Nearly all cells are covered in a carbohydrate-enriched coating called the glycocalyx, which is constructed out of various glycoconjugates and is one of the first barriers in cell-to-cell communication. Sialidases, enzymes also known as neuraminidases, cleave sialic acid from glycoconjugates and play a crucial role in the regulation of neuronal glycocalyx. Structural and chemical changes of the neuronal glycocalyx contribute to neuronal synapse formation and the removal of sialic acid is suggested as one of the mechanisms of synapse pruning, a process required to eliminate unnecessary synapses during brain development leading to healthy neuronal network [1]. There are four main sialidases, named Neu1 – Neu4, which vary in expression during the development of mammalian brain, with Neu2 being barely expressed [2]. Neuronal sialylation is an essential process for healthy neuron activity [3], however the connection between sialidase activity and synaptic function is poorly understood, and the exact role of sialidases in the process of neuronal network development and maturation remains understudied.
During this study, the expression of sialidases between various age groups (P0 to P90) was quantified by real-time PCR and the ddCT method was used to analyse the collected data. At the same postnatal age points, enzymatic sialidase activity was measured in freshly collected mouse hippocampus tissue samples. Enzymatic activity was normalised to the amount of total protein in the sample. Statistical analysis was performed to determine the differences between age groups and sex effect.
Our results demonstrated that Neu1 gene had the highest expression and the Neu2 had the lowest gene expression. Neu1 gene had the highest expression which remained consistent during the development of hippocampus, there was a slight increase in Neu2 expression, Neu3 expression decreased and Neu4 gene had a tremendous (around 600 times) increase in expression, indicating that Neu1 and Neu4 play the primary roles in brain development. As for the enzymatic activity of sialidases, barely any activity was present in the P0 mice hippocampus, suggesting that sialidases do not contribute to the prenatal development of brain. The highest enzymatic activity of sialidases, around double compared to the other age groups, was observed at age P14 with significantly higher activity in male hippocampus compared to female. This discovery suggests that the neuronal desialylation of the hippocampus may differ between male and female mice.
Overall, we showed that Neu1 gene expression is the highest in mouse hippocampus. However, at the early postnatal age development (P14) of mouse hippocampus an immense increase in the expression of Neu4 was observed. In line, sialidases demonstrate the highest activity at the age of P14, which corresponds to the timepoint of most active developmental pruning of synapses in the hippocampus. Our findings suggest that sialidases play a key role in the mechanism of synaptic pruning in the developing brain.