Target-derived matricryptins organize cerebellar synapse formation through α3β1 integrins

The role of integrins in brain health and neurodegenerative diseases

Integrins are heterodimeric membrane proteins expressed on the surface of most cells. They mediate adhesion and signaling processes relevant for a wealth of physiological processes, including nervous system development and function. Interestingly, integrins are also recognized therapeutic targets for inflammatory diseases, such as multiple sclerosis. Here, we discuss the role of integrins in brain development and function, as well as in neurodegenerative diseases affecting the brain (Alzheimer's disease, multiple sclerosis, stroke). Furthermore, we discuss therapeutic targeting of these adhesion receptors in inflammatory diseases of the brain.

Collagen XIX is required for pheromone recognition and glutamatergic synapse formation in mouse accessory olfactory bulb

In mammals, the accessory olfactory bulb (AOB) receives input from vomeronasal sensory neurons (VSN) which detect pheromones, chemical cues released by animals to regulate the physiology or behaviors of other animals of the same species. Cytoarchitecturally, cells within the AOB are segregated into a glomerular layer (GL), mitral cell layer (MCL), and granule cell layer (GCL). While the cells and circuitry of these layers has been well studied, the molecular mechanism underlying the assembly of such circuitry in the mouse AOB remains unclear. With the goal of identifying synaptogenic mechanisms in AOB, our attention was drawn to Collagen XIX, a non-fibrillar collagen generated by neurons in the mammalian telencephalon that has previously been shown to regulate the assembly of synapses. Here, we used both a targeted mouse mutant that lacks Collagen XIX globally and a conditional allele allowing for cell-specific deletion of this collagen to test if the loss of Collagen XIX causes impaired synaptogenesis in the mouse AOB. These analyses not only revealed defects in excitatory synapse distribution in these Collagen XIX-deficient mutants, but also showed that these mutant mice exhibit altered behavioral responses to pheromones. Although this collagen has been demonstrated to play synaptogenic roles in the telencephalon, those roles are at perisomatic inhibitory synapses, results here are the first to demonstrate the function of this unconventional collagen in glutamatergic synapse formation.

Extracellular matrix and synapse formation

The extracellular matrix (ECM) is a complex molecular network distributed throughout the extracellular space of different tissues as well as the neuronal system. Previous studies have identified various ECM components that play important roles in neuronal maturation and signal transduction. ECM components are reported to be involved in neurogenesis, neuronal migration, and axonal growth by interacting or binding to specific receptors. In addition, the ECM is found to regulate synapse formation, the stability of the synaptic structure, and synaptic plasticity. Here, we mainly reviewed the effects of various ECM components on synapse formation and briefly described the related diseases caused by the abnormality of several ECM components.

Proteolysis: a key post-translational modification regulating proteoglycans

Proteoglycans are composite molecules comprising a protein backbone, i.e., the core protein, with covalently attached glycosaminoglycan chains of distinct chemical types. Most proteoglycans are secreted or attached to the cell membrane. Their specialized structures, binding properties, and biophysical attributes underlie diverse biological roles, which include modulation of tissue mechanics, cell adhesion, and the sequestration and regulated release of morphogens, growth factors, and cytokines. As an irreversible post-translational modification, proteolysis has a profound impact on proteoglycan function, abundance, and localization. Proteolysis is required for molecular maturation of some proteoglycans, clearance of extracellular matrix proteoglycans during tissue remodeling, generation of bioactive fragments from proteoglycans, and ectodomain shedding of cell-surface proteoglycans. Genetic evidence shows that proteoglycan core protein proteolysis is essential for diverse morphogenetic events during embryonic development. In contrast, dysregulated proteoglycan proteolysis contributes to osteoarthritis, cardiovascular disorders, cancer, and inflammation. Proteolytic fragments of perlecan, versican, aggrecan, brevican, collagen XVIII, and other proteoglycans are associated with independent biological activities as so-called matrikines. Yet, proteoglycan proteolysis has been investigated to only a limited extent to date. Here, we review the actions of proteases on proteoglycans and illustrate their functional impact with several examples. We discuss the applications and limitations of strategies used to define cleavage sites in proteoglycans and explain how proteoglycanome-wide proteolytic mapping, which is desirable to fully understand the impact of proteolysis on proteoglycans, can be facilitated by integrating classical proteoglycan isolation methods with mass spectrometry-based proteomics.

Development of astrocyte morphology and function in mouse visual thalamus

The rodent visual thalamus has served as a powerful model to elucidate the cellular and molecular mechanisms that underlie sensory circuit formation and function. Despite significant advances in our understanding of the role of axon-target interactions and neural activity in orchestrating circuit formation in visual thalamus, the role of non-neuronal cells, such as astrocytes, is less clear. In fact, we know little about the transcriptional identity and development of astrocytes in mouse visual thalamus. To address this gap in knowledge, we studied the expression of canonical astrocyte molecules in visual thalamus using immunostaining, in situ hybridization, and reporter lines. While our data suggests some level of heterogeneity of astrocytes in different nuclei of the visual thalamus, the majority of thalamic astrocytes appeared to be labeled in Aldh1l1-EGFP mice. This led us to use this transgenic line to characterize the neonatal and postnatal development of these cells in visual thalamus. Our data show that not only have the entire cohort of astrocytes migrated into visual thalamus by eye-opening but they also have acquired their adult-like morphology, even while retinogeniculate synapses are still maturing. Furthermore, ultrastructural, immunohistochemical, and functional approaches revealed that by eye-opening, thalamic astrocytes ensheathe retinogeniculate synapses and are capable of efficient uptake of glutamate. Taken together, our results reveal that the morphological, anatomical, and functional development of astrocytes in visual thalamus occurs prior to eye-opening and the emergence of experience-dependent visual activity.

Mutations in the COL18A1 gen associated with knobloch syndrome and structural brain anomalies: a novel case report and literature review of neuroimaging findings

. COL18A1 gene mutations have been associated with Knobloch syndrome, which is characterized by ocular and brain abnormalities. Here we report a 4.5 years-old male child with autism and two novel COL18A1 mutations (NM_030582.4: c.1883_1891dup and c.1787C>T). Hypermetropic astigmatism, but not brain migration disorders, was observed. However, an asymmetric pattern of cerebellar perfusion and a smaller arcuate fascicle were found.  Low levels of collagen XVIII were also observed in the patient´s serum. Thus, biallelic loss-of-function mutations in COL18A1 may be a new cause of autism  without the brain malformations typically reported in patients with Knobloch syndrome.

Paracrine Role for Somatostatin Interneurons in the Assembly of Perisomatic Inhibitory Synapses

GABAergic interneurons represent a heterogenous group of cell types in neocortex that can be clustered based on developmental origin, morphology, physiology, and connectivity. Two abundant populations of cortical GABAergic interneurons include the low-threshold, somatostatin (SST)-expressing cells and the fast-spiking, parvalbumin (PV)-expressing cells. While SST⁺ and PV⁺ interneurons are both early born and migrate into the developing neocortex at similar times, SST⁺ cells are incorporated into functional circuits prior to PV⁺ cells. During this early period of neural development, SST⁺ cells play critical roles in the assembly and maturation of other cortical circuits; however, the mechanisms underlying this process remain poorly understood. Here, using both sexes of conditional mutant mice, we discovered that SST⁺ interneuron-derived Collagen XIX, a synaptogenic extracellular matrix protein, is required for the formation of GABAergic, perisomatic synapses by PV⁺ cells. These results, therefore, identify a paracrine mechanism by which early-born SST⁺ cells orchestrate inhibitory circuit formation in the developing neocortex.SIGNIFICANCE STATEMENT Inhibitory interneurons in the cerebral cortex represent a heterogenous group of cells that generate the inhibitory neurotransmitter GABA. One such interneuron type is the low-threshold, somatostatin (SST)-expressing cell, which is one of the first types of interneurons to migrate into the cerebral cortex and become incorporated into functional circuits. In addition, to contributing important roles in controlling the flow of information in the adult cerebral cortex, SST⁺ cells play important roles in the development of other neural circuits in the developing brain. Here, we identified an extracellular matrix protein that is released by these early-born SST⁺ neurons to orchestrate inhibitory circuit formation in the developing cerebral cortex.

Role of Endogenous Regulators of Hem- And Lymphangiogenesis in Corneal Transplantation

Under normal conditions, the cornea, being the transparent “windscreen” of the eye, is free of both blood and lymphatic vessels. However, various diseases of the eye, like infections, can interfere with the balance between promoting and inhibiting factors, which leads to ingrowth of blood and lymphatic vessels. The newly formed lymphatic vessels increase the risk of graft rejection after subsequent corneal transplantation. Corneal transplantation is one of the most commonly performed transplantations worldwide, with more than 40,000 surgeries per year in Europe. To date, various anti-hem- and anti-lymphangiogenic treatment strategies have been developed specifically for the corneal vascular endothelial growth factor (VEGF) pathway. Currently, however, no treatment strategies are clinically available to specifically modulate lymphangiogenesis. In this review, we will give an overview about endogenous regulators of hem- and lymphangiogenesis and discuss potential new strategies for targeting pathological lymphangiogenesis. Furthermore, we will review recently identified modulators and demonstrate that the cornea is a suitable model for the identification of novel endogenous modulators of lymphangiogenesis. The identification of novel modulators of lymphangiogenesis and a better understanding of the signaling pathways involved will contribute to the development of new therapeutic targets for the treatment of pathological lymphangiogenesis. This, in turn, will improve graft rejection, not only for the cornea.

Coding Capacity of Purkinje Cells With Different Schemes of Morphological Reduction

The brain as a neuronal system has very complex structures with a large diversity of neuronal types. The most basic complexity is seen from the structure of neuronal morphology, which usually has a complex tree-like structure with dendritic spines distributed in branches. To simulate a large-scale network with spiking neurons, the simple point neuron, such as the integrate-and-fire neuron, is often used. However, recent experimental evidence suggests that the computational ability of a single neuron is largely enhanced by its morphological structure, in particular, by various types of dendritic dynamics. As the morphology reduction of detailed biophysical models is a classic question in systems neuroscience, much effort has been taken to simulate a neuron with a few compartments to include the interaction between the soma and dendritic spines. Yet, novel reduction methods are still needed to deal with the complex dendritic tree. Here, using 10 individual Purkinje cells of the cerebellum from three species of guinea-pig, mouse and rat, we consider four types of reduction methods and study their effects on the coding capacity of Purkinje cells in terms of firing rate, timing coding, spiking pattern, and modulated firing under different stimulation protocols. We found that there is a variation of reduction performance depending on individual cells and species, however, all reduction methods can preserve, to some degree, firing activity of the full model of Purkinje cell. Therefore, when stimulating large-scale network of neurons, one has to choose a proper type of reduced neuronal model depending on the questions addressed. Among these reduction schemes, Branch method, that preserves the geometrical volume of neurons, can achieve the best balance among different performance measures of accuracy, simplification, and computational efficiency, and reproduce various phenomena shown in the full morphology model of Purkinje cells. Altogether, these results suggest that the Branch reduction scheme seems to provide a general guideline for reducing complex morphology into a few compartments without the loss of basic characteristics of the firing properties of neurons.

Collagen 18 and agrin are secreted by neural crest cells to remodel their microenvironment and regulate their migration during enteric nervous system development

The enteric nervous system (ENS) arises from neural crest cells that migrate, proliferate, and differentiate into enteric neurons and glia within the intestinal wall. Many extracellular matrix (ECM) components are present in the embryonic gut, but their role in regulating ENS development is largely unknown. Here, we identify heparan sulfate proteoglycan proteins, including collagen XVIII (Col18) and agrin, as important regulators of enteric neural crest-derived cell (ENCDC) development. In developing avian hindgut, Col18 is expressed at the ENCDC wavefront, while agrin expression occurs later. Both proteins are normally present around enteric ganglia, but are absent in aganglionic gut. Using chick-mouse intestinal chimeras and enteric neurospheres, we show that vagal- and sacral-derived ENCDCs from both species secrete Col18 and agrin. Whereas glia express Col18 and agrin, enteric neurons only express the latter. Functional studies demonstrate that Col18 is permissive whereas agrin is strongly inhibitory to ENCDC migration, consistent with the timing of their expression during ENS development. We conclude that ENCDCs govern their own migration by actively remodeling their microenvironment through secretion of ECM proteins.

From Structure to Phenotype: Impact of Collagen Alterations on Human Health

The extracellular matrix (ECM) is a highly dynamic and heterogeneous structure that plays multiple roles in living organisms. Its integrity and homeostasis are crucial for normal tissue development and organ physiology. Loss or alteration of ECM components turns towards a disease outcome. In this review, we provide a general overview of ECM components with a special focus on collagens, the most abundant and diverse ECM molecules. We discuss the different functions of the ECM including its impact on cell proliferation, migration and differentiation by highlighting the relevance of the bidirectional cross-talk between the matrix and surrounding cells. By systematically reviewing all the hereditary disorders associated to altered collagen structure or resulting in excessive collagen degradation, we point to the functional relevance of the collagen and therefore of the ECM elements for human health. Moreover, the large overlapping spectrum of clinical features of the collagen-related disorders makes in some cases the patient clinical diagnosis very difficult. A better understanding of ECM complexity and molecular mechanisms regulating the expression and functions of the various ECM elements will be fundamental to fully recognize the different clinical entities.

F-spondin Is Essential for Maintaining Circadian Rhythms

The suprachiasmatic nucleus (SCN) is the master pacemaker that drives circadian behaviors. SCN neurons have intrinsic, self-sustained rhythmicity that is governed by transcription-translation feedback loops. Intrinsic rhythms within the SCN do not match the day-night cycle and are therefore entrained by light-derived cues. Such cues are transmitted to the SCN by a class of intrinsically photosensitive retinal ganglion cells (ipRGCs). In the present study, we sought to identify how axons from ipRGCs target the SCN. While none of the potential targeting cues identified appeared necessary for retinohypothalamic innervation, we unexpectedly identified a novel role for the extracellular matrix protein F-spondin in circadian behavior. In the absence of F-spondin, mice lost their ability to maintain typical intrinsic rhythmicity. Moreover, F-spondin loss results in the displacement of vasoactive intestinal peptide (VIP)-expressing neurons, a class of neurons that are essential for maintaining rhythmicity among SCN neurons. Thus, this study highlights a novel role for F-spondin in maintaining circadian rhythms.

Two Classes of Secreted Synaptic Organizers in the Central Nervous System

Research in the last two decades has identified many synaptic organizers in the central nervous system that directly regulate the assembly of pre- and/or postsynaptic molecules, such as synaptic vesicles, active zone proteins, and neurotransmitter receptors. They are classified into secreted factors and cell adhesion molecules, such as neurexins and neuroligins. Certain secreted factors are termed extracellular scaffolding proteins (ESPs) because they are components of the synaptic extracellular matrix and serve as a scaffold at the synaptic cleft. These include Lgi1, Cbln1, neuronal pentraxins, Hevin, thrombospondins, and glypicans. Diffusible secreted factors, such as Wnts, fibroblast growth factors, and semaphorins, tend to act from a distance. In contrast, ESPs remain at the synaptic cleft and often help synaptic adhesion and/or accumulation of postsynaptic receptors. Many fundamental questions remain about when, how, and why various synaptic organizers establish and modify the vast numbers of connections during development and throughout life.

Fragments generated upon extracellular matrix remodeling: Biological regulators and potential drugs

The remodeling of the extracellular matrix (ECM) by several protease families releases a number of bioactive fragments, which regulate numerous biological processes such as autophagy, angiogenesis, adipogenesis, fibrosis, tumor growth, metastasis and wound healing. We review here the proteases which generate bioactive ECM fragments, their ECM substrates, the major bioactive ECM fragments, together with their biological properties and their receptors. The translation of ECM fragments into drugs is challenging and would take advantage of an integrative approach to optimize the design of pre-clinical and clinical studies. This could be done by building the contextualized interaction network of the ECM fragment repertoire including their parent proteins, remodeling proteinases, and their receptors, and by using mathematical disease models.

Optic Nerve Hypoplasia Is a Pervasive Subcortical Pathology of Visual System in Neonates

Purpose

Optic nerve hypoplasia (ONH) is the most common cause of childhood congenital blindness in developed nations, yet the fundamental pathobiology of ONH remains unknown. The objective of this study was to employ a ‘face validated' murine model to determine the timing of onset and the pathologic characteristics of ONH.

Methods

Based on the robust linkage between X-linked CASK haploinsufficiency and clinically diagnosed ONH, we hypothesized that heterozygous deletion of CASK (CASK^(+/−)) in rodents will produce an optic nerve pathology closely recapitulating ONH. We quantitatively analyzed the entire subcortical visual system in female CASK^(+/−) mice using immunohistochemistry, anterograde axonal tracing, toluidine blue staining, transmission electron microscopy, and serial block-face scanning electron microscopy.

Results

CASK haploinsuffiency in mice phenocopies human ONH with complete penetrance, thus satisfying the ‘face validity'. We demonstrate that the optic nerve in CASK^(+/−) mice is not only thin, but is comprised of atrophic retinal axons and displays reactive astrogliosis. Myelination of the optic nerve axons remains unchanged. Moreover, we demonstrate a significant decrease in retinal ganglion cell (RGC) numbers and perturbation in retinothalamic connectivity. Finally, we used this mouse model to define the onset and progression of ONH pathology, demonstrating for the first time that optic nerve defects arise at neonatally in CASK^(+/−)mice.

Conclusions

Optic nerve hypoplasia is a complex neuropathology of the subcortical visual system involving RGC loss, axonopathy, and synaptopathy and originates at a developmental stage in mice that corresponds to the late third trimester development in humans.

Region- and Cell-Specific Expression of Transmembrane Collagens in Mouse Brain

Unconventional collagens are nonfribrillar proteins that not only contribute to the structure of extracellular matrices but exhibit unique bio-activities. Although roles for unconventional collagens have been well-established in the development and function of non-neural tissues, only recently have studies identified roles for these proteins in brain development, and more specifically, in the formation and refinement of synaptic connections between neurons. Still, our understanding of the full cohort of unconventional collagens that are generated in the mammalian brain remains unclear. Here, we sought to address this gap by assessing the expression of transmembrane collagens (i.e., collagens XIII, XVII, XXIII and XXV) in mouse brain. Using quantitative PCR and in situ hybridization (ISH), we demonstrate both region- and cell-specific expression of these unique collagens in the developing brain. For the two most highly expressed transmembrane collagens (i.e., collagen XXIII and XXV), we demonstrate that they are expressed by select subsets of neurons in different parts of the brain. For example, collagen XXIII is selectively expressed by excitatory neurons in the mitral/tufted cell layer of the accessory olfactory bulb (AOB) and by cells in the inner nuclear layer (INL) of the retina. On the other hand, collagen XXV, which is more broadly expressed, is generated by subsets of excitatory neurons in the dorsal thalamus and midbrain and by inhibitory neurons in the retina, ventral thalamus and telencephalon. Not only is col25a1 expression present in retina, it appears specifically enriched in retino-recipient nuclei within the brain (including the suprachiasmatic nucleus (SCN), lateral geniculate complex, olivary pretectal nucleus (OPN) and superior colliculus). Taken together, the distinct region- and cell-specific expression patterns of transmembrane collagens suggest that this family of unconventional collagens may play unique, yet-to-be identified roles in brain development and function.

Activity-dependent proteolytic cleavage of cell adhesion molecules regulates excitatory synaptic development and function

Activity-dependent remodeling of neuronal connections is critical to nervous system development and function. These processes rely on the ability of synapses to detect neuronal activity and translate it into the appropriate molecular signals. One way to convert neuronal activity into downstream signaling is the proteolytic cleavage of cell adhesion molecules (CAMs). Here we review studies demonstrating the mechanisms by which proteolytic processing of CAMs direct the structural and functional remodeling of excitatory glutamatergic synapses during development and plasticity. Specifically, we examine how extracellular proteolytic cleavage of CAMs switches on or off molecular signals to 1) permit, drive, or restrict synaptic maturation during development and 2) strengthen or weaken synapses during adult plasticity. We will also examine emerging studies linking improper activity-dependent proteolytic processing of CAMs to neurological disorders such as schizophrenia, brain tumors, and Alzheimer's disease. Together these findings suggest that the regulation of activity-dependent proteolytic cleavage of CAMs is vital to proper brain development and lifelong function.

Loss of Interneuron-Derived Collagen XIX Leads to a Reduction in Perineuronal Nets in the Mammalian Telencephalon

Perineuronal nets (PNNs) are lattice-like supramolecular assemblies of extracellular glycoproteins that surround subsets of neuronal cell bodies in the mammalian telencephalon. PNNs emerge at the end of the critical period of brain development, limit neuronal plasticity in the adult brain, and are lost in a variety of complex brain disorders diseases, including schizophrenia. The link between PNNs and schizophrenia led us to question whether neuronally expressed extracellular matrix (ECM) molecules associated with schizophrenia contribute to the assembly of these specialized supramolecular ECM assemblies. We focused on collagen XIX-a minor, nonfibrillar collagen expressed by subsets of telencephalic interneurons. Genetic alterations in the region encoding collagen XIX have been associated with familial schizophrenia, and loss of this collagen in mice results in altered inhibitory synapses, seizures, and the acquisition of schizophrenia-related behaviors. Here, we demonstrate that loss of collagen XIX also results in a reduction of telencephalic PNNs. Loss of PNNs was accompanied with reduced levels of aggrecan (Acan), a major component of PNNs. Despite reduced levels of PNN constituents in collagen XIX-deficient mice ( col19a1-/-), we failed to detect reduced expression of genes encoding these ECM molecules. Instead, we discovered a widespread upregulation of extracellular proteases capable of cleaving Acan and other PNN constituents in col19a1-/- brains. Taken together, these results suggest a mechanism by which the loss of collagen XIX speeds PNN degradation and they identify a novel mechanism by which the loss of collagen XIX may contribute to complex brain disorders.

The anti-tumor NC1 domain of collagen XIX inhibits the FAK/ PI3K/Akt/mTOR signaling pathway through αvβ3 integrin interaction

Type XIX collagen is a minor collagen associated with basement membranes. It was isolated for the first time in a human cDNA library from rhabdomyosarcoma and belongs to the FACITs family (Fibril Associated Collagens with Interrupted Triple Helices). Previously, we demonstrated that the NC1 domain of collagen XIX (NC1(XIX)) exerts anti-tumor properties on melanoma cells by inhibiting their migration and invasion. In the present work, we identified for the first time the integrin αvβ3 as a receptor of NC1(XIX). Moreover, we demonstrated that NC1(XIX) inhibits the FAK/PI3K/Akt/mTOR pathway, by decreasing the phosphorylation and activity of the major proteins involved in this pathway. On the other hand, NC1(XIX) induced an increase of GSK3β activity by decreasing its degree of phosphorylation. Treatments targeting this central signaling pathway in the development of melanoma are promising and new molecules should be developed. NC1(XIX) seems to have the potential for the design of new anti-cancer drugs.

Collagen XVIII in tissue homeostasis and dysregulation - Lessons learned from model organisms and human patients

Collagen XVIII is a ubiquitous basement membrane (BM) proteoglycan produced in three tissue-specific isoforms that differ in their N-terminal non-collagenous sequences, but share collagenous and C-terminal non-collagenous domains. The collagenous domain provides flexibility to the large collagen XVIII molecules on account of multiple interruptions in collagenous sequences. Each isoform has a complex multi-domain structure that endows it with an ability to perform various biological functions. The long isoform contains a frizzled-like (Fz) domain with Wnt-inhibiting activity and a unique domain of unknown function (DUF959), which is also present in the medium isoform. All three isoforms share an N-terminal laminin-G-like/thrombospondin-1 sequence whose specific functions still remain unconfirmed. The proteoglycan nature of the isoforms further increases the functional diversity of collagen XVIII. An anti-angiogenic domain termed endostatin resides in the C-terminus of collagen XVIII and is proteolytically cleaved from the parental molecule during the BM breakdown for example in the process of tumour progression. Recombinant endostatin can efficiently reduce tumour angiogenesis and growth in experimental models by inhibiting endothelial cell migration and proliferation or by inducing their death, but its efficacy against human cancers is still a subject of debate. Mutations in the COL18A1 gene result in Knobloch syndrome, a genetic disorder characterised mainly by severe eye defects and encephalocele and, occasionally, other symptoms. Studies with gene-modified mice have elucidated some aspects of this rare disease, highlighting in particular the importance of collagen XVIII in the development of the eye. Research with model organisms have also helped in determining other structural and biological functions of collagen XVIII, such as its requirement in the maintenance of BM integrity and its emerging roles in regulating cell survival, stem or progenitor cell maintenance and differentiation and inflammation. In this review, we summarise current knowledge on the properties and endogenous functions of collagen XVIII in normal situations and tissue dysregulation. When data is available, we discuss the functions of the distinct isoforms and their specific domains.

Regional Regulation of Purkinje Cell Dendritic Spines by Integrins and Eph/Ephrins

Climbing fibres and parallel fibres compete for dendritic space on Purkinje cells in the cerebellum. Normally, climbing fibres populate the proximal dendrites, where they suppress the multiple small spines typical of parallel fibres, leading to their replacement by the few large spines that contact climbing fibres. Previous work has shown that ephrins acting via EphA4 are a signal for this change in spine type and density. We have used an in vitro culture model in which to investigate the ephrin effect on Purkinje cell dendritic spines and the role of integrins in these changes. We found that integrins α3, α5 and β4 are present in many of the dendritic spines of cultured Purkinje cells. pFAK, the main downstream signalling molecule from integrins, has a similar distribution, although the intenstity of pFAK staining and the percentage of pFAK+ spines was consistently higher in the proximal dendrites. Activating integrins with Mg2+ led to an increase in the intensity of pFAK staining and an increase in the proportion of pFAK+ spines in both the proximal and distal dendrites, but no change in spine length, density or morphology. Blocking integrin binding with an RGD-containing peptide led to a reduction in spine length, with more stubby spines on both proximal and distal dendrites. Treatment of the cultures with ephrinA3-Fc chimera suppressed dendritic spines specifically on the proximal dendrites and there was also a decrease of pFAK in spines on this domain. This effect was blocked by simultaneous activation of integrins with Mn2+. We conclude that Eph/ephrin signaling regulates proximal dendritic spines in Purkinje cells by inactivating integrin downstream signalling.

Collagen-derived matricryptins promote inhibitory nerve terminal formation in the developing neocortex

Inhibitory synapses comprise only ∼20% of the total synapses in the mammalian brain but play essential roles in controlling neuronal activity. In fact, perturbing inhibitory synapses is associated with complex brain disorders, such as schizophrenia and epilepsy. Although many types of inhibitory synapses exist, these disorders have been strongly linked to defects in inhibitory synapses formed by Parvalbumin-expressing interneurons. Here, we discovered a novel role for an unconventional collagen-collagen XIX-in the formation of Parvalbumin(+) inhibitory synapses. Loss of this collagen results not only in decreased inhibitory synapse number, but also in the acquisition of schizophrenia-related behaviors. Mechanistically, these studies reveal that a proteolytically released fragment of this collagen, termed a matricryptin, promotes the assembly of inhibitory nerve terminals through integrin receptors. Collectively, these studies not only identify roles for collagen-derived matricryptins in cortical circuit formation, but they also reveal a novel paracrine mechanism that regulates the assembly of these synapses.

Matricryptins Network with Matricellular Receptors at the Surface of Endothelial and Tumor Cells

The extracellular matrix (ECM) is a source of bioactive fragments called matricryptins or matrikines resulting from the proteolytic cleavage of extracellular proteins (e.g., collagens, elastin, and laminins) and proteoglycans (e.g., perlecan). Matrix metalloproteinases (MMPs), cathepsins, and bone-morphogenetic protein-1 release fragments, which regulate physiopathological processes including tumor growth, metastasis, and angiogenesis, a pre-requisite for tumor growth. A number of matricryptins, and/or synthetic peptides derived from them, are currently investigated as potential anti-cancer drugs both in vitro and in animal models. Modifications aiming at improving their efficiency and their delivery to their target cells are studied. However, their use as drugs is not straightforward. The biological activities of these fragments are mediated by several receptor families. Several matricryptins may bind to the same matricellular receptor, and a single matricryptin may bind to two different receptors belonging or not to the same family such as integrins and growth factor receptors. Furthermore, some matricryptins interact with each other, integrins and growth factor receptors crosstalk and a signaling pathway may be regulated by several matricryptins. This forms an intricate 3D interaction network at the surface of tumor and endothelial cells, which is tightly associated with other cell-surface associated molecules such as heparan sulfate, caveolin, and nucleolin. Deciphering the molecular mechanisms underlying the behavior of this network is required in order to optimize the development of matricryptins as anti-cancer agents.

Endostatin's emerging roles in angiogenesis, lymphangiogenesis, disease, and clinical applications

BACKGROUND

Angiogenesis is the process of neovascularization from pre-existing vasculature and is involved in various physiological and pathological processes. Inhibitors of angiogenesis, administered either as individual drugs or in combination with other chemotherapy, have been shown to benefit patients with various cancers. Endostatin, a 20-kDa C-terminal fragment of type XVIII collagen, is one of the most potent inhibitors of angiogenesis.

SCOPE OF REVIEW

We discuss the biology behind endostatin in the context of its endogenous production, the various receptors to which it binds, and the mechanisms by which it acts. We focus on its inhibitory role in angiogenesis, lymphangiogenesis, and cancer metastasis. We also present emerging clinical applications for endostatin and its potential as a therapeutic agent in the form a short peptide.

MAJOR CONCLUSIONS

The delicate balance between pro- and anti-angiogenic factors can be modulated to result in physiological wound healing or pathological tumor metastasis. Research in the last decade has emphasized an emerging clinical potential for endostatin as a biomarker and as a therapeutic short peptide. Moreover, elevated or depressed endostatin levels in diseased states may help explain the pathophysiological mechanisms of the particular disease.

GENERAL SIGNIFICANCE

Endostatin was once sought after as the 'be all and end all' for cancer treatment; however, research throughout the last decade has made it apparent that endostatin's effects are complex and involve multiple mechanisms. A better understanding of newly discovered mechanisms and clinical applications still has the potential to lead to future advances in the use of endostatin in the clinic.

Sex differences in protein expression in the mouse brain and their perturbations in a model of Down syndrome

Background

While many sex differences in structure and function of the mammalian brain have been described, the molecular correlates of these differences are not broadly known. Also unknown is how sex differences at the protein level are perturbed by mutations that lead to intellectual disability (ID). Down syndrome (DS) is the most common genetic cause of ID and is due to trisomy of human chromosome 21 (Hsa21) and the resulting increased expression of Hsa21-encoded genes. The Dp(10)1Yey mouse model (Dp10) of DS is trisomic for orthologs of 39 Hsa21 protein-coding genes that map to mouse chromosome 10 (Mmu10), including four genes with known sex differences in functional properties. How these genes contribute to the DS cognitive phenotype is not known.

Methods

Using reverse phase protein arrays, levels of ~100 proteins/protein modifications were measured in the hippocampus, cerebellum, and cortex of female and male controls and their trisomic Dp10 littermates. Proteins were chosen for their known roles in learning/memory and synaptic plasticity and include components of the MAPK, MTOR, and apoptosis pathways, immediate early genes, and subunits of ionotropic glutamate receptors. Protein levels were compared between genotypes, sexes, and brain regions using a three-level mixed effects model and the Benjamini-Hochberg correction for multiple testing.

Results

In control mice, levels of approximately one half of the proteins differ significantly between females and males in at least one brain region; in the hippocampus alone, levels of 40 % of the proteins are significantly higher in females. Trisomy of the Mmu10 segment differentially affects female and male profiles, perturbing protein levels most in the cerebellum of female Dp10 and most in the hippocampus of male Dp10. Cortex is minimally affected by sex and genotype. Diverse pathways and processes are implicated in both sex and genotype differences.

Conclusions

The extensive sex differences in control mice in levels of proteins involved in learning/memory illustrate the molecular complexity underlying sex differences in normal neurological processes. The sex-specific abnormalities in the Dp10 suggest the possibility of sex-specific phenotypic features in DS and reinforce the need to use female as well as male mice, in particular in preclinical evaluations of drug responses.

Electronic supplementary material

The online version of this article (doi:10.1186/s13293-015-0043-9) contains supplementary material, which is available to authorized users.

Proteases decode the extracellular matrix cryptome

The extracellular matrix is comprised of 1100 core-matrisome and matrisome-associated proteins and of glycosaminoglycans. This structural scaffold contributes to the organization and mechanical properties of tissues and modulates cell behavior. The extracellular matrix is dynamic and undergoes constant remodeling, which leads to diseases if uncontrolled. Bioactive fragments, called matricryptins, are released from the extracellular proteins by limited proteolysis and have biological activities on their own. They regulate numerous physiological and pathological processes such as angiogenesis, cancer, diabetes, wound healing, fibrosis and infectious diseases and either improve or worsen the course of diseases depending on the matricryptins and on the molecular and biological contexts. Several protease families release matricryptins from core-matrisome and matrisome-associated proteins both in vitro and in vivo. The major proteases, which decrypt the extracellular matrix, are zinc metalloproteinases of the metzincin superfamily (matrixins, adamalysins and astacins), cysteine proteinases and serine proteases. Some matricryptins act as enzyme inhibitors, further connecting protease and matricryptin fates and providing intricate regulation of major physiopathological processes such as angiogenesis and tumorigenesis. They strengthen the role of the extracellular matrix as a key player in tissue failure and core-matrisome and matrisome-associated proteins as important therapeutic targets.

Cell-specific and developmental expression of lectican-cleaving proteases in mouse hippocampus and neocortex

Mounting evidence has demonstrated that a specialized extracellular matrix exists in the mammalian brain and that this glycoprotein-rich matrix contributes to many aspects of brain development and function. The most prominent supramolecular assemblies of these extracellular matrix glycoproteins are perineuronal nets, specialized lattice-like structures that surround the cell bodies and proximal neurites of select classes of interneurons. Perineuronal nets are composed of lecticans, a family of chondroitin sulfate proteoglycans that includes aggrecan, brevican, neurocan, and versican. These lattice-like structures emerge late in postnatal brain development, coinciding with the ending of critical periods of brain development. Despite our knowledge of the presence of lecticans in perineuronal nets and their importance in regulating synaptic plasticity, we know little about the development or distribution of the extracellular proteases that are responsible for their cleavage and turnover. A subset of a large family of extracellular proteases (called a disintegrin and metalloproteinase with thrombospondin motifs [ADAMTS]) is responsible for endogenously cleaving lecticans. We therefore explored the expression pattern of two aggrecan-degrading ADAMTS family members, ADAMTS15 and ADAMTS4, in the hippocampus and neocortex. Here, we show that both lectican-degrading metalloproteases are present in these brain regions and that each exhibits a distinct temporal and spatial expression pattern. Adamts15 mRNA is expressed exclusively by parvalbumin-expressing interneurons during synaptogenesis, whereas Adamts4 mRNA is exclusively generated by telencephalic oligodendrocytes during myelination. Thus, ADAMTS15 and ADAMTS4 not only exhibit unique cellular expression patterns but their developmental upregulation by these cell types coincides with critical aspects of neural development.

Perlecan antagonizes collagen IV and ADAMTS9/GON-1 in restricting the growth of presynaptic boutons

In the mature nervous system, a significant fraction of synapses are structurally stable over a long time scale. However, the mechanisms that restrict synaptic growth within a confined region are poorly understood. Here, we identified that in the C. elegans neuromuscular junction, collagens Type IV and XVIII, and the secreted metalloprotease ADAMTS/GON-1 are critical for growth restriction of presynaptic boutons. Without these components, ectopic boutons progressively invade into the nonsynaptic region. Perlecan/UNC-52 promotes the growth of ectopic boutons and functions antagonistically to collagen Type IV and GON-1 but not to collagen XVIII. The growth constraint of presynaptic boutons correlates with the integrity of the extracellular matrix basal lamina or basement membrane (BM), which surrounds chemical synapses. Fragmented BM appears in the region where ectopic boutons emerge. Further removal of UNC-52 improves the BM integrity and the tight association between BM and presynaptic boutons. Together, our results unravel the complex role of the BM in restricting the growth of presynaptic boutons and reveal the antagonistic function of perlecan on Type IV collagen and ADAMTS protein.

Nuclei-specific differences in nerve terminal distribution, morphology, and development in mouse visual thalamus

Background

Mouse visual thalamus has emerged as a powerful model for understanding the mechanisms underlying neural circuit formation and function. Three distinct nuclei within mouse thalamus receive retinal input, the dorsal lateral geniculate nucleus (dLGN), the ventral lateral geniculate nucleus (vLGN), and the intergeniculate nucleus (IGL). However, in each of these nuclei, retinal inputs are vastly outnumbered by nonretinal inputs that arise from cortical and subcortical sources. Although retinal and nonretinal terminals associated within dLGN circuitry have been well characterized, we know little about nerve terminal organization, distribution and development in other nuclei of mouse visual thalamus.

Results

Immunolabeling specific subsets of synapses with antibodies against vesicle-associated neurotransmitter transporters or neurotransmitter synthesizing enzymes revealed significant differences in the composition, distribution and morphology of nonretinal terminals in dLGN, vLGN and IGL. For example, inhibitory terminals are more densely packed in vLGN, and cortical terminals are more densely distributed in dLGN. Overall, synaptic terminal density appears least dense in IGL. Similar nuclei-specific differences were observed for retinal terminals using immunolabeling, genetic labeling, axonal tracing and serial block face scanning electron microscopy: retinal terminals are smaller, less morphologically complex, and more densely distributed in vLGN than in dLGN. Since glutamatergic terminal size often correlates with synaptic function, we used in vitro whole cell recordings and optic tract stimulation in acutely prepared thalamic slices to reveal that excitatory postsynaptic currents (EPSCs) are considerably smaller in vLGN and show distinct responses following paired stimuli. Finally, anterograde labeling of retinal terminals throughout early postnatal development revealed that anatomical differences in retinal nerve terminal structure are not observable as synapses initially formed, but rather developed as retinogeniculate circuits mature.

Conclusions

Taken together, these results reveal nuclei-specific differences in nerve terminal composition, distribution, and morphology in mouse visual thalamus. These results raise intriguing questions about the different functions of these nuclei in processing light-derived information, as well as differences in the mechanisms that underlie their unique, nuclei-specific development.

Neural ECM and synaptogenesis

Chemical synapses allow neurons to perform complex computations and regulate other systems of the body. At a chemical synapse, pre- and postsynaptic sites are separated by a small space (the synaptic cleft) and surrounded by astrocytes. The basement membrane (BM), a sheetlike, specialized extracellular matrix (ECM), is found ubiquitously in the PNS. It has become clear that the ECMs not only play a structural role but also serve as barriers and filters in the PNS and CNS. Moreover, proteoglycans and tenascin family proteins in the ECM regulate synapse formation and synaptic plasticity. Although CNS synapses lack the BMs, recent results indicate that the BM-associated collagens are also present in the CNS synaptic cleft and affect synaptogenesis in both the CNS and the PNS. The C1q domain-containing family proteins are important components of the CNS synaptic cleft in regulating synapse formation, maintenance, and the pruning process. The ECM is regarded as a crucial component of the tetrapartite synapse, consisting of pre- and postsynaptic neurons, astrocyte, and ECM.

Contributions of VLDLR and LRP8 in the establishment of retinogeniculate projections

BACKGROUND

Retinal ganglion cells (RGCs), the output neurons of the retina, project to over 20 distinct brain nuclei, including the lateral geniculate nucleus (LGN), a thalamic region comprised of three functionally distinct subnuclei: the ventral LGN (vLGN), the dorsal LGN (dLGN) and the intergeniculate leaflet (IGL). We previously identified reelin, an extracellular glycoprotein, as a critical factor that directs class-specific targeting of these subnuclei. Reelin is known to bind to two receptors: very-low-density lipoprotein receptor (VLDLR) and low-density lipoprotein receptor-related protein 8 (LRP8), also known as apolipoprotein E receptor 2 (ApoER2). Here we examined the roles of these canonical reelin receptors in retinogeniculate targeting.

RESULTS

To assess the roles of VLDLR and LRP8 in retinogeniculate targeting, we used intraocular injections of fluorescently conjugated cholera toxin B subunit (CTB) to label all RGC axons in vivo. Retinogeniculate projections in mutant mice lacking either VLDLR or LRP8 appeared similar to controls; however, deletion of both receptors resulted in dramatic defects in the pattern of retinal innervation in LGN. Surprisingly, defects in vldlr(-/-);lrp8(-/-) double mutant mice were remarkably different than those observed in mice lacking reelin. First, we failed to observe retinal axons exiting the medial border of the vLGN and IGL to invade distant regions of non-retino-recipient thalamus. Second, an ectopic region of binocular innervation emerged in the dorsomedial pole of vldlr(-/-);lrp8(-/-) mutant dLGN. Analysis of retinal projection development, retinal terminal sizes and LGN cytoarchitecture in vldlr(-/-);lrp8(-/-) mutants, all suggest that a subset of retinal axons destined for the IGL are misrouted to the dorsomedial pole of dLGN in the absence of VLDLR and LRP8. Such mistargeting is likely the result of abnormal migration of IGL neurons into the dorsomedial pole of dLGN in vldlr(-/-);lrp8(-/-) mutants.

CONCLUSIONS

In contrast to our expectations, the development of both the LGN and retinogeniculate projections appeared dramatically different in mutants lacking either reelin or both canonical reelin receptors. These results suggest that there are reelin-independent functions of VLDLR and LRP8 in LGN development, and VLDLR- and LRP8-independent functions of reelin in class-specific axonal targeting.

Type XVIII collagen is essential for survival during acute liver injury in mice

The regenerative response to drug- and toxin-induced liver injury induces changes to the hepatic stroma, including the extracellular matrix. Although the extracellular matrix is known to undergo changes during the injury response, its impact on maintaining hepatocyte function and viability in this process remains largely unknown. We demonstrate that recovery from toxin-mediated injury is impaired in mice deficient in a key liver extracellular matrix molecule, type XVIII collagen, and results in rapid death. The type-XVIII-collagen-dependent response to liver injury is mediated by survival signals induced by α1β1 integrin, integrin linked kinase and the Akt pathway, and mice deficient in either α1β1 integrin or hepatocyte integrin linked kinase also succumb to toxic liver injury. These findings demonstrate that type XVIII collagen is an important functional component of the liver matrix microenvironment and is crucial for hepatocyte survival during injury and stress.