Sensory neurons increase keratinocyte proliferation through CGRP release in a tissue engineered in vitro model of innervation in psoriasis.

Skin denervation has been shown to cause remission of psoriatic lesions in patients, which can reappear if reinnervation occurs. This effect can be induced by the activation of dendritic cells through sensory innervation. However, a direct effect of nerves on the proliferation of keratinocytes involved in the formation of psoriatic plaques has not been investigated. We developed, by tissue engineering, a model of psoriatic skin made of patient skin cells that showed increased keratinocyte proliferation and epidermal thickness compared to healthy controls. When this model was treated with CGRP, a neuropeptide released by sensory neurons, an increased keratinocyte proliferation was observed in the psoriatic skin model, but not in the control. When a sensory nerve network was incorporated in the psoriatic model and treated with capsaicin to induce neuropeptide release, an increase of keratinocyte proliferation was confirmed, which was blocked by a CGRP antagonist while no difference was noticed in the innervated healthy control. We showed that sensory neurons can participate directly to keratinocyte hyperproliferation in the formation of psoriatic lesions through the release of CGRP, independently of the immune system. Our unique tissue-engineered innervated psoriatic skin model could be a valuable tool to better understand the mechanism by which nerves may modulate psoriatic lesion formation in humans. STATEMENT OF SIGNIFICANCE: This study shows that keratinocytes extracted from patients' psoriatic skin retain, at least in part, the disease phenotype. Indeed, when combined in a 3D model of tissue-engineered psoriatic skin, keratinocytes exhibited a higher proliferation rate, and produced a thicker epidermis than a healthy skin control. In addition, their hyperproliferation was aggravated by a treatment with CGRP, a neuropeptide released by sensory nerves. In a innervated model of tissue-engineered psoriatic skin, an increase in keratinocyte hyperproliferation was also observed after inducing neurons to release neuropeptides. This effect was prevented by concomitant treatment with an antagonist to CGRP. Thus, this study shows that sensory nerves can directly participate to affect keratinocyte hyperproliferation in psoriasis through CGRP release.

In vitro glycation of a tissue-engineered wound healing model to mimic diabetic ulcers.

Diabetic foot ulcers are a major complication of diabetes that occurs following minor trauma. Diabetes-induced hyperglycemia is a leading factor inducing ulcer formation and manifests notably through the accumulation of advanced glycation end-products (AGEs) such as N-carboxymethyl-lysin. AGEs have a negative impact on angiogenesis, innervation, and reepithelialization causing minor wounds to evolve into chronic ulcers which increases the risks of lower limb amputation. However, the impact of AGEs on wound healing is difficult to model (both in vitro on cells, and in vivo in animals) because it involves a long-term toxic effect. We have developed a tissue-engineered wound healing model made of human keratinocytes, fibroblasts, and endothelial cells cultured in a collagen sponge biomaterial. To mimic the deleterious effects induced by glycation on skin wound healing, the model was treated with 300 µM of glyoxal for 15 days to promote AGEs formation. Glyoxal treatment induced carboxymethyl-lysin accumulation and delayed wound closure in the skin mimicking diabetic ulcers. Moreover, this effect was reversed by the addition of aminoguanidine, an inhibitor of AGEs formation. This in vitro diabetic wound healing model could be a great tool for the screening of new molecules to improve the treatment of diabetic ulcers by preventing glycation.

Development of an innervated tissue-engineered skin with human sensory neurons and Schwann cells differentiated from iPS cells.

Cutaneous innervation is increasingly recognized as a major element of skin physiopathology through the neurogenic inflammation driven by neuropeptides that are sensed by endothelial cells and the immune system. To investigate this process in vitro, models of innervated tissue-engineered skin (TES) were developed, yet exclusively with murine sensory neurons extracted from dorsal root ganglions. In order to build a fully human model of innervated TES, we used induced pluripotent stem cells (iPSC) generated from human skin fibroblasts. Nearly 100% of the iPSC differentiated into sensory neurons were shown to express the neuronal markers BRN3A and ß3-tubulin after 19 days of maturation. In addition, these cells were also positive to TRPV1 and neurofilament M, and some of them expressed Substance P, TrkA and TRPA1. When stimulated with molecules inducing neuropeptide release, iPSC-derived neurons released Substance P and CGRP, both in conventional monolayer culture and after seeding in a 3D fibroblast-populated collagen sponge model. Schwann cells, the essential partners of neurons for function and axonal migration, were also successfully differentiated from human iPSC as shown by their expression of the markers S100, GFAP, p75 and SOX10. When cultured for one additional month in the TES model, iPSC-derived neurons seeded at the bottom of the sponge formed a network of neurites spanning the whole TES up to the epidermis, but only when combined with mouse or iPSC-derived Schwann cells. This unique model of human innervated TES should be highly useful for the study of cutaneous neuroinflammation. STATEMENT OF SIGNIFICANCE: The purpose of this work was to develop in vitro an innovative fully human tissue-engineered skin enabling the investigation of the influence of cutaneous innervation on skin pathophysiology. To reach that aim, neurons were differentiated from human induced pluripotent stem cells (iPSCs) generated from normal human skin fibroblasts. This innervated tissue-engineered skin model will be the first one to show iPSC-derived neurons can be successfully used to build a 3D nerve network in vitro. Since innervation has been recently recognized to play a central role in many human skin diseases, such as psoriasis and atopic dermatitis, this construct promises to be at the forefront to model these diseases while using patient-derived cells.

High yield extraction of pure spinal motor neurons, astrocytes and microglia from single embryo and adult mouse spinal cord.

Extraction of mouse spinal motor neurons from transgenic mouse embryos recapitulating some aspects of neurodegenerative diseases like amyotrophic lateral sclerosis has met with limited success. Furthermore, extraction and long-term culture of adult mouse spinal motor neurons and glia remain also challenging. We present here a protocol designed to extract and purify high yields of motor neurons and glia from individual spinal cords collected on embryos and adult (5-month-old) normal or transgenic mice. This method is based on mild digestion of tissue followed by gradient density separation allowing to obtain two millions motor neurons over 92% pure from one E14.5 single embryo and more than 30,000 from an adult mouse. These cells can be cultured more than 14 days in vitro at a density of 100,000 cells/cm(2) to maintain optimal viability. Functional astrocytes and microglia and small gamma motor neurons can be purified at the same time. This protocol will be a powerful and reliable method to obtain motor neurons and glia to better understand mechanisms underlying spinal cord diseases.

Extended Synaptotagmin Interaction with the Fibroblast Growth Factor Receptor Depends on Receptor Conformation, Not Catalytic Activity.

We previously demonstrated that ESyt2 interacts specifically with the activated FGF receptor and is required for a rapid phase of receptor internalization and for functional signaling via the ERK pathway in early Xenopus embryos. ESyt2 is one of the three-member family of Extended Synaptotagmins that were recently shown to be implicated in the formation of endoplasmic reticulum (ER)-plasma membrane (PM) junctions and in the Ca(2+) dependent regulation of these junctions. Here we show that ESyt2 is directed to the ER by its putative transmembrane domain, that the ESyts hetero- and homodimerize, and that ESyt2 homodimerization in vivo requires a TM adjacent sequence but not the SMP domain. ESyt2 and ESyt3, but not ESyt1, selectively interact in vivo with activated FGFR1. In the case of ESyt2, this interaction requires a short TM adjacent sequence and is independent of receptor autophosphorylation, but dependent on receptor conformation. The data show that ESyt2 recognizes a site in the upper kinase lobe of FGFR1 that is revealed by displacement of the kinase domain activation loop during receptor activation.

Sensory neurons accelerate skin reepithelialization via substance P in an innervated tissue-engineered wound healing model.

Keratinocytes are responsible for reepithelialization and restoration of the epidermal barrier during wound healing. The influence of sensory neurons on this mechanism is not fully understood. We tested whether sensory neurons influence wound closure via the secretion of the neuropeptide substance P (SP) with a new tissue-engineered wound healing model made of an upper-perforated epidermal compartment reconstructed with human keratinocytes expressing green fluorescent protein, stacked over a dermal compartment, innervated or not with sensory neurons. We showed that sensory neurons secreted SP in the construct and induced a two times faster wound closure in vitro. This effect was partially reproduced by addition of SP in the model without neurons, and completely blocked by a treatment with a specific antagonist of the SP receptor neurokinin-1 expressed by keratinocytes. However, this antagonist did not compromise wound closure compared with the control. Similar results were obtained when the model with or without neurons was transplanted on CD1 mice, while wound closure occurred faster. We conclude that sensory neurons play an important, but not essential, role in wound healing, even in absence of the immune system. This model is promising to study the influence of the nervous system on reepithelialization in normal and pathological conditions.

Nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 and glial-derived neurotrophic factor enhance angiogenesis in a tissue-engineered in vitro model.

Skin is a major source of secretion of the neurotrophic factors nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and glial-derived neurotrophic factor (GDNF) controlling cutaneous sensory innervation. Beside their neuronal contribution, we hypothesized that neurotrophic factors also modulate the cutaneous microvascular network. First, we showed that NGF, BDNF, NT-3, and GDNF were all expressed in the epidermis, while only NGF and NT-3 were expressed by cultured fibroblasts, and BDNF by human endothelial cells. We demonstrated that these peptides are highly potent angiogenic factors using a human tissue-engineered angiogenesis model. A 40% to 80% increase in the number of capillary-like tubes was observed after the addition of 10 ng/mL of NGF, 0.1 ng/mL of BDNF, 15 ng/mL of NT-3, and 50 ng/mL of GDNF. This is the first characterization of the direct angiogenic effect of NT-3 and GDNF. This angiogenic effect was mediated directly through binding with the neurotrophic factor receptors tropomyosin-receptor kinase A (TrkA), TrkB, GFRα-1 and c-ret that were all expressed by human endothelial cells, while this effect was blocked by addition of the Trk inhibitor K252a. Thus, if NGF, BDNF, NT-3, and GDNF may only moderately regulate the microvascular network in normal skin, they might have the potential to greatly increase angiogenesis in pathological situations.

Extended-synaptotagmin-2 mediates FGF receptor endocytosis and ERK activation in vivo.

Targeting of activated plasma membrane receptors to endocytic pathways is important in determining the outcome of growth factor signaling. However, the molecular mechanisms are still poorly understood. Here, we show that the synaptotagmin-related membrane protein E-Syt2 is essential for rapid endocytosis of the activated FGF receptor and for functional signal transduction during Xenopus development. E-Syt2 depletion prevents an early phase of activated FGF receptor endocytosis that we show is required for ERK activation and the induction of the mesoderm. E-Syt2 interacts selectively with the activated FGF receptor and with Adaptin-2, and is required upstream of Ras activation and of receptor autophosphorylation for ERK activation and the induction of the mesodermal marker Xbra. The data identify E-Syt2 as an endocytic adaptor for the clathrin-mediated pathway whose function is conserved in human and suggest a broader role for the E-Syt subfamily in growth factor signaling.