Podoplanin: a new regulator of synaptic plasticity in the brain

The sialoglycoprotein podoplanin, originally identified in the rat kidney, is widely expressed in the human body and is known to regulate cell motility and migration. Interestingly, podoplanin has been also localized in the brain, raising the question of whether podoplanin could also regulate motility and growth-related processes in brain cells thus influencing the brain function. However, the role of podoplanin in the central nervous system still remains unknown. The objective of this work is to contribute to the elucidation of the physiological role of podoplanin in the mammalian nervous system. To this end, we used the mouse as experimental model and first verified the presence of podoplanin in the brain using qPCR, Western blots and immunocytochemistry assays. We found that its transcription is several fold higher in the brain as compared to other tissues (including heart, lung and kidney). The expression of podoplanin was constrained to specific brain regions, being particularly abundant in neurons from the hippocampal dentate gyrus, a neurogenic region critical for memory formation. Additionally, using a genetically modified podoplanin-knockout mice we found that depletion of podoplanin in hippocampal neurons impaired neurotrophin-induced neuritic outgrowth. Our in vitro electrophysiological studies using hippocampal slices further showed that lack of podoplanin impaired long-term potentiation and depression in the dentate gyrus. On the grounds of these findings, and given the relevance of the hippocampus for learning and memory, we hypothesized that podoplanin might be involved in vivo in the regulation of hippocampal synaptic plasticity and memory-related behavioural functions. In the course of this project, we will test this hypothesis using molecular/functional approaches encompassing live- cell microscopy, molecular biology, electrophysiology and behavioural assays. First, we will characterize the effects of podoplanin depletion and overexpression on the morphological and electrical properties of hippocampal neuronal networks using primary cultures. Next, we will examine electrophysiologically whether the absence of podoplanin affects synaptic plasticity in the CA3-CA1 Schaffer collateral pathway using hippocampal slices. We will further investigate whether podoplanin is implicated in adult hippocampal neurogenesis. Finally, we will use a battery of behavioural tests in order to examine the potential involvement of podoplanin in hippocampus-dependent long-term memory formation in vivo. We foresee that the results to be obtained through this project will contribute to elucidating the hitherto unknown role of podoplanin in the mammalian brain and broaden our understanding of the mechanisms underlying neurogenesis and memory storage.

Podoplanin is a protein found in humans and other species, and located in many tissues critical for life support, including the lung and the heart. Studies have shown that animals with complete lack of podoplanin develop abnormal lungs and heart and die shortly after birth due to cardio-respiratory malfunctions. Conversely, the cellular abundance of podoplanin is associated to the genesis and expansion of various types of cancer. These observations demonstrate thus the importance of podoplanin in the physiology and the pathology. Interestingly, podoplanin have been also identified in neurons of the hippocampus, a brain structure critical for learning and memory functions. However, the role of podoplanin in adult brain neurons has remained for long unknown. Using the mouse as model organism and combined live-cell microscopy, molecular biology, electrophysiology and behavioral assays, researchers have now provided novel experimental evidence proposing a role for podoplanin in the modulation of brain neuronal functions. Data indicated that lack of podoplanin in the adult brain impairs hippocampus- dependent spatial learning and memory functions, whereas podoplanin overexpression promotes synaptic activity and neuritic outgrowth. Moreover, Surface Plasmon Resonance, a technique used to study molecular interactions, have further revealed that podoplanin can indeed physically interact with neurotrophins, which are small signalling molecules that act as powerful regulators of neuronal growth and memory functions. This work therefore encourages further studies examining the functional crosstalk between podoplanin and neurotrophin signaling pathways for its potential involvement in the regulation of brain neuronal activity and learning and memory functions.