Auditory Brainstem Deficits from Early Treatment with a CSF1R Inhibitor Largely Recover with Microglial Repopulation

Abstract

Signaling between neurons and glia is necessary for the formation of functional neural circuits. A role for microglia in the maturation of connections in the medial nucleus of the trapezoid body (MNTB) was previously demonstrated by postnatal microglial elimination using a colony stimulating factor 1 receptor (CSF1R). Defective pruning of calyces of Held and significant reduction of the mature astrocyte marker glial fibrillary acidic protein (GFAP) were observed after hearing onset. Here, we investigated the time course required for microglia to populate the mouse MNTB after cessation of CSF1R inhibitor treatment. We then examined whether defects seen after microglial depletion were rectified by microglial repopulation. We found that microglia returned to control levels at four weeks of age (18 d postcessation of treatment). Calyceal innervation of MNTB neurons was comparable to control levels at four weeks and GFAP expression recovered by seven weeks. We further investigated the effects of microglia elimination and repopulation on auditory function using auditory brainstem recordings (ABRs). Temporary microglial depletion significantly elevated auditory thresholds in response to 4. 8, and 12 kHz at four weeks. Treatment significantly affected latencies, interpeak latencies, and amplitudes of all the ABR peaks in response to many of the frequencies tested. These effects largely recovered by seven weeks. These findings highlight the functions of microglia in the formation of auditory neural circuits early in development. Further, the results suggest that microglia retain their developmental functions beyond the period of circuit refinement.

Segregation of Multimodal Inputs Into Discrete Midbrain Compartments During an Early Critical Period

“…In somatosensory cortex, fractalkine signaling controls entry of MGCs into barrel fields and CX3CR1-deficiency delays functional maturation of its connectivity (Hoshiko et al, 2012 ). Microglia appear to also perform similar functions in the developing auditory brainstem, evidenced by significant pruning deficits and abnormal auditory brainstem responses when MGCs are pharmacologically depleted (Milinkeviciute et al, 2019 , 2021a ). While pruning at the calyx of Held is unaffected in fractalkine receptor mutants, these mice express increased glycinergic synaptic markers in the auditory brainstem, suggesting an important role for CX3CL1–CX3CR1 crosstalk in sculpting immature inhibitory connections (Milinkeviciute et al, 2021b ).…”

Microglia as hackers of the matrix: sculpting synapses and the extracellular space

“…However, caution must be taken regarding the interpretation of this result, as both studies utilized diphtheria toxin-based models of microglial ablation, which are associated with inflammation (e.g., upregulation of TNFɑ, IL-1β [8] or an interferon response [284]) that is not seen with genetic-or inhibitor-based models due to the manner in which microglial death is achieved [11]. Accordingly, IL-1β attenuates synaptic formation induced by IL-10 [282], and postnatal CSF1R inhibitor-based microglial depletion instead results in excess synapses [285] that are normalized following microglial repopulation [286]. Interestingly, loss of CSPG-5 (neuroglycan C), which normally localizes to the perisynaptic space [222], results in impaired presynaptic maturation as well as synaptic elimination that occurs earlier than normal in cerebellar Purkinje cells [223], which microglia survey and regulate [287][288][289][290].…”

Restorative therapy using microglial depletion and repopulation for central nervous system injuries and diseases

“…Microglial depletion and repopulation did not affect the neuropathology of EAE or CNS T-cell responses ( 136 ). The repopulated microglia correct the anatomical defects, partially restore the auditory function ( 137 ), and eradicate fear memory in mice after fear conditioning ( 138 ). A recent study showed that repopulated microglia can reduce neuroinflammation, neurological deficits, and brain edema following intracerebral hemorrhage in the aged brain ( 139 ).…”