Coupled cell networks are target cells of inflammation, which can spread between different body organs and develop into systemic chronic inflammation
Astrocytes
The well-studied cells coupled into networks are astrocytes in the CNS 6], 21]. Astrocytes in networks positioned between the vasculature and synapses monitor neuronal
signaling and synapse rebuilding 6]. Astrocytes express nearly the same repertoire of receptors and ion channels as neurons,
regulate synaptic transmission via bidirectional communication with neurons, and release
gliotransmitters and other factors such as cytokines, fatty acid metabolites, and
free radicals 27], 28].
Because they do not communicate via action potentials, astrocytes are not electrically
active; however, they display a form of excitability that manifests as an increased
intracellular Ca
2+
concentration. Stimuli such as transmitters released from neurons and glial cells
can evoke Ca
2+
elevation in single astrocytes, which passes to adjacent astrocytes and leads to a
Ca
2+
wave that can propagate over long distances, albeit much more slowly than the propagation
of action potentials in neurons 19], 29], 30].
Incoming stimuli activate G protein-coupled receptors that hydrolyze phosphatidylinositol
4,5-bisphosphate (PIP
2
) and cause the release of inositol 1,4,5-trisphosphate (IP
3
) into the cytosol. IP
3
receptors located on the endoplasmic reticulum respond to this elevation of IP
3
by releasing Ca
2+
. Cytosolic Ca
2+
plays a key role as a second messenger; thus, the control of Ca
2+
signals is critical. This control involves coordinating Ca
2+
entry across the plasma membrane, Ca
2+
release from the endoplasmic reticulum, endoplasmic reticulum store refilling, and
Ca
2+
extrusion across the plasma membrane 31]. 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.This Na
+
pump, the Na
+
/K
+
-ATPase, indirectly modulates Ca
2+
signaling 32], and inflammatory stimuli influence Ca
2+
homeostasis in astrocyte networks 33]–35].
Astrocytes are directly connected to adjacent cells by gap junctions, and Cx43 is
the primary gap junction protein 25]. Astrocytes also express hemichannels that open exteriorly; Cx43 appears to also
be the main Cx found in these hemichannels 24]. Astrocytes in most parts of the CNS use two types of Ca
2+
communication: intercellular communication through gap junctions and extracellular
communication through the diffusion of ATP, which then binds to purinoceptors. Both
inter- and extracellular Ca
2+
communication occur in many parts of the cerebrum 19], 36]. In the retina, intercellular communication occurs through astrocytes, but extracellular
communication occurs between astrocytes and Müller cells 37].
The cytoskeleton is important for controlling plasma membrane microdomains and the
endoplasmic reticulum complex. The adaptor protein ankyrin B is associated with the
Na
+
pump as well as with endoplasmic reticulum proteins such as IP
3
. The primary cytoplasmic matrix proteins spectrin and actin are attached to ankyrin
B. An intact cytoskeleton is required for astrocytic Ca
2+
wave propagation 36], and cytoskeletal disruption abolishes Ca
2+
oscillations by changing the balance of the Ca
2+
regulatory processes 38].
Astrocytes contribute to the homeostasis and regulation of extracellular glutamate
levels. The glutamate-glutamine cycle is a well-known process through which glutamine
is released from astrocytes and taken up by glutamatergic or ?-aminobutyric acid (GABA)ergic
neurons. Glutamine is then converted to glutamate in neurons and released into the
synaptic space. The majority of the released glutamate is taken up by astrocytes through
the glutamate transporters; glutamate/aspartate transporter (GLAST/excitatory amino
acid transporter 1, EAAT1), and glial glutamate transporter-1 (GLT-1/EAAT2) and then
metabolized by glutamine synthetase to glutamine 39], 40].
Keratinocytes
The epidermis is a dynamic, stratified structure formed by continually proliferating
and differentiating keratinocytes that surround the sensory nerve endings of several
C- and A?-fiber subtypes. The skin and buccal membrane primarily comprise keratinocytes,
the epidermis. The cells are connected by well-developed intercellular junctions such
as gap junctions. Within these gap junctions, Cx43 is associated with the regulation
of cell proliferation and mediates forms of intercellular communication in which ions
and small molecules are allowed to pass from one cell to another. Cx43 is primarily
localized to the lower epithelial layer, the stratum basale and stratum spinosum 41]. Cx43 degradation is thought to play a role in the differentiation of the gingival
epithelium. Properly regulated gap junctions appear to be essential for efficient
wound healing and for protection against skin diseases. Human epidermal keratinocytes
use intercellular Ca
2+
signaling. G protein-coupled receptors, which activate phospholipase C (PLC) and convert
PIP
2
into diacylglycerol and IP
3
, trigger the release of Ca
2+
from intracellular Ca
2+
stores 42], 43].
In response to stress, injury or even chronic pain, keratinocytes can release ATP
through hemichannels, resulting in Cx43 upregulation. Metabotropic purinergic (P2Y
2
) receptors are then activated, resulting in increased intracellular Ca
2+44]. ATP release is an important signal for epidermal homeostasis and influences keratinocyte
proliferation and differentiation 45]. Glutamate-mediated signaling is observed in keratinocytes in the epidermis, and
different classes of glutamate receptors, including N-methyl-D-aspartate receptor
(NMDA), ?-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA) and metabotropic
glutamate receptors, as well as transporters such as EAAT1 have been identified in
the basal layer. Additionally, GLT-1 has been found in the suprabasal layer 46].
Chondrocytes
Chondrocytes are connected to each other via cell-to-cell interactions and form functional
gap junctions that express Cx43 47], 48]. They can sustain the propagation of intercellular Ca
2+
waves in rabbits 47], humans, and equines 49], 50] and can also form hemichannels that exchange signals within the extracellular space
51] (Skiöldebrand et al., unpublished). Articular chondrocytes accumulate intracellular
IP
3
following mechanical stimulation, causing the diffusion of IP
3
into adjacent cells through gap junctions and amplification of the response. In adult
articular cartilage, chondrocytes exist as individual cells embedded in the extracellular
matrix, and gap junctions are mainly expressed by the flattened chondrocytes facing
the outer cartilage layer where intercellular communication occurs 52]. The role of Cx43 in chondrocytes has not been extensively studied, but Cx43 is required
for the differentiation and metabolic homeostasis of the extracellular matrix 48]. Cx43 also functions as a hemichannel to release ATP and NAD
+
. Chondrocytes express purinergic receptors such as P2-purinoceptors that induce intracellular
Ca
2+
responses. These intracellular Ca
2+
responses are increased following stimulation with IL-1 53]. Glutamate and substance P have been identified in human articular chondrocytes.
Neurokinin 1 (NK1) and glutamate receptors are also expressed, as well as both metabotropic
and ionotropic glutamate receptors and the glutamate transporters GLT-1 and GLAST
54].
Bone cells
Gap junctional communication plays a critical role in the coordination of bone remodeling.
The bone-forming cells osteoblasts and osteocytes primarily express Cx43 but also
express Cx45 and Cx46, which form functional gap junctions 55]. Cx43 expression increases during differentiation, and inhibition of this communication
leads to retardation of the differentiation process, resulting in a reduced ability
to form mineralized extracellular matrix. Through mechanical manipulation, the osteoblasts,
which are non-excitable, produce synchronized Ca
2+
waves, which involve the release of IP
3
-sensitive intracellular Ca
2+
stores. These waves occur either via gap junction-mediated intercellular Ca
2+
signaling or as a result of the autocrine activity of released ATP, which stimulates
P2 purinoceptors. The P2Y class comprises G protein-coupled receptors that activate
PLC, resulting in IP
3
generation and intracellular Ca
2+
store release in human osteoblasts 56]. Hemichannels have also been reported in osteoblasts 55].
Connective tissue cells
Gap junctions are found in tendons, ligaments, synovium (within the synovial membrane),
and corneal stroma because the cells of these tissues are coupled to form networks.
Two adjacent cells join through Cx43, allowing direct cell-to-cell communication via
Ca
2+
signaling. In osteoarthritis, the synovial fibroblasts produce pro-inflammatory cytokines
and catabolic proteases, leading to degradation of the extracellular matrix. The role
of Cx43 in osteoarthritis involves an increase in its expression in both chondrocytes
and synovial cells, which affects catabolic and pro-inflammatory genes 57]. Tenocytes respond to mechanical signals by transforming them into biochemical signals
via a second messenger such as Ca
2+
or IP
358]. The mechanical load directly regulates gap junction permeability 59]. Some of the Cxs assemble to form hemichannels 60]. Through mechanical stimulation, ATP is released and acts in a paracrine or autocrine
manner through the stimulation of P2Y
2
purinoceptors, resulting in increased intracellular Ca
2+58]. Cx43 associates with actin to stabilize gap junctions at the plasma membrane 61].
Cardiac fibroblasts
Cardiac fibroblasts are the most abundant cell type in the heart, play a key role
in the myocardial maintenance and repair, and can transform into cardiac myofibroblasts,
which are present in valve leaflets in the adult heart. These cells express ?-smooth
muscle actin (?-SMA) and are referred to as ?-SMA-containing stress fibers 62]. The cells are joined by gap junctions that express Cx43 63], enabling Ca
2+
signaling that causes the release of Ca
2+
from the endoplasmic reticulum in response to ATP, histamine, 5-hydroxytryptamine
(5-HT) 64] (Lundqvist et al., unpublished), or bradykinin 65]. These cells produce extracellular matrix, exhibit high Na
+
/K
+
-ATPase activity levels in the extracellular matrix 66], and also produce and release a substantial number of cytokines and growth factors
into their environment, thereby regulating cell function in an autocrine and paracrine
manner 62].
Hepatocytes
Agonist-evoked Ca
2+
signals are found in the liver and are manifested as the propagation of intercellular
Ca
2+
waves through liver cells called hepatocytes. Agonist binding to plasma membrane receptors
stimulates G
q
proteins, which activate PLC and cause Ca
2+
mobilization from internal stores 67]. The intercellular propagation normally takes place through Cx43-containing gap junctions
68]. ATP release into the extracellular space stimulates purinoceptors, a paracrine signaling
pathway 69].
Glandular cells
Intercellular signaling in salivary glands has been observed when 5-HT triggers intercellular
Ca
2+
waves through gap junctions and induces Ca
2+
release via the IP
3
receptor 70]. Pancreatic acinar cells in the exocrine part of the gland also conduct intercellular
Ca
2+
signaling between cells 71].