Impaired activity-dependent presynaptic release of BDNF from Mossy fibers and postsynaptic BDNF-TRPC currents in CA3 pyramidal neurons of Mecp2 mutant mice

LI W, AMARAL MD, LI Y, CALFA G, POZZO-MILLER L

Leesburg, Virginia

Congreso; 12th Annual Rett Syndrome Symposium, International Rett Syndrome Foundation (IRSF); 2011

IRSF

Deficient BDNF signaling resulting from MeCP2 dysfunction is considered an important cause for sensory, motor and cognitive symptoms in Rett syndrome (RTT). To go beyond the assessments of overall BDNF mRNA and protein levels, we studied BDNF signaling by intracellular recordings of IBDNF, a non-selective cationic current mediated by canonical transient receptor potential channels (TRPC3; Li et al. Neuron 1999; Amaral & Pozzo-Miller J Neurosci 2007). TRPC3 currents were evoked in CA3 pyramidal neurons by theta burst stimulation (TBS) of presynaptic mossy fibers (MF), and were confirmed to be mediated by endogenously secreted BDNF by their sensitivity to the extracellular scavenger TrkB-IgG (Li et al. J Neurophysiol 2010). As predicted from biochemical measures of BDNF levels, MF-evoked TRPC3 currents (MF-IBDNF) were significantly smaller in CA3 pyramidal neurons in acute slices from male symptomatic Mecp2 mutant mice than in those from age-matched wildtype littermates (P40-60, Jaenisch strain). MF-IBDNF could be smaller due to reduced presynaptic BDNF release, impaired postsynaptic TrkB-TRPC3 signaling, or a combination of both. Testing postsynaptic signaling by local application of recombinant BDNF to apical dendrites of CA3 pyramidal neurons revealed that IBDNF was also smaller in Mecp2 mutant slices than in age-matched wildtype controls. However, this impairment of membrane currents evoked by direct application of recombinant BDNF is smaller than that of currents evoked by stimulation of presynaptic mossy fibers, which suggest that activity-dependent release of endogenous BDNF is indeed affected in Mecp2 mutant mice. Consistently, preliminary real-time qRT-PCR data indicate that both BDNF and TRPC3 mRNA expression levels are lower in Mecp2 mutant mice than in age-matched wildtype controls, suggesting a combination of pre and postsynaptic impairment in BDNF signaling. Current studies are further defining pre and postsynaptic mechanisms contributing to impaired BDNF signaling in this region of the hippocampus. For example, membrane currents evoked by direct local application of the TRPC3 activator DOG (1,2-dioctanoylsn-glycerol), a non-hydrolyzable analog of DAG (diacylglycerol), are being used to test if IBDNF is smaller due to differences in the levels of the BDNF receptor TrkB or the TRPC3 channels themselves. The full characterization of these membrane currents as a bioassay for endogenous BDNF signaling will aid in the identification and testing of novel therapeutic approaches for RTT.