Microglial depletion disrupts normal functional development of adult-born neurons in the olfactory bulb

The brain has its own population of resident immune cells known as microglia, which defend against infections and are involved in conditions such as Alzheimer’s, Parkinson’s and other diseases. In the last decade, new studies have suggested that these cells also sculpt brain circuits during early development. They can ‘eat’ weak connections between neurons, and help strong ones to mature. Most of brain ‘wiring’ happens during development, when the majority of neurons is born and connects together. However, a few brain areas can incorporate new neurons during adulthood into existing circuits. In mice for example, this process takes place in the olfactory bulb, the area that first processes smells: it is believed that new neurons connecting to existing ones helps to detect new odors. It is unclear, however, whether microglia also help to shape these connections, or if their role is confined to early development. To investigate this question, Wallace et al. gave adult mice a drug that kills only microglia, and then examined how the neurons respond when the animals are exposed to smells. The results show that the new neurons that developed without microglia responded to fewer odors. These neurons also formed weaker connections and had physical features that indicated they might not have been properly incorporated into the circuit. It may be possible to encourage new neurons to be born in brain areas that normally do not produce these cells in adulthood. Ultimately, this could potentially help to repair the damages of age or disease, but this will rely on understanding exactly how new neurons are integrated into existing brain circuits. Future work, however, is still necessary to figure out how much these new neurons could compensate for cells damaged by injury or disease.