Autism etiology includes both environemental and genetic risk factors. One genetic factor is the Engrailed 2 (En2) gene, which we found associated with Autism Spectrum Disorder in 3 different datasets, and disease associations have been reported by 6 other groups. En2is a transcription factor that is expressed in and patterns mid/hindbrain and cerebellar structures, acting both pre- and post-natally.
Surprisingly, in previous work, we found En2-knock out (En2-KO) mice exhibited deficits in the forebrain, a region outside its classical hindbrain localization. In the hippocampus, norepinephrine (NE) levels were reduced by 33%, and there were reductions in hippocampal weight (-12%), DNA content (-16%) and dentate gyrus neurons (-16%). The decrease in hippocampal neurons was associated with a 77% increase in cell death (caspase3+, pyknotic body), and while progenitor cell proliferation (BrdU+, PCNA+) was increased 2-fold, newly born (BrdU+) cells at P21 underwent excessive cell death, suggesting dysregulation of neurogenesis. These data led to the hypothesis that deficits in NE in the En2-KO may result in enhanced programmed cell death, and that neurogenesis may increase in compensatory fashion.
Behavioral studies in the En2-KO indicate deficits in social interaction and hippocampal-dependent behaviors. Significantly, both post-natal neurogenesis and synaptic plasticity are critical for learning and memory. We now explore the nature of progenitors cell death and define long term potentiation (LTP), a form of synaptic plasticity, in the En2-KO.
By defining the deficits caused by En2 mutation, we aim to identify new biological functions downstream of En2. We are using the hippocampus as a model structure to understand En2’s influence on neurogenesis and synapse function hoping to generalize finding to other brain regions and reveal new treatment avenues.
Immunohistochemical markers of neural progenitor cells (Sox2, Dcx) and proliferation (BrdU) were assessed. Field Excitatory Post-Synaptic Potential (fEPSP) recordings were performed on 1 month old hippocampus slices, stimulating the Schaffer collateral pathway while recording in the CA1 stratum-radiatum. We measured synaptic transmission, paired pulse facilitation and LTP.
The En2-KO dentate gyrus exhibited increased proliferation at P21 that reflected early neural progenitors (Sox2+) only (N=5/genotype; p<0.02). They were increased in cell number (+17%) as well as proliferative activity (+219%). In contrast, there were no changes in later progenitors (Dcx+).
In En2-KO hippocampal slices, fEPSPs were normal, including synaptic transmission and paired pulse facilitation (N=10/genotype). In contrast, preliminary data suggest increased LTP in En2-KO (+17%) compared to WT (+8%). Based on enhanced LTP, we are now examining the GABA-inhibitory neuron population, which may be decreased. Preliminary data show decreased in Parvalbumine-population interneurons (-17%).
These studies suggest that in the absence of hindbrain patterning gene En2, there are many consequences for hippocampal neuron production, survival and function. While currently speculation, changes in forebrain neurogenesis and synaptic plasticity may be due to diminished NE, which affects both processes. Further, recent behavioral studies show that a NE re-uptake inhibitor can reverse En2-KO deficits in both social and hippocampal-dependent tasks. Potentially, altering monoamine levels during development may correct forebrain structural deficts and improve functional abnormalities.