Knockout of Cyclophilin-D Provides Partial Amelioration of Intrinsic and Synaptic Properties Altered by Mild Traumatic Brain Injury

Abstract

Mitochondria are central to cell survival and Ca2+ homeostasis due to their intracellular buffering capabilities. Mitochondrial dysfunction resulting in mitochondrial permeability transition pore (mPTP) opening has been reported after mild traumatic brain injury (mTBI). Cyclosporine A provides protection against the mPTP opening through its interaction with cyclophilin-D (CypD). A recent study has found that the extent of axonal injury after mTBI was diminished in neocortex in cyclophilin-D knockout (CypDKO) mice. Here we tested whether this CypDKO could also provide protection from the increased intrinsic and synaptic neuronal excitability previously described after mTBI in a mild central fluid percussion injury mice model. CypDKO mice were crossed with mice expressing yellow fluorescent protein (YFP) in layer V pyramidal neurons in neocortex to create CypDKO/YFP-H mice. Whole cell patch clamp recordings from axotomized (AX) and intact (IN) YFP+ layer V pyramidal neurons were made 1 and 2 days after sham or mTBI in slices from CypDKO/YFP-H mice. Both excitatory post synaptic currents (EPSCs) recorded in voltage clamp and intrinsic cellular properties, including action potential (AP), afterhyperpolarization (AHP), and depolarizing after potential (DAP) characteristics recorded in current clamp were evaluated. There was no significant difference between sham and mTBI for either spontaneous or miniature EPSC frequency, suggesting that CypDKO ameliorates excitatory synaptic abnormalities. There was a partial amelioration of intrinsic properties altered by mTBI. Alleviated were the increased slope of the AP frequency vs. injected current plot, the increased AP, AHP and DAP amplitudes. Other properties that saw a reversal that became significant in the opposite direction include the current rheobase and AP overshoot. The AP threshold remained depolarized and the input resistance remained increased in mTBI compared to sham. Additional altered properties suggest that the CypDKO likely has a direct effect on membrane properties, rather than producing a selective reduction of the effects of mTBI. These results suggest that inhibiting CypD after TBI is an effective strategy to reduce synaptic hyperexcitation, making it a continued target for potential treatment of network abnormalities.

Mild Traumatic Brain Injury Evokes Pyramidal Neuron Axon Initial Segment Plasticity and Diffuse Presynaptic Inhibitory Terminal Loss

“…Pyramidal neurons are the major excitatory component in cortical networks (Fairen et al, 1984 ; DeFelipe and Fariñas, 1992 ; Somogyi et al, 1998 ). Previously we showed that intact YFP+ pyramidal neurons intrinsic and extrinsic/synaptic properties were altered post-mTBI and developed over time (Greer et al, 2012 ; Hånell et al, 2015a ; Sun and Jacobs, 2016 ). Initially after mTBI, the intrinsic properties of intact pyramidal neurons paralleled that found in the DAI population, suggesting that ion and/or neurotransmitter imbalance had an indiscriminant overriding effect on currents (Greer et al, 2012 ).…”

“…Specifically, following experimental mTBI using transgenic mice, we observed primary neocortical damage involving scattered DAI within a subset of layer 5 pyramidal neurons expressing a yellow variant of green fluorescent protein (YFP) labeling the soma as well as their dendrites and axons (Greer et al, 2011 ). Perhaps even more biologically significant was our parallel observation that the non-DAI (intact) YFP+ neuronal population showed altered electrophysiological properties without any evidence of overt structural damage (Greer et al, 2012 ; Hånell et al, 2015a ; Sun and Jacobs, 2016 ). Over time, these same intact YFP+ pyramidal neurons became hyperexcitable, entirely consistent with some form of mTBI-induced circuit disruption within local neocortical networks (Zhang and Raichle, 2010 ; Wolf and Koch, 2016 ).…”

“…To quantitatively assess for intrinsic AIS and extrinsic/synaptic perisomatic structural plasticity, for each animal, we labeled ankG, NaV1.6 and double-labeled glutamate decarboxylase-67 (GAD67) with PV in sections taken from a randomly selected and two adjacent wells, respectively, containing caudal S1BF (bregma level −1.5 to −2.0). This was done because of the consistency with which cFPI generates DAI within this well characterized area of neocortex, which was also the rational for choosing this region-of-interest (ROI) in our previously published electrophysiological studies (Greer et al, 2012 ; Hånell et al, 2015a ; Sun and Jacobs, 2016 ).…”

“…As previously described (Sun and Jacobs, 2016 ), whole-cell patch-clamp recordings were performed under infrared Dodt contrast microscopy (Zeiss AxioExaminer). A 60× water-immersion objective was used to visually identify YFP+ layer 5 pyramidal neurons of S1BF with axons descending from the soma into the white matter (intact) or ending with an axonal swelling (DAI), deep to the surface of the slice to avoid those transected by the vibratome.…”

Cyclophilin D Contributes to Anesthesia Neurotoxicity in the Developing Brain

“…The expression of CypD was elevated in brain tissues of Alzheimer's disease (AD) transgenic mice (Du et al, 2008(Du et al, , 2011(Du et al, , 2014, which promotes AD neuropathogenesis (Wang et al, 2009;Zhu et al, 2013;Adiele and Adiele, 2016). Knockout (KO) of CypD stabilized mitochondrial function (Du et al, 2008(Du et al, , 2011(Du et al, , 2014Gainutdinov et al, 2015;Gordan et al, 2016;Sun and Jacobs, 2016), decreased threshold of mPTP formation, and improved cognitive function in old AD transgenic mice (Du et al, 2008(Du et al, , 2011(Du et al, , 2014. However, the role of CypD in the developmental anesthesia neurotoxicity remains largely to be determined.…”