Investigating astrocyte-synapse interactions in amyotrophic lateral sclerosis

Abstract

Amyotrophic Lateral Sclerosis (ALS) is fatal neurodegenerative condition characterised by the loss of upper and lower motor neurons (MNs). Given that synapses and astrocytes regularly interact in the healthy spinal cord, and that pathology involving both has been reported in ALS, we posited that synaptic and astrocytic mechanisms may be intrinsically linked. One essential astrocytic function is synaptogenesis. Hyperexcitability of pre-MN networks has been observed in ALS, with a possible basis of this change being perturbed astrocytic synaptogenesis causing an early non-cell autonomous shift in excitatory : inhibitory (E:I) synaptic ratios. To look at this, we developed and validated a novel postnatal primary co-culture system of spinal astrocytes and neurons. These cultures were generated using multiple ALS mouse models bred with animals expressing a GFP tag on the postsynaptic density protein PSD95, with immunocytochemical targeting of presynaptic protein synapsin, and inhibitory postsynaptic protein gephyrin. We found that no combination of neuron or astrocyte genotypes altered E:I ratios in both SOD1ᴳ⁹³ᴬ and C9BAC500 co-cultures. This lack of E:I ratio change was also seen in SOD1ᴳ⁹³ᴬ spinal cords after the peak of postnatal spinal synaptogenesis, and in human ALS patient iPSC-derived MN / astrocyte cultures. Post-synapse formation, however, astrocytes also interact with mature synapses at structures called tripartite synapses. As dysfunction at tripartite synapses has previously been reported, we investigated these structures throughout disease progression in SOD1ᴳ⁹³ᴬ PSD95-eGFP animals. Using multiple markers for perisynaptic astrocytic processes, the motile elements that envelop synapses, we observed consistent loss of tripartite synapses at the early symptomatic stage of 16W. Non-tripartite synapses, however, remained similar in number to controls. This was replicated in human ALS post-mortem tissue. We therefore conclude that although we find no evidence that astrocytes are driving early E:I ratio shifts, we reveal that later in pathogenesis tripartite synapses appear to be a vulnerable fulcrum of disease in ALS.