginylation notably increases following nerve injury and during subsequent regeneration of the nerve, with the most likely function of arginine-modification of nerve proteins being the degradation of proteins damaged through injury. It appears that defective methyl group metabolism may be one of the leading causes of demyelination, as suggested by the observation of reduced cerebrospinal fluid concentrations of s-adenosylmethionine (SAMe) and 5-methyltetrahydrofolate, the key metabolites in methylation processes, in patients with a reduction in myelination of corticospinal tracts. Polyamine synthesis, which depends strongly on the availability of both SAMe and arginine, markedly increases in neurons soon after an injury. This "polyamine-response" has been found to be essential for the survival of the parent neurons after injury to their axons. Polyamines probably exert their effects through involvement in DNA, RNA and protein synthesis, or through post-translational modifications that are indicated as the most relevant events of the "axon reaction."

CONCLUSIONS

Nerve regeneration requires the presence of arginine, s-adenosylmethionine, and polyamines. Further studies are needed to explore the mechanisms involved in these processes.

Serine proteases and their serpin inhibitors in Alzheimer's disease

Our article documents recent studies in the proteolytic processing of the Alzheimer's beta-amyloid protein precursor (beta-APP), as well as the role of thrombin and its potent inhibitor, protease nexin I in Alzheimer's disease (AD). Since synapse loss correlates best with cognitive decline in AD, we also present in detail, our model of synapse formation and elimination, reviewing recent findings related to the subject as well as our own original data. Recent exciting findings concerning the involvement of thrombin-like activity in synapse elimination, which we feel to be important in neural plasticity are also discussed.

Cellular transglutaminases in neural development

Enzymes of the transglutaminase family catalyze the Ca(2+)-dependent covalent cross-linking of peptide-bound glutamine residues of proteins and glycoproteins to the epsilon-amino group of lysine residues to create inter- or intramolecular isopeptide bonds. Transglutaminases can also covalently link a variety of primary amines to peptide-bound glutamine residues giving rise to two possibilities; firstly, where the primary amine has two or more amino groups, further catalysis can result in the formation of cross-linked bridges between glutamine residues, and secondly, where the primary amine is a monoamine, glutamine residues are rendered inert to further modification. The products are therefore in the main, homo- or heterodimers, or extensive, metabolically-stable multimeric complexes or matrices. Ca(2+)-dependent transglutaminase activity is present in the mammalian peripheral and central nervous systems and transglutaminase-catalyzed cross-linking of endogenous substrates has been demonstrated in neurons of Aplysia and the mammalian brain. Transglutaminase activity increases in the brain during development, principally owing to the increasing preponderance of glial cell activity. In a few regions including the cerebellar cortex, activity is also high in early development. Cellular transglutaminases occur widely in differentiating cells and tissues in mammals, with more than one transglutaminase frequently associated with a single cell type. The primary protein sequences of three cellular transglutaminases have been fully determined in different species, together with that of a mammalian protein homologue (band 4.2) which shares extensive sequence homologies with transglutaminases, but lacks the active site cysteine residue. The upstream sequences of two mammalian cellular transglutaminase genes (C and K) contain numerous regulatory sites, and an invertebrate transglutaminase, annulin, is spatially regulated within homeodomains. Multiple molecular forms of transglutaminase C and possibly other cellular transglutaminases exist in mammalian brain. The emerging picture is one of a family of cytosolic and membrane-bound proteins central to several regulatory pathways whose functions is to stabilize the cellular and intercellular superstructure in growing organisms. The targeted formation of glu-lys isopeptide bonds between proteins is central to this function. Cytoskeletal proteins, membrane-associated receptors, enzymes in signal transduction pathways and extracellular glycoproteins are candidate substrates as are polyamines, but few cellular proteins have been identified as components of naturally-occurring covalently-bonded matrices. Transglutaminases participate in the programme of neuronal differentiation in some but not all classes of neurone. Both neuronal and non-neuronal expression of transglutaminases may be important for guidance of migrating neurons or growth cones and sustainment of cell shape and coordinates during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Activity and distribution of tissue transglutaminase in association with nerve-muscle synapses

We have measured, characterized, and localized calcium-dependent protein cross-linking activity in rat skeletal muscle, and in myotubes cultured independently or in coculture with spinal neurones, catalyzed by the enzyme tissue transglutaminase (tTG). The enzyme activity was present in both motor endplate and endplate-free zones of rat diaphragm muscle. tTG in the endplate zone was more tightly associated with the tissue. This form of association was absent in extracts of peripheral nerve. Cross-linking of endogenous proteins, as measured by the content of epsilon-(gamma-glutamyl)lysine isopeptide, was higher in the endplate than in the nonendplate zone. Cytosolic (C) and particulate (B) forms of tTG were separated by ion-exchange chromatography from both regions of the muscle. In the motor endplate zone, a higher proportion of tightly bound tTG was recovered as a separate (B1) particulate form. Km values for calcium activation of the three forms of tTG were in the range of 5-15 microM. Immunocytochemistry with polyclonal and monoclonal antibodies revealed the enzyme at motor endplates and at contacts between neurites of rat embryo spinal neurones and myotubes in primary cocultures. Appearance of the B1 transglutaminase could be induced by coculturing myotubes of the mouse C2C12 cell line with neurones. The results suggest that tTG is most concentrated and active at the motor endplate.

Rapid and transient alterations in transglutaminase activity in rat superior cervical ganglia following denervation or axotomy

The activity of transglutaminase (TG), a Ca(2+)-dependent enzyme contributing to cross-linkage formation of intracellular polypeptide chains decreased rapidly to ca. 25% of control level in superior cervical ganglia (SCG) within 0.5 h following denervation. The reduced level was maintained for at least 24 h. By contrast, following axotomy, ganglionic TG activity increased by ca. 50% within 1 h, maintained the increase to 4 h, and returned to control level by 24 h. When SCG were transferred to aerobic in vitro incubation conditions 3 h following denervation, the addition of the protein kinase C (PKC) inhibitor, trifluoperazine (TFP, 10 micrograms/ml), to the medium partially reversed the denervation-induced reduction in ganglionic TG activity. Addition of a PKC activator, 12-O-tetradecanoylphorbol 13-acetate (TPA, 1 microM), had no effect on the TG activity. These findings suggest that the pathway resulting in the rapid, denervation-induced inhibition of TG activity may involve the transsynaptic activation of PKC. When SCG were placed in vitro 3 h following axotomy, addition of nerve growth factor (NGF, 0.25 micrograms/ml) to the medium reversed approximately one-half of the axotomy-induced increase in TG activity. Thus, following axotomy, the reduction in delivery to the SCG of NGF, which can be transported retrogradely within the axon and is indispensable for morphological and functional survival of sympathetic neurons, may trigger the transient, axotomy-induced TG activation in the SCG.

Effects of depolarizing agents on transglutaminase activity, Ca2+ influx, and protein synthesis in superior cervical and nodose ganglia excised from rats

Rapid changes in transglutaminase (TG) activity, 45Ca(2+)-influx and [3H]leucine incorporation in superior cervical ganglia (SCG), and nodose ganglia (NG) excised from adult rats were examined following addition of membrane-depolarizing agents veratridine (Ver) or high extracellular [K+]o during aerobic incubation in vitro at 37 degrees C. Addition of KCl (50 mM) stimulated TG activity to a maximal extent (four to six-fold) in SCG and NG after 30 min. Ver (0.2 mM) also increased TG activity in both ganglia after 30 min. Kinetic studies showed that the stimulation of TG activity in both ganglia caused by each depolarization condition was associated with a decrease in Km and an increase in Vmax value. The depolarizing agents Ver and high [K+]o also caused significant increases in 45Ca2+ influx into both ganglia. The Ver-induced increases in TG activity and 45Ca2+ accumulation were antagonized by tetrodotoxin (TTX, 1 microM), a sodium channel blocker. The K(+)-induced increase in TG activity was not blocked by tetraethylammonium (TEA, 20 mM), a potassium channel antagonist, although TEA did block the K(+)-induced increase in 45Ca2+ accumulation. The membrane-perturbing, sialic acid-containing compounds, GM1-ganglioside (GM1, 5 nM) and alpha-sialyl cholesterol (alpha-SC, 20 microM), were moderate inhibitors of the K(+)-induced effects on TG activity and 45Ca2+ accumulation. The sialyl compounds had little effect on Ver-induced accumulation of 45Ca2+ but enhanced the Ver-evoked stimulation in TG activity. These results suggests that the veratridine- and K(+)-induced increases in TG activity occur via modulation of Ca2+ and Na+ channel gating mechanisms that are pharmacologically distinct for each depolarizing agent. The veratridine- and K(+)-induced decrease in [3H]leucine incorporation could be a result of stimulation of TG activity as a consequence of degenerative alterations.

Selective alterations in transglutaminase activity of rat superior cervical ganglia in response to neurotransmitters, high potassium and sialic acid-containing compounds

We examined the in vitro effects of neurotransmitters, high KCl as well as sialic acid-containing compounds (GM1; SC) on transglutaminase (TG) activity in isolated superior cervical ganglia (SCG) one week after denervation or axotomy. Following denervation, TG activity in SCG decreased to 83% of the unoperated control value, whereas that of axotomized ganglia was 28% of control. Thus, TG activity was relatively unaffected when sympathetic ganglionic neurons were preserved, but was markedly reduced under conditions where neurons were degenerating. Addition of ACh (0.1 mM) to the medium during aerobic incubation stimulated TG activity more than 3-fold in denervated ganglia but had no effect on TG activity in axotomized ganglia. Similarly, the NE (0.05 mM)-induced decrease of TG activity observed in intact SCG was also seen following denervation (-49%) but not following axotomy. In denervated SCG, the stimulatory effects of ACh were virtually abolished by co-addition of the cholinergic antagonists, atropine or hexamethonium, while the suppressant effects of NE were blocked by the adrenergic antagonists, propranolol, prazosin or yohimbine. These results imply that transmitter-induced rapid changes in TG activity occur predominantly in ganglionic neurons. When the ganglia were depolarized by high KCl (50 mM), a significant increase in TG activity in each intact, denervated and axotomized SCG was seen with qualitatively similar manner, suggesting that high KCl-induced depolarization affects both neuronal and glial components in the SCG. The marked increase in ganglionic TG activity in response to GM1 (5 nM) and synthetic SC (0.02 mM) were lost in denervated SCG but only partially reduced in axotomized SCG.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical characterization of programmed cell death in NGF-deprived sympathetic neurons

Young sympathetic neurons die when deprived of nerve growth factor (NGF). Under such circumstances, cell death is appropriate to the developing nervous system and requires RNA and protein synthesis. We have hypothesized the existence of an endogenous death program within neurons that is suppressed by trophic factors. The extent and timing of required changes in the synthetic events that comprise the death program are unknown. In an effort to characterize the biochemical events that mediate the death program further, we performed several experiments on embryonic rat sympathetic neurons in vitro. The death program was blocked with cycloheximide when total protein synthesis was inhibited > or = 80%. When protein synthesis was inhibited within 22 +/- 4 h of NGF deprivation, death was prevented in half the neurons. Hence, we define the commitment point for protein synthesis to be 22 +/- 4 h. Analogously, the commitment point for RNA synthesis was 26 +/- 4 h and that for NGF rescue, 24 +/- 4 h. We tested the ability of a wide variety of chemicals to interfere with the death program. Most compounds tested were unable to prevent neuronal death. Some treatments, however, did save NGF-deprived neurons and were subsequently characterized. These included ultraviolet light and agents that raise intracellular concentrations of cAMP. Finally, we looked for the neuronal expression in vitro and in vivo of genes that have been associated with programmed death in other cell types, including TRPM-2/SGP-2, polyubiquitin, TGF beta-1, c-fos, and c-myc. None of these genes showed significant activation associated with neuronal death.

Stimulation of transglutaminase activity by GM1-ganglioside and alpha-sialylcholesterol in superior cervical and nodose ganglia excised from adult rat

Changes in transglutaminase (TG) activity in superior cervical ganglia (SCG) and nodose ganglia (NG) excised from adult rats were examined following application of selected membrane transport-altering agents, including GM1-ganglioside (GM1) and alpha-sialylcholesterol (alpha-SC). Although TG activity of freshly dissected SCG and NG was relatively low, it increased gradually during 30 min of incubation, and it stayed at this elevated level for 2 h. Addition of alpha-SC at its maximal effective concentration, 20 microM, stimulated TG activity more than eightfold in SCG and more than twofold in NG by 30 min. Addition of GM1 at its most effective concentration, 5 nM, had similar effects, but of lesser magnitude. Cycloheximide, a potent inhibitor of protein biosynthesis, did not affect the GM1- or alpha-SC-evoked increases in ganglionic TG activity, suggesting that enzyme activation rather than synthesis of new enzyme was occurring. The stimulation of TG activity in both ganglia caused by either GM1 or alpha-SC was associated with a decrease in Km and an increase in Vmax values. Addition of cholera toxin B, which specifically masks the oligosaccharide chain of GM1, reduced the GM1-induced increase in TG activity by approximately 60% in SCG and 88% in NG. The alpha-SC-induced increase in TG activity was only partially mimicked by free cholesterol. Although application of either dibutyryl cyclic AMP or dibutyryl cyclic GMP produced little change in TG activity of either ganglion, phorbol ester clearly inhibited the enzymic activity. Because TG is a calcium-dependent enzyme, we measured 45Ca2+ influx into either ganglion, and found that it was reduced by GM1 and alpha-SC in SCG and by alpha-SC in NG.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation by acetylcholine and inhibition by norepinephrine of transglutaminase activity in superior cervical ganglia excised from adult rats

Very rapid changes in activity of transglutaminase (TG), a calcium-dependent enzyme contributing to cross-linkage formation of intracellular polypeptide chains, were observed in vitro in rat superior cervical ganglion (SCG) and nodose ganglion (NG) following application of cholinergic or adrenergic agonists and antagonists. In SCG, a tissue rich in synapses, the depolarizing agent acetylcholine (ACh, 0.1 mM) produced an 8.7-fold increase in TG activity within 5 min that lasted for 30 min and returned to control levels by 2 h. In contrast, the ACh-induced increase in TG activity in NG, a tissue containing neuronal cell bodies with few synapses, was more gradual and of smaller magnitude, reaching a peak of approximately 2.4 times control by 30 min that was maintained for at least 2 h. In both tissues the ACh-stimulation was effectively blocked by the nicotinic antagonist, hexamethonium (0.1 mM), whereas the muscarinic antagonist, atropine (0.1 mM), partially blocked the ACh effect in SCG and was without effect in NG. Addition of the hyperpolarizing adrenergic agonists norepinephrine (NE, 50 microM), isoproterenol (0.2 mM) or dopamine (0.1 mM) produced an inhibition of TG activity in SCG but had no effect in NG. The inhibitory effects of the adrenergic agonists in SCG were blocked by the beta-adrenergic antagonist, propranolol (10 microM) and alpha 2-adrenergic antagonist, yohimbine (10 microM). A kinetic study revealed that the ACh-induced stimulation of TG activity in SCG and NG was a result of decrease in apparent Km and an increase in Vmax value, whereas the NE-induced inhibition of SCG enzyme activity was a result of an increased Km and decreased Vmax. 45Ca2+ influx into excised SCG or NG was significantly reduced by the application of either ACh or NE. The ACh inhibition was effectively blocked by either hexamethonium or atropine. The NE inhibition was more effectively blocked by yohimbine than by propranolol. These results suggest that the rapid alterations of TG activity in SCG produced by cholinergic and adrenergic neurotransmitters are attributable to the processes of receptor-mediated depolarization and hyperpolarization, respectively, via modulation of nerve-impulse-induced Ca2+ fluxes during synaptic activity.

Transglutaminase activity in reversible cerebral ischemia in the rat

Transglutaminase (TG, EC 2.3.2.13) activity and levels of putrescine (a natural acyl-acceptor in the transglutaminase reaction) were measured in rat brains after 30 min ischemia and 8 or 24 h recirculation. TG activity was significantly increased in the striatum and hippocampus already during cerebral ischemia and, more pronounced, after 8 and 24 h recirculation. In the cortex, in contrast, TG activity did not change during ischemia and 8 h recirculation but was significantly increased after 24 h recirculation. Putrescine levels were sharply increased after 8 h recirculation and even further after 24 h recirculation. It is suggested that in vivo during ischemia and early recirculation, when cells are overloaded with calcium ions, a pathological increase in the TG-catalyzed cross-linking of proteins may be apparent especially in the nerve endings of the hippocampus where the intrinsic concentration of the acyl-donor (protein-bound glutamyl-moiety) has been shown to be high.

Retrograde changes in transglutaminase activity after peripheral nerve injuries

It has been previously demonstrated that transglutaminase activity in rat superior cervical ganglion is rapidly and transiently increased minutes after nerve injury. The present series of experiments sought to determine: (1) whether or not similar changes are expressed by other peripheral neuronal systems, and (2) if injury-induced changes in the enzyme activity can be detected along the injured nerve, and if so do they occur in axons or in non-neuronal cells. In the nodose ganglion transglutaminase activity increased (approximately 40%) 48 h after the vagus nerve was crushed 25 mm from the ganglion. In the vagus nerve the activity was transiently increased (approximately 100%) within 1 h, followed by a second increase (approximately 140%) after 3 h. This occurred only in the proximal nerve stump close to the injury site and not in the section of nerve closer to the ganglion. Comparable enzyme activity was found in unoperated vagus nerve and in distal stumps of previously ligated vagus nerves. In dorsal root ganglia no changes were found for up to 24 h after the sciatic nerve was crushed 40 mm from the ganglion. In the facial nucleus a transient increase was observed after the facial nerve was crushed about 14 mm distally with a peak (approximately 300) at 3 days and a decline within 14 days. A second lesion of the facial nerve made 12 days following a conditioning lesion led to a rebound of enzyme activity in the facial nucleus.(ABSTRACT TRUNCATED AT 250 WORDS)

Searching for the function of tissue transglutaminase: its possible involvement in the biochemical pathway of programmed cell death

Although several details are still missing, the biological role of two of the three well characterized transglutaminases in mammals, namely blood coagulation factor XIII and keratinocyte transglutaminase, is established. The function of the third one called the tissue type is still an enigma. Its constant localization in endothelial and smooth muscle cells of all organs, in heart muscle, in medullary interstitial and mesangial cells of kidney, and its induction in a number of other cell types under a variety of conditions suggest multiple functions. According to our results its participation in the biochemical pathway leading to programmed cell death (apoptosis), a basic cellular phenomenon of physiological significance, may be one of these functions.

Posttranslational protein modification by polyamines in intact and regenerating nerves

A 150,000-g supernatant from axoplasm of the giant axon of the stellate nerve of the squid and from rat sciatic and goldfish optic nerves was found to be able to incorporate covalently [3H]putrescine and [3H]spermidine into an exogenous protein (N,N'-dimethylcasein). Incorporation of radioactivity was inhibited by CuSO4, a specific inhibitor of transglutaminases, the enzymes mediating these reactions in other tissues. Analysis of pH and temperature range and enzyme kinetics displayed characteristics predicted for transglutaminase-mediated reactions. Transglutaminase activity increased during regeneration of both vertebrate nerves, but greater activity was found in segments of nerve containing no intact axons than in either intact segments or in segments containing regenerating axons. Polyacrylamide gel electrophoresis of endogenous modified proteins (in the absence of N,N'-dimethylcasein) showed labeling of 18-, 46- and 200-kilodalton proteins by both [3H]putrescine and [3H]spermidine. Analysis of the protein-bound radioactivity from intact and regenerating rat sciatic nerves demonstrated it to be predominantly in the form of the parent radioactive polyamine. These experiments demonstrate the covalent modification of proteins by polyamines at low levels in squid axoplasm and at relatively higher levels in rat sciatic and goldfish optic nerves. In the latter two cases, the activity of these modification reactions may be due in part to the modification of axonal proteins, but the majority of the activity occurs in nonneuronal cells of the nerve.

Changes of ornithine decarboxylase activity in dorsal root ganglion cells after axon injury: possible relationship to alterations in neuronal chromatin

An autoradiographic technique was used to detect changes in ornithine decarboxylase in rat dorsal root ganglion neurons after sciatic nerve lesions. Binding of [3H]difluoromethylornithine ([3H]DFMO) to tissue sections of L5 ganglia was compared between axotomized and unoperated ganglia at 0.5, 1, 2, 3, 4, 5, 7, 8, 9, 11, 14, and 30 days after a crush lesion of the sciatic nerve. The [3H]DFMO binding to axotomized ganglion neurons was elevated compared with the unoperated side at 0.5, 1, 5, 7, 8, and 11 days postoperation. Enzymatic measurements of ornithine decarboxylase on ganglia at 1, 4, 8, and 9 days after sciatic nerve crush confirmed basic patterns of enzyme activity comparisons derived from autoradiography. Compared with patterns of [3H]actinomycin D binding to nuclei during the same periods after axotomy, [3H]DFMO binding increased prior to increases in [3H]actinomycin D binding within the first week. After that time, changes in both variables occurred in parallel. The data suggest that increased activities of ornithine decarboxylase or concentrations of polyamines may be linked to the induction of increased RNA synthesis in the early phases of the axon reaction.

Lipids associated with tissue transglutaminase

A substantial amount of lipids (cholesterol and its esters, mono-, di- and triacylglycerols, free fatty acids and the phospholipids phosphatidylethanolamine and phosphatidylinositol) was found associated with tissue transglutaminase purified to apparent homogeneity from guinea pig liver. Removal of lipids results in an increased tendency of the enzyme for self-association and a decreased stability. Lauric acid was detected following hydroxylamine treatment of the enzyme, suggesting the occurrence of a fatty acid-type, covalent, posttranslational modification of transglutaminase. The results provide support for the idea that part of tissue transglutaminase may be localized in the cell membrane.

Transglutaminase and the neuronal cytoskeleton in Alzheimer's disease

Transglutaminase [EC 2.3.2.13, (R)-glutaminyl-peptide:amine gamma-glutamyltransferase], an enzyme that catalyzes the introduction of glutamine-lysine cross-links into proteins, was purified. Neurofilament and microtubule proteins were substrates for this enzyme but the insoluble neurofibrillary tangles (NFT) isolated from Alzheimer's disease brain were not substrates. In vitro cross-linking of neurofilaments and microtubules by the enzyme did not produce paired helical filaments (PHF), which are the major ultrastructural component of NFT. These results make it unlikely that PHF are formed by the straightforward cross-linking of neurofilaments or microtubules.

Early polyamine treatment accelerates regeneration of rat sympathetic neurons

After injury of their axons, damaged neurons shift their metabolic activity into a reparative mode aimed at survival and regeneration or, alternatively, they undergo degeneration and die. Previous reports have shown that at the initial stages of the response to axonal injury, polyamines are essential for neuronal survival and can accelerate functional recovery. In this study we examined the ability of exogenous polyamines to accelerate regeneration following crush of the pre- or postganglionic sympathetic nerves of the superior cervical ganglion in adult rats. We found that early treatment with polyamines after pre- or postganglionic nerve crush, accelerated the reappearance of choline acetyltransferase activity in the superior cervical ganglion, and of [3H]norepinephrine uptake in the iris, respectively. Functional recovery from eyelid ptosis was also accelerated. We conclude that treatment with polyamines can enhance regeneration of peripheral sympathetic neurons.

Transglutaminase activity in rat brain: characterization, distribution, and changes with age

The activity of transglutaminase was characterized in the rat brain. In adults, comparable levels of transglutaminase activity are present in all brain regions examined. The activity is present in all subcellular fractions, as studied by differential centrifugation, but the soluble fraction contains the highest specific activity. The endogenous activity (enzyme activity assayed in the absence of the exogenous substrate casein) is very low in all subcellular fractions, except in the synaptosomal fraction where its highest levels are about 40-60% of the activity assayed in the presence of casein. Furthermore, enzyme activity is present on the external surface of synaptosomes. In the soluble fraction, maximal activity can be detected between pH values of 9 and 10 when assayed in the presence of 5 mM CaCl2 (with half-maximal activity requiring 0.75 mM CaCl2) and 0.4 mM putrescine (with an apparent Km for putrescine of 0.1 mM). The activity can be partially inhibited by ZnCl2 (with an IC50 of 4.5 mM) and by AlCl3 (with an IC50 of 5.1 mM). In the cerebellum, where the full span of neuronal development can be studied after birth, the highest specific activity is observed just after birth, thereafter the activity starts to decline and by 14 days, after a reduction of about 65%, it reaches levels observed throughout life.