Control of excess body fat is one of the greatest healthcare challenges of our time (Afshin et al., 2017; WHO, 2023). With the prevalence of obesity rapidly increasing since the 1990s, China now has the highest number of overweight or obese individuals in the world(Chen, J. L. et al, 2011). Approximately half of the adults and one-fifth of the children in China face the problem of overweight or obese (Chen, J. L. et al, 2011). Globally, the number of obese individuals worldwide has nearly doubled since 1975. In 2016, over 1.9 billion adults aged 18 and above were overweight, with over 650 million of them being obese (WHO, 2018). Obesity not only causes inconvenience in daily life but also leads to a range of health issues, including increased risk of cardiovascular diseases, impaired functionality of the digestive and endocrine systems, and elevated cancer risk. Estimates of the financial burden of obesity upon modern healthcare systems are sizeable, with more than US$190 billion spent annually in the United States alone for obesity-related illnesses (Müller et al., 2022).

To date, lifestyle interventions, including dietary intervention and physical exercise, remain the main methods for controlling body weight. However, a significant number of reports have shown that short-term lifestyle interventions are frequently insufficient for long-term weight loss (Webb & Wadden, 2017) (Figure 1.a). As a result, experts have begun exploring the addition of pharmacological methods to treat obesity.

Developing anti-obesity medications (AOMs) has been a difficult task, with numerous setbacks arising from both technical and societal factors. In the past, many AOMs have failed after receiving regulatory approval, mainly due to adverse cardiovascular effects, increased risk of suicide, or higher chances of drug dependence and abuse (Venditti et al., 2014). Meanwhile, the effectiveness of inducing weight loss is another crucial factor for AOMs. Through a social study utilizing questionnaires, we surveyed 1,253 individuals and were surprised to find that over 40% (551) ranked the effects of AOMs on body weight as more important than their side effects (for detailed questionnaire design and data analysis, please visit the Human Practices page) (Figure 1.b). However, the body weight loss achievable through most registered AOM resides in a relatively narrow range of 3–7% after 6–12 months of treatment (Ricquier & Bouillaud, 2000), which remains unsatisfactory for most individuals we interviewed (see the detailed feedback of our interviews in the Human Practices pages). In recent years, synthetic biology has led to specific and effective therapies for multiple diseases, inspiring the development of new AOMs with improved safety and efficacy. We then decided to see if it was possible to design a new synthetic biology-inspired AOM candidate.

Among the weight loss drugs in clinical development, four target areas - leptin, ghrelin, mitochondrial uncouplers, and growth differentiation factor 15 (GDF15) - shown great potential in the realm of obesity treatment (Müller et al., 2022). To address the effectiveness and safety concern in most users, drug targets that directly reduces adipose cell volume while minimizing the affection on other organs are apparently favorable. Hence, the development of targeted mitochondrial uncouplers specifically increasing adipose tissue energy expenditure seems to be the most direct and natural way to safely reduce fat tissue content.

The uncoupling protein UCP1 is a natural mitochondrial uncoupler protein in brown adipose tissue that catalyzes the transport of protons across the mitochondrial membrane and, thereby, induces mitochondrial uncoupling of oxygen consumption from ATP synthesis (Venditti et al., 2014)(Figure 2). This uncoupling process leads to the dissipation of energy in the form of heat, accompanied with the increase of energy expenditure and basal metabolic rate (Ricquier & Bouillaud, 2000). Previous studies have shown that the ectopic expression of UCP1 in white adipose tissue results in a significant reduction of body weight in obese mice (Xue et al., 2016), implying the ectopic administration of UCP1 in white adipose tissue may be an interesting therapeutic approach to prevent body weight gain, decrease fat mass, and improve insulin sensitivity.

The ectopic overexpression of UCP1 in white adipose tissue can be achieved by multiple ways (Kozak & Anunciado-Koza, 2008). One clear path would be Adeno Associated Virus (AAV)-mediated gene delivery, which has been widely used in multiple diseases (Naso et al., 2017). However, since the finetuning of AAV-mediated gene expression level remains challenging, AAV-mediated UCP1 expression might results in a constitutively high UCP1 expression, which could be cytotoxic to the host cells. Also, the AAV-induced gene delivery results in a long-term existence of ectopic genes in human cells, which then raises safety and ethical concerns (Venditti, 2021).

In 2023, Kreitz et al. reported an engineered contraction injection system (eCIS), which utilized the Photorhabdus Virulence Cassette (PVC) to efficiently deliver payload proteins into mammalian cells (Kreitz et al., 2023)(Figure 3).Importantly, these PVCs can be retargeted to different cells by redesigning the tail fiber sequence. Their work provides a brand-new possibility to deliver UCP1 into the adipose tissue in a highly-specific manner.

Drawing inspiration from these works, our team intended to develop a safe and efficient novel weight-loss strategy by designing adipose-specific PVCs that specifically deliver UCP1 protein into human white adipocytes (Figure 4). Our results show that the engineered PVCs could be loaded with UCP1 and effectively assembled in E. coli (Figure 4). The UCP1-loading PVCs could effectively inject UCP1 into mammalian cells and affect cellular metabolism. With further development in the future, we firmly believe that our project has the potential to contribute to addressing the issue of obesity in China and even worldwide.