Electrophysiological changes precede morphological changes to frontal cortical pyramidal neurons in the rTg4510 mouse model of progressive tauopathy

Abstract

Whole-cell patch-clamp recordings and high resolution morphometry were used to assess functional and structural properties of layer 3 pyramidal neurons in early (< 4 months) and advanced (> 8 months) stages of tauopathy in frontal cortical slices prepared from rTg4510 tau mutant (P301L) mice. In early tauopathy, dendritic architecture is preserved. In advanced tauopathy, neurons can be categorized as either “atrophic” (58%)- exhibiting marked atrophy of the apical tuft, or “intact” (42%)- with normal apical tufts and, in some instances, proliferative sprouting of oblique branches of the apical trunk. Approximately equal numbers of atrophic and intact neurons contain neurofibrillary tangles (NFTs) or are tangle-free, lending further support to the idea that NFTs per se are not toxic. Spine density is decreased due to a specific reduction in mushroom spines, but filopodia are increased in both atrophic and intact neurons. By contrast to these morphological changes, which are robust only in the advanced stage, significant electrophysiological changes are present in the early stage and persist in the advanced stage in both atrophic and intact neurons. The most marked of these changes are: a depolarized resting membrane potential, an increased depolarizing sag potential and increased action potential firing rates- all indicative of hyperexcitability. Spontaneous excitatory postsynaptic currents are not reduced in frequency or amplitude in either stage. The difference in the time course of functionally important electrophysiological changes versus regressive morphological changes implies differences in pathogenic mechanisms underlying functional and structural changes to neurons during progressive tauopathy.

Neural Precursor Lineages Specify Distinct Neocortical Pyramidal Neuron Types

“…Previous empirical and modeling studies have shown that dendritic morphology plays a critical role in determining passive membrane properties, including input resistance, and related neuronal firing rates (Mainen and Sejnowski, 1996;Euler and Denk, 2001;Vetter et al, 2001;Bekkers and Häusser, 2007;Weaver and Wearne, 2008;Amatrudo et al, 2012;Ferrante et al, 2013). Indeed, correlation analysis in our study suggests that there is a significant negative relationship between proximal (and total) apical dendritic branching com- plexity and input resistance in both Tbr2 and non-Tbr2 lineage neurons.…”

“…A series of hyperpolarizing and depolarizing current steps (200 ms or 2 s in duration) were applied to each cell to assess passive membrane, single action potential, and repetitive action potential firing rates as described previously (Rocher et al, 2010;Crimins et al, 2012). Thirty electrophysiological variables were assessed in this study and quantified as follows.…”

“…Lineage identity was confirmed by confocal imaging of biocytin filled neurons. For assessment of electrophysiological properties and cell filling, whole-cell patch-clamp recordings were conducted as previously described (Rocher et al, 2010;Crimins et al, 2012). Electrodes were fabricated on a Flaming and Brown horizontal pipette puller (model P87, Sutter Instrument) and filled with potassium gluconate (KGlu) internal solution, concentrations (in mM) as follows: 122 KGlu, 2 MgCl 2 , 5 EGTA, 10 NaHEPES, 2 MgATP, 0.3 NaGTP, 1% biocytin, pH 7.4 (SigmaAldrich).…”

“…As such, an emerging theory proposes that the number and relative proportions of IPC subtypes are responsible for the extensive growth of the neocortex in humans (LaMonica et al, 2013;Florio and Huttner, 2014;Lewitus et al, 2014), an evolutionary step that underlies our unique cognitive abilities. However, the primate neocortex has expanded primarily laterally, due to the symmetric divisions of ventricular zone founder cells before the onset of IPC production (Caviness et al, 1995;. In addition, because every known class of IPCs has been identified in rodent neocortex, IPCs may not solely function to increase brain size but may offer alternative contributions to neocortical development.…”

Neurofibrillary tangle-bearing neurons are functionally integrated in cortical circuits in vivo

“…The assumption that mislocalization of tau into the somatodendritic compartment (6) and accumulation of fibrillar aggregates in NFTs mediates neurodegeneration underlies most current therapeutic strategies aimed at preventing NFT formation or disrupting existing NFTs (7,8). Although several disease-associated mutations cause both aggregation of tau and neurodegeneration, whether NFTs per se contribute to neuronal and network dysfunction in vivo is unknown (9). Here we used awake in vivo two-photon calcium imaging to monitor neuronal function in adult rTg4510 mice that overexpress a human mutant form of tau (P301L) and develop cortical NFTs by the age of 7-8 mo (10).…”