Two major forms of inflammatory bowel disease, Crohn’s disease and ulcerative colitis (UC), are the most common intestinal inflammatory diseases in developing countries [1,2]. Patients with colitis usually present with bloody stools and diarrhea as well as abdominal cramps with defecation; the disease is characterized by recurrent episodes and lesions confined to the intestinal mucosa [3,4]. There is no significant sex difference in the incidence of colitis, which is common in adults aged 30–40 years. As living standards improve and diets diversify, the incidence of colitis is increasing worldwide. Therefore, colitis has become a global public health challenge [5]. The specific pathogenesis of colitis is still unclear. Previous studies have suggested that the pathogenesis of colitis results from interactions between intestinal immune response disorders, intestinal flora changes, genetic susceptibility, and environmental factors. In terms of treatment, amino salicylic acid, a traditional therapeutic drug, regulates inflammatory factors by inhibiting nuclear factor kappa-B (NF-κB) pathway, thereby inhibiting the inflammatory response. However, adverse reactions to this medication may include nausea and vomiting. Adrenocortical hormone drugs are another treatment choice. They inhibit the activation of T lymphocytes to weaken the expression of inflammatory factors; however, because the hormone is absorbed in other parts of the body, it will eventually lead to femoral head necrosis and osteoporosis. Therefore, it is not suitable for long-term use [[5], [6], [7]].

APN, an insulin-sensitizing hormone secreted by the adipocytes, improves tissue insulin sensitivity, regulates glucose and lipid metabolism, and plays a protective role in metabolic syndrome and other diseases [8,9]. The full-length human APN comprises 244 amino acids, which has a carboxyl-terminal globular domain and an amino-terminal collagen domain and is structurally similar to complement 1q [10]. Studies have shown that the overexpression of APN restores normal mucosal structure and reduces the production of potent inflammatory cytokines and adipokines. APN significantly increases interlukin-10 (IL-10) levels in the colon and plasma of mice with colitis [11]. IL-10 may promote the selective activation of macrophages and alleviate chronic inflammation, suggesting that adiponectin plays a protective role against colitis. Importantly, APN receptors are present in the small intestine. APN knock-out mouse colons had markedly reduced proliferation and increased epithelial apoptosis and cellular stress. APN maintains intestinal homeostasis and protects against murine colitis through interactions with its receptor adipoR1 and by modulating adaptive immunity. Adenovirus-mediated supplementation of APN significantly attenuated the severity of colitis. This protective effect may be due to the inhibition of chemokine production and subsequent inflammatory responses in the intestinal epithelial cells, including macrophage infiltration and pro-inflammatory cytokine release [12,13]. Furthermore, it has been shown that the intraperitoneal injection of APN at a dose of 3 μg/g attenuates 2, 4, 6-trinitrobenzene sulfonic acid (TNBS)-induced CD in mice [14]. Therefore, increasing the circulating concentration of adiponectin using exogenous sources is expected to treat colitis.

In terms of drug delivery, rectal delivery is limited, for the most part, to the rectum and sigmoid colon, and is inefficient when inflammation occurs in the upper gastrointestinal tract. Moreover, rectal administration is difficult for patients to undertake by themselves, and so oral administration is usually the first choice. However, protein-derived drugs are easily degraded by gastric acids or digestive enzymes after oral administration [[15], [16], [17]]. Therefore, previous treatments of adiponectin for colitis in mice have been performed through the use of an adenovirus-mediated adiponectin expression system to increase the circulating concentrations in vivo [12]. Silk spun by the silkworm is composed of silk fibroin and silk sericin. Silk fibroin, the main structural component (75 %), has been well explored as a desirable biomaterial for tissue engineering owing to its promising mechanical properties, biocompatibility, and biodegradability [[18], [19], [20]]. Studies have shown that sericin, the other major component (25 %), did not induce an immune response in the body and had anti-oxidant [21], anti-aging [22], anti-bacterial [23], anti-tumor [24], UV resistant [25], anti-coagulation properties [26], anti-inflammatory [27], and promotion of cell growth and wound healing [28,29]. Pure sericin with high polymer properties (>300 kDa) can self-assemble into hydrogels. Sericin hydrogels are three-dimensional network-crosslinked water-soluble polymers with high-water content and good biocompatibility, widely used in wound dressing, bio-adhesion, and tissue engineering [30,31]. Sericin itself exhibits good hydrophilicity, biocompatibility, and degradability. Furthermore, sericin enhances bacterial cellulose when employed as a tissue-engineered scaffold for intestinal repair. Hydrogels made of sericin and polyvinyl alcohol that contain silver nanoparticles can effectively inhibit bacterial and fungal growth [32,33]. Meanwhile, piggyBac transposon based genetic engineering technology successfully engineered silk gland of silkworm as bioreactor to produce a large number of foreign proteins [34], mainly including fluorescent proteins [35], human serum albumin [36,37], growth factors [17,[38], [39], [40]], human lactoferrin [41], and glucose oxidase [42]. Thus, silk genetically incorporated with value foreign functional proteins could be more widely applied in fields of tissue repair and wound healing [43,44]. Compared with the existing adenovirus vector expression adiponectin delivery strategies, the expression of adiponectin binding sericin protein hydrogels in silkworms through transgenic technology has significant advantages in terms of stability and biocompatibility. Moreover, the naturalness and adjustable physicochemical properties of sericin hydrogels make them exhibit better safety and long-term stability in drug delivery.

Previous studies have shown the potential of oral administration of sericin hydrogel in mice [45], and the oral administration of recombinant human lactoferrin sericin nanospheres was effective in treating Dextran Sulfate Sodium Salt (DSS)-induced UC in mice [46]. Therefore, in this study, we combined the efficacy of adiponectin for treating UC with the potential of transgenic silkworms as adiponectin producers and sericin as a carrier to deliver recombinant human APN (rhAPN). Scheme 1 showed the strategy for fabricating rhAPN sericin hydrogel (rhAPN-sh) and the major processing route for the UC treatment in mice. Silk cocoons genetically incorporated with the rhAPN were harvested from a transgenic silkworm stain, then used as raw materials to fabricate the rhAPN-sh, which showed good biocompatibility, injectability for oral administration to treat the UC with significant therapeutic efficacy by modulating the inflammatory microenvironment and rebalancing the microbiota through interacting with the APN receptor on intestinal epithelial cells. Overall, the genetically engineered rhAPN-sh achieved the goal of oral APN therapy for UC.