Specific Replacements of Pyruvate for Trophic Support of Central and Peripheral Nervous System Neurons

When embryonic central nervous system neurons are seeded at low densities with Eagle's basal medium supplemented with the serum substitute N1, glucose, and glutamine, neuronal survival for even 24 h requires the additional supply of exogenous pyruvate–and so does the survival of many peripheral nervous system neurons. Pyruvate can be replaced by α‐ketoglutarate or oxaloacetate, but not by Krebs cycle substrates that are not keto acids. Most other α‐keto acids tested (though not β‐ or γ‐keto acids) also mimic pyruvate. The apparent equivalence to pyruvate of all these compounds includes identical ED50 values (300 μM for embryonic avian forebrain neurons, 30–40 μM for rat hippocampal neurons), and also identical susceptibilities to the pyruvate‐sparing effects of other low‐molecular‐weight agents present in Dulbecco's modified Eagle's medium or in astroglia conditioned medium. The substitute α‐keto acids, however—unlike pyruvate, α‐ketoglutarate, or oxaloacetate—support cell survival only in the presence of α‐amino acids that transaminate to α‐ketoglutarate, oxaloacetate, or pyruvate. The α‐keto acids, therefore, operate as acceptors of amino groups from appropriate donors to generate Krebs cycle‐relevant substrates. Consistent with this view, [14C]glutamate did not generate appreciable 14CO2 unless accompanied by a suitable α‐keto acid. Thus: (1) the true survival‐controlling agents for these cultured CNS neurons are pyruvate, α‐ketoglutarate, and oxaloacetate; (2) other α‐keto acids provide for neuronal support only insofar as they make it possible for exogenously supplied α‐amino acid precursors to generate intracellularly one of the three critical agents; and (3) the inability of CNS neurons to utilize glucose for their survival resides with some restriction of substrate access to the Krebs cycle.