The precise timing of formation and length of axonal connectivity in the developing brain is essential for proper neural circuit connectivity and function. As such, alterations in axonal connectivity are thought to contribute to neuropsychological impairments in autism spectrum disorder including monogenic disorders associated with autism such as tuberous sclerosis complex (TSC). TSC individuals express a silencing mutation in one TSC1 or TSC2 allele leading to hyperactivity of the mammalian target of rapamycin complex 1 and 40-60% of these individuals display mild to severe autistic traits and other neuropsychological problems that are independent of seizures or gross brain abnormalities. In TSC as well as other disorders associated with autism, the mechanisms leading to altered axonal connectivity remain unclear. One possible mechanism that remains unexplored is the contribution of astrocytes to abnormal axonal growth in TSC. Astrocytes are a subtype of macroglia that play a critical role in regulating neuronal development including axon elongation. Here, we thus propose to test the hypothesis that Tsc1+/- astrocytes contribute to increased axonal growth in TSC. We have two aims.
The first aim proposes to assess whether Tsc1+/- astrocytes accelerate axon growth and increase axonal patterning. Using iTRAQ (isobaric tags for relative and absolute quantitation)-based quantitative proteomics, we identified several molecules altered in Tsc1+/- astrocyte-conditioned medium, such as apolipoprotein E (apoE) that is a good candidate for controlling axon elongation.
The second aim proposes to examine whether reduced apoE levels in Tsc1+/- astrocytes contributes to abnormal axon growth. To address our hypothesis, we will use purified cultures of astrocytes and neurons as well as in vivo approaches using conditional transgenic mice to delete one allele of Tsc1 selectively in astrocytes combined with in utero electroporation to label selective neuronal populations or single cortical neurons for axon tracing.
Alterations in axonal connectivity are thought to contribute to neuropsychological impairments in autism spectrum disorder, including monogenic disorders associated with autism such as tuberous sclerosis complex (TSC). Here we will explore whether a subtype of glial cells, astrocytes, contribute to abnormal axonal growth in TSC through reductions in apolipoprotein E levels. Data from the proposed work will open a new avenue of investigation and offer therapeutic promise for treatment of some of the neurological symptoms in TSC.
