Based on some of these findings, Kato and Kato (2000) proposed a mitochondrial dysfunction hypothesis. In this hypothesis, mtDNA mutations or polymorphisms, either maternally inherited or secondary to mutations of nuclear genes, cause impairment of mitochondrial calcium uptake. This causes alterations in intracellular calcium signaling, which results in altered neuroplasticity, leading to recurrence of manic and depressive episodes. Altered energy metabolism may result in treatment-resistant depression.