A motoneuron-selective stop signal in the synaptic protein S-laminin

MMP-mediated modulation of ECM environment during axonal growth and NMJ development

Motor neurons, skeletal muscles, and perisynaptic Schwann cells interact with extracellular matrix (ECM) to form the tetrapartite synapse in the peripheral nervous system. Dynamic remodeling of ECM composition is essential to diversify its functions for distinct cellular processes during neuromuscular junction (NMJ) development. In this review, we give an overview of the proteolytic regulation of ECM proteins, particularly by secreted and membrane-type matrix metalloproteinases (MMPs), in axonal growth and NMJ development. It is suggested that MMP-2, MMP-9, and membrane type 1-MMP (MT1-MMP) promote axonal outgrowth and regeneration upon injury by clearing the glial scars at the lesion site. In addition, these MMPs also play roles in neuromuscular synaptogenesis, ranging from spontaneous formation of synaptic specializations to activity-dependent synaptic elimination, via proteolytic cleavage or degradation of growth factors, neurotrophic factors, and ECM molecules. For instance, secreted MMP-3 has been known to cleave agrin, the main postsynaptic differentiation inducer, further highlighting the importance of MMPs in NMJ formation and maintenance. Furthermore, the increased level of several MMPs in myasthenia gravis (MG) patient suggest the pathophysiological mechanisms of MMP-mediated proteolytic degradation in MG pathogenesis. Finally, we speculate on potential major future directions for studying the regulatory functions of MMP-mediated ECM remodeling in axonal growth and NMJ development.

Residential Proximity to Major Roadways at Birth, DNA Methylation at Birth and Midchildhood, and Childhood Cognitive Test Scores: Project Viva(Massachusetts, USA)

BACKGROUND

Epigenetic variability is hypothesized as a regulatory pathway through which prenatal exposures may influence child development and health.

OBJECTIVE

We sought to examine the associations of residential proximity to roadways at birth and epigenome-wide DNA methylation. We also assessed associations of differential methylation with child cognitive outcomes.

METHODS

We estimated residential proximity to roadways at birth using a geographic information system (GIS) and cord blood methylation using Illumina's HumanMethylation450-array in 482 mother-child pairs in Project Viva. We identified individual CpGs associated with residential-proximity-to-roadways at birth using robust linear regression [[Formula: see text]]. We also estimated association between proximity-to-roadways at birth and methylation of the same sites in blood samples collected at age 7-11 y ([Formula: see text]). We ran the same analyses in the Generation R Study for replication ([Formula: see text]). In Project Viva, we investigated associations of differential methylation at birth with midchildhood cognition using linear regression.

RESULTS

Living closer to major roadways at birth was associated with higher cord blood (and-more weakly-midchildhood blood) methylation of four sites in LAMB2. For each halving of residential-proximity-to-major-roadways, we observed a 0.82% increase in DNA methylation at cg05654765 [95% confidence interval (CI): (0.54%, 1.10%)], 0.88% at cg14099457 [95% CI: (0.56%, 1.19%)], 0.19% at cg03732535 [95% CI: (0.11%, 0.28)], and 1.08% at cg02954987 [95% CI: (0.65%, 1.51%)]. Higher cord blood methylation of these sites was associated with lower midchildhood nonverbal cognitive scores. Our results did not replicate in the Generation R Study.

CONCLUSIONS

Our discovery results must be interpreted with caution, given that they were not replicated in a separate cohort. However, living close to major roadways at birth was associated with cord blood methylation of sites in LAMB2-a gene known to be linked to axonal development-in our U.S. cohort. Higher methylation of these sites associated with lower nonverbal cognitive scores at age 7-11 y in the same children. https://doi.org/10.1289/EHP2034.

Matrix Metalloproteinases and Tissue Inhibitor of Metalloproteinases in Inflammation and Fibrosis of Skeletal Muscles

In skeletal muscles, levels and activity of Matrix MetalloProteinases (MMPs) and Tissue Inhibitors of MetalloProteinases (TIMPs) have been involved in myoblast migration, fusion and various physiological and pathological remodeling situations including neuromuscular diseases. This has opened perspectives for the use of MMPs' overexpression to improve the efficiency of cell therapy in muscular dystrophies and resolve fibrosis. Alternatively, inhibition of individual MMPs in animal models of muscular dystrophies has provided evidence of beneficial, dual or adverse effects on muscle morphology or function. We review here the role played by MMPs/TIMPs in skeletal muscle inflammation and fibrosis, two major hurdles that limit the success of cell and gene therapy. We report and analyze the consequences of genetic or pharmacological modulation of MMP levels on the inflammation of skeletal muscles and their repair in light of experimental findings. We further discuss how the interplay between MMPs/TIMPs levels, cytokines/chemokines, growth factors and permanent low-grade inflammation favor cellular and molecular modifications resulting in fibrosis.

BDNF/trkB Induction of Calcium Transients through Cav2.2 Calcium Channels in Motoneurons Corresponds to F-actin Assembly and Growth Cone Formation on β2-Chain Laminin (221)

Spontaneous Ca²⁺ transients and actin dynamics in primary motoneurons correspond to cellular differentiation such as axon elongation and growth cone formation. Brain-derived neurotrophic factor (BDNF) and its receptor trkB support both motoneuron survival and synaptic differentiation. However, in motoneurons effects of BDNF/trkB signaling on spontaneous Ca²⁺ influx and actin dynamics at axonal growth cones are not fully unraveled. In our study we addressed the question how neurotrophic factor signaling corresponds to cell autonomous excitability and growth cone formation. Primary motoneurons from mouse embryos were cultured on the synapse specific, β2-chain containing laminin isoform (221) regulating axon elongation through spontaneous Ca²⁺ transients that are in turn induced by enhanced clustering of N-type specific voltage-gated Ca²⁺ channels (Ca_v2.2) in axonal growth cones. TrkB-deficient (trkBTK^-/-) mouse motoneurons which express no full-length trkB receptor and wildtype motoneurons cultured without BDNF exhibited reduced spontaneous Ca²⁺ transients that corresponded to altered axon elongation and defects in growth cone morphology which was accompanied by changes in the local actin cytoskeleton. Vice versa, the acute application of BDNF resulted in the induction of spontaneous Ca²⁺ transients and Ca_v2.2 clustering in motor growth cones, as well as the activation of trkB downstream signaling cascades which promoted the stabilization of β-actin via the LIM kinase pathway and phosphorylation of profilin at Tyr129. Finally, we identified a mutual regulation of neuronal excitability and actin dynamics in axonal growth cones of embryonic motoneurons cultured on laminin-221/211. Impaired excitability resulted in dysregulated axon extension and local actin cytoskeleton, whereas upon β-actin knockdown Ca_v2.2 clustering was affected. We conclude from our data that in embryonic motoneurons BDNF/trkB signaling contributes to axon elongation and growth cone formation through changes in the local actin cytoskeleton accompanied by increased Ca_v2.2 clustering and local calcium transients. These findings may help to explore cellular mechanisms which might be dysregulated during maturation of embryonic motoneurons leading to motoneuron disease.

Differential roles of α-, β-, and γ-actin in axon growth and collateral branch formation in motoneurons

α-, β-, and γ-actin differentially regulate cytoskeletal dynamics and stability in axons of motoneurons. Locally translated α-actin contributes to stable actin filaments in axonal branches, whereas β- and γ-actin give rise to highly dynamic filaments that modulate growth cone dynamics.

Axonal branching and terminal arborization are fundamental events during the establishment of synaptic connectivity. They are triggered by assembly of actin filaments along axon shafts giving rise to filopodia. The specific contribution of the three actin isoforms, Actα, Actβ, and Actγ, to filopodia stability and dynamics during this process is not well understood. Here, we report that Actα, Actβ, and Actγ isoforms are expressed in primary mouse motoneurons and their transcripts are translocated into axons. shRNA-mediated depletion of Actα reduces axonal filopodia dynamics and disturbs collateral branch formation. Knockdown of Actβ reduces dynamic movements of growth cone filopodia and impairs presynaptic differentiation. Ablation of Actβ or Actγ leads to compensatory up-regulation of the two other isoforms, which allows maintenance of total actin levels and preserves F-actin polymerization. Collectively, our data provide evidence for specific roles of different actin isoforms in spatial regulation of actin dynamics and stability in axons of developing motoneurons.

The role of laminins in the organization and function of neuromuscular junctions

The synapse between motor neurons and skeletal muscle is known as the neuromuscular junction (NMJ). Proper alignment of presynaptic and post-synaptic structures of motor neurons and muscle fibers, respectively, is essential for efficient motor control of skeletal muscles. The synaptic cleft between these two cells is filled with basal lamina. Laminins are heterotrimer extracellular matrix molecules that are key members of the basal lamina. Laminin α4, α5, and β2 chains specifically localize to NMJs, and these laminin isoforms play a critical role in maintenance of NMJs and organization of synaptic vesicle release sites known as active zones. These individual laminin chains exert their role in organizing NMJs by binding to their receptors including integrins, dystroglycan, and voltage-gated calcium channels (VGCCs). Disruption of these laminins or the laminin-receptor interaction occurs in neuromuscular diseases including Pierson syndrome and Lambert-Eaton myasthenic syndrome (LEMS). Interventions to maintain proper level of laminins and their receptor interactions may be insightful in treating neuromuscular diseases and aging related degeneration of NMJs.

Review : The Role of Schwann cells in Neural Regeneration

The difference in regenerative capacity between the PNS and the CNS is not due to an intrinsic inability of central neurons to extend fibers. Rather, it is probably related to the environment in the CNS that is either repulsive to axonal outgrowth and/or nonsupportive of axonal elongation. In contrast, the PNS both supports and allows for axonal elongation after injury. The Schwann cell, which is the glial cell of the PNS, is strictly required for peripheral regeneration. Here we discuss recent work describing the biology of Schwann cell- dependent regeneration, discuss what is known of the molecular basis of this phenomenon, and how it might apply to the damaged CNS. NEUROSCIENTIST 5:208-216, 1999

Neuronal Guidance Molecules: Inhibitory and Soluble Factors

Axonal guidance factors play a central role in neural development and regeneration. The ability of cell surface adhesive proteins and extracellular matrix components to promote axonal outgrowth has been documented for some time. Recently, the existence and the importance of molecules that repulse axons and of soluble factors that attract axons have been appreciated. By virtue of their long-range diffusible action, the netrins are now well- defined, soluble axonal guidance molecules. The physiological role of repulsive mecha nisms has been best documented in the development of the retinotectal map and in the ability of CNS myelin to inhibit axonal regeneration. The collapsin/semaphorin family of axonal growth inhibitors has been characterized at the molecular level. It is now clear that an understanding of axonal guidance mechanisms must include soluble cell-surface and matrix-bound factors, which are both attractive and repulsive for axonal growth cones. The Neuroscientist 1:255-258, 1995

Presynaptic localization of Smn and hnRNP R in axon terminals of embryonic and postnatal mouse motoneurons

Spinal muscular atrophy (SMA) is caused by deficiency of the ubiquitously expressed survival motoneuron (SMN) protein. SMN is crucial component of a complex for the assembly of spliceosomal small nuclear ribonucleoprotein (snRNP) particles. Other cellular functions of SMN are less characterized so far. SMA predominantly affects lower motoneurons, but the cellular basis for this relative specificity is still unknown. In contrast to nonneuronal cells where the protein is mainly localized in perinuclear regions and the nucleus, Smn is also present in dendrites, axons and axonal growth cones of isolated motoneurons invitro. However, this distribution has not been shown invivo and it is not clear whether Smn and hnRNP R are also present in presynaptic axon terminals of motoneurons in postnatal mice. Smn also associates with components not included in the classical SMN complex like RNA-binding proteins FUS, TDP43, HuD and hnRNP R which are involved in RNA processing, subcellular localization and translation. We show here that Smn and hnRNP R are present in presynaptic compartments at neuromuscular endplates of embryonic and postnatal mice. Smn and hnRNP R are localized in close proximity to each other in axons and axon terminals both invitro and invivo. We also provide new evidence for a direct interaction of Smn and hnRNP R invitro and invivo, particularly in the cytosol of motoneurons. These data point to functions of SMN beyond snRNP assembly which could be crucial for recruitment and transport of RNA particles into axons and axon terminals, a mechanism which may contribute to SMA pathogenesis.

Cadherin-7 regulates mossy fiber connectivity in the cerebellum

To establish highly precise patterns of neural connectivity, developing axons must stop growing at their appropriate destinations and specifically synapse with target cells. However, the molecular mechanisms governing these sequential steps remain poorly understood. Here, we demonstrate that cadherin-7 (Cdh7) plays a dual role in axonal growth termination and specific synapse formation during the development of the cerebellar mossy fiber circuit. Cdh7 is expressed in mossy fiber pontine nucleus (PN) neurons and their target cerebellar granule neurons during synaptogenesis and selectively mediates synapse formation between those neurons. Additionally, Cdh7 presented by mature granule neurons diminishes the growth potential of PN axons. Furthermore, knockdown of Cdh7 in PN neurons in vivo severely impairs the connectivity of PN axons in the developing cerebellum. These findings reveal a mechanism by which a single bifunctional cell-surface receptor orchestrates precise wiring by regulating axonal growth potential and synaptic specificity.

The Leu-Arg-Glu (LRE) adhesion motif in proteins of the neuromuscular junction with special reference to proteins of the carboxylesterase/cholinesterase family

Short linear motifs confer evolutionary flexibility on proteins as they can be added with relative ease allowing the acquisition of new functions. Such motifs may mediate a variety of signalling functions. The adhesion-mediating Leu-Arg-Glu (LRE) motif is enriched in laminin beta 2, and has been observed in other proteins, including members of the carboxylesterase/cholinesterase family. It acts as a stop signal for growing axons in the developing neuromuscular junction, binding to the voltage-gated calcium channel. In this bioinformatic analysis, we have investigated the presence of the motif in proteins of the neuromuscular junction, and have also examined its structural position and potential for ligand interaction, as well as phylogenetic conservation, in the carboxylesterase/cholinesterase family. The motif was observed to occur with a significantly higher frequency than expected in the UniProt/Swiss-Prot database, as well as in four individual species (human, mouse, Caenorhabditis elegans and Drosophila melanogaster). Examination of its presence in neuromuscular junction proteins showed it to be enriched in certain proteins of the synaptic basement membrane, including laminin, agrin, acetylcholinesterase and tenascin. A highly significant enrichment was observed in cytoskeletal proteins, particularly intermediate filament proteins and members of the spectrin family. In the carboxylesterase/cholinesterase family, the motif was observed in four conserved positions in the protein structure. It is present in the majority of mammalian acetylcholinesterases, as well as acetylcholinesterases from electric fish and a number of invertebrates. In insects, it is present in the ace-2, rather than in the synaptic ace-1, enzyme. It is also observed in the cholinesterase-like adhesion molecules (neuroligins, neurotactin and glutactin). It is never seen in butyrylcholinesterases, which do not mediate cell adhesion. In conclusion, the significant enrichment of the motif in certain classes of protein, as well as its conserved presence and structural positioning in one protein family, suggests that it has specific functions both in cell adhesion in the neuromuscular junction and in maintaining the structural integrity of the cytoskeleton.

Spontaneous activity regulates Robo1 transcription to mediate a switch in thalamocortical axon growth

Developing axons must control their growth rate to follow the appropriate pathways and establish specific connections. However, the regulatory mechanisms involved remain elusive. By combining live imaging with transplantation studies in mice, we found that spontaneous calcium activity in the thalamocortical system and the growth rate of thalamocortical axons were developmentally and intrinsically regulated. Indeed, the spontaneous activity of thalamic neurons governed axon growth and extension through the cortex in vivo. This activity-dependent modulation of growth was mediated by transcriptional regulation of Robo1 through an NF-κB binding site. Disruption of either the Robo1 or Slit1 genes accelerated the progression of thalamocortical axons in vivo, and interfering with Robo1 signaling restored normal axon growth in electrically silent neurons. Thus, modifications to spontaneous calcium activity encode a switch in the axon outgrowth program that allows the establishment of specific neuronal connections through the transcriptional regulation of Slit1 and Robo1 signaling.

Molecular mechanism of active zone organization at vertebrate neuromuscular junctions

Organization of presynaptic active zones is essential for development, plasticity, and pathology of the nervous system. Recent studies indicate a trans-synaptic molecular mechanism that organizes the active zones by connecting the pre- and the postsynaptic specialization. The presynaptic component of this trans-synaptic mechanism is comprised of cytosolic active zone proteins bound to the cytosolic domains of voltage-dependent calcium channels (P/Q-, N-, and L-type) on the presynaptic membrane. The postsynaptic component of this mechanism is the synapse organizer (laminin β2) that is expressed by the postsynaptic cell and accumulates specifically on top of the postsynaptic specialization. The pre- and the postsynaptic components interact directly between the extracellular domains of calcium channels and laminin β2 to anchor the presynaptic protein complex in front of the postsynaptic specialization. Hence, the presynaptic calcium channel functions as a scaffolding protein for active zone organization and as an ion-conducting channel for synaptic transmission. In contrast to the requirement of calcium influx for synaptic transmission, the formation of the active zone does not require the calcium influx through the calcium channels. Importantly, the active zones of adult synapses are not stable structures and require maintenance for their integrity. Furthermore, aging or diseases of the central and peripheral nervous system impair the active zones. This review will focus on the molecular mechanisms that organize the presynaptic active zones and summarize recent findings at the neuromuscular junctions and other synapses.

Presynaptic calcium channels and α3-integrins are complexed with synaptic cleft laminins, cytoskeletal elements and active zone components

At chemical synapses, synaptic cleft components interact with elements of the nerve terminal membrane to promote differentiation and regulate function. Laminins containing the β2 subunit are key cleft components, and they act in part by binding the pore-forming subunit of a pre-synaptic voltage-gated calcium channel (Ca(v)α) (Nishimune et al. 2004). In this study, we identify Ca(v)α-associated intracellular proteins that may couple channel-anchoring to assembly or stabilization of neurotransmitter release sites called active zones. Using Ca(v)α-antibodies, we isolated a protein complex from Torpedo electric organ synapses, which resemble neuromuscular junctions but are easier to isolate in bulk. We identified 10 components of the complex: six cytoskeletal proteins (α2/β2 spectrins, plectin 1, AHNAK/desmoyokin, dystrophin, and myosin 1), two active zone components (bassoon and piccolo), synaptic laminin, and a calcium channel β subunit. Immunocytochemistry confirmed these proteins in electric organ synapses, and PCR analysis revealed their expression by developing mammalian motor neurons. Finally, we show that synaptic laminins also interact with pre-synaptic integrins containing the α3 subunit. Together with our previous finding that a distinct synaptic laminin interacts with SV2 on nerve terminals (Son et al. 2000), our results identify three paths by which synaptic cleft laminins can send developmentally important signals to nerve terminals.

Laminin-121--recombinant expression and interactions with integrins

Laminin-121, previously referred as to laminin-3, was expressed recombinantly in human embryonic kidney (HEK) 293 cells by triple transfection of full-length cDNAs encoding mouse laminin α1, β2 and γ1 chains. The recombinant laminin-121 was purified using Heparin-Sepharose followed by molecular sieve chromatography and shown to be correctly folded by electron microscopy and circular dichroism (CD). The CD spectra of recombinant laminin-121 were very similar to those of laminin-111 isolated from Engelbreth-Holm-Swarm tumor (EHS-laminin) but its T(m) value was smaller than EHS-laminin and recombinant lamnin-111 suggesting that the replacement of the β chain reduced the stability of the coiled-coil structure of laminin-121. Its binding to integrins was compared with EHS-laminin, laminin-3A32 purified from murine epidermal cell line and recombinantly expressed laminins-111, -211 and -221. Laminin-121 showed the highest affinity to α6β1 and α7β1 integrins and furthermore, laminin-121 most effectively supported neurite outgrowth. Together, this suggests that the β2 laminins have higher affinity for integrins than the β1 laminins.

New insights into the regulation of ion channels by integrins

By controlling cell adhesion to the extracellular matrix, integrin receptors regulate processes as diverse as cell migration, proliferation, differentiation, apoptosis, and synaptic stability. Because the underlying mechanisms are generally accompanied by changes in transmembrane ion flow, a complex interplay occurs between integrins, ion channels, and other membrane transporters. This reciprocal interaction regulates bidirectional signal transduction across the cell surface and may take place at all levels of control, from transcription to direct conformational coupling. In particular, it is becoming increasingly clear that integrin receptors form macromolecular complexes with ion channels. Besides contributing to the membrane localization of the channel protein, the integrin/channel complex can regulate a variety of downstream signaling pathways, centered on regulatory proteins like tyrosine kinases and small GTPases. In turn, the channel protein usually controls integrin activation and expression. We review some recent advances in the field, with special emphasis on hematology and neuroscience. Some oncological implications are also discussed.

Neuroprotective effects of laminin and its KDI domain

Successful treatment of neurodegenerative diseases and CNS trauma are the most intractable problems in modern medicine. Numerous reports have shown the strong role that laminins have on the survival, regeneration and development of various types of cells, including neural cells. It would be desirable to take advantage of laminin activities for therapeutic purposes. However, there are at least ten laminin variants and the trimeric molecules are of the order of 800,000 molecular weight. Furthermore, human laminins are not available in quantity. Therefore, we and others have taken the approach of determining which domains of the laminin molecules are functional in the CNS, and whether short peptides from these regions exhibit biological activities with the intent of testing their potential for therapeutic use. Understanding the role of laminins and their small biologically active peptide domains, such as the KDI (lysine–aspartic acid–isoleucine) peptide from γ1 laminin, in neuronal development, CNS trauma (spinal cord injury and stroke) and neurodegenerative disorders (amyotrophic lateral sclerosis, Alzheimer’s disease and Parkinson’s disease) may help to develop clinically applicable methods to treat the presently untreatable CNS diseases and trauma even in the near future.

Neurite outgrowth and in vivo sensory innervation mediated by a Ca(V)2.2-laminin beta 2 stop signal

Axons and dendrites of developing neurons establish distributed innervation patterns enabling precise discrimination in sensory systems. We describe the role of the extracellular matrix molecule, laminin beta2, interacting with the Ca(V)2.2 calcium channel in establishing appropriate sensory innervation. In vivo, Ca(V)2.2 is expressed on the growth cones of Xenopus laevis sensory neurites and laminin beta2 is expressed in the skin. Culturing neurons on a laminin beta2 substrate inhibits neurite outgrowth in a specific and calcium-dependent manner. Blocking signaling between laminin beta2 and Ca(V)2.2 leads to increased numbers of sensory terminals in vivo. These findings suggest that interactions between extracellular matrix molecules and calcium channels regulate connectivity in the developing nervous system.

Target-dependent inhibition of sympathetic neuron growth via modulation of a BMP signaling pathway

Target-derived factors modulate many aspects of peripheral neuron development including neuronal growth, survival, and maturation. Less is known about how initial target contact regulates changes in gene expression associated with these developmental processes. One early consequence of contact between growing sympathetic neurons and their cardiac myocyte targets is the inhibition of neuronal outgrowth. Analysis of neuronal gene expression following this contact revealed coordinate regulation of a bone morphogenetic protein (BMP)-dependent growth pathway in which basic helix-loop-helix transcription factors and downstream neurofilament expression contribute to the growth dynamics of developing sympathetic neurons. BMP2 had dose-dependent growth-promoting effects on sympathetic neurons cultured in the absence, but not the presence, of myocyte targets, suggesting that target contact alters neuronal responses to BMP signaling. Target contact also induced the expression of matrix Gla protein (MGP), a regulator of BMP function in the vascular system. Increased MGP expression inhibited BMP-dependent neuronal growth and MGP expression increased in sympathetic neurons during the period of target contact in vivo. These experiments establish MGP as a novel regulator of BMP function in the nervous system, and define developmental transitions in BMP responses during sympathetic development.

Defective Ca2+ channel clustering in axon terminals disturbs excitability in motoneurons in spinal muscular atrophy

Proximal spinal muscular atrophy (SMA) is a motoneuron disease for which there is currently no effective treatment. In animal models of SMA, spinal motoneurons exhibit reduced axon elongation and growth cone size. These defects correlate with reduced beta-actin messenger RNA and protein levels in distal axons. We show that survival motoneuron gene (Smn)-deficient motoneurons exhibit severe defects in clustering Cav2.2 channels in axonal growth cones. These defects also correlate with a reduced frequency of local Ca2+ transients. In contrast, global spontaneous excitability measured in cell bodies and proximal axons is not reduced. Stimulation of Smn production from the transgenic SMN2 gene by cyclic adenosine monophosphate restores Cav2.2 accumulation and excitability. This may lead to the development of new therapies for SMA that are not focused on enhancing motoneuron survival but instead investigate restoration of growth cone excitability and function.

Seeking long-term relationship: axon and target communicate to organize synaptic differentiation

Synapses form after growing axons recognize their appropriate targets. The subsequent assembly of aligned pre and postsynaptic specializations is critical for synaptic function. This highly precise apposition of presynaptic elements (i.e. active zones) to postsynaptic specializations (i.e. neurotransmitter receptor clusters) strongly suggests that communication between the axon and target is required for synaptic differentiation. What trans-synaptic factors drive such differentiation at vertebrate synapses? First insights into the answers to this question came from studies at the neuromuscular junction (NMJ), where axon-derived agrin and muscle-derived laminin beta2 induce post and presynaptic differentiation, respectively. Recent work has suggested that axon- and target-derived factors similarly drive synaptic differentiation at central synapses. Specifically, WNT-7a, neuroligin, synaptic cell adhesion molecule (SynCAM) and fibroblast growth factor-22 (FGF-22) have all been identified as target-derived presynaptic organizers, whereas axon-derived neuronal activity regulated pentraxin (Narp), ephrinB and neurexin reciprocally co-ordinate postsynaptic differentiation. In addition to these axon- and target-derived inducers of synaptic differentiation, factors released from glial cells have also been implicated in regulating synapse assembly. Together, these recent findings have profoundly advanced our understanding of how precise appositions are established during vertebrate nervous system development.

ROBO directs axon crossing of segmental boundaries by suppressing responsiveness to relocalized Netrin

Networks in the CNS consist of neural modules that are connected in a repetitive array. Whereas individual modules contain guidance information along which axons track within the unit, these guidance cues hinder axon extension across module boundaries. We investigated how axons solve this 'boundary problem' by analyzing the longitudinal connections of neuromeres in Drosophila melanogaster. The initial trajectory of the longitudinal axons is guided by Netrin, which is localized on commissural axons by its receptor, Frazzled. The Netrin cue on the commissure of the next segment can act as a barrier to longitudinal axons, inhibiting their extension and misguiding them contralaterally along the commissure. We show that, before reaching the segmental boundary, the longitudinal axons' responsiveness to Netrin presented on the commissure is suppressed by Roundabout (ROBO), through counteracting Gq signaling. The absence of suppression causes the robo phenotype: longitudinal axons project toward the midline, as if running around a roundabout (rotary).

Motor neurite outgrowth is selectively inhibited by cell surface MuSK and agrin

During neuromuscular development, "stop signals" present on the target myotube inhibit motor axon growth. Mice lacking either the neuronal form of agrin or the muscle-specific tyrosine kinase (MuSK) lose stop signal activity, suggesting that they are part of such signals or induce them in myotubes. To test whether MuSK complexes form stop signals in the absence of myotube signaling, we cultured ciliary ganglion (CG) neurons with nonmuscle cells expressing cell-surface MuSK. Expression of MuSK had no effect on neuronal adhesion. MuSK expression, however, inhibited neurite outgrowth from CG neurons, but not retinal ganglion cell neurons. The neurite-inhibitory effect could be completely reversed by an antibody to the MuSK extracellular domain, and partially reversed by an antibody to agrin, suggesting that inhibition is mediated by a complex of these proteins. Thus, an agrin/MuSK complex may form part of a motor neuron stop signal involved in "reverse signaling" to the motor neuron.

Beta1 integrins in muscle, but not in motor neurons, are required for skeletal muscle innervation

In vitro studies have provided evidence that beta1 integrins in motor neurons promote neurite outgrowth, whereas beta1 integrins in myotubes regulate acetylcholine receptor (AChR) clustering. Surprisingly, using genetic studies in mice, we show here that motor axon outgrowth and neuromuscular junction (NMJ) formation in large part are unaffected when the integrin beta1 gene (Itgb1) is inactivated in motor neurons. In the absence of Itgb1 expression in skeletal muscle, interactions between motor neurons and muscle are defective, preventing normal presynaptic differentiation. Motor neurons fail to terminate their growth at the muscle midline, branch excessively, and develop abnormal nerve terminals. These defects resemble the phenotype of agrin-null mice, suggesting that signaling molecules such as agrin, which coordinate presynaptic and postsynaptic differentiation, are not presented properly to nerve terminals. We conclude that Itgb1 expression in muscle, but not in motor neurons, is critical for NMJ development.

Coordinated motor neuron axon growth and neuromuscular synaptogenesis are promoted by CPG15 in vivo

We have used in vivo time-lapse two-photon imaging of single motor neuron axons labeled with GFP combined with labeling of presynaptic vesicle clusters and postsynaptic acetylcholine receptors in Xenopus laevis tadpoles to determine the dynamic rearrangement of individual axon branches and synaptogenesis during motor axon arbor development. Control GFP-labeled axons are highly dynamic during the period when axon arbors are elaborating. Axon branches emerge from sites of synaptic vesicle clusters. These data indicate that motor neuron axon elaboration and synaptogenesis are concurrent and iterative. We tested the role of Candidate Plasticity Gene 15 (CPG15, also known as Neuritin), an activity-regulated gene that is expressed in the developing motor neurons in this process. CPG15 expression enhances the development of motor neuron axon arbors by promoting neuromuscular synaptogenesis and by increasing the addition of new axon branches.

A synaptic laminin-calcium channel interaction organizes active zones in motor nerve terminals

Synapse formation requires the differentiation of a functional nerve terminal opposite a specialized postsynaptic membrane. Here, we show that laminin beta2, a component of the synaptic cleft at the neuromuscular junction, binds directly to calcium channels that are required for neurotransmitter release from motor nerve terminals. This interaction leads to clustering of channels, which in turn recruit other presynaptic components. Perturbation of this interaction in vivo results in disassembly of neurotransmitter release sites, resembling defects previously observed in an autoimmune neuromuscular disorder, Lambert-Eaton myasthenic syndrome. These results identify an extracellular ligand of the voltage-gated calcium channel as well as a new laminin receptor. They also suggest a model for the development of nerve terminals, and provide clues to the pathogenesis of a synaptic disease.

Characterization of a novel molluskan tyrosine kinase receptor that inhibits neurite regeneration

Receptor tyrosine kinases play many important roles in neuronal signaling including regulating neurite outgrowth. We have identified a novel receptor tyrosine kinase, neurite outgrowth regulating kinase (nork) from Aplysia californica. A fragment of this kinase was also identified in another mollusk, Lymnaea. The kinase domain is equally homologous to the Ret (rearranged during transformation) and fibroblast growth factor receptor families, but the extracellular domain is entirely novel, suggesting that it binds a nonconserved ligand. Overexpression of neurite outgrowth regulating kinase, but not a kinase dead form, causes a reduction in neurite outgrowth of Aplysia sensory neurons. Thus, we have identified a novel receptor tyrosine kinase implicated in regulating neurite outgrowth.

FGF22 and its close relatives are presynaptic organizing molecules in the mammalian brain

Target-derived cues promote local differentiation of axons into nerve terminals at sites of synaptic contact. Using clustering of synaptic vesicles in cultured neurons as an assay, we purified putative target-derived presynaptic organizing molecules from mouse brain and identified FGF22 as a major active species. FGF7 and FGF10, the closest relatives of FGF22, share this activity; other FGFs have distinct effects. FGF22 is expressed by cerebellar granule cells during the period when they receive synapses. Its receptor, FGFR2, is expressed by pontine and vestibular neurons when their axons (mossy fibers) are making synapses on granule cells. Neutralization of FGF7, -10, and -22 inhibits presynaptic differentiation of mossy fibers at sites of contact with granule cells in vivo. Inactivation of FGFR2 has similar effects. These results indicate that FGF22 and its relatives are presynaptic organizing molecules in the mammalian brain and suggest new functions for this family of signaling molecules.

Laminin receptors: achieving specificity through cooperation

The laminins are a large family of extracellular matrix proteins that can profoundly influence development, differentiation and disease progression. The biological effects of the laminins are mediated by surface receptors that link laminin matrices to intracellular signalling pathways. Several classes of receptors, including integrins and other molecules, may cooperate to provide the specificity apparent in the diverse array of laminin-mediated phenomena. This review assesses our current understanding of laminin receptors and discusses how such receptors could recognize structural differences among the laminins and relay these differences to the cell.

Integrins and extracellular matrix in animal models

Integrins are a family of transmembrane receptors that mediate interactions of cells with extracellular matrix (ECM) constituents and cell surface counter receptors. Each integrin mediates interactions with specific sets of ligands and regulates distinct aspects of cellular function including attachment to and organization of ECM assemblies, cell migration, proliferation and survival, and mechanical force transmission. Integrins exert their versatile functions by establishing a transmembrane link between the cell exterior and the cytoskeleton, and by activating intracellular second messenger systems. In addition, cellular signals can modulate integrin activity and ligand interactions, enabling transduction of information from the inside of the cell to the outside. Many of the basic functions of integrins and their ECM ligands have been uncovered by studying them biochemically or with cells in culture. Integrin and ECM functions have also been determined genetically, defining their essential roles in the organism. The ongoing challenge is to integrate cell biological, biochemical, and genetical evidence into a coherent picture. I will discuss here genetic findings, focusing on the murine system, that have shed light on the developmental functions of integrins and their ECM ligands. Where suitable information is available, I will relate the genetical finding to results obtained with cell biological and biochemical approaches.

Neuromuscular synapses mediate motor axon branching and motoneuron survival during the embryonic period of programmed cell death

The embryonic period of motoneuron programmed cell death (PCD) is marked by transient motor axon branching, but the role of neuromuscular synapses in regulating motoneuron number and axonal branching is not known. Here, we test whether neuromuscular synapses are required for the quantitative association between reduced skeletal muscle contraction, increased motor neurite branching, and increased motoneuron survival. We achieved this by comparing agrin and rapsyn mutant mice that lack acetylcholine receptor (AChR) clusters. There were significant reductions in nerve-evoked skeletal muscle contraction, increases in intramuscular axonal branching, and increases in spinal motoneuron survival in agrin and rapsyn mutant mice compared with their wild-type littermates at embryonic day 18.5 (E18.5). The maximum nerve-evoked skeletal muscle contraction was reduced a further 17% in agrin mutants than in rapsyn mutants. This correlated to an increase in motor axon branch extension and number that was 38% more in agrin mutants than in rapsyn mutants. This suggests that specializations of the neuromuscular synapse that ensure efficient synaptic transmission and muscle contraction are also vital mediators of motor axon branching. However, these increases in motor axon branching did not correlate with increases in motoneuron survival when comparing agrin and rapsyn mutants. Thus, agrin-induced synaptic specializations are required for skeletal muscle to effectively control motoneuron numbers during embryonic development.

Sensory neuron subtypes have unique substratum preference and receptor expression before target innervation

The factors controlling the specification and subsequent differentiation of sensory neurons are poorly understood. Data from embryological manipulations suggest that either sensory neuron fates are specified by the targets they encounter or sensory neurons are considerably more "plastic" with respect to specification than are neurons of the CNS. The prevailing view that sensory neurons are specified late in development is not consistent, however, with the directed outgrowth of sensory neurons to their targets and the characteristic spatial distribution of sensory neuron fates within the peripheral ganglia. To address when in development different classes of sensory neurons can first be distinguished, we investigated the interactions of early dorsal root ganglia neurons with the extracellular matrix before neurite outgrowth to targets. We found that subclasses of sensory neurons in early dorsal root ganglia show different patterns of neurite outgrowth and integrin expression that are predictive of their fates. In the absence of neurotrophins, presumptive proprioceptive neurons extend neurites robustly on both laminin and fibronectin, whereas presumptive cutaneous neurons show a strong preference for laminin. Cutaneous afferents that have innervated targets show a similar strong preference for laminin and show higher levels of integrin alpha7beta1 than do proprioceptive neurons. Finally, presumptive proprioceptive neurons express fibronectin receptors, integrin alpha3beta1, alpha4beta1, and alpha5beta1, at higher levels than do presumptive cutaneous neurons. Our results indicate that subtypes of sensory neurons have unique patterns of neurite outgrowth and receptor expression before target innervation.

Target-mediated control of neural differentiation

The development of the nervous system entails the coordination of the spatial and chemical development of both pre- and postsynaptic elements. This coordination is accomplished by signals passing between neurons and the target cells that they innervate. This review focuses on well-characterized examples of target-mediated neuronal differentiation in the central and peripheral nervous systems. These include control of neurogenesis in the leech by male genitalia, presynaptic differentiation induced by postsynaptic molecules expressed by skeletal muscle, postsynaptic adhesion molecules that induce presynaptic differentiation in the central nervous system (CNS), target-mediated control of neurotransmitter phenotype in peripheral neurons, and target-regulated control of neuronal nicotinic acetylcholine receptors (nAChRs) and large conductance calcium-activated potassium channels (BK). The detailed understanding of these processes will uncover signals critical for the directed differentiation of stem cells as well as identify future targets for therapies in neural regeneration that promote the reestablishment of functional connections.

Functional analysis of neurotransmission at beta2-laminin deficient terminals

beta2-Laminin is important for the formation of neuromuscular junctions in vertebrates. Previously, we have inactivated the gene that encodes for beta2-laminin in mice and observed predominantly prejunctional structural defects. In this study, we have used both intra- and extracellular recording methods to investigate evoked neurotransmission in beta2-laminin-deficient mice, from postnatal day 8 (P8) through to day 18 (P18). Our results confirmed that there was a decrease in the frequency of spontaneous release, but no change in the postjunctional response to such release. Analysis of evoked neurotransmission showed an increase in the frequency of stimuli that failed to elicit an evoked postjunctional response in the mutants compared to litter mate controls, resulting in a 50 % reduction in mean quantal content at mutant terminals. Compared to littermate controls, beta2-laminin-deficient terminals showed greater synaptic depression when subjected to high frequency stimulation. Furthermore, the paired pulse ratio of the first two stimuli was significantly lower in beta2-laminin mutant terminals. Statistical analysis of the binomial parameters of release showed that the decrease in quantal content was due to a decrease in the number of release sites without any significant change in the average probability of release. This suggestion was supported by the observation of fewer synaptic vesicle protein 2 (SV2)-positive varicosities in beta2-laminin-deficient terminals and by ultrastructural observations showing smaller terminal profiles and increased Schwann cell invasion in beta2-laminin mutants; the differences between beta2-laminin mutants and wild-type mice were the same at both P8 and P18. From these results we conclude that beta2-laminin plays a role in the early structural development of the neuromuscular junction. We also suggest that transmitter release activity may act as a deterrent to Schwann cell invasion in the absence of beta2-laminin.

Wiring of the brain by a range of guidance cues

During development of the central nervous system, growth cones navigate along specific pathways, recognize their targets and then form synaptic connections by elaborating terminal arbors. To date, a number of developmental and in vitro studies have characterized the nature of the guidance cues that underlie various types of axonal behavior, from initial outgrowth to synapse formation, including pathway selection, polarized growth, orientated growth, termination and branching. New approaches in molecular biology have identified several types of guidance cues, most of which are likely to act as local cues. Moreover, recent studies have indicated that axonal responsiveness to guidance cues changes dynamically, which appears to be elicited by environmental factors encountered by the navigating growth cones. This article addresses what molecular cues are responsible for guidance mechanisms including axonal responsiveness, focusing on axonal behavior in the developmental stages.

Molecular analysis of gene expression in the developing pontocerebellar projection system

As an approach toward understanding the molecular mechanisms of neuronal differentiation, we utilized DNA microarrays to elucidate global patterns of gene expression during pontocerebellar development. Through this analysis, we identified groups of genes specific to neuronal precursor cells, associated with axon outgrowth, and regulated in response to contact with synaptic target cells. In the cerebellum, we identified a phase of granule cell differentiation that is independent of interactions with other cerebellar cell types. Analysis of pontine gene expression revealed that distinct programs of gene expression, correlated with axon outgrowth and synapse formation, can be decoupled and are likely influenced by different cells in the cerebellar target environment. Our approach provides insight into the genetic programs underlying the differentiation of specific cell types in the pontocerebellar projection system.

Mapping sites responsible for interactions of agrin with neurons

The multidomain proteoglycan agrin is a critical organizer of postsynaptic differentiation at the skeletal neuromuscular junction. Agrin is also abundant in the brain, but its roles there are unknown. As a step toward understanding these roles, we mapped sites responsible for interactions of neurons with agrin. First, we used a series of recombinant agrin fragments to show that at least four sites on agrin interact with chick ciliary neurons. Use of blocking antibodies and peptides indicated that neurons adhere to a site in the second of three G domains by means of alphaVbeta1 integrin, and to a site in the last of four epidermal growth factor (EGF) repeats via a distinct beta1 integrin. A third, integrin-independent adhesion site is near to but distinct from the site that induces postsynaptic differentiation in muscles. These domains are insufficient, however, to account for neurite outgrowth-inhibiting properties of full-length agrin, which are mediated by the N-terminal half of the molecule. We then used a second set of agrin mutants to demonstrate and map a transmembrane domain in the amino-terminus of the SN-isoform of agrin. The extracellular matrix-bound form of agrin, called LN, bears an amino-terminus required for secretion and binding to laminin. The SN form, which is selectively expressed by neurons, bears a variant amino terminus that converts agrin from a secreted, matrix-associated protein to a type-II transmembrane protein, providing a mechanism for presenting agrin in central, as opposed to neuromuscular, synaptic clefts. The SN-amino terminus can mediate externalization and membrane anchoring of heterologous proteins, but is insufficient to target them to the synapse. Together, these studies define sites that contribute to the subcellular localization of and signaling by neuronal agrin.

Domain IV of mouse laminin beta1 and beta2 chains

Domain IV, consisting of about 230 residues, represents a particular protein module so far found only in laminin beta1 and beta2 chains. Both domains were obtained by recombinant production in mammalian cells. They showed a globular structure, as expected from electron microscopic examination of laminins. Fragment beta1IV was obtained as a monomer and a disulfide-bonded dimer, and both were modified to approximately 50% by a single chondroitin sulfate chain attached to Ser721 of an SGD consensus sequence. Dimerization is caused by an odd number of cysteines, with three of them having a partial thiol character. Whether both modifications also occur in tissue forms of laminin remains to be established. Fragment beta2IV was only obtained as a monomer, as it lacked one crucial cysteine and the SGD sequence. It required, however, the presence of two adjacent LE modules for proper folding. Polyclonal antibodies raised against both fragments showed no cross-reaction with each other and allowed establishment of beta chain-specific radioimmunoassays and light and electron microscopic immunostaining of tissues. This demonstrated a 5-25-fold lower content of beta2 compared with beta1 chains in various tissue extracts of adult mice. Tissues derived from beta2-deficient mice failed to react with the beta2-specific antibodies but showed a twofold higher content of beta1 than heterozygotes. The antibodies to beta2 showed broader tissue staining than reported previously, including in particular a distinct reaction with the extrasynaptic endomysium of skeletal muscle. Immunogold staining localized both beta chains primarily to basement membranes of kidney, muscle and various other tissues.

Fibronectin and laminin elicit differential behaviors from SH-SY5Y growth cones contacting inhibitory chondroitin sulfate proteoglycans

Neuronal growth cones integrate signals from outgrowth-promoting molecules, e.g., laminin (LN) or fibronectin (FN), and outgrowth-inhibiting molecules, e.g., chondroitin sulfate proteoglycans (CSPGs), to navigate through extracellular matrix (ECM). Sensory neurons on LN typically turn to avoid areas rich in inhibitory CSPGs, whereas neuron-like cells of human origin (SH-SY5Y) preferentially stop/stall. These different behaviors may reflect differences in neuron type, response to outgrowth-promoters, or the mechanisms involved in outgrowth vs. inhibition. We used image analysis to determine the effects of different outgrowth promoters on the response of SH-SY5Y cells to inhibitory CSPGs. LN increased neurite initiation and elongation compared to cells plated either on endogenous matrix or FN. On a patterned substratum consisting of alternating stripes of FN and CSPGs, 59.6 +/- 9.3% of SH-SY5Y growth cones turned upon CSPG contact, whereas only 31.9 +/- 8.2% of growth cones turned at a LN/CSPG border. Growth cones on LN spread more upon contact with CSPG than growth cones on FN, whereas growth cones on LN or FN not contacting CSPGs were morphologically similar. Because it is known that integrins are involved in outgrowth on promoters, we analyzed integrin expression in response to inhibitory CSPGs in a choice assay. CSPGs did not induce increases or redistribution of several integrin subunits in SH-SY5Y cells. Furthermore, an anti-beta1 integrin function-blocking antibody did not alter growth cone behavior at a CSPG border. These results indicate that significant mechanistic differences may exist between outgrowth on homogenous outgrowth promoters and growth cone turning at inhibitory molecules.

Properly formed but improperly localized synaptic specializations in the absence of laminin alpha4

Precise apposition of pre- to postsynaptic specializations is required for optimal function of chemical synapses, but little is known about how it is achieved. At the skeletal neuromuscular junction, active zones (transmitter release sites) in the nerve terminal lie directly opposite junctional folds in the postsynaptic membrane. Few active zones or junctional folds form in mice lacking the laminin beta2 chain, which is normally concentrated in the synaptic cleft. beta2 and the broadly expressed gamma1 chain form heterotrimers with alpha chains, three of which, alpha2, alpha4 and alpha5, are present in the synaptic cleft. Thus, alpha2beta2gamma1, alpha4beta2gamma1 and alpha5beta2gamma1 heterotrimers are all lost in beta2 mutants. In mice lacking laminin alpha4, active zones and junctional folds form in normal numbers, but are not precisely apposed to each other. Thus, formation and localization of synaptic specializations are regulated separately, and alpha4beta2gamma1 (called laminin-9) is critical in the latter process.

Extracellular matrix in development of the early embryo

The extracellular matrix interacts with cells and promotes and regulates cellular functions such as adhesion, migration, proliferation, differentiation, and morphogenesis. Extracellular molecules are linked to one another by multiple binding domains and form a stable, multifunctional matrix. Cells respond to the extracellular matrix through plasma membrane receptors, which include integrin and non-integrin receptors. The regulation of these interactions requires the coordination of a multiplicity of signals both spatially and temporally.

Identification of active regions for neurite outgrowth activity of neurocrescin

We previously identified and cloned a neurite outgrowth promoting protein, Neurocrescin (NC), from the extract of the chick denervated leg muscles. In this study, we explored the active region of NC for neurite outgrowth. Using the deletion mutants of NC, we tested their neurite outgrowth activity in the cultured telencephalic neurons of E5 chick embryos. We found three regions which independently had significant neurite outgrowth activity comparable with that of the extract of the chick denervated leg muscles. These regions were not homologous to any well-known active sites such as the laminin active region, IKVAV. In parallel, searching the endogenous deletion mutants of NC in the rat brain, we cloned a mutant in which the region including the larger part of one of the three active regions was deleted. The neurite outgrowth activity of the mutant was significantly lower than that of normal NC. These results suggest the physiological significance of these active regions.

The SAD-1 kinase regulates presynaptic vesicle clustering and axon termination

During synapse formation, presynaptic axon outgrowth is terminated, presynaptic clusters of vesicles are associated with active zone proteins, and active zones are aligned with postsynaptic neurotransmitter receptors. We report here the identification of a novel serine/threonine kinase, SAD-1, that regulates several aspects of presynaptic differentiation in C. elegans. In sad-1 mutant animals presynaptic vesicle clusters in sensory neurons and motor neurons are diffuse and disorganized. Sensory axons fail to terminate in sad-1 mutants, whereas overexpression of SAD-1 causes sensory axons to terminate prematurely. SAD-1 protein is expressed in the nervous system and localizes to synapse-rich regions of the axons. SAD-1 is related to PAR-1, a kinase that regulates cell polarity during asymmetric cell division. Overexpression of SAD-1 causes mislocalization of vesicle proteins to dendrites, suggesting that sad-1 affects axonal-dendritic polarity as well as synaptic development.

Laminins and human disease

The laminin protein family has diverse tissue expression patterns and is involved in the pathology of a number of organs, including skin, muscle, and nerve. In the skin, laminins 5 and 6 contribute to dermal-epidermal cohesion, and mutations in the constituent chains result in the blistering phenotype observed in patients with junctional epidermolysis bullosa (JEB). Allelic heterogeneity is observed in patients with JEB: mutations that results in premature stop codons produce a more severe phenotype than do missense mutations. Gene therapy approaches are currently being studied in the treatment of this disease. A blistering phenotype is also observed in patients with acquired cicatricial pemphigoid (CP). Autoantibodies targeted against laminins 5 and 6 destabilize epithelial adhesion and are pathogenic. In muscle cells, laminin alpha 2 is a component of the bridge that links the actin cytoskeleton to the extracellular matrix. In patients with laminin alpha 2 mutations, the bridge is disrupted and mature muscle cells apoptose. Congenital muscular dystrophy (CMD) results. The role of laminin in diseases of the nervous system is less well defined, but the extracellular protein has been shown to serve an important role in peripheral nerve regeneration. The adhesive molecule influences neurite outgrowth, neural differentiation, and synapse formation. The broad spatial distribution of laminin gene products suggests that laminin may be involved in a number of diseases for which pathogenic mechanisms are still being unraveled.