Effect of penetration enhancer with novel corneal cross-linking using recombinant human decoron in porcine eyes.

The aim of the study was to investigate the effectiveness of exogenous recombinant human decoron and an accompanying penetration-enhancing solution in stiffening ex-vivo porcine corneas both transepithelially and after de-epithelialization. Eight porcine paired eyes were treated transepithelially: one eye with a pre-treatment solution (Pre-Tx), penetration enhancing solution (PE), and decoron while the fellow eye was treated by the same protocol but without decoron. A second group included 4 de-epithelialized pairs treated identically. The final group included 4 de-epithelialized pairs with one eye treated with Pre-Tx, PE, and decoron while the fellow eye was treated without PE. Uniaxial tensile testing was used to compare the corneal stiffness between the different treatment conditions. Residual tissue underwent immunohistochemistry analysis to evaluate the depth of penetration of decoron into the corneal stroma. There was no stiffening effect exhibited among corneas treated transepithelially with decoron compared to control (P > 0.05) and poor stromal penetration was exhibited on tissue analysis. Among de-epithelialized corneas, there was a significant stiffening effect seen in those treated with decoron at 3%, 4%, 5%, & 6% strain (P < 0.05) compared to control. Among de-epithelialized corneas there was also a significant stiffening effect seen in those treated with the PE and decoron at 4%, 5%, & 6% strain (P < 0.05) with improved stromal penetration confirmed by immunohistochemistry, versus without PE. De-epithelialization is necessary for effective stromal penetration of decoron. Depth of penetration and subsequent corneal stiffening may be improved with a penetration enhancing solution. Compared to riboflavin, decoron requires shorter treatment time and spares UV light exposure.

Effects of fixatives on histomagnetic evaluation of iron in rodent spleen.

Characterizing the iron distribution in tissue sections is important for several pathologies. Iron content in excised tissue is typically analyzed via histochemical stains, which are dependent on sample preparation and staining protocols. In our recent studies, we examined how magnetic properties of iron can also be exploited to characterize iron distribution in tissue sections in a label free manner. To enable a histomagnetic characterization of iron in a wide variety of available tissues, it is important to extend it to samples routinely prepared for histochemical staining, which often involve use of chemical fixatives. In this study, we took a systematic approach to determine differences between unfixed and formalin-fixed murine spleen tissues in histomagnetic characterization of iron. Superconducting quantum interference device (SQUID) magnetometry and magnetic force microscopy (MFM) were used for macro- and micro-scale histomagnetic characterization. Perl's stain was used for histochemical characterization of ferric (Fe3+) iron on adjacent sections as that used for MFM analysis. While histochemical analysis revealed a substantial difference in the dispersion of the stain between fixed versus unfixed samples, histomagnetic characterization was not dependent on chemical fixation of tissue. The results from this study reveal that histomagnetic characterization of iron is free from staining artifacts which can be present in histochemical analysis.

DAPT, a potent Notch inhibitor regresses actively growing abdominal aortic aneurysm via divergent pathways.

Abdominal aortic aneurysm (AAA) is a localized pathological dilation of the aorta exceeding the normal diameter (∼20 mm) by more than 50% of its original size (≥30 mm), accounting for approximately 150000-200000 deaths worldwide per year. We previously reported that Notch inhibition does not decrease the size of pre-established AAA at late stage of the disease. Here, we examined whether a potent pharmacologic inhibitor of Notch signaling (DAPT (N-[N-(3,5-Difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester)), regresses an actively growing AAA. In a mouse model of an aneurysm (Apoe-/- mice; n=44); DAPT (n=17) or vehicle (n=17) was randomly administered at day 14 of angiotensin II (AngII; 1 µg/min/kg), three times a week and mice were killed on day 42. Progressive increase in aortic stiffness and maximal intraluminal diameter (MILD) was observed in the AngII + vehicle group, which was significantly prevented by DAPT (P<0.01). The regression of aneurysm with DAPT was associated with reduced F4/80+Cd68+ (cluster of differentiation 68) inflammatory macrophages. DAPT improved structural integrity of aorta by reducing collagen fibrils abnormality and restoring their diameter. Mechanistically, C-C chemokine receptor type 7 (Ccr7)+F4/80- dendritic cells (DCs), implicated in the regression of aneurysm, were increased in the aorta of DAPT-treated mice. In the macrophages stimulated with AngII or lipopolysaccharide (LPS), DAPT reverted the expression of pro-inflammatory genes Il6 and Il12 back to baseline within 6 h compared with vehicle (P<0.05). DAPT also significantly increased the expression of anti-inflammatory genes, including c-Myc, Egr2, and Arg1 at 12-24 h in the LPS-stimulated macrophages (P<0.05). Overall, these regressive effects of Notch signaling inhibitor emphasize its therapeutic implications to prevent the progression of active AAAs.

Magnetic mapping of iron in rodent spleen.

Evaluation of iron distribution and density in biological tissues is important to understand the pathogenesis of a variety of diseases and the fate of exogenously administered iron-based carriers and contrast agents. Iron distribution in tissues is typically characterized via histochemical (Perl's) stains or immunohistochemistry for ferritin, the major iron storage protein. A more accurate mapping of iron can be achieved via ultrastructural transmission electron microscopy (TEM) based techniques, which involve stringent sample preparation conditions. In this study, we elucidate the capability of magnetic force microscopy (MFM) as a label-free technique to map iron at the nanoscale level in rodent spleen tissue. We complemented and compared our MFM results with those obtained using Perl's staining and TEM. Our results show how MFM mapping corresponded to sizes of iron-rich lysosomes at a resolution comparable to that of TEM. In addition MFM is compatible with tissue sections commonly prepared for routine histology.

Collagen Fibril Ultrastructure in Mice Lacking Discoidin Domain Receptor 1.

The quantity and quality of collagen fibrils in the extracellular matrix (ECM) have a pivotal role in dictating biological processes. Several collagen-binding proteins (CBPs) are known to modulate collagen deposition and fibril diameter. However, limited studies exist on alterations in the fibril ultrastructure by CBPs. In this study, we elucidate how the collagen receptor, discoidin domain receptor 1 (DDR1) regulates the collagen content and ultrastructure in the adventitia of DDR1 knock-out (KO) mice. DDR1 KO mice exhibit increased collagen deposition as observed using Masson's trichrome. Collagen ultrastructure was evaluated in situ using transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. Although the mean fibril diameter was not significantly different, DDR1 KO mice had a higher percentage of fibrils with larger diameter compared with their wild-type littermates. No significant differences were observed in the length of D-periods. In addition, collagen fibrils from DDR1 KO mice exhibited a small, but statistically significant, increase in the depth of the fibril D-periods. Consistent with these observations, a reduction in the depth of D-periods was observed in collagen fibrils reconstituted with recombinant DDR1-Fc. Our results elucidate how DDR1 modulates collagen fibril ultrastructure in vivo, which may have important consequences in the functional role(s) of the underlying ECM.

Psychological Insulin Resistance in Patients with Type 2 Diabetes.

AIMS/HYPOTHESIS: To identify risk factors associated with psychological insulin resistance (PIR) in Indian type 2 diabetes (T2DM) population. METHODS: Patients with T2DM, aged 18 years, undergoing treatment with oral hypoglycaemic agents and providing written informed consent were considered eligible for the study. Patient's data was collected by face-to-face interaction using 5 validated diabetes questionnaires--Diabetes Attitude Scale, Diabetes Knowledge Test, Diabetes Self-Efficacy Scale, Interpersonal Processes of Care Survey-29, and Barriers to Insulin Treatment scale. Demographic variables, categories of patients based on their annual family income, education, glycosylated haemoglobin (HbA1c), occupation and type of healthcare setup were correlated with overall scores of validated questionnaires. Statistical analyses were performed using Pearson correlation coefficients, analysis of variance, two-group t-test and hierarchical multiple regression. RESULTS: One hundred ninty-eight patients with T2DM were enrolled where 63% were males, 52% had HbA1c <7% (<53 mmol/mol), 32% were in service, 35% had the annual family income between Rs 100,000-500,000, 50% were graduates and 81% were enrolled from private healthcare set ups. Significant high opposition to use insulin was observed in females, patients based at home, patients with insufficient education, and patients visiting government set-ups compared to males, service-class patients, graduates, and patients approaching private set-ups, respectively. CONCLUSIONS: In India, major factors contributing to PIR were fear of injection or fear of pain during injection, fear of hypoglycemia, social stigma and lack of education. Effective interpersonal interactions with healthcare providers could help to counteract PIR, especially in patients who are not sufficiently literate highlighting the need of skilled healthcare staffs in Indian public hospitals.

Discoidin domain receptors: unique receptor tyrosine kinases in collagen-mediated signaling.

The discoidin domain receptors (DDRs) are receptor tyrosine kinases that recognize collagens as their ligands. DDRs display unique structural features and distinctive activation kinetics, which set them apart from other members of the kinase superfamily. DDRs regulate cell-collagen interactions in normal and pathological conditions and thus are emerging as major sensors of collagen matrices and potential novel therapeutic targets. New structural and biological information has shed light on the molecular mechanisms that regulate DDR signaling, turnover, and function. This minireview provides an overview of these areas of DDR research with the goal of fostering further investigation of these intriguing and unique receptors.

Distinguishing ferritin from apoferritin using magnetic force microscopy.

Estimating the amount of iron-replete ferritin versus iron-deficient apoferritin proteins is important in biomedical and nanotechnology applications. This work introduces a simple and novel approach to quantify ferritin by using magnetic force microscopy (MFM). We demonstrate how high magnetic moment probes enhance the magnitude of MFM signal, thus enabling accurate quantitative estimation of ferritin content in ferritin/apoferritin mixtures in vitro. We envisage MFM could be adapted to accurately determine ferritin content in protein mixtures or in small aliquots of clinical samples.

Oligomerization of DDR1 ECD affects receptor-ligand binding.

Discoidin domain receptor 1 (DDR1) is a widely expressed receptor tyrosine kinase (RTK) which regulates cell differentiation, proliferation and migration and remodeling of the extracellular matrix. Collagen(s) are the only known ligand for DDR1. We have previously reported that collagen stimulation leads to oligomerization of the full length receptor. In this study we investigated the effect of oligomerization of the DDR1 extracellular domain (ECD) pre and post ligand binding. Solid phase binding assays showed that oligomers of recombinant DDR1-Fc bound more strongly to collagen compared to dimeric DDR1-Fc alone. In addition, DDR1-Fc itself could oligomerize upon in-vitro binding to collagen when examined using atomic force microscopy. Inhibition of dynamin mediated receptor endocytosis could prevent ligand induced endocytosis of DDR1b-YFP in live cells. However inhibition of receptor endocytosis did not affect DDR1 oligomerization. In summary our results demonstrate that DDR1 ECD plays a crucial role in receptor oligomerization which mediates high-affinity interactions with its ligand.

The myosin and RhoGAP MYO9B influences osteocyte dendrite growth and responses to mechanical stimuli.

Introduction: Myosin IXB (MYO9B) is an unconventional myosin with RhoGAP activity and thus is a regulator of actin cytoskeletal organization. MYO9B was previously shown to be necessary for skeletal growth and health and to play a role in actin-based functions of both osteoblasts and osteoclasts. However, its role in responses to mechanical stimulation of bone cells has not yet been described. Therefore, experiments were undertaken to determine the role of MYO9B in bone cell responses to mechanical stress both in vitro and in vivo. Methods: MYO9B expression was knocked down in osteoblast and osteocyte cell lines using RNA interference and the resulting cells were subjected to mechanical stresses including cyclic tensile strain, fluid shear stress, and plating on different substrates (no substrate vs. monomeric or polymerized collagen type I). Osteocytic cells were also subjected to MYO9B regulation through Slit-Robo signaling. Further, wild-type or Myo9b -/- mice were subjected to a regimen of whole-body vibration (WBV) and changes in bone quality were assessed by micro-CT. Results: Unlike control cells, MYO9B-deficient osteoblastic cells subjected to uniaxial cyclic tensile strain were unable to orient their actin stress fibers perpendicular to the strain. Osteocytic cells in which MYO9B was knocked down exhibited elongated dendrites but were unable to respond normally to treatments that increase dendrite length such as fluid shear stress and Slit-Robo signaling. Osteocytic responses to mechanical stimuli were also found to be dependent on the polymerization state of collagen type I substrates. Wild-type mice responded to WBV with increased bone tissue mineral density values while Myo9b -/- mice responded with bone loss. Discussion: These results demonstrate that MYO9B plays a key role in mechanical stress-induced responses of bone cells in vitro and in vivo.

Dissociation of pulse wave velocity and aortic wall stiffness in diabetic db/db mice: The influence of blood pressure.

Introduction: Vascular stiffness is a predictor of cardiovascular disease and pulse wave velocity (PWV) is the current standard for measuring in vivo vascular stiffness. Mean arterial pressure is the largest confounding variable to PWV; therefore, in this study we aimed to test the hypothesis that increased aortic PWV in type 2 diabetic mice is driven by increased blood pressure rather than vascular biomechanics. Methods and Results: Using a combination of in vivo PWV and ex vivo pressure myography, our data demonstrate no difference in ex vivo passive mechanics, including outer diameter, inner diameter, compliance (Db/db: 0.0094 ± 0.0018 mm2/mmHg vs. db/db: 0.0080 ± 0.0008 mm2/mmHg, p > 0.05 at 100 mmHg), and incremental modulus (Db/db: 801.52 ± 135.87 kPa vs. db/db: 838.12 ± 44.90 kPa, p > 0.05 at 100 mmHg), in normal versus diabetic 16 week old mice. We further report no difference in basal or active aorta biomechanics in normal versus diabetic 16 week old mice. Finally, we show here that the increase in diabetic in vivo aortic pulse wave velocity at baseline was completely abolished when measured at equivalent pharmacologically-modulated blood pressures, indicating that the elevated PWV was attributed to the concomitant increase in blood pressure at baseline, and therefore "stiffness." Conclusions: Together, these animal model data suggest an intimate regulation of blood pressure during collection of pulse wave velocity when determining in vivo vascular stiffness. These data further indicate caution should be exerted when interpreting elevated PWV as the pure marker of vascular stiffness.

Magnetic anisotropy considerations in magnetic force microscopy studies of single superparamagnetic nanoparticles.

In recent years, superparamagnetic nanoparticles (SPNs) have become increasingly important in applications ranging from solid state memory devices to biomedical diagnostic and therapeutic tools. However, detection and characterization of the small and unstable magnetic moment of an SPN at the single particle level remains a challenge. Further, depending on their physical shape, crystalline structure or orientation, SPNs may also possess magnetic anisotropy, which can govern the extent to which their magnetic moments can align with an externally applied magnetic field. Here, we demonstrate how we can exploit the magnetic anisotropy of SPNs to enable uniform, highly-sensitive detection of single SPNs using magnetic force microscopy (MFM) in ambient air. Superconducting quantum interference device magnetometry and analytical transmission electron microscopy techniques are utilized to characterize the collective magnetic behavior, morphology and composition of the SPNs. Our results show how the consideration of magnetic anisotropy can enhance the ability of MFM to detect single SPNs at ambient room temperature with high force sensitivity and spatial resolution.

Structurally abnormal collagen fibrils in abdominal aortic aneurysm resist platelet adhesion.

BACKGROUND: Platelet adhesion to the subendothelial collagen fibrils is one of the first steps in hemostasis. Understanding how structural perturbations in the collagen fibril affect platelet adhesion can provide novel insights into disruption of hemostasis in various diseases. We have recently identified the presence of abnormal collagen fibrils with compromised D-periodic banding in the extracellular matrix remodeling present in abdominal aortic aneurysms (AAA). OBJECTIVE: In this study, we employed multimodal microscopy approaches to characterize how collagen fibril structure impacts platelet adhesion in clinical AAA tissues. METHODS: Ultrastructural atomic force microscopy (AFM) analysis was performed on tissue sections after staining with fluorescently labeled collagen hybridizing peptide (CHP) to recognize degraded collagen. Second harmonic generation (SHG) microscopy was used on CHP-stained sections to identify regions of intact versus degraded collagen. Finally, platelet adhesion was identified via SHG and indirect immunofluorescence on the same tissue sections. RESULTS: Our results indicate that ultrastructural features characterizing collagen fibril abnormalities coincide with CHP staining. SHG signal was absent from CHP-positive regions. Additionally, platelet binding was primarily localized to regions with SHG signal. Abnormal collagen fibrils present in AAA (in SHG negative regions) were thus found to inhibit platelet adhesion compared to normal fibrils. CONCLUSIONS: Our investigations reveal how the collagen fibril structure in the vessel wall can serve as another regulator of platelet-collagen adhesion. These results can be broadly applied to understand the role of collagen fibril structure in regulating thrombosis or bleeding disorders.

Loss of Jagged1 in mature endothelial cells causes vascular dysfunction with alterations in smooth muscle phenotypes.

BACKGROUND: Notch signaling is an evolutionarily conserved pathway that functions via direct cell-cell contact. The Notch ligand Jagged1 (Jag1) has been extensively studied in vascular development, particularly for its role in smooth muscle cell maturation. Endothelial cell-expressed Jag1 is essential for blood vessel formation by signaling to nascent vascular smooth muscle cells and promoting their differentiation. Given the established importance of Jag1 in endothelial cell/smooth muscle crosstalk during development, we sought to determine the extent of this communication in the adult vasculature for blood vessel function and homeostasis. METHODS: We conditionally deleted Jag1 in endothelial cells of adult mice and examined the phenotypic consequences on smooth muscle cells of the vasculature. RESULTS: Our results show that genetic loss of Jag1 in endothelial cells has a significant impact on Notch signaling and vascular smooth muscle function in mature blood vessels. Endothelial cell-specific deletion of Jag1 causes a concomitant loss of JAG1 and NOTCH3 expression in vascular smooth muscle cells, resulting in a transition to a less differentiated state. Aortic vascular smooth muscle cells isolated from the endothelial cell-specific Jag1 deficient mice retain an altered phenotype in culture with fixed changes in gene expression and reduced Notch signaling. Utilizing comparative RNA-sequence analysis, we found that Jag1 deficiency preferentially affects extracellular matrix and adhesion protein gene expression. Vasoreactivity studies revealed a reduced contractile response and impaired agonist-induced relaxation in endothelial cell Jag1-deficient aortas compared to controls. CONCLUSIONS: These data are the first to demonstrate that Jag1 in adult endothelial cells is required for the regulation and homeostasis of smooth muscle cell function in arterial vessels partially through the autoregulation of Notch signaling and cell matrix/adhesion components in smooth muscle cells.

Mathematical model of atherosclerotic aneurysm.

Atherosclerosis is a major cause of abdominal aortic aneurysm (AAA) and up to 80% of AAA patients have atherosclerosis. Therefore it is critical to understand the relationship and interactions between atherosclerosis and AAA to treat atherosclerotic aneurysm patients more effectively. In this paper, we develop a mathematical model to mimic the progression of atherosclerotic aneurysms by including both the multi-layer structured arterial wall and the pathophysiology of atherosclerotic aneurysms. The model is given by a system of partial differential equations with free boundaries. Our results reveal a 2D biomarker, the cholesterol ratio and DDR1 level, assessing the risk of atherosclerotic aneurysms. The efficacy of different treatment plans is also explored via our model and suggests that the dosage of anti-cholesterol drugs is significant to slow down the progression of atherosclerotic aneurysms while the additional anti-DDR1 injection can further reduce the risk.

Intramuscular Hemangioma of Masseter Muscle: Case Report of Rare Clinical Entity.

RATIONALE: Intramuscular hemangiomas are unique benign vascular tumours of skeletal muscles; involving masseter and trapezius muscles in the majority of cases. The rationale was to emphasize that the diagnosis of asymptomatic swelling in the masseteric region is important as due to their deep anatomic location and unfamiliar presentation, they are often misdiagnosed as a parotid swelling or other muscular pathologies. PATIENT CONCERN: This report describes a rare case of a 25-year-old healthy male patient who presented with an asymptomatic swelling in the right masseteric region. The patient had cosmetic concerns due to the large size. DIAGNOSIS: Colour Doppler ultrasonography was done to assess the vascularity within the lesion. TREATMENT: Complete excision was successfully achieved using combined Risdon's and preauricular approach. OUTCOME: No signs of recurrence were observed after 6 months. TAKE-AWAY LESSONS: Appropriate selection of diagnostic modalities enables the clinician in making an accurate preoperative diagnosis of progressive swelling in the masseteric region.

Characterization of the human intervertebral disc cartilage endplate at the molecular, cell, and tissue levels.

Given the importance of the cartilage endplate (CEP) in low back pain (LBP), there is a need to characterize the human CEP at the molecular, cell, and tissue levels to inform treatment strategies that target it. The goal of this study was to characterize the structure, matrix composition, and cell phenotype of the human CEP compared with adjacent tissues within the intervertebral joint: the nucleus pulposus (NP), annulus fibrosus (AF), and articular cartilage (AC). Isolated CEP, NP, AF, and AC tissues and cells were evaluated for cell morphology, matrix composition, collagen structure, glycosaminoglycan content, and gene and protein expression. The CEP contained elongated cells that mainly produce a collagen-rich interterritorial matrix and a proteoglycan-rich territorial matrix. The CEP contained significantly fewer glycosaminoglycans than the NP tissue. Significant differences in matrix and cell marker gene expression were observed between CEP and NP or AF, with the greatest differences between CEP and AC. We were able to distinguish NP from CEP cells using collagen-10 (COLX), highlighting COLX as a potential CEP marker. Our findings suggest that at the cell and tissue levels, the CEP demonstrates both similarities and differences when compared with NP, AF, and hyaline AC. This study highlights a unique structure, matrix composition, and cell phenotype for the human CEP and can help to inform regenerative strategies that target the intervertebral disc joint in chronic LBP.

Risk of atypical femoral fracture with long-term use of alendronate (bisphosphonates) : a systemic review of literature.

Several recent reports have described "atypical" fractures of the subtrochanteric and diaphyseal femoral shaft that occur with minimal or no trauma, associated with the use of bisphosphonates. Physicians treating bone diseases with bisphosphonate need, therefore, to be aware of this potential risk and plan the prophylaxis, early diagnosis and prevention of potential consequences. We review the literature on this newly described complication, with particular focus on pathogenesis, preventive measures suggested before and during therapy with bisphosphonates, and the most frequent clinical presentation of these lesions. The recommendations for the management and care of patients who are on long-term use of alendronate (bisphosphonates) are summarized.

Collagen fibril abnormalities in human and mice abdominal aortic aneurysm.

Vascular diseases like abdominal aortic aneurysms (AAA) are characterized by a drastic remodeling of the vessel wall, accompanied with changes in the elastin and collagen content. At the macromolecular level, the elastin fibers in AAA have been reported to undergo significant structural alterations. While the undulations (waviness) of the collagen fibers is also reduced in AAA, very little is understood about changes in the collagen fibril at the sub-fiber level in AAA as well as in other vascular pathologies. In this study we investigated structural changes in collagen fibrils in human AAA tissue extracted at the time of vascular surgery and in aorta extracted from angiotensin II (AngII) infused ApoE-/- mouse model of AAA. Collagen fibril structure was examined using transmission electron microscopy and atomic force microscopy. Images were analyzed to ascertain length and depth of D-periodicity, fibril diameter and fibril curvature. Abnormal collagen fibrils with compromised D-periodic banding were observed in the excised human tissue and in remodeled regions of AAA in AngII infused mice. These abnormal fibrils were characterized by statistically significant reduction in depths of D-periods and an increased curvature of collagen fibrils. These features were more pronounced in human AAA as compared to murine samples. Thoracic aorta from Ang II-infused mice, abdominal aorta from saline-infused mice, and abdominal aorta from non-AAA human controls did not contain abnormal collagen fibrils. The structural alterations in abnormal collagen fibrils appear similar to those reported for collagen fibrils subjected to mechanical overload or chronic inflammation in other tissues. Detection of abnormal collagen could be utilized to better understand the functional properties of the underlying extracellular matrix in vascular as well as other pathologies. STATEMENT OF SIGNIFICANCE: Several vascular diseases including abdominal aortic aneurysm (AAA) are characterized by extensive remodeling in the vessel wall. Although structural alterations in elastin fibers are well characterized in vascular diseases, very little is known about the collagen fibril structure in these diseases. We report here a comprehensive ultrastructural evaluation of the collagen fibrils in AAA, using high-resolution microscopy techniques like transmission electron microscopy (TEM) and atomic force microscopy (AFM). We elucidate how abnormal collagen fibrils with compromised D-periodicity and increased fibril curvature are present in the vascular tissue in both clinical AAA as well as in murine models. We discuss how these abnormal collagen fibrils are likely a consequence of mechanical overload accompanying AAA and could impact the functional properties of the underlying tissue.

Discoidin domain receptors: Micro insights into macro assemblies.

Assembly of cell-surface receptors into specific oligomeric states and/or clusters before and after ligand binding is an important feature governing their biological function. Receptor oligomerization can be mediated by specific domains of the receptor, ligand binding, configurational changes or other interacting molecules. In this review we summarize our understanding of the oligomeric state of discoidin domain receptors (DDR1 and DDR2), which belong to the receptor tyrosine kinase family (RTK). DDRs form an interesting system from an oligomerization perspective as their ligand collagen(s) can also undergo supramolecular assembly to form fibrils. Even though DDR1 and DDR2 differ in the domains responsible to form ligand-free dimers they share similarities in binding to soluble, monomeric collagen. However, only DDR1b forms globular clusters in response to monomeric collagen and not DDR2. Interestingly, both DDR1 and DDR2 are assembled into linear clusters by the collagen fibril. Formation of these clusters is important for receptor phosphorylation and is mediated in part by other membrane components. We summarize how the oligomeric status of DDRs shares similarities with other members of the RTK family and with collagen receptors. Unraveling the multiple macro-molecular configurations adopted by this receptor-ligand pair can provide novel insights into the intricacies of cell-matrix interactions.