Reduction in endogenous KYNA synthesis contributes to the
hyperexcitability in MTLE-HS
Astrocyte-neuron interaction play a crucial role in the regulation of
glutamatergic activity. One of the regulation mechanisms is the
conversion of KYN to KYNA in astrocytes which inhibits glutamate
receptor-mediated synaptic transmission on to neurons (Moroni et al.,
2012). Pyridoxine and PNPO has long history with epilepsy treatment
(Watanabe, 1995; Ito et al., 2000; Kuo and Wang, 2002; Jaeger et al.,
2016). We observed that the mRNA and protein expression of the PNPO
enzyme were significantly downregulated in MTLE-HS patients leading to
reduced PLP synthesis (Figure 7g, h, i). Taken together, reduction in
both KAT II expression as well as its co-factor PLP might be affectingde novo synthesis of KYNA from KYN. Our observation that tissue
slices incubated with saturating concentration of KYN (200µM) showed
negligible change in the frequency and amplitude of spontaneous EPSCs in
the hippocampal samples of patients with MTLE-HS compared to that in
non-seizure control samples(Figure 4e) can be attributed to both
presynaptic as well as postsynaptic glutamatergic activity. KYNA
inhibits action potential dependent spontaneous glutamatergic activities
(Banerjee et al., 2012) and reduction of endogenous KYNA synthesis
enhances hippocampal extracellular glutamate release as evidenced in
animal models (Pocivavsek et al., 2011; Potter et al., 2010). Further
experiments involving isolation of miniature EPSCs in the presence of
KYNA may further help us understand the regulation of glutamatergic
activity by quantal currents in the hippocampal samples obtained from
patients with MTLE-HS. We suspect that the amount of KYNA synthesizedde novo in the hippocampal samples was less than IC50 value of
KYNA for α7 nAChR (1-8 µM) (Hilmas et al., 2001) and glycine site of
NMDA receptor (10-30µM) (Parsons et al., 1997). Consequently,
presynaptic α7 nAChR dependent glutamate release was not regulated and
action potential dependent presynaptic inputs were enhanced, leading to
increase in the frequency of sEPSCs (Banerjee et al., 2020). The
postsynaptic NMDA receptors were upregulated and functionally
hyperactive in MTLE-HS hippocampus (Banerjee et al., 2017). As,
postsynaptic glycine sites of NMDA receptors were also not competitively
inhibited by KYNA, the amplitude of spontaneous EPSCs was also
increased. Future studies involving incubating the brain slices with
either the KAT II or PLP or PLP synthesizing enzyme PNPO will be
beneficial to prove the central hypothesis more accurately by assessing
the extent of recovery of KYNA synthesis in MTLE-HS.
We could not see significant alterations in GABAergic activities under
resting state. This suggests that, under resting conditions,
hyperexcitatory activities were primarily mediated by glutamate
receptors in the hippocampal samples. Moreover, these MTLE-HS patients
were on AEDs (Supplementary table 1), most of which are
GABAA receptor modulators, may have compensated for the
altered GABAergic activity in the hippocampal samples.