BDNF signaling: Harnessing stress to battle mood disorder

The link between the onset of major depressive disorder (MDD) and loss of neurotrophins in the brain is of interest to clinicians and basic scientists. MDD is caused by a combination of genetic, environmental, and psychological factors. Trauma, chronic health problems, and substance abuse are risks (1), as are grief and other purely emotional/cognitive stresses (2, 3). MDD alters the expression of neurotrophins, such as brain-derived neurotrophic factor (BDNF). BDNF is required for neuronal development, survival, and plasticity (4, 5). Brain imaging has shown volumetric changes in limbic regions in depression attributed either to reduced numbers of glia and pyramidal neurons or to their reduced cell body size, accompanied by atrophy of pyramidal neuron apical dendrites and decreases in neurogenesis in dentate gyrus (6). These structural alterations most likely contribute to features of depression, including cognitive impairment, helplessness, and anhedonia. Neuronal dysfunction also affects activation of the hypothalamic–pituitary–adrenal axis (7). Studies show that structural and functional neuronal alterations in MDD are partially rescued by antidepressant treatment. Antidepressants, in addition to their effects on neurotransmitter levels, also increase BDNF and enhance expression of the receptor for BDNF, tropomyosin-related kinase B (TrkB), in the hippocampus (8). However, the intracellular mechanisms governing the relationship between BDNF, structural changes in limbic system cells, and clinical manifestations of MDD are still unclear.

In PNAS, Marshall et al. (9) demonstrate that Gαi1 and Gαi3, members of the GαI subclass of heterotrimeric G proteins, are essential for BDNF/TrkB signaling in hippocampus and are down-regulated by chronic stress. G proteins form membrane-associated heterotrimers consisting of α, β, and γ subunits (10). There are four main subclasses: Gs, Gi/o, Gq, and G12/13. GαI belongs to the Gi/o subclass and inhibits adenylyl cyclase. Marshall et al. report that reductions in Gαi1 and Gαi3 levels affect BDNF-induced TrkB endocytosis and activation of signaling pathways downstream of TrkB, resulting in depressive behaviors. In mouse embryonic fibroblasts, they show that the double knockout (DKO) of Gαi1 and Gαi3 results in inhibition of BDNF-induced activation of Akt–mTORC1 and ERK pathways. shRNA-mediated down-regulation of Gαi1 and Gαi3 also affects dendritic outgrowth and formation of dendritic spines in the hippocampus. Specific behavioral studies performed by the Marshall team reveal that shRNA knockdown of Gαi1 and Gαi3 or complete DKO cause depression-like behaviors. Their studies suggest that downstream BDNF signaling via Gαi1 and Gαi3 is necessary not only for sustaining the well-being of neurons but also for normal antidepressive behaviors (11). So, how does neurotrophin signaling inside the cell specifically contribute to the regulation of mental functioning?

Neurotrophins, a unique family of polypeptide growth factors that include BDNF, regulate proliferation, proper differentiation, survival, and death of neuronal and nonneuronal cells. Neurotrophins act through downstream signaling cascades following receptor activation. Brain neurotrophins regulate early prenatal brain development and adult central nervous system plasticity (4). BDNF is the predominant neurotrophin in the brain (12). Synthesized as a precursor proBDNF, it is cleaved to release the mature, active form (11). Mature BDNF binds preferentially to the TrkB receptor, activating downstream signaling pathways such as mitogen-activated protein kinase (MAPK), phospholipase Cγ, and phosphatidylinositol-3 kinase pathway. These signaling cascades regulate transcription and dendritic translation of proteins required for neuronal survival, differentiation, and learning and memory formation in the hippocampus (13, 14).

Upon elimination of Gαi1 and Gαi3, Marshall et al. also observed decreased dendritic branching and reduced numbers of synaptic spines. Substantial loss of CA1 pyramidal neurons was observed in the Gαi1/Gαi3 DKO mouse, which was not found when Gαi1 and Gαi3 were depleted in adolescent brain using shRNA, although this latter model produced alterations in synaptic structure and the same behavioral deficits as the DKO. It is not clear whether loss of neurons in the DKO mouse hippocampus was caused by loss of developmental targeting of neurons or by enhanced cell death during neurogenesis or synaptogenesis. Whether longer overexpression of Gαi1/Gαi3 shRNA and/or a more challenging stress paradigm might also induce neuronal death remains to be answered, but it nevertheless seems clear that negative synaptic changes are sufficient to produce the depressive features.

Chronic stress substantially lowers BDNF (15). The insult chosen by Marshall et al. to cause depression in the animals, chronic mild stress, consists of 3 wk of sequential forced swim, restraint, water and food deprivation, housing in wet sawdust, light/dark cycle reversal, and housing in constant illumination or darkness (16, 17). In normal rodents, BDNF mRNA levels increase during early postnatal days in cortex, hippocampus, and cerebellum, correlating with the peak of postnatal synaptogenesis (18). BDNF is also required in adulthood for proper brain functioning, supporting neuronal network strengthening and plasticity. Although most BDNF homozygous knockout mice die in the first 2 postnatal days, some survive for 2–4 wk (19). These surviving mice nevertheless show severe impairments in growth and in movement coordination, and have reduced numbers of cranial and spinal sensory neurons, albeit with no gross structural abnormalities of the hippocampus (19, 20). They do, however, show impaired hippocampal long-term potentiation (LTP) and reduced numbers of presynaptic vesicles docked at presynaptic active zones (21), suggesting dysfunctional synapses. Apart from BDNF binding to the TrkB receptor, the precleaved form of BDNF, proBDNF, can act as a signaling molecule in the opposite manner to mature BDNF. ProBDNF binds specifically to the p75 neurotrophin receptor (p75^NTR), a member of the tumor necrosis factor superfamily (22). Activation of p75^NTR decreases the complexity of neuronal projections, triggers apoptosis, and facilitates hippocampal long-term depression, negatively affecting learning and memory formation (Fig. 1) (4, 23). Perhaps the structural changes in the Gαi1- and Gαi3-depleted and the DKO mice, including loss of pyramidal neurons in the hippocampus, might be partially attributed to the actions of proBDNF; the observed synaptic deficiencies even in the absence of cell death might be related to apoptotic signaling in the synapses alone (24, 25).

Fig. 1.

BDNF binding to TrkB receptor activates signaling cascades responsible for neuronal survival and synaptic plasticity. ProBDNF binds p75^NTR and triggers long-term depression and apoptosis. Upon stress, including high-frequency synaptic stimulation, Bcl-2 is recruited to mitochondria to decrease the production of reactive oxygen species (ROS) and enhance effective ATP production; this supports relevant gene transcription and protein translation. Mild acute stress or high frequency synaptic activity recruit and may require BDNF signaling during learning and memory formation and for antidepressant coping strategies.

Stress also regulates intracellular signaling apart from its specific effects on BDNF (Fig. 1). Interestingly, brief stress-induced release of glucocorticoids, at low doses, supports neuronal branching and survival and positively influences performance in spatial learning and memory tasks (26, 27), implicating beneficial changes in limbic system networks. Under these conditions, corticosterone improves mitochondrial membrane potential and oxidation. The glucocorticoid receptor binds to the antiapoptotic B-cell lymphoma 2 protein (Bcl-2). This complex translocates into mitochondria to reduce the production of damaging reactive oxygen species (ROS) and prevent the opening of the prodeath permeability transition pore (28) while enhancing the supply of ATP required for proper brain function. The regulation of mitochondrial ATP production by low-dose corticosteroids may thereby shape the proper response of neurons to external stress. In contrast, high levels or chronic stress produce the opposite effects on mitochondria, depolarizing them and predisposing to activation of apoptotic mechanisms (28).

Neuronal excitability and synaptic plasticity require high mitochondrial fidelity (29, 30). Energy-dependent activities include calcium clearance in a timely manner, rapid actin cytoskeletal rearrangements, trafficking of synaptic vesicles, resetting of ion gradients after action potential firing, and new protein phosphorylation in response to synaptic stimulation (31). These events, crucial for neuroplasticity, are regulated by the timing and availability of mitochondrial energy supply. In cultured hippocampal neurons, BDNF treatment causes accumulation of mitochondria at presynaptic sites (32) where mitochondria contribute to synaptic vesicle docking and reserve pool galvanization (33–35). Specific phosphorylation events may relate to mitochondrial ATP production during hippocampal synaptic plasticity downstream of NMDA receptor stimulation. For example, phosphorylation of eukaryotic elongation factor 2 (eEF2) within the first few minutes after stimulation results in decreased overall protein synthesis (36), followed by exquisitely timed eEF2 dephosphorylation and increased translation of specific synaptic proteins. In summary (Fig. 1), the high-frequency stimulation that produces LTP is a stress to neurons and mitochondria that is alleviated by intracellular events that produce synaptic strengthening. Analogously, gene transcription and translation of specific proteins in response to synaptic activity in the limbic system may be necessary for the proper response to stress to prevent depression and enhance positive cognitive coping strategies. Loss of activation of growth factor-supported signaling pathways contributes to faulty responses to neuronal activity and stress.