Solute exchange through gap junctions lessens the adverse effects of inactivating mutations in metabolite-handling
Stefania Monterisi1, Johanna Michl1, Alzbeta Hulikova1
1Department of Physiology, Anatomy & Genetics, University of Oxford, Oxford, United Kingdom.
Abstract:
Growth of cancer cells in vitro can be attenuated by genetically inactivating selected metabolic pathways. However, loss-of-function mutations in metabolic pathways are not negatively selected in human cancers, indicating that these genes are not essential in vivo. We hypothesize that spontaneous mutations in 'metabolic genes' will not necessarily produce functional defects because mutation-bearing cells may be rescued by metabolite exchange with neighboring wild-type cells via gap junctions. Using fluorescent substances to probe intercellular diffusion, we show that colorectal cancer (CRC) cells are coupled by gap junctions assembled from connexins, particularly Cx26. Cells with genetically inactivated components of pH regulation (SLC9A1), glycolysis (ALDOA), or mitochondrial respiration (NDUFS1) could be rescued through access to functional proteins in co-cultured wild-type cells. The effect of diffusive coupling was also observed in co-culture xenografts. Rescue was largely dependent on solute exchange via Cx26 channels, a uniformly and constitutively expressed isoform in CRCs. Due to diffusive coupling, the emergent phenotype is less heterogenous than its genotype, and thus an individual cell should not be considered as the unit under selection, at least for metabolite-handling processes. Our findings can explain why certain loss-of-function mutations in genes ascribed as 'essential' do not influence the growth of human cancers.
Insights
Colorectal cancer cells can share functional proteins through gap junctions, rescuing cells with metabolic gene mutations. This intercellular metabolite exchange explains why some loss-of-function mutations do not impact human cancer growth.
Area of Science:
- Oncology
- Cell Biology
- Metabolic Engineering
Background:
- Cancer cell growth in vitro can be slowed by disabling metabolic pathways.
- Loss-of-function mutations in metabolic genes are not negatively selected in human cancers, suggesting they are not essential in vivo.
Purpose of the Study:
- To investigate if metabolite exchange via gap junctions rescues cancer cells with inactivated metabolic pathways.
- To determine the role of connexins, particularly Cx26, in this rescue phenomenon in colorectal cancer (CRC).
Main Methods:
- Utilized fluorescent substances to visualize and probe intercellular diffusion in colorectal cancer cells.
- Co-cultured genetically modified CRC cells (inactivating SLC9A1, ALDOA, or NDUFS1) with wild-type CRC cells.
- Examined rescue effects in co-culture xenografts and assessed the dependence on Cx26 channels.
Main Results:
- Colorectal cancer cells are coupled by gap junctions, primarily assembled from connexin 26 (Cx26).
- Cells with inactivated metabolic genes (pH regulation, glycolysis, mitochondrial respiration) were rescued by functional proteins from co-cultured wild-type cells.
- This rescue effect was observed in co-culture xenografts and was dependent on solute exchange through Cx26 channels.
Conclusions:
- Intercellular diffusive coupling via gap junctions allows metabolite exchange, rescuing cancer cells with metabolic defects.
- The emergent phenotype in colorectal cancer is less heterogeneous than its genotype due to this coupling.
- This mechanism explains the lack of negative selection for certain loss-of-function mutations in human cancers.
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