Tissue engineering of human salivary gland organoids

Organoid Models for Salivary Gland Biology and Regenerative Medicine

The salivary gland is composed of an elegant epithelial network that secrets saliva and maintains oral homeostasis. While cell lines and animal models furthered our understanding of salivary gland biology, they cannot replicate key aspects of the human salivary gland tissue, particularly the complex architecture and microenvironmental features that dictate salivary gland function. Organoid cultures provide an alternative system to recapitulate salivary gland tissue in vitro, and salivary gland organoids have been generated from pluripotent stem cells and adult stem/progenitor cells. In this review, we describe salivary gland organoids, the advances and limitations, and the promising potential for regenerative medicine.

Environmental Toxicology Assays Using Organ-on-Chip

Adverse effects of environmental toxicants to human health have traditionally been assayed using in vitro assays. Organ-on-chip (OOC) is a new platform that can bridge the gaps between in vitro assays (or 3D cell culture) and animal tests. Microenvironments, physical and biochemical stimuli, and adequate sensing and biosensing systems can be integrated into OOC devices to better recapitulate the in vivo tissue and organ behavior and metabolism. While OOCs have extensively been studied for drug toxicity screening, their implementation in environmental toxicology assays is minimal and has limitations. In this review, recent attempts of environmental toxicology assays using OOCs, including multiple-organs-on-chip, are summarized and compared with OOC-based drug toxicity screening. Requirements for further improvements are identified and potential solutions are suggested.

Metabolic Imaging of Head and Neck Cancer Organoids

Head and neck cancer patients suffer from toxicities, morbidities, and mortalities, and these ailments could be minimized through improved therapies. Drug discovery is a long, expensive, and complex process, so optimized assays can improve the success rate of drug candidates. This study applies optical imaging of cell metabolism to three-dimensional in vitro cultures of head and neck cancer grown from primary tumor tissue (organoids). This technique is advantageous because it measures cell metabolism using intrinsic fluorescence from NAD(P)H and FAD on a single cell level for a three-dimensional in vitro model. Head and neck cancer organoids are characterized alone and after treatment with standard therapies, including an antibody therapy, a chemotherapy, and combination therapy. Additionally, organoid cellular heterogeneity is analyzed quantitatively and qualitatively. Gold standard measures of treatment response, including cell proliferation, cell death, and in vivo tumor volume, validate therapeutic efficacy for each treatment group in a parallel study. Results indicate that optical metabolic imaging is sensitive to therapeutic response in organoids after 1 day of treatment (p<0.05) and resolves cell subpopulations with distinct metabolic phenotypes. Ultimately, this platform could provide a sensitive high-throughput assay to streamline the drug discovery process for head and neck cancer.

The role of human fibronectin- or placenta basement membrane extract-based gels in favouring the formation of polarized salivary acinar-like structures

Head and neck cancer patients treated with radiotherapy commonly experience hyposalivation and oral/tooth infections, leading to a reduced quality of life. Clinical management is currently unsatisfactory for dry mouth. Thus, there is a need for growing salivary fluid-secreting (acinar) cells for these patients. However, functionally-grown salivary acinar cells are cultured in Matrigel, a product that cannot be used clinically, owing to its source from a mouse sarcoma. Therefore, finding a gel suitable for clinical use and possessing properties similar to that of Matrigel would allow biopsied salivary cells to be expanded in vitro and transplanted into the mouths of xerostomic patients. This study tested gels made with human placenta basement membrane extract (BME) or fibronectin for the growth and differentiation of human salivary biopsies into acinar cells. We report here that, following expansion of primary human salivary gland epithelial cells (huSGs) in serum-free medium, using these gels (made from human proteins) allowed morphological and functional differentiation of salivary ductal cells into acinar-like cells. These (human) gels gave comparable results to Matrigel, such as differentiation into polarized acinar 3D units or monolayers with tight junction proteins (claudin-1, -2, -3) and exhibiting adequate transepithelial electrical resistance, acinar proteins (AQP5, α-amylase, mucin-1, NKCC1) and acinar adhesion-related cell markers (CD44, CD166). Ultrastructural, mRNA and protein analyses confirmed the formation of differentiated acinar polarized cells. The mitotic activity was highest with human placenta BME gel. This human culture model provided a reproducible approach to studying human salivary cell expansion and differentiation for tissue engineering. Copyright © 2016 John Wiley & Sons, Ltd.

Effects of fibroblasts on the function of acinar cells from the same human parotid gland

BACKGROUND

Artificial salivary gland replacement would be an ideal treatment for xerostomia. In vivo, salivary gland cells are surrounded by a complex stromal environment in which fibroblasts are the main cell type in proximity to the gland cells. However, very little is known about the relationship between these fibroblasts and the gland cells.

METHODS

Parotid gland acinar cells (PGACs) and fibroblasts from the same human gland were cocultured. PGAC function-related protein expression was investigated.

RESULTS

The expression of α-amylase in PGACs was increased in a fibroblast ratio-dependent manner. Both fibroblast-conditioned medium and direct coculture also significantly enhanced the PGAC expression of α-amylase. Basic fibroblast growth factor (bFGF) seems to be a regulator of α-amylase expression in PGACs.

CONCLUSION

An appropriate number of fibroblasts in contact with the PGACs is necessary to promote PGAC function. Fibroblast-secreted bFGF may play a paracrine signaling role in the regulation of α-amylase expression in PGACs. © 2015 Wiley Periodicals, Inc. Head Neck 38: E279-E286, 2016.

Current status of the development of an artificial salivary gland

Salivary glands (SGs) secrete more than half a liter of saliva daily. Saliva has many functions in maintaining the normal homeostasis of the oral cavity. Several causes underlie salivary impairment, where irradiation therapy to head and neck cancer patients is one of the most debilitating causes leading to considerable decrease in the patients' quality of life. In the last decade, others and we have focused on implementing tissue engineering principles combined with gene transfer and stem cell methodologies to develop an artificial SG device. This manuscript provides an overview of the current status of engineering an artificial SG.

Restoring the function of salivary glands

Salivary gland destruction occurs as a result of various pathological conditions such as radiation therapy for head and neck cancer and Sjögren's syndrome. As saliva possesses self-cleaning and antibacterial capability, hyposalivation is known to deteriorate dental caries and periodontal disease. Furthermore, hyposalivation causes mastication and swallowing problems, burning sensation of the mouth and dysgeusia. Currently available treatments for dry mouth are prescription for artificial saliva, moisturizers and medications which induce salivation from the residual tissue. Unfortunately, these treatments cannot restore the acini functions. This review focuses on various efforts to restore the function of damaged salivary gland. First, the possibility of salivary gland regeneration and tissue engineering is discussed with reference to stem cells, growth factors and scaffold materials. Second, the current status of gene transfer to salivary glands is discussed.

[Cell-based strategies for salivary gland regeneration]

Xerostomia as a side effect of radiotherapy or due to Sjögren's disease leads to considerable impairment of the quality of life of the affected patients. Preventive treatment approaches such as intensity-modulated radiotherapy, surgical transfer of a submandibular gland to a site outside the radiation field or administration of amifostin during radiation treatment are not yet completely established in clinical practice and are not applicable for all patients. Symptomatic treatment with pilocarpin or synthetic saliva leads to an improvement of the symptoms only in some patients, and in the case of pilocarpin significant systemic anticholinergic side-effects might occur. Because large numbers of patients are affected and current treatment options are not satisfactory, it is essential to develop new treatment options. In parallel with the in vitro production of functional salivary gland constructs by means of tissue engineering techniques, attempts are currently under way to experimentally restore salivary gland function by genetic treatment approaches such as transfection of the affected salivary glands with aquaporins or pro-angiogenic factors. In addition, the in vivo application of stem cells is under investigation. In the present paper, we discuss the clinical and radiobiological background of xerostomia and highlight possible innovative future treatment options.

In-vitro reconstitution of three-dimensional human salivary gland tissue structures

This study aimed to achieve functional reconstitution of salivary units from human salivary gland cells in an in vitro three-dimensional culture system. Human salivary cells were isolated from human salivary gland tissue, cultured, expanded, and placed into a three-dimensional culture system containing collagen and matrigel. Morphogenesis of reconstituted salivary structures was assessed by histomorphometry and transmission electron microscopy. Phenotypic and functional characteristics were assessed by immunohistochemistry and reverse transcription polymerase chain reaction (occludin, claudin 1, ZO-1, aquaporin 5, amylase) as well as spectrophotometric biochemical assay to measure amylase production. In a novel gel culture system, single human salivary cells divided and assembled into three-dimensional acinar and ductal structures in the presence of collagen and matrigel. All salivary gland units produced amylase and expressed aquaporin-5, a critical water channel protein. Tight junction proteins ZO-1, occludin, and claudin-1 were expressed under all culture conditions. Electron microscopy demonstrated desmosomes, microvilli, and secretory granules. This study showed that functional, differentiated salivary units containing acini and ducts formed from single salivary cells in a three-dimensional culture system. This in vitro culture system could be used to engineer functional salivary tissue in vivo.

Microcarriers in the engineering of cartilage and bone

A major problem in tissue engineering is the availability of a sufficient number of cells with the appropriate phenotype for delivery to damaged or diseased cartilage and bone; the challenge is to amplify cell numbers and maintain the appropriate phenotype for tissue repair and restoration of function. The microcarrier bioreactor culture system offers an attractive method for cell amplification and enhancement of phenotype expression. Besides serving as substrates for the propagation of anchorage-dependent cells, microcarriers can also be used to deliver the expanded undifferentiated or differentiated cells to the site of the defect. The present article provides an overview of the microcarrier culture system, its utility as an in vitro research tool and its potential applications in tissue engineering, particularly in the repair of cartilage and bone.

Growth of miniature pig parotid cells on biomaterials in vitro

Both Sjögren's syndrome and therapeutic irradiation for head and neck cancer lead to irreversible damage of the parenchyma of the salivary glands. This report describes an attempt to grow miniature pig (minipig) parotid gland cells on artificial films and tubular scaffolds with the ultimate intention of developing bio-engineered replacement tissue. Minipig parotid cells were isolated and cultured. The growth and structural and physiological features of the cells which were cultured on films and porous tubular scaffolds made from poly(ethylene glycol)-terephthalate (PEGT)/poly(butylene terephthalate) (PBT) were examined. By 9 days, the parotid cells on the films and the tubular scaffolds formed continuous monolayers. The secretory granules and nuclei of the cultured acinar cells remained polarised. Desmosomes, gap junctions and tight-like junctions were still present between the apical regions of adjacent cells. Amylase activity decreased during the culture period but was still evident in the medium after 10 days of culture. In conclusion, minipig parotid cells are well-maintained in vitro on both a flat surface and a three-dimensional (3D) scaffold. The addition of a Matrigel coating to the surface of synthetic materials aids cell growth and maintenance of a morphology that more closely resembles normal epithelium.

Tissue engineering in otorhinolaryngology

Tissue engineering is a field of research with interdisciplinary cooperation between clinicians, cell biologists, and materials research scientists. Many medical specialties apply tissue engineering techniques for the development of artificial replacement tissue. Stages of development extend from basic research and preclinical studies to clinical application. Despite numerous established tissue replacement methods in otorhinolaryngology, head and neck surgery, tissue engineering techniques opens up new ways for cell and tissue repair in this medical field. Autologous cartilage still remains the gold standard in plastic reconstructive surgery of the nose and external ear. The limited amount of patient cartilage obtainable for reconstructive head and neck surgery have rendered cartilage one of the most important targets for tissue engineering in head and neck surgery. Although successful in vitro generation of bioartificial cartilage is possible today, these transplants are affected by resorption after implantation into the patient. Replacement of bone in the facial or cranial region may be necessary after tumor resections, traumas, inflammations or in cases of malformations. Tissue engineering of bone could combine the advantages of autologous bone grafts with a minimal requirement for second interventions. Three different approaches are currently available for treating bone defects with the aid of tissue engineering: (1) matrix-based therapy, (2) factor-based therapy, and (3) cell-based therapy. All three treatment strategies can be used either alone or in combination for reconstruction or regeneration of bone. The use of respiratory epithelium generated in vitro is mainly indicated in reconstructive surgery of the trachea and larynx. Bioartificial respiratory epithelium could be used for functionalizing tracheal prostheses as well as direct epithelial coverage for scar prophylaxis after laser surgery of shorter stenoses. Before clinical application animal experiments have to prove feasability and safety of the different experimental protocols. All diseases accompanied by permanently reduced salivation are possible treatment targets for tissue engineering. Radiogenic xerostomia after radiotherapy of malignant head and neck tumors is of particular importance here due to the high number of affected patients. The number of new diseases is estimated to be over 500,000 cases worldwide. Causal treatment options for radiation-induced salivary gland damage are not yet available; thus, various study groups are currently investigating whether cell therapy concepts can be developed with tissue engineering methods. Tissue engineering opens up new ways to generate vital and functional transplants. Various basic problems have still to be solved before clinically applying in vitro fabricated tissue. Only a fraction of all somatic organ-specific cell types can be grown in sufficient amounts in vitro. The inadequate in vitro oxygen and nutrition supply is another limiting factor for the fabrication of complex tissues or organ systems. Tissue survival is doubtful after implantation, if its supply is not ensured by a capillary network.