In epilepsy, dysfunctional inhibitory neurons allow cortical neurons to become overexcited, leading to seizures. Drugs that enhance transmission of GABA, the major inhibitory neurotransmitter, help, but do not treat all forms of epilepsy, and they often come with debilitating side effects. While studies in mice suggest that more targeted approaches—such as turning on or off specific populations of neurons—quell seizures (see Krook-Magnuson et al., 2013, and Paz et al., 2013), these methods require genetic manipulation. Some scientists have been exploring stem cell therapies instead. Baraban and colleagues previously showed that precursors of inhibitory neurons, transplanted into neonatal mice before the onset of epilepsy, prevented seizures from occurring (see Baraban et al., 2009). However, developmental factors in young brains may have helped the transplanted neurons form synapses and integrate into established neural networks, the authors surmised. Would cells differentiate and integrate in mature adult mouse brains, too?

To find out, first author Robert Hunt and colleagues isolated precursors to GABA-producing inhibitory interneurons from the medial ganglionic eminence (MGE) of embryonic mice and injected them bilaterally into hippocampi of healthy, two-month-old adults. Cells, engineered to produce green fluorescent protein for ease of tracking, migrated up to 1,500 micrometers from the injection site. Most resembled mature interneurons both morphologically and electrophysiologically, and they expressed transcription factors and genetic markers unique to GABA-producing cells. The differentiated cells also formed synapses with excitatory cells and differentiated into a diversity of GABAergic cell types.

Satisfied that these neurons successfully entered and functioned in the adult mouse brain, the researchers next wanted to know if they prevented seizures. They turned to pilocarpine, a chemical convulsant that induces epilepsy when injected systemically. Treated mice typically seize up, behave abnormally, and have no response to anti-epileptic drugs, mimicking temporal lobe epilepsy in adults (see Gröticke et al., 2007). Hunt and colleagues found that if they also injected MGE cells bilaterally into the hippocampi, the mice seized 92 percent less frequently and were calmer when handled than untreated controls. The MGE-treated animals also performed comparably to non-epileptic controls in an open field test of locomotion. They also found the hidden platform as quickly as normal mice in the Morris water maze test of spatial navigation. Stem cells did not change performance on the rotarod test of motor coordination, the elevated plus maze test of general anxiety, or the forced swim test, a measure of depression.

Though these findings are specific for epilepsy, they could have implications for many diseases linked to hyperexcitability or interneuron dysfunction, such as autism, schizophrenia, and even AD, said Jorge Palop, also at Gladstone. Michela Gallagher, Johns Hopkins University, Baltimore, Maryland, reported last year that an anti-epilepsy drug dials back hippocampal hyperactivity in people with mild cognitive impairment and improves their cognition (see ARF related news story). Huang found that interneurons are damaged in mice with the ApoE4 allele, the biggest genetic risk factor for AD (see Andrews-Zwilling et al., 2010).

Gallagher pointed out that the biggest subpopulation of interneurons that survived the graft procedure expressed somatostatin. These neurons are hit particularly hard by aging and in people carrying the ApoE4 allele, which could mean this replacement therapy would be useful for AD-related disorders, she said. “It will be important to see if these findings are constrained by the model they used,” she told Alzforum. Baraban said he plans to test these transplants in mice that model other forms of epilepsy and to figure out how the interneurons integrate into the functional circuit. Huang plans to test if transplanted MGE cells benefit AD mouse models. He also suggested testing if the transplants work in older mice and whether all or just a few subtypes of interneurons are needed to improve seizures.

“The work is very interesting and provocative. The hope for a new treatment for refractory epilepsy is exciting,” said William Mobley, University of California, San Diego. However, he noted that it is essential to remember that epilepsy is a circuit disorder. "It is possible, and even likely, that only by understanding the changes in circuit structure and function can we know whether a cell-based treatment is appropriate, how to deliver it, and how to monitor the effects on circuit and clinical function," he said. He cautioned that before this type of therapy is suitable for people, researchers will have to figure out how to define interneuron status in the hippocampus of the patient to discern what benefits might accrue. They will also need to make human-derived interneurons that are safe to inject, he added. Two papers in the May 2 Cell Stem Cell report progress on the latter front, turning human pluripotent stem cells and embryonic stem cells into MGE-like cells that functionally integrate as interneurons in rodent brains (see Nicholas et al., 2013, and Maroof et al., 2013). A recent study used these human MGE-like cells to boost cholinergic cells in mouse hippocampi and reverse memory deficits (see ARF related news story).—Gwyneth Dickey Zakaib.

Reference:
Hunt RF, Girskis KM, Rubenstein JL, Alvarez-Buylla A, Baraban SC.GABA progenitors grafted into the adult epileptic brain control seizures and abnormal behavior. Nat Neurosci. 2013 May 5. Abstract