Reviewer #1 (Public Review):
This study presents novel experimental data from a mutant mouse model lacking microglia (Pu.1-/- mouse line) which indicates that these cells have an important role in the embryonic establishment of critical neural circuits in the brainstem generating breathing motor behavior in mice. Microglia are known to have important roles in shaping neural circuit assembly during development by controlling cell death, synapse refining, neurogenesis, and axon tract formation, but such roles have not been examined in the development of functional respiratory circuits. The authors examined the anatomical and functional characteristics of two main respiratory neuronal groups-in the embryonic parafacial (epF) and the preBötzinger complex (preBötC) regions that operate together in the developing brainstem to generate the rhythmic neural signals that are necessary to establish normal breathing behavior and ensure survival at birth. They present evidence that these respiratory networks become functional at typical developmental stages in the absence of microglia, but exhibit anomalies in rhythm generation (slower respiratory rhythm) and the mutants are unable to sustain breathing behavior at birth, consistent the observed neonatal death. Their data suggest that these deficits are associated with reduced cell numbers and abnormal rhythmogenesis in epF, and reduced commissural axonal projections of the preBötC circuits responsible for generating inspiratory rhythm.
Strengths of this study include the authors' use of the Pu.1-/- mutant in combination with technically well-executed, novel anatomical reconstruction of distributions of microglia in the developing hindbrain, neuronal activity imaging in the epF of the embryonic brainstem in vitro, and electrophysiological recording approaches in slices to assess aspects of the anatomical and functional status of the epF and preBötC relative to the control wild type mice. They also examine inspiratory drive transmission to phrenic motoneurons in vitro to assess the functional status of spinal respiratory motor output critical for breathing behavior at birth. Furthermore, their behavioral measurements by plethysmography document show that late-term (E18.5) Pu.1-/- embryos are unable to sustain breathing activity ex utero, which is consistent with the observed neonatal death of the mutants.
A limitation of the study is that the microglia-related mechanisms involved in regulating cell numbers in epF and the proper bilateral connectivity of preBötC circuits have not been investigated. Therefore it remains unknown if the reduced cell numbers in epF in the Pu.1-/- mutant is a defect, for example, of neurogenesis/neuronal migration or abnormal control of cell death, and if the defect of preBötC connectivity is actually related to the aggregation of microglia along the midline (possibly affecting commissural axonal tract formation), as the authors suggest.