Assessment of Sex-Related Differences of the Osteocyte Lacunar-Canalicular Network Across the Human Lifespan Using Synchrotron micro-Computed Tomography

Osteocytes, the most ubiquitous bone cell type, are responsible for bone maintenance and communication as well as mechanotransduction. Osteocytes reside in spaces within the bone matrix called lacunae, which are used as a proxy for the cells themselves in X-ray imaging. Previous studies have revealed that lacunar volume was reduced in females with increasing age, thus likely contributing to bone frailty and diseases such as osteoporosis. The implications of diseases caused by changes in the bone osteocyte lacunar-canalicular network are not yet fully understood. Since much of the past research revolved around gross bone morphology, this study investigated age and sex-related differences in the bone cellular network of the human species. Utilizing synchrotron radiation-based micro-Computed Tomography, male (n=12) and female (n=8) cortical bone specimen from the mid-diaphyses of the left femora were assessed for tissue volume (μm3), canal volume (μm3), canal surface (μm2), cortical porosity (%), canal surface to tissue volume (μm-1), canal diameter (μm), canal separation (μm), number of canals, and number of lacunae. These parameters were compared between age, sex, and the interaction between both factors. In regard to number of lacunae and their density, a statistically significant reduction was observed with age (p= 0.017) but not with sex or the interaction variable. Thus, lacunar density was reduced with age, but no significant differences between males and females were observed. Limitation in sample size prevented a more extensive result. As such, further investigation is encouraged to confirm the reduction of lacunar volume over the human lifespan

Long-latency modulation of motor cortex excitability by ipsilateral posterior inferior frontal gyrus and pre-supplementary motor area

The primary motor cortex (M1) is strongly influenced by several frontal regions. Dual-site transcranial magnetic stimulation (dsTMS) has highlighted the timing of early (<40 ms) prefrontal/premotor influences over M1. Here we used dsTMS to investigate, for the first time, longer-latency causal interactions of the posterior inferior frontal gyrus (pIFG) and pre-supplementary motor area (pre-SMA) with M1 at rest. A suprathreshold test stimulus (TS) was applied over M1 producing a motor-evoked potential (MEP) in the relaxed hand. Either a subthreshold or a suprathreshold conditioning stimulus (CS) was administered over ipsilateral pIFG/pre-SMA sites before the TS at different CS-TS inter-stimulus intervals (ISIs: 40-150 ms). Independently of intensity, CS over pIFG and pre-SMA (but not over a control site) inhibited MEPs at an ISI of 40 ms. The CS over pIFG produced a second peak of inhibition at an ISI of 150 ms. Additionally, facilitatory modulations were found at an ISI of 60 ms, with supra-but not subthreshold CS intensities. These findings suggest differential modulatory roles of pIFG and pre-SMA in M1 excitability. In particular, the pIFG-but not the pre-SMA-exerts intensity-dependent modulatory influences over M1 within the explored time window of 40-150 ms, evidencing fine-tuned control of M1 output

Regulation of Proto-Dbl through the Ubox Domain of CHIP

The Dbl protein is a product of a proto-oncogene that functions as a guanine nucleotide exchange factor for Rho-family GTPases. It is responsible for activating the GTPases by facilitating the dissociation of GDP thus allowing for the binding of GTP. Intracellular levels of Dbl are regulated by ubiquitin-mediated proteolysis. CHIP is the E3 protein-ubiquitin ligase responsible for this ubiquitination. More specifically, the Ubox domain of CHIP is critical to this interaction. Oncogenic Dbl, which lacks its spectrin domain, cannot bind CHIP and therefore escapes degradation. This causes accumulation of the oncogene product in the cell, and leads to persistent activation of its downstream pathways

Regulation of Proto-Oncogenic Dbl by Chaperone-Controlled, Ubiquitin-Mediated Degradation

The dbl proto-oncogene product is a prototype of a growing family of guanine nucleotide exchange factors (GEFs) that stimulate the activation of small GTP-binding proteins from the Rho family. Mutations that result in the loss of proto-Dbl's amino terminus produce a variant with constitutive GEF activity and high oncogenic potential. Here, we show that proto-Dbl is a short-lived protein that is kept at low levels in cells by efficient ubiquitination and degradation. The cellular fate of proto-Dbl is regulated by interactions with the chaperones Hsc70 and Hsp90 and the protein-ubiquitin ligase CHIP, and these interactions are mediated by the spectrin domain of proto-Dbl. We show that CHIP is the E3 ligase responsible for ubiquitination and proteasomal degradation of proto-Dbl, while Hsp90 functions to stabilize the protein. Onco-Dbl, lacking the spectrin homology domain, cannot bind these regulators and therefore accumulates in cells at high levels, leading to persistent stimulation of its downstream signaling pathways