The paralemmin protein family : Phosphoproteins involved in filopodia and dendritic spine formation

Abstract: Dendritic spines, which connect neurons with each other, constitute most of the excitatory synapses and undergo experience-dependent neuronal plasticity. The function of dendritic spines and their synaptic strength are highly correlated with morphological changes. Remodeling of the nervous system in terms of establishment, maintenance, and organized elimination of dendritic spines is important during neuronal development, and for learning and memory. Pathological alterations in dendritic spine morphology, number and plasticity have been associated with many neurodegenerative disorders. However, the underlying molecular mechanisms are still poorly understood. Paralemmin-1 has been shown to play a key role in spine formation through the regulation of filopodia formation. Furthermore, because paralemmin-1 is post-translationally modified it harbors the potential of serving as a spine morphology regulatory protein. Here, we have investigated the role of the paralemmin-1 spanning from molecule to whole organism.We found that all paralemmin family members, paralemmin-1, palmdelphin, paralemmin-2 and paralemmin-3, are associated with filopodia formation in young hippocampal neurons, are strongly expressed in the brain and share molecular features: (i) general gene architecture, (ii) the COOH-terminal CaaX-box lipidation motif, which anchors them to the plasma membrane, (iii) the predicted coiled-coil structure motif at the NH2-terminus and (iv) the prediction to be – in large parts – intrinsic disordered proteins.The investigation of paralemmin-1 knockout mice showed that they are fertile, survive to high age and have two mild phenotypes: impaired motor coordination in males and a tendency for obesity in females. The neuron morphology was not altered in paralemmin-1 knockout hippocampal neurons. The protein expression level of palmdelphin, paralemmin-2, and paralemmin-3 was not affected in paralemmin-1 knockout mice. However, functional redundancy of the paralemmins may mask stronger phenotypes in paralemmin-1 knockout mice. Acute knockout of Paralemmin-1 in cultured conditional hippocampal neurons revealed a decrease in the number of filopodia and dendritic spines.Paralemmin-1 and palmdelphin are highly phosphorylated, with most phosphorylation sites identified in the brain. Mutation of eight phosphorylation sites of paralemmin-1 to amino acids mimicking the nonphosphorylated state resulted in fewer mushroom-shaped dendritic spines in cultured hippocampal neurons. Our findings underscore the importance of regulation through phosphorylation in the development and function of the nervous system. Additionally, we present the first evidence that the paralemmins might be intrinsically disordered proteins, which may provide novel routes of protein regulation and interaction.