Neurons

The relationship between neuronal function and energy metabolism is a field of intense inquiry and while bioenergetic per se are well understood, we lack a good understanding of the ways in which these mechanisms overcome the challenges presented by the unique morphology of neurons and their volatile energy demands. Here we examined the extent to which these challenges can be met through strategic mitochondrial placement and the support of a phosphagen system.
We examined fluctuations in energy demand of Drosophila larval motor neurons utilizing a combination of computational modeling and empirical analysis, and uncovered a neglected aspect of cellular energy metabolism that appears to accommodate the stress of highly volatile energy demands. Our findings highlight a reliance on the phosphagen system to buffer against rapid changes in the rate of ATP consumption induced by burst firing. The knockdown of a key element in the phosphagen system of invertebrates, arginine kinase, revealed a suppression of the mitochondrial proton motive force, and a more rapid decline in the presynaptic ATP/ADP ratio during burst firing. The knock down of arginine kinase also revealed metabolic shifts that indicated a compensatory increase in glycolysis, but, surprisingly, few consequences for either presynaptic Ca2+ handling or neurotransmission. In a final effort to ensure that we were imposing a metabolic load adequate to challenge these motor neurons, we developed an ex vivo calcium clearance assay and in vivo locomotor performance assay – currently in their final stages of validation.

Olfactory Granule cells (GCs) are a population of inhibitory interneurons
responsible for maintaining normal olfactory bulb (OB) function and circuitry. Through
dendrodendritic synapses with the OBs projection neurons, the GCs regulate information
sent to the olfactory cortices. Throughout adulthood, GCs continue to integrate into the OB
and contribute to olfactory circuitry. However, only ~50% will integrate and survive longterm.
Factors aiding in the survival and morphological development of these neurons are
still being explored. The neurotrophin brain-derived neurotrophic factor (BDNF) aids in
the survival and dendritic spine maturation/maintenance in several populations of CNS
neurons. Investigators show that increasing BDNF in the adult-rodent SVZ stimulates
proliferation and increases numbers of new OB GCs. However, attempts to replicate these
experiments failed to find that BDNF affects proliferation or survival of adult-born granule
cells (abGCs). BDNFs regulation of dendritic spines in the CNS is well characterized. In
the OB, absence of BDNF’s receptor on abGCs hinders normal spine development and demonstrates a role for BDNF /TrkB signaling in abGCs development. In this study, we
use transgenic mice over-expressing endogenous BDNF in the OB (TgBDNF) to determine
how sustained increased in BDNF affect the morphology of olfactory GCs and the survival
and development of abGCs. Using protein assays, we discovered that TgBDNF mice have
higher BDNF protein levels in their OB. We employed a Golgi-cox staining technique to
show that increased BDNF expression leads to an increase in dendritic spines, mainly the
mature, headed-type spine on OB GCs. With cell birth-dating using 5-bromo-2’-
deoxyuridine (BrdU), immunofluorescent cell markers, TUNEL staining and confocal
microscopy, we demonstrate that over-expression of BDNF in the OB does not increase
survival of abGCs or reduce cell death in the GC population. Using virally labeled abGCs,
we concluded that abGCs in TgBDNF mice had similar integration patterns compared to
wild-type (WT) mice, but maintained increases in apical headed-type spine density from
12 to 60 days PI. The evidence combined demonstrates that although increased BDNF does
not promote cell survival, BDNF modifies GC morphology and abGC development
through its regulation of dendritic spine development, maturation and maintenance in vivo.