Biological roles of sulfoglycolipids and pathophysiology of their deficiency

Mammalian sulfoglycolipids are comprised of two major members, sulfatide (SO(3)-3Gal-ceramide) and seminolipid (SO(3)-3Gal-alkylacylglycerol). Sulfatide is abundant in the myelin sheath and seminolipid is expressed on the spermatogenic cells. Cerebroside sulfotransferase (CST)-deficient mice generated by gene targeting completely lack sulfatide and seminolipid all over the body. CST-null mice manifest some neurological disorders due to myelin dysfunction, an aberrant enhancement of oligodendrocyte terminal differentiation, and an arrest of spermatogenesis, indicating that sulfation of glycolipids is essential for myelin formation and spermatogenesis. Moreover, CST-deficiency ameliorates L-selectin-dependent monocyte infiltration in the kidney after ureteral obstruction, an experimental model of renal interstitial inflammation, indicating that sulfatide is an endogenous ligand of L-selectin. Studies on the molecular mechanisms by which sulfoglycolipids participate in these biological processes are ongoing.     

Plasma membrane glutathione transporters and their roles in cell physiology and pathophysiology

Reduced glutathione (GSH) is critical for many cellular processes, and both its intracellular and extracellular concentrations are tightly regulated. Intracellular GSH levels are regulated by two main mechanisms: by adjusting the rates of synthesis and of export from cells. Some of the proteins responsible for GSH export from mammalian cells have recently been identified, and there is increasing evidence that these GSH exporters are multispecific and multifunctional, regulating a number of key biological processes. In particular, some of the multidrug resistance-associated proteins (Mrp/Abcc) appear to mediate GSH export and homeostasis. The Mrp proteins mediate not only GSH efflux, but they also export oxidized glutathione derivatives (e.g., glutathione disulfide (GSSG), S-nitrosoglutathione (GS-NO), and glutathione-metal complexes), as well as other glutathione S-conjugates. The ability to export both GSH and oxidized derivatives of GSH, endows these transporters with the capacity to directly regulate the cellular thiol-redox status, and therefore the ability to influence many key signaling and biochemical pathways. Among the many processes that are influenced by the GSH transporters are apoptosis, cell proliferation, and cell differentiation. This report summarizes the evidence that Mrps contribute to the regulation of cellular GSH levels and the thiol-redox state, and thus to the many biochemical processes that are influenced by this tripeptide.     

BMP2 and BMP7 play antagonistic roles in feather induction

Feathers, like hairs, first appear as primordia consisting of an epidermal placode associated with a dermal condensation that is necessary for the continuation of their differentiation. Previously, the BMPs have been proposed to inhibit skin appendage formation. We show that the function of specific BMPs during feather development is more complex. BMP2 and BMP7, which are expressed in both the epidermis and the dermis, are involved in an antagonistic fashion in regulating the formation of dermal condensations, and thus are both necessary for subsequent feather morphogenesis. BMP7 is expressed earlier and functions as a chemoattractant that recruits cells into the condensation, whereas BMP2 is expressed later, and leads to an arrest of cell migration, likely via its modulation of the EIIIA fibronectin domain and alpha4 integrin expression. Based on the observed cell proliferation, chemotaxis and the timing of BMP2 and BMP7 expression, we propose a mathematical model, a reaction-diffusion system, which not only simulates feather patterning, but which also can account for the negative effects of excess BMP2 or BMP7 on feather formation.     

The BMP antagonists cerberus-like and noggin do not interact during mouse forebrain development

Mouse cerberus-like encodes for a secreted factor of the Cerberus/Dan family. This molecule has neural inducing capabilities and can bind to BMP-4 and nodal molecules in the extracellular space. When cerberus-like is inactivated, its function may be compensated for another molecule, since no abnormalities can be observed in the mouse mutant. Compensation mechanisms have been shown to occur between the BMP antagonists chordin and noggin. Here we report the generation of cerberus-like-/-; noggin-/- double mutants to uncover a possible compensation by noggin in cer-l-/- mutant. Double mutants were obtained and failed to show any further detectable defects beside the ones presented by the noggin-/- single mutant. Contrarily to chordin and noggin, mouse cerberus-like and noggin cannot compensate for each other during mouse embryogenesis.     

The pathophysiology of superoxide: roles in inflammation and ischemia

The superoxide radical plays major roles in the neutrophil-medicated acute inflammatory response and in postischemic tissue injury, although the sources and actions of the radical are quite different in these two pathological states. While neutrophils produce superoxide for the primary purpose of aiding in the killing of ingested microbes, a second useful function has evolved. The superoxide released from actively phagocytosing neutrophils serves to attract more neutrophils by reacting with, and activating, a latent chemotactic factor present in plasma. Superoxide dismutase, by preventing the activation of this superoxide-dependent chemotactic factor, exerts potent anti-inflammatory action. During ischemia, energy-starved tissues catabolize ATP to hypoxanthine. Calcium transients in these cells appear to activate a calmodulin regulated protease which attacks the enzyme xanthine dehydrogenase, converting it to a xanthine oxidase capable of superoxide generation. When the tissue is reperfused and reoxygenated, all the necessary components are present (xanthine oxidase, hypoxanthine, and oxygen) to produce a burst of superoxide which results in extensive tissue damage. Ischemic tissues are protected by superoxide dismutase or allupurinol, an inhibitor of xanthine oxidase.     

Isoprostanes: an overview and putative roles in pulmonary pathophysiology

Isoprostanes are produced during peroxidation of membrane lipids by free radicals and reactive oxygen species. Initially, they were recognized as being valuable markers of oxidative stress, and in the past 10 years, dozens of disease states and experimental conditions with diverse etiologies have been shown to be associated with marked increases in urinary, plasma, and tissue levels of isoprostanes. However, they are not just mere markers; they evoke important biological responses on virtually every cell type found within the lung, and these responses exhibit compound-, tissue-, and species-related variations. In fact, the isoprostanes may mediate many of the features of the disease states for which they are used as indicators. In this review, I describe the chemistry, metabolism, and pharmacology of isoprostanes, with a particular emphasis on pulmonary cell types, and the possible roles of isoprostanes in pulmonary pathophysiology.     

Viruses and the immune system: their roles in seizure cascade development

Viral encephalitis affects approximately 7.5 people/100 000 and carries a high rate of morbidity and mortality. Most patients with viral encephalitis will develop some form of seizure during the infectious process, and of those who survive encephalitic disease, approximately 4–20% will develop epilepsy. Arthropod-borne (arbo)viruses are the leading cause of viral encephalitis in the world today, with between 10% and 35% of patients infected with these viruses displaying some form of seizure. Several neurotropic DNA viruses, including Herpes and cytomegalovirus also commonly cause seizures in infected patients. In the clinical setting, the cause of seizures seen during viral encephalitis is usually attributed to acute febrile responses. However, it has become apparent that the mechanisms behind seizure generation during viral encephalitis are likely to be much more complicated. For example, CD4⁺ and CD8⁺ T cells possibly through their secretion of interferon-γ, appear to play an important role in determining neuronal responses when challenged with kainic acid. In addition, the ability of the human immunodeficiency virus, transactivating protein to modulate NMDA signaling possibly triggering seizures, highlights the fact that elements of the antiviral response and even virally derived proteins are capable of directly manipulating neuronal function. Understanding the complex relationships between the CNS, the immune system, and invading pathogens is a critical step in understanding the pathogenesis of seizures seen during viral infections and informing the development of novel therapies.     

CEACAMs: their role in physiology and pathophysiology

Carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) belong to a group of mammalian immunoglobulin-related glycoproteins. They are involved in cell-cell recognition and modulate cellular processes that range from the shaping of tissue architecture and neovascularization to the regulation of insulin homeostasis and T-cell proliferation. CEACAMs have also been identified as receptors for host-specific viruses and bacteria in mice and humans, respectively, making these proteins an interesting example of pathogen-host co-evolution. Forward and reverse genetics in the mouse now provide powerful novel models to elucidate the action of CEACAM family members in vivo.     

Genetic analysis of the roles of BMP2, BMP4, and BMP7 in limb patterning and skeletogenesis

Bone morphogenetic protein (BMP) family members, including BMP2, BMP4, and BMP7, are expressed throughout limb development. BMPs have been implicated in early limb patterning as well as in the process of skeletogenesis. However, due to complications associated with early embryonic lethality, particularly for Bmp2 and Bmp4, and with functional redundancy among BMP molecules, it has been difficult to decipher the specific roles of these BMP molecules during different stages of limb development. To circumvent these issues, we have constructed a series of mouse strains lacking one or more of these BMPs, using conditional alleles in the case of Bmp2 and Bmp4 to remove them specifically from the limb bud mesenchyme. Contrary to earlier suggestions, our results indicate that BMPs neither act as secondary signals downstream of Sonic Hedghog (SHH) in patterning the anteroposterior axis nor as signals from the interdigital mesenchyme in specifying digit identity. We do find that a threshold level of BMP signaling is required for the onset of chondrogenesis, and hence some chondrogenic condensations fail to form in limbs deficient in both BMP2 and BMP4. However, in the condensations that do form, subsequent chondrogenic differentiation proceeds normally even in the absence of BMP2 and BMP7 or BMP2 and BMP4. In contrast, we find that the loss of both BMP2 and BMP4 results in a severe impairment of osteogenesis.     

Common mechanisms in development and disease: BMP signaling in craniofacial development

BMP signaling is one of the key pathways regulating craniofacial development. It is involved in the early patterning of the head, the development of cranial neural crest cells, and facial patterning. It regulates development of its mineralized structures, such as cranial bones, maxilla, mandible, palate, and teeth. Targeted mutations in the mouse have been instrumental to delineate the functional involvement of this signaling network in different aspects of craniofacial development. Gene polymorphisms and mutations in BMP pathway genes have been associated with various non-syndromic and syndromic human craniofacial malformations. The identification of intricate cellular interactions and underlying molecular pathways illustrate the importance of local fine-regulation of Bmp signaling to control proliferation, apoptosis, epithelial-mesenchymal interactions, and stem/progenitor differentiation during craniofacial development. Thus, BMP signaling contributes both to shape and functionality of our facial features. BMP signaling also regulates postnatal craniofacial growth and is associated with dental structures life-long. A more detailed understanding of BMP function in growth, homeostasis, and repair of postnatal craniofacial tissues will contribute to our ability to rationally manipulate this signaling network in the context of tissue engineering.     

Emerging roles of Orai3 in pathophysiology

Calcium (Ca²⁺) is a ubiquitous second messenger that regulates a plethora of physiological functions. Deregulation of calcium homeostasis has been reported in a wide variety of pathological conditions including cardiovascular disorders, cancer and neurodegenerative diseases. One of the most ubiquitous pathways involved in regulated Ca²⁺ influx into cells is the store-operated Ca²⁺ entry (SOCE) pathway. In 2006, Orai1 was identified as the channel protein that mediates SOCE in immune cells. Orai1 has two mammalian homologs, Orai2 and Orai3. Although Orai1 has been the most widely studied Orai isoform, Orai3 has recently received significant attention. Under native conditions, Orai3 was demonstrated to be an important component of store-independent arachidonate-regulated Ca²⁺ (ARC) entry in HEK293 cells, and more recently of a store-independent leukotrieneC₄-regulated Ca²⁺ (LRC) entry pathway in vascular smooth muscle cells. Recent studies have shown upregulation of Orai3 in estrogen receptor-expressing breast cancers and a critical role for Orai3 in breast cancer development in immune-compromised mice. Orai3 upregulation was also shown to contribute to vascular smooth muscle remodeling and neointimal hyperplasia caused by vascular injury. Furthermore, Orai3 has been shown to contribute to proliferation of effector T-lymphocytes under oxidative stress. In this review, we will discuss the role of Orai3 in reported pathophysiological conditions and will contribute ideas on the potential role of Orai3 in native Ca²⁺ signaling pathways and human disease.     

BMP signalling in craniofacial development

The BMP signalling pathway is conserved throughout evolution and essential for mammalian embryonic and postnatal development and growth. In the vertebrate head, this signal is involved in the development of a variety of structures and shows divergent roles. During early head development, BMP signalling participates in the induction, formation, determination and migration of the cranial neural crest cells, which give rise to most of the craniofacial structures. Subsequently, it is also important for patterning and formation of facial primordia. During craniofacial skeletogenesis, BMP signalling is an early inductive signal required for committed cell migration, condensation, proliferation and differentiation. Thereafter, BMP signalling maintains regulatory roles in skeletons and skeletal growth centres. For myogenesis, BMP signalling is a negative regulator. Importantly, myostatin has been identified as a key mediator in this process. During palatogenesis, the crucial role of BMP signalling is demonstrated by mouse models with Alk2 or Alk3 (BMP type I receptors) deletion from the neural crest or craniofacial region, in which cleft palate is one of the major anomalies. BMP signalling is also an important participant for tooth development, regulating early tooth morphogenesis and subsequent odontoblast differentiation. In this review these aspects are discussed in detail with a focus on recent advances.     

BMP signaling in skeletal development

Development of the vertebrate skeleton, a complex biological event that includes diverse processes such as formation of mesenchymal condensations at the sites of future skeletal elements, osteoblast and chondrocyte differentiation, and three dimensional patterning, is regulated by many growth factors. Bone morphogenetic proteins (BMPs), members of the TGF-beta superfamily, play a pivotal role in the signaling network and are involved in nearly all processes associated with skeletal morphogenesis. BMP signals are transduced from the plasma membrane receptors to the nucleus through both Smad pathway and non-Smad pathways, and regulated by many extracellular and intercellular proteins that interact with BMPs or components of the BMP signaling pathways. To gain a better understanding of the molecular mechanisms underlying the role of BMP in early skeletal development, it is necessary to elucidate the BMP signaling transduction pathways in chondrocytes and osteoblasts. The major objective of this review was to summarize BMP signaling pathways in the context of craniofacial, axial, and limb development. In particular, this discourse will focus on recent advances of the role of different ligands, receptors, Smads, and BMP regulators in osteoblast and chondrocyte differentiation during embryonic development.     

Pax genes and their roles in cell differentiation and development

Members of the Pax gene family are expressed in various tissues during ontogenesis. Evidence for their crucial role in morphogenesis, organogenesis, cell differentiation and oncogenesis is provided by rodent mutants and human diseases. Additionally, recent experimental in vivo and in vitro approaches have led to the identification of molecules that interact with Pax proteins.     

Notochord-derived BMP antagonists inhibit endothelial cell generation and network formation

Embryonic blood vessel formation is initially mediated through the sequential differentiation, migration, and assembly of endothelial cells (ECs). While many molecular signals that promote vascular development have been identified, little is known about suppressors of this process. In higher vertebrates, including birds and mammals, the vascular network forms throughout the embryonic disk with the exception of a region along the midline. We have previously shown that the notochord is responsible for the generation and maintenance of the avascular midline and that BMP antagonists expressed by this embryonic tissue, including Noggin and Chordin, can mimic this inhibitory role. Here we report that the notochord suppresses the generation of ECs from the mesoderm both in vivo and in vitro. We also report that the notochord diminishes the ability of mature ECs to organize into a primitive plexus. Furthermore, Noggin mimics notochord-based inhibition by preventing mesodermal EC generation and mature EC network formation. These findings suggest that the mesoderm surrounding the midline is competent to give rise to ECs and to form blood vessels, but that notochord derived-BMP antagonists suppress EC differentiation and maturation processes leading to inhibition of midline vessel formation.