Biochemical alterations in inflammatory reactive chondrocytes: Evidence for intercellular network communication

Abstract

Chondrocytes are effectively involved in the pathophysiological processes of inflammation in joints. They form cellular processes in the superficial layer of the articular cartilage and form gap junction coupled syncytium to facilitate cell-to-cell communication. However, very little is known about their physiological cellular identity and communication. The aim with the present work is to evaluate the physiological behavior after stimulation with the inflammatory inducers interleukin-1β and lipopolysaccharide. The cytoskeleton integrity and intracellular Ca2+ release were assessed as indicators of inflammatory state. Cytoskeleton integrity was analyzed through cartilage oligomeric matrix protein and actin labeling with an Alexa 488-conjugated phalloidin probe. Ca2+ responses were assessed through the Ca2+ sensitive fluorophore Fura-2/AM. Western blot analyses of several inflammatory markers were performed. The results show reorganization of the actin filaments. Glutamate, 5-hydoxytryptamine, and ATP evoked intracellular Ca2+ release changed from single peaks to oscillations after inflammatory induction in the chondrocytes. The expression of toll-like receptor 4, the glutamate transporters GLAST and GLT-1, and the matrix metalloproteinase-13 increased. This work demonstrates that chondrocytes are a key part in conditions that lead to inflammation in the cartilage. The inflammatory inducers modulate the cytoskeleton, the Ca2+ signaling, and several inflammatory parameters. In conclusion, our data show that the cellular responses to inflammatory insults from healthy and inflammatory chondrocytes resemble those previously observed in astrocyte and cardiac fibroblasts networks.

Full text

Biochemical alterations in
inflammatory reactive
chondrocytes: evidence for
intercellular network
communication
Eva Skiöldebrand
a,
*, Anna Thorfve
a
, Ulrika Björklund
b
, Pegah Johansson
a
,
Ruth Wickelgren
a
, Anders Lindahl
a
, Elisabeth Hansson
b
a
Department of Clinical Chemistry and Transfusion Medicine, Institute of Biomedicine, Sahlgrenska University
Hospital, Gothenburg University, Gothenburg, Sweden
b
Department of Clinical Neuroscience, Institute of Neuroscience and Physiology, The Sahlgrenska Academy,
University of Gothenburg, Sweden
* Corresponding author.
E-mail address: eva.skioldebrand@clinchem.gu.se (E. Skiöldebrand).
Abstract
Chondrocytes are effectively involved in the pathophysiological processes of
inflammation in joints. They form cellular processes in the superficial layer of the
articular cartilage and form gap junction coupled syncytium to facilitate cell-to-cell
communication. However, very little is known about their physiological cellular
identity and communication. The aim with the present work is to evaluate the
physiological behavior after stimulation with the inflammatory inducers interleu-
kin-1βand lipopolysaccharide. The cytoskeleton integrity and intracellular Ca
2+
release were assessed as indicators of inflammatory state. Cytoskeleton integrity
was analyzed through cartilage oligomeric matrix protein and actin labeling with
an Alexa 488-conjugated phalloidin probe. Ca
2+
responses were assessed through
the Ca
2+
sensitive fluorophore Fura-2/AM. Western blot analyses of several
inflammatory markers were performed. The results show reorganization of the
actin filaments. Glutamate, 5-hydoxytryptamine, and ATP evoked intracellular Ca
2
+
release changed from single peaks to oscillations after inflammatory induction in
Received:
19 September 2017
Revised:
4 January 2018
Accepted:
23 January 2018
Cite as: Eva Skiöldebrand,
Anna Thorfve,
Ulrika Björklund,
Pegah Johansson,
Ruth Wickelgren,
Anders Lindahl,
Elisabeth Hansson.
Biochemical alterations in
inflammatory reactive
chondrocytes: evidence for
intercellular network
communication.
Heliyon 4 (2018) e00525.
doi: 10.1016/j.heliyon.2018.
e00525
http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

the chondrocytes. The expression of toll-like receptor 4, the glutamate transporters
GLAST and GLT-1, and the matrix metalloproteinase-13 increased. This work
demonstrates that chondrocytes are a key part in conditions that lead to
inflammation in the cartilage. The inflammatory inducers modulate the cytoskele-
ton, the Ca
2+
signaling, and several inflammatory parameters. In conclusion, our
data show that the cellular responses to inflammatory insults from healthy and
inflammatory chondrocytes resemble those previously observed in astrocyte and
cardiac fibroblasts networks.
Keywords: Cell biology
1. Introduction
Osteoarthritis (OA) is a chronic progressive disease accompanied by low-grade
inflammation that leads to pain and loss of joint mobility in horses and humans [1,
2,3,4,5]. The OA joint includes pro-inflammatory mediators; interleukin-1 β(IL-
1β) and tumor necrosis factor (TNF)-αand Serum amyloid A (SAA) present in the
early stages and antimicrobial peptides (AMPs) as defensines important for tissue
repair [6,7,8]. Activity of IL-1 βhas been detected in synovial fluid from joints
[4] and has been shown to induce gene expression of matrix degrading MMP:s
(matrix metallopeptidases) and ADAMTS (a distintegrin-like and metalloprotei-
nase with thrombospondin type 1 motif) in chondrocytes. MMP-13 (collagenase 3)
has been shown to be upregulated in articular cartilage explants after mechanically
induced damage [9] and are localized in chondrocytes from patients with
arthropathy of the temporomandibular joint discs and appears to play e key role in
the degradation of the collagen type II network of the cartilage matrix [10].
In adult articular cartilage, chondrocytes exist as individual cells embedded in the
extracellular matrix and gap junctions are mainly expressed by the flattened
chondrocytes forming cellular processes facing the superficial layer [11].
Chondrocytes also exist as several cells such structures are referred to as
chondrons [12]. Intercellular communication from paired chondrocytes is mediated
through the gap junction channel protein connexin 43 (Cx43) [13,14,15,16].
Cx43 enhances the expression of OA-associated genes such as matrix
metallopeptidase (MMP)-1, MMP-13, and a disintegrin-like and metalloproteinase
with thrombospondin type 1 motif (ADAMTS) in cultured rabbit synovial
fibroblasts [14]. Although it is required for differentiation, whether Cx43 plays
additional roles in chondrocytes remains unclear. Equine tenocytes are connected
to each other in a three-dimensional network through cytoplasmic processes and
linked gap junctions. Mechanical stimuli of both chondrocytes and tenocytes (such
as strain) are sensed through gap junctions, which leads to the synthesis of collagen
[15] [16]. Chondrocytes in the superficial layer unfold their cell membrane ruffles
in a load-dependent manner as a protective measure against cell membrane rupture
Article No~e00525
2http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

and necrosis [17]. Mitochondrial dysfunction and chondrocyte death of the cells in
the superficial zone has been shown to be an acute response to mechanical injury
[18]. Dynamic compression improves the synthesis of matrix molecules as
aggrecan, collagen type II and lubricin by chondrocytes in the superficial layer [19]
and physical activity improve the lubricative properties of articular cartilage by
increasing lubricin synthesis [20].
Changes in intracellular Ca
2+
levels have a profound influence on many cell
functions, including matrix synthesis and degradation [21]. An increase in
cytosolic Ca
2+
levels can promote the release of signaling molecules via the
process of regulated exocytosis. Such molecules can be transmitters, cytokines,
prostaglandins, proteins, and peptides [22]. The gap junction channels are
composed of two hemichannels, one provided by each of the joined cells [23].
Ca
2+
signaling over long distances is analogous to, but much slower than, the
propagation of action potentials [24]. Cytosolic Ca
2+
plays a key role as a second
messenger, and the control of Ca
2+
signals is therefore critical. This involves
coordination of Ca
2+
entry across the plasma membrane, Ca
2+
release from the
endoplasmic reticulum, refilling of the endoplasmic reticulum stores, and extrusion
across the plasma membrane [25]. There are two main types of Ca
2+
communication: either intercellular communication through gap junctions or
extracellular communication by diffusion of ATP, which binds to purinoceptors
[26,27]. In the presence of inflammation, an elevation of Ca
2+
signaling in the
cellular networks is observed; this is dependent on increased production and
release of ATP through opening of hemichannels in the plasma membrane. ATP
stimulates purinoceptors through autocrine or paracrine stimulation, resulting in
increased release from internal stores in the form of Ca
2+
oscillations, which may
change the balance between Ca
2+
-regulating processes [22]. This extracellular Ca
2
+
signaling attenuates the intercellular Ca
2+
signaling, resulting in reduced
communication via gap junctions [28].
The Na
+
-Ca
2+
exchanger, a Ca
2+
transporter that controls the intracellular Ca
2+
concentration, is driven by the Na
+
electrochemical gradient across the plasma
membrane. Referred to as an Na
+
/K
+
-ATPase, as it requires K
+
to promote Na
+
-Ca
2+
exchange, levels of this pump indirectly affect Ca
2+
signaling [29]. This is
since inflammatory stimuli downregulate the Na
+
/K
+
-ATPase, thereby altering
Ca
2+
homeostasis in cellular networks [30,31,32,33,34]. The endogenous
glutamate that is released by chondrocytes has important functions in both
physiological and pathological conditions that are mediated via binding to
glutamate receptors. Glutamate transport in chondrocytes is performed by GLAST
and GLT-1[35] and is involved in chondrocyte proliferation [36]. Dynamic
remodeling of the actin cytoskeleton, a critical event during inflammation, also
plays an essential role in the migration and proliferation of cells [37].
Article No~e00525
3http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Exposure to lipopolysaccharide (LPS) and IL-1βcan be used experimentally to
generate inflammatory responses both in vitro and in vivo. LPS (a bacterial
endotoxin) is a potent inflammatory activator [38] that stimulates Toll-like receptor
4 (TLR4) [34] [39]. TLR4 is upregulated in astrocytes under neuroinflammatory
conditions [40], and its activation leads to release of the pro-inflammatory
cytokines TNF-αand IL-1β[41].
A more faithful recapitulation of chondrocyte responses is ideally obtained by use
of cells from non-osteoarthritic cartilage. However, obtaining chondrocytes from
healthy joints of humans is prohibited for ethical reasons. To circumvent this issue
we turned to equine articular cartilage as an alternative source of healthy
chondrocytes.
The aim of this study was to explore how the chondrocytes coupled in cell
networks behave over time with respect to Ca
2+
signaling, and to determine if they
exhibit similar properties to those of astrocytes and cardiac fibroblasts. We
examined whether induction of an inflammatory response using IL-1βand LPS led
to changes in glutamate, 5-HT, and ATP induced Ca
2+
responses. Finally, we
examined whether other inflammatory factors were altered in chondrocytes, and
monitored actin filament remodeling during chondrocyte stimulation.
2. Material and methods
2.1. Experimental procedure
2.1.1. Materials
Macroscopically normal cartilage samples were obtained from middle carpal joints
of three horses (1, 3 and 4-years old) without known clinical history of disease in
the joints. Cartilage samples were obtained within 24 h after euthanasia. The horses
were euthanized because of reasons unrelated to this study. The Ethical Committee
on Animal Experiments, Stockholm, Sweden, approved the study protocol. (Dnr:
N378/12). Following aseptic preparation, arthrotomies were performed, cartilage
of the dorsal aspect of the radial facet of the third carpal bone was incised with a
scalpel, and full-thickness cartilage samples were collected with forceps. The tissue
was placed in a sterile saline (0.9% NaCl) solution with gentamicin sulfate (50 mg/
l) and amphotericin B (250 μg/ml). Cartilage samples were transported chilled
(approx. 5 °C) to the laboratory. Isolation and expansion of chondrocytes were
performed as previously described [42].
2.1.2. Chondrocyte culture in monolayer
Chondrocytes was expanded in monolayer as earlier described [42]. Subsequently
passage 3 cells were seeded at 20, 000 cells/cm
2
in Dulbecco’s modified Eagles
medium-high glucose (DMEM-HG) (Thermo Fisher Scientific; Waltham, MA,
Article No~e00525
4http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

USA) supplemented with 14 μg/ml ascorbic acid (Sigma-Aldrich, St. Louis, MO,
USA), 10
−7
M dexamethasone (Sigma-Aldrich), 1 mg/mL human serum albumin
(Equitech Bio, Kerville, TX, USA), 1 × insulin–transferrin–selenium (Gibco, Life
Technologies, Carlsbad, CA, USA), 5 μg/mL linoleic acid (Sigma-Aldrich), 1 ×
penicillin-streptomycin (PEST) (Sigma-Aldrich), and 10 ng/mL human transform-
ing growth factor (TGF) β-1 (R&D Systems, Abingdon, UK) on glass coverslips
(Nr 1, diameter 20 mm, BergmanLabora, Stockholm, Sweden) in 12 well culture
plates for Ca
2+
and immunohistochemistry analysis, or in 6 well culture plates for
Western blot analysis.
2.1.3. Treatment with inflammatory inducers
The cells were incubated 3 to 7 days in monolayer culture and were then stimulated
with recombinant IL-1β(5 ng/mL, R&D Systems) or LPS (10 ng/mL, Escherichia
coli 055:B5, List Biological Laboratories, CA, USA) for 24 h. Unstimulated cells
were used as controls.
2.1.4. Ca
2+
imaging
Chondrocytes incubated with IL-1β, LPS or untreated controls were incubated at
room temperature with the Ca
2+
sensitive fluorophore probe Fura-2/AM
(Invitrogen Molecular Probes, Eugene, USA) for 20 min (8 μL in 990 μL Hank
’s HEPES-buffered saline solution (HHBSS), containing 137 mM NaCl, 5.4 mM
KCl, 0.4 mM MgSO
4
, 0.4 mM MgCl
2
, 1.26 mM CaCl
2
, 0.64 mM KH
2
PO
4
, 3.0
mM NaHCO
3
, 5.5 mM glucose and 20 mM HEPES dissolved in distilled water, pH
7.4 (Sigma-Aldrich)). The fluorophore probe was dissolved in 40 μL dimethyl
sulfoxide (DMSO) and 10 μL pluronic acid (MolecularProbes, Leiden, the
Netherlands). The experiments were performed at room temperature using a Ca
2+
imaging system and Simple PCI software (CompixInc., Imaging Systems,
Hamamatsu Photonics Management Corporation, Cranberry Twp., PA, USA)
connected to an inverted epifluorescence microscope (Nikon ECLIPSE TE2000-E)
with a 20x (NA0.45) fluorescence dry lens and a Polychrome V, monochromator-
based illumination system (TILL Photonics, CA, USA). The various substances
glutamate (10
−3
M), 5-HT (10
−5
M), or ATP (10
−4
M) (Sigma-Aldrich) were
applied using a peristaltic pump (Instech Laboratories, Plymouth Meeting, PA,
USA) at an approximate rate of 600 μl/min. One minute after the start of the
experiment, the stimulating substance was pumped into the pump tubes for 30 s.
The substance took approximately 60 s to reach the cells through the tubes.
HHBSS continued to flow through the pump tubes and onto the cells throughout
the experiment. The images were captured with an ORCA-12AG High Res Digital
Cooled CCD Camera (C4742-80-12AG, Hamamatsu Photonics). The total area
under the curve (AUC), which reflects the amount of Ca
2+
released, was analyzed
in order to measure the strength of the Ca
2+
responses. The amplitude was
Article No~e00525
5http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

expressed as the maximum increase of the 340/380 ratios, and the area under the
Ca
2+
peaks was calculated in Origin (Microcal Software Inc., Northampton, MA,
USA).
2.1.5. Immunocytochemistry and actin visualization
The cells were fixed with 4% paraformaldehyde (Bie & Berntsen, Herlev,
Denmark) for 10 min and washed twice with phosphate-buffered saline (PBS,
Invitrogen) containing 1% bovine serum albumin (PBS–BSA, Sigma-Aldrich). The
cells were permeabilized with PBS–BSA containing 0.05% saponine (PBS–BSA-
–Sap, Sigma-Aldrich) for 20 min. Subsequently, the cells were incubated for 1 h
with a rabbit polyclonal COMP antibody (1:1000) provided by Professor
Heinegård (Lund University, Sweden)(2) or rabbit IgG (X0903, Dako, Glostrup,
Denmark) as isotype control using the same concentration as the primary antibody.
The cells were washed with PBS–BSA–Sap for 3 × 5 min and incubated with a
mixture of FITC conjugated F(ab′)2 fragment donkey anti-mouse IgG and a
Dylight 594 conjugated F(ab′)2 fragment donkey anti-rabbit IgG secondary
antibodies (Jackson ImmunoResearch Europe Ltd, Suffolk, UK), both diluted
1:150. The cells was counterstained with an Alexa
TM
488-conjugated phalloidin
probe (Invitrogen) and washed with PBS–BSA–Sap for 3 × 5 min and finally
rinsed with PBS. The cover slips were mounted on microscope slides with a
fluorescent mounting medium (Dako) and viewed in a Nikon Optiphot-2
microscope. Digital pictures were taken with the NIS Elements D Ver.3.2 (Nikon,
Tokyo, Japan).
2.1.6. Western blot analysis
Western blot analysis was carried out according to standard protocols. Briefly,
protein extracts were prepared by cell lysis in RIPA buffer (Sigma-Aldrich)
supplemented with a mammalian protease inhibitor cocktail (Sigma-Aldrich).
Protein concentration was determined using the Pierce BCA protein assay kit (Life
Technologies), according to the manufacturer’s instructions. Equal amounts of
extracts (5 μg) were resolved on 4–12% Bis-Tris pre-cast gels (Life Technologies)
and transferred onto nitrocellulose membrane (GE healthcare). Equal loading and
transfer of the proteins was confirmed by Ponceau staining (0.1% in acetic acid,
Sigma-Aldrich). The membranes were probed with the following primary
antibodies; polyclonal rabbit anti-TLR-4 (M-300) (sc-30002, Santa Cruz),
polyclonal rabbit anti-GLT-1 (pab0037, Covalab), polyclonal rabbit anti-GLAST
(pab0036-P, Covalab), polyclonal rabbit anti-connexin 4371–0700, Life technolo-
gies), monoclonal mouse anti-Na+/K+-ATPase (A276, Sigma-Aldrich), and
mouse monoclonal anti-β-actin (A5441, Sigma-Aldrich). The membranes were
then probed with horseradish peroxidase–conjugated secondary antibodies
(Jackson ImmunoResearch). Protein bands were detected using Immobilon
Article No~e00525
6http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Western chemiluminescent HRP substrate (Millipore) in a ChemiDoc XRS+
instrument (Bio-Rad). The relative intensity of the protein bands in the linear
exposure range were quantified using the ImageLab software (BioRad).
2.1.7. Assay for MMP-13
Concentration of MMP-13 were measured in the culture media using Fluorokine E
Human Active MMP-13Fluorescent Assay (R&D Systems) after adding APMA
(aminophenyl mercuric acetate), which activates latent MMP-13. The protein
determination assay was performed according to the manufacturer’s instructions
using a DC Protein Assay (Bio-Rad, Hercules, CA, USA). Both the standards (0–4
mg/mL BSA) and the samples were mixed with the reagents, incubate for 15 min at
22 °C and subsequently read at 750 nm using a Versa-max microplate reader, and
analyzed using SoftMax Pro 4.8 (Molecular Devices, CA, USA). Culture media
(from stimulated and unstimulated cells) was diluted 1:2 and all assay analyses
were performed on duplicate samples and were correlated to the protein
concentration. The lowest detection limit was 8 pg/ml for active MMP-13. The
concentration of activated MMP-13 in media was measured and correlated to total
protein in cell lysate.
2.1.8. Glutamate release
The concentration of glutamate was measured in the culture media using ninhydrid
reagent for photometric determination. Duplicate samples from culture media
(from stimulated and unstimulated cells) were analyzed and correlated to the
protein concentration. The lowest detection limit was 5 μmol/l of glutamate [43].
2.1.9. Statistical methods
The data were analyzed using GraphPad Prisma software version 6.05 (La Jolla,
CA). Statistical differences between sample groups were assessed using Mann-
Whitney-test. The statistical analysis was performed with SPSS v19 (IBM Corp.,
Armonk, NY, USA software). Statistical significance was determined using
Student’s paired t-test or one-way analysis of variance (ANOVA) followed by a
post hoc Tukey’s test. A significant difference was assumed at a p-value of ≤0.05.
Unless otherwise stated, the data are expressed as the mean and standard error of
the mean (SEM).
3. Results
3.1. Actin filaments and COMP immunocytochemistry
The chondrocytes were stained with Alexa
TM
488-conjugated phalloidin probe. The
unstimulated cells demonstrated F-actin organized in stress fibers (Fig. 1). The
Article No~e00525
7http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

inflammatory induced cells, incubated with IL-1βor LPS for 24 h, showed signs of
retracted actin structures and some actin ring structures. All cells, both
unstimulated cells and cells stimulated with IL-1βor LPS for 24 h, demonstrated
positive and distinct COMP-staining indicating a chondrogenic phenotype (Fig. 1).
3.2. Time-dependent Ca
2+
responses
In order to evaluate an appropriate time point for the experimental conditions, the
chondrocytes were stimulated with glutamate, 5-HT, or ATP and monitored using
Ca
2+
imaging between days 3 and 7. Chondrocytes stimulated with glutamate
showed an increase in Ca
2+
signaling at day 3, whereas no response was detectable
on the other days (Fig. 2A). The 5-HT stimulus evoked Ca
2+
signaling which
peaked at day 4 (Fig. 2B), and both day 3 and day 4 were significantly higher
compared to day 6. The ATP stimulation induced Ca
2+
signaling peaked at day 5,
where it was significantly higher compared to days 6 and 7 (Fig. 2C). Based on
these data, we chose 4-day-old cultures for carrying out the following experiments.
3.3. Intracellular evoked Ca
2+
release
Chondrocytes cultured for 4 days and then stimulated with IL-1βor LPS for further
24 h, were subjected to Ca
2+
imaging experiments. The areas under the Ca
2+
peak
(AUC) were calculated. The glutamate evoked Ca
2+
signaling was increased in
[(Fig._1)TD$FIG]
Fig. 1. Actin filament and immunostaining of COMP. Unstimulated chondrocytes and chondrocytes
stimulated with IL-1β(5 ng/ml) or LPS (10 ng/ml) for 24 h were stained with Alexa
TM
488-conjugated
(red) phalloidin probe and for the chondrogenic matrix molecule COMP (green). The nuclei visualized
with Hoescht33258 (blue). Untreated chondrocytes were dominated by F-actin organized in stress fibers
and pronounced COMP-staining (A, D). Chondrocytes stimulated with IL-1β(B, E) or LPS (C, F)
exhibited a more retracted organization and occasionally ring structures were demonstrated (white
arrows). COMP-staining similar to unstimulated chondrocytes was observed (n = 3-4). Control sections
with isotype staining were used (data not shown).
Article No~e00525
8http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

inflammatory chondrocytes and significantly higher in cells stimulated with IL-1β
compared to unstimulated controls (Fig. 3). Stimulation with 5-HT or ATP induced
[(Fig._2)TD$FIG]
Fig. 2. Time-dependent evoked Ca
2+
responses. All chondrocytes were stimulated in a Ca
2+
imaging
system over time with glutamate (10
−3
M) (A), 5-HT (10
−5
M) (B) or ATP (10
−4
M) (C). The areas
under the Ca
2+
peaks (AUC) of Ca
2+
transients were calculated. The cells were from 3–4 coverslips,
and from 3 different seeding times. One-way ANOVA followed by Turkey’s post hoc test was used for
statistical analysis (n = 43) *p <0.05, **p <0.01.
Article No~e00525
9http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

[(Fig._3)TD$FIG]
Fig. 3. Glutamate evoked Ca
2+
responses in inflammatory chondrocytes. All cells were stimulated in a
Ca
2+
imaging system, and the areas under the Ca
2+
peaks (AUC) of Ca
2+
transients were calculated.
Chondrocyte Ca
2+
responses to glutamate (10
−3
M) when stimulated with IL-1β(5 ng/ml) or LPS (10
ng/ml) for 24 h, unstimulated cells were used as control (A). The cells were from 3–4 coverslips, and
from 3 different seeding times. The appearance of the Ca
2+
transients is visualized; results are shown
from a typical experiment (B). For statistical analysis, a paired student’s t-test was used to compare
unstimulated cells and IL-1βor LPS stimulated cells (n = 30), *p <0.05, **p <0.01.
[(Fig._4)TD$FIG]
Fig. 4. 5-HT evoked Ca
2+
responses in inflammatory chondrocytes. All cells were stimulated in a Ca
2+
imaging system, and the areas under the Ca
2+
peaks (AUC) of Ca
2+
transients were calculated.
Chondrocyte Ca
2+
responses to 5-HT (10
−5
M) when stimulated with IL-1β(5 ng/ml) or LPS (10 ng/
ml) for 24 h, unstimulated cells were used as control (A). The cells were from 3–4 coverslips, and from
3 different seeding times. The appearance of the Ca
2+
transients is visualized; results are shown from a
typical experiment (B). For statistical analysis, a paired student’s t-test was used to compare
unstimulated cells and IL-1βor LPS stimulated cells (n = 30), *p <0.05, **p <0.01, ***p <0.001.
Article No~e00525
10 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

significantly higher Ca
2+
signaling in chondrocytes stimulated with IL-1βor LPS,
compared to unstimulated controls (Figs. 4and 5).
3.4. Activated MMP-13 release
The concentration of activated MMP-13 was increased after the chondrocytes had
been incubated with IL-1β. No activated MMP-13 release was obtained after the
chondrocytes had been incubated with LPS (Table 1).
[(Fig._5)TD$FIG]
Fig. 5. ATP evoked Ca
2+
responses in inflammatory chondrocytes. All cells were stimulated in a Ca
2+
imaging system, and the areas under the Ca
2+
peaks (AUC) of Ca
2+
transients were calculated.
Chondrocyte Ca
2+
responses to ATP (10
−4
M) when stimulated with IL-1β(5 ng/ml) or LPS (10 ng/ml)
for 24 h, unstimulated cells were used as control (A). The cells were from 3–4 coverslips, and from 3
different seeding times. The appearance of the Ca
2+
transients is visualized; results are shown from a
typical experiment (B). For statistical analysis, a paired student’s t-test was used to compare
unstimulated cells and IL-1βor LPS stimulated cells (n = 30), *p <0.05, **p <0.01, ***p <0.001.
Table 1. Concentrations of activated MMP-13. MMP-13 measured in culture,
mean (pg/ml) ± SEM, supernatants of unstimulated, IL-1βor LPS-stimulated
monolayer chondrocytes, compared to total amount of protein in cell lysate. A
higher MMP-13 concentration in media from IL-1βstimulated cells compared to
unstimulated control was found. One-way analysis of variance followed by the
Bonferroni test was used for multiple group comparisons. P <0.05 was considered
statistically significant. NS = Non-significant. P-value; investigated group vs
control.
Control IL-1βstimulated LPS stimulated
Mean 0,02 3.94 0,022
±SDs 0,0005 0,8041 0,0008
p-Value NS P <0,01 NS
N4 4 4
Article No~e00525
11 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

3.5. TLR4, Cx43, Na+/K+-ATPase, GLAST and GLT-1 ex-
pression in chondrocytes
The levels of several proteins involved in inflammatory and Ca
2+
signaling was
investigated in control cells and cells incubated with IL-1βor LPS. Chondrocytes
incubated with LPS showed an increased expression of TLR4. Cx43 expression
was increased both in IL-1βand LPS incubated chondrocytes compared to control
cells. The glutamate transporters, GLAST and GLT-1 also increased in expression
after LPS incubation. The Na+/K+-ATPase expression did not change markedly
(Figs. 6and 7).
3.6. Glutamate release
Glutamate release was increased in media from LPS stimulated chondrocytes
(0,033 ± 0,016 μmol/l/mg protein) compared to unstimulated cells (0,013 ± 0,002
[(Fig._6)TD$FIG]
Fig. 6. A-F. The effect of IL-1βand LPS treatment on TLR4 and some membrane transporter levels in
chondrocytes. Chondrocytes were treated with IL-1β(5 ng/ml) or LPS (10 ng/ml) for 24 h, and
unstimulated cells were used as control. A-E, The cells were analyzed by western blotting with
antibodies against TLR4, connexin 43 (Cx 43), Na
+
/K
+
ATPase, GLT-1, GLAST-1, and β-actin as
indicated on the figure. The blots are representative of three different horses. F, Representative Ponceau
staining to confirm protein loading, which correlated to β-actin levels indicating it to be an appropriate
internal control for normalization of protein levels.; Full size image is shown in supplementary
documents (Fig. 6)of(Fig. 6A-F).
Article No~e00525
12 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

μmol/l/mg protein), p= 0.03. Additionally, glutamate release was detected in
media from IL-1βstimulated cells (0,020 ± 0,009 μmol/l/mg protein), however
there was not a statistically significant difference when compared to the
unstimulated cells.
4. Discussion
The aim of the current study was to explore whether chondrocytes exhibit similar
biochemical properties as the network-coupled astrocytes and cardiac fibroblasts,
and how inflammatory stimuli affect these parameters. Network-coupled cells are
excitable, but do not express action potentials [24,27,44]. Rather, they are
equipped with Ca
2+
signaling systems that can be inter-and/or extracellular in
nature. Our results show time-dependent Ca
2+
responses of chondrocytes to
glutamate, 5-HT, and ATP; this mimics the response of astrocytes in the central
nervous system, which is one of the most well studied network-coupled cell
systems and cardiac fibroblast [27,45,46]. Cells cultured in monolayer are prone
to dedifferentiate, which could confound interpretation of results. Reassuringly, the
COMP-positive staining we observed confirmed the phenotype of the articular
[(Fig._7)TD$FIG]
Fig. 7. A-E. The relative intensities of protein bands (mean ± SE) are shown in bar graphs for TLR4,
connexin 43 (Cx 43), Na
+
/K
+
ATPase, GLT-1, GLAST-1. Intensity were quantified using the
ImageLab software in the linear exposure range and normalized to β-actin from the same blot. The
intensity of the control protein band was set to 1.
Article No~e00525
13 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

cartilage chondrocytes [42,47]. Intercellular communication gives tissues the
ability to coordinate different cellular functions, such as the regulation of cell
volume, intracellular ionic composition, and cell metabolism. A characteristic of
network-coupled cells is their passive electrical properties, which allows them to
provide the framework and metabolic support for different organs, but also to
supply computational power and modify behavioral output. Intercellular
communication takes place through gap junctions and extracellular communication
through the diffusion of ATP, which then binds to purinoceptors [26,27].
Chondrocytes are capable of sustaining propagation of intercellular Ca
2+
waves in
rabbits [13] as well as in equines [48,49].
During inflammation and when cells are stimulated with inflammatory agents, Ca
2
+
signaling in network-coupled cells is disturbed. There is an increased release of
IL-1β, which results in gap junction inhibition [50]. There is also an increased
release of ATP, which provides a parallel system for intercellular Ca
2+
communication [51] and triggers the release of glutamate [52]. Here we show
that the inflammatory inducers IL-1βand LPS changed the glutamate-, 5-HT-, and
ATP-evoked Ca
2+
transients from single peaks to Ca
2+
oscillations when the
chondrocytes were incubated with IL-1βor LPS for 24 h.
Comparable results have been obtained with astrocytes, also coupled with gap
junctions, in monoculture [41,53,54]. This suggests that the intracellular Ca
2+
release from the endoplasmatic reticulum is increased after inflammatory
induction, which leads to an overstimulation of the G
q
protein coupled mGluR5-
, 5-HT
2A
-, and P2Y-, respectively [26]. Intracellular Ca
2+
is an important
parameter due to its influences on many cell functions, including matrix synthesis
and degradation [55]. An increase in cytosolic Ca
2+
levels can induce release of
signaling molecules, via exocytosis and such molecules can be cytokines,
prostaglandins and transmitters [22].
OA is associated with a low grade inflammation and the presence of pro-
inflammatory cytokines, IL-1βand TNF-αduring the early stages of disease [56]
[5]; this milieu leads to MMP activation and destruction of cartilage [57]. Our
results showed after IL-1βstimulation of chondrocytes the concentration of
secreted MMP-13 in its active form was higher compared to control and LPS-
stimulated cells, indicating that the chondrocytes were responsive to inflammatory
stimuli. Systemic (peripheral) inflammation with high serum levels of pro-
inflammatory cytokines is associated with low-grade inflammatory diseases such
as OA [2] and may be a physiological manifestation of metabolic syndrome [58].
When peripheral tissues are injured, pro-inflammatory mediators are released into
the bloodstream. A systemic inflammatory response is then activated, resulting in
alterations in several inflammatory parameters [59,60].
Article No~e00525
14 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

During inflammation the expression and affinities of several receptors is changed
in many cell types. The expression of TLR4 increases in response to LPS [34,39].
It was also prominent in chondrocytes stimulated with LPS or IL-1β, and has been
confirmed in the biosynthetic activity in the cartilage [61].
A disruption of the actin filament with more pronounced ring-structures was found
in inflammatory stimulated chondrocytes. F-actin reorganization engenders
upregulation of inflammatory cytokines in many different cell types. Following
LPS exposure, the production of pro-inflammatory cytokines, is high in pulmonary
monocytes [62] as well as in astrocytes [41]. Increased IL-1βproduction is also
observed in astrocytes after LPS stimulation, which seems to be initiated through
TLR4 activation [63]. An upstream activator of IL-1βis a key component of
immune response; the inflammasomes, which is an intracellular multiprotein
complex involved in the response of inflammation [64].
Chondrocytes are connected to each other via cell-cell interactions and form
functional gap junctions expressing Cx43 [13,65]. In the adult articular cartilage,
chondrocytes exists as individual cells embedded in the extracellular matrix, and
gap junctions are mainly expressed by the flattened chondrocytes facing the
superficial cartilage layer [11]. In diseased cartilage, chondrocytes divide and
forms cluster of cells referred to as chondron. There is a possibility that the
chondrocytes laying close to each other in the chondron are connected to each
other via gap junctions. In our study confluent monolayer cells express the major
gap junction protein Cx43 and there was an increased protein expression when the
cells were stimulated with IL-1 βor LPS. This is in accordance to recent data [13]
showing increased expression of Cx43 in IL-1βstimulated chondrocytes.
Chondrocytes release and accumulate glutamate in a Ca
2+
dependent manner [66].
We show that chondrocytes express the glutamate transporters GLAST and GLT-1,
which are upregulated after inflammatory induction. We also observed glutamate
release in the media.
The present study show that chondrocytes are connected into gap junction coupled
networks, responding to Ca
2+
-evoked release from intracellular stores, which
change in behavior after inflammatory induction. As a consequence of these
cellular inflammatory changes in chondrocytes the actin filaments are reorganized
and several biochemical parameters changed.
In conclusion, our data show that response of chondrocytes to pro-inflammatory
stimuli is very similar to that of astrocytes and cardiac fibroblasts, and
demonstrates that these cells are another example of a coupled-cell network.
Article No~e00525
15 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Declarations
Author contribution statement
Eva Skiöldebrand: Conceived and designed the experiments; Performed the
experiments; Analyzed and interpreted the data; Contributed reagents, materials,
analysis tools or data; Wrote the paper.
Anna Thorfve: Performed the experiments; Analyzed and interpreted the data;
Wrote the paper.
Ulrika Björklund, Pegah Johansson, Ruth Wickelgren: Performed the experiments;
Wrote the paper.
Anders Lindahl: Analyzed and interpreted the data; Wrote the paper.
Elisabeth Hansson: Conceived and designed the experiments; Analyzed and
interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote
the paper.
Funding statement
This work was supported by the Swedish Research Council grant No 2012–2517,
ALF-Research grant 432251 Western Region, AFA Insurance, Stockholm,
Sweden, Edit Jacobson’s Foundation and the Sahlgrenska University Hospital
(LUA/ALF GBG-11587), Gothenburg, Sweden.
Competing interest statement
The authors declare no conflict of interest.
Additional information
Supplementary content related to this article has been published online at http://dx.
doi.org/10.1016/j.heliyon.2018.e00525.
Acknowledgements
We thank Dr Cecilia Ley (Section of Pathology, Department of Biomedical
Sciences and Veterinary Public Health, Swedish University of Agricultural
Sciences, Uppsala) for harvesting of the equine chondrocytes.
References
[1] C.J. Secombe, et al., Pathophysiology and diagnosis of third carpal bone
disease in horses: a review, NZ Vet. J. 50 (1) (2002) 2–8.
Article No~e00525
16 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

[2] M.B. Goldring, M. Otero, Inflammation in osteoarthritis, Curr. Opin.
Rheumatol. 23 (5) (2011) 471–478.
[3] R.F. Loeser, Osteoarthritis year in review 2013: biology, Osteoarthritis
Cartilage 21 (10) (2013) 1436–1442.
[4] J.L. Kamm, A.J. Nixon, T.H. Witte, Cytokine and catabolic enzyme
expression in synovium, synovial fluid and articular cartilage of naturally
osteoarthritic equine carpi, Equine Vet. J. 42 (8) (2010) 693–699.
[5] F. Berenbaum, Osteoarthritis as an inflammatory disease (osteoarthritis is not
osteoarthrosis!), Osteoarthritis Cartilage 21 (1) (2013) 16–21.
[6] M.B. Goldring, et al., Roles of inflammatory and anabolic cytokines in
cartilage metabolism: signals and multiple effectors converge upon MMP-13
regulation in osteoarthritis, Eur. Cell Mater. 21 (2011) 202–220.
[7] S. Jacobsen, P.H. Andersen, The acute phase protein serum amyloid A (SAA)
as a marker of inflammation in horses, Equine Veterinary Education 19 (1)
(2007) 38–46.
[8] G. Musumeci, et al., Beta-Defensin-4 (HBD-4) is expressed in chondrocytes
derived from normal and osteoarthritic cartilage encapsulated in PEGDA
scaffold, Acta Histochem. 114 (8) (2012) 805–812.
[9] E.E. van Haaften, K. Ito, C.C. van Donkelaar, The initial repair response of
articular cartilage after mechanically induced damage, J. Orthop. Res. 35 (6)
(2017) 1265–1273.
[10] R. Leonardi, et al., MMP-13 (collagenase 3) localization in human
temporomandibular joint discs with internal derangement, Acta Histochem.
110 (4) (2008) 314–318.
[11] S.S. Chi, J.B. Rattner, J.R. Matyas, Communication between paired
chondrocytes in the superficial zone of articular cartilage, J. Anat. 205 (5)
(2004) 363–370.
[12] C.A. Poole, Articular cartilage chondrons: form, function and failure, J. Anat.
191 (Pt 1) (1997) 1–13.
[13] R. Tonon, P. D'Andrea, Interleukin-1beta increases the functional expression
of connexin 43 in articular chondrocytes: evidence for a Ca2+-dependent
mechanism, J. Bone Miner. Res. 15 (9) (2000) 1669–1677.
[14] A. Gupta, et al., Connexin43 enhances the expression of osteoarthritis-
associated genes in synovial fibroblasts in culture, BMC Musculoskelet.
Disord. 15 (1) (2014) 425.
Article No~e00525
17 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

[15] C.J. O'Conor, et al., TRPV4-mediated mechanotransduction regulates the
metabolic response of chondrocytes to dynamic loading, Proc. Natl. Acad.
Sci. USA 111 (4) (2014) 1316–1321.
[16] A.J. Banes, et al., Gap junctions regulate responses of tendon cells ex vivo to
mechanical loading, Clin. Orthop. Relat. Res. 1999 (367 Suppl) (2016)
S356–S370.
[17] E.K. Moo, W. Herzog, Unfolding of membrane ruffles of in situ chondrocytes
under compressive loads, J. Orthop. Res. 35 (2) (2017) 304–310.
[18] M.L. Delco, et al., Mitochondrial dysfunction is an acute response of articular
chondrocytes to mechanical injury, J. Orthop. Res. (2017).
[19] J.E. Jeon, et al., Dynamic compression improves biosynthesis of human zonal
chondrocytes from osteoarthritis patients, Osteoarthritis Cartilage 20 (8)
(2012) 906–915.
[20] G.C. Musumeci, P. Trovato, F.M. Imbesi, R., giunta, S. Szychlinska, M.A.
Loreto, C. Castorina, S. Mobasheri, A., Physical activity ameliorates cartilage
degeneration in a rat model of aging: A study on lubricin expression, Scand.
J. Med. Sci. Sports 25 (2015) 222–230.
[21] M.J. Berridge, M.D. Bootman, P. Lipp, Calcium–a life and death signal,
Nature 395 (6703) (1998) 645–648.
[22] R. Zorec, et al., Astroglial excitability and gliotransmission: an appraisal of
Ca2+ as a signalling route, ASN Neuro. 4 (2012) (2).
[23] C. Giaume, K.D. McCarthy, Control of gap-junctional communication in
astrocytic networks, Trends Neurosci. 19 (8) (1996) 319–325.
[24] A.H. Cornell-Bell, et al., Glutamate induces calcium waves in cultured
astrocytes: long-range glial signaling, Science 247 (4941) (1990) 470–473.
[25] L. Lencesova, et al., Plasma membrane-cytoskeleton-endoplasmic reticulum
complexes in neurons and astrocytes, J. Biol. Chem. 279 (4) (2004)
2885–2893.
[26] M.L. Cotrina, et al., Connexins regulate calcium signaling by controlling ATP
release, Proc. Natl. Acad. Sci. USA 95 (26) (1998) 15735–15740.
[27] F. Blomstrand, et al., 5-Hydroxytryptamine and glutamate modulate velocity
and extent of intercellular calcium signalling in hippocampal astroglial cells
in primary cultures, Neuroscience 88 (4) (1999) 1241–1253.
Article No~e00525
18 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

[28] N. Karpuk, et al., Neuroinflammation leads to region-dependent alterations in
astrocyte gap junction communication and hemichannel activity, J. Neurosci.
31 (2) (2011) 414–425.
[29] X. Liu, et al., Ankyrin B modulates the function of Na,K-ATPase/inositol
1,4,5-trisphosphate receptor signaling microdomain, J. Biol. Chem. 283 (17)
(2008) 11461–11468.
[30] E. Hansson, Could chronic pain and spread of pain sensation be induced and
maintained by glial activation? Acta Physiologica 187 (1–2) (2006) 321–327.
[31] C. Lundborg, et al., Glial cell line-derived neurotrophic factor is increased in
cerebrospinal fluid but decreased in blood during long-term pain, J.
Neuroimmunol. 220 (1-2) (2010) 108–113.
[32] E. Hansson, L. Ronnback, Glial neuronal signaling in the central nervous
system, FASEB J. 17 (3) (2003) 341–348.
[33] D. Delbro, et al., In inflammatory reactive astrocytes co-cultured with brain
endothelial cells nicotine-evoked Ca(2 +) transients are attenuated due to
interleukin-1beta release and rearrangement of actin filaments, Neuroscience
159 (2) (2009) 770–779.
[34] J. Forshammar, et al., Naloxone and ouabain in ultralow concentrations
restore Na+/K+-ATPase and cytoskeleton in lipopolysaccharide-treated
astrocytes, J. Biol. Chem. 286 (36) (2011) 31586–31597.
[35] Y.H. Jean, et al., Increase in excitatory amino acid concentration and
transporters expression in osteoarthritic knees of anterior cruciate ligament
transected rabbits, Osteoarthritis Cartilage 16 (12) (2008) 1442–1449.
[36] E. Hinoi, et al., Functional expression of particular isoforms of excitatory
amino acid transporters by rodent cartilage, Biochem. Pharmacol. 70 (1)
(2005) 70–81.
[37] E. Hansson, Actin Filament Reorganization in Astrocyte Networks is a Key
Functional Step in Neuroinflammation Resulting in Persistent Pain: Novel
Findings on Network Restoration, Neurochem. Res. (2014).
[38] T. Nakamura, M. Shiozaki, S. Kurakata, Synthesis of ceramidated GLA-60
derivatives, Carbohydr. Res. 337 (15) (2002) 1343–1349.
[39] T. Kielian, Toll-like receptors in central nervous system glial inflammation
and homeostasis, J. Neurosci. Res 83 (5) (2006) 711–730.
[40] M.R. Hutchinson, et al., Non-stereoselective reversal of neuropathic pain by
naloxone and naltrexone: involvement of toll-like receptor 4 (TLR4), Eur. J.
Neurosci. 28 (1) (2008) 20–29.
Article No~e00525
19 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

[41] L. Block, et al., A new concept affecting restoration of inflammation-reactive
astrocytes, Neuroscience 250 (2013) 536–545.
[42] C. Ley, et al., Effects of high mobility group box protein-1, interleukin-1beta,
and interleukin-6 on cartilage matrix metabolism in three-dimensional equine
chondrocyte cultures, Connect. Tissue Res. 52 (4) (2011) 290–300.
[43] S. More, W.H. Stein, Modified ninhydrid reagent for the photometric
detemination of aminoacids and related compounds, J. Biol. Chem. 211
(1954) 907–913.
[44] E. Scemes, C. Giaume, Astrocyte calcium waves: what they are and what they
do, Glia 54 (7) (2006) 716–725.
[45] M. Nilsson, E. Hansson, L. Ronnback, Adrenergic and 5-HT2 receptors on
the same astroglial cell: A microspectrofluorimetric study on cytosolic Ca2+
responses in single cells in primary culture, Brain Res. Dev. Brain Res. 63 (1-
2) (1991) 33–41.
[46] E. Skioldebrand, et al., Inflammatory activation of human cardiac fibroblasts
leads to altered calcium signaling, decreased connexin 43 expression and
increased glutamate secretion, Heliyon 3 (10) (2017) e00406.
[47] T. Tallheden, et al., Gene expression during redifferentiation of human
articular chondrocytes, Osteoarthritis Cartilage 12 (7) (2004) 525–535.
[48] F. Vittur, et al., Energy metabolism: replicative ability, intracellular calcium
concentration, and ionic channels of horse articular chondrocytes, Exp. Cell.
Res. 210 (1) (1994) 130–136.
[49] R.J. Wilkins, et al., Homeostasis of intracellular Ca2+ in equine
chondrocytes: response to hypotonic shock, J. Equine. Vet. 35 (5) (2003)
439–443.
[50] J.E. Kim, et al., Levetiracetam inhibits interleukin-1 beta inflammatory
responses in the hippocampus and piriform cortex of epileptic rats, Neurosci.
Lett. 471 (2) (2010) 94–99.
[51] R.D. Fields, G. Burnstock, Purinergic signalling in neuron-glia interactions,
Nat. Rev. Neurosci. 7 (6) (2006) 423–436.
[52] M. Domercq, et al., P2Y1 receptor-evoked glutamate exocytosis from
astrocytes: control by tumor necrosis factor-alpha and prostaglandins, J. Biol.
Chem. 281 (41) (2006) 30684–30696.
[53] E. Hansson, et al., PACAP attenuates 5-HT: histamine, and ATP-evoked Ca2
+ transients in astrocytes, Neuroreport 20 (10) (2009) 957–962.
Article No~e00525
20 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).

[54] L. Block, et al., Ultralow concentrations of bupivacaine exert anti-
inflammatory effects on inflammation-reactive astrocytes, Eur. J. Neurosci.
38 (11) (2013) 3669–3678.
[55] M.J. Berridge, Inositol trisphosphate and calcium oscillations, Biochem. Soc.
Symp. 2007 (74) (2016) 1–7.
[56] M.B. Goldring, M. Otero, Inflammation in osteoarthritis, Curr. Opin.
Rheumatol. 23 (5) (2011) 471.
[57] P. Wojdasiewicz, L.A. Poniatowski, D. Szukiewicz, The role of inflammatory
and anti-inflammatory cytokines in the pathogenesis of osteoarthritis,
Mediators Inflamm. 2014 (2014) 561459.
[58] J.D. Katz, S. Agrawal, M. Velasquez, Getting to the heart of the matter:
osteoarthritis takes its place as part of the metabolic syndrome, Curr. Opin.
Rheumatol. 22 (5) (2010) 512–519.
[59] H. Machelska, Dual peripheral actions of immune cells in neuropathic pain,
Arch. Immunol. Ther. Exp. (Warsz) 59 (1) (2011) 11–24.
[60] E. Huang, C.A. Wells, The ground state of innate immune responsiveness is
determined at the interface of genetic, epigenetic, and environmental
influences, J. Immunol. 193 (1) (2014) 13–19.
[61] K. Bobacz, G.I. Sunk, J.G. Hofstaetter, L. Amoyo, C.D. Toma, S. Akira, T.
Weichhart, M. Aemann, J.S. Smolen, Toll-like receptors and chondrocytes,
Ahritis Rheum. 56 (6) (2007) 1880–1893.
[62] L. Du, et al., Actin filament reorganization is a key step in lung inflammation
induced by systemic inflammatory response syndrome, Am. J. Respir. Cell
Mol. Biol. 47 (5) (2012) 597–603.
[63] A.M. Blanco, et al., Involvement of TLR4/type I IL-1 receptor signaling in
the induction of inflammatory mediators and cell death induced by ethanol in
cultured astrocytes, J. Immunol. 175 (10) (2005) 6893–6899.
[64] J.P. de Rivero Vaccari, D.W. Dietrich, R.W. Keane, Activation and regulation
of cellular inflammasomes: gaps in our knowledge for central nervous system
injury, J. Cereb. Blood Flow Metab. 34 (3) (2014) 369–375.
[65] M.D. Mayan, et al., Human articular chondrocytes express multiple gap
junction proteins: differential expression of connexins in normal and
osteoarthritic cartilage, Am. J. Pathol. 182 (4) (2013) 1337–1346.
[66] T. Piepoli, et al., Glutamate signaling in chondrocytes and the potential
involvement of NMDA receptors in cell proliferation and inflammatory gene
expression, Osteoarthritis Cartilage 17 (8) (2009) 1076–1083.
Article No~e00525
21 http://dx.doi.org/10.1016/j.heliyon.2018.e00525
2405-8440/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).