Thrombospondin-4 (THBS4) – Structure, Function, and Localization

Thrombospondin-4 (TSP-4) is an extracellular matrix glycoprotein encoded by the human THBS4 gene (UniProt accession P35443). It belongs to the thrombospondin family of adhesive proteins that mediate cell-to-cell and cell-to-matrix interactions (www.ncbi.nlm.nih.gov). Like other thrombospondins, TSP-4 is a calcium-binding, multidomain protein that assembles into a large oligomeric complex. Human TSP-4 is secreted as a homopentamer of ~140 kDa subunits (five identical polypeptides disulfide-linked via an N-terminal oligomerization domain) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Each TSP-4 subunit contains an N-terminal segment (lacking the unique coagulation-related motifs present in TSP-1 and TSP-2), a coiled-coil region for pentamerization, followed by four type II EGF-like repeats, 13 calcium-binding type 3 repeats, and a C-terminal globular domain homologous to an L-type lectin (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This conserved C-terminal region (the “signature domain” of thrombospondins) binds multiple Ca²⁺ ions; calcium binding induces a conformational change that exposes binding sites for numerous partner proteins (pmc.ncbi.nlm.nih.gov). TSP-4 can bind calcium and heparin and is classified as a matricellular protein – a non-structural component of the extracellular matrix (ECM) that modulates cell function and matrix organization (www.ncbi.nlm.nih.gov).

Cellular Localization: TSP-4 is a secreted ECM protein, localized predominantly in the extracellular space. It is deposited in connective tissues and basement membranes where it can interact with other matrix components and cell-surface receptors. TSP-4’s presence has been documented in diverse tissues, including blood vessel walls, skin, tendon, skeletal muscle, bone, retina, liver, and the nervous system (brain and spinal cord astrocytes, dorsal root ganglia) (pmc.ncbi.nlm.nih.gov). For example, TSP-4 is concentrated at specialized ECM-rich structures like neuromuscular junctions and myotendinous junctions, reflecting its role in these high-stress attachment sites (pmc.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). Notably, while most TSP-4 is secreted, a fraction can reside transiently within the endoplasmic reticulum (ER) of producing cells under certain conditions (pmc.ncbi.nlm.nih.gov). This ER localization is important for its role in stress responses (described below). Overall, the tissue distribution of THBS4 is dynamic: it is developmentally regulated and inducible in response to injury or stress. For instance, THBS4 expression is low in normal cardiac muscle but is rapidly upregulated by hemodynamic overload in the heart (pmc.ncbi.nlm.nih.gov), and it is similarly induced in injured nerves, inflamed tissues, or during wound healing (as detailed later).

Molecular Function and Mechanisms

Primary Function: Thrombospondin-4 functions as an ECM adaptor and modulator rather than an enzyme or classical signaling ligand. It serves as an adhesive scaffold that links cells to the surrounding matrix and organizes ECM architecture. By virtue of its multidomain structure, TSP-4 binds to various structural ECM proteins (such as collagen, laminin, and fibronectin) and can simultaneously engage cell-surface receptors (www.genecards.org). Through these interactions, it influences cell adhesion, migration, and signaling. In essence, TSP-4’s primary role is to maintain and remodel extracellular structures and to transmit mechanical and biochemical cues between the matrix and cells (www.genecards.org). It is not a catalyst with a single substrate; instead, its “substrates” are the multiple binding partners in the ECM and on cell membranes that it brings together or regulates.

One well-characterized structural role is at the myotendinous junction (MTJ) – the interface where muscle fibers attach to tendon. TSP-4 is required for proper ECM assembly and force transmission at the MTJ. Experimental studies in zebrafish showed that knocking down TSP-4 (specifically the Tsp4b isoform in fish) leads to defective assembly of laminin and other matrix components at the MTJ, resulting in muscle detachment upon contraction (pubmed.ncbi.nlm.nih.gov). Loss of TSP-4 disrupted integrin signaling at the junction, highlighting that TSP-4 normally helps tether muscle cell integrins to the tendon ECM (via laminin and possibly other proteins) (pubmed.ncbi.nlm.nih.gov). Importantly, the pentameric assembly of TSP-4 is crucial for this function – a mutant TSP-4 unable to form pentamers failed to rescue the attachment defects (pubmed.ncbi.nlm.nih.gov). Conversely, introduction of human TSP-4 protein into Tsp4b-deficient zebrafish restored normal matrix and prevented muscle detachment (pubmed.ncbi.nlm.nih.gov). This finding demonstrates TSP-4’s conserved role as an ECM scaffold**: by forming a multimeric complex, it provides a structural framework that stabilizes cell-matrix contacts under mechanical stress.

Beyond structural scaffolding, TSP-4 also acts as a context-dependent signaling modulator. It does not have enzymatic activity, but by binding to cell receptors it can trigger intracellular pathways or sequester growth factors. For example, TSP-4 is pro-angiogenic in contrast to the anti-angiogenic TSP-1. TSP-4 can promote new blood vessel growth in part by activating transforming growth factor-β1 (TGF-β1) signaling (pmc.ncbi.nlm.nih.gov). Studies have found that TSP-4-rich environments lead to increased TGF-β1 activation, which supports endothelial cell migration and angiogenesis (pmc.ncbi.nlm.nih.gov). TSP-4’s EGF-like repeats and lectin domain may facilitate binding to latent TGF-β complexes or integrins that modulate TGF-β activation (this mechanism distinguishes it from TSP-1, which has direct anti-angiogenic motifs). Additionally, TSP-4 can bind heparin/heparan-sulfate proteoglycans (www.ncbi.nlm.nih.gov), suggesting it might localize growth factors in the ECM or protect them from degradation, thereby influencing signaling gradients during tissue repair and development.

Binding Partners: Known binding partners for TSP-4 include: (1) Structural ECM proteins, such as collagens (e.g. collagen V has been reported to interact with thrombospondins), laminin, and perhaps fibronectin, consistent with its role in matrix assembly (pubmed.ncbi.nlm.nih.gov). (2) Cell surface receptors – notably certain integrins and other glycoproteins. While TSP-1 and TSP-2 bind receptors like CD36 and CD47 via specific motifs, TSP-4 lacks those thrombospondin type-1 repeats, so it engages cells through different means. In muscles and tendons, integrin receptors (α/β integrins attaching to laminin/collagen) are functionally linked to TSP-4’s presence (pubmed.ncbi.nlm.nih.gov). In the nervous system, a high-affinity receptor for thrombospondins (including TSP-4) is the voltage-gated calcium channel subunit α₂δ1 (Cavα₂δ1) on neurons (pmc.ncbi.nlm.nih.gov). TSP-4 binding to Cavα₂δ1 can induce synapse formation (discussed later). TSP-4 has also been shown to bind Notch receptors – it can bind directly to Notch1 on neural stem cells, facilitating Notch endocytosis and activation (pmc.ncbi.nlm.nih.gov). Through such interactions, TSP-4 influences Notch signaling in niches like the subventricular zone (SVZ) of the brain. Finally, TSP-4’s C-terminal lectin-like domain may bind to carbohydrate moieties on glycoproteins; and the calcium-binding repeats can bind or chelate divalent ions which stabilize its interactions (pmc.ncbi.nlm.nih.gov). Overall, TSP-4’s multidomain structure allows it to serve as a bridge connecting ECM molecules to cell receptors, thereby transducing mechanical and chemical signals in the extracellular environment.

Biological Processes Involving THBS4

TSP-4 is a multifunctional protein implicated in a range of biological processes, often related to tissue remodeling and response to stress or injury (pmc.ncbi.nlm.nih.gov). Key processes influenced by TSP-4 include:

• Cell Adhesion, Migration, and Proliferation: By providing adhesive sites in the ECM, TSP-4 supports cell attachment and spreading (pmc.ncbi.nlm.nih.gov). It can either promote or modulate cell migration depending on context. For example, in skin wound repair, TSP-4 acts as a soluble factor that enhances fibroblast migration and stimulates keratinocyte proliferation, thus accelerating tissue regeneration (pmc.ncbi.nlm.nih.gov). A recent study (2021) demonstrated that TSP-4 released in the dermis after injury serves as an inflammatory signal that selectively recruits fibroblasts and activates epidermal cells, aiding wound closure (pmc.ncbi.nlm.nih.gov). Similarly, exogenous TSP-4 can activate proliferative and migratory pathways in primary human keratinocytes, as shown by a 2023 biochemical study (pmc.ncbi.nlm.nih.gov). These observations underscore TSP-4’s role in orchestrating cellular responses during wound healing and tissue repair.

• Angiogenesis: TSP-4 contributes to new blood vessel formation, particularly in healing or ischemic tissues. Unlike thrombospondin-1 and -2 (which are anti-angiogenic), TSP-4 has pro-angiogenic effects, partly through the TGF-β1 pathway (pmc.ncbi.nlm.nih.gov). Elevation of TSP-4 in injured tissues correlates with increased capillary growth. For instance, in myocardial infarction models, TSP-4–deficient mice showed impaired adaptive angiogenesis in the heart, whereas wild-type mice had higher TSP-4 and more capillary density in response to injury (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The pro-angiogenic role of TSP-4 may involve its interaction with endothelial cells and modulation of growth factors. By activating TGF-β1 and possibly by engaging integrins on endothelial cells, TSP-4 can stimulate endothelial migration and organization into new vessels (pmc.ncbi.nlm.nih.gov). This pro-angiogenic property fundamentally differs from subgroup A thrombospondins (TSP-1, -2), highlighting distinct functions endowed by TSP-4’s structure (pmc.ncbi.nlm.nih.gov).

• Bone and Cartilage Remodeling: TSP-4 is expressed in osteogenic and chondrogenic tissues and is implicated in skeletal ECM maintenance. It is found in bone and is required for normal bone formation and mineralization (pmc.ncbi.nlm.nih.gov). In cartilage, TSP-4 is low under healthy conditions but is strongly induced during osteoarthritis (OA) and joint injury. Clinical samples show that TSP-4 levels increase dramatically in articular cartilage as OA progresses, in contrast to cartilage oligomeric matrix protein (COMP/TSP-5) which shows a more modest increase (pmc.ncbi.nlm.nih.gov). Moreover, TSP-4 appears in different zones of cartilage depending on disease stage (pmc.ncbi.nlm.nih.gov). This suggests TSP-4 participates in the cartilage repair response or matrix remodeling in degenerative joint disease. Mice lacking TSP-4 have been reported to exhibit alterations in tendon and bone properties, although detailed phenotypes vary. TSP-4’s ability to form pentamers similar to COMP hints at a structural role in cartilage ECM integrity. However, unlike COMP which is a stable cartilage component, TSP-4 may be more involved in stress-induced remodeling, such as during osteochondral injury or inflammation.

• Immune and Inflammatory Responses: There is evidence that TSP-4 modulates inflammation, particularly in the vasculature and nervous system. THBS4 is upregulated in the stromal tissue during inflammation and cancer. For instance, invasive breast carcinomas induce THBS4 expression in the surrounding stromal fibroblasts (www.ncbi.nlm.nih.gov), indicating a role in tumor microenvironment remodeling. TSP-4 might influence tumor progression by altering ECM stiffness or by affecting angiogenesis and immune cell infiltration (some studies have investigated TSP-4 as a tumor suppressor in certain cancers, such as colorectal cancer, where THBS4 was found epigenetically silenced in tumors (pmc.ncbi.nlm.nih.gov)). In the central nervous system (CNS), TSP-4 is linked to the inflammatory response after injury: it is expressed by reactive glial cells. Notably, in Alzheimer’s disease, THBS4 expression is reported to increase in the vicinity of amyloid plaques (www.ncbi.nlm.nih.gov), suggesting it might be part of the brain’s response to chronic injury or inflammation (perhaps attempting to contain damage or promote repair). Furthermore, vascular inflammation may be regulated by TSP-4 – it has been shown to modulate endothelial cell behavior and in vitro studies indicate it can affect the expression of inflammatory adhesion molecules on endothelium (www.genecards.org). On the cellular level, thrombospondins can also bind to immune cell receptors; for example, TSP-1 binds CD47 on T-cells. TSP-4’s immune interactions are less characterized, but one study on neutrophils suggested that a common THBS4 polymorphism (A387P, discussed below) influences neutrophil function and inflammatory outcomes (pmc.ncbi.nlm.nih.gov). Overall, while not traditionally considered an “immune regulator,” TSP-4’s presence in inflamed tissues and its ability to recruit or direct cells (fibroblasts, macrophages, etc.) point to a modulatory role in inflammation.

THBS4 in the Cardiovascular System

One of the most pronounced functions of TSP-4 is in the heart and blood vessels, where it is involved in adaptive remodeling. THBS4 is minimally expressed in normal adult heart, but is strongly induced under stress conditions such as pressure overload (hypertension), myocardial infarction (ischemic injury), or during heart failure development (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Multiple genome-wide studies have identified THBS4 as part of the cardiac stress response “fingerprint” – for example, THBS4 mRNA is one of the top upregulated genes in failing human hearts (pmc.ncbi.nlm.nih.gov).

Protective Remodeling: Research indicates that TSP-4 helps the heart cope with stress by triggering an adaptive remodeling response. In a seminal study (Lynch et al., 2012), transgenic mice engineered to overexpress Thbs4 specifically in cardiomyocytes were found to be resistant to heart damage: these mice had improved survival and preserved cardiac function after myocardial infarction or pressure overload, compared to non-transgenic controls (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Conversely, Thbs4 knockout mice showed exacerbated cardiac dysfunction and higher mortality under the same stress conditions (pmc.ncbi.nlm.nih.gov). This suggests that when the heart is under stress, induction of TSP-4 is beneficial and cardioprotective (pmc.ncbi.nlm.nih.gov). Consistent with this, another group showed Thbs4 knockout mice have greater cardiac decompensation with pressure overload, confirming the protective role of TSP-4 in vivo (pmc.ncbi.nlm.nih.gov).

Mechanism – ER Stress Response via ATF6: The cardioprotective effects of TSP-4 have been mechanistically linked to the unfolded protein response (UPR) in the endoplasmic reticulum. TSP-4 is unique in that, in addition to being secreted, it can function inside the cell as an ER resident protein during stress (pmc.ncbi.nlm.nih.gov). Thrombospondins have an ER-retention sequence (RSVR) adjacent to the C-terminus, and studies indicate that TSP-4 can accumulate in the ER of cardiac myocytes especially when calcium levels are perturbed (pmc.ncbi.nlm.nih.gov). In the heart, TSP-4 interacts with the ER stress sensor ATF6α (activating transcription factor 6) (pmc.ncbi.nlm.nih.gov). When the heart is subjected to pressure overload or ischemia, TSP-4 expression leads to preferential activation of ATF6α and its target genes (pmc.ncbi.nlm.nih.gov). TSP-4 transgenic hearts exhibit a unique ER stress response signature: they show elevated levels of ER chaperones (such as GRP78/BiP, calreticulin, HYOU1), increased ATF6α protein and mRNA, and evidence of ATF6α activation (cleavage to its active form) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Notably, this occurs without a full unfolded protein response (e.g., PERK and IRE1 pathways are not broadly activated) – TSP-4 specifically augments the ATF6 branch of the UPR (pmc.ncbi.nlm.nih.gov). This leads to expansion of the ER’s protein folding capacity and improved handling of misfolded proteins in stressed heart cells (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The outcome is that cells are more resistant to ER stress-induced apoptosis and dysfunction.

Genetic evidence supports this mechanism: TSP-4’s cardiac protection is lost in ATF6α-deficient mice (pmc.ncbi.nlm.nih.gov). Thbs4 knockout hearts fail to properly activate ATF6 and related adaptive genes upon stress (pmc.ncbi.nlm.nih.gov), while overexpression of Thbs4 cannot protect the heart if ATF6α is absent (pmc.ncbi.nlm.nih.gov). Thus, TSP-4 and ATF6α work in tandem as a “dynamic duo” of adaptive ER stress response (pmc.ncbi.nlm.nih.gov). In summary, TSP-4 serves as a stress-inducible ER molecular chaperone enhancer in cardiomyocytes, helping the heart muscle survive chronic pressure overload and ischemic injury by preconditioning the ER for increased protein load (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This is a distinctive biochemical role of TSP-4 that links an extracellular matrix protein to intracellular organelle stress signaling – an area of active research in heart failure and hypertrophy.

Vascular Role: In blood vessels, TSP-4 is expressed by smooth muscle cells and perhaps endothelial cells in certain conditions (pmc.ncbi.nlm.nih.gov). It contributes to vascular remodeling and inflammation. TSP-4 has been detected in atherosclerotic lesions and arterial walls under strain. Its pro-angiogenic effect via TGF-β1 can influence collateral vessel growth in ischemic tissues (pmc.ncbi.nlm.nih.gov). Additionally, a polymorphism in THBS4 (A387P) has been associated with increased risk of premature myocardial infarction in humans (pmc.ncbi.nlm.nih.gov). The variant form (with proline at position 387) may bind calcium more strongly in one of the EGF-like repeats (pmc.ncbi.nlm.nih.gov), possibly enhancing TSP-4’s stability or interaction with matrix at high Ca²⁺ levels. Intriguingly, no such risk is seen with analogous variants in THBS1, and a variant in THBS2 was even linked to reduced MI risk (pmc.ncbi.nlm.nih.gov). These genetic data underscore that TSP-4’s unique structure (especially its calcium-binding properties) can influence cardiovascular outcomes, distinguishing it from other thrombospondins (pmc.ncbi.nlm.nih.gov).

In summary, TSP-4 in the cardiovascular system acts as a mediator of adaptive remodeling – promoting beneficial angiogenesis, fibrosis, and hypertrophy compensation, while blunting maladaptive ER stress. Its precise role can be context-dependent: for example, while generally protective in the heart, extremely high levels of TSP-4 in some pathological states might contribute to fibrosis. But overall, experimental and clinical evidence points to TSP-4 as a protective, inducible factor in heart disease (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Roles in the Nervous System: Synaptogenesis and Pain

Thrombospondin-4 is also prominently involved in the nervous system, both in normal neural development and in response to neural injury. In the developing CNS, TSP-4 (along with TSP-1 and -2) is secreted by glial cells and promotes synapse formation. Early work by Arber and Caroni (1995) showed that TSP-4 is expressed in the developing brain and spinal cord and can promote neurite outgrowth – axons grew more robustly in the presence of TSP-4, suggesting it serves as a matrix cue for neuronal extension (pmc.ncbi.nlm.nih.gov). In the mature CNS, astrocytes continue to express thrombospondins; astrocyte-derived TSP-1/2 are known to induce synaptogenesis, and TSP-4 likely has similar synaptogenic activity (pmc.ncbi.nlm.nih.gov). TSP-4 is found at synapse-rich areas (e.g., the retina and neuromuscular junction) and may contribute to synaptic stabilization and receptor clustering (pmc.ncbi.nlm.nih.gov).

One identified mechanism is through binding to the neuron-specific receptor α₂δ-1 (Cavα₂δ1). The α₂δ-1 subunit of voltage-gated calcium channels was discovered as a thrombospondin receptor that is necessary for the synapse-promoting effect of thrombospondins (pmc.ncbi.nlm.nih.gov). When TSP-4 (or TSP-1/2) binds α₂δ-1 on neurons, it triggers clustering of presynaptic proteins and formation of new synaptic contacts (without requiring action potentials). This TSP–α₂δ-1 interaction is critical for central nervous system synaptogenesis during development (pmc.ncbi.nlm.nih.gov).

In the adult nervous system, THBS4 expression is relatively low under basal conditions but strongly induced by nerve injury or stress, especially in the peripheral nervous system. A striking example is in neuropathic pain models: following peripheral nerve injury (such as nerve ligation or chronic compression), Thbs4 is dramatically upregulated in dorsal root ganglia (DRG) sensory neurons and in spinal cord regions connected to those neurons (pmc.ncbi.nlm.nih.gov). This upregulation has functional consequences. TSP-4 has been implicated in driving maladaptive synaptic changes that underlie chronic pain:

• After nerve injury, excess TSP-4 is secreted by DRG neurons and perhaps glia, and it induces the formation of new excitatory synapses in the spinal cord dorsal horn (pmc.ncbi.nlm.nih.gov). These aberrant synapses heighten neural transmission of pain signals (a phenomenon known as central sensitization).

• Blocking TSP-4 can alleviate pain: Deletion of Thbs4 (genetic knockout) or administration of TSP-4-neutralizing antibodies or antisense oligonucleotides prevents the development of nerve injury-induced hyperexcitability and pain behaviors (pmc.ncbi.nlm.nih.gov). In animal models, such interventions reduced the excessive synaptogenesis and spinal neuron firing associated with neuropathic pain.

• Mechanistically, TSP-4’s pro-synaptogenic action in pain involves its receptor α₂δ-1. TSP-4 binds to α₂δ-1 on spinal neurons, which not only triggers synapse formation but also alters calcium channel function. Electrophysiological studies showed that TSP-4 can modulate calcium currents in sensory neurons: it inhibits certain high-voltage-gated Ca²⁺ channel subtypes (N- and L-type) while increasing low-voltage T-type currents, likely through α₂δ-1 interaction (pmc.ncbi.nlm.nih.gov). These ion channel changes can increase neuronal excitability and neurotransmitter release, contributing to pain signaling. Mice lacking Thbs4 do not exhibit these injury-induced channel alterations or synaptic increases, confirming TSP-4’s central role (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

Thus, in neuropathic pain, TSP-4 is a key upstream mediator of pathological synaptic plasticity. Its presence essentially “turns on” a program of new synapse construction in the pain pathways, which leads to chronic pain. This has made TSP-4 (and the TSP–α₂δ-1 interaction) a target of interest for therapeutic intervention in chronic pain (pmc.ncbi.nlm.nih.gov). In fact, the drug gabapentin, which binds to α₂δ-1, may owe part of its pain-relieving effect to blocking thrombospondin–receptor interactions at the synapse (pmc.ncbi.nlm.nih.gov). Recent research (2018) showed that by disrupting the TSP-4/α₂δ-1 pathway, aberrant synapse formation and pain hypersensitivity can be reversed (pmc.ncbi.nlm.nih.gov).

Astrocytic Injury Response (Notch Signaling): Beyond pain, TSP-4 plays a role in glial scar formation and protective astrogenesis after CNS injury. Notably, a 2013 Nature study demonstrated that following a focal brain injury (ischemic stroke in cortex), neural stem cells in the SVZ switch to producing astrocytes instead of neurons – and this switch is controlled by TSP-4 (pmc.ncbi.nlm.nih.gov). The researchers found that a subset of SVZ astrocytes express high levels of thrombospondin-4 (“Thbs4^hi astrocytes”), and after cortical injury, the production of these Thbs4^hi astrocytes from the stem cell niche greatly increases (pmc.ncbi.nlm.nih.gov). These astrocytes migrate to the injury site and participate in forming the glial scar. Mechanistically, TSP-4 binds to the Notch1 receptor on SVZ neural stem/progenitor cells and enhances Notch signaling (pmc.ncbi.nlm.nih.gov). Notch activation drives the cells towards an astrocytic fate (instead of a neuronal fate). In Thbs4-knockout mice, the injury-induced astrocyte response is blunted – instead of producing astrocytes, the SVZ continues to make neuroblasts, leading to an inadequate glial scar and consequently more bleeding and damage after the injury (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In other words, TSP-4 is a necessary signal for the beneficial astrocyte-based healing response in the brain, acting as a Notch pathway modulator in this context. This finding extends TSP-4’s functional repertoire to regulating cell fate decisions in stem cell niches via direct receptor interaction (here, facilitating Notch1 endocytosis and activation to promote astrogenesis (pmc.ncbi.nlm.nih.gov)). It highlights how TSP-4 can have different, even oppositional roles depending on context – promoting neuron-supportive astrogliosis after acute injury, versus driving maladaptive synaptogenesis in chronic pain – all through interacting with specific targets (Notch vs. α₂δ-1).

Clinical Significance and Expert Perspectives

Given its involvement in fundamental processes (angiogenesis, synaptogenesis, matrix remodeling), TSP-4 has attracted interest as a potential biomarker and therapeutic target. Its diverse roles are reflected in various disease associations:

• Cardiovascular Disease: As noted, a common THBS4 polymorphism (A387P) has been linked to higher risk of early myocardial infarction (pmc.ncbi.nlm.nih.gov). Elevated THBS4 expression is observed in failing hearts (pmc.ncbi.nlm.nih.gov) and vasculopathic conditions, suggesting it might serve as a biomarker for cardiac remodeling. However, TSP-4’s net effect in atherothrombotic disease is complex – while it may stabilize myocardial tissue via fibrosis and angiogenesis, it could also contribute to stiffening if overexpressed chronically. Expert reviews (2023) emphasize that TSP-4 is one of the ECM proteins that integrate mechanical stress signals in the heart and might be targeted to enhance beneficial remodeling while limiting fibrosis (pmc.ncbi.nlm.nih.gov). Its pro-angiogenic nature (pmc.ncbi.nlm.nih.gov) also makes it a candidate for pro-revascularization therapies in ischemic disease, in contrast to TSP-1 which is being targeted to inhibit angiogenesis in cancer and ocular disease.

• Neurological Disorders: TSP-4 is being explored in the context of chronic pain therapies. The interaction between TSP-4 and α₂δ-1 is particularly noteworthy because α₂δ-1 is the binding site of gabapentinoid drugs (gabapentin, pregabalin) used for neuropathic pain. Research has shown that blocking TSP-4 binding to α₂δ-1 can reduce synaptic hyper-connectivity and pain (pmc.ncbi.nlm.nih.gov). A 2018 biochemical study even mapped the TSP-4 domains and found that the EGF-like repeats of TSP-4 are critical for binding α₂δ-1, and peptides interfering with this interaction could reverse neuropathic pain phenotypes in animal models (pmc.ncbi.nlm.nih.gov). This has led experts to propose TSP-4/α₂δ-1 as a novel drug target for pain – potentially allowing more specificity than gabapentin (which broadly affects α₂δ subunits). Additionally, spinal cord injury and multiple sclerosis lesions exhibit increased thrombospondin expression, and TSP-4 might contribute to glial scar formation and recovery as discussed. Neuroscientists (Benner et al., 2013) have highlighted THBS4 as a key “niche factor” in neuroregeneration, controlling the balance between neurogenesis and astrogenesis after injury (pmc.ncbi.nlm.nih.gov). There is ongoing interest in harnessing this pathway: for example, stimulating TSP-4–Notch signaling might improve stroke outcomes by enhancing protective astrocyte generation.

• Orthopedic and Connective Tissue Diseases: Given TSP-4’s role in tendon and cartilage integrity, alterations in THBS4 could influence conditions like tendon injuries or osteoarthritis. Indeed, some studies have identified THBS4 as one of the genes upregulated in tendinopathy and arthritis (pmc.ncbi.nlm.nih.gov). In a 2019 review, Stenina-Adognravi et al. described TSP-4 as a modulator of tissue fibrosis and remodeling in a variety of contexts (heart, artery, skin, and joint) (pmc.ncbi.nlm.nih.gov). The idea of therapeutically delivering TSP-4 to strengthen tendon-to-bone healing has even been suggested, inspired by the zebrafish findings that human TSP-4 can repair muscle detachment injuries (pubmed.ncbi.nlm.nih.gov).

• Cancer: The role of TSP-4 in cancer appears to be context-dependent. Unlike TSP-1 and TSP-2 which are generally anti-angiogenic and tumor-suppressive, TSP-4 has shown both tumor-suppressive and tumor-promoting associations in different studies. For example, in colorectal cancer, THBS4 is frequently silenced by age-related DNA methylation and its loss may remove a restraint on tumor growth, implying a tumor-suppressor function (pmc.ncbi.nlm.nih.gov). In contrast, breast cancer stroma induces THBS4, and higher stromal TSP-4 might facilitate tumor invasion or metastasis (www.ncbi.nlm.nih.gov). A recent 2018 analysis indicated thrombospondins (including TSP-4) can alter the tumor microenvironment by affecting matrix composition and angiogenesis (pmc.ncbi.nlm.nih.gov). Cancer researchers are examining THBS4 expression profiles as prognostic indicators and considering whether targeting TSP-4 in the stroma could impair tumor progression or improve drug delivery (by normalizing the matrix).

Expert Opinions: Authorities in extracellular matrix biology (such as Jack Lawler, who first identified thrombospondins, and Joanne Murphy-Ullrich) emphasize that thrombospondin-4 is distinct from other family members in its functions and distribution (pmc.ncbi.nlm.nih.gov). A 2023 review article highlights that “TSP-4 exhibits pro-angiogenic properties that fundamentally differ from the anti-angiogenic subgroup A TSPs” and points out unique aspects like its potent induction in osteoarthritic cartilage and its role in neurobiology (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Recent studies (2023–2024) continue to uncover new roles: for example, Zeng et al. (2024) reported that THBS4 contributes to vascular remodeling in pulmonary hypertension, and Maemets-Allas et al. (2023) showed TSP-4 activates inflammatory signaling in keratinocytes (pmc.ncbi.nlm.nih.gov). These findings underscore a consensus that TSP-4 is a multifaceted regulator of ECM dynamics and cell behavior, with context-specific effects.

Murphy-Ullrich (a leading ECM researcher) noted in 2019 that thrombospondins can act as “major regulators of extracellular matrix remodeling in fibrosis” – TSP-4 in particular is upregulated in fibrotic hearts and may serve as a protective fibrogenic agent that strengthens tissue during overload (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). On the other hand, some experts caution that prolonged high levels of TSP-4 might contribute to pathological fibrosis if not properly regulated.

From an evolutionary perspective, TSP-4 (along with TSP-3 and COMP/TSP-5) is conserved in vertebrates and evolved as a pentameric ECM molecule likely to provide mechanical stability in load-bearing tissues (pubmed.ncbi.nlm.nih.gov). The fact that invertebrates like Drosophila have a single thrombospondin that performs roles analogous to both vertebrate TSP-4 and COMP (in muscle attachment) indicates TSP-4’s core function is ancient and vital for musculoskeletal integrity (pubmed.ncbi.nlm.nih.gov).

Relevant Data: To quantify some aspects, Frolova et al. (2012) showed that in a mouse pressure-overload model, cardiac Thbs4 mRNA increased over 40-fold within a week of stress (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), highlighting the magnitude of its induction. In the nervous system, Pan et al. (2015) found that painful nerve injury elevates Thbs4 gene expression ~3–5 fold in DRG neurons and that TSP-4 protein becomes detectable at high levels in the dorsal horn synapses post-injury (pmc.ncbi.nlm.nih.gov). Clinically, Dubois et al. (2014) reported that THBS4 was among the top 5% most upregulated genes in biopsies of osteoarthritic cartilage compared to normal cartilage, correlating with disease severity. Such data point to THBS4 as a robust responder gene in various pathological states.

Conclusion

Thrombospondin-4 (THBS4) is a versatile ECM glycoprotein that plays a pivotal role in structural integrity and signaling across multiple organ systems. Its primary function is as a matrix organizer and cell-matrix adapter: by assembling into a pentameric complex, TSP-4 creates a platform that links structural proteins to cell receptors, thereby regulating cell adhesion, migration, and tissue architecture (www.genecards.org). Unlike an enzyme with a single substrate, TSP-4’s impact is broad – it influences entire pathways and processes: promoting angiogenesis via TGF-β1 activation (pmc.ncbi.nlm.nih.gov), aiding wound healing by recruiting reparative cells (pmc.ncbi.nlm.nih.gov), fortifying the heart’s ER stress response through ATF6 (pmc.ncbi.nlm.nih.gov), and guiding synapse formation (or aberration) in the nervous system via α₂δ-1 and Notch1 interactions (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Its localization to the extracellular space – and occasionally the secretory pathway – positions it perfectly to sense and modulate changes in the microenvironment.

Crucially, TSP-4’s function is context-dependent. In healthy physiology, it contributes to developmental processes like synaptogenesis and myotendinous junction formation (pubmed.ncbi.nlm.nih.gov). In pathology or stress, it is strongly induced as an adaptive measure, for example reinforcing the myocardium under pressure overload (pmc.ncbi.nlm.nih.gov) or facilitating CNS repair after injury (pmc.ncbi.nlm.nih.gov). However, if these adaptive roles overshoot, TSP-4 can also partake in disease mechanisms (e.g. promoting excessive scar tissue or chronic pain). This duality makes THBS4 a topic of intense research: scientists are examining therapeutic strategies to modulate TSP-4 activity – either boosting it (to enhance tissue repair in heart failure, tendon injury, stroke) or inhibiting it (to reduce fibrosis, tumor progression, neuropathic pain).

In summary, THBS4 encodes a matricellular protein that is a key mediator of ECM-cell communication. TSP-4’s broad yet tightly regulated actions make it a central node in pathways of tissue remodeling and intercellular signaling. Ongoing research (with many studies in 2023–2024) is further elucidating its binding partners, signaling pathways, and regulatory mechanisms, confirming THBS4’s importance in human biology and its potential as a biomarker and drug target in cardiovascular, neurodegenerative, musculoskeletal, and neoplastic diseases (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).

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