Cell-cell nanotubes: Tunneling through several types of synapses

Perspective on nanochannels as cellular mediators in different disease conditions

Tunnelling nanotubes (TNTs), also known as membrane nanochannels, are actin-based structures that facilitate cytoplasmic connections for rapid intercellular transfer of signals, organelles and membrane components. These dynamic TNTs can form de novo in animal cells and establish complex intercellular networks between distant cells up to 150 μm apart. Within the last decade, TNTs have been discovered in different cell types including tumor cells, macrophages, monocytes, endothelial cells and T cells. It has also been further elucidated that these nanotubes play a vital role in diseased conditions such as cancer, where TNT formation occurs at a higher pace and is used for rapid intercellular modulation of chemo-resistance. Viruses such as HIV, HSV and prions also hijack the existing TNT connections between host cells for rapid transmission and evasion of the host immune responses. The following review aims to describe the heterogeneity of TNTs, their role in different tissues and disease conditions in order to enhance our understanding on how these nanotubes can be used as a target for therapies.

Imaging Tunneling Membrane Tubes Elucidates Cell Communication in Tumors

Intercellular communication is a vital yet underdeveloped aspect of cancer pathobiology. This Opinion article reviews the importance and challenges of microscopic imaging of tunneling nanotubes (TNTs) in the complex tumor microenvironment. The use of advanced microscopy to characterize TNTs in vitro and ex vivo, and related extensions called tumor microtubes (TMs) reported in gliomas in vivo, has propelled this field forward. This topic is important because the identification of TNTs and TMs fills the gap in our knowledge of how cancer cells communicate at long range in vivo, inducing intratumor heterogeneity and resistance to treatment. Here we discuss the concept that TNTs/TMs fill an important niche in the ever-changing microenvironment and the role of advanced microscopic imaging to elucidate that niche.

Does cell polarity matter during spermatogenesis?

Cell polarity is crucial to development since apico-basal polarity conferred by the 3 polarity protein modules (or complexes) is essential during embryogenesis, namely the Par (partition defective)-, the CRB (Crumbs)-, and the Scribble-based polarity protein modules. While these protein complexes and their component proteins have been extensively studied in Drosophila and C. elegans and also other mammalian tissues and/or cells, their presence and physiological significance in the testis remain unexplored until the first paper on the Par-based protein published in 2008. Since then, the Par-, the Scribble- and the CRB-based protein complexes and their component proteins in the testis have been studied. These proteins are known to confer Sertoli and spermatid polarity in the seminiferous epithelium, and they are also integrated components of the tight junction (TJ) and the basal ectoplasmic specialization (ES) at the Sertoli cell-cell interface near the basement membrane, which in turn constitute the blood-testis barrier (BTB). These proteins are also found at the apical ES at the Sertoli-spermatid interface. Thus, these polarity proteins also play a significant role in regulating Sertoli and spermatid adhesion in the testis through their actions on actin-based cytoskeletal function. Recent studies have shown that these polarity proteins are having antagonistic effects on the BTB integrity in which the Par6- and CRB3-based polarity complexes promotes the integrity of the Sertoli cell TJ-permeability barrier, whereas the Scribble-based complex promotes restructuring/remodeling of the Sertoli TJ-barrier function. Herein, we carefully evaluate these findings and provide a hypothetic model regarding their role in the testis in the context of the functions of these polarity proteins in other epithelia, so that better experiments can be designed in future studies to explore their significance in spermatogenesis.

Ezrin: a regulator of actin microfilaments in cell junctions of the rat testis

Ezrin, radixin, moesin and merlin (ERM) proteins are highly homologous actin-binding proteins that share extensive sequence similarity with each other. These proteins tether integral membrane proteins and their cytoplasmic peripheral proteins (e.g., adaptors, nonreceptor protein kinases and phosphatases) to the microfilaments of actin-based cytoskeleton. Thus, these proteins are crucial to confer integrity of the apical membrane domain and its associated junctional complex, namely the tight junction and the adherens junction. Since ectoplasmic specialization (ES) is an F-actin-rich testis-specific anchoring junction-a highly dynamic ultrastructure in the seminiferous epithelium due to continuous transport of germ cells, in particular spermatids, across the epithelium during the epithelial cycle-it is conceivable that ERM proteins are playing an active role in these events. Although these proteins were first reported almost 25 years and have since been extensively studied in multiple epithelia/endothelia, few reports are found in the literature to examine their role in the actin filament bundles at the ES. Studies have shown that ezrin is also a constituent protein of the actin-based tunneling nanotubes (TNT) also known as intercellular bridges, which are transient cytoplasmic tubular ultrastructures that transport signals, molecules and even organelles between adjacent and distant cells in an epithelium to coordinate cell events that occur across an epithelium. Herein, we critically evaluate recent data on ERM in light of recent findings in the field in particular ezrin regarding its role in actin dynamics at the ES in the testis, illustrating additional studies are warranted to examine its physiological significance in spermatogenesis.

Nanotube-Enabled Vesicle-Vesicle Communication: A Computational Model

Cell-to-cell communications via the tunneling nanotubes or gap junction channels are vital for the development and maintenance of multicellular organisms. Instead of these intrinsic communication pathways, how to design artificial communication channels between cells remains a challenging but interesting problem. Here, we perform dissipative particle dynamics (DPD) simulations to analyze the interaction between rotational nanotubes (RNTs) and vesicles so as to provide a novel design mechanism for cell-to-cell communication. Simulation results have demonstrated that the RNTs are capable of generating local disturbance and promote vesicle translocation toward the RNTs. Through ligand pattern designing on the RNTs, we can find a suitable nanotube candidate with a specific ligand coating pattern for forming the RNT-vesicle network. The results also show that a RNT can act as a bridged channel between vesicles, which facilitates substance transfer. Our findings provide useful guidelines for the molecular design of patterned RNTs for creating a synthetic channel between cells.

Toxicants target cell junctions in the testis: Insights from the indazole-carboxylic acid model

There are numerous types of junctions in the seminiferous epithelium which are integrated with, and critically dependent on the Sertoli cell cytoskeleton. These include the basal tight junctions between Sertoli cells that form the main component of the blood–testis barrier, the basal ectoplasmic specializations (basal ES) and basal tubulobulbar complexes (basal TBC) between Sertoli cells; as well as apical ES and apical TBC between Sertoli cells and the developing spermatids that orchestrate spermiogenesis and spermiation. These junctions, namely TJ, ES, and TBC interact with actin microfilament-based cytoskeleton, which together with the desmosomal junctions that interact with the intermediate filament-based cytoskeleton plus the highly polarized microtubule-based cytoskeleton are working in concert to move spermatocytes and spermatids between the basal and luminal aspect of the seminiferous epithelium. In short, these various junctions are structurally complexed with the actin- and microtubule-based cytoskeleton or intermediate filaments of the Sertoli cell. Studies have shown toxicants (e.g., cadmium, bisphenol A (BPA), perfluorooctanesulfonate (PFOS), phthalates, and glycerol), and some male contraceptives under development (e.g., adjudin, gamendazole), exert their effects, at least in part, by targeting cell junctions in the testis. The disruption of Sertoli–Sertoli cell and Sertoli–germ cell junctions, results in the loss of germ cells from the seminiferous epithelium. Adjudin, a potential male contraceptive under investigation in our laboratory, produces loss of spermatids from the seminiferous tubules through disruption of the Sertoli cell spermatid junctions and disruption of the Sertoli cell cytoskeleton. The molecular and structural changes associated with adjudin administration are described, to provide an example of the profile of changes caused by disturbance of Sertoli-germ cell and also Sertoli cell-cell junctions.

Ezrin is an actin binding protein that regulates sertoli cell and spermatid adhesion during spermatogenesis

During spermatogenesis, the transport of spermatids and the release of sperms at spermiation and the remodeling of the blood-testis barrier (BTB) in the seminiferous epithelium of rat testes require rapid reorganization of the actin-based cytoskeleton. However, the mechanism(s) and the regulatory molecule(s) remain unexplored. Herein we report findings that unfold the functional significance of ezrin in the organization of the testis-specific adherens junction at the spermatid-Sertoli cell interface called apical ectoplasmic specialization (ES) in the adluminal compartment and the Sertoli cell-cell interface known as basal ES at the BTB. Ezrin is expressed at the basal ES/BTB in all stages, except from late VIII to IX, of the epithelial cycle. Its knockdown by RNA interference (RNAi) in vitro perturbs the Sertoli cell tight junction-permeability barrier via a disruption of the actin microfilaments in Sertoli cells, which in turn impeded basal ES protein (eg, N-cadherin) distribution, perturbing the BTB function. These findings were confirmed by a knockdown study in vivo. However, the expression of ezrin at the apical ES is restricted to stage VIII of the cycle and limited only between step 19 spermatids and Sertoli cells. A knockdown of ezrin in vivo by RNAi was found to impede spermatid transport, causing defects in spermiation in which spermatids were embedded deep inside the epithelium, and associated with a loss of spermatid polarity. Also, ezrin was associated with residual bodies and phagosomes, and its knockdown by RNAi in the testis also impeded the transport of residual bodies/phagosomes from the apical to the basal compartment. In summary, ezrin is involved in regulating actin microfilament organization at the ES in rat testes.

The gametic synapse: RNA transfer to the bovine oocyte

Even after several decades of quiescent storage in the ovary, the female germ cell is capable of reinitiating transcription to build the reserves that are essential to support early embryonic development. In the current model of mammalian oogenesis, there exists bilateral communication between the gamete and the surrounding cells that is limited to paracrine signaling and direct transfer of small molecules via gap junctions existing at the end of the somatic cells' projections that are in contact with the oolemma. The purpose of this work was to explore the role of cumulus cell projections as a means of conductance of large molecules, including RNA, to the mammalian oocyte. By studying nascent RNA with confocal and transmission electron microscopy in combination with transcript detection, we show that the somatic cells surrounding the fully grown bovine oocyte contribute to the maternal reserves by actively transferring large cargo, including mRNA and long noncoding RNA. This occurrence was further demonstrated by the reconstruction of cumulus-oocyte complexes with transfected cumulus cells transferring a synthetic transcript. We propose selective transfer of transcripts occurs, the delivery of which is supported by a remarkable synapselike vesicular trafficking connection between the cumulus cells and the gamete. This unexpected exogenous contribution to the maternal stores offers a new perspective on the determinants of female fertility.

Morphing structures and signal transduction in Mimosa pudica L. induced by localized thermal stress

Leaf movements in Mimosa pudica, are in response to thermal stress, touch, and light or darkness, appear to be regulated by electrical, hydrodynamical, and chemical signal transduction. The pulvinus of the M. pudica shows elastic properties. We have found that the movements of the petiole, or pinnules, are accompanied by a change of the pulvinus morphing structures. After brief flaming of a pinna, the volume of the lower part of the pulvinus decreases and the volume of the upper part increases due to the redistribution of electrolytes between these parts of the pulvinus; as a result of these changes the petiole falls. During the relaxation of the petiole, the process goes in the opposite direction. Ion and water channel blockers, uncouplers as well as anesthetic agents diethyl ether or chloroform decrease the speed of alert wave propagation along the plant. Brief flaming of a pinna induces bidirectional propagation of electrical signal in pulvini. Transduction of electrical signals along a pulvinus induces generation of an action potential in perpendicular direction between extensor and flexor sides of a pulvinus. Inhibition of signal transduction and mechanical responses in M. pudica by volatile anesthetic agents chloroform or by blockers of voltage gated ion channels shows that the generation and propagation of electrical signals is a primary effect responsible for turgor change and propagation of an excitation. There is an electrical coupling in a pulvinus similar to the electrical synapse in the animal nerves.

Chirality and homochirality of lipids is necessary to form LD-supramolecular structures of cell-cell communications

The processes of cell-cell commutations via anisometric supramolecular structures (strings) in vitro and in physicochemical models of lipids (trifluoroacetylated amino alcohols) were studied. It was shown that biological commutation through strings is more efficient than the diffusion mechanism of transfer of bioactive molecules at distances typical of populations. Strings kinetics was constructed and the signal turnover rate in these systems was evaluated. It was demonstrated that the condition of chirality of lipids forming the cell biomembrane stems from experimentally observed cell commutation through anisometric supramolecular structures.

Multifaceted roles of tunneling nanotubes in intercellular communication

Cell-to-cell communication and exchange of materials are vital processes in multicellular organisms during cell development, cell repair, and cell survival. In neuronal and immunological cells, intercellular transmission between neighboring cells occurs via different complex junctions or synapses. Recently, long distance intercellular connections in mammalian cells called tunneling nanotubes (TNTs) have been described. These structures have been found in numerous cell types and shown to transfer signals and cytosolic materials between distant cells, suggesting that they might play a prominent role in intercellular trafficking. However, these cellular connections are very heterogeneous in both structure and function, giving rise to more questions than answers as to their nature and role as intercellular conduits. To better understand and characterize the functions of TNTs, we have highlighted here the latest discoveries regarding the formation, structure, and role of TNTs in cell-to-cell spreading of various signals and materials. We first gathered information regarding their formation with an emphasis on the triggering mechanisms observed, such as stress and potentially important proteins and/or signaling pathways. We then describe the various types of transfer mechanisms, in relation to signals and cargoes that have been shown recently to take advantage of these structures for intercellular transfer. Because a number of pathogens were shown to use these membrane bridges to spread between cells we also draw attention to specific studies that point toward a role for TNTs in pathogen spreading. In particular we discuss the possible role that TNTs might play in prion spreading, and speculate on their role in neurological diseases in general.

The role of connexins in prostate cancer promotion and progression

Prostate cancer is a prevalent disease that is characterized by a presumably long latency period and a moderate propensity to metastasize. Although a range of mechanisms have been implicated in prostate carcinogenesis, the factors determining the initiation of metastasis remain obscure. The synchronized function of prostate cells depends on their metabolic and electrical coupling; disturbance of these functions has long been suggested to be integral to prostate carcinogenesis. However, although connexins form intercellular channels involved in gap-junction-mediated intercellular coupling (GJIC), whether these proteins also have GJIC-independent roles in cancer progression and metastasis remains a matter of debate. Some data indicate a correlation between connexin expression and the invasive potential of prostate cancer cells, which points to stage-specific functions of connexins during prostate cancer development. For example, restoration of connexin expression seems to be crucial for the formation of invasive cell subsets within heterogeneous prostate cancer cell populations that have undergone aberrant differentiation. Consequently, the clinical application of therapeutic and prophylactic approaches focused on the modulation of connexin expression in prostate cancer cells should be reconsidered.