Discrete clusters of axonin-1 and NgCAM at neuronal contact sites: facts and speculations on the regulation of axonal fasciculation

Neuronal post-developmentally acting SAX-7S/L1CAM can function as cleaved fragments to maintain neuronal architecture in C. elegans

Whereas remarkable advances have uncovered mechanisms that drive nervous system assembly, the processes responsible for the lifelong maintenance of nervous system architecture remain poorly understood. Subsequent to its establishment during embryogenesis, neuronal architecture is maintained throughout life in the face of the animal's growth, maturation processes, the addition of new neurons, body movements, and aging. The C. elegans protein SAX-7, homologous to the vertebrate L1 protein family of neural adhesion molecules, is required for maintaining the organization of neuronal ganglia and fascicles after their successful initial embryonic development. To dissect the function of sax-7 in neuronal maintenance, we generated a null allele and sax-7S-isoform-specific alleles. We find that the null sax-7(qv30) is, in some contexts, more severe than previously described mutant alleles, and that the loss of sax-7S largely phenocopies the null, consistent with sax-7S being the key isoform in neuronal maintenance. Using a sfGFP::SAX-7S knock-in, we observe sax-7S to be predominantly expressed across the nervous system, from embryogenesis to adulthood. Yet, its role in maintaining neuronal organization is ensured by post-developmentally acting SAX-7S, as larval transgenic sax-7S(+) expression alone is sufficient to profoundly rescue the null mutants' neuronal maintenance defects. Moreover, the majority of the protein SAX-7 appears to be cleaved, and we show that these cleaved SAX-7S fragments together, not individually, can fully support neuronal maintenance. These findings contribute to our understanding of the role of the conserved protein SAX-7/L1CAM in long-term neuronal maintenance, and may help decipher processes that go awry in some neurodegenerative conditions.

The role of cell adhesion molecules in brain wiring and neuropsychiatric disorders

Cell adhesion molecules (CAMs) in the nervous system have long been a research focus, but many mice lacking CAMs show very subtle phenotypes, giving an impression that CAMs may not be major players in constructing the nervous system. However, recent human genetic studies suggest CAM involvement in many neuropsychiatric disorders, implicating that they must have significant functions in nervous system development, namely in circuitry formation. As CAMs can provide specificity through their molecular interactions, this review summarizes possible mechanisms on how alterations of CAMs can result in neuropsychiatric disorders through circuitry modification.

Analysis of cell-cell contact mediated by Ig superfamily cell adhesion molecules

Cell-cell adhesion is a fundamental requirement for all multicellular organisms. The calcium-independent cell adhesion molecules of the immunoglobulin superfamily (IgSF-CAMs) represent a major subgroup. They consist of immunoglobulin folds alone or in combination with other protein modules, often fibronectin type-III folds. More than 100 IgSF-CAMs have been identified in vertebrates and invertebrates. Most of the IgSF-CAMs are cell surface molecules that are membrane-anchored either by a single transmembrane segment or by a glycosylphosphatidylinositol (GPI) anchor. Some of the IgSF-CAMs also occur in soluble form, e.g., in the cerebrospinal fluid or in the vitreous fluid of the eye, due to naturally occurring cleavage of the GPI anchor or the membrane-proximal peptide segment. Some IgSF-CAMs, such as NCAM, occur in various forms that are generated by alternative splicing. This unit contains a series of protocols that have been used to study the function of IgSF-CAMs in vitro and in vivo.

The maturation of photoreceptors in the avian retina is stimulated by thyroid hormone

During retinal development, the cell-fate of photoreceptors is committed long before maturation, which entails the expression of opsins and functional transduction of light. The mechanisms that delay the maturation of photoreceptors remain unknown. We have recently reported that immature photoreceptors express the LIM domain transcription factors Islet2 and Lim3, as well as the cell-surface glycoprotein axonin1 [Fischer et al., (2008a) J Comp Neurol 506:584-603]. As the photoreceptors mature to form outer segments and express photopigments, the expression of the Islet2, Lim3 and axonin1 is diminished. The purpose of this study was to investigate whether thyroid hormone (TH) influences the maturation of photoreceptors. We studied the maturation of photoreceptors across the gradient of maturity that exists in far peripheral regions of the post-natal chicken retina [Ghai et al., (2008) Brain Res 1192:76-89]. We found that intraocular injections of TH down-regulated Islet2, Lim3 and axonin1 in photoreceptors in far peripheral regions of the retina. By contrast, TH stimulated the up-regulation of red-green opsin, violet opsin, rhodopsin and calbindin in photoreceptors. We found a correlation between the onset of RLIM (RING finger LIM-domain binding protein) and down-regulation of Islet2 and Lim3 in maturing photoreceptors; RLIM is known to interfere with the transcriptional activity of LIM-domain transcription factors. We conclude that TH stimulates the maturation of photoreceptors in the avian retina. We propose that TH inhibits the expression of Islet2 and Lim3, which thereby permits photoreceptor maturation and the onset of photopigment-expression.

Transient expression of LIM-domain transcription factors is coincident with delayed maturation of photoreceptors in the chicken retina

In the retina of warm-blooded vertebrates, photoreceptors are specified many days before the onset of synaptogenesis and the expression of photopigments. The factors that regulate the maturation of photoreceptors in the developing retina remain unknown. We report here that photoreceptors transiently express LIM-domain transcription factors during the development of the chicken retina. We examined the differentiation of photoreceptors through the normal course of embryonic development and at the far periphery of the postnatal retina, where the differentiation of photoreceptors is slowed and persists across a spatial gradient. In the embryonic retina, we find visinin-positive photoreceptors that transiently express Islet2 and Lim3 starting at E8 and ending around E15, but persisting in far peripheral regions of the retina through the first 2 weeks of postnatal development. During early stages of photoreceptor maturation, there is coincident and transient expression of the LIM-domain factors with axonin1, a cell surface glycoprotein that is a member of the immunoglobulin superfamily. Coincident with the downregulation of Islet2 and Lim3, we find the upregulation of calbindin, red/green opsin, rhodopsin, and a synaptic marker in the outer plexiform layer (OPL; dystrophin). In the periphery of the postnatal retina, photoreceptors that express Islet2, Lim3, and axonin1 do not overlap with photoreceptors that express calbindin, red/green opsin, rhodopsin, and dystrophin. We propose that Islet2 and Lim3 may promote the expression of genes that are involved in the early stages of differentiation but may suppress the expression of genes that are required in the mature photoreceptors.

The SALM family of adhesion-like molecules forms heteromeric and homomeric complexes

Synaptic adhesion-like molecules (SALMs) are a newly discovered family of adhesion molecules that play roles in synapse formation and neurite outgrowth. The SALM family is comprised of five homologous molecules that are expressed largely in the central nervous system. SALMs 1-3 contain PDZ-binding domains, whereas SALMs 4 and 5 do not. We are interested in characterizing the interactions of the SALMs both among the individual members and with other binding partners. In the present study, we focused on the interactions formed by the five SALM members in rat brain and heterologous cells. In brain, we found that SALMs 1-3 strongly co-immunoprecipitated with each other, whereas SALMs 4 and 5 did not, suggesting that SALMs 4 and 5 mainly form homomeric complexes. In heterologous cells transfected with SALMs, co-immunoprecipitation studies showed that all five SALMs form heteromeric and homomeric complexes. We also determined if SALMs could form trans-cellular associations between transfected heterologous cells. Both SALMs 4 and 5 formed homophilic, but not heterophilic associations, whereas no trans associations were formed by the other SALMs. The ability of SALM4 to form trans interactions is due to its extracellular N terminus because chimeras of SALM4 N terminus and SALM2 C terminus can form trans interactions, whereas chimeras of SALM2 N terminus and SALM4 C terminus cannot. Co-culture experiments using HeLa cells and rat hippocampal neurons expressing the SALMs showed that SALM4 is recruited to points of contact between the cells. In neurons, these points of contact were seen in both axons and dendrites.

Expression of axonin-1 in developing amacrine cells in the chick retina

This study focused on the temporal and spatial pattern of expression of the cell adhesion molecule axonin-1 in amacrine cells and the identification of these cells in the developing chick retina. We analyzed 5-20-day-old chick embryos. The antigen was localized and visualized by the indirect immunogold and the immunofluorescence technique. Colocalization studies with antibodies against tyrosine hydroxylase, acetylcholinesterase, choline acetyltransferase, parvalbumin, calbindin, and calretinin served to characterize these cells further and to explore whether they have other properties in common. Axonin-1 was expressed in amacrine cells from E8 onward in the inner nuclear, in the inner plexiform, and in the ganglion cell layer. Their maturation showed a gradient similar to that found for amacrinogenesis. Expression was closely correlated with the period when the cells develop and shape their processes. The interneurons were classified with reference to Cajal, and most of the morphological types described by him were found. In addition, some cells were considered as axon-bearing amacrine cells. However, the total number of labeled cells was rather small. At least two morphologically different types terminated in each of the inner plexiform sublayers. Narrow- and wide-field arbors indicated the existence of a diversified network. The colocalization studies revealed that the neurotransmitters and neuropeptides overlapped partially with axonin-1 expression. This indicated that axonin-1-immunoreactive amacrine cells were also functionally diverse.

Factors controlling axonal and dendritic arbors

The sculpting and maintenance of axonal and dendritic arbors is largely under the control of molecules external to the cell. These factors include both substratum-associated and soluble factors that can enhance or inhibit the outgrowth of axons and dendrites. A large number of factors that modulate axonal outgrowth have been identified, and the first stages of the intracellular signaling pathways by which they modify process outgrowth have been characterized. Relatively fewer factors and pathways that affect dendritic outgrowth have been described. The factors that affect axonal arbors form an incompletely overlapping set with those that affect dendritic arbors, allowing selective control of the development and maintenance of these critical aspects of neuronal morphology.

Adhesion proteins for a tight neuron-electrode contact

The neural cell adhesion molecules axonin-1 and NgCAM have been genetically engineered and covalently immobilized on glass and silicon oxide surfaces in their correct orientation. Surfaces treated with these adhesion molecules were used as substrates for culturing dorsal root ganglion neurons. The cleft between the neuron cell membrane and the surface was determined using fluorescence interference contrast (FLIC) microscopy. For comparison, cell--material distances on laminin, RGDC, polylysine and amino-terminated surfaces were measured. When the neurons grow on axonin-1 the cell--surface distance is at a minimum (37 nm) probably because the glycocalyx hinders a closer contact. A selective treatment of extracellular electrodes with axonin-1 could be used to improve the cell-material contact and thus increase extracellularly recorded signals.

The tenascin family of ECM glycoproteins: structure, function, and regulation during embryonic development and tissue remodeling

The determination of animal form depends on the coordination of events that lead to the morphological patterning of cells. This epigenetic view of development suggests that embryonic structures arise as a consequence of environmental influences acting on the properties of cells, rather than an unfolding of a completely genetically specified and preexisting invisible pattern. Specialized cells of developing multicellular organisms are surrounded by a complex extracellular matrix (ECM), comprised largely of different collagens, proteoglycans, and glycoproteins. This ECM is a substrate for tissue morphogenesis, lends support and flexibility to mature tissues, and acts as an epigenetic informational entity in the sense that it transduces and integrates intracellular signals via distinct cell surface receptors. Consequently, ECM-receptor interactions have a profound influence on major cellular programs including growth, differentiation, migration, and survival. In contrast to many other ECM proteins, the tenascin (TN) family of glycoproteins (TN-C, TN-R, TN-W, TN-X, and TN-Y) display highly restricted and dynamic patterns of expression in the embryo, particularly during neural development, skeletogenesis, and vasculogenesis. These molecules are reexpressed in the adult during normal processes such as wound healing, nerve regeneration, and tissue involution, and in pathological states including vascular disease, tumorigenesis, and metastasis. In concert with a multitude of associated ECM proteins and cell surface receptors that include members of the integrin family, TN proteins impart contrary cellular functions, depending on their mode of presentation (i.e., soluble or substrate-bound) and the cell types and differentiation states of the target tissues. Expression of tenascins is regulated by a variety of growth factors, cytokines, vasoactive peptides, ECM proteins, and biomechanical factors. The signals generated by these factors converge on particular combinations of cis-regulatory elements within the recently identified TN gene promoters via specific transcriptional activators or repressors. Additional complexity in regulating TN gene expression is achieved through alternative splicing, resulting in variants of TN polypeptides that exhibit different combinations of functional protein domains. In this review, we discuss some of the recent advances in TN biology that provide insights into the complex way in which the ECM is regulated and how it functions to regulate tissue morphogenesis and gene expression.

Use of lipophilic dyes in studies of axonal pathfinding in vivo

Fluorescent lipophilic dyes are an ideal tool to study axonal pathfinding. Because these dyes do not require active axonal transport for their spreading, they can be used in fixed tissue. Here, we describe the method we have used to study the molecular mechanisms of commissural axon pathfinding in the embryonic chicken spinal cord in vivo. Based on in vitro studies, different families of molecules had been suggested to play a role in the guidance of developing axons. In order to test their function in vivo, we used the commissural neurons that are located at the dorsolateral border of the chicken spinal cord as a model system [Stoeckli and Landmesser (1995) Neuron 14:1165-1179]. Axonin-1, NgCAM, and NrCAM, three members of the immunoglobulin (Ig) superfamily of cell adhesion molecules (CAMs), were shown to be important for the correct growth pattern of commissural axons. We studied the effect of perturbations of specific CAM/CAM interactions by injection of function-blocking antibodies into the central canal of the spinal cord in ovo. After 2 days, the embryos were sacrificed and fluorescent tracers, such as Fast-DiI, were used to visualize commissural axons, and thus, to analyze their response to these perturbations in two different types of fixed preparations: transverse vibratome sections and whole-mount preparations of the spinal cord. Both pathfinding errors and defasciculation of axons were observed as a result of the perturbation of CAM/CAM interactions.