Dendritic spines are the receptive contacts at most excitatory synapses in the central nervous system. Spines are dynamic in the developing brain, changing shape as they mature as well as appearing and disappearing as they make and break connections. Spines become much more stable in adulthood, and spine structure must be actively maintained to support established circuit function. At the same time, adult spines must retain some plasticity so their structure can be modified by activity and experience. As such, the regulation of spine stability and remodeling in the adult animal is critical for normal function, and disruption of these processes is associated with a variety of late onset diseases including schizophrenia and Alzheimer’s disease. The extracellular matrix (ECM), composed of a meshwork of proteins and proteoglycans, is a critical regulator of spine and synapse stability and plasticity. While the role of ECM receptors in spine regulation has been extensively studied, considerably less research has focused directly on the role of specific ECM ligands. Here, we review the evidence for a role of several brain ECM ligands and remodeling proteases in the regulation of dendritic spine and synapse formation, plasticity, and stability in adults.

Fibrolamellar carcinoma (FLC) is a rare liver cancer. FLCs uniquely produce DNAJ-PKAc, a chimeric enzyme consisting of a chaperonin-binding domain fused to the Cα subunit of protein kinase A. Biochemical analyses of clinical samples reveal that a unique property of this fusion enzyme is the ability to recruit heat shock protein 70 (Hsp70). This cellular chaperonin is frequently up-regulated in cancers. Gene-editing of mouse hepatocytes generated disease-relevant AML12DNAJ-PKAc cell lines. Further analyses indicate that the proto-oncogene A-kinase anchoring protein-Lbc is up-regulated in FLC and functions to cluster DNAJ-PKAc/Hsp70 sub-complexes with a RAF-MEK-ERK kinase module. Drug screening reveals Hsp70 and MEK inhibitor combinations that selectively block proliferation of AML12DNAJ-PKAc cells. Phosphoproteomic profiling demonstrates that DNAJ-PKAc biases the signaling landscape toward ERK activation and engages downstream kinase cascades. Thus, the oncogenic action of DNAJ-PKAc involves an acquired scaffolding function that permits recruitment of Hsp70 and mobilization of local ERK signaling.

No abstract

Figure 2. Active ABL assembles the RUNX2-TAZ transcription factor complex required for osteoblast differentiation. (A) Primary murine osteoblasts were cultured in osteogenic medium for 14 days, and phosphotyrosine immune complexes were probed with an anti-ABL antibody. WCLs were probed with the indicated antibodies for Western blot analysis. (B) HEK293T cells were cotransfected with Flag-TAZ and RUNX2, with or without ABL (PP or KD). Flag-TAZ immune complexes were probed with an anti-RUNX2 antibody. (C) Luciferase activity from a BGLAP reporter assay in HEK293T cells cotransfected with the indicated constructs. n = 3. (D and E) HEK293T cells were cotransfected with the indicated constructs, and RUNX2 (D) or ABL (E) immune complexes were probed with an anti-ABL (D) or anti-Flag (E) antibody. (F) HEK293T cells infected with shGFP or shABL were cotransfected with RUNX2 and Flag-TAZ. The nuclear compartment was extracted from the cells, and Flag-TAZ immune complexes were probed with an anti-RUNX2 antibody. (G and H) HEK293T cells were cotransfected with WT or YF RUNX2 (G) or Flag-TAZ (H), with or without ABL (PP). RUNX2 (G) or Flag-TAZ (H) immune complexes were probed with an anti-p-Tyr antibody. (I) Primary murine osteoblasts infected with shGFP or shABL were cultured in osteogenic medium. p-Tyr or RUNX2 immune complexes were probed with an anti-RUNX2 or anti-TAZ antibody. (J) Luciferase activity from a Bglap reporter assay in primary murine osteoblasts in the presence of shGFP or shABL. n = 3. (K and L) Saos-2 cells infected with an empty vector control or an FKBP-ABL-expressing retroviral vector in the presence of shGFP (K and L), shRUNX2 (K), or shTAZ (L) were cultured in growth medium containing 50 nM FK1012. Cells were stained with alizarin red S solution. n = 3. *P < 0.05, by ANOVA with a Tukey-Kramer post-hoc test. Data represent the mean ± SEM. YAP has previously been reported to be stabilized following tyrosine phosphorylation by ABL (25). In distinction, we observed that all-tyrosine-to-phenylalanine-mutant TAZ (YF) was also stabilized by active ABL (PP) (Supplemental Figure 4F), suggesting that TAZ stabilization by ABL is not mediated by tyrosine phosphorylation. TAZ is degraded following the phosphorylation of Ser311 by LATS, which primes for the phosphorylation of Ser314 by CK1ε (26). Phosphorylated Ser314 lies in the TAZ phosphodegron and triggers binding to and subsequent ubiquitylation of TAZ by the E3-ubiquitin ligase β-TrCP (26). We hypothesized that ABL suppressed TAZ ubiquitylation through disruption of the interaction between TAZ and β-TrCP and observed that ABL (PP), but not ABL (KD), diminished the interaction between these proteins ( Figure 4F). These data demonstrate that ABL stabilizes TAZ through the suppression of its ubiquitin-mediated degradation pathway initiated by β-TrCP. The Journal of Clinical Investigation R E S E A R C H A R T I C L EABL stabilizes the TAZ-TEAD complex required for osteoblast expansion. We have shown that ABL mediated stabilization of TAZ protein and asked whet...