Abstract
Increasing evidence suggests that synapse dysfunctions are a major determinant of several neurodevelopmental and neurodegenerative diseases. Here we identify protein kinase N1 (PKN1) as a novel key player in fine-tuning the balance between axonal outgrowth and presynaptic differentiation in the parallel fiber–forming (PF-forming) cerebellar granule cells (Cgcs). Postnatal Pkn1–/– animals showed a defective PF–Purkinje cell (PF-PC) synapse formation. In vitro, Pkn1–/– Cgcs exhibited deregulated axonal outgrowth, elevated AKT phosphorylation, and higher levels of neuronal differentiation-2 (NeuroD2), a transcription factor preventing presynaptic maturation. Concomitantly, Pkn1–/– Cgcs had a reduced density of presynaptic sites. By inhibiting AKT with MK-2206 and siRNA-mediated knockdown, we found that AKT hyperactivation is responsible for the elongated axons, higher NeuroD2 levels, and reduced density of presynaptic specifications in Pkn1–/– Cgcs. In line with our in vitro data, Pkn1–/– mice showed AKT hyperactivation, elevated NeuroD2 levels, and reduced expression of PF-PC synaptic markers during stages of PF maturation in vivo. The long-term effect of Pkn1 knockout was further seen in cerebellar atrophy and mild ataxia. In summary, our results demonstrate that PKN1 functions as a developmentally active gatekeeper of AKT activity, thereby fine-tuning axonal outgrowth and presynaptic differentiation of Cgcs and subsequently the correct PF-PC synapse formation.
Highlights
Protein kinase N1 (PKN1/PRK1) is the most abundantly expressed isoform of the PKN family in the central nervous system and accounts for 0.01% of total brain protein [1]
Considering the high expression levels of protein kinase N1 (PKN1) in cerebellar granule cells (Cgcs) and Purkinje cells (PCs) [4], we investigated the effect of Pkn1 deletion on the formation of parallel fibers (PFs)–Purkinje cell (PF-PC) as well as climbing fibers (CFs)-PC synapses during cerebellar development
There were no differences between WT and Pkn1–/– animals in vesicular glutamate transporter 2 (VGlut2)-stained CF terminals at postnatal day 8 (P8), where they were mainly found around the PC somata (Figure 1, A and B)
Summary
Protein kinase N1 (PKN1/PRK1) is the most abundantly expressed isoform of the PKN family in the central nervous system and accounts for 0.01% of total brain protein [1] It is widely studied for its involvement in cancer [2]; surprisingly little is known about the brain-specific function of this kinase, even though it was first isolated from human hippocampal cDNA in 1994 [3] and is enriched in certain brain areas [4]. Activation of PKN1 is achieved by caspase-3–mediated cleavage, resulting in a constitutively active protein product missing the regulatory N-terminus [9]. This form of deregulated PKN1 activation occurs during apoptosis [10] and has been linked to various
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
