Localization of the extracellular matrix protein SC1 to synapses in the adult rat brain

Glioma-derived SPARCL1 promotes the formation of peritumoral neuron-glioma synapses

PURPOSE

Gliomas are the most common type of primary malignant brain tumor with high degree of malignancy and rapid progression, and patients often have complications such as epilepsy and cognitive impairment. Thus, identifying related therapeutic targets, prolonging patient survival time and improving patient quality of life are urgently needed. Recent studies have shown that glutamatergic neurons around tumors and glioma cells form synapses, neuron-glioma synapses (NGSs), which have electrophysiological properties and participate in the proliferation, infiltration and invasion of tumors. Therefore, we aimed to explore the molecular mechanisms underlying NGS formation.

METHODS

We used bioinformatic analysis to screen for the expression of SPARCL1, which may play a role in promoting NGS formation, and we evaluated clinical samples through immunofluorescence, Western blot, and reverse transcriptase polymerase chain reaction (RT‒PCR) assays to validate the bioinformatic analysis results. Vitro neuron-glioma cell coculture model was established and allowed us to edit SPARCL1 expression in glioma cells, further allowing us to investigate the role of SPARCL1 in NGS formation.

RESULTS

Bioinformatic analysis revealed that SPARCL1 is highly expressed in glioma cells and is associated with synaptogenesis. Clinical samples were evaluated to verify the bioinformatics results, and SPARCL1 was found to be highly distributed in the tumor peripheral region. In the vitro neuron-glioma cell coculture model, NGSs were clearly observed, and SPARCL1 overexpression promoted NGS formation.

CONCLUSION

Taken together, these findings suggest that SPARCL1 is one of the molecules that promotes NGS formation in the tumor peripheral region.

Astrocytes Control Cocaine-Induced Synaptic Plasticity and Reward Through the Matricellular Protein Hevin

BACKGROUND

Astrocytes in the nucleus accumbens (NAc) play a dynamic role in regulating synaptic plasticity induced by drugs of abuse through modulation of glutamatergic neurotransmission. Astrocyte-secreted factors may also contribute to the reprogramming of brain circuitry that leads to drug-seeking behavior. Here, we investigated the role of astrocyte Ca2+ signals in vivo and of the astrocyte-secreted protein hevin in the rewarding properties of cocaine.

METHODS

Ca2+ signals in NAc astrocytes were measured by in vivo fiber photometry during conditioned place preference (CPP) to cocaine. Depletion of Ca2+ and chemogenetic activation were used to evaluate the contribution of astrocyte Ca2+ signals to cocaine CPP. The effects of cocaine in hevin-null mice and after hevin knockdown in NAc astrocytes were evaluated by imaging of medium spiny neuron spines, electrophysiology, and CPP. Hevin secretion was monitored by light-sheet imaging in brain slices.

RESULTS

Cocaine increased the amplitude of Ca2+ signals in astrocytes during conditioning. Attenuating Ca2+ signals in astrocytes prevented cocaine CPP, whereas augmenting these signals potentiated this conditioning. Astrocyte activation induced a surge in hevin secretion ex vivo. Hevin knockdown in NAc astrocytes led to a decrease in CPP and in structural and synaptic plasticity induced by cocaine in medium spiny neurons.

CONCLUSIONS

These findings reveal a fine-tuning by cocaine of in vivo Ca2+ signals in NAc astrocytes. Astrocyte Ca2+ signals are sufficient and necessary for the acquisition of cocaine-seeking behavior. Hevin can be released upon astrocyte activation and is a major effector of the action of cocaine and Ca2+ signals on reward and neuronal plasticity.

Microglia and Astrocytes in Postnatal Neural Circuit Formation

Over the past two decades, microglia and astrocytes have emerged as critical mediators of neural circuit formation. Particularly during the postnatal period, both glial subtypes play essential roles in orchestrating nervous system development through communication with neurons. These functions include regulating synapse elimination, modulating neuronal density and activity, mediating synaptogenesis, facilitating axon guidance and organization, and actively promoting neuronal survival. Despite the vital roles of both microglia and astrocytes in ensuring homeostatic brain development, the extent to which the postnatal functions of these cells are regulated by sex and the manner in which these glial cells communicate with one another to coordinate nervous system development remain less well understood. Here, we review the critical functions of both microglia and astrocytes independently and synergistically in mediating neural circuit formation, focusing our exploration on the postnatal period from birth to early adulthood.

Astrocytes regulate neuronal network activity by mediating synapse remodeling

Based on experience during our life, neuronal connectivity continuously changes through structural remodeling of synapses. Recent studies have shown that the complex interaction between astrocytes and synapses regulates structural synapse remodeling by inducing the formation and elimination of synapses, as well as their functional maturation. Defects in this astrocyte-mediated synapse remodeling cause problems in not only neuronal network activities but also animal behaviors. Moreover, in various neurological disorders, astrocytes have been shown to play central roles in the initiation and progression of synaptic pathophysiology through impaired interactions with synapses. In this review, we will discuss recent studies identifying the novel roles of astrocytes in neuronal circuit remodeling, focusing on synapse formation and elimination. We will also discuss the potential implication of defective astrocytic function in evoking various brain disorders.

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.

Cholecystokinin octapeptide improves hippocampal glutamatergic synaptogenesis and postoperative cognition by inhibiting induction of A1 reactive astrocytes in aged mice

Aims

Delayed neurocognitive recovery (dNCR) is a common postoperative complication in geriatric surgical patients for which there is no efficacious therapy. Cholecystokinin octapeptide (CCK‐8), an immunomodulatory peptide, regulates memory and learning. Here, we explored the effects and mechanism of action of CCK‐8 on dNCR.

Methods

We applied laparotomy to establish a model of dNCR in aged mice. Morris water maze and fear conditioning tests were used to evaluate cognition. Immunofluorescence was used to detect the density of CCK‐8, A1 reactive astrocytes, glutamatergic synapses, and activation of microglia in the hippocampus. Quantitative PCR was performed to determine mRNA levels of synapse‐associated factors. A1 reactive astrocytes, activated microglia, and glutamatergic synapse‐associated protein levels in the hippocampus were assessed by western blotting.

Results

Administration of CCK‐8 suppressed the activation of microglia, the induction of A1 reactive astrocytes, and the expression of tumor necrosis factor alpha, complement 1q, and interleukin 1 alpha in the hippocampus. Furthermore, it promoted glutamatergic synaptogenesis and neurocognitive recovery in aged dNCR model mice.

Conclusion

Our findings indicated that CCK‐8 alleviated cognitive impairment and promoted glutamatergic synaptogenesis by inhibiting the induction of A1 reactive astrocytes and the activation of microglia. CCK‐8 is, therefore, a potential therapeutic target for dNCR.

Characterization of Hevin (SPARCL1) Immunoreactivity in Postmortem Human Brain Homogenates

Hevin is a matricellular glycoprotein that plays important roles in neural developmental processes such as neuronal migration, synaptogenesis and synaptic plasticity. In contrast to other matricellular proteins whose expression decreases when development is complete, hevin remains highly expressed, suggesting its involvement in adult brain function. In vitro studies have shown that hevin can have different post-translational modifications. However, the glycosylation pattern of hevin in the human brain remains unknown, as well as its relative distribution and localization. The present study provides the first thorough characterization of hevin protein expression by Western blot in postmortem adult human brain. Our results demonstrated two major specific immunoreactive bands for hevin: an intense band migrating around 130 kDa, and a band migrating around 100 kDa. Biochemical assays revealed that both hevin bands have a different glycosylation pattern. Subcellular fractionation showed greater expression in membrane-enriched fraction than in cytosolic preparation, and a higher expression in prefrontal cortex (PFC) compared to hippocampus (HIP), caudate nucleus (CAU) and cerebellum (CB). We confirmed that a disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS4) and matrixmetalloproteinase 3 (MMP-3) proteases digestion led to an intense double band with similar molecular weight to that described as SPARC-like fragment (SLF). Finally, hevin immunoreactivity was also detected in human astrocytoma, meningioma, cerebrospinal fluid and serum samples, but was absent from any blood cell type.

Interplay between hevin, SPARC, and MDGAs: Modulators of neurexin-neuroligin transsynaptic bridges

Hevin is secreted by astrocytes and its synaptogenic effects are antagonized by the related protein, SPARC. Hevin stabilizes neurexin-neuroligin transsynaptic bridges in vivo. A third protein, membrane-tethered MDGA, blocks these bridges. Here, we reveal the molecular underpinnings of a regulatory network formed by this trio of proteins. The hevin FS-EC structure differs from SPARC, in that the EC domain appears rearranged around a conserved core. The FS domain is structurally conserved and it houses nanomolar affinity binding sites for neurexin and neuroligin. SPARC also binds neurexin and neuroligin, competing with hevin, so its antagonist action is rooted in its shortened N-terminal region. Strikingly, the hevin FS domain competes with MDGA for an overlapping binding site on neuroligin, while the hevin EC domain binds the extracellular matrix protein collagen (like SPARC), so that this trio of proteins can regulate neurexin-neuroligin transsynaptic bridges and also extracellular matrix interactions, impacting synapse formation and ultimately neural circuits.

Role of astrocytes in synapse formation and maturation

Astrocytes are the most abundant glial cells in the mammalian brain and directly participate in the proper functioning of the nervous system by regulating ion homeostasis, controlling glutamate reuptake, and maintaining the blood-brain barrier. In the last two decades, a growing body of work also identified critical roles for astrocytes in regulating synaptic connectivity. Stemming from the observation that functional and morphological development of astrocytes occur concurrently with synapse formation and maturation, these studies revealed that both developmental processes are directly linked. In fact, astrocytes both physically contact numerous synaptic structures and actively instruct many aspects of synaptic development and function via a plethora of secreted and adhesion-based molecular signals. The complex astrocyte-to-neuron signaling modalities control different stages of synaptic development such as regulating the initial formation of structural synapses as well as their functional maturation. Furthermore, the synapse-modulating functions of astrocytes are evolutionarily conserved and contribute to the development and plasticity of diverse classes of synapses and circuits throughout the central nervous system. Importantly, because impaired synapse formation and function is a hallmark of many neurodevelopmental disorders, deficits in astrocytes are likely to be major contributors to disease pathogenesis. In this chapter, we review our current understanding of the cellular and molecular mechanisms by which astrocytes contribute to synapse development and discuss the bidirectional secretion-based and contact-mediated mechanisms responsible for these essential developmental processes.

SPARCL1 Promotes Excitatory But Not Inhibitory Synapse Formation and Function Independent of Neurexins and Neuroligins

Emerging evidence supports roles for secreted extracellular matrix proteins in boosting synaptogenesis, synaptic transmission, and synaptic plasticity. SPARCL1 (also known as Hevin), a secreted non-neuronal protein, was reported to increase synaptogenesis by simultaneously binding to presynaptic neurexin-1α and to postsynaptic neuroligin-1B, thereby catalyzing formation of trans-synaptic neurexin/neuroligin complexes. However, neurexins and neuroligins do not themselves mediate synaptogenesis, raising the question of how SPARCL1 enhances synapse formation by binding to these molecules. Moreover, it remained unclear whether SPARCL1 acts on all synapses containing neurexins and neuroligins or only on a subset of synapses, and whether it enhances synaptic transmission in addition to boosting synaptogenesis or induces silent synapses. To explore these questions, we examined the synaptic effects of SPARCL1 and their dependence on neurexins and neuroligins. Using mixed neuronal and glial cultures from neonatal mouse cortex of both sexes, we show that SPARCL1 selectively increases excitatory but not inhibitory synapse numbers, enhances excitatory but not inhibitory synaptic transmission, and augments NMDAR-mediated synaptic responses more than AMPAR-mediated synaptic responses. None of these effects were mediated by SPARCL1-binding to neurexins or neuroligins. Neurons from triple neurexin-1/2/3 or from quadruple neuroligin-1/2/3/4 conditional KO mice that lacked all neurexins or all neuroligins were fully responsive to SPARCL1. Together, our results reveal that SPARCL1 selectively boosts excitatory but not inhibitory synaptogenesis and synaptic transmission by a novel mechanism that is independent of neurexins and neuroligins.SIGNIFICANCE STATEMENT Emerging evidence supports roles for extracellular matrix proteins in boosting synapse formation and function. Previous studies demonstrated that SPARCL1, a secreted non-neuronal protein, promotes synapse formation in rodent and human neurons. However, it remained unclear whether SPARCL1 acts on all or on only a subset of synapses, induces functional or largely inactive synapses, and generates synapses by bridging presynaptic neurexins and postsynaptic neuroligins. Here, we report that SPARCL1 selectively induces excitatory synapses, increases their efficacy, and enhances their NMDAR content. Moreover, using rigorous genetic manipulations, we show that SPARCL1 does not require neurexins and neuroligins for its activity. Thus, SPARCL1 selectively boosts excitatory synaptogenesis and synaptic transmission by a novel mechanism that is independent of neurexins and neuroligins.

Spinal hevin mediates membrane trafficking of GluA1-containing AMPA receptors in remifentanil-induced postoperative hyperalgesia in mice

INTRODUCTION

Hevin, a matricellular protein involved in tissue repair and remodeling, is crucial for initiation and development of excitatory synapses. Besides, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA) is an ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system (CNS). This study aimed to investigate the correlation between spinal Hevin and AMPA receptors in remifentanil-induced postoperative hyperalgesia in mice.

METHODS

Remifentanil (1.33 μg/kg/min for 60 min) was subcutaneously injected into a mouse model of postoperative pain. The von Frey and hot plate tests were performed to assess mechanical and thermal hyperalgesia. The gene and protein expression of Hevin and the membrane trafficking of GluA1-containing AMPA receptors in the dorsal horn of spinal cord were detected by quantitative reverse transcription polymerase chain reaction (RT-qPCR) and Western blot analysis. In addition, Hevin-shRNA, exogenous Hevin, and 1-naphtylacetyl-spermine (NASPM) were administrated intrathecally to assess the relationship between spinal Hevin and AMPA receptors.

RESULTS

Perioperative administration of remifentanil can aggravate and prolong incision-induced mechanical and thermal hyperalgesia. Treatment with remifentanil increased the expression of spinal Hevin and the membrane trafficking of AMPA receptors. Additionally, knockdown of spinal Hevin attenuated pain hypersensitivity and downregulated membrane trafficking of AMPA receptors after treatment with remifentanil. Meanwhile, preadministration of NASPM reversed spontaneous pain and membrane trafficking of spinal GluA1-containing AMPA receptors induced by exogenous Hevin in naïve mice.

CONCLUSIONS

Spinal Hevin was involved in the maintenance of remifentanil-induced postoperative hyperalgesia via modulating membrane trafficking of AMPA receptors.

The effects of peripherally-subacute treatment with irisin on hippocampal dendritogenesis and astrocyte-secreted factors

[Purpose]

Fibronectin type III domain containing 5 (FNDC5)/irisin is an exercise-induced myokine, which contributes to cognitive functions. However, the relationship between the neuroprotective effects of FNDC5/irisin and hippocampal dendritic remodeling and astrocyte-secreted factors remains unclear. Therefore, we explored whether subchronic recombinant irisin treatment affected hippocampal morphology and some astrocyte-derived molecules.

[Methods]

Mice were intraperitoneally injected with irisin (0.5 μg/kg/day) for seven days, followed by their sacrifice two days later. Hippocampal morphometric parameters were analyzed and pgc-1a, fndc5, bdnf, and some astrocyte-derived factors mRNA levels were measured.

[Results]

Dendritic length, arborization, and spine density were enhanced by irisin regimen in hippocampal CA1 and CA3 areas. Hippocampal pgc-1a, fndc5, and bdnf mRNA levels were significantly increased by irisin treatment. Moreover, hevin mRNA levels were significantly enhanced, whereas tgf-b1 levels downregulated by irisin treatment.

[Conclusion]

FNDC5/irisin has dendritogenic activity probably through hevin induction and TGF-β1 suppression.

Olfactory ensheathing cells facilitate neurite sprouting and outgrowth by secreting high levels of hevin

Transplantation of olfactory ensheathing cells (OECs) has been shown to enhance synapse formation. However, the mechanisms underlying this effect are not completely understood. We performed profiling of the OEC and astrocyte secretomes via a proteomics approach, in case hevin secreted by astrocytes might be involved in the formation of synapses. Semi-quantitative proteomic analysis revealed that 25 proteins were highly expressed, and 22 were weakly expressed in OEC conditioned medium compared with astrocyte conditioned medium. These molecules are highly associated with neural differentiation and regeneration, enzyme regulatory activity, and growth factor binding. The quantification data of clusterin, fibronectin, hevin, insulin-like growth factor binding protein 2 and secreted protein acidic and rich in cysteine were further confirmed by western blotting. Moreover, the addition of hevin in the culture medium improved neurite sprouting and outgrowth of differentiated neural stem cells. The greater expression of hevin in OEC conditioned medium than in astrocyte conditioned medium was associated with a greater capacity of synaptic formation. Thus, our results indicate that soluble factors secreted by OECs provide a permissive environment for nerve repair, and hevin is one of the key molecules facilitating neurite sprouting and outgrowth.

Shaping Synapses by the Neural Extracellular Matrix

Accumulating data support the importance of interactions between pre- and postsynaptic neuronal elements with astroglial processes and extracellular matrix (ECM) for formation and plasticity of chemical synapses, and thus validate the concept of a tetrapartite synapse. Here we outline the major mechanisms driving: (i) synaptogenesis by secreted extracellular scaffolding molecules, like thrombospondins (TSPs), neuronal pentraxins (NPs) and cerebellins, which respectively promote presynaptic, postsynaptic differentiation or both; (ii) maturation of synapses via reelin and integrin ligands-mediated signaling; and (iii) regulation of synaptic plasticity by ECM-dependent control of induction and consolidation of new synaptic configurations. Particularly, we focused on potential importance of activity-dependent concerted activation of multiple extracellular proteases, such as ADAMTS4/5/15, MMP9 and neurotrypsin, for permissive and instructive events in synaptic remodeling through localized degradation of perisynaptic ECM and generation of proteolytic fragments as inducers of synaptic plasticity.

Cell adhesion and matricellular support by astrocytes of the tripartite synapse

Astrocytes contribute to the formation, function, and plasticity of synapses. Their processes enwrap the neuronal components of the tripartite synapse, and due to this close interaction they are perfectly positioned to modulate neuronal communication. The interaction between astrocytes and synapses is facilitated by cell adhesion molecules and matricellular proteins, which have been implicated in the formation and functioning of tripartite synapses. The importance of such neuron-astrocyte integration at the synapse is underscored by the emerging role of astrocyte dysfunction in synaptic pathologies such as autism and schizophrenia. Here we review astrocyte-expressed cell adhesion molecules and matricellular molecules that play a role in integration of neurons and astrocytes within the tripartite synapse.

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.

SPARCL1-containing neurons in the human brainstem and sensory ganglion

Secreted protein, acidic and rich in cysteine-like 1 (SPARCL1) is a member of the osteonectin family of proteins. In this study, immunohistochemistry for SPARCL1 was performed to obtain its distribution in the human brainstem, cervical spinal cord, and sensory ganglion. SPARCL1-immunoreactivity was detected in neuronal cell bodies including perikarya and proximal dendrites, and the neuropil. The motor nuclei of the IIIrd, Vth, VIth, VIIth, IXth, Xth, XIth, and XIIth cranial nerves and spinal nerves contained many SPARCL1-immunoreactive (-IR) neurons with medium-sized to large cell bodies. Small and medium-sized SPARCL1-IR neurons were distributed in sensory nuclei of the Vth, VIIth, VIIIth, IXth, and Xth cranial nerves. In the medulla oblongata, the dorsal column nuclei also had small to medium-sized SPARCL1-IR neurons. In addition, SPARCL1-IR neurons were detected in the nucleus of the trapezoid body and pontine nucleus within the pons and the arcuate nucleus in the medulla oblongata. In the cervical spinal cord, the ventral horn contained some SPARCL1-IR neurons with large cell bodies. These findings suggest that SPARCL1-containing neurons function to relay and regulate motor and sensory signals in the human brainstem. In the dorsal root (DRG) and trigeminal ganglia (TG), primary sensory neurons contained SPARCL1-immunoreactivity. The proportion of SPARCL1-IR neurons in the TG (mean ± SD, 39.9 ± 2.4%) was higher than in the DRG (30.6 ± 2.1%). SPARCL1-IR neurons were mostly medium-sized to large (mean ± SD, 1494.5 ± 708.3 μm(2); range, 320.4-4353.4 μm(2)) in the DRG, whereas such neurons were of various cell body sizes in the TG (mean ± SD, 1291.2 ± 532.8 μm(2); range, 209.3-4326.4 μm(2)). There appears to be a SPARCL1-containing sensory pathway in the ganglion and brainstem of the spinal and trigeminal nervous systems.

SPARCL1 is a novel predictor of tumor recurrence and survival in hilar cholangiocarcinoma

Secreted protein acidic and rich in cysteines-like protein 1 (SPARCL1) has been implicated in tumor initiation, formation, and progression of various cancers, yet its role in hilar cholangiocarcinoma remains largely uncharacterized. In the present study, tissue microarrays containing resected hilar cholangiocarcinoma specimens from 92 patients were used to evaluate the expression of SPARCL1 protein by immunohistochemistry (IHC). In vitro assays were used to determine the effect of SPARCL1 overexpression on cell growth and migration. Loss of SPARCL1 expression was observed in 46 (50.0 %) of the 92 primary tumors. SPARCL1 expression is inversely associated with poorly or undifferentiation specimens (P = 0.030) in addition to lymph node metastasis (P = 0.047). Survival analysis demonstrated that SPARCL1 is an independent factor in predicting the outcome of patients with hilar cholangiocarcinoma. SPARCL1 overexpression suppressed tumor cell migration in vitro by inhibiting MMP-9, MMP-2, Vimentin, and Fibronectin expression, whereas did not inhibit cell proliferation in vitro. Our results suggest that loss of SPARCL1 is involved in the tumorigenesis of hilar cholangiocarcinoma and may serve as a novel molecular biomarker for patients' outcome.

Astrocytes refine cortical connectivity at dendritic spines

During cortical synaptic development, thalamic axons must establish synaptic connections despite the presence of the more abundant intracortical projections. How thalamocortical synapses are formed and maintained in this competitive environment is unknown. Here, we show that astrocyte-secreted protein hevin is required for normal thalamocortical synaptic connectivity in the mouse cortex. Absence of hevin results in a profound, long-lasting reduction in thalamocortical synapses accompanied by a transient increase in intracortical excitatory connections. Three-dimensional reconstructions of cortical neurons from serial section electron microscopy (ssEM) revealed that, during early postnatal development, dendritic spines often receive multiple excitatory inputs. Immuno-EM and confocal analyses revealed that majority of the spines with multiple excitatory contacts (SMECs) receive simultaneous thalamic and cortical inputs. Proportion of SMECs diminishes as the brain develops, but SMECs remain abundant in Hevin-null mice. These findings reveal that, through secretion of hevin, astrocytes control an important developmental synaptic refinement process at dendritic spines.

Mechanisms of otoconia and otolith development

BACKGROUND

Otoconia are bio-crystals that couple mechanic forces to the sensory hair cells in the utricle and saccule, a process essential for us to sense linear acceleration and gravity for the purpose of maintaining bodily balance. In fish, structurally similar bio-crystals called otoliths mediate both balance and hearing. Otoconia abnormalities are common and can cause vertigo and imbalance in humans. However, the molecular etiology of these illnesses is unknown, as investigators have only begun to identify genes important for otoconia formation in recent years.

RESULTS

To date, in-depth studies of selected mouse otoconial proteins have been performed, and about 75 zebrafish genes have been identified to be important for otolith development.

CONCLUSIONS

This review will summarize recent findings as well as compare otoconia and otolith development. It will provide an updated brief review of otoconial proteins along with an overview of the cells and cellular processes involved. While continued efforts are needed to thoroughly understand the molecular mechanisms underlying otoconia and otolith development, it is clear that the process involves a series of temporally and spatially specific events that are tightly coordinated by numerous proteins. Such knowledge will serve as the foundation to uncover the molecular causes of human otoconia-related disorders.

Mechanisms of otoconia and otolith development

BACKGROUND

Otoconia are bio-crystals that couple mechanic forces to the sensory hair cells in the utricle and saccule, a process essential for us to sense linear acceleration and gravity for the purpose of maintaining bodily balance. In fish, structurally similar bio-crystals called otoliths mediate both balance and hearing. Otoconia abnormalities are common and can cause vertigo and imbalance in humans. However, the molecular etiology of these illnesses is unknown, as investigators have only begun to identify genes important for otoconia formation in recent years.

RESULTS

To date, in-depth studies of selected mouse otoconial proteins have been performed, and about 75 zebrafish genes have been identified to be important for otolith development.

CONCLUSIONS

This review will summarize recent findings as well as compare otoconia and otolith development. It will provide an updated brief review of otoconial proteins along with an overview of the cells and cellular processes involved. While continued efforts are needed to thoroughly understand the molecular mechanisms underlying otoconia and otolith development, it is clear that the process involves a series of temporally and spatially specific events that are tightly coordinated by numerous proteins. Such knowledge will serve as the foundation to uncover the molecular causes of human otoconia-related disorders.

Differences in brain transcriptomes of closely related Baikal coregonid species

The aim of this work was to get deeper insight into genetic factors involved in the adaptive divergence of closely related species, specifically two representatives of Baikal coregonids—Baikal whitefish (Coregonus baicalensis Dybowski) and Baikal omul (Coregonus migratorius Georgi)—that diverged from a common ancestor as recently as 10–20 thousand years ago. Using the Serial Analysis of Gene Expression method, we obtained libraries of short representative cDNA sequences (tags) from the brains of Baikal whitefish and omul. A comparative analysis of the libraries revealed quantitative differences among ~4% tags of the fishes under study. Based on the similarity of these tags with cDNA of known organisms, we identified candidate genes taking part in adaptive divergence. The most important candidate genes related to the adaptation of Baikal whitefish and Baikal omul, identified in this work, belong to the genes of cell metabolism, nervous and immune systems, protein synthesis, and regulatory genes as well as to DTSsa4 Tc1-like transposons which are widespread among fishes.

SPARC-like 1 (SC1) is a diversely expressed and developmentally regulated matricellular protein that does not compensate for the absence of SPARC in the CNS

SPARC-like 1 (SC1) is a member of the SPARC family of matricellular proteins that has been implicated in the regulation of processes such as cell migration, proliferation, and differentiation. Here we show that SC1 exhibits remarkably diverse and dynamic expression in the developing and adult nervous system. During development, SC1 localizes to radial glia and pial-derived structures, including the vasculature, choroid plexus, and pial membranes. SC1 is not downregulated in postnatal development, but its expression shifts to distinct time windows in subtypes of glia and neurons, including astrocytes, large projection neurons, Bergmann glia, Schwann cells, and ganglionic satellite cells. In addition, SC1 expression levels and patterns are not altered in the SPARC null mouse, suggesting that SC1 does not compensate for the absence of SPARC. We conclude that SC1 and SPARC may share significant homology, but are likely to have distinct but complementary roles in nervous system development.

SC1/hevin identifies early white matter injury after ischemia and intracerebral hemorrhage in young and aged rats

The progression of white matter damage after ischemic and hemorrhagic strokes can exacerbate the initial injury, but little is known about the processes involved. We show that the antiadhesive matricellular glycoprotein SC1 is a novel early marker of white matter damage in 3 models of acute injury in the rat striatum: transient focal ischemia, intracerebral hemorrhage, and a needle penetration wound. SC1 was restricted to the damaged portions of axon bundles that bordered stroke lesions in young-adult and aged rats. SC1 peaked at 1 and 3 days after intracerebral hemorrhage and at 7 days after ischemia. The SC1-positive bundles usually expressed degraded myelin basic protein and amyloid precursor protein, a marker of axonal injury. At the hematoma edge, SC1 was seen in a few axon bundles that retained myelin basic protein staining. In these bundles, punctate SC1 staining filled individual axons, extended beyond a core of pan-axonal neurofilament and NF200 and was inside or overlapped with myelin basic protein staining when it was present. Aged rats had less SC1 (and amyloid precursor protein) after both types of stroke, suggesting a reduced axonal response. SC1 also labeled amyloid precursor protein-positive axon bundles along the needle penetration tract of saline-injected rats; thus, SC1 appears to characterize damaged striatal white matter damage after multiple types of injury.

SC1/hevin and reactive gliosis after transient ischemic stroke in young and aged rats

SC1 is a member of the SPARC family of glycoproteins that regulate cell-matrix interactions in the developing brain. SC1 is expressed in astrocytes, but nothing is known about the expression in the aged or after stroke. We found that after focal striatal ischemic infarction in adult rats, SC1 increased in astrocytes surrounding the infarct and in the glial scar, but in aged rats, SC1 was lower at the lesion edge. Glial fibrillary acidic protein (GFAP) also increased, but it was less prominent in reactive astrocytes further from the lesion in the aged rats. On the basis of their differential expression of several molecules, 2 types of reactive astrocytes with differing spatiotemporal distributions were identified. On Days 3 and 7, SC1 was prevalent in cells expressing markers of classic reactive astrocytes (GFAP, vimentin, nestin, S100β), as well as apoliprotein E (ApoE), interleukin 1β, aggrecanase 1 (ADAMTS4), and heat shock protein 25 (Hsp25). Adjacent to the lesion on Days 1 and 3, astrocytes with low GFAP levels and a "starburst" SC1 pattern expressed S100β, ApoE, and Hsp32 but not vimentin, nestin, interleukin 1β, ADAMTS4, or Hsp25. Neither cell type was immunoreactive for NG2,CC-1, CD11b, or ionized calcium-binding adapter-1. Their differing expression of inflammation-related and putatively protective molecules suggests different roles for starburst and classic reactive astrocytes in the early glial responses to ischemia.

Control of excitatory CNS synaptogenesis by astrocyte-secreted proteins Hevin and SPARC

Astrocytes regulate synaptic connectivity in the CNS through secreted signals. Here we identified two astrocyte-secreted proteins, hevin and SPARC, as regulators of excitatory synaptogenesis in vitro and in vivo. Hevin induces the formation of synapses between cultured rat retinal ganglion cells. SPARC is not synaptogenic, but specifically antagonizes synaptogenic function of hevin. Hevin and SPARC are expressed by astrocytes in the superior colliculus, the synaptic target of retinal ganglion cells, concurrent with the excitatory synaptogenesis. Hevin-null mice had fewer excitatory synapses; conversely, SPARC-null mice had increased synaptic connections in the superior colliculus. Furthermore, we found that hevin is required for the structural maturation of the retinocollicular synapses. These results identify hevin as a positive and SPARC as a negative regulator of synapse formation and signify that, through regulation of relative levels of hevin and SPARC, astrocytes might control the formation, maturation, and plasticity of synapses in vivo.

Expression, functional, and structural analysis of proteins critical for otoconia development

Otoconia, developed during late gestation and perinatal stages, couple mechanic force to the sensory hair cells in the vestibule for motion detection and bodily balance. In the present work, we have investigated whether compensatory deposition of another protein(s) may have taken place to partially alleviate the detrimental effects of Oc90 deletion by analyzing a comprehensive list of plausible candidates, and have found a drastic increase in the deposition of Sparc-like 1 (aka Sc1 or hevin) in Oc90 null versus wt otoconia. We show that such up-regulation is specific to Sc1, and that stable transfection of Oc90 and Sc1 full-length expression constructs in NIH/3T3 cells indeed promotes matrix calcification. Analysis and modeling of Oc90 and Sc1 protein structures show common features that may be critical requirements for the otoconial matrix backbone protein. Such information will serve as the foundation for future regenerative purposes.

Processing of the matricellular protein hevin in mouse brain is dependent on ADAMTS4

The matricellular SPARC family member hevin (SPARC-like 1/SPARCL-1/SC1/Mast9) contributes to neural development and alters tumor progression in a range of mammalian models. The distribution of hevin in mouse tissues was reexamined with a novel monoclonal antibody that discriminates between hevin and its ortholog SPARC. We now report proteolysis of hevin in many tissues, with the most extensive processing in the brain. We demonstrate a cleavage site within the hevin sequence for the neural tissue proteinase ADAMTS4. Digestion of hevin by ADAMTS4 in vitro produced fragments similar to those present in brain lysates. Monoclonal antibodies revealed a SPARC-like fragment generated from hevin that was co-localized with ADAMTS4 in vivo. We show that proteolysis of hevin by ADAMTS4 in the mouse cerebellum is important for the normal development of this tissue. In conclusion, we have identified the fragmentation of hevin by ADAMTS4 in the mouse brain and propose that this specific proteolysis is integral to cell morphology and extracellular matrix deposition in the developing brain.

The role of astrocyte-secreted matricellular proteins in central nervous system development and function

Matricellular proteins, such as thrombospondins (TSPs1-4), SPARC, SPARC-like1 (hevin) and tenascin C are expressed by astrocytes in the central nervous system (CNS) of rodents. The spatial and temporal expression patterns of these proteins suggest that they may be involved in important developmental processes such as cell proliferation and maturation, cell migration, axonal guidance and synapse formation. In addition, upon injury to the nervous system the expression of these proteins is upregulated, suggesting that they play a role in tissue remodeling and repair in the adult CNS. The genes encoding these proteins have been disrupted in mice. Interestingly, none of these proteins are required for survival, and furthermore, there are no evident abnormalities at the gross anatomical level in the CNS. However, detailed analyses of some of these mice in the recent years have revealed interesting CNS phenotypes. Here we will review the expression of these proteins in the CNS. We will discuss a newly described function for thrombospondins in synapse formation in the CNS in detail, and speculate whether other matricellular proteins could play similar roles in nervous system development and function.

The extracellular matrix protein SC1/Hevin localizes to multivesicular bodies in Bergmann glial fibers in the adult rat cerebellum

SC1 is an extracellular matrix molecule prominent in the mammalian brain. In the cerebellum, SC1 localizes to Bergmann glial cells and perisynaptic glial processes that envelop synapses in the molecular layer. In the present study, confocal microscopy revealed a punctate distribution of SC1 along Bergmann glial fibers that colocalized with the intermediate filament GFAP when fibers were viewed in cross-section. Immunoelectron microscopy showed that the punctate SC1 pattern corresponded to the localization of SC1 in multivesicular bodies situated within Bergmann glial fibers. The pattern of SC1 localization was not disrupted following hyperthermia or pilocarpine-induced status epilepticus. The present study suggests that SC1 protein may reach its destination in perisynaptic glial processes and glial endfeet by transport along Bergmann glial fibers in multivesicular bodies and that this process is preserved following stress.

The extracellular matrix protein SC1/hevin localizes to excitatory synapses following status epilepticus in the rat lithium-pilocarpine seizure model

The epileptic brain is characterized by increased susceptibility to neuronal hyperexcitability. The rat lithium-pilocarpine model, which mimics many features of temporal lobe epilepsy, has been used to study processes leading to the development of recurrent seizures. After a prolonged seizure episode, termed status epilepticus (SE), neural changes occur during a period known as epileptogenesis and include neuronal cell death, reactive gliosis, axonal sprouting, and synaptogenesis. Extracellular matrix adhesion molecules are important regulators of synaptogenesis and axonal sprouting resulting from SE. SC1, also known as hevin, is an antiadhesive extracellular matrix molecule that localizes to synapses in the mammalian brain. In this study, the distribution of SC1 protein in neurons following SE was examined using the lithium-pilocarpine model. SC1 protein levels in neuronal cell bodies showed a transient decrease at 1 day post-SE, which coincided with an increase of SC1 in the synapse-rich neuropil that was identified with the synaptic marker synaptophysin. Immunoelectron microscopy confirmed the decrease of SC1 signal in neurons at 1 day post-SE and showed that SC1 remained localized to postsynaptic elements throughout the seizure time course. Increased colocalization of SC1 was detected with the excitatory synaptic markers vesicular glutamate transporter 1 (VGLUT1), AMPA receptor subunit GluR1, and N-methyl-D-aspartate receptor subunit NR1, but not with the inhibitory synaptic markers vesicular gamma-aminobutyric acid (GABA) transporter (VGAT) and GABA(A) receptor subunit beta2 (GABA(A) beta2), which could reflect enhanced association of SC1 with excitatory synapses. These findings suggest that SC1 may be involved in synaptic modifications underlying epileptogenesis.

Differential expression patterns of occ1-related genes in adult monkey visual cortex

We have previously revealed that occ1 is preferentially expressed in the primary visual area (V1) of the monkey neocortex. In our attempt to identify more area-selective genes in the macaque neocortex, we found that testican-1, an occ1-related gene, and its family members also exhibit characteristic expression patterns along the visual pathway. The expression levels of testican-1 and testican-2 mRNAs as well as that of occ1 mRNA start of high in V1, progressively decrease along the ventral visual pathway, and end of low in the temporal areas. Complementary to them, the neuronal expression of SPARC mRNA is abundant in the association areas and scarce in V1. Whereas occ1, testican-1, and testican-2 mRNAs are preferentially distributed in thalamorecipient layers including “blobs,” SPARC mRNA expression avoids these layers. Neither SC1 nor testican-3 mRNA expression is selective to particular areas, but SC1 mRNA is abundantly observed in blobs. The expressions of occ1, testican-1, testican-2, and SC1 mRNA were downregulated after monocular tetrodotoxin injection. These results resonate with previous works on chemical and functional gradients along the primate occipitotemporal visual pathway and raise the possibility that these gradients and functional architecture may be related to the visual activity–dependent expression of these extracellular matrix glycoproteins.

A BAC transgenic mouse model to analyze the function of astroglial SPARCL1 (SC1) in the central nervous system

Extracellular matrix associated Sparc-like 1 (SC1/SPARCL1) can influence the function of astroglial cells in the developing and mature central nervous system (CNS). To examine SC1's significance in the CNS, we generated a BAC transgenic mouse model in which Sc1 is expressed in radial glia and their astrocyte derivatives using the astroglial-specific Blbp (Brain-lipid binding protein; [Feng et al., (1994) Neuron 12:895-908]) regulatory elements. Characterization of these Blbf-Sc1 transgenic mice show elevated Sc1 transcript and protein in an astroglial selective pattern throughout the CNS. This model provides a novel in vivo system for evaluating the role of SC1 in brain development and function, in general, and for understanding SC1's significance in the fate and function of astroglial cells, in particular.

Extracellular matrix protein SC1/hevin in the hippocampus following pilocarpine-induced status epilepticus

Pilocarpine-induced status epilepticus (SE) mimics many features of temporal lobe epilepsy and is a useful model to study neural changes that result from prolonged seizure activity. In this study, distribution of the anti-adhesive extracellular matrix protein SC1 was examined in the rat hippocampus following SE. Western blotting showed decreased levels of SC1 protein in the week following SE. Immunohistochemistry demonstrated that the decrease in overall SC1 protein levels was reflected by a reduction of SC1 signal in granule cells of the dentate gyrus. Interestingly, levels of SC1 protein in neurons of the seizure-resistant CA2 sector of the hippocampus did not change throughout the seizure time course. However, at 1 day post-SE, a subset of neurons of the hippocampal CA1, CA3, and hilar regions, which are noted for extensive neuronal degeneration after SE, exhibited a transient increase in SC1 signal. Neurons exhibiting enhanced SC1 signal were not detected at 7 days post-SE. The cellular stress response was also examined. A prominent induction of heat-shock protein (Hsp70) and Hsp27 was detected following SE, while levels of constitutively expressed Hsp40, Hsp90, Hsp110, and Hsc70 showed little change at the time points examined. The subset of neurons that demonstrated a transient increase in SC1 colocalized with the cellular stress marker Hsp70, the degeneration marker Fluoro-Jade B, and the neuron activity marker activity-regulated cytoskeleton-associated protein (Arc). Taken together, these findings suggest that SC1 may be a component of the 'matrix response' involved in remodeling events associated with neuronal degeneration following neural injury.

The phenomics and expression quantitative trait locus mapping of brain transcriptomes regulating adaptive divergence in lake whitefish species pairs (Coregonus sp.)

We used microarrays and a previously established linkage map to localize the genetic determinants of brain gene expression for a backcross family of lake whitefish species pairs (Coregonus sp.). Our goals were to elucidate the genomic distribution and sex specificity of brain expression QTL (eQTL) and to determine the extent to which genes controlling transcriptional variation may underlie adaptive divergence in the recently evolved dwarf (limnetic) and normal (benthic) whitefish. We observed a sex bias in transcriptional genetic architecture, with more eQTL observed in males, as well as divergence in genome location of eQTL between the sexes. Hotspots of nonrandom aggregations of up to 32 eQTL in one location were observed. We identified candidate genes for species pair divergence involved with energetic metabolism, protein synthesis, and neural development on the basis of colocalization of eQTL for these genes with eight previously identified adaptive phenotypic QTL and four previously identified outlier loci from a genome scan in natural populations. Eighty-eight percent of eQTL-phenotypic QTL colocalization involved growth rate and condition factor QTL, two traits central to adaptive divergence between whitefish species pairs. Hotspots colocalized with phenotypic QTL in several cases, revealing possible locations where master regulatory genes, such as a zinc-finger protein in one case, control gene expression directly related to adaptive phenotypic divergence. We observed little evidence of colocalization of brain eQTL with behavioral QTL, which provides insight into the genes identified by behavioral QTL studies. These results extend to the transcriptome level previous work illustrating that selection has shaped recent parallel divergence between dwarf and normal lake whitefish species pairs and that metabolic, more than morphological, differences appear to play a key role in this divergence.

Localization of the extracellular matrix protein SC1 coincides with synaptogenesis during rat postnatal development

SC1 is an extracellular matrix protein that belongs to the SPARC family of matricellular molecules. This anti-adhesive protein localizes to synapses in the adult rat brain and has been postulated to modulate synapse shape. In this study, increased levels of SC1 were detected from postnatal days 10-20, with a peak at postnatal day 15, a period of intense synaptogenesis. During this time, increased colocalization of SC1 with the synaptic marker synaptophysin was observed in synapse-rich regions of the cerebellum and the cerebral cortex. These findings indicate that the pattern of SC1 localization coincided with synaptogenesis during rat postnatal development.

Analysis of the extracellular matrix protein SC1 during reactive gliosis in the rat lithium-pilocarpine seizure model

When the nervous system is subjected to stressful stimuli, reactive gliosis often ensues. This phenomenon consists of the hypertrophy of astrocyte processes as well as the proliferation of these cells. In this study, the lithium-pilocarpine model of temporal lobe epilepsy was employed to study the effects of status epilepticus (SE) on the localization of SC1 protein in reactive astrocytes. SC1 is an anti-adhesive extracellular matrix protein strongly expressed in the mammalian brain. At 1 day following SE, SC1 transiently localized to hypertrophied astrocyte processes that were closely associated with neurons and blood vessels. SC1 was also detected at 7 days post-SE in proliferating astrocytes labeled with the cell division marker PCNA. These findings indicate that the anti-adhesive protein SC1 is ideally localized to create an environment conducive to process extension and cellular proliferation in reactive astrocytes.