AbstractAll animal cells maintain cell size homeostasis, where cell growth (increase in size) is balanced with proliferation (reduction in size via cell division). Yet, different cell types have different sizes and there are physiologically relevant situations where animal cells undergo major cell size changes. So how is cell size regulated? And why is cell size regulated? Are there specific cellular processes that have different functionality in different sized cells? This thesis investigates these questions from the perspective of cellular metabolism.
Using a Cyclin dependent kinase 1 knockout mouse model with different degrees of hepatocytes enlargement, gene expression levels were correlated with cell size in vivo. This revealed that the relative expression of mitochondrial and lipid biosynthesis genes are downregulated with increasing cell size. However, mitochondrial content of the liver samples was not decreased, suggesting that cell functions and cell contents scale differently with cell size. To better investigate how mitochondrial functions scale with cell size in non-mutant cells, a novel and high throughput flow cytometry based single-cell analysis method called CoSRA was developed. Using fluorescence mitochondrial probes CoSRA revealed that, while mitochondrial content increases linearly with cell size, mitochondrial membrane potential is decreased in the very smallest and the largest cells. These effects were independent of cell cycle and all animal cell types examined displayed similar effects. Similar nonlinearity was observed in mitochondrial respiration. Furthermore, cell-to-cell variability in mitochondrial membrane potential was minimised in cells which are close to the median cell size of the whole population. The cell size dependence of mitochondrial functions was regulated by mitochondrial dynamics.
It was also investigated if mitochondrial functions or lipid biosynthesis are capable of regulating cell size in human cell culture models. Various mitochondrial inhibitions increased cell size by reducing proliferation. Similar results were seen with inhibitions on lipid biosynthesis and especially with inhibitions of mevalonate pathway. Systematic dissection of the mevalonate pathway revealed that protein geranylgeranylation is required for maintaining normal cell size and proliferation ratio. Geranylgeranylation of the recycling endosome regulating protein RAB11 was identified to be at least partially responsible for the cell size regulation by the mevalonate pathway. Furthermore, the link from the mevalonate pathway to RAB11 was found to regulate basal autophagic flux, thus providing a novel connection from lipid biosynthesis to other growth regulating processes.
In conclusion, this thesis provides evidence for cell size dependent metabolism, where mitochondrial functions do not increase linearly with cell size. This provides conceptual insights into organelle scaling with cell size and a potential mechanism for maintenance of cell size homeostasis. In addition, mitochondria and lipid synthesis are identified as critical processes for normal cell size homeostasis.
|Date of Award||2015|
|Sponsors||Biotechnology and Biological Sciences Research Council|
|Supervisor||Mikael Bjorklund (Supervisor)|
- Cell size
- Lipid synthesis
- Mevalonate pathway