Circadian regulation of adipose tissue; the role of the myeloid system

Importance: Maintenance of energy balance and metabolic health is emerging as a major challenge of our time. The consequences of obesity include cardiovascular and metabolic disease (e.g. type 2 diabetes), as well as increased cancer, musculoskeletal (e.g. osteoarthritis), and infectious disease risk, notably coronavirus mortality. Therapeutic approaches to body weight regulation have focussed on appetitive pathways in the brain or nutrient absorption in the gut; with neither approach yielding adequate success. Therefore, new, effective and safe strategies are needed.

Most systemic and cellular energy metabolism is closely tied into the regulatory network of the circadian clock. Notably, ribosome biogenesis shows strong daily oscillations that are regulated by both circadian and feeding rhythms, acting through the translation control (e.g. ribosomal protein and translation factor biosynthesis). Rhythmicity also affects mitochondrial dynamics (synthesis, fission/fusion) and oxidative activity, in part due to the close coupling between the clock and regulators of mitochondrial biogenesis including Pgc1a. Given the role played by timing in energy metabolism it is essential to consider circadian factors in analysis of adipose function.

Hypotheses: Regulation of timing in adipose tissue beds reflects inputs from the sympathetic nervous system, which is closely linked to the central brain clock in the suprachiasmatic nucleus, and flows external light-dark cycles; and the clock in the adipocyte which follows feeding and fasting cycles. This opens the possibility of internal desynchrony, with circadian oscillators running out of phase in adipose tissue, with unknown consequences.

Resources: we will use mouse as a pre-clinical model. We have a range of transgenic mice with circadian clock components floxed, to allow tissue-specific gene knockout. We can manipulate light cycles, and feeding times, in order to drive changes in circadian phase in adipocytes, and the sympathetic nervous system. We can use tissue specific TNFa expression to drive inflammation to adipose beds, to model conditions of inflammation seen in human obesity, or in human adipose beds in patients with systemic inflammation eg rheumatoid arthritis.

Plans: We will analyse the impact of circadian misalignment on adipocyte and myeloid cell compartments in adipose tissue, focussed on circadian function, gene expression, and energy metabolic function (lipolysis/lipogenesis, mitochondrial function).
We will initially focus on disrupting circadian function in macrophages, to target the adipose tissue associated macrophage, and the newly-identified sympathetic nervous system associated macrophages (discovered by Dr A Domingos).
We will then move to light cycle manipulation, and time-restricted feeding protocols to drive misalignment. We will block sympathetic innervation to the adipose beds using surgical denervation, and transgenic directed loss of the circadian clock in the sympathetic neurones. We can also use beta adrenergic system pharmacological blockade. We will use a new mouse model of conditional TNFa expression, confined to adipose beds by crossing adiponectin-creERET mice with a ROSA26 stop-flox TNFa mouse. This model will test the prevalent condition of adipose bed inflammation, which we can test under standard and high fat obesity conditions.
We will seek to investigate cellular heterogeneity in the adipose beds and will make selective use of single cell RNA-seq and ATAC-seq, which can be done in the same cells, to establish the consequences of such misalignment for cell type-specific differentiation in adipose beds.
Our links with NN will mainly focus on scientific discussions, and the commercial context of our models, and our findings. This will be facilitated by mutually agreed project update meetings. Depending on progress we may seek specific collaborative arrangements, on a case-by-case basis