Genetic causes of autism
Proteins associated with synapses and cellular signaling have been identified as genetic drivers of autism. Vogt and his research lab focus on cell signaling to identify links between events that occur on the cell surface and cytoplasm (inside the cell) (2:00). The speaker introduces two signaling cascades critical to cell growth, division and migration (3:00):
- mTOR (Mammalian target of rapamycin): leads to translational regulation; inhibited by known autism risk genes Pten in early development and Tsc1 in late development
- MAPK (Mitogen-activated protein kinase): leads to transcriptional regulation; inhibited by known autism risk gene Nf1 at early stages of development
Cortical interneurons (CINs)
To determine what, if any, mutations or effects are common across pathways, Vogt and colleagues study GABAergic cortical interneuron ratios in mice (5:15). Cortical interneurons (CINs) shape neural networks in the brain and originate from embryonic brain structures called the medial and caudal ganglionic eminences (MGE/CGE) (6:20). The speaker explains that the MGE develops around 70% of CINs via four major events:
- Cell proliferation (birth) and apoptosis (death)
- Cell migration across long distances to final targets in the brain
- Cell fate, or the acquisition of specific properties related to cell function
- Electrical properties used to communicate across the microcircuit synapses
Vogt describes a study that found significantly decreased numbers of Parvalbumin (PV+) CINs (important for brain circuitry and memory) in autistic participants compared to controls (9:08). He touches on the Paravalbumin hypothesis of autism, which asserts that disruption of PV+ CINs may be a root cause of autism symptoms (10:14). Somatostatin (SST+) cells are another subgroup of CINs with a slower electric firing frequency. The presenter outlines mouse model methods for assessing the impact of mutant pathways on CIN subgroup ratios (11:15).
For more information on brain development in autism, view Transcranial Magnetic Stimulation and Autism, a free webinar presented by Manuel Casanova, MD.
mTOR signaling pathway
The speaker asserts that converging evidence across multiple phenotypes associated with the loss of Pten suggests the electric cell communication circuit may be impaired in autism (14:11). Vogt outlines the process by which Pten is adjusted in mouse MGE cells to test for different variants and their functions (15:45). Mice with a Pten deletion showed no changes in SST expression, but had significantly reduced PV expression. However, mice with a human variant of Pten were rescued (returned) to control/wild-type levels (17:00). These findings, Vogt claims, show that variations of Pten cannot maintain the pathway, meaning that it is a loss-of-function gene (18:00). Tsc1 mutations also cause increased PV expression, suggesting a hyperactive mTOR pathway (19:30). The speaker reviews that mTOR activity promotes PV+ CINs, that the loss of Pten and Tsc1 lead to hyperactive signaling (e.g., higher expression of PV and lower expression of SST), and that most genetic variants of Pten and Tsc1 are loss-of-function genes (20:10).
MAPK signaling pathway
Deletion of the Nf1 gene or consecutive expression of bRaf leads to hyperactivation of the MAPK pathway. The speaker explains that hyperactive MAPK signaling causes increased levels of SST+ CINs and the repression of the ARX gene, which is critical for MGE and CGE development (20:50). Hyperactive MAPK pathways also have decreased PV expression, further evidencing circuit-based phenotypic variations (23:30). Vogt outlines a study that found decreased SST expression and normal PV levels after deletion of ERK1/2 in a mouse model. He explains that the pathway’s simultaneous hyperactivity and reduced activity suggest a dosage problem that corresponds to SST levels (25:07).
The speaker reviews MAPK loss-of-function mutations, highlighting that a hyperactive pathway leads to higher SST and lower PV expression. Conversely, loss of ERK1/2 produces fewer SST+ and more PV+ cells (26:20). The speaker continues that some data suggest overproduction of SST in early development may outweigh the ability to make enough Pten later.
Electrical properties
The presenter explains that by assessing the electrical properties of CINs, we can determine pathways of cell fate and communication (27:35). As noted previously, PV+ CINs have faster electrical spike patterns compared to SST+ CINs. Therefore, Vogt explains, hyperactive mTOR pathways have higher firing properties (more PV), and hyperactive MAPK mutants have slower firing properties (more SST) (30:00). He asserts that these findings show common changes across these pathways that are important for cellular fates, properties, and functionality (31:20).
What’s next
Researchers have observed that mice with hyperactive MAPK (Nf1 deletion) present behaviors that resemble hyperactivity and a reduced sense of danger, suggesting a link between molecular changes and behaviors (32:40). The presenter discusses changes to transcription factors in the MGE in hyperactive MAPK (Nf1) mice, highlighting that more research is needed to understand the connections between cellular circuitry and behavior (35:25).
Vogt outlines unpublished research that uses Selumetinib, an FDA-approved MEK inhibitor, to test molecular changes in mice. Preliminary results show that ERK remains active in controls. At the same time, blot test bands were rescued in mice who consumed the drug (36:50). The same study also found that systemic delivery of Selumetinib led to some behavior rescue in Nf1 adult mice. The speaker notes that ARX markers also seem to respond to the drug (38:50). Future directions include switching mTOR mutants with known molecular and cellular changes and using rapamycin to inhibit mTOR and search for other changes that may correlate with behaviors (40:15). Vogt provides thanks and acknowledgments (42:40) before the Q&A (44:00).
