Mechanisms of Cdc42 Polarization in Yeast

Polarization is important for the function and morphology of many different cell types. The keys regulators of polarity in eukaryotes are the Rho-family GTPases. In the budding yeast Saccharomyces cerevisiae, which must polarize in order to bud and to mate, the master regulator is the highly conserved Rho GTPase, Cdc42. During polarity establishment, active Cdc42 accumulates at a site on the plasma membrane characterizing the “front” of the cell where the bud will emerge. The orientation of polarization is guided by upstream cues that dictate the site of Cdc42 clustering. However, in the absence of upstream cues, yeast can still polarize in a random direction during symmetry breaking. Symmetry breaking suggests cells possess an autocatalytic polarization mechanism that can amplify stochastic fluctuations of polarity proteins through a positive feedback mechanism.

Two different positive feedback mechanisms have been proposed to polarize Cdc42 in budding yeast. One model posits that Cdc42 activation must be localized to a site at the plasma membrane. Another model posits that Cdc42 delivery must be localized to a particular site at the plasma membrane. Although both mechanisms could work in parallel to polarize Cdc42, it is unclear which mechanism is critical to polarity establishment. We directly tested the predictions of the two positive feedback models using genetics and live microscopy. We found that localized Cdc42 activation is necessary for polarity establishment.

While this explains how active Cdc42 localizes to a particular site at the plasma membrane, it does not address how Cdc42 concentrates at that site. Several different mechanisms have been proposed to concentrate Cdc42. The GDI can extract Cdc42 from membranes and selective mobilize GDP-Cdc42 in the cytoplasm. It was proposed that selectively mobilizing GDP-Cdc42 in combination with local activation could locally concentrate total Cdc42 at the polarity site. Although the GDI is important for rapid Cdc42 accumulation at the polarity site, it is not essential to Cdc42 concentration. It was proposed that delivery of Cdc42 by actin-mediated vesicle can act as a backup pathway to concentrate Cdc42. However, we found no evidence for an actin-dependent concentrating pathway. Live microscopy experiments reveal that prenylated proteins are not restricted to membranes, and can enter the cytoplasm. We found that the GDI-independent concentrating pathway still requires Cdc42 to exchange between the plasma membrane and the cytoplasm, which is supported by computational modeling. In the absence of the GDI, we found that Cdc42 GAP became essential for polarization. We propose that the GAP limits GTP-Cdc42 leak into the cytoplasm, which would be prohibitive to Cdc42 polarization.

Screening for Sexual Trauma, Intimate Partner Violence and Mental Health Symptoms among HIV Positive South African Women

Background: The psychological sequelae of sexual trauma and physical intimate partner violence (IPV) exposure can lead to poor HIV care outcomes, including poor treatment adherence. This study aimed to estimate the prevalence of and factors associated with mental health symptoms and trauma among HIV positive women. Additionally, the study aimed to assess the feasibility and acceptability of screening for trauma and mental health symptoms among HIV positive South African women. Finally, the study aimed to elicit healthcare workers’ perceptions related to sexual trauma and the provision of care and services for HIV positive women with trauma histories.

Methods: The study utilized a mixed-methods approach that included a cross-sectional survey of 70 HIV positive women recruited through referral sampling and key informant interviews with seven healthcare workers (HCWs). A study-screening instrument consisting of 24 items from standard measures was used to screen women for sexual trauma, physical intimate partner violence (IPV), depression and PTSD. Sexual trauma and IPV were assessed across the lifetime, while depression and PTSD were current assessments. Logistic regression models were used to explore the relationship between trauma exposure and mental health symptoms, while controlling for age and education. Interview transcripts were coded and analyzed for emergent themes on HCWs perceptions on sexual trauma and HIV care.

Results: Among participants, 51% had sexual trauma experience and 75% had intimate partner violence (IPV) experience. Among participants, 36% met screening criteria for major depression; among those with traumatic experiences (n=57), 70% met screening criteria for post-traumatic stress disorder (PTSD). Compared to having no sexual trauma or IPV exposure, having both sexual trauma and IPV was significantly associated with higher odds of depression (OR = 8.11; 95% CI 1.48-44.34), while having either IPV or sexual trauma individually was not significantly associated with increased odds of depression. Compared to having either IPV or sexual trauma, having both sexual trauma and IPV was not significantly associated with PTSD. Responses from participants’ feedback on screening process suggest that screening was feasible and acceptable to participants. Some of the health care workers (HCWs) did not perceive dealing with trauma to be part of their duties, but instead viewed social workers or psychologists as the appropriate health cadre to provide care related to trauma and mental health.

Conclusions: High levels of sexual trauma, IPV and mental health distress were reported among HIV positive women in this setting. Screening for trauma and mental health symptoms was acceptable to the participants, but several challenges were encountered in implementing screening. Given the potential impact of trauma and mental health on HIV care engagement, interventions to address trauma and its psychological sequelae are needed.