Wanted, dead and alive: Why a multidisciplinary approach is needed to unlock hair treatment potential.

Human recorded history is littered with attempts to improve the perceived appearance of scalp hair. Throughout history, treatments have included both biological and chemical interventions. Hair "quality" or "perceived appearance" is regulated by multiple biological intervention opportunities: adding more hairs by flipping follicles from telogen to anagen, or delaying anagen follicles transiting into catagen; altering hair "apparent amount" by modulating shaft diameter or shape; or, in principle, altering shaft physical properties changing its synthesis. By far the most common biological intervention strategy today is to increase the number of hairs, but to date this has proven difficult and has yielded minimal benefits. Chemical intervention primarily consists of active material surface deposition to improve shaft shine, fibre-fibre interactions and strength. Real, perceptible benefits will best be achieved by combining opportunity areas across the three primary sciences: biology, chemistry and physics. Shaft biogenesis begins with biology: proliferation in the germinative matrix, then crossing "Auber's Critical Line" and ceasing proliferation to synthesize shaft components. Biogenesis then shifts to oxidative chemistry, where previously synthesized components are organized and cross-linked into a shaft. We herein term the crossing point from biology to chemistry as "The Orwin Threshold." Historically, hair biology and chemistry have been conducted in different fields, with biological manipulation residing in biomedical communities and hair shaft chemistry and physics within the consumer care industry, with minimal cross-fertilization. Detailed understanding of hair shaft biogenesis should enable identification of factors necessary for optimum hair shaft production and new intervention opportunities.

Hair Shaft Formation and Mitochondrial Bioenergetics: Combining Biology, Chemistry, and Physics.

Research into biological manipulation of hair "quality" has ebbed and waned but today is in a resurgence. Hair appearance is regulated by multiple intervention opportunities-adding more hairs; increasing hair "amount" by modulating shaft diameter or shape; or, in principle, by altering shaft physical properties by changing its synthesis. It is likely that improved benefits may be achieved by combining multiple areas-minimizing follicle loss and miniaturization, maximizing shaft production, and treating the existing shaft. A previously overlooked opportunity is follicle metabolism: building "better" hairs. Hair production is energy intensive, and it is known that follicle metabolism influences shaft diameter. Multiphoton microscopy enables metabolic investigation of live, growing, human, hair follicles. This allows definition of multiple "zones" with vastly different metabolism: proliferation-where keratinocytes proliferate and migrate into specialized layers; production-proliferation ceases, and synthesis and patterning begin; construction and elongation-the structural framework is seeded and cells extend to create the nascent fiber; and maturation-gradual hardening and transformation into mature shaft. Recent investigations into the transition from construction to maturation reinforce this as a key developmental threshold, where shaft production transforms from a biologically driven into a biochemically driven process. We now name this "Orwin's transition."

Isolation of a Cyclic (Alkyl)(amino)germylene.

A 1,4-addition of a dichlorogermylene dioxane complex with α,ß-unsaturated imine 1 gave a dichlorogermane derivative 2 bearing a GeC3N five-membered ring skeleton. By reducing 2 with KC8, cyclic (alkyl)(amino)germylene 3 was synthesized and fully characterized. Germylene 3 readily reacted with TEMPO, N2O and S8, producing the 1:2 adduct 4, the oxo-bridged dimer 5 and the sulfido-bridged dimer 6, respectively.

A role for Rab23 in the trafficking of Kif17 to the primary cilium.

The small GTPase Rab23 is an antagonist of sonic hedgehog (Shh) signaling during mouse development. Given that modulation of Shh signaling depends on the normal functioning of the primary cilium, and overexpression of Evi5L, a putative Rab23 GTPase-activating protein (GAP), leads to reduced ciliogenesis, Rab23 could have a role at the primary cilium. Here, we found that wild-type Rab23 and the constitutively active Rab23 Q68L mutant were enriched at the primary cilium. Therefore, we tested the role of Rab23 in the ciliary targeting of known cargoes and found that ciliary localization of the kinesin-2 motor protein Kif17 was disrupted in Rab23-depleted cells. Co-immunoprecipitation and affinity-binding studies revealed that Rab23 exists in a complex with Kif17 and importin ß2 (the putative Kif17 ciliary import carrier), implying that Kif17 needs to bind to regulatory proteins like Rab23 for its ciliary transport. Although a ciliary-cytoplasmic gradient of nuclear Ran is necessary to regulate the ciliary transport of Kif17, Rab23 and Ran appear to have differing roles in regulating the ciliary entry of Kif17. Our findings have uncovered a hitherto unknown effector of Rab23 and demonstrate how Rab23 could mediate the transport of Kif17 to the primary cilium.

Getting into the cilia: nature of the barrier(s).

The primary cilium that protrudes from the plasma membrane of many eukaryotic cell types is very much a cellular organelle in its own right. Its unique membrane and luminal composition is effectively compartmentalized by diffusion barrier at its base, known as the transition zone. Recent works have now shed light on the molecular components of this diffusion barrier, and revealed intriguing functional similarities with other better characterized cellular barriers.

The Evi5 family in cellular physiology and pathology.

The Ecotropic viral integration site 5 (Evi5) and Evi5-like (Evi5L) belong to a small subfamily of the Tre-2/Bub2/Cdc16 (TBC) domain-containing proteins with enigmatically divergent roles as modulators of cell cycle progression, cytokinesis, and cellular membrane traffic. First recognized as a potential oncogene and a cell cycle regulator, Evi5 acts as a GTPase Activating Protein (GAP) for Rab11 in cytokinesis. On the other hand, its homologue Evi5L has Rab-GAP activity towards Rab10 as well as Rab23, and has been implicated in primary cilia formation. Recent genetic susceptibility analysis points to Evi5 as an important factor in susceptibility to multiple sclerosis. We discuss below the myriad of cellular functions exhibited by the Evi5 family members, and their associations with disease conditions.

Intercellular organelle trafficking by membranous nanotube connections: a possible new role in cellular rejuvenation?

Cells could make actin-based filopodial extensions that connect up with other cells. Such close-ended, actin-based filopodial bridges, or cytonemes, have been observed during developmental and pathological processes. On the other hand, tunneling nanotubes (TNTs) form conduits with open ends that allow transfer of cytoplasmic materials and organelles between cells. The past years have witnessed the description of TNTs in multiple cell types, with a range of interesting physiological and pathophysiological activities. Some hints of the molecular components that drive their formations are now emerging. Recent work has further suggested that TNTs could be important in regenerative transfer of large cellular components, including organelles such as mitochondria and lysosomes between senescing and younger cell types (at least for cells in culture). These findings have intriguing implications in cell biology and regenerative medicine.

Non-classical membrane trafficking processes galore.

Dogmatic views of how proteins and other cellular components may traffic within and between eukaryotic cells have been challenged in the past few years. Beyond the classical secretory/exocytic pathway and its established players, other pathways of cell surface membrane transport, generally termed "unconventional secretion," are now better understood. More insights have also been gleaned on the roles of secreted or shedding microvesicles, either exosomal or ectosomal in origin, in unconventional secretion. Recent works have also revealed key molecular components, particularly the Golgi reassembly stacking protein (GRASP), and the importance of stress-induced autophagy, in unconventional exocytic transport. This GRASP and autophagy-dependent (GAD) mode appears to underlie the unconventional exocytosis of many soluble and membrane cargoes. Likewise, recent findings have revealed transport processes that contrast the classically known mitochondria import, namely vesicular transport from the mitochondria to peroxisomes and lysosomes. Mitochondria-peroxisomal targeting of mitochondria-derived vesicles appears to involve the retromer complex, which was classically associated with endosome-Golgi membrane traffic. The routes of intracellular membrane transport and communications between eukaryotic organelles now appear far more complex that one would have imagined 10 years ago.

Rabs and other small GTPases in ciliary transport.

The non-motile primary cilium is a single, microtubule-based hair-like projection that emanates from most, if not all, non-dividing mammalian cells. Enriched in a variety of signalling receptors and accessories, the cilium mediates crucial sensory and regulatory functions during development and postnatal tissue homoeostasis. Maintenance of ciliary morphology and function requires continuous IFT (intraflagellar transport), and recent findings have shed light on some molecular details of how ciliogenesis is dependent on targeted exocytic membrane trafficking from the Golgi. The ARL [Arf (ADP ribosylation factor)-related] small GTPase Arf4 functions in TGN (trans-Golgi network) sorting of cilia-targeted rhodopsin into carrier vesicles, while Arl6 (Arf-like 6) and Arl13b regulate aspects of ciliary transport and IFT. Ciliogenesis and ciliary functions are also regulated by small Rabs. Rab8a, in conjunction with Rab11a, and via its interaction with a multitude of proteins associated with the ciliary basal body and axoneme/membrane, appears to be critical for ciliogenesis. Rab8's close homologue Rab10 may also play a ciliogenic role in some cells. Rab23, the depletion or inactivation of which affects cilia formation, may regulate specific ciliary protein targeting and turnover, particularly those involved in Shh (Sonic hedgehog) signalling. Recent findings have also implicated Ran, a small GTPase better known for nuclear import, in ciliary targeting of the KIF17 motor protein. We highlight and discuss recent findings on how Rabs and other small GTPases mediate ciliogenesis and ciliary traffic.

Rab35--a vesicular traffic-regulating small GTPase with actin modulating roles.

The Rab family of GTPases are regulators of eukaryotic vesicular membrane traffic, while modulation of actin dynamics is a function conventionally associated with the Rho family of GTPases. Rab35 is a Rab protein with both plasma membrane and endosomal localization, and has been implicated in diverse processes that include T-cell receptor recycling, oocyte yolk protein recycling and cytokinesis. Rab35 regulates neurite outgrowth in neuronal-like cells, and can induce protrusions even in typically non-adherent Jurkat T-cells. Recent evidence indicates that Rab35's activity, particularly the ability to mediate protrusive outgrowths, is due to its direct influence on actin dynamics. This can occur via activation of the Rho family of GTPases, or through the engagement of its effector fascin, an actin bundling protein.