Brain-specific BNIP-2-homology protein Caytaxin relocalises glutaminase to neurite terminals and reduces glutamate levels

Abstract

Human Cayman ataxia and mouse or rat dystonia are linked to mutations in the genes ATCAY (Atcay) that encode BNIP-H or Caytaxin, a brain-specific member of the BNIP-2 family. To explore its possible role(s) in neuronal function, we used protein precipitation and matrix-assisted laser desorption/ionisation mass spectrometry and identified kidney-type glutaminase (KGA) as a novel partner of BNIP-H. KGA converts glutamine to glutamate, which could serve as an important source of neurotransmitter. Co-immunoprecipitation with specific BNIP-H antibody confirmed that endogenous BNIP-H and KGA form a physiological complex in the brain, whereas binding studies showed that they interact with each other directly. Immunohistochemistry and in situ hybridisation revealed high BNIP-H expression in hippocampus and cerebellum, broadly overlapping with the expression pattern previously reported for KGA. Significantly, BNIP-H expression was activated in differentiating neurons of the embryonic carcinoma cell line P19 whereas its overexpression in rat pheochromocytoma PC12 cells relocalised KGA from the mitochondria to neurite terminals. It also reduced the steady-state levels of glutamate by inhibiting KGA enzyme activity. These results strongly suggest that through binding to KGA, BNIP-H could regulate glutamate synthesis at synapses during neurotransmission. Thus, loss of BNIP-H function could render glutamate excitotoxicity or/and deregulated glutamatergic activation, leading to ataxia, dystonia or other neurological disorders.

BNIP2 extra long inhibits RhoA and cellular transformation by Lbc RhoGEF via its BCH domain

“…Furthermore, the BCH domain of BPGAP1, a close homolog of Cdc42GAP/p50RhoGAP, also promotes Cdc42-dependent short pseudopodia, which are necessary for cell migration (Shang et al, 2003;Lua and Low, 2004) and Ras/MAPK activation (Lua and Low, 2005). More recently, we demonstrated that the BCH domain of the brain-specific BNIP-H/Caytaxin is involved in glutamate production and glutaminase trafficking (Buschdorf et al, 2006). These findings suggest that the BCH domain exerts an additional regulatory control over the cellular activity of their cognate small GTPases or other specific proteins.…”

Cayman ataxia protein caytaxin is transported by kinesin along neurites through binding to kinesin light chains

“…Other proteins isolated several times were RNA polymerase II polypeptide G (four clones) and Bcl-2-associated transcription factor (two clones). Considering the nuclear localization of these two proteins, it seemed implausible that they were interacting partners for caytaxin, which is a cytoplasmic protein (Buschdorf et al, 2006;Hayakawa et al, 2007). Another 17 genes were isolated only once.…”

“…So far, no biological activities have been assigned to the Nterminal regions of caytaxin, BNIP-2 and BNIP-S (Buschdorf et al, 2006;Qin et al, 2003;Zhou et al, 2005;Zhou et al, 2006). However, if the binding of caytaxin to KLCs has physiological significance, overexpression of the caytaxin N-terminal fragment might exert dominant-negative effects on the endogenous caytaxin.…”

“…This activity depends on the interaction with Cdc42. Although it is not known whether the CRAL-TRIO domain of caytaxin binds small lipophilic molecules, the caytaxin CRAL-TRIO domain has been reported to associate with a neurotransmitter-producing enzyme, kidney-type glutaminase (KGA) and the peptidyl-prolyl isomerase Pin1 in neurons (Buschdorf et al, 2006;Buschdorf et al, 2008). The physiological and etiological significance of this binding remains to be elucidated.…”

[P2–202]: Dapk1 Signaling Pathways in Stroke: From Mechanisms to Therapies

“…Many studies reported that caytaxin plays an important role in axonal transporting [65], glutamate synthesis [66], synaptic apoptosis, and neurodegenerative diseases [67]. By using coimmunoprecipitation and mass spectrum detection, our unpublished preliminary results showed that DAPK1 could also interact and phosphorylate caytaxin on serine 46 at the presynaptic area in the mice model of middle cerebral artery occlusion (MCAO).…”