3. Altered hippocampal glia-neuron interactions in ASD model
animals
Multiple lines of evidence have demonstrated the essential roles of
glial cells, particularly astrocytes and microglia, in maintaining the
physiological function of the hippocampus \cite{RN63,RN64,RN65,RN66}. Astrocytes, the most abundant cell type in the human
brain, coordinate synapse formation and elimination, neuronal survival,
and axon guidance in the developing brain \cite{RN67,RN68,RN69,RN70}. Moreover, astrocytes form tripartite synapse, actively
regulating synaptic transmission by interacting with presynaptic axon
terminal and postsynaptic dendritic spines of neurons \cite{RN71,RN70}. On the other hand, microglia, as the
dominant immune cells in the brain, possess phagocytic capacity,
enabling them to engulf dead cells and detrimental substances in the
brain \cite{RN72}. They also participate in synaptic
pruning by engulfing excessive synapses through interactions with
neuron \cite{RN66,RN74,RN73}. These glial cells work in
concert to maintain brain homeostasis \cite{RN75,RN70}.
Consequently, morphological and functional alterations of glia have been
widely observed in many brain disorders, including neurodevelopmental
disorders such as ASD \cite{RN76} and dysfunction in
glial cells has been recognized as an etiological factor in their
pathogenesis \cite{RN76}.
Due to the highly heterogeneous nature of ASD, many different transgenic
mice modelling the disorder have been developed, and how hippocampal
glia are altered substantially varies depending on the specific model.
Nonetheless, most previous studies have commonly reported that,
regardless of the model, astrocytes and microglia exhibit morphological,
molecular, and/or functional alterations in the hippocampus.
In the Fmr1-KO mouse model multiple studies have reported astrocytic
changes \cite{RN46,RN78,RN77}. During early postnatal
development, dynamic alterations in the expression of Hevin, a protein
secreted at excitatory synapses, are observed, suggesting the
involvement of astrocytic alterations in abnormal synaptic development
in FXS \cite{RN77}. In 2-month-old Fmr1-KO mice,
astrocytes and microglia were reported to show reduced function at
tripartite synapse and synaptic pruning respectively \cite{RN46}, while astrocytes showed increased GFAP
expression in 3-month-old mice \cite{RN78}. Another
notable study has developed astrocyte-specific Fmr1 conditional KO mice
and observed that astrocytic GABA synthesis increased alongside a modest
elevation in GFAP expression \cite{RN79}, demonstrating
a positive correlation between astrocytic GABA synthesis and reactivity \cite{RN82,RN81,RN80}.
In the Mecp2-KO mouse model astrocytes undergo cytoskeletal atrophy
during severe symptomatic time point, but not at earlier time points \cite{RN83}. The reduced ramification of astrocytes was
also observed in postnatal Mecp2-conditional KO mice \cite{RN84}. Microglia in these mice also exhibit
reduced branch complexity in the late-phenotypic stage, whereas no such
changes occur during the pre-phenotypic period \cite{RN85}. Furthermore, CA1 astrocytes in the same
mouse model have been implicated in the reduction of tonic inhibition
due to decreased expression of the GABA transporter 3 (GAT3) within
astrocytes \cite{RN86}, a phenotype linked to the
hyperexcitability and increased seizure susceptibility in these mice. A
similar finding was also observed in the astrocyte-specific Mecp2-KO
mice \cite{RN86}. Collectively, these studies suggest
that hippocampal glia are significantly impacted by, as well as
contribute to, the pathology of Rett syndrome.
At the level of synaptic proteins, mice with C-terminal deleted Shank3
(deletion of exon 21, which includes the Homer- and Cortactin-binding
domains; Shank3+/ΔC) exhibit morphological changes in astrocytes and
microglia within the hippocampus, with no observable alterations in the
number or size of astrocytes (GFAP+ and S100+) in most hippocampal
subregions. However, there was a noteworthy reduction in the GFAP+ area
specifically within the CA1 stratum radiatum. In contrast, microglia
exhibited no discernible changes in their morphological characteristics \cite{RN87}. On the other hand, a separate
transcriptomics study conducted with Shank3-KO mice revealed an increase
in the expression of gene sets related to astrocytes, microglia, and
oligodendrocytes \cite{RN88}. However, Shank2-KO
(lacking exon 6 and 7) mice showed that astrocytic and microglial genes
are negatively enriched \cite{RN88}. These findings
implicate the Shank2 and Shank3 deletions lead to differential
transcriptomic changes of glial cells in the hippocampus.
A recent study, utilizing NLG4-KO mice, revealed sex-dependent
morphological and functional alterations in microglia \cite{RN89}. These microglial changes were more
pronounced in males than in females. Specifically, male Nlgn4-KO mice,
aged 13 weeks and 20 weeks, exhibited reduced microglial density and
branching, diminished phagocytic activity, decreased expression of MHC1
and CD54, impaired response to injury, and disrupted energy metabolism
in the CA3 region of the hippocampus. In contrast, in the
NGL3R451C point-mutant knock-in mouse model, DG
microglia showed no discernible morphological alterations, while DG
astrocytes showed a shrunken morphology \cite{RN90}.
In Cntnap2-KO mice there were only subtle morphological changes observed
in astrocytes in the DG molecular layer and CA1 stratum radiatum, with
no alterations in the numbers or area of GFAP+ cells. Notably, in the
ventral hippocampus, there was a significant reduction in the number of
S100+ astrocytes, although the area of S100+ pixels remained unaltered.
Similarly, microglia exhibited minimal alterations with no change in the
numbers or area of Iba1+ cells. There was a slight, non-significant
increase in the CD68+ area observed in the dorsal molecular layer and
ventral stratum radiatum, but this was not observed in other hippocampal
subregions \cite{RN87}. Intriguingly, there was a
report with a mouse model that underwent a plasma exchange operation
using plasma from from two male patients with CASPR2 (contactin
associated protein 2, encoded by Cntnap2 gene)-positive encephalitis. In
this mouse model, regardless of their age, hippocampal microglia showed
no morphological alterations, while there was a significant increase in
microglial numbers in the cortex \cite{RN91}. The
modest change or the absence of the alterations in hippocampal glial
morphology in the Cntnap2-KO model may distinguish it from many other
ASD-like mouse models.
In SCN1A haplodeficient (-/+) mice, a Dravet syndrome model, one recent
study reported increased GFAP expression and an increased number of
Iba1-positive microglia in DG, implying increased reactivity of
astrocytes and microglia \cite{RN92}. Additionally,
this study reported a reduction in tonic GABA current in CA1 pyramidal
neurons, which needs further investigation due to its inconsistency with
existing evidence showing the positive correlation between astrocytic
reactivity and tonic GABA current \cite{RN92}. In
SCN2A-deficient mice, another study reported partially activated
microglia, evidenced by increased cell bodies and reduced branches \cite{RN93}. These activated microglia showed excessive
phagocytic pruning of synapses, which occurs during development and
continues into adulthood. These changes in microglia led to a reduction
in spine density and glutamatergic synaptic transmission in CA1
pyramidal neurons \cite{RN93}. Collectively, these
studies suggest that glial alterations in both SCN1A and SCN2A
deficiency-induced ASD-like mouse models could impact E/I balance in the
hippocampus.