The ability to silence the activity of genetically specified neurons in a temporally precise fashion would open up the ability to investigate the causal role of specific cell classes in neural computations, behaviors, and pathologies. Here we show that members of the class of light-driven outward proton pumps can mediate very powerful, safe, multiple-color silencing of neural activity. The gene archaerhodopsin-31 (Arch) from Halorubrum sodomense enables near-100% silencing of neurons in the awake brain when virally expressed in mouse cortex and illuminated with yellow light. Arch mediates currents of several hundred picoamps at low light powers, and supports neural silencing currents approaching 900 pA at light powers easily achievable in vivo. In addition, Arch spontaneously recovers from light-dependent inactivation, unlike light-driven chloride pumps that enter long-lasting inactive states in response to light. These properties of Arch are appropriate to mediate the optical silencing of significant brain volumes over behaviourally-relevant timescales. Arch function in neurons is well tolerated because pH excursions created by Arch illumination are minimized by self-limiting mechanisms to levels comparable to those mediated by channelrhodopsins2,3 or natural spike firing. To highlight how proton pump ecological and genomic diversity may support new innovation, we show that the blue-green light-drivable proton pump from the fungus Leptosphaeria maculans4 (Mac) can, when expressed in neurons, enable neural silencing by blue light, thus enabling alongside other developed reagents the potential for independent silencing of two neural populations by blue vs. red light. Light-driven proton pumps thus represent a high-performance and extremely versatile class of “optogenetic” voltage and ion modulator, which will broadly empower new neuroscientific, biological, neurological, and psychiatric investigations.

SUMMARYThe presence of dark melanin (eumelanin) within human epidermis represents one of the strongest predictors of low skin cancer risk. Topical rescue of eumelanin synthesis, previously achieved in “redhaired” Mc1r-deficient mice, demonstrated significant protection against UV damage. However, application of a topical strategy for human skin pigmentation has not been achieved, largely due to the greater barrier function of human epidermis. Salt-inducible kinase (SIK) has been demonstrated to regulate MITF, the master regulator of pigment gene expression, through its effects on CRTC and CREB activity. Here, we describe the development of small-molecule SIK inhibitors that were optimized for human skin penetration, resulting in MITF upregulation and induction of melanogenesis. When topically applied, pigment production was induced in Mc1r-deficient mice and normal human skin. These findings demonstrate a realistic pathway toward UV-independent topical modulation of human skin pigmentation, potentially impacting UV protection and skin cancer risk.

Warmth is a characteristic but nondiagnostic feature of cellulitis. We assessed the diagnostic utility of skin surface temperature in differentiating cellulitis from pseudocellulitis. Adult patients presenting to the emergency department of a large urban hospital with presumed cellulitis were enrolled. Patients were randomized to dermatology consultation (n = 40) versus standard of care (n = 32). Thermal images of affected and unaffected skin were obtained for each patient. Analysis was performed on dermatology consultation patients to establish a predictive model for cellulitis, which was then validated in the other cohort. Of those evaluated by dermatology consultation, pseudocellulitis was diagnosed in 28%. Cellulitis patients had an average maximum affected skin temperature of 34.1°C, which was 3.7°C warmer than the corresponding unaffected area (95% confidence interval = 2.7-4.8°C, P < 0.00001). Pseudocellulitis patients had an average maximum affected temperature of 31.5°C, which was 0.2°C warmer than the corresponding unaffected area (95% confidence interval = -1.1 to 1.5°C, P = 0.44). Temperature differences between sites were greater in cellulitis patients than in pseudocellulitis patients (3.7 vs. 0.2°C, P = 0.002). A logistic regression model showed that a temperature difference of 0.47°C or greater conferred a 96.6% sensitivity, 45.5% specificity, 82.4% positive predictive value, and 83.3% negative predictive value for cellulitis diagnosis. When validated in the other cohort, this model gave the correct diagnosis for 100% of patients with cellulitis and 50% of those with pseudocellulitis. A difference threshold of 0.47°C or greater between affected and unaffected skin showed an 87.5% accuracy in cellulitis diagnosis.