Autotypic Junctions Research Articles

Sensory axons induce epithelial lipid microdomain remodeling and determine the distribution of junctions in the epidermis.

Epithelial cell properties are determined by the polarized distribution of membrane lipids, the cytoskeleton, and adhesive junctions. Epithelia are often profusely innervated, but little work has addressed how neurites affect epithelial organization. We previously found that basal keratinocytes in the zebrafish epidermis enclose axons in ensheathment channels sealed by autotypic junctions. Here we characterized how axons remodel cell membranes, the cytoskeleton, and junctions in basal keratinocytes. At the apical surface of basal keratinocytes, axons organized lipid microdomains quantitatively enriched in reporters for PI(4,5)P2 and liquid-ordered (Lo) membranes. Lipid microdomains supported the formation of cadherin-enriched, F-actin protrusions, which wrapped around axons, likely initiating ensheathment. In the absence of axons, cadherin-enriched microdomains formed on basal cells but did not organize into contiguous domains. Instead, these isolated domains formed heterotypic junctions with periderm cells, a distinct epithelial cell type. Thus, axon endings dramatically remodel polarized epithelial components and regulate epidermal adhesion.

A conserved morphogenetic mechanism for epidermal ensheathment of nociceptive sensory neurites.

Interactions between epithelial cells and neurons influence a range of sensory modalities including taste, touch, and smell. Vertebrate and invertebrate epidermal cells ensheath peripheral arbors of somatosensory neurons, including nociceptors, yet the developmental origins and functional roles of this ensheathment are largely unknown. Here, we describe an evolutionarily conserved morphogenetic mechanism for epidermal ensheathment of somatosensory neurites. We found that somatosensory neurons in Drosophila and zebrafish induce formation of epidermal sheaths, which wrap neurites of different types of neurons to different extents. Neurites induce formation of plasma membrane phosphatidylinositol 4,5-bisphosphate microdomains at nascent sheaths, followed by a filamentous actin network, and recruitment of junctional proteins that likely form autotypic junctions to seal sheaths. Finally, blocking epidermal sheath formation destabilized dendrite branches and reduced nociceptive sensitivity in Drosophila. Epidermal somatosensory neurite ensheathment is thus a deeply conserved cellular process that contributes to the morphogenesis and function of nociceptive sensory neurons.

Methylglyoxal Disrupts Paranodal Axoglial Junctions via Calpain Activation.

Nodes of Ranvier and associated paranodal and juxtaparanodal domains along myelinated axons are essential for normal function of the peripheral and central nervous systems. Disruption of these domains as well as increases in the reactive carbonyl species methylglyoxal are implicated as a pathophysiology common to a wide variety of neurological diseases. Here, using an ex vivo nerve exposure model, we show that increasing methylglyoxal produces paranodal disruption, evidenced by disorganized immunostaining of axoglial cell-adhesion proteins, in both sciatic and optic nerves from wild-type mice. Consistent with previous studies showing that increase of methylglyoxal can alter intracellular calcium homeostasis, we found upregulated activity of the calcium-activated protease calpain in sciatic nerves after methylglyoxal exposure. Methylglyoxal exposure altered clusters of proteins that are known as calpain substrates: ezrin in Schwann cell microvilli at the perinodal area and zonula occludens 1 in Schwann cell autotypic junctions at paranodes. Finally, treatment with the calpain inhibitor calpeptin ameliorated methylglyoxal-evoked ezrin loss and paranodal disruption in both sciatic and optic nerves. Our findings strongly suggest that elevated methylglyoxal levels and subsequent calpain activation contribute to the disruption of specialized axoglial domains along myelinated nerve fibers in neurological diseases.

Analgesic drug delivery via recombinant tissue plasminogen activator and microRNA-183-triggered opening of the blood-nerve barrier

The peripheral nerve contains three barriers which include the blood-nerve barrier consisting of endoneurial vessels and the perineurium as well as autotypic junctions in Schwann cells. The perineurium prevents diffusion of perineurally injected drugs that can be used for selective regional pain control. It is composed of a basal membrane and layers of perineurial cells sealed by tight junction proteins like claudin-1. Claudin-1 expression and barrier function are regulated via low-density lipoprotein receptor-related protein (LRP-1). Perisciatic application of recombinant tissue plasminogen activator (rtPA) or the catalytically inactive rtPAi – both agonists of LRP-1 – reduced claudin-1 mRNA and protein expression in the rat nerve. This facilitated an increase of nociceptive thresholds after local application of hydrophilic opioids or the voltage gated sodium channel blocker (NaV1.7) ProToxin-II without apparent nerve toxicity. RtPA-induced barrier opening was mediated by LRP-1 and intracellularly by Erk phosphorylation. In silico, microRNA (miR)-rno-29b-2-5p and rno-miR-183-5p were identified as potential regulators of claudin-1 transcription in the rat. RtPA application increased miR-183-5p in the sciatic nerve. MiR-183-5p mimics functionally opened the perineurium and downregulated claudin-1 expression in vivo. In vitro, hsa-miR-183-3p mimics reduced claudin-1 expression in human HT-29/B6 cells. Overall, rtPA regulates perineurial barrier tightness via LRP-1, Erk phosphorylation and miR-183-5p/3p. This mechanism might serve as a new principle to facilitate drug delivery to peripheral nerves in humans.

수초화 슈반세포 autotypic 세포연접의 PDZ 도메인 보유 단백질

자가밀착연접 단백질들은 세포, 특히 수초화된 신경교세포막의 층상구조 사이에 존재하는 밀착연접에 존재한다. 그들 중 일부는 다른 단백질의 C-말단의 PDZ 결합 모티프에 붙는 postsynaptic density-95/Disks large/Zonula occludens-1 (PDZ) 도메인을 가진다. PDZ domain은 박테리아, 식물, 세균, 후생동물, Drosophila에 존재하여 거대한 단백복합체를 형성할 수 있게 해준다. 이러한 단백복합체들은 세포 내 신호전달, 단백질 표적화, 그리고 세포막 극화 작용을 한다. ZO-1, ZO-2, AF-6, PATJ, MUPP1, PAR-3는 자가밀착연접에 존재한다고 확인되었다. PAR-3는 atypical protein kinase C와 PAR-6와 반응하여 세포의 극성 형성에 중요한 역할을 하는 3차원 단백질복합체를 형성하는데 이는 Caenorhabditis elegans와 Drosophila 종에서 척추동물에까지 보존되었다. MAGI2는 흥분성 시냅스에서 <TEX>${\alpha}$</TEX>-amino-3-hydroxyl-5-methyl-4-isoxazole propionate (AMPA) 수용체와 반응한다. PATJ는 claudin-1과 함께 마디곁 루프에서 발견되는 반면, MUPP1은 claudin-5와 함께 축삭사이막과 Schmidt-Lanterman 절흔에서 찾을 수 있다. ZO-1, ZO-2 그리고 PAR-3의 경우에는 세 장소 모두에서 발견된다. PDZ 도메인을 보유한 단백질들의 서로 다른 분포는 자가밀착연접의 발생에 영향을 준다. 이 총설에서는 수초화된 슈반 세포의 자가밀착연접에 존재하는 PDZ 도메인을 가진 단백질들과 그들의 기능을 알아볼 것이다. A type of cell junction that is formed between different parts within the same cell is called autotypic cell junction. Autotypic junction proteins form tight junctions found between membrane lamellae of a cell, especially in myelinating glial cells. Some of them have postsynaptic density-95/disks large/zonula occludens-1 (PDZ) domains, which interact with the carboxyl (C)-terminal PDZ-binding motif of other proteins. PDZ domains are protein-protein interaction modules that play a role in protein complex assembly. The PDZ domain, which is widespread in bacteria, plants, yeast, metazoans, and Drosophila, allows the assembly of large multi-protein complexes. The multi-protein complexes act in intracellular signal transduction, protein targeting, and membrane polarization. The identified PDZ domain-containing proteins located at autotypic junctions include zonula occludens-1 (ZO-1), ZO-2, pals-1-associated tight junction protein (PATJ), multi-PDZ domain proteins (MUPPs), membrane-associated guanylate kinase inverted 2 (MAGI2), and protease-activated receptor (PAR)-3. PAR-3 interacts with atypical protein kinase C and PAR-6, forming a ternary complex, which plays an important role in the regulation of cell polarity. MAGI2 interacts with <TEX>${\alpha}$</TEX>-amino-3-hydroxyl-5-methyl-4-isoxazole propionate (AMPA) receptor at excitatory synapses. PATJ is detected in paranodal loops associated with claudin-1. On the other hand, MUPP1 is found in mesaxons and Schmidt-Lanterman incisures with claudin-5. ZO-1, ZO-2, and PAR-3 are found at all three sites. Different distributions of PDZ domain-containing proteins affect the development of autotypic junctions. In this review, we will describe PDZ domain-containing proteins at autotypic tight junctions in myelinating Schwann cells and their roles.

Tight junction proteins in human Schwann cell autotypic junctions.

Tight junctions (TJs) form physical barriers in various tissues and regulate paracellular transport of ions, water, and molecules. Myelinating Schwann cells form highly organized structures, including compact myelin, nodes of Ranvier, paranodal regions, Schmidt-Lanterman incisures, periaxonal cytoplasmic collars, and mesaxons. Autotypic TJs are formed in non-compacted myelin compartments between adjacent membrane lamellae of the same Schwann cell. Using indirect immunofluorescence and RT-PCR, we analyzed the expression of adherens junction (E-cadherin) and TJ [claudins, zonula occludens (ZO)-1, occludin] components in human peripheral nerve endoneurium, showing clear differences with published rodent profiles. Adult nerve paranodal regions contained E-cadherin, claudin-1, claudin-2, and ZO-1. Schmidt-Lanterman incisures contained E-cadherin, claudin-1, claudin-2, claudin-3, claudin-5, ZO-1, and occludin. Mesaxons contained E-cadherin, claudin-1, claudin-2, claudin-3, ZO-1, and occludin. None of the proteins studied were associated with nodal inter-Schwann cell junctions. Fetal nerve expression of claudin-1, claudin-3, ZO-1, and occludin was predominantly punctate, with a mesaxonal labeling pattern, but paranodal (ZO-1, claudin-3) and Schmidt-Lanterman incisure (claudins-1 and -3) expression profiles typical of compact myelin were visible by gestational week 37. The clear differences observed between human and published rodent nerve profiles emphasize the importance of human studies when translating the results of animal models to human diseases.

E‐cadherin expression in postnatal Schwann cells is regulated by the cAMP‐dependent protein kinase a pathway

Expression of E-cadherin in the peripheral nervous system is a highly regulated process that appears postnatally in concert with the development of myelinating Schwann cell lineage. As a major component of autotypic junctions, E-cadherin plays an important role in maintaining the structural integrity of noncompact myelin regions. In vivo, the appearance of E-cadherin in postnatal Schwann cell is accompanied by the disappearance of N-cadherin, suggesting reciprocal regulation of the two cadherins during Schwann cell development. The molecular signal that regulates the cadherin switch in Schwann cell is unclear. Using a neuron-Schwann cell co-culture system, here we show that E-cadherin expression is induced by components on the axonal membrane. We also show that the axonal effect is mediated through cAMP-dependent protein kinase A (cAMP-PKA) activation in the Schwann cell: (1) inhibition of cAMP-PKA blocks axon-induced E-cadherin expression and (2) cAMP elevation in the Schwann cell is sufficient to induce E-cadherin expression. In addition, cAMP-dependent E-cadherin expression is promoted by contact between adjacent Schwann cell membranes, suggesting its role in autotypic junction formation during myelination. Furthermore, cAMP-induced E-cadherin expression is accompanied by suppression of N-cadherin expression. Therefore, we propose that axon-dependent activation of cAMP-PKA serves as a signal that promotes cadherin switch during postnatal development of Schwann cells.

Cellular junctions of myelinated nerves (Review).

Myelinated nerves are specifically designed to allow the efficient and rapid propagation of action potentials. Myelinating glial cells contain several types of cellular junctions that are found between the myelin lamellas themselves in specialized regions of non-compact myelin and between the myelin membrane and the underlying axon. These include most of the junctional specializations found in epithelial cells, including tight, gap and adherens junctions. However, whereas in epithelial cells these junctions are formed between different cells, in myelinating glia these so called autotypic junctions are found between membrane lamellae of the same cell. In addition, myelinating glial cells form a heterotypic septate-like junction with the axon around the nodes of Ranvier and, in the peripheral nerve system, contact the basal lamina, which surrounds myelinating Schwann cells. This short review discusses the structure, molecular composition and function of the junctions present in myelinating cells, concentrating on the axo-glial junction.