Microglial physiological properties and interactions with synapses are altered at presymptomatic stages in a mouse model of Huntington’s disease pathology

Abstract

Background: Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder that affects cognitive and motor abilities by primarily targeting the striatum and cerebral cortex. HD is caused by a mutation elongating the CAG repeats within the Huntingtin gene, resulting in HTT protein misfolding. Although the genetic cause of HD has been established, the specific susceptibility of neurons within various brain structures has remained elusive. Microglia, which are the brain's resident macrophages, have emerged as important players in neurodegeneration. Nevertheless, few studies have examined their implication in HD. Methods: To provide novel insights, we investigated the maturation and dysfunction of striatal microglia using the R6/2 mouse model of HD. This transgenic model, which presents with 120+/-5 CAG repeats, displays progressive motor deficits beginning at 6 weeks of age, with full incapacitation by 13 weeks. We studied microglial morphology, phagocytic capacity, and synaptic contacts in the striatum of R6/2 versus wild-type (WT) littermates at 3, 10, and 13 weeks of age, using a combination of light and transmission electron microscopy. We also reconstructed dendrites and determined synaptic density within the striatum of R6/2 and WT littermates, at nanoscale resolution using focused ion beam scanning electron microscopy. Results: At 3 weeks of age, prior to any known motor deficits, microglia in R6/2 animals displayed a more mature morphological phenotype than WT animals. Microglia from R6/2 mice across all ages also demonstrated increased phagocytosis, as revealed by light microscopy and transmission electron microscopy. Furthermore, microglial processes from 10-week-old R6/2 mice made fewer contacts with synaptic structures than microglial processes in 3week-old R6/2 mice and age-matched WT littermates. Synaptic density was not affected by genotype at 3 weeks of age but increased with maturation in WT mice. The location of synapses was lastly modified in R6/2 mice compared with WT controls, from targeting dendritic spines to dendritic trunks at both 3 and 10 weeks of age. Conclusions: These findings suggest that microglia may play an intimate role in synaptic alteration and loss during HD pathogenesis.

CK2 alpha prime and alpha-synuclein pathogenic functional interaction mediates synaptic dysregulation in Huntington’s disease

“…Nevertheless, our IF data are consistent with recent reports using Iba1 IF in zQ175 mice at this same age that also reported the absence of significant microgliosis (Diaz-Castro et al 2019). Similar results were obtained in the R6/2 mice where an increased number of Iba1+ cell bodies as well as Iba1+ body area using IF were only significant at pre-symptomatic stages (Savage et al 2020). Therefore, although microglia may play an important role during early HD pathogenesis, their role could become less relevant at later stages of the disease.…”

“…Our data is consistent with a recent report suggesting the absence of significant microgliosis in symptomatic zQ175 mice (Diaz-Castro et al 2019). Similar results were obtained in the R6/2 mice using a combination of light and transmission electron microscopy, where increased reactive microglia and microglia phagocytic activity were preferentially induced at pre-symptomatic stages (Savage et al 2020). Therefore, although microglia may play an important role during early HD pathogenesis, their role could become less relevant at later stages of the disease (Petkau et al 2019).…”

“…The accumulation of pro-inflammatory cytokines, other inflammatory mediators, and molecules secreted from damaged neurons often result in the activation of microglia, the resident macrophages of the central nervous system (CNS) that respond to degenerated neurons and facilitate immunomodulatory and phagocytic activities. Activation of microglia, which is characterized by swollen shapes and the retraction of processes, has been observed using positron emission tomography (PET) in pre-symptomatic patients with HD and reported in numerous HD mouse models (Pavese et al 2006, Tai et al 2007, Savage et al 2020). There is mounting evidence that microglia play an important role in HD pathogenesis, but the extent of their contribution and the exact timing at which activated microglia are essential in HD is still under debate.…”

“…Neuroinflammation is a coordinated response to brain toxicity involving the activation of microglia and astrocytes. Increased reactive microglia were reported in numerous HD mouse models (Pavese et al 2006, Savage et al 2020, Franciosi et al 2012), in pre-symptomatic patients with HD (Pavese et al 2006, Tai et al 2007), and are often associated with increased pro-inflammatory cytokines (Crotti and Glass 2015). Although there is mounting evidence that microglia play an important role in HD pathogenesis, the extent of their contribution and the exact timing at which activated microglia are essential in HD is still unclear.…”

Imaging the Neuroimmune Dynamics Across Space and Time

“…As illustrated in Figures 1G,H, this nanoscale resolution allows to identify signs of 'intracellular' activity (e.g., organellar changes, phagosomal inclusions) and extracellular activity (e.g., extracellular digestion of cellular debris, intercellular contacts with other glial cells, neurons, and synapses). The quantification of events for each cell type studied is essential to assess the situation of the cell, whether it is in steady state or overwhelmed by phagocytic inclusions requiring digestion, accumulated misfolded proteins in the endoplasmic reticulum or Golgi apparatus, stressed mitochondria or making increased synaptic contacts as previously evaluated for microglia across a range of contexts, including Huntington's disease pathology (Savage et al, 2020). Additionally, an increasing body of evidence supports the idea that the microglial population is composed of diverse subpopulations endowed with unique intrinsic properties that perform different functions, and display a high degree of spatial and temporal specialization (Stratoulias et al, 2019).…”