Estrogen and testosterone supplementation improves tendon healing and functional recovery after rotator cuff repair

Failure of healing after rotator cuff repair (RCR) is common. The purpose of the current study was to evaluate the effect of systemic estrogen or testosterone supplementation on tendon healing after RCR. Seventy-two adult male mice were utilized for all experiments. The supraspinatus tendon was transected and repaired with 6-0 Prolene suture on the left shoulder of 51 animals. Mice were segregated into three groups postoperative: (1) vehicle group (VG; n = 18), (2) estrogen group (EST; n = 17), and (3) testosterone group (TST; n = 16). An unrepaired control group (unrepaired, n = 21) did not have surgery. Utilizing these animals, histological analysis, activity testing, biomechanical testing and RNA sequencing (RNA-seq) was performed. At 8 weeks post-RCR, TST, and EST supplementation improved the overall histologic structure of the repaired enthesis site. No differences in ultimate failure loads or stiffness were detected between VG, EST, and TST groups after biomechanical testing. RCR caused a reduction in wheel activity compared to unrepaired controls and supplementation with TST restored wheel activity. RNA-seq analysis indicated that estrogen and testosterone regulated different pathways associated with enthesis healing, including a suppression of inflammatory signaling. Supplementation with sex hormones improved the structure of the repaired tendon enthesis and significantly regulated expression of diverse pathways regulating multiple biological processes. Testosterone administration following RCR restored wheel activity without having a detrimental impact on biomechanical strength. Future human studies of sex hormone supplementation after RCR are warranted as supplementation in an animal model may improve tendon enthesis healing.

Meta-analysis of erosive hand osteoarthritis identifies four common variants that associate with relatively large effect

OBJECTIVES

Erosive hand osteoarthritis (EHOA) is a severe subset of hand osteoarthritis (OA). It is unclear if EHOA is genetically different from other forms of OA. Sequence variants at ten loci have been associated with hand OA but none with EHOA.

METHODS

We performed meta-analysis of EHOA in 1484 cases and 550 680 controls, from 5 populations. To identify causal genes, we performed eQTL and plasma pQTL analyses, and developed one zebrafish mutant. We analysed associations of variants with other traits and estimated shared genetics between EHOA and other traits.

RESULTS

Four common sequence variants associated with EHOA, all with relatively high effect. Rs17013495 (SPP1/MEPE, OR=1.40, p=8.4×10^-14) and rs11243284 (6p24.3, OR=1.35, p=4.2×10^-11) have not been associated with OA, whereas rs11631127 (ALDH1A2, OR=1.46, p=7.1×10^-18), and rs1800801 (MGP, OR=1.37, p=3.6×10^-13) have previously been associated with hand OA. The association of rs1800801 (MGP) was consistent with a recessive mode of inheritance in contrast to its additive association with hand OA (OR homozygotes vs non-carriers=2.01, 95% CI 1.71 to 2.37). All four variants associated nominally with finger OA, although with substantially lower effect. We found shared genetic components between EHOA and other OA measures, grip strength, urate levels and gout, but not rheumatoid arthritis. We identified ALDH1A2, MGP and BMP6 as causal genes for EHOA, with loss-of-function Bmp6 zebrafish mutants displaying EHOA-like phenotypes.

CONCLUSIONS

We report on significant genetic associations with EHOA. The results support the view of EHOA as a form of severe hand OA and partly separate it from OA in larger joints.

Summary Report of the Arthritis Foundation and the American Orthopaedic Foot & Ankle Society's Symposium on Targets for Osteoarthritis Research: Part 1: Epidemiology, Pathophysiology, and Current Imaging Approaches

This first of a 2-part series of articles recounts the key points presented in a collaborative symposium sponsored jointly by the Arthritis Foundation and the American Orthopaedic Foot & Ankle Society with the intent to survey the state of scientific knowledge related to incidence, diagnosis, pathologic mechanisms, and injection treatment options for osteoarthritis (OA) of the foot and ankle. A meeting was held virtually on December 3, 2021. A group of experts were invited to present brief synopses of the current state of knowledge and research in this area. Part 1 overviews areas of epidemiology and pathophysiology, current approaches in imaging, diagnostic and therapeutic injections, and genetics. Opportunities for future research are discussed. The OA scientific community, including funding agencies, academia, industry, and regulatory agencies, must recognize the needs of patients that suffer from arthritis of foot and ankle. The foot and ankle contain a myriad of interrelated joints and tissues that together provide a critical functionality. When this functionality is compromised by OA, significant disability results, yet the foot and ankle are generally understudied by the research community. Level of Evidence: Level V - Review Article/Expert Opinion.

Summary Report of the Arthritis Foundation and the American Orthopaedic Foot & Ankle Society's Symposium on Targets for Osteoarthritis Research: Part 2: Treatment Options

This second of a 2-part series of articles recounts the key points presented in a collaborative symposium sponsored jointly by the Arthritis Foundation and the American Orthopaedic Foot & Ankle Society with the intent to survey current treatment options for osteoarthritis (OA) of the foot and ankle. A meeting was held virtually on December 10, 2021. A group of experts were invited to present brief synopses of the current state of knowledge and research in this area. Topics were chosen by meeting organizers, who then identified and invited the expert speakers. Part 2 overviews the current treatment options, including orthotics, non-joint destructive procedures, as well as arthroscopies and arthroplasties in ankles and feet. Opportunities for future research are also discussed, such as developments in surgical options for ankle and the first metatarsophalangeal joint. The OA scientific community, including funding agencies, academia, industry, and regulatory agencies, must recognize the importance to patients of addressing the foot and ankle with improved basic, translational, and clinical research.

LEVEL OF EVIDENCE

Level V, review article/expert opinion.

Familial Clustering and Genetic Analysis of Severe Thumb Carpometacarpal Joint Osteoarthritis in a Large Statewide Cohort

PURPOSE

Our goals were to identify individuals who required surgery for thumb carpometacarpal (CMC) joint osteoarthritis (OA), determine if CMC joint OA clusters in families, define the magnitude of familial risk of CMC joint OA, identify risk factors associated with CMC joint OA, and identify rare genetic variants that segregate with familial CMC joint OA.

METHODS

We searched the Utah Population Database to identify a cohort of CMC joint OA patients who required surgery. Affected individuals were mapped to pedigrees to identify high-risk families with excess clustering of CMC joint OA. Cox regression models were used to calculate familial risk of CMC joint OA in related individuals. Risk factors were evaluated using logistic regression models. Whole exome sequencing was used to identify rare coding variants associated with familial CMC joint OA.

RESULTS

We identified 550 pedigrees with excess clustering of severe CMC joint OA. The relative risk of CMC joint OA requiring surgical treatment was elevated significantly in first- and third-degree relatives of affected individuals, and significant associations with advanced age, female sex, obesity, and tobacco use were observed. We discovered candidate genes that dominantly segregate with severe CMC joint OA in 4 independent families, including a rare variant in Chondroitin Sulfate Synthase 3 (CHSY3).

CONCLUSIONS

Familial clustering of severe CMC joint OA was observed in a statewide population. Our data indicate that genetic and environmental factors contribute to the disease process, further highlighting the multifactorial nature of the disease. Genomic analyses suggest distinct biological processes are involved in CMC joint OA pathogenesis.

CLINICAL RELEVANCE

Awareness of associated comorbidities may guide the diagnosis of CMC joint OA in at-risk populations and help identify individuals who may not do well with nonoperative treatment. Further pursuit of the genes associated with severe CMC joint OA may lead to assays for detection of early stages of disease and have therapeutic potential.

NOD/RIPK2 signalling pathway contributes to osteoarthritis susceptibility

OBJECTIVES

How inflammatory signalling contributes to osteoarthritis (OA) susceptibility is undetermined. An allele encoding a hyperactive form of the Receptor Interacting Protein Kinase 2 (RIPK2) proinflammatory signalling intermediate has been associated with familial OA. To test whether altered nucleotide-binding oligomerisation domain (NOD)/RIPK2 pathway activity causes heightened OA susceptibility, we investigated whether variants affecting additional pathway components are associated with familial OA. To determine whether the Ripk2104Asp disease allele is sufficient to account for the familial phenotype, we determined the effect of the allele on mice.

METHODS

Genomic analysis of 150 independent families with dominant inheritance of OA affecting diverse joints was used to identify coding variants that segregated strictly with occurrence of OA. Genome editing was used to introduce the OA-associated RIPK2 (p.Asn104Asp) allele into the genome of inbred mice. The consequences of the Ripk2104Asp disease allele on physiology and OA susceptibility in mice were measured by histology, immunohistochemistry, serum cytokine levels and gene expression.

RESULTS

We identified six novel variants affecting components of the NOD/RIPK2 inflammatory signalling pathway that are associated with familial OA affecting the hand, shoulder or foot. The Ripk2104Asp allele acts dominantly to alter basal physiology and response to trauma in the mouse knee. Whereas the knees of uninjured Ripk2Asp104 mice appear normal histologically, the joints exhibit a set of marked gene expression changes reminiscent of overt OA. Although the Ripk2104Asp mice lack evidence of chronically elevated systemic inflammation, they do exhibit significantly increased susceptibility to post-traumatic OA (PTOA).

CONCLUSIONS

Two types of data support the hypothesis that altered NOD/RIPK2 signalling confers susceptibility to OA.

The effect of estrogen-like compound on rotator cuff tendon healing in a murine model

Estrogen deficiency has been shown to negatively influence rotator cuff tendon healing. Therefore, the addition of an estrogen-like-compound (ELC) in a nonestrogen-deficient animal may improve the quality of a rotator cuff repair. The purpose of this study was to evaluate the effects of an ELC, diethylstilbestrol (DES), on tendon healing in a murine rotator cuff repair model. Thirty-three male wild-type mice (C57BL/6NJ) were randomly divided into three study groups. Group 1-unoperated mice with normal rotator cuff tendons. Groups 2 and 3 consisted of surgically repaired rotator cuff tendons; Group 2 (repair-only) was the standard repair group (no DES injected), whereas Group 3 (repair + DES) was the experimental repair group (injected with DES). Comparing the maximal thickness of calcified fibrocartilage to uncalcified fibrocartilage, the ratios for the control (intact tendon), repair-only, and repair + DES groups were 2:1, 0.9:1, and 1.7:1. RNA expression data demonstrated upregulation of chondrogenic, angiogenic, and tendon modulation genes in the repair- only group compared to the control (intact tendon) group (p < 0.04 for all), and that addition of DES further increased the osteogenic, angiogenic, and tendon modulation gene expression compared to the repair-only group (p < 0.02). Immunohistochemical analysis indicated that the addition of DES further increased osteogenic, angiogenic, and tendon maturation protein expression at the enthesis compared to standard repairs.

Familial Clustering of Erosive Hand Osteoarthritis in a Large Statewide Cohort

OBJECTIVE

Erosive hand osteoarthritis (OA) is a severe and rapidly progressing subset of hand OA. Its etiology remains largely unknown, which has hindered development of successful treatments. This study was undertaken to test the hypothesis that erosive hand OA demonstrates familial clustering in a large statewide population linked to genealogical records, and to determine the association of potential risk factors with erosive hand OA.

METHODS

Patients diagnosed as having erosive hand OA were identified by searching 4,741,840 unique medical records from a comprehensive statewide database, the Utah Population Database (UPDB). Affected individuals were mapped to pedigrees to identify high-risk families with excess clustering of erosive hand OA as defined by a familial standardized incidence ratio (FSIR) of ≥2.0. The magnitude of familial risk of erosive hand OA in related individuals was calculated using Cox regression models. Association of potential erosive hand OA risk factors was analyzed using multivariate conditional logistic regression and logistic regression models.

RESULTS

We identified 703 affected individuals linked to 240 unrelated high-risk pedigrees with excess clustering of erosive hand OA (FSIR ≥2.0, P < 0.05). The relative risk of developing erosive hand OA was significantly elevated in first-degree relatives (P < 0.001). There were significant associations between a diagnosis of erosive hand OA and age, sex, diabetes, and obesity (all P < 0.05).

CONCLUSION

Familial clustering of erosive hand OA observed in a statewide database indicates a potential genetic contribution to the etiology of the disease. Age, sex, diabetes, and obesity are risk factors for erosive hand OA. Identification of causal gene variants in these high-risk families may provide insight into the genes and pathways that contribute to erosive hand OA onset and progression.

A role for the MEGF6 gene in predisposition to osteoporosis

Osteoporosis is a common skeletal disorder characterized by deterioration of bone tissue. The set of genetic factors contributing to osteoporosis is not completely specified. High-risk osteoporosis pedigrees were analyzed to identify genes that may confer susceptibility to disease. Candidate predisposition variants were identified initially by whole exome sequencing of affected-relative pairs, approximately cousins, from 10 pedigrees. Variants were filtered on the basis of population frequency, concordance between pairs of cousins, affecting a gene associated with osteoporosis, and likelihood to have functionally damaging, pathogenic consequences. Subsequently, variants were tested for segregation in 68 additional relatives of the index carriers. A rare variant in MEGF6 (rs755467862) showed strong evidence of segregation with the disease phenotype. Predicted protein folding indicated the variant (Cys200Tyr) may disrupt structure of an EGF-like calcium-binding domain of MEGF6. Functional analyses demonstrated that complete loss of the paralogous genes megf6a and megf6b in zebrafish resulted in significant delay of cartilage and bone formation. Segregation analyses, in silico protein structure modeling, and functional assays support a role for MEGF6 in predisposition to osteoporosis.

Evaluation for Kienböck Disease Familial Clustering: A Population-Based Cohort Study

PURPOSE

Kienböck disease (KD) is rare and its etiology remains unknown. As a result, the ideal treatment is also in question. Our primary purpose was to test the hypothesis that KD would demonstrate familial clustering in a large statewide population with comprehensive genealogical records, possibly suggesting a genetic etiologic contribution. Our secondary purpose was to evaluate for associations between KD and known risk factors for avascular necrosis.

METHODS

Patients diagnosed with KD were identified by searching medical records from a comprehensive statewide database, the Utah Population Database. This database contains pedigrees dating back to the early 1800s, which are linked to 31 million medical records for 11 million patients from 1996 to the present. Affected individuals were then mapped to pedigrees to identify high-risk families with an increased incidence of KD relative to control pedigrees. The magnitude of familial risk of KD in related individuals was calculated using Cox regression models. Association of risk factors related to KD was analyzed using conditional logistic regression.

RESULTS

We identified 394 affected individuals linked to 194 unrelated high-risk pedigrees with increased incidence of KD. The relative risk of developing KD was significantly elevated in first-degree relatives. There was a significant correlation between alcohol, glucocorticoid, and tobacco use and a history of diabetes, and the diagnosis of KD.

CONCLUSIONS

Familial clustering of KD observed in the Utah Population Database cohort indicates a potential genetic contribution to the etiology of the disease. Identification of causal gene variants in these high-risk families may provide insight into the genes and pathways that contribute to the onset and progression of KD.

CLINICAL RELEVANCE

This study suggests that there is a potential genetic contribution to the etiology of KD and that the disease has a significant association with several risk factors.

The Paf1 complex and P-TEFb have reciprocal and antagonist roles in maintaining multipotent neural crest progenitors

Multipotent progenitor populations are necessary for generating diverse tissue types during embryogenesis. We show the RNA polymerase-associated factor 1 complex (Paf1C) is required to maintain multipotent progenitors of the neural crest (NC) lineage in zebrafish. Mutations affecting each Paf1C component result in near-identical NC phenotypes; alyron mutant embryos carrying a null mutation in paf1 were analyzed in detail. In the absence of zygotic paf1 function, definitive premigratory NC progenitors arise but fail to maintain expression of the sox10 specification gene. The mutant NC progenitors migrate aberrantly and fail to differentiate appropriately. Blood and germ cell progenitor development is affected similarly. Development of mutant NC could be rescued by additional loss of positive transcription elongation factor b (P-TEFb) activity, a key factor in promoting transcription elongation. Consistent with the interpretation that inhibiting/delaying expression of some genes is essential for maintaining progenitors, mutant embryos lacking the CDK9 kinase component of P-TEFb exhibit a surfeit of NC progenitors and their derivatives. We propose Paf1C and P-TEFb act antagonistically to regulate the timing of the expression of genes needed for NC development.

Highly Efficient CRISPR-Cas9-Based Methods for Generating Deletion Mutations and F0 Embryos that Lack Gene Function in Zebrafish

Inconsistent activity limits the use of CRISPR-Cas9 in zebrafish. We show supernumerary guanine nucleotides at the 5' ends of single guide RNAs (sgRNAs) account for diminished CRISPR-Cas9 activity in zebrafish embryos. Genomic sequences can be targeted consistently with extremely high efficiency using Cas9 ribonucleoproteins (RNPs) containing either a sgRNA molecule or a synthetic crRNA:tracrRNA duplex that perfectly matches the protospacer target site. Following injection of zebrafish eggs with such RNPs, virtually every copy of a targeted locus harbors an induced indel mutation. Loss of gene function is often complete, as F0 embryos closely resemble true null mutants without detectable non-specific effects. Mosaicism is sufficiently low in F0 embryos that cell non-autonomous gene functions can be probed effectively and redundant activities of genes can be uncovered when two genes are targeted simultaneously. Finally, heritable deletion mutations of at least 50 kbp can be readily induced using pairs of duplex guide RNPs targeted to a single chromosome.

A hyperactivating proinflammatory RIPK2 allele associated with early-onset osteoarthritis

Osteoarthritis (OA) is a common debilitating disease characterized by abnormal remodeling of the cartilage and bone of the articular joint. Ameliorating therapeutics are lacking due to limited understanding of the molecular pathways affecting disease initiation and progression. Notably, although a link between inflammation and overt OA is well established, the role of inflammation as a driver of disease occurrence is highly disputed. We analyzed a family with dominant inheritance of early-onset OA and found that affected individuals harbored a rare variant allele encoding a significant amino acid change (p.Asn104Asp) in the kinase domain of receptor interacting protein kinase 2 (RIPK2), which transduces signals from activated bacterial peptidoglycan sensors through the NF-κB pathway to generate a proinflammatory immune response. Functional analyses of RIPK2 activity in zebrafish embryos indicated that the variant RIPK2104Asp protein is hyperactive in its signaling capacity, with augmented ability to activate the innate immune response and the NF-κB pathway and to promote upregulation of OA-associated genes. Further we show a second allele of RIPK2 linked to an inflammatory disease associated with arthritis also has enhanced activity stimulating the NF-κB pathway. Our studies reveal for the first time the inflammatory response can function as a gatekeeper risk factor for OA.

Precise genome editing by homologous recombination

Simple and efficient methods are presented for creating precise modifications of the zebrafish genome. Edited alleles are generated by homologous recombination between the host genome and double-stranded DNA (dsDNA) donor molecules, stimulated by the induction of double-strand breaks at targeted loci in the host genome. Because several kilobase-long tracts of sequence can be exchanged, multiple genome modifications can be generated simultaneously at a single locus. Methods are described for creating: (1) alleles with simple sequence changes or in-frame additions, (2) knockin/knockout alleles that express a reporter protein from an endogenous locus, and (3) conditional alleles in which exons are flanked by recombinogenic loxP sites. Significantly, our approach to genome editing allows the incorporation of a linked reporter gene into the donor sequences so that successfully edited alleles can be identified by virtue of expression of the reporter. Factors affecting the efficiency of genome editing are discussed, including the finding that dsDNA products of I-SceI meganuclease enzyme digestion are particularly effective as donor molecules for gene-editing events. Reagents and procedures are described for accomplishing efficient genome editing in the zebrafish.

Hadp1, a newly identified pleckstrin homology domain protein, is required for cardiac contractility in zebrafish

The vertebrate heart is one of the first organs to form, and its early function and morphogenesis are crucial for continued embryonic development. Here we analyze the effects of loss of Heart adaptor protein 1 (Hadp1), which we show is required for normal function and morphogenesis of the embryonic zebrafish heart. Hadp1 is a pleckstrin homology (PH)-domain-containing protein whose expression is enriched in embryonic cardiomyocytes. Knockdown of hadp1 in zebrafish embryos reduced cardiac contractility and altered late myocyte differentiation. By using optical mapping and submaximal levels of hadp1 knockdown, we observed profound effects on Ca²⁺ handling and on action potential duration in the absence of morphological defects, suggesting that Hadp1 plays a major role in the regulation of intracellular Ca²⁺ handling in the heart. Hadp1 interacts with phosphatidylinositol 4-phosphate [PI4P; also known as PtdIns(4)P] derivatives via its PH domain, and its subcellular localization is dependent upon this motif. Pharmacological blockade of the synthesis of PI4P derivatives in vivo phenocopied the loss of hadp1 in zebrafish. Collectively, these results demonstrate that hadp1 is required for normal cardiac function and morphogenesis during embryogenesis, and suggest that hadp1 modulates Ca²⁺ handling in the heart through its interaction with phosphatidylinositols.

SHIP2, a factor associated with diet-induced obesity and insulin sensitivity, attenuates FGF signaling in vivo

SH2-domain-containing inositol phosphatase 2 (SHIP2) belongs to a small family of phosphoinositide 5-phosphatases that help terminate intracellular signaling initiated by activated receptor tyrosine kinases. Mammalian SHIP2 is viewed primarily as an attenuator of insulin signaling and has become a prominent candidate target for therapeutic agents that are designed to augment insulin signaling. Despite this view, no signaling pathway has yet been demonstrated as being affected directly by SHIP2 function in vivo, and in vitro studies indicate that the protein may function in multiple signaling pathways. Here, we analyze the role of a SHIP2 family member in the early zebrafish embryo where developmental and gene expression defects can be used to assay specific signaling pathways. The zebrafish ship2a transcript is maternally supplied, and inhibiting the expression of its protein product results in the expansion of dorsal tissue fates at the expense of ventral ones. We show that the developmental defects are the result of perturbation of fibroblast growth factor (FGF) signaling in the early embryo. Loss of Ship2a leads to an increased and expanded expression of outputs of FGF-mediated signaling, including FGF-dependent gene expression and activated mitogen-activated protein kinase (MAPK) signaling. Our findings demonstrate that Ship2a attenuates the FGF signaling pathway in vivo and functions in the establishment of normal tissue patterning in the early embryo. We suggest that modulation of FGF signaling may be a principal function of SHIP2 in mammals.

SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans

Lighter variations of pigmentation in humans are associated with diminished number, size, and density of melanosomes, the pigmented organelles of melanocytes. Here we show that zebrafish golden mutants share these melanosomal changes and that golden encodes a putative cation exchanger slc24a5 (nckx5) that localizes to an intracellular membrane, likely the melanosome or its precursor. The human ortholog is highly similar in sequence and functional in zebrafish. The evolutionarily conserved ancestral allele of a human coding polymorphism predominates in African and East Asian populations. In contrast, the variant allele is nearly fixed in European populations, is associated with a substantial reduction in regional heterozygosity, and correlates with lighter skin pigmentation in admixed populations, suggesting a key role for the SLC24A5 gene in human pigmentation.

Two divergent slit1 genes in zebrafish

Members of the Slit family regulate axon guidance and cell migration. To date, three vertebrate slit1 genes have been identified in mammals and orthologs of two, slit2 and slit3, have been identified in zebrafish. Here, we describe the cloning of full-length cDNAs for two zebrafish slit orthologs, slit1a and slit1b. Both predicted proteins contain the conserved motifs that characterize other vertebrate Slits. slit1a and slit1b are both expressed in the midline, hypochord, telencephalon, and hindbrain. Apart from these shared expression domains, however, their expression patterns largely differ. Whereas slit1a is expressed broadly in the central nervous system (CNS) and in the somites, pectoral fin buds, tail bud, and caudal fin folds, slit1b is expressed in the olfactory system throughout embryonic and larval development, and in the retina during larval stages. Their expression patterns, particularly that of slit1a, suggest that Slit proteins may have roles in tissue morphogenesis in addition to their established roles in axon guidance and cell migration.

Increased adenine nucleotide translocator 1 in reactive astrocytes facilitates glutamate transport

A hallmark of central nervous system (CNS) pathology is reactive astrocyte production of the chronic glial scar that is inhibitory to neuronal regeneration. The reactive astrocyte response is complex; these cells also produce neurotrophic factors and are responsible for removal of extracellular glutamate, the excitatory neurotransmitter that rises to neurotoxic levels in injury and disease. To identify genes expressed by reactive astrocytes, we employed an in vivo model of the glial scar and differential display PCR and found an increase in the level of Ant1, a mitochondrial ATP/ADP exchanger that facilitates the flux of ATP out of the mitochondria. Ant1 expression in reactive astrocytes is regulated by transforming growth factor-beta1, a pluripotent CNS injury-induced cytokine. The significance of increased Ant1 is evident from the observation that glutamate uptake is significantly decreased in astrocytes from Ant1 null mutant mice while a specific Ant inhibitor reduces glutamate uptake in wild-type astrocytes. Thus, the astrocytic response to CNS injury includes an apparent increase in energy mobilization capacity by Ant1 that contributes to neuroprotective, energy-dependent glutamate uptake.

TIGR is upregulated in the chronic glial scar in response to central nervous system injury and inhibits neurite outgrowth

Reactive astrocytes respond to central nervous system (CNS) injury and disease by elaborating a glial scar that is inhibitory to axonal regeneration. To identify genes that may be involved in the astrocytic response to injury, we used differential display polymerase chain reaction and an in vivo model of the CNS glial scar. Expression of the trabecular meshwork inducible glucocorticoid response (TIGR) gene was increased in gliotic tissue compared with the uninjured cerebral cortex. Increased TIGR expression by reactive astrocytes was confirmed by in situ hybridization, quantitative reverse transcriptase-polymerase chain reaction, immunoblot analysis, and immunohistochemistry. Although mutations of the TIGR gene have been implicated in glaucoma, a function for TIGR has not been reported. Since TIGR is secreted, we assessed a possible role in inhibition of neuronal regeneration with an in vitro bioassay and found that this protein is a potent inhibitor of neurite outgrowth. Thus, TIGR is a newly identified component of the CNS glial scar that is likely to contribute to neuronal regenerative failure characteristic of the mammalian CNS.

The chondroitin sulfate proteoglycans neurocan and phosphacan are expressed by reactive astrocytes in the chronic CNS glial scar

Chondroitin sulfate proteoglycans (CS-PGs) expressed by reactive astrocytes may contribute to the axon growth-inhibitory environment of the injured CNS. The specific potentially inhibitory CS-PGs present in areas of reactive gliosis, however, have yet to be thoroughly examined. In this study, we used immunohistochemistry, combined immunohistochemistry-in situ hybridization, immunoblot analysis, and reverse transcription-PCR to examine the expression of specific CS-PGs by reactive astrocytes in an in vivo model of reactive gliosis: that is, the glial scar, after cortical injury. Neurocan and phosphacan can be localized to reactive astrocytes 30 d after CNS injury, whereas brevican and versican are not expressed in the chronic glial scar. Neurocan is also expressed by astrocytes in primary cell culture. Relative to the amount present in cultured astrocytes or uninjured cortex, neurocan expression increases significantly in the glial scar resulting from cortical injury, including the re-expression of the neonatal isoform of neurocan. In contrast, phosphacan protein levels are decreased in the glial scar compared with the uninjured brain. Because these CS-PGs are capable of inhibiting neurite outgrowth in vitro, our data suggest that phosphacan and neurocan in areas of reactive gliosis may contribute to axonal regenerative failure after CNS injury.