Huntington's disease is caused by an expanded CAG repeat in the gene encoding huntingtin (HTT), resulting in loss of striatal and cortical neurons. Given that the gene product is widely expressed, it remains unclear why neurons are selectively targeted. Here we show the relationship between synaptic and extrasynaptic activity, inclusion formation of mutant huntingtin protein (mtHtt) and neuronal survival. Synaptic N-methyl-D-aspartate-type glutamate receptor (NMDAR) activity induces mtHtt inclusions via a T complex-1 (TCP-1) ring complex (TRiC)-dependent mechanism, rendering neurons more resistant to mtHtt-mediated cell death. In contrast, stimulation of extrasynaptic NMDARs increases the vulnerability of mtHtt-containing neurons to cell death by impairing the neuroprotective cyclic AMP response element-binding protein (CREB)-peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) cascade and increasing the level of the small guanine nucleotide-binding protein Rhes, which is known to sumoylate and disaggregate mtHtt. Treatment of transgenic mice expressing a yeast artificial chromosome containing 128 CAG repeats (YAC128) with low-dose memantine blocks extrasynaptic (but not synaptic) NMDARs and ameliorates neuropathological and behavioral manifestations. By contrast, high-dose memantine, which blocks both extrasynaptic and synaptic NMDAR activity, decreases neuronal inclusions and worsens these outcomes. Our findings offer a rational therapeutic approach for protecting susceptible neurons in Huntington's disease.

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Targeting amino acid metabolism has therapeutic implications for aggressive brain tumors. Asparagine is an amino acid that is synthesized by normal cells. However, some cancer cells lack asparagine synthetase (ASNS), the key enzyme for asparagine synthesis. Asparaginase (ASNase) contributes to eradication of acute leukemia by decreasing asparagine levels in serum and cerebrospinal fluid. However, leukemic cells may become ASNase-resistant by upregulating ASNS. High expression of ASNS has also been associated with biologic aggressiveness of other cancers, including gliomas. Here, the impact of enzymatic depletion of asparagine on proliferation of brain tumor cells was determined. ASNase was used as monotherapy or in combination with conventional chemotherapeutic agents. Viability assays for ASNase-treated cells demonstrated significant growth reduction in multiple cell lines. This effect was reversed by glutamine in a dose-dependent manner--as expected, because glutamine is the main amino group donor for asparagine synthesis. ASNase treatment also reduced sphere formation by medulloblastoma and primary glioblastoma cells. ASNase-resistant glioblastoma cells exhibited elevated levels of ASNS mRNA. ASNase cotreatment significantly enhanced gemcitabine or etoposide cytotoxicity against glioblastoma cells. Xenograft tumors in vivo showed no significant response to ASNase monotherapy and little response to temozolomide alone. However, combinatorial therapy with ASNase and temozolomide resulted in significant growth suppression for an extended duration of time. Taken together, these findings indicate that amino acid depletion warrants further investigation as adjunctive therapy for brain tumors.

Implications

Findings have potential impact for providing adjuvant means to enhance brain tumor chemotherapy.