Expression of osteopontin in the inner ear

Molecular differences between neonatal and adult stria vascularis from organotypic explants and transcriptomics

Summary

The stria vascularis (SV), part of the blood-labyrinth barrier, is an essential component of the inner ear that regulates the ionic environment required for hearing. SV degeneration disrupts cochlear homeostasis, leading to irreversible hearing loss, yet a comprehensive understanding of the SV, and consequently therapeutic availability for SV degeneration, is lacking. We developed a whole-tissue explant model from neonatal and adult mice to create a robust platform for SV research. We validated our model by demonstrating that the proliferative behaviour of the SV in vitro mimics SV in vivo, providing a representative model and advancing high-throughput SV research. We also provided evidence for pharmacological intervention in our system by investigating the role of Wnt/β-catenin signaling in SV proliferation. Finally, we performed single-cell RNA sequencing from in vivo neonatal and adult mouse SV and revealed key genes and pathways that may play a role in SV proliferation and maintenance. Together, our results contribute new insights into investigating biological solutions for SV-associated hearing loss.

Significance

Hearing loss impairs our ability to communicate with people and interact with our environment. This can lead to social isolation, depression, cognitive deficits, and dementia. Inner ear degeneration is a primary cause of hearing loss, and our study provides an in depth look at one of the major sites of inner ear degeneration: the stria vascularis. The stria vascularis and associated blood-labyrinth barrier maintain the functional integrity of the auditory system, yet it is relatively understudied. By developing a new in vitro model for the young and adult stria vascularis and using single cell RNA sequencing, our study provides a novel approach to studying this tissue, contributing new insights and widespread implications for auditory neuroscience and regenerative medicine.

Highlights

- We established an organotypic explant system of the neonatal and adult stria vascularis with an intact blood-labyrinth barrier. - Proliferation of the stria vascularis decreases with age in vitro , modelling its proliferative behaviour in vivo . - Pharmacological studies using our in vitro SV model open possibilities for testing injury paradigms and therapeutic interventions. - Inhibition of Wnt signalling decreases proliferation in neonatal stria vascularis.- We identified key genes and transcription factors unique to developing and mature SV cell types using single cell RNA sequencing.

Patterns of gene expression associated with Pten deficiency in the developing inner ear

In inner ear development, phosphatase and tensin homolog (PTEN) is necessary for neuronal maintenance, such as neuronal survival and accurate nerve innervations of hair cells. We previously reported that Pten conditional knockout (cKO) mice exhibited disorganized fasciculus with neuronal apoptosis in spiral ganglion neurons (SGNs). To better understand the genes and signaling networks related to auditory neuron maintenance, we compared the profiles of differentially expressed genes (DEGs) using microarray analysis of the inner ear in E14.5 Pten cKO and wild-type mice. We identified 46 statistically significant transcripts using significance analysis of microarrays, with the false-discovery rate set at 0%. Among the DEGs, expression levels of candidate genes and expression domains were validated by quantitative real-time RT-PCR and in situ hybridization, respectively. Ingenuity pathway analysis using DEGs identified significant signaling networks associated with apoptosis, cellular movement, and axon guidance (i.e., secreted phosphoprotein 1 (Spp1)-mediated cellular movement and regulator of G-protein signaling 4 (Rgs4)-mediated axon guidance). This result was consistent with the phenotypic defects of SGNs in Pten cKO mice (e.g., neuronal apoptosis, abnormal migration, and irregular nerve fiber patterns of SGNs). From this study, we suggest two key regulatory signaling networks mediated by Spp1 and Rgs4, which may play potential roles in neuronal differentiation of developing auditory neurons.

Osteopontin: A novel regulator at the cross roads of inflammation, obesity and diabetes

Since its first description more than 20 years ago osteopontin has emerged as an active player in many physiological and pathological processes, including biomineralization, tissue remodeling and inflammation. As an extracellular matrix protein and proinflammatory cytokine osteopontin is thought to facilitate the recruitment of monocytes/macrophages and to mediate cytokine secretion in leukocytes. Modulation of immune cell response by osteopontin has been associated with various inflammatory diseases and may play a pivotal role in the development of adipose tissue inflammation and insulin resistance. Here we summarize recent findings on the role of osteopontin in metabolic disorders, particularly focusing on diabetes and obesity.

Differential expression of secreted phosphoprotein 1 in the motor cortex among primate species and during postnatal development and functional recovery

We previously reported that secreted phosphoprotein 1 (SPP1) mRNA is expressed in neurons whose axons form the corticospinal tract (CST) of the rhesus macaque, but not in the corresponding neurons of the marmoset and rat. This suggests that SPP1 expression is involved in the functional or structural specialization of highly developed corticospinal systems in certain primate species. To further examine this hypothesis, we evaluated the expression of SPP1 mRNA in the motor cortex from three viewpoints: species differences, postnatal development, and functional/structural changes of the CST after a lesion of the lateral CST (l-CST) at the mid-cervical level. The density of SPP1-positive neurons in layer V of the primary motor cortex (M1) was much greater in species with highly developed corticospinal systems (i.e., rhesus macaque, capuchin monkey, and humans) than in those with less developed corticospinal systems (i.e., squirrel monkey, marmoset, and rat). SPP1-positive neurons in the macaque monkey M1 increased logarithmically in layer V during postnatal development, following a time course consistent with the increase in conduction velocity of the CST. After an l-CST lesion, SPP1-positive neurons increased in layer V of the ventral premotor cortex, in which compensatory changes in CST function/structure may occur, which positively correlated with the extent of finger dexterity recovery. These results further support the concept that the expression of SPP1 may reflect functional or structural specialization of highly developed corticospinal systems in certain primate species.

A novel biological function for CD44 in axon growth of retinal ganglion cells identified by a bioinformatics approach

The failure of CNS regeneration and subsequent motor and sensory loss remain major unsolved questions despite massive accumulation of experimental observations and results. The sheer volume of data and the variety of resources from which these data are generated make it difficult to integrate prior work to build new hypotheses. To address these challenges we developed a prototypic suite of computer programs to extract protein names from relevant publications and databases and associated each of them with several general categories of biological functions in nerve regeneration. To illustrate the usefulness of our data mining approach, we utilized the program output to generate a hypothesis for a biological function of CD44 interaction with osteopontin (OPN) and laminin in axon outgrowth of CNS neurons. We identified CD44 expression in retinal ganglion cells and when these neurons were plated on poly-l-lysine 3% of them initiated axon growth, on OPN 15%, on laminin-111 (1x) 41%, on laminin-111 (0.5x) 56%, and on a mixture of OPN and laminin (1x) 67% of neurons generated axon growth. With the aid of a deoxyribozyme (DNA enzyme) to CD44 that digests the target mRNA, we demonstrated that a reduction of CD44 expression led to reduced axon initiation of retinal ganglion cells on all substrates. We suggest that such an integrative, applied systems biology approach to CNS trauma will be critical to understand and ultimately overcome the failure of CNS regeneration.

Regular expression of osteopontin in granular cell tumor: distinctive feature among Schwannian cell tumors

For further characterization of S-100 protein-positive granular cell tumor (GCT), the expression of osteopontin (OPN) in the tumor cells was compared with other types of Schwannian cell tumors. Twenty GCT, three amputation neuromas, 12 neurilemmomas (NM) and three malignant peripheral nerve sheath tumors (MPNST) were used. In addition, two granular cell epulides were employed because of histological similarity to that of GCT. Immunohistochemistry by anti-OPN-monoclonal antibody revealed OPN-immunoreactivity in tumor cells of all GCT and stromal macrophages in neurilemmomas and one MPNST, but neither in amputation neuromas nor in granular cell epulides. All nine GCT studied by in situ hybridization (ISH) and all three GCT analyzed by reverse transcription-polymerase chain reaction (RT-PCR) were positive for OPN mRNA. In contrast, seven NM and three amputation neuromas were negative for OPN mRNA by either ISH or RT-PCR, while macrophages infiltrated in these tumors had OPN mRNA consistently. Double immunostaining for OPN and CD68, a lysosome-associated glycoprotein, showed their colocalization in tumor cells of GCT, suggesting a possible degradation of OPN by lysosomes. In conclusion, GCT among Schwannian cell tumors consistently express OPN, but its biological significance requires further investigation.

Ganglion cells of the rat retina show osteopontin-like immunoreactivity

Osteopontin (OPN) is a negatively charged, highly acidic glycosylated phosphoprotein that contains an GRGDS amino acid sequence, characteristic of proteins that bind to integrin receptors, thereby playing crucial roles in a number of physiological processes. This study was conducted to examine the expression of OPN in the rat retina by Northern blot analysis, Western blot analysis and immunocytochemistry. The expression of OPN was identified in the retina and OPN-like immunoreactivity was present in a number of ganglion cells. Thus, OPN appears to be important in the retinal homeostasis.

Expression of osteopontin mRNA in the adult rat brain

Examining the distribution of osteopontin (OPN) in the rat brain using in situ hybridization, it was found that OPN mRNA was restricted to likely neurons in the olfactory bulb and the brain stem; it was not detected in the telencephalon and the diencephalon. In addition, Northern blot analysis showed a confined expression only in the brain stem with higher level in the pons and the medulla than in the midbrain. In the brain stem, it was found in functionally diverse areas including motor-related areas, sensory system and reticular formation. This specific but widespread distribution suggests that OPN may play an important role in the adult rat brain.

Detection of various epitopes of murine osteopontin by monoclonal antibodies

We immunized rats with recombinant murine osteopontin protein and obtained four monoclonal antibodies recognizing distinct epitopes of murine osteopontin. OPN1.2 recognized the amino-terminal half of OPN, while OPN2.2, OPN2.3, and OPN3.1 recognized the carboxy-terminal half of OPN. The epitope recognized by OPN2.2 was destroyed by further cleavage of the carboxy half of OPN. The epitope recognized by OPN2.3 was located in the amino-terminal end of the carboxy half of OPN, whereas that recognized by OPN3.1 was located in the carboxy-terminal end of the carboxy half of OPN. OPN1.2 and OPN2.2 recognized thrombin-cleaved osteopontin, whereas thrombin-cleaved osteopontin was not recognized by OPN2.3 and OPN3.1. Thus, these monoclonal antibodies will be useful in structure/function studies of the role of osteopontin in murine models of disease.