Syndecan regulates cell migration and axon guidance in C. elegans

C. elegans touch receptor neurons direct mechanosensory complex organization via repurposing conserved basal lamina proteins

The sense of touch is conferred by the conjoint function of somatosensory neurons and skin cells. These cells meet across a gap filled by a basal lamina, an ancient structure found in metazoans. Using Caenorhabditis elegans, we investigate the composition and ultrastructure of the extracellular matrix at the epidermis and touch receptor neuron (TRN) interface. We show that membrane-matrix complexes containing laminin, nidogen, and the MEC-4 mechano-electrical transduction channel reside at this interface and are central to proper touch sensation. Interestingly, the dimensions and spacing of these complexes correspond with the discontinuous beam-like extracellular matrix structures observed in serial-section transmission electron micrographs. These complexes fail to coalesce in touch-insensitive extracellular matrix mutants and in dissociated neurons. Loss of nidogen reduces the density of mechanoreceptor complexes and the amplitude of the touch-evoked currents they carry. Thus, neuron-epithelium cell interfaces are instrumental in mechanosensory complex assembly and function. Unlike the basal lamina ensheathing the pharynx and body wall muscle, nidogen recruitment to the puncta along TRNs is not dependent upon laminin binding. MEC-4, but not laminin or nidogen, is destabilized by point mutations in the C-terminal Kunitz domain of the extracellular matrix component, MEC-1. These findings imply that somatosensory neurons secrete proteins that actively repurpose the basal lamina to generate special-purpose mechanosensory complexes responsible for vibrotactile sensing.

The matrisome landscape controlling in vivo germ cell fates

The developmental fate of cells is regulated by intrinsic factors and the extracellular environment. The extracellular matrix (matrisome) delivers chemical and mechanical cues that can modify cellular development. However, comprehensive understanding of how matrisome factors control cells in vivo is lacking. Here we show that specific matrisome factors act individually and collectively to control germ cell development. Surveying development of undifferentiated germline stem cells through to mature oocytes in the Caenorhabditis elegans germ line enabled holistic functional analysis of 443 conserved matrisome-coding genes. Using high-content imaging, 3D reconstruction, and cell behavior analysis, we identify 321 matrisome genes that impact germ cell development, the majority of which (>80%) are undescribed. Our analysis identifies key matrisome networks acting autonomously and non-autonomously to coordinate germ cell behavior. Further, our results demonstrate that germ cell development requires continual remodeling of the matrisome landscape. Together, this study provides a comprehensive platform for deciphering how extracellular signaling controls cellular development and anticipate this will establish new opportunities for manipulating cell fates.

Heparan sulfate proteoglycans mediate prion-like α-synuclein toxicity in Parkinson's in vivo models

This study offers new Caenorhabditis elegans models in which feeding with α-synuclein pre-formed fibrils results in prion-like aggregation of host α-synuclein and dopamine neuron degeneration, and these disease phenotypes are regulated by the heparan sulfate proteoglycan pathway.

Parkinson’s disease (PD) is a debilitating neurodegenerative disorder characterized by progressive motor decline and the aggregation of α-synuclein protein. Growing evidence suggests that α-synuclein aggregates may spread from neurons of the digestive tract to the central nervous system in a prion-like manner, yet the mechanisms of α-synuclein transmission and neurotoxicity remain poorly understood. Animal models that are amenable to high-throughput investigations are needed to facilitate the discovery of disease mechanisms. Here we describe the first Caenorhabditis elegans models in which feeding with α-synuclein preformed fibrils (PFFs) induces dopaminergic neurodegeneration, prion-like seeding of aggregation of human α-synuclein expressed in the host, and an associated motor decline. RNAi-mediated knockdown of the C. elegans syndecan sdn-1, or other enzymes involved in heparan sulfate proteoglycan synthesis, protected against PFF-induced α-synuclein aggregation, motor dysfunction, and dopamine neuron degeneration. This work offers new models by which to investigate gut-derived α-synuclein spreading and propagation of disease.

The regulatory landscape of neurite development in Caenorhabditis elegans

Neuronal communication requires precise connectivity of neurite projections (axons and dendrites). Developing neurites express cell-surface receptors that interpret extracellular cues to enable correct guidance toward, and connection with, target cells. Spatiotemporal regulation of neurite guidance molecule expression by transcription factors (TFs) is critical for nervous system development and function. Here, we review how neurite development is regulated by TFs in the Caenorhabditis elegans nervous system. By collecting publicly available transcriptome and ChIP-sequencing data, we reveal gene expression dynamics during neurite development, providing insight into transcriptional mechanisms governing construction of the nervous system architecture.

A somatic proteoglycan controls Notch-directed germ cell fate

Communication between the soma and germline optimizes germ cell fate programs. Notch receptors are key determinants of germ cell fate but how somatic signals direct Notch-dependent germ cell behavior is undefined. Here we demonstrate that SDN-1 (syndecan-1), a somatic transmembrane proteoglycan, controls expression of the GLP-1 (germline proliferation-1) Notch receptor in the Caenorhabditis elegans germline. We find that SDN-1 control of a somatic TRP calcium channel governs calcium-dependent binding of an AP-2 transcription factor (APTF-2) to the glp-1 promoter. Hence, SDN-1 signaling promotes GLP-1 expression and mitotic germ cell fate. Together, these data reveal SDN-1 as a putative communication nexus between the germline and its somatic environment to control germ cell fate decisions.

Neural Tissue Homeostasis and Repair Is Regulated via CS and DS Proteoglycan Motifs

Chondroitin sulfate (CS) is the most abundant and widely distributed glycosaminoglycan (GAG) in the human body. As a component of proteoglycans (PGs) it has numerous roles in matrix stabilization and cellular regulation. This chapter highlights the roles of CS and CS-PGs in the central and peripheral nervous systems (CNS/PNS). CS has specific cell regulatory roles that control tissue function and homeostasis. The CNS/PNS contains a diverse range of CS-PGs which direct the development of embryonic neural axonal networks, and the responses of neural cell populations in mature tissues to traumatic injury. Following brain trauma and spinal cord injury, a stabilizing CS-PG-rich scar tissue is laid down at the defect site to protect neural tissues, which are amongst the softest tissues of the human body. Unfortunately, the CS concentrated in gliotic scars also inhibits neural outgrowth and functional recovery. CS has well known inhibitory properties over neural behavior, and animal models of CNS/PNS injury have demonstrated that selective degradation of CS using chondroitinase improves neuronal functional recovery. CS-PGs are present diffusely in the CNS but also form denser regions of extracellular matrix termed perineuronal nets which surround neurons. Hyaluronan is immobilized in hyalectan CS-PG aggregates in these perineural structures, which provide neural protection, synapse, and neural plasticity, and have roles in memory and cognitive learning. Despite the generally inhibitory cues delivered by CS-A and CS-C, some CS-PGs containing highly charged CS disaccharides (CS-D, CS-E) or dermatan sulfate (DS) disaccharides that promote neural outgrowth and functional recovery. CS/DS thus has varied cell regulatory properties and structural ECM supportive roles in the CNS/PNS depending on the glycoform present and its location in tissue niches and specific cellular contexts. Studies on the fruit fly, Drosophila melanogaster and the nematode Caenorhabditis elegans have provided insightful information on neural interconnectivity and the role of the ECM and its PGs in neural development and in tissue morphogenesis in a whole organism environment.

Syndecan Transmembrane Domain Specifically Regulates Downstream Signaling Events of the Transmembrane Receptor Cytoplasmic Domain

Despite the known importance of the transmembrane domain (TMD) of syndecan receptors in cell adhesion and signaling, the molecular basis for syndecan TMD function remains unknown. Using in vivo invertebrate models, we found that mammalian syndecan-2 rescued both the guidance defects in C. elegans hermaphrodite-specific neurons and the impaired development of the midline axons of Drosophila caused by the loss of endogenous syndecan. These compensatory effects, however, were reduced significantly when syndecan-2 dimerization-defective TMD mutants were introduced. To further investigate the role of the TMD, we generated a chimera, 2eTPC, comprising the TMD of syndecan-2 linked to the cytoplasmic domain of platelet-derived growth factor receptor (PDGFR). This chimera exhibited SDS-resistant dimer formation that was lost in the corresponding dimerization-defective syndecan-2 TMD mutant, 2eT(GL)PC. Moreover, 2eTPC specifically enhanced Tyr 579 and Tyr 857 phosphorylation in the PDGFR cytoplasmic domain, while the TMD mutant failed to support such phosphorylation. Finally, 2eTPC, but not 2eT(GL)PC, induced phosphorylation of Src and PI3 kinase (known downstream effectors of Tyr 579 phosphorylation) and promoted Src-mediated migration of NIH3T3 cells. Taken together, these data suggest that the TMD of a syndecan-2 specifically regulates receptor cytoplasmic domain function and subsequent downstream signaling events controlling cell behavior.

The HSPG syndecan is a core organizer of cholinergic synapses

The extracellular matrix has emerged as an active component of chemical synapses regulating synaptic formation, maintenance, and homeostasis. The heparan sulfate proteoglycan (HSPG) syndecans are known to regulate cellular and axonal migration in the brain. They are also enriched at synapses, but their synaptic functions remain more elusive. Here, we show that SDN-1, the sole orthologue of syndecan in C. elegans, is absolutely required for the synaptic clustering of homomeric α7-like acetylcholine receptors (AChRs) and regulates the synaptic content of heteromeric AChRs. SDN-1 is concentrated at neuromuscular junctions (NMJs) by the neurally secreted synaptic organizer Ce-Punctin/MADD-4, which also activates the transmembrane netrin receptor DCC. Those cooperatively recruit the FARP and CASK orthologues that localize α7-like-AChRs at cholinergic NMJs through physical interactions. Therefore, SDN-1 stands at the core of the cholinergic synapse organization by bridging the extracellular synaptic determinants to the intracellular synaptic scaffold that controls the postsynaptic receptor content.

Specific heparan sulfate modifications stabilize the synaptic organizer MADD-4/Punctin at C. elegans neuromuscular junctions

Heparan sulfate proteoglycans contribute to the structural organization of various neurochemical synapses. Depending on the system, their role involves either the core protein or the glycosaminoglycan chains. These linear sugar chains are extensively modified by heparan sulfate modification enzymes, resulting in highly diverse molecules. Specific modifications of glycosaminoglycan chains may thus contribute to a sugar code involved in synapse specificity. Caenorhabditis elegans is particularly useful to address this question because of the low level of genomic redundancy of these enzymes, as opposed to mammals. Here, we systematically mutated the genes encoding heparan sulfate modification enzymes in C. elegans and analyzed their impact on excitatory and inhibitory neuromuscular junctions. Using single chain antibodies that recognize different heparan sulfate modification patterns, we show in vivo that these two heparan sulfate epitopes are carried by the SDN-1 core protein, the unique C. elegans syndecan orthologue, at neuromuscular junctions. Intriguingly, these antibodies differentially bind to excitatory and inhibitory synapses, implying unique heparan sulfate modification patterns at different neuromuscular junctions. Moreover, while most enzymes are individually dispensable for proper organization of neuromuscular junctions, we show that 3-O-sulfation of SDN-1 is required to maintain wild-type levels of the extracellular matrix protein MADD-4/Punctin, a central synaptic organizer that defines the identity of excitatory and inhibitory synaptic domains at the plasma membrane of muscle cells.

Functions of the extracellular matrix in development: Lessons from Caenorhabditis elegans

Cell-extracellular matrix interactions are crucial for the development of an organism from the earliest stages of embryogenesis. The main constituents of the extracellular matrix are collagens, laminins, proteoglycans and glycosaminoglycans that form a network of interactions. The extracellular matrix and its associated molecules provide developmental cues and structural support from the outside of cells during development. The complex nature of the extracellular matrix and its ability for continuous remodeling poses challenges when investigating extracellular matrix-based signaling during development. One way to address these challenges is to employ invertebrate models such as Caenorhabditis elegans, which are easy to genetically manipulate and have an invariant developmental program. C. elegans also expresses fewer extracellular matrix protein isoforms and exhibits reduced redundancy compared to mammalian models, thus providing a simpler platform for exploring development. This review summarizes our current understanding of how the extracellular matrix controls the development of neurons, muscles and the germline in C. elegans.

Roles of glycoconjugates in neural patterning in C. elegans

Establishment of neural circuits requires reproducible and precise interactions between growing axons, dendrites and their tissue environment. Cell adhesion molecules and guidance factors are involved in the process, but how specificity is achieved remains poorly understood. Glycans are the third major class of biopolymers besides nucleic acids and proteins, and are usually covalently linked to proteins to form glycoconjugates. Common to most glycans is an extraordinary level of molecular diversity, making them attractive candidates to contribute specificity during neural development. Indeed, many genes important for neural development encode glycoproteins, or enzymes involved in synthesizing or modifying glycans. Glycoconjugates are classified based on both the types of glycans and type of attachment that link them to proteins. Here I discuss progress in understanding the function of glycans, glycan modifications and glycoconjugates during neural development in Caenorhabditis elegans. I will also highlight relevance to human disease and known roles of glycoconjugates in regeneration.

A complement factor H homolog, heparan sulfation, and syndecan maintain inversin compartment boundaries in C. elegans cilia

Age-related macular degeneration (AMD) is a leading cause of blindness among the elderly. Canonical disease models suggest that defective interactions between complement factor H (CFH) and cell surface heparan sulfate (HS) result in increased alternative complement pathway activity, cytolytic damage, and tissue inflammation in the retina. Although these factors are thought to contribute to increased disease risk, multiple studies indicate that noncanonical mechanisms that result from defective CFH and HS interaction may contribute to the progression of AMD as well. A total of 60 ciliated sensory neurons in the nematode Caenorhabditis elegans detect chemical, olfactory, mechanical, and thermal cues in the environment. Here, we find that a C. elegans CFH homolog localizes on CEP mechanosensory neuron cilia where it has noncanonical roles in maintaining inversin/NPHP-2 within its namesake proximal compartment and preventing inversin/NPHP-2 accumulation in distal cilia compartments in aging adults. CFH localization and maintenance of inversin/NPHP-2 compartment integrity depend on the HS 3-O sulfotransferase HST-3.1 and the transmembrane proteoglycan syndecan/SDN-1. Defective inversin/NPHP-2 localization in mouse and human photoreceptors with CFH mutations indicates that these functions and interactions may be conserved in vertebrate sensory neurons, suggesting that previously unappreciated defects in cilia structure may contribute to the progressive photoreceptor dysfunction associated with CFH loss-of-function mutations in some AMD patients.

Syndecan receptors: pericellular regulators in development and inflammatory disease

The syndecans are the major family of transmembrane proteoglycans, usually bearing multiple heparan sulfate chains. They are present on virtually all nucleated cells of vertebrates and are also present in invertebrates, indicative of a long evolutionary history. Genetic models in both vertebrates and invertebrates have shown that syndecans link to the actin cytoskeleton and can fine-tune cell adhesion, migration, junction formation, polarity and differentiation. Although often associated as co-receptors with other classes of receptors (e.g. integrins, growth factor and morphogen receptors), syndecans can nonetheless signal to the cytoplasm in discrete ways. Syndecan expression levels are upregulated in development, tissue repair and an array of human diseases, which has led to the increased appreciation that they may be important in pathogenesis not only as diagnostic or prognostic agents, but also as potential targets. Here, their functions in development and inflammatory diseases are summarized, including their potential roles as conduits for viral pathogen entry into cells.

Molecular insights and immune responses of big belly seahorse syndecan-2 (CD362): Involvement of ectodomain in regulating cell survival, proliferation, and wound healing

Syndecan-2, also known as CD362, is a transmembrane heparan sulfate proteoglycan which regulates cell growth, proliferation, cell adhesion, wound healing, and recruits immune cells. In the present study, we performed bioinformatics, spatial and temporal expression analyses of Hippocampus abdominalis syndecan-2 (HaSDC-2). Additionally, functional assays were conducted. HaSDC-2 has five major domains; an extracellular heparan sulfate attachment domain, a co-receptor binding domain, a transmembrane domain, two conserved domains (C1 domain, C2 domain), and a variable (V) domain. The ectodomain contained a signal peptide and GAG attachment sites. In-silico analysis revealed that HaSDC-2 contained a 798 bp long ORF and protein sequence of 265 amino acid residues. Further analysis of the amino acid sequence predicted a 28.9 kDa molecular weight and a 4.13 theoretical isoelectric point. The spatial expression of HaSDC-2 was ubiquitous in all tested tissues. HaSDC-2 expression in the liver was upregulated 24 h post-injection in response to all stimuli. Further, HaSDC-2 expression in blood cells was upregulated at 12 and 72 h post-injection in response to all the stimuli. HaSDC-2 + pcDNA™3.1(+) transfected cells exhibited significant survival in response to cell stressors such as H₂O₂ and HED. The ectodomain of recombinant HaSDC-2 treated cells showed significant cell proliferation in a concentration-dependent manner. The scratch wound healing assay showed significant Δ gap closures with increasing concentrations of HaSDC-2. Collectively, these results indicated that syndecan-2 was involved in regulating immune responses and cell stress conditions.

Genome wide analysis reveals heparan sulfate epimerase modulates TDP-43 proteinopathy

Pathological phosphorylated TDP-43 protein (pTDP) deposition drives neurodegeneration in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD-TDP). However, the cellular and genetic mechanisms at work in pathological TDP-43 toxicity are not fully elucidated. To identify genetic modifiers of TDP-43 neurotoxicity, we utilized a Caenorhabditis elegans model of TDP-43 proteinopathy expressing human mutant TDP-43 pan-neuronally (TDP-43 tg). In TDP-43 tg C. elegans, we conducted a genome-wide RNAi screen covering 16,767 C. elegans genes for loss of function genetic suppressors of TDP-43-driven motor dysfunction. We identified 46 candidate genes that when knocked down partially ameliorate TDP-43 related phenotypes; 24 of these candidate genes have conserved homologs in the human genome. To rigorously validate the RNAi findings, we crossed the TDP-43 transgene into the background of homozygous strong genetic loss of function mutations. We have confirmed 9 of the 24 candidate genes significantly modulate TDP-43 transgenic phenotypes. Among the validated genes we focused on, one of the most consistent genetic modifier genes protecting against pTDP accumulation and motor deficits was the heparan sulfate-modifying enzyme hse-5, the C. elegans homolog of glucuronic acid epimerase (GLCE). We found that knockdown of human GLCE in cultured human cells protects against oxidative stress induced pTDP accumulation. Furthermore, expression of glucuronic acid epimerase is significantly decreased in the brains of FTLD-TDP cases relative to normal controls, demonstrating the potential disease relevance of the candidate genes identified. Taken together these findings nominate glucuronic acid epimerase as a novel candidate therapeutic target for TDP-43 proteinopathies including ALS and FTLD-TDP.

The protein TDP-43 forms aggregates in disease-affected neurons in patients with ALS and FTLD-TDP. In addition, mutations in the human gene coding for TDP-43 can cause inherited ALS. By expressing human mutant TDP-43 protein in C. elegans neurons, we have modelled aspects of ALS pathobiology. This animal model exhibits severe motor dysfunction, progressive neurodegeneration, and accumulation of abnormally modified TDP-43 protein. To identify genes controlling TDP-43 neurotoxicity in C. elegans, we have conducted a genome-wide reverse genetic screen and found 46 genes that participate in TDP-43 neurotoxicity. We demonstrated that one of them, glucuronic acid epimerase, is decreased in patients with FTLD-TDP suggesting inhibitors of glucuronic acid epimerase could have therapeutic value for ALS and FTLD.

New SDC function prediction based on protein-protein interaction using bioinformatics tools

The precise roles for SDC have been complex to specify. Assigning and reanalyzing protein and peptide identification to novel protein functions is one of the most important challenges in postgenomic era. Here, we provide SDC molecular description to support, contextualize and reanalyze the corresponding protein-protein interaction (PPI). From SDC-1 data mining, we discuss the potential of bioinformatics tools to predict new biological rules of SDC. Using these methods, we have assembled new possibilities for SDC biology from PPI data, once, the understanding of biology complexity cannot be capture from one simple question.

The Caenorhabditis elegans SMOC-1 Protein Acts Cell Nonautonomously To Promote Bone Morphogenetic Protein Signaling

Bone morphogenetic protein (BMP) signaling regulates many different developmental and homeostatic processes in metazoans. The BMP pathway is conserved in Caenorhabditis elegans, and is known to regulate body size and mesoderm development. We have identified the C. elegans smoc-1 (Secreted MOdular Calcium-binding protein-1) gene as a new player in the BMP pathway. smoc-1(0) mutants have a small body size, while overexpression of smoc-1 leads to a long body size and increased expression of the RAD-SMAD (reporter acting downstream of SMAD) BMP reporter, suggesting that SMOC-1 acts as a positive modulator of BMP signaling. Using double-mutant analysis, we showed that SMOC-1 antagonizes the function of the glypican LON-2 and acts through the BMP ligand DBL-1 to regulate BMP signaling. Moreover, SMOC-1 appears to specifically regulate BMP signaling without significant involvement in a TGFβ-like pathway that regulates dauer development. We found that smoc-1 is expressed in multiple tissues, including cells of the pharynx, intestine, and posterior hypodermis, and that the expression of smoc-1 in the intestine is positively regulated by BMP signaling. We further established that SMOC-1 functions cell nonautonomously to regulate body size. Human SMOC1 and SMOC2 can each partially rescue the smoc-1(0) mutant phenotype, suggesting that SMOC-1's function in modulating BMP signaling is evolutionarily conserved. Together, our findings highlight a conserved role of SMOC proteins in modulating BMP signaling in metazoans.

Making Connections: Guidance Cues and Receptors at Nonneural Cell-Cell Junctions

The field of axon guidance was revolutionized over the past three decades by the identification of highly conserved families of guidance cues and receptors. These proteins are essential for normal neural development and function, directing cell and axon migration, neuron-glial interactions, and synapse formation and plasticity. Many of these genes are also expressed outside the nervous system in which they influence cell migration, adhesion and proliferation. Because the nervous system develops from neural epithelium, it is perhaps not surprising that these guidance cues have significant nonneural roles in governing the specialized junctional connections between cells in polarized epithelia. The following review addresses roles for ephrins, semaphorins, netrins, slits and their receptors in regulating adherens, tight, and gap junctions in nonneural epithelia and endothelia.

Diverse roles for glycosaminoglycans in neural patterning

The nervous system coordinates the functions of most multicellular organisms and their response to the surrounding environment. Its development involves concerted cellular interactions, including migration, axon guidance, and synapse formation. These processes depend on the molecular constituents and structure of the extracellular matrices (ECM). An essential component of ECMs are proteoglycans, i.e., proteins containing unbranched glycan chains known as glycosaminoglycans (GAGs). A defining characteristic of GAGs is their enormous molecular diversity, created by extensive modifications of the glycans during their biosynthesis. GAGs are widely expressed, and their loss can lead to catastrophic neuronal defects. Despite their importance, we are just beginning to understand the function and mechanisms of GAGs in neuronal development. In this review, we discuss recent evidence suggesting GAGs have specific roles in neuronal patterning and synaptogenesis. We examine the function played by the complex modifications present on GAG glycans and their roles in regulating different aspects of neuronal patterning. Moreover, the review considers the function of proteoglycan core proteins in these processes, stressing their likely role as co-receptors of different signaling pathways in a redundant and context-dependent manner. We conclude by discussing challenges and future directions toward a better understanding of these fascinating molecules during neuronal development. Developmental Dynamics 247:54-74, 2018. © 2017 Wiley Periodicals, Inc.

Coordination of Heparan Sulfate Proteoglycans with Wnt Signaling To Control Cellular Migrations and Positioning in Caenorhabditis elegans

Heparan sulfates (HS) are linear polysaccharides with complex modification patterns, which are covalently bound via conserved attachment sites to core proteins to form heparan sulfate proteoglycans (HSPGs). HSPGs regulate many aspects of the development and function of the nervous system, including cell migration, morphology, and network connectivity. HSPGs function as cofactors for multiple signaling pathways, including the Wnt-signaling molecules and their Frizzled receptors. To investigate the functional interactions among the HSPG and Wnt networks, we conducted genetic analyses of each, and also between these networks using five cellular migrations in the nematode Caenorhabditis elegans We find that HSPG core proteins act genetically in a combinatorial fashion dependent on the cellular contexts. Double mutant analyses reveal distinct redundancies among HSPGs for different migration events, and different cellular migrations require distinct heparan sulfate modification patterns. Our studies reveal that the transmembrane HSPG SDN-1/Syndecan functions within the migrating cell to promote cellular migrations, while the GPI-linked LON-2/Glypican functions cell nonautonomously to establish the final cellular position. Genetic analyses with the Wnt-signaling system show that (1) a given HSPG can act with different Wnts and Frizzled receptors, and that (2) a given Wnt/Frizzled pair acts with different HSPGs in a context-dependent manner. Lastly, we find that distinct HSPG and Wnt/Frizzled combinations serve separate functions to promote cellular migration and establish position of specific neurons. Our studies suggest that HSPGs use structurally diverse glycans in coordination with Wnt-signaling pathways to control multiple cellular behaviors, including cellular and axonal migrations and, cellular positioning.

Reduced Levels of Tear Lacritin Are Associated With Corneal Neuropathy in Patients With the Ocular Component of Sjögren's Syndrome

Purpose

To determine whether levels of endogenous tear protein, lacritin, are linked to altered corneal innervation and dry eye severity in patients with Sjögren's syndrome (SS).

Methods

Clinical data were obtained from 10 SS and 10 age-matched controls. Enzyme-linked immunosorbent assay was used to assess total tear lacritin extracted from Schirmer strips. Western blot was used to detect active lacritin monomer (∼25 kDa), active lacritin fragment (∼12–15 kDa), and inactive tissue transglutaminase–generated lacritin (≥40 kDa). In vivo confocal microscopy was used to assess nerve fiber density (NFD) and length (NFL). Relationships between nerve morphology and tear lacritin were examined by Spearman correlation. Diagnostic performance of tear lacritin was analyzed using receiver operating characteristic.

Results

Active tear lacritin was significantly reduced in SS patients (3.72 ± 5.62 [SS] versus 18.17 ± 4.57 ng/100 ng total tear protein [controls]; P < 0.001), while inactive lacritin was increased (84.99% ± 11.15% [SS] versus 51.04% ± 12.03% [controls]; P < 0.001). Nerve fiber density (21.70 ± 18.93 vs. 31.80 ± 9.35; P = 0.03) and NFL (4.18 ± 3.44 vs. 6.54 ± 2.47; P < 0.05) were significantly decreased in SS patients compared to controls. Reduced NFL (r = 0.74, P < 0.01) and NFD (r = 0.70, P < 0.01) were highly correlated with reduced tear lacritin. Similarly, total tear lacritin was highly correlated with Schirmers (r = 0.77, P < 0.01), ocular staining (r = −0.80, P < 0.01), and corneal sensitivity (r = 0.81, P < 0.01). Tear lacritin showed equivalent or better diagnostic performance compared to traditional clinical measures for SS (100.00% sensitivity, 85.71% specificity, cutoff = 14.50 ng/100 ng tear protein).

Conclusions

Reduced tear lacritin levels in SS patients are highly correlated with clinical signs of dry eye, as well as decreased NFD and NFL. Lacritin and its components provide excellent diagnostic sensitivity and specificity in SS.

The NCLX-type Na+/Ca2+ Exchanger NCX-9 Is Required for Patterning of Neural Circuits in Caenorhabditis elegans

NCLX is a Na⁺/Ca²⁺ exchanger that uses energy stored in the transmembrane sodium gradient to facilitate the exchange of sodium ions for ionic calcium. Mammals have a single NCLX, which has been shown to function primarily at the mitochondrion and is an important regulator of neuronal physiology by contributing to neurotransmission and synaptic plasticity. The role of NCLX in developmental cell patterning (e.g. in neural circuits) is largely unknown. Here we describe a novel role for the Caenorhabditis elegans NCLX-type protein, NCX-9, in neural circuit formation. NCX-9 functions in hypodermal seam cells that secrete the axon guidance cue UNC-129/BMP, and our data revealed that ncx-9^-/- mutant animals exhibit development defects in stereotyped left/right axon guidance choices within the GABAergic motor neuron circuit. Our data also implicate NCX-9 in a LON-2/heparan sulfate and UNC-6/netrin-mediated, RAC-dependent signaling pathway to guide left/right patterning within this circuit. Finally, we also provide in vitro physiology data supporting the role for NCX-9 in handling calcium exchange at the mitochondrion. Taken together, our work reveals the specificity by which the handling by NCLX of calcium exchange can map to neural circuit patterning and axon guidance decisions during development.

Functional Requirements for Heparan Sulfate Biosynthesis in Morphogenesis and Nervous System Development in C. elegans

The regulation of cell migration is essential to animal development and physiology. Heparan sulfate proteoglycans shape the interactions of morphogens and guidance cues with their respective receptors to elicit appropriate cellular responses. Heparan sulfate proteoglycans consist of a protein core with attached heparan sulfate glycosaminoglycan chains, which are synthesized by glycosyltransferases of the exostosin (EXT) family. Abnormal HS chain synthesis results in pleiotropic consequences, including abnormal development and tumor formation. In humans, mutations in either of the exostosin genes EXT1 and EXT2 lead to osteosarcomas or multiple exostoses. Complete loss of any of the exostosin glycosyltransferases in mouse, fish, flies and worms leads to drastic morphogenetic defects and embryonic lethality. Here we identify and study previously unavailable viable hypomorphic mutations in the two C. elegans exostosin glycosyltransferases genes, rib-1 and rib-2. These partial loss-of-function mutations lead to a severe reduction of HS levels and result in profound but specific developmental defects, including abnormal cell and axonal migrations. We find that the expression pattern of the HS copolymerase is dynamic during embryonic and larval morphogenesis, and is sustained throughout life in specific cell types, consistent with HSPGs playing both developmental and post-developmental roles. Cell-type specific expression of the HS copolymerase shows that HS elongation is required in both the migrating neuron and neighboring cells to coordinate migration guidance. Our findings provide insights into general principles underlying HSPG function in development.

During animal development, cells and neurons navigate long distances to reach their final target destinations. Migrating cells are guided by extracellular molecular cues, and cellular responses to these cues are regulated by heparan sulfate proteoglycans. Heparan sulfate proteoglycans are proteins with long heparan sulfate polysaccharide chains attached. Here we identify and study previously unavailable viable mutants that disrupt the elongation of the heparan sulfate chains in the nematode C. elegans. Our analysis shows that these HS-chain-elongation mutations affect the development of the nervous system as they result in misguided migrations of neurons and axons. Furthermore, we find that heparan sulfate chain elongation occurs in numerous cell types during development and that the coordinated production of heparan sulfate proteoglycans, in both the migrating cell and neighboring tissues, ensures proper migration. Our findings highlight the critical roles of heparan sulfate proteoglycans in nervous system development and the evolutionary conservation of the molecular mechanisms driving guided migrations.

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

The correct wiring of neuronal circuits depends on outgrowth and guidance of neuronal processes during development. In the past two decades, great progress has been made in understanding the molecular basis of axon outgrowth and guidance. Genetic analysis in Caenorhabditis elegans has played a key role in elucidating conserved pathways regulating axon guidance, including Netrin signaling, the slit Slit/Robo pathway, Wnt signaling, and others. Axon guidance factors were first identified by screens for mutations affecting animal behavior, and by direct visual screens for axon guidance defects. Genetic analysis of these pathways has revealed the complex and combinatorial nature of guidance cues, and has delineated how cues guide growth cones via receptor activity and cytoskeletal rearrangement. Several axon guidance pathways also affect directed migrations of non-neuronal cells in C. elegans, with implications for normal and pathological cell migrations in situations such as tumor metastasis. The small number of neurons and highly stereotyped axonal architecture of the C. elegans nervous system allow analysis of axon guidance at the level of single identified axons, and permit in vivo tests of prevailing models of axon guidance. C. elegans axons also have a robust capacity to undergo regenerative regrowth after precise laser injury (axotomy). Although such axon regrowth shares some similarities with developmental axon outgrowth, screens for regrowth mutants have revealed regeneration-specific pathways and factors that were not identified in developmental screens. Several areas remain poorly understood, including how major axon tracts are formed in the embryo, and the function of axon regeneration in the natural environment.

The Caenorhabditis elegans Q neuroblasts: A powerful system to study cell migration at single-cell resolution in vivo

During development, cell migration plays a central role in the formation of tissues and organs. Understanding the molecular mechanisms that drive and control these migrations is a key challenge in developmental biology that will provide important insights into disease processes, including cancer cell metastasis. In this article, we discuss the Caenorhabditis elegans Q neuroblasts and their descendants as a tool to study cell migration at single-cell resolution in vivo. The highly stereotypical migration of these cells provides a powerful system to study the dynamic cytoskeletal processes that drive migration as well as the evolutionarily conserved signaling pathways (including different Wnt signaling cascades) that guide the cells along their specific trajectories. Here, we provide an overview of what is currently known about Q neuroblast migration and highlight the live-cell imaging, genome editing, and quantitative gene expression techniques that have been developed to study this process.

Heparan sulfate proteoglycans: a sugar code for vertebrate development?

Heparan sulfate proteoglycans (HSPGs) have long been implicated in a wide range of cell-cell signaling and cell-matrix interactions, both in vitro and in vivo in invertebrate models. Although many of the genes that encode HSPG core proteins and the biosynthetic enzymes that generate and modify HSPG sugar chains have not yet been analyzed by genetics in vertebrates, recent studies have shown that HSPGs do indeed mediate a wide range of functions in early vertebrate development, for example during left-right patterning and in cardiovascular and neural development. Here, we provide a comprehensive overview of the various roles of HSPGs in these systems and explore the concept of an instructive heparan sulfate sugar code for modulating vertebrate development.

Summary: This Review article examines the role of heparan sulfate proteoglycans in vertebrate development and explores the concept of an instructive 'sugar code' for modulating development.

Orchestrating A/P and D/V guidance - A Wnt/Netrin tale

While ample information was gathered in identifying guidance cues and their downstream mediators, very little is known about how the information from multiple extracellular cues is intracellularly to generate normal patterning. Netrin and Wnt signaling pathways play key roles in normal development as well as in malignancies. In C. elegans, as in vertebrates, dorso-ventral (D/V) graded distributions of UNC-6/Netrin and antero-posterior (A/P) graded distributions of Wnts provide instructive polarity information to guide cells and axons along their respective gradients. In this commentary, I will discuss recent findings demonstrating that these 2 signaling pathways also function redundantly to regulate polarity orthogonal to the axis of their gradation. Thus, Wnt signaling components contribute to D/V polarity, while Netrin signaling components contribute to A/P polarity and their joint action collaboratively governs migratory transitions from one axis to the other. These findings pave the way to unraveling broader roles of Wnt and Netrin signaling pathways, roles that are masked due to their redundant nature, and provide a conceptually novel view of how antero-posterior and dorso-ventral guidance mechanisms are orchestrated to establish polarity in multiple biological processes.

Redefining the role of syndecans in C. elegans biology

Cytosolic calcium is an important factor during fertilization, development and differentiation. Hence, the control of cytosolic calcium levels has been studied extensively for several decades. Numerous calcium channels have been identified and their mechanism of action elucidated. However, the mode of calcium channel regulation remains elusive. Here we discuss our recent findings regarding the role of syndecans in the regulation of cytosolic calcium levels. Syndecans are transmembrane proteoglycans present in both vertebrates and invertebrates that interact with extracellular ligands resulting in the activation of several downstream signaling pathways. We identified a previously unappreciated role of syndecans in cytosolic calcium regulation in mammals that is conserved in C. elegans. We concluded that calcium regulation is the basic, evolutionarily conserved role for syndecans, which enables them to be integral for multiple cellular functions.

The Caenorhabditis elegans Ephrin EFN-4 Functions Non-cell Autonomously with Heparan Sulfate Proteoglycans to Promote Axon Outgrowth and Branching

The Eph receptors and their cognate ephrin ligands play key roles in many aspects of nervous system development. These interactions typically occur within an individual tissue type, serving either to guide axons to their terminal targets or to define boundaries between the rhombomeres of the hindbrain. We have identified a novel role for the Caenorhabditis elegans ephrin EFN-4 in promoting primary neurite outgrowth in AIY interneurons and D-class motor neurons. Rescue experiments reveal that EFN-4 functions non-cell autonomously in the epidermis to promote primary neurite outgrowth. We also find that EFN-4 plays a role in promoting ectopic axon branching in a C. elegans model of X-linked Kallmann syndrome. In this context, EFN-4 functions non-cell autonomously in the body-wall muscle and in parallel with HS modification genes and HSPG core proteins. This is the first report of an epidermal ephrin providing a developmental cue to the nervous system.

Transmembrane proteoglycans control stretch-activated channels to set cytosolic calcium levels

Syndecans regulate members of the transient receptor potential family to control cytosolic calcium levels with impact on cell adhesion, junction formation, and neuronal guidance.

Transmembrane heparan sulfate proteoglycans regulate multiple aspects of cell behavior, but the molecular basis of their signaling is unresolved. The major family of transmembrane proteoglycans is the syndecans, present in virtually all nucleated cells, but with mostly unknown functions. Here, we show that syndecans regulate transient receptor potential canonical (TRPCs) channels to control cytosolic calcium equilibria and consequent cell behavior. In fibroblasts, ligand interactions with heparan sulfate of syndecan-4 recruit cytoplasmic protein kinase C to target serine714 of TRPC7 with subsequent control of the cytoskeleton and the myofibroblast phenotype. In epidermal keratinocytes a syndecan–TRPC4 complex controls adhesion, adherens junction composition, and early differentiation in vivo and in vitro. In Caenorhabditis elegans, the TRPC orthologues TRP-1 and -2 genetically complement the loss of syndecan by suppressing neuronal guidance and locomotory defects related to increases in neuronal calcium levels. The widespread and conserved syndecan–TRPC axis therefore fine tunes cytoskeletal organization and cell behavior.

Glypican Is a Modulator of Netrin-Mediated Axon Guidance

Netrin is a key axon guidance cue that orients axon growth during neural circuit formation. However, the mechanisms regulating netrin and its receptors in the extracellular milieu are largely unknown. Here we demonstrate that in Caenorhabditis elegans, LON-2/glypican, a heparan sulfate proteoglycan, modulates UNC-6/netrin signaling and may do this through interactions with the UNC-40/DCC receptor. We show that developing axons misorient in the absence of LON-2/glypican when the SLT-1/slit guidance pathway is compromised and that LON-2/glypican functions in both the attractive and repulsive UNC-6/netrin pathways. We find that the core LON-2/glypican protein, lacking its heparan sulfate chains, and secreted forms of LON-2/glypican are functional in axon guidance. We also find that LON-2/glypican functions from the epidermal substrate cells to guide axons, and we provide evidence that LON-2/glypican associates with UNC-40/DCC receptor-expressing cells. We propose that LON-2/glypican acts as a modulator of UNC-40/DCC-mediated guidance to fine-tune axonal responses to UNC-6/netrin signals during migration.

SDN-1/Syndecan Acts in Parallel to the Transmembrane Molecule MIG-13 to Promote Anterior Neuroblast Migration

The Q neuroblasts in Caenorhabditis elegans display left-right asymmetry in their migration, with QR and descendants on the right migrating anteriorly, and QL and descendants on the left migrating posteriorly. Initial QR and QL migration is controlled by the transmembrane receptors UNC-40/DCC, PTP-3/LAR, and the Fat-like cadherin CDH-4. After initial migration, QL responds to an EGL-20/Wnt signal that drives continued posterior migration by activating MAB-5/Hox activity in QL but not QR. QR expresses the transmembrane protein MIG-13, which is repressed by MAB-5 in QL and which drives anterior migration of QR descendants. A screen for new Q descendant AQR and PQR migration mutations identified mig-13 as well as hse-5, the gene encoding the glucuronyl C5-epimerase enzyme, which catalyzes epimerization of glucuronic acid to iduronic acid in the heparan sulfate side chains of heparan sulfate proteoglycans (HSPGs). Of five C. elegans HSPGs, we found that only SDN-1/Syndecan affected Q migrations. sdn-1 mutants showed QR descendant AQR anterior migration defects, and weaker QL descendant PQR migration defects. hse-5 affected initial Q migration, whereas sdn-1 did not. sdn-1 and hse-5 acted redundantly in AQR and PQR migration, but not initial Q migration, suggesting the involvement of other HSPGs in Q migration. Cell-specific expression studies indicated that SDN-1 can act in QR to promote anterior migration. Genetic interactions between sdn-1, mig-13, and mab-5 suggest that MIG-13 and SDN-1 act in parallel to promote anterior AQR migration and that SDN-1 also controls posterior migration. Together, our results indicate previously unappreciated complexity in the role of multiple signaling pathways and inherent left-right asymmetry in the control of Q neuroblast descendant migration.

A Synthetic Lethal Screen Identifies a Role for Lin-44/Wnt in C. elegans Embryogenesis

BACKGROUND

The C. elegans proteins PTP-3/LAR-RPTP and SDN-1/Syndecan are conserved cell adhesion molecules. Loss-of-function (LOF) mutations in either ptp-3 or sdn-1 result in low penetrance embryonic developmental defects. Work from other systems has shown that syndecans can function as ligands for LAR receptors in vivo. We used double mutant analysis to test whether ptp-3 and sdn-1 function in a linear genetic pathway during C. elegans embryogenesis.

RESULTS

We found animals with LOF in both sdn-1 and ptp-3 exhibited a highly penetrant synthetic lethality (SynLet), with only a small percentage of animals surviving to adulthood. Analysis of the survivors demonstrated that these animals had a synergistic increase in the penetrance of embryonic developmental defects. Together, these data strongly suggested PTP-3 and SDN-1 function in parallel during embryogenesis. We subsequently used RNAi to knockdown ~3,600 genes predicted to encode secreted and/or transmembrane molecules to identify genes that interacted with ptp-3 or sdn-1. We found that the Wnt ligand, lin-44, was SynLet with sdn-1, but not ptp-3. We used 4-dimensional time-lapse analysis to characterize the interaction between lin-44 and sdn-1. We found evidence that loss of lin-44 caused defects in the polarization and migration of endodermal precursors during gastrulation, a previously undescribed role for lin-44 that is strongly enhanced by the loss of sdn-1.

CONCLUSIONS

PTP-3 and SDN-1 function in compensatory pathways during C. elegans embryonic and larval development, as simultaneous loss of both genes has dire consequences for organismal survival. The Wnt ligand lin-44 contributes to the early stages of gastrulation in parallel to sdn-1, but in a genetic pathway with ptp-3. Overall, the SynLet phenotype provides a robust platform to identify ptp-3 and sdn-1 interacting genes, as well as other genes that function in development, yet might be missed in traditional forward genetic screens.

Genome-wide functional analysis of CREB/long-term memory-dependent transcription reveals distinct basal and memory gene expression programs

Induced CREB activity is a hallmark of long-term memory, but the full repertoire of CREB transcriptional targets required specifically for memory is not known in any system. To obtain a more complete picture of the mechanisms involved in memory, we combined memory training with genome-wide transcriptional analysis of C. elegans CREB mutants. This approach identified 757 significant CREB/memory-induced targets and confirmed the involvement of known memory genes from other organisms, but also suggested new mechanisms and novel components that may be conserved through mammals. CREB mediates distinct basal and memory transcriptional programs at least partially through spatial restriction of CREB activity: basal targets are regulated primarily in nonneuronal tissues, while memory targets are enriched for neuronal expression, emanating from CREB activity in AIM neurons. This suite of novel memory-associated genes will provide a platform for the discovery of orthologous mammalian long-term memory components.

Regulation of axonal midline guidance by prolyl 4-hydroxylation in Caenorhabditis elegans

Neuronal wiring during development requires that the growth cones of axons and dendrites are correctly guided to their appropriate targets. As in other animals, axon growth cones in Caenorhabditis elegans integrate information in their extracellular environment via interactions among transiently expressed cell surface receptors, their ligands, and the extracellular matrix (ECM). Components of the ECM undergo a wide variety of post-translational modifications that may affect efficacy of binding to neuronal guidance molecules. The most common modification of the ECM is prolyl 4-hydroxylation. However, little is known of its importance in the control of axon guidance. In a screen of prolyl 4-hydroxylase (P4H) mutants, we found that genetic removal of a specific P4H subunit, DPY-18, causes dramatic defects in C. elegans neuroanatomy. In dpy-18 mutant animals, the axons of specific ventral nerve cord neurons do not respect the ventral midline boundary and cross over to the contralateral axon fascicle. We found that these defects are independent of the known role of dpy-18 in regulating body size and that dpy-18 acts from multiple tissues to regulate axon guidance. Finally, we found that the neuronal defects in dpy-18 mutant animals are dependent on the expression of muscle-derived basement membrane collagens and motor neuron-derived ephrin ligands. Loss of dpy-18 causes dysregulated ephrin expression and this is at least partially responsible for the neurodevelopmental defects observed. Together, our data suggest that DPY-18 regulates ephrin expression to direct axon guidance, a role for P4Hs that may be conserved in higher organisms.

Analyzing the role of heparan sulfate proteoglycans in axon guidance in vivo in zebrafish

One of the most fascinating questions in the field of neurobiology is to understand how neuronal connections are properly formed. During development, neurons extend axons that are guided along defined paths by attractive and repulsive cues to reach their brain target. Most of these guidance factors are regulated by heparan sulfate proteoglycans (HSPGs), a family of cell-surface and extracellular core proteins with attached heparan sulfate (HS) glycosaminoglycans. The unique diversity and structural complexity of HS sugar chains, as well as the variety of core proteins, have been proposed to generate a complex "sugar code" essential for brain wiring. While the functions of HSPGs have been well characterized in C. elegans or Drosophila, relatively little is known about their roles in nervous system development in vertebrates. In this chapter, we describe the advantages and the different methods available to study the roles of HSPGs in axon guidance directly in vivo in zebrafish. We provide protocols for visualizing axons in vivo, including precise dye labeling and time-lapse imaging, and for disturbing the functions of HS-modifying enzymes and core proteins, including morpholino, DNA, or RNA injections.

Syndecan defines precise spindle orientation by modulating Wnt signaling in C. elegans

Wnt signals orient mitotic spindles in development, but it remains unclear how Wnt signaling is spatially controlled to achieve precise spindle orientation. Here, we show that C. elegans syndecan (SDN-1) is required for precise orientation of a mitotic spindle in response to a Wnt cue. We find that SDN-1 is the predominant heparan sulfate (HS) proteoglycan in the early C. elegans embryo, and that loss of HS biosynthesis or of the SDN-1 core protein results in misorientation of the spindle of the ABar blastomere. The ABar and EMS spindles both reorient in response to Wnt signals, but only ABar spindle reorientation is dependent on a new cell contact and on HS and SDN-1. SDN-1 transiently accumulates on the ABar surface as it contacts C, and is required for local concentration of Dishevelled (MIG-5) in the ABar cortex adjacent to C. These findings establish a new role for syndecan in Wnt-dependent spindle orientation.

Complex cooperative functions of heparan sulfate proteoglycans shape nervous system development in Caenorhabditis elegans

The development of the nervous system is a complex process requiring the integration of numerous molecular cues to form functional circuits. Many cues are regulated by heparan sulfates, a class of linear glycosaminoglycan polysaccharides. These sugars contain distinct modification patterns that regulate protein-protein interactions. Misexpressing the homolog of KAL-1/anosmin-1, a neural cell adhesion molecule mutant in Kallmann syndrome, in Caenorhabditis elegans causes a highly penetrant, heparan sulfate-dependent axonal branching phenotype in AIY interneurons. In an extended forward genetic screen for modifiers of this phenotype, we identified alleles in new as well as previously identified genes involved in HS biosynthesis and modification, namely the xylosyltransferase sqv-6, the HS-6-O-sulfotransferase hst-6, and the HS-3-O-sulfotransferase hst-3.2. Cell-specific rescue experiments showed that different HS biosynthetic and modification enzymes can be provided cell-nonautonomously by different tissues to allow kal-1-dependent branching of AIY. In addition, we show that heparan sulfate proteoglycan core proteins that carry the heparan sulfate chains act genetically in a highly redundant fashion to mediate kal-1-dependent branching in AIY neurons. Specifically, lon-2/glypican and unc-52/perlecan act in parallel genetic pathways and display synergistic interactions with sdn-1/syndecan to mediate kal-1 function. Because all of these heparan sulfate core proteins have been shown to act in different tissues, these studies indicate that KAL-1/anosmin-1 requires heparan sulfate with distinct modification patterns of different cellular origin for function. Our results support a model in which a three-dimensional scaffold of heparan sulfate mediates KAL-1/anosmin-1 and intercellular communication through complex and cooperative interactions. In addition, the genes we have identified could contribute to the etiology of Kallmann syndrome in humans.

Combinatorial roles of heparan sulfate proteoglycans and heparan sulfates in Caenorhabditis elegans neural development

Heparan sulfate proteoglycans (HSPGs) play critical roles in the development and adult physiology of all metazoan organisms. Most of the known molecular interactions of HSPGs are attributed to the structurally highly complex heparan sulfate (HS) glycans. However, whether a specific HSPG (such as syndecan) contains HS modifications that differ from another HSPG (such as glypican) has remained largely unresolved. Here, a neural model in C. elegans is used to demonstrate for the first time the relationship between specific HSPGs and HS modifications in a defined biological process in vivo. HSPGs are critical for the migration of hermaphrodite specific neurons (HSNs) as genetic elimination of multiple HSPGs leads to 80% defect of HSN migration. The effects of genetic elimination of HSPGs are additive, suggesting that multiple HSPGs, present in the migrating neuron and in the matrix, act in parallel to support neuron migration. Genetic analyses suggest that syndecan/sdn-1 and HS 6-O-sulfotransferase, hst-6, function in a linear signaling pathway and glypican/lon-2 and HS 2-O-sulfotransferase, hst-2, function together in a pathway that is parallel to sdn-1 and hst-6. These results suggest core protein specific HS modifications that are critical for HSN migration. In C. elegans, the core protein specificity of distinct HS modifications may be in part regulated at the level of tissue specific expression of genes encoding for HSPGs and HS modifying enzymes. Genetic analysis reveals that there is a delicate balance of HS modifications and eliminating one HS modifying enzyme in a compromised genetic background leads to significant changes in the overall phenotype. These findings are of importance with the view of HS as a critical regulator of cell signaling in normal development and disease.

Topical administration of lacritin is a novel therapy for aqueous-deficient dry eye disease

PURPOSE

Lacritin is a tear glycoprotein with prosecretory, prosurvival, and mitogenic properties. We examined lacritin levels in the tears of Sjögren's syndrome (SS) patients and explored the therapeutic potential of topical lacritin for the treatment of keratoconjunctivitis sicca.

METHODS

Tears from healthy controls (n = 14) and SS patients (n = 15) were assayed for lacritin using a C-terminal antibody. In a paired-eye study, autoimmune regulator (Aire) knockout (KO) mice (n = 7) were treated three times daily for 21 days with 10 μL of 4 μM lacritin (left eye) or vehicle (PBS) control (right eye). Tear secretion and ocular surface integrity were assessed at baseline and after treatment. Immunohistochemical staining of CD4+ T cells, cytokeratin-10 (K10), and cytokeratin-12 (K12) expression in the cornea and CD4+ T cell infiltration in the lacrimal glands were assessed.

RESULTS

Lacritin monomer (421.8 ± 65.3 ng [SS] vs. 655.8 ± 118.9 ng [controls]; P = 0.05) and C-terminal fragment protein (125 ± 34.1 ng [SS] vs. 399.5 ± 84.3 ng [controls]; P = 0.008) per 100 μL of tear eluate were significantly lower in SS patients. In Aire KO mice treated with lacritin, tear secretion increased by 46% (13.0 ± 3.5 mm vs. 8.9 ± 2.9 mm; P = 0.01) and lissamine green staining score significantly decreased relative to baseline (-0.417 ± 0.06 vs. 0.125 ± 0.07; P = 0.02). Expression of K10 but not K12 in the cornea was significantly decreased in lacritin-treated eyes. Focal CD4+ T cell infiltration of the lacrimal glands was significantly reduced on the lacritin-treated side versus the untreated side.

CONCLUSIONS

Lacritin is significantly reduced in the tears of SS patients. Topically administered lacritin has therapeutic potential for the treatment of aqueous-deficient dry eye disease.

Hs3st-A and Hs3st-B regulate intestinal homeostasis in Drosophila adult midgut

Intrinsic and extrinsic signals as well as the extracellular matrix (ECM) tightly regulate stem cells for tissue homeostasis and regenerative capacity. Little is known about the regulation of tissue homeostasis by the ECM. Heparan sulfate proteoglycans (HSPGs), important components of the ECM, are involved in a variety of biological events. Two heparin sulfate 3-O sulfotransferase (Hs3st) genes, Hs3st-A and Hs3st-B, encode the modification enzymes in heparan sulfate (HS) biosynthesis. Here we demonstrate that Hs3st-A and Hs3st-B are required for adult midgut homeostasis. Depletion of Hs3st-A in enterocytes (ECs) results in increased intestinal stem cell (ISC) proliferation and tissue homeostasis loss. Moreover, increased ISC proliferation is also observed in Hs3st-B null mutant alone, or in combination with Hs3st-A RNAi. Hs3st-A depletion-induced ISC proliferation is effectively suppressed by simultaneous inhibition of the EGFR signaling pathway, suggesting that tissue homeostasis loss in Hs3st-A-deficient intestines is due to increased EGFR signaling. Furthermore, we find that Hs3st-A-depleted ECs are unhealthy and prone to death, while ectopic expression of the antiapoptotic p35 is able to greatly suppress tissue homeostasis loss in these intestines. Together, our data suggest that Drosophila Hs3st-A and Hs3st-B are involved in the regulation of ISC proliferation and midgut homeostasis maintenance.

Syndecan promotes axon regeneration by stabilizing growth cone migration

Growth cones facilitate the repair of nervous system damage by providing the driving force for axon regeneration. Using single-neuron laser axotomy and in vivo time-lapse imaging, we show that syndecan, a heparan sulfate (HS) proteoglycan, is required for growth cone function during axon regeneration in C. elegans. In the absence of syndecan, regenerating growth cones form but are unstable and collapse, decreasing the effective growth rate and impeding regrowth to target cells. We provide evidence that syndecan has two distinct functions during axon regeneration: (1) a canonical function in axon guidance that requires expression outside the nervous system and depends on HS chains and (2) an intrinsic function in growth cone stabilization that is mediated by the syndecan core protein, independently of HS. Thus, syndecan is a regulator of a critical choke point in nervous system repair.

GLCE regulates PC12 cell neuritogenesis induced by nerve growth factor through activating SMAD/ID3 signalling

Depletion of glucuronic acid epimerase causes arrested PC12 cell growth and promotes the neuritogenesis and differentiation induced by nerve growth factor through activating SMAD/ID3 signalling. This action is independent of its enzymatic activity.

Characterization of the interaction between Robo1 and heparin and other glycosaminoglycans

Roundabout 1 (Robo1) is the cognate receptor for secreted axon guidance molecule, Slits, which function to direct cellular migration during neuronal development and angiogenesis. The Slit2-Robo1 signaling is modulated by heparan sulfate, a sulfated linear polysaccharide that is abundantly expressed on the cell surface and in the extracellular matrix. Biochemical studies have further shown that heparan sulfate binds to both Slit2 and Robo1 facilitating the ligand-receptor interaction. The structural requirements for heparan sulfate interaction with Robo1 remain unknown. In this report, surface plasmon resonance (SPR) spectroscopy was used to examine the interaction between Robo1 and heparin and other GAGs and determined that heparin binds to Robo1 with an affinity of ~650 nM. SPR solution competition studies with chemically modified heparins further determined that although all sulfo groups on heparin are important for the Robo1-heparin interaction, the N-sulfo and 6-O-sulfo groups are essential for the Robo1-heparin binding. Examination of differently sized heparin oligosaccharides and different GAGs also demonstrated that Robo1 prefers to bind full-length heparin chains and that GAGs with higher sulfation levels show increased Robo1 binding affinities.

A network of HSPG core proteins and HS modifying enzymes regulates netrin-dependent guidance of D-type motor neurons in Caenorhabditis elegans

Heparan sulfate proteoglycans (HSPGs) are proteins with long covalently attached sugar side chains of the heparan sulfate (HS) type. Depending on the cellular context HS chains carry multiple structural modifications such as sulfate residues or epimerized sugars allowing them to bind to a wide range of molecules. HSPGs have been found to play extremely diverse roles in animal development and were shown to interact with certain axon guidance molecules. In this study we describe the role of the Caenorhabditis elegans HSPG core proteins Syndecan (SDN-1) and Glypican (LON-2) and the HS modifying enzymes in the dorsal guidance of D-type motor axons, a process controlled mainly by the conserved axon guidance molecule UNC-6/Netrin. Our genetic analysis established the specific HS code relevant for this axon guidance event. Using two sensitized genetic backgrounds, we isolated novel components influencing D-type motor axon guidance with a link to HSPGs, as well as new alleles of several previously characterized axon guidance genes. Interestingly, the dorsal axon guidance defects induced by mutations in zfp-1 or lin-35 depended on the transgene oxIs12 used to visualize the D-type motor neurons. oxIs12 is a large multi-copy transgene that enlarges the X chromosome by approximately 20%. In a search for genes with a comparable phenotype we found that a mutation in the known dosage compensation gene dpy-21 showed similar axon guidance defects as zfp-1 or lin-35 mutants. Thus, derepression of genes on X, where many genes relevant for HS dependent axon guidance are located, might also influence axon guidance of D-type motor neurons.

Nonautonomous regulation of neuronal migration by insulin signaling, DAF-16/FOXO, and PAK-1

Neuronal migration is essential for nervous system development in all organisms and is regulated in the nematode, C. elegans, by signaling pathways that are conserved in humans. Here, we demonstrate that the insulin/IGF-1-PI3K signaling pathway modulates the activity of the DAF-16/FOXO transcription factor to regulate the anterior migrations of the hermaphrodite-specific neurons (HSNs) during embryogenesis of C. elegans. When signaling is reduced, DAF-16 is activated and promotes migration; conversely, when signaling is enhanced, DAF-16 is inactivated, and migration is inhibited. We show that DAF-16 acts nonautonomously in the hypodermis to promote HSN migration. Furthermore, we identify PAK-1, a p21-activated kinase, as a downstream mediator of insulin/IGF-1-DAF-16 signaling in the nonautonomous control of HSN migration. Because a FOXO-Pak1 pathway was recently shown to regulate mammalian neuronal polarity, our findings indicate that the roles of FOXO and Pak1 in neuronal migration are most likely conserved from C. elegans to higher organisms.

Novel insight into the biological functions of syndecan ectodomain core proteins

Syndecans are a four member family of multifunctional transmembrane heparan sulphate bearing cell surface receptors. Each family member has common molecular architecture but a distinct expression profile. Numerous molecular interactions between syndecan heparan sulphate chains, growth factors, cytokines, and extracellular matrix molecules have been reported and syndecans are intimately associated with cell adhesion and migration. Here, we describe the important emerging concept that contained within syndecan extracellular core proteins are "adhesion regulatory domains." Cell adhesion is driven by the integrins and syndecan ectodomain adhesion regulatory domains can alter integrin driven cellular responses. Cell adhesion and migration is central to numerous pathologies and an understanding of how syndecan ectodomains influence integrins will lead to novel therapeutic strategies.