Caveolin-1 regulates γ-secretase-mediated AβPP processing by modulating spatial distribution of γ-secretase in membrane

Unraveling the Cave: A Seventy-Year Journey into the Caveolar Network, Cellular Signaling, and Human Disease

In the mid-1950s, a groundbreaking discovery revealed the fascinating presence of caveolae, referred to as flask-shaped invaginations of the plasma membrane, sparking renewed excitement in the field of cell biology. Caveolae are small, flask-shaped invaginations in the cell membrane that play crucial roles in diverse cellular processes, including endocytosis, lipid homeostasis, and signal transduction. The structural stability and functionality of these specialized membrane microdomains are attributed to the coordinated activity of scaffolding proteins, including caveolins and cavins. While caveolae and caveolins have been long appreciated for their integral roles in cellular physiology, the accumulating scientific evidence throughout the years reaffirms their association with a broad spectrum of human disorders. This review article aims to offer a thorough account of the historical advancements in caveolae research, spanning from their initial discovery to the recognition of caveolin family proteins and their intricate contributions to cellular functions. Furthermore, it will examine the consequences of a dysfunctional caveolar network in the development of human diseases.

Caveolin-1 inhibition mediates the opposing effects of alcohol on γ-secretase activity in arterial endothelial and smooth muscle cells

Notch is important to vessel homeostasis. We investigated the mechanistic role of caveolin-1 (Cav-1) in mediating the effects of alcohol (Ethanol/EtOH) on the γ-secretase proteolytic activity necessary for Notch signaling in vascular cells. Human coronary artery endothelial cells (HCAEC) were treated with EtOH (0-50 mM), Notch ligand delta-like ligand 4 (Dll4), and the γ-secretase inhibitor DAPT. EtOH stimulated Notch signaling in HCAEC as evidenced by increased Notch receptor (N1, N4) and target gene (hrt2, hrt3) mRNA levels with the most robust response achieved at 25 mM EtOH. Ethanol (25 mM) stimulated γ-secretase proteolytic activity, to the same extent as Dll4, in HCAEC membranes. Ethanol inhibited Cav-1 mRNA and protein levels in HCAEC. Caveolin-1 negatively regulated γ-secretase activity in HCAEC as Cav-1 knockdown stimulated it, while Cav-1 overexpression inhibited it. Moreover, Cav-1 overexpression blocked the stimulatory effect of EtOH on γ-secretase activity in HCAEC. Although EtOH also inhibited Cav-1 expression in human coronary artery smooth muscle cells (HCASMC), EtOH inhibited γ-secretase activity in HCASMC in contrast to its effect in HCAEC. The inhibitory effect of EtOH on γ-secretase in HCASMC was mimicked by Cav-1 knockdown and prevented by Cav-1 overexpression, suggesting that in these cells Cav-1 positively regulates γ-secretase activity. In conclusion, EtOH differentially regulates γ-secretase activity in arterial EC and SMC, being stimulatory and inhibitory, respectively. These effects are both mediated by caveolin-1 inhibition which itself has opposite effects on γ-secretase in the two cell types. This mechanism may underlie, in part, the effects of moderate drinking on atherosclerosis.

The Role of Intracellular Trafficking of Notch Receptors in Ligand-Independent Notch Activation

Aberrant Notch signaling has been found in a broad range of human malignancies. Consequently, small molecule inhibitors and antibodies targeting Notch signaling in human cancers have been developed and tested; however, these have failed due to limited anti-tumor efficacy because of dose-limiting toxicities in normal tissues. Therefore, there is an unmet need to discover novel regulators of malignant Notch signaling, which do not affect Notch signaling in healthy tissues. This review provides a comprehensive overview of the current knowledge on the role of intracellular trafficking in ligand-independent Notch receptor activation, the possible mechanisms involved, and possible therapeutic opportunities for inhibitors of intracellular trafficking in Notch targeting.

Synapsin-caveolin-1 gene therapy preserves neuronal and synaptic morphology and prevents neurodegeneration in a mouse model of AD

Alzheimer's disease (AD) is the most common form of neurodegeneration and cognitive dysfunction in the elderly. Identifying molecular signals that mitigate and reverse neurodegeneration in AD may be exploited therapeutically. Transgenic AD mice (PSAPP) exhibit learning and memory deficits at 9 and 11 months, respectively, with associated decreased expression of caveolin-1 (Cav-1), a membrane/lipid raft (MLR) scaffolding protein necessary for synaptic and neuroplasticity. Neuronal-targeted gene therapy using synapsin-Cav-1 cDNA (SynCav1) was delivered to the hippocampus of PSAPP mice at 3 months using adeno-associated virus serotype 9 (AAV9). Bilateral SynCav1 gene therapy was able to preserve MLRs profile, learning and memory, hippocampal dendritic arbor, synaptic ultrastructure, and axonal myelin content in 9- and 11-month PSAPP mice, independent of reducing toxic amyloid deposits and astrogliosis. Our data indicate that SynCav1 gene therapy may be an option for AD and potentially in other forms of neurodegeneration of unknown etiology.

The DLC-1 tumor suppressor is involved in regulating immunomodulation of human mesenchymal stromal /stem cells through interacting with the Notch1 protein

Background

Immunomodulatory activities of human mesenchymal stromal /stem cells (hMSCs) has been widely recognized as the most critical function of hMSCs for exerting its therapeutic effects. However, the detailed mechanisms responsible for regulating the immunomodulation of hMSCs still remain largely unknown. Previous studies revealed that the Notch1 protein exerted a pro-immunomodulatory function probably through interacting with the protein(s) subjective to proteasome-mediated protein degradation. The DLC-1 protein represents a well characterized tumor suppressor subjective to proteasome-mediated degradation. However, the detailed signaling pathway of Notch1 and the involvement of DLC-1 in regulating the immunomodulation of hMSCs have not been studied before.

Methods

The transfection with cDNA or siRNA into hMSCs assisted by co-culture of hMSCs with peripheral blood mononuclear cells and small molecule inhibitors of signaling proteins, followed by immunoprecipitation, Western blotting, RT-PCR, and flowcytometry, were employed to characterize the Notch1 signaling, to identify DLC-1 as a candidate proteasome-targeted protein, and to characterize DLC-1 signaling pathway and its interaction with the Notch1 signaling, in the regulation of immunomodulation of hMSCs, specifically, the inhibition of pro-inflammatory CD4⁺-Th1 lymphocytes, and the release of immunomodulatory molecule IDO1.

Statistical analysis

One-way ANOVA was utilized as a statistical tool to analyze the data presented as means ± SEM of at least three separate experiments.

Results

The present study revealed that the Notch1-Hey1 axis, but not the Notch1-Hes1 axis, was likely responsible for mediating the pro-immunomodulatory function of the Notch1 signaling. The DLC-1 protein was found subjective to proteasome-mediated protein degradation mediated by the DDB1 and FBXW5 E3 ligases and served as an inhibitor of the immunomodulation of hMSCs through inhibiting Rock1, but not Rock2, downstream the DLC-1 signaling. The Notch1 signaling in the Notch1-Hey1 pathway and the DLC-1 signaling in the DLC-1-Rock1-FBXW5 pathway exhibited a mutual exclusion interaction in the regulation of immunomodulation of hMSCs.

Conclusions

The present study uncovers a novel function of DLC-1 tumor suppressor in regulating the immunomodulation of hMSCs. It also proposes a novel mutual exclusion mechanism between the DLC-1 signaling and the Notch1 signaling that is possibly responsible for fine-tuning the immunomodulation of hMSCs with different clinical implications in hMSCs therapy.

Supplementary Information

Supplementary information accompanies this paper at 10.1186/s12885-020-07542-5.

Phosphatidylinositol-4-phosphate 5-kinase type 1α attenuates Aβ production by promoting non-amyloidogenic processing of amyloid precursor protein

Alzheimer's disease (AD) is characterized by a chronic decline in cognitive function and is pathologically typified by cerebral deposition of amyloid-β peptide (Aβ). The production of Aβ is mediated by sequential proteolysis of amyloid precursor protein (APP) by β- and γ-secretases, and has been implicated as the essential determinant of AD pathology. Previous studies have demonstrated that the level of phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] in the membrane may potentially modulate Aβ production. Given that PI(4,5)P2 is produced by type 1 phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks), we sought to determine whether the level of PIP5K type Iα (PIP5K1A) can affect production of Aβ by modulating the lipid composition of the membrane. Using a HEK-derived cell line that constitutively expresses yellow fluorescent protein-tagged APP (APP-YFP), we demonstrated that overexpression of PIP5K1A results in significant enhancement of non-amyloidogenic APP processing and a concomitant suppression of the amyloidogenic pathway, leading to a marked decrease in secreted Aβ. Consistently, cells overexpressing PIP5K1A exhibited a significant redistribution of APP-YFP from endosomal compartments to the cell surface. Our findings suggest that PIP5K1A may play a critical role in governing Aβ production by modulating membrane distribution of APP, and as such, the pathway may be a valuable therapeutic target for AD.

Intra- and intergenerational changes in the cortical DNA methylome in response to therapeutic intermittent hypoxia in mice

Recent evidence from our laboratory documents functional resilience to retinal ischemic injury in untreated mice derived from parents exposed to repetitive hypoxic conditioning (RHC) before breeding. To begin to understand the epigenetic basis of this intergenerational protection, we used methylated DNA immunoprecipitation and sequencing to identify genes with differentially methylated promoters (DMGPs) in the prefrontal cortex of mice treated directly with the same RHC stimulus (F0-RHC) and in the prefrontal cortex of their untreated F1-generation offspring (F1-*RHC). Subsequent bioinformatic analyses provided key mechanistic insights into how changes in gene expression secondary to promoter hypo- and hypermethylation might afford such protection within and across generations. We found extensive changes in DNA methylation in both generations consistent with the expression of many survival-promoting genes, with twice the number of DMGPs in the cortex of F1*RHC mice relative to their F0 parents that were directly exposed to RHC. In contrast to our hypothesis that similar epigenetic modifications would be realized in the cortices of both F0-RHC and F1-*RHC mice, we instead found relatively few DMGPs common to both generations; in fact, each generation manifested expected injury resilience via distinctly unique gene expression profiles. Whereas in the cortex of F0-RHC mice, predicted protein-protein interactions reflected activation of an anti-ischemic phenotype, networks activated in F1-*RHC cortex comprised networks indicative of a much broader cytoprotective phenotype. Altogether, our results suggest that the intergenerational transfer of an acquired phenotype to offspring does not necessarily require the faithful recapitulation of the conditioning-modified DNA methylome of the parent.

Age-related focal loss of contractile vascular smooth muscle cells in retinal arterioles is accelerated by caveolin-1 deficiency

Cerebral microcirculation is critical for the preservation of brain health, and vascular impairment is associated with age-related neurodegenerative diseases. Because the retina is a component of the central nervous system, cellular changes that occur in the aging retina are likely relevant to the aging brain, and the retina provides the advantage that the entire vascular bed is visible, en face. In this study, we tested the hypothesis that normal, healthy aging alters the contractile vascular smooth muscle cell (VSMC) coverage of retinal arterioles. We found that aging results in a significant reduction of contractile VSMCs in focal patches along arterioles. Focal loss of contractile VSMCs occurs at a younger age in mice deficient in the senescence-associated protein, caveolin-1. Age-related contractile VSMC loss is not exacerbated by genetic depletion of insulin-like growth factor-1. The patchy loss of contractile VSMCs provides a cellular explanation for previous clinical studies showing focal microirregularities in retinal arteriolar responsiveness in healthy aged human subjects and is likely to contribute to age-related retinal vascular complications.

Curcumin enhances vascular contractility via induction of myocardin in mouse smooth muscle cells

A variety of cardiovascular diseases is accompanied by the loss of vascular contractility. This study sought to investigate the effects of curcumin, a natural polyphenolic compound present in turmeric, on mouse vascular contractility and the underlying mechanisms. After mice were administered curcumin (100 mg·kg-1·d-1, ig) for 6 weeks, the contractile responses of the thoracic aorta to KCl and phenylephrine were significantly enhanced compared with the control group. Furthermore, the contractility of vascular smooth muscle (SM) was significantly enhanced after incubation in curcumin (25 μmol/L) for 4 days, which was accompanied by upregulated expression of SM marker contractile proteins SM22α and SM α-actin. In cultured vascular smooth muscle cells (VSMCs), curcumin (10, 25, 50 μmol/L) significantly increased the expression of myocardin, a "master regulator" of SM gene expression. Curcumin treatment also significantly increased the levels of caveolin-1 in VSMCs. We found that as a result of the upregulation of caveolin-1, curcumin blocked the activation of notch1 and thereby abolished Notch1-inhibited myocardin expression. Knockdown of caveolin-1 or activation of Notch1 signaling with Jagged1 (2 μg/mL) diminished these effects of curcumin in VSMCs. These findings suggest that curcumin induces the expression of myocardin in mouse smooth muscle cells via a variety of mechanisms, including caveolin-1-mediated inhibition of notch1 activation and Notch1-mediated repression of myocardin expression. This may represent a novel pathway, through which curcumin protects blood vessels via the beneficial regulation of SM contractility.

New evidence of the relative protective effects of neurodegenerative diseases and cancer against each other

BACKGROUND

Cancer and degenerative diseases share some pathogenic mechanisms which act in opposition to one another to produce either uncontrolled cell proliferation or cell death. According to several studies, patients with Alzheimer disease have a lower risk of neoplasia, and vice versa. This study describes the prevalence of tumours (active or successfully treated) in a series of patients with and without a dementing degenerative disease treated at a cognitive neurology unit.

PATIENTS AND METHOD

We analysed the frequency and topography of tumours and the presence or absence of a neurodegenerative disease in a group of 1,164 patients. Neurodegenerative diseases were classified in 4 groups: Alzheimer disease, synucleinopathies, Pick complex, and polyglutamine complex. We subsequently compared tumour frequency in patients with and without a degenerative disease, and prevalence of neurodegenerative diseases in patients with and without tumours.

RESULTS

Tumours were detected in 12.1% of the patients with a neurodegenerative disease and in 17.3% of the remaining patients. Around 14.8% of the patients with a history of neoplasia and 20.8% of the patients with no history of neoplasia were diagnosed with a neurodegenerative disease. Except for these differences and the differences between subgroups (type of degenerative disease and tumour location) were not statistically significant, except when comparing neurodegenerative diseases to central nervous system tumours, and synucleinopathies to neoplasms.

CONCLUSION

Dementing degenerative diseases and neoplastic disorders are not mutually exclusive. Nevertheless, the rate of co-occurrence is lower than would be expected given the prevalence rate for each group.

Ethanol inhibits γ-secretase proteolytic activity in vascular smooth muscle cells

BACKGROUND

Ethanol (EtOH) inhibits Notch-mediated vascular smooth muscle cell (SMC) proliferation, an event that is key in vessel remodeling and atherogenesis. The object of this study was to determine whether EtOH inhibits Notch signaling in SMC at the level of γ-secretase, a protease that in concert with α-secretase catalyzes the release of the intracellular domain of the Notch receptor necessary for signaling.

METHODS

Human coronary artery SMCs (HCASMCs) were treated with a recombinant soluble Notch ligand, Delta-like ligand 4 (DLL4) (2 μg/ml), or transfected with a constitutively active Notch 1 intracellular domain (N1ICD), in the absence or presence of EtOH. EtOH (25 mM) treatment inhibited DLL4-stimulated CBF-1/RBP-Jk-dependent promoter activity (determined by luciferase assay) and downstream target gene HRT-3 mRNA levels. In contrast, EtOH had no effect on N1ICD-driven CBF-1/RBP-Jk-dependent promoter activity or HRT-3 expression.

RESULTS

These data suggest that EtOH inhibits Notch signaling at, or prior to, Notch intracellular domain (NICD) generation. γ-Secretase activity was determined in solubilized membrane preparations from HCASMC treated with/without EtOH (25 mM) or the γ-secretase inhibitor DAPT (20 μM) using (i) a fluorometric assay and (ii) Western blot detection of cleavage products using a Flag-tagged Notch-based substrate, N100Flag. EtOH inhibited basal and DLL4-stimulated γ-secretase activity, and SMC growth to a similar extent as DAPT, whereas it had no effect on α-secretase (TACE/ADAM17) activity also determined by fluorometric assay. Moreover, EtOH treatment inhibited the expression of caveolin-1, a lipid raft protein implicated in regulating γ-secretase activity, and altered its cellular distribution in HCASMC.

CONCLUSIONS

EtOH inhibits Notch signaling in vascular SMCs at the level of γ-secretase activity, possibly by affecting lipid raft function. Such a response might be expected to result in attenuation of pathologic vessel remodeling and thus may contribute to moderate alcohols' cardioprotective effects.

Alzheimer's disease: relevant molecular and physiopathological events affecting amyloid-β brain balance and the putative role of PPARs

Alzheimer’s disease (AD) is the most common form of age-related dementia. With the expected aging of the human population, the estimated morbidity of AD suggests a critical upcoming health problem. Several lines of research are focused on understanding AD pathophysiology, and although the etiology of the disease remains a matter of intense debate, increased brain levels of amyloid-β (Aβ) appear to be a critical event in triggering a wide range of molecular alterations leading to AD. It has become evident in recent years that an altered balance between production and clearance is responsible for the accumulation of brain Aβ. Moreover, Aβ clearance is a complex event that involves more than neurons and microglia. The status of the blood-brain barrier (BBB) and choroid plexus, along with hepatic functionality, should be considered when Aβ balance is addressed. Furthermore, it has been proposed that exposure to sub-toxic concentrations of metals, such as copper, could both directly affect these secondary structures and act as a seeding or nucleation core that facilitates Aβ aggregation. Recently, we have addressed peroxisomal proliferator-activated receptors (PPARs)-related mechanisms, including the direct modulation of mitochondrial dynamics through the PPARγ-coactivator-1α (PGC-1α) axis and the crosstalk with critical aging- and neurodegenerative-related cellular pathways. In the present review, we revise the current knowledge regarding the molecular aspects of Aβ production and clearance and provide a physiological context that gives a more complete view of this issue. Additionally, we consider the different structures involved in AD-altered Aβ brain balance, which could be directly or indirectly affected by a nuclear receptor (NR)/PPAR-related mechanism.

Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling

The plasma membrane in eukaryotic cells contains microdomains that are enriched in certain glycosphingolipids, gangliosides, and sterols (such as cholesterol) to form membrane/lipid rafts (MLR). These regions exist as caveolae, morphologically observable flask-like invaginations, or as a less easily detectable planar form. MLR are scaffolds for many molecular entities, including signaling receptors and ion channels that communicate extracellular stimuli to the intracellular milieu. Much evidence indicates that this organization and/or the clustering of MLR into more active signaling platforms depends upon interactions with and dynamic rearrangement of the cytoskeleton. Several cytoskeletal components and binding partners, as well as enzymes that regulate the cytoskeleton, localize to MLR and help regulate lateral diffusion of membrane proteins and lipids in response to extracellular events (e.g., receptor activation, shear stress, electrical conductance, and nutrient demand). MLR regulate cellular polarity, adherence to the extracellular matrix, signaling events (including ones that affect growth and migration), and are sites of cellular entry of certain pathogens, toxins and nanoparticles. The dynamic interaction between MLR and the underlying cytoskeleton thus regulates many facets of the function of eukaryotic cells and their adaptation to changing environments. Here, we review general features of MLR and caveolae and their role in several aspects of cellular function, including polarity of endothelial and epithelial cells, cell migration, mechanotransduction, lymphocyte activation, neuronal growth and signaling, and a variety of disease settings. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Retinoic acid-elicited RARα/RXRα signaling attenuates Aβ production by directly inhibiting γ-secretase-mediated cleavage of amyloid precursor protein

Retinoic acid (RA)-elicited signaling has been shown to play critical roles in development, organogenesis, and the immune response. RA regulates expression of Alzheimer's disease (AD)-related genes and attenuates amyloid pathology in a transgenic mouse model. In this study, we investigated whether RA can suppress the production of amyloid-β (Aβ) through direct inhibition of γ-secretase activity. We report that RA treatment of cells results in significant inhibition of γ-secretase-mediated processing of the amyloid precursor protein C-terminal fragment APP-C99, compared with DMSO-treated controls. RA-elicited signaling was found to significantly increase accumulation of APP-C99 and decrease production of secreted Aβ40. In addition, RA-induced inhibition of γ-secretase activity was found to be mediated through significant activation of extracellular signal-regulated kinases (ERK1/2). Treatment of cells with the specific ERK inhibitor PD98059 completely abolished RA-mediated inhibition of γ-secretase. Consistent with these findings, RA was observed to inhibit secretase-mediated proteolysis of full-length APP. Finally, we have established that RA inhibits γ-secretase through nuclear retinoic acid receptor-α (RARα) and retinoid X receptor-α (RXRα). Our findings provide a new mechanistic explanation for the neuroprotective role of RA in AD pathology and add to the previous data showing the importance of RA signaling as a target for AD therapy.

Lipid raft disarrangement as a result of neuropathological progresses: a novel strategy for early diagnosis?

Lipid rafts are the preferential site of numerous membrane signaling proteins which are involved in neuronal functioning and survival. These proteins are organized in multiprotein complexes, or signalosomes, in close contact with lipid classes particularly represented in lipid rafts (i.e. cholesterol, sphingolipids and saturated fatty acids), which may contribute to physiological responses leading to neuroprotection. Increasing evidence indicates that alteration of lipid composition in raft structures as a consequence of neuropathologies, such as Alzheimer's disease (AD) and Parkinson's disease (PD), causes a dramatic increase in lipid raft order. These phenomena may correlate with perturbation of signalosome activities, likely contributing to neurodegenerative progression. Interestingly, significant disruption of stable raft microenvironments has been already observed in the first stages of either AD or PD, suggesting that these alterations may represent early events in the neuropathological development. In this regard, the search for biochemical markers, such as specific metabolic products altered in the brain at the first steps of the disease, presently represents an important challenge for early diagnostic strategies. Alterations of these biomarkers may be reflected in either plasma or cerebrospinal fluid, thus representing a potential strategy to predict an accurate diagnosis. We propose that pathologically-linked lipid raft markers may be interesting candidates to be explored at this level, although it has not been studied so far to what extent alteration of different signalosome components may be reflected in peripheral fluids. In this mini-review, we will discuss on relevant aspects of lipid rafts that contribute to the modulation of neuropathological events related to AD and PD. An interesting hypothesis is that anomalies on raft biomarkers measured at peripheral fluids might mirror the lipid raft pathology observed in early stages of AD and PD.

Lipid rafts and Alzheimer's disease: protein-lipid interactions and perturbation of signaling

Lipid rafts are membrane domains, more ordered than the bulk membrane and enriched in cholesterol and sphingolipids. They represent a platform for protein-lipid and protein–protein interactions and for cellular signaling events. In addition to their normal functions, including membrane trafficking, ligand binding (including viruses), axonal development and maintenance of synaptic integrity, rafts have also been implicated in the pathogenesis of several neurodegenerative diseases including Alzheimer’s disease (AD). Lipid rafts promote interaction of the amyloid precursor protein (APP) with the secretase (BACE-1) responsible for generation of the amyloid β peptide, Aβ. Rafts also regulate cholinergic signaling as well as acetylcholinesterase and Aβ interaction. In addition, such major lipid raft components as cholesterol and GM1 ganglioside have been directly implicated in pathogenesis of the disease. Perturbation of lipid raft integrity can also affect various signaling pathways leading to cellular death and AD. In this review, we discuss modulation of APP cleavage by lipid rafts and their components, while also looking at more recent findings on the role of lipid rafts in signaling events.

The role of endocytosis in activating and regulating signal transduction

Endocytosis is increasingly understood to play crucial roles in most signaling pathways, from determining which signaling components are activated, to how the signal is subsequently transduced and/or terminated. Whether a receptor-ligand complex is internalized via a clathrin-dependent or clathrin-independent endocytic route, and the complexes' subsequent trafficking through specific endocytic compartments, to then be recycled or degraded, has profound effects on signaling output. This review discusses the roles of endocytosis in three markedly different signaling pathways: the Wnt, Notch, and Eph/Ephrin pathways. These offer fundamentally different signaling systems: (1) diffusible ligands inducing signaling in one cell, (2) membrane-tethered ligands inducing signaling in a contacting receptor cell, and (3) bi-directional receptor-ligand signaling in two contacting cells. In each of these systems, endocytosis controls signaling in fascinating ways, and comparison of their similarities and dissimilarities will help to expand our understanding of endocytic control of signal transduction across multiple signaling pathways.

Matrikines and the lungs

The extracellular matrix is a complex network of fibrous and nonfibrous molecules that not only provide structure to the lung but also interact with and regulate the behaviour of the cells which it surrounds. Recently it has been recognised that components of the extracellular matrix proteins are released, often through the action of endogenous proteases, and these fragments are termed matrikines. Matrikines have biological activities, independent of their role within the extracellular matrix structure, which may play important roles in the lung in health and disease pathology. Integrins are the primary cell surface receptors, characterised to date, which are used by the matrikines to exert their effects on cells. However, evidence is emerging for the need for co-factors and other receptors for the matrikines to exert their effects on cells. The potential for matrikines, and peptides derived from these extracellular matrix protein fragments, as therapeutic agents has recently been recognised. The natural role of these matrikines (including inhibitors of angiogenesis and possibly inflammation) make them ideal targets to mimic as therapies. A number of these peptides have been taken forward into clinical trials. The focus of this review will be to summarise our current understanding of the role, and potential for highly relevant actions, of matrikines in lung health and disease.