Energy supply per neuron is constrained by capillary density in the mouse brain

Neuronal densities vary enormously across sites within a brain. Does the density of the capillary bed accompany the presumably larger energy requirement of sites with more neurons, or with larger neurons, or is energy supply constrained by a mostly homogeneous capillary bed? Here we find evidence for the latter across various sites in the mouse brain and show that as a result, the ratio of capillary cells per neuron, and thus presumably blood and energy supply per neuron, decreases uniformly with increasing neuronal density and therefore smaller average neuronal size across sites. Additionally, we find that local capillary density is also not correlated with local synapse densities, although there is a small but significant correlation between lower neuronal density (and therefore larger neuronal size) and more synapses per neuron within the restricted range of 6,500-9,500 across cortical sites. Further, local variations in the glial/neuron ratio are also not correlated with local variations in number of synapses per neuron or local synaptic densities. These findings suggest that it is not that larger neurons, neurons with more synapses, or even sites with more synapses demand more energy, but simply that larger (and thus fewer) neurons have more energy available per cell, and to its synapses as a whole, than smaller (and thus more numerous) neurons due to competition for limited resources supplied by a capillary bed of fairly homogeneous density throughout the brain.Significance StatementThe brain is an expensive organ and at rest already uses nearly as much energy as during sensory activation. To ultimately determine whether the high energy cost of the brain is driven by an unusually high energy demand by neurons or constrained by capillary density in the organ, we examine whether sites in the mouse brain with more neurons, larger neurons, or more synapses have more capillary supply, and find instead that capillary density is mostly homogeneous across brain structures. We propose that neurons are constrained to using what energy is available, with little evidence for adjustments according to local demand, which explains its high risk of ischemia and vulnerability to states of compromised metabolism, including normal aging.

Reduced neuronal size and mTOR pathway activity in the Mecp2 A140V Rett syndrome mouse model

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutation in the X-linked MECP2 gene, encoding methyl-CpG-binding protein 2. We have created a mouse model (Mecp2 A140V “knock-in” mutant) expressing the recurrent human MECP2 A140V mutation linked to an X-linked mental retardation/Rett syndrome phenotype. Morphological analyses focused on quantifying soma and nucleus size were performed on primary hippocampus and cerebellum granule neuron (CGN) cultures from mutant (Mecp2A140V/y) and wild type (Mecp2+/y) male mice. Cultured hippocampus and cerebellar granule neurons from mutant animals were significantly smaller than neurons from wild type animals. We also examined soma size in hippocampus neurons from individual female transgenic mice that express both a mutant (maternal allele) and a wild type Mecp2 gene linked to an eGFP transgene (paternal allele). In cultures from such doubly heterozygous female mice, the size of neurons expressing the mutant (A140V) allele also showed a significant reduction compared to neurons expressing wild type MeCP2, supporting a cell-autonomous role for MeCP2 in neuronal development. IGF-1 (insulin growth factor-1) treatment of neuronal cells from Mecp2 mutant mice rescued the soma size phenotype. We also found that Mecp2 mutation leads to down-regulation of the mTOR signaling pathway, known to be involved in neuronal size regulation. Our results suggest that i) reduced neuronal size is an important in vitro cellular phenotype of Mecp2 mutation in mice, and ii) MeCP2 might play a critical role in the maintenance of neuronal structure by modulation of the mTOR pathway. The definition of a quantifiable cellular phenotype supports using neuronal size as a biomarker in the development of a high-throughput, in vitro assay to screen for compounds that rescue small neuronal phenotype (“phenotypic assay”).