The actin filament is a fundamental part of the cytoskeleton defining cell morphology and regulating various physiological processes, including filopodia formation and dendritic spinogenesis of neurons. Serine/threonine-protein kinase Pak4, an essential effector, links Rho GTPases to control actin polymerization. Previously, we identified the Inka2 gene, a novel mammalian protein exhibiting sequence similarity to Inka1, which serves as a possible inhibitor for Pak4. Although Inka2 is dominantly expressed in the nervous system and involved in focal-adhesion dynamics, its molecular role remains unclear. Here, we found that Inka2-iBox directly binds to Pak4 catalytic domain to suppress actin polymerization. Inka2 promoted actin depolymerization and inhibited the formation of cellular protrusion caused by Pak4 activation. We further generated the conditional knockout mice of the Inka2 gene. The beta-galactosidase reporter indicated the preferential Inka2 expression in the dorsal forebrain neurons. Cortical pyramidal neurons of Inka2-/- mice exhibited decreased density and aberrant morphology of dendritic spines with marked activation/phosphorylation of downstream molecules of Pak4 signal cascade, including LIMK and cofilin. These results uncovered the unexpected function of endogenous Pak4 inhibitor in neurons. Unlike Inka1, Inka2 is a critical mediator for actin reorganization required for dendritic spine development.

Engagement of neural stem/progenitor cells (NSPCs) into proper neuronal differentiation requires the spatiotemporally regulated generation of metabolites. Purines are essential building blocks for many signaling molecules. Enzymes that catalyze de novo purine synthesis are assembled as a huge multienzyme complex called "purinosome." However, there is no evidence of the formation or physiological function of the purinosome in the brain. Here, we showed that a signal transduction ATPases with numerous domains (STAND) protein, NACHT and WD repeat domain-containing 1 (Nwd1), interacted with Paics, a purine-synthesizing enzyme, to regulate purinosome assembly in NSPCs. Altered Nwd1 expression affected purinosome formation and induced the mitotic exit and premature differentiation of NSPCs, repressing neuronal migration and periventricular heterotopia. Overexpression/knockdown of Paics or Fgams, other purinosome enzymes, in the developing brain resulted in a phenocopy of Nwd1 defects. These findings indicate that strict regulation of purinosome assembly/disassembly is crucial for maintaining NSPCs and corticogenesis.

Cell migration is essential for many physiological and pathological processes, including embryonic development, wound healing, immune response and cancer metastasis. Inka2 transcripts are observed in migrating cells during embryonic development, suggesting the involvement of inka2 in cell migration. However, its precise role remains unclear. Here, we found that inka2 controlled focal adhesion dynamics and cell migration, likely by regulating protein phosphatase-2A (PP2A) function. A scratch assay revealed that inka2 shRNA-transfected NIH3T3 cells showed rapid wound closure, indicating an inhibitory effect by inka2 on cell migration. Live-cell imaging of NIH3T3 cells expressing EGFP-paxillin using total internal reflection fluorescence microscopy revealed that inka2 knockdown increased the turnover rate of focal adhesions. Given that PP2A, which consists of catalytic (C), regulatory (B) and scaffolding (A) subunits, is known to regulate focal adhesions, we examined the inka2-PP2A interaction. Immunoprecipitation revealed an association between inka2 and the PP2A C subunit. Binding of Inka2 to the C subunit prevented the association between the A and C subunits, suggesting that inka2 can inhibit PP2A function. Furthermore, both inka2 expression and PP2A inhibition decreased focal adhesion kinase-paxillin interaction, resulting in reduced formation of focal adhesions. We assessed the effect of pharmacological PP2A inhibition on the inka2 knockdown-induced increase in cell migration speed and found that treatment with a PP2A inhibitor negated the accelerated migration of inka2 knockdown cells. These results suggest that inka2 knockdown exerts its effects through PP2A-dependent regulation of focal adhesions. Our findings contribute to a better understanding of the molecular mechanisms underlying cell migration.

The orchestrated events required during brain development, as well as the maintenance of adult neuronal plasticity, highly depend on the accurate responses of neuronal cells to various cellular stress or environmental stimuli. Recent studies have defined a previously unrecognized, broad class of multidomain proteins, designated as signal transduction ATPases with numerous domains (STAND), which comprises a large number of proteins, including the apoptotic peptidase activating factor 1 (Apaf1) and nucleotide-binding oligomerization domain-like receptors (NLRs), central players in cell death and innate immune responses, respectively. Although the involvement of STANDs in the central nervous system (CNS) has been postulated in terms of neuronal development and function, it remains largely unclear. Here, we identified Nwd1 (NACHT and WD repeat domain-containing protein 1), as a novel STAND protein, expressed in neural stem/progenitor cells (NSPCs). Structurally, Nwd1 was most analogous to the apoptosis regulator Apaf1, also involved in mitosis and axonal outgrowth regulation in the CNS. Using a specific antibody, we show that, during the embryonic and postnatal period, Nwd1 is expressed in nestin-positive NSPCs in vivo and in vitro, while postnatally it is found in terminally differentiated neurons and blood vessels. At the subcellular level, we demonstrate that Nwd1 is preferentially located in the cytosolic compartment of cultured NSPCs, partially overlapping with cytochrome c. These observations imply that Nwd1 might be involved in the neuronal lineage as a new STAND gene, including having a pro-apoptotic or nonapoptotic role, similar to Apaf1.

正常な脳発達においてプリン代謝産物の産生量の制御は不可欠である。近年、プリン新生のための連続的な酵素反応は、巨大タンパク質複合体プリノソームで行われることが示された。プリノソームの形成は、細胞の増殖に必須の役割を持つと推定されるが、プリノソーム形成の分子メカニズムは不明である。申請者が新規同定したNwd１は神経幹細胞で豊富に発現し、プリノソーム形成に関与することで増殖を制御する。本研究ではプリノソーム形成におけるNwd1の役割を明らかにするため、Nwd1と相互作用するタンパク質群の同定を試みた。プルダウンアッセイとLC-MS解析の結果から、Nwd1と相互作用する因子群の同定に成功した。