A multidisciplinary team of scientists has discovered the mechanism that controls synaptic pruning of new neurons in the adult brain. The team discovered that microglial cells control the number of synapses by "eating" excessive synapses. Microglia recognized phosphatidylserine (PS) present on the cell surface of developing synapses, causing phagocytosis of the extra synapses. The full findings of the study are published in the March 17, 2022, online edition of the Journal of Experimental Medicine.

Adult mammalian brains can generate new functional neurons. These new neurons form structures called synapses that connect them to other neurons to create highly functional neural circuits. Synaptic pruning, the process of synapse elimination, helps maintain an appropriate number of synapses for the normal development and proper function of the brain.

PS is exposed on the outer plasma membrane of apoptotic cells and serves as an eat-me signal for phagocytes. During dead cell clearance by phagocytosis, targeted cells are first touched, then wrapped, and finally phagocytosed by phagocytes. PS is involved in the wrapping and subsequent phagocytosis of targeted cells by promoting the spreading of phagocytes' plasma membrane.

Using an electron microscope, the authors observed that microglia engulf synapses. Upon examining the localization of PS in the adult mouse brain, the authors found that PS is exposed outside the cell membrane at synapses, especially at less active synapses. PS acted as a synaptic eat-me signal and eliminated excess synapses.

Senior author Kazunobu Sawamoto told BioWorld Science that "we generated a transgenic mouse line in this study which enabled us to mask PS specifically in adult-born new neurons and inhibit their synaptic elimination by microglia specifically. What was surprising was the fact that PS preferentially labeled spines (small membrane protrusion formed at the synapse) that receive lower neuronal input. We also found that microglial spreading on and wrapping of spines of new neurons were suppressed when PS was masked."

Sawamoto is a professor at Institute of Brain Science, Nagoya City University Graduate School of Medical Sciences and National Institute for Physiological Sciences. The first author, Chihiro Kurematsu, is a fourth-year student at Nagoya City University School of Medicine.

The authors found that in the transgenic mice model with masked PS, microglia could not phagocytose synapses properly, resulting in extra synapses left behind. Furthermore, neurons in these mice showed electrophysiological abnormalities. These results indicated that the synaptic pruning of new neurons by microglia in the adult brain is PS-dependent, and that this mechanism was seen to be important for the correct maturation of these newborn neurons. Sawamoto also added that, "the present study provides new insights into the mechanisms of synaptic pruning by microglia, which is an essential process for the formation of functional neural circuits in the postnatal and adult brain."

Previous studies have indicated that neural stem cells in human infants also generate new neurons, and microglial synaptic pruning is believed to be important for postnatal brain development. Sawamoto explained that "therefore, the PS-dependent spine pruning by microglia could be involved in the functional integration of newborn neurons in the postnatal human brain, and conversely, defective pruning may be related to neurodevelopmental disorders such as autism spectrum disorders."

He added that "the novel genetic approach described here constitutes a powerful tool for understanding the biology of PS-dependent phagocytosis in vivo. By analyzing the phenotypes of the masking of PS in various organs and tissues, novel PS-dependent biological processes may be identified."

Sawamoto's team now aims to investigate PS-dependent synaptic elimination in mouse models of brain diseases to develop new therapeutic strategies for neurodevelopmental disorders such as autism, where abnormalities in microglia and synaptic density have been observed