Coordinated interaction of Down syndrome cell adhesion molecule and deleted in colorectal cancer with dynamic TUBB3 mediates Netrin-1-induced axon branching

Netrin-1 as A neural guidance protein in development and reinnervation of the larynx

Neural guidance proteins participate in motor neuron migration, axonal projection, and muscle fiber innervation during development. One of the guidance proteins that participates in axonal pathfinding is Netrin-1. Despite the well-known role of Netrin-1 in embryogenesis of central nervous tissue, it is still unclear how the expression of this guidance protein contributes to primary innervation of the periphery, as well as reinnervation. This is especially true in the larynx where Netrin-1 is upregulated within the intrinsic laryngeal muscles after nerve injury and where blocking of Netrin-1 alters the pattern of reinnervation of the intrinsic laryngeal muscles. Despite this consistent finding, it is unknown how Netrin-1 expression contributes to guidance of the axons towards the larynx. Improved knowledge of Netrin-1's role in nerve regeneration and reinnervation post-injury in comparison to its role in primary innervation during embryological development, may provide insights in the search for therapeutics to treat nerve injury. This paper reviews the known functions of Netrin-1 during the formation of the central nervous system and during cranial nerve primary innervation. It also describes the role of Netrin-1 in the formation of the larynx and during recurrent laryngeal reinnervation following nerve injury in the adult.

The role of microtubule-associated protein tau in netrin-1 attractive signaling

Direct binding of netrin receptors with dynamic microtubules (MTs) in the neuronal growth cone plays an important role in netrin-mediated axon guidance. However, how netrin-1 (NTN1) regulates MT dynamics in axon turning remains a major unanswered question. Here, we show that the coupling of netrin-1 receptor DCC with tau (MAPT)-regulated MTs is involved in netrin-1-promoted axon attraction. Tau directly interacts with DCC and partially overlaps with DCC in the growth cone of primary neurons. Netrin-1 induces this interaction and the colocalization of DCC and tau in the growth cone. The netrin-1-induced interaction of tau with DCC relies on MT dynamics and TUBB3, a highly dynamic β-tubulin isotype in developing neurons. Netrin-1 increased cosedimentation of DCC with tau and TUBB3 in MTs, and knockdown of either tau or TUBB3 mutually blocked this effect. Downregulation of endogenous tau levels by tau shRNAs inhibited netrin-1-induced axon outgrowth, branching and commissural axon attraction in vitro, and led to defects in spinal commissural axon projection in vivo. These findings suggest that tau is a key MT-associated protein coupling DCC with MT dynamics in netrin-1-promoted axon attraction.

TUBB3 and KIF21A in neurodevelopment and disease

Neuronal migration and axon growth and guidance require precise control of microtubule dynamics and microtubule-based cargo transport. TUBB3 encodes the neuronal-specific β-tubulin isotype III, TUBB3, a component of neuronal microtubules expressed throughout the life of central and peripheral neurons. Human pathogenic TUBB3 missense variants result in altered TUBB3 function and cause errors either in the growth and guidance of cranial and, to a lesser extent, central axons, or in cortical neuronal migration and organization, and rarely in both. Moreover, human pathogenic missense variants in KIF21A, which encodes an anterograde kinesin motor protein that interacts directly with microtubules, alter KIF21A function and cause errors in cranial axon growth and guidance that can phenocopy TUBB3 variants. Here, we review reported TUBB3 and KIF21A variants, resulting phenotypes, and corresponding functional studies of both wildtype and mutant proteins. We summarize the evidence that, in vitro and in mouse models, loss-of-function and missense variants can alter microtubule dynamics and microtubule-kinesin interactions. Lastly, we highlight additional studies that might contribute to our understanding of the relationship between specific tubulin isotypes and specific kinesin motor proteins in health and disease.

With the Permission of Microtubules: An Updated Overview on Microtubule Function During Axon Pathfinding

During the establishment of neural circuitry axons often need to cover long distances to reach remote targets. The stereotyped navigation of these axons defines the connectivity between brain regions and cellular subtypes. This chemotrophic guidance process mostly relies on the spatio-temporal expression patterns of extracellular proteins and the selective expression of their receptors in projection neurons. Axon guidance is stimulated by guidance proteins and implemented by neuronal traction forces at the growth cones, which engage local cytoskeleton regulators and cell adhesion proteins. Different layers of guidance signaling regulation, such as the cleavage and processing of receptors, the expression of co-receptors and a wide variety of intracellular cascades downstream of receptors activation, have been progressively unveiled. Also, in the last decades, the regulation of microtubule (MT) assembly, stability and interactions with the submembranous actin network in the growth cone have emerged as crucial effector mechanisms in axon pathfinding. In this review, we will delve into the intracellular signaling cascades downstream of guidance receptors that converge on the MT cytoskeleton of the growing axon. In particular, we will focus on the microtubule-associated proteins (MAPs) network responsible of MT dynamics in the axon and growth cone. Complementarily, we will discuss new evidences that connect defects in MT scaffold proteins, MAPs or MT-based motors and axon misrouting during brain development.

Role of DSCAM in the Development of Neural Control of Movement and Locomotion

Locomotion results in an alternance of flexor and extensor muscles between left and right limbs generated by motoneurons that are controlled by the spinal interneuronal circuit. This spinal locomotor circuit is modulated by sensory afferents, which relay proprioceptive and cutaneous inputs that inform the spatial position of limbs in space and potential contacts with our environment respectively, but also by supraspinal descending commands of the brain that allow us to navigate in complex environments, avoid obstacles, chase prey, or flee predators. Although signaling pathways are important in the establishment and maintenance of motor circuits, the role of DSCAM, a cell adherence molecule associated with Down syndrome, has only recently been investigated in the context of motor control and locomotion in the rodent. DSCAM is known to be involved in lamination and delamination, synaptic targeting, axonal guidance, dendritic and cell tiling, axonal fasciculation and branching, programmed cell death, and synaptogenesis, all of which can impact the establishment of motor circuits during development, but also their maintenance through adulthood. We discuss herein how DSCAM is important for proper motor coordination, especially for breathing and locomotion.

Unraveling Axon Guidance during Axotomy and Regeneration

During neuronal development and regeneration axons extend a cytoskeletal-rich structure known as the growth cone, which detects and integrates signals to reach its final destination. The guidance cues “signals” bind their receptors, activating signaling cascades that result in the regulation of the growth cone cytoskeleton, defining growth cone advance, pausing, turning, or collapse. Even though much is known about guidance cues and their isolated mechanisms during nervous system development, there is still a gap in the understanding of the crosstalk between them, and about what happens after nervous system injuries. After neuronal injuries in mammals, only axons in the peripheral nervous system are able to regenerate, while the ones from the central nervous system fail to do so. Therefore, untangling the guidance cues mechanisms, as well as their behavior and characterization after axotomy and regeneration, are of special interest for understanding and treating neuronal injuries. In this review, we present findings on growth cone guidance and canonical guidance cues mechanisms, followed by a description and comparison of growth cone pathfinding mechanisms after axotomy, in regenerative and non-regenerative animal models.

Axonal Growth Abnormalities Underlying Ocular Cranial Nerve Disorders

Abnormalities in cranial motor nerve development cause paralytic strabismus syndromes, collectively referred to as congenital cranial dysinnervation disorders, in which patients cannot fully move their eyes. These disorders can arise through one of two mechanisms: (a) defective motor neuron specification, usually by loss of a transcription factor necessary for brainstem patterning, or (b) axon growth and guidance abnormalities of the oculomotor, trochlear, and abducens nerves. This review focuses on our current understanding of axon guidance mechanisms in the cranial motor nerves and how disease-causing mutations disrupt axon targeting. Abnormalities of axon growth and guidance are often limited to a single nerve or subdivision, even when the causative gene is ubiquitously expressed. Additionally, when one nerve is absent, its normal target muscles attract other motor neurons. Study of these disorders highlights the complexities of axon guidance and how each population of neurons uses a unique but overlapping set of axon guidance pathways. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Congenital Mirror Movements Associated With Brain Malformations

BACKGROUND

Congenital mirror movements are involuntary movements of a side of the body imitating intentional movements on the opposite side, appearing in early childhood and persisting beyond 7 years of age. Congenital mirror movements are usually idiopathic but have been reported in association with various brain malformations.

METHODS

We describe clinical, genetic, and radiologic features in 9 individuals from 5 families manifesting congenital mirror movements.

RESULTS

The brain malformations associated with congenital mirror movements were: dysplastic corpus callosum in father and daughter with a heterozygous p.Met1* mutation in DCC; hypoplastic corpus callosum, dysgyria, and malformed vermis in a mother and son with a heterozygous p.Thr312Met mutation in TUBB3; dysplastic corpus callosum, dysgyria, abnormal vermis, and asymmetric ventricles in a father and 2 daughters with a heterozygous p.Arg121Trp mutation in TUBB; hypoplastic corpus callosum, dysgyria, malformed basal ganglia and abnormal vermis in a patient with a heterozygous p.Glu155Asp mutation in TUBA1A; hydrocephalus, hypoplastic corpus callosum, polymicrogyria, and cerebellar cysts in a patient with a homozygous p.Pro312Leu mutation in POMGNT1.

CONCLUSION

DCC, TUBB3, TUBB, TUBA1A, POMGNT1 cause abnormal axonal guidance via different mechanisms and result in congenital mirror movements associated with brain malformations.

FEZ1 Forms Complexes with CRMP1 and DCC to Regulate Axon and Dendrite Development

Elaboration of neuronal processes is an early step in neuronal development. Guidance cues must work closely with intracellular trafficking pathways to direct expanding axons and dendrites to their target neurons during the formation of neuronal networks. However, how such coordination is achieved remains incompletely understood. Here, we characterize an interaction between fasciculation and elongation protein zeta 1 (FEZ1), an adapter involved in synaptic protein transport, and collapsin response mediator protein (CRMP)1, a protein that functions in growth cone guidance, at neuronal growth cones. We show that similar to CRMP1 loss-of-function mutants, FEZ1 deficiency in rat hippocampal neurons causes growth cone collapse and impairs axonal development. Strikingly, FEZ1-deficient neurons also exhibited a reduction in dendritic complexity stronger than that observed in CRMP1-deficient neurons, suggesting that the former could partake in additional developmental signaling pathways. Supporting this, FEZ1 colocalizes with VAMP2 in developing hippocampal neurons and forms a separate complex with deleted in colorectal cancer (DCC) and Syntaxin-1 (Stx1), components of the Netrin-1 signaling pathway that are also involved in regulating axon and dendrite development. Significantly, developing axons and dendrites of FEZ1-deficient neurons fail to respond to Netrin-1 or Netrin-1 and Sema3A treatment, respectively. Taken together, these findings highlight the importance of FEZ1 as a common effector to integrate guidance signaling pathways with intracellular trafficking to mediate axo-dendrite development during neuronal network formation.

DSCAM Mutation Impairs Motor Cortex Network Dynamic and Voluntary Motor Functions

While it is well known that netrin-1 and its receptors UNC5 and UNC40 family members are involved in the normal establishment of the motor cortex and its corticospinal tract, less is known about its other receptor Down syndrome cell adherence molecule (DSCAM). DSCAM is expressed in the developing motor cortex, regulates axonal outgrowth of cortical neurons, and its mutation impairs the dendritic arborization of cortical neurons, thus suggesting that it might be involved in the normal development and functioning of the motor cortex. In comparison to WT littermates, DSCAM2J mutant mice slipped and misplaced their paw while walking on the rungs of a horizontal ladder, and exhibited more difficulties in stepping over an obstacle while walking at slow speed. Anterograde tracing showed a normal pyramidal decussation and corticospinal projection, but a more dorsal distribution of their axonal terminals in the spinal gray matter. Intracortical microstimulations showed a reduced corticospinal and intracortical efficacy, whereas stimulations of the pyramidal tract revealed a normal spinal efficacy and excitability of corticospinal tract axons, thus arguing for a dysfunctional cortical development. Our study reveals impairment of the network dynamics within the motor cortex, reducing corticospinal drive and impairing voluntary locomotor functions upon DSCAM2J mutation.

Expression of Genes Involved in Axon Guidance: How Much Have We Learned?

Neuronal axons are guided to their target during the development of the brain. Axon guidance allows the formation of intricate neural circuits that control the function of the brain, and thus the behavior. As the axons travel in the brain to find their target, they encounter various axon guidance cues, which interact with the receptors on the tip of the growth cone to permit growth along different signaling pathways. Although many scientists have performed numerous studies on axon guidance signaling pathways, we still have an incomplete understanding of the axon guidance system. Lately, studies on axon guidance have shifted from studying the signal transduction pathways to studying other molecular features of axon guidance, such as the gene expression. These new studies present evidence for different molecular features that broaden our understanding of axon guidance. Hence, in this review we will introduce recent studies that illustrate different molecular features of axon guidance. In particular, we will review literature that demonstrates how axon guidance cues and receptors regulate local translation of axonal genes and how the expression of guidance cues and receptors are regulated both transcriptionally and post-transcriptionally. Moreover, we will highlight the pathological relevance of axon guidance molecules to specific diseases.

Disease-associated mutations in human TUBB3 disturb netrin repulsive signaling

Missense mutations in the human TUBB3 gene cause a variety of neurological disorders associated with defects in axon guidance and neuronal migration, but the underlying molecular mechanisms are not well understood. Recent studies have shown that direct coupling of dynamic TUBB3 in microtubules with netrin receptors is required for netrin-1-mediated axon guidance, and the interaction of netrin-1 repulsive receptor UNC5C with TUBB3 is involved in netrin-1 mediated axonal repulsion. Here, we report that TUBB3 mutations perturb netrin-1/UNC5C repulsive signaling in the developing nervous system. Among twelve mutants reported in previous studies, five of them show significantly reduced interaction with UNC5C in comparison to the wild-type TUBB3. TUBB3 mutants R262C and R62Q exhibit decreased subcellular colocalization with UNC5C in the peripheral area of the growth cone of primary mouse neurons. Netrin-1 reduces the colocalization of UNC5C with wild-type TUBB3, but not TUBB3 mutants R262C or R62Q, in the growth cone. Results from the in vitro cosedimentation assay indicate that netrin-1 inhibits cosedimentation of UNC5C with polymerized microtubules in primary mouse neurons expressing the wild-type TUBB3, but not R262C or R62Q. Expression of either R262C or R62Q not only blocks netrin-1-induced growth cone collapse and axonal repulsion of primary EGL cells in vitro, but also results in axon projections defects of chicken dorsal root ganglion neurons in ovo. Our study reveals that human TUBB3 mutations specifically perturb netrin-1/UNC5C-mediated repulsion.

Nuclear import of the DSCAM-cytoplasmic domain drives signaling capable of inhibiting synapse formation

DSCAM and DSCAML1 are immunoglobulin and cell adhesion-type receptors serving important neurodevelopmental functions including control of axon growth, branching, neurite self-avoidance, and neuronal cell death. The signal transduction mechanisms or effectors of DSCAM receptors, however, remain poorly characterized. We used a human ORFeome library to perform a high-throughput screen in mammalian cells and identified novel cytoplasmic signaling effector candidates including the Down syndrome kinase Dyrk1a, STAT3, USP21, and SH2D2A. Unexpectedly, we also found that the intracellular domains (ICDs) of DSCAM and DSCAML1 specifically and directly interact with IPO5, a nuclear import protein of the importin beta family, via a conserved nuclear localization signal. The DSCAM ICD is released by γ-secretase-dependent cleavage, and both the DSCAM and DSCAML1 ICDs efficiently translocate to the nucleus. Furthermore, RNA sequencing confirms that expression of the DSCAM as well as the DSCAML1 ICDs alone can profoundly alter the expression of genes associated with neuronal differentiation and apoptosis, as well as synapse formation and function. Gain-of-function experiments using primary cortical neurons show that increasing the levels of either the DSCAM or the DSCAML1 ICD leads to an impairment of neurite growth. Strikingly, increased expression of either full-length DSCAM or the DSCAM ICD, but not the DSCAML1 ICD, significantly decreases synapse numbers in primary hippocampal neurons. Taken together, we identified a novel membrane-to-nucleus signaling mechanism by which DSCAM receptors can alter the expression of regulators of neuronal differentiation and synapse formation and function. Considering that chromosomal duplications lead to increased DSCAM expression in trisomy 21, our findings may help uncover novel mechanisms contributing to intellectual disability in Down syndrome.

miR-92 Suppresses Robo1 Translation to Modulate Slit Sensitivity in Commissural Axon Guidance

Temporospatial regulation of guidance signaling is essential for axon outgrowth and pathfinding in the developing nervous system. Regulation of Robo1 levels in commissural neurons modulates Slit sensitivity facilitating proper axon guidance. The mechanisms underlying this regulation in the vertebrate nervous system are not well understood. Here, we report that miR-92, a highly conserved microRNA (miRNA), regulates chicken Robo1 expression in commissural neurons by binding to the 3' untranslated region (3' UTR) of Robo1 mRNA. miR-92 and Robo1 are differentially expressed in the developing spinal cord. miR-92 interacts with the Robo1 3'UTR to cause translational repression, but not mRNA degradation. Disruption of the miR-92/Robo1 3' UTR interaction induces premature responsiveness to Slit2 repulsion of precrossing commissural axons (CAs) in vitro and causes CA projection defects in vivo. These results indicate that miR-92 represses Robo1 expression thereby regulating Slit sensitivity to control CA projection and midline crossing.

Revisiting Netrin-1: One Who Guides (Axons)

Proper patterning of the nervous system requires that developing axons find appropriate postsynaptic partners; this entails microns to meters of extension through an extracellular milieu exhibiting a wide range of mechanical and chemical properties. Thus, the elaborate networks of fiber tracts and non-fasciculated axons evident in mature organisms are formed via complex pathfinding. The macroscopic structures of axon projections are highly stereotyped across members of the same species, indicating precise mechanisms guide their formation. The developing axon exhibits directionally biased growth toward or away from external guidance cues. One of the most studied guidance cues is netrin-1, however, its presentation in vivo remains debated. Guidance cues can be secreted to form soluble or chemotactic gradients or presented bound to cells or the extracellular matrix to form haptotactic gradients. The growth cone, a highly specialized dynamic structure at the end of the extending axon, detects these guidance cues via transmembrane receptors, such as the netrin-1 receptors deleted in colorectal cancer (DCC) and UNC5. These receptors orchestrate remodeling of the cytoskeleton and cell membrane through both chemical and mechanotransductive pathways, which result in traction forces generated by the cytoskeleton against the extracellular environment and translocation of the growth cone. Through intracellular signaling responses, netrin-1 can trigger either attraction or repulsion of the axon. Here we review the mechanisms by which the classical guidance cue netrin-1 regulates intracellular effectors to respond to the extracellular environment in the context of axon guidance during development of the central nervous system and discuss recent findings that demonstrate the critical importance of mechanical forces in this process.

Human TUBB3 Mutations Disrupt Netrin Attractive Signaling

Heterozygous missense mutations in human TUBB3 gene result in a spectrum of brain malformations associated with defects in axon guidance, neuronal migration and differentiation. However, the molecular mechanisms underlying mutation-related axon guidance abnormalities are unclear. Recent studies have shown that netrin-1, a canonical guidance cue, induced the interaction of TUBB3 with the netrin receptor deleted in colorectal cancer (DCC). Furthermore, TUBB3 is required for netrin-1-induced axon outgrowth, branching and pathfinding. Here, we provide evidence that TUBB3 mutations impair netrin/DCC signaling in the developing nervous system. The interaction of DCC with most TUBB3 mutants (eight out of twelve) is significantly reduced compared to the wild-type TUBB3. TUBB3 mutants R262C and A302V exhibit decreased subcellular colocalization with DCC in the growth cones of primary neurons. Netrin-1 increases the interaction of endogenous DCC with wild-type human TUBB3, but not R262C or A302V, in primary neurons. Netrin-1 also increases co-sedimentation of DCC with polymerized microtubules (MTs) in primary neurons expressing the wild-type TUBB3, but not R262C or A302V. Expression of either R262C or A302V not only suppresses netrin-1-induced neurite outgrowth, branching and attraction in vitro, but also causes defects in spinal cord commissural axon (CA) projection and pathfinding in ovo. Our study reveals that missense TUBB3 mutations specifically disrupt netrin/DCC-mediated attractive signaling.

Shift in the function of netrin-1 from axon outgrowth to axon branching in developing cerebral cortical neurons

BACKGROUND

Netrin-1, a multifunctional axon guidance cue, elicits axon outgrowth via one of its receptors deleted in colorectal cancer (DCC) in several types of neurons, including cerebral cortical neurons of embryonic mice. However, we and others have observed de novo formation of axon branches without axon outgrowth induced by netrin-1 in cortical culture of neonatal hamsters. These previous reports suggested the possibility that netrin-1 function might alter during development, which we here investigated using dissociated culture prepared from cerebral cortices of embryonic mice.

RESULTS

Imaging analysis revealed netrin-1-induced outgrowth in embryonic day (E) 14 axons and netrin-1-induced branching in E16 axons. Netrin-1-evoked filopodial protrusions, which sprouted on the shafts of E16 axons preceding branch formation, were visualized by a novel method called atmospheric scanning electron microscopy. Treatment with an anti-DCC function-blocking antibody affected both axon outgrowth and branching.

CONCLUSIONS

Morphological analyses suggested a possibility of a shift in the function of netrin-1 in cortical axons during development, from promotion of outgrowth to promotion of branch formation starting with filopodial protrusion. Function-blocking experiments suggested that DCC may contribute not only to axon outgrowth but branching.

Uncoupling of UNC5C with Polymerized TUBB3 in Microtubules Mediates Netrin-1 Repulsion

Modulation of microtubule (MT) dynamics is a key event of cytoskeleton remodeling in the growth cone (GC) during axon outgrowth and pathfinding. Our previous studies have shown that the direct interaction of netrin receptor DCC and DSCAM with polymerized TUBB3, a neuron-specific MT subunit in the brain, is required for netrin-1-mediated axon outgrowth, branching, and attraction. Here, we show that uncoupling of polymerized TUBB3 with netrin-1-repulsive receptor UNC5C is involved in netrin-1-mediated axonal repulsion. TUBB3 directly interacted with UNC5C and partially colocalized with UNC5C in the peripheral area of the GC of primary neurons from the cerebellar external granule layer of P2 mouse pups of both sexes. Netrin-1 reduced this interaction as well as the colocalization of UNC5C and TUBB3 in the GC. Results from the in vitro cosedimentation assay indicated that UNC5C interacted with polymerized TUBB3 in MTs and netrin-1 decreased this interaction. Knockdown of either TUBB3 or UNC5C blocked netrin-1-promoted axon repulsion in vitro and caused defects in axon projection of DRG toward the spinal cord in vivo Furthermore, live-cell imaging of end-binding protein 3 tagged with EGFP (EB3-GFP) in primary external granule layer cells showed that netrin-1 differentially increased MT dynamics in the GC with more MT growth in the distal than the proximal region of the GC during repulsion, and knockdown of either UNC5C or TUBB3 abolished the netrin-1 effect. Together, these data indicate that the disengagement of UNC5C with polymerized TUBB3 plays an essential role in netrin-1/UNC5C-mediated axon repulsion.SIGNIFICANCE STATEMENT Proper regulation of microtubule (MT) dynamics in the growth cone plays an important role in axon guidance. However, whether guidance cues modulate MT dynamics directly or indirectly is unclear. Here, we report that dissociation of UNC5C and polymerized TUBB3, the highly dynamic β-tubulin isoform in neurons, is essential for netrin-1/UNC5C-promoted axon repulsion. These results not only provide a working model of direct modulation of MTs by guidance cues in growth cone navigation but also help us to understand molecular mechanisms underlying developmental brain disorders associated with TUBB3 mutations.

Netrin-1: A regulator of cancer cell motility?

Netrins form a family of secreted and membrane-associated proteins, netrin-1 being the prototype and most investigated member of the family. The major physiological functions of netrin-1 lie in the regulation of axonal development as well as morphogenesis of different branched organs, by promoting the polarity of migratory/invasive front of the cell. On the other hand, netrin-1 acts as a factor preventing cell apoptosis. These events are mediated via a range of different receptors, including UNC5 and DCC-families. Cancer cells often employ developmental pathways to gain survival and motility advantage. Within recent years, there has been increasing number of observations of upregulation of netrin-1 expression in different forms of cancer, and the increased expression of netrin-1 has been linked to its functions as a survival and invasion promoting factor. We review here recent advances in the netrin-1 related developmental processes that may be of special interest in tumor biology, in addition to the known functions of netrin-1 in tumor biology with special focus on cancer cell migration.

Dependence receptor involvement in subtilisin-induced long-term depression and in long-term potentiation

The serine protease subtilisin induces a form of long-term depression (LTD) which is accompanied by a reduced expression of the axo-dendritic guidance molecule Unco-ordinated-5C (Unc-5C). One objective of the present work was to determine whether a loss of Unc-5C function contributed to subtilisin-induced LTD by using Unc-5C antibodies in combination with the pore-forming agents Triton X-100 (0.005%) or streptolysin O in rat hippocampal slices. In addition we have assessed the effect of subtilisin on the related dependence receptor Deleted in Colorectal Cancer (DCC) and used antibodies to this protein for functional studies. Field excitatory postsynaptic potentials (fEPSPs) were analyzed in rat hippocampal slices and protein extracts were used for Western blotting. Subtilisin produced a greater loss of DCC than of Unc-5C, but the antibodies had no effect on resting excitability or fEPSPs and did not modify subtilisin-induced LTD. However, antibodies to DCC but not Unc-5C did reduce the amplitude of theta-burst long-term potentiation (LTP). In addition, two inhibitors of endocytosis - dynasore and tat-gluR2(3Y) - were tested and, although the former compound had no effect on neurophysiological responses, tat-gluR2(3Y) did reduce the amplitude of subtilisin-induced LTD without affecting the expression of DCC or Unc-5C but with some loss of PostSynaptic Density Protein-95. The results support the view that the dependence receptor DCC may be involved in LTP and suggest that the endocytotic removal of a membrane protein or proteins may contribute to subtilisin-induced LTD, although it appears that neither Unc-5C nor DCC are involved in this process.

Overexpressed Down Syndrome Cell Adhesion Molecule (DSCAM) Deregulates P21-Activated Kinase (PAK) Activity in an In Vitro Neuronal Model of Down Syndrome: Consequences on Cell Process Formation and Extension

In humans, Down syndrome (DS) is caused by the presence of an extra copy of autosome 21. The most striking finding in DS patients is intellectual disability and the onset of Alzheimer's disease (AD)-like neuropathology in adulthood. Gene overdose is most likely to underlie both developmental impairments, as well as altered neuronal function in DS. Lately, the disruption of cellular signaling and regulatory pathways has been implicated in DS pathophysiology, and many of such pathways may represent common targets for diverse DS-related genes, which could in turn represent attractive therapeutical targets. In this regard, one DS-related gene Down Syndrome Cell Adhesion Molecule (DSCAM), has important functions in neuronal proliferation, maturation, and synaptogenesis. p21-associated kinases (PAKs) appear as a most interesting possibility for study, as DSCAM is known to regulate the PAKs pathway. Hence, in DS, overexpressed DSCAM could deregulate PAKs activity and affect signaling pathways that regulate synaptic plasticity such as dendritic spine dynamics and axon guidance and growth. In the present work, we used an immortalized cell line derived from the cerebral cortex of an animal model of DS such as the trisomy 16 (Ts16) fetal mouse (named CTb), and a similar cell line established from a normal littermate (named CNh), to study the effect of DSCAM in the PAKs pathway. The present study shows that DSCAM is overexpressed in CTb cells by approximately twofold, compared to CNh cells. Congruently, PAK1, as well as its downstream effectors LIMK and cofilin, stay phosphorylated for longer periods after DSCAM activation in the CTb cells, leading to an altered actin dynamics, expressed as an increased basal F/G ratio and reduced neurite growth, in the trisomic condition. The present work presents the correlation between DSCAM gene overexpression and a dysregulation of the PAK pathway, resulting in altered morphological parameters of neuronal plasticity in the trisomic cell line, namely decreased number and length of processes.

Netrin-1 induces local translation of down syndrome cell adhesion molecule in axonal growth cones

Down syndrome cell adhesion molecule (DSCAM) plays an important role in many neurodevelopmental processes such as axon guidance, dendrite arborization, and synapse formation. DSCAM is located in the Down syndrome trisomic region of human chromosome 21 and may contribute to the Down syndrome brain phenotype, which includes a reduction in the formation of long-distance connectivity. The local translation of a select group of mRNA transcripts within growth cones is necessary for the formation of appropriate neuronal connectivity. Interestingly, we have found that Dscam mRNA is localized to growth cones of mouse hippocampal neurons, and is dynamically regulated in response to the axon guidance molecule, netrin-1. Furthermore, netrin-1 stimulation results in an increase in locally translated DSCAM protein in growth cones. Deleted in colorectal cancer (DCC), a netrin-1 receptor, is required for the netrin-1-induced increase in Dscam mRNA local translation. We also find that two RNA-binding proteins-fragile X mental retardation protein (FMRP) and cytoplasmic polyadenylation element binding protein (CPEB)-colocalize with Dscam mRNA in growth cones, suggesting their regulation of Dscam mRNA localization and translation. Finally, overexpression of DSCAM in mouse cortical neurons results in a severe stunting of axon outgrowth and branching, suggesting that an increase in DSCAM protein results in a structural change having functional consequences. Taken together, these results suggest that netrin-1-induced local translation of Dscam mRNA during embryonic development may be an important mechanism to regulate axon growth and guidance in the developing nervous system. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 799-816, 2016.