Effective communication between cells is fundamental to the success of multicellular life. For multicellularity to persist and function, cells need to coordinate their activities to support growth, development, differentiation, organ formation, and overall physiological regulation. This coordination relies on cell-cell communication, involving the generation of signalling molecules and a cascade of molecular events that relay these signals within cells to elicit precise and timely cellular responses.
One such pathway, phosphoinositide (PI) signalling, is evolutionarily conserved in eukaryotes and controls processes such as cell division, vesicular transport, cytoskeletal reorganisation and plasma membrane function in eukaryotic cells. Phosphoinositides, a group of phospholipids, serve as key signalling molecules in this pathway, and their levels are controlled by phosphoinositide kinases and phosphatases. While most enzymes in this pathway are conserved in all eukaryotes, a few enzymes are only found in multicellular eukaryotes or metazoans.
A recent study by Prof. Raghu Padinjat’s and Prof. R. Sowdhamini’s group at NCBS has found that the genes encoding phosphatidylinositol 5-phosphate 4-kinase (PIP4K), a key enzyme that controls the levels of phosphatidylinositol 3,4,5-trisphosphate (PIP3), are found exclusively in metazoans. “What we aimed to do was identify the earliest metazoan in which PIP4K appears and examine whether its activity has remained conserved throughout evolution. Since this enzyme also regulates growth, we believe it could play a crucial role in the evolution of multicellularity”, said Harini K, lead author of the study.
A key aspect of PIP4K function is its role in regulating the insulin signalling pathway, a pathway central to growth and metabolism in metazoans. Insulin receptors, located on cell membranes, bind to insulin and initiate a cascade of intracellular signals. One of the first steps is activation of Class I PI3 kinase (PI3K), a kinase unique to metazoans. PI3K then activates mTOR, a master regulator of cell growth. mTOR, in turn, activates specific transcription factors that enter the nucleus and control gene expression, ultimately driving growth and metabolism. Dysfunction in mTOR signalling is linked to diseases like cancer and diabetes. To trace the evolutionary history of PIP4K, the group developed a detailed bioinformatics pipeline. They identified PIP4K homologs in different species that evolved from a common ancestor and likely perform similar functions across all metazoan phyla. They also searched databases of marine unicellular organisms and parasites, but found no gene sequence for this enzyme in any of their genomes, except the choanoflagellates.
“Although choanoflagellates are unicellular eukaryotes and not obligate metazoans, they do spend some part of their life cycle in a transient multicellular state. We think that PIP4K may mediate hormone signalling during this transient multi-cellular state.”
Earlier studies on mice and from Padinjat’s group on fruit flies had shown that knocking out PIP4K led to reduced growth and enhanced insulin signalling, indicating its critical role in controlling growth and metabolism. To test whether the function of PIP4K was retained through evolution, the researchers cloned a PIP4K gene from a sponge, Amphimedon queenslandica (AqPIP4K), one of the earliest branching metazoans. They expressed AqPIP4K in Drosophila lacking their own PIP4K. These knockout flies display reduced growth and body size due to impaired insulin signalling; the presence of sponge PIP4K completely rescued the growth defect, restoring normal body size and development.
Further biochemical analysis revealed that both the sponge and human PIP4K enzymes are very similar in their enzymatic activity: they bind only to their substrate, PI5P, and do not phosphoarylate other phosphoinositides. Sequence analysis showed that all PIP4K proteins contain a catalytic domain and conserved motifs important for substrate and ATP binding, supporting the idea that substrate recognition and function are similar across evolution. PIP4K’s emergence in metazoans may reflect the increasing complexity of multicellular life, where cell-cell communication through signalling pathways becomes critical. Since PIP4K is involved in growth regulation, its presence in even the earliest animals hints at a fundamental role in the coordination and control required for multicellularity.
In mammals, there exist three different isoforms of PIP4K. “Each isoform is expressed in different tissues and cell types, and they have distinct roles, though all are linked to regulating growth factor signalling. Changes in their levels are known to result in tumorigenesis, metabolic changes and immune function under different conditions”, says Harini.
“Being a successful multicellular organism requires many attributes: cell division and number have to be controlled; cells need to stick to each other, and it is necessary for the various cells in a multicellular organism to communicate with each other to ensure coordinated behaviour. This last feature, communication, is typically mediated by chemicals such as hormones or growth factors”, says Raghu Padinjat, senior author of the study. “In order for them to influence target cells, there needs to be chemical signals in the target cells that detect hormones. Receptor tyrosine kinases and Class I PI3K mediate this, and these two classes of molecules are a feature of metazoans only. PIP4K is a regulator of Class I PI3K signalling and, therefore, was probably important for the evolution of multicellularity”, he adds.
0 Comments