Proteolytic activity, synapse elimination, and the Hebb synapse

The Evolving Concept of Neuro-Thromboinflammation for Neurodegenerative Disorders and Neurotrauma: A Rationale for PAR1-Targeting Therapies

Interest in the role of coagulation and fibrinolysis in the nervous system was active in several laboratories dating back before cloning of the functional thrombin receptor in 1991. As one of those, our attention was initially on thrombin and plasminogen activators in synapse formation and elimination in the neuromuscular system, with orientation towards diseases such as amyotrophic lateral sclerosis (ALS) and how clotting and fibrinolytic pathways fit into its pathogenesis. This perspective is on neuro-thromboinflammation, emphasizing this emerging concept from studies and reports over more than three decades. It underscores how it may lead to novel therapeutic approaches to treat the ravages of neurotrauma and neurodegenerative diseases, with a focus on PAR1, ALS, and parmodulins.

Thrombin and the Coag-Inflammatory Nexus in Neurotrauma, ALS, and Other Neurodegenerative Disorders

This review details our current understanding of thrombin signaling in neurodegeneration, with a focus on amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease) as well as future directions to be pursued. The key factors are multifunctional and involved in regulatory pathways, namely innate immune and the coagulation cascade activation, that are essential for normal nervous system function and health. These two major host defense systems have a long history in evolution and include elements and regulators of the coagulation pathway that have significant impacts on both the peripheral and central nervous system in health and disease. The clotting cascade responds to a variety of insults to the CNS including injury and infection. The blood brain barrier is affected by these responses and its compromise also contributes to these detrimental effects. Important molecules in signaling that contribute to or protect against neurodegeneration include thrombin, thrombomodulin (TM), protease activated receptor 1 (PAR1), damage associated molecular patterns (DAMPs), such as high mobility group box protein 1 (HMGB1) and those released from mitochondria (mtDAMPs). Each of these molecules are entangled in choices dependent upon specific signaling pathways in play. For example, the particular cleavage of PAR1 by thrombin vs. activated protein C (APC) will have downstream effects through coupled factors to result in toxicity or neuroprotection. Furthermore, numerous interactions influence these choices such as the interplay between HMGB1, thrombin, and TM. Our hope is that improved understanding of the ways that components of the coagulation cascade affect innate immune inflammatory responses and influence the course of neurodegeneration, especially after injury, will lead to effective therapeutic approaches for ALS, traumatic brain injury, and other neurodegenerative disorders.

Long-term depression-associated signaling is required for an in vitro model of NMDA receptor-dependent synapse pruning

Activity-dependent pruning of synaptic contacts plays a critical role in shaping neuronal circuitry in response to the environment during postnatal brain development. Although there is compelling evidence that shrinkage of dendritic spines coincides with synaptic long-term depression (LTD), and that LTD is accompanied by synapse loss, whether NMDA receptor (NMDAR)-dependent LTD is a required step in the progression toward synapse pruning is still unknown. Using repeated applications of NMDA to induce LTD in dissociated rat neuronal cultures, we found that synapse density, as measured by colocalization of fluorescent markers for pre- and postsynaptic structures, was decreased irrespective of the presynaptic marker used, post-treatment recovery time, and the dendritic location of synapses. Consistent with previous studies, we found that synapse loss could occur without apparent net spine loss or cell death. Furthermore, synapse loss was unlikely to require direct contact with microglia, as the number of these cells was minimal in our culture preparations. Supporting a model by which NMDAR-LTD is required for synapse loss, the effect of NMDA on fluorescence colocalization was prevented by phosphatase and caspase inhibitors. In addition, gene transcription and protein translation also appeared to be required for loss of putative synapses. These data support the idea that NMDAR-dependent LTD is a required step in synapse pruning and contribute to our understanding of the basic mechanisms of this developmental process.

Examination of the genetic basis for sexual dimorphism in the Aedes aegypti (dengue vector mosquito) pupal brain

Background

Most animal species exhibit sexually dimorphic behaviors, many of which are linked to reproduction. A number of these behaviors, including blood feeding in female mosquitoes, contribute to the global spread of vector-borne illnesses. However, knowledge concerning the genetic basis of sexually dimorphic traits is limited in any organism, including mosquitoes, especially with respect to differences in the developing nervous system.

Methods

Custom microarrays were used to examine global differences in female vs. male gene expression in the developing pupal head of the dengue vector mosquito, Aedes aegypti. The spatial expression patterns of a subset of differentially expressed transcripts were examined in the developing female vs. male pupal brain through in situ hybridization experiments. Small interfering RNA (siRNA)-mediated knockdown studies were used to assess the putative role of Doublesex, a terminal component of the sex determination pathway, in the regulation of sex-specific gene expression observed in the developing pupal brain.

Results

Transcripts (2,527), many of which were linked to proteolysis, the proteasome, metabolism, catabolic, and biosynthetic processes, ion transport, cell growth, and proliferation, were found to be differentially expressed in A. aegypti female vs. male pupal heads. Analysis of the spatial expression patterns for a subset of dimorphically expressed genes in the pupal brain validated the data set and also facilitated the identification of brain regions with dimorphic gene expression. In many cases, dimorphic gene expression localized to the optic lobe. Sex-specific differences in gene expression were also detected in the antennal lobe and mushroom body. siRNA-mediated gene targeting experiments demonstrated that Doublesex, a transcription factor with consensus binding sites located adjacent to many dimorphically expressed transcripts that function in neural development, is required for regulation of sex-specific gene expression in the developing A. aegypti brain.

Conclusions

These studies revealed sex-specific gene expression profiles in the developing A. aegypti pupal head and identified Doublesex as a key regulator of sexually dimorphic gene expression during mosquito neural development.

Protein kinase C isoforms at the neuromuscular junction: localization and specific roles in neurotransmission and development

The protein kinase C family (PKC) regulates a variety of neural functions including neurotransmitter release. The selective activation of a wide range of PKC isoforms in different cells and domains is likely to contribute to the functional diversity of PKC phosphorylating activity. In this review, we describe the isoform localization, phosphorylation function, regulation and signalling of the PKC family at the neuromuscular junction. Data show the involvement of the PKC family in several important functions at the neuromuscular junction and in particular in the maturation of the synapse and the modulation of neurotransmission in the adult.

Activity-dependent plasticity of spinal circuits in the developing and mature spinal cord

Part of the development and maturation of the central nervous system (CNS) occurs through interactions with the environment. Through physical activities and interactions with the world, an animal receives considerable sensory information from various sources. These sources can be internally (proprioceptive) or externally (such as touch and pressure) generated senses. Ample evidence exists to demonstrate that the sensory information originating from large diameter afferents (Ia fibers) have an important role in inducing essential functional and morphological changes for the maturation of both the brain and the spinal cord. The Ia fibers transmit sensory information generated by muscle activity and movement. Such use or activity-dependent plastic changes occur throughout life and are one reason for the ability to acquire new skills and learn new movements. However, the extent and particularly the mechanisms of activity-dependent changes are markedly different between a developing nervous system and a mature nervous system. Understanding these mechanisms is an important step to develop strategies for regaining motor function after different injuries to the CNS. Plastic changes induced by activity occur both in the brain and spinal cord. This paper reviews the activity-dependent changes in the spinal cord neural circuits during both the developmental stages of the CNS and in adulthood.

Synapse elimination accompanies functional plasticity in hippocampal neurons

A critical component of nervous system development is synapse elimination during early postnatal life, a process known to depend on neuronal activity. Changes in synaptic strength in the form of long-term potentiation (LTP) and long-term depression (LTD) correlate with dendritic spine enlargement or shrinkage, respectively, but whether LTD can lead to an actual separation of the synaptic structures when the spine shrinks or is lost remains unknown. Here, we addressed this issue by using concurrent imaging and electrophysiological recording of live synapses. Slices of rat hippocampus were cultured on multielectrode arrays, and the neurons were labeled with genes encoding red or green fluorescent proteins to visualize presynaptic and postsynaptic neuronal processes, respectively. LTD-inducing stimulation led to a reduction in the synaptic green and red colocalization, and, in many cases, it induced a complete separation of the presynaptic bouton from the dendritic spine. This type of synapse loss was associated with smaller initial spine size and greater synaptic depression but not spine shrinkage during LTD. All cases of synapse separation were observed without an accompanying loss of the spine during this period. We suggest that repeated low-frequency stimulation simultaneous with LTD induction is capable of restructuring synaptic contacts. Future work with this model will be able to provide critical insight into the molecular mechanisms of activity- and experience-dependent refinement of brain circuitry during development.

Matrix metalloproteinase-7 modulates synaptic vesicle recycling and induces atrophy of neuronal synapses

Matrix metalloproteinase-7 (MMP-7) belongs to a family of zinc dependent endopeptidases that are expressed in a variety of tissues including the brain. MMPs are known to be potent mediators of pericellular proteolysis and likely mediators of dynamic remodelling of neuronal connections. While an association between proteases and the neuronal synapse is emerging, a full understanding of this relationship is lacking. Here, we show that MMP-7 alters the structure and function of presynaptic terminals without affecting neuronal survival. Bath application of recombinant MMP-7 to cultured rat neurons induced long-lasting inhibition of vesicular recycling as measured by synaptotagmin 1 antibody uptake assays and FM4-64 optical imaging. MMP-7 application resulted in reduced abundance of vesicular and active zone proteins locally within synaptic terminals although their general levels remained unaltered. Finally, chronic application of the protease resulted in synaptic atrophy, including smaller terminals and fewer synaptic vesicles, as determined by electron microscopy. Together these results suggest that MMP-7 is a potent modulator of synaptic vesicle recycling and synaptic ultrastructure and that elevated levels of the enzyme, as may occur with brain inflammation, may adversely influence neurotransmission.

Tissue inhibitor of metalloproteinase-2(TIMP-2)-deficient mice display motor deficits

The degradation of the extracellular matrix is regulated by matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). Matrix components of the basement membrane play critical roles in the development and maintenance of the neuromuscular junction (NMJ), yet almost nothing is known about the regulation of MMP and TIMP expression in either the pre- or postsynaptic compartments. Here, we demonstrate that TIMP-2 is expressed by both spinal motor neurons and skeletal muscle. To determine whether motor function is altered in the absence of TIMP-2, motor behavior was assessed using a battery of tests (e.g., RotaRod, balance beam, hindlimb extension, grip strength, loaded grid, and gait analysis). TIMP-2(-/-) mice fall off the RotaRod significantly faster than wild-type littermates. In addition, hindlimb extension is reduced and gait is both splayed and lengthened in TIMP-2(-/-) mice. Motor dysfunction is more pronounced during early postnatal development. A preliminary analysis revealed NMJ alterations in TIMP-2(-/-) mice. Juvenile TIMP-2(-/-) mice have increased nerve branching and acetylcholine receptor expression. Adult TIMP-2(-/-) endplates are enlarged and more complex. This suggests a role for TIMP-2 in NMJ sculpting during development. In contrast to the increased NMJ nerve branching, cerebellar Purkinje cells have decreased neurite outgrowth. Thus, the TIMP-2(-/-) motor phenotype is likely due to both peripheral and central defects. The tissue specificity of the nerve branching phenotype suggests the involvement of different MMPs and/or extracellular matrix molecules underlying the TIMP-2(-/-) motor phenotype.

Nitric Oxide and Synaptic Dynamics in the Adult Brain: Physiopathological Aspects

The adult brain retains the capacity to rewire mature neural circuits in response to environmental changes, brain damage or sensory and motor experiences. Two plastic processes, synaptic remodeling and neurogenesis, have been the subject of numerous studies due to their involvement in the maturation of the nervous system, their prevalence and re-activation in adulthood, and therapeutic relevance. However, most of the research looking for the mechanistic and molecular events underlying synaptogenic phenomena has been focused on the extensive synaptic reorganization occurring in the developing brain. In this stage, a vast number of synapses are initially established, which subsequently undergo a process of activity-dependent refinement guided by target-derived signals that act as synaptotoxins or synaptotrophins, promoting either loss or consolidation of pre-existing synaptic contacts, respectively. Nitric oxide (NO), an autocrine and/or paracrine-acting gaseous molecule synthesized in an activity-dependent manner, has ambivalent actions. It can act by mediating synapse formation, segregation of afferent inputs, or growth cone collapse and retraction in immature neural systems. Nevertheless, little information exists about the role of this ambiguous molecule in synaptic plasticity processes occurring in the adult brain. Suitable conditions for elucidating the role of NO in adult synaptic rearrangement include physiopathological conditions, such as peripheral nerve injury. We have recently developed a crush lesion model of the XIIth nerve that induces a pronounced stripping of excitatory synaptic boutons from the cell bodies of hypoglossal motoneurons. The decline in synaptic coverage was concomitant with de novo expression of the neuronal isoform of NO synthase in motoneurons. We have demonstrated a synaptotoxic action of NO mediating synaptic withdrawal and preventing synapse formation by cyclic GMP (cGMP)-dependent and, probably, S-nitrosylation-mediated mechanisms, respectively. This action possibly involves the participation of other signaling molecules working together with NO. Brain-derived neurotrophic factor (BDNF), a target-derived synaptotrophin synthesized and released postsynaptically in an activity-dependent form, is a potential candidate for effecting such a concerted action. Several items of evidence support an interrelationship between NO and BDNF in the regulation of synaptic remodeling processes in adulthood: i) BDNF and its receptor TrkB are expressed by motoneurons and upregulated by axonal injury; ii) they promote axon arborization and synaptic formation, and modulate the structural dynamics of excitatory synapses; iii) NO and BDNF each control the production and activity of the other at the level of individual synapses; iv) the NO/cGMP pathway inhibits BDNF secretion; and finally, v) BDNF protects F-actin from depolymerization by NO, thus preventing the collapsing and retracting effects of NO on growth cones. Therefore, we propose a mechanism of action in which the NO/BDNF ratio regulates synapse dynamics after peripheral nerve lesion. This hypothesis also raises the possibility that variations in this NO/BDNF balance constitute a common hallmark leading to synapse loss in the progression of diverse neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's and Parkinson's diseases.

Critical period plasticity in local cortical circuits

Neuronal circuits in the brain are shaped by experience during 'critical periods' in early postnatal life. In the primary visual cortex, this activity-dependent development is triggered by the functional maturation of local inhibitory connections and driven by a specific, late-developing subset of interneurons. Ultimately, the structural consolidation of competing sensory inputs is mediated by a proteolytic reorganization of the extracellular matrix that occurs only during the critical period. The reactivation of this process, and subsequent recovery of function in conditions such as amblyopia, can now be studied with realistic circuit models that might generalize across systems.

Experience-dependent pruning of dendritic spines in visual cortex by tissue plasminogen activator

Sensory experience physically rewires the brain in early postnatal life through unknown processes. Here, we identify a robust anatomical consequence of monocular deprivation (MD) in layer II/III of visual cortex that corresponds to the rapid, functional loss of responsiveness preceding any changes in axonal input. Protrusions on pyramidal cell apical dendrites increased steadily after eye opening, but were transiently lost through competitive mechanisms after brief MD only during the physiological critical period. Proteolysis by tissue-type plasminogen activator (tPA) conversely declined with age and increased with MD only in young mice. Targeted disruption of tPA release or its upstream regulation by glutamic acid decarboxylase (GAD65) prevented MD-induced spine loss that was pharmacologically rescued concomitant with critical period plasticity. An extracellular mechanism for structural remodeling that is limited to the binocular zone upon proper detection of competing inputs thus links early sensory experience to visual function.

Differential expression of neuropsin and protease M/neurosin in oligodendrocytes after injury to the spinal cord

Neuropsin and protease M/neurosin are serine proteases expressed by neurons and glial cells, and serve a variety of functions in the central nervous system (CNS). The current study demonstrates changes in the expression of these proteases following hemisection of the mouse spinal cord. Within unlesioned spinal cord, neuropsin mRNA expression was occasionally observed in the gray but not white matter, while the level of protease M/neurosin mRNA was higher in the white matter. After injury to the spinal cord, neuropsin mRNA expression was induced in the white matter in the area immediately adjacent to the lesion, peaking at 4 days post-injury and disappearing by 14 days. Enhanced expression of protease M/neurosin mRNA was observed throughout the white and gray matter surrounding the lesion, peaking at 4 days and persisting for 14 days. Neuropsin mRNA was expressed predominantly by CNPase-positive oligodendrocytes. Furthermore, most of these cells were also associated with immunoreactivity for protease M/neurosin protein. Within unlesioned spinal cord, most protease M/neurosin mRNA-expressing cells were CNPase-positive oligodendrocytes, and a substantial fraction of these cells also showed immunoreactivity for NG2, a marker for oligodendrocyte progenitors. After injury, protease M/neurosin mRNA expression within NG2-positive cells was significantly decreased, while the constitutive expression in CNPase-positive oligodendrocytes appeared to be preserved. These findings suggest that each subpopulation of oligodendrocytes based on the expression of neuropsin and protease M/neurosin has different roles in the response of the spinal cord to injury as well as in normal homeostasis.

Critical Period Mechanisms in Developing Visual Cortex

Binocular vision is shaped by experience during a critical period of early postnatal life. Loss of visual acuity following monocular deprivation is mediated by a shift of spiking output from the primary visual cortex. Both synaptic and network explanations have been offered for this heightened brain plasticity. Direct experimental control over its timing, duration, and closure has now been achieved through a consideration of balanced local circuit excitation-inhibition. Notably, canonical models of homosynaptic plasticity at excitatory synapses alone (LTP/LTD) fail to produce predictable manipulations of the critical period in vivo. Instead, a late functional maturation of intracortical inhibition is the driving force, with one subtype in particular standing out. Parvalbumin-positive large basket cells that innervate target cell bodies with synapses containing the alpha1-subunit of GABA(A) receptors appear to be critical. With age, these cells are preferentially enwrapped in peri-neuronal nets of extracellular matrix molecules, whose disruption by chondroitinase treatment reactivates ocular dominance plasticity in adulthood. In fact, critical period plasticity is best viewed as a continuum of local circuit computations ending in structural consolidation of inputs. Monocular deprivation induces an increase of endogenous proteolytic (tPA-plasmin) activity and consequently motility of spines followed by their pruning, then re-growth. These early morphological events faithfully reflect competition only during the critical period and lie downstream of excitatory-inhibitory balance on a timescale (of days) consistent with the physiological loss of deprived-eye responses in vivo. Ultimately, thalamic afferents retract or expand accordingly to hardwire the rapid functional changes in connectivity. Competition detected by local inhibitory circuits then implemented at an extracellular locus by proteases represents a novel, cellular understanding of the critical period mechanism. It is hoped that this paradigm shift will lead to novel therapies and training strategies for rehabilitation, recovery from injury, and lifelong learning in adulthood.

Activity-dependent synaptic competition at mammalian neuromuscular junctions

Synapse elimination is a widespread developmental process in the peripheral and central nervous system that brings about refinement of neural connections through epigenetic mechanisms. Here we describe recent advances concerning the role of the pattern of motoneuronal firing, synchronous or asynchronous, in neuromuscular synapse elimination.

Proteinase-activated receptors in the nervous system

Recent data point to important roles for proteinases and their cognate proteinase-activated receptors (PARs) in the ontogeny and pathophysiology of the nervous system. PARs are a family of G-protein-coupled receptors that can affect neural cell proliferation, morphology and physiology. PARs also have important roles in neuroinflammatory and degenerative diseases such as human immunodeficiency virus-associated dementia, Alzheimer's disease and pain. These receptors might also influence the pathogenesis of stroke and multiple sclerosis, conditions in which the blood-brain barrier is disrupted. The diversity of effects of PARs on neural function and their widespread distribution in the nervous system make them attractive therapeutic targets for neurological disorders. Here, we review the roles of PARs in the central and peripheral nervous systems during health and disease, with a focus on neuroinflammatory and degenerative disorders.

The role of ECM molecules in activity-dependent synaptic development and plasticity

Growth and guidance of neurites (axons and dendrites) during development is the prerequisite for the establishment of functional neural networks in the adult organism. In the adult, mechanisms similar to those used during development may regulate plastic changes that underlie important nervous system functions, such as memory and learning. There is now ever-increasing evidence that extracellular matrix (ECM)-associated factors are critically involved in the formation of neuronal connections during development, and their plastic changes in the adult. Here, we review the current literature on the role of ECM components in activity-dependent synaptic development and plasticity, with the major focus on the thrombospondin type I repeat (TSR) domain-containing proteins. We propose that ECM components may modulate neuronal development and plasticity by: 1) regulating cellular motility and morphology, thus contributing to structural alterations that are associated with the expression of synaptic plasticity, 2) coordinating transsynaptic signaling during plasticity via their cell surface receptors, and 3) defining the physical parameters of the extracellular space, thereby regulating diffusion of soluble signaling molecules in the extracellular space (ECS).

Synapse number and synaptic efficacy are regulated by presynaptic cAMP and protein kinase A

The mechanisms by which neurons regulate the number and strength of synapses during development and synaptic plasticity have not yet been defined fully. This lack of fundamental knowledge in the fields of neurodevelopment and synaptic plasticity can be attributed, in part, to compensatory mechanisms by which neurons accommodate for the loss of function in their synaptic partners. This is generally achieved either by scaling up neuronal transmitter release capabilities or by enhancing the postsynaptic responsiveness. Here, we demonstrate that regulation of synaptic strength and number between identified Lymnaea neurons visceral dorsal 4 (VD4, the presynaptic cell) and left pedal dorsal 1 (LPeD1, the postsynaptic cell) requires presynaptic activation of a cAMP-PKA-dependent signal. Experimental activation of the cAMP-PKA pathway resulted in reduced synaptic efficacy, whereas inhibition of the cAMP-PKA cascade permitted hyperinnervation and an overall enhancement of synaptic strength. Because synaptic transmission between VD4 and LPeD1 does not require a cAMP-PKA pathway, our data show that these messengers may play a novel role in regulating the synaptic efficacy during early synaptogenesis and plasticity.

Protein kinases and Hebbian function

The Hebb synapse, in which the strength of synapses is affected by activity in presynaptic and postsynaptic nerve cells, is a widely used model for developmental and learning-related neuroplasticity. Presynaptic and postsynaptic firing that is correlated in time is postulated to increase synaptic strength while activity in presynaptic and postsynaptic neurons that is not correlated results in weakening. The authors describe a cell biologic, mechanistic model for activity-dependent modification of synapse strength that selectively weakens inactive inputs to activated targets. Differentially localized protein kinase A and protein kinase C molecules are activated by spike and synaptic activity. Subsequent kinase-specific phosphorylation and stabilization or destabilization of synaptic receptors are molecular and cell biologic substrates of the Hebb synapse.

Calcium-dependent maintenance of agrin-induced postsynaptic specializations

Although much progress has been made in understanding synapse formation, little is known about the mechanisms underlying synaptic maintenance and loss. The formation of agrin-induced AChR clusters on cultured myotubes requires both activation of the receptor tyrosine kinase MuSK and intracellular calcium fluxes. Here, we provide evidence that such AChR clusters are maintained by agrin/MuSK-induced intracellular calcium fluxes. Clamping intracellular calcium fluxes after AChR clusters have formed leads to rapid MuSK and AChR tyrosine dephosphorylation and cluster dispersal, even in the continued presence of agrin. Both the dephosphorylation and the dispersal are inhibited by the tyrosine phosphatase inhibitor pervanadate. In contrast, clamping intracellular calcium at the time of initial agrin stimulation has no effect on agrin-induced MuSK or AChR phosphorylation, but blocks AChR cluster formation. These findings suggest an avenue by which postsynaptic stability can be regulated by modification of intracellular signaling pathways that are distinct from those used during synapse formation.

Activity of a newly identified serine protease in CNS demyelination

We have identified a novel serine protease, myelencephalon-specific protease (MSP), which is preferentially expressed in the adult CNS, and therein, is abundant in both neurones and oligodendroglia. To determine the potential activity of MSP in CNS demyelination, we examined its expression in multiple sclerosis lesions and in two animal models of multiple sclerosis: Theiler's murine encephalomyelitis virus (TMEV) and myelin oligodendrocyte glycoprotein (MOG)-induced experimental allergic encephalomyelitis (EAE) in marmosets. High levels of MSP were present within infiltrating mononuclear cells, including macrophages and T cells, which characteristically fill sites of demyelination, both in multiple sclerosis lesions and in animal models of this disease. The functional consequence of excess MSP on oligodendroglia was determined in vitro by evaluating the effects of recombinant MSP (r-MSP) on oligodendrocyte survival and process number. Application of excess r-MSP resulted in a dramatic loss of processes from differentiated oligodendrocytes, and a parallel decrease in process outgrowth from immature cells. Transfection of oligodendrocyte progenitors with an MSP-green fluorescent protein construct produced similar changes in oligodendrocyte process number. Importantly, r-MSP did not affect oligodendrocyte survival or differentiation towards the sulphatide-positive lineage. We further demonstrate that myelin basic protein, and to a lesser extent myelin oligodendrocyte glycoprotein, can serve as MSP substrates. These studies support the hypothesis that excess MSP, as is present in inflammatory CNS lesions, promotes demyelination.

Pertussis toxin-sensitive G-protein and protein kinase C activity are involved in normal synapse elimination in the neonatal rat muscle

Individual skeletal muscle fibers in most new-born rodents are innervated at a single endplate by several motor axons. During the first postnatal weeks, the polyneuronal innervation decreases in a process of synaptic elimination. Previous studies showed that the naturally occurring serine-protease thrombin mediates the activity-dependent synapse reduction at the neuromuscular junction (NMJ) in vitro and that thrombin-receptor activation may modulate nerve terminal consolidation through a protein kinase mechanism. To test whether these mechanisms may be operating in vivo, we applied external thrombin and its inhibitor hirudin, and several substances affecting the G protein-protein kinase C system (GP-PKC) directly over the external surface of the neonatal rat Levator auris longus muscle. Muscles were processed for immunocytochemistry to simultaneously detect acetylcholine receptors (AChRs) and axons for counting the percentage of polyinnervated NMJ. We found that exogenous thrombin accelerated synapse loss and hirudin blocked axonal removal. Phorbol-12-myristate-13-acetate, a potent PKC activator, had a similar effect as thrombin, whereas the PKC inhibitors, calphostin C and staurosporine, prevented axonal removal. Pertussis toxin, an effective blocker of GP function, blocked synapse elimination. These findings suggest that the normal synapse elimination in the neonatal rat muscle may be modulated, at least in part, by the pertussis-sensitive G-protein and PKC activity and that thrombin could play a role in the postnatal synaptic maturation in vivo.

Schwann cell dependence of regenerating rat sensory neurons is inversely related to the quality of axon growth substratum

It is still controversial to what extent elongation of regenerating sensory axons depends on proliferating Schwann cells (SCs) in an injured peripheral nerve. We hypothesized that such regeneration was independent of SC support early after nerve injury, but later became SC-dependent. The sural nerve in rats was crushed, and freezing destroyed cells but not their basal laminae (BL) in the distal nerve segment. Sensory axon elongation was assessed by the nerve pinch test and their abundance was examined immunohistochemically. Sensory axons regenerated fairly rapidly during the first week even if SC migration was prevented. Thereafter, they ceased to elongate and withdrew until their terminals contacted the SCs migrating from the proximal nerve segment. Intrinsic neuronal capacity for growth without cell support, however, had not been lost. Rather, progressive degradation of the former SC BL and loss of laminin in the acellular segment arrested axon growth. Further elongation occurred only when SC migration was possible, corroborating our hypothesis. Sensory neurons continued to elongate and maintain their axons in spite of deteriorating growth substratum if, prior to injury the axons had been allowed to sprout into the denervated skin. Previous sprouting exposed the sensory neurons to high levels of NGF.

Preferential expression of myelencephalon-specific protease by oligodendrocytes of the adult rat spinal cord white matter

Myelencephalon-specific protease (MSP) is a novel serine protease that is expressed predominantly in the nervous system. In the adult rat spinal cord, MSP mRNA expression was dramatically upregulated, in both the white and gray matter, after systemic exposure to the glutamate receptor agonist, kainic acid (KA) (Scarisbrick et al. J Neurosci 17: 8156-8168, 1997b). To determine the cell-specific expression patterns of MSP, we generated MSP-specific monoclonal antibodies. These have been used in immunohistochemical and in situ hybridization colocalization studies, to demonstrate that MSP mRNA and protein are produced predominantly by CNP-immunoreactive oligodendroglia, but not by GFAP-immunoreactive astrocytes, in the white matter of the normal adult cord. In vitro, the soma of oligodendrocytes were also densely MSP immunoreactive, as were their growth tips, while astrocytes were associated with lower levels. These findings suggest that the enzymatic activity of MSP is likely to be important in the biology of oligodendrocytes and/or in the maintenance of the nerve fiber tracts of the adult spinal cord.

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Electrical activity and gene expression in the development of vertebrate neural circuits

Over the past several decades, anatomical and electrophysiological analyses have demonstrated that the electrical activity of neurons is required for development of the precise patterns of synaptic connectivity found in the adult central nervous system. However, knowledge of the molecular cascades that underlie activity-dependent synaptic development remains rudimentary. As a result, many fundamental issues remain unresolved. Recent advances in differential cloning have begun to provide the tools and insight necessary to bring a molecular level of understanding to principles of activity-dependent synaptic development established via classic systems approaches.

Metalloprotease MP100: a synaptic protease in rat brain

Proteases are expressed widely throughout the nervous system and perform essential functions. We have earlier characterized and cloned the metalloprotease MP100, an enzyme originally described as a beta-amyloid precursor protein (beta-APP) processing candidate. In the present study we describe the cellular and subcellular localization of MP100 in rat brain. A punctuate intracellular immunostaining in cortical, hippocampal and cerebellar neurons suggests its high abundance in vesicular intracellular structures. The MP100 staining pattern resembled that of the presynaptic protein synaptophysin. In gel filtration chromatography of isolated rat brain synaptosomal membranes, MP100 co-fractionated with synaptophysin and beta-APP. Furthermore, pre-embedding immunoelectron microscopy of the cerebellum revealed MP100 to be localized at synaptic sites. All together, these data might indicate a role for MP100 in functions such as proteolytic modification of synaptic proteins.

Axonal atrophy: the retraction reaction

Recent studies indicate that morphological alterations of axon branches that are removed during normal development are similar to those that occur following ablation of postsynaptic cells in adult animals. In both situations, axons retract (rather than degenerate), the calibers of withdrawing axon branches are markedly reduced, and spherical swellings near (or at) the axon terminations appear. The similarity between naturally occurring and target-deprived axon withdrawal suggests that both developing and adult axons withdraw from target cells that no longer provide support.

Implications of a neural network model of early sensori-motor development for the field of developmental neurology

This paper reports on a neural network model for early sensori-motor development and on the possible implications of this research for our understanding and, eventually, treatment of motor disorders like cerebral palsy. We recapitulate the results we published in detail in a series of papers [1-4]. The neural circuits in the model self-organize on the basis of rhythmic activity spontaneously generated in the model. This indicates the importance of endogenously generated activity in the developing brain. We also show that afferent feed-back from the mechanical part of the model is easily incorporated in the neural part of the model. In this way the model acquires reflex-related properties which have long been demonstrated in man. In the discussion we relate these experimental findings to the variability concept from developmental neurology and show how variable motor performance is important for motor learning. We also discuss possible implications of our modelling effort for movement disorders, specifically spastic cerebral palsy.

Brain waves and brain wiring: the role of endogenous and sensory-driven neural activity in development

Neural activity is critical for sculpting the intricate circuits of the nervous system from initially imprecise neuronal connections. Disrupting the formation of these precise circuits may underlie many common neurodevelopmental disorders, ranging from subtle learning disorders to pervasive developmental delay. The necessity for sensory-driven activity has been widely recognized as crucial for infant brain development. Recent experiments in neurobiology now point to a similar requirement for endogenous neural activity generated by the nervous system itself before sensory input is available. Here we use the formation of precise neural circuits in the visual system to illustrate the principles of activity-dependent development. Competition between the projections from lateral geniculate nucleus neurons that receive sensory input from the two eyes shapes eye-specific connections from an initially diffuse projection into ocular dominance columns. When the competition is altered during a critical period for these changes, by depriving one eye of vision, the normal ocular dominance column pattern is disrupted. Before ocular dominance column formation, the highly ordered projection from retina to lateral geniculate nucleus develops. These connections form before the retina can respond to light, but at a time when retinal ganglion cells spontaneously generate highly correlated bursts of action potentials. Blockade of this endogenous activity, or biasing the competition in favor of one eye, results in a severe disruption of the pattern of retinogeniculate connections. Similar spontaneous, correlated activity has been identified in many locations in the developing central nervous system and is likely to be used during the formation of precise connections in many other neural systems. Understanding the processes of activity-dependent development could revolutionize our ability to identify, prevent, and treat developmental disorders resulting from disruptions of neural activity that interfere with the formation of precise neural circuits.

Expression of a serine protease (motopsin PRSS12) mRNA in the mouse brain: in situ hybridization histochemical study

Serine proteases are considered to play several important roles in the brain. In an attempt to find novel brain-specific serine proteases (BSSPs), motopsin (PRSS-12) was cloned from a mouse brain cDNA library by polymerase chain reaction (PCR). Northern blot analysis demonstrated that the postnatal 10-day mouse brain contained the most amount of motopsin mRNA. At this developmental stage, in situ hybridization histochemistry showed that motopsin mRNA was specifically expressed in the following regions: cerebral cortical layers II/III, V and VIb, endopiriform cortex and the limbic system, particularly in the CA1 region of the hippocampal formation. In addition, in the brainstem, the oculomotor nucleus, trochlear nucleus, mecencephalic and motor nuclei of trigeminal nerve (N), abducens nucleus, facial nucleus, nucleus of the raphe pontis, dorsoral motor nucleus of vagal N, hypoglossal nucleus and ambiguus nucleus showed motopsin mRNA expression. Expression was also found in the anterior horn of the spinal cord. The above findings strongly suggest that neurons in almost all motor nuclei, particularly in the brainstem and spinal cord, express motopsin mRNA, and that motopsin seems to have a close relation to the functional role of efferent neurons.

Blockade of N-methyl-D-aspartate receptor activation suppresses learning-induced synaptic elimination

Auditory filial imprinting in the domestic chicken is accompanied by a dramatic loss of spine synapses in two higher associative forebrain areas, the mediorostral neostriatum/hyperstriatum ventrale (MNH) and the dorsocaudal neostriatum (Ndc). The cellular mechanisms that underlie this learning-induced synaptic reorganization are unclear. We found that local pharmacological blockade of N-methyl-D-aspartate (NMDA) receptors in the MNH, a manipulation that has been shown previously to impair auditory imprinting, suppresses the learning-induced spine reduction in this region. Chicks treated with the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV) during the behavioral training for imprinting (postnatal day 0-2) displayed similar spine frequencies at postnatal day 7 as naive control animals, which, in both groups, were significantly higher than in imprinted animals. Because the average dendritic length did not differ between the experimental groups, the reduced spine frequency can be interpreted as a reduction of the total number of spine synapses per neuron. In the Ndc, which is reciprocally connected with the MNH and not directly influenced by the injected drug, learning-induced spine elimination was partly suppressed. Spine frequencies of the APV-treated, behaviorally trained but nonimprinted animals were higher than in the imprinted animals but lower than in the naive animals. These results provide evidence that NMDA receptor activation is required for the learning-induced selective reduction of spine synapses, which may serve as a mechanism of information storage specific for juvenile emotional learning events.

Developmental regulation of the serpin, protease nexin I, localization during activity-dependent polyneuronal synapse elimination in mouse skeletal muscle

During vertebrate neuromuscular development, all muscle fibers are transiently innervated by more than one neuron. Among the numerous factors shown to potentially influence the passage from poly- to mononeuronal innervation, serine proteases and their inhibitors appear to play important roles. In this regard, protease nexin I (PNI), a potent inhibitor of the serine protease, thrombin, is highly localized to the neuromuscular junction (NMJ). In turn, thrombin is responsible for activity-dependent synapse elimination both in an in vitro model, and in vivo. In the present study, we used a monospecific anti-PNI polyclonal antibody to study the developmental kinetics of PNI expression in mouse leg skeletal muscle. By using immunoblotting, we detected PNI at embryonic day 16 (E16), as a 48-kDa band. This 48-kDa PNI band became prominent in leg muscle extracts at postnatal day 5 (P5) and remained so in extracts from adult muscle. In contrast, a higher molecular weight immunoreactive PNI band, which was sodium dodecyl sulfate- and beta-mercaptoethanol-resistant, was first detected at E16, increased at birth (P0), and then decreased at P15, i.e., after the wave of polyneuronal synapse elimination had occurred in these muscles. The results of an enzyme-linked immunosorbent assay, measuring active, complexed, and truncated PNI, correlated with Western blot data. We used immunocytochemistry to probe the localization of PNI at the NMJ and found that PNI was present in the cytoplasm of myotubes at E16, but neither then nor at birth did it colocalize with acetylcholine receptors. PNI became localized at NMJs by P5 and increased by P15, after which it remained stably concentrated there in the adult. Finally, we studied the gene expression of PNI mRNA, by using Northern blotting, and showed that PNI mRNA was present in skeletal muscle and remained stable throughout the time-course studies, suggesting that developmental regulation of muscle PNI occurs principally at the translational and/or post-translational levels. These results suggest that the localization of PNI, through a binding site or "receptor" may play an important role in differentiation and maintenance of synapse.

Characterization of recombinant and brain neuropsin, a plasticity-related serine protease

Activity-dependent changes in neuropsin gene expression in the hippocampus implies an involvement of neuropsin in neural plasticity. Since the deduced amino acid sequence of the gene contained the complete triplet (His-Asp-Ser) of the serine protease domain, the protein was postulated to have proteolytic activity. Recombinant full-length neuropsin produced in the baculovirus/insect cell system was enzymatically inactive but was readily converted to active enzyme by endoprotease processing. The activational processing of prototype neuropsin involved the specific cleavage of the Lys32-Ile33 bond near its N terminus. Native neuropsin that was purified with a purity of 1,100-fold from mouse brain had enzymatic characteristics identical to those of active-type recombinant neuropsin. Both brain and recombinant neuropsin had amidolytic activities cleaving Arg-X and Lys-X bonds in the synthetic chromogenic substrates, and the highest specific activity was found against Boc-Val-Pro-Arg-4-methylcoumaryl-7-amide. The active-type recombinant neuropsin effectively cleaved fibronectin, an extracellular matrix protein. Taken together, these results indicate that this protease, which is enzymatically novel, has significant limbic effects by changing the extracellular matrix environment.

Involvement of protease-activated receptor-1 in the in vitro development of mesencephalic dopaminergic neurons

In situ hybridization studies have revealed high levels of protease (thrombin)-activated receptor-1 messenger RNA in the mesencephalon of rats, suggesting that dopaminergic neurons are a target for thrombin's actions. We have evaluated the effect of thrombin receptor activation, either by thrombin or by thrombin receptor agonist peptide, a 14 amino acid agonist of protease-activated receptor-1, on tyrosine hydroxylase-positive neurons. Pure cultures of rat mesencephalic neurons or co-cultures of mesencephalic neurons and glial cells were treated with either thrombin or thrombin receptor agonist peptide the day after plating. Tyrosine hydroxylase-positive cell counting, [3H]dopamine uptake and morphometric analysis were performed on day 5. Thrombin and thrombin receptor agonist peptide influenced neurite elongation, branching and the number of primary, secondary and tertiary neurites of tyrosine hydroxylase-positive neurons. In pure cultures, the most significant effects of thrombin and thrombin receptor agonist peptide were to delay branching and to increase the centrifugal growth of neurites without affecting the total neuritic length. Thrombin (up to 10 nM) and thrombin receptor agonist peptide did not affect the number of tyrosine hydroxylase-positive neurons or [3H]dopamine uptake. Neurotrophin-4 also influenced the morphology of tyrosine hydroxylase-positive neurons. The increase of neuritic length initiated by this neurotrophin is complementary to the radial elongation induced by protease-activated receptor-1 activation. When neurons were cultured in the presence of glial cells, the effects of thrombin and thrombin receptor agonist peptide on most of these parameters were larger than those observed with pure cultures. Thus, thrombin is able to initiate a complex remodelling of the architecture of tyrosine hydroxylase-positive neurons through the activation of protease-activated receptor-1. These results provide further support for the involvement of protease-activated receptor-1 activation in the development and differentiation of the central nervous system.

Blockade of neuropsin, a serine protease, ameliorates kindling epilepsy

The behavioural and electrographical abnormalities associated with seizures in epileptic (kindled) mice correspond with those of human epilepsy. In kindled mice, neuropsin was markedly increased in the hippocampus and cerebral cortices. A single intraventricular injection of monoclonal antibodies specific to neuropsin reduced or eliminated the epileptic pattern noted on electroencephalograms and, as a result markedly inhibited the progression of kindling. Therefore, neuropsin appears to be a key protein controlling pathogenic events in the hippocampus, and thus neuropsin inhibitors might be useful for treatment of epilepsy.

Retrograde signaling in the development and modification of synapses

Retrograde signaling from the postsynaptic cell to the presynaptic neuron is essential for the development, maintenance, and activity-dependent modification of synaptic connections. This review covers various forms of retrograde interactions at developing and mature synapses. First, we discuss evidence for early retrograde inductive events during synaptogenesis and how maturation of presynaptic structure and function is affected by signals from the postsynaptic cell. Second, we review the evidence that retrograde interactions are involved in activity-dependent synapse competition and elimination in developing nervous systems and in long-term potentiation and depression at mature synapses. Third, we review evidence for various forms of retrograde signaling via membrane-permeant factors, secreted factors, and membrane-bound factors. Finally, we discuss the evidence and physiological implications of the long-range propagation of retrograde signals to the cell body and other parts of the presynaptic neuron.

Nervous system-specific expression of a novel serine protease: regulation in the adult rat spinal cord by excitotoxic injury

A full-length cDNA clone of a previously unidentified serine protease, myelencephalon-specific protease (MSP), has been isolated by using a PCR cloning strategy and has been shown to be expressed in a nervous system and spinal cord-specific pattern. Sequence analysis demonstrated that MSP is most similar in sequence to neuropsin, trypsin, and tissue kallikrein and is predicted to have trypsin-like substrate specificity. MSP mRNA was found to be approximately 10-fold greater in the CNS of the rat and human, as compared with most peripheral tissues, and within the CNS was found to be highest by a factor of four in the medulla oblongata and spinal cord. Levels of mRNA encoding tissue plasminogen activator (tPA) also were elevated in the spinal cord but were more widespread in peripheral tissues as compared with MSP. In the adult rat lumbosacral spinal cord, in situ localization of MSP mRNA demonstrated 2-fold higher levels in the white, as compared with the gray, matter. MSP mRNA expression was shown to increase 3-fold in the white matter and 1.5-fold in the gray laminae at 72 hr after intraperitoneal injection of the AMPA/kainate glutamate receptor-specific agonist, kainic acid (KA). MSP mRNA remained elevated in the ventral gray matter, including expression associated with the motor neurons of lamina IX, at 7 d after the initial excitotoxic insult. Together, these observations indicate that MSP is in a position to play a fundamental role in normal homeostasis and in the response of the spinal cord to injury.

Quantitative PCR analysis reveals novel expression of prothrombin mRNA and regulation of its levels in developing mouse muscle

Precise determination of mRNA levels is an essential element in any investigation of complex regulatory systems. Classical methodologies such as Northern hybridization suffer from requirements for significant samples of material and also a degree of nonspecificity. Recently, quantitative techniques involving PCR amplification have been devised. We have developed and applied such procedures to the determination of prothrombin messages in skeletal muscle cells during development. In addition to its role in the blood coagulation cascade, the serine protease thrombin has been shown to participate in several signaling events in the neuromuscular system. The inactive precursor, prothrombin, primarily produced in the liver, has also been shown to be synthesized and developmentally-regulated in the brain. In skeletal muscle, thrombin is a mediator of activity-dependent polyneuronal synapse elimination (ADPSE) which occurs in early postnatal development. Recent experiments showing that thrombin is released from myotubes in culture under the influence of acetylcholine suggest that locally-synthesized prothrombin may be the source of this Hebbian synaptic interaction. We have determined that prothrombin is expressed in skeletal muscle, as the likely source of thrombin involved in ADPSE, and the current results show the quantitative expression of muscle prothrombin during this time of intense synapse remodeling.

mRNAs encoding urokinase-type plasminogen activator and plasminogen activator inhibitor-1 are elevated in the mouse brain following kainate-mediated excitation

Urokinase-type plasminogen activator (uPA) is an inducible extracellular serine protease implicated in fibrinolysis and in tissue remodeling. Recently, we have localized uPA mRNA strictly in limbic structures and the parietal cortex of the adult mouse brain. Here, we tested whether the systemic treatment of mice with kainic acid (KA), an amino acid inducing limbic seizures, could elevate in the brain mRNAs encoding uPA and its specific inhibitor, plasminogen activator inhibitor-1 (PAI-1), a major antifibrinolytic agent. Brain sections encompassing the hippocampus were tested through in situ hybridization using radiolabeled riboprobes specific for the two mRNA species. The results showed that KA greatly enhanced both mRNA species in sites of limbic structures and cortex. However, in the hypothalamus and brain blood vessels only PAI-1 mRNA was elevated. Those were also the only two locations where PAI-1 mRNA was detected in the non-treated control brain, although at a low level. For both mRNAs, KA enhancement was first evident 2-4 h after treatment, and it was most prolonged in the hippocampal area, where prominent hybridization signals persisted for three days. Here, both mRNAs were initially elevated in the hilar region of the dentate gyrus and in the molecular and oriens layers; however, PAI-1 mRNA became evident throughout the area, while uPA mRNA became especially pronounced in the CA3/CA4 subfield. In the cortex both mRNA types were induced, but only uPA mRNA was elevated in the retrosplenial cortex, and also in the subiculum. In the amygdaloid complex, uPA mRNA was restricted to the basolateral nucleus, whereas PAI-1 mRNA was seen throughout the structure, however, excluding this nucleus. These data show that seizure activity enhances the expression of uPA and PAI-1 genes in the brain; the patterns of enhancement suggest that the protease and its inhibitor may act in brain plasticity in synchrony, however, also independently of each other. Furthermore, the results suggest that by elevating PAI-1 mRNA in brain blood vessels, limbic seizures generate a risk for stroke.

Intercellular communication that mediates formation of the neuromuscular junction

Reciprocal signals between the motor axon and myofiber induce structural and functional differentiation in the developing neuromuscular junction (NMJ). Elevation of presynaptic acetylcholine (ACh) release on nerve-muscle contact and the correlated increase in axonal-free calcium are triggered by unidentified membrane molecules. Restriction of axon growth to the developing NMJ and formation of active zones for ACh release in the presynaptic terminal may be induced by molecules in the synaptic basal lamina, such as S-laminin, heparin binding growth factors, and agrin. Acetylcholine receptor (AChR) synthesis by muscle cells may be increased by calcitonin gene-related peptide (CGRP), ascorbic acid, and AChR-inducing activity (ARIA)/heregulin, which is the best-established regulator. Heparin binding growth factors, proteases, adhesion molecules, and agrin all may be involved in the induction of AChR redistribution to form postsynaptic-like aggregates. However, the strongest case has been made for agrin's involvement. "Knockout" experiments have implicated agrin as a primary anterograde signal for postsynaptic differentiation and muscle-specific kinase (MuSK), as a putative agrin receptor. It is likely that both presynaptic and postsynaptic differentiation are induced by multiple molecular signals. Future research should reveal the physiological roles of different molecules, their interactions, and the identity of other molecular participants.

Cholinergic stimulation increases thrombin activity and gene expression in cultured mouse muscle

Activity-dependent synapse reduction is a major determinant of neuromuscular innervation. Previous research has shown that nanomolar concentrations of hirudin, a specific thrombin antagonist, significantly attenuates this reduction, and protease nexin 1 (PN1), an endogenous thrombin inhibitor closely localized to the neuromuscular synapse, can inhibit synapse reduction at similar concentrations. Protease inhibitors which do not inhibit thrombin, including cystatin and aprotinin, had no effect on synapse reduction. We present a series of experiments examining whether prothrombin and/or PN1 gene expression, as well as thrombin activity, are regulated in muscle cultures by acetylcholine (ACh) receptor activation. We also studied the effect of exogenous thrombin on synapse elimination in co-cultures of muscle and cholinergic neurons. Cultured muscle cells were electrically blocked with tetrodotoxin (TTX), or co-treated with ACh in order to isolate ACh receptor activation. Electrical blockade resulted in a decrease in thrombin release to about two-thirds of control values. The application of ACh to electrically blocked muscle cultures resulted in a 2.5-fold increase in thrombin activity released into the medium and a 2-fold increase in prothrombin gene expression. In contrast, ACh treatment in the presence of TTX had no effect on PN1 gene expression compared to treatment with TTX alone. In addition, exogenous thrombin significantly increased synapse elimination in unstimulated muscle/cholinergic neuron co-cultures. These results suggest that thrombin or a thrombin-like molecule released from muscle is required for activity-dependent synapse elimination and is regulated by neuromuscular activity.

Nip and tuck at the neuromuscular junction: a role for proteases in developmental synapse elimination

During late embryonic and early postnatal development, synaptic connections are extensively modified so that some functional connections are weakened and eliminated from a neural circuit while others are strengthened and maintained. The mechanisms that underlie synapse elimination are beginning to be understood from studies of the neuromuscular junction. A recent paper provides some intriguing insights into the role proteases may play in the developmental disassembly of neuromuscular synapses.

Neural correlates of sensitive periods in avian song learning

A wide range of neural events takes place during avian song learning, including changes in receptor systems and synaptic organization and the wholesale production and growth of new cells and their connections. Any (or all) of these events may be involved in song learning, and therefore their timing may restrict when auditory memories are laid down in the song system and when these memories are used to pattern vocal behavior. In particular, mounting evidence indicates that NMDA receptor activation participates in the acquisition of auditory memories during song learning, and developmental changes in the number and function of NMDA receptors within the song system may regulate the capacity for song learning. These developmental changes in NMDA receptors occur in conjunction with a period of synapse elimination, and NMDA receptor activation during song learning may regulate this regressive event. In addition, both developmental and seasonal periods of vocal learning correlate well with neuronal addition and turnover within the vocal motor pathway. This temporal overlap provides the opportunity for auditory experience to shape the initial organization of naive motor circuitry. Thus, a variety of cellular substrates present themselves as candidates for providing the neural plasticity necessary for song learning and may together regulate how and when experience is stored in the nervous system.