The physiological role of TrkB.T1 is still unknown. Here we examined the consequences of TrkB.T1 deletion in mouse development as well as the role of the endogenous TrkB.T1 receptor on BDNF signaling in vivo. We found that loss of TrkB.T1 led to increased anxiety related behavior that is associated with structural alterations in neurites of neurons of the amygdala. Moreover, we show that reducing TrkB.T1 levels in vivo partially rescues the phenotypes caused by loss of one BDNF allele.
Truncated TrkB.T1 receptors were first described almost twenty years ago but to date very little is known about their physiological role in neurotrophin signaling and development. One of the major obstacles in identifying these functions has been the absence of a suitable animal model lacking only this receptor isoform. Previous animal models have been generated targeting either the full-length isoform or all TrkB isoforms (Klein et al., 1993
; Rohrer et al., 1999
). TrkB.FL receptor exerts strong pro-survival functions in neurons and, consequently, its deletion causes extreme phenotypes, which have made it impossible to evaluate any long-term functional roles the truncated TrkB isoform might have. Recently, we targeted the TrkB locus to specifically delete the TrkB.T1 isoform without affecting the level or the spatiotemporal pattern of expression of TrkB.FL (Dorsey et al., 2006
). These animals are viable and fertile and no obvious phenotype has been detected by simple observation. The lack of strong developmental phenotypes in our model supports the idea that the predominant role of TrkB.T1 is not to support neuronal survival. Instead it might be involved in the regulation of BDNF signaling and in the differentiation and/or function of neurons. Alternatively, it suggests that other truncated TrkB (e.g TrkB.T2) or TrkC receptors may compensate for TrkB.T1 deficiency. However, we did not find any up-regulation of other truncated TrkB or TrkC receptor isoform in either neurons nor glia lacking TrkB.T1. Moreover, truncated TrkC receptors are present at a significantly lower level than TrkB.T1 suggesting that, at least at the expression level other truncated Trk receptors may not be able to compensate for the loss of the most expressed of all truncated Trk receptors [(Dorsey et al., 2006
) and data not shown].
A role for TrkB.T1 in regulating BDNF signaling was first proposed when it was cloned and such a function has been supported by its pattern of expression and a number of in vitro and in vivo overexpression experiments (Klein et al., 1990
; Middlemas et al., 1991
; Biffo et al., 1995
; Eide et al., 1996
; Saarelainen et al., 2000b
; Haapasalo et al., 2001
). However, whether physiological levels of this receptor isoform could exert such activity has never been definitively proven. Our study, by showing partial rescue of BDNF haploinsufficiency by TrkB.T1 deletion proves that physiologically TrkB.T1 indeed limits BDNF signaling in vivo (). A key question is why would an organism require a negative modulator of BDNF signaling such as TrkB.T1? A number of studies have suggested that excessive BDNF is involved in the pathogenesis of epilepsy, mania and autism [reviewed in (Tsai, 2007
)]. Moreover, it has been shown that TrkB.FL is required for epileptogenesis in the kindling model and that zinc, a metal abundantly present in the brain can transactivate synaptic TrkB by a neuronal activity-regulated mechanism (He et al., 2004
; Huang et al., 2008
). Thus, while TrkB.FL signaling is important for synaptic plasticity, it appears that excessive activation of this receptor could be one of the causes leading to hyperexcitability of specific brain areas which in turn could cause epilepsy. The finding that physiological TrkB.T1 limits BDNF signaling in vivo suggests that TrkB.T1 may be part of a mechanism critical to prevent pathological activation of the TrkB.FL. It will be of interest to investigate whether TrkB.T1 may represent an important buffer to prevent overactivation of TrkB.FL during neuronal activity.
Alternatively, the primary function of TrkB.T1 could be the modulation of other cellular functions independent of the TrkB kinase receptor (Baxter et al., 1997
; Rose et al., 2003
; Ohira et al., 2005
). For example, TrkB.T1 has been reported to regulate astrocytic morphology by directly interacting with Rho GDP dissociation inhibitor 1 and modulate calcium release from intracellular stores in astrocytes (Rose et al., 2003
; Ohira et al., 2005
). However, the lack of a more dramatic phenotype in this model also indicates that TrkB.T1 does not have a critical widespread function in CNS as might be suggested by its potential role in astroglia calcium homeostasis (Reichardt, 2003
TrkB.FL and TrkB.T1 expression are tightly regulated during development. While TrkB.FL is the highest expressed isoform in early CNS development, TrkB.T1 is dramatically upregulated during post-natal brain development (Allendoerfer et al., 1994
; Escandon et al., 1994
; Fryer et al., 1996
). The reason for this tight regulation of TrkB receptor isoforms expression is unknown. In addition to the above-discussed role in the control of TrkB.FL activation, it has been suggested that TrkB.T1 and TrkB.FL can regulate distinct modes of dendritic growth in visual cortical neurons. Specifically, TrkB.FL promotes the addition of short branches in dendritic regions proximal to the cell body whereas TrkB.T1 induces the extension of dendrites in regions more distal to the soma. These data suggest that expression of the correct set of TrkB isoforms is essential for normal dendritic development (Yacoubian and Lo, 2000
). Indeed, we found that TrkB.T1 deficiency does affect neurite complexity, as well as dendrite length of neurons of the amygdala. Although we can not assess whether this effect is caused by a dominant/negative inhibition of TrkB.FL or by a different mechanism, these data provide definitive evidence that in certain neuronal populations physiological TrkB.T1 is important in regulating neuronal branching. This effect is not widespread since in contrast with the amygdala, our behavioral and structural analysis of the hippocampus has not shown any change so far, suggesting that there are different regional TrkB.T1 requirements during neuronal development.
TrkB.T1 is also present at cortical glutamatergic synapses together with TrkB.FL suggesting that it may play a role in synaptic plasticity (Gomes et al., 2006
). Surprisingly, we have so far failed to detect any clear electrophysiological abnormality in the hippocampus, a region whose neurons express both full-length and TrkB.T1 receptors. Lack of an effect on induction or maintenance of LTP has been reported also in transgenic mice overexpressing TrkB.T1 in the cortex and the hippocampus suggesting that TrkB.T1 is not limiting for the induction of LTP (Saarelainen et al., 2000a
). A previous study had shown LTP inhibition in hippocampal slices overexpressing TrkB.T1 delivered by adenoviral infection but it has been suggested that different levels of expression at synapses were responsible for the discrepancy (Li et al., 1998
) (Saarelainen et al., 2000a
). Nevertheless, our data strongly indicates that, at least in young animals, endogenous TrkB.T1 is not limiting the induction of LTP.
In conclusion, we have shown that truncated TrkB.T1 receptor is indeed an important regulator of BDNF signaling in vivo, it is involved in the control of complex behaviors and it affects neurite complexity in the amygdala. Further analysis of aged animals and the employment of paradigms to challenge these mutants will help to shed further light into other physiological roles of TrkB.T1, the highest expressed TrkB isoform in the mature mammalian brain.