Circulating Tumor DNA Assessment for Treatment Monitoring Adds Value to PSA in Metastatic Castration-Resistant Prostate Cancer.

PURPOSE: Enzalutamide after abiraterone progression is commonly used in metastatic castration-resistant prostate cancer despite a low rate of clinical benefit. Analyzing IMbassador250, a phase III trial assessing enzalutamide with or without atezolizumab after abiraterone, we hypothesized that baseline and early changes in circulating tumor DNA (ctDNA) tumor fraction (TF) may identify patients more likely to exhibit survival benefit from enzalutamide. EXPERIMENTAL DESIGN: ctDNA was quantified from plasma samples using a tissue-agnostic assay without buffy coat sequencing. Baseline ctDNA TF, changes in ctDNA TF from baseline to cycle 3 day 1 (C3D1), and detection at C3D1 alone were compared with overall response rate, radiographic progression-free survival (rPFS), median OS (mOS), and 50% reduction in PSA. RESULTS: ctDNA TF detection at baseline and/or C3D1 was associated with shorter rPFS and OS in 494 evaluable patients. Detection of ctDNA TF at C3D1, with or without detection at cycle 1 day 1, was associated with worse rPFS and mOS than lack of detection. When ctDNA TF and PSA response at C3D1 were discordant, patients with (ctDNA TF undetected/PSA not reduced) had more favorable outcomes than (ctDNA TF detected/PSA reduced; mOS 22.1 vs. 16 months; P < 0.001). CONCLUSIONS: In a large cohort of patients with metastatic castration-resistant prostate cancer receiving enzalutamide after abiraterone, we demonstrate the utility of a new tissue-agnostic assay for monitoring molecular response based on ctDNA TF detection and dynamics. ctDNA TF provides a minimally invasive, complementary biomarker to PSA testing and may refine personalized treatment approaches.

Growth Factors as Axon Guidance Molecules: Lessons From in vitro Studies.

Growth cones at the tips of extending axons navigate through developing organisms by probing extracellular cues, which guide them through intermediate steps and onto final synaptic target sites. Widespread focus on a few guidance cue families has historically overshadowed potentially crucial roles of less well-studied growth factors in axon guidance. In fact, recent evidence suggests that a variety of growth factors have the ability to guide axons, affecting the targeting and morphogenesis of growth cones in vitro. This review summarizes in vitro experiments identifying responses and signaling mechanisms underlying axon morphogenesis caused by underappreciated growth factors.

Familiar growth factors have diverse roles in neural network assembly.

The assembly of neuronal circuits during development depends on guidance of axonal growth cones by molecular cues deposited in their environment. While a number of families of axon guidance molecules have been identified and reviewed, important and diverse activities of traditional growth factors are emerging. Besides clear and well recognized roles in the regulation of cell division, differentiation and survival, new research shows later phase roles for a number of growth factors in promoting neuronal migration, axon guidance and synapse formation throughout the nervous system.

RHOA signaling defects result in impaired axon guidance in iPSC-derived neurons from patients with tuberous sclerosis complex.

Patients with Tuberous Sclerosis Complex (TSC) show aberrant wiring of neuronal connections formed during development which may contribute to symptoms of TSC, such as intellectual disabilities, autism, and epilepsy. Yet models examining the molecular basis for axonal guidance defects in developing human neurons have not been developed. Here, we generate human induced pluripotent stem cell (hiPSC) lines from a patient with TSC and genetically engineer counterparts and isogenic controls. By differentiating hiPSCs, we show that control neurons respond to canonical guidance cues as predicted. Conversely, neurons with heterozygous loss of TSC2 exhibit reduced responses to several repulsive cues and defective axon guidance. While TSC2 is a known key negative regulator of MTOR-dependent protein synthesis, we find that TSC2 signaled through MTOR-independent RHOA in growth cones. Our results suggest that neural network connectivity defects in patients with TSC may result from defects in RHOA-mediated regulation of cytoskeletal dynamics during neuronal development.

Environmental Elasticity Regulates Cell-type Specific RHOA Signaling and Neuritogenesis of Human Neurons.

The microenvironment of developing neurons is a dynamic landscape of both chemical and mechanical cues that regulate cell proliferation, differentiation, migration, and axon extension. While the regulatory roles of chemical ligands in neuronal morphogenesis have been described, little is known about how mechanical forces influence neurite development. Here, we tested how substratum elasticity regulates neurite development of human forebrain (hFB) neurons and human motor neurons (hMNs), two populations of neurons that naturally extend axons into distinct elastic environments. Using polyacrylamide and collagen hydrogels of varying compliance, we find that hMNs preferred rigid conditions that approximate the elasticity of muscle, whereas hFB neurons preferred softer conditions that approximate brain tissue elasticity. More stable leading-edge protrusions, increased peripheral adhesions, and elevated RHOA signaling of hMN growth cones contributed to faster neurite outgrowth on rigid substrata. Our data suggest that RHOA balances contractile and adhesive forces in response to substratum elasticity.

Division of labor in the growth cone by DSCR1.

Local protein synthesis directs growth cone turning of nascent axons, but mechanisms governing this process within compact, largely autonomous microenvironments remain poorly understood. In this issue, Wang et al. (2016. J. Cell Biol http://dx.doi.org/10.1083/jcb.201510107) demonstrate that the calcineurin regulator Down syndrome critical region 1 protein modulates both basal neurite outgrowth and growth cone turning.